Blockade of electrical activity promotes the death of mammalian retinal ganglion cells in culture.

Abstract

During the first 2 postnatal weeks, up to 50% of the ganglion cells in the mammalian retina normally die. Natural cell death may result from several factors, and electrical activity has been proposed as one critical element. Recent experiments in vivo using intraocular injection of tetrodotoxin (TTX) have suggested that competition for survival between ganglion cells from the two eyes is mediated by their degree of neuronal activity. In addition, the level of activity of afferents to the ganglion cells has been postulated to be an important variable in determining their survival. To investigate the mechanism of cell death engendered by altered activity, I studied the effect of electrical blockade with TTX (to block sodium channels and thus action potentials) or low Ca/high Mg (to block transmitter release and hence synaptic activity) on individual neurons in vitro. For this purpose, identified retinal ganglion cells (RGCs) from postnatal rats were maintained in culture. Unlike the previous in vivo experiments, this approach permitted the exact concentration of each agent to be controlled and the electrical activity of the RGCs to be recorded. In cultures from animals of postnatal day 2-10 (P2-10), 1 microM TTX or 0.2 mM Ca/20 mM Mg resulted in the death of about 50% of the RGCs, representing those cells that had displayed spontaneous electrical activity, but did not affect RGCs that lacked activity. However, the death of RGCs with spontaneous activity from P11-13 animals was not influenced by these drugs. These findings suggest that during a critical period of development neurons become dependent upon electrical activity, and the cessation of this activity can result in their death. In addition, conditioned medium, collected from cultures lacking TTX, rescued from death a large proportion of TTX-treated RGCs. Thus, the critical element for survival may represent modulation of a trophic factor related to the level of activity rather than electrical activity itself. Since, in vivo, natural cell death occurs in neurons of similar type and age, and in the same proportion as that induced by the artificial blockade of electrical activity in culture, these findings may be germane to the mechanism of natural cell death in the retina.