CHOLINERGIC CONTROL OF THE WALKING NETWORK IN THE CRAYFISH PROCAMBARUS-CLARKII

The output of a neuronal network results generally from both the prope rties of the component neurons and their synaptic relationships. This article aims at synthesizing various results obtained on the neural ne twork generating locomotion in vitro. In the preparation used, consist ing of the last three thoracic ganglia (3-5) along with motor nerves f rom the 5th leg ganglion to the promotor, remotor, levator and depress or muscles, motor nerve recordings generally revealed only tonic activ ity in several different motoneurons (MNs). However, rhythmic activity can be obtained by the use of cholinergic agents such as the oxotremo rine (Oxo) superfused in the bath (5 x 10(-5) M). If Oxo is pressure-e jected locally in the ganglion, it is possible, depending upon the loc us 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 d epolarization (1-3 mV). These responses persisted in the presence of T TX and Co2+. The transient initial depolarization is a mixed nicotinic and muscarinic voltage-independent response during which the input re sistance decreases by 20 to 40%. In contrast, the long lasting compone nt is voltage-dependent, exclusively muscarinic and associated to a 5- 10% increase of input resistance due to the closing of a K+ conductanc e that is active at the resting Vm, and totally suppressed at holding potentials below -70 mV. More generally, K+ currents activated at rest ing potential are responsible for membrane potential stability. The in jection of TEA, a blocker of the K+ currents, through the recording el ectrode is able to unmask plateaus above a threshold depolarization Th ese plateaus are TTX-sensitive but persist in the presence of Ca2+ cha nnel blockers. Moreover, in 10% of TEA-filled MNs a spontaneous pacema ker activity was revealed. The organization of the locomotor network i s also based upon connections between MNs and INs. Within a MN pool, c onnections are only loosely established, appearing to consist mainly o f electrical coupling. Inhibitory synaptic connections between MNs of opposite pools are mediated by chloride channels. However, the neurotr ansmitter involved could be either GABA or glutamate. Therefore, at th e level of a given joint, a basic rhythm occurs due to both motoneuron al membrane properties and motoneuronal connectivity. However, the coo rdination of all MNs of an entire leg during fictive walking activity requires the involvement of Ws. Based upon these data, we propose a tw o-stage model of the locomotor network organization: a joint motoneuro nal level and a whole leg interneuronal level.