Phenyl-substituted normethadones: synthesis and pharmacology

The new generation of GABA enhancers. Potential in the treatment of epilepsy

gamma-Aminobutyric acid (GABA) is considered to be the major inhibitory neurotransmitter in the brain and loss of GABA inhibition has been clearly implicated in epileptogenesis. GABA interacts with 3 types of receptor: GABAA, GABAB and GABAC. The GABAA receptor has provided an excellent target for the development of drugs with an anticonvulsant action. Some clinically useful anticonvulsants, such as the benzodiazepines and barbiturates and possibly valproic acid (sodium valproate), act at this receptor. In recent years 4 new anticonvulsants, namely vigabatrin, tiagabine, gabapentin and topiramate, with a mechanism of action considered to be primarily via an effect on GABA, have been licensed. Vigabatrin elevates brain GABA levels by inhibiting the enzyme GABA transaminase which is responsible for intracellular GABA catabolism. In contrast, tiagabine elevates synaptic GABA levels by inhibiting the GABA uptake transporter, GAT1, and preventing the uptake of GABA into neurons and glia. Gabapentin, a cyclic analogue of GABA, acts by enhancing GABA synthesis and also by decreasing neuronal calcium influx via a specific subunit of voltage-dependent calcium channels. Topiramate acts, in part, via an action on a novel site of the GABAA receptor. Although these drugs are useful in some patients, overall, they have proven to be disappointing as they have had little impact on the prognosis of patients with intractable epilepsy. Despite this, additional GABA enhancing anticonvulsants are presently under development. Ganaxolone, retigabine and pregabalin may prove to have a more advantageous therapeutic profile than the presently licensed GABA enhancing drugs. This anticipation is based on 2 characteristics. First, they act by hitherto unique mechanisms of action in enhancing GABA-induced neuronal inhibition. Secondly, they act on additional antiepileptogenic mechanisms. Finally, CGP 36742, a GABAB receptor antagonist, may prove to be particularly useful in the management of primary generalised absence seizures. The exact impact of these new GABA-enhancing drugs in the treatment of epilepsy will have to await their licensing and a period of postmarketing surveillance. As to clarification of their role in the management of epilepsy, this will have to await further clinical trials, particularly direct comparative trials with other anticonvulsants.

Naturally occurring antinociceptive substances from plants

Despite the progress that has occurred in recent years in the development of therapy, there is still a need for effective and potent analgesics, especially for the treatment of chronic pain. One of the most important analgesic drugs employed in clinical practice today continues to be the alkaloid morphine. In this review, emphasis will be given to the important contribution and the history of Papaver somniferum, Salix species, Capsicum species and Cannabis sativa in the development of new analgesics and their importance in the understanding of the complex pathways related to electrophysiological and molecular mechanisms associated with pain transmission. Recently discovered antinociceptive substances include alkaloids, terpenoids and flavonoid. Plant-derived substances have, and will certainly continue to have, a relevant place in the process of drug discovery, particularly in the development of new analgesic drugs.

Central benzodiazepine involvement in clonidine cardiovascular actions

It is well known that the GABAergic and noradrenergic systems play an important role in blood pressure and heart rate regulation. Benzodiazepines and beta-carbolines, respectively, increase or decrease the probability of chloride-channel opening induced by GABA. The aim of this study was to determine, in conscious rats, the interaction existing between the central alpha2-adrenoceptor stimulation induced by clonidine and the facilitation or impairment of benzodiazepine receptor activity through the administration of either diazepam, a benzodiazepine receptor agonist, or methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), an inverse benzodiazepine agonist. Clonidine (5-10 µg, intracerebroventricularly) reduced heart rate and increased mean blood pressure by activation of central alpha2-adrenoceptors. Diazepam (2 mg/kg, intravenously (i.v.)) induced an increase in heart rate, while DMCM (0.3 mg/kg, i.v.) elicited a bradycardic effect. The bradycardic effects induced by both clonidine and DMCM were antagonized by the prior administration of methylatropine (1.5 mg/kg, i.v.). DMCM (0.3 mg/kg, i.v.) prevented the clonidine effects on heart rate and mean blood pressure, while diazepam (2 mg/kg, i.v.) failed to modify these effects. Our results suggest that the bradycardic effects of clonidine are mediated by a vagal stimulation and are related to the activation of a GABAergic pathway.Key words: blood pressure, clonidine, diazepam, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), heart rate.