C fibres facilitate proprioceptive afferent transmission to motoneurons

Abstract

Activation of α5 GABAA receptors located on nodes of primary afferents have been found to produce a long lasting tonic primary afferent depolarization in the spinal cord (tonic PAD). In particular, C fibres have been found to be especially effective in eliciting this tonic PAD. Here by activating C fibres either chemically (via icilin) or electrically (at 50 x T, threshold), we were able to investigate the effects of this C fibre induced tonic PAD on sensory transmission to motoneurons by measuring monosynaptic reflexes (MSRs), extracellular field potentials from afferents and spike transmission in afferents themselves (spikes evoked by PAD: dorsal root reflexes; DRRs). We also used the 5HT1D receptor agonist, zolmitriptan, to inactivate C fibres because these 5HT receptors are found exclusively on C fibres. Consequently, we also investigated the effects of inactivating C fibres on sensory transmission. We found that C fibre induced tonic PAD facilitated spike transmission within the spinal cord resulting in increased MSRs, extracellular fields and DRRs. As we expected, when we inactivated C fibres using zolmitriptan, this reduced spike transmission resulting in decreased MSRs, extracellular fields and DRRs. These findings demonstrate the effects C fibres have on sensory transmission via their ability to elicit a long lasting tonic PAD. We also show that C fibres make direct contacts with GABAergic neurons, demonstrating how C fibres can indirectly produce tonic PAD.