Biology Reference
In-Depth Information
following Ca
2
þ
influx during an action potential; both delays had the same
temperature dependence. Thus photolysis of DM-nitrophen caused a [Ca
2
þ
]
i
tran-
sient resembling that occurring normally at transmitter release sites in the vicinity
of Ca
2
þ
channels that open briefly during an action potential. After the secretory
burst, a moderate phase of transmitter release persisted for 15 ms, corresponding
to a relaxation in [Ca
2
þ
]
i
measured with fura-2 that probably reflected slow Ca
2
þ
displacement of Mg
2
þ
bound to unphotolyzed DM-nitrophen.
Similar responses to partial flash photolysis of lightly Ca
2
þ
-loaded DM-nitro-
phen were observed at crayfish neuromuscular junctions (
Land ` and Zucker,
1994
). Transmitter release evoked by slow photolysis of Ca
2
þ
-DM-nitrophen
using steady illumination also has been studied at this junction (
Mulkey and
Zucker, 1993
). The rate of quantal transmitter release, measured as the frequency
of miniature excitatory junctional potentials (MEJPs), was increased
1000-fold
during the illumination. Brief illumination (0.3-2 s) evoked a rise in MEJP fre-
quency that dropped abruptly back to normal when the light was extinguished, as
would be expected from the reversible rise in [Ca
2
þ
]
i
that should be evoked by such
illumination, which leaves most of the DM-nitrophen unphotolyzed (
Zucker,
1993
). Longer light exposures caused an increase in MEJP frequency that outlasted
the light signal, as would be expected from the rise in resting [Ca
2
þ
]
i
after photoly-
sis of most DM-nitrophen. These experiments illustrate the utility of steady pho-
tolysis of partially Ca
2
þ
-loaded DM-nitrophen in generating reversible changes in
[Ca
2
þ
]
i
in cells.
At cultured snail synapses, FMRFamide inhibits asynchronous transmitter
release elicited by [Ca
2
þ
]
i
elevated by photolysis of presynaptic nitr-5 (
Man-Son-
Hing et al., 1989
), and blocks synchronous release to partial flash photolysis of
partially Ca
2
þ
-loaded DM-nitrophen (
Haydon et al., 1991
). As at crayfish and
squid synapses, these flash-evoked postsynaptic responses resembled the spike-
evoked responses and were triggered by the spike in [Ca
2
þ
]
i
that results when DM-
nitrophen is used in this fashion.
At leech serotonergic synapses, a presynaptic Ca
2
þ
uptake process may be
activated by photolysis of presynaptic DM-nitrophen; blocking it with zimelidine
or by external Na
þ
removal eliminated the presynaptic transport current and
prolonged the postsynaptic response, uncovering a contribution of this process
to the termination of transmitter release (
Bruns et al., 1993
).
DM-nitrophen has been used extensively to probe the steps involved in exocyto-
sis in endocrine cells. Measuring [Ca
2
þ
]
i
changes with furaptra, we and others
(
Heinemann et al., 1994; Neher and Zucker, 1993; Thomas et al., 1993
) observed—
in bovine chroma
n cells and rat melanotrophs—three kinetic secretory phases in
response to [Ca
2
þ
]
i
steps to
Y
erent vesicle
pools. Prior exposure to a modest [Ca
2
þ
]
i
rise primed phasic responses to a
subsequent step in [Ca
2
þ
]
i
, indicating that [Ca
2
þ
]
i
not only triggers exocytosis but
also mobilizes vesicles into a docked or releasable status. After exocytosis, another
[Ca
2
þ
]
i
stimulus often evoked a rapid reduction in membrane capacitance signaling
a[Ca
2
þ
]
i
-dependent compensatory endocytosis.
100
m
M, reflecting release from di
V
