[Function and therapeutic potential of the dopamine D3 receptor]

The D3 receptor is recognized with high affinity by all antipsychotics and selectively expressed in limbic brain areas participating in the central of emotions, motivation and reward. In transfected cultured cells, stimulation of the D3 receptor inhibits cAMP formation and increases mitogenesis, which, in turn, is potentiated by activation of the cAMP cascade. This suggests that both opposite and synergistic interactions occur between the D3 receptor and the cydic AMP pathway, possibly underlying D1/D3 receptor interactions. In fact, D1 and D3 receptors colocalize in the islands of Calleja, in which they interact in opposition on c-fos mRNA expression, and in the shell of nucleus accumbens, in which they interact in synergy on substance P mRNA expression. The expression of the D3 receptor is highly dependent of the dopamine innervation: lesion of ascending dopamine neurons reduces D3 receptor mRNA and binding in the shell of nudeus accumbens, by deprivation of an unknown factor of dopamine neurons, distinct form dopamine and its cotransmitters. In agreement, expression of the D3 receptor in neurons during rat brain development starts after the settlement of dopamine innervation during the first postnatal week. However, in adult rats with a unilateral lesion of dopamine neurons, repeated treatment with levodopa rescues D3 receptor expression in the shell of nudeus accumbens and induces this expression in the dorsal striatum, a region controlling movements in which the D3 receptor is normally absent. This induction seems responsible for the behavioral sensitisation, i.e. increased responsiveness to levodopa. These observations suggest a role of the D3 receptor in the progressive increase in the therapeutic efficacy of levodopa in the initial treatment of Parkinson's disease, and/or its adversive motor and psychopathological effects during long-term treatment. Finally, various pharmacological and genetic data suggest a role of the D3 receptor in drug addiction and schizophrenia, the treatment of which could benefit from selective D3R agents.