Co-localization of histamine with GABA but not with galanin in the human tuberomamillary nucleus

The presence of GABA and galanin in histaminergic neurons was previously reported in the rodent brain but whether such co-localizations also occur in the human brain was not known. We used in situ hybridization histochemistry and immunohistochemistry to study the co-localization of histamine with GABA and galanin in neurons of the tuberomamillary nucleus of adult human posterior hypothalamus. On consecutive formalin-fixed paraffin-embedded sections, co-localization was assessed using the in situ hybridization for L-histidine decarboxylase mRNA and immunocytochemistry for glutamate decarboxylase-67 kDa or galanin in the two profiles of same cell. The pattern of distribution and number of histaminergic neurons identified by in situ hybridization of the synthesizing enzyme gene transcripts were in accordance with data reported for histamine immunoreactivity. The great majority of neurons within the main divisions of the tuberomamillary nucleus containing L-histidine decarboxylase mRNA was also immunoreactive for glutamate decarboxylase-67 kDa. The range of co-localization of the two markers varied from 72% in the lateral part, to 75-87% in the medial part and 83-88% in the ventral part. In contrast, no cell containing L-histidine decarboxylase mRNA was immunoreactive for galanin. We conclude that tuberomamillary neurons in human co-express histamine with GABA but, unlike the neurons in rodents, do not express galanin, indicating that neurotransmitter co-localization patterns differ in the two species.

Effects of histamine H3 receptor agonist and antagonist on histamine co-transmitter expression in rat brain

The histaminergic H3-receptor (H3R) controls histamine synthesis and release in the tuberomamillary nucleus. We evaluated the effects of stimulating or blocking of H(3)R on glutamate-decarboxylase 67 kDa (GAD-67) and galanin mRNA expression, two histamine co-transmitters.After in situ hybridization histochemistry (ISHH), we observed a colocalization of 100% between histidine decarboxylase (HDC) and GAD-67 or H3R and of 80 to 97% with galanin. Adult rats received an H3R agonist ((R)alpha-Methylhistamine) or antagonist (ciproxifan) and were sacrificed 1 or 3 hours later. Treatment effects on HDC, galanin and GAD-67 mRNA were studied by quantitative ISHH on serial sections. Treatment with the H3R agonist known to decrease histamine neuron activity initially reduced HDC and galanin gene expression but an inverse change, presumably reflecting a compensatory mechanism, was observed after 3 h on both markers. In contrast, the H3R antagonist known to activate histamine neurons, had opposite effects on the two markers, suggesting that co-transmitters are submitted to independent control mechanisms. Furthermore, GAD-67 mRNA levels were not significantly modified by these treatments.