Estradiol benzoate potentiates neuroactive steroids' effects on pain sensitivity

Progesterone (P), its metabolites, and other neuroactive steroids alter pain thresholds consistent with their efficacies at modulating gamma-aminobutyric acid (GABA(A)) receptor complexes. We investigated whether estradiol benzoate (EB) potentiates low dosages of neuroactive steroids' effects on pain. Subcutaneous EB (10 mu g) or sesame oil vehicle was administered to ovariectomized Long-Evans rats(n = 40) 48 h before intracerebroventricular (ICV) infusion of a neuroactive steroid (0.0, 0.1, 0.3, or 0.5 mu g) in cyclodextrin vehicle. Neuroactive steroids (listed from greatest to least efficacious at GABA(A) receptor complexes) were THP [5 alpha-pregnan-3 alpha-ol-20-one], THDOC [5 alpha-pregnan-3 alpha, 21-diol-20-one], DHP [5 alpha-pregnan-3,20-dione], P [4-pregnen-3,20-dione], and DHEAS [5-androsten-3 beta-ol-17-one sulfate]. Pain sensitivity was assessed using the radiant heat tail-flick method before and 20 and 60 min following infusion. Estradiol benzoate interacted with the neuroactive steroids to alter tail-flick latencies. In particular, EB potentiated the antinociceptive effect of THP and DHP by significantly increasing tail-flick latencies above those of non-EB-treated animals. A similar pattern of increased tail-flick latencies occurred in EB-primed animals that received THDOC. Estradiol benzoate less consistently altered the pain threshold of animals administered P, which is less effective at modulating GABAergic activity. Conversely, EB increased the nociceptive effect of the neurosteroid DHEAS, an allosteric antagonist of GABA(A) receptor complexes, by significantly decreasing tail-flick latencies of EB- compared to vehicle-primed rats. Thus, EB priming potentiated neuroactive steroids' effects on pain threshold.