Selective dysfunction of the vagal component of the baroreflex following cerebral ischemia: protection by ifenprodil and flunarizine

A systematic review of currently available pharmacological neuroprotective agents as a sole intervention before anticipated or induced cardiac arrest

We conducted a Medline search for controlled studies evaluating currently available drugs for pharmacological neuroprotection. They had to be administered prior to transient global cerebral ischaemia without further non-pharmacological measures. We deliberately excluded focal ischaemia since its pathophysiology is substantially different from global ischaemia. A total of 45 articles conducted exclusively in laboratory animals met these criteria. The following classes of agents were evaluated: anaesthetics, GABAergic drugs, calcium-antagonists, anticonvulsives, sodium-channel blockers, potassium-channel activators, NMDA-receptor antagonists, hormones, vasodilators, dopamine- and alpha2-agonists, magnesium, xanthine oxidase- and cyclooxygenase inhibitors, a nootropic, a protease inhibitor, and immunosuppressants. Some of them were applied chronically and others administered via clinically impracticable routes. The available literature favours isoflurane, phenytoin, lamotrigine, magnesium, and potentially, nimodipine, and flunarizine. If factors like costs, toxicity, side effects, route and mode of application are considered, isoflurane and MgSO4 that have also been safely applied to patients with compromised left ventricular pump function are advantageous but their true role in human neuroprotection remains unclear.

The influence of nicardipine and ifenprodil on the brain free arachidonic acid level and behavior in hypoxia-exposed rats

1. The effects of the calcium channel blockers, nicardipine and ifenprodil, on the brain free arachidonic acid level and learning ability in rats exposed to hypoxia were examined. 2. Adult rats were injected with 0.003; 0.01; 0.03; 0.1; 0.3 or 1.0 mg/kg of tested drugs i.p. Thirty min later the learning ability was tested in a passive avoidance task according to the step-through procedure. Immediately after the training trial, the animals were subjected to a period of oxygen deprivation hypoxia until the loss of the righting reflex. The retention trial was carried out 24 hr later. 3. The other groups of animals were pretreated with mentioned substances before hypoxia-exposure. Fifteen min after the loss of the righting reflex they were decapitated and brains were frozen in liquid nitrogen. The brain free arachidonic acid level was quantified by gas chromatography. 4. Both nicardipine and ifenprodil were effective in preventing a memory decline in hypoxia-exposed rats but did not prevent the accumulation of the brain free arachidonic acid in hypoxia-exposed rats. 5. The protective effects of both substances in behavioral studies during acute brain damage caused by hypoxia could not be explained by the prevention of the increase of the brain free arachidonic acid, but by some other mechanism.

Influence of transient global cerebral ischemia on the facilitatory modulation of the vagal baroreflex in dogs

The influence of 5-min global cerebral ischemia on the facilitatory modulation of the vagal baroreflex through central alpha 2-adrenoceptors or by the electrical stimulation of the septum was investigated in anesthetized dogs. Reflex bradycardia was produced by a bolus injection of phenylephrine at a dose which produces about a 25-mmHg increase in mean blood pressure. The ischemia was produced by the occlusion of the brachiocephalic and the left subclavian arteries with preceding ligation of the intercostal arteries. Clonidine at 10 micrograms, administered intracisternally, decreased the blood pressure and heart rate and facilitated the vagal reflex bradycardia. During the reperfusion period following ischemia, however, clonidine failed to affect the reflex bradycardia. Electrical stimulation of the septal region facilitated the reflex bradycardia without marked influences on the basal blood pressure and heart rate. The facilitatory effect was dependent on the frequency (10 to 75 Hz) and amplitude (3 to 15 V) of stimulation and was not observed after vagotomy or ischemic insult. These results suggest that 5-min global cerebral ischemia may produce the dysfunction of the neurons which are closely related to the baroreflex loop and receive the facilitatory modulation through alpha 2-adrenoceptors and/or from the forebrain structures, leading to the dysfunction of the vagal baroreflex.

Blockade of alpha 2-adrenoceptors protects the vagal baroreflex system from transient global cerebral ischemia in dogs

Ifenprodil, phentolamine, yohimbine or idazoxan, 1 mg/kg i.v., administered 5 min prior to 5-min global cerebral ischemia, completely prevented the post-ischemic dysfunction of the vagal baroreflex in anesthetized dogs. Idazoxan, 0.1 mg/kg i.v., was cerebroprotective against mild ischemia, but ineffective against severe ischemia. The post-ischemic administration of idazoxan, 1 mg/kg i.v., failed to restore the damaged vagal baroreflex. These results suggest that blockade of alpha 2-adrenoceptors during ischemia and the early reperfusion period may protect the vagal component of baroreflex from cerebral ischemia.

Protection by nicorandil against the dysfunction of the central vagal baroreflex system following transient global cerebral ischaemia in dogs

1. A possible cerebroprotective effect of nicorandil was investigated in a canine model of 5 min global cerebral ischaemia, and compared with protective effects of nitroglycerin and nicardipine. 2. Cerebral ischaemia was produced by occlusion of the left subclavian and the brachiocephalic arteries with preceding ligation of the intercostal arteries. The decrease in baroreflex sensitivity (BRS), measured by phenylephrine-induced reflex bradycardia, was used to assess the cerebroprotective effect. 3. Nicorandil (10 or 30 micrograms kg-1 min-1, i.v.), nitroglycerin (3 micrograms kg-1 min-1, i.v.) or nicardipine (0.3 micrograms kg-1 min-1, i.v.) were infused for 60 min just before ischaemia. Nitroglycerin and nicardipine decreased mean arterial blood pressure to an extent similar to that induced by the lower dose of nicorandil. Blood flow in the dorsal medulla oblongata was increased by nicorandil and nicardipine, but not by nitroglycerin. 4. Nicorandil at both doses and nitroglycerin prevented the post-ischaemic decrease in BRS. In these cases, bilateral vagotomy during the reperfusion period decreased BRS, indicating that the vagal component of BRS was protected from ischaemia. On the other hand, nicardipine failed to exert a cerebroprotective effect. 5. The present study suggests that nicorandil may possess a direct cerebroprotective effect and that its property as a nitrate might, at least in part, be important for the observed cerebral protection.

MK-801 prevents the post-ischemic cerebral hypoperfusion, but not the dysfunction of the vagal baroreflex in dogs

Pretreatment with MK-801, a non-competitive N-methyl-D-aspartate (NMDA) antagonist, failed to protect the vagal component of reflex bradycardia from 5-min global cerebral ischemia in dogs under pentobarbital anesthesia. On the other hand, MK-801 completely prevented the development of the post-ischemic cerebral hypoperfusion without affecting the cerebral blood flow in sham-operated animals. The results suggest that NMDA receptors may participate in the development of the secondary disturbance of the cerebral circulation, but are not involved in the post-ischemic dysfunction of the baroreflex system.