Evidence for the existence of inhibitory postsynaptic potentials in dendrites and their functional significance in hippocampal pyramidal cells of adult rabbits.

(1) Hyperpolarizing potentials were studied in hippocampal pyramidal cells of normal, adult rabbits, anesthetized with Nembutal and, in addition, curarized. (2) Single shock stimulation of the fornix produced in these cells long-lasting hyperpolarizing potentials (400-2000 msec). As a rule, the early part of these potentials (about 200 msec from the onset) was influenced by the diffusion of Cl ions into the cell and current injections, whereas their late part was not. The early part exhibited impedance changes whereas the late part did not. From these results it was concluded that the former was the inhibitory postsynaptic potential (IPSP) occurring in the soma while the latter originated in the dendrites. Evidence was obtained suggesting that the latter was also a Cl-mediated IPSP. The hyperpolarization elicited in these cells by mesencephalic stimulation was similar in properties to the latter. (3) Disfacilitation in the dendrites and the functional significance of the dendritic IPSP were discussed.