The Hcrt peptides were originally thought to be synthesized exclusively in the brain, as peripheral tissues such as liver, spleen, thymus, testes and ovaries did not show hcrt mRNA expression.1 A number of studies have since reported Hcrt immunoreactivity in non-neural brain tissues including the pineal gland50 and olfactory epithelium,15 although hcrt mRNA has not been localized in these structures to date. Hcrt mRNA was also detected in the ventricular ependymal cell lining.57 Hcrt peptides and receptors have also been reported in several peripheral tissues including plasma,58'59 sympathetic ganglia, myenteric plexus, endocrine cells of the gastrointestinal tract,60 adrenal gland,61-63 islet cells of the pancreas,60,61 and placenta.61 In the gut, the gastrin-containing endocrine cells of the stomach were Hcrtl immunoreactive64,65 and several Hcrtl-immunoreactive neurons were seen in the intestinal mucosa, a subpopulation of which also expressed serotonin.64 Hcrt mRNA was shown by RT-PCR to be expressed in the testes,63'66'67 but not in the ovaries.63 The testes also apparently contain hcrtrl mRNA.63'67 Merkel cells in the pig snout have also been reported to be Hcrt-immunoreactive.68 Expression of hcrtl mRNA was also seen in the pituitary, kidney, adrenal, thyroid, jejunum,63 olfactory epithelium,15 and Merkel cells.68 High levels of hcrtr2 mRNA was seen in the adrenal gland (higher in male), and low levels were seen in the lung and pituitary.63 HcrtR2 immunoreactivity was detected in the olfactory epithelium15 and Merkel cells.68
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