9.2 Sex differences in learning processes of rats  (Page 6/9)

Sex differences in conditioned taste aversion:

In conditioned taste aversion, animals are fed a novel sweet solution which either contains an aversive compound which makes the animal ill or an aversive compound is administered immediately after the animal consumes the solution. Eventually, animals learn to associate the solution with illness and avoid it (Dalla, 2009).

Randall-Thomson (2003) performed conditioned taste aversion on rats using morphine and lithium chloride (LiCl) as the aversive compound. Each day, liquid-deprived subjects were given a 20 minute access period to a saccharine solution and every four days were injected with either morphine or LiCl, for a total of four trials (16 days). Acquisition of the aversive behavior did not differ by sex, and both males and females typically learned to avoid the solution by the fifth day. Memory extinction rates were then measured for this learned behavior. Although the rates did not differ between males and females for morphine, the female LiCl-induced rats went back to the saccharin solution sooner than males. This suggests that females forget learned taste aversion before males (Randall-Thompson, 2003).

Hormonal differences

Although the non-reproductive effects of gonadal hormones have been given significantly less attention than the reproductive effects, the magnitude of sex-differences in some learned responses seems to be heavily influenced by sex hormones. It appears that testosterone, typically present in higher levels in males, has little effect on sex-differences. Estrogen, a typically feminine hormone, does play some role. For example, the difference in performance between male and females in CEC is greater when females begin training in proestrus , when estrogen levels are high (Dalla, 2009).

In both CEC and fear conditioning, although castration of adult male rats does not alter the conditioned response, removing the ovaries of adult females did. Research suggests that removal of the ovarian tubes, and thus of female-specific hormones, can reduce the learning bias, causing ovariectomized females to perform at the same level as males (Gupta, 2000, Wood, 1998). The differences could not be re-established by administering normal doses of estrogen to females who had the ovaries removed, although very high doses did increase performance (Gupta, 2000, Leuner, 2004). However, some studies show that removal of the ovaries in females and castration in males has no effect on the learning differences (Dalla, 2008). It is only fair to note that it is highly unlikely that there can be a total dependence of basic learning processes on one or even a few hormones. The adaptive value of learning is too great. Because males and females of all species undergo drastic changes in sex hormone availability throughout their lifetime, learning cannot be exclusively influenced by hormone levels.