Intranigral injection of the H3 agonist immepip and systemic apomorphine elicit ipsilateral turning behaviour in naive rats, but reduce contralateral turning in hemiparkinsonian rats.

There is evidence that histamine H3 receptors co-localise with dopamine D1 receptors on the terminals of striato-nigral neurones. In this work we studied the effect of the local activation of H3 receptors present in substantia nigra pars reticulata (SNr) on turning behaviour following apomorphine administration to either naive or hemiparkinsonian rats. In naive rats the intranigral (SNr) injection of the H3 receptor agonist immepip (3.2 or 32 ng/1 microl) resulted in ipsilateral turning following systemic apomorphine (0.5 mg/kg, subcutaneous). The effect of immepip was related to the dose and prevented by the H3 antagonist thioperamide (5 mg/kg, intraperitoneal). Conversely, in rats with 6-hydroxydopamine (6-OHDA) lesions to either substantia nigra pars compacta or the medial forebrain bundle (mfb), apomorphine-induced contralateral turning was reduced by intranigral immepip, an effect prevented by systemic thioperamide. Our data show that H3 receptors present in SNr regulate the synaptic output of the basal ganglia, most likely by reducing GABA release from striato-nigral terminals. These results may be relevant for the understanding of the role of histamine and H3 receptors in the control of motor behaviour both in normal and pathophysiological conditions, such as Parkinson's disease in which histaminergic innervation and histamine levels in substantia nigra have been shown to increase.