Excitatory connection from lateral hypothalamic self-stimulation sites to escape sites in medullary reticular formation

Abstract

Behavioral tests show that noncontingent stimulation of self-stimulation sites in the lateral hypothalamus increases escape and avoidance from central or peripheral stimuli. The present study investigated the possibility that this phenomenon was mediated by an excitatory connection from the lateral hypothalamus to neurons in aversive regions of the brainstem. Most of the 24 rats participated in two phases of the experiment: unit recording under anesthesia; and behavioral tests with intracranial stimulation in the waking state. Recordings of 120 units were obtained from the n. gigantocellularis reticularis (NGC) since stimulation of this region in the awake rat consistently elicited escape behavior. Unit responses could be divided into two catagories: 61% responded to deep pinch or foot shock (or both) but not to light brushing of the hair; and 39% responded to light brushing of the hair or were not responsive. Stimulation of behaviorally confirmed self-stimulation sites in lateral hypothalamus evoked short-latency (2 and 4 msec) excitation of 88% of the NGC units in the first category and only 15% in the second. Nearly all hypothalamic sites not eliciting self-stimulation also failed to excite NGC units. The excitatory hypothalamic-bulboreticular connection may serve to increase the excitability of a nociceptive system during self-stimulation.