Histamine H3 receptor ligands modulate L-dopa-evoked behavioral responses and L-dopa-derived extracellular dopamine in dopamine-denervated rat striatum

To explore a recently established association between histaminergic and dopaminergic neuronal phenotypic systems in brain, we determined the effect of the respective histaminergic H(3) receptor agonist and antagonist/inverse agonist, imetit and thioperamide, on L-DOPA - derived tissue and extracellular DA and metabolite levels in the striatum of 6-hydroxydopamine (6-OHDA) - lesioned rats (i.e., parkinsonian rats). We also examined the influence of histamine H(3) ligands on L-DOPA evoked behavioral responses (locomotor activity, number of rearings, stereotyped behavior and motor coordination). Using HPLC/ED and in vivo microdialysis technique imetit (5 mg/kg, i.p.) but not thioperamide (5 mg/kg, i.p.) was shown to attenuate an L-DOPA-evoked (15 mg/kg, i.p.; carbidopa, 30 min pretreatment) increase in extracellular DA in the neostriatum of 6-OHDA-lesioned rats. However, both imetit and thioperamide increased microdialysate levels of DOPAC and HVA, probably by enhancing intraneuronal DA utilization. As indicated by neurochemical analysis of the striatum imetit produced a decrease in tissue DA content. These findings support the hypothesis that central H(3) histaminergic receptors have a modulatory role in the storage, metabolism and release of DA derived from exogenous L-DOPA challenge. Furthermore, evidence from behavioral studies indicate that histamine H3 receptor blockade markedly improved motor coordination. Conversely, histamine H(3) receptor stimulation, being without effect on motor coordination, enhanced vertical activity in rats. From the above we conclude that the histamine H(3) agonism may augment motor dyskinesia in Parkinson's disease (PD) patients and presumably worsen L-DOPA therapy. Consequently, the histaminergic system represents a viable target for modulating the effectiveness of L-DOPA therapy in Parkinson's disease.