Endogenous opioids
Suggested to facilitate and reinforce stereotypic behavior
Administering opioid antagonists reduces crib-biting
Measurement of plasma -endorphin concentrations in
crib-biting horses has produced conflicting results
Hypothalamus–pituitary–adrenal (HPA) axis
Crib-biting hypothesized as an adaptive response to stress
Bouts of crib-biting have been shown to lower plasma
cortisol concentration, heart rate, and nociceptive
threshold
Measurement of cortisol concentrations in crib-biting horses
has produced conflicting results and remains controversial
Dopaminergic system
Dopamine receptor densities have been shown to differ
between crib-biting and non-crib-biting horses
Dopamine pathways are important in goal-directed
behavior and crib-biting horses have demonstrated
difficulties in learning response-outcome contingencies