Experimental Brain Research

, Volume 129, Issue 2, pp 260–268

A comparative study of the effects of morphine in the dorsal periaqueductal gray and nucleus accumbens of rats submitted to the elevated plus-maze test

Authors

  • Vanessa C. Z. Anseloni
    • Laboratório de Psicobiologia, FFCLRP, campus USP, Av. Bandeirantes 3900, 14049–901, Ribeirão Preto, SP, Brazil, Fax: +55-16-6335015
  • Norberto C. Coimbra
    • Laboratório de Psicobiologia, FFCLRP, campus USP, Av. Bandeirantes 3900, 14049–901, Ribeirão Preto, SP, Brazil, Fax: +55-16-6335015
  • S. Morato
    • Laboratório de Psicobiologia, FFCLRP, campus USP, Av. Bandeirantes 3900, 14049–901, Ribeirão Preto, SP, Brazil, Fax: +55-16-6335015
  • M. L. Brandão
    • Laboratório de Psicobiologia, FFCLRP, campus USP, Av. Bandeirantes 3900, 14049–901, Ribeirão Preto, SP, Brazil, Fax: +55-16-6335015
Research Article

DOI: 10.1007/s002210050896

Cite this article as:
Anseloni, V., Coimbra, N., Morato, S. et al. Exp Brain Res (1999) 129: 260. doi:10.1007/s002210050896

Abstract 

We studied the effects of morphine injected either systemically or into the dorsal periaqueductal gray (DPAG) or nucleus accumbens (NA) using conventional and ethological analyses of behavior of rats submitted to the elevated plus-maze test with transparent walls. Intraperitoneal morphine (0.1 mg/kg and 0.3 mg/kg) increased both standard and ethological measures, expressing general exploratory activity such as total arm entries, end-exploration, scanning, head-dipping, and rearing. Morphine 10 (7.6 µg/µl) and 30 nmol (23 µg/µl) injected into nucleus accumbens produced similar effects, which were blocked by i.p. naltrexone (2.0 mg/kg), an opioid antagonist with good affinity for µ-opioid receptors. Morphine injected into the DPAG produced either antiaversive (10 nmol) or aversive effects (30 nmol), which respectively reduced and increased entries and time spent in the open arms and behaviors associated with risk assessment (peeping out, stretched attend postures, and flat back approach). The proaversive effects were inhibited by i.p. norbinaltorphimine (2.0 mg/kg), a selective inhibitor for κ-opioid receptors. These findings support the contention that at least some of the motivational effects of morphine may be due to activation of opioid mechanisms in nucleus accumbens, and DPAG has neural substrates for antiaversive and aversive effects of morphine. Moreover, on the basis of previous and present data obtained in this laboratory, it is suggested that stimulation of µ-opioid receptors inhibits and stimulation of κ-receptors activates the neural substrate of aversion in the DPAG. On the other hand, the increase in exploratory behavior due to interaction of morphine with µ-opioid receptors in the nucleus accumbens may be due to the stimulation of the interface between neural substrates of motivation and motor output in this structure.

Key words Opioidsµ-Receptorsκ-ReceptorsAversionRat

Copyright information

© Springer-Verlag Berlin Heidelberg 1999