Behavioral effects of opiates: a pharmacogenetic analysis

A number of studies are reviewed in relation to a pharmacogenetic approach to the effect of opiates. The behavioral effects of morphine and heroin in different species or strains of animals are considered. In particular, a number of behavioral, neurophysiological, and biochemical correlates of the opiates in different inbred strains of mice are cited. Recent studies concerning the effects of opiates on behavior have utilized the C57BL/6J and the DBA/2J strains, which are characterized by different brain levels and turnover of cholinergic and adrenergic mediators. It has been shown that the effects of opiates on running activity and on analgesia are strain-dependent, and a negative correlation is evident between the two measures in the strains considered. Experiments carried out on mice with septal lesions and on normal mice have confirmed that the motor and analgesic effects of morphine in the mouse are two distinct phenomena, which may be explained through different neurophysiological and biochemical models. Differences between the strains considered have also been observed when the ECoG response to morphine administration has been investigated. The results of these experiments have suggested: 1. The existence of a correlation between behavioral activation and sleep-like ECoG patterns. 2. The existence of a similarity between the effects of the anticholinergic drugs and those of morphine, since a dissociation between ECoG and behavior became evident following morphine administration. Some studies have also suggested that the environmental factors play an important role in determining the stimulating effects of morphine. This effect was absent in "experience" mice (i.e., in subjects already tested in the apparatus), as compared with naive "inexperienced" animals.