Intracerebroventricular Injection of Rats: A Sensitive Assay Method for Endogenous Pyrogen Circulating in Rats 1

Abstract Intracerebroventricular (icv) injection of endogenous pyrogen (EP) into rats causes dose-related fever. To compare this procedure with the usual assay method, iv injection into rabbits, a preparation of crude rabbit EP was titrated by both methods. The rat icv injection procedure gave results similar in linearity and magnitude of response to the conventional rabbit iv method, but was found to be at least 50-fold more sensitive. The icv assay worked for human as well as rabbit EP and for partially purified as well as crude preparations. Plasma from human or animal sources did not interfere. The rat icv and rabbit iv assay methods were also compared in their ability to detect a pyrogen in filter-sterilized plasma from rats infected with the live vaccine strain (LVS) of Francisella tularensis. The rat icv method gave fevers of 1° with 0.05-ml samples of the filtered plasma from infected rats, but no fever with plasma from control rats. The pyrogen was partially purified from the infected rat plasma. It was found to be heat labile and nondialyzable, and was tentatively identified as rat EP. The less sensitive rabbit assay was unable to detect the plasma pyrogen, using 0.05- and 1.00-ml samples. These observations suggest that icv injection into rats is a useful and sensitive assay method for EP, particularly when only small volumes are available for testing.