Neuroactive steroid stereospecificity of ethanol-like discriminative stimulus effects in monkeys

Abstract

Positive modulation of GABA(A) and antagonism of N-methyl-D-aspartate receptors mediate the discriminative stimulus effects of ethanol. Endogenous neuroactive steroids produce effects similar to ethanol, suggesting that these steroids may modulate ethanol addiction. The four isomers of the functional esters at C-3 of the 3-hydroxy metabolites of 4-pregnene-3,20-dione (progesterone) [allopregnanolone (3alpha,5alpha-P), pregnanolone (3alpha,5beta-P), epiallopregnanolone (3beta,5alpha-P), and epipregnanolone (3beta,5beta-P)], a synthetic analog of steroids modified by endogenous sulfation [pregnanolone hemisuccinate (3alpha,5beta-P HS)], and a structurally similar, adrenally derived steroid [3alpha-hydroxy-5-androstan-17-one (3alpha,5alpha-A, androsterone)] were assessed for ethanol-like discriminative stimulus effects at 30 or 60 min after administration in male (n = 9) and female (n = 8) cynomolgus monkeys (Macaca fascicularis) trained to discriminate 1.0 or 2.0 g/kg ethanol (i.g.) with a 30-min pretreatment interval. The 3alpha-hydroxysteroids completely substituted for ethanol (80% of cases), whereas the 3beta-hydroxysteroids and 3alpha,5beta-P HS rarely substituted for ethanol (6% of cases). There were no sex differences. Compared with monkeys trained to discriminate 2.0 g/kg ethanol, 3alpha,5beta-P and 3alpha,5alpha-A substituted more potently in monkeys trained to discriminate 1.0 g/kg ethanol. Compared with the 5beta-reduced isomer (3alpha,5beta-P), the 5alpha isomer of pregnanolone (3alpha,5alpha-P) substituted for ethanol with 3 to 40-fold greater potency but was least efficacious in female monkeys trained to discriminate 2.0 g/kg ethanol. The data suggest that the discriminative stimulus effects of lower doses (1.0 g/kg) of ethanol are mediated to a greater extent by 3alpha,5beta-P- and 3alpha,5alpha-A-sensitive receptors compared with higher doses (2.0 g/kg). Furthermore, the discriminative stimulus effects of ethanol appear to be mediated by activity at binding sites that are particularly sensitive to 3alpha,5alpha-P.

Fig. 1

Structures of pregnanolone isomers. The –OH group is axial for the 5β and equatorial for the 5α steroids. Shown below is the mean (± SEM) ethanol-appropriate responding (1.0 and 2.0 g/kg training groups shown separately) produced by the four neuroactive steroid isomers collapsed across the 30- and 60-min pre-treatment intervals. Means include data from both male and female cynomolgus monkeys trained to discriminate 1.0 or 2.0 g/kg ethanol (i.g.) from water. Each point represents data from substitution tests in 4–10 monkeys.

Fig. 2

Mean (± SEM) percentage of total session responding on the ethanol-appropriate lever 30- (closed circles) or 60-min (open circles) after administration of 3α,5α-pregnanolone (P) in cynomolgus monkeys trained to discriminate 1.0 (female, n=2–4; male, n=1–5) or 2.0 g/kg (female, n=2–4; male, n=1–3) ethanol from water with a 30-min pre-treatment interval. Additional doses were tested, but are not shown, as indicated by the dose-response curves for individual monkeys shown in Supplemental Figure 1.

Fig. 3

Mean (± SEM) percentage of total session responding on the ethanol-appropriate lever 30- (closed circles) or 60-min (open circles) after administration of 3α,5β-pregnanolone (P) in cynomolgus monkeys trained to discriminate 1.0 (female, n=1–4; male, n=4–6) or 2.0 (female, n=2–4; male, n=2–3) g/kg ethanol from water with a 30-min pre-treatment interval. Additional doses were tested, but are not shown, as indicated by the dose-response curves for individual monkeys shown in Supplemental Figure 2.

Fig. 4

Mean (± SEM) percentage of total session responding on the ethanol-appropriate lever 30- (closed circles) or 60-min (open circles) after administration of 3α,5α-androsterone (A) in female and male cynomolgus monkeys trained to discriminate 1.0 (female, n=3–4; male, n=2–6) or 2.0 (female, n=3–4; male, n=2–3) g/kg ethanol from water with a 30-min pre-treatment interval. Dose-response curves for individual monkeys are shown in Supplemental Figure 3.