The effect of p-chlorophenylalanine on the pethidine- or methadone-induced decrease in locomotor activity of rats

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

Methadone effects on locomotor activity of young and aged mice

The effect of age on the behavioral response to methadone hydrochloride was examined in male C57BL/6J mice. After 1 hr habituation to an oval runway, locomotor activity of young (6-8 months) and aged (30-32 months) mice was monitored for 3 hr following injections of saline or methadone hydrochloride (2.5, 7.5, 15.0 or 22.5 mg/kg). The three highest doses initially elevated activity which then declined over the next 1 1/2 to 2 1/2 hr. Compared to young mice, activity of aged mice was elevated less extensively, but remained elevated for a longer period of time. The attenuated elevation could be interpreted in terms of reduced physical capacity of aged mice; however, the more prolonged elevation was clearly an age-related difference in reaction to the drug. Although the mechanism accountable for this effect is unknown, it is not likely related to age differences in drug absorption or distribution to active sites.

Role of 5-hydroxytryptamine in morphine-, pethidine-, and methadone-induced hypothermia in rats at low ambient and room temperature

1 The effect of morphine (10 or 20 mg/kg s.c.), pethidine (25 or 50 mg/kg s.c.) or methadone (4 or 8 mg/kg s.c.) on the body temperature of nontreated and p-chlorophenylalanine-pretreated rats was studied at room (21+/-0.2 degrees C) or low ambient (12+/-0.2 degrees C) temperature. 2 Neither pethidine nor smaller doses of morphine and methadone altered the mean rectal temperature of rats kept at room temperature but larger doses of morphine and methadone produced significant hypothermia. 3 All narcotic analgesics at doses used in the present investigation produced significant hypothermia in rats maintained in a low ambient temperature. The hypothermia was prevented by naloxone (1 mg/kg s.c.). 4 The administration of p-chlorophenylalanine (PCPA, 320 mg/kg i.p.) 48 h before the narcotic injection prevented the fall in body temperature both at room and low ambient temperature. 5 The administration of narcotic analgesics at doses, which when administered by themselves did not alter the body temperature of rats, produced significant hyperthermia in rats pretreated with PCPA. 6 When rats pretreated with PCPA were given 5-hydroxytryptophan (75 mg/kg s.c.) 30 min before narcotic administration, the usual response to narcotics was restored. 7 It appears that pethidine and methadone as well as morphine have both hyperthermic and hypothermic actions in rats and that 5-hydroxytryptamine may be involved in the narcotic-induced hypothermia not only at room temperature but also at low ambient temperature.

Effects of narcotic analgesics on serotonin metabolism in brain of rats and mice

The effects of narcotic analgesics on the brain 5-hydroxytryptamine (5-Ht) and 5-hydroxyindoleacetic acid (5-HIAA) levels of rats and mice were investigated in relation to our preceding data on the effect of humoral modulatorents. The results suggest that morphine accelerates the release of brain 5-HT both in rats and mice, and that neither methadone nor pethidine alters the brain 5-HT and 5-HIAA levels in rats. The morphine-induced increase in brain 5-HT turnover is likely to be involved in the morphine-induced decrease in locomotor activity and hypothermia in rats. The activity-decreasing effects of methadone or pethidine, on the other hand, are mediated by mechanisms different from those which mediate the effects of morphine. In contrast, an increase in brain 5-HT turnover in mice apparently does not play an important role on activity-increasing effects of morphine but rather participates in other pharmacological effects of morphine.