Thermal Hyperalgesia via Supraspinal Mechanisms in Mice Lacking Glutamate Decarboxylase 65

Abstract

␥-Aminobutyric acid, which is synthesized by two isoforms of glutamate decarboxylase (GAD), inhibits the transfer of nociceptive signals from primary afferent fibers to the central nervous system. However, the roles of a 65-kDa isoform of GAD (GAD65)-mediated GABA in nociceptive processing are less clear. This study tested whether partial reductions in GABAergic inhibitory tone by GAD65 gene knockout [GAD65(Ϫ/Ϫ)] would contribute to the regulation of pain threshold in mice. Experiments were performed on male wild-type (WT) mice and GAD65(Ϫ/Ϫ) mice. Acute nociception and inflammatory pain tests were compared between WT mice and GAD65(Ϫ/Ϫ) mice. GABA A receptor-mediated inhibitory postsynaptic currents were also examined by use of the whole-cell patch-clamp method in somatosensory cortical neurons in brain slices. In the hot plate test, which reflects supraspinal sensory integration, a significant reduction in the latency was observed for GAD65(Ϫ/Ϫ) mice. Intraperitoneal administration of the GABAethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (C 21 H 22 N 2 O 3 ⅐HCl; NO-711), dose-dependently prolonged the latency in both genotypes, suggesting that GABA concentration contributes to acute thermal nociception. However, there was no genotype difference in responses to the tailimmersion test or the von Frey test, indicating that spinal reflex and mechanical nociception are kept intact in GAD65(Ϫ/Ϫ) mice. There was no genotype difference in responses to chemical inflammatory nociception (formalin test and carrageenan test). Although properties of the phasic component of inhibitory postsynaptic currents were similar in both genotypes, tonic inhibition was significantly reduced in GAD65(Ϫ/Ϫ) mice. These results support the hypothesis that GAD65-mediated GABA synthesis plays relatively small but significant roles in nociceptive processing via supraspinal mechanisms.

A Subconvulsive Dose of Kainate Selectively Compromises Astrocytic Metabolism in the Mouse Brain In Vivo

“…23 This was based on the observation that kainate did not lead to hypometabolism in a GAD65 knockout mouse model exhibiting impairments in tonic inhibition. 23,33 As alterations in tonic inhibition are expected to rely on changes in neuronal GABA release, a convulsive dose of kainate, which affects both neurons and astrocytes, is required to mediate an augmented tonic inhibition and the associated hypometabolism. Moreover, the effect of tonic inhibition appears to be dramatic, as it is able to override the substantial metabolic changes observed in astrocytes induced by the subconvulsive dose.…”

Possible Loss of GABAergic Inhibition in Mice With Induced Adenomyosis and Treatment With Epigallocatechin-3-Gallate Attenuates the Loss With Improved Hyperalgesia

“…This is consistent with the report that in rat persistent inflammatory and neuropathic pain epigenetically suppresses gad65 activities in the NRM through HDACmediated histone hypoacetylation, resulting in impaired GABA synaptic inhibition. 36 In addition, gad65 knockout mice exhibit thermal hyperalgesia 17 and display exaggerated pain behavior in conjunction with an impaired GABA synaptic function in the brainstem neurons. 36 This may also explain, at least in part, that the use of an HDAC inhibitor such as valproic acid can reduce hyperalgesia resulting from induced adenomyosis in mice, 9,20 since HDAC inhibitors overwhelmingly increase gad65 activities, restore GABA synaptic function, and relieve the sensitized pain behavior.…”

“…16 Mice lacking GAD65 exhibit thermal hyperalgesia, suggesting that GAD65-mediated GABA synthesis plays a significant role in nociceptive processing via supraspinal mechanisms. 17 Indeed, supraspinal GABAergic inhibition, usually through modulation of specific GABA A receptor subtypes, plays an important role in neuronal hyperexcitability and spinal/supraspinal nociceptive neurons. 18,19 We have previously shown that mice with induced adenomyosis exhibit increased uterine hyperactivity, general hyperalgesia, and elevated plasma corticosterone levels.…”

Transcriptional control ofGad2

“…Gad2 KO mice display sensitized pain behaviors, 27,36 through histone hypoacetylation-induced downregulation of Gad2. 27 Viral delivery of the Gad2 gene or histone hyperacetylation-induced upregulation of Gad2 inhibits pain.…”