Altered neurovascular coupling and brain arginine metabolism in endothelial nitric oxide synthase deficient mice.

Nitric oxide (NO) produced by endothelial NO synthase (eNOS) is a key regulator of cerebral blood flow (CBF) dynamics. Mice with eNOS deficiency (eNOS-/-) display age-related increases in amyloid beta in the brain and memory deficits, implicating eNOS dysfunction in the neuropathogenesis and/or development of Alzheimer's disease (AD). The present study systematically investigated behavioural, CBF and brain arginine metabolic profile changes in male and female wildtype (WT) and eNOS-/- mice at 14 months of age. eNOS-/- mice displayed altered behaviour in the Y-maze and open field tests. A real-time microcirculation imager revealed a significant sex difference in the basal CBF and significantly increased perfusion response to whisker stimulations in the Barrel cortex in both male and female eNOS-/- mice relative to their sex-matched WT controls. The treatment of 7-nitroindazole blocked the increased perfusion response to whisker stimulations in eNOS-/- mice. Neurochemically, the most intriguing changes were markedly reduced glutamine levels in both male and female eNOS-/- mice in the frontal cortex, hippocampus, parahippocampal region and cerebellum. These findings demonstrate altered behavioural function, neurovascular coupling and brain arginine metabolism (glutamine in particular) under the condition of eNOS deficiency, which further supports the role of eNOS dysfunction in the AD neuropathogenesis.

Effects of withdrawal from repeated phencyclidine administration on behavioural function and brain arginine metabolism in rats.

Phencyclidine (PCP) induces behavioural changes in humans and laboratory animals that resemble positive and negative symptoms, and cognitive impairments in schizophrenia. It has been shown repeated treatment of PCP leading to persistent symptoms even after the drug discontinuation, and there is a growing body of evidence implicating altered arginine metabolism in the pathogenesis of schizophrenia. The present study investigated the effects of withdrawal from repeated daily injection of PCP (2mg/kg) for 12 consecutive days on animals'behavioural performance and arginine metabolism in the hippocampus and prefrontal cortex in male young adult rats. Repeated PCP treatment reduced spontaneous alternations in the Y-maze and exploratory and locomotor activities in the open field under the condition of a washout period of 24h, but not 4days. Interestingly, the PCP treated rats also displayed spatial working memory deficits when tested 8-10days after withdrawal from PCP and showed altered levels of arginase activities and eight out of ten l-arginine metabolites in neurochemical- and region-specific manner. Cluster analyses showed altered relationships among l-arginine and its three main metabolites as a function of withdrawal from repeated PCP treatment in a duration-specific manner. Multiple regression analysis revealed significant neurochemical-behavioural correlations. Collectively, the results suggest both the residual and long-term effects of withdrawal from repeated PCP treatment on behavioural function and brain arginine metabolism. These findings demonstrate, for the first time, the influence of the withdrawal duration on animals' behaviour and brain arginine metabolism.

Altered arginine metabolism in Alzheimer's disease brains.

L-arginine is a semi-essential amino acid with a number of bioactive metabolites. Accumulating evidence suggests the implication of altered arginine metabolism in the pathogenesis of Alzheimer's disease (AD). The present study systematically compared the metabolic profile of L-arginine in the superior frontal gyrus, hippocampus, and cerebellum from AD (mean age 80 years) and normal (mean age 80 or 60 years) cases. The activity and protein expression of nitric oxide synthase and arginase were altered with AD and age in a region-specific manner. There were also AD- and age-related changes in the tissue concentrations of L-arginine and its downstream metabolites (L-citrulline, L-ornithine, agmatine, putrescine, spermidine, spermine, glutamate, γ-aminobutyric acid, and glutamine) in a metabolite- or region-specific manner. These findings demonstrate that arginine metabolism is dramatically altered in diverse regions of AD brains, thus meriting further investigation to understand its role in the pathogenesis and/or progression of the disease.

Effects of acute phencyclidine administration on arginine metabolism in the hippocampus and prefrontal cortex in rats.

Phencyclidine (PCP), a non-competitive N-methyl-d-aspartate glutamate receptor antagonist, induces schizophrenic symptoms in healthy individuals, and altered arginine metabolism has been implicated in schizophrenia. The present study investigated the effects of a single subcutaneous injection of PCP (2, 5 or 10 mg/kg) on arginine metabolism in the sub-regions of the hippocampus and prefrontal cortex in male young adult Sprague-Dawley rats. Animals' general behaviour was assessed in the open field apparatus 30 min after the treatment, and the brain tissues were collected at the time point of 60 min post-treatment. Behaviourally, PCP resulted in reduced exploratory activity in a dose-dependent manner, and severe stereotype behaviour and ataxia at the highest dose. Neurochemically, PCP significantly altered the nitric oxide synthase and arginase activities, the l-arginine, agmatine, spermine, glutamate and GABA levels, and the glutamine/glutamate and glutamate/GABA ratios in a dose-dependent and/or region-specific manner. Cluster analyses showed that l-arginine and its main metabolites l-citrulline, l-ornithine and agmatine formed distinct groups, which changed as a function of PCP mainly in the hippocampus. Multiple regression analysis revealed significant neurochemical-behavioural correlations. These results demonstrate, for the first time, that a single acute administration of PCP affects animals' behaviour and arginine metabolism in the brain.

Scopolamine impairs behavioural function and arginine metabolism in the rat dentate gyrus.

Alzheimer's disease (AD) is a neurodegenerative disorder with progressive memory loss. It has been shown that the cholinergic neurotransmission deficit is one of the neurochemical characteristics of AD, and that L-arginine and its metabolites also play a prominent role in AD pathogenesis. Scopolamine, a non-selective muscarinic receptor antagonist, blocks cholinergic neurotransmission and impairs behavioural function, including learning and memory. This study investigated the effects of scopolamine on animals' behavioural performance and L-arginine metabolism in the hippocampus and prefrontal cortex. Rats were given intraperitoneal injections of scopolamine (0.8 mg/kg) or saline (1 ml/kg) and tested in the Y-maze, open field, water maze and elevated plus maze 30 min post-treatment. After completion of the behavioural testing, the CA1, CA2/3 and dentate gyrus (DG) sub-regions of the hippocampus and the prefrontal cortex were harvested to measure the activity and protein expression of nitric oxide synthase (NOS) and arginase, and the levels of L-arginine, L-citrulline, L-ornithine, agmatine, putrescine, spermidine, spermine, glutamate and GABA. Scopolamine treated rats displayed reduced alternation and exploratory behaviour, increased swimming speed and impaired spatial learning and memory. There were significantly decreased NOS activity, increased arginase activity, and increased L-ornithine and putrescine levels in the DG, but not other regions examined, in the scopolamine treated rats as compared to the controls. These findings suggest that scopolamine impairs behavioural function and alters L-arginine metabolism in the DG sub-region of the hippocampus specifically. The underlying mechanisms of it remain to be explored further.

Pre-aggregated Aß(25-35) alters arginine metabolism in the rat hippocampus and prefrontal cortex.

Amyloid beta (Aß) has been proposed to play a central and causative role in the development of Alzheimer's disease. Aß(25-35), the neurotoxic domain of the full-length Aß, causes learning and memory impairments in rodents. The present study investigated the effects of a single bilateral i.c.v. infusion of pre-aggregated Aß(25-35) (30 nmol/rat) on animals' performance in the open field, and on arginine metabolic enzymes and metabolites in the CA1, CA2/3, and dentate gyrus (DG) sub-regions of the hippocampus and prefrontal cortex (PFC) at the time point of 6-8 days after Aß infusion. Aß(25-35) rats displayed reduced exploratory activity in the open field relative to the Aß(35-25) (reverse peptide; 30 nmol) rats. Aß(25-35) resulted in significantly decreased nitric oxide synthase (NOS) activity and endothelial NOS protein expression, but increased arginase activity, arginase II protein expression, and ornithine and putrescine levels, in hippocampal CA2/3. There were increased glutamate and putrescine levels in the DG, but decreased agmatine levels in the DG and PFC, in the Aß(25-35) group relative to the Aß(35-25) one. Cluster analyses were performed to determine if the nine related neurochemical variables (arginine, citrulline, ornithine, agmatine, putrescine, spermidine, spemine, glutamate, and GABA) formed distinct groups, and whether it changed as a function of Aß(25-35). There were substantially different clusters between the two groups in the hippocampus and PFC. These results demonstrate that Aß(25-35) alters arginine metabolism, which further supports the prominent role of arginine and its metabolites in Alzheimer's disease (AD) pathogenesis.

Memory-related changes in L-citrulline and agmatine in the rat brain.

L-citrulline, L-ornithine, and agmatine are the metabolites of L-arginine by nitric oxide synthase (NOS), arginase, and arginine decarboxylase (ADC), respectively. In contrast to the NOS and arginase pathways, the role of the ADC-agmatine pathway in learning and memory has only been paid attention lately. Recent evidence suggests a potential involvement of agmatine in learning and memory processing. The present study further addressed this issue by comparing the levels of agmatine, as well as L-arginine, L-citrulline, and L-ornithine, in the hippocampus, parahippocampal region, prefrontal cortex, vestibular nucleus, and cerebellum in rats that were trained in the delayed nonmatch to position task in the T-maze with their yoked controls. There were significantly increased agmatine levels in the prefrontal, entorhinal, and perirhinal cortices and increased L-citrulline concentrations in the dentate gyrus (DG) and prefrontal cortex in the T-maze training group relative to the control one. L-arginine and L-ornithine levels were not significantly different between groups in the brain regions examined. These results demonstrate T-maze training-induced region-specific increases in L-citrulline and agmatine. Significant positive correlations between prefrontal and perirhinal agmatine levels and animals' performance in the T-maze further suggest the direct involvement of agmatine in learning and memory processing.

Spatial learning results in elevated agmatine levels in the rat brain.

Accumulating evidence suggests that agmatine, a metabolite of L-arginine by arginine decarboxylase, is a novel neurotransmitter, and exogenous agmatine can modulate behavior functions including learning and memory. However, direct evidence of its involvement in learning and memory processes is currently lacking. This study measured agmatine levels in the hippocampus, parahippocampal region, cerebellum, and vestibular nucleus in rats that were trained to find a hidden escape platform in the water-maze task, or forced to swim in the pool with no platform presented, or kept in the holding-box, using liquid chromatography/mass spectrometry. Compared with the swimming only group and holding-box group, agmatine levels were significantly increased in the CA1 and dentate gyrus subregions of the hippocampus, the entorhinal cortex and the vestibular nucleus in the water-maze training group. These results, for the first time, demonstrate spatial learning-induced region-specific elevation in agmatine, and raise a novel issue of the involvement of agmatine in the processes of learning and memory.

Behavioral effects of intracerebroventricular microinfusion of agmatine in adult rats.

The present study investigated the behavioral effects of intracerebroventricular microinfusion of agmatine. Rats with low dose (10 microg), but not high dose (100 microg), of agmatine spent significantly less time in the enclosed arm and more time in the open arm in the elevated plus maze. In the water maze task, the high dose group displayed a transient impairment in searching for a hidden platform, whereas the low dose group had reduced latency in the first probe test. In the object recognition task, all groups could detect the novel object, but the low dose group spent significantly more time exploring displaced objects. Furthermore, the low dose group made significantly fewer errors in the working, but not the reference, memory version of the radial arm maze task. These results suggest that the behavioral effects of agmatine are task- and dose-dependent, and agmatine may be an anxiolytic and memory modulator.