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Received March 17, 2004
Revised April 14, 2004
Accepted after revision April 26, 2004
1 University of Manitoba
2 410 BMSB
* To whom correspondence should be addressed. E-mail: brent{at}scrc.umanitoba.ca.
Previous work has shown there is an increase in motoneuron excitability produced by hyperpolarization of the threshold potential at which an action potential is elicited (Vth) at the onset, and throughout brainstem induced fictive locomotion in the decerebrate cat. This represents a moment-by-moment facilitation in the membrane potential for activation dependent on the presence of fictive locomotion. The present study tests the hypothesis that a similar neuromodulatory mechanism facilitating neuronal recruitment also exists in the neonatal rat, and the endogenous pathway mediating the Vth hyperpolarization can be activated by electrical stimulation of the neonatal brainstem. Isolated brainstem/spinal cord preparations from 1 to 5 day old neonatal rats, and whole-cell recording techniques were used to examine the patterns of ventral root (VR) activity produced, and the effect of electrical stimulation of the ventromedial medulla on lumbar spinal neurons. Hyperpolarization of Vth was seen in 10/11 (range -2 to -18 mV) neurons recorded during locomotor-like VR activity, and appeared analogous to the locomotor-dependent Vth hyperpolarization previously described in the cat. However, in the present study, Vth hyperpolarization was also seen during electrical brainstem stimulation that evoked alternating, rhythmic, or tonic VR activity; or failed to evoke VR activity. 36/71 neurons were antidromically identified as lumbar motoneurons and 33/36 showed a hyperpolarization of Vth (-2 to -14 mV) during electrical brainstem stimulation. 31/35 of the unidentified lumbar ventral horn neurons also showed hyperpolarization of Vth (-2 to -20 mV) during brainstem stimulation. The hyperpolarization of Vth and VR activity induced by brainstem stimulation was reversibly blocked by cooling of the cervical cord, indicating it is mediated by descending fibers, and application of the serotonergic antagonist ketanserin to the spinal cord was effectively able to block the brainstem evoked hyperpolarization of Vth. These results demonstrate a previously unknown action of the endogenous descending serotonergic system, to facilitate spinal motoneuronal recruitment and firing by inducing a hyperpolarization of Vth. This modulatory process can be examined in the neonatal rat brainstem/spinal cord preparation without the requirement for ongoing locomotor activity.
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