HIGH-LEVELS OF EXTRACELLULAR GLUTAMATE ARE PRESENT IN RETINA DURING NEONATAL DEVELOPMENT

The three major classes of neurons which comprise the primary visual p athway in retina are glutamatergic. These cells are generated in two s eparate developmental stages, with one subclass of photoreceptors (con es) and ganglion cells generated before birth; and the other subclass of photoreceptors (rods) and bipolar cells generated during the first week after birth. Gas chromatography/mass spectroscopy analysis couple d with a new method for collecting small samples of extracellular flui ds from retina were used to determine the levels of endogenous glutama te present during differentiation and synaptogenesis of these differen t cell types. As expected the total retinal content of glutamate incre ased during the postnatal period in synchrony with the generation and maturation of glutamatergic cells. However, a significant proportion o f the endogenous pool was found extracellularly at birth. Intracellula r glutamate is localized within cell bodies and growing processes of c ones and ganglion cells at this time but few glutamatergic synapses ar e present. The extracellular concentration of glutamate actually decli ned during the most active period of synaptogenesis, reaching very low levels in the adult. The high concentrations of extracellular glutama te in neonatal retina could play an important role in a variety of dev elopmental events such as dendritic pruning, programmed cell death and neurite sprouting.