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1Hotchkiss Brain Institute, 2Department of Physiology and Biophysics, and 3Department of Clinical Neurosciences, University of Calgary, Calgary, Alberta, Canada
Submitted 9 June 2006; accepted in final form 5 September 2006
Monoaminergic projections are among the first supraspinal inputs to innervate spinal networks. Little is known regarding the role of monoamines in modulating ongoing locomotor patterns evoked by endogenous release of neurotransmitter. Here we activate a locomotor-like rhythm by electrical stimulation of afferents and then test the modulatory effects of monoamines on the frequency, pattern, and quality of the rhythm. Stimulation of the cauda equina induced a rhythm consisting of left-right and ipsilateral alternation indicative of locomotor-like activity. First, we examined the effects of noradrenaline (NA), serotonin (5-HT), or dopamine (DA) at dose levels that did not elicit locomotor activity. Bath application of NA and DA resulted in a depression of the cauda-equina-evoked rhythm. Conversely, bath-applied 5-HT increased both the amplitude and cycle period of the evoked rhythm, an effect that was mimicked by the addition of 5-HT2 agonists to the bath. Application of 5-HT7 agonists disrupted the evoked rhythmic behavior. Next, we examined the effects of NA 
1 and 2 agonists and found that the suppressive effects of NA on the rhythm could be reproduced by adding the 2 agonist, clonidine, to the bath. In contrast, bath applying the 1 agonist, phenylephrine, increased the amplitude and duration of the cycle period. Finally, the suppressive effects of DA were not replicated by the administration of D1, D2, or D3 agonists although application of NA 2 antagonists reversed the effects of DA. Application of D1 agonists, increased the amplitude of the bursts but did not affect the cycle period. Our results indicate that monoamines can control the expression, pattern, and timing of cauda-equina-evoked locomotor patterns in developing mice.
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