Neuronal-glial-endothelial interactions and cell plasticity in the postnatal hypothalamus: implications for the neuroendocrine control of reproduction.

It is becoming increasingly apparent that non-neuronal cells play a critical role in generating and regulating the flow of information within the brain. Among these non-neuronal cells, astroglial cells have been shown to play important roles in the control of both synaptic transmission and neurosecretion. In addition to modulating neuronal activity, astroglial cells interact with endothelial cells throughout the central nervous system to define specific functional domains. In the hypothalamus, neurons that release gonadotropin-releasing hormone (GnRH), the neurohormone that controls both sexual development and adult reproductive function, offer an attractive model system in which to study glial-neuronal-endothelial interactions. Within the median eminence of the hypothalamus, alterations of the anatomical relationship that exists between GnRH axon terminals and ependymoglial cell processes belonging to tanycytes regulate the direct access of GnRH neurosecretory axons to the vascular wall. This cell plasticity presumably modulates the release of GnRH into the portal vasculature during the reproductive cycle. Both structural changes and GnRH secretory activity appear to be modulated, at least in part, by specific cell-cell signalling molecules secreted by astrocytes, tanycytes and endothelial cells. It is becoming increasingly clear that among the different factors that may be involved, glial cells use growth factor members of the epidermal growth factor (EGF) family, acting via receptors endowed with tyrosine kinase activity, to produce morphological changes and release neuroactive substances that directly excite nearby neurons, whereas endothelial cells of the median eminence employ nitric oxide to induce neuroglial plasticity and facilitate GnRH release.