Protective effect of synaptic inhibition during cerebral ischemia in rats and rabbits

Postischemic diazepam is neuroprotective in the gerbil hippocampus

In this study, we address the hypothesis that enhancement of gamma-aminobutyric acid (GABA) neurotransmission following an ischemic episode is neuroprotective in the hippocampus. Mongolian gerbils were subjected to transient forebrain ischemia for 5 min by occlusion of the carotid arteries and then administered diazepam (10 mg/kg i.p.) 30 min or 30 and 90 min following ischemia. Diazepam produced a significant decrease in both rectal and brain temperature (4-6 degrees C) in the sham and ischemic gerbils. 1 day following the onset of reperfusion, diazepam substantially reduced the hyperactivity normally induced by the ischemic episode. 7 days later, neuronal viability in the hippocampus was assessed. The single dose of diazepam completely protected the CA1 pyramidal cells of the hippocampus in 62% of the gerbils and the double dose of diazepam completely protected CA1 pyramidal neurons in 67% of the gerbils. There was a significant correlation between the degree of pyramidal cell degeneration in the CA1 area of the hippocampus measured 7 days following ischemia and the degree of hyperactivity measured 1 day following ischemia. Diazepam also prevented the loss of [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding to GABA-gated chloride channels in the dendritic fields of the CA1 area of the hippocampus. Our findings support the hypothesis that enhancement of GABA neurotransmission following an ischemic event may offset neuronal excitability and prevent neuronal death in specific brain regions. We conclude that GABA-enhancing drugs, such as diazepam, are attractive candidates as neuroprotective agents following ischemic insults.

Muscimol prevents neuronal injury induced by NMDA

The effect of muscimol on N-methyl-D-aspartate (NMDA)-induced injury of primary cultured cerebral cortical neurons was examined. NMDA induced a dose-dependent leakage of LDH activity, which was significantly inhibited by (+-)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]cyclopentan-5,10-imine (MK-801). Muscimol significantly reduced the NMDA-induced increase of lactic dehydrogenase (LDH) leakage, and bicuculline abolished this protective effect of muscimol. Similarly, muscimol reduced the NMDA-induced increase in trypan blue staining of the cells, and bicuculline suppressed this inhibitory action of muscimol. These results suggest that GABAA-receptor stimulation exerts a protective action against the neuronal injury induced by NMDA-receptor activation.

Combination therapy protects ischemic brain in rats. A glutamate antagonist plus a gamma-aminobutyric acid agonist

BACKGROUND AND PURPOSE

The excitotoxic effects of glutamate can be blocked almost completely with gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, in cell culture, tissue slices, and in some animal models. After stroke in rats, we showed previously that an agonist of GABA, muscimol, was as neuroprotective as MK-801, an antagonist of glutamate. To obtain further neuroprotection and to avoid the side effects associated with high doses of MK-801, we wanted to assess the efficacy of the two agents in combination.

METHODS

Treatment was administered 5 minutes after embolic cerebral ischemia in Sprague-Dawley rats. The subjects were rated using a neurological evaluation 48 hours later. Visual-spatial learning was measured 8 to 10 weeks after stroke, after which we measured the volume of each cerebral hemisphere and several large cerebral compartments. Treatment groups included saline (n = 27), MK-801 1.0 mg/kg (n = 23), muscimol 1.0 mg/kg (n = 17), and both agents together using a dose of 0.5 mg/kg each (n = 25).

RESULTS

A probit analysis of the neurological ratings revealed a protective effect of muscimol used alone (MK-801 potency ratio, 2.0; P = NS; muscimol potency ratio, 4.0; P < .05) and a protective effect of the combination (potency ratio, 5.0; P < .05). Focal ischemia caused a moderate to severe delay in the acquisition of visual-spatial information, which was completely eliminated by the combination treatment but only partially ameliorated with MK-801 or muscimol alone. Ischemia reduced the cerebral hemisphere volume from 0.42 mm3 to 0.34 mm3 (P < .0001), the volume density of cortex from 22% to 17% of total cerebral volume (P < .01), and that of hippocampus from 4.3% to 3.0% (P < .05). Only the combination was neuroprotective, as measured by the ratio of the lesioned to the contralateral hemisphere volume (P = .013). The combination treatment and MK-801 protected the hemisphere volume, the cortex, and the hippocampus and reduced the size of visible infarction.

CONCLUSIONS

Combination therapy, using a glutamate antagonist and a GABA-A agonist, appeared to protect the brain and ameliorate a defect in learning behavior after stroke. The combination may have been more effective than either agent used alone, although further study of higher doses is needed.

Reversed alterations of hippocampal parvalbumin and protein kinase C-gamma immunoreactivity after stroke in spontaneously hypertensive stroke-prone rats

BACKGROUND AND PURPOSE

Aging spontaneously hypertensive stroke-prone rats (SHR-SP) were previously shown to develop neocortical strokes. Because the hippocampal CA1 is selectively vulnerable to abnormal brain perfusion, the neuropathological effects of spontaneous strokes were investigated on specific neurochemical alterations in two major cell types of the hippocampal CA1 in SHR-SP.

METHODS

The immunoreactivity for the gamma-isoform of protein kinase C (in pyramidal cells) and parvalbumin (in interneurons) was determined in the hippocampal CA1 by applying monoclonal antibodies. Because chronic treatment with the calcium antagonist nimodipine prevents the development of strokes in SHR-SP, we compared SHR-SP (stroke) with age-matched nimodipine-treated rats (nonstroke).

RESULTS

After stroke in control animals, we observed a strikingly enhanced immunoreactivity for protein kinase C-gamma in CA1 pyramidal cells compared with nimodipine-treated rats, which can be interpreted as the result of an increased activation of these cells. The pathological increase of protein kinase C-gamma immunoreactivity was accompanied by a reduced parvalbuminergic innervation of these pyramidal cells in symptomatic SHR-SP.

CONCLUSIONS

Because parvalbumin is present in a subset of GABAergic inhibitory interneurons, these data suggest that increased activity of CA1 pyramidal cells after spontaneous stroke may partially be related to a decreased inhibitory input on these cells.