Modifications of arterial baroreflexes: obligatory roles in cardiovascular regulation in stress and poststress recovery.

Despite the physiological importance of arterial baroreflexes as a powerful stabilizer of blood pressure, their functions themselves are not always stable and there are a variety of circumstances in which they are significantly modulated. During stressful conditions, including fight/flight, defense/attack, somatic nociception, visceral nociception, exercise, and mental stress, arterial baroreflexes are generally inhibited. The inhibition is purposeful for achieving dynamic readjustment of circulation needed for the animal's reaction to cope with these conditions. Central sites which are proposed to be involved in the inhibition include the motor cortex, amygdala, hypothalamus, dorsolateral part of the periaqueductal gray matter, parabrachial nucleus, cerebellar vermis, etc. On the other hand, arterial baroreflexes are occasionally facilitated, during sleep, following endurance exercise, etc. The facilitation favors restoration of energy exhausted during a stressful phase in which the animal reacts actively to changing environment. The central sites proven to elicit the facilitation are the medial prefrontal cortex, the preoptic/anterior hypothalamus, the ventrolateral part of the periaqueductal gray matter, and the nucleus raphe magnus. The inhibition and facilitation of arterial baroreflexes, which probably occur alternatively, are essential mechanisms supporting reactions to stressful conditions and poststress recovery, respectively. This review describes when, how, and why the arterial baroreflexes are so modulated.