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High-frequency stimulation of pyramidal cell inputs to developing (P9-12) hippocampal stratum radiatum interneurons expressing GluR2-lacking, Ca(2+)-permeable AMPA receptors produces long-term depression of synaptic transmission, if N-methyl-d-aspartate (NMDA) receptors are blocked. Here we show that these same synapses display a remarkably versatile signal integration if postsynaptic NMDA receptors are activated. At synapses expressing GluR2-deficient AMPA receptors, tetanic stimulation that activates NMDA receptors triggered long-term potentiation or depression (LTP or LTD) depending on membrane potential. LTP was elicited at most synapses when the interneuron was held at -30 mV during the stimulus train but was typically prevented by postsynaptic hyperpolarization to -70 mV, by strong depolarization to 0 mV, by d-2-amino-5-phosphonovaleric acid, or by postsynaptic injection of the Ca2+ chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. At synapses with predominantly GluR2-containing AMPA receptors, repetitive stimulation did not change synaptic strength regardless of whether NMDA receptors were activated. The interactions among GluR2 expression, NMDA receptor expression, and membrane potential thus confer on hippocampal interneurons a distinctive means for differential decoding of high-frequency inputs, resulting in enhanced or depressed transmission depending on the functional state of the interneuron.

Regulation of synaptic transmission by metabotropic glutamate receptors (mGluRs) was examined at two excitatory inputs to interneurons with cell bodies at the granule cell-hilus border in hippocampal slices taken from neonatal rats. Subgroup-selective mGluR agonists altered the reliability, or probability of transmitter release, of evoked minimal excitatory synaptic inputs and decreased the amplitudes of excitatory postsynaptic currents (EPSCs) evoked with conventional stimulation. The group II-selective agonist, (2S,1R',2R',3R')-2-(2, 3-dicarboxylcyclopropyl) glycine (DCG-IV; 1 microM), reversibly depressed the reliability of EPSCs evoked by stimulation of the dentate granule cell layer. However, DCG-IV had no significant effect on EPSCs evoked by CA3 stimulation in the majority (82%) of hilar border interneurons. Both the group III-selective agonist, -(+)-2-amino-4-phosphonobutyric acid (-AP4; 3 microM), and the group I-selective agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG; 20 microM) reversibly depressed synaptic input to interneurons from both CA3 and the granule cell layer. We conclude that multiple pharmacologically distinct mGluRs presynaptically regulate synaptic transmission at two excitatory inputs to hilar border interneurons. Further, the degree of mGluR-meditated depression of excitatory drive is greater at synapses from dentate granule cells onto interneurons than at synapses from CA3 pyramidal cells.

The role of metabotropic glutamate receptors (mGluRs) and adenosine receptors in hypoxia-induced suppression of excitatory synaptic input to interneurons residing at the granule cell-hilus border in the dentate gyrus was investigated with the use of whole cell electrophysiological recording techniques in thin (250 microns) slices of immature rat hippocampus. Minimal stimulation evoked glutamatergic excitatory postsynaptic currents (EPSCs) in dentate interneurons in 68 +/- 4% (mean +/- SE) of trials during stimulation in the dentate granule cell layer (GCL) and 48 +/- 3% of trials during stimulation in CA3. Hypoxic episodes, produced by switching the perfusing solution from 95% O2-5% CO2 to a solution containing 95% N2-5% CO2 for 3-5 min, rapidly and reversibly decreased the synaptic reliability, or probability of evoking an EPSC, from either input without reducing EPSC amplitude, consistent with a presynaptic suppression of transmitter release. The mGluR antagonist (+)-alpha-methyl-4-carboxyphenylglycine [(+) MCPG; 500 microM] did not alter synaptic reliability or mean EPSC amplitude in either pathway. However, (+) MCPG significantly attenuated hypoxic suppression of input from both pathways, suggesting that mGluRs activated by release of glutamate partially mediate hypoxic suppression of EPSCs to dentate interneurons. The mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100 microM) rapidly decreased the reliability of excitatory transmission from both the GCL (19 +/- 5% of control) and CA3 (39 +/- 15% of control). ACPD also increased the frequency of spontaneous EPSCs and evoked a slow inward current in dentate interneurons. Exogenous adenosine (10-300 microM) decreased synaptic reliability for both pathways and reduced the frequency of spontaneous EPSCs, but did not cause a decrease in the mean amplitude of evoked EPSCs, consistent with a presynaptic suppression of excitatory input to dentate interneurons. Conversely, the selective adenosine A1 receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (200 nM to 1 microM) and N6-cyclopentyl-9-methyladenine (1 microM) enhanced excitatory input to dentate interneurons by increasing synaptic reliability for both the GCL and CA3 inputs. Adenosine A1 receptor antagonists did not, however, reduce hypoxic suppression of excitatory input to dentate interneurons. These results indicate that hypoxia induces a presynaptic inhibition of excitatory input to dentate interneurons mediated in part by activation of mGluRs, but not adenosine A1 receptors, whereas both mGluRs and adenosine A1 receptors can depress excitatory input to dentate interneurons during normoxic stimulation. Regulation of excitatory input to dentate interneurons provides a mechanism to shape excitatory input to the hippocampus under both normal and pathological conditions.