Selective actions of certain neuroactive pregnanediols at the gamma-aminobutyric acid type A receptor complex in rat brain.
Certain endogenous pregnanediols (5 alpha-pregnan-3 alpha,20 alpha-diol and 5 beta-pregnan-3 alpha,20 beta-diol) were observed to have limited efficacy as allosteric modulators of t-[35S]butylbicyclophosphorothionate ([35S]TBPS) and [3H]flunitrazepam binding to sites on the gamma-aminobutyric acid (GABA)A receptor complex in rat brain. In contrast, 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) and 3 alpha-hydroxy-5 beta-pregnan-20-one (3 alpha,5 beta-P) have full efficacy. Moreover, 3 alpha,5 beta-P but not 3 alpha,5 alpha-P recognizes high (nanomolar) and low (micromolar) affinity neuroactive steroid sites in these allosteric modulatory assays. The concentration-response curve for 3 alpha,5 alpha-P modulation of [35S]TBPS binding was shifted rightward in the presence of these pregnanediols and GABA. The maximum shift produced by these pregnanediols never exceeded the concentration-response curve obtained with 3 alpha,5 alpha-P alone in the absence of GABA. Additionally, neither 5 alpha-pregnan-3 alpha,20 alpha-diol nor 5 beta-pregnan-3 alpha,20 beta-diol had any effect on the site recognized by 3 alpha,5 alpha-P in the absence of GABA. The difference in the affinities of the two apparent sites (29 nM versus 152 nM in the presence and absence of GABA, respectively) recognized by 3 alpha,5 alpha-P is only approximately 5-fold. In contrast, the difference between the high (30 nM) and low (7 microM) affinity sites discriminated by 3 alpha,5 beta-P is > 200-fold. Thus, the selective interaction between the high affinity site recognized by 3 alpha,5 beta-P and these pregnanediols can be clearly observed. A saturating concentration of 5 beta-pregnan-3 alpha,20 beta-diol selectively eliminated the high affinity component recognized by 3 alpha,5 beta-P, whereas 5 alpha-pregnan-3 alpha,20 alpha-diol did not completely abolish the high affinity site. 5 alpha-Pregnan-3 alpha,20 alpha-diol recognized only a portion of the high affinity sites discriminated by 3 alpha,5 beta-P, relative to 5 beta-pregnan-3 alpha,20 beta-diol, whereas the two pregnanediols recognized a similar population of sites mediating 3 alpha,5 alpha-P inhibition of [35S]TBPS binding. Collectively, these studies provide evidence that the limited efficacy of certain pregnanediols as allosteric modulators of [35S]TBPS binding may be explained in part by selectivity for the high affinity site recognized by 3 alpha,5 beta-P.(ABSTRACT TRUNCATED AT 400 WORDS)
McCauley
L D
LD
Department of Pharmacology, College of Medicine, University of California, Irvine 92717.
Liu
V
V
Chen
J S
JS
Hawkinson
J E
JE
Lan
N C
NC
Gee
K W
KW
D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D001643:Bridged Bicyclo Compounds / Q000378:metabolism; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002460:Cell Line; D005445:Flunitrazepam / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D008297:Male; D011276:Pregnanediol / Q000378:metabolism / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid; D005445:Flunitrazepam; C037476:tert-butylbicyclophosphorothionate; D011276:Pregnanediol