Determination of the inhibition dose-response profile of the chick a4^2 nAChR reconstituted in Xenopus oocytes yielded a distinct sensitivity profile as a function of the steroid tested. PROG is the most powerful steroid. Half-inhibition concentrations were, respectively, 6 ^M for PROG, 46 ^M for testosterone (T) and 62 ^M for the synthetic steroid dexamethasone. Exposure to cholesterol, pregnenolone (PREG), or allo-pregnanolone (3a-hydroxy-5a-pregnan-20-one) induced no significant reduction of the ACh-evoked currents (68). Comparison of steroids effective on nAChR and GABAaR reveals a clear difference in the pharmacological profile of these two families of receptors. For example neuronal nAChRs are insensitive to pregnenolone sulfate (PREGS), whereas this compound is a potent modulator of the GABAARs (69,70).
In addition, it was shown that PROG coupled to bovine serum albumin (BSA) in position 3 (P-3-BSA) or position 11 (P-11-BSA) was as efficient as unmodified PROG. Thus, addition of a molecule as large as BSA in positions 3 or 11 of the molecule had no detectable influence on its biophysiological action, which suggests that PROG is interacting in the extracellular domain of the receptor.
Results obtained with ion flux assays on cell lines expressing nAChRs show pharmacological profiles comparable to those obtained by electrophysiological measurements (71). Namely, rubidium efflux assays of TE671/RD cells, which express muscle nAChR, or SH-SY5Y cells, which are known to express ganglionic nAChR are inhibited by low concentrations of steroids. These cells display an IC50 of 6.1 and 11 ^M, respectively for PROG. In agreement with results obtained for the chick a4^2, higher values of IC50 were found for dexamethasone or T (45 and 100 ^M) on cells expressing a3^4. Incubation with P-3-BSA induced a slightly larger inhibition of the 86Rb+ efflux than PROG itself (71). Corticosterone induced a small but significant reduction of cation flux with an IC50 in the 100 ^M range (71).
It is interesting to note that all data available so far suggest that steroids predominant in females are more active than those found in males and, in turn, might lead to more profound modulation of the neuronal nAChRs under physiological conditions. These fundamental observations, however, still await clinical confirmation.
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