We thank Drs. Lu and Yu for their comments on our article1and for bringing up an important discussion about the impact of different transmitter systems in the neuromuscular junction. Although the neuromuscular junction has been extensively studied, the interplay between cholinergic and purinergic transmitters is not fully understood.
In our article, we demonstrate that nondepolarizing neuromuscular blocking agents inhibit human neuronal nicotinic acetylcholine receptors (nAChRs), including the α3β2subtype, expressed in Xenopus oocytes.1Notably, there has for a long time been physiologic and pharmacologic evidence for presynaptic nAChRs at the motor nerve ending in the neuromuscular junction; however, the exact nature of these nicotinic receptors has not been known (for reviews, see Bowman et al. 2and Vizi and Lendvai3). Immunohistochemical analysis has demonstrated presence of the α3nAChR subunit at the presynaptic motor nerve ending.4Recently, it was demonstrated that a selective block of the α3β2nAChR in an in vitro nerve-muscle preparation reduced the presynaptic acetylcholine release, and furthermore caused tetanic fade after a magnesium-induced reduction of the safety factor of synaptic transmission.5Moreover, nicotinic inhibitors with a mixed antagonist profile (i.e. , both α3β2and α1β1δϵ antagonists) both inhibited acetylcholine release and caused tetanic fade.5Altogether, a selective inhibition of the α3β2nAChR reduces the presynaptic acetylcholine release, thus working as an autoreceptor, and during conditions of reduced safety factor in the neuromuscular junction, produced a tetanic fade.5Based on this and our recent findings,1an inhibition of the presynaptic α3β2nAChR is a possible molecular explanation for the mechanism behind tetanic and train-of-four fade because the safety factor indeed is reduced during a nondepolarizing neuromuscular blockade.
As we point out in our article,1there is evidence that adenosine and adenosine triphosphate, acting via purinergic receptors, also play an important role in the modulation of acetylcholine release and tetanic fade.6In addition, muscarinic M1 and M2 receptors are present in the neuromuscular junction and are involved in the modulation of acetylcholine release, but do not seem to be key players in the mechanism(s) behind tetanic fade.2,3
In conclusion, inhibition of presynaptic α3β2nAChRs inhibits acetylcholine release and causes tetanic fade and most likely train-of-four fade.1–5We do agree with Drs. Lu and Yu that purinergic and/or muscarinic receptors are important players in the synaptic transmission in the neuromuscular junction, but to what extent clinically used neuromuscular blocking agents interact with these receptor systems remains to be evaluated.
*Karolinska University Hospital and Karolinska Institutet, Stockholm, Sweden. email@example.com