Gain control of primary afferent neurotransmission at their intraspinal terminals occurs by several mechanisms including primary afferent depolarization (PAD). PAD produces presynaptic inhibition via a reduction in transmitter release. While it is known that descending monoaminergic pathways complexly regulate sensory processing, the extent these actions include modulation of afferent-evoked PAD remains uncertain. We investigated the effects of serotonin (5HT), dopamine (DA) and noradrenaline (NA) on afferent transmission and PAD. Responses were evoked by stimulation of myelinated hindlimb cutaneous and muscle afferents in the isolated neonatal mouse spinal cord. Monosynaptic responses were examined in the deep dorsal horn either as population excitatory synaptic responses (recorded as extracellular field potentials; EFPs) or intracellular excitatory postsynaptic currents (EPSCs). The magnitude of PAD generated intraspinally was estimated from electrotonically back-propagating dorsal root potentials (DRPs) recorded on lumbar dorsal roots. 5HT depressed the DRP by 76%. Monosynaptic actions were similarly depressed by 5HT (EFPs 54%; EPSCs 75%) but with a slower time course. This suggests that depression of monosynaptic EFPs and DRPs occurs by independent mechanisms. DA and NA had similar depressant actions on DRPs but weaker effects on EFPs. IC50 values for DRP depression were 0.6, 0.8 and 1.0 µM for 5HT, DA and NA, respectively. Depression of DRPs by monoamines was nearly-identical in both muscle and cutaneous afferent-evoked responses, supporting a global modulation of the multimodal afferents stimulated. 5HT, DA and NA produced no change in the compound antidromic potentials evoked by intraspinal microstimulation indicating that depression of the DRP is unrelated to direct changes in the excitability of intraspinal afferent fibers, but due to metabotropic receptor activation. In summary, both myelinated afferent-evoked DRPs and monosynaptic transmission in the dorsal horn are broadly reduced by descending monoamine transmitters. These actions likely integrate with modulatory actions elsewhere to reconfigure spinal circuits during motor behaviors.

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT 7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT 2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.

Somatosensory input strength can be modulated by primary afferent depolarization (PAD) generated via presynaptic GABAA receptors on afferent terminals. We investigated whether acetylcholine (ACh) also provides modulatory actions on PAD via nicotinic acetylcholine receptors (nAChRs) using in vitro murine spinal cord nerve-attached models. Primary afferent stimulation-evoked dorsal root potentials (DRPs) were used as an indirect measure of PAD while evoked afferent transmission was recorded in the deep dorsal horn as extracellular field potentials (EFPs). Changes in afferent membrane excitability were inferred from DC-shifts in recorded dorsal roots or peripheral nerves. Of nAChR antagonists tested, D-tubocurarine (D-TC) depressed DRP amplitude the most (43% of control) and actions were restricted to the A-fiber-evoked DRP and selective depression of Aδ-evoked synaptic EFPs (36% of control). These actions occurred centrally as afferent excitability was unchanged. In comparison, ACh depressed evoked responses by different mechanisms. ACh produced coincident depolarizing DC-shifts in peripheral axons and intraspinally that corresponded temporally with reductions in the DRP and all afferent-evoked synaptic actions (31-37% of control). DC-shifts were produced via nAChRs on primary afferents: they were also seen with nAChR agonists (epibatidine and nicotine), blocked with D-TC but not GABAA receptor blockers, and retained after block of voltage-gated Na+ channels. Notably, prominent actions on evoked responses were comparably altered between two mouse strains, in rat, and when performed in different labs. Thus, nAChRs can regulate afferent excitability via two distinct mechanisms: by modulating central Aδ-afferent actions, and by broadly changing membrane polarization of all classes of primary afferents.