Circadian variability of body temperature responses to methamphetamine

Vital parameters of living organisms exhibit circadian rhythmicity. Although rats are nocturnal animals, most of the studies involving rats are performed during the day. The objective of this study was to examine the circadian variability of the body temperature responses to methamphetamine (Meth). Body temperature was recorded in male Sprague-Dawley rats received Meth (1 or 5 mg/kg) or saline at 10am or 10pm. The baseline body temperature at night was 0.8ºC higher than during the day. Both during the day and at night 1 mg/kg of Meth induced monophasic hyperthermia. However, the maximal temperature increase at night was 50% smaller than during the daytime. Injection of 5 mg/kg of Meth during the daytime caused a delayed hyperthermic response. In contrast, the same dose at night produced responses with a tendency towards a decrease of body temperature. Using mathematical modeling, we previously showed that the complex dose-dependence of the daytime temperature responses to Meth results from an interplay between inhibitory and excitatory drives. Here, using the model, we explain the suppression of the hyperthermia in response to Meth at night. First, we found that the baseline activity of the excitatory drive is greater at night. It appears partially saturated, and, thus, is additionally activated by Meth to a lesser extent. Therefore, the excitatory component causes lesser hyperthermia or becomes overpowered by the inhibitory drive in response to the higher dose. Second, at night the injection of Meth results in reduction of the equilibrium body temperature leading to gradual cooling counteracting hyperthermia.

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