Gustavo Baiardi

Involvement of the brain renin-angiotensin system (RAS) in the neuroadaptive responses induced by amphetamine in a two-injection protocol.

A single or repeated exposure to psychostimulants induces long-lasting neuroadaptative changes. Different neurotransmitter systems are involved in these responses including the neuropeptide angiotensin II. Our study tested the hypothesis that the neuroadaptative changes induced by amphetamine produce alterations in brain RAS components that are involved in the expression of the locomotor sensitization to the psychostimulant drug. Wistar male rats, pretreated with amphetamine were used 7 or 21 days later to study AT1 receptors by immunohistochemistry and western blot and also angiotensinogen mRNA and protein in caudate putamen and nucleus accumbens. A second group of animals was used to explore the possible role of Ang II AT1 receptors in the expression of behavioral sensitization. In these animals treated in the same way, bearing intra-cerebral cannula, the locomotor activity was tested 21 days later, after an amphetamine challenge injection and the animals received an AT1 blocker, losartan, or saline 5min before the amphetamine challenge. An increase of AT1 receptor density induced by amphetamine was found in both studied areas and a decrease in angiotensinogen mRNA and protein only in CPu at 21 days after treatment; meanwhile, no changes were established in NAcc. Finally, the increased locomotor activity induced by amphetamine challenge was blunted by losartan administration in CPu. No differences were detected in the behavioral sensitization when the AT1 blocker was injected in NAcc. Our results support the hypothesis of a key role of brain RAS in the neuroadaptative changes induced by amphetamine.

A previous history of repeated amphetamine exposure modifies brain angiotensin II AT1 receptor functionality

UNLABELLED Previous results from our laboratory showed that angiotensin II AT1 receptors (AT1-R) are involved in the neuroadaptative changes induced by amphetamine. The aim of the present work was to study functional and neurochemical responses to angiotensin II (ANG II) mediated by AT1-R activation in animals previously exposed to amphetamine. For this purpose male Wistar rats (250-320 g) were treated with amphetamine (2.5mg/kg/day intraperitoneal) or saline for 5 days and implanted with intracerebroventricular (i.c.v.) cannulae. Seven days after the last amphetamine administration the animals received ANG II (400 pmol) i.c.v. One group was tested in a free choice paradigm for sodium (2% NaCl) and water intake and sacrificed for Fos immunoreactivity (Fos-IR) determinations. In a second group of rats, urine and plasma samples were collected for electrolytes and plasma renin activity determination and then they were sacrificed for Fos-IR determination in Oxytocinergic neurons (Fos-OT-IR). RESULTS Repeated amphetamine exposure (a) prevented the increase in sodium intake and Fos-IR cells in caudate-putamen and accumbens nucleus induced by ANG II i.c.v. (b) potentiated urinary sodium excretion and Fos-OT-IR in hypothalamus and (c) increased the inhibitory response in plasma renin activity, in response to ANG II i.c.v. Our results indicate a possible functional desensitisation of AT1-R in response to ANG II, induced by repeated amphetamine exposure. This functional AT1-R desensitisation allows to unmask the effects of ANG II i.c.v. mediated by oxytocin. We conclude that the long lasting changes in brain AT1-R functionality should be considered among the psychostimulant-induced neuroadaptations.

Neurovascular unit alteration in somatosensory cortex and enhancement of thermal nociception induced by amphetamine involves central AT1 receptor activation

The use of psychostimulants, such as amphetamine (Amph), is associated with inflammatory processes, involving glia and vasculature alterations. Brain Angiotensin II (Ang II), through AT1‐receptors (AT1‐R), modulates neurotransmission and plays a crucial role in inflammatory responses in brain vasculature and glia. Our aim for the present work was to evaluate the role of AT1‐R in long‐term alterations induced by repeated exposure to Amph. Astrocyte reactivity, neuronal survival and brain microvascular network were analysed at the somatosensory cortex. Thermal nociception was evaluated as a physiological outcome of this brain area. Male Wistar rats (250–320 g) were administered with AT1‐R antagonist Candesartan/vehicle (3 mg/kg p.o., days 1–5) and Amph/saline (2.5 mg/kg i.p., days 6–10). The four experimental groups were: Veh‐Sal, CV‐Sal, Veh‐Amph, CV‐Amph. On day 17, the animals were sacrificed and their brains were processed for Nissl staining and immunohistochemistry against glial fibrillary acidic protein (GFAP) and von Willebrand factor. In another group of animals, thermal nociception was evaluated using hot plate test, in the four experimental groups, on day 17. Data were analysed with two‐way anova followed by Bonferroni test. Our results indicate that Amph exposure induces an increase in: neuronal apoptosis, astrocyte reactivity and microvascular network, evaluated as an augmented occupied area by vessels, branching points and their tortuosity. Moreover, Amph exposure decreased the thermal nociception threshold. Pretreatment with the AT1‐R blocker prevented the described alterations induced by this psychostimulant. The decreased thermal nociception and the structural changes in somatosensory cortex could be considered as extended neuroadaptative responses to Amph, involving AT1‐R activation.

Neurovascular Cognitive Alterations: Implication of Brain Renin–Angiotensin System

The neurovascular unit which comprises the microenvironment within small blood vessels in the brain parenchyma is responsible for the maintenance of normal neuronal function by a continuous supply of nutrients. Inflammatory processes and loss of brain–blood-barrier (BBB) integrity can lead to vascular dysfunction and pathological interactions between microvasculature, neurons, and astrocytes. These events have been closely related to the development of brain disorders such as cognitive decline, supported by numerous studies using hypertension animal models. There is a large body of evidence showing the implication of circulating and local renin angiotensin system in cerebral microvasculature function. Angiotensin II, trough AT1 receptor activation, has been related to elevated reactive oxygen species production, endothelial dysfunction, elevated permeability, inflammatory events, and vascular structure alterations. The angiotensin receptor blockers, used in antihypertensive treatments, are an important pharmacologic tool with neuroprotective effects because they can modify vascular damage and improve cognitive alterations. The development of vascular diseases can be influenced and promoted by external factors such as stress and drug abuse. Stress is related to induction of structural changes in arteries and cytokine production leading to endothelial damage and inflammation. It is known that psychostimulants have cardiovascular stimulant effects that can promote cerebral vasculitis and intracranial hemorrhage by direct and indirect mechanisms on the vasculature. The brain renin–angiotensin system is becoming an interesting new therapeutic target for vascular and related cognitive disorders.

Mechanisms Involved in Memory Processes: Alterations Induced by Psychostimulants—Targeting the Central AT 1 Receptors

Learned experiences are indispensable for adaptation and survival of every living organism. The generation of a memory trace is an active physiological process which implies association and organization of the new impressions with already stored ones. Therefore, memory is explained as activity-dependent synaptic plasticity, involving electrophysiological, biochemical and morphological changes in functional synapse.

The Extent of Neuroadaptive Responses to Psychostimulants: Focus on Brain Angiotensin System

Amphetamine and cocaine are drugs of abuse worldwide consumed for their stimulant properties in the central nervous system. They mainly potentiate noradrenergic and dopaminergic neurotransmission and induce long-term changes in multiple neuronal circuits, modifying the future responses to pharmacological or non-pharmacological challenges. The altered neuronal connectivity induced by psychostimulants has long been studied in reward processing brain areas and in behavioral responses. Different neurotransmitter systems are involved in these responses, including the neuropeptide angiotensin II. Locally produced brain angiotensin II, acting through AT1 receptors, plays an important role in the modulation of central dopaminergic neurotransmission. Dopamine-innervated areas such as caudate putamen, nucleus accumbens, substantia nigra, hypothalamus, and ventral pallidum express high AT1 receptor density. Our recent studies show the role of angiotensin II AT1 receptors in the development of neuroadaptative behavioral and neurochemical changes induced by amphetamine. Moreover, we found alterations in the components of the renin angiotensin system (RAS) and in the functionality of AT1 receptors after amphetamine exposure. The evidence presented in this chapter highlight the RAS as a neuromodulatory system of superior brain activities, and further validate Angiotensin II involvement in amphetamine-induced alterations through AT1 receptor activation. The AT1 receptor blockers are currently and safely used in clinic for different pathologies, so they would be prominent candidates for pharmacological treatment in pathologies related to altered dopamine neurotransmission, such as drug addiction, schizophrenia, or even depression.

Brain Renin-Angiotensin System: A Novel Therapeutic Target for Psychostimulant and Alcohol Related Disorders?

The renin angiotensin system (RAS) is involved not only in the regulation of blood pressure and fluid homeostasis, but also in the modulation of multiple additional functions in the brain. In this sense, it was found to be involved in many neuroadaptive responses induced by drugs such as cocaine, amphetamines, alcohol, as well as others.

Role of the Neuropeptide Angiotensin II in Stress and Related Disorders

Angiotensin II (Ang II) was described as a peripheral hormone; its synthesis and metabolism were characterized and it is currently known as the renin-angiotensin system (RAS). All the components of the RAS, including the receptors, have been found in brain tissue, indicating a role as a hormone or neuromodulator in the central nervous system. Ang II exerts its principal known actions at the AT1 receptor. Its functions related to AT2 receptors are controversial and associated with AT1 opposite effects, although there is evidence showing cross-talk between both receptors. The metabolism of Ang II generates other active peptides, such as Angiotensin 1–7 and Angiotensin IV, which will not be discussed. Neurobiological research has explained many of the different neuroendocrine and behavioral responses to stressors. Stress is a complex phenomenon in response to physical, environmental, or psychological stimulus. Stress triggers important adaptive functions improving health and survival. Meanwhile, excessive stress can be deleterious, therefore, individuals unable to cope with stress are highly vulnerable to a variety of diseases. Stress is a major contributor of cardiovascular disorders and psychiatric illness such as anxiety and depression. Many studies have confirmed that stress also increases the vulnerability to drug abuse.