HYPERPOLARIZING SYNAPTIC POTENTIALS-EVOKED IN CA1 PYRAMIDAL CELLS BY GLUTAMATE STIMULATION OF INTERNEURONS FROM THE ORIENS-ALVEUS BORDER OFRAT HIPPOCAMPAL SLICES .2. SENSITIVITY TO GABA ANTAGONISTS

The receptor type mediating the inhibitory postsynaptic potentials (gl ut-IPSPs), recorded in CA] pyramidal cells, as a result of glutamate s timulation of interneurons in stratum oriens near the alveus (O/A) was assessed and compared to the type mediating recurrent IPSPs evoked by recurrent activation of interneurons through glutamate stimulation of pyramidal cells in stratum pyramidale (PYR). In response to repetitiv e electrical stimulation, the peak amplitude of both the O/A glut-IPSP and the PYR glut-IPSP was attenuated (n = 5) in parallel to the reduc tion in amplitude of the early and late components of the electrically evoked response (stimulus-evoked disinhibition). This suggested the i nvolvement of GABAergic receptors and attested that the interneurons a ctivated during glut-IPSPs were also involved in the circuitry of the electrically evoked IPSPs. The local application of the selective GABA (A) antagonist bicuculline (100-200 muM) to the slice resulted in a si gnificant reduction in the amplitude of both the O/A (by 76.5%; n - 9) and PYR (by 86.2%; n = 5) glut-IPSPs, in parallel to a decrease of th e electrically evoked early IPSP, but not of the late IPSP. The presen ce of the GABA(B) antagonist 2-hydroxy-saclofen (1 mM) was able to sig nificantly reduce the amplitude of the O/A glut-IPSPs (by 27.5%; n - 7 ) and of the electrically evoked late IPSP. but not the PYR glut-IPSP (n = 3). Although the application of phaclofen (20 mM) to the slice re duced the amplitude of the O/A glut-IPSPs (n = 3), the reduction was n ot statistically significant. These results suggest that recurrent IPS Ps elicited from activation of interneurons by stimulation of pyramida l cells are mediated solely via GABA(A) receptors. Inhibitory postsyna ptic potentials elicited from stimulation of interneurons in O/A were also mediated mostly by GABA(A) receptors, but in addition, displayed a minor component mediated by GABA(B) receptors. Therefore, since a la rge proportion of interneurons in O/A are recurrently excited by pyram idal cells (Lacaille J-C et al., 1987, J Neurosci 7: 1979-1993), and s ince recurrent IPSPs appeared mediated by GABA(A) receptors, a subpopu lation of interneurons activated from O/A might exist that do not rece ive recurrent excitation but can inhibit pyramidal cells via GABA(B) r eceptors.