Effects of histamine on spontaneous electrical activity of neurons in rat suprachiasmatic nucleus

Abstract

The hypothalamic suprachiasmatic nucleus (SCN) is thought to be a light-entrained pacemaker in mammals, inducing a wide range of endogenous circadian events. In rat brain, histaminergic (HAergic) fibres are particulary rich in the hypothalamus. This prompted an investigation of the influence of bath-applied HAergic compounds on the spontaneous electrical activity of SCN neurons, recorded extracellularly in the hypothalamic slice preparation. Cells activated by bath application of HA ( n = 28) outnumbered those inhibited by HA ( n = 6). 48% of cells tested ( n = 28) were unresponsive to HA application. HA-induced activation of SCN neurons' discharge rate could be suppressed by the H 1-antagonist mepyramine, but not by the H 2-antagonist cimetidine. HAergic effects were still present when synaptic transmission was blocked, indicating a postsynaptic site of action for HA within SCN. Due to the anatomical distribution of HA-responsive SCN neurons and the independence of HA-effects with respect to circadian time, HA seems not to endow a pivotal role within generation and maintenance of circadian rhythm. Although data were obtained from SCN deprived of neural input, results show clear evidence for a modulatory impact of HA on the spontaneous electrical activity of part of SCN neurons.