Effects of lipopolysaccharide, antipyrogen, and body condition on the chronopharmacology of fever in Dipsosaurus dorsalis.

Abstract

I examined the effects of two doses (2.5 mg kg-1 and 25 mg kg-1) of a pyrogen (lipopolysaccharide, LPS) independently and in combination with an antipyrogen (acetylsalicylic acid) at two times of injection (noon and midnight) on behavioral thermoregulation of adult desert iguanas in linear thigmothermal gradients (3.60 +/- 0.19--75.91 +/- 1.14° C). I also described some aspects of the basic febrile response (latency period and duration of the response), the chronopharmacology of the febrile response, and the effects of morphological parameters on thermoregulation.

After acclimation for 4 to 5 days at 30° C with a 12:12 LD photoperiod, I recorded body temperatures in lizards that received a low or high dose of pyrogen either in the absence or presence of an antipyrogen, and whether at noon or midnight. Overall mean Tb for 48 hours after injection, for day 1, and for day 2 from these animals were compared with each other and to the Tb of control animals to elucidate the effects of these agents and time of injection on thermoregulation. Variance in Tb for these groups was analyzed to compare thermoregulatory precision under the influence of pyrogen dose, time of injection and presence or absence of an antipyrogen. Skewness of Tb response was compared among all treatment groups to compare latency period. Kurtosis of Tb response was compared among all treatment groups to compare response duration. Comparisons of Tbs within each treatment group against initial and final mass, initial and final body condition, and change in body mass revealed an influence of energy reserves on thermoregulatory decisions.

Dose and time of injection in the presence of only pyrogen affected Tb for the total run period and day 2. Animals receiving the high dose had Tbs higher than lizards receiving the low dose. Lizards injected at noon had higher Tbs than lizards injected at midnight. All lizards that received an antipyrogen with the pyrogen exhibited Tbs similar to the Tbs of the controls on day 1 and the total run period. On day 2, lizards receiving pyrogen + antipyrogen showed a dose effect.

Time of injection affected whether or not energy reserves are the most important factor determining Tb in the face of a pyrogen + antipyrogen. Lizards injected with the low dose of pyrogen + antipyrogen at midnight exhibited positive correlations between Tb and body condition, mass, and SVL at various times during the trial. Lizards injected with the high dose of LPS and antipyrogen at midnight showed positive correlations between Tb and SVL, and lizards injected with the high dose of LPS at midnight showed positive correlations between Tb and mass change. Lizards subjected to the control treatment exhibited positive correlations between Tb and mass and body condition.

Animals that received the high dose of pyrogen + antipyrogen at midnight had higher thermoregulatory precision than those injected at noon on day 1. Control animals had higher precision than those injected with the high dose of pyrogen + antipyrogen and those injected with the high dose of pyrogen only on day 1. Midnight injections produced longer responses than did noon injections, and higher doses induced faster responses.