SENSITIVITY OF THE NICTITATING MEMBRANE AFTER VARIOUS PROCEDURES AND AGENTS

¹ Department of Pharmacology, Harvard Medical School, Boston, Massachusetts

The sensitivity of the nictitating membrane of the spinal cat to norepinephrine, tyramine and acetylcholine was determined after various procedures known to affect the sensitivity and/or the norepinephrine content of this organ. The epinephrine and norepinephrine content of various organs (heart, aorta, iris, nictitating membrane and adrenal medulla) was determined after some of these procedures.

No relation was found between the sensitivity to norepinephrine and the norepinephrine content of the nictitating membrane. Supersensitivity to norepinephrine appeared whenever, for a period of 7 to 14 days, the nictitating membrane was deprived of the usual influence of tonic impulses. This was observed after chronic denervation, chronic decentralization, 7 days of pretreatment with reserpine, TM 10 or chlorisondamine. Depletion of brief duration (caused by pretreatment with a single large dose of reserpine) did not cause supersensitivity.

Supersensitivity to intraarterial injections of acetylcholine paralleled that to norepinephrine after the various procedures employed.

Subsensitivity to tyramine was observed after all procedures which resulted in depletion of the norepinephrine stores (denervation and both types of reserpine-pretreatment), whereas supersensitivity to this substance was found when the norepinephrine stores were normal or only slightly affected (decentralization, TM 10 and chlorisondamine). This provides further support for the view that tyramine acts by liberating endogenous norepinephrine.

In contrast to treatment with an intravenous injection of cocaine, prolonged pretreatrnent with reserpine does not increase the plasma half-life of injected norepinephrine.