DIRECT IMMUNOCYTOCHEMICAL EVIDENCE FOR THE TRANSFER OF GLUTAMINE FROMGLIAL-CELLS TO NEURONS - USE OF SPECIFIC ANTIBODIES DIRECTED AGAINST THE D-STEREOISOMERS OF GLUTAMATE AND GLUTAMINE

We have raised antibodies against D-stereoisomers of the amino acids g lutamate and glutamine. These stereoisomers are not naturally occurrin g in mammals but can be taken up into cells by transporters that norma lly handle the endogenous L-amino acids. Exposure of isolated rabbit r etinae to 50 mu M D-glutamate resulted in a strong accumulation of D-g lutamate, and hence immunoreactivity for D-glutamate in radial glial c ells (Muller cells). By contrast the glutamatergic ganglion cells exhi bited no immunoreactivity for D-glutamate. D-Glutamate can be converte d into D-glutamine by the glial enzyme glutamine synthetase. Immunolab elling for D-glutamine revealed the presence of D-glutamine in somata of subsets of neurons including the glutamatergic ganglion cells. Labe lling was also present in the inner plexiform layer, possibly indicati ng labelling of neuronal processes. These data indicate that after D-g lutamate has been taken up into glial cells it is converted into D-glu tamine. This D-glutamine is then exported from the glial cells and tak en up by a subset of neurons, including the glutamatergic ganglion cel ls.