THE JOURNAL OF COMPARATIVE NEUROLOGY 350:412-438 (1994) Mu, Delta, and Kappa Opioid Receptor mRNA Expression in the Rat CNS: An In Situ Hybridization Study ALFRED MANSOUR, CHARLES A. FOX, SHARON BURKE, FAN MENG, ROBERT C. THOMPSON, HUDA AKIL, AND STANLEY J. WATSON Mental Health Research Institute and Department of Psychiatry, University of Michigan, Ann Arbor, Michigan 48109-0720 ABSTRACT The p, 6, and K opioid receptors are the three main types of opioid receptors found in the central nervous system (CNS) and periphery. These receptors and the peptides with which they interact are important in a number of physiological functions, including analgesia, respiration, and hormonal regulation. This study examines the expression of p, 6, and K receptor mRNAs in the rat brain and spinal cord using in situ hybridization techniques. Tissue sections were hybridized with 35S-labeledcRNA probes to the rat IJ. (744-1,064 b), 6 (304-1,287 b), and K (1,351-2,124 b) receptors. Each mRNA demonstrates a distinct anatomical distribution that corresponds well to known receptor binding distributions. Cells expressing p receptor mRNA are localized in such regions as the olfactory bulb, caudate-putamen, nucleus accumbens, lateral and medial septum, diagonal band of Broca, bed nucleus of the stria terminalis, most thalamic nuclei, hippocampus, amygdala, medial preoptic area, superior and inferior colliculi, central gray, dorsal and median raphe, raphe magnus, locus coeruleus, parabrachial nucleus, pontine and medullary reticular nuclei, nucleus ambiguus, nucleus of the solitary tract, nucleus gracilis and cuneatus, dorsal motor nucleus of vagus, spinal cord, and dorsal root ganglia. Cellular localization of 6 receptor mRNA varied from p or K, with expression in such regions as the olfactory bulb, allo- and neocortex, caudate-putamen, nucleus accumbens, olfactory tubercle, ventromedial hypothalamus, hippocampus, amygdala, red nucleus, pontine nuclei, reticulotegmental nucleus, motor and spinal trigeminal, linear nucleus of the medulla, lateral reticular nucleus, spinal cord, and dorsal root ganglia. Cells expressing K receptor mRNA demonstrate a third pattern of expression, with cells localized in regions such as the claustrum, endopiriform nucleus, nucleus accumbens, olfactory tubercle, medial preoptic area, bed nucleus of the stria terminalis, amygdala, most hypothalamic nuclei, median eminence, infundibulum, substantia nigra, ventral tegmental area, raphe nuclei, paratrigeminal and spinal trigeminal, nucleus of the solitary tract, spinal cord, and dorsal root ganglia. These findings are discussed in relation to the physiological functions associated with the opioid receptors. D 1% Wiley-Liss, Inc. Key words: endorphins,endorphin receptors, localization, opiates, pept,ide receptors Several lines of research have demonstrated that the opioid receptors can be classified into three main types referred to as mu (p), delta (61, and kappa (K; Martin et al., 1976; Lord et al., 1977; Chang and Cuatrecasas, 1979; Smith and Simon, 1980). The IJ. and K receptor types were initially differentiated by Martin and colleagues (1976) in the chronic spinal dog preparation and are the sites at which alkaloids, such as morphine, and benzomorphans, such as ethylketazocine, bind to produce their respective physiological effects. The 6 receptors, named for the mouse vas deferens preparation from which they were initially described (Lord et al., 1977),differ from the p and K binding sites and have a higher affinity for proenkephalin-derived peptides. These early studies suggesting multiple opioid receptor types have been extended by numerous anatomi- cal, pharmacological, biochemical, and behavioral studies (Gillan and Kosterlitz, 1982; Magnun et al., 1982; Wood, 1982; Pasternak et al., 1983; Goldstein and James, 1984; James and Goldstein, 1984; Zukin and Zukin, 1984; McLean et al., 1986; Mansour et al., 1987; Goldstein and Naidu, Accepted June 27, 1994. Address reprint requests to Dr. Alfred Mansour, Mental Health Research Institute,, University of Michigan, 205 Zina Pitcher Place, Ann Arbor, MI 48109-0720. O 1994 WILEY-LISS, INC.