Assessing glutamatergic function and dysfunction in peripheral tissues.

Glutamate is the major mediator of excitatory signaling in the mammalian central nervous system (CNS) and it has recently been described to have a central role in the transduction of sensory input in the peripheral nervous system (PNS), too. However, functional glutamatergic systems are expressed by peripheral non-neural tissues as well, such as heart, kidney, lungs, ovary, testis, blood and skin. Interestingly, glutamatergic alterations have been repeatedly described in these tissues in various neuropsychiatric diseases. Here we will review evidence suggesting that glutamate measurements obtained from sampling ex vivo peripheral cells can permit the assessment of the dynamics of glutamate release, uptake, receptor-mediated signaling, synthesis and degradation, and mirror homologous dysfunctions operative within the CNS in each single patient. Among all the available cell types we will focus on leukocytes, platelets and fibroblasts that can be easily obtained from patients multiple times without concerns related to post-mortem changes. Finally, we will briefly review another possibility, offered by testing plasma (and CSF) glutamate levels, allowing the indirect investigation of glutamate-mediated crosstalk between central and peripheral compartments. Technical pitfalls of these biomarkers will be contextually emphasized.

Expression, distribution and glutamate uptake activity of high affinity-excitatory aminoacid transporters in in vitro cultures of embryonic rat dorsal root ganglia.

Glutamate is the major mediator of excitatory signalling in the mammalian central nervous system, but it has recently been shown to play a role in the transduction of sensory input at the periphery and in peripheral neuropathies. New advances in research have demonstrated that rat peripheral sensory terminals and dorsal root ganglia (DRG) express molecules involved in glutamate signalling, including high-affinity membrane-bound glutamate transporters (GLAST [glutamate aspartate transporter], GLT1 [glutamate transporter 1], EAAC1 [excitatory aminoacid transporter 1]) and that alterations in their expression and/or functionality can be implicated in several models of peripheral neuropathy, neuropathic pain and hyperalgesia. Here we describe, through immunoblotting, immunofluorescence assays and ß-counter analysis of [(3)H] l-glutamate uptake, the expression, distribution and activity of the glutamate transporters in in vitro cultures of embryonic dorsal root ganglia sensory neurons, sensory neurons+satellite cells and satellite cells. In this work we demonstrated that glutamate transporters are expressed in all cultures with a peculiar pattern of distribution. Even if GLAST is strongly detected in satellite cells, it is slightly expressed also in sensory neurons. GLT1 immunostaining is very weak in DRG neurons, but it was evident in the satellite cells. Finally, EAAC1 is localized in the soma and in the neuritis of sensory neurons, while it is not detectable in satellite cells. Moreover, all the cell cultures showed a strong sodium-energy-dependent glutamate uptake activity and it is more marked in neurons alone or in co-culture with satellite cells compared to satellite cells alone. Finally, we show that the complete or partial pharmacological inhibition of glutamate transporters virtually completely or partially abolish glutamate uptake in all cell culture. These results, that demonstrate that functionally active glutamate transporters can be studied in dorsal root ganglia cell cultures, provide further evidence for a role of glutamatergic transport in the peripheral nervous system and will be useful for testing whether any changes occur in in vitro models of peripheral nervous system damage.