Spatial memory in spontaneously hypertensive rats (SHR)

Effects of Gestational Exercise on Nociception, BDNF, and Irisin Levels in an Animal Model of ADHD

Abnormal cognitive and sensorial properties have been reported in patients with psychiatric and neurodevelopmental conditions, such as attention deficit hyperactivity disorder (ADHD). ADHD patients exhibit impaired dopaminergic signaling and plasticity in brain areas related to cognitive and sensory processing. The spontaneous hypertensive rat (SHR), in comparison to the Wistar Kyoto rat (WKY), is the most used genetic animal model to study ADHD. Brain neurotrophic factor (BDNF), critical for midbrain and hippocampal dopaminergic neuron survival and differentiation, is reduced in both ADHD subjects and SHR. Physical exercise (e.g. swimming) promotes neuroplasticity and improves cognition by increasing BDNF and irisin. Here we investigate the effects of gestational swimming on sensorial and behavioral phenotypes, striatal dopaminergic parameters, and hippocampal FNDC5/irisin and BDNF levels observed in WKY and SHR. Gestational swimming improved nociception in SHR rats (p = 0.006) and increased hippocampal BDNF levels (p = 0.02) in a sex-dependent manner in adolescent offspring. Sex differences were observed in hippocampal FNDC5/irisin levels (p = 0.002), with females presenting lower levels than males. Our results contribute to the notion that swimming during pregnancy is a promising alternative to improve ADHD phenotypes in the offspring.

Memory deficits and hippocampal cytokine expression in a rat model of ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a complex behavioral disorder characterized by hyperactivity, impulsivity, inattention, and deficits in working memory and time perception. While animal models have advanced our neurobiological understanding of this condition, there are limited and inconsistent data on working and elapsed time memory function. Inflammatory signaling has been identified as a key factor in memory and cognitive impairments, but its role in ADHD remains unclear. Additionally, the disproportionate investigation of male subjects in ADHD research has contributed to a poor understanding of the disorder in females. This study sought to investigate the potential connections between memory, neuroimmunology, and ADHD in both male and female animals. Specifically, we utilized the spontaneously hypertensive rat (SHR), one of the most extensively studied animal models of ADHD. Compared to their control, the Wistar-Kyoto (WKY) rat, male SHR are reported to exhibit several behavioral phenotypes associated with ADHD, including hyperactivity, impulsivity, and poor sustained attention, along with impairments in learning and memory. As the hippocampus is a key brain region for learning and memory, we examined the behavior of male and female SHR and WKY rats in two hippocampal-dependent memory tasks. Our findings revealed that SHR have delay-dependent working memory deficits that were similar to, albeit less severe than, those seen in hippocampal-lesioned rats. We also observed impairments in elapsed time processing in female SHR, particularly in the discrimination of longer time durations. To investigate the impact of inflammatory signaling on memory in these rats, we analyzed the levels of several cytokines in the dorsal and ventral hippocampus of SHR and WKY. Although we found some sex and genotype differences, concentrations were generally similar between groups. Taken together, our results indicate that SHR exhibit deficits in spatial working memory and memory for elapsed time, as well as some differences in hippocampal cytokine concentrations. These findings contribute to a better understanding of the neurobiological basis of ADHD in both sexes and may inform future research aimed at developing effective treatments for the disorder. Nonetheless, the potential mediating role of neuroinflammation in the memory symptomatology of SHR requires further investigation.

Spontaneously hypertensive rats vs. Wistar Kyoto and Wistar rats: an assessment of anxiety, motor activity, memory performance, and seizure susceptibility

Spontaneously hypertensive rats (SHRs) are widely accepted for modeling essential hypertension and Attention deficit hyperactivity disorder (ADHD). However, data concerning central nervous system changes associated with behavioral responses of this strain and usage of Wistar Kyoto (WKY) rats as controls are confounding. The objective of the present study was to assess the impact of anxiety and motor activity on the cognitive responses of SHRs compared to Wistar and WKY rats. In addition, the role of brain-derived neurotrophic factor (BDNF) in the hippocampus on cognitive behavior and seizure susceptibility in the three strains was evaluated. In Experiment#1, SHR demonstrated impulsive responses in the novelty suppression feeding test accompanied by impaired spatial working and associative memory in the Y maze and object recognition test compared with the Wistar rat but not WKY rats. In addition, the WKY rats exhibited diminished activity compared to Wistar rats in an actimeter. In Experiment#2, the seizure susceptibility was assessed by 3-min electroencephalographic (EEG) recording after two consecutive injections of pentylenetetrazol (PTZ) (20+40 mg/kg). The WKY rats were more vulnerable to rhythmic metrazol activity (RMA) than the Wistar rats. In contrast, Wistar rats were more prone to generalized tonic-clonic seizures (GTCS) than WKY rats and SHRs. Control SHR had lower BDNF expression in the hippocampus compared to Wistar rats. However, while the BDNF levels were elevated in the Wistar and WKY rats after PTZ injection, no change in this signaling molecule was observed in the SHR in the seizure condition. The results suggest Wistar rats as a more appropriate control of SHR than WKY rats for studying memory responses mediated by BDNF in the hippocampus. The higher vulnerability to seizures in Wistar and WKY rats compared to SHR might be linked to PTZ-induced decreased expression of BDNF in the hippocampus.

The Wistar Kyoto Rat: A Model of Depression Traits

There is an ongoing debate about the value of animal research in psychiatry with valid lines of reasoning stating the limits of individual animal models compared to human psychiatric illnesses. Human depression is not a homogenous disorder; therefore, one cannot expect a single animal model to reflect depression heterogeneity. This limited review presents arguments that the Wistar Kyoto (WKY) rats show intrinsic depression traits. The phenotypes of WKY do not completely mirror those of human depression but clearly indicate characteristics that are common with it. WKYs present despair- like behavior, passive coping with stress, comorbid anxiety, and enhanced drug use compared to other routinely used inbred or outbred strains of rats. The commonly used tests identifying these phenotypes reflect exploratory, escape-oriented, and withdrawal-like behaviors. The WKYs consistently choose withdrawal or avoidance in novel environments and freezing behaviors in response to a challenge in these tests. The physiological response to a stressful environment is exaggerated in WKYs. Selective breeding generated two WKY substrains that are nearly isogenic but show clear behavioral differences, including that of depression-like behavior. WKY and its substrains may share characteristics of subgroups of depressed individuals with social withdrawal, low energy, weight loss, sleep disturbances, and specific cognitive dysfunction. The genomes of the WKY and WKY substrains contain variations that impact the function of many genes identified in recent human genetic studies of depression. Thus, these strains of rats share characteristics of human depression at both phenotypic and genetic levels, making them a model of depression traits.

From attention-deficit hyperactivity disorder to sporadic Alzheimer's disease-Wnt/mTOR pathways hypothesis

Alzheimer's disease (AD) is the most common neurodegenerative disorder with the majority of patients classified as sporadic AD (sAD), in which etiopathogenesis remains unresolved. Though sAD is argued to be a polygenic disorder, apolipoprotein E (APOE) ε4, was found three decades ago to pose the strongest genetic risk for sAD. Currently, the only clinically approved disease-modifying drugs for AD are aducanumab (Aduhelm) and lecanemab (Leqembi). All other AD treatment options are purely symptomatic with modest benefits. Similarly, attention-deficit hyperactivity disorder (ADHD), is one of the most common neurodevelopmental mental disorders in children and adolescents, acknowledged to persist in adulthood in over 60% of the patients. Moreover, for ADHD whose etiopathogenesis is not completely understood, a large proportion of patients respond well to treatment (first-line psychostimulants, e.g., methylphenidate/MPH), however, no disease-modifying therapy exists. Interestingly, cognitive impairments, executive, and memory deficits seem to be common in ADHD, but also in early stages of mild cognitive impairment (MCI), and dementia, including sAD. Therefore, one of many hypotheses is that ADHD and sAD might have similar origins or that they intercalate with one another, as shown recently that ADHD may be considered a risk factor for sAD. Intriguingly, several overlaps have been shown between the two disorders, e.g., inflammatory activation, oxidative stress, glucose and insulin pathways, wingless-INT/mammalian target of rapamycin (Wnt/mTOR) signaling, and altered lipid metabolism. Indeed, Wnt/mTOR activities were found to be modified by MPH in several ADHD studies. Wnt/mTOR was also found to play a role in sAD and in animal models of the disorder. Moreover, MPH treatment in the MCI phase was shown to be successful for apathy including some improvement in cognition, according to a recent meta-analysis. In several AD animal models, ADHD-like behavioral phenotypes have been observed indicating a possible interconnection between ADHD and AD. In this concept paper, we will discuss the various evidence in human and animal models supporting the hypothesis in which ADHD might increase the risk for sAD, with common involvement of the Wnt/mTOR-pathway leading to lifespan alteration at the neuronal levels.

Curcumin for attention-deficit-hyperactivity disorder: a systematic review and preliminary behavioral investigation

Curcumin has protective actions in neuropsychiatric disorders, acting as a neuroprotective agent. As a first approach, the study aimed at a systematic review of the potential effects of curcumin on cognitive performance for attention-deficit-hyperactivity disorder (ADHD). This research was carried out in the databases of PubMed, Embase, SciELO, the Cochrane Central Register of Controlled Trials (CENTRAL), the Web of Science, and the Grey literature. Upon discovering the scarcity of relevant studies, and knowing that curcumin might have an ADHD hyperactive and anxious behavior, the study proposed to evaluate the effects of curcumin in an ADHD phenotype of spontaneously hypertensive Wistar rats (SHR). No studies were found that related to curcumin and ADHD. Fifteen SHRs were then divided into separate groups that received water (1 mg/kg/day), curcumin (50 mg/kg/day), or methylphenidate (1 mg/kg/day) for 42 days. Behavioral tests to assess activity (Open Field Test), anxiety and impulsivity (Elevated Plus-Maze, and Social Interaction), and memory (Y-Maze, and the Object Recognition Test) were all performed. The animals that were treated with curcumin showed less anxious and hyperactive behavior, as seen in the Open Field Test and the Social Interaction Test. Anxious behavior was measured by the EPM and was not modulated by any treatment. The results of the Y-Maze Test demonstrated that curcumin improved spatial memory. In the Object Recognition Test, neither the short nor the long-term memory was improved. The treatments that were used in this study beneficially modulated the anxious and hyperactive behavior of the SHR.

Memory impairment in spontaneously hypertensive rats is associated with hippocampal hypoperfusion and hippocampal vascular dysfunction

We investigated the effect of chronic hypertension on hippocampal arterioles (HippAs) and hippocampal perfusion as underlying mechanisms of memory impairment, and how large artery stiffness relates to HippA remodeling. Using male spontaneously hypertensive rats (SHR) and normotensive Wistar rats (n = 12/group), long-term (LTM) and spatial memory were tested using object recognition and spontaneous alternation tasks. Hippocampal blood flow was measured via hydrogen clearance basally and during hypercapnia. Reactivity of isolated and pressurized HippAs to pressure and pharmacological activators and inhibitors was investigated. To determine large artery stiffness, distensibility and elastin content were measured in thoracic aorta. SHR had impaired LTM and spatial memory associated with decreased basal blood flow (68 ± 12 mL/100 g/min) vs. Wistar (111 ± 28 mL/100 g/min, p < 0.01) that increased during hypercapnia similarly between groups. Compared to Wistar, HippAs from SHR had increased tone at 60 mmHg (58 ± 9% vs. 37 ± 7%, p < 0.01), and decreased reactivity to small- and intermediate-conductance calcium-activated potassium (SK/IK) channel activation. HippAs in both groups were unaffected by NOS inhibition. Decreased elastin content correlated with increased stiffness in aorta of SHR that was associated with increased stiffness and hypertrophic remodeling of HippAs. Hippocampal vascular dysfunction during hypertension could potentiate memory deficits and may provide a therapeutic target to limit vascular cognitive impairment.

Hypertension and sympathetic nervous system overactivity rely on the vascular tone of pial vessels of the rostral ventrolateral medulla in spontaneously hypertensive rats

NEW FINDINGS

What is the central question of this study? Is purinergic signalling in the pial vessels involved in the control of vascular tone in the ventral surface of the brainstem, affecting high blood pressure and sympathetic overactivity in spontaneously hypertensive rats? What is the main finding and its importance? The regulation of vascular tone in the ventral surface of the brainstem is tailored to support neuronal functions, arterial pressure and sympathetic activity. This adds one more piece in the complex puzzle to understand the central mechanisms underlying the genesis of hypertension.

ABSTRACT

Evidence suggests the rostral ventrolateral medulla (RVLM) region is chronically hypoperfused and hypoxic in spontaneously hypertensive rats (SHR), which can facilitate ATP release throughout the brainstem. Thus, we hypothesized that purinergic signalling plays a key role in the increased vascular tone in the RVLM region, which in turn could be responsible for the high sympathetic tone and blood pressure in the SHR. The application of an antagonist of P2 receptors, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (10 µm), or of P2Y1a receptors, MRS2179 (100 µm), on the surface of RVLM pial vessels of SHR produced an increase in the diameter of blood vessels (PPADS: 31 ± 1.4 µm or MRS2179: 32 ± 0.78 µm vs. saline: 27 ± 1.2 µm), an effect not observed in normotensive Wistar rats. In addition, the antagonism of P2 receptors was able to evoke a significant decrease in the arterial pressure, heart rate and splanchnic nerve activity in SHR, but not in Wistar rats. Our data show that SHR have higher vascular tone of pial vessels in the RVLM region when compared to the normotensive Wistar rats, a mechanism that relies on purinergic signalling through P2 receptors, suggesting a possible association with higher activity of sympathoexcitatory neurones, and sustained increases in blood pressure.

Differences in serum steroid hormones concentrations in Spontaneously Hypertensive Rats (SHR) - an animal model of Attention-Deficit/Hyperactivity Disorder (ADHD)

Spontaneously hypertensive rats are the most common animal model used to study attention deficit hyperactivity disorder (ADHD). The present study investigated the levels of steroid hormones in the bloodstream of hypertensive rats and its normotensive control strain, Wistar-Kyoto rats, to check if there are any hormonal differences between both strains at the onset of ADHD. Plasma samples were collected from young (5-week-old) and mature (10-week-old) male hypertensive and normotensive rats to determine the serum level of testosterone, 17beta-estradiol, free estriol, progesterone, corticosterone and cortisol using ELISA kits. The results showed statistically significant increases in serum levels of testosterone and free estriol in 10-week-old hypertensive and normotensive rats when compared to 5-week-old animals. Moreover, the concentrations of progesterone, corticosterone and cortisol were significantly elevated in 10-week-old hypertensive rats when compared to 5-week-old animals of both strains as well as 10-week-old normotensive rats. Hormonal differences observed between 10-week-old hypertensive and normotensive rats were also accompanied by differences in the volumes of lateral ventricles as well as the third ventricle and cerebral aqueduct. In conclusion, elevated contents of progesterone, corticosterone and cortisol in hypertensive rats may be associated not only with ADHD but also with developing hypertension. This question needs further study.

Hypertension-induced cognitive impairment: insights from prolonged angiotensin II infusion in mice

The causal relation between hypertension and cerebral small vessel disease (cSVD) remains elusive, and appropriate animal models are scarce. We aimed to assess the relevance of prolonged angiotensin II-induced hypertension in mice for the study of cSVD.Adult male C57BL/6 mice were continuously infused for 3 months with Angiotensin II (Ang II; 2 µg/kg/min, sc) or saline (control) via osmotic minipumps. Blood pressure, neurological function, locomotor activity, and working memory (Y-maze alternation task) were assessed throughout the study. Short-term memory performance (object location task) was measured after 3 months of infusion. Blood-brain barrier (BBB) function was assessed by the presence of IgG leakage and quantified in each brain area of interest. Microglial activation and myelin loss were studied in the areas of leakage.Systolic blood pressure increased and remained elevated over the 3 months of Ang II infusion, while neurological scores and locomotor activity did not change. Working memory performance was also not changed, yet short-term memory performance was impaired in Ang II-treated mice compared to controls. While BBB leakages were present in both groups, mainly in the neocortex, hippocampus, and cerebral nuclei, Ang II-treated mice showed greater leakage than control mice, along with greater microglial density and soma size. Myelin loss was observed for the largest leaks.Prolonged Ang II-induced hypertension is associated with large BBB leaks, microglial activation, myelin loss, and memory dysfunction in the absence of stroke.

Differential Behavioral and Biochemical Responses to Caffeine in Male and Female Rats from a Validated Model of Attention Deficit and Hyperactivity Disorder

Epidemiological studies suggest sex differences in attention deficit and hyperactivity disorder (ADHD) symptomatology. The potential benefits of caffeine have been reported in the management of ADHD, but its effects were not properly addressed with respect to sex differences. The present study examined the effects of caffeine (0.3 g/L) administered since childhood in the behavior and brain-derived neurotrophic factor (BDNF) and its related proteins in both sexes of a rat model of ADHD (spontaneously hypertensive rats-SHR). Hyperlocomotion, recognition, and spatial memory disturbances were observed in adolescent SHR rats from both sexes. However, females showed lack of habituation and worsened spatial memory. Although caffeine was effective against recognition memory impairment in both sexes, spatial memory was recovered only in female SHR rats. Besides, female SHR rats showed exacerbated hyperlocomotion after caffeine treatment. SHR rats from both sexes presented increases in the BDNF, truncated and phospho-TrkB receptors and also phospho-CREB levels in the hippocampus. Caffeine normalized BDNF in males and truncated TrkB receptor at both sexes. These findings provide insight into the potential of caffeine against fully cognitive impairment displayed by females in the ADHD model. Besides, our data revealed that caffeine intake since childhood attenuated behavioral alterations in the ADHD model associated with changes in BDNF and TrkB receptors in the hippocampus.

Animal Models of Vascular Cognitive Impairment and Dementia (VCID)

Vascular cognitive impairment and dementia (VCID) is the most common etiology of dementia in the elderly. Both, vascular and Alzheimer's disease, pathologies work synergistically to create neurodegeneration and cognitive impairments. The main causes of VCID include hemorrhage/microbleed (i.e., hyperhomocysteinemia), cerebral small vessel disease, multi-infarct dementia, severe hypoperfusion (i.e., bilateral common carotid artery stenosis), strategic infarct, angiopathy (i.e., cerebral angiopathy), and hereditary vasculopathy (i.e., cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy). In this review, we will discuss the experimental animal models that have been developed to study these pathologies. We will discuss the limitations and strengths of these models and the important research findings that have advanced the field through the use of the models.

Ultrastructural Correlates of Enhanced Norepinephrine and Neuropeptide Y Cotransmission in the Spontaneously Hypertensive Rat Brain

The spontaneously hypertensive rat (SHR) replicates many clinically relevant features of human essential hypertension and also exhibits behavioral symptoms of attention-deficit/hyperactivity disorder and dementia. The SHR phenotype is highly complex and cannot be explained by a single genetic or physiological mechanism. Nevertheless, numerous studies including our own work have revealed striking differences in central catecholaminergic transmission in SHR such as increased vesicular catecholamine content in the ventral brainstem. Here, we used immunolabeling followed by confocal microscopy and electron microscopy to quantify vesicle sizes and populations across three catecholaminergic brain areas—nucleus tractus solitarius and rostral ventrolateral medulla, both key regions for cardiovascular control, and the locus coeruleus. We also studied colocalization of neuropeptide Y (NPY) in norepinephrine and epinephrine-containing neurons as NPY is a common cotransmitter with central and peripheral catecholamines. We found significantly increased expression and coexpression of NPY in norepinephrine and epinephrine-positive neurons of locus coeruleus in SHR compared with Wistar rats. Ultrastructural analysis revealed immunolabeled vesicles of 150 to 650 nm in diameter (means ranging from 250 to 300 nm), which is much larger than previously reported. In locus coeruleus and rostral ventrolateral medulla, but not in nucleus tractus solitarius, of SHR, noradrenergic and adrenergic vesicles were significantly larger and showed increased NPY colocalization when compared with Wistar rats. Our morphological evidence underpins the hypothesis of hyperactivity of the noradrenergic and adrenergic system and increased norepinephrine and epinephrine and NPY cotransmission in specific brain areas in SHR. It further strengthens the argument for a prohypertensive role of C1 neurons in the rostral ventrolateral medulla as a potential causative factor for essential hypertension.

D-amphetamine improves attention performance in adolescent Wistar, but not in SHR rats, in a two-choice visual discrimination task

The validity of spontaneous hypertensive rat (SHR) as a model of attention deficit hyperactivity disorder (ADHD) has been explored by comparing SHR with Wistar rats in a test of attention, the two-choice visual discrimination task (2-CVDT). Animals were 4-5 weeks old during the training phase of the experiment and 6-7 weeks old during the testing phase in which they were tested with D-amphetamine, a stimulant drug used for the treatment of ADHD. As compared to Wistar, SHR showed a slightly better attention performance, a slightly lower impulsivity level, and a lower general activity during the training phase, but these differences disappeared or lessened thereafter, during the testing phase. D-amphetamine (0.5, 1 mg/kg) improved attention performance in Wistar, but not in SHR, and did not modify impulsivity and activity in the two strains. In conclusion, the present study did not demonstrate that SHR represents a valid model of ADHD, since it did not show face validity regarding the behavioral symptoms of ADHD and predictive validity regarding the effect of a compound used for the treatment of ADHD. On the other hand, this study showed that the 2-CVDT may represent a suitable tool for evaluating in adolescent Wistar rats the effect on attention of compounds intended for the treatment of ADHD.

Characterization of cognitive deficits in spontaneously hypertensive rats, accompanied by brain insulin receptor dysfunction

Background

The spontaneously hypertensive rat (SHR) has been used to model changes in the central nervous system associated with cognitive-related disorders. Recent human and animal studies indicate a possible relationship between cognitive deficits, insulin resistance and hypertension. We aimed to investigate whether cognitively impaired SHRs develop central and/or peripheral insulin resistance and how their cognitive performance is influenced by the animal’s sex and age as well as strains used for comparison (Wistar and Wistar-Kyoto/WKY).

Methods

Three and seven-month-old SHR, Wistar, and WKY rats were studied for their cognitive performance using Morris Water Maze (MWM) and Passive Avoidance tests (PAT). Plasma glucose and insulin were obtained after oral glucose tolerance tests. Cerebral cortex, hippocampus, and striatum status of insulin-receptor (IR) β-subunit and glycogen synthase kinase-3β (GSK3β) and their phosphorylated forms were obtained via ELISA.

Results

SHRs performed poorly in MWM and PAT in comparison to both control strains but more pronouncedly compared to WKY. Females performed poorer than males and 7-month-old SHRs had poorer MWM performance than 3-month-old ones. Although plasma glucose levels remained unchanged, plasma insulin levels were significantly increased in the glucose tolerance test in 7-month-old SHRs. SHRs demonstrated reduced expression and increased activity of IRβ-subunit in cerebral cortex, hippocampus, and striatum with different regional changes in phospho/total GSK3β ratio, as compared to WKYs.

Conclusion

Results indicate that cognitive deficits in SHRs are accompanied by both central and peripheral insulin dysfunction, thus allowing for the speculation that SHRs might additionally be considered as a model of insulin resistance-induced type of dementia.

Electronic supplementary material

The online version of this article (doi:10.1186/s40303-015-0012-6) contains supplementary material, which is available to authorized users.