Differential inhibition by propranolol of feeding induced in rats by various stimuli

Thermoregulation and performance of heat-reinforced autoshaped keypecking in chicks

Performance of autoshaped keypecking reinforced by heat was found to increase and then to decrease when chicks were exposed to training under a constant thermal load on a daily basis over the first 1-2 weeks after hatching. The decline in performance was not affected by variation in US duration (Experiment 1). No decline in performance was observed when food reinforcement was used (Experiment 2), suggesting that the effect depended on the use of thermal motivation. When thermoregulation was disrupted by pharmacological blockade using propranolol hydrochloride (10 mg/kg), keypecking was reinstated (Experiment 3). The increase in keypecking produced by propranolol was not due to a nonspecific energization of behavior inasmuch as the drug depressed autoshaped keypecking established with food reinforcement (Experiment 4). These findings support the conclusion that the decline in autoshaped keypecking reinforced by heat is due to development of the chick's thermoregulatory system. Various physical changes (e.g., feathering) and the increasing effectiveness of autonomic and behavioral thermoregulatory responses reduce the net motivational value of external heat sources, thus leading to a decline in the likelihood of approach and contact with signals for heat.

Opioid and non-opioid components of insulin-induced feeding

The present study was initiated to clarify the involvement of endogenous opioids in insulin-induced feeding. Naloxone (3 mg/kg) was injected in male Sprague Dawley rats every hour for 2 hours after insulin injection (10 U/kg). Only the first hour food intake was depressed (68% reduction). When naloxone was given only 1 hour after insulin administration, depression of food intake was not noted. When food was withheld for 2 hours after insulin injection, both naloxone and its long acting congener, naltrexone (3 mg/kg) were able to depress only the first hour feeding subsequent to food presentation. These data suggest that insulin-induced feeding can be divided into two pharmacologically distinct phases: the early phase being naloxone-sensitive while the late phase is naloxone-insensitive. Furthermore, the early phase begins with the presentation of food and not with the administration of insulin.

Effects of 2% sodium chloride imbibition on various opiate related hyperphagic conditions

Dynorphin is one of the most potent appetite stimulants among the endogenous opioids. In this study, we describe the anorexic effects of 5 days of forced 2% NaCl drinking in rats, a regimen which depletes vasopressin as well as dynorphin in the neurohypophysis. Feeding induced by direct activation of kappa-opioid receptors with ketocyclazocine was unaffected by the NaCl regimen. However, 2% NaCl imbibition reduced 2-deoxy-D-glucose (2-DG) induced feeding by 65% and spontaneous nocturnal feeding by 38%. Feeding subsequent to 24 hour food deprivation was not decreased. Naloxone-resistant hyperphagia induced by insulin and spontaneous daytime feeding were also not reduced. The combination of naloxone (3.0 mg/kg) and the NaCl regimen produced an additional 50% reduction in 2-DG induced feeding and an extra 40% decrease in nocturnal feeding. Naloxone, given with 2% NaCl to food deprived animals, retained its appetite suppressing activity, indicating that the NaCl regimen did not deplete the endogenous opioid which mediates food deprivation hyperphagia. These results demonstrate that 2% NaCl imbibition suppresses certain opioid mediated hyperphagias. However, the failure of 2% NaCl to affect all of the naloxone-sensitive types of feeding and the independence of naloxone-sensitive and NaCl-sensitive components suggests that NaCl drinking does not deplete dynorphin in the brain areas which mediate opiate-sensitive hyperphagias.