Muscarinic modulation of the Xenopus laevis tadpole spinal mechanosensory pathway

Muscarinic acetylcholine receptors (mAChRs) mediate effects of acetylcholine (ACh) in many systems, including those involved in spinal functions like locomotion. In Xenopus laevis tadpoles at two days old, a model vertebrate for motor control research, we investigated the role of mAChRs in the skin mechanosensory pathway. We found that mAChR activation by carbachol did not affect the sensory Rohon-Beard neuron properties. However, carbachol could hyperpolarise sensory interneurons and decrease their voltage responses to outward currents. Carbachol could increase the threshold for the mechanosensory pathway to start swimming, preventing the initiation of swimming at higher concentrations altogether. Recording from the sensory interneurons in carbachol showed that their spiking after skin stimulation was depressed. However, the general muscarinic antagonist atropine did not have a clear effect on the swimming threshold or the modulation of sensory interneuron membrane conductance. Our results suggest the skin mechanosensory pathway may be subject to muscarinic modulation in this simple vertebrate system.