Ischemic flow threshold for striatal dopamine release in rats

Neurochemical changes and laser Doppler flowmetry in the endothelin-1 rat model for focal cerebral ischemia

Generalized neurotransmitter overflow into the extracellular space, after cerebral ischemia, has been suggested to contribute to subsequent neuronal death. This study aims to investigate the striatal release of the neurotransmitters dopamine (DA), glutamate (Glu) and gamma-aminobutyric acid (GABA) by means of microdialysis, in a rat model for focal transient cerebral ischemia. Ischemia was induced by the application of 120 pmol endothelin-1 (Et-1), adjacent to the middle cerebral artery (MCA) in freely moving rats. Ischemia produced a large increase in extracellular striatal DA concentrations (2400%), Glu (5500%) and GABA (800%) concentrations. Laser Doppler flowmetry in anaesthetized rats, indicated that the blood flow within the striatum decreased by 75+/-11%. The period of sustained drop of blood flow, was dose-dependently related to the concentration Et-1 injected. Histological analysis of brain slices, taken from anaesthetized and conscious animals, indicated a 500 pmol dose of Et-1 was required to produce a similar infarct in anaesthetized rats to a 120 pmol dose of Et-1 in freely moving rats. The immediate drop in striatal blood flow, and the prompt increase of extracellular DA, after the micro-application of Et-1, were quite striking. This suggests that the DA release, rather than the Glu overflow may be the primary event initiating the cascade of processes ultimately leading to cell death and neurological deficits.

Changes in striatal dopamine metabolism after microsphere embolism in rats

BACKGROUND AND PURPOSE

Dopamine plays an important role in striatal function. The present study was undertaken to elucidate the pathophysiological changes in striatal dopamine metabolism after microsphere embolism.

METHODS

Microspheres (48 microns) were injected into the right internal carotid artery of rats. Extracellular levels of dopamine and its metabolites were measured by in vivo microdialysis with the aid of high-performance liquid chromatography. In vivo striatal tyrosine hydroxylation and turnover (catabolism) rate of dopamine were estimated on the first and third days after the embolism. These were estimated by measuring tissue dopa or dopamine content in the presence of either an aromatic L-amino acid decarboxylase inhibitor or a tyrosine hydroxylase inhibitor, respectively.

RESULTS

In the microdialysis study, a 190-fold increase in the release of dopamine from the right striatum was observed 40 minutes after microsphere embolism, whereas the striatal dopamine metabolites decreased during the first 180 minutes after the embolism. Microsphere embolism decreased the striatal dopamine content throughout the experiment (28 days), whereas it increased tissue dopamine metabolites on the first day, followed by a decline in the metabolites on the third day or later. The in vivo turnover rate of dopamine decreased both on the first and third days, whereas the in vivo tyrosine hydroxylation decreased only on the third day after the embolism.

CONCLUSIONS

The results suggest that microsphere embolism induces severe damage to striatal dopaminergic metabolism 3 to 28 days after the embolism. Dopamine synthesis may be more resistant to the embolism-induced ischemic insults than its catabolism.

Evaluation of monoaminergic neurotransmitters in the rat striatum during varied global cerebral ischemia

Neurotransmitter release during cerebral ischemia has been extensively studied and is thought to play a key role in excitotoxic neuronal death. The changes in neurotransmitter release and its metabolism may reflect changes in cellular metabolism during ischemia. The purpose of this study is to assess alterations in extracellular dopamine and serotonin and their metabolites under varied degrees of ischemia in rat striatum to elucidate the pathophysiology of cerebral ischemia. Twenty rats were used to induce varied forebrain ischemia by means of bilateral common carotid artery occlusion along with hemorrhagic hypotension. Cerebral blood flow (CBF) in the striatum was measured every 40 minutes by methods of hydrogen clearance and maintained within certain ranges for 6 hours. Dopamine, serotonin, and their metabolites were measured every 20 minutes by in vivo microdialysis. Varying degrees of ischemia were obtained, ranging from 9.4 to 81.3% of control CBF. The animals were divided into three groups according to CBF levels measured 20 minutes after the induction of ischemia. In the mild ischemia group (n = 5), CBF ranged from 65 to 88% of baseline levels and resulted in only a slight increase of dopamine. In the moderate ischemia group (n = 10), CBF ranged from 21 to 48% of baseline levels and resulted in transient increases of dopamine (24-fold) and serotonin (5-fold). In the severe ischemia group (n = 5), CBF was below 14% of baseline levels and resulted in marked increases in dopamine (462-fold) and serotonin (225-fold). These alterations remained elevated for 3 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

A previous potassium stimulation enhances the increases of striatal extracellular dopamine and 5-hydroxytryptamine during global ischaemia under simulated penumbral conditions

The effect of a previous K+ stimulation on striatal extracellular monoamine levels during global ischaemia, under simulated penumbral conditions, was investigated. Rats were implanted with microdialysis probes in both striata, monoamine release was stimulated unilaterally by adding K+ (100 mM, 20 min) to the artificial CSF perfused through one probe, and bilateral partial ischaemia was imposed after monoamine levels had returned to basal values or below. Resultant increases in dialysate levels of dopamine and 5-hydroxytryptamine were markedly and significantly greater on the side previously exposed to K+, even though electrophysiological measurements indicated similarly severe ischaemia on both sides. Associated monoamine metabolite changes did not differ significantly between the two sides. There was no evidence of greater neuronal loss in the K(+)-stimulated striata 7 days after ischaemia. However, striatal tissue probably exposed to the highest concentrations of K+ could not be examined because of extensive gliosis around the probe.

Effect of global ischaemia, under simulated penumbral conditions, on brain monoamine neurochemistry and subsequent neurological and histological deficits

We have measured changes in the levels of dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites in striatal dialysates during 30 min of global ischaemia under simulated penumbral conditions, and compared these with neurological assessments over the following 7 days and histological damage at the end of this period. On the basis of dialysate DA levels during ischaemia, the animals fell into two subgroups; group I, with little or no DA increase (less than three times basal); and group II, with a much larger increase (greater than 30 times basal). Changes in 5-HT, though of lesser magnitude, showed a similar pattern. These findings may indicate that the amine changes depend on a critical reduction of blood flow within the range obtained by our experimental procedure. Levels of deaminated metabolites fell in all ischaemic animals, with comparable decreases of 3,4-dihydroxyphenylacetic acid plus homovanillic acid in both groups. Decreases of 5-hydroxyindoleacetic acid were greater in group II than in group I, but the relative differences between the groups were much less marked than those of 5-HT. These neurochemical findings suggest that moderate ischaemia affects extracellular amine and deaminated metabolite levels by different mechanisms. Only one of the ischaemic rats (a member of group II) showed a marked neurological deficit, but histological damage, as indicated by neuronal loss and gliosis in vulnerable structures, was apparent in all ischaemic animals. Although damage tended to be greater in animals with marked increases in extracellular monoamines, differences were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral blood flow and ischemia-induced neurotransmitter release in the striatum of aged spontaneously hypertensive rats

BACKGROUND AND PURPOSE

We found age-related vulnerability to cerebral ischemia in the hippocampus and striatum in spontaneously hypertensive rats. Further study revealed that ischemia-induced release of hippocampal taurine, an inhibitory amino acid, was reduced by 40% in aged rats compared with adult rats, which suggested an impaired inhibitory function against excitotoxicity in aged rats. The purpose of this study was to examine whether ischemia-induced neurotransmitter release is altered in the striatum of aged spontaneously hypertensive rats.

METHODS

Five adult (5-6 months) and five aged (18-22 months) female spontaneously hypertensive rats were subjected to 20 minutes of cerebral ischemia induced by bilateral carotid artery occlusions and 120 minutes of recirculation under amobarbital anesthesia (100 mg/kg i.p.). Cerebral blood flow was determined using the hydrogen clearance method, and extracellular concentrations of neurotransmitters were determined with the brain dialysis technique in the striatum.

RESULTS

During ischemia, cerebral blood flow in aged rats decreased to 8.7 +/- 1.2 (mean +/- SEM) mL/100 g per minute (11% of the resting), which was not different from 5.2 +/- 1.7 mL/100 g per minute (8% of the resting) in adult rats, and extracellular dopamine and amino acids (glutamate, aspartate, and taurine) increased by approximately 170- and 10-30-fold, respectively, and returned to baseline after 20-40 minutes of recirculation. These values of neurotransmitters, however, were not different between aged and adult rats during ischemia and reperfusion.

CONCLUSIONS

It is unlikely that a presynaptic mechanism is involved in age-related vulnerability in the striatum of hypertensive rats.

Isradipine attenuates the ischemia-induced release of dopamine in the striatum of the rat

We examined the effect of isradipine, a blocker of L-type voltage-sensitive Ca2+ channels (VSCCs), on the ischemia-induced release of dopamine in the rat striatum. Perfusion of 200 micrograms/ml isradipine in the striatum did not alter extracellular dopamine concentrations monitored by microdialysis. However, a marked increase (145-fold) in dopamine level during forebrain ischemia, developed by bilateral carotid artery occlusion, was attenuated significantly by 37% by isradipine whereas the intensity of ischemia, monitored by striatal blood flow, was unchanged. These results suggest that isradipine attenuates the ischemia-induced release of dopamine via blockade of L-type VSCCs on dopaminergic neurons.

Inhibition of ischemia-induced dopamine release by omega-conotoxin, a calcium channel blocker, in the striatum of spontaneously hypertensive rats: in vivo brain dialysis study

The effect of omega-conotoxin GVIA (CgTX), an N-and L-type voltage-sensitive calcium channel (VSCC) blocker, on the release of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum before and during transient cerebral ischemia in spontaneously hypertensive rats was studied using an in vivo brain dialysis technique. Continuous perfusion of CgTX in the striatum was started 20 min before ischemia and concentrations of dopamine and DOPAC in the dialysate were measured using HPLC with an electro-chemical detector. Before ischemia, both 10 and 100 microM CgTX significantly lowered the concentration of dopamine, to 49% of the basal values. DOPAC concentrations also decreased significantly, by 28 and 17%, respectively. Forebrain ischemia, produced by bilateral carotid artery occlusion, reduced striatal blood flow to less than 6% of the resting value in each group. During 20 min of ischemia, the vehicle group showed a marked increase in dopamine (175 times the basal concentration). In the 10 or 100 microM CgTX perfusion group, in contrast, dopamine release was significantly attenuated, to 38 or 29% of the vehicle group, respectively. DOPAC concentrations decreased during ischemia to 58% of the basal value in the vehicle group and 49% in both CgTX groups. These results indicate that the massive release of striatal dopamine during ischemia depends largely on the influx of extracellular calcium via CgTX-sensitive VSCCs.

Age-related vulnerability to cerebral ischemia in spontaneously hypertensive rats

BACKGROUND AND PURPOSE

We sought to determine the effects of aging on regional cerebral blood flow and ischemic brain damage in transient cerebral ischemia in rats.

METHODS

Five adult (5-6 months) and five aged (18-22 months) female spontaneously hypertensive rats were subjected to 20 minutes of bilateral carotid occlusion and 60 minutes of recirculation under amobarbital anesthesia (100 mg/kg i.p.). Regional cerebral blood flow in the hippocampus and striatum was measured using the hydrogen clearance method. Nine adult and 14 aged rats were subjected to 20 minutes of bilateral carotid occlusion or were sham-operated under ether anesthesia. Seven days after 20 minutes of cerebral ischemia, the rats' brains were perfusion fixed. Ischemic damage in the hippocampus and striatum was graded (0 [normal] to 3 [majority of neurons damaged]).

RESULTS

After 20 minutes of bilateral carotid occlusion, striatal cerebral blood flow decreased to 9.1 +/- 1.5 and 3.9 +/- 2.0 ml/100 g/min in aged and adult rats, respectively, and hippocampal cerebral blood flow decreased to 8.6 +/- 2.4 and 5.7 +/- 2.4 in aged and adult rats, respectively. Although these ischemic cerebral blood flow values were not significantly different between the two age groups, scores for ischemic damage in the hippocampus CA-1 subfield and striatum were significantly higher in aged than in adult rats (p less than 0.05, Kruskal-Wallis' h test with Bonferroni correction).

CONCLUSIONS

We conclude that aging may be a primary factor in the development of greater ischemic neuronal damage observed in aged hypertensive rats.