HETEROLOGOUS MODULATION OF INHIBITORY SYNAPTIC TRANSMISSION BY METABOTROPIC GLUTAMATE RECEPTORS IN CULTURED HIPPOCAMPAL-NEURONS

1. Whole cell patch-clamp recordings of monosynaptically connected pai rs of hippocampal neurons in very low-density culture were performed t o determine the effects of the activation of metabotropic glutamate re ceptors (mGluRs) on inhibitory terminals. The mGluR agonist (1S, 3R)-1 -aminocyclopentane-1,3-dicarboxylic acid [(1S, 3R)-ACPD] and the recen tly described mGluR antagonist(RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) were used. In addition, the glutamate uptake inhibitors L-tran s-pyrrolidine-2,4-dicarboxylate (L-trans-PDC) and D,L-beta-threo-hydro xyaspartate (THA) were used to determine whether endogenous agents (pr esumably glutamate) could activate mGluRs at inhibitory terminals. Pre vious reports of the role of mGluRs on inhibitory terminals were perfo rmed in slice preparations; our use of patch-clamp recordings from iso lated pairs of hippocampal neurons is uniquely useful for the study of inhibitory synaptic transmission in the absence of polysynaptic conne ctivity. 2. The mGluR agonist (1S, 3R)-ACPD (100 mu M) reversibly decr eased the amplitude of evoked inhibitory postsynaptic currents (IPSCs) in all pairs tested; this effect was completely blocked by coapplicat ion of the mGdluR antagonist MCPG (500 mu M) with (1S, 3R)-ACPD. MCPG (500 mu M) alone had no effect on IPSC amplitude. These results indica te that all inhibitory neurons in our cultures express functional mGlu Rs in their terminals. 3. Examination of the frequency and the distrib ution of amplitudes of miniature IPSCs (mIPSCs) provide indications of changes in the sensitivity of postsynaptic receptors and/or of change s in the process of presynaptic transmitter release. Recordings of min iature currents from hippocampal neurons cultured at very low density makes possible the analysis of mIPSCs that arise from a single input, whereas in high density or slice preparations, spontaneous miniature c urrents reflect numerous synaptic inputs. No change in the amplitudes or frequency of the mIPSCs were observed upon application of (1S, 3R)- ACPD (100 mu M). Thus we conclude that the depression of the evoked IP SC amplitude by (1S, 3R)-ACPD is mediated by a presynaptic mechanism i n these isolated pairs of hippocampal neurons. 4. The glutamate uptake inhibitor L-trans-PDC also reduced IPSC amplitude in 8 of 13 pairs. I n these eight pairs, an increase in N-methyl-D-aspartate (NMDA) recept or-mediated membrane noise indicated an increase in ambient concentrat ions of glutamate induced by L-trans-PDC. In the remaining five pairs, membrane noise remained unaffected by L-trans-PDC, and IPSCs were not attenuated. Similar results were observed with the use of the uptake inhibitor THA. The mGluR antagonist MCPG blocked the effects of L-tran s-PDC and THA on IPSC amplitude. We propose that inhibition of glutama te uptake mechanisms results in activation of mGluRs on GABAergic term inals via endogenous sources of glutamate and that the uptake inhibito rs (L-trans-PDC and THA) do not directly activate the metabotropic rec eptor. 5. Presynaptic receptors and active modulation of uptake mechan isms are clearly involved in a wide range of physiological and patholo gical synaptic events. The data presented here suggest that heterosyna ptic modulation of inhibitory synaptic transmission by metabotropic gl utamate receptors may be important for the maintenance and plasticity of the balances between excitatory and inhibitory synaptic transmissio n in the CNS.