SEPARATE NEURAL SUBSTRATES MEDIATE THE MOTIVATING AND DISCRIMINATIVE PROPERTIES OF MORPHINE

A previous study (T. V. Jaeger & D. van der Kooy, 1993) has implicated a visceral and taste region (parabrachial nucleus), but not mesolimbi c dopamine terminal fields (nucleus accumbens), as a substrate for opi ate discriminative effects. The authors now show that (a) morphine's d iscriminative effects in the parabrachial nucleus (PEN) require the ac tivation of opiate receptors; (b) in rats trained to discriminate morp hine from saline, infusions of morphine into the ventral tegmental are a (VTA) do not generalize to the systemic training condition; (c) infu sions of morphine into the PBN, but not the VTA, serve as a stimulus f or the acquisition of discrimination learning; and (d) morphine applie d to the VTA, but not the PEN, is motivating. The data show that the m otivating and discriminative effects of morphine are processed separat ely by the brain. Further, discriminative drug effects are neither nec essary nor sufficient for opiate motivational effects.