Noradrenergic control of locomotor networks in the in vitro spinal cord of the neonatal rat

In this paper, we address the role of noradrenaline (NA) in the control of locomotor networks in the neonatal rat. Using an in vitro isolated spinal cord preparation, we observed that bath application of NA elicited an extremely slow alternating motor pattern (period around 80-90 s) alternating between contralateral sides, which was recorded in the lumbar ventral roots but not between flexor and er;tensor units. These effects of NA were mimicked by the alpha 1 agonists methoxamine and phenylephrine, whereas alpha 2 and beta receptors bath-applied alone did not elicit any activity. NA slightly affected the locomotor-like activity induced by the activation of NMDA receptors, whereas the alpha 1 agonists speeded up the locomotor activity. Both the activation of alpha 2 and beta agonist receptors slowed down the motor rhythm while simultaneously increasing the burst amplitude in the case of the beta agonists. NA depolarized the motoneurones and increased their input membrane resistance. It was concluded that NA does not trigger locomotor activity, but modulates the activity of the locomotor networks. This multimodal control is mediated by the activation of a variety of receptor types. (C) 2000 Elsevier Science B.V. All rights reserved.