Heterologous modulation of inhibitory synaptic transmission by metabotropic glutamate receptors in cultured hippocampal neurons

1. Whole cell patch-clamp recordings of monosynaptically connected pairs of hippocampal neurons in very low-density culture were performed to determine the effects of the activation of metabotropic glutamate receptors (mGluRs) on inhibitory terminals. The mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid [(1S, 3R)-ACPD] and the recently described mGluR antagonist(RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) were used. In addition, the glutamate uptake inhibitors L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC) and D,L-beta-threo-hydroxyaspartate (THA) were used to determine whether endogenous agents (presumably glutamate) could activate mGluRs at inhibitory terminals. Previous reports of the role of mGluRs on inhibitory terminals were performed in slice preparations; our use of patch-clamp recordings from isolated pairs of hippocampal neurons is uniquely useful for the study of inhibitory synaptic transmission in the absence of polysynaptic connectivity. 2. The mGluR agonist (1S, 3R)-ACPD (100 mu M) reversibly decreased the amplitude of evoked inhibitory postsynaptic currents (IPSCs) in all pairs tested; this effect was completely blocked by coapplication 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 indicate that all inhibitory neurons in our cultures express functional mGluRs in their terminals. 3. Examination of the frequency and the distribution of amplitudes of miniature IPSCs (mIPSCs) provide indications of changes in the sensitivity of postsynaptic receptors and/or of changes in the process of presynaptic transmitter release. Recordings of miniature 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 currents 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 IPSC amplitude by (1S, 3R)-ACPD is mediated by a presynaptic mechanism in these isolated pairs of hippocampal neurons. 4. The glutamate uptake inhibitor L-trans-PDC also reduced IPSC amplitude in 8 of 13 pairs. In these eight pairs, an increase in N-methyl-D-aspartate (NMDA) receptor-mediated membrane noise indicated an increase in ambient concentrations 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-trans-PDC and THA on IPSC amplitude. We propose that inhibition of glutamate uptake mechanisms results in activation of mGluRs on GABAergic terminals via endogenous sources of glutamate and that the uptake inhibitors (L-trans-PDC and THA) do not directly activate the metabotropic receptor. 5. Presynaptic receptors and active modulation of uptake mechanisms are clearly involved in a wide range of physiological and pathological synaptic events. The data presented here suggest that heterosynaptic modulation of inhibitory synaptic transmission by metabotropic glutamate receptors may be important for the maintenance and plasticity of the balances between excitatory and inhibitory synaptic transmission in the CNS.