Hippocampal CA1 Interneurons: an in Vivo Intracellular Labeling Study

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The Journal of Neuroscience, October 1995, 75(10): 6651-6665

Hippocampal CA1 Interneurons: An in vivo Intracellular Labeling Study

Attila Sik,a Markku Penttonen,b Aarne Ylinen,” and Gyiirgy Buzsiki Center for Molecular and Behavioral Neuroscience, Rutgers, The State University of New Jersey, Newark, New Jersey 07102

Fast spiking interneurons in the CA1 area of the dorsal hip- bition, axonal tree, network, parvalbumin, calbindin, calre- pocampus were recorded from and filled with biocytin in tinin, synapses, GA BA,, GABAJ anesthetized rats. The full extent of their dendrites and axonal arborizations as well as their calcium binding protein Although considerableknowledge has accumulated about the content were examined. Based on the spatial extent of connectivity and major molecular and channelproperties of hip- axon collaterals, local circuit cells (basket and 0-LM neu- pocampal principal cells (Tamamaki et al., 1988; Amaral and rons) and long-range cells (bistratified, trilaminar, and Witter, 1989; Lopes da Silva et al., 1990; Tamamaki and Nojyo, backprojection neurons) could be distinguished. Basket 1990, 1991; Traub and Miles, 1991; Seeburg, 1993; Li et al., cells were immunoreactive for parvalbumin and their axon 1994), it is becomingclear that their network interactionsin real collaterals were confined to the pyramidal layer. A single brains cannot be understoodwithout knowledge of their inhibi- basket cell contacted more than 1500 pyramidal neurons tory interneuronal control. Traditionally, interneuronshave been and 60 other parvalbumin-positive interneurons. Commis- believed to contact neuronswithin a local region of the cortex, sural stimulation directly discharged basket cells, followed in contrast to principal cells, which sendtheir axons to distant by an early and late IPSPs, indicating interneuronal inhi- regionsof the brain (Ramon y Cajal, 1911; Nicoll, 1994). Recent bition of basket cells. The dendrites of another local circuit works suggest,however, that there are several groups of hip- neuron (0-LM) were confined to stratum oriens and it had pocampal inhibitory cells with different circuit, physiological a small but high-density axonal terminal field in stratum and biochemicalproperties, and hypothesized functions (Struble lacunosum-moleculare. The fastest firing cell of all inter- et al., 1978; Alonso and Kohler, 1982; Somogyi et al., 1983, neurons was a calbindin-immunoreactive bistratified neu- 1984; Misgeld and Frotscher, 1986; Ribak et al., 1986; Michel- ron with axonal targets in stratum oriens and radiatum. son and Wong, 1991; Miettinen et al., 1992; T&h and Freund, Two neurons with their cell bodies in the alveus innervated 1992; Toth et al., 1993; Sik et al., 1994a). Interneuronal groups the CA3 region (backprojection cells), in addition to rich may receive different local and extrinsic inputs, possessvarious axon collaterals in the CA1 region. The trilaminar interneu- intrinsic molecularand functional properties, and target different ron had axon collaterals in strata radiatum, oriens and pyr- somatodendriticsegments of the principal cells (Somogyi et al., amidale with its dendrites confined to stratum oriens. Com- 1983; Freund et al., 1990; Gulyas et al., 1993b; McBain and missural stimulation evoked an early EPSP-IPSP-late de- Dingledine, 1993; Miles and Poncer, 1993; Buhl et al., 1994; polarizing potential sequence in this cell. All interneurons Miles et al., 1994; Sik et al., 1994a,b). Although interneurons formed symmetric synapses with their targets at the elec- numerically representa minority in the hippocampus,they can tron microscopic level. These findings indicate that inter- regulate various aspectsof the operational modes of principal neurons with distinct axonal targets have differential func- cells through their widespreadand strategically wired axon col- tions in shaping the physiological patterns of the CA1 net- lateral systems. work. To date, the variety of the anatomicalclasses of interneurons [Key words: hippocampus, interneurons, biocytin, inhi- may be contrasted to the paucity of information about their dis- tinctive physiological roles. Recent observations suggest that Received Mar. 23, 1995; revised June 9, 1995; accepted June 15, 1995. distinct classesof interneuronsmay be involved in the induction We thank Drs. T. E Freund, C. J. McBain, 1. Mody, F! A. Schwartzkroin, P. and maintenanceof network oscillations in the hippocampusin Somogyi, I. SoltBsz, R. D. Traub, and X.-J. Wang for their comments on the the theta, gamma(40-100 Hz) and ultrafast (200 Hz) frequency manuscript. We also thank K. G. Baimbridge and J. J. Rogers for their gifts of antibodies. This work was supported by NIH (34994), HFSP, the Whitehall ranges (Soltesz and Deschenes, 1993; Buzsaki and Chrobak, Foundation, the Finnish Academy of Sciences, and the University of Jyvlskyll. 1995; Bragin et al., 1995; Wittington et al., 1995; Ylinen et al., A.Y. and M.P. were visiting scholars at Rutgers University, supported by the 1995a,b) as well as in the regulation of dendritic plasticity and Finnish Academy of Sciences. Correspondence should be addressed to Gyiirgy BuzsBki, Center for Molec- fast neuronal transmission(Traub et al., 1994). Identification of ular and Behavioral Neuroscience, Rutgers University, 197 University Avenue, the various subgroupsof interneuronsis a prerequisitefor study- Newark, NJ 07102. ing their selective, behavior-dependentcontrol of hippocampal “Permanent address: Institute of Experimental Medicine, Hungarian Acade- my of Sciences, Budapest, Hungary. networks and for understandingthe functional consequencesof “Permanent address: Department of Psychology, University of Jyv%skyll, their impairment in disease.Furthermore, quantitative data on Jyv?iskyl& Finland. cPermanent address: Department of Neurology, University of Kuopio, Kuo- the connectivity of single neuronsis an absoluteprerequisite for pio, Finland. meaningful computational models of cortical function. In this Copyright 0 1995 Society for Neuroscience 0270-6474/95/156651-15$05.00/O study we recorded from interneurons of the hippocampalCA1 6652 Sik et al. - Hippocampal CA1 Interneurons region of the intact brain, filled them with the tracer biocytin, known distribution of chemically different subgroups of interneurons. The second antiserum was anti-rabbit IgG conjugated with fluorescein and reconstructed their whole dendritic and axonal arbors, and (FITC). Cell bodies and neuronal processes in the vicinity of the intra- in some cases identified their calcium binding protein contents cellularly labeled cell were photographed and videotaped. In case of a as well as their synaptic targets. negative result, the section was immunoreacted against another calcium binding protein. In the last step, these sections were also developed for Materials and Methods biocytin (DAB-Ni) and FITC-labeled videoframes were compared for possible overlap with the intracellularly filled cell. The antisera have One hundred and eighty-three Sprague-Dawley (250-350 gm) rats were been extensively tested for specificity by the producers: rabbit anti- anesthetized with urethane (1.3-1.5 g/kg) and placed in a stereotaxic calbindin D28K (R202; Baimbridge and Miller, 1982), rabbit anti-par- apparatus. Other information obtained on this animal group have been valbumin (R301; Bairnbridge and Miller, 1982), and rabbit anti-calre- published (Li et al., 1994; Sik et al., 1994; Ylinen et al., 1995a,b). The tinin (Rogers, 1989). Additional naive rats were used to count the num- body temperature of the rat was kept constant by a small animal ther- ber of parvalbumin-immunoreactive (four rats) neurons in the different moregulation device. The scalp was removed and a small (1.2 X 0.8 layers of the CA1 region. These numbers were used to calculate con- mm) bone window was drilled above the hippocampus (anteromedial vergence of thesecell typesfrom the ratio of immunolabeledcells and edge at AP = 3.3 and L = 2.2 mm from bregma) for extra- and intra- pyramidal cells and the information obtained from the intracellularly cellular recordings. The cistema magna was opened and the cerebro- labeled cells using the formula: N,R X D = total number of links = N, spinal fluid was drained to decrease pulsation of the brain. A pair of X C, where N,R, numberof immunoreactivecells; N,, numberof py- stimulating electrodes (100 km each, with 0.5 mm tip separation) was ramidal neurons in the same volume, D, divergence, and C, conver- inserted into the right fimbria-fornix (AP = 1.3, L = 1.0, V = 4.1) to gence. The absolute number of pyramidal cells in a given section was stimulate the commissural inputs. Extracellular recording electrodes calculated by multiplying the number of parvalbumin-immunoreactive (three 20 pm insulated tungsten wires) were inserted at the medial edge cells by the ratio of parvalbumin-immunoreative interneurons and py- of the bone window. The uppermost wire was placed into the CA1 ramidal cells (Aika et al., 1994). pyramidal layer and the remaining two wires aimed at the hippocampal Visualization of parvalbumin-immunoreactive target interneurons. fissure and the hilus, respectively. After the intracellular recording elec- The DAB-Ni stained sections were incubated in rabbit anti-parvalbumin trode was inserted into the brain, the bone window was covered by a (1: 1500 ) antiserum for 2 d. The second antiserum (overnight) was anti- mixture of paraffin and paraffin oil in order to prevent drying of the rabbit IgG (1:50, ICN), and the third layer was rabbit peroxidase anti- brain and decrease pulsation. The distance of the intracellular and ex- peroxidase complex (DAKOPATTS, 1: 100, overnight). The second im- tracellular electrodes was 0.5-1.5 mm in the anteroposterior and O.O- munoperoxidase reaction was developed with DAB alone, giving a 0.5 mm in the lateral directions. brown reaction product. During the entire ABC staining and immuno- Micropipettes for intracellular recordings were pulled from 2.0 mm cytochemical procedure 50 mM Tris-buffered saline (TBS, pH 7.4) con- diameter capillary glass. They were filled with 1 M potassium acetate taining 1% normal goat serum was used for washing and for dilution in 50 mM Tris buffer, containing also 3% biocytin for intracellular la- of the antisera. For light microscopic preparations, all the solutions con- beling. In vivo electrode impedances varied from 60 to 100 MR Once tained 0.5% Triton X-100 to enhance the penetration of antibodies. stable intracellular recordings were obtained, evoked and passive phys- Axon tracing. Sections were viewed at 40X magnification and axon iological properties of the cell were determined. Field activity recorded collaterals were traced with the aid of a drawing tube (Sik et al., 1993). through the extracellular electrode was filtered between 1 Hz and 5 kHz. Potential contact points with immunolabeled postsynaptic cells in dou- The intracellular signal from the amplifier (Axoclamp-2B) was digitized ble-labeled sections were marked on the drawings and reexamined with both as a DC signal and after further amplification (5X) and filtering oil immersion (100X). Interbouton intervals were also measured with (0.1 Hz to 5 kHz). The direct and amplified intracellular activity and oil immersion in different layers. Random samples of boutons and bou- the extracellular field/unit were digitized at 10 kHz with 12 bit precision ton contacts with neurons were examined with an electron microscope. (ISC-16 board, RC Electronics). The data were stored on optical disks. The length of the axon collaterals were measured from the paper trac- Spontaneous membrane oscillations were examined in the frequency ings with a digitizing table and the two-dimensional axon length was domain by calculating the fast Fourier transform of frequency (Buzs&i calculated for each coronal section. These data were then used to de- et al., 1983). scribe the axon length distribution in the septo-temporal axis, relative After the completion of the physiological data collection or when the to the location of the cell bodv (Figs. 2B. 5B, 6B, and 7B). The axon membrane potential of the recorded neuron began to deteriorate at any length was divided by the aveiage kterbouton int&val to calculate the phase of the experiment, biocytin was injected through a bridge circuit total number of boutons and the number of boutons per section. (Axoclamp-2B) using 300 msec depolarizing pulses at 2-5 nA at 1 Hz Preparation for electron microscopy. For electron microscopy, sec- for 8 to 60 min. Neuronal activity was followed throughout the proce- tions were treated with 1% 0~0, for 1 hr, dehydrated in ethanol and dure. Interneurons with incomplete penetration or shorter than 8 min propylene oxide, counterstained with uranil-acetate, and embedded in periods of biocytin injection are not included in this study, since these Durcupan. Areas innervated by biocytin-labeled axons were selected short injections never resulted in complete labeling of the axonal tree and reembedded for ultrathin sectioning. Serial ultrathin sections were of the interneurons or other cells. After 2-12 hr postinjection survival cut and mounted on single-slot Formvar-coated copper grids (Sigma, times the animals were given an urethane overdose and then perfused St. Louis, MO). The ultrathin sections were counterstained by lead- intracardially with 100 ml physiological saline followed by 400 ml of citrate and examined by a Philips CM 10 electron microscope. 4% paraformaldehyde, 0.1% glutaraldehyde, and 15% saturated picric acid dissolved in 0.1 M phosphate buffer (pH = 7.3). The brains were Results then removed and stored in the fixative solution overnight. Thick (60 or 100 pm) coronal sections were cut and processed for light and elec- Our active searchfor interneurons(nonpyramidal cells) was con- tron microscopy. fined to the alveus/stratumoriens and stratum pyramidale of the Double labeling of intracellularly filled cells. A three-step procedure CA1 region. In accordance with the anatomical distribution of was used for double labeling of biocytin labeled cells to avoid nonspe- cific crossreaction of antibodies. Every third section was washed several the interneurons(Aika et al., 1994), most of the cells penetrated times in 0.1 M PB, immersed in cryoprotective solution (25% sucrose, were pyramidal cells. Interneuron recordings were identified by 10% glycerol in 0.01 M PB), freeze thawed in liquid nitrogen, and their distinct physiological characteristics, including fast spon- washed apain in several changes of 0.1 M PB, before incubated in ABC taneousactivity (1O-400 Hz), short-duration action potentials(< solution 2 hr to overnight).-The peroxidase reaction was developed with ammonium-nickel sulfate-intensified 3,3’-diaminobenzidine (DAB- 0.5 msec at 50% amplitude), pronounced spike afterhyperpolar- Ni) as a chromogene, to produce a deep blue to black end product. ization after spontaneousaction potentials and sustainedhigh After microscopic examination of the already stained sections, the po- frequency firing in responseto depolarizing intracellular current sition of the soma and/or main dendrites could be predicted from the pulses (Schwartzkroin and Mathers, 1978; Kawaguchi and identified dendrites. Next, the neighboring sections were immunostained with one to three antibodies against parvalbumin, calretinin, or calbin- Hama, 1987a; Lacaille and Schwartzkroin 1988a,b). The input din. Antibody selection was based on location, physiological properties, resistanceof the interneurons (range: 30 to 90 Ma) was larger spine density, and axonal arbor of the labeled interneuron and the than in pyramidal cells recorded under the sameconditions (20 The Journal of Neuroscience, October 1995, 75(10) 6653

+ 3.0 Ma, Li et al., 1994). On the basis of histological identi- der between stratum oriens and stratum pyramidale. The contacts fication, six cells were basket cells, two interneurons arborized formed by the biocytin-filled and parvalbumin double-labeled primarily in the basal and apical dendritic layers, two neurons basket cell (UR32) on other parvalbumin-positive targets were innervated exclusively the distal apical dendrites of pyramidal investigated using 100X oil-immersion objective. When contacts cells, and the remaining two neurons innervated cells beyond were identified closely to each other in neighboring sections, the CA1 region (backprojection interneurons). part of the dendritic arborization of the PV-positive target neuron was reconstructed. This was necessary in only one case; thus, Basket cells the error from this source is negligible. Four light microscopi- Anatomical properties. Two of the six basket cells were only cally identified contacts were examined by correlated electron partially filled as evidenced by the gradual fading and disap- microscopy. In all four cases, electron microscopy revealed sym- pearance of distal axon collaterals. Nevertheless, these cells ful- metric synapses between biocytin-filled presynaptic terminals filled the histological criteria of basket neurons, since their axon and parvalbumin-immunoreactive postsynaptic somata (Fig. arborization was visualized in sufficient detail to conclude that lC,D) or dendrites. Overall, 99 boutons in contact with 64 par- areas other than the pyramidal layer were not innervated. The valbumin-immunoreactive neurons were counted (Fig. 2). Thir- somata of all basket cells were located in or close to the CA1 ty-five contacts were on somata and the remaining ones on thick pyramidal layer. Dendrites were aspiny or covered with a few proximal dendrites. The majority of the targets were contacted spines with long necks, arborized in strata oriens, radiatum and by a single bouton, whereas 13 neurons received two to four lacunosum-moleculare. In contrast to chandelier cells (Somogyi boutons from the biocytin filled cell. For each section a ratio et al., 1983; Li et al., 1992), dendritic arborization of basket (probability) was obtained by dividing the number of contacted cells in stratum radiatum was considerably larger than in lacu- parvalbumin cells by the total number of parvalbumin-immu- nosum-moleculare. noreactive neurons in the area innervated by the axon collaterals. The complete axon arborizations of the basket neurons was A similar calculation was made for the pyramidal cells (Fig. 2, reconstructed in the hippocampus from 19 (UR32), 29 (UR80B), right panel). These plots indicate that in the vicinity of the filled 18 (M63), and 13 (M69) 60 p,rn sections. The majority of the cell body 40 to 60% of the potential targets were contacted but axon collaterals were found in stratum pyramidale with very few this value decreased to 10 to 20% at the periphery of the inner- collaterals entering into either the stratum radiatum or stratum vation zone. Overall, these findings indicate that the basket cells oriens. The total three-dimensional axon lengths for these cells innervate parvalbumin-immunoreactive and pyramidal cell tar- were: 40,491 pm (UR32), 53,516 pm (UR80B), 48,982 pm gets with equal probability, although pyramidal cells are inner- (M63), and 41,714 pm (M69), respectively. The axonal arbori- vated by more boutons than basket and chandelier cells. Whether zation covered a circular or slightly elliptical area of the pyramidal the numerically less number of boutons in target intemeurons cell layer occupying 1140 by 1154 p,m* (UR32), 1740 by 1154 translates to smaller size IPSPs than in pyramidal cells should pm2 (URSOB), 1080 by 940 Frn* (M63), and 780 by 920 km* be determined by double recording of cell pairs. (M69). The septo-temporal and medio-lateral directions of axon Physiological properties. Although the general physiological collateral distributions, relative to the cell body, were symmetrical. properties of the interneuron cell types were similar, some no- On the basis of bouton density, measured in random samples at table differences were also observed. Basket cells had smaller different distances from the soma (22.6 ? 3.91100 km, n = 50), amplitude afterhyperpolarizations (6.5 mV IT 1.8 SE) than other the total number of bouton in these cells were 9151 (UR32), intemeurons (range: 11 to 17 mV, p < 0.02; Student’s t test). 12,095 (UR80B), 11,070 (M63), and 9427 (M69). Bouton density In addition, accommodation of current-induced action potentials was independent from the distance of the axon collateral from the was more pronounced in basket cells than in the other cells. cell body. Since the majority of the basket cells targets are py- Indeed, spike accommodation observed in basket cells was often ramidal cells and form an average of six synapses on each of their comparable to that seen in CA1 pyramidal neurons. targets (Gulyas et al., 1993a; Buhl et al., 1994), we estimate that The threshold for evoking an action potential by commissural basket cells innervate between 1500 and 2000 pyramidal cells stimulation was always lower in basket cells than in pyramidal (divergence). Because in the pyramidal layer 1 out of 50 neurons cells recorded in the same experiment or the threshold for elic- are immunoreactive for parvalbumin (basket cells and chandelier iting a population spike. At intensities suprathreshold to evoking cells; Aika et al., 1994) and because the divergence of chandelier a population spike basket cells fired two to four action potentials cells is similar to that of the basket cells (Li et al., 1992), the of decreasing amplitude, which emanated from a large depolar- present findings indicate that 30 to 40 parvalbumin-immunoreac- izing potential. The first action potential always preceded the tive cells converge on a single CA1 pyramidal neuron. extracellular population spike (Fig. 3A), similar to putative bas- Three basket cells were examined for the presence of parval- ket neurons recorded in extracellular studies (Buz&ki and Ei- bumin in their somata and dendrites by double staining. Two of delberg, 1982). This initial burst was followed by early and late these were also tested for calretinin, and one of the two also for hyperpolarizing potentials associated with the cessation of firing. calbindin (UR32). All three cells showed immunoreactivity for The duration of this suppression and the amplitude of the early parvalbumin in both the soma and dendrites (Fig. 1). Often, even (15-25 msec to peak) and late (150-250 msec to peak) IPSPs axon collaterals could be clearly identified in corresponding correlated with the intensity of stimulation (Fig. 30). The am- FITC and DAB-Ni sections. None of the basket cells was im- plitude of the early IPSP reached its amplitude maximum (10 munoreactive for calretinin and calbindin. mV) with relatively low stimulus intensity. The late IPSP be- Parvalbumin-positive targets of basket cells. In the CA1 re- came obvious only at higher stimulation intensities but with gion, parvalbumin-immunoreactive cell bodies were found in stronger stimulation the amplitude of the late IPSP was much strata oriens and pyramidale, with radially running dendrites that bigger than that of the early one (Fig. 30). The early and late span all layers. Axon terminals of parvalbumin-positive cells IPSPs could also be differentiated by varying the membrane were largely restricted to the stratum pyramidale and to the bor- potential (Fig. 3B,C). Reversal of the early IPSP occurred at 6664 Sik et al. l Hippocampal CA1 interneurons

Figure 1. Basket cells are parvalbumin immunoreactive and innervate other parvalbumin-positive interneurons. A, Biocytin-labeled basket neuron (M69). B, Fluorescent view of parvalbumin immunoreactivity of the same section. Arrows in A and B point to the biocytin-filled cell body. C, Parvalbumin-positive target of a filled basket neuron (UR32). The white arrow in the inset(C) indicates a putative synaptic contact between the biocytin-filled terminal and the cell body of a parvalbumin-immunoreactive neuron. 0, Correlated electron microscopic analysis of the bouton indicates a symmetric synapse (arrow in inset) on the cell body. o, stratum oriens; p, CA1 pyramidal layer; r, stratum radiatum. approximately -70 mV and of the late IPSP between -90 mV laterals of of M50 arborized and contained boutons almost ex- and 110 mV. Repetitive stimulation at 1 Hz or higher could clusively in the stratum lacunosum-moleculare(91.5%) with attenuate both the early and especially the late IPSPs, and the only few local collaterals in stratum oriens (7%) and a few fibers late IPSP could be converted into a late depolarizing potential enteredthe subiculum(1.5%). Most collateralsprojected septally from which action potentials emanated(not shown). The fre- from the cell body (Fig. 5A,B). Another important feature of this quency recruitment of the late dischargeswas similar to that cell was its limited septo-temporaland medio-lateralextent of describedearlier in extracellular studiesof putative basket cells the termination field (840 pm by 500 pm). The total axon length (Andersenet al., 1963; Buzs&i and Eidelberg, 1982). of the 0-LM cell was 63,436 pm. Basedon bouton density (26.6 Basket cells had a relatively high level of spontaneousfiring 2 4.0/100 Fm), the calculated total number of boutons was (5 to 60 Hz) when rhythmic theta activity was present in the 16,874.Thus, even though the spatial extent of the terminal field background (Fig. 4). The membranepotential displayed rhyth- was limited, the total number of putative synapseswas higher mic modulationand bursts of two to six action potentials, phase than in the basket cells, indicating that the incidence of target locked to the extracellular theta waves. These fast bursts could innervation was similar or higher than in basket neurons.Elec- be prevented by hyperpolarization of the neuron, and this pro- tron microscopic examination confirmed that boutons represent- cedure revealed a 20 to 60 Hz oscillation of the membranesu- ed synapseson spinesof presumedpyramidal cells as well as perimposedon the slow theta oscillation. In the absenceof ex- on small-diameterdendritic shafts of pyramidal cells and/or intracellular theta EEG activity, the dischargerate of the basket terneurons (Fig. 5E,F). The axonal distribution of M247 was cells decreasedand both the fast and theta membraneoscillation very similar to M50. However, very weak immunolabeling of diminishedor disappeared(Fig. 4B,C). the axon collaterals of this cell prevented reconstruction of the Alveudoriens interneuron with lacunosum-moleculareaxon axonal tree. arborization (0-LM) An important physiological feature of these neuronswas the The cell bodies and dendrites of these neurons (M50; M247) large time-dependentinward rectification (sag in the response), were confined to the stratum oriens and alveus. The axon col- which was visible even at small current injections (Fig. 5C), The Journal of Neuroscience, October 1995, 75(10) 6655

Figure 2. Neuronaltargets of an intracellularlylabeled basket cell (UR32). A, Reconstructeddendtitic tree and axon arborizationfrom sevenneighboring XC- tions(out of 19 sectionscontaining axon collaterals).Large circles indicatecon- tacts with other parvalbuminintemeu- rons.Inset positionof the neuronin the coronalplane. In this andin subsequent figuresthe septotemporallevel of coronal drawingscorrespond to the positionof the soma.B, Distributionof pyramidal cell and parvalbumin-positivecell (PV) targetsof the intracellularlylabeled basket cell in the septotemporaldirection (orthogonalto the planein A). Left or- dinateof the pyramidalcell graphindi- catesnumber of boutonsper 60 pm section.Right panel: probabilityof pyramidal (pyr) and parvalbumin-immunoreactive (Pv) neuronsinnervated by the filled cell.For eachsection a ratio(probability) wasobtained by dividingthe numberof contactedparvalbumin (or pyramidal) cellsby thetotal number of parvalbumin- immunoreactive(or pyramidal)neurons in the areainnervated by the axon collaterals.S, positionof the soma. consistentwith in vitro observations (McBain, 1994). Low in- caudally projecting collaterals reached and terminated in sub- tensity stimulation of the commissuralinput evoked only short icular region (3.5%). Few axon collaterals crossedthe pyramidal latency (< 5 msec) IPSPs. Strong stimulus intensities elicited layer (4.7%), but the layer was practically devoid of terminal only a single action potential and only after the occurrence of boutons. As shown in Figure 6B, the axon terminal cloud was the extracellularly recorded population spikes. These observa- asymmetric, with most of the axon collaterals projecting caudal tions suggestthat 0-LM neuronsare innervated by the nearby to the cell body. The area occupied by the axon collaterals was CA1 pyramidal cells but not by commissuralfibers. 1860 Frn (septo-temporal)by 2090 p.rn (medio-lateral).The total Neurons with a similar confinement of their axonal trees to axon length was 78,800 p,rn with 16,600 boutons (bouton den- the stratum lacunosum-molecularehave been described in the sity 21 t 5.61100pm; n = 100). Since a single bistratified cell CA3 region of the guinea pig (GulyBs et al., 1993) and the CA1 contacts pyramidal targets by six synapses(Buhl et al., 1994), region of the rat (McBain et al., 1994). Our in vivo finding our findings indicate that an individual neuron may innervate as confirms that the restricted arborization of the axon collaterals many as 2500 pyramidal cells (divergence). of 0-LM cells in vitro is not due to the slicing-induced loss of Of all cells in these experiments, this neuron maintainedthe collaterals in other layers. highest firing frequency (> 300 Hz) upon current injection (1 nA) into the soma and displayed some spike frequency accom- Calbindin positive alveus/oriensinterneuron with bistratijed modation (Fig. 6C). Since stimulation electrodeswere not im- axon arborization planted in this rat, evoked properties of the cell could not be In contrast to basket cells, axon collaterals of this neuron type studied. However, previous work indicates that bistratified cells only crossedthe pyramidal layer and most of its axon collaterals are strongly activated by the commissural/Schaffercollateral in- arborized and terminated in strata oriens and radiatum. The cell puts (Buhl et al., 1994). body of the interneuron (M83) was located in stratum oriens and possessedboth radially and horizontally oriented dendrites.The Trilaminar interneuron “apical” dendritesprotruded into the stratum radiatum but did The cell body was located in the stratum oriens/alveolarborder not enter stratum lacunosum-moleculare(Fig. 6A). The dendrite of the septal third of the hippocampus(Fig. 7A). The somawas surfaceswere smooth. Earlier in vitro studiestermed this class tested for calbindin immunoreactivity and proved negative. The of cells bistratified interneuron (Buhl et al., 1994). The section dendritic arborization occupied a large area of stratum oriens, containing the cell body was immunoreactedagainst the calcium running parallel with the stratum pyramidale. The dendritesex- binding protein calbindin and proved calbindin positive (Fig. hibited only a few dendritic spines. The main axon emerged 60). from the soma and bifurcated, giving rise to collateralsin strata The principal axon arose from the cell body and ascended oriens, radiatum, and pyramidale. One secondary axon became through stratum pyramidale into the stratum radiatum and de- myelinated in the fimbria and could not be traced further. An- scended into the stratum oriens and formed dense terminal other main collateral traveled toward the septal pole of hippo- clouds in both layers. The entire axon tree was reconstructed campus,turned back to give rise to a large number of collaterals from 31 (60 p,m) consecutive sections.The axon collaterals in- close to the level of the cell body. The axon terminals richly nervated the entire mediolateral (CA3-subiculum) extent of the innervated the stratum radiatum (68.4%), lessaxons were found CA1 region with most collaterals terminating in the stratum or- in stratum oriens (12.8%) and pyramidale (16.7%). The axon iens (53% of total length) and stratum radiatum (39%). Some collaterals were not simply crossingthe pyramidal layer, as ev- 6656 Sik et al. * Hippocampal CA1 Interneurons

-.A 10 mV -93 p--- - 100 ms 120 mV 50 ms -127 +------! \- ____---- ____------_--

A early IPSP b-x mlate IPSP &w;~a

s! -140-120-100-80 -60 -40 membrane potential (mV) A m

Figure 3. Evoked response properties of identified basket cells. A, Simultaneous recording of extracellular activity in the CA1 pyramidal layer (j&&f) and intracellulti response (basket) to stimulation of the commissural path (dot). Note that the basket cell (URSOB) began to fire prior to the population spike in the field. B and C, Voltage dependence and reversal potentials of IPSPs. B, Responses at RMP and two hyperpolarization levels. Triangle and square indicate early and late IPSPs, respectively. C, Response amplitudes as a function of the membrane potential. D, Six superimposed responses to increasing stimulation intensities of the commissural input (40, 60, and 100 PA) in another basket cell (M63). Note long-lasting cessation of spontaneous firing. Bottom: response of a pyramidal cell to 100 pA stimulation. Note similar latencies of the early (triangle) and late The Journal of Neuroscience, October 1995, 15(10) 6657

j?OmV I I I I 50 ms 0 20 40 60 Hz

theta

r extracellular Figure 4. Intracellular theta activity in a CA1 basket cell (UR80B). A, Re- non-theta sponses of the neuron to depolarizing (0.4 nA) and hyperpolarizing (-0.2, -0.4, and -0.8 nA) current steps. B, Power spectrum of intracellular membrane fluctuation in the presence and absence of extracellular theta. Action potentials were digitally removed for the calculation of power. C, Simulta- neous recording of intracellular activity 20 mV of the basket neuron and extracellular 0.3 mV activity in the CA1 pyramidal layer i during theta activity (theta, above) and 500 ms its absence (non-theta, below). Note rhythmic firing of the basket cell during theta. No current was applied to the interneuron. idenced by both “en passant” and terminal boutons (Fig. 8C). collaterals was 2600 pm (septo-temporal)by 2450 pm (medio- A few caudally coursing collaterals entered the subiculum lateral). The total axon length was 55,913 Frn. Basedon bouton (2.1%). Since the arearatio between strataradiatum, pyramidale, density (28.2 ? 4.91100km; II = 50), the calculated total num- and oriens is about 5:1:2 in CA1 region of the hippocampus ber of boutons was 15,767. (Aika et al., 1994), the findings suggestthat this neuronal type Of all interneurons,this cell (M96) had the lowest threshold can contact their targets in the pyramidal layer with a higher to commissuralstimulation. It displayed long bursts of sponta- probability than in stratum oriens.The areaoccupied by the axon neousaction potentials (Fig. 8A) and large EPSPs (Fig. 8A,G). t (square) IPSPs in the basket cell and pyramidal cell. Note also that the basket cell fired prior to the action potential of the pyramidal cell (inset) and that the second action potential of the basket cell preceded the afterdepolarization of the pyramidal neuron. 6658 Sik et al. l Hippocampal CA1 Interneurons

axon distribution

S septal 60um temporal

J20mV C 50 ms -b/l/f f (4 f JJ fh=---

Figure5. Stratumoriens interneuron (M50) with lacunosum-molecularetermination field (0-LM cell). A, Cameralucida reconstruction from only threeconsecutive 60 ym sections.Black dot indicatesthe somalocation. B, Septotemporaldistribution of axon collateralsin consecutive60 pm sections.The axon tree did not exceed800 pm in diameterin eitherthe mediolateralor septotemporaldirections. C, Responses of the neuron to depolarizing(0.4 nA) and hyperpolarizing(-0.4, -0.8, and - 1.2 nA) currentsteps. Note the largetime-dependent inward rectification (sag in the response), visible even at small current injections. Inset: Strong commissural stimulation (black square) elicited a single spike at long latency (9 msec). D, Light microscopic picture of the dense terminal field in stratum lacunosum-moleculare (boxed area in A). E and F, At the electron microscopic level boutons terminated (arrows) on small diameter dendrites (d) as well as on spines (s) (E).

By increasing stimulus intensity, the neuron respondedwith a duced depolarization threshold for the burst responsewas ap- burst of action potentials with decreasingamplitude spikes and proximately -60 mV. However, when this or more positive short interspike intervals (> 300 Hz), emanating from a large membranepotential was achieved by injection of an intracellular depolarizing potential. The burst was followed by an IPSP, current (Fig. 8F) or occurred spontaneously(Fig. 8A), only sin- which in turn, was followed by a late depolarizing potential and gle fast spikeswere observed.Even when repetitive activity oc- associatedburst of lower frequency action potentials (100-200 curred together with a fast membraneoscillation, the successive Hz; Fig. 8B). With current-injected membranehyperpolarization spikes were of similar amplitude. Such local depolarization of the IPSP became null at approximately -70 mV (Fig. K), the dendrite presumably could not be achieved by intrasomatic whereasboth the early EPSP and the late depolarizing potential current injection. In this rat, no extracellular electrode was im- increased.Further hyperpolarization of the membraneresulted planted. However, when the strength of commissuralstimulation in further increaseof the amplitude of the early EPSP and the was increasedto a level sufficient to dischargea subsequently late depolarizing potential but with decreasingnumber of action recorded pyramidal cell or to evoke population spikesin other potentials(Fig. 7B). At the sametime, the amplitudeof the IPSP, experiments(> 100 PA), commissuralactivation evoked a large which now becamedepolarizing, continued to increase.The re- and long-lastingdepolarization, as if the early EPSP and the late versal potential of the IPSP suggestedthat it was mediated by depolarizing potential merged and obliterated the intervening GABA, receptors. Unidentified neurons with similar location early IPSP (Fig. 8D). After the initial burst, the large depolar- and late depolarizing responsewere observed in the CA1 region ization plateau blocked the occurrence of Na+ spikes and the in vitro (Lacaille, 1991). membraneshowed a small amplitude but fast (400-500 Hz) os- The initial burst responsecould be elicited with low intensity cillation until the spikes recovered. These observations suggest (< 25 PA) stimulation, which in most other neuronswas sub- that synaptic activation perhapsinduced a dendritic spike, which thresholdfor evoking an action potential. The stimulation-in- in turn, triggered fast inactivating Na+ spikes(LlinBs, 1988). The The Journal of Neuroscience, October 1995, 75(10) 6659

1 OOpm

axon distribution

septal s 60pm temporal

Figure 6. B&ratified calbindin-positive cell. A, Reconstruction of the entire axon collateral system (thin lines) and dendritic arborization. Axon collaterals from all coronal sections (n = 33) were collapsed into one. o, stratum oriens; p, CA1 pyramidal layer; r, stratum radiatum; J hippocampal fissure. B, Septotemporal distribution of the axon collaterals in successive 60 pm sections (orthogonal to the plane in A). S, position of the soma. C, Fast firing of the interneuron in response to intracellular current injection (1 nA). D, Photograph of the biocytin-labeled cell body (arrow) and main dendrites. Inset: Calbindin-immunoreactivity in the same section (FITC). White arrow indicates the immunoreactive soma of the intracellularly filled neuron. Asterisks: blood vessel. neuron could be antidromically activated by stimulation of the cells (SR) describedin the CA3 region of the guinea pig (Gulyas contralateral fimbria-fornix at 4.1 msec latency, indicating 1.0 et al., 1993), although the dendritesof that cell type were con- to 1.5 -m/set spike propagation in the myelinated commissural fined to the strata radiatum, lucidum, and pyramidale. axon collateral (Fig. 8E). In summary, both the physiological and anatomical features Backprojection cells of this neuron were distinct from basket cells. It was also dif- Axon collaterals of two cells (UR37 and Ml 13) left the CA1 ferent from the bistratified interneuronsof the CA1 region (Buhl region and innervated the CA3 field, and one of them also the et al., 1994), since its dendrites were confined to the stratum hilar region. The position and dendritic arborization of the two oriens, most of its collaterals terminated in strata radiatum and neurons were similar The first cell had a total of 101,342 pm pyramidale, and the neuron was immunonegativefor calbindin. long axon collaterals in the CAl, CA3, and the hilar region. The It showedsome similarity to the stratum radiatum nonpyramidal anatomical and physiological details of this neuron have been 6660 Sik et al. * Hippocampal CA1 Interneurons

I3 axon distribution v *,” -_I z5ooc 5 : 5 z n A 0

Figure 7. Trilaminar interneuron (M96). A, Reconstruction of the axon collaterals and dendrites of the interneuron. Axon collaterals from all sections were collapsed into a two-dimensional display. o, stratum oriens; p, CA1 pyramidal layer; r, stratum radiatum; j hippocampal fissure. Some collaterals falsely appear in the stratum lacunosum-moleculare because of the two-dimensional projection of curving layer boundaries. B, Septotemporal distribution of the axon collaterals in successive 100 pm sections. S, soma. C and 0, Biocytin-labeled boutons (a) form symmetric synapse (arrow) on dendrites (d) in the pyramidal layer (C) and the stratum oriens (D). p in C indicates pyramidal cell bodies.

publishedearlier (Sik et al., 1994). The cell body of Ml13 was the interneuron, suggestingthat thC pyramidal cell have formed at the border of the alveus and stratum oriens and its long den- only a single synapseon its interneuron target (Fig. 9D). drites were confined to the stratum oriens. Reconstructedtwo- dimensional display of the axon arbor of the second cell is Discussion shown in Figure 9. The total length of collaterals was 20,642 Recent in vitro intracellular labeling studiesrevealed three main km, 59.4% of which was in the CA1 region and 40.6% coursed classesof inhibitory cells in the hippocampalCA1 region: bas- back to CA3 a-b areainnervating the strata oriens and radiatum. ket cells, chandelier cells, and bistratified cells (Buhl et al., Several boutons were found in the immediateproximity of cap- 1994). The present findings confirmed these observations and illaries. Under the electron microscope theseproved to be reg- extended them in three important ways. First, additional inter- ular synapses,but they were separatedfrom the capillary en- neuronal types were visualized. Second, the axonal trees of our dothelium by only the lamina basalisand a thin process of a cells were completely filled, allowing us to quantitatively assess capillary pericyte (Fig. 9D). their divergence and convergence. Third, the calcium binding After the completion of the intracellular injection, the pipette protein content of the interneuronswas revealed in somecases. was withdrawn from the cell and a CA1 pyramidal neuron was Fourth, physiological properties of the interneurons were con- also penetrated and filled, approximately 150 pm from the in- trasted to their anatomicalfeatures. terneuron. The axon collaterals of the pyramidal cell and the backprojection interneuron could be clearly separated.Axon col- Interneuronal types in the CA1 region laterals of the interneuron did not have any bouton contactswith Interneurons in the hippocampushave been classifiedaccording the pyramidal cell. On the other hand, a recurrent collateral of to their calcium binding protein and peptide content (Somogyi the pyramidal cell terminated with a bouton on the dendrite of et al., 1984; Sloviter and Nilaver, 1987; Kosaka et al., 1988; The Journal of Neuroscience, October 1995, 15(10) 6661

-60 500 ms -80 -1 u -x mV Figure 8. Physiologicalproperties of a trilaminarinterneuron. A, Spontane- ous activity. Note long train of action potentials (60 to 100 Hz) and surround- ing silence. Action potentials were clipped. Znset reveals fast membrane oscillation. B, Burst responses to commissural stimulation (40 PA; dot) at -62 2 two different membrane potential levels. The early EPSP and action poten- I 120mV tial burst was followed by an early IPSP (triangle) and a late EPSP. C, Re- 20 ms sponse amplitudes of the IPSP (triangle in B) as a function of the membrane potential. The linear regression line in- tercepts the reversal potential line (dot- mV , . ted) at approximately -70 mV. D, Evoked responses to stronger (100 p,A) commissural stimulation (dot) at rest- ing membrane potential. Note depolarization block of action potentials and fast membrane oscillation. E, Anti- dromic activation of the cell by commissural stimulation (30 p,A: dot). Three superimposed responses. The cell was slightly depolarized (-60 mV). Note that the early (antidromic) spike occurs without a depolarizing potential, followed by an early EPSP and burst firing. F, Responses of the neuron -I I I I I I I to depolarizing (0.4 nA) and hyperpolarizing (-0.4, -0.8, and -1.2 nA) -100 -80 -60 -40 current steps. Arrows indicate large membrane potential (mV) spontaneous EPSPs.

Sloviter, 1989; Gulyas et al., 1991, 1992; Miettinen et al., 1992; the somatostatin neurons and becauseof the presenceof so- T&h and Freund, 1992), inputs (Schwarzkroin and Mathers, matostatin-immunoreactiveaxons in the stratum lacunosum-mo- 1978; Alger and Nicoll, 1982; Buz&i and Eidelberg, 1982; leculare (Sloviter and Nilaver, 1987; Kosaka et al., 1988). The Buzs&i 1984; Freund et al., 1990), location of their somaden- trilaminar cell is consideredto be a new cell type not only be- dritic trees (Kunkel et al., 1987; Lacaille et al., 1987; Lacaille causeof its distinct innervation of the somata,basal, and prox- and Schwartzkroin, 1988a,b; Scharfman et al., 1990; La&lie, imal apical dendrites of pyramidal cells, but becauseits physi- 1991; Gulyas et al., 1993a,b), glutamate receptor properties ological features and dendritic arbor were characteristically dif- (Baude et al., 1993; McBain and Dingledine, 1993; McBain et ferent from other types. Backprojection neuronsare also treated al., 1994), and firing properties (Kawaguchi and Hama 1987a,b; as a new subgroup becausethey exert their inhibitory effect Lacaille and Schwartzkroin, 1988a,b; Fraser and MacVicar, almostequally on CA1 and CA3 pyramidal cells. Backprojection 1991). Recent intracellular labeling studiesbegan to classify in- neuronsmay contain the enzyme nitric oxide synthase,because terneurons in terms of their target domainson the somatoden- axons of some neuronsproducing nitric oxide in stratum oriens dritic surfaceof principal cells (Gulyas et al., 1993a,b; Hahn et course back from the CA1 to the CA3 and hilar regions (Sik et al., 1993; Halasy and Somogyi, 1993; Buhl et al., 1994; Buck- al., 1994a). Direct evidence, i.e., demonstrationof nitric oxide master and Schwartzkroin, 1995), a distinction most pertinent to synthase in backprojection neurons, is still lacking, however. their physiological role. The precisechemical content of the other interneuronalsubclass- In addition to chandelier cells, basket cells, and bistratified es has yet to be determined and it is of special importance to cells (Finch et al., 1983; Li et al., 1992; Buhl et al., 1994), we reveal how the distinct chemical nature of thesesubclasses con- found three additional inhibitory cell types in the CA1 region. tribute to their function. Although the above six inhibitory neu- The 0-LM cell terminated most prominently in the stratum la- ronal groups in the stratum oriens and pyramidal layer are suf- cunosum-moleculareand to someextent in stratum oriens, with ficient to differentially affect the orderly arrangedexcitatory af- rather limited septotemporaland mediolateral extent. A similar ferents to the CA1 region, it is expected that further research cell type has recently been described in the CA3 (GulyBs et al., will add to this list. 1993b) and CA1 (McBain et al., 1994) regions in vitro. 0-LM Local circuit and long-range inhibition in the CA1 region neuronslikely belong to the somatostatin-immunoreactiveclass On the basisof the spatial extent of axon collaterals,two major becauseof the similarity of its somadendriticlocation to that of groups could be distinguished: local circuit and long-range in- Sk et al. l Hippocampal CA1 Interneurons

f r \ \ \ m

100pm

I- s septal 60pm temporal

Figure 9. Backprojection interneuron I(Ml 13). A, Reconstructed two-dimensional display of the axon tree of the interneuron and dendrites of both interneuron and a labeled pyramidal cell. o, stratum oriens; p, CA1 pyramidal layer; r, stratum radiatum; j hippocampal fissure; m, molecular layer; g, granule cell layer; h, hilus. B, Axon collateral distribution in the septotemporal axis (60 km sections). C, Electron microscopic picture of a bouton in the vicinity of a capillary (c) in the CA3 stratum oriens. The bouton is separated from the lamina basalis of the capillary by a thin process of a pericyte (white arrow). A symmetric synapse on dendrite is indicated by the black arrow. D, Light microscopic photo image of the boxed area from the reconstruction (A). White arrow shows the interneuron soma in stratum oriens. A filled pyramidal cell close to the interneuron is also visible (arrow). The inset shows enlargement of the boxed area. The arrow indicates a putative synaptic contact on the interneuron dendrite from an axon collateral of the labeled CA1 pyramidal cell. terneurons. The local circuit group contains basket cells, chan- in the CA1 region establish 2 to 10 releasesites on the axon delier cells and 0-LM cells, with cylindrical axon trees of less initial segmentof a single postsynaptic neuron and also inner- than a millimeter in diameter. A single basket cell innervated vate more than 1000 pyramidal cells (Li et al., 1992). The spatial more than 1500 pyramidal neuronsand 60 other parvalbumin- extent of the 0-LM cell was smaller than that of the basket immunoreactive(basket and chandelier) cells. Chandelier cells neuronsbut with a higher density of axon collaterals. In this The Journal of Neuroscience, October 1995, 15(10) 6663

basket cell chandeker cell 0-LM cell was completely absentin the backprojection cells and very small in the other types. These preliminary observations suggestthat

0 a reliable relationship may exist between intrinsic properties, network behavior, and axonal targets of the different interneu- P ronal types. Kawaguchi et al. (1987) have shown previously that fast spiking neuronscontain parvalbumin, and it was hypothesized that 0i this calcium binding protein is causally related to the fast dis- Im 1Imm charge of interneurons by reducing their accommodation and afterhyperpolarization (Celio, 1986). Although all neurons in this study were fast spiking, only basket cells were immuno- bistratified cell trilaminaf cell backprojection cell reactive for parvalbumin, suggestingthat parvalbumin is not related to the fast neuronal discharge.

CA3 Previous physiological and anatomicalworks have already indicated connectivity among interneurons. Both inhibitory and excitatory connectionshave been suggestedto occur among interneurons (Misgeld and Frotscher, 1986; Lacaille et al., 1987; CA3 Michelson and Wong, 1991). A striking observation of the present study was the high probability of contactsamong basket cells with approximately 60 parvalbumin-immunoreactive interneurons converging on a single basket cell. In line with the anatom- Figure 10. Three-dimensional distribution of the axonal trees of CA1 ical arrangement,inhibition of basket cell firing and the appear- interneurons in alveus, strata oriens, and pyramidale. The intensity of ance of the early (putative GABA,) and late (putative GABA,) gray is proportional to the axon collateral density. Upper row: local IPSPs occurred at similar stimulusintensities in both basketand circuit cells. Bottom TOW: long-range interneurons. For the local circuit interneurons, the axon arbor was almost cylindrical. Axon collaterals of pyramidal neurons. At low frequency stimulation basket cells the long-range cells were more extensive in the septotemporal direction did not fire more than three action potentials even at high stim- than in the medio-lateral (subiculo-fimbrial) direction. Data for the ulus intensities. However, the duration of postactivation sup- chandelier cell are from Li et al. (1992). Soma location and approximate pressionof cell dischargeincreased and the amplitude of both denritic extent are also shown. Note different calibration in the y and x-z direction. o,, stratum oriens; p, CA1 pyramidal layer; r, stratum the early and especially the late IPSPs was augmentedwith radiatum; lm, stratum lacunosum-moleculare. stronger stimuli. These observations suggestthat activation-dependent inhibition of pyramidal cells and basket cells cannot be explained by increasednumber of action potentials of the basket context, it is important to emphasizethat the entorhinal cortex- neurons.Rather, increasedinhibition is likely due to recruitment CA1 projection is similarly circumscribed (Tamamaki and No- of further basket cells and/or other interneurons terminating on jyo, 1995); thus, inhibition of an 0-LM cell by its afferents may the sametarget neurons.This explanation is in line with a pre- allow activation of a selectedgroup of CA1 pyramidal cells by vious suggestionthat synaptically releasedGABA quickly sat- the entorhinal input. urates GABA, receptor channels(Faber et al., 1992); therefore, The long-rangeinterneuronal group includesbistratified cells, inhibition can be most efficiently augmentedby synchronizing trilaminar, and backprojection interneurons. Long-range inter- nearby synaptic terminals (Otis et al., 1994). neuronsarborized in a much larger volume than local circuit With stimulation frequenciesfaster than 1 Hz, the number of cells. The remarkably similar bouton density of all interneurons action potentials in basket cells increased(Andersen et al., 1963; (21-28/100 pm), and the overlapping rangesof the total axonal Buzs&i and Eidelberg, 1982; Sah et al., 1990; Lacaille, 1991; lengthsin the two groups (20 to 100 mm) suggestthat although McBain, 1994). This may be due to presynaptic inhibition of the volume and the number of neuronsinnervated by the long- GABA releaseor to the decreasedeffectiveness of the released range interneurons is larger, the density of innervated postsyn- GABA on basket cells as a result of increasingintracellular Cl- aptic targets is less than for the local circuit group. Long-range concentration(Thompson and Gahwiler, 1989).Against this sce- interneuronstherefore may exert a weaker but moreglobal effect nario, our observation that the trilaminar neuron had a late de- on their targets than local circuit interneurons. polarizing potential even at low frequency stimulation suggests that the density of GABA receptors on trilaminar neurons is Functional implications quite low. Overall, thesephysiological observationsindicate that Despiteof the small number of cells in each group, clear distin- GABAergic innervation of interneuronsmay vary extensively. quishingphysiological propertiesof theseclasses have emerged. Traditionally, inhibitory interneuronshave been assumedto Basket cells had significantly lower afterhyperpolarizationsthan provide stability to the principal cell population by feed-back that of any other cell type. They respondedto commissuralstim- and feed-forward inhibition. Becauseof their expected spatial ulation with a burst responsefollowed by suppressionof firing. selectivity, interneurons were believed to have restricted axon In contrast, the trilaminar neuronfired both early and late bursts. collateral systems;hence, the frequently usedterm “local circuit 0-LM cells and the backprojection neurons on the other hand interneurons.” The widespreadand interregional projection of were inhibited by commissuralstimulation and dischargedonly interneurons, however, suggestnew functions for interneurons. after the emergenceof CA1 population spikes, indicating that It has been hypothesized that ligand- or voltage-dependentos- they are innervated by local CA1 pyramidal cell collaterals but cillation of interneuronsand their linking into cooperative en- not by commissuralfibers. Hyperpolarization of 0-LM neurons semblepatterns may be critical for theta, gamma,and ultrafast evoked a prominent sag current (seealso McBain, 1994), which (200 Hz) population oscillationsin the hippocampus(Fraser and 6664 Sik et al. l Hippocampal CA1 Interneurons

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