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Journal of Neuroscience, Vol 11, 746-759, Copyright © 1991 by Society for Neuroscience
Calcium currents and graded synaptic transmission between heart interneurons of the leech
JD Angstadt and RL Calabrese
Department of Biology, Emory University, Atlanta, Georgia 30322.
Synaptic transmission between reciprocally inhibitory heart interneurons
(HN cells) of the medicinal leech was examined in the absence of
Na-mediated action potentials. Under voltage clamp, depolarizing steps from
a holding potential of -60 mV elicited 2 kinetically distinct components of
inward current in the presynaptic HN cell: an early transient current that
inactivates within 200 msec and a persistent current that only partially
decays over several seconds. Both currents begin to activate near -60 mV.
Steady-state inactivation occurs over the voltage range between -70 and -45
mV and is completely removed by 1-2-sec hyperpolarizing voltage steps to
-80 mV. The inward currents are carried by Ca2+, Ba2+, or Sr2+ ions, but
not by Co2+, Mn2+, or Ni2+. These same inward currents underlie the
burst-generating plateau potentials previously described in HN cells (Arbas
and Calabrese, 1987a,b). With a presynaptic holding potential of -60 mV,
the threshold for transmitter release is near -45 mV. Postsynaptic currents
in the contralateral HN cell have a reversal potential near - 60 mV. The
largest postsynaptic currents (300-400 pA) exhibit an initial peak response
that is followed by a more slowly decaying component. The persistent
component of Ca2+ current in the presynaptic neuron is strongly correlated
with the prolonged component of the postsynaptic current, while the
transient presynaptic Ca2+ current appears to correspond to the early peak
of postsynaptic current. These data are consistent with the hypothesis that
voltage-dependent calcium currents contribute to the oscillatory capability
of reciprocally inhibitory HN cells by (1) generating the plateau potential
that drives the burst of action potentials and (2) underlying the release
of inhibitory transmitter onto the contralateral cell.
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