Astrocyte Glutamate Transporters Regulate Metabotropic Glutamate Receptor-Mediated Excitation of Hippocampal Interneurons

Abstract

Clearance of extracellular glutamate is essential for limiting the activity of metabotropic glutamate receptors (mGluRs) at excitatory synapses; however, the relative contribution of transporters found in neuronal and glial membranes to this uptake is poorly understood. Hippocampal interneurons located at the oriens-alveus border express mGluR1␣, a metabotropic glutamate receptor that regulates excitability and synaptic plasticity. To determine which glutamate transporters are essential for removing glutamate at these excitatory synapses, we recorded mGluR1-mediated EPSCs from oriens-lacunosum moleculare (O-LM) interneurons in acute hippocampal slices. Stimulation in stratum oriens reliably elicited a slow mGluR1-mediated current in O-LM interneurons if they were briefly depolarized to allow Ca 2ϩ entry before stimulation. Selective inhibition of GLT-1 [for glutamate transporter; EAAT2 (for excitatory amino acid transporter)] with dihydrokainate increased the amplitude of these responses approximately threefold, indicating that these transporters compete with mGluRs for synaptically released glutamate. However, inhibition of all glutamate transporters with TBOA (DL-threo-bbenzyloxyaspartic acid) increased mGluR1 EPSCs Ͼ15-fold, indicating that additional transporters also shape activation of these receptors. To identify these transporters, we examined mGluR1 EPSCs in mice lacking GLAST (for glutamate-aspartate transporter; EAAT1) or EAAC1 (for excitatory amino acid carrier; EAAT3). A comparison of responses recorded from wild-type and transporter knock-out mice revealed that the astroglial glutamate transporters GLT-1 and GLAST, but not the neuronal transporter EAAC1, restrict activation of mGluRs in O-LM interneurons. Transporter-dependent potentiation of mGluR1 EPSCs led to a dramatic increase in interneuron firing and enhanced inhibition of CA1 pyramidal neurons, suggesting that acute or prolonged disruption of transporter activity could lead to changes in network activity as a result of enhanced interneuron excitability.

Disynaptic Amplification of Metabotropic Glutamate Receptor 1 Responses in the Olfactory Bulb

“…Likewise, a depolarizing step delivered before stimulation of the ON did not potentiate mGluR1-mediated EPSPs isolated in the presence of AP-5 and NBQX (n ϭ 4; data not shown), suggesting that the glutamate released from one mitral cell did not significantly facilitate the activation of mGluR1s. These data are also consistent with a recent study (Yuan and Knopfel, 2006) indicating that elevation of intracellular calcium does not potentiate the metabotropic EPSP as in other pathways (Batchelor and Garthwaite, 1997;Huang et al, 2004).…”

“…4b). Thus, in the absence of glutamate release from dendrites, ON-stimulated mGluR1 responses appear similar to other pathways where mGluR1s are located at the periphery of the postsynaptic density (Baude et al, 1993;Batchelor and Garthwaite, 1997;Brasnjo and Otis, 2001;Dzubay and Otis, 2002;Huang et al, 2004). Our data are also consistent with ultrastructural data (van den Pol, 1995) suggesting that mitral cell mGluR1s are located on postsynaptic membranes facing ON terminals.…”

“…Our results were somewhat puzzling because synaptic activation of mGluR1 in other pathways typically requires trains of stimuli (Batchelor and Garthwaite, 1997;Brasnjo and Otis, 2001;Dzubay and Otis, 2002;Huang et al, 2004). Furthermore, as discussed above, the mitral cell response evoked by ON stimulation is usually completely blocked by NMDA and AMPA receptor antagonists (Chen and Shepherd, 1997;Carlson et al, 2000;Schoppa and Westbrook, 2001;De Saint Jan and Westbrook, 2005).…”

“…The metabotropic glutamate receptor 1 (mGluR1) is expressed in mitral cell postsynaptic dendrites (van den Pol, 1995) and can affect mitral cell excitability (Schoppa and Westbrook, 2001;Heinbockel et al, 2004). However, mGluR1-mediated EPSPs/EPSCs have only been reported after high-frequency stimulation of presynaptic neurons or when glutamate transporters are blocked (Batchelor and Garthwaite, 1997;Brasnjo and Otis, 2001;Dzubay and Otis, 2002;Huang et al, 2004;De Saint Jan and Westbrook, 2005;Ennis et al, 2006;Yuan and Knopfel, 2006). Such protocols presumably are required because they facilitate glutamate spillover onto perisynaptic loci where mGluRs are generally expressed (Baude et al, 1993).…”

Dynamic Regulation of Synaptic GABA Release by the Glutamate-Glutamine Cycle in Hippocampal Area CA1

“…To test this hypothesis, we applied DHK (250 M), together with MSOP (50 M) and MCPG (100 M) (mGluR I and III antagonists, respectively) during burst stimulation. mGluR antagonists were included in these experiments to prevent the effects of DHK-induced elevation of extracellular glutamate from acting on mGluRs located on GABA interneurons, which could confound our analyses (Huang et al, 2004). As shown in Figure 2 D, the presence of DHK produced the same effects as MSO and MeAIB; the eIPSC amplitude was significantly decreased to 53 Ϯ 10% ( p Ͻ 0.05; from 360.2 Ϯ 89.1 to 161.0 Ϯ 31.8 pA; n ϭ 6) but returned to 99 Ϯ 20% of baseline within 30 min (n ϭ 4).…”