Nicotinic receptor modulation of primary afferent excitability with selective regulation of Aδ-mediated spinal actions

Abstract

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.

The corticospinal tract primarily modulates sensory inputs in the mouse lumbar cord

“…We evaluated the proportion of GAD65+ neurons among these neurons ( Figure 1D,E ): 16.4% ± 3.2% of spinal Td-Tomato-positive neurons were GAD65-GFP+ (69 neurons out of 427, n = 3 mice), suggesting that the cortically evoked DRP may be mediated directly by the activation of a GAD65+ spinal target of the CST. In the spinal dorsal horn, cholinergic neurons are a subpopulation of GABAergic neurons ( Todd, 1991 ; Mesnage et al, 2011 ), and cholinergic terminals are known to be presynaptic to primary afferents ( Ribeiro-da-Silva and Cuello, 1990 ; Pawlowski et al, 2013 ) and have recently been shown to modulate primary afferent inputs ( Hochman et al, 2010 ; Shreckengost et al, 2021 ). By performing a similar experiment in ChAT-EGFP mice, we showed that only 2.2% ± 1.8% of CST targets were ChAT-EGFP positive (1 out of 45 neurons, n = 4 mice), limiting the likelihood of a direct major involvement of a cholinergic mechanism ( Figure 1—figure supplement 3 ).…”

“…At the spinal cord level, sensory gating can be mediated by presynaptic inhibition of sensory inputs through primary afferent depolarization (PAD), a powerful mechanism controlled both by supraspinal centers and by adjacent primary afferents ( Eccles et al, 1962 ). While multiple mechanisms have been demonstrated ( Shreckengost et al, 2021 ), the most studied cause of PAD is presynaptic GABA-A-mediated depolarization. Primary afferents have indeed a high chloride content and respond to GABA by a depolarization that has been shown to reduce the amplitude of incoming action potentials and thus inhibit synaptic transmission ( Prescott and De Koninck, 2003 ; Doyon et al, 2011 ).…”

Lumbar corticospinal tract in rodents modulates sensory inputs but does not convey motor command

“…In TdTomato-flex mice crossed with GAD65::GFP mice, we injected a monosynaptic anterograde transynaptic virus (AAV1-CBA::WGA-Cre) encoding the Cre-recombinase fused to the wheat germ agglutinin (WGA) (Libbrecht et al, 2017) 16.4±3.2% of spinal Td-Tomato positive neurons were GAD65::GFP+ (69 neurons out of 427, n=3 mice), suggesting that the cortically-evoked DRP may be mediated directly by the activation of a GAD65+ spinal target of the CST. In the spinal dorsal horn, cholinergic neurons are a subpopulation of GABAergic neurons (Todd, 1991;Mesnage et al, 2011), and cholinergic terminals are known to be presynaptic to primary afferents (Ribeiro-da-Silva and Cuello, 1990;Pawlowski et al, 2013) and have recently been shown to modulate primary afferent inputs (Hochman et al, 2010;Shreckengost et al, 2021). By performing a similar experiment in ChAT::EGFP mice, we showed that only 2.2±1.8% of CST targets were ChAT::EGFP positive (1 out of 45 neurons, n=4 mice), limiting the likelihood of a direct major involvement of a cholinergic mechanism (Figure 1 -Figure Supplement 3).…”

Cholinergic Modulation of Locomotor Circuits in Vertebrates

“…In rodents nAChRs are also present on primary afferent terminals in the dorsal horn [139,140], and a nicotinic modulation of somatosensory information has been found both in the periphery [141] and centrally on the primary afferent terminals [142,143]. The level of polarization of the terminals is now known to be continuously modulated and different mechanisms are involved.…”