-Substances produced in one body region that
have effects in some other region: chemical messengers
Examples of animal hormones
• Endocrine hormones: produced in specialized endocrine (‘ductless’) glands, distributed in blood
• Neurotransmitters, neurohormones: released from nerve cells directly onto other cells (neurotransmitters) or into blood (neurohormones)
• “local messengers:” not produced in specialized glands; tend to have local effects on nearby cells -- paracrine signaling (histamines, prostaglandins, nitrous oxide, cytokines, etc.)
• species recognition
• group recognition
• territory marking
• social interactions (aggression, cooperation, alarm, etc.)
• reproduction (sex recognition, attractants, receptivity indication, fertility control, etc.)
• parent offspring interactions
• Homeostatic regulation, especially blood-born substances (aldosterone, vasopressin, insulin, glucagon, leptin)
• Emergency responses ("fight or flight"), in concert with the nervous system to prepare for emergencies (epinephrine)
• Development, growth, maturation (growth hormones in vertebrates; molting hormones in arthropods)
• Reproduction: maturation and functioning of gonads and other reproductive organs, secondary sexual characteristics, reproductive and parental behavior (testosterone, estrogens, progesterone, prolactin, oxytocin...)
• Biological rhythms, including circadian, monthly, or annual rhythms. These are often mediated by hormone activity (melatonin, etc. )
• proteins or short polypeptides
• modified amino acids
• lipid-derived hormones
5-100 amino acids (e.g. insulin, growth hormone, vasopressin, endorphins, leptin) or glycoproteins (FSH, lutenizing hormone). A few amino acid substitutions can produce hormones with very different effects (e.g., oxytocin and vasopressin).
Amine hormones, such as catecholamines (epinephrine, norepinephrine), histamine, melatonin, serotonin, thyroid hormone (contains iodine)
steroids: corticosteroids involved in homeostasis and stress responses (aldosterone, corticosterone, cortisol) and sex hormones that affect primary and secondary sexual characteristics (testosterone, estrogens; often chemically similar). Prostaglandins are chemically different but also lipid based.
Hormonal function involves reception, signal transduction, and response.
requires binding of hormone to a receptor protein :
• like enzymes binding substrates
• can be on cell surface or in the cytoplasm or nucleus
• different receptors for the same hormone may produce different responses, as for epinephrine:
• dilation of blood vessels with beta receptors (skeletal muscle)
• contraction of blood vessels with alpha receptors (intestine)
• glycogen breakdown in liver cells with beta receptors
‘carry’ message to effectors that produce the response.
Hormones influence their target cells in two general ways
-This is the mode of action for most peptide hormones and amine hormones; fairly fast.
In many regulatory mechanisms mediated by hormones, there are multiple levels of feedback:
• sensing the status of the parameter of interest -- blood glucose, blood pressure, osmolarity, etc.
• sensing the levels of the hormones themselves.
-Gonadotropins may have some systemic effects.
-Sex hormones have negative feedbac on pituitary and on hypothalamus
-Gonadotropins also feed back on hypothalamic production of RFs
• social situation
• biological clock
• food supply
• body condition