An Analysis of Adrenergic Influences on the Sural-gastrocnemius Reflex of the Decerebrated Rabbit

The sural-gastrocnemius reflex was observed in decerebrated rabbits during intrathecal application of four alpha-adrenoceptor antagonists. Idazoxan and yohimbine, which are antagonists at the alpha 2-receptor, caused facilitation of the reflex, although idazoxan was more potent and produced a larger overall increase in the reflex response. However, when given after yohimbine, idazoxan elicited no further increase in reflex responses. The differences between the two drugs may result from the interaction of yohimbine with receptors for 5-hydroxytryptamine. The selective alpha 1-receptor antagonist prazosin had no consistent effects when given alone, but reduced the facilitatory effects of idazoxan. The putative selective post-junctional alpha 2-receptor blocker SK&F 104078 had no significant effects when given alone, nor did it influence the facilitatory action of a subsequent dose of idazoxan. Section of the spinal cord in the presence of idazoxan always caused a decrease in gastrocnemius responses to sural nerve stimulation. These data show that the facilitatory effects of idazoxan are almost certainly mediated at the spinal cord and that they do not involve blockade of alpha 1-receptors. It appears that idazoxan acts by blockade of adrenergic descending inhibition in combination with increased descending facilitation. The inhibition is probably mediated through noradrenaline acting at alpha 2-receptors, and the facilitation may be the result of release of noradrenaline (acting at alpha 1-receptors) and 5-hydroxytryptamine in the spinal cord.