Correlation between electroencephalogram isoelectric time and hippocampal norepinephrine levels, measured by microdialysis, during ischemia in rats

Progressive multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory, demyelinating disease of the central nervous system, which starts in the majority of patients with a relapsing/remitting MS (RRMS) course , which after several years of disease duration converts into a progressive disease. Since anti-inflammatory therapies and immune modulation exert a beneficial effect at the relapsing/remitting stage of the disease, but not in the progressive stage, the question was raised whether inflammation drives tissue damage in progressive MS at all. We show here that also in progressive MS, inflammation is the driving force for brain injury and that the discrepancy between inflammation-driven tissue injury and response to immunomodulatory therapies can be explained by different pathomechanisms acting in RRMS and progressive MS.

Chronic sustained hypoxia enhances both evoked EPSCs and norepinephrine inhibition of glutamatergic afferent inputs in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) receives inputs from both arterial chemoreceptors and central noradrenergic neural structures activated during hypoxia. We investigated norepinephrine (NE) modulation of chemoreceptor afferent integration after a chronic exposure to sustained hypoxia (CSH) (7-8 d at 10% FIO(2)). Whole-cell recordings of NTS second-order neurons identified by DiA (1,1'-dilinoleyl-3,3,3',3'-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate) labeling of carotid bodies were obtained in a brain slice. Electrical stimulation of the solitary tract was used to evoke EPSCs. CSH exposure increased evoked EPSC (eEPSC) amplitude via both presynaptic and postsynaptic mechanisms. NE dose dependently decreased the amplitude of eEPSCs. NE increased the paired-pulse ratio of eEPSCs and reduced the frequency of miniature EPSCs, suggesting a presynaptic mechanism. EC(50) of NE inhibition of eEPSCs was lower in CSH cells (3.0 +/- 0.9 microM; n = 5) than in normoxic (NORM) cells (7.6 +/- 1.0 microM; n = 7; p < 0.01). NE (10 microM) elicited greater inhibition of eEPSCs in CSH cells (63 +/- 2%; n = 16) than NORM cells (45 +/- 3%; n = 21; p < 0.01). The alpha-adrenoreceptor antagonist phentolamine abolished NE inhibition of eEPSCs. CSH enhanced the alpha2-adrenoreceptor agonist clonidine-mediated inhibition (3 microM; NORM, 23 +/- 2%, n = 5 vs CSH, 44 +/- 5%, n = 4; p < 0.05) but attenuated alpha1-adrenoreceptor agonist phenylephrine-mediated inhibition (40 microM; NORM, 36 +/- 2%, n = 11 vs CSH, 26 +/- 4%, n = 6; p < 0.05). The alpha2-adrenoreceptor antagonist yohimbine abolished CSH-induced enhancement of NE inhibition of eEPSCs. These results demonstrate that CSH increases evoked excitatory inputs to NTS neurons receiving arterial chemoreceptor inputs. CSH also enhances NE inhibition of glutamate release from inputs to these neurons via presynaptic alpha2-adrenoreceptors. These changes represent central neural adaptations to CSH.

Effects of the noradrenergic system in rat white matter exposed to oxygen-glucose deprivation in vitro

Norepinephrine (NE) is released in excess into the extracellular space during oxygen-glucose deprivation (OGD) in brain, increasing neuronal metabolism and aggravating glutamate excitoxicity. We used isolated rat optic nerve and spinal cord dorsal columns to determine whether the noradrenergic system influences axonal damage in white matter. Tissue was studied electrophysiologically by recording the compound action potential (CAP) before and after exposure to 60 min of OGD at 36 degrees C. Depleting catecholamine stores with reserpine was protective and improved CAP recovery after 1 h of reperfusion from 17% (control) to 35%. Adding NE during OGD decreased CAP recovery to 8%, and adding NE to reserpine during OGD eliminated the protective effect of the latter. Selective inhibitors of Na(+)-dependent norepinephrine transport desipramine and nisoxetine improved recovery to 58% and 44%, respectively. alpha2 adrenergic receptor agonists UK14,304 and medetomidine improved CAP recovery to 41% and 46% after 1 h of OGD. Curiously, alpha2 antagonists alone were also highly protective (e.g., atipamezole: 86% CAP recovery), at concentrations that did not affect baseline excitability. The protective effect of alpha2 receptor modulation was corroborated by imaging fluorescent Ca(2+) and Na(+) indicators within axons during OGD. Both agonists and antagonists significantly reduced axonal Ca(2+) and Na(+) accumulation in injured axons. These data suggest that the noradrenergic system plays an active role in the pathophysiology of axonal ischemia and that alpha2 receptor modulation may be useful against white matter injury.

Cerebral ischemia and brain histamine

Cerebral ischemia induces excess release of glutamate and an increase in the intracellular Ca(2+) concentration in neurons, which provokes enzymatic process leading to irreversible neuronal injury. Histamine plays a role as a neurotransmitter in the mammalian brain, and histamine release from nerve endings is enhanced in ischemia by facilitation of histaminergic activity. Dissimilar to ischemia-induced release of glutamate, histamine release is gradual and long lasting. The enhancement may contribute to neuroprotection against ischemic damage, because suppression of histaminergic activity aggravates the histologic outcome caused by ischemia. Preischemic administration of histamine (i.c.v.) suppresses ischemic release of glutamate and ameliorates neuronal damage, whereas blockade of central histamine H(2) receptors aggravates ischemic injury. These suggest that histamine provides beneficial effects against ischemic damage through histamine H(2) receptors, when administered before induction of ischemia. Postischemic loading with histidine, a precursor of histamine, alleviates both brain infarction and delayed neuronal death. Since the alleviation is abolished by blockade of central histamine H(2) receptors, facilitation of central histamine H(2) action caused by histidine may prevent reperfusion injury after ischemic events. Because the ischemia-induced increase in the glutamate level rapidly resumes after reperfusion of cerebral blood flow, beneficial effects caused by postischemic loading with histidine may be due to other mechanisms besides suppression of excitatory neurotransmitter release. Anti-inflammatory action by histamine H(2) receptor stimulation is a likely mechanism responsible for the improvement.

Adrenoceptor subtype-specific acceleration of the hypoxic depression of excitatory synaptic transmission in area CA1 of the rat hippocampus

The depression of excitatory synaptic transmission by hypoxia in area CA1 of the hippocampus is largely dependent upon the activation of adenosine A(1) receptors on presynaptic glutamatergic terminals. As well as adenosine, norepinephrine levels increase in the hypoxic/ischemic hippocampus. We sought to determine the influence of alpha- and beta-adrenoceptor (AR) activation on the hypoxic depression of synaptic transmission utilizing electrophysiological, pharmacological and adenosine sensor techniques. Norepinephrine depressed synaptic transmission and significantly accelerated the hypoxic depression of synaptic transmission. The alpha-AR agonist 6-fluoronorepinephrine mimicked both of these effects whilst the alpha(2)-AR antagonist yohimbine, but not the alpha(1)-AR antagonist urapidil, prevented the actions of 6-fluoronorepinephrine. In contrast, the beta-AR agonist isoproterenol enhanced synaptic transmission and only accelerated the hypoxic depression of transmission in hypoxia-conditioned slices in which the hypoxic release of adenosine is reduced. The effects of isoproterenol were blocked by the non-selective beta-AR antagonist propranolol and the selective beta(1)-AR antagonist betaxolol. Using an enzyme-based adenosine sensor we observed that the application of the beta-AR agonist resulted in increased extracellular adenosine during repeated hypoxia. Our results suggest that alpha(2)-AR activation facilitates the hypoxic depression of synaptic transmission probably via the known alpha(2)-AR-mediated inhibition of presynaptic calcium channels whereas beta(1)-AR activation does so via increased extracellular adenosine and greater activation of inhibitory adenosine A(1) receptors.

Blockade of central histaminergic H2 receptors facilitates catecholaminergic metabolism and aggravates ischemic brain damage in the rat telencephalon

Blockade of central H(2) receptors aggravates ischemic neuronal damage. Since changes in the activity of the monoaminergic system are contributing factors in the development of ischemic neuronal damage, the authors evaluated the effects of ranitidine on the monoaminergic system and ischemic neuronal damage in the middle cerebral artery (MCA) occlusion model of rats. Wistar rats pretreated with saline or ranitidine (3 and 30 nmol, i.c.v.) were subjected to reversible occlusion of MCA for 2 h. The total infarct volume was determined 24 h after reperfusion. The relationship between dopaminergic activity and the histologic outcome was estimated by lesioning the substantia nigra 2 days before MCA occlusion. In a second experiment, the animals were subjected to 15 min of MCA occlusion, and the effects of ranitidine on the histologic outcome was evaluated 7 days after ischemia. In a third experiment, the tissue concentrations of monoamines and their metabolites were determined in the cerebral cortex and striatum 2 h after reperfusion following MCA occlusion for 2 h. The turnover of norepinephrine and dopamine was compared between animals treated with saline and those treated with ranitidine by estimating the alpha-methyl-p-tyrosine-induced depletion of norepinephrine and dopamine, respectively. The turnover of 5-hydroxytryptamine was evaluated by the probenecid-induced accumulation of 5-hydroxyindoleacetic acid. Treatments with ranitidine markedly increased the infarct volume 24 h after reperfusion. Ranitidine also aggravated delayed neuronal death 7 days after ischemia. The aggravation was abolished by the lesion of the substantia nigra before MCA occlusion. The MCA occlusion increased the turnover of cortical norepinephrine and striatal dopamine. The turnover was further facilitated by ranitidine. Although ranitidine suppressed the 5-hydroxytryptamine turnover in the cerebral cortex, the extent of this effect was similar in both the ischemic and non-ischemic sides. These results suggest that facilitation of the catecholaminergic systems is involved in the aggravation of ischemic neuronal damage by H(2) blockade.

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

Effect of intracerebral norepinephrine depletion on outcome from severe forebrain ischemia in the rat

Manipulations of plasma catecholamine concentrations influence outcome from ischemic brain insults. It has been suggested that these effects are mediated by influences on brain catecholamine concentrations. This study examined whether major changes in brain norepinephrine concentrations can alter outcome from severe forebrain ischemia. Sprague-Dawley rats were administered 50 mg/kg i. p. N-(chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) or were left untreated (control). One week later, these rats were subjected to either 7 or 8 min of normothermic forebrain ischemia (bilateral carotid occlusion and MABP=30 mmHg) and allowed to recover for 4 days. Histologic damage was then evaluated. In other control and DSP-4-treated animals, hippocampal microdialysate norepinephrine concentrations were measured before, during and after 8 min of forebrain ischemia. Norepinephrine concentrations were also determined in brain homogenates from non-ischemic DSP-treated and control rats. A 95% depletion of norepinephrine was observed in brain homogenates from non-ischemic DSP-4-treated rats compared with control. During ischemia, microdialysate norepinephrine concentrations increased in control but not in DSP-4-treated rats (P=0.002). For plasma, intra-ischemic epinephrine concentrations increased 8-10-fold and returned to baseline values post-ischemia with no differences between groups. Plasma norepinephrine values remained unchanged in both groups. Histologic damage resulting from either 7 or 8 min of ischemia in hippocampal structures, caudoputamen, and neocortex was similar between DSP-4-treated and control groups. This study could not identify any effect of major changes in brain norepinephrine concentrations on ischemic brain damage. These data indicate that peripheral catecholamine effects on near-complete forebrain ischemic outcome are unlikely to be mediated by effects on central catecholamine concentrations.

Neuroprotective effect of L-lysine monohydrochloride on acute iterative anoxia in rats with quantitative analysis of electrocorticogram

Lysine is one of the indispensible amino acids and L-lysine monohydrochloride (LMH) is widely available to public as a nonprescription oral supplement. Potential clinical usefulness of oral LMH supplements has been indicated in stroke, hypertension, and seizure induced by pentylenetetrazole (PTZ), etc. We compared the effects of LMH and flunarizine on the Electrocorticogram (EcoG) of rats intragastrically administered 1 hour before anoxia. LMH dose-dependently prolonged the time reaching the lowest ECoG average amplitude after anoxia and decreased the recovery time after recirculation in both 0.63 g/kg and 1.26 g/kg groups. There was significant difference between the LMH- and saline-pretreated groups but no significant difference between the 1.26 g/kg LMH- and 2.5 mg/kg flunarizine-pretreated groups. The difference was not significant in the 2.52 g/kg group. The ECoG average amplitude did not reach isoelectric point and the lowest average amplitude was 10 percent of that of nomoxia in the 1.26 g/kg LMH-pretreated group during 2-min anoxia. The average amplitude pretreated with LMH (0.63 and 1.26 g/kg) was lower than that of those pretreated with saline and flunarizine. The results tend to indicate that LMH can protect brain cells against anoxia by means of providing energy, reducing cerebral metabolic rate and inhibiting the effect of the excitable amino acids.

Effects of isoflurane, ketamine, and fentanyl/N2O on concentrations of brain and plasma catecholamines during near-complete cerebral ischemia in the rat

We postulated that adrenergic responses to global cerebral ischemia are anesthetic-dependent and similar in both brain and arterial blood. Rats were anesthetized with isoflurane (1.4%), ketamine (1 mg x kg(-1) x min(-1)), or fentanyl (25 microg x kg(-1) x h(-1))/70% N2O. The carotid arteries were occluded for either 20 min with mean arterial pressure (MAP) 50 mm Hg (incomplete ischemia) or 10 min with MAP 30 mm Hg (near-complete ischemia). Norepinephrine was measured in hippocampal microdialysate. Norepinephrine and epinephrine were measured in arterial plasma. In both hippocampus and plasma, basal norepinephrine was similar among anesthetics. During incomplete ischemia, hippocampal norepinephrine was twofold greater with fentanyl/N2O than with isoflurane (P = 0.037), but plasma norepinephrine and epinephrine were similar and unchanged among all three anesthetics. During near-complete ischemia, hippocampal norepinephrine was threefold greater with ketamine than fentanyl/N2O (P = 0.005), whereas plasma norepinephrine and epinephrine were markedly greater with fentanyl/N2O than with ketamine (P < 0.0005) or isoflurane (P = 0.05). There was no correlation between norepinephrine concentrations in hippocampus and plasma for either incomplete or near-complete ischemia. This study demonstrates that adrenergic responses to global ischemia are anesthetic-dependent, particularly during more severe insults. The absence of a correlation between plasma and brain catecholamine concentrations indicates that adrenergic responses to ischemia are independent in brain and blood.

IMPLICATIONS

It has been proposed that anesthetics modulate cerebral ischemic outcome by influencing peripheral adrenergic responses to ischemia. This experiment demonstrates that anesthetics differentially modulate adrenergic responses to ischemia but that effects in plasma and brain are independent. This suggests that events detected in the peripheral circulation do not implicate direct mechanisms of action of catecholamines at the neuronal/glial level.

The effect of hypoxia and catecholamines on regional expression of heat-shock protein-72 mRNA in neonatal piglet brain

The present study has shown that hypoxia leads to expression of heat-shock protein in the brain of newborn piglets and this process is almost completely abolished by depletion of catecholamines prior to the hypoxic episode. The piglets were anesthetized and mechanically ventilated. One hour of hypoxia was generated by decreasing the oxygen fraction in the inspired gas (FiO2) from 22% to 6%-10%. FiO2 was then returned to the control value for a period of 2 h. Following the 2 h of reoxygenation, regional expression of the 72-kDa heat-shock protein (hsp72) mRNA was determined using in situ hybridization and autoradiography. The hypoxic insult (cortical pO2 = 3-10 mmHg) induced expression of hsp72 mRNA in regions of both white and gray matter, with strong expression occurring in the cerebral cortex of individual animals. Depleting the brain of catecholamines prior to hypoxia, by treating the animals with alpha-methyl-p-tyrosine (AMT), resulted in a major change in the hsp72 mRNA expression. In the catecholamine depleted group of animals, the intensity of hsp72 mRNA expression was greatly decreased or almost completely abolished relative to the nondepleted hypoxic group. These results suggest that the catecholamines play a significant role in the expression of the hsp72 gene in response to hypoxic insult in neonatal brain.

Age-related electrocorticographic and respiratory adaptation to repeated hypoxia

Severe hypoxia is known to produce depression in electrical brain activity and perturbation of respiratory pattern. In piglets undergoing chronic recording of brain and respiratory muscle activities, a depressed electrocorticogram (ECoG) was observed in response to rapidly induced (< 30 s), brief (10 min), and moderate hypoxia (10% O2 in 90% N2) in 16 out of 42 study sessions in young (3- to 11-day-old) animals only. Responses to hypoxia were monitored over 4 consecutive days. In five cases, the latency to the onset of the ECoG depression increased progressively over the 4 test days, and its duration decreased progressively. An associated respiratory gasping pattern also exhibited gradual remission over consecutive days. These changes in the responses to repeated hypoxia demonstrate adaptation of mechanisms underlying neuronal perturbation by oxygen deprivation.