Differential distribution of endomorphin 1- and endomorphin 2-like immunoreactivities in the CNS of the rodent

Endomorphins are endogenous peptides that have high affinity and selectivit y for the mu-opiate receptor and potent analgesic activity. The distributio ns of endomorphin 1 (Tyr-Pro-Trp-Phe-NH2; EM1) and endomorphin 2 (Tyr-Pro-P he-Phe-NH2; EM2) in the rat central nervous system mere determined by immun ocytochemistry with two antisera, each demonstrating clear preference for t he target antigen. Perikarya expressing EM2-like immunoreactivity were pres ent in the posterior hypothalamus, whereas those expressing EM1-like immuno reactivity were present in both the posterior hypothalamus and the nucleus of the solitary tract (NTS). EM1-like immunoreactivity was more widely and densely distributed throughout the brain than was EM2-like immunoreactivity , whereas EM2-like immunoreactivity was more prevalent in the spinal cord t han was EM1-like immunoreactivity. The greatest density of EM1-like-immunor eactive fibers was detected in the parabrachial nucleus and the NTS, with n otable staining in the septum, diagonal band, bed nucleus of the stria term inalis, organum vasculosum, nucleus of Meynert, paraventricular thalamic nu cleus, posterior hypothalamic nucleus, periaqueductal gray, locus coeruleus , nucleus accumbens, and amygdala. The greatest density of EM2-like-immunor eactive fibers was detected in the superficial laminae of the spinal cord d orsal horn and the nucleus of the spinal trigeminal tract. The overall patt ern of immunoreactivities was similar in rat, mouse, and guinea pig, but so me differences were observed. In many but not in all locations, immunoreact ive fibers were prominently present in regions in which mu p receptors are reported to be concentrated. The neuroanatomical results suggest that endom orphins participate in modulating nociceptive and autonomic nervous system processes and responsiveness to stress. Published 1999 Wiley-Liss, Inc.dagg er