REGULATION OF ASTROGLIAL RESPONSIVENESS TO NEUROLIGANDS IN PRIMARY CULTURE

Previous studies from this laboratory indicate that type-1 astroglia i n primary culture are pharmacologically heterogeneous. Two competing h ypotheses were proposed to explain the development of glial heterogene ity. First, that the heterogeneity may reflect stable subclasses of as troglia that express a set of receptor-signalling systems. Second, tha t astroglia can undergo qualitative changes in their expression of rec eptor-signalling systems with time in vitro. To distinguish between th ese two hypotheses, experiments were designed to examine neuroligand-e voked calcium responses within clones of type-I astroglia. If stable a nd distinct subsets of astroglial were present, a clone derived from a single cell would exhibit uniform responses to a given set of neuroli gands. Alternatively, if the pharmacological properties of astroglia u nderwent qualitative changes, astroglial ones should contain pharmacol ogically distinct cells. A video-based imaging system and the Ca2+ ind icator dye Fura-2 were used to monitor receptor-mediated increases in Ca(i)2+ upon receptor activation. Interestingly, only a fraction of th e cells within a given clone responded to carbachol or histamine with an increase in Ca(i)2+, whereas treatment with a P2Y purinergic recept or agonist generally increased Ca(i)2+ in 100% of the cells within the clone. To examine the stability of the receptor signalling over time, individual astroglia within a number of clones were tested on differe nt days for their ability to respond to neuroligands. The results of t hese experiments indicated that individual astroglial cells tended to lose their responsiveness to certain ligands such as carbachol and his tamine as they developed responsiveness to others such as norepinephri ne. Our data indicate that during development neurotransmitter recepto rs on astroglial cells are regulated by both internal and external mec hanisms. Glial proliferation produces a variety of pharmacologically d istinct astroglial cells. Exposure to neurotransmitters can qualitativ ely turn off some, but not all, astroglial receptor systems.