Abstracts

RATIONALE: Previously we have shown that strychnine-senstive glycine-gated chloride channels (GlyRs) are functionally and anatomically expressed by CA1 and CA3 hippocampal neurons in slices prepared from 3-4 week old rats. In this study, we tested the hypothesis that 1) functional GlyRs are also expressed in dentate gyrus and 2) activation of GlyRs can suppress neuronal hyperexcitability in the dentate, CA3 and CA1 subfields of rat hippocampus.
METHODS: A combination of immunohistochemical techniques, whole-cell patch clamp and extracellular popspike recordings were performed in acutely prepared hippocampal slices from 3-4 week old rats. Synaptic transmission was electrically evoked via a stimulating electrode placed in the perforant path or stratum radiatum. Agonists and antagonists were applied via bath perfusion or by pressure ejection.
RESULTS: The presence of GlyRs was confirmed in dentate by observing specific GlyR labeling of cell bodies and dendrites of granule cells and hilar interneurons and by observing robust glycine-mediated currents (I[sub]gly[/sub]) recorded from both dentate cell types (n=6). Application of 300 [mu]M glycine elicited I[sub]gly[/sub] that were unaffected by blockade of Na⁺ channels and ionotropic glutamate and GABA[sub]A[/sub] receptors, but was reversibly blocked by the GlyR antagonist, strychnine (1 [mu]M). Activation of GlyRs depressed the dentate synaptic network as indicated by 1) reducing suprathreshold EPSPs to subthreshold events, 2) decreasing the amplitude of EPSCs by ~35% and IPSCs by ~20% and 3) decreasing the amplitude of granule cell popspikes by ~35%. Blockade of GlyRs with strychnine (1[mu]M) increased neuronal excitability as demonstrated by an increase in the amplitude of the granule cell popspike as well as an increase in EPSC amplitude recorded from granule cells. These data indicate that basally active GlyRs contribute to the ambient level of neuronal activity. Application of glycine (0.3-1 mM) to hyperexcitable slices induced by 8.5 mM extracellular K⁺ depressed the excitability of granule cells and pyramidal cells in whole cell current clamp (figure) and popspike recordings. Blockade of GlyRs with strychnine reversed the anti-epileptic effect of glycine.
CONCLUSIONS: Our results show that GlyRs are expressed in all subfields of hippocampus and suggest that activation of GlyRs can limit the activity in synaptic circuits and suppress neuronal hyperexcitability.[figure]
Support: The Epilepsy Foundation (SC).