Inhibitory potentials produced in cortical cells by stimulation of the lateral hypothalamus in rabbits

(1) Intracellular potentials were recorded from pyramidal tract (PT) and non-pyramidal tract (non-PT) cells of the frontal cortex in urethane-anesthetized rabbits and the effects of electrical stimulation of the lateral hypothalamus (LH) were examined on the ipsilateral side. (2) Latencies of antidromic spikes of PT cells evoked by stimulation of the medullary pyramidal tract (PYR) had a unimodal distribution with the mean at 4.0 msec. The mean conduction velocity of the pyramidal tract fibers was 10.5 m/sec. (3) Prolonged IPSPs were produced in PT and non-PT cells by single shock stimulation of LH (LH-IPSP). They were significantly longer lasting than those produced by PYR stimulation (PYR-IPSP). (4) The latencies of LH-IPSPs ranged from 1.7 to 25.0 msec and were divided into two groups. The latencies of PYR-IPSPs had a unimodal distribution ranging from 2.5 to 30 msec. (5) In a few non-PT cells, a sequence of brief depolarization and prolonged hyperpolarization occurred in response to LH stimulation. (6) In some non-PT cells, EPSPs which occasionally resulted in spike discharges were observed after LH stimulation. However, no excitation of PT cells was observed by LH stimulation.