Differential control of GABA release at synapses from distinct interneurons in rat hippocampus

1. Paired recordings from monosynaptically connected CA3 interneurons and p yramidal cells of rat hippocampal slice cultures were used to compare the m odulation of GABA release at synapses from distinct interneurons. 2. The group II metabotropic glutamate receptor (mGluR) agonist (2S,2'R,3'R )-2-(2',3'-dicarboxylcyclopropyl) glycine (DCG-IV, 5 muM) reduced the ampli tude of IPSPs originating from stratum radiatum hut not stratum oriens inte rneurons. In contrast, the GABA(B) receptor agonist (-)baclofen (10 muM) re duced the amplitude of unitary IPSPs elicited by all interneurons. 3. IPSPs mediated by stratum oriens interneurons were unaffected by the N-t ype calcium channel blocker omega -conotoxin MVIIA (1 muM),ut were suppress ed by the P/Q-type blocker omega -agatoxin IVA (200 nM). In contrast, IPXPs mediated by stratum radiatum interneurons were abolished by omega -conotox in MVIIA. 4. Transmission dynamics were different at synapses from the two groups of interneurons. IPSPs mediated by stratum oriens interneurons showed marked p aired-pulse depression (PPD) at intervals of 50-400 ms. IPXPs mediated by s tratum radiatum interneurons showed paired pulse facilitation (PPF) at 50 m s and PPD at longer intervals. 5. The amplitude of unitary IPSPs from all interneurons was unaffected by t he GABA(B) receptor antagonist CGP52432 (2 muM) as was PPD at both 50 and 4 00 ms intervals. However, CGP52432 did reduce PPD of extracellularly evoked IPSPs. 6. Our results show that two groups of inhibitory synapses impinging onto C A3 pyramidal cells can be distinguished according to their dynamic and modu latory properties.