The development of a conditioned place preference to morphine: effects of lesions of various CNS sites.

This study examined the neural substrates underlying the development of a conditioned place preference (CPP) to morphine (2 mg/kg x 3 pairings) by testing whether lesions of 7 different neural sites block a morphine-induced CPP. Lesions of the pedunculopontine tegmental nucleus (PPTg), the periaqueductal gray (PAG), or the fornix reduced the preference for a morphine-paired compartment. When they were retested following morphine administration, fornix- or PAG-lesioned animals exhibited a CPP indicating that lesions did not block morphine-induced reward or the ability to associate this effect with salient environmental cues. PPTg-lesioned animals did not express a CPP during state-dependent testing, suggesting that the lesions may attenuate the rewarding effect of the drug. Lesions of the mesolimbic dopamine system, the ventral pallidum, the lateral nucleus of the amygdala, or the caudate putamen had no effect on a morphine-induced CPP.