6-cyano-7-nitroquinoxaline-2-3-dione and cytisine

The GABA/glycine-mediated inhibitory activity in the substantia gelatinosa (SG) of the spinal cord is critical in the control of nociceptive transmission. We examined whether and how SG inhibitory activity might be regulated by neuronal nicotinic receptors (nAChRs). Patch-clamp recordings were performed in SG neurons of spinal slice preparations from adult rats. We provided electrophysiological evidence that inhibitory presynaptic terminals in the SG expressed nAChRs and their activation resulted in large increases in the frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in over 90% SG neurons tested. The enhancement of inhibitory activity was mediated by increases in the release of GABA/glycine, and direct Ca(2+) entry through SG presynaptic nAChRs appeared to be involved. Miniature IPSC frequency could be enhanced by the nAChR agonists nicotine or cytisine. Nicotine could still elicit large increases in mIPSC frequency in the presence of the alpha4beta2 nAChR antagonist dihydro-beta-erythroidine (5 microM) and the alpha7 nAChR-selective antagonist methyllycaconitine (40 nM). However, nicotine did not produce a significant enhancement of mIPSC frequency in the presence of the broad spectrum nAChR antagonist mecamylamine (5 microM). Nicotinic agonist-evoked whole-cell currents from SG neurons and the antagonist profiles also indicated the presence of a subtype of nAChRs, which were different from the major central nervous system nAChR subtypes, i.e. alpha4beta2* or alpha7 nAChRs. Together, our results suggest that a subtype of nAChR, possibly alpha3beta4* nAChR or a new nAChR type, is highly expressed at the inhibitory presynaptic terminals in SG of adult rats and play a role in the control of inhibitory activity in SG.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Age Factors; Alkaloids; Animals; Azocines; Choline; Excitatory Amino Acid Antagonists; gamma-Aminobutyric Acid; Glycine; Neural Inhibition; Nicotine; Nicotinic Agonists; Nootropic Agents; Patch-Clamp Techniques; Quinolizines; Rats; Rats, Sprague-Dawley; Receptors, Nicotinic; Substantia Gelatinosa; Synaptic Transmission

Whole-cell patch clamp recordings were performed on neurones in the lamina X of rat spinal cord slices in order to characterize glycinergic synaptic currents and their modulation by nicotinic acetylcholine receptors. In the presence of TTX, bicuculline and kynurenic acid, glycine-induced currents and miniature glycinergic postsynaptic currents (mIPSCs) were recorded. These currents reversed near the chloride ion equilibrium potential and were blocked by strychnine (1 microM). A selective nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenyl-piperazinium (DMPP), increased the frequency of glycinergic mIPSCs without altering significantly their amplitude distributions or their kinetic properties. The effects of DMPP were mimicked by different nAChRs agonists with the following apparent order of potency: ACh > DMPP > nicotine > cytisine. The effect of DMPP on mIPSCs was blocked by both d-tubocurarine and hexamethonium, and was reduced by dihydro-beta-erythroidine and methyllycaconitine (MLA), antagonists of non alpha7- and alpha7-containing nAChRs, respectively. In the absence of TTX, strychnine-sensitive glycinergic electrically evoked postsynaptic currents (eIPSCs) could be recorded. DMPP blocked the appearance of electrically evoked IPSCs while still inducing the appearance of spontaneous glycine IPSCs. These data demonstrate that neurones surrounding the central canal of the spinal cord present a glycinergic synaptic transmission which is modulated by terminal nAChRs.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Aconitine; Alkaloids; Anesthetics, Local; Animals; Animals, Newborn; Azocines; Bicuculline; Dihydro-beta-Erythroidine; Dimethylphenylpiperazinium Iodide; Drug Interactions; Excitatory Amino Acid Antagonists; Glycine; Glycine Agents; Hexamethonium; In Vitro Techniques; Kynurenic Acid; Membrane Potentials; Neurons; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Quinolizines; Rats; Rats, Wistar; Receptors, Nicotinic; Spinal Cord; Strychnine; Tetrodotoxin; Time Factors; Tubocurarine

Presynaptic nicotinic acetylcholine receptors (nAChRs) are present in many regions of the brain and potentially serve as targets for the pharmacological action of nicotine in vivo. To investigate their mechanism of action, we performed patch-clamp recordings in relay neurons from slices of thalamus sensory nuclei. In these nuclei, nAChR activation facilitated the release of the inhibitory neurotransmitter GABA. Micromolar concentrations of nicotinic agonists increased the frequency of miniature GABAergic synaptic currents and decreased the failure rate of evoked synaptic currents. These actions of nicotinic agonists were not observed in knock-out mice lacking the beta 2 nAChR subunit gene. Nicotinic effects were dependent on extracellular calcium ions, and they persisted when calcium was replaced by strontium or barium but not by magnesium. Furthermore, in high extracellular calcium concentrations, nicotinic agonists evoked an increase in spontaneous release lasting for minutes after removal of the agonist. This supports the view that presynaptic nAChRs facilitate the release of neurotransmitter by increasing the calcium concentrations in presynaptic nerve endings. With use of cadmium and nickel ions as selective blockers, it was found that in different sensory nuclei the presynaptic influx of calcium could result either from the activation of voltage-dependent calcium channels or from a direct influx through nAChR channels. Finally, we propose that the nicotinic facilitation of GABAergic transmission may contribute to the increase of signal-to-noise ratio observed in the thalamus in vivo during arousal.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Aconitine; Action Potentials; Alkaloids; Animals; Arousal; Atropine; Azocines; Bacterial Toxins; Bridged Bicyclo Compounds, Heterocyclic; Calcium; Calcium Channel Blockers; Calcium Channels; Carbachol; Cations, Divalent; Cyanobacteria Toxins; Dihydro-beta-Erythroidine; Dimethylphenylpiperazinium Iodide; GABA Antagonists; gamma-Aminobutyric Acid; Ion Transport; Lobeline; Marine Toxins; Mice; Mice, Inbred C57BL; Mice, Knockout; Microcystins; Neurons, Afferent; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Potassium; Pyridazines; Pyridines; Quinolizines; Receptors, GABA-A; Receptors, Nicotinic; Tetrodotoxin; Thalamic Nuclei