Download ABSTRACT - Thermal Physiology and Pharmacology

Abst. 100
SELECTIVE BRAIN COOLING: A MULTIPLE REGULATORY
MECHANISM
M. Caputa
Department of Animal Physiology, Institute of General and Molecular Biology,
N. Copernicus University, Toruń, Poland, email: [email protected]
The mechanism of selective brain cooling (SBC) allows the brain to remain cooler than the
rest of the body. This paper aims to provide new ideas to better understand SBC, emphasizing
how it works, how it is controlled and what its role is.
Various mechanisms of SBC evolved in birds and in a large variety of mammalian orders,
which makes this physiological response a common ability in homeotherms. There are two
distinct types of SBC: (1) using precooling of arterial blood destined for the brain, with cool
venous blood returning from the nose and head skin, (2) using venous blood to cool the brain
directly. Both SBC systems use powerful effectors of evaporative heat loss.
There is a common mechanism of control of SBC intensity. Reduced sympathetic activity
leads to simultaneous dilation of the angular oculi veins, supplying the intracranial heat
exchangers, and constriction of the facial veins, supplying the heart. Therefore, SBC is
enhanced during heat exposure, endurance exercise, relaxed wakefulness and NREM sleep,
and vanishes in the cold and during emotional distress. Similar to other thermolytic responses,
SBC starts whenever actual body temperature exceeds its set-point value. Accordingly, SBC
is turned, on or off, parallel to the respective immediate decreases or increases in the set-point
of temperature regulation.
SBC may be regarded as a multifunctional effector mechanism, which integrates not only the
thermoregulatory system itself (thereby stabilizing brain temperature) but, paradoxically, also
serves to destabilize brain temperature (thereby modulating e.g. vigilance states). Emotional
turning SBC off, which abruptly warms cerebral emotional centres, might be used to reinforce
the emotional response in a short-term positive feed-back-control of alertness, excitement,
fear and flight-or-fight responses. Moreover, SBC helps animals endure prolonged, extreme
hyperthermia, dehydration, long-distance races (SBC helps delay the onset of fatigue) or
asphyxia (it is used in diving ducks and seals to drop cerebral temperature much below its
normal level, expanding diving capacity and protecting the brain from asphyxic damage). In
case of emergency, such as sudden asphyxia, SBC allows the brain to decrease its temperature
rapidly enough, a result that would not be possible by means of general body cooling.
Altogether, SBC integrates both thermal and nonthermal regulatory functions.
Because emotional inhibition of SBC is a source of errors in SBC research, animals used in
such studies should be perfectly adapted to experimental conditions. On the other hand, too
many uncontrolled variables interfere with SBC in free-ranging animals in their natural
habitat. Therefore, a thorough investigation of SBC might be impossible under such
conditions.