Journal of Physiology (2002), 543.3, pp. 779–793 DOI: 10.1113/jphysiol.2002.024588 © The Physiological Society 2002 www.jphysiol.org A model of atropine-resistant theta oscillations in rat hippocampal area CA1 M. J. Gillies *, R. D. Traub ‡, F. E. N. LeBeau *, C. H. Davies †, T. Gloveli *, E. H. Buhl * and M. A. Whittington * * School of Biomedical Sciences, University of Leeds, Leeds LS2 9NQ, UK, † GlaxoSmithKline, Harlow, Essex, CM19 5AW, UK and ‡ Department of Physiology and Pharmacology, SUNY Health Science Center, Brooklyn, NY 11203, USA Theta frequency oscillations are a predominant feature of rhythmic activity in the hippocampus. We demonstrate that hippocampal area CA1 generates atropine-resistant theta population oscillations in response to metabotropic glutamate receptor activation under conditions of reduced Journal of Physiology AMPA receptor activation. This activity occurred in the absence of inputs from area CA3 and extra- ammonic areas. Field theta oscillations were co-expressed with pyramidal distal apical dendritic burst spiking and were temporally related to trains of IPSPs with slow kinetics. Pyramidal somatic responses showed theta oscillations consisted of compound inhibitory synaptic potentials with initial IPSPs with slow kinetics followed by trains of smaller, faster IPSPs. Pharmacological modulation of IPSPs altered the theta oscillation suggesting an inhibitory network origin. Somatic IPSPs, dendritic burst firing and stratum pyramidale interneuron activity were all temporally correlated with spiking in stratum oriens interneurons demonstrating intrinsic theta-frequency oscillations. Disruption of spiking in these interneurons was accompanied by a loss of both field theta and theta frequency IPSP trains. We suggest that population theta oscillations can be generated as a consequence of intrinsic theta frequency spiking activity in a subset of stratum oriens interneurons controlling electrogenesis in pyramidal cell apical dendrites. (Resubmitted 20 May 2002; accepted after revision 2 July 2002; first published online 19 July 2002) Corresponding author M. A. Whittington: School of Biomedical Sciences, The Worsley Building, University of Leeds, Leeds LS2 9NL, UK. Email: [email protected] Field potential oscillations at theta frequencies (4–12 Hz) In vitro models of theta frequency oscillations in are readily observed in the hippocampal formation in vivo hippocampal slices have demonstrated a number of and have been implicated in various cognitive processes situations in which the hippocampal formation alone may including processing of visuospatial information (O’Keefe generate rhythmic activity within the theta range. Most & Nadel, 1978) and in models of memory formation and of this work has used application of carbachol (with or retrieval (Larson & Lynch, 1986; Buzsáki, 1989). Theta without bicuculline) to generate theta-like discharges, frequency oscillations are prominent in all areas of the particularly in area CA3, where they are attenuated by hippocampal formation and, from lesion experiments, AMPA receptor blockers (MacVicar & Tse, 1989; Williams have been variously proposed to originate from reciprocal & Kauer, 1997). However, in this form of model, unlike in interactions between rhythmic inputs from the medial vivo, interneuron activity is not essential for rhythmo- septum-diagonal band of Broca (MSDB), the entorhinal genesis (Traub et al. 1992). With inhibition intact, by far cortex and other subcortical structures (Petsche et al. 1962; the most robust form of oscillation generated by carbachol Buzsáki et al. 1983; Vertes & Kocsis, 1997). in area CA3 is within the gamma band, with only weak activity at theta frequencies (Fisahn et al. 1998). In vivo, The majority of activity at theta frequencies observed in theta frequency oscillations are accompanied by a large the hippocampal formation in vivo appears dependent amount of inhibitory interneuronal activity, with rhythmic on cholinergic activity and, more so, inputs from MSDB. trains of spikes from fast-spiking interneurons and suggested However, recent findings concerning the MSDB have cast specific involvement of a number of interneuron subtypes doubt on the notion that the MSDB alone has the capacity to in cornu ammonis oriens and lacunosum moleculare impose a coherent theta frequency oscillation on the hippo- laminae and the dentate gyrus (Buzsáki, 2002). campus (King et al. 1998). In addition data demonstrating the timing of hippocampal output activity with reference Hippocampal interneurons are differentially sensitive to to excitatory and inhibitory inputs has revealed a complex metabotropic glutamate receptor (mGluR)-dependent drive relationship which is difficult to explain in terms of extrinsic (e.g. van Hooft et al. 2000), and positive interactions between control alone (see Buzsáki, 2002 for review). these receptors and the NMDA subtype of glutamate 780 M. J. Gillies and others J. Physiol. 543.3 receptor critical for theta genesis in vivo (Vanderwolf & thiopental, 50 mM. Drugs were obtained from Tocris (Bristol, UK) Leung, 1983) have been reported (Awad et al. 2000). In or Sigma (Poole, UK). addition co-activation of mGluRs and metabotropic Peak frequency of recorded activity was determined off-line from cholinergic receptors has been reported to generate robust digitised data (digitisation frequency 10 kHz) from power spectra theta frequency oscillations in the hippocampus in vitro of 60 s epochs of recorded activity. Patterns of spike timing were (Cobb et al. 2000). From this it was hypothesised that illustrated by superimposition of concurrently recorded stratum mGluR activation may also play a role in generating theta pyramidale field potential and intracellular electrode signal. frequency oscillations in the hippocampus devoid of Average spike timing profiles were quantified by combining cross correlograms of 3 s epochs of data from concurrently recorded extrinsic, phasic inputs. We have previously shown that stratum pyramidale field potentials and intracellular signals. transient activation of metabotropic glutamate receptors in Correlation values were calculated by measuring the y-axis area CA1 of the hippocampus generates gamma frequency distance between the first negative and first positive peaks to the oscillations when AMPA receptor activation is blocked right of the central peak of the correlograms. IPSP rise times were (Whittington et al. 1995). Here we demonstrate that tonic measured using Minianalysis (Synaptosoft Inc.). More than 600 activation of metabotropic glutamate receptors also IPSPs were measured from each slice used in the analysis. Decay Journal of Physiology generates prominent, inhibition-based, theta frequency time measurements could only be estimated owing to the compound nature of the inhibitory events. oscillations when AMPA receptor activation is attenuated or abolished. RESULTS METHODS AMPA receptor blockade transforms gamma into theta frequency oscillation Male Wistar rats (~150 g) were fully anaesthetised with isoflurane Gamma frequency oscillations were generated in area CA1 prior to injection of ketamine (> 100 mg kg_1) and xylazine (10 mg kg_1), then cardioperfused with artificial cerebrospinal of rat hippocampal slices by bath application of DHPG (20–200 M). Stratum pyramidale field recordings showed fluid (ACSF, containing (mM): NaCl, 126; KCl, 3; NaH2PO4, 1.25; m 2 _1 CaCl2, 2; MgSO4, 2; NaHCO3, 24; glucose, 10) in which NaCl had a peak in power spectra of 37 ± 9 mV Hz at a mean been replaced with an equiosmolar concentration of sucrose. All frequency of 37 ± 2 Hz (n = 29). In each case the resulting procedures were carried out in accordance with the UK Animals gamma oscillations were modulated at theta frequencies (Scientific Procedures) Act 1986. Transverse slices of mid hippo- either weakly or not at all (Fig. 1A). However, bath campus, 450 m thick, were cut horizontally and maintained at m application of NBQX (20 M) abolished field power in the the interface between oxygenated ACSF and humidified 95 % m gamma band (P < 0.001, n = 29) and increased dramatically O2–5 % CO2 at 35 °C. CA1 minislices were obtained by dissecting transverse slices between CA3 and CA1 subfields. the power in the upper theta band (6–12 Hz). Peak power increased from 22 ± 7 mV2 Hz_1 to 115 ± 43 mV2 Hz_1 at a Extracellular recordings were made via Neurolog amplifiers mean peak frequency of 6.7 ± 0.3 Hz (n = 29). Auto- (Digitimer, UK) using glass microelectrodes containing ACSF correlations demonstrated clear rhythmicity at theta (resistance < 3 MV). Potentials were band pass filtered on-line (0.1 Hz–2 kHz). Intracellular recordings were made via Axoclamp frequencies (Fig. 1A). Laminar profiles (n = 4) revealed a amplifiers (Axon Instruments, Inc., Union City, CA, USA) using clear phase reversal between distal stratum radiatum/ glass microelectrodes containing either potassium methylsulfate lacunosum moleculare and other laminae (Fig. 1B). or potassium acetate (resistance 60–100 MV). Stratum oriens inter- Similar AMPA receptor blockade-dependent expression neurons demonstrated diverse electrophysiological characteristics. of field theta oscillations in area CA1 were seen with bath The data presented are from a subset of oriens interneurons which application of carbachol (20–40 mM) and NBQX or kainate showed brief action potentials, large, compound afterhyper- polarisations (AHPs) and intrinsic theta frequency membrane (1.0–1.5 mM) and the specific AMPA receptor blocker potential oscillations in the