The influence of single daily heroin injections on drinking patterns in the rat

The licking behavior of male Sprague-Dawley rats individually housed in a controlled environment with a 12-h lighting schedule and food and water available ad libitum was recorded every 30 min. Body weight and 24-h fluid intake were measured daily at 0800 hours, after which subcutaneous injection was given. When saline (0.3 mL/100 g) was injected as a control for 5 days, only a small percentage of total daily licking behavior occurred during the light hours. Heroin hydrochloride (5 or 20 mg/kg) was then injected for the next 10 days. The first injection of heroin caused a cessation of licking behavior followed by a bout of daytime drinking so that the percentage occurring during the light hours increased. This "stimulated" licking occurred progressively earlier as heroin injections were repeated. Total daily fluid intake and rate of weight gain during heroin treatment were not significantly different from those in the control period, although they tended to be less with the higher dose. Withdrawal from this dose led to mild transient hyperirritability (Teiger scores) during the first 36 h and another change of licking behavior. The results suggest that the diurnal licking pattern of rats drinking ad libitum is especially vulnerable to disruption by heroin.