Blockade of neuronal facilitatory nicotinic receptors containing α3β2 subunits contribute to tetanic fade in the rat isolated diaphragm

Abstract

Nicotinic receptor (nAChR) subtypes involved in pre- and postjunctional actions underlying tetanic fade were studied in rat phrenic-nerve hemidiaphragms. We investigated the ability of subtype-specific nAChR antagonists to depress nerve-evoked contractions and [(3)H]-acetylcholine ([(3)H]-ACh) release. Muscle tension was transiently increased during brief high frequency trains (50 Hz for 5 sec). The rank potency order of nAChR antagonists to reduce tetanic peak tension was alpha-bungarotoxin > d-tubocurarine >> mecamylamine > hexamethonium. Reduction of maximal tetanic tension produced by dihydro-beta-erythroidine (0.03-10 microM), methyllycaconitine (0.003-3 microM), and alpha-conotoxin MII (0.001-0.3 microM) did not exceed 30%. Besides reduction of peak tension d-tubocurarine (0.1-0.7 microM), mecamylamine (0.1-300 microM), and hexamethonium (30-3,000 microM) also caused tetanic fading. With alpha-conotoxin MII (0.001-0.3 microM) and dihydro-beta-erythroidine (0.03-10 microM), tetanic fade was evident only after decreasing the safety factor of neuromuscular transmission (with high magnesium ions, 6-7 mM). The antagonist rank potency order to reduce evoked (50 Hz for 5 sec) [(3)H]-ACh release from motor nerve terminals was alpha-conotoxin MII (0.1 microM) > dihydro-beta-erythroidine (1 microM) approximately d-tubocurarine (1 microM) > mecamylamine (100 microM) > hexamethonium (1,000 microM). When applied in a concentration (0.3 microM) above that producing tetanic paralysis, alpha-bungarotoxin failed to affect [(3)H]-ACh release. Data obtained suggest that postjunctional neuromuscular relaxants interact with alpha-bungarotoxin-sensitive nicotinic receptors containing alpha1-subunits, whereas blockade of neuronal alpha3beta2-containing receptors produce tetanic fade by breaking nicotinic autofacilitation of acetylcholine release.

Tetanic Facilitation of Neuromuscular Transmission by Adenosine A_2A and Muscarinic M₁ Receptors is Dependent on the Uptake of Choline via High-Affinity Transporters

“…4) applied in conditions known to block α3β2-subunit containing neuronal nicotinic receptors [12] and muscarinic M 2 receptors respectively [34].…”

“…The preparations were superfused (1 mL/min) in a 3-mL organ bath at 24 ° C with Tyrode's solution containing (mmol/L): NaCl 137, KCl 2.7, CaCl 2 1.8, MgCl 2 1, NaH 2 PO 4 0.4, NaHCO 3 11.9 and glucose 11.2, which was continuously gassed with 95% O 2 and 5% CO 2 . After a 30-min period, muscle stimulation was established at a supramaximal voltage of 14.5 ± 2 V; at this point, after the equilibration period, phrenic nerve-hemidiaphragm preparations were incubated with α-BTX (4 μmol/L, during 15-20 min), which prevents nerve-evoked muscle fiber contractions, since it irreversibly blocks muscle-type nicotinic ACh receptors containing α1 subunits, but not nicotinic receptors present on motor nerve terminals [12,40]. After a 10-min incubation period with FM4-64 (5 μmol/L) made up in Tyrode's solution, loading of synaptic vesicles (high-probability release pool) was achieved upon phrenic nerve trunk stimulation with 250 pulses of supramaximal intensity (0.04 ms duration, 8 mA) applied at a frequency of 50 Hz, followed by an additional 10-min period of rest with the dye [31].…”

“…Neuronal nicotinic receptors are fast desensitizing α3β2-subunit containing receptors involved in the short-term positive feedback mechanism of ACh release [12]. Tetanic release facilitation mediated by nicotinic α3β2 autoreceptors is cut-short by activation of adenosine A 2A receptors by endogenously generated adenosine [13] from ATP co-released with ACh [14].…”

Sugammadex: novos questionamentos sobre a reversão

“…In the first, the greatest emphasis is on the alpha motor neurons in which neuronal nicotinic receptors can be identified 4 , as well as voltage-gated calcium and potassium channels, which are fundamental structures to control the entry of calcium into neuron. These receptors have characteristics that distinguish them from muscle receptors, such as the presence of only two types of subunits, α2-10 and β2-4, and the lack of safety margin [4][5][6][7][8] . This last characteristic is related to the additional release of acetylcholine in the presence of high-intensity stimuli, by a positive feedback mechanism from stimulation of α3β2 nicotinic receptor.…”

Protein Kinase A and Ca_v1 (L-Type) Channels Are Common Targets to Facilitatory Adenosine A_2A and Muscarinic M₁ Receptors on Rat Motoneurons

“…19] . Neuronal nicotinic autoreceptors containing ␣ 3 ␤ 2 subunits [20] mediate a short-term positive feedback mechanism, which is terminated by rapid autodesensitization [21,22] . Dual modulation (facilitation and inhibition) of ACh release has been demonstrated using muscarinic receptor agonists and antagonists [2,23,24] .…”