Action of biogenic amines on the isolated toad spinal cord

Abstract

1. The effects of noradrenaline, adrenaline, dopamine and 5-hydroxytryptamine have been tested on isolated hemisected spinal cords in which synaptic transmission or action potential propagation were blocked by perfusion with high Mg 2+—or tetrodotoxin—containing solutions respectively. 2. The catecholamines at a concentration of 10 −3M consistently depolarized dorsal root terminals, and in most preparations elicited a small depolarization of motoneurons. At lower concentrations their effects were less consistent. The threshold concentration for discernable effects on the dorsal roots was ∼10 −4M. 3. Desensitization to its own effects and those of the other catecholamines occurred following catecholamine application. Recovery from desensitization took up to 60 min. 4. Noradrenaline evoked depolarizations of both roots were antagonized by phentolamine (2 × 10 −5M) and propranolol (10 −4M). There may be both α- and β-receptors on amphibian neurons and axon terminals. 5. 5-Hydroxytryptamine (10 −4M) consistently depolarized dorsal root terminals, usually with a small depolarization of the ventral roots. Higher concentrations elicited pronounced depolarizations in both roots. Desensitization to the effects of 5-hydroxytryptamine was observed with repeated applications. Cross-desensitization between the indole-amine and catecholamines was not observed.