Neurosteroid modulation of the benzodiazepine receptor in developing chick optic lobe

The modulation of [3H]flunitrazepam binding by 3 alpha-hydroxy-5 alpha-dihydroprogesterone (3 alpha-OH-DHP) and 3 alpha, 5 alpha-tetrahydrodeoxicorticosterone (3 alpha-THDOC) was investigated in synaptic membranes isolated from chick optic lobe at three developmental stages, in order to evaluate the role of neurosteroids in central nervous system functional maturation. It was demonstrated that both steroids modulate [3H]flunitrazepam binding in a concentration-dependent manner at embryonic day 14, hatching and adult stage, producing maximal [3H]flunitrazepam binding enhancement at early stages of development and declining thereafter. EC50 values for 3 alpha-OH-DHP were lower than for 3 alpha-THDOC at all stages examined. On the other hand, Emax values were higher with 3 alpha-OH-DHP than with 3 alpha-THDOC. Scatchard analysis performed at embryonic day 14, hatching and adult stage showed that dissociation constants (Kd) were of 22.23 +/- 0.15, 18.38 +/- 1.15 and 19.86 +/- 0.62 nM, and maximal number of binding sites (Bmax) were 1.95 +/- 0.15, 3.13 +/- 0.21 and 2.25 +/- 0.13 pmol/mg protein, respectively. By adding 4 microM of either 3 alpha-OH-DHP or 3 alpha-THDOC, Kd values decreased significantly to 10.65 +/- 0.62, 9.71 +/- 0.85 and 13.25 +/- 0.74 nM or 9.54 +/- 0.65, 11.20 +/- 1.27 and 12.96 +/- 1.38 nM, at the above mentioned stages, respectively. Thus, either drug at the given concentration increased the affinity of benzodiazepine receptor sites for [3H]flunitrazepam, while the density of receptor sites remained unchanged. Our results suggest that these steroids display a positive allosteric modulation of benzodiazepine receptor sites which is inversely related to age.