Immunocytochemical mapping of endomorphin-2-immunoreactivity in rat brain

Endomorphin-2 (Tyr-Pro-Phe-Phe-NH₂) is a novel endogenous opioid with high affinity and selectivity for the μ-opioid receptor. Immunocytochemical studies have located this peptide in spinal cord, brainstem and selected brain regions. However, there are disagreements regarding its distribution between published reports. Furthermore, the distributions reported for the endomorphins resemble that of neuropeptide FF, suggesting that some of the previous findings might be due to cross-reactivity with the latter substance. In the present study, the distribution of endomorphin-2-immunoreactivity (ir) was examined throughout the entire rat brain using an affinity-purified antiserum that appeared not to cross-react with neuropeptide FF. Endomorphin- 2-ir cell somata were most prominent in the hypothalamus and the nucleus of the solitary tract (NTS). Endomorphin-2-ir varicose fibers were observed in such areas as the bed nucleus of the stria terminalis, the septal nuclei, the periaqueductal gray, the locus coeruleus, the lateral parabrachial nucleus, the NTS, and the substantia gelatinosa of the medulla. More modest immunoreactivity was seen in substantia nigra, nucleus raphe magnus, the ventral tegmental area, the pontine nuclei and the amygdala. Fibers were also observed in the ventral cerebellum. Of note was the negligible immunoreactivity in the striatum, a region known to express high levels of μ-opioid receptors. Thus, endomorphin-2-ir was widely, but not uniformly, distributed throughout the central nervous system and was associated largely, but not exclusively, with regions expressing μ-opioid receptors. Based on its distribution, it may have a role in the control of neuroendocrine, cardiovascular and respiratory functions, and mood, feeding, sexual behavior and pain.