Fetal cortical transplants into neonatal rats respond to thalamic and peripheral stimulation in the adult. An electrophysiological study of single-unit activity.

Plates of presumptive sensorimotor neocortex obtained from fetal rats at 14-16 days gestation were grafted into the cerebral hemisphere of 0- to 1-day-old newborn rats. The transplants were placed into small lesion cavities in the sensorimotor cortical area made immediately prior to grafting. At 4-5 months of age, single-unit activity in the transplants (TP) or normal cortex (NCX) was recorded under ketamine HCl anesthesia using standard electrophysiological techniques. Over 90% of cells in both the transplants (n = 63) and normal cortex (n = 39) responded to ventral thalamic stimulation. Their average response latencies were similar (11.48 +/- 0.56 ms in TP. 12.77 +/- 1.20 ms in NCX, mean +/- S.E.M., no significant difference), as were their average responses of slightly more than 1 spike per thalamic stimulus (1.24 +/- 0.08 in TP, 1.07 +/- 0.01 in NCX, no significant difference). In addition, 64% (30/47) of transplant cells and 44% (14/32) of normal cells also responded to electrical stimulation of the contralateral forepaw, and here also the latencies were similar (16.11 +/- 0.67 ms in TP, 14.30 +/- 0.78 ms in NCX). The spontaneous neuronal activity observed within the transplants was also comparable to that seen in normal cortex, as measured by comparison of spontaneous interspike interval (ISI) histograms (median ISI of 84.5 for TP and 80.5 for NCX) and comparison of the burst index (% of intervals less than 5 ms, 14.7% for TP and 10.5% for NCX). These results are compared to those found in several recent studies where fetal cortex is grafted into adult hosts.