Excitatory effect of histamine on neuronal activity of rat cerebellar fastigial nucleus in vitro

Histamine evokes excitatory response of neurons in the cerebellar dentate nucleus via H2 receptors

Previous studies have shown an excitatory effect of histamine on neurons in two cerebellar nuclei, the fastigial nucleus and the interposed nucleus. Here we investigated action of histamine on the dentate nucleus (DN), another nucleus of the cerebellum, and provided more evidence for motor control by histamine via the cerebellum. Spontaneous unitary discharge of neurons in the DN was extracellularly recorded by use of cerebellar slice preparations. In total 79-recorded neurons, which were from 53 cerebellar slices, 67 neurons (84.8%) had an excitatory response to histamine stimulation, and the rest (15.2%) were not reactive. The histamine-induced excitation of the DN neurons was not blocked by low-Ca(2+)/high-Mg(2+) medium, demonstrating that this effect of histamine was postsynaptic. Triprolidine, an antagonist of histamine H(1) receptors, did not block the excitatory effect of histamine, but ranitidine, an antagonist for H(2) receptors, blocked the excitatory response to histamine in a concentration-dependent manner. Further, histamine H(1) receptor agonist 2-pyridylethylamine did not elicit any response of DN neurons, but H(2) receptor agonist dimaprit had an excitatory action on the DN cells and this action was blocked by ranitidine. These results indicate that histamine excites cerebellar DN neurons via histamine H(2) receptors. Since the DN receives hypothalamocerebellar histaminergic projections and plays a role in initiation and planning of somatic movement, the postsynaptic excitation of the DN neurons by histamine suggests the possibility that the initiation and planning of movement may be modulated by the histaminergic projections.

Histamine excites rat lateral vestibular nuclear neurons through activation of post-synaptic H2 receptors

Through whole-cell patch recordings in brainstem slices, the effects of histamine on neuronal activity of the lateral vestibular nucleus (LVN) were investigated. Bath application of histamine elicited a concentration-dependent excitation of both spontaneous firing (n=19) and silent (n=7) LVN neurons. Moreover, histamine induced a stable inward current in the LVN neurons (n=5) and the histamine-induced depolarization of membrane potential persisted in the presence of tetrodotoxin (n=4), indicating a direct post-synaptic effect of the histamine on the LVN neurons. Selective histamine H2 receptor antagonist ranitidine effectively blocked the histamine-evoked excitatory responses on the LVN neurons (n=4), but selective histamine H1 receptor antagonist triprolidine did not (n=4). In addition, selective histamine H2 receptor agonist dimaprit (n=3) rather than 2-pyridylethylamine (n=4), a selective histamine H1 receptor agonist, mimicked the excitatory action of histamine on LVN neurons. The results demonstrate that histamine excites the LVN neurons via post-synaptic histamine H2 receptors and suggest that the central histaminergic projection arising from the hypothalamus may modulate LVN neurons activity and actively influence the vestibular reflexes and functions.