Histamine excites rat cerebellar granule cells in vitro through H 1 and H 2 receptors Wen-Chang Li*, Xiao-Han Tang**, Hong-Zhao Li, Jian-Jun Wang*** Department of Biological Science and Technology, Nanjing University, 22 Hankou Road, Nanjing 210093, China (Received 26 June 1998; accepted 5 January 1999) Abstract — The effects of histamine on the firing of cerebellar granule cells were investigated in vitro. Histamine predominantly produced excitatory (117/123, 95.1%) and in a few cases inhibitory (6/123, 4.9%) responses in granule cells. The histamine-induced excitation was not blocked by perfusing the slice with low Ca 2+ /high Mg 2+ medium, supporting a direct postsynaptic action of histamine. The H 1 receptor antagonists triprolidine and chlorpheniramine significantly diminished the histamine-induced excitation, but the H 2 receptor antagonist ranitidine did not significantly reduce the excitation. On the other hand, the H 2 receptor agonist dimaprit could elicit a weak excitation of granule cells. This dimaprit-induced excitation was blocked by ranitidine but not triprolidine. These results reveal that the excitatory effect of histamine on cerebellar granule cells is mediated by both H 1 and H 2 receptors with a predominant contribution of H 1 receptors. The relevance of these findings to the possible function of the hypothalamocerebellar histaminergic fibers in cerebellum is discussed. © Elsevier, Paris histamine / cerebellar granule cells / histamine receptors / hypothalamocerebellar histaminergic fibers 1. Introduction Over the last 10 years neuroanatomic studies have revealed that hypothalamus and cerebellum are recip- rocally connected by hypothalamocerebellar and cer- ebellohypothalamic projections in the mammalian brain [3]. Immunocytochemical experiments have in- dicated that some of the hypothalamocerebellar fibers are histaminergic [3, 15, 20]. The histamine- containing fibers project from the tuberomammillary nucleus of the hypothalamus to cerebellar cortex and deep cerebellar nuclei, and the highest density of histaminergic terminations is in the granule cell (GrC) layer of vermis and flocculus [3, 15, 20]. The presence of H 1 and H 2 receptors was also demonstrated in rat cerebellar cortex and deep nuclei, but H 3 receptors are scarce in the cerebellum [2, 17]. In primary culture of isolated cerebellar GrCs, the turnover of phosphati- dylinositol induced by the activation of histamine H 1 receptors was also observed [24]. These studies sug- gest that histamine may be the transmitter of some of cerebellar afferent fibers projecting to the GrC, and that the turnover of phosphatidylinositol induced by H 1 receptors may have an important role in modulating the excitability of GrC. However, it is not clear what effects histamine may have on the electrical activity of GrCs. In this study, we examined the effects of histamine on the unitary activity of GrCs in rat cerebellar slice in order to obtain initial insights into the action of this putative neurotransmitter on the cerebellar neurons. The results indicate that histamine exerts an excitatory effect on GrCs through H 1 and H 2 receptors with a predominant contribution of H 1 re- ceptors. 2. Materials and methods Under ether anesthesia, 33 Sprague-Dawley rats (200–300 g) were decapitated, and their cerebella were quickly taken out and washed in ice-cold oxygenated artificial cerebrospinal fluid (ACSF, composition in mM: NaCl 124, KCl 5, MgSO 4 1.3, KH 2 PO 4 1.2, NaHCO 3 26, CaCl 2 2.4 and D-glucose 10). Cemented onto the stage of a Vibroslice, the vermis of cerebellum was cut in the saggital plane (300 μm thick), and the slices were transferred to a recording chamber which was continuously perfused with ACSF equilibrated with 95% O 2 /5% CO 2 (pH 7.4, 33 ± 0.2 °C, flow rate 2 mL /min). All slices were incubated for a minimum of 40 min before recording. In some experiments, the low Ca 2+ /high Mg 2+ medium was used to decrease the presynaptic transmitter release. In these cases, the concentration of Ca 2+ was lowered to 0.25 mM and the Mg 2+ was raised to 4.0 mM respectively. Spontaneous unitary activity of GrCs in lobule X of the cerebellar slices was recorded extracellularly using glass microelectrodes filled with 2 M NaCl (impe- dence 2–20 MΩ). The criteria for determining that the * Present address: Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, China ** Present address: Department of Animal Sciences, Univer- sity of Illinois at Urbana-Champaign, Urbana, IL 61801, USA *** Correspondence and reprints J. Physiol. (Paris) 93 (1999) 239-244 © Elsevier, Paris