Convulsant

Abstract: Certain steroids are potent barbiturate-like modulators of the gamma-aminobutyric acidA (GABA) receptor-chloride ionophore complex in rat brain membranes. At nanomolar to low micromolar concentrations, these steroids stimulate [3H]flunitrazepam and [3H] muscimol binding and displace the convulsant [35S]t-butylbicyclophosphorothionate from its binding site in an allosteric manner, in addition to enhancing Cl- conductance responses to GABA recorded in cultured rat hippocampal and spinal neurons. A stringent structure-activity relationship exists for these interactions of steroids with the GABAA receptor complex. Comparison of the structure-activity relationship data obtained in this study with those for steroid-induced general anesthesia strongly suggests that steroidal anesthesia may result from the interaction between steroids and the GABAA receptor. The essential features of the active structures are a 5 alpha or 5 beta-reduced pregnane skeleton with a hydroxyl at C3 in the alpha-position and a ketone group at C20. These features are all present in some naturally occurring steroids, including metabolites of deoxycorticosterone and progesterone, that show potent activity at the GABAA receptor complex. Two of the compounds investigated are known to be formed in vivo as reduced metabolites of endogenous steroid hormones: 5 alpha-pregnane-3 alpha -ol-20-one and 5 alpha-pregnane-3 alpha,21-diol-20-one, which are derived from progesterone and deoxycorticosterone, respectively. These two steroids produce a striking prolongation of GABA-mediated inhibitory postsynaptic currents recorded at synapses between rat hippocampal neurons in culture and could conceivably regulate GABA-mediated inhibition under some physiologic and pathologic conditions.

Abstract: The gamma-aminobutyric acid (GABA)-benzodiazepine receptor complex, which is composed of distinct proteins embedded in the neuronal plasma membrane, is important for several effects of benzodiazepines, including protection afforded against convulsions During structural modification of ethyl beta-carboline-3-carboxylate an agent was discovered which has high affinity for brain benzodiazepine receptors but which is a potent convulsant Also in contrast to benzodiazepines, this type of benzodiazepine receptor ligand favors benzodiazepine receptors in the non-GABA-stimulated conformation, which may explain the convulsive properties

Abstract: Pentylenetetrazole (PTZ) is a central nervous system convulsant that is thought, based on binding studies, to act at the picrotoxin (PTX) site of the gamma-aminobutyric acid type A (GABA(A)) receptor. In the present study, we have investigated the mechanism and site of action of PTZ in recombinant GABA(A) receptors. In rat alpha 1 beta 2 gamma 2 receptors, PTZ inhibited GABA-activated Cl(-) current in a concentration-dependent, voltage-independent manner, with an IC(50) of 0.62 +/- 0.13 mM. The mechanism of inhibition appeared competitive with respect to GABA in both rat and human alpha 1 beta 2 gamma 2 receptors. Varying subunit configuration (change or lack of alpha subunit isoform or lack of gamma 2 subunit) had modest effects on PTZ-induced inhibition, as evidenced by comparable IC(50) values (0.6-2.2 mM) in all receptor configurations tested. This contrasts with PTX and other PTX-site ligands, which have greater affinity in receptors lacking an alpha subunit. Using a one-site model for PTZ interaction with alpha 1 beta 2 gamma 2 receptors, the association rate (k(+1)) was found to be 1.14 x 10(3) M(-1) s(-1) and the dissociation rate (k(-1)) was 0.476 s(-1), producing a functional k(d) of 0.418 mM. PTZ could only gain access to its binding site extracellularly. Single-channel recordings demonstrated that PTZ decreased open probability by increasing the duration of closed states but had no effect on single-channel conductance or open state duration. alpha-Isopropyl-alpha-methyl-gamma-butyrolactone, a compound known to antagonize effects of PTX, also diminished the effects of PTZ. Taken together, our results indicate that pentylenetetrazole and picrotoxin interact with overlapping but distinct domains of the GABA(A) receptor.