Inhibition of ischemia-induced brain catecholamine alterations by clonidine

The Effect of Cyclohexyladenosine on the Penischemic Increases of Hippocampal Glutamate and Glycine in the Rabbit

We investigated the ability of N6-cyclohexyladenosine (CHA), a potent and selective agonist of the adenosine A1 receptor, to attenuate elevations of levels of extracellular hippocampal glutamate and glycine that result from episodes of transient global cerebral ischemia (TGCI). A total of 30 New Zealand white rabbits were randomly assigned to receive 0 (n = 5), 0.1 (n = 8), 1.0 (n = 6), 10 (n = 6), or 100 (n = 5) microM CHA. The drug was dissolved in artificial CSF (vehicle) and administered via a microdialysis probe placed stereotactically into the dorsal hippocampus. A second microdialysis probe placed into the contralateral hippocampus of each animal was perfused with vehicle alone. Ten minutes of TGCI was induced by neck tourniquet inflation and deliberate hypotension from 0 to 10 min. Microdialysis samples were collected as follows: every 20 min preischemia (at -80, -60, -40, -20, and 0 min); every 5 min during ischemia and in the immediate reperfusion period (at 5, 10, 15, and 20 min); and every 20 min for the remainder of the reperfusion period (at 40, 60, and 80 min). Samples were then analyzed for their concentration of glutamate and glycine by HPLC. Following 10 min of ischemia, glutamate levels increased to a peak of 3.28 +/- 0.55 times baseline and returned to preischemic levels by 40 min, i.e., during reperfusion. Glycine concentrations increased to 5.41 +/- 0.91 times over baseline and remained elevated for the duration of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of clonidine on cell survival, glutamate, and aspartate release in normo- and hyperglycemic rats after near complete forebrain ischemia

The present study was undertaken to investigate the effects of the alpha2 adrenergic agonist, clonidine, on the near complete cerebral ischemia (NCFI) evoked release of glutamate and aspartate from normo- and hyperglycemic rodent brain tissue using microdialysis tissue techniques. Hemodynamic variables, blood lactate, and glucose levels were monitored throughout the 40 min NCFI occlusion period. After 48 h, rats were killed and the extent of neuronal injury was determined in the cortex, striatum, and hippocampus. Hemodynamic variables recorded during ischemia improved with clonidine treatment in both normo- and hyperglycemic groups. Glutamate and aspartate levels were greatly increased over control values during normo- and hyperglycemic NCFI treatment. Clonidine pretreatment suppressed the release of both glutamate and aspartate during NCFI in normo- and hyperglycemic rodents when compared with NCFI-treated normo- and hyperglycemic rats without the drug. Significant neuroprotection of cells in the cortex, striatum, and hippocampus was also observed in drug-treated animals 48 h postischemia. The combined effects of diminished glutamate release after NCFI and reduced neuronal injury in both normo- and hyperglycemic states suggests that clonidine treatment during NCFI is neuroprotective. The neuroprotective effect of clonidine during ischemia may be ascribed to both a sensitization of central sympathetic activity and a reduced release of glutamate thereby reducing NMDA receptor activation and neuronal damage.

alpha2-Adrenoceptor agonists: shedding light on neuroprotection?

Although alpha(2)-adrenoceptor agonists are widely used for analgesia, anxiolysis, sedation, sympatholysis and as anaesthetic-adjuncts for many years, their potential use as neuroprotectants has so far been confined to laboratory experiments. Despite the large body of evidence from both in vivo and in vitro studies, their exact neuroprotective mechanisms remain elusive. Herein, we review the available literature pertaining to the neuroprotective effect of alpha(2)-adrenoceptor agonists and the possible biochemical and physiological cascades involved in their mechanisms of action. The remarkable safety profile of alpha(2)-adrenoceptor agonists and their high potency of neuroprotection should prompt clinical trials to evaluate their neuroprotective efficacy in humans.

Effect of in vivo treatment of clonidine on ATP-ase's enzyme systems of synaptic plasma membranes from rat cerebral cortex

The effects on energy-consuming ATP-ases were studied in two types of synaptic plasma membranes from rat cerebral cortex after in vivo injection of clonidine. To study the mechanism of action of clonidine at subcellular level, the enzyme activities of Na+, K+-ATP-ase, Ca2+, Mg2+-ATP-ase, Low- and High-affinity Ca2+-ATP-ase, and Mg2+-ATP-ase were evaluated on synaptic plasma membranes of control and treated animals with clonidine (5 microg x kg(-1); i.p. 30 minutes). Acute treatment with clonidine decreased the catalytic activity of Ca2+, Mg2+-ATP-ase and of low-affinity Ca2+-ATP-ase only in synaptic plasma membranes of II type, that is the fraction enriched in synaptic plasma membranes. The decreases of these enzymatic activities are related to the interference of the drug on Ca2+ homeostasis in synaptoplasm. The reductions of these enzyme-consuming ATP-ases give further evidence that clonidine has not only neuroreceptorial effects, but that the drug also affects the energy metabolism of cerebral tissue, improving the knowledges about the pharmacology of clonidine. Because the elevation of [Ca2+]i, during ischemia/hypoxia contributes to cellular injury, these findings may suggest that the prevention of calcium overload may be the key mechanism of protection by alpha2-agonist.

Perioperative Stroke, Encephalopathy, and Central Nervous System Dysfunction

The leading cause of mortality in adult populations throughout the world is atherosclerosis, which results in cardiovascular and cerebrovascular complications and consumes substantive health care resources. The impact of atherosclerosis on patients undergoing surgery is also considerable, given the multiple stresses occurring during, and especially following, the surgical procedures, thereby precipitating vascular morbidity. Perioperative cerebrovascular morbidity and mortality occur in approximately 10% of the 600,000 patients who undergo cardiac surgery annually, consuming approximately $13 billion, which is expended on in-hospital, intensive care unit (ICU), and long-term specialized care for these neurological complications of stroke, encephalopathy, and cognitive dysfunction. Furthermore, risk of these outcomes will continue to increase as the surgical population ages. Principal among the etiologies of focal stroke and encephalopathy appear to be perioperative hypotension and precipitation of macroemboli and microemboli. As a result, new detection techniques for these events have been instituted, including (1) continuous hemodynamic monitoring, for detection of hypotensive episodes; (2) transesophageal echocardiography, for detection of aortic atherosclerosis, a potential source for emboli; and (3) transcranial Doppler sonography, for detection of cerebral emboli, as well as determination of cerebral blood flow. Recent large-scale multicenter studies have identified risk factors and indices for perioperative central nervous system (CNS) morbidity. Regarding therapy, a number of pharmacological approaches are currently under consideration; principal among these approaches are agents that can modulate the excitotoxic response, including glutamate receptor antagonists (NMDA, AMPA, metabotrophic), calcium channel blockers, free radical scavengers, and agents that modify the inflammatory white cell response. Although a number of laboratory, animal, and smaller clinical trials have been conducted, only one large-scale multicenter program to date has been conducted to assess the efficacy of adenosine modulation. These data, collected in more than 4,000 patients undergoing cardiac surgery, suggest that in addition to mitigation of myocardial injury, stroke also may be modulated by enhancing adenosine concentration in the area of cerebral ischemia. However, these preliminary findings must be validated in appropriately powered clinical trials. Finally, postoperative stroke and encephalopathy consume substantive resources, resulting in prolonged length-of-stay (17 days in-hospital 10 days for patients suffering Q-wavc infarction, vs 7 days for patients having no adverse outcome) and prolonged length-of-stay in the ICU following surgery (5 vs 3 vs 2 days, respectively). Hospital costs increase by approximately 3- to 4-fold in patients who suffer CNS outcomes following surgery. In conclusion, perioperative CNS morbidity and mortality is a critical problem that affects a substantial portion of the surgical population and consumes considerable health care resources. Over the next several years, attention must be focused on this important problem, and clinical and research resources should be redirected toward the solution of perioperative CNS morbidity.

Alpha 2-adrenergic agonists reduce glutamate release and glutamate receptor-mediated calcium changes in hippocampal slices during hypoxia

The mechanisms by which alpha 2-adrenergic agonists reduce ischemic brain damage are not clear. In ischemia-vulnerable hippocampal neurons we tested whether alpha 2-agonists reduce glutamate efflux and glutamate receptor-mediated increase of cytosolic free calcium. Brain slices (300 microns thick) from rat hippocampal were located with fura-2 for measurements of cytosolic free calcium with a microscope fluorometer. Change of cytosolic calcium in CA1 neurons during application of N-methyl-D-aspartate (NMDA) was measured, as were calcium changes during simulated ischemia (hypoxia, NaCN, iodoacetate) of hypoxia plus high glutamate concentration (pO2 = 25 mmHg, 3 mM glutamate). In order slices, glutamate efflux evoked by anoxia (pO2 = 25 mmHg, 100 microM NaCN) was measured. The selective alpha 2-agonist mivazerol (1 microM) decreased NMDA receptor-mediated calcium changes in hippocampal CA1 neurons by 28% (p = 0.0079). With hypoxia and 3 mM glutamate, 1 microM mivazerol reduced early peak calcium changes in CA1 neurons by 57% (p = 0.0007). An alpha 2-antagonist (rauwolscine, 1 microM) blocked this. Mivazerol did not reduce the rate of calcium change during simulated ischemia. Clonidine (0.1 microM), a partial alpha 2-agonist, decrease glutamate/hypoxia-mediated calcium changes in CA1 (p = 0.01), but 1 microM clonidine, which stimulates alpha 1-receptors, did not. Mivazerol decreased hypoxia and KCl1-evoked glutamate release by 50% and 75% (p < 0.01), respectively. In addition, 1 microM mivazerol reduced lactate dehydrogenase leakage rate from brain slices during anoxia by 61% (p = 0.018). Thus, alpha 2-receptors influence glutamate release, calcium changes, and cell damage in ischemia-vulnerable hippocampal neurons. These effects may contribute to the cerebroprotective actions of alpha 2-agonists.

Sedation for the critically ill neurologic patient

OBJECTIVE

To review the scientific basis for sedation of critically ill neurologic patients by summarizing the distinct neurophysiologic disturbances present in this population and presenting the central nervous system effects of sedative agents to permit optimal drug therapy.

DATA SOURCES

Review of the English language clinical and scientific literature using MEDline data search.

STUDY SELECTION

Literature references were selected through a key word search of sedative therapy, drugs used for sedation, and specific neurologic disorders and processes to provide an in-depth overview of sedative drug mechanisms of action, effects on neurophysiology and intracranial dynamics, pharmacokinetics, and toxicity profile. Special emphasis was placed on neurologic side effects.

DATA EXTRACTION

Clinical and scientific literature was reviewed and data relevant to neurophysiologic effects of sedative drug therapy were summarized. Recommendations for institution of sedative therapy and of particular agents were made as a result of analysis of all pooled data.

DATA SYNTHESIS

Critically ill patients with neurologic pathology present as a unique subset of individuals cared for in an acute care setting. Because monitoring of neurologic patients requires frequent assessment of the neurologic examination, the goal of sedative therapy should be to enhance, or to minimally perturb elicitation of the examination. Neurophysiologic disturbances introduce distinct risks for sedation and require their identification and understanding before the initiation of any sedative therapy. Sedative drugs, in particular, act to disturb central nervous system function and their effects may result in diagnostic confusion and further neurologic deterioration. The pharmacokinetic and neurophysiologic actions of the common classes of sedative agents, such as benzodiazepines, opioids, barbiturates, and neuroleptics, as well as ketamine, propofol, and clonidine are discussed. Recommendations are presented based on the specific type of sedation required and the underlying neurologic disturbance. Several specific examples, including head trauma, neuromuscular disease, and alcohol withdrawal, are provided.

CONCLUSIONS

Preservation of the neurologic examination is paramount in documenting clinical improvement or deterioration in the critically ill neurologic patient. Pharmacologic sedation in this unique population of acute care patients requires careful consideration of the underlying neurophysiologic disturbances and potential adverse effects introduced by sedative drugs.