Chang, P; Fabrizi, L; Fitzgerald, M; (2020) Distinct age dependent C fibre driven oscillatory activity in the rat somatosensory cortex. eNeuro , 7 (5) , Article ENEURO.0036-20.2020. 10.1523/ENEURO.0036-20.2020.

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Abstract

When skin afferents are activated, the sensory signals are transmitted to the spinal cord and eventually reach the primary somatosensory cortex (SI), initiating the encoding of the sensory percept in the brain. While subsets of primary afferents mediate specific somatosensory information from an early age, the subcortical pathways that transmit this information undergo striking changes over the first weeks of life, reflected in the gradual emergence of specific sensory behaviours. We therefore hypothesised that this period is associated with differential changes in the encoding of incoming afferent volleys in SI. To test this, we compared SI responses to A fibre skin afferent stimulation and A+C skin afferent fibre stimulation in lightly anaesthetised male rats at postnatal day (P) 7, 14, 21 and 30. Differences in SI activity following A and A+C fibre stimulation changed dramatically over this period. At P30, A+C fibre stimulation evoked significantly larger gamma, beta and alpha energy increases compared to A fibre stimulation alone. At younger ages, the changes in S1 oscillatory activity evoked by the two afferent volleys were not significantly different. Silencing TRPV1+ C fibres with QX-314 significantly reduced the gamma and beta SI oscillatory energy increases evoked by A+C fibres, at P30 and P21, but not at younger ages. Thus, C fibres differentially modulate SI oscillatory activity only from the third postnatal week, well after the functional maturation of the somatosensory cortex. This age-related change in afferent evoked S1 oscillatory activity may underpin the maturation of sensory discrimination in the developing brain.Significance Statement Behavioural responses to sensory stimulation of the skin undergo major developmental changes over the first postnatal weeks. Here we show that this is accompanied by a shift in the differential frequency encoding of sensory A fibre and C fibre afferent inputs into the developing rat somatosensory cortex. The results demonstrate major postnatal changes in the ability of the cortex to differentiate between afferent sensory inputs arriving in the mammalian brain.

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