Naloxone enhances the expression of morphine-induced conditioned place preference.
The present study examined the effects of naloxone on acquisition and expression of morphine-induced conditioned place preference (CPP). Three groups of rats were given morphine (5 mg/kg, SC), both morphine and naloxone (1 mg/kg, SC), or saline paired with a distinctive environment. On alternating days they were given saline paired with another distinctive environment. After four exposures to each environment, the animals were given a preference test in which they had access to both environments simultaneously while under the influence of either naloxone (1 mg/kg, SC) or saline. Morphine-conditioned animals showed CPP evident as an increased amount of time spent in the drug-associated environment relative to saline controls. Rats given both naloxone and morphine during conditioning, and saline on the test day, did not show CPP. In contrast, morphine-conditioned animals given naloxone on the test day showed stronger CPP than morphine-conditioned animals given saline. These findings indicate that naloxone blocks the acquisition, but enhances the expression of morphine-induced CPP. In a separate experiment, the effects of naloxone on locomotor activity were determined during the CPP test. The results indicated that naloxone decreased locomotor activity. In morphine-conditioned animals only, naloxone also produced an increase in the amount of time per entry in the drug-associated environment. The results suggest that naloxone may enhance morphine-induced CPP by decreasing locomotor activity that may otherwise compete with expression of CPP.
Neisewander
J L
JL
Department of Pharmacology, University of Pennsylvania, Philadelphia 19104.
Pierce
R C
RC
Bardo
M T
MT
D000818:Animals; D002755:Choice Behavior / Q000187:drug effects; D003216:Conditioning, Operant / Q000187:drug effects*; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects; D009270:Naloxone / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D012055:Reinforcement Schedule
D009270:Naloxone; D009020:Morphine