Muscarinic excitatory and inhibitory mechanisms involved in afferent fibre‐evoked depolarization of motoneurones in the neonatal rat spinal cord

Abstract

1 The involvement of acetylcholine and muscarinic receptors in spinal synaptic responses evoked by electrical and noxious sensory stimuli was investigated in the neonatal rat spinal cord in vitro. 2 Potentials were recorded extracellularly from a ventral root (L3-L5) of the isolated spinal cord, spinal cord-cutaneous nerve, and spinal cord-skin preparations of 1-to 4-day-old rats. Spinal reflexes were elicited by electrical stimulation of the ipsilateral dorsal root or the cutaneous saphenous nerve, or by noxious skin stimulation.3 Single shock stimulation of supramaximum intensity of a dorsal root induced a mono-synaptic reflex in the corresponding ventral root. Bath-application of the muscarinic agonists, muscarine (0.3-30 9M) and (+ )-cis-dioxolane (0.1-100 9AM), produced an inhibition of the mono-synaptic reflex and a depolarization of motoneurones. Other muscarinic agonists, arecoline (10 nM-10 AM) and oxotremorine (1OnM-1 tM), inhibited the mono-synaptic reflex with little or no depolarization of motoneurones.Repetitive stimulation of the saphenous nerve at C-fibre strength induced a slow depolarizing response lasting about 30 s of the L3 ventral root. This slow ventral root potential (VRP) was also inhibited by arecoline (10 nM-1I0 AM) and oxotremorine (10 nM-1 9AM). 4 In the spinal cord-saphenous nerve-skin preparation, a slow VRP was evoked by application of capsaicin (0.5 9AM), bradykinin (3 9AM), or noxious heat (47°C) to skin. This slow VRP was depressed by the muscarinic agonists, arecoline (3 9AM) and oxotremorine (1 9AM). 5 Of the (+)-cis-dioxolane-induced inhibition of mono-synaptic reflex and motoneurone depolarization, the M2 antagonists, AF-DX 116 (0.1-1I9M) and methoctramine (100-300 nM), preferentially blocked the former response, whereas the M3 antagonists, 4-DAMP (3-10 nM) and p-F-HHSiD (0.3-3 AM), preferentially blocked the latter response. AF-DX 116 (0.1-1I9M) and methoctramine (100-300 nM) also effectively antagonized the arecoline-and oxotremorine-induced inhibition of the slow VRP. The pA2 values of AF-DX 116 and methoctramine against the arecoline-induced inhibition of the mono-synaptic reflex were both 6.79, and that of 4-DAMP against the (+)-cis-dioxolane-induced motoneurone depolarization was 8.16. 6 In the spinal cord-cutaneous nerve preparation, the saphenous nerve-evoked slow VRP was augmented by the anticholinesterase, edrophonium (5 9AM). (1 AM) and methoctramine (100 nM) also potentiated the slow VRP, whereas 4-DAMP (10 nM) depressed the response. 4-DAMP (5-10 nM) depressed the capsaicin-induced slow VRP in the spinal cord-skin preparation. 7 Oxotremorine (0.3 9lM) and arecoline (1 AM) markedly depressed the depolarization of motoneurones evoked by application of capsaicin (3 9AM) to the spinal cord, whereas they depressed only slightly the depolarization induced by substance P (10 nM). 8 The present study suggests that both excitatory (via M3-type receptors) and inhibitory (via M2-type receptors) muscarinic mechanisms are involved in afferent fibre-evoked nociceptive tr...

Functional Activity of the M2 and M4 Receptor Subtypes in the Spinal Cord Studied with Muscarinic Acetylcholine Receptor Knockout Mice

“…Thus, the M 2 and M 4 subtypes likely are the mAChRs measured in the present study since they are both coupled to G i/o proteins. The recently developed mAChR subtype KO mice are particularly useful experimental tools because inactivation of one specific subtype does not seem to affect the expression levels of the remaining mAChR subtypes (Wess, 2004 (Kurihara et al, 1993;Duttaroy et al, 2002;Chen and Pan, 2004). Thus, these data reinforce the concept that agonist-stimulated […”

Muscarinic control of the excitability of hindlimb motoneurons in chronic spinal‐transected salamanders

“…Immediately after a complete spinal transection (acute spinal transection condition), exogenous application of acetylcholine can increase the excitability of spinal motoneurons (MNs) through the activation of muscarinic receptors (Kurihara et al. , 1993; Delgado‐Lezama et al.…”

Identified spinal motoneurons of young rats possess nicotinic acetylcholine receptors of the heteromeric family

“…Spinal motoneurons are contacted by choline acetyltransferase (ChAT)‐ or vesicular acetylcholine transporter (VAChT)‐immunoreactive axon terminals (Nagy et al ., 1993; Schafer et al ., 1998), part of which probably originates from axon collaterals of nearby motoneurons (Cullheim et al ., 1977). Muscarinic agonists can depolarize spinal motoneurons (Kurihara et al ., 1993), suggesting that cholinergic input to motoneurons may be, at least in part, mediated by muscarinic ACh receptors. However, ACh also acts on nicotinic receptors (MacDermott et al ., 1999; Karlin, 2002; Hogg et al ., 2003), and recent studies suggest that ACh receptors may mediate cholinergic excitation of spinal motoneurons.…”