Biopsychology Midterm 2: Thirst

Homeostasis

the maintenance of a relatively constant internal physiological environment despite ever-changing internal and external stimuli

• Commonly referred to as separate “drives” where we have to fight to satisfy these drives once their set point is met– tightly linked with motivation

• Difficult to maintain

  • Simply existing, even in a stable room challenges our body as a result of obligatory losses (the body is perpetually drained of salt, liquid, food…)

• There are two primary forms of feedback that regulate our physiology– negative and positive feedback

Negative Feedback— stimulus, sensor, controller, effector

• The ability of a system to be sensitive to its output to suppress future output 

• Stimulus: the variable that is changing

• Sensor: monitors the values of the variable

• Controller: contrasts data with normal values, the set point and determines if change is needed

• Effector: organ, gland muscle, other structure that acts on signal from control center

Allostasis

the behavioral and physiological adjustments an individual makes to maintain optimal (as opposed to unchanging) function of a regulated system in the face of current or anticipated environmental stressors

Ex: heart racing before a stressful event

Hypothalamus— 6 things it controls

1. Energy expenditure (feeding, digestion, metabolism)

2. Blood pressure and electrolyte composition (drinking, salt)

   1. Variations in the salt concentration of our bodily fluids is lethal

   2. Almost every organism have homeostatic mechanisms that keep the concentration of salt in their bodies close to that of sea water 

3. Reproduction (hormonal regulation)

4. Body temperature

5. Emergency responses– stress hormones

6. Circadian rhythms and sleep

Injecting small amounts of salt directly into the hypothalamus induces water drinking in otherwise stated animals

Osmosis

 the movement of water molecules that occurs to equalize the concentration of two solutions across a semipermeable membrane– usually flow of water through channels called aquaporins

Osmolality

• the concentration of salt in our bodies extracellular fluid is 0.9%

  • 0.9% salt in water is saline

Isotonic

Solutions that are as salty as saline are called isotonic

Hypertonic

more salty than the body

Hyptonic

Less salty than the body

Osmotic pressure

physical force that pushes or pulls water across a membrane to equalize its osmotic gradient

What are the two types of thirst?

Osmotic and hypovolemic

Osmotic thirst

• a high extracellular concentration of solute, salt, drives water intake

  • Neurons in the brain detect the increase saltiness of extracellular fluid

  • A change in the balance of water to salt pulls water out of intracellular compartment

Hypovolemic thirst

• low extracellular volume due to the loss of bodily fluids 

  • Baroreceptors in major blood vessels detect any pressure drop from fluid loss

    • Baroreceptors line the blood vessels and heart and signal changes in blood pressure

    • Baroreceptors signal to the brain to induce fluid intake– drops in blood pressure cause the heart to make less atrial natriuretic peptide

  • Hypovolemic thirst is driven by a loss of Ifuioid and not a change in the actual concentration of fluid in the body 

Osmosensory neurons

• neurons that specifically monitor for the concentration of extracellular fluid– are spread through the hypothalamus, detect salt levels

  • Have been found in the anterior hypothalamus, the preoptic area, the supraoptic nucleus

  • Highly concentrated in the circumventricular organs

Subfornical Organ

• Subfornical glutamatergic neurons promote thirst— led to drinking water

• SFO GABA neurons suppress thirst

• Consuming water rapidly suppresses SFO activity

  • The basal activity of SFO neurons increases as thirst increases, and the consumption of water, the restoration of normal osmolality, returns SFO activity to the baseline

• Subfornical neurons directly sense and are altered by the osmolality of extracellular fluid

SFO Glutamate

• Glutamate neurons in the SFO were made to express either Channelrhodopsin (ChR2) or GFP

• The experimenter could then turn on these glutamate producing SFO neurons by shining blue light on them→ led to drinking of water

• Experiencing thirst by directly activating SFO glutamate neurons is aversive

  • Evidence that these neurons cause water intake and engage the psychological state of thirst

SFO GABA

• Turning on these neurons stop water drinking in thirsty mice

Cold Water

• Researchers compared water consumption of different temperatures–cold works best

• Cold metal on the tongue turned down SFO activity 

Importance of salt regulation

• We aren’t able to effectively get rid of excess salt fast

• Salt excretion is governed by the kidneys and is controlled by the hormone aldosterone (secreted by the adrenal glands– tells the kidney to retain salt (more aldosterone means we are needing to consume salt))

  • Marine birds compensate for drinking seawater by having salt glands that pull excess salt out of plasma and release it out the nostrils

Dehydration sensing in the brain

• As blood pressure drops, neurons in the hypothalamus release vasopressin into the bloodstream via the posterior pituitary 

• Vasopressin causes the restriction of blood vessels and instructs the kidneys to slow fluid output to the kidney

• Vasopressin naturally increases at night during sleep because it suppresses urine production and prevents dehydration

Distinct cortical areas associated with thrist in humans

• Humans scanned in an MRI show increased bloodflow to two cortical regions selectively for thirst– the anterior cingulate cortex (a prefrontal cortical area) and the insula (which holds the gustatory cortex)

Oral experience and thirst

• If you do an experiment where you deliver water directly into the stomach– bypassing the mouth and throat– it isn’t as reinforcing or “good”

• Oral experience plays a role in the experience of thirst suggesting some sort of sensor for water or osmolality in the mouth and throat

Circumventricular organs (the ones to know)

Ventricular system and subfornical organ

Subfornical Sensing of Extracellular fluid

• SFO neurons directly sense and are altered by the osmolality of extracellular fluid

  • Researchers recorded SFO neurons using calcium imaging (calcium is increased as neurons fire– more calcium=more neural activity), we express a fluorescent sensor for calcium in neurons and record how bright it is as a proxy for neural activity 

  • Injections of salt into the mice immediately caused activation of the SFO

• Consuming water rapidly suppresses subfornical activity