Astrocytes exhibit spontaneous calcium oscillations that could induce the release of glutamate as gliotransmitter in rat hippocampal slices. However, it is unknown whether this spontaneous release of astrocytic glutamate may contribute to determining the basal neurotransmitter release probability in central synapses. Using whole-cell recordings and Ca(2+) imaging, we investigated the effects of the spontaneous astrocytic activity on neurotransmission and synaptic plasticity at CA3-CA1 hippocampal synapses. We show here that the metabolic gliotoxin fluorocitrate (FC) reduces the amplitude of evoked excitatory postsynaptic currents and increases the paired-pulse facilitation, mainly due to the reduction of the neurotransmitter release probability and the synaptic potency. FC also decreased intracellular Ca(2+) signalling and Ca(2+) -dependent glutamate release from astrocytes. The addition of glutamine rescued the effects of FC over the synaptic potency; however, the probability of neurotransmitter release remained diminished. The blockage of group I metabotropic glutamate receptors mimicked the effects of FC on the frequency of miniature synaptic responses. In the presence of FC, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N ',N '-tetra-acetate or group I metabotropic glutamate receptor antagonists, the excitatory postsynaptic current potentiation induced by the spike-timing-dependent plasticity protocol was blocked, and it was rescued by delivering a stronger spike-timing-dependent plasticity protocol. Taken together, these results suggest that spontaneous glutamate release from astrocytes contributes to setting the basal probability of neurotransmitter release via metabotropic glutamate receptor activation, which could be operating as a gain control mechanism that regulates the threshold of long-term potentiation. Therefore, endogenous astrocyte activity provides a novel non-neuronal mechanism that could be critical for transferring information in the central nervous system.

The cellular mechanisms that mediate spike timing-dependent plasticity (STDP) are largely unknown. We studied in vitro in CA1 pyramidal neurons the contribution of AMPA and N-methyl-D-aspartate (NMDA) components of Schaffer collateral (SC) excitatory postsynaptic potentials (EPSPs; EPSP AMPA and EPSP NMDA ) and of the back-propagating action potential (BAP) to the long-term potentiation (LTP) induced by a STDP protocol that consisted in pairing an EPSP and a BAP. Transient blockade of EPSP AMPA with 7-nitro-2,3-dioxo-1,4-dihydroquinoxaline-6-carbonitrile (CNQX) during the STDP protocol prevented LTP. Contrastingly LTP was induced under transient inhibition of EPSP AMPA by combining SC stimulation, an imposed EPSP AMPA -like depolarization, and BAP or by coupling the EPSP NMDA evoked under sustained depolarization (approximately Ϫ40 mV) and BAP. In Mg 2ϩ -free solution EPSP NMDA and BAP also produced LTP. Suppression of EPSP NMDA or BAP always prevented LTP. Thus activation of NMDA receptors and BAPs are needed but not sufficient because AMPA receptor activation is also obligatory for STDP. However, a transient depolarization of another origin that unblocks NMDA receptors and a BAP may also trigger LTP. I N T R O D U C T I O NLong-term potentiation (LTP) is an activity-dependent modification in synaptic efficacy that is thought to be the cellular substrate of the learning machinery in the brain (Bliss and Collingridge 1993;Malenka and Bear 2004; Nicoll and Malenka 1999). LTP may be induced by different protocols, but spike timing-dependent plasticity (STDP) seems to be closer to physiological conditions. STDP is a form of synaptic plasticity that obeys the associative "Hebbian" learning rule (Hebb 1949) and induces LTP by pairing at short delays a subthreshold excitatory postsynaptic potential (EPSP) with a back-propagating action potential (BAP) (Abbott and Nelson 2000;Bell et al. 1997;Bi and Poo 1998;Campanac and Debanne 2008;Debanne et al. 1996Debanne et al. , 1998Markram et al. 1997;Nevian and Sakmann 2006;Sjostrom and Nelson 2002;Sjostrom et al. 2001).We have shown that the waveform and amplitude of the Schaffer collateral (SC) EPSP determines the EPSP-BAP delay (i.e., the "temporal window") needed to induce and regulate the magnitude of the LTP . A likely interpretation of these results is that the initial depolarization provided by the activation of AMPA receptors (AMPARs) plays a key role in the regulation of the induction threshold and mag- We studied the relative role of EPSP AMPA , EPSP NMDA , and BAP to the LTP induced by STDP in hippocampal CA1 pyramidal neurons in vitro that consisted in pairing at 10 ms a subthreshold SC EPSP and BAP. Transient inhibition of EPSP AMPA with 7-nitro-2,3-dioxo-1,4-dihydroquinoxaline-6-carbonitrile (CNQX) during the STDP protocol prevented LTP. In contrast, LTP was induced under transient blockade of EPSP AMPA by pairing SC stimulation and a brief depolarization that precisely reproduced the EPSP AMPA waveform (i.e., "simulated EPSP AMPA ") and BAP or by coupling a sl...

The precise timing of pre-postsynaptic activity is vital for the induction of long-term potentiation (LTP) or depression (LTD) at many central synapses. We show in synapses of rat CA1 pyramidal neurons in vitro that spike timing dependent plasticity (STDP) protocols that induce LTP at glutamatergic synapses can evoke LTD of inhibitory postsynaptic currents or STDP-iLTD. The STDP-iLTD requires a postsynaptic Ca(2+) increase, a release of endocannabinoids (eCBs), the activation of type-1 endocananabinoid receptors and presynaptic muscarinic receptors that mediate a decreased probability of GABA release. In contrast, the STDP-iLTD is independent of the activation of nicotinic receptors, GABAB Rs and G protein-coupled postsynaptic receptors at pyramidal neurons. We determine that the downregulation of presynaptic Cyclic adenosine monophosphate/protein Kinase A pathways is essential for the induction of STDP-iLTD. These results suggest a novel mechanism by which the activation of cholinergic neurons and retrograde signaling by eCBs can modulate the efficacy of GABAergic synaptic transmission in ways that may contribute to information processing and storage in the hippocampus.

REPLY: In their letter Meredith and Groen (2010) emphasize the role of GABAergic inhibition in controlling dendritic excitability and depolarization in pyramidal neurons and, consequently, in regulating the induction threshold of long-term potentiation (LTP). In particular they stress the requirement of a burst of postsynaptic back-propagating action potentials in the induction of spike timing-dependent plasticity (STDP) when inhibition is functional in vitro. We totally agree with their view. Indeed countless reports show that LTP is more readily induced when dendritic excitability and depolarization are not regulated by inhibition.In our experiments GABAergic inhibition was blocked to analyze the role of glutamatergic excitatory postsynaptic potentials (EPSPs) and back-propagating action potentials in the genesis of LTP by STDP (Fuenzalida et al. 2007(Fuenzalida et al. , 2010. In these reports a single action potential was sufficient to induce LTP with STDP protocols.We point out that recently Kwag and Paulsen (2009) reported that an STDP-like protocol-which simulates the theta rhythm and generates a single back-propagating action potential per theta cycle added to an EPSP-was able to induce LTP not only with intact inhibition but also without inhibition in CA1 pyramidal cells in vitro.Therefore the degree of postsynaptic depolarization needed for back-propagation of action potentials and dendritic Ca 2ϩ spike generation required for STDP may be attained in special conditions without the need of postsynaptic action potential bursts. R E F E R E N C E S Fuenzalida M, Fernández de Sevilla D, Buño W. Changes of the EPSP waveform regulate the temporal window for spike-timing-dependent plasticity. J Neurosci 27: 11940 -11948, 2007. Fuenzalida M, Fernández de Sevilla D, Couve A, Buño W. Role of AMPA and NMDA receptors and back-propagating action potentials in spike timing-dependent plasticity. J Neurophysiol 103: 47-54, 2010. Kwag J, Paulsen O. The timing of external input controls the sign of plasticity at local synapses. Nat Neurosci 12: 1219 -1221, 2009. Meredith RM, Groen MR. Inhibition of action potential backpropagation during postnatal development of the hippocampus.