Pharmacological characterization of glycine-gated chloride currents recorded in rat hippocampal slices

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus has been overlooked, largely due to the misconception that GlyR expression ceases early during development and to few functional studies demonstrating their presence. As a result, little is known regarding the physiological and pharmacological properties of native GlyRs expressed by hippocampal neurons. In this study, we used pharmacological tools and whole cell patch-clamp recordings of CA1 pyramidal cells and interneurons in acutely prepared hippocampal slices from 3- to 4-wk old rats to characterize these understudied receptors. We show that glycine application to recorded pyramidal cells and interneurons elicited strychnine-sensitive chloride-mediated currents (I(gly)) that did not completely desensitize in the continued presence of agonist but reached a steady state at 45-60% of the peak amplitude. Additionally, the inhibitory amino acid, taurine, which has been shown to activate GlyRs in other systems, activated GlyRs expressed by both pyramidal cells and interneurons, although with much less potency than glycine, having an EC(50) 10-fold higher. To examine the potential subunit composition of hippocampal GlyRs, we tested the effect of the GABA(A) receptor antagonist, picrotoxin, on I(gly) recorded from both cell types. At low micromolar concentrations of picrotoxin (< or =100 microM), which selectively block alpha homomeric GlyRs, I(gly) was partially attenuated in both cell types, indicating that alpha homomeric receptors are expressed by pyramidal cells and interneurons. At picrotoxin concentrations < or =1 mM, approximately 10-20% of the whole cell current remained, suggesting that alphabeta heteromeric GlyRs are also expressed because this subtype of GlyR is relatively resistant to picrotoxin antagonism. Finally, we examined whether hippocampal GlyRs are modulated by zinc. Consistent with previous reports in other preparations, zinc elicited a bidirectional modulation of GlyRs, with physiological zinc concentrations (1-100 microM) increasing whole cell currents and concentrations >100 microM depressing them. Furthermore, the same concentration of zinc that potentiates I(gly) suppressed currents mediated by the N-methyl-D-aspartate subtype of the glutamate receptor. Thus we provide a pharmacological characterization of native GlyRs expressed by both major neuron types in hippocampus and show that these receptors can be activated by taurine, an amino acid that is highly concentrated in hippocampus. Furthermore, our data suggest that at least two GlyR subtypes are present in hippocampus and that GlyR-mediated currents can be potentiated by zinc at concentrations that suppress glutamate-mediated excitability.