Anorexia Nervosa as an Auto-Addiction Clinical and Basic Studies

The role of multiple opioid receptors in the potentiation of reward by food restriction

Chronic food restriction and weight loss were previously shown to produce a naltrexone-reversible facilitation of perifornical lateral hypothalamic self-stimulation. In the present study, high affinity receptor-selective antagonists were used to determine the particular opioid receptor type(s) that mediates the facilitation of reward by food restriction. Separate groups of food-restricted and ad libitum fed rats were used to conduct i.c.v. dose-response studies with TCTAP (mu), norbinaltorphimine (kappa), and naltrindole (delta). The highest dose of naltrindole (50.0 nmol) raised self-stimulation threshold independently of feeding condition. This suggests that delta opioid activity is involved in self-stimulation under basal conditions and may explain previous findings that high systemic doses of naloxone or naltrexone reduce self-stimulation. The highest doses of TCTAP and norbinaltorphimine (5.0 and 50.0 nmol, respectively) reversed the lowering of self-stimulation threshold produced by food restriction while having no effect on thresholds of ad libitum fed rats. These results suggest that state-dependent mu and kappa opioid activity facilitate reward. Since food restriction is known to increase the rewarding effect of food and drugs of abuse, the opioid mechanism identified in the present study may mediate adaptive behavior and, under some circumstances, pathological behavior. The possible relation of state-dependent opioid activity to Anorexia Nervosa, binge eating, and the high comorbidity of eating disorders and substance abuse is discussed.

Development of tolerance to endogenous opiates activated by 24-h food deprivation

Four experiments assessed the effects of exposure to 24-h food deprivation on the tail-flick latency of rats exposed to a temperature stimulus. Confirming previous studies, Experiment 1 showed that food deprivation gave rise to analgesia, as indicated by increased tail-flick latencies, that was antagonized by naloxone. Experiment 2 found that analgesia was greatly reduced after five exposures to periods of 24-h food deprivation (alternating with 24-h free access to food), indicating the development of tolerance. Experiments 3 and 3a examined the development of tolerance to the analgesic effects of morphine following repeated morphine injections, saline injections, and exposure to 24-h food deprivation plus saline injections. The combined results of both experiments provided evidence that repeated exposures to either morphine or food deprivation, produced greater tolerance to morphine than did exposures to saline. That food deprivation was cross-tolerant with morphine indicated that tolerance to food deprivation-induced analgesia involved opioid mechanisms. The relevance of opioid tolerance to psychobiological models of feeding and to the development of an animal model of anorexia nervosa was discussed.