The development of activity evoked by fine diameter cutaneous fibres in the spinal cord of the newborn rat.

The cutaneous afferent volley on the dorsal root was recorded in normal rat pups and in pups whose fine diameter afferents had been destroyed with capsaicin at birth. From postnatal day 2 (P2) it was possible to identify a long latency wave in the afferent volley that was reduced or absent in capsaicin treated pups and therefore attributable to A delta and C fibers. Despite this, no difference between the evoked activity of dorsal horn cells to electrical skin stimulation in normal and capsaicin treated rat pups could be detected at P2 or P5. Only at P9 was a clear long latency burst of spikes evoked in dorsal horn cells that was absent in capsaicin treated rats. The results support the idea that the postsynaptic activity evoked by small diameter A and C fibres in the rat dorsal horn in the first postnatal week is very immature.