Regional cyclic AMP systems during secondary ischemia in gerbils: influence of anesthetic agents

Cerebral blood flow and EEG changes in preterm infants with patent ductus arteriosus

It is unknown whether the decreased cerebral blood flow seen in infants with a large patent ductus arteriosus is associated with cerebral dysfunction. Decreased cerebral blood flow in adult human and animal models has been associated with altered electroencephalography (EEG), spectral-analyzed EEG, and EEG response to photic stimulation. Cerebral blood flow velocity, EEG, spectral analysis of EEG, and photic alteration of EEG spectra were evaluated in 8 infants before and after closure of a significant patent ductus arteriosus and in 10 control infants without a patent ductus arteriosus. All infants with patent ductus arteriosus had moderate or large shunts associated with a 25% mean reduction in cerebral blood flow velocity. There were no differences, however, in EEG, spectral analysis of EEG, or photic alteration of the spectral analysis for these infants before and after patent ductus arteriosus closure as compared to controls. It is concluded that the degree of decreased cerebral blood flow in infants with a significant patent ductus arteriosus is not sufficient to cause measurable alteration in electrocortical activity.

Regional profiles of steady-state levels of cyclic nucleotides, cyclic AMP phosphodiesterase, and guanylate cyclase activities during late stages of unilateral ischemia in gerbil forebrain

The present study was an extension of earlier work regarding the role of cyclic nucleotides and related enzymes during cerebral ischemia in the gerbil. Following unilateral carotid occlusion, levels of cyclic AMP and cyclic GMP were measured in four rapidly inactivated brain regions at 3, 6, and 24 hr after permanent occlusion and at 2 hr of occlusion plus 1 hr of reflow. An analysis of variance indicated significant minor fluctuations in the steady-state levels of the two cyclic nucleotides within the frontal cortex, the hippocampus, the striatum, and especially the olfactory tubercle with respect to occlusion time (3 and 24 hr) but not when comparing control vs ischemic hemispheres (except at 3 hr). Changes occurred only in animals developing neurological symptoms of ischemia. At 24 hr postocclusion the specific activity of the low-Km form of cyclic AMP phosphodiesterase was elevated especially on the ischemic side when determined in homogenates of the four brain regions. Alternatively, the high-Km form of the enzyme in the presence or absence of Ca2+-calmodulin was unchanged. Guanylate cyclase activity in tissue homogenates was not influenced by the conditions of ischemia until 24 hr had elapsed, an event likewise unique to symptomatic gerbils. The sensitivity of the enzyme to hematin-catalase was decreased in the ischemic hemispheres of the hippocampus, striatum, and olfactory tubercle. In addition, further activation of the hematin-catalase response by NaN3 was depressed in the ischemic side of the hippocampus and striatum. Taken together these and previous studies indicate that fluctuations in the steady-state levels of cyclic nucleotides that occur rather prominently during acute and to a lesser degree during prolonged ischemia are not correlated with associated changes in enzymes responsible for their synthesis and/or degradation.

Protective action of calcium channel blockers on Na+,K+-ATPase in gerbil cerebral cortex following ischemia

The present study was initiated to determine whether pretreatment of gerbils with calcium channel blockers, flunarizine and verapamil would prevent injury to cerebral ATPases following secondary ischemia consisting of 60-min bilateral clamping of the carotids followed by 40 min of reperfusion. The sequence of ischemia produced a deficit in Na+,K+-ATPase activity without influencing Ca++,Mg++- or Mn++-sensitive ATPases. Pretreatment with flunarizine significantly prevented the damage to Na+,K+-ATPase while the effect of verapamil was marginal. Verapamil, along with dimethylsulfoxide (DMSO), reduced the mortality rate of gerbils subjected to the paradigm of ischemia. When added directly to the cerebral fractions in vitro the two calcium channel blockers inhibited Na+,K+-ATPase alone. Flunarizine was more potent in vitro than verapamil.

Free radicals generated by xanthine oxidase-hypoxanthine damage adenylate cyclase and ATPase in gerbil cerebral cortex

The generation of superoxide radicals from xanthine oxidase-hypoxanthine in a particulate fraction of gerbil cerebral cortex influenced the activity of the synaptic enzyme adenylate cyclase, as well as Mn2+- and Na+,K+-sensitive forms of ATPase. Low concentrations of xanthine oxidase actually elevated the sensitivity of adenylate cyclase to GTP, GTP + norepinephrine (NE), and forskolin but not significantly to Mn2+. Higher levels of xanthine oxidase elicited a marked inhibition of these responses. The stimulation of adenylate cyclase mechanisms requiring GTP (GTP, forskolin, and NE) was more susceptible than was Mn2+, suggesting that the guanine nucleotide stimulatory protein was more vulnerable to free radical attack than the catalytic site of adenylate cyclase. Superoxide dismutase (SOD), but not catalase, partially protected the forskolin-sensitive enzyme from the action of xanthine oxidase-hypoxanthine. A combination of SOD plus catalase preserved enzyme responses to forskolin. In comparison, additions of SOD plus mannitol or catalase plus flunarizine were less effective. The sensitivity of the particulate ATPase to Mn2+ was more labile to the consequence of superoxide formation than Na+, K+ -ATPase. In this regard the Ca2+,Mg2+ sensitivity of the enzyme was reduced only to a marginal extent. The findings might be analogous to in vivo data in which cerebral adenylate cyclase and Na+, K+-ATPase are damaged following postischemic reperfusion in gerbils, a process thought to be mediated by free radicals.

Modulation of ischemic-induced damage to cerebral adenylate cyclase in gerbils by calcium channel blockers

It has been previously established that prolonged bilateral carotid occlusion followed by recirculation produces damage to the synaptic enzyme adenylate cyclase in the frontal cortex of the gerbil. Since calcium entrance into the brain may account in part for the deleterious consequences of stroke, the present study examined whether pretreatment with calcium channel blockers would modify the effects of 60 min of bilateral ischemia plus 40 min of reflow on various parameters of cortical adenylate cyclase activation. In this context activation of cerebral homogenates by norepinephrine with or without 5'-guanylyl imidodiphosphate was preserved by pretreatment of ischemic gerbils with verapamil but worsened by flunarizine. In contrast, in particulate fractions (treated with EGTA to reduce metallic ion levels) the damage to the Mn2+-sensitive catalytic site of adenylate cyclase was prevented only by flunarizine. Pretreatment with the two calcium channel blockers resulted in an elevated basal activity of the enzyme, thereby reducing the response in the homogenate preparation to forskolin. Gerbils pretreated with verapamil tended to have an increased ability for survival resulting from the ischemic episode. Under in vitro conditions the enzyme preparations were not markedly influenced by either drug.

Protective effect of prostaglandins D2, E1 and I2 against cerebral hypoxia/anoxia in mice

The protective effect of prostaglandins (PGs) against cerebral hypoxia/anoxia was investigated with a variety of experimental models in relation to their CNS depressant effects in mice. Furthermore, the effect of PGs on the changes of cerebral energy metabolites and cyclic nucleotide was examined in hypoxic mice. Mice were given s.c. doses of PGs 30 min before tests. Among the PGs tested, treatment with PGD2, PGE1 and PGI2 Na showed a consistent and dose-dependent protection against cerebral anoxia induced by all models studied: histotoxic anoxia by KCN, hypobaric hypoxia, normobaric hypoxia and decapitation-induced gasping. However, PGA1, PGA2, PGB1, PGB2, PGE2, PGF1 alpha, PGF2 alpha and 6-keto-PGF1 alpha at a dose of 3 mg/kg were without effect against normobaric hypoxia and gasping duration. The three PGs, i.e. PGD2, PGE1 and PGI2 which showed anti-hypoxic effects decreased locomotor activity and potentiated hexobarbital-induced sleep. On the other hand, PGE2, PGA1, PGA2 and PGB2 also caused a decrease in locomotor activity. Similarly, PGE2 and PGA1 caused a potentiation of hexobarbital-induced sleep, but interestingly they did not cause clear-cut increase in cerebral resistance to hypoxia, in contrast with the former three PGs. Thus general depression of CNS function appears not to be responsible for the PGD2-, PGE1- and PGI2-induced increase in cerebral resistance to hypoxia. The levels of Cr-P and ATP were significantly reduced and those of ADP and AMP were markedly elevated in hypoxic brain, resulting in a decrease in a calculated energy charge potential. The lactate level and lactate/pyruvate ratio increased and the glucose level decreased markedly.(ABSTRACT TRUNCATED AT 250 WORDS)

Further probes into the molecular sites of damage to cerebral adenylate cyclase following postischemic reperfusion

A variety of pharmacological agents were used as experimental probes to determine with greater precision the site(s) of damage to cerebral adenylate cyclase as a consequence of postischemic reperfusion in the gerbil. A paradigm of 60-min bilateral ischemia followed by 40-min reperfusion results in a decreased sensitivity of the catalytic site of adenylate cyclase to Mn2+. Likewise, the GTP-transducer site (guanine nucleotide regulatory or G protein) revealed depressed responses to GTP in the absence or presence of norepinephrine, dopamine agonists, substance P, yohimbine, and cholera and pertussis toxins. Moreover, a crude preparation of GTPase disclosed that damage elicited by postischemic reperfusion was directed to the higher-affinity form of this enzyme, which is associated with the overall function of the guanine nucleotide regulatory protein. Injury to adenylate cyclase was unrelated either to the ability of adrenergic ligands to bind to associated receptor sites or to the capacity of the brain to generate visual evoked potentials in response to visual stimuli.

Effects of naloxone and morphine on cerebral ischemia in gerbils

A therapeutic role for naloxone during stroke has been suggested, but a neurochemical site of action remains to be determined. Previous work with the gerbil cerebral cortex has shown that either bilateral secondary ischemia (60-min occlusion of the carotid arteries followed by 40-min reflow) or unilateral primary ischemia (permanent ligation of one carotid artery for 6 hr in symptomatic animals) produced deficits in both Na+, K+-ATPase (EC 3.6.1.3) activity and various parameters of activation of adenylate cyclase (EC 4.6.1.1). Pretreatment of gerbils with either naloxone or morphine failed to ameliorate or exacerbate, respectively, the neurological signs of ischemia; however, morphine did reduce mortality. Infusion of naloxone prior to ischemia afforded varying degrees of protection to forskolin, GTP analogs, and NE (norepinephrine) activation of adenylate cyclase, as well as to Na+, K+-ATPase (bilateral ischemia only). Similarly, morphine inhibited damage to basal activity of adenylate cyclase and to stimulation by NE, forskolin, and Gpp (NH)p (5'-guanylyl imidodiphosphate). Under in vitro conditions morphine increased the basal activity of adenylate cyclase but reduced responses to NE and forskolin. Furthermore, morphine injected into control gerbils elevated basal- and forskolin-elicited activities but reduced the activation of adenylate cyclase by NE.

Adenylate cyclase activity and motor behavior following cerebral ischemia in the unanesthetized gerbil

Five minutes of bilateral carotid occlusion in unanesthetized gerbils produced substantial changes in spontaneous locomotor activity. Behavior was decreased after 1 hr of reperfusion and was increased at 24 hrs post-ischemia. Adenylate cyclase activity was measured in homogenates of frontal cortex and hippocampus at 90 min and 24 hrs following 5 min of cerebral ischemia. Enzyme activity was determined in the absence and presence of the activators guanosine-5'-triphosphate (GTP), guanylyl-5'-imidodiphosphate (GppNHp), isoproterenol (Iso) plus GTP, and forskolin (Fors) plus GTP. Homogenates responded with expected increases over basal adenylate cyclase activity with addition of all activators. An additional small increase in isoproterenol-stimulated activity was observed in frontal cortex homogenates at 90 min post-ischemia. No other significant changes in adenylate cyclase activity were observed after either 90 min or 24 hrs of reperfusion. The substantial increases in locomotor activity evident at 24 hrs after transient ischemia are not associated with measurable changes in adenylate cyclase activity in homogenates of frontal cortex or hippocampus.

An unanesthetized-gerbil model of cerebral ischemia-induced behavioral changes

A surgical procedure has been developed to study the effects of cerebral ischemia in the unanesthetized Mongolian gerbil. The methodology is based upon the surgical isolation and instrumentation of both common carotid arteries. A loop of dental floss is placed around each carotid artery and passed through double lumen catheter material; this allows for later occlusion of the carotid arteries and their release in unanesthetized subjects. Functional changes following transient carotid artery occlusion are readily demonstrated by the occurrence of altered spontaneous locomotor activity at various times postischemia. This model should be useful in the evaluation of potential therapeutic agents in the treatment of cerebral ischemia.

Adenylate cyclase and histopathological changes in the gerbil brain following prolonged unilateral ischemia and recirculation

This study was designed to correlate histopathological changes in gerbil brain following unilateral primary and secondary ischemia to enzymatic-adenylate cyclase damage. At three hrs permanent occlusion of the right common carotid artery only minimal histological changes were evident in cerebrum, hippocampus, striatum and olfactory tubercle while the enzyme responses were unremarkable. Severe histological and enzymatic alterations were present at one hour of recirculation subsequent to 3 hrs of unilateral occlusion. Similar damage was evident at 6 and 24 hrs permanent occlusion. Principal enzyme damage was directed toward basal activity, as well as stimulation of the catalytic (forskolin-sensitive) sites on the enzyme complex. For the most part the transducer (GTP-sensitive) site was unaffected by ischemia until 24 hr ligation. These changes were observed in only those gerbils developing severe symptoms of stroke.

Classification of ischemic-induced damage to Na+, K+-ATPase in gerbil forebrain. Modification by therapeutic agents

The activity of three forms of ATPase were examined in fractions of the brain of the gerbil treated with ethylene glycol-N-N-tetra-acetic acid (EGTA) under a variety of conditions of primary and secondary (reflow) ischemia. In animals which were unilateral ischemic (ligation of the right common carotid), damage to Na+, K+-ATPase alone was observed only after at least 6 hr of ischemia had elapsed. The phenomenon occurred in only symptomatic gerbils and was absent in animals which were either asymptomatic or only displayed partial neurological symptoms. Under conditions of bilateral cerebral ischemia, in which both carotid arteries were clamped, only irreversible ischemia (60 min) followed by reflow, was associated with highly significant damage to cerebral Na+, K+-ATPase. In regional studies of the forebrain involving ischemia for 60 min plus 30 min reflow, damage to Na+, K+-ATPase was evident in the cerebrum, hippocampus, striatum and thalamus, while the hypothalamus and olfactory bulb were spared. Pretreatment of gerbils with allopurinol, clonazepam or combinations of thiopental plus either indomethacin or methylprednisolone offered protection to cerebral Na+, K+-ATPase subsequent to secondary ischemia. With only minor exceptions (striatum) neither Ca2+, Mg2+- nor Mn2+-ATPase were altered by stroke or treatment with drugs.

Cyclic nucleotides in stroke and related cerebrovascular disorders

Evidence has steadily accumulated to indicate that the rapid fluctuations in cyclic nucleotides during primary and secondary stroke are more than epiphenomena of the disease. During acute phases of ischemia, anoxia or hypoxia cyclic AMP rapidly accumulates in cerebral tissue, cerebrospinal fluid (CSF) and venous plasma, while cyclic GMP either remains unchanged or declines. The massive release of transmitters (catecholamines and adenosine) or ionic fluxes (Na+ and K+) may account for these observations. If reflow is established through a previously occluded vessel cyclic AMP content rises even higher in conjunction with a sharp rise in cyclic GMP. It is during this reflow period subsequent to longer term stroke (30-60 min) that the synaptic membrane enzyme, adenylate cyclase, is especially vulnerable. Presumably the cause of injury to cell membrane systems results from excess lactic acid accumulation and/or Ca++ entry through the damaged blood-brain barrier. The latter initiates breakdown of membrane phospholipids with resultant synthesis of vasoactive prostaglandins and formation of free radicals causing further insult to membrane phospholipids. Thus drugs acting to inhibit formation of prostaglandins, scavenge free radicals, reduce lactate formation, inhibit Ca++ entry or stabilize cell membranes have been shown to possess varying degrees of protective action toward adenylate cyclase. Moreover, cyclic AMP has been found to reverse stroke-induced vasospasm in central vessels. Reduced cyclic AMP content in CSF has been used to monitor the severity of coma, whereas clinical improvement was associated with predictable increases in the cyclic nucleotide. Therefore, cyclic nucleotides and related membrane enzyme systems might be used as target molecules in which to develop future therapeutic strategies for prevention or treatment of stroke.