Age-dependent and cell class-specific modulation of retinal ganglion cell bursting activity by GABA

Competition for postsynaptic targets during development is thought to be driven by differences in temporal patterns of neuronal activity. In the ferret visual system, retinal ganglion cells that are responsive either to the onset (On) or to the offset (Off) of light exhibit similar patterns of spontaneous bursting activity early in development but later develop different bursting rhythms during the period when their axonal arbors segregate to occupy spatially distinct regions in the dorsal lateral geniculate nucleus. Here, we demonstrate that GABAergic transmission plays an important, although not exclusive, role in regulating the bursting patterns of morphologically identified On and Off ganglion cells. During the first and second postnatal weeks, blocking GABAA receptors leads to a decrease in the bursting activity of all ganglion cells, suggesting that GABA potentiates activity at the early ages. Subsequently, during the period of On-Off segregation in the geniculate nucleus, GABA suppresses ganglion cell bursting activity. In particular, On ganglion cells show significantly higher bursting rates when GABAergic transmission is blocked, but the bursting rates of Off ganglion cells are not affected systematically. Thus, developmental differences in the bursting rates of On and Off ganglion cells emerge as GABA becomes inhibitory and as it consistently and more strongly inhibits On compared with Off ganglion cells. Because in many parts of the CNS GABAergic circuits appear early in development, our results also implicate a potentially important and possibly general role for local inhibitory interneurons in creating distinct temporal patterns of presynaptic activity that are specific to each developmental period.