Gliotransmitter

Gliotransmitters are chemicals released from glial cells that facilitate neuronal communication between neurons and other glial cells and are usually induced from Ca²⁺ signaling, although recent research has questioned the role of Ca²⁺ in gliotransmitters and may require a revision of the relevance of gliotransmitters in neuronal signalling in general.

While gliotransmitters can be released from any glial cell, including oligodendrocytes, astrocytes, and microglia, they are primarily released from astrocytes. Astrocytes rely on gap junctions for coupling, and are star-like in shape, which allows them to come into contact with many other synapses in various regions of the brain. Their structure also makes them capable of bidirectional signaling. It is estimated that astrocytes can make contact with over 100,000 synapses, allowing them to play an essential role in synaptic transmission. While gliotransmission primarily occurs between astrocytes and neurons, gliotransmission is not limited to these two cell types. Besides the central nervous system, gliotransmission also occurs among motor nerve terminals and Schwann cells in the peripheral nervous system. Another occurrence of gliotransmission takes place between glial cells in the retina, called Müller cells, and retinal neurons.

The word “glia” illustrates the original belief among scientists that these cells play a passive role in neural signaling, being responsible only for neuronal structure and support within the brain. Glial cells cannot produce action potentials and therefore were not suspected as playing an important and active communicative role in the central nervous system, because synaptic transmission between neurons is initiated with an action potential. However, research shows that these cells express excitability with changes in the intracellular concentrations of Ca²⁺. Gliotransmission occurs because of the ability of glial cells to induce excitability with variations in Ca²⁺ concentrations. Changes in the concentration of Ca²⁺ correlate with currents from NMDA receptor-mediated neurons which are measured in neighboring neurons of the ventrobasal (VB) thalamus. Because glial cells greatly outnumber neurons in the brain, accounting for over 70% of all cells in the central nervous system, gliotransmitters released by astrocytes have the potential to be very influential and important within the central nervous system, as well as within other neural systems throughout the body. These cells do not simply carry functions of structural support, but can also take part in cell-to-cell communication with neurons, microglia, and other astrocytes by receiving inputs, organizing information, and sending out chemical signals. The Ca²⁺ signal from the astrocyte may also participate in controlling blood flow in the brain.

...
Wikipedia