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Glutamate-Gated Inhibitory Currents of Central Pattern Generator Neurons in the Lobster Stomatogastric Ganglion

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Glutamate-Gated Inhibitory Currents of Central Pattern Generator Neurons in the Lobster Stomatogastric Ganglion The Journal of Neuroscience, October 1995, 75(10): 6631-6639 Glutamate-Gated Inhibitory Currents of Central Pattern Generator Neurons in the Lobster Stomatogastric Ganglion Thomas A. Cleland and Allen I. Selverston Biology Department, University of California at San Diego, La Jolla, California 92093-0357 Inhibitory glutamatergic neurotransmission is an elemental (Johnsonand Hooper, 1992), and have served as model systems “building block” of the oscillatory networks within the for understanding the function of biological neural networks crustacean stomatogastric ganglion (STG). This study con- (Kristan, 1980; Getting, 1989). Inhibitory glutamatergicsynaptic stitutes the initial characterization of glutamatergic cur- transmissionis a major building block of the reciprocal inhibi- rents in isolated STG neurons in primary culture. Super- tory pairs and recurrent cyclic inhibitory chains of the neurons fusion of 1 mM L-glutamate evoked a current response in that comprise these oscillatory CPGs (Marder and Paupardin- 45 of 65 neurons examined. The evoked current incorpo- Tritsch, 1978; Marder and Eisen, 1984). The chemical modula- rated two kinetically distinct components in variable pro- tion of these glutamatergic synapsescontributes heavily to con- portion: a fast desensitizing component and a slower com- trol of oscillatory phase relationships among the participating ponent. The current was mediated by an outwardly recti- neurons (Johnson et al., 1994), and changesin such phasere- fying conductance increase and reversed at -46.6 + 5.3 lationshipsin turn directly mediate changesin behavioral output mV. Reducing the external chloride concentration by 50% (Heinzel et al., 1993). deflected the glutamate equilibrium potential (E,,,) by +14 The glutamatergicIPSP in situ is primarily dependenton chlo- mV, while increasing external potassium threefold shifted ride, but also sometimesexhibits a smaller potassium depen- E,,, by up to +6 mV. lbotenic acid fully activated both com- dence which has not been separablefrom the chloride-mediated ponents of the glutamate response. Saturating concentra- responsein Panulirus interruptus (Marder, 1987). These “fast” tions of glutamate completely occluded neuronal re- glutamatergic IPSPs are blocked by picrotoxin (Eisen and Mar- sponses to ibotenic acid, indicating that ibotenic acid was der, 1982; Marder and Eisen, 1984), and are similar in reversal activating the same receptor(s) as glutamate. Millimolar potential and ionic dependenceto the responseselicited by ion- concentrations of quisqualic acid, kainate, AMPA, and tophoretically applied glutamate (Marder and Paupardin-Tiitsch, NMDA each failed to evoke any response. Picrotoxin (1O-4 1978; Bidaut, 1980; Eisen and Marder, 1982). The ionic and M) completely blocked the glutamate response. Niflumic pharmacological profile of this receptor clearly distinguish it acid (100 PM) blocked >60% of the desensitizing compo- from the major vertebrate glutamate receptor families; however, nent and -50% of the sustained component. Reduction or the chloride-mediated current resemblesthe currents mediated elimination of extracellular calcium did not abolish the re- by glutamate-gatedchloride channels in Aplysia neurons (Ike- sponse. moto and Akaike, 1988; Sawada et al., 1984; King and Carpen- This study extends the ionic and pharmacological anal- ter, 1989), extrajunctional glutamate receptors in locust and ysis of glutamatergic conductances in STG neurons. The American lobster muscles(Cull-Candy, 1976; Lingle and Mar- currents described are consistent with glutamatergic inhib- der, 1981), and the cloned GluCl ionotropic glutamate receptor itory synaptic and agonist-evoked responses previously from C. elegans (Cully et al., 1994). described in situ. We discuss their pharmacology, ionic This study describesthe glutamate-gatedmembrane currents mechanisms, and functional significance. in isolated, cultured STG neurons under two-electrode voltage [Key words: niflumic acid, lobster, stomatogastric, crus- clamp in order to lay the groundwork for detailed studiesof the tacean, reciprocal inhibition, glutamate, chloride current, glutamate receptor and its modulation at the cellular and network potassium current, central pattern generator, voltage de- levels. Cultured neurons are advantageousfor these studies in pendence] that they are electrotonically compact, isolated from other neu- rons which might evoke secondary effects, and accessibleto The crustacean stomatogastricganglion (STG) contains neural manipulations such as focal perfusion. In this initial character- elementsof at least two central pattern generator (CPG) circuits ization, we describereceptor pharmacology and ionic dependen- (Fig. lA), which control rhythmic movements of the foregut cies, as well as an intrinsic voltage dependenceof receptor ac- tivation which carries possible implications for its role in the stomatogastricneural network. Received Mar. 28, 1995; revised May 15, 1995; accepted June 6, 1995. Some of thesedata were previously publishedin abstract form Our thanks to R. C. Elson, W.-D. Krenz, C. M. E. Hempel, and Y. I. Ar- shavsky for critical reading of the manuscript, and I. Hsieh for valuable com- (Cleland and Selverston, 1994). puter support. This work was supported by the National Institutes of Health Program Project Grant POlNS25916 to A.I.S. and a National Science Foun- Materials and Methods dation predoctoral fellowship to T.A.C. Correspondence should be addressed to Thomas A. Cleland, UCSD Biology Cell culture. Adult Pacific spiny lobsters, Panulirus interruptus, were Department 0357, 9500 Gilman Drive, La Jolla, CA 92093.0357. captured wild and kept in running seawater until use. The stomatogastric Copyright 0 1995 Society for Neuroscience 0270-6474/95/156631-09$05.00/O ganglion (STG) was removed, desheathed, pinned to a Sylgard-lined 6632 Cleland and Selverston l Glutamate-Gated Inhibitory Currents in Lobster Neurons 0CD2 Gastric Pyloric ILP ; J 20 mV 300 ms 1 set Figure 1. Functional role of the inhibitory glutamate receptor. A, Circuit diagram of the gastric and pyloric networks and other cells of the stomatoeastric ganglion (STG). Most neurons in this central nattern generator are glutamatergic (AB, IC, PY, LP, LG, MG, DG, AM, and orobablv I&). A?1 pylo& ieurons and nearly all gastric neurons receive glu~amatergic inhibitory synapses from other STG neurons in situ. S&i circle; indicate inhibitory glutamatergic synapses; open circles indicate inhibitory cholinergic synapses. Question marks indicate presumed glutamatergic synapses, both inhibitory (circles) and excitatory (triangles). Electrotonic synapses are depicted by the resistor symbol; rectifying electrotonic synapses by the diode symbol. Note that there are two PD, two LPG, four GM, and eight PY neurons within the STG. The stomatogastric ganglion also contains one CD2 neuron (a member of the interganglionic cardiac sac network) and an estimated four EX neurons which are not well understood. B, Recording from an in situ ganglionic preparation from Punulirus interruptus demonstrating the effect of a glutamatergic synapse between identified neurons in the STG. The LP neuron (depicted by the spikes recorded from its output nerve, the LPn) normally inhibits the PD neuron phasically via a glutamatergic synapse. This inhibition can be removed by hyperpolarizing LP (note the injection of hyperpolarizing current in the ZLp trace), and a depolarizing pulse into LP can evoke a glutamatergic “forced IPSP” in PD (figure modified from Herterich, 1992). C, Glutamate application hyperpolarizes a tonically spiking, isolated, cultured STG neuron, thereby terminating action potential generation. A steady current injection of +2 nA was required in order to elicit tonic spiking in the neuron depicted. dish, and incubated at room temperature for 1 hr in 2 mg/ml subtilisin was prepared from full-strength Leibovitz-15 medium (Sigma powder) (Subtilisin Carlsberg, a nonspecific protease; Sigma) dissolved in Pan- with all common ions supplemented to Punulirus saline concentrations, ulirus saline. After proteolysis, the ganglion was washed for l-2 hr in except NaCl which was slightly reduced (to 446 mu) from its Panulirus maintenance medium or Panulirus saline, at 15°C or room temperature. saline concentration in order to osmotically balance the added ingredi- These variations in wash parameters did not correlate with any observed ents of L-15. The final recipe for 1 liter of PMM was: 14.8 gm L-15 variabiIity of cellular properties. Individual neurons were removed from powder, 18.07 gm NaCl, 0.55 gm KCl, 1.82 gm CaC1,.2H,O, 0.612 gm the ganglion by gentle suction and plated individually into 35 mm Fal- MgSO,, 0.56 gm Na,SO,, 1.19 gm HEPES acid, 1.15 .gm TES, and 3 con Primaria culture dishes in sterile maintenance medium. Recordings gm o-glucose. Final salt concentrations (including the contributions were made from cells after 34 d in culture, by which time cells had from L-15 powder) were 446 mu NaCl, 12.7 mu KCl, 13.7 mu adhered securely to the substrate and most had extended short processes CaC1,.2H,O, 5.9 mu MgSO,, 3.91 mM NqSO,, 5.0 mM HEPES acid, from the neurite stump, or occasionally from the soma. Incubation tem- 5.0 mu TES, and 16.7 mu D-glucose. Penicillin-streptomycin (final perature (15°C or room temperature) did not noticeably influence glu- concentrations: 100 U/ml penicillin, 0.1 mg/ml streptomycin) was also tamate response properties. added to the stock solution. PMM was brought to pH = 7.5 with HCl/ Media. Panulirus saline
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