Abundant expression of mu and delta opioid receptor mRNA and protein in the cerebellum of the fetal, neonatal, and adult rat.

Opioid receptor proteins and mRNAs have been localized to a variety of regions within the rat brain. It is generally accepted that within the lobes of the rat cerebellum, only delta opioid receptor (DOR) is expressed. This is in contrast to that observed in humans and rabbits which express both mu opioid receptor (MOR) and DOR. In this study, we report detection of MOR as well as DOR protein by immunohistochemical localization, and mRNA by fluorescent in situ hybridization (FISH) within Purkinje cells (PK) and the granular layer of neonatal (P6) and adult rat cerebellum. Expression of MOR mRNA was also detected within cells of the molecular layer, but at lower levels than those seen within the PK cells. Abundant expression of MOR and DOR mRNA was detected in the external germinal layer of the immature cerebellum of the fetal (E16) rat, supporting a role for MOR and DOR in regulating neurogenesis of the cerebellum. In addition, using exon-specific cRNA probes, exons 1 and 4, which are both found in the MOR-1 splice variant mRNA, were detected in PK cells in the cerebellum and also within deep cerebellar nuclei in the adult.