Biology Reference
In-Depth Information
The release of synaptic transmitter by action potentials is often enhanced for about
a second after short bouts of presynaptic activity (synaptic facilitation), and for much
longer after sustained activity lasting minutes (posttetanic potentiation, PTP). We
have used photolytic release of presynaptic Ca
2
þ
from DM-nitrophen to induce
facilitation without electrical activity (
Kamiya and Zucker, 1994
). Photolysis of
diazo-2 or diazo-4 to terminate the [Ca
2
þ
]
i
increase lingering briefly after a short
spike train abolished facilitation immediately at crayfish neuromuscular junctions.
PTP induced by longer stimulation, both in this preparation and in Aplysia neuronal
synapses (
Fischer et al.,1997
), was abolished more slowly by rapid reduction of the
prolonged residual [Ca
2
þ
]
i
resulting frommitochondrial overload (
Tang and Zucker,
1997
). Thus, facilitation and PTP arise from residual [Ca
2
þ
]
i
acting on distinct
molecular targets di
erent from the secretory trigger, which is also activated by
Ca
2
þ
. We compared responses to [Ca
2
þ
]
i
steps on flash photolyzing DMNPE-4 at
weakly transmitting but strongly facilitating neuromuscular junctions to responses at
strongly transmitting but depressible junctions (
Millar et al.,2005
), and concluded
that the di
V
erence in the state
of Ca
2
þ
-dependent priming, such that strongly transmitting synapses were already
preprimed at rest by a priming target tuned to have a higher Ca
2
þ
-sensitivity. This
led, in turn, to the development of a comprehensive model of synaptic transmission,
facilitation, and depression that comprised three Ca
2
þ
-dependent processes—vesicle
mobilization to docking sites, priming of docked vesicles, and activation of mem-
brane fusion (
Pan and Zucker, 2009
).
We (
Land
`
and Zucker, 1989
) and
Heidelberger et al. (1994)
were the first to use
DM-nitrophen photolysis to characterize the Ca
2
þ
-cooperativity of secretion at
neuromuscular junctions and retinal bipolar neurons; subsequently, we (
Ohnuma
et al., 2001
) used NP-EGTA and
Kasai et al. (1999)
used DM-nitrophen to show
di
V
erence in response kinetics was best explained by a di
V
erences in the Ca
2
þ
-dependence and sensitivity of peptidergic or aminergic
large dense core vesicle fusion and cholinergic small clear vesicle fusion at central
molluscan synapses and in PC12 cells.
Hsu et al. (1996)
reported that transmitter
release at squid giant synapses decayed to step [Ca
2
þ
]
i
increases produced by
NP-EGTA photolysis; subsequent higher steps evoked more release, indicating
transmitter stores had not been depleted, suggesting either an adaptation of the
release process, as the authors proposed, or possibly vesicle heterogeneity in
sensitivity to release, or the operation of mobilization or priming processes
enabling release of previously undocked or unprimed vesicles at higher [Ca
2
þ
]
i
.
Caged Ca
2
þ
photolysis has been used extensively in the last decade in many
elegant experiments, especially from the laboratories of Erwin Neher and Bert
Sakmann, to kinetically characterize in detail the secretory trigger for neurosecre-
tion, primarily at the giant synapse of the calyx of Held (
Bollmann and Sakmann,
2005; Bollmann et al., 2000; Felmy et al., 2003a,b; Hosoi et al., 2007; Sakaba et al.,
2005; Schneggenburger and Neher, 2000; Wadel et al., 2007; Wang et al., 2004;
Young and Neher, 2009
). Ca
2
þ
uncaging from DM-nitrophen has been used to
probe the kinetics and cooperativity of Ca
2
þ
binding to the secretory trigger,
kinetic consequences of SNARE protein and synaptotagmin mutation, e
V
V
ects of
