Dark-reared cats: unresponsive cells become visually responsive with microiontophoresis of an excitatory amino acid.

The visual system of kittens reared in total darkness is grossly abnormal. Although estimates vary, substantial proportions of cells in the visual cortex of these animals are unresponsive to visual stimulation. Additional cells are weakly responsive or erratic. We have considered the possibility that these neurons receive subthreshold input which might be evident if an excitatory neurochemical agent is applied during extracellular recording with a microelectrode. To test this notion, we have recorded from cells in the striate cortex of dark-reared kittens during microiontophoretic application of an excitatory amino acid, DL-homocysteate (DLH). Using this technique, we find that virtually all cells in the visual cortex of dark-reared kittens are responsive to visual stimulation. Prior to application of DLH, 27% of the cells were unresponsive to visual stimuli. Following iontophoresis of DLH, half of these cells responded with excitatory discharge to visual stimuli and the other half exhibited an inhibitory response in that the elevated maintained activity was suppressed during presentation of a visual stimulus. Additional cells from these animals, which were initially visually responsive, were also studied. For some of these units, responses were weak prior to administration of DLH and we were able to obtain a more clear estimate of selectivity for stimulus orientation during microiontophoresis of the drug. In these cases, and for the few cells which were initially responsive and orientation selective, we observed no major differences in selectivity before and after DLH application.