Abstract

The modification by various substances of the dose-response curve of tyramine was studied in spinal cats; recorded were the nictitating membrane, the blood pressure and the heart rate.

Pretreatment of short duration with reserpine (previously found to cause no supersensitivity to norepinephrine) reduced the effect of tyramine; the mode of action of reserpine was that typical for an "unsurmountable" antagonist. After pretreatment with reserpine the response of the nictitating membrane to tyramine was significantly related to the response of the nictitating membrane to nerve stimulation.

The main effect of injections of increasing amounts of cocaine was a graded parallel shift of the dose-response curve of tyramine to the right; the mode of action was that typical for a "surmountable" antagonist.

2,6-Xylyl ether of choline (TM 10), morphine and methadone, given in amounts sufficient to reduce or abolish the response of the nictitating membrane to nerve stimulation, failed to reduce or abolish the response to tyramine.

The phenomenon of subsensitivity to tyramine can be experimentally separated from that of supersensitivity to norepinephrine; it is likely that two different mechanisms are responsible for the two phenomena. The evidence is compatible with the view that tyramine liberates norepinephrine from tissue stores, and a scheme is discussed which accounts for the available evidence concerning norepinephrine storage sites.