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Journal of Neuroscience, Vol 16, 723-729, Copyright © 1996 by Society for Neuroscience
Characterization of a descending system that enables crossed group II inhibitory reflex pathways in the cat spinal cord
NC Aggelopoulos, MJ Burton, RW Clarke and SA Edgley
Department of Anatomy, University of Cambridge, United Kingdom.
In the cat, stimulation of group II afferents from hindlimb muscles evokes
different crossed reflex actions depending on the integrity of the spinal
cord: with the cord intact, extensor motoneurons are inhibited by
activation of contralateral group II afferents; after spinal transection,
the same stimuli excite these neurons (crossed extension reflex). We have
investigated the mechanisms underlying this descending control. To delimit
the descending pathway, the effects of funicular lesions of the thoracic
cord on the crossed actions on motoneurons were examined. Bilateral lesions
of the dorsolateral funiculi abolished the crossed IPSPs as effectively as
complete spinal section. If either dorsolateral funiculus was spared, the
IPSPs remained. To examine whether serotonergic fibers were involved, the
effects of agents selective for 5-hydroxytryptamine (5-HT)1A receptors were
examined. After abolishing the crossed IPSPs by spinal transection,
systemic administration of the 5-HT1A receptor agonist (+/-
)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 0.1- 1.3
mg/kg, i.v.) restored the crossed inhibition. This effect was antagonized
by the selective 5-HT1A receptor antagonist WAY-100135 (0.7- 3.7 mg/kg,
i.v.). The simplest explanation of these findings is that a serotonergic
pathway, descending via the dorsolateral funiculi and acting via 5-HT1A
receptors, is involved: with the spinal cord intact, the pathway would be
tonically active and thus enable the crossed inhibition. This raises the
possibility that a serotonergic pathway is involved in the selection of
specific spinal reflex patterns via 5-HT1A receptors.
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