Environment alters sensitivity to naloxone-induced suppression of food intake

We evaluated the effect of environment on naloxone-induced suppression of feeding in streptozotocin rats and sham injected controls. Naloxone was administered to animals fasted for 24 hours and food intake was measured at 30, 60 and 120 minutes. Diabetic rats, in their home cages, were insensitive to naloxone's suppressive effect for the first 30 min and only the 5 mg/kg dose suppressed feeding at 60 and 120 min. In control rats feeding was suppressed at 0.1, 1 and 5 mg/kg naloxone during the first 30 min. In contrast, when animals were placed in novel plastic cages, control animals were insensitive to naloxone at all time points at doses as high as 5 mg/kg. In novel cages, diabetic rats responded to doses of 1 and 5 mg/kg during the first 30 min period by lowering food intake by 57%. It should also be noted that basal food intake was suppressed (40-53%) when animals were placed in novel cages. These data suggest that stress of a novel environment alters the neuroregulatory system involved in inducing feeding. Lack of response of normal rats to naloxone's suppressive effect in a novel environment suggests that a non-opioid feeding system operates under these conditons or opioid receptors are occupied as a result of the release of endogenous opioids due to stress. The opposite result observed in the diabetics indicates that glucose has a modulating effect on opioid effects.