Morphology and electrophysiological properties of hamster spinal dorsal horn neurons that express VGLUT2 and enkephalin

Abstract

The excitatory amino acid glutamate mediates transmission at spinal synapses, including those formed by sensory afferent fibers and by intrinsic interneurons. The identity and physiological properties of glutamatergic dorsal horn neurons are poorly characterized despite their importance in spinal sensory circuits. Moreover, many intrinsic spinal glutamatergic synapses colocalize the opioid peptide enkephalin (ENK), but the neurons to which they belong are yet to be identified. Therefore, we used immunohistochemistry and confocal microscopy to investigate expression of the VGLUT2 vesicular glutamate transporter, an isoform reported in nonprimary afferent spinal synapses, and ENK in electrophysiologically identified neurons of hamster spinal dorsal horn. VGLUT2 immunoreactivity was localized in restricted fashion to axon varicosities of neurons recorded from laminae II-V, although the occurrence of immunolabeling in individual varicosities varied widely between cells (39 +/- 36%, n = 31 neurons). ENK colocalized with VGLUT2 in up to 77% of varicosities (17 +/- 21%, n = 21 neurons). The majority of neurons expressing VGLUT2 and/or ENK had axons with dense local terminations or projections consistent with propriospinal functions. VGLUT2 and ENK labeling were not correlated with cellular morphology, intrinsic membrane properties, firing patterns, or synaptic responses to sensory afferent stimulation. However, VGLUT2 expression was significantly higher in neurons with depolarized resting membrane potential. The results are new evidence for a population of dual-function dorsal horn interneurons that might provide another mechanism for limiting excitation within dorsal horn circuits during periods of strong sensory activation.

Local Circuit Connections Between Hamster Laminae III and IV Dorsal Horn Neurons

“…In this study, simultaneous whole cell recordings were made between synaptically connected pairs of neurons in spinal lamina III and IV where collateral branches of slowly and rapidly adapting mechanoreceptor afferent fibers terminate. The present results are consistent with our previous studies suggesting that LIII/IV interneurons are interconnected via excitatory synapses using glutamate as a transmitter or via inhibitory linkages mediated by release of GABA (Schneider and Lopez 2002;Schneider and Walker 2007). Synaptic connections between LIII and IV neurons are predominantly unidirectional, and the majority of these are inhibitory.…”

“…The present results are consistent with our previous finding that the VGLUT2 vesicular glutamate transporter, a specific and reliable marker of glutamatergic synapses, is localized within axon terminals of dorsal horn interneurons (Schneider and Walker 2007). Together these studies show that glutamate, acting on AMPA type receptors, mediates excitatory synaptic linkages between lamina III and IV interneurons.…”

“…Transmission failures between LIII and IV cells could reflect impairment of presynaptic action potential invasion at the branch points of complex axon terminations (Henneman et al 1984;Lüscher and Shiner 1990) that are characteristic of interneurons in this region (Schneider 1992;Schneider et al 1995). It is also possible that the high failure rates seen in the present study result from co-release of enkephalin at synaptic terminals containing glutamate or GABA (Schneider and Walker 2007;Todd et al 1992Todd et al , 2003 via activation of inhibitory presynaptic opioid receptors (Grudt and Henderson 1998;Hori et al 1992).…”

Development of “Pinceaux” formations and dendritic translocation of climbing fibers during the acquisition of the balance between glutamatergic and γ‐aminobutyric acidergic inputs in developing Purkinje cells

“…The pattern of immunohistochemical labeling obtained with the guinea pig anti-VGLUT1 in rodent brain tissue was similar to that obtained with the rabbit anti-VGLUT1 (Persson et al, 2006;Bogen et al, 2006). Preabsorption of antisera with the immunogen synthetic peptide utilized to produce guinea pig anti-VGLUT2 antibodies eliminated the immunostaining in tissue sections from rat CNS (manufacturer's technical information) and in hamster spinal cord (Schneider and Walker, 2007). The pattern of rodent CNS staining was identical to that obtained by Fremeau et al (2001) with a polyclonal rabbit anti-VGLUT2 antibody.…”

Coexpression of the mu‐opioid receptor splice variant MOR1C and the vesicular glutamate transporter 2 (VGLUT2) in rat central nervous system

“…Many neurons in the spinal cord are interneurons with processes that extend only few spinal segments (Kostyuk and Vasilenko, 1979). At least some of these interneurons express VGLUT2 (Hinckley et al, 2005;Bannatyne et al, 2006;Schneider and Walker, 2007) and thus could be the source of some of the VGLUT2-ir we observed in the spinal cord. As mentioned above, we and others (Pan et al, 1999;Schnell and Wessendorf, 2004) have found abundant MOR 1C mRNA throughout the spinal cord.…”