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This Article Full Text Full Text (PDF) All Versions of this Article: 558/1/213    most recent jphysiol.2004.064717v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gilmore, J. Articles by Fedirchuk, B. Search for Related Content PubMed PubMed Citation Articles by Gilmore, J. Articles by Fedirchuk, B.

Department of Physiology, University of Manitoba, 730 William Avenue, Winnipeg, MB, Canada R3E 3J7

Previous work has shown there is an increase in motoneurone excitability produced by hyperpolarization of the threshold potential at which an action potential is elicited (V_th) at the onset, and throughout brainstem-induced fictive locomotion in the decerebrate cat. This represents a transient 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 V_th 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 neurones. Hyperpolarization of V_th was seen in 10/11 (range –2 to –18 mV) neurones recorded during locomotor-like VR activity, and appeared analogous to the locomotor-dependent V_th hyperpolarization previously described in the cat. However, in the present study, V_th hyperpolarization was also seen during electrical brainstem stimulation that evoked alternating, rhythmic, or tonic VR activity, or failed to evoke VR activity. Thirty-six of 71 neurones were antidromically identified as lumbar motoneurones and 33/36 showed a hyperpolarization of V_th (–2 to –14 mV) during electrical brainstem stimulation. Of the unidentified lumbar ventral horn neurones, 31/35 also showed hyperpolarization of V_th (–2 to –20 mV) during brainstem stimulation. The hyperpolarization of V_th and VR activity induced by brainstem stimulation was reversibly blocked by cooling of the cervical cord, indicating it is mediated by descending fibres, and application of the serotonergic antagonist ketanserin to the spinal cord was effectively able to block the brainstem-evoked hyperpolarization of V_th. 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 V_th. This modulatory process can be examined in the neonatal rat brainstem–spinal cord preparation without the requirement for ongoing locomotor activity.

(Received 17 March 2004; accepted after revision 26 April 2004; first published online 30 April 2004)
Corresponding author B. Fedirchuk: Department of Physiology, University of Manitoba, 730 William Avenue, Winnipeg, MB, Canada R3E 3J7. Email: brent{at}scrc.umanitoba.ca

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