Abstract
Anatomical studies have shown that the globus pallidus receives abundant 5‐hydroxytryptamine (5‐HT) innervations from raphe nuclei. 5‐HT may occupy an important position in the modulation of motor function through its affect on the activity of globus pallidus. In the present study, intrapallidal microinjection of 5‐HT (0.1 mM) alone did not induce any motor behavior or postural asymmetry in the unilateral 6‐hydroxydopamine (6‐OHDA)‐lesioned rats. However, when infused concomitantly with a low dose of 3, 4‐dihydroxyphenylalanine (L‐DOPA, 3 mg/kg i.p.), which itself can induce modest contralateral rotational behavior, 5‐HT significantly potentiated the number of contralateral rotations. To elucidate the cellular mechanism, in vivo extracellular recordings were performed to examine the effects of 5‐HT on globus pallidus neurons. In normal rats, the predominant effect of micropressure ejection of 5‐HT on pallidal neurons was excitation. In 6‐OHDA‐lesioned rats, although 5‐HT increased the firing rate in most pallidal neurons, 5‐HT‐induced inhibitory effects was stronger than that on the unlesioned side as well as normal rats. Furthermore, 5‐HT~1B~ receptors are mainly involved in 5‐HT‐induced excitation while 5‐HT~1A~ receptors are involved in 5‐HT‐induced inhibition. The results suggest that 5‐HT may potentiate the antiparkinsonian effect of L‐DOPA through modulating the activity of globus pallidus. © 2009 Wiley‐Liss, Inc.