Control of motor activity, via motoneuron excitability and sensory-motor integration, by the steroid hormone 20-hydroxyecdysone in crayfish

Abstract

Summary We studied the effects of molting hormone, 20-hydroxyecdysone (20E), on leg sensory-motor networks of the red swamp crayfish Procambarus clarkii. The hormone was injected in isolated crayfishes and network activity was analyzed three days after 20E injection, using electrophysiology on an in vitro preparation of the leg locomotor network. We observed that this 20E treatment deeply reduced motor activity, by affecting both motoneuron (MN) intrinsic properties and sensory-motor integration. Indeed, we noticed a general decrease in motor nerve tonic activities, principally in depressor and promotor nerves. Moreover, intracellular recordings of depressor motoneurons confirmed a decrease of motoneuron excitability due to a drop in input resistance. In parallel, sensory inputs originating from a proprioceptor, which codes joint movements controlled by these motoneurons, were also reduced. The shape of excitatory post-synaptic potentials (PSPs) triggered in motoneurons by sensory activity of this proprioceptor showed a reduction of polysynaptic components, whereas inhibitory PSPs were suppressed, demonstrating that 20E acted also on interneurons relaying sensory to motor inputs. Consequently, 20E injection modified the whole sensory-motor loop, as demonstrated by the alteration of the resistance reflex amplitude. These locomotor network changes induced by 20E were consistent with the decrease of locomotion observed in a behavioral test. In summary, 20E controls locomotion during crayfish premolt by acting both on MN excitability and sensory-motor integration. Among these cooperative effects, the drop of input resistance of motoneurons seems to be mostly responsible for the reduction of motor activity.