Effects of an inhibitor of GABA-aminotransferase (gamma-vinyl-GABA) on the spatial navigation deficit induced by muscarinic blockade

Tiagabine, a gamma-amino-butyric acid transporter inhibitor impairs spatial learning of rats in the Morris water-maze

gamma-Amino-butyric acid (GABA) is cleaved from the synaptic cleft by uptake via specific transporters. Inhibition of such transporters increases the effectiveness of physiologically released GABA. Increased GABAergic neurotransmission has an impact on learning and memory. Therefore, effects of tiagabine, a GABA-transporter inhibitor, were investigated on spatial orientation in the Morris water-maze. Rats were given four training trials per day for 4 days and a probe trial without platform on the 5th day. Compared to saline treated rats, rats treated daily with 20 mg/kg tiagabine showed impaired learning during the acquisition trials. Retrieval was impaired in rats treated only at the probe trial with tiagabine. These results further elucidate the role of GABA in learning and memory.

Chronic elevation of brain GABA levels beginning two days after status epilepticus does not prevent epileptogenesis in rats

Vigabatrin (VGB) treatment is neuroprotective in various models of status epilepticus (SE) and delays the development of kindling via mechanisms that are assumed to relate to the elevation of GABA levels in the brain. Here, we tested the hypothesis that a chronic elevation of brain GABA levels obtained by VGB treatment prevents the development of spontaneous seizures (i.e. epilepsy) following SE in rats. Self-sustained SE (SSSE) was induced by stimulating the lateral nucleus of the amygdala. Two days later, chronic VGB (75 mg/kg/day) or saline treatment was started via subcutaneous osmotic minipumps. The development of spontaneous seizures was monitored once a week (24 h at a time) using video-EEG recording. Rats were perfused for histology either at the end of the 10-week drug treatment, or later at the end of an 8-week drug-free follow-up period. Before perfusion for histology, spatial learning and memory perform was tested in the Morris water-maze. Spontaneous seizures were observed in 55% (6/11) of the saline-treated and 73% (8/11) of the VGB-treated rats during the 10-week treatment period. Seizure frequency, severity, and duration were similar in VGB-treated rats and controls during and after the drug-treatment period. VGB treatment did not decrease neuronal damage in various temporal lobe regions or mossy fiber sprouting. VGB treatment also did not attenuate spatial learning or memory impairments. These findings indicate that the augmentation of GABAergic neurotransmission by VGB does not prevent the development of epilepsy when treatment is started 2 days after SE.

Chronic administration of flumazenil increases life span and protects rats from age-related loss of cognitive functions: a benzodiazepine/GABAergic hypothesis of brain aging

Under barrier condition and with ad lib access to food and water, 20 Fischer-344 rats were chronically treated for 10 months with the benzodiazepine (BDZ) antagonist, flumazenil (FL; 4 mg/kg/day in drinking water acidified to pH = 3.0), beginning at the age of 13 months, while the group of 20 control age-matched rats received plain acidified water. The life span of the first 8 deceased rats treated with FL was significantly longer than that of the first 8 deceased rats in the age-matched control group. In tests for spontaneous ambulation and exploratory behavior in the Holeboard apparatus, conducted during the 3rd and the 8th month of treatment, the FL group, relative to controls, had significantly higher scores for the ambulation and exploratory behavior. In tests for unrewarded spontaneous alternation in the T maze, conducted at days 7, 39, 42, and 47 through 54 after drug withdrawal, i.e., at the age of 24-25 months, the FL-exposed group, compared to age-matched controls, showed a significantly higher percent of alternating choices, a behavior that was statistically comparable to that of the "young" 6-month-old controls. In the Radial Maze tests conducted 2 months after drug withdrawal, the FL group made significantly less "working memory" errors and "reference memory" errors, relative to the age-matched 25-month-old control group, a performance that was comparable to that of the young 7-month-old control group. In conclusion, chronic FL significantly protected rats from age-related loss of cognitive functions. It is postulated that the age-related alterations in brain function may be attributable to the negative metabolic/trophic influences of the "endogenous" benzodiazepine (BDZ) ligands and/or those ingested with food. A BDZ/GABAergic hypothesis of brain aging has been formulated which assumes that age-related and abnormally strong BDZ/GABAergic influences promote neurodegeneration by suppressing trophic functions of the aminergic and peptidergic neurons through opening of chloride channels in soma membrane and axon terminals, causing excessive hyperpolarizing and depolarizing inhibition, respectively. The review of human clinical and animal data indicates that FL has nootropic actions by enhancing vigilance cognitive and habituation processes.

Neocortical, hippocampal and septal parvalbumin- and somatostatin-containing neurons in young and aged rats: correlation with passive avoidance and water maze performance

Aged (26-month-old) rats were impaired compared with young (three-month-old) rats in passive avoidance and water maze tasks. In order to study whether changes in inhibitory circuits are involved in these age-related cognitive impairments, the number of two different subpopulations of GABAergic neurons, i.e. somatostatin- and parvalbumin-containing neurons, were counted in the hippocampal formation, septum and neocortex. We found that the number of parvalbumin-containing neurons was decreased in the entorhinal, somatosensory and motor cortex as well as in the medial septum and vertical limb of the diagonal band of Broca, but not in the hippocampus of aged rats. Somatostatin-containing neurons were affected in the somatosensory and motor cortex, and in the dorsolateral septum, but not in the hippocampus or in the entorhinal cortex. The decreased number of parvalbumin-containing neurons in the entorhinal cortex of the aged rats correlated with their performance deficits in passive avoidance and spatial learning. We propose that impaired functioning of the entorhinal cortex parvalbumin-containing inhibitory neurons may, to some extent, be responsible for the learning and memory defects found in aged rats.

Effects of an inhibitor of GABA-aminotransferase (gamma-vinyl-GABA) on the spatial navigation deficit induced by nicotinic blockade

1. The present study investigated whether stimulation of the GABA-ergic system affects spatial navigation (water-maze, WM) deficit induced by nicotinic blockade (mecamylamine). 2. The effects of various doses of gamma-vinyl-GABA (GVG: 50, 150 and 300 mg/kg) and mecamylamine (2.5 and 10 mg/kg) were examined alone and in combination. 3. GVG at the dose 150 mg/kg alone did not impair the performance of rats in the WM task. 4. Mecamylamine at the dose 2.5 and 10 mg/kg clearly impaired the performance of rats in WM task. 5. When the two drugs were co-administered, no interaction between mecamylamine and GVG was observed. 6. Combined nicotinic and muscarinic blockade did not interact as well with GVG administration. 7. Our results do not provide support for any interaction between cholinergic and GABA-ergic mechanisms.

Attenuation by the benzodiazepine receptor antagonist, ZK 93 426, of the deficit in spatial navigation induced by nucleus basalis lesions

The effects of the benzodiazepine receptor antagonist, beta-carboline ZK 93,426 treatment were studied both in NB-lesioned (ibotenic acid) and in unoperated Kuo-Wistar rats in a water maze task. The ZK 93,426 administered in the doses of 1 and 5 mg/kg, 30 min prior to the testing in a water maze apparatus, attenuated the NB lesion-induced spatial navigation deficit, although it had no effect on the performance of unoperated rats. The results suggest functional interactions between GABAergic system and ibotenic acid-induced lesion of the basal forebrain in rats.