Changes in diurnal temperature and feeding patterns of rats during repeated injections of heroin and withdrawal

Rectral temperatures and feeding activity were measured in adult male rats every 30 min for 4 hr, then every 4 hr following daily s.c. injections at 08:00 or 20:00hr or saline or heroin. Initial heroin injections disrupted diurnal temperature and feeding rhythms: 5 mg/kg heroin HCl induced hyperthermia and abolished feeding for 2 hr; 20 mg/kg led to hypothermia, then hyperthermia and abolished feeding for 4 hr. By the 5th heroin day hyperthermia and increased feeding occurred in all groups with a shorter latency to onset. Total food intake was higher than on the 1st heroin day but the diurnal patterns remained disrupted. Changes in both diurnal rhythms again occurred on the 1st withdrawal day as hypothermia, sporadic feeding and hyper-irritability were observed. By the 5th withdrawal day diurnal temperature and feeding rhythms resembled those of the control period. Monitoring diurnal temperature and feeding patterns of rats reveals characteristic dose-related disruptions after heroin which are modified by repeated doses as tolerance develops and which eventually disappear on withdrawal.