Modulation of synaptic transmission from primary afferents to spinal substantia gelatinosa neurons by group III mGluRs in GAD65-EGFP transgenic mice

Abstract

Group III metabotropic glutamate receptors (mGluRs) are involved in nociceptive transmission in the spinal cord. However, the cellular mechanism underlying the modulation of synaptic transmission from nociceptive primary afferents to dorsal horn neurons by group III mGluRs has yet to be explored. In this study, we used transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the glutamate decarboxylase (GAD) 65 promoter to identify specific subpopulations of GABAergic inhibitory interneurons. By GABA immunolabeling, we confirmed the majority of GAD65-EGFP-expressing neurons were GABAergic. Because GAD65-EGFP-expressing neurons have not been examined in detail before, we first investigated the physiological properties of GAD65-EGFP- and non-EGFP-expressing neurons in substantia gelatinosa (SG) of the spinal dorsal horn. Membrane properties, such as the resting membrane potential, membrane capacitance, action potential threshold, and action potential height, differed significantly between these two groups of neurons. Most EGFP-expressing neurons displayed a tonic firing pattern (73% of recorded neurons) and received monosynaptic Aδ and/or C primary afferent inputs (85% of recorded neurons). In contrast, we observed a delayed firing pattern in 53% of non-EGFP-expressing neurons. After identifying the physiological properties of EGFP-expressing neurons, we tested the effects of group III mGluRs on synaptic transmission pharmacologically. A group III mGluR agonist, L-AP4, attenuated Aδ fiber-evoked synaptic transmission but did not affect C fiber-evoked synaptic transmission to EGFP-expressing neurons. Similar primary afferent-specific inhibition by L-AP4 was also observed in non-EGFP-expressing neurons. Moreover, Aδ fiber-evoked synaptic transmission was suppressed by a selective mGluR7 agonist, AMN082. These results suggest that modulation of the synaptic transmission from primary afferents to SG neurons by group III mGluR agonist is specific to the type of nociceptive primary afferents but not to the type of target neurons.

Morphological, biophysical and synaptic properties of glutamatergic neurons of the mouse spinal dorsal horn

“…; Cui et al . ). Corresponding marker genes for glutamatergic neurons belong to the family of vesicular glutamate transporters, which comprises three members, designated vGluT1 to vGluT3 (Chaudhry et al .…”

“…A more recently developed and in general more efficient approach is the use of reporter mice that express fluorescent proteins in defined neuronal subpopulations. Mice expressing enhanced green fluorescent protein (eGFP) in GABAergic neurons under the transcriptional control of the Gad67 or Gad65 gene, or in glycinergic neurons under the control of the GlyT2 (Slc6a5) gene have been successfully used to characterise dorsal horn inhibitory interneurons (Heinke et al 2004;Zeilhofer et al 2005;Gassner et al 2009;Labrakakis et al 2009;Cui et al 2011). Corresponding marker genes for glutamatergic neurons belong to the family of vesicular glutamate transporters, which comprises three members, designated vGluT1 to vGluT3 (Chaudhry et al 2008).…”

Characteristics of GABAergic and cholinergic neurons in perinuclear zone of mouse supraoptic nucleus

“…A description of these mice can be found in López-Bendito et al (2004), and numerous articles have been published on brain GABAergic anatomy and function using this line (e.g., Bali et al 2005;Betley et al 2009;Cui et al 2011;Parrish-Aungst et al 2007;Shin et al 2007Shin et al , 2011Wierenga et al 2010;Zhang et al 2006). The Szabó lab generated several lines of GAD65 mice-those used here were from line 30 and have been found to substantially overlap in hypothalamus and elsewhere with the known distribution of neurons immunoreactive for GAD or GABA (e.g., Mugnaini and Oertel 1985).…”

Ionotropic Glutamate Receptors and Voltage-Gated Ca2+Channels in Long-Term Potentiation of Spinal Dorsal Horn Synapses and Pain Hypersensitivity

“…The primary candidates for iSG neurons are γ -aminobutyric acid- (GABA-) ergic neurons in the spinal DH, an electrophysiologically heterogeneous group [28, 29] that make up approximately 30% of neurons in lamina II (SG) of the spinal cord [30]. It has been suggested, using combined single/paired whole-cell patch-clamp recordings and biocytin labeling, that ~75% of all SG neurons fall into five groups that differ in firing behavior and other electrophysiological properties and most prominently, in the structure of their dendritic arbors: islet, radial, central, medial-lateral, and vertical cells [31–36].…”