A9 and A10 dopamine nuclei as a site of action for effects of 8-OH-DPAT on locomotion in the rat.

The 5-hydroxytryptamine (5-HT) 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was applied locally (0-5 microg bilaterally) into either the substantia nigra (A9) or the ventral tegmental area (A10) of adult male Wistar rats, and 10 min later spontaneous motor activity was observed in an open field ( approximately 0.5 m(2)) for 30 min. The rate of dopamine synthesis was estimated in neostriatal areas, the amygdala, and the prefrontal cortex, by measuring the accumulation of DOPA, following inhibition of cerebral decarboxylase by means of 3-hydroxybenzylhydrazine (NSD-1015). The A10 application of 8-OH-DPAT resulted in an increase in all aspects of spontaneous motor activity in the open field. A9 application of 8-OH-DPAT produced a stereotyped forward locomotion, characterized by a modest decrease in total horizontal activity, almost complete inhibition of rearing activity and an increase in proportion forward locomotion along the perimeter of the open-field arena. The injection of 8-OH-DPAT into the A9 was accompanied by an increased neostriatal DA rate of synthesis, whereas the A10 injection was followed by a decreased DA rate of synthesis in the amygdala and in the prefrontal cortex. It is concluded that mesencephalic dopaminergic mechanisms are involved in the stereotyped forward locomotion characteristically seen after systemic administration of the 5-HT(1A) receptor agonist 8-OH-DPAT.