Stress through Life

Effects of stress depends on

• Intensity of the stressor

• Duration of the stressor

• Timing of the stress exposure

  • brain is particularly sensitive to stress early and late in life

  • early life has a larger welfare impact on domesticated animals (few actually make it to old age)

  • stress experienced during critical early-life period can have long-lasting or permanent changes on the brain

Brain maturation involves multiple dynamic processes that are regulated by

• genetic factors

• environmental input    

  • e.g. visual lateralization in chickens

    • if the chick/egg is incubated in darkness, the brain does not become lateralized

Developmental processes in the early postnatal brain

• Axonal and dendritic growth

• Synaptic stabilisation

• Synaptic pruning

Epigenetics

= how our environment changes the gene expression

• heritable, but no actual changes in the DNA

Two specific phenomena of interest:

• Histone modification

  • how DNA wraps around histones (proteins)

• DNA methylation

  • Moves histones to prevent/allow gene expression (on/off switch basically)

    • can be permanent

E.g. by cross-fostering, scientists found out that anxious parents raise anxious kids, while caring mothers causes permanent changes in the genetically anxious kids to become more secure adults.

• Hippocampus is involved

• The different maternal cares caused differences in DNA methylation in brains of the offspring

Hippocampus

• Complex brain structure embedded deep into a temporal lobe

• Major role in learning and memory

Discuss how early care of offspring can affect the offspring in adult life

(Using a cross-fostering experiment on rats):

• Type of maternal care caused distinctive differences in DNA methylation in brains of offspring

  • Differences still present in adult offspring, affecting their stress responses

    • Corticosterone increase after physical restraint was less pronounced in adult offspring of more caring mothers, which demonstrated long-lasting effects of the epigenetic modifications

• Female offspring having received more licking and grooming by their mother also behaved similarly towards their own offspring

• Transgenerational effects may therefore remain for a long time by means of a reinstatement of the behavioural phenotype in every generation

Early life can be divided into:

• Prenatal period

  • Before birth/hatch

• Early postnatal

  • Right after birth/hatch

• Adolescence

  • Just before sexual maturity

Prenatal stress

• Mediated via mother (maternal stress)

  • a mother’s stress affects her daughter AND granddaughter

• Very sensitive period due to the complex and rapid development of the embryo

  • a LOT happens from day to day

• Effects on offspring:

  • Increased HPA-axis

  • Structural and functional changes in the limbic system and prefrontal cortex

  • Learning/memory deficit and increased anxiety levels

  • Decreased immune system

  • Lower birthweight (humans)

From when is the HPA-axis functional in chickens?

• From day 15 of incubation (21 days in total)

• Cort can be measured in the down-feathers of newly hatched chicks,

  • (but no cort in the egg yolk ??)

Discuss how CORT affects the reproductive system

• Glucocorticoids can influence the synthesis of gonadal steroids both by downregulation of the gonadotrophins and via direct effects on the testis and the ovaries

• Stress impacts the reproductive axis — the hypothalamus (to affect GnRH secretion) and the pituitary gland (to affect gonadotrophin secretion)

• Elevated plasma CORT levels lead to

  • reduced plasma/egg testosterone and progesterone concentrations

  • reduced yolk/egg mass

How is offspring (fetus) protected by the mother’s cort levels ?

• Mammalian fetus is partially protected from elevated cort exposure by placental 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2)

  • (11 beta-HSD2) plays a crucial role in converting hormonally active cortisol to inactive cortisone

• Extra embryonic membranes in birds eggs have same function as placenta in mammals

• 11β-HSD2 has been found in chicken oviduct/gonads

• In vitro studies on chicken yolk follicles - Yolk follicles degrades corticosterone to 20β-dihydrocorticosterone

Effect of prenatal stress on the adult chicken brain findings:

• Brain size not affected

• Brain region size not affected

• Neuronal density not affected

• Non-neural density reduced

Altricial vs precocial

• Altricial

  • Useless baby (underdeveloped)

• Precocial

  • Functional baby (developed)

How does stress during early postnatal period affect adults?

• E.g. rats who were separated earlier from mothers become more fearful

• E.g. cats who were weaned earlier tend to wool-suck more

Examples of early life stress in production systems

• Transport stress

• Nutrition stress

• Heat stress

• Mixing stress

• Antigen toxin stress

• Weaning stress

• Cold stress

• Crowding stress

Why is the adolescence period sensitive for stress?

• Large hormone alterations

  • HPA axis interacts with the HPG axis

• Large behavioural alterations

  • Brain maturation

State some hypotheses why animals are so sensitive to stress

• PAR hypothesis

  • Organism uses environmental cues during early life to forecast future conditions and permanently adjust to its long-term physiology

• Developmental-mismatch hypothesis

  • Early experiences shape the brain and behaviour

    • major differences in stressors between the early years of development and later life stages are mismatches that can cause diseases (both physically and mentally)