Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn

Abstract

Studies indicate that metabotropic glutamate receptors (mGluRs) may play a role in spinal sensory transmission. We examined the cellular and subcellular distribution of the mGluR subtype 4a in spinal tissue by means of a specific antiserum and immunocytochemical techniques for light and electron microscopy. A dense plexus of mGluR4a-immunoreactive elements was seen in the dorsal horn, with an apparent accumulation in lamina II. The immunostaining was composed of sparse immunoreactive fibres and punctate elements. No perikaryal staining was seen. Immunostaining for mGluR4a was detected in small to medium-sized cells but not in large cells in dorsal root ganglia. At the electron microscopic level, superficial dorsal horn laminae demonstrated numerous immunoreactive vesicle-containing profiles. Labelling was present in the cytoplasmic matrix, but accretion of immunoreaction product to presynaptic specialisations was commonly observed. Axolemmal labelling was confirmed by using a preembedding immunogold technique, which revealed distinctive deposits of gold immunoparticles along presynaptic thickenings with an average centre-to-centre distance of 41 nm (41.145 +/- 13.59). Immunoreactive terminals often formed synaptic contacts with dendritic profiles immunonegative for mGluR4a. Immunonegative dendritic profiles were observed in apposition to both mGluR4a-immunoreactive and immunonegative terminals. Diffuse immunoperoxidase reaction product was also detected in dendritic profiles, some of which were contacted by mGluR4a-immunoreactive endings, but only occasionally were they observed to accumulate immunoreaction product along the postsynaptic density. Terminals immunoreactive for mGluR4a also formed axosomatic contacts. The present results reveal that mGluR4a subserves a complex spinal circuitry to which the primary afferent system seems to be a major contributor.

Alleviating Pain Hypersensitivity through Activation of Type 4 Metabotropic Glutamate Receptor

“…By contrast, mGlu4 receptor labeling does not significantly overlap with NF200 (Ͻ1%), a marker of myelinated A␤ or A␦ fibers. In accordance with previous electronic microscopy data indicating that mGlu4 is commonly observed in presynaptic terminals in the dorsal spinal cord (Azkue et al, 2001), most mGlu4 receptor immunoreactivity merges with that of synaptophysin, a membrane glycoprotein characteristic of presynaptic vesicles (Wiedenmann and Franke, 1985), and bassoon, a protein expressed at the presynaptic nerve terminals (tom Dieck et al, 1998).…”

“…These results are consistent with the antihyperalgesia observed after activation of Group III mGluRs with Group III selective agonists . Antihyperalgesic effects of Group III agonists could be mediated by mGlu4, mGlu7, and/or mGlu8 receptors, which are expressed either in the spinal cord (Ohishi et al, 1995b;Azkue et al, 2001) or in cell bodies of sensory neurons (Carlton and Hargett, 2007). This notwithstanding, the use of selective mGlu4 agonists in this study or allosteric enhancers suggests that this particular subtype plays a pivotal role in pain modulation.…”

“…Previous immunocytochemistry studies revealed the presence of mGlu4 receptors in presynaptic elements from afferent fibers in lamina II of the dorsal horn in the rat spinal cord (Azkue et al, 2001). However, the nature of the neurons expressing this receptor was not determined.…”

“…Silencing of spinal mGlu4 was achieved using AS oligonucleotides injected intrathecally according to the method described by Mestre et al (1994). This method allows the AS oligonucleotides to reach the DRG neurons (Bourinet et al, 2005). Rats were treated by vehicle, scrambled control oligonucelotides (MM), or mGlu4 receptor targeting AS oligonucleotides (AS) injected intrathecally twice daily for 4 d. On the fourth day, mechanical sensitivity was assessed by the paw pressure test be- fore killing the animals and quantifying the degree of knockdown.…”

“…Chronic pain is among the most debilitating and costly afflictions in North America and Europe, seriously affecting the quality of life of Ͼ19% of adult Europeans (Verhaak et al, 1998;Ospina and Harstall, 2002;Breivik et al, 2006;Bouhassira et al, 2008). With regard to neuropathic pain, the burden of disease is further worsened by the large proportion of patients whose treatment does not offer significant relief (Micó et al, 2006;Finnerup et al, 2010), highlighting the crucial need of progress in the management of pain.…”