Opiates and deer mouse behaviour: differences between island and mainland populations

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Abstract
A number of the behavioural and ecological characteristics of island populations of small mammals differ from their mainland counterparts. Little is known, however, about possible neurochemical correlates of these behavioural differences. Substantial evidence indicates that endogenous opioid peptides are differentially involved in the regulation of the expression of fundamental behavioural and physiological functions, including responses to aversive stimuli and enhanced nociceptive thresholds (analgesia). In the present study, we compared the effects of peripherally administered δ- κ-, and μ-opiate receptor directed agonists and antagonists on the analgesic responses and locomotor activity of four different populations of male and female deer mice: Peromyscus maniculatus artemisiae and P. m. nebrascensis from mainlands, and P. m. angustus and P. m. triangularis from small islands. The insular deer mice displayed markedly greater μ-opiate and significantly lower δ- and κ-opiate mediated responses than the mainland animals. In all of the populations males displayed significantly greater opiate-induced analgesic responses and locomotory changes than females. These results demonstrate that there are marked population and sex differences in the opiate-mediated behavioural responses of deer mice. These "pharmaco-ecological" findings also suggest that the behavioural differences between island and mainland populations of deer mice may, in part, be related to differences in opioid activity.