Sweetness, reward and analgesia

Comparison of mu opioid receptors in brains of rats bred for high or low rate of self-stimulation

Opiates and endogenous opioid peptides play an important role in reward-mediated behaviors, including self-stimulation. Two strains of rats, LC2-Hi and LC2-Lo, selectively bred for high vs. low rate of lateral hypothalamic self-stimulation, were employed in the present study. Quantitative autoradiography was performed on brains of adult male rats of each strain, using the mu opioid receptor agonist 3H-DAGO. Strain differences in receptor density were observed in the nucleus accumbens and in ventral areas of the hippocampus.

Opiate antagonists and self-stimulation: extinction-like response patterns suggest selective reward deficit

The present study investigated the response decrement patterns produced by opiate antagonists on intracranial self-stimulation behavior, in order to determine if these drugs affect the reinforcement value of the stimulation or interfere with the ability of the animal to respond. Male rats lever-pressed in 60-min sessions on a continuous reinforcement schedule for self-stimulation of the nucleus accumbens. Naloxone (2.0 and 20 mg/kg) and naltrexone (2.0 and 20 mg/kg) suppressed self-stimulation only after a significant delay, in an extinction-like response decrement pattern, mimicking the effects of reductions in current intensity (75% and 50% of baseline). The increasing behavioral effects characteristic of the extinction pattern were observed despite the fact that testing began after the time point at which maximal suppression of self-stimulation occurs with these drugs, and when brain concentrations of these drugs were declining. Since normal responding was observed for several minutes after the beginning of the session, the results may explain why long sessions are necessary to observe suppression of self-stimulation by opiate antagonists. The extinction-like pattern produced by these drugs suggests that opiate antagonists suppress self-stimulation by reducing the reinforcement value of the stimulation, rather than by interfering with the ability of the animal to respond. These findings are consistent with a role for endogenous opioid peptides in brain stimulation reward.

Naloxone suppression of self-stimulation is independent of response difficulty

The action of the opiate antagonist naloxone on relatively easy (nose-poke) and relatively difficult (lever-press) self-stimulation behaviors was compared, in order to determine if opiate antagonists suppress self-stimulation by interfering with the ability of the animal to respond, or by reducing the reinforcement value of the stimulation. Naloxone (0.2, 2.0 and 20 mg/kg) significantly suppressed both nose-poking and lever-pressing self-stimulation rates, and the degree of suppression was virtually identical for both tasks at all doses examined. If naloxone had interfered with the ability of the animal to respond, then lever-pressing--which requires more motor output than nose-poking--should have been more suppressed than nose-poking. The results suggest that opiate antagonists do not interfere with the ability of the animal to respond, and are therefore consistent with the hypothesis that these drugs reduce the reinforcement value of the stimulation.

Multiple opiate receptors and pharmacological response to morphine in rats maintained on diets differing in protein concentration

1. The influence of diets differing in protein concentration on the characteristics of mu, delta, and kappa-opiate receptors and on the analgesic and hyperthermic responses to morphine was examined in rats. Three groups of male Sprague-Dawley rats were maintained for four weeks on isocaloric diet containing either 4, 20 or 50% protein. 2. The animals maintained on 4% protein diet weighed 92 +/- 2% of the initial weight at the end of the fourth week, whereas animals maintained on 20% and 50% protein diet weighed 222 +/- 2% and 221 +/- 2%, respectively. The average food intake per 100 g body weight on day 1 of the study in 4, 20 and 50% protein diet group was 5.0 +/- 2.2 g, 10.4 +/- 1.4 g, and 10.0 +/- 1.2 g, respectively. This difference in food intake was not observed during rest of the period of the study. 3. Water intake was higher for the animals maintained on diet containing 50% protein as compared to the other two groups. 4. The analgesic and the hyperthermic response to morphine varied in direct relation to the concentration of protein in the diet. The concentration of morphine in the brain and plasma of animals maintained on the three diets following challenge dose of morphine did not differ. Similarly the Bmax and Kd values for the binding of [3H]naltrexone, [3H]D-Ser2-Thr6-leucine enkephalin and [3H]ethylketocyclazocine to brain membranes prepared from rats kept on 4, 20 and 50% protein concentration did not differ. 5. It is concluded that the diet differing in protein concentration can alter the responses to morphine, and that such altered effects can not be accounted for by the changes in the distribution of morphine in brain and plasma or to the changes in the characteristics of the mu, delta and kappa opiate receptors in the brain.