title: Spinal glutamate uptake is critical for maintaining normal sensory transmission in rat spinal cord.
abstract: Glutamate is a major excitatory neurotransmitter in primary afferent terminals and is critical for normal spinal excitatory synaptic transmission. However, little is known about the regulation of synaptically released glutamate in the spinal cord under physiologic conditions. The sodium-dependent, high-affinity glutamate transporters are the primary mechanism for the clearance of synaptically released glutamate. In the present study, we found that intrathecal injection of glutamate transporter blockers DL-threo-beta-benzyloxyaspartate (TBOA) and dihydrokainate produced significant and dose-dependent spontaneous nociceptive behaviors, such as licking, shaking, and caudally directed biting, phenomena similar to the behaviors caused by intrathecal glutamate receptor agonists. Intrathecal TBOA also led to remarkable hypersensitivity in response to thermal and mechanical stimuli. These behavioral responses could be significantly blocked by intrathecal injection of the NMDA receptor antagonists MK-801 and AP-5, the non-NMDA receptor antagonist CNQX or the nitric oxide synthase inhibitor L-NAME. In vivo microdialysis analysis showed short-term elevation of extracellular glutamate concentration in the spinal cord after intrathecal injection of TBOA. Furthermore, topical application of TBOA on the dorsal surface of the spinal cord resulted in a significant elevation of extracellular glutamate concentration demonstrated by in vivo glutamate voltametry. The present study indicates that defective spinal glutamate uptake caused by inhibition of glutamate transporters leads to excessive glutamate accumulation in the spinal cord. The latter results in persistent over-activation of synaptic glutamate receptors, producing spontaneous nociceptive behaviors and sensory hypersensitivity. Our results suggest that glutamate uptake through spinal glutamate transporters is critical for maintaining normal sensory transmission under physiologic conditions.
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University School of Medicine, 355 Ross, 720 Rutland Ave., Baltimore, MD 21205, USA.
authors.forename: Wen-Jinn
authors.initials: WJ
authors.lastname: Liaw
authors.forename: Robert L
authors.initials: RL
authors.lastname: Stephens
authors.forename: Brian C
authors.initials: BC
authors.lastname: Binns
authors.forename: Yachun
authors.initials: Y
authors.lastname: Chu
authors.forename: Jehuda P
authors.initials: JP
authors.lastname: Sepkuty
authors.forename: Roger A
authors.initials: RA
authors.lastname: Johns
authors.forename: Jeffery D
authors.initials: JD
authors.lastname: Rothstein
authors.forename: Yuan-Xiang
authors.initials: YX
authors.lastname: Tao
mesh_terms: D000818:Animals; D001224:Aspartic Acid / Q000008:administration & dosage; D004305:Dose-Response Relationship, Drug; D018698:Glutamic Acid / Q000378:metabolism*; D008297:Male; D009475:Neurons, Afferent / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism; D009619:Nociceptors / Q000187:drug effects / Q000503:physiopathology*; D010146:Pain / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000187:drug effects / Q000503:physiopathology*; D009435:Synaptic Transmission
chemical_list: D018377:Neurotransmitter Agents; C120673:benzyloxyaspartate; D001224:Aspartic Acid; D018698:Glutamic Acid