Endocrine System Flashcards
Hormone
chemical molecule that is produced in a minute quantity (10-12 M) in one part of an organism and is transported to another part of a organism where it exerts an effect
Endocrine (ductless) gland
a gland containing a high concentration of secretory cells that produce hormones
Neurohormone
hormone produced by a neuroendocrine cell and released into the blood
Paracrine signals
travel short distances to adjacent cells and do not enter the blood stream
Autocrine signals
travel short distances and act on the secretory cell directly
Influence functions of the same cell that secretes them
Endocrine signals
travel great distances within the body via the blood stream
Endocrine cells and neuroendocrine cells secrete hormone molecules that diffuse into blood capillaries
hormones come in contact with contact cells they interact with receptor molecules on the cell surface to elicit a response. The system relies on the target cells having the appropriate receptor cells.
Effect of neurons, Paracrine and autocrine cells
Localised
Small populations of target cells
Effects of endocrine secretions
Potentially large populations of target cells remote from the point of secretion
Why do endocrine signals often have a time delay
Travel to target cells remote
Often response is by gene transcription and protein production
Steroid hormones
Synthesised from cholesterol
Secreted by gonads, adrenal cortex, skin and placenta
Ecdysone (moulting hormone in arthropods) is also a steroid
Lipid soluble and pass through plasma-lemma to intra-cellular receptor molecules
Some cells transport steroidal hormones (e.g. oestrogen) across the membrane using protein receptor molecules – e.g. megalin
Peptide hormones
Synthesised from amino acids organised into chains
Widespread sites of secretion (but not adrenals, thyroid, or pineal glands, or skin)
Anti-diuretic hormone, insulin (shown to the right) and growth hormone
Invertebrates: gamete-shedding hormone of sea stars and diuretic hormones of insects
Vary in size from a tripeptide (e.g. thyrotropin-releasing hormone) to large proteins with ~200 amino acids (e.g. growth hormones)
Soluble in aqueous solutions
Amine hormones
Synthesised from modified amino acids
Limited sites of secretion
Catecholamines derived from tyrosine: dopamine produced in the hypothalamus,
Adrenal medulla produces norepinephrine (noradrenalin) and epinephrine (adrenalin)
Thyroid hormones
Melatonin
Catecholamines
derived from the amino acid tyrosine which is converted to dopamine by replacing a –CO2H group with –H and adding an –OH group (see circled parts of the molecule).
Norepnephrine and epinephrine are catecholamines produced in the adrenal medulla.
Thyroid hormone
thyroxine and triiodothyronine) derived from tyrosine and secreted by thyroid gland
Melatonin
derived from tryptophan and secreted by the pineal gland
Types of hormones
Steroid
Peptide
Amine
Cholecystokinin
Cholecystokinin is a peptide secreted by I-cells in the small intestine
Induces contraction of the gallbladder, relaxes the sphincter of Oddi, reduces of gastric acid secretion, increases bile acid production in the liver, delays gastric emptying, and induces digestive enzyme production in the pancreas
Also a neurotransmitter in the central nervous system
Testosterone
Testosterone is a steroid secreted by Leydig cells in the testis
Widespread endocrine function throughout the body
Affects gonads, skin, brain, bone, immune system
Also has paracrine function in testis supporting spermatogenesis
Hormone synthesis and release
generally synthesised and released all of the time but in minute quantities
Rate of release is variable – controlled by neurons or other hormones
Hormones half-life
the time it takes to reduce its concentration by 50% - is a function of the rate of removal
How do steroid hormones work
easily cross the cell membranes and interact by binding with an intracellular receptor. This complex then acts as a transcription factor that interacts with the cell’s DNA to alter gene expression and so protein synthesis.
How do peptide and amide hormones work
bind to cell-surface receptors that face the extracellular fluid
Receptors regulate ion-channel permeability or activating secondary intracellular messaging system
Insulin binds to enzyme-linked cell-surface binding site
Peptide hormones mainly regulate activity of existing proteins and so effects can be quite rapid
How are hormone levels detected
Radioimmunoassay
Radioimmunoassay
An antigen is produced that reacts to produce antibodies. Some of the antigen is then labelled with radioactivity and a known amount is added to a solution of the unlabelled antigen. The cells react and produce antibodies that bind either labelled or unlabelled antigens. The proportion of radioactive antigen bound to the antibodies can be measured and compared to the know proportion in the original mixture. This then allows calculation of the concentration of the unknown antigens.
Standard curve for the relationship between radio-labelled hormones and known concentrations
Different forms of hormone function
Concerted/ Additive
Non-additive
Synergistic
Permissive
Consider two hormones A & B. For concerted or additive effects the individual effects of A & B are just combined. For non-additive effect B has no really effect on the effect of A, whereas for synergistic effects the combined effect of A&B is greater than the sum of A & B. For permissive effects hormone A has an effect but B does not but when A is in the presence of B the effect is magnified.
Adrenal gland
Located above kidneys different hormones are produced by different parts of the gland
The cortex is multi-layered and each layer produces different hormone types
Zona reticularis – androgen steroids
Zona fasciculata – glucocorticoids, e.g. cortisol – mediates stress response
Zona glomerulosa – aldosterone – regulates Na+ and K + in kidney
The medulla produces the catecholamines epinephrine and norepinepherine
Zona reticularis
Androgen steroids
Formation of peptide hormones
Insulin is produced within the endoplasmic reticulum and Golgi apparatus
Vesicles containing the hormone bud off the Golgi and can act as a store
Release to the blood stream is via exocytosis
Formation of steroid hormones
synthesised on demand and released via diffusion across the plasma-lemma
Hormones in insects
many hormones are produced by neurosecretory cells in the brain
but there are also endocrine glands such as the prothoracic gland that produces ecdysone
Zona fasciculata
Glucocorticoids eg cortisol
Zona glomerulosa
Aldosterone
Adrenal medulla
Catecholamines
Epinephrine
Norepinephrine
Pineal gland
The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates.
Associated with exposure to light produces melatonin, a serotonin-derived hormone which modulates sleep patterns in both circadian and seasonal cycles.
It is likely that the common ancestor of all vertebrates had a pair of photo-sensory organs on the top of its head, similar to the arrangement in modern lampreys, and many lizards
Pheromones
produced by one animal - released outside the body into the environment - received by other animal/s typically by their sense of smell
Evolved to enable communication between members of the same species
Pheromone signalling elicits a specific reaction in the receiver:
stereotyped behaviour
developmental process
Not all smells are pheromones
Counter regulatory feedback
2 or more hormones controlling the same thing
Hypothalamic-pituitary-thyroid axis
Thyrotropin-releasing hormone (TRH) is secreted by the hypothalamus in the brain, which stimulates production of thyrotropin-stimulating hormones (TSH) from the nearby pituitary gland.
This is released into the blood and travels to the thyroid glands where T3 and T4 are produced in response.
These hormones act on tissue metabolism and feedback via the blood on the hypothalamus and the pituitary to reduce TRH and TSH secretion.
Hormonal positive feedback
Certain hormones can ‘self prime’ own production - gland makes even more hormone
e.g. oxytocin and control of suckling
Oxytocin positive feedback
The act of suckling triggers nerves in the nipple which trigger milk production by lactocytes in the breast and oxytocin release in the hypothalamus and posterior pituitary. The oxytocin then stimulates myoepithelial cells lining the ducts of the milk glands to push the milk to the nipple. This release of milk encourages suckling and so increases production of milk and oxytocin.
Basic behavioural drivers in vertebrates
Feeding
Fleeing
Securing a mate
Reproducing
Reproduction - key organs
Hypothalamus
Pituitary
Gonads
Control of reproduction
Environmental factors interact with hormones to stimulate secretion of hormones such as gonadotropin-releasing hormones and gonadotropin-inhibiting hormones, which interact with the pituitary to stimulate (or inhibit) secretion of luteinising hormones and follicle stimulating hormone.
These are produced in both males and females alike. These travel via the blood to interact with the gonads (testis and ovary) where they stimulate sex steroids – male androgens and female oestrogens.
Vasoactive intestinal protein is secreted in the hypothalamus and promotes secretion of prolactin. Arginine vasotocin (AVT) has direct actions on factors such as oviposition in birds.
Androgens
Testosterone
Androstenedione
Oestrogens
Progesterone
Oestrone
Oestrodiol 17(beta)
Sexual behaviours
Mate searching
Courtship
Copulation
Parental care
Mate searching in salamanders
Japanese red-bellied newt (Cynops pyrrhogaster) 7α-Hydroxypregnenolone produced by neurones in parts of the brain stimulates locomotor activity during mate searching
Hormone interaction affecting mate attraction in frogs by calling
Plasma androgens activate calling behaviour and increase in response to calling
Increased calling effort leads to higher energetic demands and rise in plasma corticosterone
Plasma corticosterone reaches threshold and inhibits androgen secretion by negative feedback on the hypothalamic-gonadal axis
Plasma androgen levels drop and calling decreases allowing males to increase energy stores
Restored energy stores cause corticosterone levels to drop and androgen levels increase and calling begins again
Phonotaxis
Movement of females attracted to calling males
Prolactin in vertebrates - location
Prolactin is formed in the pars distalis of the pituitary and is under the control of vasoactive intestinal protein (VIP) produced in endocrine neurons in the hypothalamus
Prolactin in vertebrates role
stimulates the mammary glands to produce milk (lactation): increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands and prepare for milk production, which normally starts when levels of progesterone fall by the end of pregnancy and a suckling stimulus is present.
Prolactin in fish
implicated in migration behaviour, reproductive development and attainment of sexual maturity, steroidogenesis and gonadogenesis, and brood care behaviour
Mouth brooding
Nest building
Egg fanning
Pit-digging behaviour
Nest guarding and defence
Guarding of schooling fry
‘Pregnancy’
Nutrient provision to offspring
Mouth brooding in fish
Several lineages of fish incubate eggs and larvae in their oral cavity (mouth brooding).
In cichlids a combination of PRL and oestrogen promotes oral egg carrying behaviours
Pituitary tiPRL177 concentrations increase in female Mozambique tilapia brooding eggs or early stage embryos
Serum concentrations of tiPRL177 also increasing in females brooding late-stage larvae
Prolactin in salamanders
Japanese red-bellied newt Prolactin released in response to cold temperature
Prolactin is important in the physiological transition of salamanders from a terrestrial refugia to the aquatic habitat of the breeding pond
Prolactin is important in courtship behaviour
Injected anti-newt prolactin receptor antibody decreased the incidence and frequency of courtship tail-fanning behaviours
Prolactin effects in reproductive behaviour in mammals
Stimulates maternal behaviours
Prolactin levels are only elevated in the males that are group-living and breeding (black bar) and the females (white bar in graph). Outside of the breeding season males and females exhibit no differences in prolactin levels (right graph).
Prolactin levels in mammals - short term
Wild groups of cooperatively breeding meerkats, male helpers that decided to babysit had significantly higher levels of prolactin, coupled with lower levels of cortisol, before initiating a babysitting session compared with similarly aged individuals that decided to forage.
Hormonal differences disappeared over the course of the day, suggesting that hormone levels changed in a fundamentally different way in meerkats that babysat versus those that foraged.
In contrast, long-term contributions to babysitting were not significantly associated with plasma levels of prolactin, cortisol, or testosterone in individual male helpers.
Prolactin levels in mammals- long term
Long-term contributions to pup provisioning were significantly and positively correlated with plasma levels of cortisol rather than prolactin, while plasma levels of testosterone were not related to individual patterns of pup-feeding..
A playback experiment used pup begging calls to increase the feeding rates of male helpers gave rise to parallel increases in plasma cortisol levels, whilst prolactin and testosterone levels remained unchanged.
Reproductive hormones in birds
Bird lay eggs that require contact incubation to raise temperature to promote embryonic development through to hatching
Reproductive behaviours involve incubation of the eggs and subsequent care of chicks
Gonadotropin Releasing Hormone and Luteinising Hormone, and sex steroids all implicated in courtship and nest-building
Female ring doves (Streptopelia risoria) require oestrogen and progesterone for nest building and incubation
Males require testosterone to show nest-building behaviour
Prolactin in birds
Prolactin levels are high during incubation and chick rearing
Both male and female Adelie penguin (Pygoscelus adeilie) show rise in Prolactin during courtship
Prolactin secretion stimulates incubation behaviour but incubation behaviour is stimulated by prolactin secretion
Difficult to tease apart cause and effect
Prolactin in doves and pigeons
Pigeons and doves differ from other birds
Prolactin not usually elevated until incubation is well established
Oestrogen and progesterone seem to act synergistically to initiate incubation behaviour, which is then maintained by prolactin secretion
Ring doves removed a nest but injected with prolactin maintain nest attachment and incubation drive but controls don’t
In chickens chemical suppression of prolactin secretion limits broody behaviour
Inhibition of AVT secretion in turkeys prevents incubation behaviour
Control of prolactin secretion in birds
Vasoactive Intestinal Protein and Dopamine produced in the brain express antagonistic roles in stimulating prolactin release from the pituitary
The brood patch is typically characterised as being an area of featherless, swollen skin, rich in blood vessels on the ventral side of the thorax and abdomen.
Good evidence of feedback loop between brood patch and prolactin secretion
Denervation of the brood patch leads to drop in prolactin levels and loss of incubation behaviour
Brood patch
typically characterised as being an area of featherless, swollen skin, rich in blood vessels on the ventral side of the thorax and abdomen.
Prolactin and parental care in birds
Post-hatching parental care seems to be regulated by prolactin – high correlation between level of parental care and prolactin
Varies from independent precocial species to highly dependent altricial species
Prolactin levels fall by ~50% at hatching but persist during rearing if the chicks require considerable parental care
Patterns are variable according to individual species life histories – seabirds that spend a lot of time out to sea have persistence of prolactin
Testosterone and male parental behaviour in bird
stosterone is high during courtship and mating but there is a dramatic decline when the eggs are laid in species where males contribute to parental care
Challenge hypothesis….
Low levels of testosterone promote courtship and gonadal development but high levels promote aggression
Drop in testosterone leads to improved fitness as aggression can be in conflict with parental care
Supplementation with androgens decreases or disrupts parental care – promotes other behaviours that conflict with parental care
Ghrelin
Peptide hormone released from endocrine glands in mucosal lining of stomach and upper intestines
Key role in stimulating hunger
Stomach empty – ghrelin secreted.
Stomach stretched – secretion stops.
Ghrelin action
Acts on hypothalamic cells both to increase hunger, and to increase gastric acid secretion and gastrointestinal motility to prepare the body for food intake
Loss of weight affects magnitude of ghrelin signal so increases secretion in advance of a meal
Adjusts both energy input (hunger signals) and energy output – by adjusting the allocation of energy to usage or storage
Hormones that terminate eating
Stretching of the stomach wall leads to secretion of peptides and neuronal signally with the vagus nerve
CCK
Peptide YY
Glucagon-like peptide-1 (GIP)
Glucagon
Amylin
Leptin
Polypeptide hormone produced by white adipose tissue
Secretion is in proportion of stored fat in the body – ‘adiposity signal’ of body reserves
Peptide YY secreted for a long time after meal and so suppresses appetite
Receptor for ghrelin, the ghrelin/growth hormone secretagogue receptor (GHS-R), is found on same cells in the brain as the receptor for leptin
Flight response
In response to acute stress, the body’s sympathetic nervous system is activated due to the sudden release of hormones.
The sympathetic nervous system stimulatestheadrenal glandstriggering the release of catecholamines, which include epinephrine and norepinephrine (adrenaline and noradrenaline)
This results in an increase in heart rate, blood pressure, and breathing rate
After the threat is gone, it takes between 20 to 60 minutes for the body to return to its pre-arousal levels
Different types of aggression
to kill for food,
to defend offspring,
to win a mate,
to scare off a potential adversary
Predatory aggression
to kill for food, involves attacks against a member of a different species for the purpose of obtaining food;
NOT associated with high levels of activity in the sympathetic division of the ANS;
Affective aggression
for show, involves vocalisation and threatening or defensive posture high levels of activity in the sympathetic division.
Territorial aggression
regulated by androgens, and that aggression itself can modulate androgen levels, is well established in males.
This response has been interpreted as a mechanism for adjusting aggressive motivation to a changing social environment.
Adaptive to anticipate social challenges and reacting to their clues with an anticipatory androgen response to adjust agonistic motivation to an imminent social challenge.
Direct affects of hormones on territorial aggression
Some evidence for gonadotropins influencing social aggression
Postneonatal gonadectomy removes major source of sex steroids – suppresses aggression in a range of vertebrates
Therapy with aromatisable androgens (testosterone) increases aggression in gonadectomised individuals
Therapy with oestrogens – increases aggression in gonadectomised individuals but suppresses aggression in intact individuals
Aggression and testosterone
The “challenge” hypothesis asserts that testosterone and aggression correlate only during periods of heightened interactions between males.
Under more stable social conditions relationships among males are maintained by other factors (e.g. social inertia, individual recognition of status, and territorial boundaries) and testosterone levels remain low.
Testosterone is produced by the gonads and is transported to the brain – influences expression of reproductive behaviours
Territorial aggression in birds
Correlations between levels of testosterone and aggression are convincing in birds but social context and environmental influences must be taken into account
If testosterone is associated with territorial aggression then blood levels should parallel behaviour
Free-living, monogamous species
Testosterone is highest when territories are first established and aggressive interactions between males are highest – declines thereafter
Nest site availability often drives competition – if low then testosterone is reduced
Polygynous bird species
Males spend longer periods of time defending territories so testosterone is elevated when territories are first established but are maintained for longer
Pied flycatchers have both monogamous (grey) and polygynous (blue) males and exhibit both patterns
Brood parasite cowbirds defend females rather than territories
Monogamous bird species
Testosterone is highest when territories are first established and aggressive interactions between males are highest – declines thereafter
Nest site availability often drives competition – if low then testosterone is reduced
Female aggression in starlings,
Males gain from polygyny, whereas monogamy increases female fitness.
Cost of polygyny to females lead to intense female–female competition
Physiological regulation of such female aggression in starlings is not yet known.
What controls levels of circulating testosterone in birds - environmental cues
Environmental cues…
Light, territory or signals from rival male
Increasing day length stimulates Luteinising Hormone (LH) release from the pituitary, which stimulates testosterone secretion from the gonads
High circulating levels of testosterone are not required for expression of sexual behaviour
Free-living males – high testosterone is associated with aggression
What controls levels of circulating testosterone in birds - sensory cues
Visual, auditory, tactile(??) or chemical (??)
Males exposed to only a song playback (auditory only) or to a devocalised male (visual only), do not exhibit rise in hormone levels
Both cues required and it is species-specific
Are there hormone specific receptors on the neurons that allow for rapid effects?
Organisational effect – affects song control nuclei in brain
Immediate aggressive behaviour is not associated with a rapid peak in Testosterone
