Train-of-Four and Tetanic Fade Are Not Always a Prejunctional Phenomenon as Evaluated by Toxins Having Highly Specific Pre- and Postjunctional Actions

Abstract

Background Nerve-stimulated fade in muscle is generally accepted as a prejunctional phenomenon mediated by block of prejunctional acetylcholine receptors (AChRs) at the nerve terminal, while decrease of twitch tension is considered a postjunctional effect due to block of muscle AChRs. Using ligands with specific pre or postjunctional effects only, we tested the hypothesis that fade is not necessarily a prejunctional phenomenon. Methods Neuromuscular function in rats was evaluated after intramuscular (2.5U) or IV (12.0U) injection of botulinum toxin (Botx), or IV (250µg/kg) alpha-bungarotoxin (α-BTX) alone. The acute neuromuscular effects of IV 2mg/kg dihydro-β-erythroidine (DHβE), alone and in combination with α-BTX were also tested. Botx decreases vesicular release of acetylcholine (ACh), and α-BTX binds to postjunctional nicotinic AChRs only, while DHβE binds specifically to prejunctional α3β2 AChRs only. In view of the lack of acute effects of Botx even at 2 hours after IV injection, its neuromuscular effects were also evaluated at 24 hours after intramuscular injection (0.6 U) and compared to intramuscular injection of α-BTX (25 µg/kg) or saline also given 24 hours earlier. The sciatic nerve-tibialis muscle preparation, during train-of-four and tetanic stimulation, was used to test neuromuscular effects in vivo. Results Intravenous and intramuscular Botx had no observable neuromuscular effects at 2 hours. Intravenous α-BTX caused twitch depression within a few minutes, and significant fade (P=0.002) at 75% of baseline twitch tension; these effects persisted until the end of the observation period of 2 hours. Intravenous DHβE alone caused no significant change in single twitch (p=0.899) or train-of-four ratio (P=0.394), but significantly enhanced the fade of IV α-BTX (P=0.001 at 75% of baseline twitch tension). Intramuscular Botx or α-BTX, at 24 hours after their injection, resulted in a significant decrease of single twitch and tetanic tensions (p<0.0001), but Botx did not cause fade, while α-BTX caused significant (P<0.0001) fade at 24 hours. The tibialis muscle weights and protein expression of α1 subunit of AChR (Western blots) did not differ between Botx, α-BTX and saline-injected groups at 24 hours but increased in denervated muscle (positive control). Conclusions Botx-induced decreased ACh release in and of itself does not cause fade but does cause decrease of absolute tensions. Decrease of available (functional) postjunctional AChRs by α-BTX did induce fade. The prejunctional fade effects of DHβE on α3β2 AChRs become manifest only when the margin of safety was decreased by concomitant administration of α-BTX. Thus, fade during repetitive stimulation is not always a prejunctional phenomenon, and may also reflect the decreased margin of safety of neurotransmission, which can be due to a pure postjunctional AChRs block or to a combination of both pre and postjunctional AChRs block. Block of prejunctional α3β2 AChRs alone is not necessary and sufficient to cause fade.

Anesthesia with Disuse Leads to Autophagy Up-regulation in the Skeletal Muscle

“…19 Tibialis anterior muscle was also studied with and without stimulation of the sciatic nerve with bipolar electrode at the supramaximal voltage. 23 …”

Block of postjunctional muscle-type acetylcholine receptors in vivo causes train-of-four fade in mice

“…16 For example, waglerin-1, a potent antagonist of muscle nAChRs in mice has minimal effects in rats. 16 Therefore, fade observed during postsynaptic nAChRs block 15 may be restricted to rats, and not observed in another rodent species.…”

“…On the other hand, specific block of postjunctional muscle nAChRs with α-bungarotoxin (α-BTX) alone also induced TOF fade in rats. 15 Thus, the exact role of prejunctional vs postjunctional nAChRs in the mediation of fade is unclear. The response to nAChR antagonists can differ between species.…”

Backbone Cyclization Turns a Venom Peptide into a Stable and Equipotent Ligand at Both Muscle and Neuronal Nicotinic Receptors

“…The inhibition of the presynaptic facilitatory α3β2 nAChR autoreceptor at motor nerve terminals and the resulting inhibition of autofacilitatory ACh release has been suggested as an explanation for the train-of-four fade seen during a non-depolarizing neuromuscular block 28,29 . However, this hypothesis was recently challenged by using ligands with different selectivities for pre-and post-synaptic receptors 34 . Therefore, due to their original dual muscular/α3β2 nAChRs antagonist property, it was of interest to determine whether α-CIA and the cCIA analogues were able to produce TOF fade.…”