Colocalization of metabotropic glutamate receptors in rat dorsal root ganglion cells

Abstract

Glutamate is the main excitatory transmitter in both central and peripheral nervous systems. Discovery of metabotropic glutamate receptors (mGluRs) made it clear that glutamate can have excitatory or inhibitory effects on neuronal function, with group I mGluRs enhancing cell excitability but group II and III mGluRs decreasing excitability. The present study investigated the colocalization of mGluR subtypes representing groups I, II, or III in rat L5 dorsal root ganglion (DRG) cells. The analyses show that group III has the highest expression, with 75.0% of DRG cells expressing mGluR8, followed by group II, with 51.6% expressing mGluR2/3, followed by group I, with only 6.8% expressing mGluR1alpha. mGluR8 is expressed by small, medium, and large diameter cells. In contrast, mGluR1alpha and mGluR2/3 are expressed by mainly small and medium cells. Approximately half of cells expressing group I mGluR1alpha also express either group II mGluR2/3 or group III mGluR8. These mGluR1alpha double-labeled populations are not likely to overlap since >1.0% of mGluR1alpha are triple-labeled. As expected from the high percentage of single-labeled mGluR2/3 and mGluR8 cells, there is a considerable population of double-labeled cells with approximately 30% of each population expressing both receptors. Due to the fact that the number of mGluR1alpha-expressing cells in the DRG is low, the percentage of triple-labeled cells is also low ( approximately 1-2%). The prevalence of groups II and III indicate that glutamate could have a substantial inhibitory effect of primary afferent function, reducing and/or fine-tuning sensory input before transmission to the spinal cord. These anatomical data highlight the potential inhibitory role glutamate may play in peripheral sensory transmission.

Group II metabotropic glutamate receptor activation attenuates peripheral sensitization in inflammatory states

“…Furthermore, the concentrations of APDC (0.1 μM and 0.5 μM) needed to achieve these effects are very close to the ED 50 (0.4 μM, based on an assay analyzing inhibition of forskolin-stimulated cAMP in non-neural cells, [Schoepp et al, 1999]) and these concentrations are at least 100-fold less than that used in previous studies (Yang and Gereau, 2002;Yang and Gereau, 2003). Although Group I and III mGluRs are also expressed by primary afferents (Ohishi et al, 1995;Zhou et al, 2001; Azkue et al, 2001;Carlton and Hargett, 2007), it is unlikely that they contribute to these actions since APDC, used at these low concentrations, does not bind to Group I or Group III mGluRs (Schoepp et al, 1996(Schoepp et al, , 1999. The Group II antagonist LY does have some affinity for Group I and III mGluRs (Schoepp et al, 1999); however, 100 μM LY has no significant inhibitory effect on forskolin-activated cAMP in mGluR8-expressing cells and LY was shown to enhance forskolin-activated cAMP in mGluR7-and mGluR4-expressing cells (Kingston et al, 1998).…”

“…Group II mGluRs have been localized in dorsal root ganglion cells (Petralia et al, 1996) and on central processes of presumed primary afferent fibers (Jia et al, 1999). Our lab has localized Group II receptors on peripheral cutaneous processes and lumbar DRG cell bodies (Carlton et al, 2001b;Carlton and Hargett, 2007). Thus, it is highly likely that Group II mGluRs can modulate primary afferent mechanisms that lead to peripheral sensitization, as well as spinal mechanisms that lead to central sensitization.…”

“…Approximately 40-50% of lumbar DRG cells express Group II mGluRs and these are mainly small to medium sized cells (mean diameter = 23μM) (Carlton et al, 2001b;Carlton and Hargett, 2007). The majority of DRG cells expressing Group II mGluRs (76%) stain positively for isolectin Griffonia Simplicifolia (I-B4) (Carlton et al, 2001b), suggesting that these cells either express low levels of peptides or they are non-peptidergic (Silverman and Kruger, 1990).…”

Capsaicin-Induced Glutamate Release Is Implicated in Nociceptive Processing Through Activation of Ionotropic Glutamate Receptors and Group I Metabotropic Glutamate Receptor in Primary Afferent Fibers

“…There are evidences that virtually all DRG cells as well as their central and peripheral terminals are positively labeled for iGluRs (3) and mGluRs (4). Electron microscope studies demonstrate that Glu receptors are transported from the DRG cell bodies into central and/ or peripheral primary afferent terminals (5).…”

Interactions Between Glutamate Receptors and TRPV1 Involved in Nociceptive Processing at Peripheral Endings of Primary Afferent Fibers

“…The ionotropic, and metabotropic subunits of Glu receptors are present in DRG cell bodies and on unmyelinated fibers in the glabrous skin of the mammalian foot (Carlton et al, 1995;Bhave et al, 2001;Carlton et al, 2001;Sato et al, 1993;Carlton et al, 2007). It is well established that the excitatory amino acids in the peripheral endings of small-diameter afferent fibers contribute to development and/or maintenance of pain in humans (Nordlind et al, 1993;Warncke et al, 1997) and in laboratory animals (Davidson et al, 1997;Cairns et al, 1998;Davidson et al, 1998).…”