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

The receptor type mediating the inhibitory postsynaptic potentials (glut-IPSPs), recorded in CA] pyramidal cells, as a result of glutamate stimulation 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 repetitive 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 reduction in amplitude of the early and late components of the electrically evoked response (stimulus-evoked disinhibition). This suggested the involvement of GABAergic receptors and attested that the interneurons activated 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 significant 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 the electrically evoked early IPSP, but not of the late IPSP. The presence of the GABA(B) antagonist 2-hydroxy-saclofen (1 mM) was able to significantly 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 reduced the amplitude of the O/A glut-IPSPs (n = 3), the reduction was not statistically significant. These results suggest that recurrent IPSPs elicited from activation of interneurons by stimulation of pyramidal cells are mediated solely via GABA(A) receptors. Inhibitory postsynaptic 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 large proportion of interneurons in O/A are recurrently excited by pyramidal cells (Lacaille J-C et al., 1987, J Neurosci 7: 1979-1993), and since recurrent IPSPs appeared mediated by GABA(A) receptors, a subpopulation of interneurons activated from O/A might exist that do not receive recurrent excitation but can inhibit pyramidal cells via GABA(B) receptors.