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Calcium currents and graded synaptic transmission between heart interneurons of the leech
1Department of Biology, Emory University, Atlanta, Georgia 30322.
Insights
Calcium currents in leech heart interneurons (HN cells) generate plateau potentials and drive inhibitory neurotransmitter release, enabling oscillatory activity in these neurons without sodium action potentials.
Area of Science:
- Neuroscience
- Electrophysiology
- Computational Biology
Background:
- Reciprocally inhibitory heart interneurons (HN cells) in medicinal leeches exhibit oscillatory activity.
- Understanding the ionic mechanisms underlying HN cell function is crucial for comprehending neural circuits.
Purpose of the Study:
- To investigate the role of voltage-dependent calcium currents in synaptic transmission between HN cells.
- To elucidate the contribution of these currents to the oscillatory behavior of HN cells.
Main Methods:
- Voltage clamp electrophysiology was used to record currents in HN cells.
- Depolarizing voltage steps were applied to study inward currents and transmitter release.
- Pharmacological agents were used to identify the ion species carrying the currents.
Main Results:
- Two distinct components of inward calcium current (transient and persistent) were identified in presynaptic HN cells.
- These currents activate near -60 mV and inactivate between -70 and -45 mV.
- The persistent current correlated with prolonged postsynaptic currents, while the transient current correlated with early postsynaptic currents.
Conclusions:
- Voltage-dependent calcium currents generate plateau potentials that drive action potential bursts in HN cells.
- These calcium currents mediate the release of inhibitory neurotransmitters onto contralateral HN cells.
- Calcium currents are essential for the oscillatory capability of reciprocally inhibitory HN cells.
Abstract:
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.