The post‐natal development of cutaneous afferent fibre input and receptive field organization in the rat dorsal horn.

Abstract

The responses evoked in lumbar dorsal horn cells by both natural and electrical hind‐limb skin stimulation were recorded in the spinal cord of rat pups aged 0‐15 days under urethane anaesthesia. The input volley was recorded on the L4 dorsal root and consisted of two separate waves from birth. Latency and threshold measurements were consistent with these two waves being immature A (myelinated fibre) waves and C (non‐myelinated fibre) waves. On the first 3 days of life background activity of cells in the dorsal horn was low and evoked discharges were sluggish. On electrical stimulation of the skin, neonatal dorsal horn cells frequently responded with only 1 or 2 impulses per input volley with long central delays of up to 20 ms. Synaptic linkage appeared weak and many cells failed to follow stimulation rates of 5 Hz. Natural skin stimulation showed that the majority of cells at days 0‐3 responded to pinching the skin only. The development of responses evoked by C fibres in the dorsal horn was delayed compared to that of responses evoked by A fibres. Short and long latency responses corresponding to the early A and late C afferent input volleys could be recorded in the superficial laminae (I, II and III) of the dorsal horn from day 0, but in the deeper laminae only early short latency A responses were evoked until the age of day 7‐8. After this time, a long latency C response also appeared and increased in strength with age. Convergence of low and high threshold inputs onto dorsal horn cells was rare at birth but increased gradually over the following two weeks. Receptive field areas, mapped by natural mechanical stimulation of skin, were large at birth and decreased in size with age. At birth the mean receptive field area was 14.2% of the total hind‐limb area whereas at day 15 it was 3.6%. This fall in size was particularly marked in cells of the deep dorsal horn. Pinching or brushing the receptive field of many neonatal dorsal horn cells resulted in long‐lasting after‐discharges (30‐90 s) which on days 0‐3 could be more pronounced than the initial evoked response. The duration and amplitude of these responses decreased with age. Repetitive electrical skin stimulation of the receptive fields of these cells produced 'wind‐up' and prolonged after‐discharge. Ipsilateral, contralateral and distant inhibitory components to receptive fields were observed from day 0.(ABSTRACT TRUNCATED AT 400 WORDS)