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1 Hotchkiss Brain Institute, University of Calgary, Calgary, Canada
* To whom correspondence should be addressed. E-mail: whelan{at}ucalgary.ca.
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. 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 nor 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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