Brown fat temperature decrease by electrical stimulation of in and around retrorubral field in the golden hamster

Hypothalamic or Extrahypothalamic Modulation and Targeted Temperature Management After Brain Injury

Targeted temperature management (TTM) has been recognized to protect tissue function and positively influence neurological outcomes after brain injury. While shivering during hypothermia nullifies the beneficial effect of TTM, traditionally, antishivering drugs or paralyzing agents have been used to reduce the shivering. The hypothalamic area of the brain helps in controlling cerebral temperature and body temperature through interactions between different brain areas. Thus, modulation of different brain areas either pharmacologically or by electrical stimulation may contribute in TTM; although, very few studies have shown that TTM might be achieved by activation and inhibition of neurons in the hypothalamic region. Recent studies have investigated potential pharmacological methods of inducing hypothermia for TTM by aiming to maintain the TTM and reduce the shivering effect without using antiparalytic drugs. Better survival and neurological outcome after brain injury have been reported after pharmacologically induced TTM. This review discusses the mechanisms and modulation of the hypothalamus with other brain areas that are involved in inducing hypothermia through which TTM may be achieved and provides therapeutic strategies for TTM after brain injury.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Influence of the hypothalamus on the midbrain tonic inhibitory mechanism on metabolic heat production in rats

Influence of the hypothalamus on increased body temperature was examined in male rats. Body temperature was increased by removing the midbrain tonic inhibitory mechanism (TIM) on heat production from brown adipose tissue (BAT) by microinjections of a local anesthetic, procaine, into the midbrain. Procaine microinjections in unanesthetized rats increased rectal temperature that was followed by a strong tail skin temperature rise. Procaine microinjections in unanesthetized and decerebrated rats also increased rectal temperature but without skin temperature rise. These decerebrated animals fatally developed hyperthermia. In anesthetized rats, procaine microinjections increased temperature of the interscapular BAT (IBAT) higher with shorter onset for temperature rise than rectal temperature. Increased IBAT temperature by procaine microinjections in anesthetized rats was attenuated during hypothalamic warming, and enhanced during hypothalamic cooling when compared with that observed during thermoneutral hypothalamic temperature. These results suggest that the midbrain TIM is able to function in unanesthetized conscious rats, and that the integrity of the midbrain mechanism to tonically inhibit metabolic heat production does not require the presence of intact hypothalamus. These results also suggest that the hypothalamus modulates directly or indirectly IBAT heat production that was induced by removal of the midbrain TIM.

Metabolic responses of chicken and muscovy duck embryos to high incubation temperatures

Oxygen consumption, heat production (HP) and core temperature (T(af)) were measured over 3 h in 20-34-day-old Muscovy duck and 12-21-day-old chicken embryos at ambient temperature (T(a)) of 37.5 degrees C and thereafter for 3 h at T(a) of 39.0 degrees C. At 37.5 degrees C T(a), HP increased with age in avian embryos of both species, following an exponential function. In muscovy duck embryos, a plateau phase occurred between D29 and D32; in chicken embryos, a similar plateau occurred between D19 and D21. T(af) rose in accordance with HP, and the relationships between T(af) and HP could be described by significant linear regressions in both species. Mostly, HP increased in embryos of both species during heat load, but by less than calculated by the van't Hoff rule; however, there was often also a decrease in HP under these conditions. Obviously, in avian embryos high T(a) causes a down-regulation of HP mediated by active thermoregulatory mechanisms. This is in agreement with data describing the influence of hyperthermia on HP in the postnatal period of birds and mammals. Because of this, the term 'second chemical thermoregulation' defined by Gelineo [C. R. Soc. Biol. (1936) 122 337] for birds and mammals should also be used for avian embryos.