Effect of alpha 2-adrenergic drugs dexmedetomidine and atipamezole on extracellular amino acid levels in vivo

The Anticonvulsant Effects of Alpha-2 Adrenoceptor Agonist Dexmedetomidine on Pentylenetetrazole-Induced Seizures in Rats

Alpha2-adrenoreceptor (α₂-AR) is a noradrenergic receptor that is frequently studied for modulation of seizure activity. However, the precise role of this receptor agonists in regulating seizure activity is still unclear. Our aim in this study was to investigate the effects of α₂-AR agonist dexmedetomidine (DEX) and atipamezole (α₂-AR antagonist, ATI) on seziures in rats. In the study, 32 adult male Wistar Albino rats (weighing 220-260 g) were used. To induce seizures in rats, pentylenetetrazole (PTZ, 35 mg/kg) was injected intraperitoneally (i.p.) and seizure stages were determined according to the Racine scale. After induction of seizures, DEX (0.1 mg/kg, i.p.) and ATI (1 mg/kg, i.p.) were administered to rats and their effects determined on seizures. GABA levels of the brain hippocampal tissue sample were measured using an ELISA kit and c-Fos positive cells of the dentate gyrus and hippocampal regions were quantitatively analyzed with Image J software. The results showed that DEX decreased the seizure stages according to the Racine scale, significantly prolonged the onset time of first myoclonic jerk (FMJ) and reduced the number of spikes and percentage seizure duration (p < 0.05). In contrast, ATI increased the seizure stage, the number of spikes and percentage seizure duration. The hippocampal GABA level was significantly decreased in rats with only PTZ injection (p < 0.05). In addition, DEX reduced the number of c-Fos positive cells in dentate gyrus and the hippocampal CA1 and CA3 regions. In conclusion, our findings showed that α2-AR agonist DEX had a reducing activity on PTZ-induced seizure, while α2-AR antagonist ATI facilitated seizure formation.

Molecular targets and mechanism of action of dexmedetomidine in treatment of ischemia/reperfusion injury

Dexmedetomidine (DEX), a highly specific α2-adrenergic agonist, which exhibits anaesthetic-sparing, analgesia and sympatholytic properties. DEX modulates gene expression, channel activation, transmitter release, inflammatory processes and apoptotic and necrotic cell death. It has also been demonstrated to have protective effects in a variety of animal models of ischemia/reperfusion (I/R) injury, including the intestine, myocardial, renal, lung, cerebral and liver. The broad spectrum of biological activities associated with DEX continues to expand, and its diverse effects suggest that it may offer a novel therapeutic approach for the treatment of human diseases with I/R involvement.

Decreased GABA-A binding on FMZ-PET in succinic semialdehyde dehydrogenase deficiency

OBJECTIVE

Succinic semialdehyde dehydrogenase (SSADH) deficiency is an autosomal recessive disorder of GABA metabolism characterized by elevated levels of GABA and gamma-hydroxybutyric acid. Clinical findings include intellectual impairment, hypotonia, hyporeflexia, hallucinations, autistic behaviors, and seizures. Autoradiographic labeling and slice electrophysiology studies in the murine model demonstrate use-dependent downregulation of GABA(A) receptors. We studied GABA(A) receptor activity in human SSADH deficiency utilizing [(11)C]-flumazenil (FMZ)-PET.

METHODS

FMZ binding was measured in 7 patients, 10 unaffected parents, and 8 healthy controls. Data analysis was performed using a reference region compartmental model, with time-activity curve from pons as the input function. Relative parametric binding potential (BP(ND)) was derived, with MRI-based pixel by pixel partial volume correction, in regions of interest drawn on coregistered MRI.

RESULTS

In amygdala, hippocampus, cerebellar vermis, frontal, parietal, and occipital cortex, patients with SSADH deficiency had significant reductions in FMZ BP(ND) compared to parents and controls. Mean cortical values were 6.96 +/- 0.79 (controls), 6.89 +/- 0.71 (parents), and 4.88 +/- 0.77 (patients) (F ratio 16.1; p < 0.001). There were no differences between controls and parents in any cortical region.

CONCLUSIONS

Succinic semialdehyde dehydrogenase (SSADH) deficient patients show widespread reduction in BZPR binding on [(11)C]-flumazenil-PET. Our results suggest that high endogenous brain GABA levels in SSADH deficiency downregulate GABA(A)-BZPR binding site availability. This finding suggests a potential mechanism for neurologic dysfunction in a serious neurodevelopmental disorder, and suggests that PET may be useful to translate studies in animal models to human disease.

GABA(A)-mediated toxicity of hippocampal neurons in vitro

In the present study, we examined whether the elevation of GABA by gamma-vinyl-GABA protects cultured rat fetal hippocampal neurons against toxicity induced by a 20-min incubation with 100 microM L-glutamate. Neither a 24-h pretreatment nor posttreatment with gamma-vinyl-GABA (100 microM) had any neuroprotective effects, as determined by counting microtubule-associated protein-2 positive cells and lactate dehydrogenase assay 24 h after the glutamate treatment. Unexpectedly, gamma-vinyl-GABA alone induced a 20% loss of microtubule-associated protein-2-positive cells in a culture that was grown in medium containing 25 mM KCl. The toxic effect of gamma-vinyl-GABA was mimicked by a 24-h treatment with GABA (100 microM) and the GABA(A) receptor agonist, muscimol (10 microM), but not the GABA(B) receptor agonist, baclofen (10 microM). The GABA(A) receptor antagonist, bicuculline (10 microM), protected against gamma-vinyl-GABA and GABA-evoked toxicity. Neither gamma-vinyl-GABA nor GABA was toxic in culture medium containing 15 mM KCl. These data indicate that, under depolarizing conditions, an increased GABA level is toxic for a subpopulation of developing hippocampal neurons in vitro. The effect is GABA(A) receptor-mediated. These data provide a new view for understanding neurodegenerative processes, and raise a question of the safety of therapies aimed at increasing GABA concentration following brain insults, especially in immature brains.

Effects of vigabatrin treatment on status epilepticus-induced neuronal damage and mossy fiber sprouting in the rat hippocampus

Selective neuronal damage and mossy fiber sprouting may underlie epileptogenesis and spontaneous seizure generation in the epileptic hippocampus. It may be beneficial to prevent their development after cerebral insults that are known to be associated with a high risk of epilepsy later in life in humans. In the present study, we investigated whether chronic treatment with an anticonvulsant, vigabatrin (gamma-vinyl GABA), would prevent the damage to hilar neurons and the development of mossy fiber sprouting. Vigabatrin treatment was started either 1 h, or 2 or 7 days after the beginning of kainic acid-induced (9 mg/kg, i.p.) status epilepticus and continued via subcutaneous osmotic minipumps for 2 months (75 mg/kg per day). Thereafter, rats were perfused for histological analyses. One series of horizontal sections was stained with thionine to estimate the total number of hilar neurons by unbiased stereology. One series was prepared for somatostatin immunohistochemistry and another for Timm histochemistry to detect mossy fiber sprouting. Our data show that vigabatrin treatment did not prevent the decrease in the total number of hilar cells, nor the decrease in hilar somatostatin-immunoreactive (SOM-ir) neurons when SOM-ir neuronal numbers were averaged from all septotemporal levels. However, when vigabatrin was administered 2 days after the onset of status epilepticus, we found a mild neuroprotective effect on SOM-ir neurons in the septal end of the hippocampus (92% SOM-ir neurons remaining; P < 0.05 compared to the vehicle group). Vigabatrin did not prevent mossy fiber sprouting regardless of when treatment was started. Rather, sprouting actually increased in the septal end of the hippocampus when vigabatrin treatment began 1 h after the onset of status epilepticus (P < 0.05 compared to the vehicle group). Our data show that chronic elevation of brain GABA levels after status epilepticus does not have any substantial effects on neuronal loss or mossy fiber sprouting in the rat hippocampus.

Mivazerol inhibits intrathecal release of glutamate evoked by halothane withdrawal in rats

BACKGROUND

Mivazerol is a new and selective alpha 2-adrenergic receptor agonist devoid of hypotensive effects (1, 2). Previous studies have demonstrated that mivazerol prevents hemodynamic instability during emergence from halothane anesthesia in rats (3). The present study was to determine whether glutamate and aspartate are involved in this action of mivazerol, at the second to third thoracic segments (T2-T3) of the spinal cord.

METHODS

In vivo microdialysis in combination with high-performance liquid chromatography (HPLC) was employed in the study. Blood pressure (BP) and heart rate (HR) were recorded along with intrathecal (i.t.) microdialysis perfusion.

RESULTS

BP, HR and i.t. release of glutamate (GLU, pmol/microliter) were stable in the rats under 1.1% halothane anesthesia. However, halothane withdrawal immediately increased BP, HR, and i.t. release of GLU, and remained elevated for at least 2 h after withdrawal of halothane. Thirty minutes prior to halothane withdrawal, intravenous (i.v.) infusion of mivazerol (15 micrograms.kg-1.h-1) almost completely prevented the increases in HR (delta 18 +/- 7 vs delta 79 +/- 7 beats/min), and in the i.t. release of GLU (delta 10.3 +/- 3.7 vs delta 30.6 +/- 5.9; 112% vs 167%). Local i.t. microinjection of mivazerol (2.5 micrograms/kg) 2 min prior to withdrawal of halothane also blocked the HR responses, as well as on the i.t. release of GLU following halothane withdrawal.

CONCLUSION

The present study demonstrates that emergence from halothane anesthesia increases i.t. release of GLU, and that mivazerol has an inhibitory effect on the above, through its direct action on the spinal cord.

High levels of cerebrospinal fluid glutamate in Rett syndrome

Rett syndrome is a neurodevelopmental disease affecting girls. The cause is not known. Roles for trophic factors and excitatory neurotransmitters have been postulated. To study the significance of excitatory amino acids in Rett syndrome, we determined glutamate and aspartate concentrations in the cerebrospinal fluid from 11 girls with Rett syndrome (age 8 years 4 months +/- 5.7 years, mean +/- SD) and 11 controls (age 7 +/- 4.2 years). In the patients with Rett syndrome, the mean of cerebrospinal fluid glutamate concentration was 355.2 nmol/L (SD +/- 109.2 nmol/L). In the controls it was 203.9 nmol/L (SD +/- 55.5 nmol/L). In Rett syndrome cases, cerebrospinal fluid glutamate concentrations were significantly higher (P = 0.0006) than in the controls. In the Rett syndrome group, the mean cerebrospinal fluid aspartate concentration was 119.4 nmol/L (SD +/- 43.5 nmol/L). In the control group, it was 90.9 nmol/L (SD +/- 20.9 nmol/L). The difference between the cerebrospinal fluid aspartate values was not significant. Glutamate may therefore play an important role in the primary pathogenesis in Rett syndrome. Further investigations are needed, with recognition of possible actions of neuronal growth factors and excitatory neurotransmitters in the damage mechanisms of Rett syndrome.

Tiagabine prevents seizures, neuronal damage and memory impairment in experimental status epilepticus

A novel antiepileptic drug, tiagabine ((R)-N-[4,4-di-(3-methylthien-2-yl) but-3-enyl] nipecotic acid hydrochloride), was studied in rats in order to determine its efficacy in preventing seizures, seizure-induced neuronal damage and impairment of spatial memory in the perforant pathway stimulation model of status epilepticus. In pilot experiments, administration of tiagabine (50, 100 or 200 mg/kg/day) with subcutaneously implanted Alzet osmotic pumps led to a dose-dependent increase in tiagabine concentrations in the serum and brain. Two days of tiagabine treatment at a dose range of 50-200 mg/kg/day did not change the levels of gamma-aminobutyric acid (GABA), glutamate or aspartate in cisternal cerebrospinal fluid (CSF) compared to the controls. In the pentylenetetrazol test, the maximal anticonvulsive effect of tiagabine administered via osmotic pumps was achieved already with a dose of 50 mg/kg/day. In the perforant pathway model of status epilepticus, subchronic treatment with tiagabine (Alzet pumps, 50 mg/kg/day) completely prevented the appearance of generalized clonic seizures during stimulation (P < 0.001). In the same rats, tiagabine treatment reduced the loss of pyramidal cells in the CA3c and CA1 fields of the hippocampus (P < 0.05) but not the loss of somatostatin immunoreactive neurons in the hilus. Two weeks after perforant pathway stimulation, the tiagabine-treated rats performed better in the Morris water-maze test than the vehicle-treated rats did (P < 0.001). Our results show that tiagabine treatment reduces the severity of seizures in the perforant pathway stimulation model of status epilepticus. Possibly associated with the reduction in seizure number and severity, tiagabine treatment also reduced seizure-induced damage to pyramidal cells in the hippocampus as well as the impairment of the spatial memory associated with hippocampal damage.