AVP release is triggered by osmoreceptors which activate circumventricular organs which cause production of AVP in the hypothalamus where it is then stored in the posterior pituitary gland and at some point a signal will be sent to the posterior pituitary gland to release AVP. AVP then binds to V2R receptors (G-protein coupled receptors) and this binding causes cAMP production which then activates protein kinase A upregulation, this upregulation causes Aquaporin 2 phosphorolation which activates these channels which allows water reabsorption from the apical membrane.
When plasma osmolality increases (indicating dehydration or high solute concentration), osmoreceptors are activated, leading to the release of AVP.