Effects of moxonidine on corticocerebral blood flow under normal and ischemic conditions in conscious rabbits

Protection in glutamate-induced neurotoxicity by imidazoline receptor agonist moxonidine

In the present study we investigated the effects of mixed imidazoline-1 and alpha(2)-adrenoceptor agonist, moxonidine, in glutamate-induced neurotoxicity in frontal cortical cell cultures of rat pups by dye exclusion test. Also, phosphorylated p38 mitogen activated protein kinases (p-p38 MAPK) levels were determined from rat frontal cortical tissue homogenates by two dimensional gel electrophoresis and semidry western blotting. Glutamate at a concentration of 10(-6) M was found neurotoxic when applied for 16 hr in cell cultures. Dead cell mean scores were 12.8 +/- 0.5 for control and 52.3 +/- 4.8 for glutamate (p < .001). On the other hand, p-p38 MAPK levels start to increase at a glutamate concentration of 10(-7) M for 20 min application. Moxonidine was found to have an U-shape neuroprotective effect in glutamate-induced neurotoxicity in neuronal cell culture experiments. Even though moxonidine did not induce neurotoxicity alone between the doses of 10(-8) to 10(-4) M concentrations in cell culture series, it caused the reduction of glutamate-induced dead cell population 23.07 +/- 3.6% in 10(-6) M and 26.7 +/- 2.1% in 10(-5) M concentrations (p <.001 for both, in respect to control values). The protective effect of moxonidine was confirmed in 10(-8) and 10(-7) M, but not in higher concentrations in glutamate neurotoxicity in gel electrophoresis and western blotting of p-p38 MAPK levels. In addition to other studies that revealed an antihypertensive feature of moxonidine, we demonstrated a possible partial neuroprotective role in lower doses for it in glutamate-mediated neurotoxicity model.

Effects of citalopram and fluoxetine on the corticocerebral blood flow in conscious rabbits

Depression, which is associated with an increased incidence of vascular events, frequently occurs following stroke. Selective serotonin reuptake inhibitory drugs (SSRIs) as antidepressants, are well tolerated, and also seem to be effective in post-stroke depression. The aim of this study was to investigate the effects of the SSRIs citalopram and fluoxetine, on the corticocerebral blood flow (cCBF) in rabbits with unilateral carotid occlusion induced cerebral ischemia. The cCBF was measured by the hydrogen clearance technique. After determination of the mean baseline cCBF, the effects of individual doses (0.1, 0.3 and I mg/kg) of citalopram or fluoxetine on the cCBF were investigated. Following the induction of an impaired cCBF, the changes in cCBF after drug treatments in this condition were likewise measured. The mean arterial blood pressure (MABP) and the heart rate (HR) from the electrocardiogram (ECG) were also determined. Neither citalopram nor fluoxetine influenced the cCBF in the control group. Fluoxetine improved the cCBF only very slightly in the ischemic animals. In contrast, all the doses of citalopram exerted pronounced and dose-dependent cCBF-increasing effects in the animals with unilateral carotid occlusion (maximal mean ACBF: 10, 16 and 27 ml/min/100 g tissue). The HR was decreased in both groups. Only citalopram treatment led to a slight MABP-decreasing effect. Besides enhancement of the serotonergic transmission in the brain, the cCBF-increasing effect of citalopram under ischemic conditions may be of benefit in post-stroke and vascular depression.

Comparison of regional vasomotor responses to acetazolamide and CO2 in rabbit cerebrum and cerebellum, measured by a hydrogen clearance method

AIM

Many investigators have proved the usefulness of acetazolamide provocation and the carbon dioxide test for assessment of the local cerebrovascular reactivity by measurement of the regional cerebral blood flow in patients with occlusive cerebrovascular disease. Data originating from a comparison of these two different vasomotor stimuli as concerns the differences in sensitivity to them in various parts of the central nervous system are scarce. Our aim was to compare the cerebral blood flow responses to hypercapnic and acetazolamide stimuli in different brain regions.

METHODS

The cerebral blood flow was measured in the cerebrum (cortex and caudate nucleus) and cerebellum (cortex), as measured by a hydrogen clearance method in anaesthetized, artificially ventilated rabbits.

RESULTS

In normocapnia, the cerebral blood flow values in the cerebrum and the cerebellum differed significantly. The cerebral blood flow responses to both vasodilatory stimuli were to be significantly higher in the cerebrum than in the cerebellum, but the relative increases, i.e. the mean relative reactivities, were similar in the different regions measured.

CONCLUSION

The regional dissimilarity might explain to some extent the different sensitivities of the various brain areas to sudden blood pressure changes (infarction or haemorrhage). The results further suggest that heterogeneity in cerebrovascular reactivity should be considered in the assessment of vasoreactivity in patients with occlusive cerebrovascular disease. Since the comparison of the carbon dioxide and acetazolamide-induced cerebrovascular reactivities revealed a strong linear relationship, it was concluded that acetazolamide provocation is equivalent to the carbon dioxide test in the evaluation of cerebrovascular reactivity.

Electrocerebral Silence by Intracarotid Anesthetics Does Not Affect Early Hyperemia After Transient Cerebral Ischemia in Rabbits

Evidence suggests that early postischemic hyperemia is mediated by both neurological and vascular mechanisms. We hypothesized that if neuronal activity were primarily responsible for reperfusion hyperemia, then electrocerebral silence induced by intracarotid anesthetics (propofol and pentothal) would attenuate the hyperemic response. New Zealand white rabbits were subjected to 10 min of cerebral ischemia using bilateral carotid occlusion and systemic hypotension. Subsequently, carotid occlusion was released, and the mean arterial blood pressure was increased to baseline values. In the control group, intracarotid saline was periodically injected during reperfusion. In the treatment groups, intracarotid propofol or thiopental was administered to maintain electrocerebral silence for 10 min. Physiological data were measured at baseline, during ischemia, and at reperfusion. Satisfactory data were available for 16 of 19 rabbits. Mean arterial blood pressure, end-tidal CO(2), and cerebral blood flows decreased significantly in both groups during carotid occlusion. During early reperfusion, a similar percent increase in cerebral blood flow from baseline values was observed in all 3 groups (192% +/- 76%, 218% +/- 84%, and 185% +/- 101% for saline, propofol, and pentothal, respectively). These results suggest that suppression of neuronal activity during reperfusion does not affect early hyperemia after transient cerebral ischemia.

IMPLICATIONS

Intracarotid injection of anesthetic drugs in doses that are sufficient to produce electrocerebral silence do not obtund early cerebral hyperemia after transient cerebral ischemia. This suggests that vascular, not neuronal mechanisms, are primarily responsible for early postischemic cerebral hyperperfusion.

Effect of systemic administration of L-kynurenine on corticocerebral blood flow under normal and ischemic conditions of the brain in conscious rabbits

Kynurenic acid, the only known endogenous antagonist of the excitatory amino acid receptors, exerts neuroprotective effect in focal cerebral ischemia. Kynurenic acid poorly while its bioprecursor, l-kynurenine (L-KYN) completely crosses the blood-brain barrier. The aim of our study was to investigate the effect of intravenous l-KYN (0.3, 1, and 3 mg/kg) on the normal and the unilateral carotid artery occlusion induced ischemic corticocerebral blood flow (cCBF) measured by hydrogen polarography in conscious rabbits. Administration of l-KYN produced a significant increase in the normal cCBF; the peak values were recorded at the dose of 1 mg/kg (187% at 120 and 150 mins. respectively). The cCBF-improving effect of l-KYN was immediate and highly pronounced also in rabbits with carotid occlusion (peak value was 192% at 120 mins. at the dose of 1 mg/kg). Pretreatment with either atropine or Nomega-nitro-L-arginine-methyl-ester (L-NAME) prevented the l-KYN induced enhancement of the normal and the ischemic cCBF alike. It is suggested that the cCBF-increasing effect of l-KYN might be mediated by activation of cholinergic and nitric oxide pathways.

Moxonidine: some controversy

Initially it was considered that moxonidine, like clonidine, acted at central (2)-adrenoceptors to reduce blood pressure. With the characterisation of imidazoline binding sites distinct from (2)-adrenoceptors, the consensus became that moxonidine was acting predominantly at imidazoline I(1) receptors in the rostral ventrolateral medulla to lower blood pressure. Moxonidine acts at prejunctional (2)-adrenoceptors on sympathetic nerve endings to decrease noradrenaline release and this may contribute to its ability to lower blood pressure. The predominant site of action of moxonidine may also depend on route of administration, with imidazoline I(1) receptors being predominant after central, and (2)-adrenoceptors predominant after systemic administration. The controversy over the mechanism and site of action with moxonidine is ongoing. In animal models, moxonidine lowers blood pressure, reduces cardiac hypertrophy and remodelling, reduces cardiac arrhythmias and increases blood flow in cerebral ischaemia. Moxonidine also has beneficial effects in animal models of diabetes and kidney disease. Moxonidine increases sodium and water excretion in rats, but not humans. Animal studies indicate that moxonidine may be useful in the treatment of glaucoma by reducing intra-ocular pressure. Animal studies show that moxonidine may also be effective in pain and in ethanol withdrawal. In humans, the pharmacokinetics of moxonidine are of the one-compartment model with first-order absorption. Renal elimination is the major route of elimination and individual titration of moxonidine is needed in patients with renal impairment. There is overwhelming evidence that moxonidine is a safe and effective antihypertensive. A large clinical trial of moxonidine in heart failure, MOXCON, was stopped because of excessive deaths in the moxonidine group. Moxonidine should not be used in patients with heart failure, but there are no obvious reasons to stop its use as an antihypertensive, or its development for other clinical uses.