Effects of the 5-hydroxytryptamine on spinal reflexes of the cat

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

The effects of serotonergic drugs on the locomotor pattern and on cutaneous reflexes of the adult chronic spinal cat

The effects of serotonergic substances on the locomotor pattern and cutaneous reflexes were studied in 3 adult chronic spinal cats trained for 1-3 months to walk with their hindlimbs on a treadmill. The 5-hydroxytryptamine (5-HT) precursor, 5-hydroxytryptophan (5-HTP), and two 5-HT agonists, 5-methoxy-N,N-dimethyltryptamine and quipazine, were found to generally increase the step length and augment the amplitude of hindlimb extensors and flexors as well as axial muscles. Correspondingly, the excursion of the hip, the knee and the ankle joints was increased, mainly in the flexion direction. Cyproheptadine, a 5-HT antagonist, partially or completely antagonised these effects. The threshold current needed to elicit a flexion reflex by stimulating the dorsum of the paw through implanted wires, was lower after the injection of 5-HT agonists than in the immediately preceding control period. Fast paw shaking initiated by dipping the paw in water was unchanged after quipazine and was not abolished by cyproheptadine. In accordance with others, our results suggest that serotonergic drugs may increase the excitability of several types of spinal neurones, including motoneurones, and consequently influence the locomotor pattern as well as the reflex responsiveness. The changes observed with serotonergic agonists were different in many respects from those obtained with noradrenergic agonists and these differences are discussed. This may indicate specific roles for these classes of substances on locomotor function and reflex activity and also provide a basis for further clinical investigations.