Question

he figure below shows the results from an experiment by Llinas and Heuser, 1977 using the squid giant synapse. The voltage-gated sodium and potassium channels were blocked with TTX and TEA, respectively. The presynaptic terminal was clamped at different voltage steps (bottom traces) and presynaptic calcium current (middle) and postsynaptic voltage (top) were measured. Explain what this experiment demonstrated about the mechanism of neurotransmitter release. Does this particular experiment address necessity or sufficiency, or both?

Postsynaptic potential 2 mV Presynaptic Ca2 current 500 nA Presynaptic command potential 20 mV 2 ms

Answer 1

Following presynaptic and post-synaptic axon impalements and the determination of normal synaptic transmission (Llinas et al., 1985, 1994; Lin et al., 1990), the effect of human tau on synaptic release was evaluated by presynaptic microinjections administered under direct visualization using a fluorescent dye/protein mix, reaching a final concentration after diffusion of approximately 80 nM (see Materials and Methods). Presynaptic and post-synaptic potentials were recorded simultaneously under current-clamp configuration. Presynaptic spikes were activated every 5 min (low-frequency protocol). With this paradigm, it was determined that 5–10 min after an injection of h-tau42, a reduction of transmitter release could be observed. With further time, a total block of transmission resulted, within 30–40 min depending on the length of the release zone in the preterminal axon (Figures ​(Figures1A1A and ​and22 blue dots, n = 8). No modification of presynaptic spike amplitude or duration ensued (Figure ​(Figure1A).1A). By contrast, following T-817MA administration to the squid (see bellow) h-tau42-dependent transmitter block was prevented (Figures ​(Figures1B1B and ​and22 red dots, n = 7).

results indicate that h-tau42 affects synaptic release by modifying intracellular phosphorylation homeostasis as a resultant of the h-tau42 hyperphosphorylation. This dynamic change, we propose, leads to a marked reduction of synaptic vesicle availability, most probably due to the reduction of synapsin 1 phosphorylation, known to be a powerful modulator of synaptic release (Llinas et al., 1985). Beyond affecting synaptic release the reduction of such vesicular fusion on constitutive vesicular dynamics would also result in a disconnection event ultimately generating a “dying-back” phenomenon (Stamer et al., 2002; Pigino et al., 2007; Serulle et al., 2007). Systemic administration of T-817MA, a proposed neuro-protective agent, rescued tau-induced synaptic abnormalities, markedly reduced h-tau42 hyperphosphorylation and prevented synaptic vesicle clustering, as determine by ultrastructural analysis.