A large body of evidence indicates that astrocytes play an important role in a range of brain functions through calcium (Ca 2+ ) signaling. Experimentally evoking Ca 2+ signaling is a useful technique for investigating the functions of astrocytes. However, conventional stimulation methods typically have poor spatio-temporal precision, and some are invasive. Our group has developed a technique to overcome these problems, in which astrocytes are photostimulated with a femtosecond laser. In the current study, we applied this method to a hippocampal neural network to explore astrocytic functions in detail. The results revealed that applying photostimulation to astrocytes in a cultured hippocampal astrocyte-neuron network caused the following changes: (i) Synchronous Ca 2+ oscillations in neurons were induced; (ii) spontaneous Ca 2+ synchrony instantaneously emerged; and (iii) high-frequency spontaneous Ca 2+ synchrony was regulated. Thus, astrocytic Ca 2+ signaling evoked by photostimulation was found to modulate synchronous Ca 2+ oscillations in hippocampal neurons. We propose that photostimulation with a femtosecond laser will serve as a powerful tool in investigating astrocytic functions at the network level. femtosecond laser, photostimulation, astrocyte, hippocampal neuron, synchronous calcium oscillation Citation: Zhao Y, Liu X L, Zhang Y, et al. Modulation of synchronous calcium oscillations in hippocampal neurons by photostimulation of astrocytes with femtosecond laser.The neural cells comprising neural systems include neurons and neuroglial cells (glials). The neuron is the primary cell in the neural system. The number of glials in the brain is 10-50 times greater than that of neurons. There are several subtypes of glials, and, of these, astrocytes are the most numerous. Astrocytes are widely distributed in the mammalian brain. It was traditionally believed that the function of astrocytes was limited to supporting and nurturing neurons by clearing neurotransmitters and balancing ion homeostasis. With the development of new techniques, an increasing number of studies have demonstrated that astrocytes play a range of more complicated and important roles in the brain [1-5] than previously believed. Unlike excitable cells, such as the neuron, astrocytes cannot generate action potentials. However, they express various kinds of neurotransmitter receptors on the plasma membrane [6]. Therefore, they can respond to outside stimulation, especially synaptic neurotransmitters. As the astrocyte is activated, an intracellular Ca 2+ increase is induced. This can subsequently induce Ca 2+ increases in other astrocytes, generating a form of spatial propagation termed the Ca 2+ wave [7]. During Ca 2+ signaling, many physiological reactions are triggered in the astrocyte. The most important one is the release of transmitters, including neurotransmitters and vascular regulation factors. Neurotransmitters, such as ATP [8], Glu [9] and D-serine [10], can act on synapses, with diverse influences on synaptic electrical and Ca 2+ ac...