Cholinergic control of the walking network in the crayfish Procambarus clarkii

Abstract

The output of a neuronal network results generally from both the properties of the component neurons and their synaptic relationships. This article aims at synthesizing various results obtained on the neural network generating locomotion in vitro. In the preparation used, consisting of the last three thoracic ganglia (3–5) along with motor nerves from the 5th leg ganglion to the promotor, remotor, levator and depressor muscles, motor nerve recordings generally revealed only tonic activity in several different motoneurons (MNs). However, rhythmic activity can be obtained by the use of cholinergic agents such as the oxotremorine (Oxo) superfused in the bath (5 × 10 −5 M). If Oxo is pressure-ejected locally in the ganglion, it is possible, depending upon the locus where the drug is applied, to elicit a rhythmic activity restricted to a group of antagonistic MNs. To analyze how cholinergic agents are able to induce such rhythmic activity, very small volumes of drug (50–200 pl), were applied close to the recording electrode. Two types of depolarizing response occurred: a fast large amplitude depolarization (5–20 mV) and a long lasting (10 s to several minutes) low amplitude depolarization (1–3 mV). These responses persisted in the presence of TTX and Co 2+. The transient initial depolarization is a mixed nicotinic and muscarinic voltage-independent response during which the input resistance decreases by 20 to 40%. In contrast, the long lasting component is voltage-dependent, exclusively muscarinic and associated to a 5–10% increase of input resistance due to the closing of a K + conductance that is active at the resting Vm, and totally suppressed at holding potentials below −70 mV. More generally, K + currents activated at resting potential are responsible for membrane potential stability. The injection of TEA, a blocker of the K + currents, through the recording electrode is able to unmask plateaus above a threshold depolarization. These plateaus are TTX-sensitive but persist in the presence of Ca 2+ channel blockers. Moreover, in 10% of TEA-filled MNs a spontaneous pacemaker activity was revealed. The organization of the locomotor network is also based upon connections between MNs and INs. Within a MN pool, connections are only loosely established, appearing to consist mainly of electrical coupling. Inhibitory synaptic connections between MNs of opposite pools are mediated by chloride channels. However, the neurotransmitter involved could be either GABA or glutamate. Therefore, at the level of a given joint, a basic rhythm occurs due to both motoneuronal membrane properties and motoneuronal connectivity. However, the coordination of all MNs of an entire leg during fictive walking activity requires the involvement of INs. Based upon these data, we propose a two-stage model of the locomotor network organization: a joint motoneuronal level and a whole leg interneuronal level.