Effects of bilateral clamping of carotid arteries on hippocampal kindling in rats

Effects of cadmium on memory processes in mice exposed to transient cerebral oligemia

The purpose of the present study was to examine whether the effects of chronic or acute exposure to cadmium on memory processes in mice could be exacerbated by transiently reducing cerebral oxygen supply. Adult mice were subjected to bilateral clamping of the carotid artery (BCCA) for 30 min under anesthesia. Cadmium chloride was administered intraperitoneally after surgery at single doses of: 0.7 mg/kg (low dose), 1.4 mg/kg (high dose), or at a prolonged dose of 0.7 mg/kg for up to 10 days. Long-term memory was evaluated in a step-through passive avoidance task while spatial working memory was evaluated using a Y-maze spontaneous alternation task. BCCA mice injected with the 1.4 mg/kg dose of cadmium exhibited recall deficits in the step-through passive avoidance task. Combined treatment at either dose had no effect on the acquisition of passive avoidance. In the Y-maze task, spontaneous alternation behavior was only impaired in BCCA mice treated with the prolonged cadmium dose. These results indicate that cerebral oligemic hypoxia may alter cadmium neurotoxicity and potentiate the tendency for cadmium-induced memory impairments in the passive avoidance task and spontaneous alternation deficits.

Neuro-overprotection? A functional evaluation of clomethiazole-induced neuroprotection following hypoxic-ischemic injury

Hypoxic-ischemic (H-I) injury produces extensive damage to the hippocampus of young rats. We have recently shown that administration of 125 mg kg-1 clomethiazole (CMZ), a GABA(A)-agonist, provides complete histological protection against H-I injury if administered 3 h post-H-I (Brain Res 1035 (2005) 194). However, whether that histological protection translates into lasting functional preservation is unclear. To determine whether hippocampal-based circuits remain functionally intact in CMZ-protected H-I rats, we administered 125 mg kg-1 (high dose [CMZ-HD]) or 65 mg kg-1 (low dose [CMZ-LD]) CMZ, 3 h post-H-I, and examined numerous kindling parameters in the dorsal hippocampus 60 days following H-I. Kindling parameters included afterdischarge (AD) thresholds (ADTs), AD durations and kindling rates. Additional groups assessed included vehicle-injected H-I (VIH), hypoxic, ligated and naive rats. VIH, CMZ-HD, CMZ-LD and hypoxic rats all exhibited significantly faster kindling rates than naive rats. Thus, a previous traumatic event, even hypoxia alone, facilitated subsequent seizure propagation. Still, a significantly slower kindling rate was evident in CMZ-HD rats than in hypoxic, VIH or CMZ-LD rats. Moreover, while longer pre-kindling AD durations were observed in the damaged hippocampus of VIH compared with naive rats, this was not true for either CMZ-treated groups, hypoxic or ligated rats. Collectively, these findings suggest CMZ can suppress the epileptogenic effects of H-I. Surprisingly, however, both groups of CMZ-treated rats exhibited a four to nine times greater ADT than any other group and this effect was most profound in the CMZ-protected hippocampus. Thus, CMZ administration protected local neurons against terminal insult and left network excitability relatively normal with respect to seizure offset mechanisms but also caused profound elevation of local ADTs, which suggests a local hypoexcitability/increased inhibition. Finally, this study demonstrates, for the first time, that the kindling model can serve as a sensitive measure of function-related neuroprotective efficacy in animal models of ischemia.

The effect of CGP-40116 on pilocarpine evoked seizures in mice exposed to transient episode of brain ischemia

The objective of the study was to examine the role of N-methyl-D-aspartate (NMDA) receptors in the modulation of a brain tolerance after a transient cerebral ischemia. Adult mice were exposed for 30 min to bilateral clamping of common carotid arteries (BCCA) under anaesthesia. The competitive NMDA antagonist CGP-40116 was administered intraperitoneally (i.p.) in two experimental paradigms, (a) acute treatment: twice, 4.0 mg/kg; 1.5 h before the clamping of vessels and 6 h after re-circulation and (b) chronic treatment in a dose of 1.0 mg/kg; started 24 h after re-circulation and continued once daily for 13 days with the last injection 24 h before the induction of convulsions. Seizures were evoked with pilocarpine (400 mg/kg, i.p.) 14 days after BCCA. The preliminary study showed that BCCA induced protection against pilocarpine toxicity. The acute treatment with CGP-40116 partially diminished the anticonvulsant phenomenon. In contrast, the chronic treatment with the drug led to a marked potentiation of the effect. The whole brain gamma-aminobutyric acid (GABA) analysis performed 14 days after BCCA showed a moderate increase in vehicle-treated mice and a significant elevation after chronic treatment with CGP-40116. It can be concluded that NMDA antagonists may exert the opposite effects on the brain tolerance against pilocarpine toxicity after BCCA. The acute treatment with CGP-40116 diminished its induction while the chronic low-dose treatment enhanced a brain tolerance, possibly through the mechanism of chemical preconditioning.

The b-wave of the electroretinogram as an index of retinal ischemia

1. The b-wave of the electroretinogram (ERG) is a particularly sensitive index of retinal ischemia. The present paper summarizes the changes in the b-wave observed in five in vivo models of retinal ischemia. 2. Although the amount of reduction in b-wave amplitude during ischemia corresponds to the severity of the insult, the degree of recovery of the b-wave during reperfusion depends on the duration of ischemia. 3. A massive release of glutamate, intracellular overload with calcium and enhanced production of free radicals are suggested to be three major pathophysiological processes that contribute to retinal ischemic damage. The b-wave of the ERG represents a functional measure for potential therapeutic efficacy of drugs interacting with these pathophysiological processes. 4. Several glutamate antagonists, such as MK-801, memantine, flupirtine or GYKI 52466, along with the free radical scavengers vitamin E, lipoate, superoxide dismutase and catalase, all reduce the depression of the b-wave during ischemia or accelerate the recovery of the b-wave during reperfusion or both. The calcium channel antagonists nimodipine and levemopamil exert only a slight beneficial effect on the recovery of the amplitude of the b-wave during reperfusion, provided that the blood pressure is not potently reduced.

Induction of tolerance and mossy fibre neuropeptide-Y expression in the contralateral hippocampus following a unilateral intrahippocampal kainic acid injection in the rat

We have previously reported an ectopic expression of neuropeptide-Y (NPY) immunoreactivity in mossy fibres (MFs) in the contralateral hippocampus following a unilateral intrahippocampal (IH) injection of kainic acid (KA). In the present study we report that, in addition to MF NPY expression, unilateral IH KA injections also induce tolerance towards a subsequent intracerebroventricular (ICV) contralateral KA injection, resulting in a reduction in the number of overt seizures and degree of cell loss.

Hypoxia protects against the neurotoxicity of kainic acid

A normobar hypoxia (9% oxygen) of 8 h reduces the neurotoxicity of a subcutaneous injection of 10 mg/kg kainic acid given one week later. Both seizures and degenerative changes, including cell death of hippocampal and cortical neurons are markedly decreased by hypoxia. It is also shown that hypoxia also markedly reduced the extensive depletion of zinc from mossy fiber terminals normally induced by kainic acid. This suggests that a protective mechanism induced by hypoxia may affect the glutamatergic transmission in these synapses and prevent excessive synaptic excitation. The possible involvement of adenosine and/or GABA in this protective mechanism is discussed.

Influence of short-lasting bilateral clamping of carotid arteries (BCCA) on GABA turnover in rat brain structures

We have previously shown that short-lasting reduction of cerebral blood flow by bilateral clamping of carotid arteries (BCCA) results in long-lasting increase in regional GABA concentration and decrease in seizure susceptibility in rats. In the present experiments, the effect of BCCA on GABA turnover and the enzymes involved in GABA synthesis and degradation were studied in rats. Regional GABA turnover was measured by means of GABA accumulation induced by the GABA-transaminase (GABA-T) inhibitor aminooxyacetic acid (AOAA). Fourteen days after BCCA, GABA turnover was significantly increased in hippocampus, substantia nigra and cortex, but not different from sham-operated controls in several other brain regions, including striatum, hypothalamus and cerebellum. The activity of glutamate decarboxylase (GAD) measured ex vivo did not show any changes in investigated structures, while the activity of GABA-T was slightly increased in hippocampus. The increased GABA turnover in some brain regions may explain our previous findings of increased GABA content in these brain regions and decreased sensitivity of BCCA treated animals to the GABAA-receptor antagonist bicuculline.

Levemopamil reduces spatial learning deficit following transient occlusion of common carotid arteries in normotensive rats

The effect of the calcium channel blocker and 5-HT2 antagonist levemopamil on spatial learning impairment of rats subjected to transient bilateral clamping of carotid arteries (BCCA) was investigated. In addition, the acute effect of BCCA on local cerebral blood flow was measured in the presence and absence of levemopamil in a separate group of rats. Pretreatment with levemopamil prevented the BCCA-induced increase in escape latency during the test trials and ameliorated the BCCA-induced decrease in spatial bias during a probe trial. Under these experimental conditions, local cerebral blood flow fell to near ischaemic values in all mid- and forebrain regions during occlusion of the carotid arteries. However, pretreatment with levemopamil affected the BCCA-induced blood flow changes only in one brain area of 34 investigated. The present data suggest that pretreatment with levemopamil reduces impairment in spatial behaviour and that this effect seems not related to the compound's cerebral vasodilatory action, but to direct neuronal mechanisms.

Effect of transient reduction of cerebral blood flow in normotensive rats on striatal dopamine-release

Bilateral Clamping of both Carotid Arteries (BCCA) in normotensive rats is known to cause a transient reduction in cerebral blood flow. Using in vivo trans-striatal microdialysis and HPLC/ECD we measured the release of dopamine and DA-metabolites under these oligemic conditions. BCCA caused a substantial stimulation of striatal DA-release (40-fold) and a decrease of the outflow of DA-metabolites. The elevated DA-release returned to baseline levels before the onset of reperfusion. Upon reperfusion, DA-metabolites rose above their initial baseline values. Trans-striatal administration of glutamate-diethylester (GDEE, 10 mM) attenuated the oligemia-induced DA-release. A sudden reduction of blood flow appears to disrupt the compartmentation of dopamine in striatal dopaminergic nerve endings in a similar but more moderate manner as compared to ischemia.

Metabolic changes during and after transient clamping of carotid arteries in normotensive rats

In the present experiments changes of local metabolism in the striatum, hippocampus, and frontal cortex during and after transient clamping of carotid arteries in normotensive rats (BCCA) were studied by continuous measurement of local cerebral temperature, partial oxygen tension (PO2), and extracellular levels of lactate. Local temperature in the striatum, hippocampus, and frontal cortex fell between 1.7 and 2.3 degrees C upon occlusion and quickly returned to preocclusion values after free flow had been established. Local PO2 was reduced in the striatum, hippocampus, and frontal cortex to values between 70 and 30% during BCCA. Immediately after termination of BCCA the PO2 showed a tendency to recover in the striatum and frontal cortex, whereas in the hippocampus, this process started later. Extracellular levels of lactate within these three structures increased during BCCA and went down to preocclusion values within the observed period of reperfusion. The results suggest that BCCA induces a transient anaerobic metabolism that seems to be sufficient to evoke functional changes without neuronal damage.

Retinal ischemia induced by occlusion of both common carotid arteries in rats as demonstrated by electroretinography

In 2 models of reduced cerebral blood flow-permanent occlusion of the vertebral arteries plus transient occlusion of the common carotid arteries (4VO) and transient clamping of the common carotid arteries (BCCA)-the acute effects on the electrical function of the retina were monitored by recording the photopic electroretinogram. During both 4VO and BCCA the amplitude of the b-wave was reduced. Within 30 min of reperfusion after 4VO and after BCCA the b-wave had fully recovered. In contrast, the a-wave was not affected by either treatment. The data suggest that occlusion of common carotid arteries leads to retinal ischemia and might represent a useful model of amaurosis fugax.

Transient reduction of cerebral blood flow leads to longlasting increase in GABA content in vulnerable structures and decreased susceptibility to bicuculline induced seizures

Rats were exposed for 24 min to bilateral clamping of the common carotid arteries (BCCA) in pentobarbital anaesthesia. The GABA content was measured 24 hours, 48 hours, 4 days, 14 days and 3 months after BCCA. In other groups of rats seizures were elicited by i.p. injection of (+)-bicuculline (3 mg/kg) 24 hours, 48 hours, 4 days, 14 days and 3 months after BCCA. Analysis of the GABA content revealed significant increase compared with controls in the hippocampus, frontal cortex and substantia nigra from 24 hours up to 3 months. Bicuculline treatment induced tonic/clonic seizures and status epilepticus in sham operated animals; these effects were drastically diminished at various time points after BCCA. The present results suggest that BCCA produces a longlasting increase in GABA content and as a consequence protection from bicuculline-induced seizures.