Jennings, Ernest Albert; (1999) Cutaneous afferent evoked activity in the postnatal rat spinal cord. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The postnatal development of afferent evoked activity in single extracellularly recorded dorsal horn cells, was examined in urethane anaesthetised rats aged 3, 6, 10 and 21 days old. Responses to single and repeated stimuli to primary afferent fibres, and "natural stimulation" were investigated. Cells responded to natural stimulation from the first age tested, but the convergence of inputs increased with age. A fibre strength evoked long lasting, variable, responses in the youngest animals, particularly in superficial cells. Mean latencies decreased with age, from 33.1±2.78ms at P3 to 7.3±0.3 ms at P21. No long latency spike responses were evoked in response to C fibre stimulation in pups aged P3 (postnatal day 3) or P6 while by PIO, 35% of cells had a C fibre response. The size of the peripheral receptive field decreased with age. At P3 the mean receptive field occupied 50% of the plantar hindpaw, this decreased to 15% by P21. Repeated stimulation of cutaneous A fibres at 0.5 Hz at twice threshold produced sensitisation in a population of dorsal horn cells in the neonate. The direct A fibre evoked activity did not increase but the background activity increased during repetitive stimulation leading to a prolonged afterdischarge beyond the stimulation period. At P6, 33% cells were sensitised displaying a mean afterdischarge of 70.6±18 s. At PIO, only 6% were sensitised with a mean afterdischarge of 63 s and by P21, it was no longer observed. Injection of carageenan (an inflammatory agent) into the hindpaw of PIO rats caused an increase in background activity which peaked at under 100mins, less than in the adult. Peripheral receptive fields were larger and stimulation thresholds lower, as is the case in adults. These results demonstrate that dorsal horn activity undergoes considerable postnatal development, this will affect sensory processing in the newborn.

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