of the hypothalamus, rapid changes in astrocyte morphology have been shown to affect heterosynaptic transmission between neurons. In the hippocampus, astrocytes suppress synaptic transmission by releasing ATP, which is hydrolyzed by ectonucleotidases to yield adenosine. Adenosine acts on neuronal adenosine receptors to inhibit synaptic transmission, thereby increasing the dynamic range available for LTP. Vasomodulation: Astrocytes may serve as intermediaries in neuronal regulation of blood flow. Promotion of the myelinating activity of oligodendrocytes: Electrical activity in neurons causes them to release ATP, which serves as an important stimulus for myelin to form. However, the ATP does not act directly on oligodendrocytes. Instead, it causes astrocytes to secrete cytokine leukemia inhibitory factor (LIF), a regulatory protein that promotes the myelinating activity of oligodendrocytes. This suggests that astrocytes have an executive-coordinating role in the brain. Nervous system repair: Upon injury to nerve cells within the central nervous system, astrocytes fill up the space to form a glial scar, and may contribute to neural repair. The role of astrocytes in CNS regeneration following injury is not well understood though. The glial scar has traditionally been described as an impermeable barrier to regeneration, thus implicating a negative role in axon regeneration. However, recently, it was found through genetic ablation studies that astrocytes are actually required for regeneration to occur. More importantly, the authors found that the astrocyte scar is actually essential for stimulated axons (that axons that have been coaxed to grow via neurotrophic supplementation) to extend through the injured spinal cord. Astrocytes that have been pushed into a reactive phenotype (termed astrogliosis, defined by upregulation of GFAP expression, a definition still under debate) may actually be toxic to neurons, releasing signals that can kill neurons. Much work, however, remains to elucidate their role in nervous system injury. Long-term potentiation: Scientists debate whether astrocytes integrate learning and memory in the hippocampus. Recently it has been shown that engrafting human glial progenitor cell in the nascent mice brains will cause the cells to differentiate into astrocytes. After differentiation these cells increase LTP and improve memory performance in the mice. Circadian clock: Astrocytes alone are sufficient to drive the molecular oscillations in the SCN and circadian behavior in mice, and thus can autonomously initiate and sustain complex mammalian behavior. The switch of the nervous system: Based on the evidence listed below, it has been recently conjectured in, that macro glia (and astrocytes in particular) act both as