Lysine enhancement of 3H-flunitrazepam binding: interaction with GABA, pentobarbital and Ro5-3663

L-lysine is a barbiturate-like anticonvulsant and modulator of the benzodiazepine receptor

Our earlier observations showed that L-lysine enhanced the activity of diazepam against seizures induced by pentylenetetrazol (PTZ), and increased the affinity of benzodiazepine receptor binding in a manner additive to that caused by gamma-aminobutyric acid (GABA). The present paper provides additional evidence to show that L-lysine has central nervous system depressant-like characteristics. L-lysine enhanced [3H]flunitrazepam (FTZ) binding in brain membranes was dose-dependent and stimulated by chloride, bromide and iodide, but not fluoride. Enhancement of [3H]FTZ binding by L-lysine at a fixed concentration was increased by GABA but inhibited by pentobarbital between 10(-7) to 10(-3)M. While GABA enhancement of [3H]FTZ binding was inhibited by the GABA mimetics imidazole acetic acid and tetrahydroisoxazol pyridinol, the enhancement by pentobarbital and L-lysine of [3H]FTZ binding was dose-dependently increased by these two GABA mimetics. The above results suggest that L-lysine and pentobarbital acted at the same site of the GABA/benzodiazepine receptor complex which was different from the GABA binding site. The benzodiazepine receptor antagonist imidazodiazepine Ro15-1788 blocked the antiseizure activity of diazepam against PTZ. Similar to pentobarbital, the anti-PTZ effect of L-lysine was not blocked by Ro15-1788. Picrotoxinin and the GABA, receptor antagonist bicuculline partially inhibited L-lysine's enhancement of [3H]FTZ binding with the IC50s of 2 microM and 0.1 microM, respectively. The convulsant benzodiazepine Ro5-3663 dose-dependently inhibited the enhancement of [3H]FTZ binding by L-lysine. This article shows the basic amino acid L-lysine to have a central nervous system depressant characteristics with an anti-PTZ seizure activity and an enhancement of [3H]FTZ binding similar to that of barbiturates but different from GABA.

Chronic L-lysine develops anti-pentylenetetrazol tolerance and reduces synaptic GABAergic sensitivity

L-Lysine 10 mmol/kg given to mice for 1 to 10 days significantly increased clonic and tonic seizure latencies caused by 60 mg/kg pentylenetetrazol (PTZ). On day 1 the clonic and tonic seizure latencies were increased from 160.4 +/- 26.3 and 828.6 +/- 230.8 s to 286.1 +/- 103.3 and 982.3 +/- 98.6 s, respectively. Both clonic and tonic seizure latencies increased steadily with additional L-lysine treatment without significant change in survival rate. On day 10, the anticonvulsant effect reached its highest level with a block of tonic seizures and a survival rate of 100% without tolerance developing. Acute L-lysine treatment significantly increased the mean clonic latency from 85.8 +/- 5.24 to 128.2 +/- 9.0 s and the mean tonic seizure latency from 287.2 +/- 58.7 to 313.5 +/- 42.2 s with 80 mg/kg PTZ. On the day 10 of treatment, the anticonvulsant effect of L-lysine was highest, with a significant increase of 155 and 184% in clonic and tonic latencies over the control, respectively. After 15- and 20-day treatment, clonic and tonic seizures latencies and survival rate decreased, suggesting development of tolerance. Brain membranes from tolerant mice showed decreased enhancement by gamma-aminobutyric acid of specific [3H]flunitrazepam binding from 210 +/- 8 to 169 +/- 5% with EC50 values of 4.1 +/- 1.4 and 7.8 +/- 1.5 microM, respectively. Scatchard analysis of [3H] flunitrazepam binding showed no significant change of apparent binding affinity (KD) or binding density (Bmax) after chronic L-lysine exposure. L-Lysine enhanced the specific [35S]tert-butyl bicyclophosphorothionate (TBPS) binding in brain membranes dose dependently.(ABSTRACT TRUNCATED AT 250 WORDS)