Associative and limbic regions of monkey striatum express high levels of dopamine D3 receptors:: effects of MPTP and dopamine agonist replacement therapies

The role of the dopamine Dg receptor subtype in the central nervous system is still not well understood. It has a distinct and restricted distribution, mostly associated with limbic territories of the striatum (olfactory tubercle and the shell of nucleus accumbens) in rat brain, Dopaminergic denervation induced by a 6-hydroxydopamine lesion of the nigrostriatal system in rat down-regulates the expression of the D-3 receptor. In the present study, we investigated the functional neuroanatomy of the dopamine Dg receptor subtype in the monkey (Macaca fascicularis) basal ganglia. We also studied the effect of administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic D-1-like (SKF 82958) or D-2-like (cabergoline) agonist treatments on dopamine D3 receptor levels using receptor autoradiography. Our results clearly show that the distribution of D3 receptors in the monkey is more closely related to associative and limbic components of the striatum (caudate-putamen), as compared with its sensorimotor counterpart. Hence, D-3 receptors may be more specifically involved in cognitive and motivational aspects of striatal functions, which are elaborated in prefrontal, temporal, parietal, cingulate and limbic cortices. Moreover, MPTP administration significantly decreased levels of D-3 receptors and this effect was reversed or compensated by a chronic treatment with a D-1-like, but not a D-2- like, receptor agonist. The D-3 receptor may represent an important target for adjunct or direct therapy designed to improve cognitive deficits observed in patients with Parkinson's disease, schizophrenia and other illnesses with frontal lobe cognitive disturbances.