A system for measuring the noradrenaline receptor contribution to the flexor reflex

A system is described for recording psoas muscle contractions in response to stimuli delivered to the sole of the foot of the acute spinalized unanaesthetized rat. Clonidine, an alpha adrenergic receptor agonist, caused a major increase in this flexor reflex amplitude. The increase was blocked by phenoxybenzamine at a dosage level of 20 mg/kg which did not, of itself, reduce flexor reflex amplitude. Older, larger rats responded significantly more forcefully to clonidine than did younger, smaller rats. However, these differences between rats of different size tended to disappear when the data were expressed in terms of 0/0 of original baseline amplitude. L-dopa, 75 mg/kg, increased the flexor reflex amplitude more so than did L(+)erythro dihydroxyphenylserine (DOPS) at twice the disage in rats pretreated with reserpine and nialamide. Other DOPS isomers were less effective, and L(-)threo DOPS was toxic. These results support the view that spinal noradrenaline receptors exist that increase the psoas flexor reflex. The advantages of the present system are twofold: first, the results may be quantitated; second, small amounts of potential central noradrenaline receptor agonists and antagonists may be tested for their effect on the flexor reflex using relatively few unanaesthetized rats.