Differential effects of intracerebral microinjection of morphine on approach and escape responses induced by lateral hypothalamic stimulation in the mouse

Potentiation of brain stimulation reward by morphine: effects of neurokinin-1 receptor antagonism

RATIONALE

The abuse potential of opioids may be due to their reinforcing and rewarding effects, which may be attenuated by neurokinin-1 receptor (NK1R) antagonists.

OBJECTIVE

This study was conducted to measure the effects of opioid and NK1R blockade on the potentiation of brain stimulation reward (BSR) by morphine using the intracranial self-stimulation method.

METHODS

Adult male C57BL/6J mice (n = 15) were implanted with unipolar stimulating electrodes in the lateral hypothalamus and trained to respond for varying frequencies of rewarding electrical stimulation. The BSR threshold (θ(0)) and maximum response rate (MAX) were determined before and after intraperitoneal administration of saline, morphine (1.0-17.0 mg/kg), or the NK1R antagonists L-733,060 (1.0-17.0 mg/kg) and L-703,606 (1.0-17.0 mg/kg). In morphine antagonism experiments, naltrexone (0.1-1.0 mg/kg) or 10.0 mg/kg L-733,060 or L-703,606 was administered 15 min before morphine (1.0-10.0 mg/kg) or saline.

RESULTS

Morphine dose-dependently decreased θ(0) (maximum effect = 62% of baseline) and altered MAX when compared to saline. L-703,606 and L-733,060 altered θ(0); 10.0 mg/kg L-733,060 and L-703,606, which did not affect θ(0) or MAX, attenuated the effects of 3.0 and 10.0 mg/kg morphine, and 1.0 and 0.3 mg/kg naltrexone blocked the effects of 10.0 mg/kg morphine. Naltrexone given before saline did not affect θ(0) or MAX.

CONCLUSIONS

The decrease in θ(0) by morphine reflects its rewarding effects, which were attenuated by NK1R and opioid receptor blockade. These results demonstrate the importance of substance P signaling during limbic reward system activation by opioids.

Differential effects of naloxone on approach and escape responses induced by electrical stimulation of the lateral hypothalamus or the mesencephalic central gray area in mice

BALB/c mice implanted with a bipolar electrode were trained in a shuttle-box to initiate and to terminate a continuous electrical stimulation applied in the lateral hypothalamus (LH) or in the mesencephalic central gray area (CG). Following stabilization of the baseline response latencies, the subjects were subcutaneously injected with isotonic NaCl or with naloxone HCl (0.5, 2 or 10 mg/kg) 15 min or 45 min before a test session. In LH-stimulated animals no modification of the behavioral responses was observed after injection of 0.5 mg/kg of naloxone. The 2 mg/kg dose increased the value of escape latency (ON time) but had no effect on approach latency (OFF time). The 10 mg/kg dose also increased ON time. At this dose, an increase of OFF time was simultaneously observed but only 15 min after the injection. In CG-stimulated mice an increase of OFF time and a reduction of ON time were recorded 15 min after the injection of 0.5 mg/kg. Only the reduction of ON time was detected for the 45-min delay. The 2 mg and 10 mg/kg doses simultaneously increased OFF time and reduced ON time for the two delays. These results demonstrate 1) that the effects of naloxone on self-stimulation varied as a function of the structure considered 2) that the predominant characteristic of the considered structure (essentially "rewarding" as the LH or "aversive" as the CG) governs the modulations induced by naloxone.