Morphine-induced effects on the hypothalamic stimulation-evoked emotional/behavioral reactions and peculiarities of abstinent syndrome in adult and old rats

Experiments were carried out on adult young (6- to-9-month-old) and old (28- to-30-month-old) rats. The effects of a single i.p. injection of morphine on self-stimulation of the lateral hypothalamic region and on active avoidance responses evoked by stimulation of the ventromedial hypothalamic nucleus were studied. In a separate series of the experiments we studied age-related specificities of the abolition syndrome after a course of intraventricular injections of morphine. In most old rats single injections of morphine resulted in intensification of self-stimulation, while in most adult rats such injections suppressed this reaction. In old animals injections of morphine resulted in more pronounced increases in the threshold and latency of the active avoidance responses, as compared with similar changes in adult rats. Natural abolition of a 15-day-long course of injections of morphine resulted in the development of a characteristic abstinent syndrome that was more intensively manifested in old rats. Our results show that “hedonic” and anti-aversive effects of morphine increase with aging; this phenomenon probably facilitates the development of morphine dependence in old organisms.