Hypothalamus and Homeostasis

PSY391S

March 13, 2006

John Yeomans

Hypothalamus Functions

• “We are the accumulation of mechanisms that allowed our ancestors to survive and reproduce their genes” (Sagan)

• Hypothalamus is an accumulation of tiny nuclei for survival and reproduction.

• Hormones and neural signals to and from viscera.

• Motivated behaviors to respond to internal challenges.

• Homeostasis maintains internal environment. Reproduction changes environment.

Hormonal and Neural Signals• Hypothalamus controls pituitary hormones

acting on many organs.

• Hypothalamus receives hormonal inputs by way of circumventricular organs.

• Hypothalamus coordinates neural and hormonal signals for visceral control (Autonomic NS) and motivated behaviors.

• Hypothalamus receives neural inputs from descending limbic/olfactory systems and ascending taste/visceral systems.

Posterior Pituitary

Hormones to Brain

• Blood-brain barrier (glia surrounding brain vessels) prevents entry of large molecules in most brain regions.

• Leaky capillaries in 6 tiny nuclei near ventricles allow entry of hormones.

• Circumventricular organs have receptors for many circulating hormones (e.g. leptin, angiotensin II, gonadal hormones).

Circumventricular Organs

Arcuate N.Posterior Pituitary

Subcommissuralorgan

Endotherms

• Birds and mammals control body temperature near 37ºC.

• Advantages: Constant physiology, better brain function, bigger brains, and adaptation to extreme environments.

• Costs: More insulation needed (fat, feathers, fur), more energy needed, more internal controls.

Temperature Control Systems

Temperature

Hypothalamus maintainstemperature best.

Fluid Regulation

• Body made up of ~65% H2O, for cell chemistry, and for movement of nutrients and cells (e.g. blood and lymph).

• Salt concentration maintained at 0.9%.

• Intracellular and extracellular compartments separated by semipermeable membranes that control ions and other chemicals.

Water and Salt Regulation

Thirst

Water Loss:respiration,sweating,urination,defecation,bleeding.

Salt loss.

Water Intake:drinking,feeding,

Salt content.

Renin-Angiotensin

Kidney

Blood

Adrenals

14 AA

10 AA

8 AA

7AA

Neural controlof blood pressure Hormonal control of

blood pressure and tonicity

Behavioral controlof water and salt.

Vasoconstriction &H20 reuptake in kidney

Energy Sources

• Carbohydrates-->Glucose• Proteins-->Amino acids• Fats-->Fatty acids• These are stored in the body:• Fat cells (long-term) 80-90% of total• Muscle and liver glycogen (starch)• Blood glucose, fatty acids and amino acids.

(short-term)• Ketones used if blood glucose low.

Insulin and Glucagon

• Pancreas hormones made by beta and alpha cells of Islets of Langerhans.

• High blood glucose activates insulin; low blood glucose activates glucagon.

• Insulin activates transport of glucose into body cells (not brain).

• Glucagon converts glycogen into glucose.

• This keeps blood glucose levels stable.

Brain Energy

• Uses glucose and O2 only (Ketones in starvation).

• Does not need insulin so always gets glucose.

• Brain uses over 20% of glucose and O2.

• Fainting helps brain get these when blood pressure drops.

Feeding and Obesity

John Yeomans

PSY391S

March 15, 2006

Hypothalamus and Feeding

Brain Lesions and Stimulation

• LH and PVN lesions--> less eating; VMH and arcuate lesions--> more eating.

• LH stimulation-->eating; VMH-->aversion.

• PVN-->NPY and NE increases feeding; 5HT decreases feeding.

• Therefore, LH and PVN for feeding, VMH for satiety.

ob/ob or db/db mice

- (-/-) (+/+)

Leptin and Obesity

• ob/ob and db/db mice are obese, and eat fats as if starving.

• ob/ob mice have mutation in leptin gene.

• Leptin is peptide produced in fat cells in proportion to size.

• db/db mice have mutation in leptin receptor gene.

• Leptin receptors in arcuate n. and LH.

Short-Term Feeding Signals• Taste and smell (accept or reject).• Stomach--ghrelin, distension, vomiting (area

postrema).• Intestines, pancreas, gall bladder--hormones

(CCK, insulin, PYY3-36),• Liver-->vagus to N. Solitary Tract, area

postrema-->parasympathetic and sympathetic.• Hypothalamus--hormone receptors, neural

systems, motivated behaviors.

Human Obesity

• Starvation-->hunger, depression and poor health. Exercise healthier, but harder.

• Surgery--liposuction, stomach resection. Still hungry.

• Leptin insensitivity. Leptin is high, but hypothalamus doesn't respond.

• Serotonin (fenfluramine, SSRIs) works, but has side effects.