Effect of dorsomedial hypothalamic nuclei knife cuts on ingestive behavior.

Previous findings show that rats with electrolytic or excitotoxic lesions in the dorsomedial hypothalamic nucleus (DMN) are hypophagic and hypodipsic and have reduced ponderal and linear growth but normal body composition. DMN-lesioned (DMNL) rats also show altered ingestive responses to naloxone. The present study investigated the intrahypothalamic nerve pathways involved in these DMNL effects and the response of the pathways to deprivation challenges by placing knife cuts posterior (Post), lateral (Lat), ventral (Vent), dorsal, or anterior to the DMN or by administering sham operations. One major finding was that rats with Post or Vent were hypophagic (P < 0. 05) and had reduced body weight but responded normally to deprivation challenges. Post and Lat groups were hypodipsic (P < 0. 05), but plasma Na+, K+, and osmolality and 24-h post-water-deprivation drinking responses were similar in all groups. Naloxone did not suppress the intake of Post rats. It appears that the hypophagia and the reduced body weight after DMNL involve fibers entering or leaving the DMN from ventral and posterior directions, and they may be part of an opioid feeding system.