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Differential distribution in rat brain of mu opioid receptor carboxy terminal splice variants MOR‐1C‐like and MOR‐1‐like immunoreactivity: Evidence for region‐specific processing

The present study examined immunohistochemically the regional distribution of the mu opioid receptor splice variant MOR‐1C by using a rabbit antisera generated against the C‐terminal peptide sequences and compared it with MOR‐1. Overall, the distribution of MOR‐1C–like immunoreactivity (–LI) differed from MOR‐1–LI. Both MOR‐1C–LI and MOR‐1–LI were prominent in a few central nervous system regions, including the lateral parabrachial nucleus, the periaqueductal gray, and laminae I‐II of the spinal trigeminal nuclei and the spinal cord. In the striatum, hippocampal formation, presubiculum and parasubiculum, amygdaloid nuclei, thalamic nuclei, locus coeruleus, and nucleus ambiguous MOR‐1–LI predominated, whereas MOR‐1C–LI was absent or sparse. Conversely, MOR‐1C–LI exceeded MOR‐1–LI in the lateral septum, the deep laminae of the spinal cord, and most hypothalamic nuclei such as the median eminence, periventricular, suprachiasmatic, supraoptic, arcuate, paraventricular, ventromedial, and dorsomedial nuclei. Double‐labeling studies showed colocalization of the two receptors in neurons of the lateral septum, but not in the median eminence or in the arcuate nucleus, even though both MOR‐1 isoforms were expressed. Because both MOR‐1 and MOR‐1C are derived from the same gene, these differences in regional distribution represent region‐specific mRNA processing. The regional distributions reported in this study involve the epitope seen by the combinations of exons 7, 8, and 9. However, if other MOR‐1 variants containing exons 7, 8, and 9 exist, the antisera would not distinguish between them and MOR‐1C. J. Comp. Neurol. 419:244–256, 2000. © 2000 Wiley‐Liss, Inc.