Thirst is the sensation of needing and/or wanting to drink. It has
been variously described as “the perception of one's need for drink”
[11], as “the consequence of the need to moisten the mouth” [9] and as
“a physiological state linked to fluid deficit” [12]. Physiologically, thirst is part of the mechanism that controls osmolality or volume of the
extracellular liquid, and is mainly regulated by the brain in the
hypothalamus [13], however other associated and peripheral organs
also play a role (reviewed by Sewards and Sewards [14]). Thirst and/
or drinking can be triggered by increases in osmolality, hypovolemia
or hypotension, which are signals of fluid depletion in the body.
Osmoreceptors present in the brain and the periphery (kidneys) and
baroreceptors present in the cardiopulmonary system and arterial
walls can detect changes in osmolality or volume and transmit a
neural input to the brain. Sodium channels [15] and cation
transporters [16] have been implicated in this signalling. Osmo- and
baroreceptors are innervated by neurons terminating in the nucleus of
the solitary tract (NST) in the brain or adjacent regions connecting
with the NST, which in turn connect with the hypothalamus. In
addition to this neural pathway, hyperosmolality, hypovolemia or
hypotension induces the secretion of fluid-regulating hormones into
the bloodstream. Thus, angiotensin II is released after activation of
renin (secreted by the kidney in response to central nervous system
activation) and is the main hormone involved in stimulating drinking.
Another hormone, vasopressin is synthesized in the paraventricular
and supraoptic nuclei of the hypothalamus and is secreted through the
pituitary gland into the bloodstream to stimulate water intake. The
action of both hormones seems to be additive (reviewed by [14]). The
secretion of these hormones is the first line of defense against fluid
depletion. This line seems to exist on top of the neural pathway to
sustain a stronger response, whichwould result in awareness of thirst.
Angiotensin and vasopressin cannot cross the blood brain barrier
though.