The Effects of Chronic Section of Dorsal Roots on the Responsiveness of Motoneurones to 5‐hydroxytryptamine and a Substance P Analogue

Abstract

1 Experiments were performed on rats 14-21 days after unilateral dorsal root section in order to determine if the effects on motoneurone excitability of a substance P analogue, 5-hydroxytryptamine (5-HT) and noradrenaline were altered by section of primary afferents.2 The effects of iontophoretic applications of these agents on motoneurone excitability were quantified by measuring the change in amplitude of the short latency field potentials evoked antidromically, by ventral root stimulation. 3 lontophoretic application of the substance P analogue (eledoisin-related peptide, ERP) always produced an increase in the amplitude of the field potential. These increases in amplitude were 25.9% larger on the sides of the cords with sectioned dorsal roots. This was not a statistically significant difference (P>0.05). 4 Section of dorsal roots did not alter responses to noradrenaline. 5 Responses to 5-HT were significantly larger following section of dorsal roots. There is very little evidence for the release of 5-HT by primary afferents and denervation supersensitivity is an improbable explanation. It is possible that descending 5-HT systems directly excite motoneurones and indirectly inhibit primary afferent transmission. Dorsal root section would alter the balance between these actions of 5-HT in favour of an excitatory effect.

Serotonin, and Mouse Spinal Neurones in Cell Culture

“…Serotonin is present in the spinal cord, being highest in the motoneurone area of the ventral horn and also in the lateral part of the substantia gelatinosa (Oliveras, Bourgoin, Hery, Besson & Hamon, 1978), most being associated with terminals of descending fibres. Activity in these fibres may be responsible for modulating segmental reflexes and nociception (Yaksh & Wilson, 1979;Roberts & Wright, 1981).…”

“…Activity in these fibres may be responsible for modulating segmental reflexes and nociception (Yaksh & Wilson, 1979;Roberts & Wright, 1981).…”