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Fast synaptic transmission mediated by -bungarotoxin-sensitive nicotinic acetylcholine receptors in lamina X neurones of neonatal rat spinal cord

A. Bradaïa and J.Trouslard

Using patch clamp recordings on neonatal rat spinal cord slices, we have looked for the presence of -bungarotoxin-sensitive nicotinic ACh receptors (nAChRs) on sympathetic preganglionic neurones (SPNs) surrounding the central canal of the spinal cord (lamina X) and examined whether they were implicated in a fast cholinergic synaptic transmission. SPNs were identified either by their morphology using biocytin in the recording electrode and/or by antidromic stimulation of the ventral rootlets. The selective 7-containing nAChR (7*nAChR) agonist choline (10 mM) induced a fast, rapidly desensitizing inward current, which was fully blocked by -bungarotoxin (-BgT; 50 nM) and strychnine (1 µM), two antagonists of 7*nAChRs. The I-V relationship of the choline-induced current showed a strong inward-going rectification. Electrically evoked excitatory postsynaptic currents (eEPSCs) could be recorded. At -60 mV, eEPSCs peaked at -26.2 pA and decayed monoexponentially with a mean time constant of 8.5 ms. The current-voltage relationship for eEPSCs exhibited a strong inward rectification and a reversal potential close to 0 mV, compatible with a non-selective cationic current. The appearance of eEPSCs was entirely suppressed by the application of 100 µM ACh or nicotine. Choline (10 mM) and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP; 100 µM) both reduced the amplitude of eEPSCs, whereas cytisine (100 µM) had no effect. Strychnine (1 µM) and -BgT (50 nM) both suppressed the eEPSCs. Blocking the P2X purinergic and 5-HT₃ receptors had no effect on eEPSCs. DMPP induced four types of current, which differed in their onset and desensitization rate. The most frequently encountered responses were insensitive to the action of strychnine and -BgT, and were reproduced by ACh and nicotine but not by cytisine. We conclude that SPNs of the lamina X express several classes of nAChRs and in particular -BgT-sensitive nAChRs. This is the first demonstration in a mammalian spinal cord preparation of a fast cholinergic neurotransmission in which -BgT-sensitive nicotinic receptors are involved.

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