OBSERVATIONS ON THE ANALGESIC EFFECT OF MORPHINE DURING CONTINUED DAILY ADMINISTRATION OF SMALL AND UNIFORM DOSES TO DOGS

¹ From the Department of Physiology, Northwestern University Medical School, and the Northwestern University Dental School, Chicago

Eight dogs were each given 3.0 mg. of morphine sulfate per kilogram body weight subcutaneously daily for fifty days. General reactions were observed and changes in tooth-pain thresholds were determined by electrical stimulation. The tooth-pain thresholds were determined for twelve weeks before and after the period of morphine administration.

Reactions such as vomiting, salivation and defecation were reduced after the first few injections and thereafter varied irregularly.

Elevation of pain threshold by the morphine was at first marked, becoming less on successive injection and then again greater, the effect tending to vary periodically. The pre-administration pain threshold tended to be higher for the group during the period of daily administration of morphine.

It is concluded that continued daily administration of a uniform dose of morphine produces tolerance to the analgesic effect of morphine in the dog but that the tolerance is not lasting, disappearing and reappearing periodically as daily administration is continued.