Activation of neuronal nicotinic receptors induces an increase of calcium level in motor nerve terminal
Since the process of evoked release of a neurotransmitter is triggered by the entry of calcium ions into the nerve ending (Katz B., 1969; Crawford, 1974), it was suggested that the inhibitory effect of nicotine on the ACh release could be related to a decrease in Ca2+ influx.
The amplitude changes of the optical signal (ΔF/F0) from the calcium dye loaded into the nerve terminal in response to a single stimulus (with the same characteristics as during EPP registration) averaged about 30% (Figure 3). Nicotine application did not lead to a decrease, as expected, but caused a significant increase in the amplitude of the calcium transient by 13.7 ± 4.3% (n = 5, 8 NMJs; Figure 4). Thus, in the presence of a nicotinic receptor agonist, the presynaptic calcium level in response to nerve stimulation increases more strongly than in its absence. Is this increase indeed triggered by nNAChRs, which activation of leads to a decrease in subsequent ACh release? The answer to this question was obtained in the experiments with an antagonist of nicotinic receptors, DHβE.
Application of the antagonist alone led to a decrease in the calcium transient significantly by 12.9 ± 1.5% (n = 5, 15 NMJs; Figure 4), however, after pretreatment with DHβE, the calcium signal-enhancing effect of nicotine was completely abolished (100.0 ± 0.8%; n = 5; 15 NMJs; Figure 4).