Elsevier

Neuroscience

Volume 102, Issue 2, 15 January 2001, Pages 473-479
Neuroscience

Inhibition of spinal N-acetylated-α-linked acidic dipeptidase produces an antinociceptive effect in the rat formalin test

https://doi.org/10.1016/S0306-4522(00)00502-9Get rights and content

Abstract

N-acetyl-aspartyl-glutamate is a putative neurotransmitter and acts as a weak agonist at the N-methyl-d-aspartate receptor. N-acetyl-aspartyl-glutamate also acts as an agonist at the metabotropic glutamate receptor 3. N-acetyl-aspartyl-glutamate is hydrolyzed by N-acetylated-α-linked acidic dipeptidase to liberate N-acetyl-aspartate and glutamate. Recently, a specific inhibitor of N-acetylated-α-linked acidic dipeptidase, 2-(phosphonomethyl)pentanedioic acid, has been reported. In the present study, we examined the effect of i.t. administered 2-(phosphonomethyl)pentanedioic acid in the rat formalin test (a model of inflammatory pain) and the rat hot plate test. In the formalin test, drugs were administered 10 min before (pre-treatment study) or 7 min after (post-treatment study) the formalin injection. The paw formalin injection induces biphasic flinching (phase 1: 0–2 min; phase 2: 10–60 min) of the injected paw. In the pre-treatment study, i.t. administered 2-(phosphonomethyl)pentanedioic acid depressed both phases 1 and 2 flinching behavior in a dose-dependent manner but 2-(phosphonomethyl)pentanedioic acid had no effect on the flinching behavior in the post-treatment study. In the pre-treatment study, the potency of 2-(phosphonomethyl)pentanedioic acid in depressing the phase 2 response is greater than that in depressing the phase 1 response. Intrathecal injection of 2-(phosphonomethyl)pentanedioic acid had no effect in the hot plate test.

We suggest that N-acetylated-α-linked acidic dipeptidase plays an important role in spinal nociceptive transmission and that inhibition of spinal N-acetylated-α-linked acidic dipeptidase produces an antinociceptive effect during the rat formalin test but not during the hot plate test.

Section snippets

Experimental procedures

The following investigations were carried out under a protocol approved by the Institutional Animal Care Committee, Chiba University. All efforts were made to minimize animal suffering and to reduce the number of animals used. Male Sprague–Dawley rats (250–300 g, Japan SLC, Shizuoka, Japan) were prepared with i.t. catheters and examined for the effects of the agent on the formalin test and the hot plate test.

Behavioral analysis

After the i.t. injection of either 2-PMPA or saline, all animals scored 2 (normal motor function) in the placing/stepping reflex and righting reflex tests.

Formalin test

In the saline-treated rats, a s.c. injection of formalin resulted in a highly reliable biphasic display of flinching of the injected paw (Fig. 1), and this behavior was comparable to that previously reported.24., 27.

In the pre-treatment study, i.t. administered 2-PMPA decreased the level of the sum of instances of flinching in both phases 1

Discussion

In the present study, we clearly demonstrated that pre-treatment with 2-PMPA depressed both the phase 1 and 2 flinching behavior in a dose dependent manner at a dose between 1 and 100 μg. 2-PMPA inhibited phase 2 responses with a greater potency than phase 1; at a dose of 10 μg inhibition of phase 2 responses was significantly greater than that of phase 1 responses, whilst a comparable inhibition of both phases was produced by 100 μg. Intraperitoneal injection of 100 μg of 2-PMPA had no effect on

Conclusions

Intrathecal injection of 2-PMPA, an inhibitor of NAALADase, depressed the phase 1 and 2 flinching behavior in the rat formalin test. Intrathecal injection of 2-PMPA may produce NMDA receptor antagonism and inhibition of transmitter release via activation of mGluR3. These two mechanisms may produce a pronounced inhibition of both phases of the formalin response. These data suggest a new potential dual therapeutic approach to treating pain caused by inflammation.

Acknowledgements

We thank professor Takashi Nishino, Department of Anesthesiology, School of Medicine, Chiba University for his generous support of our study. This study was supported in part by a Grant-in-Aid for Scientific Research (B) of Japan 12470315.

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