Prostaglandin fevers in rats: regulated change in body temperature or change in regulated body temperature?

Experiments examining the effects of central injections of E-series prostaglandins (PGE) on body temperature have only been done in the light part of a light-dark cycle. The present experiments examined the characteristics of fevers in rats after intraventricular PGE2 injections in both light and dark in a 12:12 h photoperiod. In the light, the change in body temperature (Tb) after 0.5 microgram was not significantly different from the change after vehicle injection. After injection of PGE2 (1, 2, 4, and 8 micrograms), Tb rose in a dose-dependent fashion. Mean initial Tb in the light was 36.4-36.6 degrees C. Tb rose a mean of 1.5 degrees C after 1 microgram, 1.9 degrees C after 2 micrograms, 2.7 degrees C after 4 micrograms, and 3.5 degrees C after 8 micrograms PGE2. A dose of 16 micrograms gave almost identical results as 8 micrograms. In the dark, mean initial Tb was 37.4-37.7 degrees C. Tb rose less than 0.8, 1.1, 1.4, and 2.3 degrees C after 1-8 micrograms PGE2, respectively. Thus there were two distinct dose-response curves for day and night. Nevertheless, peak Tb values attained in the two conditions were not significantly different from each other at any given dose. These results show that a particular dose of PGE2 raises Tb to a particular level, largely independent of either the Tb at the time of the injection or the phase of the light-dark cycle. However, the change in Tb at any dose depends strongly on initial Tb. Therefore, we urge researchers in the pharmacology of thermoregulation to report initial and final Tb values as well as changes in Tb.