Two types of medial hypothalamic inhibition of lateral hypothalamic reward.

Electrode-implanted rats pressed a lever to self-stimulate a reward mechanism in their lateral hypothalamus (LH) each time with a brief train of pulses. Interdigitation of a medial hypothalamus (MH) train into the ipsilateral LH train inhibited the reward as indicated by a reduced lever pressing rate. Such interdigitation into the contralateral LH train resulted in only a minimal inhibition accounted for by an across-midline current spread, thus indicating only an ipsilateral MH-to-LH route for inhibiting reward. By varying the phase of interdigitation according to a pulse-pair technique (cf. 20), the time course of this ipsilateral reward inhibition proved bimodal. The peripheral administration of strychnine, a glycine transmitter antagonist, resulted in disinhibition of the fast rising, fast decay (0.1-2.0 ms) intervals, while peripheral administration of picrotoxin, a GABA transmitter antagonist, increased bar pressing only at the slow rising, slow decay (2-20 ms) intervals. The placement of the MH electrodes was then varied dorsoventrally and the strength of the MH current was reduced to minimize dorsal-ventral current spread. From the resulting differential reductions in the two modes it was inferred that the neural elements in the MH responsible for short duration inhibitory effects on LH reward are concentrated more dorsally whereas those responsible for the longer duration inhibitory effects are concentrated more ventrally. These findings demonstrate the relevance of the pulse-pair technique for visualizing congruences between overt behavior and underlying neurophysiological, neuroanatomical and molecular events.