Responses of vibrissa-sensitive cortical neurons in normal and prenatally X-irradiated rat.

1. Rats were irradiated by 200 R of X ray on day 17 (the 18th day) of gestation through the body wall of the mother. When they underwent the following electrophysiological tests at the age of 3-4 mo, the somatosensory cortex showed a lack of layers II, III, IV, and Va, as described by previous investigators (3, 4). 2. Spike responses to quick whisker deflections were recorded from single cells in the somatosenory cortex of normal and prenatally X-irradiated rats. 3. In normal animals, single units often responded to several whiskers, but the magnitudes (response latency and firing probability) of the responses were graded around a whisker in the "center" of the field that gave rise to the strongest response. During electrode penetration perpendicular to the cortical surface, such a center remained unchanged or shifted to another whisker that was immediately adjacent. As for the large-sized, initially positive unitary spikes (typically 2 mV or higher) sampled in this study, no systematic change in the response latency, the size of receptive field, or response probability was detected during the penetration. 4. The above description holds also for the irradiated rats except that the response latency was prolonged when compared to the normal controls. 5. Cortical laminar analysis of field potentials revealed that while the earliest electric sink appeared in layer IV in the controls, that in the irradiated rats was observed in layer Vb. However, there was no difference in the latency of these potentials between the two groups, suggesting that vibrissal sensory signals reach the cortical level normally even in the irradiated rats. The prolonged latency of the irradiated cortical neuronal response could thus be ascribed to an abnormal intracortical delay, which was most likely associated with the failure of development of layer IV stellate cells in these preparations. 6. A possibility as to how the layer IV stellate cells are involved in input-output relationships in the normal somatosensory cortex is presented and discussed.