Central thermoregulatory effects of lactate in the toad Bufo paracnemis

Hypoxia induces a regulated decrease in body temperature (Tb; anapyrexia) in organisms ranging from protozoans to mammals, but very little is known about the mechanisms involved. Several candidates have been suggested to mediate hypoxia-induced anapyrexia, among them lactate, which is a classical companion of hypoxic stress in vertebrates. The present study was designed to assess the central thermoregulatory effects of lactate in Bujo paracnemis. Toads equipped with a temperature probe were tested over a thermal gradient (10–40°C). Lactate injected systemically (4.0 mmol kg⁻¹) caused a significant reduction of Tb from 24.6±2.1 to 17.4±3.9°C. To assess the role of central thermoregulatory mechanisms, a lower dose (0.4 mmol kg⁻¹) of lactate was injected into the fourth cerebral ventricle or systemically. Intracerebroventricular injection of lactate caused a similar decrease in Tb, whereas systemic injection caused no change. The data indicate that lactate may play a role in hypoxia-induced anapyrexia in central rather than peripheral sites.