HYPERPOLARIZING SYNAPTIC POTENTIALS-EVOKED IN CA1 PYRAMIDAL CELLS BY GLUTAMATE STIMULATION OF INTERNEURONS FROM THE ORIENS-ALVEUS BORDER OFRAT HIPPOCAMPAL SLICES .1. ELECTROPHYSIOLOGICAL RESPONSE PROPERTIES

To examine the inhibitory postsynaptic potentials (IPSPs) elicited in pyramidal cells by interneurons situated at the stratum oriens/alveus border (O/A), glutamate was applied by micropressure to this area duri ng intracellular recordings from CA1 pyramidal cells. Glutamate stimul ation evoked IPSPs (glut-IPSPs) of small amplitude (4 mV). delayed pea k latency (100-110 ms), and long duration (300-400 ms). Recurrent acti vation of interneurons via glutamate stimulation of pyramidal cells by local application in stratum pyramidale (PYR) evoked recurrent IPSPs (PYR glut-IPSPs) with similar amplitude and time course as O/A glut-IP SPs. The mean equilibrium potential of O/A glut-IPSPs ( - 77 mV) was s ignificantly different from that of the PYR glut-IPSPs (-71 mV), howev er, neither equilibrium potential was significantly different from tha t of the electrically evoked early IPSP in the same cells. Glutamate-e voked IPSPs elicited from O/A displayed some response reversal (27% re versal) like those evoked from PYR (41% reversal). The early IPSP evok ed by electrical stimulation displayed significantly more response rev ersal (67% reversal) than glut-IPSPs. Both types of glut-IPSPs (O/A an d PYR) were associated with moderate increases in membrane conductance (5.9 and 6.6 nS. respectively), which were significantly less than th e conductance change associated with the early IPSP (45.8 nS). In inte rneurons within PYR, glutamate stimulation in PYR readily elicited a f lurry of excitatory postsynaptic potentials, whereas glutamate stimula tion in O/A elicited IPSPs. The electrophysiological properties of IPS Ps elicited in pyramidal cells by glutamate stimulation of interneuron s in O/A were similar to those of recurrent IPSPs evoked from PYR. Giv en that both of these types of glutamate-evoked IPSPs were mostly medi ated via GABA(A) receptor channels (Samulack DD, Lacaille J-C, 1993, H ippocampus 3:345-358), the small differences observed between equilibr ium potentials. response reversals, and conductance changes could be d ue to a more electrotonically distant location from the soma of the sy napses involved in O/A glut-IPSPs as compared to those of recurrent IP SPs elicited from PYR.