Up‐regulation of GLT‐1 severely impairs LTD at mossy fibre–CA3 synapses

Abstract

Glutamate transporters are responsible for clearing synaptically released glutamate from the extracellular space. By this action, they maintain low levels of ambient glutamate, thus preventing excitotoxic damage, and contribute to shaping synaptic currents. We show that up-regulation of the glutamate transporter GLT-1 by ceftriaxone severely impaired mGluR-dependent long-term depression (LTD), induced at rat mossy fibre (MF)-CA3 synapses by repetitive stimulation of afferent fibres. This effect involved GLT-1, since LTD was rescued by the selective GLT-1 antagonist dihydrokainate (DHK). DHK per se produced a modest decrease in fEPSP amplitude that rapidly regained control levels after DHK wash out. Moreover, the degree of fEPSP inhibition induced by the low-affinity glutamate receptor antagonist γ-DGG was similar during basal synaptic transmission but not during LTD, indicating that in ceftriaxone-treated rats LTD induction did not alter synaptic glutamate transient concentration. Furthermore, ceftriaxone-induced GLT-1 up-regulation significantly reduced the magnitude of LTP at MF-CA3 synapses but not at Schaffer collateral-CA1 synapses. Postembedding immunogold studies in rats showed an increased density of gold particles coding for GLT-1a in astrocytic processes and in mossy fibre terminals; in the latter, gold particles were located near and within the active zones. In both CEF-treated and untreated GLT-1 KO mice used for verifying the specificity of immunostaining, the density of gold particles in MF terminals was comparable to background levels. The enhanced expression of GLT-1 at release sites may prevent activation of presynaptic receptors, thus revealing a novel mechanism by which GLT-1 regulates synaptic plasticity in the hippocampus.

The mGluR2/3 Agonist LY379268 Blocks the Effects of GLT-1 Upregulation on Prepulse Inhibition of the Startle Reflex in Adult Rats

“…Rothstein et al (2005) showed that CEF robustly stimulates GLT-1 expression, but not that of the other glutamate transporters, through increased GLT-1 gene transcription. This finding has been replicated in different laboratories (eg, Chu et al, 2007;Ouyang et al, 2007;Lee et al, 2008), including ours Omrani et al, 2009). We also showed that CEF increases GLT-1a expression in forebrain regions that regulate PPI of the startle .…”

“…mGluR2/3 activation depends on the spread of glutamate from release sites to the perisynaptic zone, where these receptors are predominantly located (Scanziani et al, 1997;Min et al, 1998;Mitchell and Silver, 2000;Ferraguti and Shigemoto, 2006); and GLT-1 controls extracellular levels of glutamate and it is mainly localized perisynaptically (Melone et al, 2009a). Thus, by modulating glutamate spillover, GLT-1 influences activation of mGluR2/3 Barbour, 2001;Diamond, 2002;Tzingounis and Wadiche, 2007;Zheng et al, 2008;Omrani et al, 2009). Our western blotting and immunocytochemical studies are consistent with this view.…”

“…GLT-1 upregulation induced by CEF determines a strong, DHK-sensitive, impairment of mGluR-dependent LTD (Omrani et al, 2009). The possibility that a widespread impairment of LTD can induce cognitive alterations, including PPI abnormalities, has been investigated in animal models of schizophrenia.…”

“…CEF sodium (CEF, Rocefin, Roche, Milano, Italy) was used to upregulate GLT-1 (Rothstein et al, 2005;Bellesi et al, 2009;Omrani et al, 2009). CEF was dissolved in saline (0.9% NaCl) and administered intraperitoneally (i.p.)…”

“…The selective mGluR2/3 agonist LY379268 attenuates behavioral abnormalities in animal models of schizophrenia, and its analog LY404039 is effective in a schizophrenia phase 2 clinical trial (Cartmell et al, 1999(Cartmell et al, , 2000Clark et al, 2002;Imre et al, 2006;Imre, 2007;Patil et al, 2007;Harrison, 2008). Recently, we have shown that ceftriaxone (CEF)-induced GLT-1 upregulation impairs hippocampal long-term depression (LTD) in a DHK-reversible manner by limiting activation of mGluR2/3, thus indicating that a reduced function of mGluR2/3 mediates the effects of GLT-1 upregulation (Omrani et al, 2009).…”