Detailed mapping of the histamine H 2 receptor and its gene transcripts in guinea-pig brain

Abstract

Autoradiographic studies of the distribution of the histamine H 2 receptor and its messenger RNAs were performed on serial frontal and a few sagittal sections of guinea-pig brain using [ 125I]iodoaminopotentidine for radioligand binding and a 33P-labelled complementary RNA probe for in situ hybridization, respectively. Both probes were validated by assessing non-specific labelling using non-radioactive competing H 2 receptor ligands and a sense probe for binding sites and gene transcripts, respectively. In some areas, e.g., cerebral cortex, hippocampal complex or cerebellum, such studies were completed by identification of neurons expressing the H 2 receptor messenger RNAs on emulsion-dipped sections. Nissl-stained sections from comparable levels were used to localize brain structures. In many brain areas, the distribution of the H 2 receptor and its messenger RNAs appeared to parallel that known for histaminergic axons. For instance, high levels of both H 2 receptor markers were detected in striatal and limbic areas known to receive abundant histaminergic projections. In contrast, in septum, hypothalamic, pontine and several thalamic nuclei, a comparatively low density of both H 2 receptor markers was detected, suggesting that histamine actions in these areas are mediated by H 1 and/or H 3 receptors. Generally, the distribution of H 2 receptor messenger RNA correlates well with that of [ 125I]iodoaminopotentidine binding sites, although some differences were observed. In a few regions (e.g., substantia nigra, locus coeruleus) high or moderate densities of binding sites were accompanied by a much more restricted expression of H 2 receptor transcripts. Conversely, the mammillary region and the pontine nucleus exhibited higher levels of hybridization than of binding sites. In hippocampus, cerebral and cerebellar cortex there was a selective localization of the H 2 receptor messenger RNA in the granule cells of dentate gyrus, pyramidal cells of the Ammon's horn and cerebral cortex, and Purkinje cells of cerebellum, whereas [ 125I]iodoaminopotentidine binding sites were located in layers where the dendritic trees of these messenger RNA-expressing neurons extend. The same discrepancy between messenger RNAs and binding sites suggests that striatonigral endings are endowed with the H 2 receptor. The histamine H 1 and H 2 receptors both appear to be present in several brain areas, in some cases in a way suggesting their potential co-expression by the same neuronal populations, e.g., in granule and pyramidal cells in the hippocampal formation. This co-expression accounts for synergic responses, e.g., on cAMP generation, previously observed upon co-stimulation of both receptor subtypes. The widespread distribution of the H 2 receptor, namely in thalamic nuclei or in telencephalic areas such as most layers of the cerebral cortex, together with its excitatory role previously established in electrophysiological studies, support its alleged function in mediating the histamine-driven control of arousal mechanisms. In addition, the detection of H 2 receptor expression in brainstem areas from which other monoaminergic pathways involved in the control of states of sleep and wakefulness emanate, e.g., several raphe nuclei, locus coeruleus or substantia innominata, suggests possible interrelationships between all of these systems with highly divergent projections to the thalamus and telencephalon. The present mapping of the H 2 receptor and its gene transcripts should facilitate neurochemical, neurophysiological and behavioural studies aimed at clarifying the role of histaminergic systems in brain.