Activity-dependent development of the locomotor network in Xenopus laevis larvae

Abstract

The impact of activity in Xenopus embryonic and larval development was studied with regards to the locomotor system and developmental rate. The data suggest that pharmacologically suppressing neuronal and muscular activity decreases, and enhancing swimming activity, by raising tadpoles in a rotating water column, increases the rate of development. Moreover the latter adapted their swimming behaviour in accordance to the treatment: their swimming was characterized by longer episode durations and more frequent turning manoeuvres. Patch clamp recordings revealed that spinal neurons of agitated animals receive more synaptic drive, probably deriving from descending INs, enabling the animals to maintain swimming for long time periods. An increase in the incidence of miniature potentials suggests that synaptic connections are strengthened. Probably in response to the increase in synaptic excitation the intrinsic excitability, and thereby the probability of signal transmission, decreased. I argue that this represents a case of homeostatic plasticity and serves to prevent over-excitation and maintain pattern generation in the network. Stronger bursts might be explained by enhanced signal transmission from motoneurons to muscle cells. The amount of extrasynaptic AChRs at the NMJ appeared reduced. In contrast, when activity was suppressed behavioural, ventral root and patch data were comparable to those of control animals, suggesting that the locomotor system can develop normally up to stage 42 in the absence of electrical activity. However, animals appeared less excitable, indicating that the establishment of sensory systems might indeed require extrinsic inputs.