Contribution ofIhand GABABto Synaptically Induced Afterhyperpolarizations in CA1: A Brake on the NMDA Response

Abstract

CA1 pyramidal cells receive two major excitatory inputs: the perforant path (PP) terminates in the most distal dendrites, whereas the Schaffer collaterals (SC) terminate more proximally. We have examined the mechanism of the afterhyperpolarization (AHP) that follows single subthreshold excitatory postsynaptic potentials (EPSPs) in these inputs. The AHPs were not reduced by a GABA_A antagonist or by agents that block Ca²⁺ entry. Application of the I_h blocker, ZD7288, partially blocked the AHP in the PP; the substantial remaining component was blocked by 2-hydroxysaclofen, a GABA_B antagonist. In contrast, the AHP in the SC depends nearly completely on I_h, with almost no GABA_B component. Thus postsynaptic GABA_B receptors appear to be preferentially involved at distal synapses, consistent with the spatial distribution of GABA_B receptors and g protein-coupled inward rectifying potassium (GIRK) channels. GABA_B does, however, play a role at proximal synapses through presynaptic suppression of glutamate release, a mechanism that is much weaker at distal synapses. Experiments were conducted to explore the functional role of the AHP in the PP, which has a higher N-methyl-d-aspartate (NMDA)/AMPA ratio than the SC. Blockade of the AHP converted a response that had a small NMDA component to one that had a large component. These results indicate that the I_h and postsynaptic GABA_B conductances act as a brake on distally generated NMDA responses.