NAAG peptidase inhibitor increases dialysate NAAG and reduces glutamate, aspartate and GABA levels in the dorsal hippocampus following fluid percussion injury in the rat

Abstract

Traumatic brain injury (TBI) produces a rapid and excessive elevation in extracellular glutamate that induces excitotoxic brain cell death. The peptide neurotransmitter N-acetylaspartylglutamate (NAAG) is reported to suppress neurotransmitter release through selective activation of presynaptic group II metabotropic glutamate receptors. Therefore, strategies to elevate levels of NAAG following brain injury could reduce excessive glutamate release associated with TBI. We hypothesized that the NAAG peptidase inhibitor, ZJ-43 would elevate extracellular NAAG levels and reduce extracellular levels of amino acid neurotransmitters following TBI by a group II metabotropic glutamate receptor (mGluR)-mediated mechanism. Dialysate levels of NAAG, glutamate, aspartate and GABA from the dorsal hippocampus were elevated after TBI as measured by in vivo microdialysis. Dialysate levels of NAAG were higher and remained elevated in the ZJ-43 treated group (50 mg/kg, i.p.) compared with control. ZJ-43 treatment also reduced the rise of dialysate glutamate, aspartate, and GABA levels. Co-administration of the group II mGluR antagonist, LY341495 (1 mg/kg, i.p.) partially blocked the effects of ZJ-43 on dialysate glutamate and GABA, suggesting that NAAG effects are mediated through mGluR activation. The results are consistent with the hypothesis that inhibition of NAAG peptidase may reduce excitotoxic events associated with TBI.

Phencyclidine and Dizocilpine Induced Behaviors Reduced by N-acetylaspartylglutamate Peptidase Inhibition via Metabotropic Glutamate Receptors

“…Consistent with our behavioral data with ZJ43 and LY341495 and in the context of this model, a group II mGluR agonist reduces PCP-induced glutamate efflux as well as motor and cognitive behaviors (9). Similarly, we also found that ZJ43 induced elevation of synaptic NAAG levels reduces glutamate efflux (38).…”

“…Given the IC 50 and Ki of ZJ43 of 2.4 nM and 0.9 nM respectively when tested against human GCPII (37), these behaviorally effective doses of ZJ43 are clearly within the range required to substantially inhibit the activity of NAAG peptidases. Consistent with this conclusion, we have demonstrated that this dose range of ZJ43 significantly elevates extracellular NAAG levels and decreases glutamate release in the rat hippocampus following traumatic brain injury (38). While we do not yet know the full time course of inhibition of NAAG peptidases that is achieved by a single i.p.…”

“…While we do not yet know the full time course of inhibition of NAAG peptidases that is achieved by a single i.p. dose of ZJ43, we found that 150 mg/kg of ZJ43 retained near maximal effectiveness for at least 2 hours in vivo (unpublished) and that this effect was fully blocked by a single dose of LY431495 (38).…”

“…We directly tested this hypothesis in a brain injury paradigm and found that systemic injection of ZJ43 results in increased levels of extracellular NAAG and that this effect was completely blocked by the group II antagonist (38). One model of the role of glutamate in schizophrenia and PCP-induced behaviors proposes that hypoactivity of NMDA receptors, as is modeled by PCP, leads to reduced GABA-ergic inhibition, and excess release of glutamate and dopamine, particularly in the prefrontal cortex (45;46).…”

Neuroprotective Effects of Selective N-Type VGCC Blockade on Stretch-Injury-Induced Calcium Dynamics in Cortical Neurons

“…Microdialysis was initiated 10 min prior to injury and treatment, resulting in one sample before stretch injury and two samples collected at 10 and 20 min after stretch injury. Dialysates were then frozen, and glutamate levels were later analyzed by HPLC as described previously (Zhong et al, 2006).…”