Morphine administration or sexual segregation in infancy affect the response to the same drug in adult mice

Several experiments indicate that CNS opioid regulatory systems show a remarkable plasticity during development. The same systems respond to a wide range of environmental stimuli, particularly those which can affect the threshold of pain sensitivity (e.g., Environmentally Induced Analgesia). This paper summarizes a series of studies using outbred CD-1 mice, aimed at assessing: a) morphine effects on pain sensitivity and locomotor activity at two ages during development, namely, before and after weaning, and b) the consequences of such exposure on adult sensitivity to the same drug. The development of hot-plate response consisted mainly of a progressive decrease of latencies and of a parallel reduction of sensitivity to morphine. While morphine depressed activity on day 14, it increased or had apparently no effect on day 21. With respect to carry-over consequences of early drug and test exposure, the animals with a history of testing at the preweanling stage were more sensitive to the depressant effect of morphine (10 mg/kg) than those pretested at a later stage. By contrast, morphine analgesia was attenuated by drug pre-exposure, independently of the age of previous testing. In sum, the age of early exposure and type of early treatment interacted to determine the level of adult pain sensitivity in the no-drug state. Finally, the long-term effects of sexual segregation in infancy on the response to painful stimulation and morphine were assessed. Adult male mice-reared from birth to weaning in litters containing either only male pups (MM), or both male and female pups (MF)--were challenged in a hot-plate test upon morphine or saline injection.(ABSTRACT TRUNCATED AT 250 WORDS)