Stuart C. Taylor

Acute and chronic effects of the benzodiazepine receptor ligand FG 7142: proconvulsant properties and kindling.

1 The effects of the acute and chronic administration of the β‐carboline benzodiazepine receptor ligand, FG 7142 were studied in mice. 2 On acute administration FG7142 (at doses between 10 and 40 mgkg−1) lowered seizure thresholds to infused pentylenetetrazol (PTZ) but showed an unusual dose‐response curve in that higher doses had less effect. The duration of action was considerably longer than that of other β‐carbolines, such as ethyl‐β‐carboline‐3‐carboxylate (βCCE). 3 During repeated administration, doses of FG7142 which were initially proconvulsant subsequently produced generalized seizures on average in 60% of animals after 12 once daily treatments. This seemed to be a form of chemical kindling. 4 The effects of different drug administration regimes were studied. Once daily dosage was shown to be the optimum for kindling production, and was therefore used for subsequent experiments. 5 Kindling lasted for at least one month after 12 single once daily doses of 40 mgkg−1 (FG 7142). 6 The administration of the benzodiazepine antagonist Ro 15–1788 concurrent with FG7142 prevented kindling. When Ro 15–1788 was given to kindled animals along with a challenge dose of FG7142, it prevented the expression of kindled seizures. These data show that kindling is mediated via the benzodiazepine receptor.

A benzodiazepine agonist and contragonist have hypothermic effects in rodents

The effects on body temperature of the benzodiazepine contragonists FG 7142 (N-methyl-beta-carboline-3-carboxamide) and DMCM (methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate), the benzodiazepine antagonist Ro 15-1788 (ethyl-8-fluoro-5,6-dihydro-5-methyl-7-oxo-4H-imidazo-(1,5-a)(1,4)benzod iazepin e -3-carboxylate) and the benzodiazepine agonist, flurazepam, were investigated in mice. Both of the contragonists and flurazepam reduced the body temperature. The antagonist Ro 15-1788 did not alter body temperature alone but significantly antagonised the effects of FG 7142 and of flurazepam. The effects of the latter two drugs on locomotor activity in animals in a familiar environment were also investigated. Flurazepam reduced the activity counts in these circumstances but FG 7142 did not cause a significant change, at doses which considerably reduced body temperature. The drug FG 7142 also reduced the body temperature in rats. The effects on body temperature of the benzodiazepine contragonists and flurazepam were unusual in that they were changes in the same direction, both antagonised by Ro 15-1788. However, they differed in that while the effects of flurazepam on locomotor activity might contribute to its hypothermic effect this did not appear to be the case for FG 7142.

Kindling and withdrawal changes at the benzodiazepine receptor.

Drugs acting at benzodiazepine receptors can have two types of pharmacological profile: benzodiazepine agonists are anxiolytic, anticonvulsant and sedative, whilst benzo diazepine inverse agonists cause anxiety and convulsions. In 1982 we showed that a benzo diazepine antagonist, Ro 15-1788, prevented the effects of both types of compound at doses without intrinsic activity in the tests used. We put forward the hypothesis that the benzo diazepine receptor complex could undergo two possible conformational changes, resulting in increases (benzodiazepine agonists) or decreases (benzodiazepine inverse agonists) in the effects of the inhibitory transmitter γ-aminobutyric acid (GABA). This concept has been widely accepted. We have now studied the effects of inverse agonists after chronic treatment with inverse agonists themselves and with benzodiazepine agonists, in order to see if tolerance develops (as seen with the agonists) or whether an opposite change occurs.

Chronic benzodiazepine treatment increases the effects of the inverse agonist FG7142.

A schedule of treatment with the benzodiazepine, flurazepam, in mice for 7 days caused a significant enhancement of the convulsive effects of the partial inverse agonist FG7142. Full convulsions were seen with FG7142 after the chronic administration of flurazepam, although this compound does not cause convulsions in normal mice of the strain used. The change appeared to be maximal at 24 hr after the last dose of flurazepam and lasted for up to a week. The chronic treatment with flurazepam caused tolerance to the effects of flurazepam, but the tolerance was of shorter duration than the increase in the effects of FG7142. When the benzodiazepine antagonist, Ro 15-1788, was given with the flurazepam, the incidence of convulsions induced by FG7142 was no longer significant. Repeated administration of midazolam also slightly increased the effects of FG7142. Single doses of flurazepam or midazolam did not significantly alter the effects of FG7142, although some convulsions were seen.

The effects of drugs acting at the GABAA-receptor/ionophore after chemical kindling with the benzodiazepine receptor ligand FG 7142

1 Repeated administration of the β‐carboline benzodiazepine receptor ligand FG 7142 produces sensitization to its effects so that full seizures develop (chemical kindling); initially it is only proconvulsant. The present study investigated alterations in the function of drugs which act at the different sites at the γ‐aminobutyric acid (GABA) benzodiazepine receptor complex, after repeated administration of FG 7142. 2 In FG 7142 kindled mice decreased anticonvulsant and hypothermic effects of the GABA agonist muscimol were observed. The hypothermic effects of the GABA agonist progabide were reduced. In contrast a small increase in the hypothermic effect of pentobarbitone was seen. 3 The convulsant effects of bicuculline and picrotoxin were unaltered when they were given intravenously but marginally increased when they were given by the intraperitoneal route. No changes were seen in the hypothermic effects of these drugs. 4 No significant changes were seen in the convulsant or hypothermic effects of pentylenetetrazol. 5 These results suggest that kindling with FG 7142 may alter GABA receptor function.

A single dose of FG 7142 causes long-term increases in mouse cortical β-adrenoceptors

There is evidence that central monoamines are involved in the actions of benzodiazepines. We have investigated the effects of a single dose of the benzodiazepine partial inverse agonist, FG 7142, on radioligand binding to alpha 2- and beta-adrenoceptors in mouse cerebral cortex. We found that seven days after a single injection of FG 7142 there was a large increase in the density of beta-adrenoceptors. This rise was not detectable either 15-30 min, or 24 h after the injection and no statistically significant changes in alpha 2-adrenoceptor binding were found at any of these times. Administration of the benzodiazepine antagonist Ro 15-1788 at the same time as FG 7142 prevented the rise in beta-adrenoceptor density. We discuss the possibility that the beta-adrenoceptor upregulation is related to the behavioural effects of FG 7142.

Kindling with the β-Carboline FG7142 Suggests Separation between Changes in Seizure Threshold and Anxiety-Related Behaviour

The aim of this paper was to determine whether the prolonged decrease in seizure threshold produced by chemical kindling was accompanied by behavioural changes in tests of anxiety and aggression. The responses of rats in five tests were examined after chronic treatment with the benzodiazepine inverse agonist FG7142. This treatment caused chemical kindling, so that the originally proconvulsant drug caused full seizures. This effect is very long-lasting, and our previous work with mice had suggested that it might be accompanied by an increase in anxiety-related behavior. In the present work no significant differences were found between the behaviour of FG7142-kindled rats and vehicle-treated controls in social interaction test, elevated plus maze, or the Vogel conflict test of anxiety or in tests of home cage aggression or startle responses. The results therefore show that prolonged changes in seizure threshold can occur without alterations in the apparent level of anxiety.

The benzodiazepine antagonist, Ro 15-1788 does not decrease ethanol withdrawal convulsions in rats.

The effects of the benzodiazepine antagonist, Ro 15-1788 on ethanol withdrawal convulsions were investigated in rats. The study originated from recent reports of benzodiazepine binding activity in urine of alcoholics during withdrawal. No alleviation of convulsions was found with Ro 15-1788. This suggested that this component of the withdrawal syndrome is not due to endogenous production of a benzodiazepine inverse agonist (contragonist), as Ro 15-1788 prevents the action of this type of compound.

Investigating benzodiazepine receptor function in vivo using an intravenous infusion of DMCM.

A method for measuring seizure thresholds using an intravenous infusion of a convulsant beta-carboline benzodiazepine receptor ligand (DMCM) is reported. Seizure thresholds to DMCM are elevated by the benzodiazepine receptor agonist, flurazepam and antagonist, Ro 15-1788, and the proconvulsant beta-carboline, FG 7142. Various other anticonvulsant also antagonise DMCM seizures. Selectivity for the benzodiazepine/GABA receptor complex is demonstrated since bicuculline and pentylenetetrazol lowered thresholds whereas strychnine and N-methyl DL-aspartate did not.

Effects of chronic treatment with benzodiazepine receptor ligands on cortical adrenoceptors.

We report the effects of kindling with the benzodiazepine partial inverse agonist, FG 7142 on adrenoceptor binding in mouse cerebral cortex. Twelve once-daily injections of FG 7142 caused a statistically significant increase in the density of both alpha 2- and beta-adrenoceptor binding sites, seven days after the last injection. Despite the marked effects of FG 7142 on adrenoceptors, there were no changes in either alpha 2- or beta-adrenoceptor binding after prolonged treatment with the benzodiazepine, flurazepam.