Blockade of micro-opioid receptors in the medial thalamus inhibits acquisition, but not expression, of morphine-induced conditioned place preference

Neuroscience. 2008 Feb 19;151(4):948-54. doi: 10.1016/j.neuroscience.2007.10.058. Epub 2007 Dec 14.


The medial thalamus contains abundant mu-opioid receptors and is activated by acute morphine administration. However, the role of the medial thalamus in the rewarding effects of morphine is unclear. The present study examined whether mu-opioid receptors of the medial thalamus influenced the acquisition and expression of morphine-induced conditioned place preference (CPP) in rats. An unbiased apparatus and biased subject assignment were used. Administration of morphine in increasing doses (2 mg/kg, 4 mg/kg, 6 mg/kg, 10 mg/kg, s.c.) was paired with an initially non-preferred chamber and saline administration was paired with an initially preferred chamber. Conditioning trials were conducted twice daily for 4 days. Microinjection of the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (5 microg/rat), into the medial thalamus 23 h prior to each morphine conditioning completely blocked the acquisition of CPP. However, microinjection of beta-funaltrexamine into the medial thalamus after morphine conditioning trials, but 23 h prior to a test session, had no effect on the expression of CPP. It is concluded that mu-opioid receptors in the rat medial thalamus are involved in the acquisition, but not expression, of morphine-induced CPP.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Behavior, Animal / drug effects
  • Conditioning, Operant / drug effects*
  • Conditioning, Operant / physiology
  • Drug Administration Routes
  • Drug Interactions
  • Male
  • Microinjections / methods
  • Morphine / pharmacology*
  • Naltrexone / analogs & derivatives
  • Naltrexone / pharmacology
  • Narcotic Antagonists / pharmacology
  • Narcotics / pharmacology*
  • Rats
  • Rats, Sprague-Dawley
  • Receptors, Opioid / physiology*
  • Thalamus / drug effects*
  • Thalamus / physiology


  • Narcotic Antagonists
  • Narcotics
  • Receptors, Opioid
  • Naltrexone
  • beta-funaltrexamine
  • Morphine