CHARACTERIZATION OF A DESCENDING SYSTEM THAT ENABLES CROSSED GROUP-IIINHIBITORY REFLEX PATHWAYS IN THE CAT SPINAL-CORD

In the cat, stimulation of group II afferents from hindlimb muscles ev okes different crossed reflex actions depending on the integrity of th e spinal cord: with the cord intact, extensor motoneurons are inhibite d by activation of contralateral group II afferents; after spinal tran section, the same stimuli excite these neurons (crossed extension refl ex). We have investigated the mechanisms underlying this descending co ntrol. To delimit the descending pathway, the effects of funicular les ions of the thoracic cord on the crossed actions on motoneurons were e xamined. Bilateral lesions of the dorsolateral funiculi abolished the crossed IPSPs as effectively as complete spinal section. If either dor solateral funiculus was spared, the IPSPs remained. To examine whether serotonergic fibers were involved, the effects of agents selective fo r 5-hydroxytryptamine (5-HT)(1A) receptors were examined. After abolis hing the crossed IPSPs by spinal transection, systemic administration of the 5-HT1A receptor agonist (+)-8-hydroxy-2-(di-n-propylamino)tetra lin 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 recep tor antagonist WAY-100135 (0.7-3.7 mg/kg, i.v.). The simplest explanat ion of these findings is that a serotonergic pathway, descending via t he dorsolateral funiculi and acting via 5-HT1A receptors, is involved: with the spinal cord intact, the pathway would be tonically active an d thus enable the crossed inhibition. This raises the possibility that a serotonergic pathway is involved in the selection of specific spina l reflex patterns via 5-HT1A receptors.