The development of Inhibition in the neonatal dorsal horn

Spinal nociceptive processing undergoes extensive maturation in the postnatal period. Dorsal horn neurons are more easily excited by peripheral stimuli in young animals and nociceptive reflexes are less well directed. Since circuits in the central nervous system require a balance of excitation and inhibition, I hypothesised that the maturation of inhibitory circuitry is responsible for changing nociceptive processing in postnatal animals. To test this I have investigated the maturation of segmental and descending inhibition in the anaesthetised and decerebrate rat dorsal horn using in vivo electrophysiological methods. First, I examined the developmental regulation of inhibitory signalling in spinal nociceptive circuits. Single cells in the lumbar dorsal horn, characterised by their responses to hindpaw cutaneous stimulation, were recorded at two postnatal ages over a prolonged period in the presence and absence of a GABAaR antagonist. The results show that at the circuit level, GABAergic signalling does not change between postnatal days 3 and 21 and that blocking GABA activity is equally excitatory at both ages. Next, I examined the role of descending activity from supraspinal centres upon single dorsal horn cell activity at different ages. The results show that while descending inhibition of spinal nociceptive activity is present in young animals, there is also an excitatory descending influence upon young dorsal hom cells that is not observed in older animals. Finally, I mapped the spatial pattern of inhibitory receptive fields at different ages. The results show that contralateral inhibitory fields are less spatially restricted in young animals and are activated by both low and high intensity stimulation. A model is proposed to show how inhibitory receptive field organisation may determine higher-level sensory processing. In conclusion, the postnatal maturation of spinal nociceptive processing is likely due to maturation of local and descending inhibitory activity within spinal circuits.