Glycine-Binding Site Stimulants of NMDA Receptors Alleviate Extrapyramidal Motor Disorders by Activating the Nigrostriatal Dopaminergic Pathway

Abstract

Dysfunction of the N-methyl-d-aspartate (NMDA) receptor has been implicated in the pathogenesis of schizophrenia. Although agonists for the glycine-binding sites of NMDA receptors have potential as new medication for schizophrenia, their modulation of antipsychotic-induced extrapyramidal side effects (EPS) has not yet been clarified. We herein evaluated the effects of glycine-binding site stimulants of NMDA receptors on antipsychotic-induced EPS in mice and rats. d-cycloserine (DCS) and d-serine significantly improved haloperidol (HAL)-induced bradykinesia in mice, whereas glycine showed no effects. Sodium benzoate, a d-amino acid oxidase inhibitor, also attenuated HAL-induced bradykinesia. Improvements in HAL-induced bradykinesia by DCS were antagonized by the NMDA antagonist dizocilpine or nitric oxide synthase inhibitor L-N^G-Nitro-l-arginine methyl ester. In addition, DCS significantly reduced HAL-induced Fos expression in the dorsolateral striatum without affecting that in the nucleus accumbens. Furthermore, a microinjection of DCS into the substantia nigra pars compacta significantly inhibited HAL-induced EPS concomitant with elevations in dopamine release in the striatum. The present results demonstrated for the first time that stimulating the glycine-binding sites of NMDA receptors alleviates antipsychotic-induced EPS by activating the nigrostriatal dopaminergic pathway, suggesting that glycine-binding site stimulants are beneficial not only for efficacy, but also for side-effect management.

Keywords: ">d-cycloserine; antipsychotics; extrapyramidal side effects; glycine-binding site agonists of NMDA receptors; nigrostriatal dopaminergic system; schizophrenia.

Figure 1

Effects of glycine-site agonists of N-methyl-d-aspartate (NMDA) receptors and the d-amino acid oxidase inhibitor on haloperidol (HAL)-induced bradykinesia. (A–D) Glycine-site agonists of NMDA receptors, d-cycloserine (3–30 mg/kg, i.p.), d-serine (100–1000 mg/kg, i.p.), and glycine (30–300 mg/kg, i.p.), and the d-amino acid oxidase inhibitor, sodium benzoate (10–600 mg/kg, i.p.), were administered to animals 15 min before the HAL injection. The pole test was performed 30 min after the HAL injection. Each column represents the mean ± S.E.M. of 5–13 mice. These data were analyzed using the Kruskal−Wallis and Steel−Dwass tests. * p < 0.05, ** p < 0.01, significantly different from the value with HAL alone.

Figure 2

Effects of dizocilpine or L-N^G-Nitro-l-arginine methyl ester (L-NAME) on the ameliorative action of d-cycloserine (DCS) against haloperidol (HAL)-induced bradykinesia. (A,B) Mice received dizocilpine (0.01 mg/kg, i.p.), L-NAME (10 mg/kg, i.p.), or vehicle simultaneously with DCS (30 mg/kg, i.p.) or vehicle 15 min before the HAL injection (1 mg/kg, i.p.). The pole test was performed 30 min after the HAL injection. Each column represents the mean ± S.E.M. of 12–23 mice. These data were analyzed using the Mann-Whitney U-test. * p < 0.05: Significantly different from the value with HAL alone. ^# p < 0.05, significantly different from the value with HAL + DCS.

Figure 3

(A) Effects of d-cycloserine (DCS) on haloperidol (HAL)-induced Fos expression in the dorsolateral striatum (dlST) and accumbens shell (AcS). (A) DCS (30 mg/kg, i.p.) or vehicle was administered to animals 15 min before the HAL injection, which was followed by the pole test 30 min later; (B) Photographs illustrating Fos-IR-positive cells in the dlST and AcS (left panel: vehicle + HAL (1 mg/kg, i.p.)-treated mice, right panel: DCS (30 mg/kg, i.p.) + HAL (1 mg/kg, i.p.)-treated mice). Scale bar: 100 μm; (C,D) Effects of DCS (30 mg/kg, i.p.) on HAL (1 mg/kg, i.p.)-induced Fos expression in the dlST (C) and AcS (D). The brain was removed from animals 2 h after the HAL injection. Each column represents the mean ± S.E.M. of 6–7 mice. These data were analyzed using the Kruskal−Wallis and Steel−Dwass tests (behavioral test) or one-way ANOVA and Tukey’s test (Fos analysis). * p < 0.05, ** p < 0.01, significantly different from the value for vehicle + vehicle. ^# p < 0.05, ^## p < 0.01, significantly different from the value for vehicle + HAL.

Figure 4

Effects of intranigral and intrastriatal microinjections of d-cycloserine (DCS) on haloperidol (HAL)-induced catalepsy in rats. (A,B) The effects of DCS (10 µg/1 µL/side) microinjected into the substantia nigra compacta (SNc) or dorsolateral striatum (dlST) against HAL-induced catalepsy were examined. Each dose of HAL was administered 15 min after each DCS microinjection and, 30 min later (45 min after the DCS microinjection), the catalepsy time was measured. Schematic drawings of a rat brain section illustrating DCS or vehicle injection sites (filled circles) in the SNc (A) or dlST (B) are shown at the top. Each column represents the mean ± S.E.M. of 6–11 rats. These data were analyzed by the Mann-Whitney U-test. * p < 0.05, significantly different from the control value with vehicle + HAL.

Figure 5

Effects of an intranigral microinjection of d-cycloserine (DCS) on dopamine release in the rat striatum. (A) Schematic drawing illustrating the microinjection site of DCS in the substantia nigra pars compacta (SNc) and microdialysis site in the dorsolateral striatum (dlST); (B) Extracellular levels of dopamine were analyzed in 10-min dialysate samples. Data were normalized to the mean value of the first six 10-min samples (basal value). Each column shows the mean ± S.E.M. of 6 rats. These data were analyzed by the Student’s t-test. * p < 0.05, ** p < 0.01, significantly different from the basal value.