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Journal of Neuroscience, Vol 9, 2846-2857, Copyright © 1989 by Society for Neuroscience
A hyperpolarization-activated inward current in heart interneurons of the medicinal leech
JD Angstadt and RL Calabrese
Emory University, Atlanta, Georgia 30322.
Heart interneurons (HN cells) in isolated ganglia of the medicinal leech
were voltage-clamped with single microelectrodes. Hyperpolarizing voltage
steps elicited a slow inward current (Ih), which underlies the
characteristic depolarizing response of HN cells to injection of prolonged
hyperpolarizing current pulses (Arbas and Calabrese, 1987a). The
conductance underlying Ih begins to activate near -mV and is fully
activated between -70 and -80 mV. The activation kinetics of Ih are slow
and voltage dependent. The activation time constant (tau h) ranges from
approximately 2 sec at -60 mV to near 700 msec at -100 mV. Ih persists in
low Ca2+ (0.1 mM), 5 mM Mn2+ saline and exhibits a reversal potential of
-21 +/- 5 mV. The reversal potential is shifted by altering [Na+]o or [K+]o
but is unaffected by changes in [Cl-]o. Ih is blocked by extracellular Cs+
(1-5 mM) but not Ba2+ (5 mM) or TEA (25 mM). Low concentrations of Cs+
(100-200 microM) cause a partial block that exhibits strong voltage
dependence. Temperature changes were also shown to affect Ih. Both the rate
of activation and the steady-state amplitude of Ih are enhanced by
temperature increases. HN cells are interconnected by inhibitory chemical
synapses, and their normal electrical activity consists of bursts of action
potentials separated by periods of inhibition. During the inhibitory phase
of rhythmic bursting activity, HN cells hyperpolarize to a voltage range
where Ih is activated. Block of Ih with extracellular Cs+ (4 mM) disrupted
the normal bursting activity of HN cells. These results are consistent with
the hypothesis that Ih contributes to escape from inhibitory inputs during
normal bursting activity.
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