Astroglial modulation of neurotransmitter peptide release from the neurohypophysis: present status

Reviewed in this article are those studies that have contributed heavily to our current conceptualizations of glial participation in the functioning o f the magnocellular hypothalamo-neurohypophysial system. This system underg oes remarkable morphological and functional reorganization induced by incre ased demand for peptide synthesis and release, and this reorganization invo lves the astrocytic elements in primary roles. Under basal conditions, thes e glia appear to be vested with the responsibility of controlling the neuro nal microenvironment in ways that reduce neuronal excitability, restrict ac cess to neuronal membranes by neuroactive substances and deter neuron-neuro n interactions within the system. With physiological activation, the glial elements, via receptor-mediated mechanisms, take up new positions. This per missively facilitates neuron-neuron interactions such as the exposure of ne uronal membranes to released peptides and the formation of gap junctions an d new synapses, enhances and prolongs the actions of those excitatory neuro transmitters for which there are glial uptake mechanisms, and facilitates t he entry of peptides into the blood. In addition, subpopulations of these g lia either newly synthesize or increase synthesis of neuroactive peptides f or which their neuronal neighbors have receptors. Release of these peptides by the glia or their functional roles in the system have not yet been demo nstrated. (C) 1999 Elsevier Science B.V. All rights reserved.