Effect of differential rearing environments on morphine-induced behaviors, opioid receptors and dopamine synthesis

M. T. Bardo, P. M. Robinet, R. F. Hammer

Research output: Contribution to journalArticlepeer-review

44 Scopus citations


Rats were raised from weaning (21 days old) to young adulthood (50-60 days old) in either an enriched or impoverished stimulus environment. In the enriched condition (EC), rats were group-housed with various novel objects that were re-arranged daily. In the impoverished condition (IC), rats were housed individually without any objects. As adults, a four-trial conditioned place preference (CPP) test was used to assess locomotor activity and reward produced by morphine (0, 0.1, 1 or 10 mg/kg). On morphine conditioning day 1, both EC and IC rats displayed an inverted U-shaped dose-effect curve for locomotor activity and the locomotor stimulant effect of acute morphine was greater in IC than EC rats. Across morphine conditioning days 1-4, both EC and IC rats displayed locomotor sensitization; the locomotor sensitization following repeated morphine injections was greater in IC than EC rats. In contrast to the enhanced locomotor stimulant effect of morphine observed in IC rats, morphine-induced CPP was attenuated in IC rats relative to EC rats, indicating that the locomotor and rewarding effects of opioids depend upon different neural substrates. Measurement of μ opioid receptor density and rates of morphine-stimulated dopamine synthesis in the mesolimbic and nigrostriatal systems of EC and IC rats revealed no reliable differences between groups. Therefore, the ability of μ opioid receptors to modulate mesolimbic dopamine neurotransmission does not account for the differential behavioral effects of morphine in EC and IC rats.

Original languageEnglish
Pages (from-to)251-259
Number of pages9
Issue number2
StatePublished - Feb 1997

Bibliographical note

Funding Information:
This research was supported by USPHS grants DA-05312 and DA-06924.


  • conditioned place preference
  • dopamine
  • drug reward
  • environmental enrichment
  • locomotor activity
  • morphine
  • opiate receptors

ASJC Scopus subject areas

  • Pharmacology
  • Cellular and Molecular Neuroscience


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