Modulation of presynaptic ion channels

One interesting aspect of biology concerns how presynaptic nerve terminals function activity levels are high. Herein are described two separate lines of investigation into compensatory mechanisms in nerve terminals which may be recruited during such periods.; It has been proposed that syntaxin might influence Ca2+ channel function in presynaptic endings, in a reversal of the conventional flow of information from Ca2+ channels to the release machinery. Accordingly, we have examined whether syntaxin or other SNARE proteins affect Ca2+ influx in isolated rat neocortical nerve terminals (synaptosomes). Cleavage of syntaxin with botulinum neurotoxin type C1 (BoNtC1) altered the pattern of Ca2+ entry monitored with fura-2. Whereas the initial Ca2+ rise induced by 4-aminopyridine or high K+ was unchanged, late Ca2+ entry was strongly augmented by syntaxin cleavage. Cleavage of VAMP with BoNtD or SNAP-25 with BoNtE failed to produce a significant change in Ca2+ entry. BoNtC1 did not affect Ca 2+ extrusion or buffering, involving only N-, P/Q- and R-type Ca 2+ channels, the high voltage-activated channels most intimately associated with presynaptic release machinery. The modulatory effect of syntaxin developed with a delay even when synaptosomes had been predepolarized in the absence of external Ca2+, suggesting that significant vesicular turnover as seen during periods of high activity may be necessary to make syntaxin available for its stabilizing effect on Ca2+ channel inactivation.; Models of the synaptic cleft as well as recordings in the mammalian brain and single avian synapses predict that synaptic [Ca2+] _o decreases significantly during the course of an action potential. We have made electrical recordings from synaptosomes and have found that a major component of the membrane current is supported by a novel non-specific cation channel which is activated by decreases in [Ca2+] and membrane depolarization. The responsiveness to [Ca2+] _o is mediated by a distinct receptor that is sensitive to Ca 2+, Mg2+, Gd3+ and spermidine. These results suggest that the non-specific cation channel may serve to regulate release probability during physiological decreases in [Ca2+] _o at the synaptic cleft, possibly by broadening subsequent action potentials and opposing the decreased driving force for Ca2+ entry.