Functional neuronal-glial anatomical remodelling in the hypothalamus

The supraoptic nucleus (SON) of the hypothalamus undergoes a striking anatomical remodelling under conditions of intense stimulations like chronic dehydration, parturition and lactation. This morphological plasticity modifies the astrocytic coverage of magnocellular neurons and their synaptic afferent inputs. These changes occur within a few hours and are completely reversible upon the cessation of the stimulation. By comparing synaptic transmission and diffusion properties before and during this neuroglial remodelling, we have been able to show that the astrocytic environment of neurons contributes to the regulation of synaptic and extrasynaptic transmission. It appears that the presence of fine astrocytic processes enveloping synapses and neuronal elements ensures two important functions. First, they control the level of activation of presynaptic metabotropic glutamate autoreceptors located on glutamatergic terminals, thereby regulating synaptic strength at excitatory synapses. Second, they constitute a physical barrier to diffusion, limiting spatially and temporally spill-over of neurotransmitters and, as a consequence, extrasynaptic transmission, a process essential for intercellular communication. Using the neuroglial anatomical remodelling of the SON as an experimental model has brought new insights into the role of glial cells in the regulation of synaptic transmission and signal processing in the brain.