Adrenoreceptor-mediated modulation of the spinal locomotor pattern during swimming inXenopus laevistadpoles

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Abstract
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 α1 adrenoreceptors increased the duration of episodes of fictive swimming, whilst in larvae, 24 h after hatching, they were decreased. Activation of α2 adrenoreceptors, however, markedly reduced episode duration at both developmental stages. Cycle periods in both stages were increased by the activation of α1 and/or α2 receptor subclasses, whereas β adrenoreceptors were not apparently involved in the modulation of cycle periods or the duration of swim episodes. However, both β and α1 receptor activation decreased the intersegmental delay in the head‐to‐tail propagation of swimming activity, while α2 receptors did not influence these rostro‐caudal delays. Activation of neither α, nor β, 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 α and β 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.