A Presynaptic K+ Channel-Independent Neurotransmitter Release Mechanism Caused by Serotonin May Exist in Crayfish

Abstract

Recent studies on the sea-slug Aplysia have shown that the neurotransmitter serotonin (5-HT) may trigger an additional ion-channel-independent mechanism that contributes to the presynaptic release of neurotransmitter thereby enhancing excitatory post-synaptic potential (EPSP) sizes and facilitation in chemical synapses beyond what is already known to be caused by 5-HTs inhibition of K+ channels. Though numerous studies of this mechanism have been performed on Aplysia, few or none have been done on crayfish. In order to test the universality of this additional K+ channel-independent mechanism in other organisms, we devised a system of experiments attempting to isolate the mechanism in the neuromuscular junction (NMJ) of the superficial crayfish extensor muscle. By blocking presynaptic K+ channels through application of the drug 3-4 diaminopyridine (3-4 DAP), we tried to account for how much the K+ channels contributed to overall EPSP enhancement and facilitation when exposed to 5-HT. Our results, however, contradicted our predictions; 3-4 DAP decreased both EPSP amplitude and facilitation in both test conditions. This, along with some apparently inconsistent behavior between experimentation days, prevents us from being able to support the mechanisms existence. Nevertheless, the broadened shapes of the experimental EPSPs hint towards presynaptic action potential lengthening under our test conditions, a sign that this K+ channel-independent process may in fact exist in crayfish.