Activity-dependent morphological synaptic plasticity in an adult neurosecretory system: magnocellular oxytocin neurons of the hypothalamus

Oxytocin and vasopressin neurons, located in the supraoptic and paraventric ular nuclei of the hypothalamus, send their axons to the neurohypophysis wh ere the neurohormones are released directly into the general circulation. H ormone release depends on the electrical activity of the neurons, which in turn is regulated by different afferent inputs. During conditions that enha nce oxytocin secretion (parturition, lactation, and dehydration), these aff erents undergo morphological remodelling which results in an increased numb er of synapses contacting oxytocin neurons. The synaptic changes are revers ible with cessation of stimulation. Using quantitative analyses on immunola belled preparations, we have established that this morphological synaptic p lasticity affects both inhibitory and excitatory afferent inputs to oxytoci n neurons. This review describes such synaptic modifications, their functio nal significance, and the cellular mechanisms that may be responsible.