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 simil ar 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 do rsal lateral geniculate nucleus. Here, we demonstrate that GABAergic t ransmission plays an important, although not exclusive, role in regula ting the bursting patterns of morphologically identified On and Off ga nglion cells. During the first and second postnatal weeks, blocking GA BA(A) receptors leads to a decrease in the bursting activity of all ga nglion cells, suggesting that GABA potentiates activity at the early a ges. Subsequently, during the period of On-Off segregation in the geni culate nucleus, GABA suppresses ganglion cell bursting activity. In pa rticular, On ganglion cells show significantly higher bursting rates w hen GABAergic transmission is blocked, but the bursting rates of Off g anglion cells are not affected systematically. Thus, developmental dif ferences in the bursting rates of On and Off ganglion cells emerge as GABA becomes inhibitory and as it consistently and more strongly inhib its On compared with Off ganglion cells. Because in many parts of the CNS GABAergic circuits appear early in development, our results also i mplicate a potentially important and possibly general role for local i nhibitory interneurons in creating distinct temporal patterns of presy naptic activity that are specific to each developmental period.