Excitatory effect of histamine on neuronal activity of rat globus pallidus by activation of H 2 receptors in vitro

Abstract

Previous studies have revealed distribution of histaminergic fibers and presence of histamine receptors in globus pallidus (GP). In this study, the brain slice preparation of adult rats was used to examine the effect of histamine on the spontaneous unitary discharge of GP neurons and the underlying receptor mechanism. Ninety-five GP neurons were extracellularly recorded from 42 slices containing the GP, of which 87 (91.6%) were excited by the stimulation of histamine. The histamine-induced excitation was concentration-dependent and persisted in low Ca 2+/high Mg 2+ medium ( n = 9), demonstrating that the action of histamine on the GP neurons was postsynaptic. The excitatory effect of histamine on the GP neurons was not blocked by selective histamine H 1 receptor antagonist triprolidine ( n = 16) or chlorpheniramine ( n = 6), but was effectively suppressed by ranitidine, a highly selective histamine H 2 receptor antagonist ( n = 21). On the other hand, highly selective histamine H 2 receptor agonist dimaprit mimicked the excitatory effect of histamine on the GP neurons ( n = 23), while histamine H 1 receptor agonists, including 2-pyridylethylamine ( n = 22), 2-thiazolyethylamine ( n = 9) and betahistine ( n = 9), did not cause GP neurons any response. The dimaprit-induced GP neuronal excitation was effectively antagonized by selective histamine H 2 receptor antagonist ranitidine ( n = 14) but not influenced by selective histamine H 1 receptor antagonist triprolidine ( n = 12). Moreover, adenylate cyclase (AC) activator forskolin ( n = 7) was observed to evoke GP neurons an excitatory response, whereas the histamine-induced excitation was effectively reduced by H-89 ( n = 9), a selective and potent inhibitor of protein kinase A (PK A). Finally, it was noted that neurons of both subdivisions of the GP, the internal (GPi, n = 35) and external (GPe, n = 60) segment, showed no differences in their responses to stimulations of the tested histaminergic reagents. These results demonstrated that histamine excited GP (including GPi and GPe) neurons via histamine H 2 receptors and H 2 receptors linked intracellular G-protein-AC-PK A signaling pathway, suggesting that the hypothalamic histaminergic afferent fibers innervating GP may play an important modulatory role in motor control through its excitatory effect on GP neurons.