Localization of 5-HT2C and 5-HT1B Serotonin Receptors in Orexinergic Neurons of the Hypothlamic Perifornical Area of Rodents

Abstract

Both the orexin peptides produced from prepro-orexin in orexinergic neurons of the lateral hypothalamus and hypothalamic perifornical area (PFA) and serotonin coming to these hypothalamic areas from synaptic endings of serotonergic neurons of the midbrain raphe nuclei play an important role in the regulation of feeding behavior and circadian rhythms. This indicates a close relationship between the serotoninergic and orexinergic systems in the lateral hypothalamus and PFA which, however, remains poorly studied. The aim of this study was to identify immunohistochemically and quantify the 2C-subtype (5-HT2CR) and the 1B-subtype (5-HT1BR) serotonin receptors in orexinergic neurons of the hypothalamic PFA in mice and rats. An immunohistochemical reaction for orexin-A was used to identify orexinergic neurons, and the double fluorescence immunolabeling was applied to assess the colocalization of serotonin receptors and orexin-A. Using double immunolabeling, the colocalization of orexin-A and 5-HT2CR and 5-HT1BR in the hypothalamic PFA of different rodent species, such as C57Bl/6J mice and Wistar rats, was demonstrated. It was shown that all orexinergic neurons expressed both types of serotonin receptors. At the same time, only 32–35% of 5-HT2CR and 5-HT1BR were localized in the orexinergic neurons, while the remaining part of them was expressed in the other types of PFA neurons. Thus, the immunohistochemical evidence was obtained for the serotonin-mediated regulation of functional activity of orexinergic neurons in the rodent hypothalamic PFA; also it was established that these effects of serotonin can be realized via 5-HT2CR and 5-HT1BR.