The effects of dark-rearing on the electrophysiology of the rat visual cortex

Our previous two studies have shown that dark-rearing affects the morphology and chemistry of adult rat primary visual cortex (area 17). In this study we demonstrate correlated physiological alterations with single unit recordings in the same preparation. Rats were raised from birth in either 14 h light/10 h dark (Lt/Dk) or in total darkness (Dk). At the age of 3 months, single units were recorded in area 17 of both groups. The cortical cells of Dk animals showed significantly more spontaneous activity during ambient lighting. The mean rate of randomly appearing spontaneous activity was greatly increased in Dk animals. Moreover, many cells in Dk animals also exhibited a particular type of spontaneous activity which occurred as 'bursts' of spikes, i.e. quantified groupings of fast firing spikes, separated by randomly appearing spontaneous activity. The mean number of bursts per min seen in Dk animals was also significantly more than any such activity seen in Lt/Dk animals. Visual stimuli consisted of white or dark bars moving with different orientations and directions at slow and fast speeds, and full field flashes. In response to moving stimuli, notably fewer cells were orientation- or direction tuned in dark-reared animals, and when they did respond to moving bar stimuli, the responses were of relatively longer duration. The pathologically high spontaneous activity rate, as well as lack of tuning and relatively prolonged duration of responses to moving stimuli indicate that intracortical inhibitory mechanisms are seriously compromised in both the unstimulated and stimulated states and is in agreement with our previous findings (Bakkum, B.W., Port, J.D., Cohen, R.S. and Benevento, L.A., Soc. Neursci. Abst., 15 (1989) 797) of a decreased number of synapses and GABA-containing cells in the visual cortex of the same preparation. Other evidence suggests that there may be a decrease in stimulus-bound excitatory drive. Significantly fewer cells in Dk animals were excited by all visual stimuli, and responses elicited by flashes had relatively longer 'on' latencies, relatively shorter durations, and were generally weaker. This may correlate with our finding of a significantly smaller number of perforated postsynaptic densities in the cortex of the same preparation (Bakkum, B.W., Benevento, L.A. and Cohen, R.S., J. Neurosci. Res., 23 (1991) 65-80).