Mechanisms that limit the light stimulus frequency following through the DL-2-amino-4-phosphonobutyric acid sensitive and insensitive rod Off-pathways.

In the retina, rod signal pathways process scotopic visual information. Light decrements are mediated by two distinct groups of rod pathways in the dark-adapted retina that can be differentiated on the basis of their sensitivity to the glutamate agonist dl-2-amino-4-phosphonobutyric acid (APB). We have found that the APB sensitive and insensitive rod Off-pathways signal different light decrement information: the APB sensitive rod Off-pathway conveys slow and low frequency light signals, whereas the APB insensitive rod Off-pathways mediate fast and high frequency light signals [Wang GY (2006) Unique functional properties of the APB sensitive and insensitive rod pathways signaling light decrements in mouse retinal ganglion cells. Vis Neurosci 23:127-135]. However, the mechanisms which limit the frequency following through the APB sensitive and insensitive rod Off-pathways remain unknown. In the current study, whole-cell patch-clamp recordings were made from ganglion cells in dark and light adapted mouse retina to identify the mechanisms that limit the frequency following through the APB sensitive and insensitive rod Off-pathways. The results showed that the sites from AII amacrine cells to Off cone bipolar cells are the major mechanisms that limit the frequency following through the APB sensitive rod Off-pathway. In the APB insensitive rod Off-pathways, rods themselves limited the frequency following through these pathways. Moreover, ganglion cells were able to follow higher frequencies under photopic conditions than under scotopic conditions. The Off responses followed lower frequencies than On responses under photopic conditions. This finding was observed in cells that yielded On or Off responses only as well as in On-Off cells.