Disruption of diurnal feeding patterns of rats by heroin

Adult male rats receiving 5 or 20 mg/kg heroin HCl by single injections (08:00 or 20:00 hr) or in 3 equal injections (8 hr intervals) showed a disruption in the normal diurnal pattern of behavior. Initially, heroin abolished feeding for several hr after the injection, reduced the total daily food consumption in a dose-related manner, due primarily to decreased night-time feeding, and prevented or slowed weight gain. Subsequent heroin injections led to a phase of vigorous feeding following the period of depression. Magnitude and duration of the depression decreased, but the stimulatory phase of feeding became more pronounced as tolerance developed. Total daily food intake and body weight returned towards control levels, but the proportion eaten during daylight hr became elevated. Sporadic feeding occurred on the first withdrawal day with abolition of the stimulatory phase which had followed each heroin injection. Subsequently, the normal diurnal pattern of behavior gradually returned. Close measurement of 24 hr food consumption may be a sensitive and valuable measure of the disruptive effects of narcotic analgesics.