Is the motivational effect of opiate withdrawal reflected by common somatic indices of precipitated withdrawal? A place conditioning study in the rat.

Rats implanted for 5 days with a morphine pellet were dosed once with naltrexone (subcutaneous, s.c.) or methylnaltrexone (intraventricular, i.c.v.). Then the rats were observed for several somatic signs of precipitated withdrawal and tested for aversion to the place of the withdrawal. The two antagonist treatments produced different withdrawal syndromes, but both were associated with a place aversion which followed a simple monophasic function of the dose of antagonist. More importantly, there was an absence of any overall relation between individual withdrawal signs (jumping, writhing, shaking, diarrhea, and weight loss) and the aversive effect seen. It was concluded, therefore, that the motivational impact of opiate abstinence is not always addressed in conventional models of withdrawal.