Glutamate released from astrocytes mediates slow inward currents in rat spinal cord dorsal horn.

Activation of glial cells (microglia and astrocytes) in spinal cord dorsal horn is involved in the maintenance and possibly the generation of inflammatory and neuropathic pain (Wieseler-Frank et al., 2004). In other brain regions, activated astrocytes release glutamate that acts on NMDA receptors, preferentially at the extrasynaptic membrane, to generate in nearby neurons slow inward currents (SICs). We used here patch-clamp recording and calcium imaging techniques to investigate whether astrocytes exert a similar action on dorsal horn neurons. In spinal cord slices from young postnatal rats, in the absence of extracellular Mg2+ and in the presence of tetrodotoxin, we observed spontaneous SICs in 5 of 42 lamina II neurons (12%), while occasional recordings revealed SICs also in laminae III-IV neurons. The percentage of cells exhibiting SICs increased by application of stimuli that induced Ca2+ oscillations in astrocytes, such as the purinerigic receptor agonist 2-& 3-O-(4-benzoylbenzoyl)ATP (100 µM) and low extracellular Ca2+. These stimuli triggered SICs in 33% and 61 % of neurons, respectively. These events had the typical slow kinetics of SICs: a rise time of 70 ± 14 ms, a decay time of 372 ± 102 ms and a mean amplitude of 175 ± 37 pA. By using pharmacological tools, we confirmed the identity of these events as SICs. Indeed, they resulted to be mediated by NMDA receptors since they were blocked by 100 µM D-APV (only one SIC was observed in 1 out of 9 cells in low extracellular Ca2+), and the AMPA receptor blocker NBQX (10 µM) had no effects (under these conditions, 55% of neurons displayed SICs upon low Ca2+ stimulation). Furthermore, in 1 mM Mg2+, astrocytic glutamate was still capable of activating neuronal NMDA receptors since low Ca2+ induced SICs in 36% of lamina II neurons tested. Confocal microscope Ca2+ imaging in spinal cord slices loaded with the Ca2+ indicator Oregon Green BAPTA-1 revealed that astrocytic glutamate, as previously observed in CA3 and CA1 hippocampal regions, evoked synchronous Ca2+ elevations in small groups of dorsal horn neurons.Our observations reveal that NMDA receptor-mediated signals characterize the action of astrocytes also in the dorsal horn and raise questions about their possible pathophysiological role.