Extracellular ATP released from axons is known to assist activity-dependent signaling between neurons and Schwann cells in the peripheral nervous system. Here we report that ATP released from astrocytes as a result of neuronal activity can also modulate central synaptic transmission. In cultures of hippocampal neurons, endogenously released ATP tonically suppresses glutamatergic synapses via presynaptic P2Y receptors, an effect that depends on the presence of cocultured astrocytes. Glutamate release accompanying neuronal activity also activates non-NMDA receptors of nearby astrocytes and triggers ATP release from these cells, which in turn causes homo- and heterosynaptic suppression. In CA1 pyramidal neurons of hippocampal slices, a similar synaptic suppression was also produced by adenosine, an immediate degradation product of ATP released by glial cells. Thus, neuron-glia crosstalk may participate in activity-dependent synaptic modulation.

Glial cells constitute nearly 50% of the cells in the human brain1. Astrocytes, which make up the largest glial population, are crucial to the regulation of synaptic connectivity during postnatal development2. Because defects in astrocyte generation are associated with severe neurological disorders such as brain tumours3, it is important to understand how astrocytes are produced. Astrocytes reportedly arise from two sources4–6: radial glia in the ventricular zone and progenitors in the subventricular zone, with the contribution from each region shifting with time. During the first three weeks of postnatal development, the glial cell population, which contains predominantly astrocytes, expands 6–8-fold in the rodent brain7. Little is known about the mechanisms underlying this expansion. Here we show that a major source of glia in the postnatal cortex in mice is the local proliferation of differentiated astrocytes. Unlike glial progenitors in the subventricular zone, differentiated astrocytes undergo symmetric division, and their progeny integrate functionally into the existing glial network as mature astrocytes that form endfeet with blood vessels, couple electrically to neighbouring astrocytes, and take up glutamate after neuronal activity.

A strocytes, the most abundant glial cells in the brain, are found to associate closely with neuronal synapses (1, 2). Although astrocytes lack excitability in terms of action potentials, they express various neurotransmitter receptors and ion channels and can create Ca 2ϩ waves in response to various stimuli, including neuronal activity. Furthermore, astrocytes can secrete many factors that modulate neuronal activity (3-5). Recent studies have shown that astrocytes play important roles in neurogenesis (6), synaptogenesis (7,8), and synaptic modulation (3-5, 9-11). There is evidence that astrocytes may also contribute to high-level brain functions such as motivation, learning, and behavior (12). Given the potential role of glial factors in neuronal functions, it is of interest to know whether and how astrocytes may contribute to the induction of long-term potentiation (LTP), the most extensively studied form of synaptic plasticity that may serve as a cellular mechanism underlying learning and memory (13,14). Deletion of glial fibrillary acidic protein (GFAP) or S-100B in the mouse (15, 16) resulted in an enhancement of LTP, but how these cytoplasmic proteins in glial cells affect synaptic plasticity remains unknown. Studies of LTP are usually carried out in slice preparations, in which glial cells are intimately associated with neurons and synapses. It is thus difficult to separate the role of glia in synaptogenesis from a direct modulatory role of glial factors in synaptic plasticity. To address this problem, we have established in the present study two types of hippocampal neuronal cultures: conventional mixed cultures, in which neurons were grown on a layer of astrocytes, and cultures in which the neurons were fed with glial conditioned medium (GCM) but grown without contacting astrocytes (see Materials and Methods). In the latter GCM cultures, apparently normal synaptogenesis occurs among hippocampal neurons. By using correlated stimulation of pre-and postsynaptic neurons to induce LTP (17), we were able to analyze directly the role of immediate astrocyte-derived factors in the induction of LTP. We found that LTP could be induced in mixed cultures, but not in GCM cultures, and that glia-derived D-serine plays a key role in LTP induction by its action on the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptors. Furthermore, we showed that in hippocampal slices, the induction of LTP at Schaffer collateral-CA1 pyramidal cell synapses also critically depends on the presence of intact glial cells and extracellular D-serine. Taken together, we have identified an astrocyte-derived factor that contributes to activity-induced long-term synaptic plasticity. Materials and MethodsCell Cultures. Primary cultures of astrocytes were prepared as described (18, 19) with some modifications. In brief, cortices were prepared from 1-d-old rat and were dissociated by trypsin, plated on polyD-lysine-coated glass coverslips, cultured with Eagle's MEM (GIBCO) containing 10% FBS, and maintained at 37°C in a 5% CO 2 incubato...