NEUROMODULATION OF RECIPROCAL GLUTAMATERGIC INHIBITION BETWEEN ANTAGONISTIC MOTONEURONS BY 5-HYDROXYTRYPTAMINE (5-HT) IN CRAYFISH WALKING SYSTEM

In an in vitro preparation of the crayfish thoracic locomotor system, paired intracellular recordings were performed from antagonistic depre ssor (Dep) and levator (Lev) motoneurons (MNs) that control the second joint of walking legs, Connections between these two groups of MNs co nsist mainly of inhibitory connections and weak electrotonic synapses. Injection of depolarizing current into a Lev MN results in a hyperpol arization in a Dep MN, and vice versa. This reciprocal glutamatergic i nhibition, is not changed in the presence of the sodium channel blocke r tetrodotoxin (TTX) and therefore is likely supported by a direct con nection between MNs, By contrast, reciprocal inhibition is largely red uced in the presence of Ei-hydroxytryptamine (5-HT; 10 mu M). Direct m icro-application of glutamate pressure-ejected close to an intracellul arly recorded MN, evoked an inhibitory response in that MN, accompanie d by a decrease of input resistance. These two effects were dramatical ly reduced in the presence of 5-HT, Thus 5-HT could be involved in mec hanisms of dynamic reconfigurations of the neural network controlling leg movements in crayfish, (C) 1998 Elsevier Science Ireland Ltd.