Diurnal variation of cerebral blood flow in rat hippocampus

Circadian and Diurnal Regulation of Cerebral Blood Flow

Circadian and diurnal variation in cerebral blood flow directly contributes to the diurnal variation in the risk of stroke, either through factors that trigger stroke or due to impaired compensatory mechanisms. Cerebral blood flow results from the integration of systemic hemodynamics, including heart rate, cardiac output, and blood pressure, with cerebrovascular regulatory mechanisms, including cerebrovascular reactivity, autoregulation, and neurovascular coupling. We review the evidence for the circadian and diurnal variation in each of these mechanisms and their integration, from the detailed evidence for mechanisms underlying the nocturnal nadir and morning surge in blood pressure to identifying limited available evidence for circadian and diurnal variation in cerebrovascular compensatory mechanisms. We, thus, identify key systemic hemodynamic factors related to the diurnal variation in the risk of stroke but particularly identify the need for further research focused on cerebrovascular regulatory mechanisms.

Hypertension: Causes and Consequences of Circadian Rhythms in Blood Pressure

Hypertension is extremely common, affecting approximately 1 in every 2 adults globally. Chronic hypertension is the leading modifiable risk factor for cardiovascular disease and premature mortality worldwide. Despite considerable efforts to define mechanisms that underlie hypertension, a potentially major component of the disease, the role of circadian biology has been relatively overlooked in both preclinical models and humans. Although the presence of daily and circadian patterns has been observed from the level of the genome to the whole organism, the functional and structural impact of biological rhythms, including mechanisms such as circadian misalignment, remains relatively poorly defined. Here, we review the impact of daily rhythms and circadian systems in regulating blood pressure and the onset, progression, and consequences of hypertension. There is an emphasis on the impact of circadian biology in relation to vascular disease and end-organ effects that, individually or in combination, contribute to complex phenotypes such as cognitive decline and the loss of cardiac and brain health. Despite effective treatment options for some individuals, control of blood pressure remains inadequate in a substantial portion of the hypertensive population. Greater insight into circadian biology may form a foundation for novel and more widely effective molecular therapies or interventions to help in the prevention, treatment, and management of hypertension and its related pathophysiology.

Behavioral and histopathological consequences of transient ischemic stroke in the Flinders Sensitive Line rat, a genetic animal model of depression

Patients with depression have an increased risk for stroke, higher mortality rates following stroke and worse functional outcomes among survivors. Preclinical studies may help to better understand the underlying mechanisms linking these two diseases, but only a few animal studies have investigated the effects of prestroke depression. The present study investigates whether Flinders Sensitive Line (FSL) rats, a genetic depression model, respond differently to focal ischemic stroke compared to control strains (Flinders Resistant Line [FRL] and Sprague-Dawley [SD]). Male adult FSL, FRL and SD rats received a unilateral injection of either vehicle or Endothelin-1 (ET-1) adjacent to the middle cerebral artery (MCA). Motor function was assessed at 48 h followed by euthanasia and infarct volume measurement using 2,3,5-triphenyltetrazolium chloride (TTC) staining and image analysis. In a separate cohort behavior was assessed using standard tests for motor function, locomotor activity, cognition, anxiety- and depression-like behavior beginning at 10 days post-injection followed by infarct quantification. We found that ET-1-induced MCA occlusion produced significant infarcts in all three strains. Stroke animals had slightly impaired motor function, but there was no clear interaction effects between strain and stroke surgery on behavioral outcomes. We conclude that FSL rats show no increased susceptibility to brain damage or behavioral deficits following ET-1-induced focal ischemic stroke compared to controls.

Rodent models for intravascular ischemic cerebral infarction: a review of influencing factors and method optimization

Rodent models for cerebral infarction are useful for studying human focal ischemic cerebral infarction, by simulating etiological and pathophysiological mechanisms. However, differences in the selection of anesthetic drugs, surgical methods and other factors may affect the extent to which preclinical models reflect the human condition. This review summarizes these factors. We searched pertinent literature from the MEDLINE and Web of Science databases, and reviewed differences in rodent strain, anesthesia method, sex, surgical method, timing of surgery, and factors influencing postoperative evaluation. In particular, circadian rhythm was found to have a significant impact on the outcome of cerebral infarction in rodent models. This information will enable researchers to quickly and clearly select appropriate modeling methods, acquire reliable quantitative experimental results, and obtain basic data for fundamental mechanism research.

In vivo variation in same-day estimates of metabotropic glutamate receptor subtype 5 binding using [11C]ABP688 and [18F]FPEB

Positron emission tomography tracers [11C]ABP688 and [18F]FPEB target the metabotropic glutamate receptor subtype 5 providing quantification of the brain glutamatergic system in vivo. Previous [11C]ABP688 positron emission tomography human test-retest studies indicate that, when performed on the same day, significant binding increases are observed; however, little deviation is reported when scans are >7 days apart. Due to the small cohorts examined previously (eight and five males, respectively), we aimed to replicate the same-day test-retest studies in a larger cohort including both males and females. Results confirmed large within-subject binding differences (ranging from -23% to 108%), suggesting that measurements are greatly affected by study design. We further investigated whether this phenomenon was specific to [11C]ABP688. Using [18F]FPEB and methodology that accounts for residual radioactivity from the test scan, four subjects were scanned twice on the same day. In these subjects, binding estimates increased between 5% and 39% between scans. Consistent with [11C]ABP688, mean absolute test-retest variability was previously reported as <12% when scans were >21 days apart. This replication study and pilot extension to [18F]FPEB suggest that observed within-day binding variation may be due to characteristics of mGluR5; for example, diurnal variation in mGluR5 may affect measurement of this receptor.

Long Term Amperometric Recordings in the Brain Extracellular Fluid of Freely Moving Immunocompromised NOD SCID Mice

We describe the in vivo characterization of microamperometric sensors for the real-time monitoring of nitric oxide (NO) and oxygen (O₂) in the striatum of immunocompromised NOD SCID mice. The latter strain has been utilized routinely in the establishment of humanized models of disease e.g., Parkinson's disease. NOD SCID mice were implanted with highly sensitive and selective NO and O₂ sensors that have been previously characterized both in vitro and in freely moving rats. Animals were systemically administered compounds that perturbed the amperometric current and confirmed sensor performance. Furthermore, the stability of the amperometric current was investigated and 24 h recordings examined. Saline injections caused transient changes in both currents that were not significant from baseline. l-NAME caused significant decreases in NO (p < 0.05) and O₂ (p < 0.001) currents compared to saline. l-Arginine produced a significant increase (p < 0.001) in NO current, and chloral hydrate and Diamox (acetazolamide) caused significant increases in O₂ signal (p < 0.01) compared against saline. The stability of both currents were confirmed over an eight-day period and analysis of 24-h recordings identified diurnal variations in both signals. These findings confirm the efficacy of the amperometric sensors to perform continuous and reliable recordings in immunocompromised mice.

Does antenatal betamethasone have negative effects on fetal activities and hemodynamics in cases of preeclampsia without severe features? A prospective, placebo-controlled, randomized study

AIM

To evaluate whether antenatal betamethasone affects the fetal biophysical profile (BPP) and Doppler indices of umbilical and middle cerebral arteries (MCAs) in cases of preeclampsia without severe features.

MATERIALS AND METHODS

Forty singleton preeclamptic pregnancies without severe features at gestational ages of 28-34 weeks were randomly divided into two groups of 20 patients: betamethasone and control groups. Patients in the betamethasone group were administered two consecutive doses of 12 mg betamethasone intramuscularly, 24 h apart, and patients in the control group were administered the same volume of saline as a placebo. All participants were evaluated before (0 h) and at hours 24, 48, and 72 of betamethasone/placebo administration using BPP scoring and umbilical and MCA Doppler examinations.

RESULTS

Total BPP scores were significantly lower in the betamethasone group across the three time points during the follow-up period (p < 0.001). None of the Doppler indices differed significantly between the groups (p > 0.05).

CONCLUSION

Antenatal betamethasone negatively affects fetal BPP score parameters, including the non-stress test, fetal body and breathing movements, without affecting vascular indices of umbilical arteries and MCAs. Clinician awareness of this transient drug-induced effect might be valuable for preventing iatrogenic preterm delivery for fetuses in preeclamptic pregnancies without severe features.

Influence of Chronobiology on the Nanoparticle-Mediated Drug Uptake into the Brain

Little attention so-far has been paid to the influence of chronobiology on the processes of nanoparticle uptake and transport into the brain, even though this transport appears to be chronobiologically controlled to a significant degree. Nanoparticles with specific surface properties enable the transport across the blood–brain barrier of many drugs that normally cannot cross this barrier. A clear dependence of the central antinociceptive (analgesic) effects of a nanoparticle-bound model drug, i.e., the hexapeptide dalargin, on the time of day was observable after intravenous injection in mice. In addition to the strongly enhanced antinociceptive effect due to the binding to the nanoparticles, the minima and maxima of the pain reaction with the nanoparticle-bound drug were shifted by almost half a day compared to the normal circadian nociception: The maximum in the pain reaction after i.v. injection of the nanoparticle-bound dalargin occurred during the later rest phase of the animals whereas the normal pain reaction and that of a dalargin solution was highest during the active phase of the mice in the night. This important shift could be caused by an enhanced endo- and exocytotic particulates transport activity of the brain capillary endothelial cells or within the brain during the rest phase.

Diurnal rhythmicity in biological processes involved in bioavailability of functional food factors

In the past few decades, many types of functional factors have been identified in dietary foods; for example, flavonoids are major groups widely distributed in the plant kingdom. However, the absorption rates of the functional food factors are usually low, and many of these are difficult to be absorbed in the intact forms because of metabolization by biological processes during absorption. To gain adequate beneficial effects, it is therefore mandatory to know whether functional food factors are absorbed in sufficient quantity, and then reach target organs while maintaining beneficial effects. These are the reasons why the bioavailability of functional food factors has been well investigated using rodent models. Recently, many of the biological processes have been reported to follow diurnal rhythms recurring every 24 h. Therefore, absorption and metabolism of functional food factors influenced by the biological processes may vary with time of day. Consequently, the evaluation of the bioavailability of functional food factors using rodent models should take into consideration the timing of consumption. In this review, we provide a perspective overview of the diurnal rhythm of biological processes involved in the bioavailability of functional food factors, particularly flavonoids.

Death by a thousand cuts in Alzheimer's disease: hypoxia--the prodrome

A wide range of clinical consequences may be associated with obstructive sleep apnea (OSA) including systemic hypertension, cardiovascular disease, pulmonary hypertension, congestive heart failure, cerebrovascular disease, glucose intolerance, impotence, gastroesophageal reflux, and obesity, to name a few. Despite this, 82 % of men and 93 % of women with OSA remain undiagnosed. OSA affects many body systems, and induces major alterations in metabolic, autonomic, and cerebral functions. Typically, OSA is characterized by recurrent chronic intermittent hypoxia (CIH), hypercapnia, hypoventilation, sleep fragmentation, peripheral and central inflammation, cerebral hypoperfusion, and cerebral glucose hypometabolism. Upregulation of oxidative stress in OSA plays an important pathogenic role in the milieu of hypoxia-induced cerebral and cardiovascular dysfunctions. Strong evidence underscores that cerebral amyloidogenesis and tau phosphorylation--two cardinal features of Alzheimer's disease (AD), are triggered by hypoxia. Mice subjected to hypoxic conditions unambiguously demonstrated upregulation in cerebral amyloid plaque formation and tau phosphorylation, as well as memory deficit. Hypoxia triggers neuronal degeneration and axonal dysfunction in both cortex and brainstem. Consequently, neurocognitive impairment in apneic/hypoxic patients is attributable to a complex interplay between CIH and stimulation of several pathological trajectories. The framework presented here helps delineate the emergence and progression of cognitive decline, and may yield insight into AD neuropathogenesis. The global impact of CIH should provide a strong rationale for treating OSA and snoring clinically, in order to ameliorate neurocognitive impairment in aged/AD patients.

Effects of bilateral jugular vein ligation on local cerebral blood flow

The authors examined the effect of bilateral jugular vein ligation (JVL) on local cerebral blood flow (CBF). Local CBF was measured in the neocortex of the frontal lobe over a 1-week period in 10 freely moving male rats. Measurements were taken with the hydrogen clearance method before and after the rat underwent either an operation of bilateral JVL or a sham operation. After the sham operation, blood flow values did not change significantly compared with preoperative values. Blood flow values decreased rapidly in rats with JVL. For 3 hours after surgery, JVL values were significantly lower (p <0.05) than preoperative values were, then gradually they recovered and reached preoperative values within 48 hours. Histopathologic study showed no remarkable parenchymal damages in the brain of rats with JVL. These results show that bilateral JVL produces a reversible decrease in the CBF and does not cause histopathologic brain damages.

Circadian periodicity of cerebral blood flow revealed by laser-Doppler flowmetry in awake rats: relation to blood pressure and activity

Cardiovascular parameters such as arterial blood pressure (ABP) and heart rate display pronounced circadian variation. The present study was performed to detect whether there is a circadian periodicity in the regulation of cerebral perfusion. Normotensive Sprague-Dawley rats (SDR, approximately 15 wk old) and hypertensive (mREN2)27 transgenic rats (TGR, approximately 12 wk old) were instrumented in the abdominal aorta with a blood pressure sensor coupled to a telemetry system for continuous recording of ABP, heart rate, and locomotor activity. After 5-12 days, a laser-Doppler flow (LDF) probe was attached to the skull by means of a guiding device to measure changes in brain cortical blood flow (CBF). After the animals recovered from anesthesia, measurements were taken for 3-4 days. The time series were analyzed with respect to the midline estimating statistic of rhythm (i.e., mean value of a periodic event after fit to a cosine function), amplitude, and acrophase (i.e., phase angle that corresponds to the peak of a given period) of the 24-h period. The LDF signal displayed a significant circadian rhythm, with the peak occurring at around midnight in SDR and TGR, despite inverse periodicity of ABP in TGR. This finding suggests independence of LDF periodicity from ABP regulation. Furthermore, the acrophase of the LDF was consistently found before the acrophase of the activity. From the present data, it is concluded that there is a circadian periodicity in the regulation of cerebral perfusion that is independent of circadian changes in ABP and probably is also independent of locomotor activity. The presence of a circadian periodicity in CBF may have implications for the occurrence of diurnal alterations in cerebrovascular events in humans.

Cerebral blood flow during spontaneous and cholinergically induced behavioral states in the sheep fetus

The sleep-wake cycle has been studied extensively in both adult and fetal mammalian species with emphasis in different areas. Fetal studies have focused on characterization of behavioral states and responses to challenges such as hypoxia, and there have been relatively fewer studies that have investigated the control of fetal behavioral state. The objective of this study was to determine whether cerebral blood flow during cholinergically induced fetal behavioral states was similar to that during spontaneous fetal behavioral states in chronically catheterized near-term sheep fetuses. Injection of carbachol (1.25 microg) into the cisterna magna increased the duration of the subsequent low-voltage electrocortical epoch. Scopolamine infusion (0.3 mg) increased the duration of the subsequent high-voltage electrocortical activity epoch. Cerebral blood flow and oxygen delivery were higher during both spontaneous and carbachol-induced low-voltage/rapid eye movement behavioral state than during spontaneous and scopolamine-induced high-voltage/non-rapid eye movement behavioral state. Thus, pharmacologic manipulation of fetal behavioral state induced a state that resembled spontaneous fetal behavioral state both electrophysiologically and metabolically. This study shows that inducing extended periods of a desired fetal behavioral state is possible and that this method may be used to study their function.

Betamethasone effects on fetal sheep cerebral blood flow are not dependent on maturation of cerebrovascular system and pituitary-adrenal axis

Synthetic glucocorticoids are administered to pregnant women in premature labour to accelerate fetal lung maturation at a time when fetal cerebrovascular and endocrine systems are maturing. Exposure to glucocorticoids at 0.8-0.9 of gestation increases peripheral and cerebrovascular resistance (CVR) in fetal sheep. We examined whether the increase of CVR and its adverse effect on cerebral blood flow (CBF) depend on the current level of maturation of the pituitary-adrenal axis and the cerebrovascular system. Using fluorescent microspheres, regional CBF was measured in 11 brain regions before and 24 h and 48 h after the start of 3.3 microg kg(-1) h(-1) betamethasone (n = 8) or vehicle (n = 7) infusions to fetal sheep at 0.73 of gestation. Hypercapnic challenges were performed before and 24 h after the onset of betamethasone exposure to examine betamethasone effects on cerebrovascular reactivity. Betamethasone exposure decreased CBF by approximately 40% in all brain regions after 24 h of infusion (P < 0.05). The decline in CBF was mediated by a CVR increase of 111 +/- 16% in the cerebral cortex and 129 +/- 29% in subcortical regions (P < 0.05). Hypercapnic cerebral vasodilatation and associated increase in CBF were blunted (P < 0.05). Fetal CBF recovered after 48 h of betamethasone administration. There were no differences in glucocorticoid induced CBF and CVR changes compared with our previous findings at 0.87 of gestation. We conclude that the cerebrovascular effects of antenatal glucocorticoids are independent of cerebrovascular maturation and preparturient increase in activity of the fetal pituitary-adrenal axis.

Rats living in small cages respond to restraint stress with adrenocortical corticosterone release but not with hippocampal acetylcholine release

We previously reported that the restriction of environmental space attenuated the hippocampal acetylcholine release and impaired spatial learning function. To examine the effect of the restriction of environmental space on the stress response of the hippocampal acetylcholine release, an in vivo microdialysis study was performed in male rats after 4 days of housing in a large cylindrical cage (diameter=35 cm) or a small cylindrical cage (diameter=19 cm). Significant stress response of the hippocampal acetylcholine release was observed in rats in the large cages (N=5), but it was not observed in rats in the small cages (N=5). The corticosterone concentration in serum was significantly increased by the restraint stress in both groups of rats. Although cage size does not influence stress-induced secretion of corticosterone, rats housed in a small cage exhibit lower levels of stress-induced ACh release than rats living in a large cage.

Circadian pattern of ventilation during acute and chronic hypercapnia in conscious adult rats

Because metabolism is a determinant of the ventilatory chemosensitivity, we tested the hypothesis that the ventilatory response to acute and prolonged hypercapnia is adjusted to the circadian oscillations in oxygen consumption (VO2). Adult rats were instrumented for measurements of body temperature (T(b)) and activity by telemetry. Pulmonary ventilation (VE) was measured by the barometric method and VO2 by the flow-through method. In the acute experiments, 16 conscious rats entrained to a 12:12-h light (L)-dark (D) cycle (lights on 7:00 AM) were exposed to air, 2%, and then 5% CO2 in normoxia (30-45 min each) at 11:00 AM and 11:00 PM. In a separate group of seven rats, simultaneous recordings of all variables were made continuously for 3 consecutive days in air followed by 3 days in 2% CO2 in normoxia, in a 12:12-h L-D cycle (lights on 7:00 AM). In air, all variables were significantly higher at night, whether rats were studied acutely or chronically. Acute CO2 exposure had similar significant effects at 11:00 AM and 11:00 PM on VE (approximately 25 and 100% increase with 2 and 5% CO2, respectively) and VO2 (approximately 8% drop with 5% CO2), such that the hyperventilatory response (% increase in VE/VO2 from air) was similar at both times. Chronic CO2 breathing increased VE at all times of the day, but less so during the L phase (approximately 15 vs. 22% increase in L and D, respectively), when activity was lower. However, VO2 was reduced from the air level (approximately 10% drop) in the L, such that the VE/VO2 response was similar between L and D. The same result was obtained when the VE/VO2 response was compared between the L and D phases for the same level of activity. These results suggest that, throughout the day, the hypercapnic hyperpnea, whether during acute or prolonged CO2, is perfectly adjusted to the metabolic level.

Effects of betamethasone administration to the fetal sheep in late gestation on fetal cerebral blood flow

Glucocorticoid administration to women at risk of preterm delivery to accelerate fetal lung maturation has become standard practice. Antenatal glucocorticoids decrease the incidence of intraventricular haemorrhage as well as accelerating fetal lung maturation. Little is known regarding side effects on fetal cerebral function. Cortisol and synthetic glucocorticoids such as betamethasone increase fetal blood pressure and femoral vascular resistance in sheep. We determined the effects of antenatal glucocorticoid administration on cerebral blood flow (CBF) in fetal sheep. Vehicle (n = 8) or betamethasone (n = 8) was infused over 48 h via the jugular vein of chronically instrumented fetal sheep at 128 days gestation (term 146 days). The betamethasone infusion rate was that previously shown to produce fetal plasma betamethasone concentrations similar to human umbilical vein concentrations during antenatal glucocorticoid therapy. Regional CBF was measured in 10 brain regions, using coloured microspheres, before and 24 and 48 h after onset of treatment, and during hypercapnic challenges performed before and 48 h after onset of betamethasone exposure. Betamethasone exposure decreased CBF in all brain regions measured except the hippocampus after 24 h of infusion (P < 0.05). The CBF decrease was most pronounced in the thalamus and hindbrain (45-50% decrease) and least pronounced in the cortical regions (35-40% decrease). It was mediated by an increase in cerebral vascular resistance (CVR, P < 0.05) and led to a decrease in oxygen delivery to subcortical and hindbrain structures of 30-40%, to 8.6 +/- 1.1 ml x (100 g)(-1) x min(-1), and 40-45 %, to 11.0 +/- 1.6 ml x 100 g(-1) x min(-1), respectively (P < 0.05). After 48 h of betamethasone treatment, the reduction in CBF was diminished to about 25-30 %, but was still significant in comparison to vehicle-treated fetuses in all brain regions except three of the five measured cortical regions (P < 0.05). CVR and oxygen delivery were unchanged in comparison to values at 24 h of treatment. The CBF increase in response to hypercapnia was diminished (P < 0.05). These observations demonstrate for the first time that glucocorticoids exert major vasoconstrictor effects on fetal CBF. This mechanism may protect the fetus against intraventricular haemorrhage both at rest and when the fetus is challenged. Betamethasone exposure decreased the hypercapnia-induced increase in CBF (P < 0.05) due to decreased cerebral vasodilatation (P < 0.05).

Circadian phase-dependent antinociceptive reaction in mice determined by the hot-plate test and the tail-flick test after intravenous injection of dalargin-loaded nanoparticles

Peptides normally do not cross the blood-brain barrier (BBB). Previously, it has been shown that the hexapeptide enkephalin analogue dalargin with polysorbate-80-coated nanoparticles (DAL/NP) can be transported across the BBB and is able to exhibit an antinociceptive effect in mice. In the present study, the circadian time and dose dependencies of the antinociceptive effect of different dalargin preparations were investigated. The active preparation (DAL/NP, 5 mg/kg, 10 mg/kg), as well as a dalargin solution in phosphate buffered saline (DAL/SOL, 10 mg/kg) were injected intravenously to groups of 10-12 inbred DBA/2 mice at 12 different circadian times; mice were synchronized to a light-dark (LD) 12:12 regimen. The antinociceptive effect was determined 15 minutes postinjection by the hot-plate test. Experiments with DAL/NP were repeated using the tail-flick test system at two selected times (08:00 and 20:00) to test for dose dependency (2.5, 5, 7.5, 10 mg/kg). Hot-plate latencies were rhythmic under baseline and after DAL/SOL, with acrophases in the dark phase; DAL/SOL did not influence latency time. In contrast, DAL/NP significantly increased reaction time dose dependently; the maximal possible effect was rhythmic with the 10 mg/kg preparation, with a peak effect in the early light phase. Results were confirmed by the tail-flick test. The experiments demonstrate that an enkephalin analogue coated with nanoparticles can easily cross the BBB and is able to display a dose- and time-dependent antinociceptive effect.

Chronic stress exposure influences local cerebral blood flow in the rat hippocampus

To examine the influence of chronic stress on the brain, we measured local cerebral blood flow in the hippocampus of rats which had been exposed to chronic stress by the hydrogen clearance method in the freely moving status. Rats were exposed, once a day for 12 weeks, to stress of a 15-min immersion in cold water at 4 degrees C (the stress group) or slightly handled for about 1 min (the control group). Local cerebral blood flow values in the hippocampus, which were measured after a 12-week recovery period, were lower in rats in the stress group than those of rats in the control group only in the dark cycle, but not in the light cycle. Accordingly, local cerebral blood flow in the hippocampus of rats in the stress group did not have a daily fluctuation, i.e. lower in the light cycle and higher in the dark cycle, as was shown in rats in the control group. There were no significant changes in motor activity in rats in the stress group as compared to those in the control group. Severe structural damages were observed in the CA2 and CA3 cell fields of the hippocampus of rats in the stress group. We found that an increase in local cerebral blood flow in the hippocampus in the dark cycle was blunted following chronic stress exposure, suggesting that chronic stress exposure caused hippocampal neurons to be less responsive to environmental stimuli derived from motor activity during the dark cycle.

Long-term glucocorticoid treatments decrease local cerebral blood flow in the rat hippocampus, in association with histological damage

The present study examined the influence of a long-term treatment with glucocorticoid on local cerebral blood flow of the hippocampus in rats, estimated with the hydrogen clearance method. Either a cholesterol (100 mg, as a control) or corticosterone (100 mg) bead was implanted subcutaneously in rats for a period of three months, beginning at 12 weeks of age. The effects of the treatments on the local circulation of the hippocampus were evaluated three to four months after the termination of the treatments. Hippocampal cerebral blood flow in corticosterone-treated rats was significantly lower (P<0.05) than that in control rats, and fluctuated over a day in lower amplitude than the controls. Severe histological damage was observed in the CA1 and CA3 cell fields of the hippocampus in corticosterone-treated rats. These neuropathological changes were characterized by soma shrinkage and condensation, or nuclear pyknosis, as reported previously. We concluded that a long-term glucocorticoid exposure resulted in an impairment of the hippocampal functions, accompanied by neuronal damage similar to that found in aged hippocampus. The present results support the hypothesis that glucocorticoids accelerate age-related changes in the brain.

Hippocampal and striatal blood flow during behavior in rats: chronic laser Doppler flowmetry study

A technique is described for the chronic measurement of cerebral blood flow in conscious, unrestrained rodents, utilizing laser doppler flowmetry (LDF) removably coupled to an optical fiber permanently implanted into brain tissue by established stereotaxic procedures. Changes in relative blood flow in response to a range of pharmacological and behavioral challenges were measured in the hippocampus (HBF) and striatum (StBF) 24-72 h and up to 28-32 days after surgical implantation of the optical fiber. Intraseptal microinfusion of L-glutamate in artificial cerebrospinal fluid 48-96 h and 28-32 days after surgery increased HBF. Pentobarbital (Nembutal) and urethane anesthesia decreased HBF. On the day of euthanasia under urethane anesthesia, HBF was demonstrated to be responsive to alteration of blood CO2 via hyper/hypocapnia, and autoregulation was demonstrated in response to hypovolemic hypotension. In behavioral experiments, blood flow was found to increase with activity and locomotion, as well as during paradoxical (PS) and slow-wave sleep (SWS). The greatest increase in CBF was measured during PS. Although basal levels of blood flow were similar between regions, the increase in blood flow during PS was greater in the hippocampus. This simple procedure enables real-time measurement of qualitative changes in regional cerebral blood flow during behaviors in conscious, unrestrained animals. The observation that constancy of measurements was obtained for 1 month enables within-subject analysis in longitudinal studies and reduces the number of animals required for investigations.

Measurement of brain tissue oxygen at a carbon past electrode can serve as an index of increases in regional cerebral blood flow

Simultaneous monitoring of tissue O(2) and regional cerebral blood flow (rCBF) was performed in the striatum of freely-moving rats. Differential pulse amperometry and constant potential amperometry were used to monitor O(2) levels at a carbon paste electrode (CPE), while rCBF values were obtained using the H2 clearance technique. Two forms of behavioural activation were studied and the resultant changes in tissue O(2) and blood flow compared. Both tail pinch and induced grooming produced immediate and parallel increases in O(2) and blood flow which returned to baseline on cessation of activity. These findings indicate that under conditions of physiological stimulation the direct voltammetric measurement of O(2) in brain tissue with a CPE can be used as a reliable index of increases in rCBF, resulting in an improvement in time resolution from 5 min (H2 clearance) to <1 s (amperometry). Because tissue O(2) is a balance between supply by the blood stream and utilisation by the cells, increases in O(2) current are an index of increased blood flow only when supply significantly exceeds utilisation.

Medial septal injection of naloxone elevates acetylcholine release in the hippocampus and induces behavioral seizures in rats

The effects of injections of naloxone, a universal opioid receptor antagonist, into the medial septal nucleus on hippocampal acetylcholine (ACh) release and behavior were investigated in freely moving rats by means of the microdialysis method. The injection of naloxone (2, 10 and 20 micrograms) produced a marked increase in hippocampal ACh release in a dose-dependent manner. These effects of naloxone were reversed by the post-injection of [D-Ala2, N-Me-Phe4, Gly-ol]-enkephalin (DAGO; 10 micrograms), an opioid mu receptor agonist. Furthermore, basal release of hippocampal ACh was significantly reduced by the injection of DAGO alone. It was also found that rats given an injection of naloxone showed an increase in motor activity and occasionally exhibited behavioral seizures. These effects of naloxone were also reversed by the post-injection of DAGO. The present results suggest that endogenous opioids ionically inhibit the activity of septo-hippocampal cholinergic neurons via mediation of mu opioid receptors in the medial septal nucleus. They also suggest that endogenous opioids modulate the incidence of seizures, at least in part, through opioid mu receptors in the medial septal nucleus.

Spontaneous acetylcholine release in the hippocampus exhibits a diurnal variation in both young and old rats

Extracellular levels of acetylcholine (ACh) in the hippocampus were measured by the microdialysis method in freely moving young (3-4 months old) and old (18-24 months old) female rats over a period of 24 h to examine the effect of aging on hippocampal ACh release. Hippocampal ACh release during a 24-h period exhibited a diurnal variation with higher levels during the dark cycle than during the light cycle in old rats as well as young rats. The present study suggests that a diurnal variation in ACh release is maintained fairly well until the rats are at least 24 months old.

Adrenalectomy increases local cerebral blood flow in the rat hippocampus

The present study examined the effect of glucocorticoid manipulations on local cerebral blood flow in the hippocampus. We measured local cerebral blood flow in the hippocampus at 1-h intervals over a 1-day period in freely moving rats, by means of the H2 clearance method, before and after sham adrenalectomy, adrenalectomy or adrenalectomy with corticosterone replacement. We also measured local cerebral blood flow in the prefrontal cortex before and after adrenalectomy. Four weeks after the adrenalectomy, hippocampal blood flow at each time of day was an average of 47% greater than before the operation, showing diurnal variation as before. After the sham adrenalectomy or adrenalectomy with corticosterone replacement, hippocampal blood flow did not change significantly with respect to either its level or its diurnal variation. Local cerebral blood flow in the prefrontal cortex increased by only 19% after adrenalectomy. The present study demonstrates that adrenalectomy causes a remarkable increase in hippocampal blood flow, probably due to a lack of corticosterone.

Rapid changes in extracellular glucose levels and blood flow in the striatum of the freely moving rat

The dynamics of regional cerebral blood flow and brain extracellular glucose were studied in the freely moving rat. These two variables were measured in the striatum during and following both mild tail pinch and restraint stress. Blood flow was monitored using a refinement of the hydrogen clearance technique that allowed repeated measurements at 5-min intervals. A slow stream of hydrogen was directed at the rat's snout for 10-20 s through lightweight tubing attached to the animal's head and detected at a chronically implanted platinum electrode. Extracellular glucose was monitored with microdialysis in a separate group of animals using an on-line, enzyme-based assay that provided 2.5-min time resolution. Mean striatal blood flow 24 h following implantation was 89.9 +/- 2.5 ml.(100 g)-1.min-1. A 5-min tail pinch caused flow to increase immediately to 169.5 +/- 20 ml.(100 g)-1.min-1. In contrast, there was no change in blood flow during restraint stress, although there was a small increase following the end of the stress. Significant increases in blood flow were also observed in the striatum during periods of eating and grooming. Extracellular glucose levels increased following both forms of stress, to a maximum of 170 +/- 22% of baseline with restraint compared to 110 +/- 2% with tail pinch. In both cases, the increase occurred after the stress had ended and persisted while blood flow returned to basal levels.

Acetylcholine release in the rat hippocampus as measured by the microdialysis method correlates with motor activity and exhibits a diurnal variation

Extracellular levels of acetylcholine were measured by the microdialysis method coupled to high performance liquid chromatography in the dorsal hippocampus of freely moving rats over a period of 24 h to examine whether the acetylcholine release in the hippocampus exhibited a diurnal variation. Spontaneous motor activity was simultaneously measured with an automatic animal activity monitor. The amount of acetylcholine collected per 20-min sample varied markedly, in a range from about 5 to 90 pmol. There appeared to be variations in the amount with a 2-4 h periodicity as well as an apparent diurnal periodicity. In all five rats studied, the overall mean value for the dark cycle (11.1-34.5, average 20.9 pmol/20 min) was significantly greater than that for the light cycle (5.1-21.3, average 12.3 pmol/20 min), showing a 70% average increase. Cross-correlation analysis performed between the amount of acetylcholine and the motor activity count for the animal during the sampling revealed a significant positive correlation coefficient in four rats studied. The present study demonstrates for the first time that the acetylcholine release shows a diurnal variation.