Negative GABA(A) modulators attenuate the discriminative stimulus effects of benzodiazepines and the neuroactive steroid pregnanolone in rhesus monkeys.
BACKGROUND
Negative GABA(A) modulators (i.e., inverse agonists) might be useful for identifying mechanisms at the GABA(A) receptor complex that mediate the effects of positive GABA(A) modulators, especially those for which there are no available competitive antagonists.


OBJECTIVE
Drug discrimination was used to examine antagonism of a 5-beta neuroactive steroid (pregnanolone) and a benzodiazepine (midazolam) by several negative GABA(A) modulators in rhesus monkeys.


METHODS
One group of monkeys (n=5) received 5.6 mg kg(-1) day(-1) of diazepam (p.o.) and discriminated the benzodiazepine antagonist flumazenil (0.1 or 0.32 mg/kg s.c.); another group of monkeys (n=5) discriminated the benzodiazepine midazolam (0.32 mg/kg s.c.).


RESULTS
In diazepam-treated monkeys, negative GABA(A) modulators with increasing efficacy, including Ro 15-4513, ethyl beta-carboline-3-carboxylate (beta-CCE), methyl beta-carboline-3-carboxylate (beta-CCM) and methyl-6,7-dimethoxyl-4-ethyl-beta-carboline-3-carboxylate (DMCM), substituted for flumazenil. In monkeys discriminating midazolam, pregnanolone occasioned high levels of midazolam-lever responding, and these effects were attenuated by beta-CCE and beta-CCM, but not by flumazenil, Ro 15-4513, or DMCM. The midazolam discriminative stimulus also was attenuated by beta-CCM and DMCM; Schild analysis was consistent with a simple competitive interaction between midazolam and beta-CCM but not between midazolam and DMCM.


CONCLUSIONS
Negative modulators are qualitatively similar to neutral modulators in diazepam-treated animals; however, interactions between negative modulators and midazolam suggest that different receptors mediate the effects of some (DMCM) and not other (beta-CCM) negative modulators. Negative modulators at benzodiazepine sites exert efficacy-dependent antagonism of positive modulators at neuroactive steroid sites. Without competitive antagonists at neuroactive steroid or barbiturate sites, negative modulators could prove useful for examining the mechanism of action of different classes of positive GABA(A) modulator.
Department of Pharmacology, The University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA.
Lance R
LR
McMahon
Charles P
CP
France
D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology*; D002243:Carbolines / Q000494:pharmacology; D004193:Discrimination Learning / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005260:Female; D005442:Flumazenil / Q000494:pharmacology; D008253:Macaca mulatta; D008297:Male; D008874:Midazolam / Q000494:pharmacology; D011280:Pregnanolone / Q000494:pharmacology*; D011963:Receptors, GABA-A / Q000187:drug effects*; D012702:Serotonin Antagonists / Q000494:pharmacology*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology*
D002243:Carbolines; D011963:Receptors, GABA-A; D012702:Serotonin Antagonists; D017366:Serotonin Receptor Agonists; D001569:Benzodiazepines; C034818:methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate; D005442:Flumazenil; D011280:Pregnanolone; C036150:beta-carboline-3-carboxylic acid methyl ester; D008874:Midazolam