Antagonist displacement of brain morphine during precipitated abstinence

Effect of naloxone on the morphine concentration in the central nervous system and plasma in rats

We investigated whether the distribution (concentration) of morphine in the central nervous system (CNS) after systemic administration could be an index for the in vivo binding of morphine. Neither the morphine concentration nor its decline correlated with the density of opiate receptors. Naloxone decreased the morphine concentration in some CNS regions and plasma after a high dose of morphine, but not after a small dose of morphine. The CNS regions in which naloxone decreased the morphine concentration did not correlate with the density of opiate receptors, and the concentration ratios (CNS regions versus plasma) of morphine were not affected by naloxone. These results suggested that the morphine concentration in the CNS did not reflect the in vivo binding of morphine and that the naloxone-induced decrease in morphine concentration was not due to a displacement of morphine from its receptor sites but due to a change in morphine kinetics. Pharmacokinetic studies suggested that naloxone decreased the morphine concentration through an increased volume of morphine distribution. This naloxone-induced decrease in morphine concentration may contribute to the naloxone-induced inhibition of morphine action in addition to the competitive antagonism at opiate receptors.

Naltrexone-induced alterations of the distribution of morphine in brain regions and spinal cord of the rat

The effects of naltrexone injected intravenously (i.v.) on the pharmacological actions and distribution of i.v. injected morphine in brain regions and spinal cord of male Sprague-Dawley rats were determined. Naltrexone (0.625- and 2.5-mg/kg doses) antagonized the analgesic and hyperthermic effects of morphine (10-mg/kg dose). For distribution studies, naltrexone (0.625- and 2.5-mg/kg doses) was co-administered with morphine via indwelling catheters. Rats were sacrificed at various times after drug injection and the concentration of morphine in brain regions (hypothalamus, hippocampus, cortex, pons and medulla, amygdala, midbrain and corpus striatum), spinal cord and serum was determined by radioimmunoassay. The concentration of morphine in various brain regions was found to be time dependent. Initially, at 5 min, the highest concentration of morphine was found in the hypothalamus and the lowest in the striatum. In cortex and spinal cord, the concentration of morphine was significantly higher in comparison to the other brain regions at 30- and 60-min time points. Co-administration of lower dose of naltrexone (0.625 mg/kg) did not significantly alter the distribution of morphine in brain regions and spinal cord with some exceptions. The higher dose of naltrexone (2.5 mg/kg) increased the concentration of morphine in several brain regions and spinal cord. The ratio of the concentration of morphine in brain region or spinal cord to serum was decreased by naltrexone. It is concluded that naltrexone also alters the distribution of morphine in the central nervous system.

Receptor binding, antagonist, and withdrawal precipitating properties of opiate antagonists

A number of opiate antagonists and the dextro isomers of some of these drugs were studied for antagonism of acute opiate effects on ilea isolated from opiate-naive guinea pigs, precipitation of a withdrawal contraction of ilea isolated from morphine-dependent guinea pigs, precipitation of withdrawal in morphine-dependent rhesus monkeys and stereospecific displacement of 3H-etorphine binding to rat-brain membranes. With the exception of d-naloxone, all of the compounds displaced 3H-etorphine. With the exception of d-naloxone, nalorphine, and quaternary nalorphine, all of the antagonists caused a contraction of ilea isolated from morphine-dependent guinea pigs. Moreover, the IC 50 values of the compounds for displacing 3H-etorphine binding were well correlated with both their Ke values for antagonism in the ileum (r = 0.95) and with their EC 50 values for precipitating a contraction in this preparation (r = 0.92). Generally, the concentration of antagonist necessary to precipitate half maximal contracture was 30-fold greater than the Ke value of the antagonist. Most of the opiate antagonists also precipitated withdrawal when administered to morphine-dependent rhesus monkeys and their in vivo potencies were well correlated with their in vitro potencies in ileum (with Ke: r = 0.95; with EC 50: r = 0.99) and in displacing 3H-etorphine (r = 0.95). The quaternary derivative of naltrexone, however, was an effective opiate antagonist only in vitro, and was ineffective in precipitating withdrawal in morphine-dependent rhesus monkeys. These results suggest that the receptor sites labeled by 3H-etorphine are the same as those involved in antagonism of acute opiate actions and in precipitation of withdrawal.

In vivo opiate binding unchanged in tolerant/dependent mice

The in vivo receptor binding was measured for an opiate agonist and antagonist in the brains of naive mice and of mice made tolerant to and dependent on opiates by morphine pretreatment. There was significant differences between the receptor binding of these substances in the naive and in the tolerant/dependent animals. However, these differences disappeared 8 h after interruption of the morphine supply, although tolerance showed no decline. Therefore, the differences in the binding were obviously not produced by changes in the opiate receptors related to tolerance. Rather, the differences were probably caused by morphine in the brain of the tolerant/dependent mice pretreated with morphine and disappeared because the concentration of the morphine in the brains of these animals dropped during abrupt withdrawal. The fact that tolerance persisted for hours beyond this time indicated that tolerance is a time-dependent phenomenon, not directly dependent on receptor occupation by an agonist.

Opiate antagonist--receptor interaction unchanged by acute or chronic opiate treatment

The apparent pA2 of naloxone, a measure of the apparent receptor affinity for the antagonist, was estimated in naive rats, in rats after acute and various schedules of chronic morphine pretreatment, and in rats during abrupt withdrawal. The results were adjusted to compensate for the amount of pretreatment drug in the rat brains as, otherwise, such a drug could have inflated the apparent pA2 values (see appendix). After this adjustment, a tendency of the apparent pA2 values to be increased after chronic pretreatment disappeared. The apparent pA2 of naloxone in acutely pretreated rats was also unchanged from that in naive rats, while the results in abruptly withdrawn rats were ambiguous. Thus, no change in the affinity of the antagonist for opiate receptors was found and theories suggesting that a conformational change takes place in the opiate receptor during the development of tolerance/dependence could not be supported.

The effects of naltrexone on the development of physical dependence on morphine

A single i.p. injection of naltrexone (20 mg/kg) partially inhibited the development of physical dependence upon morphine in mice rendered dependent on morphine by implantation of a pellet containing 75 mg of morphine free base for three days. This was evidenced by an increase in the dose of naloxone (ED50) required to precipitate withdrawal jumping response. The increase in naloxone ED50 was much more pronounced when naltrexone was given prior to and during the course of pellet implantation. Inhibition was also observed when naltrexone was administered one day after the morphine pellet implantation, i.e., after some dependence had already developed. Naltrexone administration prior to and during the development of dependence also inhibited, but only partially, the loss of body weight and hypothermic response observed during abrupt withdrawal of morphine in morphine-dependent mice. The inhibitory effect of naltrexone on morphine dependence development was not associated with changes in brain morphine concentration.

Inability of naloxone to change brain morphine levels in tolerant mice

The effect of naloxone on brain morphine concentrations was measured in naive and morphine-dependent mice using radioimmunoassay and gas-liquid chromatography. No displacement of morphine from the brain by naloxone could be observed in naive mice acutely injected with morphine or in pellet-implanted mice at increasing intervals after removal of the morphine pellets. The suggestion of a change in affinity of opiate receptors during the development of tolerance/dependence, which had been made on the basis of the displacement of morphine by naloxone found by other workers, is thus not supported by the present results.

Recent developments in opiate research and their implications for psychiatry

Considerable progress in opiate research has been made during the last few years regarding the identification and localization of opiate receptors in vitro and in vivo, the analysis of drug-receptor interactions and the characterization of an endogenous ligand of the opiate receptor. There is little evidence that effects induced by chronic exposure to opiates - development of tolerance and dependence -are due to changes in opiate receptor mechanisms; it is supposed that the adaptive changes occur mainly in the chain of events triggered by the drug-receptor interaction. Such changes may be directly or indirectly related to the metabolism of neurotransmitters and/or cyclic nucleotides. The obvious links between physical and psychic equivalents of opiate dependence are discussed. Present data points to the significance of brain stem and limbic structures in both these processes, monoamines probably playing an important role. Relations between psychic manifestations of opiate addiction and mental disorders are pointed out.