Blockade of micro-opioid receptors in the medial thalamus inhibits acquisition, but not expression, of morphine-induced conditioned place preference.
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.
Guo
N
N
Department of Structural and Cellular Biology, Tulane University School of Medicine, 1430 Tulane Avenue, SL-49, New Orleans, LA 70112, USA. nguo@tulane.edu
Garcia
M M
MM
Taylor
B K
BK
Zadina
J E
JE
Harlan
R E
RE
D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D003216:Conditioning, Operant / Q000187:drug effects* / Q000502:physiology; D004333:Drug Administration Routes; D004347:Drug Interactions; D008297:Male; D008845:Microinjections / Q000379:methods; D009020:Morphine / Q000494:pharmacology*; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011957:Receptors, Opioid / Q000502:physiology*; D013788:Thalamus / Q000187:drug effects* / Q000502:physiology
D009292:Narcotic Antagonists; D009294:Narcotics; D011957:Receptors, Opioid; D009271:Naltrexone; C024524:beta-funaltrexamine; D009020:Morphine