Cutaneous nerve-evoked cholinergic inhibition of monosynaptic reflex in the neonatal rat spinal cord: Involvement of M 2 receptors and tachykininergic primary afferents

Abstract

The mechanisms of a cutaneous nerve-evoked inhibition of monosynaptic reflex were investigated in an isolated spinal cord-peripheral nerve preparation of the neonatal rat. Conditioning stimulation of the saphenous nerve, with five pulses at 50 Hz and a strength sufficient to activate C fibers, evoked an inhibition lasting about 20 s of the monosynaptic reflex that was elicited by stimulation of the nerve branch to quadriceps femoris muscle and recorded from the L3 ventral root. This inhibition of monosynaptic reflex was potentiated by an anticholinesterase, edrophonium, and mostly blocked by atropine. Application of acetylcholine, muscarine, bethanechol, carbachol, arecoline and oxotremorine induced an inhibition of monosynaptic reflex. From the effects of muscarinic antagonists, pirenzepine, AF-DX 116, and 4-diphenylacetoxy- N-methylpiperidine on the agonist-evoked and primary afferent-evoked inhibition of monosynaptic reflex it was concluded that the muscarinic receptors involved in the cutaneous nerve-evoked inhibition of monosynaptic reflex are of M 2 type. When monosynaptic reflexes were evoked by two successive stimuli with intervals of 15 ms to 1 s, the second response was smaller than the first. This depression of monosynaptic reflex became less pronounced when the reflex was reduced by application of oxotremorine or arecoline or by conditioning stimulation of primary afférents, suggesting that the inhibition of monosynaptic reflex is presynaptic in nature. The late phase of the cutaneous nerve-evoked inhibition of monosynaptic reflex (5–20 s after conditioning stimulation) was markedly depressed by a tachykinin antagonist, spantide. Perfusion of the spinal cord with capsaicin (1 μM) for 1 h also abolished the late phase of the inhibition. Application of capsaicin or substance P to the spinal cord induced an inhibition of monosynaptic reflex, and the inhibition was antagonized by spantide and atropine. Measurements of acetylcholine content in the perfusate of isolated rat spinal cords demonstrated that bath-applied substance P evoked a release of acetylcholine from spinal neurons. These results suggest that tachykininergic primary afferent C fibers excite cholinergic spinal neurons and the released acetylcholine inhibits the monosynaptic reflex by acting on the presynaptic muscarinic receptors on group Ia fibers.