Oscillatory behaviors in pharmacologically isolated heart interneurons from the medicinal leech

The central motor pattern for heartbeat in the medicinal leech is based upo n the alternating bursting activity of mutually inhibitory pairs of heart i nterneurons (HNs). When pharmacologically isolated, these neurons spike ton ically. Using a canonical model of an HN cell (Nadim et al., J. Comput. Neu rosci. 2 (1995) 215-235) as a starting point, we generated three models, po ssessing different subsets of the currents exhibited by HN cells, that are capable of autonomous bursting behavior. These three models represent three different experimentally testable mechanisms that make HN cells potentiall y capable of bursting or generating slow oscillations under different ionic and pharmacological conditions. The three different mechanisms for produci ng bursting involve the interplay of: (1) fast Na+ current, leakage current and outward currents, (2) rapidly inactivating low-threshold Ca2+ current, leakage current and outward currents, and (3) slowly inactivating low-thre shold Ca2+ current, leakage current and outward currents. (C) 2000 Elsevier Science B.V. All rights reserved.