Bidirectional effects of opioids in motivational processes and the involvement of D1 dopamine receptors.

Exogenous and endogenous opioids significantly affect motivational processes; depending on the particular opioid receptor type with which they interact opposite effects are induced: activation of mu and delta opioid receptors is rewarding whereas activation of kappa opioid receptors induces aversive effects. Antagonism of mu-opioid receptors also induces aversion, suggesting the existence of a tonically active opioidergic reward pathway. There is evidence that beta-endorphin pathways arising from the hypothalamus play an important role in this respect. Opioid-induced reward as well as aversion seem to be mediated by the mesolimbic dopamine system, reward by increased, aversion by decreased transmission at D1 dopamine receptors.