POSTNATAL EXPRESSION OF H-1-RECEPTOR MESSENGER-RNA IN THE RAT-BRAIN -CORRELATION TO L-HISTIDINE DECARBOXYLASE EXPRESSION AND LOCAL UP-REGULATION IN LIMBIC SEIZURES

Histamine is implicated in the regulation of brain functions through t hree distinct receptors. Endogenous histamine in the brain is derived from mast cells and neurons, but the importance of these two pools dur ing early postnatal development is still unknown. The expression of hi stamine H-1-receptor in the rat brain was examined using in situ hybri dization during postnatal development and in adults. For comparison, t he expression of L-histidine decarboxylase (HDC) in the two pools was revealed. H-1-receptor was evenly expressed throughout the brain on th e first postnatal days, but resembled the adult, uneven pattern alread y on postnatal day 5 (P5). HDC was expressed in both mast cells and tu beromammillary neurons from birth until P5, after which the mast cell expression was no more detectable. In adult rat brain, high or moderat e levels of H-1-receptor expression were found in the hippocampus, zon a incerta, medial amygdaloid nucleus and reticular thalamic nucleus. I n most areas of the adult brain the expression of H-1-receptor mRNA co rrelates well with binding data and histaminergic innervation. A notab le exception is the hypothalamus, with high fibre density but moderate or low H-1-receptor expression. Systemic kainic acid administration i nduced increased expression of H-1-receptor mRNA in the caudate-putame n and dentate gyrus, whereas no change was seen in the hippocampal sub fields CA1-CA3 or in the entorhinal cortex 6 h after kainic acid injec tions. This significant increase supports the concept that histaminerg ic transmission, through H-1-receptor is involved in the regulation of seizure activity in the brain.