PRIMARY ASTROGLIAL CULTURES DERIVED FROM SEVERAL RAT-BRAIN REGIONS DIFFERENTIALLY EXPRESS MU-OPIOID, DELTA-OPIOID AND KAPPA-OPIOID RECEPTORMESSENGER-RNA

The existence of opioid receptors within glial cell membranes has been proposed by several laboratories based on biochemical and radioligand binding data. The recent cloning of the mu, delta and kappa receptors has enabled us to directly examine the issue of opioid receptor expre ssion in rat brain astroglia by using solution hybridization/ribonucle ase protection assays to analyze the total RNA obtained from primary c ultures of cortical, striatal, cerebellar, hippocampal and hypothalami c astrocytes. The results indicate that all five glial cultures expres sed mu, delta and kappa receptor mRNA. The rank order of receptor mRNA abundance, expressed collectively across all five cultures, was deter mined to be delta greater than or equal to kappa much greater than mu. An analysis of the glial distribution profile for each receptor type revealed that mu receptor mRNA levels were the most abundantly express ed in cortical cultures, while the greatest levels of delta receptor m RNA were found in the cortical and hypothalamic cultures, and signific ant kappa receptor mRNA levels were produced by the cortical, hypothal amic and cerebellar cultures. Furthermore, the five glial cultures eac h expressed different levels of total opioid receptor (mu + delta + ka ppa) mRNA. The rank order of total opioid receptor mRNA expression acr oss different astroglial cultures was found to be cortex > hypothalamu s > cerebellum = hippocampus > striatum. An analysis of the relative e xpression profiles for mu, delta and kappa receptor mRNA within each c ulture revealed that all cultures manifested relatively high levels of delta and kappa receptor mRNA, but relatively low levels of mu recept or mRNA. Generally, cortical, hippocampal and hypothalamic cultures we re characterized by comparable levels of delta and kappa receptor mRNA , and little, if any, mu receptor mRNA. However, striatal cultures wer e characterized by a high level of delta receptor mRNA which was appro ximately twice and four times that of the kappa and mu receptor mRNA, respectively. In contrast, cerebellar cultures expressed predominantly kappa receptor mRNA at a level which was almost twice that of the del ta receptor mRNA, and expressed very little mu receptor mRNA. These da ta show that primary astroglial cultures not only express mu, delta an d kappa receptor mRNAs, but they do so in a manner dependent upon rece ptor type and brain region. This suggests a regional heterogeneity of astrocytes with respect to opioid receptor expression, a characteristi c previously described only for neurons. Furthermore, it suggests the existence of an additional anatomical component in CNS opioid systems.