Localization of metabotropic glutamate receptors 2/3 on primary afferent axons in the rat.

The goal of the present study is to determine the relationship of metabotropic glutamate receptors 2/3 (mGluR2/3) to dorsal root ganglion cells, peripheral primary afferent fibers in digital nerves and central primary afferent fibers in the spinal cord. We demonstrate that approximately 40% of L4 and L5 dorsal root ganglion cells contain mGluR2/3-like immunoreactivity. These mGluR2/3-positive cells are small in diameter (23 microm) and 76% stain for the isolectin Griffonia simplicifolia (I-B4), while 67% of I-B4 cells have mGluR2/3-like immunoreactivity. Electron microscopic analyses of mGluR2/3-like immunoreactivity in axons in digital nerves indicate that 32% of unmyelinated and 28% of myelinated axons are labeled. In the lumbar dorsal horn, mGluR2/3-like immunoreactivity is localized preferentially in lamina IIi with lighter staining in laminae III and IV. The dense mGluR2/3-like immunoreactivity in lamina IIi is consistent with the localization of these receptors in I-B4-labeled dorsal root ganglion cells. Elimination of primary afferent input following unilateral dorsal rhizotomies significantly decreases the mGluR2/3-like immunoreactivity density in the dorsal horn although some residual staining does remain, suggesting that many but not all of these receptors are located on primary afferent processes. The finding that mGluR2/3s are located on peripheral sensory axons suggests that they are involved in peripheral sensory transduction and can modulate transmission of sensory input before it reaches the spinal cord. This offers the possibility of altering sensory input, particularly noxious input, at a site that would avoid CNS side effects. Since many but not all of these receptors are located on primary afferent terminals, these receptors may also influence primary afferent transmission in the dorsal horn through presynaptic mechanisms and glutamatergic transmission in general through both presynaptic and postsynaptic mechanisms. Since these receptors are concentrated in lamina IIi and also largely co-localized with I-B4, they may have considerable influence on nociceptive processing by what are considered to be non-peptidergic primary afferent neurons.