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

• Published in: Neuroscience Vol. 151; no. 4; p. 948
• Main Authors: Guo, N, Garcia, M M, Taylor, B K, Zadina, J E, Harlan, R E
• Format: Journal Article
• Language: English
• Published: United States 19.02.2008
• Subjects: 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
• ISSN: 0306-4522
• DOI: 10.1016/j.neuroscience.2007.10.058

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.