Neuronal-glial and synaptic remodelling in the adult hypothalamus in response to physiological stimuli

Activation of certain neurosecretory systems of the mammalian hypothalamus induces remodelling of the conformation of their neurons and glial cells. During stimulation of the hypothalamo-neurohypophysial system, astrocytic coverage of oxytocinergic somata and dendrites diminishes and their surfaces become extensively juxtaposed. In the neurohypophysis and median eminence, stimulation evokes a retraction of glial processes and an increase in the contact area between neurosecretory terminals and the perivascular space. These changes are reversible and glial coverage returns to normal upon cessation of stimulation. Neuronal-astrocytic rearrangements also occur in the arcuate nucleus in response to changes in sex steroid levels. The significance of such modifications is a matter of speculation. In the hypothalamic nuclei they may permit synaptic remodelling that takes place concurrently; in the neurohaemal structures they may facilitate neuropeptide release. We know little about the cellular mechanisms involved but glia and neurons of these systems express certain molecules implicated in cell-cell interactions in the developing central nervous system, such as the polysialylated isoform of the neural cell adhesion molecule; this may allow them to manifest their capacity for morphological plasticity in adulthood. The factors inducing the changes vary in the different structures: while oxytocin, in synergy with steroids, appears essential to the induction of the changes in the oxytocinergic system, oestrogen alone is critical in the arcuate nucleus; in the neurohypophysis noradrenaline appears important.