State-dependent regulation of sensory-motor transmission: role of muscarinic receptors in sensory-motor integration in the crayfish walking system.

The aim of this study was to investigate a potential mechanism for state-dependent regulation of sensory-motor transmission from sensory afferents of a proprioceptor to motoneurons (MNs) in the walking system of the crayfish. This study was performed using an in vitro preparation of thoracic ganglia including motor nerves and the proprioceptor that codes movements of the second joint (coxo-basal chordotonal organ - CBCO) of the leg. Application of movements to the CBCO elicits resistance reflex responses intracellularly recorded from Dep MNs. This reflex response is enhanced when Dep MNs are depolarized either spontaneously or by current injection. This enhancement is abolished in the presence of scopolamine (an antagonist of muscarinic acetylcholine receptors). Using pharmacology, we demonstrate that the monosynaptic connection from CBCO sensory neurons to the Dep MNs includes both nicotinic and muscarinic components. In addition, the shape of monosynaptic excitatory postsynaptic potentials (EPSPs) depends on the membrane potential: at a subthreshold depolarizing membrane potential, the time constant of the falling phase of the EPSPs is significantly increased compared with its value at resting potential. This change is suppressed in the presence of scopolamine, indicating that the muscarinic component may contribute to the activation of the Dep MN pool by sensory activity. This state-dependent amplification of the sensory input may be important for increasing the strength of sensory feedback at times when central activation of the Dep MNs is very strong (e.g. during walking).