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Volume 17, Number 11, Issue of June 1, 1997 pp. 4461-4472
Copyright ©1997 Society for Neuroscience

A Slow Outward Current Activated by FMRFamide in Heart Interneurons of the Medicinal Leech

Received Nov. 27, 1996; revised March 17, 1997; accepted March 21, 1997.

Farzan Nadim and Ronald L. Calabrese

Department of Biology, Emory University, Atlanta, Georgia 30322

The endogenous neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH₂) can accelerate the oscillation of reciprocally inhibitory pairs of interneurons that pace heartbeat in the medicinal leech. A model based on all available biophysical data of a two-cell heart interneuron oscillator provides a theoretical basis for understanding this modulation. Previously observed modulation of K⁺ currents by FMRFamide cannot account for this acceleratory effect in the model. This observation prompted the present reexamination of K⁺ currents in heart interneurons. We devised better methods for separation of the various components of K⁺ current and more accurately measured their activation and deactivation kinetics. Moreover, we demonstrated that FMRFamide activates a previously undetected K⁺ current (I_KF), which has very slow activation and deactivation kinetics. Addition of physiologically measured amounts of I_KF to the model two-cell oscillator can account for the acceleratory effect of FMRFamide.

Key words: FMRFamide; slow outward currents; neural oscillator; conductance-based model; leech; K⁺

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