PSYC202 Foundations of Cognitive Neuroscience – Week 15b: Motivated behaviour:
Hunger and thirst
In this lecture, we will consider some of the mechanisms involved in controlling food and
water intake. Thirst will be considered briefly, mainly as a way of demonstrating a simpler
homeostatic (self-regulating) system relative to hunger and eating. The differences between
the two systems will be highlighted, as a possible way of understanding deregulation of
eating, currently a very pressing health problem, with 25% of the UK population considered
obese (Public Health England data until 2012). It is no accident that these topics are within
the broader set of lectures on motivation and decision-making, as there is clear overlap in the
brain systems controlling eating and drinking with other reward-related stimuli.
Thirst and drinking
In any homeostatic system, we need to understand which aspects of the system are sensed
(for a boiler, this would be temperature for example), which system detects the change (the
thermostat), and how behaviour is initiated to correct the system to optimal levels (the boiler
acting through radiators). [slide 4]There are two aspects of the level of fluid in the body that
are monitored as signals for thirst. The first is the level of fluid inside cells (making up two
thirds of total fluid levels; Rolls, 2005), and the second is the level of fluid in the blood,
sensed indirectly through blood pressure and salt levels. [slide 5 & 6] The term used for the
system that monitors the level of fluid inside cells is osmotic thirst, and Blass and colleagues
(Blass & Epstein, 1971) showed in the rat that an area of the hypothalamus, called the lateral
preoptic area is involved in sensing this intracellular fluid level (by sensing the level within
the neurons themselves). Just altering fluid balance by micro-injection of saline in that area
initiates drinking behaviour, and lesions disrupt drinking. The hypothalamus (just below and
in front of the thalamus) is approximately the size on an almond in humans, and consists in a
collection of nuclei and areas, involved in diverse homeostatic functions via sensing, and
communicating with, the endocrine (hormonal) system. The lateral preoptic area is also
involved in temperature regulation (Kalat, 2012).
The term used for the sensing of fluid levels in the blood is hypovolemic thirst (with hypo =
low, and volemic = blood volume). The areas responsible for detecting blood pressure and
fluid levels are around the third ventricle in the brain, in particular two areas called the
subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT). The
latter area receives information about blood pressure from the branches of the vagus nerve
coming from the heart. Both areas are also able to sense blood concentration via a peptide
released by the kidneys called angiotensin. Normally, angiotensin would not be able to cross
the blood-brain barrier (the walls of capillaries are much less permeable in the brain than they
are in the body, to ensure that pathogens, for example bacteria, are not able to attack brain
cells), but these specialised regions around the third ventricle have normal capillaries and are
outside the blood-brain barrier. A micro-dose of angiotensin administered in these areas
promotes drinking (Epstein, Fitzsimons & B. J. Rolls, 1970).
These signals that sense the need for drinking transmit information to areas of the brain we
are now familiar with, involved with reward related behaviour, such as the orbitofrontal
cortex and the ventral striatum. De Araujo et al. (2003) measured fMRI responses in human
orbitofrontal cortex that correlated with pleasantness ratings of water, as a function of how
thirsty/sated participants were. Thus these represent reward value for the water stimulus in
the mouth. It is interesting to note that the “taste” of water probably derives from a collective
1
Hunger and thirst
In this lecture, we will consider some of the mechanisms involved in controlling food and
water intake. Thirst will be considered briefly, mainly as a way of demonstrating a simpler
homeostatic (self-regulating) system relative to hunger and eating. The differences between
the two systems will be highlighted, as a possible way of understanding deregulation of
eating, currently a very pressing health problem, with 25% of the UK population considered
obese (Public Health England data until 2012). It is no accident that these topics are within
the broader set of lectures on motivation and decision-making, as there is clear overlap in the
brain systems controlling eating and drinking with other reward-related stimuli.
Thirst and drinking
In any homeostatic system, we need to understand which aspects of the system are sensed
(for a boiler, this would be temperature for example), which system detects the change (the
thermostat), and how behaviour is initiated to correct the system to optimal levels (the boiler
acting through radiators). [slide 4]There are two aspects of the level of fluid in the body that
are monitored as signals for thirst. The first is the level of fluid inside cells (making up two
thirds of total fluid levels; Rolls, 2005), and the second is the level of fluid in the blood,
sensed indirectly through blood pressure and salt levels. [slide 5 & 6] The term used for the
system that monitors the level of fluid inside cells is osmotic thirst, and Blass and colleagues
(Blass & Epstein, 1971) showed in the rat that an area of the hypothalamus, called the lateral
preoptic area is involved in sensing this intracellular fluid level (by sensing the level within
the neurons themselves). Just altering fluid balance by micro-injection of saline in that area
initiates drinking behaviour, and lesions disrupt drinking. The hypothalamus (just below and
in front of the thalamus) is approximately the size on an almond in humans, and consists in a
collection of nuclei and areas, involved in diverse homeostatic functions via sensing, and
communicating with, the endocrine (hormonal) system. The lateral preoptic area is also
involved in temperature regulation (Kalat, 2012).
The term used for the sensing of fluid levels in the blood is hypovolemic thirst (with hypo =
low, and volemic = blood volume). The areas responsible for detecting blood pressure and
fluid levels are around the third ventricle in the brain, in particular two areas called the
subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT). The
latter area receives information about blood pressure from the branches of the vagus nerve
coming from the heart. Both areas are also able to sense blood concentration via a peptide
released by the kidneys called angiotensin. Normally, angiotensin would not be able to cross
the blood-brain barrier (the walls of capillaries are much less permeable in the brain than they
are in the body, to ensure that pathogens, for example bacteria, are not able to attack brain
cells), but these specialised regions around the third ventricle have normal capillaries and are
outside the blood-brain barrier. A micro-dose of angiotensin administered in these areas
promotes drinking (Epstein, Fitzsimons & B. J. Rolls, 1970).
These signals that sense the need for drinking transmit information to areas of the brain we
are now familiar with, involved with reward related behaviour, such as the orbitofrontal
cortex and the ventral striatum. De Araujo et al. (2003) measured fMRI responses in human
orbitofrontal cortex that correlated with pleasantness ratings of water, as a function of how
thirsty/sated participants were. Thus these represent reward value for the water stimulus in
the mouth. It is interesting to note that the “taste” of water probably derives from a collective
1
