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 mesolimbic dopamine terminal fields (nucleus accumbens), as a substrate for opiate discriminative effects. The authors now show that (a) morphine's discriminative effects in the parabrachial nucleus (PBN) require the activation of opiate receptors; (b) in rats trained to discriminate morphine from saline, infusions of morphine into the ventral tegmental area (VTA) do not generalize to the systemic training condition; (c) infusions of morphine into the PBN, but not the VTA, serve as a stimulus for the acquisition of discrimination learning; and (d) morphine applied to the VTA, but not the PBN, is motivating. The data show that the motivating and discriminative effects of morphine are processed separately by the brain. Further, discriminative drug effects are neither necessary nor sufficient for opiate motivational effects.