Spinal and supraspinal functions of noradrenaline in the frog embryo: consequences for motor behaviour

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Abstract
The monoamine noradrenaline (NA) can initiate and/or modulate locomotion in a variety of vertebrates. Here we report that exogenous NA application can facilitate two completely different fictive behaviours in embryos of the common frog Rana temporaria, depending on whether spinal networks are connected to supraspinal centres. When the nervous system is intact, NA elicits a non-rhythmic coiling motor response, reminiscent of a spontaneous behaviour appropriate to drive hatching movements, but has only minor effects on evoked swimming activity. After the spinal cord has been severed from the brain, spontaneous coiling is no longer observed, nor can NA elicit it, but the amine can ‘release’ swimming rhythm generation in response to electrical skin stimulation. The rhythm is similar, but relatively inflexible when compared to fictive swimming recorded from intact animals. Our pharmacological tests indicate that α1-adrenoreceptors are involved in the permissive role of NA during spinalised rhythmic swimming and that the fictive coiling response to NA in intact animals involves descending inputs and the activation of β1-adrenoreceptors. Furthermore, the subtle effects of NA on evoked swimming in intact animals were mimicked by either α1- or α2-adrenoreceptor activation, reversibly decreasing motor burst durations and increasing their frequency. We discuss our results with reference to the known synergistic actions of NA with another aminergic neuromodulator, serotonin, and raise the possibility that these amines may actively regulate the release of one another during locomotion, in addition to their respective post-synaptic targets in the spinal cord.