Ischemia-induced seizures and cortical monoamine levels

The role of dopamine in epilepsy

The clinical benefits of dopamine agonists in the management of epilepsy can be traced back over a century, whilst the introduction of neuroleptics into psychiatry practice 40 years ago witnessed the emergence of fits as a side effect of dopamine receptor blockade. Epidemiologists noticed a reciprocal relationship between the supposed dopaminergic overactivity syndrome of schizophrenia and epilepsy, which came to be regarded as a dopamine underactivity condition. Early pharmacological studies of epilepsy employed nonselective drugs, that often did not permit dopamine's antiepileptic action to be clearly dissociated from that of other monoamines. Likewise, the biochemical search for genetic abnormalities in brain dopamine function, as predeterminants of spontaneous epilepsy, proved largely inconclusive. The discovery of multiple dopamine receptor families (D1 and D2), mediating opposing influences on neuronal excitability, heralded a new era of dopamine-epilepsy research. The traditional anticonvulsant action of dopamine was attributed to D2 receptor stimulation in the forebrain, while the advent of selective D1 agonists with proconvulsant properties revealed for the first time that dopamine could also lower the seizure threshold from the midbrain. Whilst there is no immediate prospect of developing D2 agonists or D1 antagonists as clinically useful antiepileptics, there is a growing awareness that seizures might be precipitated as a consequence of treating other neurological disorders with D2 antagonists (schizophrenia) or D1 agonists (parkinsonism).

Transient hypoxia alters striatal catecholamine metabolism in immature brain: an in vivo microdialysis study

Microdialysis probes were inserted bilaterally into the striatum of 7-day-old rat pups (n = 30) to examine extracellular fluid levels of dopamine, its metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA). The dialysis samples were assayed by HPLC with electrochemical detection. Baseline levels, measured after a 2-h stabilization period, were as follows: dopamine, not detected; DOPAC, 617 +/- 33 fmol/min; HVA, 974 +/- 42 fmol/min; and 5-HIAA, 276 +/- 15 fmol/min. After a 40-min baseline sampling period, 12 animals were exposed to 8% oxygen for 120 min. Hypoxia produced marked reductions in the striatal extracellular fluid levels of both dopamine metabolites (p less than 0.001 by analysis of variance) and a more gradual and less prominent reduction in 5-HIAA levels (p less than 0.02 by analysis of variance), compared with controls (n = 12) sampled in room air. In the first hour after hypoxia, DOPAC and HVA levels rose quickly, whereas 5-HIAA levels remained suppressed. The magnitude of depolarization-evoked release of dopamine (elicited by infusion of potassium or veratrine through the microdialysis probes for 20 min) was evaluated in control and hypoxic animals. Depolarization-evoked dopamine efflux was considerably higher in hypoxic pups than in controls: hypoxic (n = 7), 257 +/- 32 fmol/min; control (n = 12), 75 +/- 14 fmol/min (p less than 0.001 by analysis of variance). These data demonstrate that a brief exposure to moderate hypoxia markedly disrupts striatal catecholamine metabolism in the immature rodent brain.

Inhibition of ischemia-induced brain catecholamine alterations by clonidine

The effect of clonidine, an alpha 2-agonist, on ischemia-induced alterations in brain catecholamine and metabolite levels was studied in Mongolian gerbils subjected to 180 min of unilateral cerebral ischemia. The gerbils were randomly assigned to four treatment groups: sham-operated or unilateral carotid lesion; each pretreated with clonidine 0.4 mg/kg IP, or untreated. All animals were neurologically assessed and categorized as asymptomatic, neurological deficit or seizure activity at the time of sacrifice. Hemispheric levels of noradrenaline (NA), dopamine (DA), homovanillic acid (HVA), and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured using high pressure liquid chromatography with electrochemical detection. No changes from control were found in animals that remained asymptomatic regardless of treatment. In untreated gerbils that exhibited neurological deficits, marked reductions in both NA and DA and increases in HVA occurred in the ischemic hemisphere. These alterations were greater in gerbils that developed seizures during the observation period. Ischemic animals pretreated with clonidine did not show any significant alterations in catecholamine or metabolite levels from clonidine-treated, sham-operated controls in spite of the presence of neurological deficits. Although significant reductions in NA and DA still occurred in pretreated animals that developed seizures, the changes were markedly less than in untreated gerbils. These results indicate that alpha 2-adrenoceptor stimulation is an effective approach for inhibition of ischemia-induced brain catecholamine alterations, and thus may provide a useful method for assessing the role of catecholamine release in the production of acute ischemic neuronal damage.

Effect of ischemia on hydroxylase cofactor (tetrahydrobiopterin) and monoamine neurotransmitters in rat brain

Hydroxylase cofactor, monoamine neurotransmitters and their metabolites were measured in ischemic rat brain produced by four-vessel occlusion for 30 and 60 min periods. Slight reduction of hydroxylase cofactor activity was observed in the ischemic cortex after 60 min. Dopamine increased in the brainstem, and its metabolites, 3,4-dihydroxyphenylacetic acid and homovanillic acid, increased throughout the brain. Decrease in norepinephrine was observed in the whole brain. Decrease in serotonin and increase in 5-hydroxyindoleacetic acid, a metabolite of serotonin, was observed in the ischemic cerebral cortex. The present study has revealed that there appears to be no significant relationship between hydroxylase cofactor activity and monoamine levels in the ischemic brain. Thus, the hydroxylase cofactor does not play a main role in regulating monoamine synthesis in the acute phase of brain ischemia.

Epileptic seizures due to thrombotic and embolic cerebrovascular disease in older patients

Thromboembolic vascular disease is a frequent precipitant of seizures, and is the most common etiology in older patients. The occurrence of seizures shortly after a stroke, however, does not necessarily indicate that the patient will continue to have seizures following initial recovery. This is true even when patients present in epileptic status. This may be because early and late seizures are produced by different pathophysiologic mechanisms.

Effects of hypoxia-ischemia on monoamine metabolism in the immature brain

We measured acute changes in monoamine metabolites in corpus striatum of immature rat pups exposed to hypoxia-ischemia, hypoxia alone, or total global ischemia. Carotid ligations and two hours of 8% oxygen environment in 7-day-old pups led to asymmetrical turning behavior, a 70% decrease in endogenous striatal dopamine levels, and a 125% increase in homovanillic acid (HVA) concentrations on the side of ligation. In contrast, hypoxia alone and total global ischemia alone were not associated with HVA level elevation. Elevation of HVA level with hypoxia-ischemia showed a threshold effect between 1 and 1.5 hours, and this time course paralleled that for production of gross morphological changes in rats raised to maturity. The data suggest that dopamine release from striatal nerve terminals is associated with events causing brain injury during perinatal hypoxia-ischemia. Tissue HVA in the animal model appears to be a quantitative marker for the effects of the insult on a population of nerve terminals.

Differential effect of cerebral ischemia on monoamine content of discrete brain regions of the Mongolian gerbil (Meriones unguiculatus)

The effect of bilateral cerebral ischemia on noradrenaline, dopamine, and serotonin concentrations in six brain regions (i.e., the cerebral cortex, striatum, hippocampus, midbrain-diencephalon, cerebellum, and pons-medulla oblongata) was examined in the gerbil stroke model. The relative changes in regional cerebral blood flow after bilateral common carotid occlusion were also assessed using the radioactive microsphere technique. At 1 h after bilateral carotid occlusion, a significant decrease of monoamine concentration was observed in the cerebral cortex, striatum, hippocampus, and midbrain-diencephalon whereas no significant change was detected in the cerebellum and pons-medulla oblongata. The fall in NA content was most prominent in the cerebral cortex and hippocampus and percentage reductions of dopamine and serotonin were greatest in the striatum and cerebral cortex, respectively. These results suggest that the monoamine neurons in various brain regions might have different vulnerabilities to ischemic insult and show no evidence of transtentorial diaschisis.

The effect of ischemia on biogenic amine concentrations in the central nervous system

A rabbit spinal cord ischemia model was used to study the effects of focal ischemia on the tissue concentrations of serotonin, 5-hydroxyindole acetic acid, and norepinephrine. Ischemia induced by abdominal aorta occlusion caused both serotonin and norepinephrine concentrations to decline in the most ischemic areas of the spinal cord by 55 minutes. In marginally perfused adjacent areas, serotonin concentrations transiently declined at 14 and 20 min. After the onset of ischemia and then returned to normal. The minimum was reached at the same time when previous studies showed damage had become irreversible in more ischemic regions. Concentrations of 5-hydroxyindole acetic acid did not change at any time and norepinephrine declined only in the most ischemic areas after damage was irreversible. Thus, permanent serotonin and norepinephrine decreases occur only in areas destined to be destroyed by infarction, but the serotonin returns to normal in marginal tissue that remains viable. These studies suggest that serotonin may be involved in the early stages of irreversible changes during central nervous system ischemia.

Effect of omental transposition on to the brain on the cortical content of norepinephrine, dopamine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in experimental cerebral ischaemia

Local cerebral ischaemia causes a significant decrease in norepinephrine, dopamine, 5-hydroxytryptamine in the cortical brain tissue of rabbits, associated with an increase in 5-hydroxyindoleacetic acid. Previous transposition of the omentum on to the brain surface maintains, to a large extent, physiological levels of these metabolites. This study stresses the role of the transposed omentum in reducing the effects of experimental occlusion of a major cerebral artery.

Cerebral microembolization in the rat: changes in blood-brain barrier permeability and cerebral blood flow as related to the degree of ischemia

Unilateral cerebral microembolism was performed in the rat by injecting calibrated, 50 micrometers in diameter, carbonized microspheres into the internal carotid artery. The events that follow brain ischemia due to cerebral embolization were studied by the analysis of the blood-brain barrier (BBB) function, the degree of regional cerebral blood flow (CBF) and the development of brain edema. Two hours after embolization there was no change in the brain water content. The local CBF (14C-ethanol technique) was only reduced in the ipsilateral hemisphere. Twenty-four hours after embolization the brain water content was increased significantly in the ipsilateral, but not in the contralateral hemisphere. Local CBF further decreased in the ipsilateral hemisphere and a reduction in flow was also observed in the contralateral hemisphere. Embolization led to an increase in the BBB permeability, analysed as regional penetrability of 3H-dextran and of Evans blue-albumin complexes, which was restricted to the side of the injection of the microspheres.

The dissociation of cerebral blood flow, metabolism, and function in the early stages of developing cerebral infarction

Temporal and site correlation of local cerebral blood flow (1-CBF), tissue redox state, energy metabolism, tissue pH, and cerebral electrophysiological activity in induced cerebral ischemia was performed in rats in an effort to obtain helpful clues for the management of occlusive cerebrovascular disease. CBF decreased acutely in both the embolized and nonembolized hemispheres but returned toward normal in 5 minutes. However, total cerebral oxidative metabolism remained depressed throughout the 30-minute observation period despite improved perfusion. The change in CBF correlated with the development and resolution of tissue acidosis, which was maximal 3 minutes after embolization but became alkaline after 30 minutes, possibly due to accumulation of sodium lactate. Oxidized form of nicotinamide-adenine dinucleotide and cytochrome a,a3 quickly became reduced in the ischemic core, but a tardyspontaneous postischemic tissue perfusion resulted in their hyperoxidation. The CBF-metabolism uncoupling as well as postischemic hyperoxidation of the electron transport system, which is associated with accumulation of pyruvate and lactate, probably resulted from stagnation of electron flow at the entrance to the mitochondrial respiratory processes. Seizures could not account for these results, as paroxysmal changes in the EEG usually appeared only in the nonembolized hemisphere and were not dependent upon lack of energy. These studies confirm that metabolic failure may persist in ischemic tissue despite adequate reperfusion, which may, in fact, contribute to tissue damage through hyperoxidation.

Catecholamine levels and turnover during brain ischemia in the rat

Unilateral brain ischemia was induced in the rat by injecting radioactive microspheres into the left internal carotid artery. The microspheres were mainly distributed in the left cerebral hemisphere which contained 8 to 10 times more microspheres than the contralateral hemisphere. Embolization caused dopamine (DA) and noradrenaline (NA) depletion only in the left hemisphere. NA levels were already reduced 2 hours after injury while DA was still unaltered after 6 hours. A 30--40% depletion was observed for the two amines after 24 hours. Catecholamine turnover was estimated by measuring the amine depletion after synthesis inhibition with alpha-methyl-p-tyrosine. During the first 2 hours following embolization, DA and NA depletions were slightly increased only in the left hemisphere, indicating an increase in catecholamine efflux. At times 24 hours, an important retardation in amine disappearance after synthesis inhibition was found for DA and NA in the left hemisphere and to a lesser extent for DA in the right hemisphere, suggesting a reduction of the physiological activity of catecholaminergic neurons. These biochemical alterations can be related to the post-stroke behavioural changes of the embolized animals which exhibited an initially increased motor activity followed by a lethargic state.

Influence of cerebral embolism on brain monoamines

In baboons the right cerebral hemisphere was embolised by a shower of microemboli, immediately followed by one large embolus designed to occlude the middle cerebral artery (MCA). One hour after embolism a significant, though small, reduction in blood flow and oxygen consumption of the embolised hemisphere was recorded, at which time the animals were killed and brain monoamines measured. Dopamine was reduced in the ipsilateral caudate nucleus, the reported site of maximal ischaemic damage in this model. Dopamine levels were increased in frontal and occipital grey matter sampled from areas surrounding the occluded MCA territory and in similar brain areas of the opposite non-embolised hemisphere. Noradrenaline was increased in grey matter from both cerebral hemispheres, as well as subcortical structures bilaterally. Brain 5-hydroxytryptamine levels were unaltered, but increased 5-hydroxyindoleacetic acid in cisternal cerebrospinal fluid suggested transient alteration in 5-hydroxytryptamine metabolism after embolism. The effects of cerebral embolism on brain monoamine metabolism appear to be different from the effects of permanent surgical occlusion of major cerebral vessels. The bilaterality of effects after unilateral hemispheric embolism might be related to diaschisis. The mechanisms of the observed changes, as well as their relevance to the progression of cerebral ischaemia and the complications associated with cerebral embolism, still require to be established.

Effect of experimental ischemia on neurotransmitter amines in the gerbil brain

In the gerbil cerebral infarction was produced by unilateral carotid ligation. 3.5 hours later, when the neurological deficit was fully developed, hemisphere dopamine (DA) showed little change from normal. It seems unlikely that changes in DA are the direct cause of the turning behavior shown by these animals. Slight changes in norepinephrine (NE) occurred on the operated side but 4 hydroxy-3-methoxy phenyl-ethyleneglycol sulphate (MOPEG-SO4) levels were not affected. Significant falls in 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid (5-HIAA) were found on the operated side but there was also a trend for both 5HT and 5-HIAA to fall on the unoperated side. These changes occurred in clincally affected and unaffected animals and their clinical significance is unproven.