mGluR5 metabotropic glutamate receptor distribution in rat and human spinal cord: a developmental study

By combining biochemical, molecular and immunohistochemical approaches, we have investigated the presence of metabotropic glutamate receptors (mGluRs) belonging to the subtype 5 in rat and human spinal cords and the developmental changes in their expression. A polyclonal antibody raised against the carboxy-terminal portion of mGluR5 was used to study the distribution of the receptor in rat foetal (Et15), neonatal (P8) and adult spinal cords and dorsal root ganglia (DRG). mGluR5 appeared to be predominantly expressed in regions containing the primary sensory afferents. Immunoblotting with anti-mGluR5 antibody revealed lower receptor protein levels in rat adult spinal cord when compared with P8 rat spinal cord. Reverse transcriptase-polymerase chain reaction showed both mGluR5a and mGluR5b mRNAs expression in rat spinal cord. The mGluR5a variant was found more abundant in young animals than in adults. The pattern of mGluR5 immunostaining was also studied in foetal (6-8, 10, 12 and 22 weeks of gestation) and adult human spinal cord. At all stages of human development, a strong mGluR5 immunoreactivity was observed in the dorsal roots and in the dorsal and dorsolateral funiculi with maximum levels of staining at week 12 of gestation. Foetal DRG neurons were heterogeneously labeled. mGluR5 was also diffusely detectable in the mantle layer. In adult human spinal cords, immunoreactivity was confined to laminae I and II of the dorsal horns. These results demonstrate for the first time the presence of mGluR5 in human spinal cord. The distribution of this receptor suggests a role in the development of somatosensory pathways and in the control of nociceptive neurotransmission.