Expression of the vesicular glutamate transporters-1 and -2 in adult mouse dorsal root ganglia and spinal cord and their regulation by nerve injury

Abstract

The expression of two vesicular glutamate transporters (VGLUTs), VGLUT 1 and VGLUT 2, was studied with immunohistochemistry in lumbar dorsal root ganglia (DRGs), the lumbar spinal cord and the skin of the adult mouse. About 12% and 65% of the total number of DRG neuron profiles (NPs) expressed VGLUT 1 and VGLUT 2, respectively. VGLUT 1-immunoreactive (IR) NPs were usually medium- to large-sized, in contrast to a majority of small- or medium-sized VGLUT 2-IR NPs. Most VGLUT 1-IR NPs did not coexpress calcitonin gene-related peptide (CGRP) or bound isolectin B4 (IB4). In contrast, ∼31% and ∼42% of the VGLUT 2-IR DRG NPs were also CGRP-IR or bound IB4, respectively. Conversely, virtually all CGRP-IR and IB4-binding NPs coexpressed VGLUT 2. Moderate colocalization between VGLUT 1 and VGLUT 2 was also observed. Sciatic nerve transection induced a decrease in the overall number of VGLUT 1- and VGLUT 2-IR NPs (both ipsi- and contralaterally) and, in addition, a parallel, unilateral increase of VGLUT 2-like immunoreactivity (LI) in a subpopulation of mostly small NPs. In the spinal cord, strong VGLUT 1-LI was detected, particularly in deep dorsal horn layers and in the ventral horns. VGLUT 2-LI was abundant throughout the gray spinal matter, ‘radiating’ into/from the white matter. A unilateral dorsal rhizotomy reduced VGLUT 1-LI, while apparently leaving unaffected the VGLUT 2-LI. Transport through axons for both VGLUTs was confirmed by their accumulation after compression of the sciatic nerve or dorsal roots. In the hind paw skin, abundant VGLUT 2-IR nerve fibers were observed, sometimes associated with Merkel cells. Lower numbers of VGLUT 1-IR fibers were also detected in the skin. Some VGLUT 1-IR and VGLUT 2-IR fibers were associated with hair follicles. Based on these data and those by Morris et al. [Morris JL, Konig P, Shimizu T, Jobling P, Gibbins IL (2005) Most peptide-containing sensory neurons lack proteins for exocytotic release and vesicular transport of glutamate. J Comp Neurol 483:1–16], we speculate that virtually all DRG neurons in adult mouse express VGLUTs and use glutamate as transmitter.