Motor Neurones of the Crayfish Walking System Possess Tea+-revealed Regenerative Electrical Properties

In crustaceans, some motor neurones (MNs) have been shown to be part of the central pattern generator in the stomatogastric system (Harris-Warrick et al. 1992; Moulins, 1990), the swimmeret system (Heitler, 1978) or the walking system (Chrachri and Clarac, 1990). These MNs induce changes in the central rhythm when depolarized and are conditional oscillators in the stomatogastric ganglion. Moreover, in the walking system, rhythmic activity can be triggered by muscarinic cholinergic agonists (Chrachri and Clarac, 1987). We have recently analyzed the role of muscarinic receptors in crayfish walking leg MNs (D. Cattaert and A. Araque, in preparation) and demonstrated that oxotremorine, a muscarinic agonist, evoked long-lasting depolarizing responses associated with an increased input resistance. The outward current blocked by oxotremorine is likely to be carried by K+, as is the case for the M current (IM) in vertebrates (Brown and Adams, 1980). In most neurones, K+ conductances play a principal role in maintaining the membrane potential at rest: for example, IM is active at the resting membrane potential, thus contributing to its maintenance, and the 'delayed-rectifier' (IK) assists the fast repolarization after an action potential. Some K+ conductances are Ca2+-dependent (IK,Ca) and are activated by an increase in internal Ca2+ concentration. In such cases, Ca2+ currents may result in hyperpolarization of the neurone through activation of IK,Ca. In opposition to these K+ currents, the direct effect of Na+ and Ca2+ conductances is to depolarize the neurone. For example, the persistant Na+ current (INap) that is responsible for the slow subthreshold depolarization termed slow pre-potentials (Gestrelius et al. 1983; Leung and Yim, 1991) participates in the formation of pacemaker depolarization (Barrio et al. 1991) and generates plateau-type responses in control conditions (Barrio et al. 1991; Llinas and Sugimori, 1980). Similarly Ca2+ or non-specific (Na+/Ca2+) conductances generate such events in Aplysia californica burster neurones (Adams and Benson, 1985), crustacean cardiac ganglion (Tazaki and Cooke, 1990), insect neurones (Hancox and Pitman, 1991) and crustacean stomatogastric ganglion (Kiehn and Harris-Warrick, 1992). Since crustacean MNs can participate in rhythm production, such depolarizing conductances may exist in most of them and may contribute to the long-lasting MN depolarizations and spike bursts present during locomotion.