N-methyl-d-aspartate receptor dysfunction in the prefrontal cortex of stroke-prone spontaneously hypertensive rat/Ezo as a rat model of attention deficit/hyperactivity disorder

Central amygdala is related to the reduction of aggressive behavior by monosodium glutamate ingestion during the period of development in an ADHD model rat

Introduction

Monosodium glutamate (MSG), an umami substance, stimulates the gut-brain axis communication via gut umami receptors and the subsequent vagus nerves. However, the brain mechanism underlying the effect of MSG ingestion during the developmental period on aggression has not yet been clarified. We first tried to establish new experimental conditions to be more appropriate for detailed analysis of the brain, and then investigated the effects of MSG ingestion on aggressive behavior during the developmental stage of an ADHD rat model.

Methods

Long-Evans, WKY/Izm, SHR/Izm, and SHR-SP/Ezo were individually housed from postnatal day 25 for 5 weeks. Post-weaning social isolation (PWSI) was given to escalate aggressive behavior. The resident-intruder test, that is conducted during the subjective night, was used for a detailed analysis of aggression, including the frequency, duration, and latency of anogenital sniffing, aggressive grooming, and attack behavior. Immunohistochemistry of c-Fos expression was conducted in all strains to predict potential aggression-related brain areas. Finally, the most aggressive strain, SHR/Izm, a known model of attention-deficit hyperactivity disorder (ADHD), was used to investigate the effect of MSG ingestion (60 mM solution) on aggression, followed by c-Fos immunostaining in aggression-related areas. Bilateral subdiaphragmatic vagotomy was performed to verify the importance of gut-brain interactions in the effect of MSG.

Results

The resident intruder test revealed that SHR/Izm rats were the most aggressive among the four strains for all aggression parameters tested. SHR/Izm rats also showed the highest number of c-Fos + cells in aggression-related brain areas, including the central amygdala (CeA). MSG ingestion significantly decreased the frequency and duration of aggressive grooming and attack behavior and increased the latency of attack behavior. Furthermore, MSG administration successfully increased c-Fos positive cell number in the intermediate nucleus of the solitary tract (iNTS), a terminal of the gastrointestinal sensory afferent fiber of the vagus nerve, and modulated c-Fos positive cells in the CeA. Interestingly, vagotomy diminished the MSG effects on aggression and c-Fos expression in the iNTS and CeA.

Conclusion

MSG ingestion decreased PWSI-induced aggression in SHR/Izm, which was mediated by the vagus nerve related to the stimulation of iNTS and modulation of CeA activity.

Learning and Memory Impairments With Attention-Deficit/Hyperactivity Disorder

ADHD is a common chronic neurodevelopmental disorder and is characterized by persistent inattention, hyperactivity, impulsivity and are often accompanied by learning and memory impairment. Great evidence has shown that learning and memory impairment of ADHD plays an important role in its executive function deficits, which seriously affects the development of academic, cognitive and daily social skills and will cause a serious burden on families and society. With the increasing attention paid to learning and memory impairment in ADHD, relevant research is gradually increasing. In this article, we will present the current research results of learning and memory impairment in ADHD from the following aspects. Firstly, the animal models of ADHD, which display the core symptoms of ADHD as well as with learning and memory impairment. Secondly, the molecular mechanism of has explored, including some neurotransmitters, receptors, RNAs, etc. Thirdly, the susceptibility gene of ADHD related to the learning and impairment in order to have a more comprehensive understanding of the pathogenesis. Key words: Learning and memory, ADHD, Review.

Role of Glutamate Receptor-related Biomarkers in the Etiopathogenesis of ADHD

Objective

: Pathways associated with glutamate receptors are known to play a role in the pathophysiology of attention-deficit hyperactivity disorder (ADHD). However, cyclin-dependent kinase 5 (CDK5), microtubule-associated protein-2 (MAP2), guanylate kinase-associated protein (GKAP), and postsynaptic density 95 (PSD95), all of which are biomarkers involved in neurodevelopmental processes closely related to glutamatergic pathways, have not previously been studied in patients with ADHD. The main purpose of this study was to evaluate the plasma levels of CDK5, MAP2, GKAP, and PSD95 in children with ADHD and investigate whether these markers have a role in the etiology of ADHD.

Methods

: Ninety-six children with ADHD between 6 and 15 years of age and 72 healthy controls were included in the study. Five milliliters of blood samples were taken from all participants. The samples were stored at -80°C until analyzed by the enzyme-linked immunosorbent assay method.

Results

: Statistically significantly lower CDK5 levels were observed in children with ADHD than in healthy controls (p = 0.037). The MAP2, GKAP, and PSD95 levels were found to be statistically significantly higher in the ADHD group than in healthy controls (p = 0.012, p = 0.009, and p = 0.024, respectively). According to binary regression analysis, CDK5 and MAP2 levels were found to be predictors of ADHD.

Conclusion

: In conclusion, we found that a close relationship existed between ADHD and glutamatergic pathways, and low levels of CDK5 and high levels of MAP2 and GKAP played a role in the etiopathogenesis of ADHD.

Impaired monoamine neural system in the mPFC of SHRSP/Ezo as an animal model of attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is the most common childhood-onset psychiatric disorder. We investigated the effects of systemic administration of monoamine reuptake inhibitors on long-term potentiation (LTP) formation and monoamine release in the medial prefrontal cortex (mPFC) of the stroke-prone spontaneously hypertensive rat (SHRSP)/Ezo, an animal model of ADHD, and its genetic control, Wistar Kyoto (WKY)/Ezo, to elucidate the functional changes in the mPFC monoamine neural system. Methylphenidate (dopamine (DA) and noradrenaline (NA) reuptake inhibitor) and desipramine (NA reuptake inhibitor) improved LTP formation defects in the mPFC of SHRSP/Ezo, suggesting that NA or both DA and NA are required for improvement of impaired LTP. Methylphenidate increased mPFC DA in both WKY/Ezo and SHRSP/Ezo, but the increase was greater in the former. GBR-12909 (DA reuptake inhibitor) increased mPFC DA in WKY/Ezo but had no effect in SHRSP/Ezo. This may be because DA transporter in SHRSP/Ezo is functionally impaired and contributes less to DA reuptake, so its inhibition did not increase DA level. Meanwhile, basal DA levels in the mPFC of SHRSP/Ezo were paradoxically decreased. These results suggest that functional changes in the DA and NA neural system in the frontal lobe are involved in the pathology of ADHD.

Molecular Mechanisms Underlying NMDARs Dysfunction and Their Role in ADHD Pathogenesis

Attention deficit hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, although the aetiology of ADHD is not yet understood. One proposed theory for developing ADHD is N-methyl-D-aspartate receptors (NMDARs) dysfunction. NMDARs are involved in regulating synaptic plasticity and memory function in the brain. Abnormal expression or polymorphism of some genes associated with ADHD results in NMDAR dysfunction. Correspondingly, NMDAR malfunction in animal models results in ADHD-like symptoms, such as impulsivity and hyperactivity. Currently, there are no drugs for ADHD that specifically target NMDARs. However, NMDAR-stabilizing drugs have shown promise in improving ADHD symptoms with fewer side effects than the currently most widely used psychostimulant in ADHD treatment, methylphenidate. In this review, we outline the molecular and genetic basis of NMDAR malfunction and how it affects the course of ADHD. We also present new therapeutic options related to treating ADHD by targeting NMDAR.

Methyl Palmitate Modulated NMDA-Induced Cerebral Hyperemia in Hypertensive Rats

N-methyl-D-aspartate (NMDA) receptors were found to be dysfunctional in hypertensive rats. Methyl palmitate (MP) has been shown to diminish the nicotine-induced increase in blood flow in the brainstem. The aim of this study was to determine how MP modulated NMDA-induced increased regional cerebral blood flow (rCBF) in normotensive (WKY), spontaneously hypertensive (SHR), and renovascular hypertensive (RHR) rats. The increase in rCBF after the topical application of experimental drugs was measured using laser Doppler flowmetry. Topical NMDA application induced an MK-801-sensitive increase in rCBF in anesthetized WKY rats, which was inhibited by MP pretreatments. This inhibition was prevented by pretreatment with chelerythrine (a PKC inhibitor). The NMDA-induced increase in rCBF was also inhibited by the PKC activator in a concentration-dependent manner. Neither MP nor MK-801 affected the increase in rCBF induced by the topical application of acetylcholine or sodium nitroprusside. Topical application of MP to the parietal cortex of SHRs, on the other hand, increased basal rCBF slightly but significantly. MP enhanced the NMDA-induced increase in rCBF in SHRs and RHRs. These results suggested that MP had a dual effect on the modulation of rCBF. MP appears to play a significant physiological role in CBF regulation.

D-serine metabolism in the medial prefrontal cortex, but not the hippocampus, is involved in AD/HD-like behaviors in SHRSP/Ezo

Attention-deficit/hyperactivity disorder (AD/HD) is a mild neurodevelopmental disorder with inattention, hyperactivity, and impulsivity as its core symptoms. We previously revealed that an AD/HD animal model, juvenile stroke-prone spontaneously hypertensive rats (SHRSP/Ezo) exhibited functional abnormalities in N-methyl-D-aspartate (NMDA) receptors in the prefrontal cortex. D-serine is an endogenous co-ligand that acts on the glycine-binding site of NMDA receptors, which is essential for the physiological activation of NMDA receptors. We herein performed neurochemical and pharmacological behavioral experiments to elucidate dysfunctions in D-serine metabolism (namely, biosynthesis and catabolism) associated to AD/HD. The serine enantiomers ratio (D-serine/D-serine + L-serine, DL ratio) in the medial prefrontal cortex (mPFC) and hippocampus (HIP) was lower in SHRSP/Ezo than in its genetic control. The level of D-amino acid oxidase (DAAO, D-serine degrading enzyme) was higher in the mPFC, and the level of serine racemase (SR, D-serine biosynthetic enzyme), was lower in the HIP in SHRSP/Ezo. Thus, changes in these enzymes may contribute to the lower DL ratio of SHRSP/Ezo. Moreover, a microinjection of a DAAO inhibitor into the mPFC in SHRSP/Ezo increased DL ratio and attenuated AD/HD-like behaviors, such as inattention and hyperactivity, in the Y-maze test. Injection into the HIP also increased the DL ratio, but had no effect on behaviors. These results suggest that AD/HD-like behaviors in SHRSP/Ezo are associated with an abnormal D-serine metabolism underlying NMDA receptor dysfunction in the mPFC. These results will contribute to elucidating the pathogenesis of AD/HD and the development of new treatment strategies for AD/HD.

N-methyl-d-aspartate receptor dysfunction in the prefrontal cortex of stroke-prone spontaneously hypertensive rat/Ezo as a rat model of attention deficit/hyperactivity disorder

AIM

We previously reported that stroke-prone spontaneously hypertensive rat/Ezo (SHRSP/Ezo) has high validity as an attention deficit/hyperactivity disorder (AD/HD) animal model, based on its behavioral phenotypes, such as inattention, hyperactivity, and impulsivity. Fronto-cortical dysfunction is implicated in the pathogenesis of AD/HD. In this study, we investigated prefrontal cortex (PFC) function in SHRSP/Ezo rats by electrophysiological methods and radioreceptor assay.

METHODS

We recorded excitatory postsynaptic potential in layer V pyramidal neurons in the PFC by intracellular recording method to assess synaptic plasticity in the form of long-term potentiation (LTP). We also performed N-methyl-d-aspartate acid (NMDA) receptor binding assay in the PFC and hippocampus using radiolabeled NMDA receptor antagonist [³ H]MK-801.

RESULTS

Theta-burst stimulation induced LTP in the PFC of genetic control, WKY/Ezo, whereas failed to induce LTP in that of SHRSP/Ezo. The K_d value of [³ H]MK-801 binding for NMDA receptors in the PFC of SHRSP/Ezo was higher than in the WKY/Ezo. Neither the B_max nor K_d of [³ H]MK-801 binding in the SHRSP/Ezo hippocampus was significantly different to WKY/Ezo.

CONCLUSION

These results suggest that the AD/HD animal model SHRSP/Ezo has NMDA receptor dysfunction in the PFC.