HETEROGENEITY IN GAP JUNCTION EXPRESSION IN ASTROCYTES CULTURED FROM DIFFERENT BRAIN-REGIONS

Heterogeneity among astrocytes suggests that their role in the central nervous system is more complex than is commonly recognized. This paper describes just such a functional difference, comparing gap junctions in astrocytes derived from two brain regions. Astrocytes, both in situ and in culture, employ gap junctions as a means of intercellular communication. Recent evidence utilizing cultured rat cortical and striatal astrocytes has shown that these channels consist of subunits of connexin 43, the same protein as that composing cardiac gap junctions. Here we report that astrocytes cultured from neonatal rat hypothalamus contain a greater number of functional channels than astrocytes from the striatum, a difference reflected in both connexin 43 protein and mRNA. Specifically, in hypothalamic astrocytes the level of connexin 43 protein was approximately four times that found in comparable cultures from the striatum, as determined by immunoblotting. Complementary results from immunocytochemical experiments using an antibody specific for connexin 43 reveal significantly greater fluorescence in astrocytes cultured from the hypothalamus as compared to those from the striatum. Northern blot analysis showed that connexin 43 mRNA levels were also approximately 4-fold greater in the hypothalamic cultures, consistent with the difference seen by immunoblotting. Finally, dye coupling studies using confluent cultures consistently showed that within 1 min Lucifer Yellow injected into striatal astrocytes spread to immediately surrounding cells while in hypothalamic astrocytes dye often spread to apparent third or fourth order neighbors within the same time period. Thus, the higher level of connexin 43 expression seen in hypothalamic astrocytes results in cells with greater numbers of functional channels. Interestingly, both the immunocytochemical and dye coupling experiments also revealed heterogeneity among cells derived from each brain area, indicating that both regions contain subpopulations of astrocytes. Taken together, our data show differences in gap junction expression, both within and between astrocytes derived from two brain regions. Further, this heterogeneity appears to derive from differences in either transcriptional regulation or in mRNA stability, and not from changes in the relative levels of phosphorylated forms of connexin 43. Thus, these data, which are consistent with light microscopic immunocytochemical studies in tissue sections of rat brain, predict that, in vivo, region-specific coupling subserves functional specialization within the central nervous system.