Adrenoreceptor-mediated modulation of the spinal locomotor pattern during swimming in Xenopus laevis tadpoles

This study focused on the contribution of different adrenoreceptor subtypes to the modulation of fictive swimming activity in a relatively simple, yet intact, lower vertebrate system, the immobilized Xenopus laevis tadpole and explored their possible role in mediating the noradrenergic modulation of spinal motor networks. In Xenopus embryos, near the time of hatching, activation of alpha (1) adrenoreceptors increased the duration of episodes of fictive swimming, whilst in larvae, 24 h after hatching, they were decreased. Activation of alpha (2) adrenoreceptors, however, markedly reduced episode duration at both developmental stages. Cycle periods in both stages were increased by the activation of alpha (1) and/or alpha (2) receptor subclasses, whereas beta adrenoreceptors were not apparently involved in the modulation of cycle periods or the duration of swim episodes. However, both beta and alpha (1) receptor activation decreased the intersegmental delay in the head-to-tail propagation of swimming activity, while alpha (2) receptors did not influence these rostro-caudal delays. Activation of neither alpha, nor beta, receptor subclasses had any consistent effect on the duration of ventral motor bursts. Our findings suggest that noradrenergic modulation of the swim-pattern generator in Xenopus tadpoles is mediated through the activation of alpha and beta adrenoreceptors. In addition, activation of particular receptor subclasses might enable the selective modulation of either the segmental rhythm generating networks, the intersegmental coordination of those networks or control at both levels simultaneously.