Protective effects of diazepam and valproate on beta-vinyllactic acid-induced seizures

How chronically administered valproate increases chlordiazepoxide transfer through the blood-brain barrier

Sodium valproate (VPA) is a drug widely used in the treatment of epileptics often in association with benzodiazepines. Recent animal studies have shown that the addition of valproate increases diazepam levels in the cortex and the cerebellum (Hariton et al, 1985). The aim of our study was to determine the effect of VPA on the transfer of benzodiazepines through the blood-brain barrier. They were investigated using the intracarotid injection technique in rats as described by Oldendorf (1971). Our results show that the 14C-chlordiazepoxide brain extraction is significantly higher in rats on prolonged valproate treatment than in controls. With regard to plasma protein binding effects on chlordiazepoxide transport, our data indicate that a fraction of the protein-bound chlordiazepoxide could transfer from the intracapillary space to the brain tissue space because of enhanced drug dissociation from albumin in the brain microcirculation (Kd in vitro = 74.1 microM; Kd in vivo = 793.7 microM). Two distinct mechanisms can be deduced from this study: 1) chlordiazepoxide is displaced from HSA by valproate, 2) in addition, this fatty acid could increase drug permeation through the blood brain barrier (PS/F (chlordiazepoxide) = 0.60 in controls, PS/F (chlordiazepoxide) = 0.97 in treated rats). On the contrary, the washout of the benzodiazepine from the rat brain does not seem to be modified by the addition of valproate.

Evaluation of the anticonvulsant and biochemical activity of CGS 8216 and CGS 9896 in animal models

CGS 8216, a benzodiazepine-receptor ligand with inverse agonistic properties, and CGS 9896, which possesses partial agonistic or mixed agonist-antagonist properties were compared in a number of epilepsy models. The effect of CGS 9896 on the decrease in GABA levels induced by isoniazid was also investigated. CGS 9896 inhibited the kindling process in rats in that it delayed the development of overt seizures and the increase in the duration of afterdischarges. In a genetic rat model characterized by absence-like EEG patterns, CGS 9896 dose-dependently suppressed these spontaneously occurring discharges, while CGS 8216 had no effect. However, CGS 8216 antagonized the anticonvulsant action of CGS 9896. CGS 9896 protected mice against seizures induced by beta-vinyllactic acid, whereas CGS 8216 shortened the latency period before convulsions occurred. CGS 9896 retarded the onset of convulsive fits caused by isoniazid without preventing the decrease in GABA levels produced by that drug. These results confirm the anticonvulsant activity of CGS 9896 and demonstrate the inverse agonistic activity of CGS 8216. The profile of CGS 9896 in the above tests suggests that it might be an effective anticonvulsant, primarily in absence-type seizures.

Dose pipecolic acid interact with the central GABA-ergic system?

Several previous studies have suggested a strong GABA-mimetic action of the endogenous brain imino acid, L-pipecolic acid (L-PA). In the present study, these observations were evaluated using electrophysiological and neurochemical methods. In contrast to published data our electrophysiological studies on rat cortical neurones in situ showed only a weak, but bicuculline-sensitive depressant action of L-PA on cortical neurones. Furthermore, L-PA proved to have no affinity for any of the three components of the GABA-benzodiazepine-chloride channel receptor complex. However, using a modification of published methods a weak affinity for the GABA-B receptor site was demonstrated (IC50 = 1.8 X 10(-3) M). L-PA showed no anticonvulsive activity in several tests; in particular, it did not protect mice from seizures induced by inhibition of L-glutamate-1-decarboxylase (EC 4.1.1.15: GAD). L-PA had a very weak action on brain GABA levels of mice, and did not modify the rate of GABA synthesis. In conclusion, these results are not compatible with a strong in vivo interaction between L-PA and GABA-mediated inhibitory transmission.

The specific protective effect of diazepam and valproate against isoniazid-induced seizures is not correlated with increased GABA levels

Amino acid concentrations were measured in the cortex, cerebellum and hippocampus of the mouse brain before and during seizures induced by isoniazid (250 mg/kg i.p.), an inhibitor of L-glutamate-1-decarboxylase (EC 4.1.1.15: GAD). Valproate sodium and diazepam dose-dependently delay the onset of convulsive fits caused by isoniazid. However, neither diazepam nor valproate prevented the decrease in GABA concentrations produced by isoniazid alone. Also, these antiepileptic drugs did not modify the rate of GABA depletion elicited by isoniazid. These results, observed in four different brain structures, strengthen those first obtained with beta-vinyllactic acid, another inhibitor of GAD.