Suppressive rod-cone interaction in distal vertebrate retina: intracellular records from Xenopus and Necturus.

The influence of dim diffuse adapting fields upon the sensitivity to focal photic stimulation was studied by means of intracellular recording in retinal neurons of the south african clawed frog, Xenopus and the mudpuppy, Necturus. In cones and in most horizontal and bipolar cells lacking color opponency, dim diffuse backgrounds have little influence upon the response to diffuse flicker of low (less than 2 Hz) temporal frequencies; however, with small diameter test probes of higher temporal frequencies, presentation of dim backgrounds enhance the peak-to-peak amplitude of responses to sinusoidal flicker by as much as 800%. This background enhancement effect adheres to the spectral sensitivity of the green-absorbing rod photopigment, and appears to be largely independent of the influence of the adapting field upon cone photopigment or ambient membrane potential in the recorded neuron. This effect cannot be obtained with rod-driven flicker responses. We designate this background influence on flicker, suppressive rod-cone interaction (SRCI) and attribute it to a tonic suppressive (probably inhibitory) influence of rods upon cone pathways that is removed by rod light adaptation. SRCI is also observed in the response of most sustained ON and OFF ganglion cells. However, no corresponding effect occurs in rods, color-opponent second-order neurons, ON-OFF amacrine cells, or most ON-OFF ganglion cells. The spatial and temporal limitations of SRCI observed by means of intracellular recording in amphibians are very similar to those documented by means of psychophysical or electroretinogram (ERG) procedures in a wide variety of species including humans (2, 4, 11, 22, 23, 29). SRCI most probably reflects a process that is mediated by horizontal cells. The specifics of the underlying mechanism remain unclear.