The effects of theophylline and caffeine on thermoregulatory functions of rats at different ambient temperatures

Both systemic and central administration of theophylline and caffeine produced a dose-dependent rise in rectal temperature at ambient temperatures of 8, 22 and 30 degrees C. The hyperthermia in response to either xanthine was brought about by an increase in metabolic heat production. In addition, their administration produced behavioral excitation, cutaneous vasodilation (as estimated by an increase in the foot and tail skin temperatures) and diuresis. There was no change in respiratory evaporative heat loss. Probably, the hyperthermia induced by the two drugs was due to behavioral excitation leading to an increased metabolism at the ambient temperatures studied. Furthermore, either destruction of central catecholaminergic nerve fibres (with 6-hydroxydopamine) or blockade of alpha-adrenergic and dopaminergic (with phentolamine and haloperidol) receptors antagonized the xanthine-induced hyperthermia. The data suggest that these xanthines elicit a central activation of both adrenergic and dopaminergic receptors via release of endogenous catecholamines that leads to behavioral excitation and hyperthermia in rats.

Serotoninergic mechanisms of beta-endorphin- and clonidine-induced analgesia in rats

Both beta-endorphin and clonidine proved to have statistically significant analgesic activity (increase in latency to hind-paw lick in hot plate test) in rats. Furthermore, the pain inhibition induced by beta-endorphin and clonidine could be antagonized by prior treatment of animals with either naloxone (a narcotic antagonist) or the depletors of central serotonin pathways such as 5,6-dihydroxytryptamine, 5,7-dihydroxytryptamine and p-chlorophenylalanine have not effect on latency to hind-paw lick. The data indicate that serotoninergic activity in the brain plays a role in the elaboration or modulation of beta-endorphin and clonidine analgesia in rats.

Effects of gamma-hydroxybutyric acid on metabolic, respiratory and vasomotor activities and body temperature in rats

The effects of gamma-hydroxybutyric acid (GHBA) on metabolic, respiratory and vasomotor activities and body temperature were assessed in unanesthetized rats at three different ambient temperatures (Ta) of 8, 22 and 30 degrees C. Intraperitoneal administration of GHBA produced dose-dependent hypothermia in rats at both 8 and 22 degrees C Ta. At Ta = 8 degrees C the hypothermia was brought about solely by a decrease in metabolic heat production, while at Ta = 22 degrees C the hypothermia was due to both a decrease in metabolic heat production and an increase in cutaneous circulation (as indicated by changes in tail and foot skin temperatures). However, at Ta = 30 degrees C GHBA administration produced no changes in rectal temperature or other thermoregulatory parameters. Respiratory evaporative heat loss was not affected by GHBA application. Furthermore, it was found that the GHBA-induced hypothermia was antagonized by haloperidol (a selective blocker of dopamine receptors), but not by p-chlorophenylalanine (an inhibitor of serotonin synthesis). The data suggest that GHBA elicits a central dopamine receptor activation mainly via release of endogenous dopamine and leads to a hypothermia.