Diazepam modulates lateral hypothalamic self-stimulation but not stimulation-escape in rats

Rats electrically stimulated via chronically implanted lateral hypothalamic (LH) electrodes were assessed with and without diazepam (DZ), for thresholds of stimulation-bound feeding (SBF) and for barpressing rates to administer and to escape from the same current. Six pure-reward rats, who self-stimulated but did not escape LH stimulation, exhibited SBF. Their electrode tips lay in medial forebrain bundle (MFB) and zona inserta along the entire rostral-caudal extent of the ventromedial nucleus of the hypothalamus (VMH). Six reward-escape rats, who self-stimulated and escaped from LH stimulation, did not (with one histologically deviant exception) show SBF. Reward-escape electrode tips were anterior to all the pure-reward placements. They lay in MFB rostral to the VMH up to the level of the bed nucleus of the stria terminalis (with the deviant electrode tip located on the zona inserta/ventral thalamic border). After i.p. injections of DZ, self-stimulation (SS) rates increased for both groups of animals and SBF thresholds decreased. Stimulation-escape (SE) rates, however, remained unchanged by the drug. The results are consistent with the existence of dual substrates: a DZ-sensitive reward system, present in both groups of animals, and a simultaneously stimulated, drug-resistant aversion system which is powerfully engaged in reward-escape animals only.