Age-related development of gamma-aminobutyric acid (GABA)B receptor functions in various brain regions of spontaneously hypertensive rats

Hippocampus under Pressure: Molecular Mechanisms of Development of Cognitive Impairments in SHR Rats

Data from clinical trials and animal experiments demonstrate relationship between chronic hypertension and development of cognitive impairments. Here, we review structural and biochemical alterations in the hippocampus of SHR rats with genetic hypertension, which are used as a model of essential hypertension and vascular dementia. In addition to hypertension, dysfunction of the hypothalamic-pituitary-adrenal system observed in SHR rats already at an early age may be a key factor of changes in the hippocampus at the structural and molecular levels. Global changes at the body level, such as hypertension and neurohumoral dysfunction, are associated with the development of vascular pathology and impairment of the blood-brain barrier. Changes in multiple biochemical glucocorticoid-dependent processes in the hippocampus, including dysfunction of steroid hormones receptors, impairments of neurotransmitter systems, BDNF deficiency, oxidative stress, and neuroinflammation are accompanied by the structural alterations, such as cellular signs of neuroinflammation micro- and astrogliosis, impairments of neurogenesis in the subgranular neurogenic zone, and neurodegenerative processes at the level of synapses, axons, and dendrites up to the death of neurons. The consequence of this is dysfunction of hippocampus, a key structure of the limbic system necessary for cognitive functions. Taking into account the available results at various levels starting from the body and brain structure (hippocampus) levels to molecular one, we can confirm translational validity of SHR rats for modeling mechanisms of vascular dementia.

Evidence for reduced tonic levels of GABA in the hippocampus of an animal model of ADHD, the spontaneously hypertensive rat

Recent studies have investigated the role of γ-aminobutyric acid (GABA) in the behavioural symptoms of attention-deficit/hyperactivity disorder (ADHD), specifically in behavioural disinhibition. Spontaneously hypertensive rats (SHR) are widely accepted as an animal model of ADHD, displaying core symptoms of the disorder. Using an in vitro superfusion technique, we have shown that glutamate-stimulated release of radio-actively labelled norepinephrine ([(3)H]NE) from prefrontal cortex and hippocampal slices is greater in SHR than in their normotensive control strain, Wistar-Kyoto rats (WKY), and/or a standard control strain, Sprague-Dawley rats (SD). In the present study, we investigated how the level of extracellular (tonic) GABA affects release of [(3)H]NE in hippocampal slices of male and female SHR, WKY and SD rats, in response to 3 glutamate stimulations (S1, S2, and S3). The hippocampal slices were prelabelled with [(3)H]NE and superfused with buffer containing 0μM, 1μM, 10μM, or 100μM GABA. Three consecutive glutamate stimulations were achieved by exposing slices to 3 pulses of glutamate (1mM), each separated by 10min. Increasing tonic levels of GABA increased basal and stimulated release of [(3)H]NE in all strains. When GABA was omitted from the superfusion buffer used to perfuse SHR hippocampal slices, but present at 100µM in the buffer used to perfuse WKY and SD hippocampal slices, glutamate-stimulated release of [(3)H]NE was similar in all three strains. In these conditions, the decrease in [(3)H]NE release from S1 to S2 and S3 was also similar in all three strains. These findings suggest that extracellular concentrations of GABA may be reduced in SHR hippocampus, in vivo, compared to WKY and SD. An underlying defect in GABA function may be at the root of the dysfunction in catecholamine transmission noted in SHR, and may underlie their ADHD-like behaviours.

Fever induction by systemic stimulation with macrophage-activating lipopeptide-2 depends upon TLR2 but not CD36

This study was designed to test the responses of TLR2-knockout mice (TLR2-KO) and wild- type mice (C57/BL-6), and of CD36 deficient spontaneously hypertensive rats (SHR) and their genetic controls [Wistar Kyoto (WKY) rats] to systemic stimulations with the TLR2/6 agonist MALP-2 and the TLR4 agonist LPS. Fever and formation of TNF-α and IL-6 induced by intraperitoneal injections of MALP-2 (1000 µg/kg) were completely blunted in TLR2-KO, while LPS (100 µg/kg)-induced responses were not abolished in these animals. In SHR lacking CD36, a reduction of fever was observed in response to MALP-2 (100 µg/kg), but LPS-fever was even more attenuated in SHR when compared with WKY controls. Concentrations of circulating IL-6 tended to be lower in SHR after stimulation with both pyrogens. However, the IL-6-mediated activation of the transcription factor STAT3 in the brain was identical in both strains, indicating that the brain-controlled inflammatory response to MALP-2 (and LPS) is not impaired in the absence of CD36. In addition, stimulation of peritoneal macrophages with LPS and MALP-2 (10 µg/ml) caused the appearance of similar concentrations of bioactive cytokines in the supernatants from cells of both rat strains. These results demonstrate that TLR2 is essential for the manifestation of MALP-2, but not LPS-induced inflammatory responses. A moderate participation of CD36 in MALP-2-induced sickness- and cytokine-responses can not be ruled out but is unlikely as LPS-induced inflammatory responses were also attenuated in SHR.

Increased GABA B receptor subtype expression in the nucleus of the solitary tract of the spontaneously hypertensive rat

Expression of GABA(B) receptor messenger RNA (mRNA) in the central nervous system was compared between the spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rat. Polymerase chain reaction (PCR) revealed all the isoforms except B1e in cortex, hypothalamus, and medulla oblongata. In the nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM), the B1a-c and 1 g isoforms were present as well as B2. Real-time PCR detected significantly higher levels of B1a (p < 0.01) and B2 (p < 0.05) mRNA in the NTS of SHR compared to WKY. A significant increase in B1a expression (p < 0.05) was detected in VLM. Immunolabeling suggested presynaptic and postsynaptic expression of B1a, B1b, and B2 subtypes throughout the NTS, with significant differences in distribution patterns and labeling between subtypes and between SHR and WKY. These findings suggest that GABA(B) receptors expressed by neurones in NTS may be involved in cardiovascular regulation and that changes in GABA(B) mRNA expression levels may contribute to the hypertensive state in SHR.

Does hypertension alone lead to cognitive decline in spontaneously hypertensive rats?

Spontaneously hypertensive rats (SHR) represent an animal model of cognitive decline associated with hypertension. Few studies have systematically investigated this decline in aging. We assessed spatial learning performances of SHR, the genetically similar Wistar-Kyoto rats (WKY), and Sprague-Dawley rats (SD), in a water maze (WM) task. The following age ranges were studied: 6, 12, and 20 months. The results demonstrated that all three strains show the same level of impairment at the age of 20 months. The rate of cognitive decline, however, is different: both SHR and WKY show moderate degrees of impairment at all age ranges, while SD display good cognitive abilities at 6 months, declining at 12, and reaching the performance level of the other two groups at 20 months. Besides, the nine groups showed substantial differences in swim velocity and WKY exhibited a peculiar motor behavior. These results suggest the following: (a) the decline in cognitive level exhibits different trends in the three strains; (b) learning impairment of aged SHR might not be entirely explained by hypertension; (c) WKY should be used cautiously as normotensive control for SHR, due to their unusual behavior and low learning abilities; (d) analysis of escape distances is mandatory for the comparison of different strains in the WM test.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.