The selectivity of rostroventral medulla descending control of spinal sensory inputs shifts postnatally from A fibre to C fibre evoked activity.

Brainstem descending control is crucial in maintaining the balance of excitation and inhibition in spinal sensory networks. In the adult, descending inhibition of spinal dorsal horn circuits arising from the brainstem rostroventral medial medulla (RVM) is targeted to neurons with a strong nociceptive C fibre input. Before the fourth postnatal week, the RVM exerts a net facilitation of spinal networks but it is not known if this is targeted to specific dorsal horn neuronal inputs. As the maturation from descending facilitation to inhibition occurs only after C fibre central synaptic maturation is complete, we hypothesized that RVM facilitation in young animals is targeted to A fibre afferent inputs. To test this, the RVM was stimulated while recording dorsal horn neuronal activity in vivo under isoflurane anaesthesia at postnatal day (P) 21 and P40 (adult). Electrical thresholds for A and C fibre evoked activity, spike counts and wind-up characteristics at baseline and during RVM stimulation (10-100 µA, 10 Hz) were compared. In adults, RVM stimulation selectively increased the threshold for C fibre evoked activity while at P21, it selectively decreased the threshold for A fibre evoked activity and these effects were correlated to the wind-up characteristics of the neuron. Thus, the postnatal shift in RVM control of dorsal horn circuits is not only directional but also modality specific, from facilitation of A fibre input in the young animal to inhibition of nociceptive C input in the adult, with additional contextual factors. The descending control of spinal sensory networks serves very different functions in young and adult animals.