Juan M. Saavedra

Norepinephrine and dopamine content of hypothalamic nuclei of the rat

Abstract The concentrations of norepinephrine and dopamine of 27 isolated rat hypothalamic nuclei or nuclear subdivisions have been determined using an enzymatic-isotopic assay. The hypothalamic nuclei contain 3–20 times more NE and DA than is measured in the cortex. They are distributed unevenly throughout the hypothalamus and individual hypothalamic nuclei are heterogenous as regards their content of the amines. The dopamine content of the median eminence is among the highest measured in the brain (65 ng/mg protein). The norepinephrine content is about half that of dopamine. All of the hypothalamic nuclei contain dopamine. It is highly concentrated in the rostral subdivisions of the arcuate nucleus, the paraventricular and dorsomedial nuclei, the retrochiasmatic area, the medial posterior subdivision of the ventromedial nucleus and in the medial forebrain bundle at the posterior hypothalamic level. The highest concentrations of norepinephrine were found in the paraventricular and dorsomedial nuclei and in the retrochiasmatic area. The rostal subdivision of the arcuate nucleus and the periventricular and preoptic suprachiasmatic nuclei are also rich in norepinephrine. The posterior hypothalamus (premammillary nuclei, caudal subdivision of the arcurate nucleus, posterior hypothalamic nucleus) contains norepinephrine and dopamine in relatively low concentrations.

Serotonin content of the brain stem nuclei in the rat.

Abstract The serotonin (5-hydroxytryptamine; 5-HT) concentrations of 55 individual brain stem regions were determined by use of a specific and sensitive enzymatic isotopic technique in the rat. Serotonin was unevenly distributed throughout all the brain stem areas examined. The highest concentrations of 5-HT (above 20 ng/mg protein) were found in certain of the raphe nuclei. A high amount of 5-HT was also measured in the substantia nigra and in the globus pallidus. The central and tegmental gray matters of the mesencephalon and pons contained moderate amounts of 5-HT. The nuclei of reticular formation all contained about the same amount of 5-HT. There was more than a 5-fold difference between the highest and lowest concentrations of 5-HT measured in the cranial nerve nuclei. The motor nerve nuclei seemed to contain more of the amine than the sensory nerve nuclei.

Thyrotropin-Releasing Hormone in Specific Nuclei of Rat Brain

The regional distribution of thyrotropin-releasing hormone (TRH) in rat brain was studied. The greatest concentration of TRH was found in the median eminence. High concentrations were also found in several hypothalamic nuclei. Outside the hypothalamus, relatively large amounts of TRH were found in the septal and preoptic areas.

Angiotensin II AT 1 Blockade Normalizes Cerebrovascular Autoregulation and Reduces Cerebral Ischemia in Spontaneously Hypertensive Rats

BACKGROUND AND PURPOSE Angiotensin II, through stimulation of AT(1) receptors, not only controls blood pressure but also modulates cerebrovascular flow. We sought to determine whether selective AT(1) antagonists could be therapeutically advantageous in brain ischemia during chronic hypertension. METHODS We pretreated spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto controls with the AT(1) antagonist candesartan (CV-11974), 0.5 mg/kg per day, for 3 to 14 days, via subcutaneously implanted osmotic minipumps. We analyzed cerebral blood flow by laser-Doppler flowmetry, cerebral stroke in SHR after occlusion of the middle cerebral artery with reperfusion, and brain AT(1) receptors by quantitative autoradiography. RESULTS Candesartan treatment normalized blood pressure and the shift toward higher blood pressures at both the upper and lower limits of cerebrovascular autoregulation in SHR. Candesartan pretreatment of SHR for 14 days partially prevented the decrease in blood flow in the marginal zone of ischemia and significantly reduced the volume of total and cortical infarcts after either 1 or 2 hours of middle cerebral artery occlusion with reperfusion, relative to untreated SHR, respectively. This treatment also significantly reduced brain edema after 2 hours of middle cerebral artery occlusion with reperfusion. In SHR, candesartan markedly decreased AT(1) binding in areas inside (nucleus of the solitary tract) and outside (area postrema) the blood-brain barrier and in the middle cerebral artery. CONCLUSIONS Pretreatment with an AT(1) antagonist protected hypertensive rats from brain ischemia by normalizing the cerebral blood flow response, probably through AT(1) receptor blockade in cerebral vessels and in brain areas controlling cerebrovascular flow during stroke.

Changes in central catecholaminergic neurons in the spontaneously (genetic) hypertensive rat.

Catecholamines and catecholamine-synthesizing enzymes have been examined in specific brain areas during the development of spontaneously (genetic) hypertensive (SH) rats. Changes in catecholamine metabolism were localized to regions of the brain implicated in the regulation of blood pressure. Norepinephrine levels and dopamine-β-hydroxylase (DBH) activities were decreased in specific nuclei of the hypothalamus and in the nucleus interstitialis striae terminalis ventralis, in both young and adult rats. The decrease in the formation of norepinephrine can result in a reduced activation of central α-adrenergic receptors which may be related causally to the onset of hypertension. The activity of the epinephrine-forming enzyme, phenylethanolamine-A+—methyltransferase (PNMT), was increased in the A, and A2 areas of the brainstem in young SH rats, but it was normal in adult hypertensive animals. These results implicate adrenergic neurons in the brainstem and noradrenergic neurons in the hypothalamus in the development of spontaneous (genetic) hypertension in rats.

Serotonin distribution in the nuclei of the rat hypothalamus and preoptic region

Abstract Serotonin was measured in individual nuclei of the rat hypothalamic and preoptic regions by a specific and sensitive enzymatic-isotopic technique. This amine was unevenly distributed throughout the areas examined. Highest serotonin concentrations were found in nuclei of the basal and posterior hypothalamus. The high concentration of serotonin in the hypothalamus and median eminence suggests that this amine plays a role in the regulation of neuroendocrine mechanisms.

Norepinephrine and dopamine in the limbic system of the rat

Abstract Norepinephrine and dopamine, which were assayed by means of a sensitive enzymatic isotopic method, were found to be unevenly distributed among the areas which make up the limbic system. Both catecholamines could be detected in all of the regions studied. Many regions of the limbic system were richly endowed with one or both of these compounds. The concentration of norepinephrine was particularly high in the dorsal part of the nucleus interstitialis striae terminalis, the nucleus interstitialis striae medullaris, and the nucleus tractus diagonalis; the concentration of dopamine was very high in the olfactory tubercle and in the area amygdaloidea anterior.

Brain angiotensin II: new developments, unanswered questions and therapeutic opportunities.

Abstract1. There are two Angiotensin II systems in the brain. The discovery of brain Angiotensin II receptors located in neurons inside the blood brain barrier confirmed the existence of an endogenous brain Angiotensin II system, responding to Angiotensin II generated in and/or transported into the brain. In addition, Angiotensin II receptors in circumventricular organs and in cerebrovascular endothelial cells respond to circulating Angiotensin II of peripheral origin. Thus, the brain responds to both circulating and tissue Angiotensin II, and the two systems are integrated.2. The neuroanatomical location of Angiotensin II receptors and the regulation of the receptor number are most important to determine the level of activation of the brain Angiotensin II systems.3. Classical, well-defined actions of Angiotensin II in the brain include the regulation of hormone formation and release, the control of the central and peripheral sympathoadrenal systems, and the regulation of water and sodium intake. As a consequence of changes in the hormone, sympathetic and electrolyte systems, feed back mechanisms in turn modulate the activity of the brain Angiotensin II systems. It is reasonable to hypothesize that brain Angiotensin II is involved in the regulation of multiple additional functions in the brain, including brain development, neuronal migration, process of sensory information, cognition, regulation of emotional responses, and cerebral blood flow.4. Many of the classical and of the hypothetical functions of brain Angiotensin II are mediated by stimulation of Angiotensin II AT1 receptors.5. Brain AT2 receptors are highly expressed during development. In the adult, AT2 receptors are restricted to areas predominantly involved in the process of sensory information. However, the role of AT2 receptors remains to be clarified.6. Subcutaneous or oral administration of a selective and potent non-peptidic AT1 receptor antagonist with very low affinity for AT2 receptors and good bioavailability blocked AT1 receptors not only outside but also inside the blood brain barrier. The blockade of the complete brain Angiotensin II AT1 system allowed us to further clarify some of the central actions of the peptide and suggested some new potential therapeutic avenues for this class of compounds.7. Pretreatment with peripherally administered AT1 antagonists completely prevented the hormonal and sympathoadrenal response to isolation stress. A similar pretreatment prevented the development of stress-induced gastric ulcers. These findings strongly suggest that blockade of brain AT1 receptors could be considered as a novel therapeutic approach in the treatment of stress-related disorders.8. Peripheral administration of AT1 receptor antagonists strongly affected brain circulation and normalized some of the profound alterations in cerebrovascular structure and function characteristic of chronic genetic hypertension. AT1 receptor antagonists were capable of reversing the pathological cerebrovascular remodeling in hypertension and the shift to the right in the cerebral autoregulation, normalizing cerebrovascular compliance. In addition, AT1 receptor antagonists normalized the expression of cerebrovascular nitric oxide synthase isoenzymes and reversed the inflammatory reaction characteristic of cerebral vessels in hypertension. As a consequence of the normalization of cerebrovascular compliance and the prevention of inflammation, there was, in genetically hypertensive rats a decreased vulnerability to brain ischemia. After pretreatment with AT1 antagonists, there was a protection of cerebrovascular flow during experimental stroke, decreased neuronal death, and a substantial reduction in the size of infarct after occlusion of the middle cerebral artery. At least part of the protective effect of AT1 receptor antagonists was related to the inhibition of the Angiotensin II system, and not to the normalization of blood pressure. These results indicate that treatment with AT1 receptor antagonists appears to be a major therapeutic avenue for the prevention of ischemia and inflammatory diseases of the brain.9. Thus, orally administered AT1 receptor antagonists may be considered as novel therapeutic compounds for the treatment of diseases of the central nervous system when stress, inflammation and ischemia play major roles.10. Many questions remain. How is brain Angiotensin II formed, metabolized, and distributed? What is the role of brain AT2 receptors? What are the molecular mechanisms involved in the cerebrovascular remodeling and inflammation which are promoted by AT1 receptor stimulation? How does Angiotensin II regulate the stress response at higher brain centers? Does the degree of activity of the brain Angiotensin II system predict vulnerability to stress and brain ischemia? We look forward to further studies in this exiting and expanding field.

Protection Against Ischemia and Improvement of Cerebral Blood Flow in Genetically Hypertensive Rats by Chronic Pretreatment With an Angiotensin II AT1 Antagonist

Background and Purpose— Pretreatment with angiotensin II AT1 receptor antagonists protects against cerebral ischemia. We studied whether modulation of cerebral blood flow (CBF) and morphometric changes in brain arteries participated in this protective mechanism. Methods— We pretreated adult spontaneously hypertensive rats with equally antihypertensive doses of candesartan (0.1 or 0.3 mg/kg per day), nicardipine (0.1 mg/kg per day), or captopril (3.0 mg/kg per day) for 3 or 28 days via subcutaneous osmotic minipumps followed by permanent left middle cerebral artery (MCA) occlusion distal to the origin of the lenticulostriate arteries. We measured CBF by autoradiography with 4-iodo-[N-methyl-14C]antipyrine 3 hours after operation and the areas of infarct and tissue swelling 24 hours after operation. Morphometric changes in the MCA were studied after antihypertensive treatment. Results— Twenty-eight days of candesartan pretreatment decreased the infarct area by 31%; reduced the CBF decrease at the peripheral area of ischemia and the cortical volume of severe ischemic lesion, where CBF was <0.50 mL/g per minute; increased the MCA external diameter by 16%; and reduced the media thickness of the MCA by 23%. Captopril pretreatment for 28 days decreased the infarct area by 25%. Pretreatment with candesartan for 3 days or nicardipine for 28 days was ineffective. Conclusions— Angiotensin II system inhibition protects against neuronal injury more effectively than calcium channel blockade. Protection after AT1 receptor blockade is not directly correlated with blood pressure reduction but with normalization of MCA media thickness, leading to increased arterial compliance and reduced CBF decrease during ischemia at the periphery of the lesion.

Changes in expression of angiotensin receptor subtypes in the rat aorta during development

Quantitative autoradiography was used to characterize angiotensin AT1 and AT2 receptors, in the rat aorta at three developmental ages; embryonic day 18 (E18), and postnatal weeks 2 and 8. The expression of angiotensin receptors was higher in the aorta of E18 and 2-week-old rat. A major proportion of the angiotensin receptors expressed in the aorta at these two ages was AT2 (84 and 81% respectively). Conversely, in the aorta of 8-week-old rats, AT1 was the predominant angiotensin receptor subtype (71%). In 8-week-old rats, the AT2 subtype was also present (28%). In pre- and postnatal rats, [125I]Sar1-angiotensin II binding to AT1 receptors was sensitive to GTP gamma S whereas binding to AT2 receptors was not. AT2 receptors may serve an important role during stages of rapid growth of the aorta, and also have a significant function in the adult vasculature.