One of the functions of glial receptors is to regulate synthesis and release of a variety of neuropeptides and growth factor peptides, which in turn act on neurons or other glia. Because of the potential importance of these interactions in injured brain, we have examined the role of two different receptors in the regulation of astrocyte neuropeptide synthesis. Stimulation of 6-adrenergic receptors on type 1 astrocytes resulted in increased mRNA and protein for the proenkephalin (PE) and somatostatin genes. This receptor also increased expression of nerve growth factor (NGF) and brainderived neurotrophic factor (BDNF). The potential role of opiate receptors was examined in several ways. Treatment of newborn rats for 7 days with the opiate antagonist naltrexone, prior t o preparation of astrocytes, had no effect on PE mRNA or met-enkephalin content but resulted in a significant increase in NGF content. However, treatment of astrocytes in culture with met-enkephalin, morphine, or naltrexone had no effect on any of these parameters. No opiate binding could be detected, using either etorphine or bremazocine, to membranes of astrocytes prepared from cortex, cerebellum, striatum, or hippocampus of 1-day, 7-day, or 14-day postnatal rats. Thus we conclude that type 1 astrocytes do not express opiate receptors and that the in vivo effects of naltrexone are mediated indirectly via some other cell typeheceptor. o 1994 Wiley-Liss, Inc.
1NTRODUCTION
Expression of almost all the known neurotransmitter and neuropeptide receptors on astrocytes is well established: as clearly indicated by this issue of Glia, the questions now being addressed relate to the functions of astrocyte receptors. The presence of functional receptors on astrocytes has suggested that they can respond to neuronally released signals. The recent discoveries of neuropeptide synthesis by astrocytes further support the long-standing concept (Varon and Somjen, 1979) that intercellular communication through neurotransmittersheuropeptides can go in several directions, not only from neurons to glia but also from glia back to neurons, as well as to other glia (reviewed in Schwartz, 1993). Our understanding of neuropeptide-mediated intercellular communication is therefore dependent on determining which receptors regulate synthesis and release of specific astrocyte peptides.
Among the peptides synthesized by astrocytes are