Reduction in motor responding of the mouse by actions of dopamine agonists in the midbrain

Mice were implanted stereotaxically with chronically indwelling bilateral guide cannulae to allow the intracerebral injection of dopamine or the dopamine agonist 2-di-n-propylamino-5,6-dihydroxytetralin into the centre of the substantia nigra, above the substantia nigra or anterior and posterior to the nigra. The intranigral injection of dopamine and the tetralin compound could be shown to dose-dependently reduce the spontaneous locomotor activity of mice. The effectiveness of dopamine decreased with a delayed onset when injections were made anterior or posterior to the substantia nigra; this spectrum of reduced and delayed responding was also apparent when dopamine and the tetralin compound were injected 1 or 2 mm above the nigra. The inhibitory action of dopamine on motor activity, when injected into the substantia nigra, was antagonised by a small dose of spiroperidol which did not influence spontaneous locomotor responding in its own right; prazosin and yohimbine were ineffective. It is suggested that the inhibitory actions of dopamine and 2-di-n-propylamino-5,6-dihydroxytetralin on motor activity may reflect an ability to stimulate dopamine "autoreceptors" in the midbrain area containing the dopamine cell bodies which innervate striatal and mesolimbic forebrain regions.