Acute stimulation of feeding with repeated injections of morphine sulphate to non-obese and fatty Zucker rats

Abstract

1. 1. Food intake studies with genetically obese rodents show that these hyperphagic animals, which have increased central and peripheral levels of endogenous opioid peptides (E.O.P.), have an increased sensitivity to the suppressive feeding effects of narcotic antagonists compared to lean controls. Feeding experiments were conducted to determine if genetically obese rats, with enhanced E.O.P., have a reduced sensitivity toward the narcotic agonist property of stimulated feeding seen in non-obese rats. 2. 2. Food intake was monitored continuously over each experimental day in groups of female Sprague-Dawley (S.D.,), fatty Zucker (fa/fa) and their lean heterozygote littermates (Fa/ fa) following subcutaneous a.m. injections of sterile saline, morphine sulphate (5 or 10 mg/kg) or naloxone HCl (10 mg/kg) and during recovery. 3. 3. Acute 4-h post-injection feeding was reduced in all groups with the first 10 mg/kg injection of morphine sulphate. With repeated morphine administration, a phase of stimulated feeding occurred in both obese and non-obese groups. 4. 4. Due to the post-injection phase of vigorous feeding with repeated morphine injections, the circadian pattern of day/night food intake of all groups was altered such that daytime feeding increased from saline control levels. 5. 5. Naloxone HCl abolished the post-injection phase of stimulated feeding seen with chronic morphine injections and reduced 4-h post-injection food intakes. 6. 6. Plasma glucose and serum insulin levels were decreased in non-obese rats from saline controls of blood samples taken 2-h following the 7th daily M.S. injection. These levels increased again by the end of the recovery period. No blood glucose or insulin changes were seen in the obese Zucker rats with morphine administration. 7. 7. The results show that daily morphine injections can cause acute stimulated feeding in obese, fatty Zucker rats as well as in non-obese rats.