Metabotropic glutamate receptors in acutely isolated hippocampal astrocytes: developmental changes of mGluR5 mRNA and functional expression

We previously found that 82% of glial fibrillary acidic protein (GFAP)-positive hippocampal astrocytes acutely isolated from P1-10 rats responded to glutamate (Glu) with transient intracellular calcium increases via activation of a Group I metabotropic glutamate receptor (mGluR). Fewer cells responded to ATP and none to serotonin (5-HT). In this study we asked the question whether hippocampal astrocytes in older animals retain this relative pattern of expression. We have found that 77% of GFAP (+) cells from P11-20 rats responded to 50 microM Glu, 43% to ATP, and none to 5-HT. Thirty-three percent of GFAP (+) cells from P25-35 rats responded to Glu, 12% to ATP and 3% to 5-HT. In the case of the responses to Glu, pharmacological characterization and single-cell RT-PCR data confirmed that these responses were mediated by the mGluR5 subtype of group I mGluRs. Also, fewer (36%) GFAP mRNA (+) cells from P25-35 rats expressed detectable mGluR5 mRNA than those from P11-20 rats (77%). This number essentially corresponds to the number of GFAP(+) showing a Ca(2+) response to Glu. Both mGluR5a and b were expressed with equal frequency in cells from P11-20 rats, but the b form predominated in cells from older animals. Overall, our studies show that expression of mGluR5 in hippocampal astrocytes decreases with increasing age and the "a" splice variant declines to a greater extent than the "b" splice variant, corresponding to the developmental changes shown in total tissue for mGluR5.