PURPOSE. Alpha-2 adrenoceptor (AR) agonists are currently used in the medical management of glaucoma for their ocular hypotensive properties. Recent evidence suggests that these compounds can also prevent neuronal death in models of retinal and optic nerve injury, but the neuroprotective mechanisms are not fully understood. This work investigated the action of an alpha-2 AR agonist (UK14,304) on the glutamate-induced calcium influx in isolated rat retinal neurons. METHOD. Retinae were obtained from neonatal rats and dissociated enzymatically into a suspension of single cells. The retinal cells were plated onto glass coverslips and cultured in serum-free medium containing neurotrophic factors and forskolin. Isolated retinal neurons were loaded with the ratiometric calcium-indicator dye Fura-2 and exposed to a short application (20 s) of 10 micromolar glutamate, both with and without the presence of UK14,304. Changes in fluorescence, indicative of changes in intracellular calcium, were imaged using a Sensicam cooled CCD camera and analyzed using Axon Imaging Workbench.
RESULTS. In the presence of 10 micromolar (n=21) and 100 micromolar (n=30) UK14,304, the glutamate-induced calcium current was reduced to 85.68% and 68.39%, respectively, of that of the pre-treatment controls. The decrease in calcium influx, relative to controls, was statistically significant (p<0.01) at both doses.
CONCLUSIONS. These results suggest that the alpha-2 AR agonist UK14,304 can reduce glutamate-induced calcium influx in cultured rat retinal neurons in a dose-dependent manner. Since increases in intracellular calcium levels are associated with glutamate excitotoxic neuronal death, a decrease in calcium influx may contribute to the apparent neuroprotective effect of alpha-2 AR agonists.