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

Role of tissue plasminogen activator/plasmin cascade in delayed neuronal death after transient forebrain ischemia

We studied the possible involvement of the tissue plasminogen activator (t-PA)/plasmin system on both delayed neuronal death in the hippocampus and the associated enhancement of locomotor activity in rats, after transient forebrain ischemia induced by a four-vessel occlusion (FVO). Seven days after FVO, locomotor activity was abnormally increased and, after 10 days, pyramidal cells were degraded in the CA1 region of the hippocampus. FVO increased the t-PA antigen level and its activity in the hippocampus, which peaked at 4 h. Both the enhanced locomotor activity and the degradation of pyramidal cells were significantly suppressed by intracerebroventricular injection of aprotinin, a plasmin inhibitor, at 4 h but not during FVO. These results suggest the importance of the t-PA/plasmin cascade during the early pathological stages of delayed neuronal death in the hippocampus following transient forebrain ischemia.

Impairment of adenylyl cyclase and of spatial memory function after microsphere embolism in rats

The purpose of the present study was to characterize alterations in the adenylyl cyclase (AC), cyclic adenosine 3',5'-monophosphate (cAMP), and spatial memory function after sustained cerebral ischemia. Sustained cerebral ischemia was induced by injection of 900 microspheres (48 microm in diameter) into the right (ipsilateral) hemisphere of rats. Alterations in the AC and cAMP in the cerebral cortex and hippocampus were examined up to 7 days after the embolism. A decrease in the cAMP content was seen in the ipsilateral hemisphere throughout the experiment. Microsphere embolism (ME) decreased the activity of Ca(2+)/calmodulin (CaM)-sensitive AC in the ipsilateral hemisphere throughout the experiment, whereas the basal and 5'-guanylyl imidodiphosphate (Gpp(NH)p)-sensitive AC activities were not altered. Immunoblotting analysis of AC subtypes with specific antibodies showed a decrease in the immunoreactivity of AC-I in the ipsilateral hemisphere during these periods. No significant differences in the immunoreactivity of AC-V/VI and AC-VIII were observed after ME. The levels of GTP-binding proteins Galpha(s), Galpha(i), and Gbetawere unchanged. Furthermore, microsphere-embolized rats showed prolongation of the escape latency in the water maze task determined on the seventh to ninth day after the operation. These results suggest that sustained cerebral ischemia may induce the impairment of the AC, particularly a selective reduction in the AC-I level and activity, coupled with the decrease in cAMP content. This reduction may play an appreciable role in the disturbance in cAMP-mediated signal transduction system, possibly leading to learning and memory dysfunction.

Selective reduction in type I adenylyl cyclase after microsphere embolism in rat brain

Alterations of adenylyl cyclase (AC) subtypes after cerebral ischemia remain unclear. The purpose of the present study was to characterize alterations in AC after sustained cerebral ischemia. Sustained cerebral ischemia was induced by injection of 900 microspheres into the right (ipsilateral) internal carotid artery of rats. Microsphere embolism (ME) decreased the Ca(2+)/calmodulin-sensitive AC activity in the ipsilateral hippocampus examined up to 7 days after the embolism, whereas basal and 5'-guanylyl imidodiphosphate-sensitive AC activities were not altered. An immunoreactivity of type I adenylyl cyclase (AC-I) was decreased in the ipsilateral hippocampus during these periods, whereas type V/VI AC and VIII AC immunoreactivities were not altered. These results suggest that a selective reduction in the AC-I level and activity is induced by ME, which may lead to dysfunction of AC signal transduction.

Motor dysfunction in a photothrombotic focal ischaemia model

The study of behavioural deficits resulting from cerebral infarction in animal models of stroke has in the past taken second place to histological assessment. This is particularly true of the photothrombotic lesion model. Most tests currently used to measure motor deficits use a scoring system to quantify parameters such as beam walking. The present study set out to characterise a simple and objective assessment for motor impairment in the photothrombotic cortical lesion model. Rats were assessed on a number of motor function tests, i.e. gross locomotor activity, rotarod, and grip strength. After the establishment of stable baselines, cortical photothrombotic lesions were induced, after which the animals were re-tested for a further 18 days. The presence of cortical photothrombotic lesions significantly imparied the rats' performance on the rotarod and grip-strength tests. The deficit observed with the grip-strength task appeared 24 h postsurgery, but was much reduced by day 18 postsurgery. The rotarod test revealed an effect that took longer to establish, but which was more persistent. Gross locomotor activity was not affected. These data suggest that bilateral photothrombotic lesions of the prefrontal cortex produce deficits that can be detected by rotarod and grip-strength tasks.

Age influence on oxidative events during brain ischemia/reperfusion

Brain readily undergoes oxidative damage if it is organizationally disrupted. Iron plays a catalytic role in mediating oxidative damage in brain homogenate. Utilizing several methods to assess oxidative events, including salicylate trapping of hydroxyl free radicals, spin-trapping of free radicals and quantitation of protein oxidation, we have clearly demonstrated that oxidative events do occur in the ischemia/ reperfusion lesioned Mongolian gerbil brain. In the gerbil model, ischemia/reperfusion insult (IRI) mediated protein oxidative damage is accompanied by a concomitant loss of glutamine synthetase activity. The loss of glutamine synthetase activity may possibly lead to the build-up of glutamate, an excitotoxic amino acid which has been strongly implicated in the neuronal death observed in hippocampus following an IRI. The brain homogenate from an IRI-lesioned animal is more susceptible to peroxidation than comparable sham-operated control animals. We have discovered that an IRI causes lethality in older gerbils more readily than in younger gerbils. We have also found that the spin-trapping agent alpha-phenyl-tert-butyl nitrone (PBN) protects gerbils from an IRI. Data are presented which implicate that an IRI may seriously impair the energy production capacity of older gerbils more than comparable oxidative stresses in younger animals. We hypothesize that energy production capacity and its utilization for the synthesis of new gene products may be key events which help mediate survival from an IRI. In addition we hypothesize that higher brain functions, including memory and learning are inversely correlated with oxidative damage in the brain. The elucidation of the mechanisms involved in this fascinating area of research and the design of new protective agents to prevent oxidative damage in brain is a most exciting challenge.

Cerebral ischemia model with conscious mice. Involvement of NMDA receptor activation and derangement of learning and memory ability

During anesthesia in mice, both common carotid arteries were tied loosely with an overhand knot suture (an occluder), while two snares (releasers) were placed in the knot so that it could be repeatedly tightened to occlude the arteries and loosened again to allow for reperfusion while the mice were conscious and unrestrained. The incidence of mortality as well as impairment of brain metabolism depended upon the length of cerebral ischemia. Cortical electroencephalogram (EEG) clearly reflected the regional ischemia as evidenced by electrical quiescence. Less mortality was observed with ischemic mice treated with dextrorphan (30 mg/kg p.o.). On day 1 (24 hr after ischemia), there were impairments in complex motor coordination, multichoice swim performance, and step-through or thermal pain-motivated avoidance responses. Thereafter, the battery of tests progressively improved. This improvement depended on the period of resumption of cerebral blood flow; the 7-day, postischemic lapse significantly reduced the deficit observed. Reduction in the degree of habituation of exploratory activity was also clearly observed following ischemic insult. Dextrorphan (1-30 mg/kg i.p.) given to ischemic mice was effective in the habituation and step-through-type passive avoidance test paradigms. In conclusion, 1) the decline in cognition as observed with ischemic mice is due to the temporal and reversible derangement of their neuronal networks; 2) excessively released glutamate is probably of major pathogenic importance in the consequences of cerebral ischemia based on the positive results of the N-methyl-D-aspartate receptor antagonist, dextrorphan; 3) the simple technique could be useful in elucidating the pathophysiologic mechanisms of ischemically elicited derangement of the cerebral organization; and 4) the model could be used to assess the efficiency of drugs with high clinical predictivity.

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.

Increased in vitro lipid peroxidation of gerbil cerebral cortex as compared with rat

The in vitro thiobarbituric acid test was used as a measure of lipid peroxidation in the gerbil and rat. Synaptosomal preparations were isolated from the cerebral cortex of each species and incubated with a free radical generating system. Varying concentrations of ADP-Fe3+, with ascorbate and oxygenated incubation medium were used to generate hydroxy-radicals. Peroxidation of the synaptosomal membrane lipids was determined using malondialdehyde (MDA) accumulation. Both the gerbil and rat demonstrated significant increases in MDA in the presence of the generating system, while the gerbil P2 fraction consistently showed an increased level of MDA accumulation as compared with rat at each of the concentrations of ADP-Fe3+. Across a range of concentrations, there was a 2-2.6-fold greater increase in MDA accumulation in gerbil as compared with rat. Free radical generation is currently thought to be involved in the associated damage following cerebral ischemia. An in vitro model capable of producing biochemically similar damage to membrane systems by means of a controlled free-radical generating system may prove useful in studying possible mechanisms of ischemic damage.

Acute tryptophan pretreatment protects against behavioral changes caused by cerebral ischemia

Male gerbils (Meronies ungulata) were treated with various doses of tryptophan and the changes in spontaneous motor activity determined. Tryptophan decreased behavior at a dose of 200 mg/kg. Cerebral ischemia was produced by bilateral carotid occlusion for 5 min. This duration of ischemia produced a large increase in activity at both 6 h and 24 h postischemia. Tryptophan (200 mg/kg) prevented the ischemia-induced increases in locomotor activity. These data suggest that dietary amino acids may play a role in determining the effects of ischemia.