Role of neurosteroids on memory formation in the chick

Abstract

In this study the presence and possible synthesis of the neurosteroids pregnenolone- sulphate (PREG-S), pregnenolone (PREG), dehydroepiandosterone-sulphate (DHEA-S) and dehydroepiandosterone (DHEA) in the day-old chick brain and their possible role in memory formation have been investigated. Radioimmunoassay analyses indicated that PREG-S, PREG, DHEA-S and DHEA are present in the chick brain at higher concentrations than in the plasma. Immunocytochemical studies also showed that the enzyme that catalyses the synthesis of PREG, cytochrome P450scc, is present in the chick brain and is located in neurons but not in astrocytes. Training on the strong version of the passive avoidance task (100% methylanthranilate; MeA) increased the concentration of PREG-S in the LPO but not in the IMHV, in females only. The concentrations of PREG, DHEA-S or DHEA in both the IMHV and LPO remained unchanged in both sexes. Basal levels of PREG and DHEA in the IMHV were much higher than in the LPO and the whole forebrain. However, there was a difference in the basal concentration of PREG-S in the IMHV between the sexes; females had higher concentrations of PREG-S than males. When injected bilaterally into the brain PREG-S (0.004-400 ng) and PREG (0.003-300 ng) enhanced memory retention for the weak version of the passive avoidance task (10% MeA) but the range of effective doses and the time windows for their memory enhancing effect were different for females and males. DHEA-S (0.004-400 ng) and DHEA (0.003-300 ng) also enhanced memory retention but, unlike PREG-S and PREG, their effects were similar for both sexes.