Astrocytes are known to be the most important neural cell type for the maintenance of brain homeostasis, support neurons in many different ways, energy production of the brain, ion and pH balance, and they regulate synapse creation, nervous tissue repair, and the formation of the blood-brain barrier and defense against oxidative stress.

Astrocytes do these important roles in the following way:

Glutamate uptake and recycling:

Since astrocytic rays surround synaptic apparatus express transporters for neurotransmitters and neuromodulators including glutamate, y-aminobutyric acid (GABA), glycine, and histamine. These transporters contribute in the rapid elimination of neurotransmitters released into the synaptic cleft, which is essential for the termination of synaptic transmission and maintenance of neuronal excitability. This is primarily achieved by the astrocyte-specific sodium-dependent high-affinity glutamate transporters GLT-1 and G LAST and to a lesser extent by the neuronal glutamate transporters EAAT3 and EAAT4.

K+ buffering:

The substantial local increases of extracellular potassium ions (K+) in the restricted extracellular space occur as a result of propagation of action potentials during impulse conduction. Due to lack of tight regulatory mechanisms, this alters the neuronal membrane potential, leading to neuronal hyperexcitability and seriously compromise CNS function. This is prevented by the buffering of extracellular K+ by glial cells. Indeed, astrocytes have a strongly negative resting potential and express a number of potassium channels, resulting in a high membrane permeability to K+. These features, along with the action of the Na+/K+ ATPase, enable astrocytes to accumulate the excess extracellular K+, which can then travel in the astrocytic syncitium through gap junctions down its concentration gradient. This allows for the spatial dispersion of K+ from areas of high concentration to areas of lower concentration where it can be extruded either into the extracellular space or the circulation, thus maintaining the overall extracellular K+ concentration within the physiological range.

Supply of energy:

Brain requires highest energy to perform normally. Astrocytes play an instrumental role in coupling neuronal activity and brain glucose uptake through a mechanism referred to as the astrocyte-neuron lactate shuttle.

pH regulation of the brain micro-environment:

The main feature of glial cells is a high pH buffering capacity, is their enriched expression of carbonic anhydrase (CA) which converts CO2 into H+ and HCO3 - - effectively allowing them to act as a CO2 sink.

Defense against oxidative stress:

Astrocytes possess significantly higher levels of a variety of antioxidant molecules (including glutathione, ascorbate, and vitamin E) and display greater activities for ROS-detoxifying enzymes (including glutathione S-transferase, glutathione peroxidase, and catalase).

Astrocytes play diverse roles in CNS from being supportive cells (older view) to recently discovered new functions like regulating neural plasticity and firing of neurons. Astrocytes have plethora of receptors important for many glial-neuron communications like purinergic (P2X7) receptors, beta adrenergic receptors etc. Recently it has been shown that HIV-induced neuro and cognitive deficits are mediated by astrocyte mediated killing of neurons where Tet proteins of HIV has been shown to bind to P2X7 receptors and induce changes in gene expressions in astrocytes. Astrocytes are also important for glutamine-glutamate shuttle which is important for neurotransmission.