The impact of ovarian hormones and cholecystokinin on palatability and satiety

The ovarian hormone estradiol, and the gastrointestinal peptide cholecystokinin (CCK), reduce food intake in several species by decreasing meal size rather than meal frequency. It has been hypothesized that estradiol and CCK might interact in the regulation of feeding by modulating palatability and inducing satiety. Based on this premise, the present thesis used the taste reactivity paradigm to examine the selective impact of, and the relationship between, gonadal hormones and cholecystokinin on palatability and satiety in a rodent model. The first study examined the relationship between endogenous hormones and taste responsivity by examining sex differences in taste reactivity responses, as well as changes these responses during the reproductive cycle. Three qualitatively different stimuli, a sucrose solution, a quinine solution, and a sucrose-quinine solution, were used. Results revealed a sex difference in taste responsivity in the rat, which is predicated on the reproductive status of females. The second study examined sex differences in CCK-induced modification of the palatability of a sucrose solution. This study revealed that CCK-octapeptide (CCK-8) was able to alter the palatability of the sucrose tastant by producing a decrease in ingestive responses and an increase in passive drips elicited by this taste stimulus. This study also revealed a sex difference in taste reactivity in the vehicle-injected control rats, with females displaying more ingestive responses than males. CCK-8 eliminated this sex difference. Thus, the second study, provided evidence of sexual dimorphism in the suppressive effect of CCK-8 on ingestive taste reactivity responses. The third study further investigated the relationship between natural fluctuations in hormonal levels during the estrous cycle and CCK, and the effects on satiety and palatability. The ability of CCK-8 to modify the palatability of three tastants (a sucrose solution, a quinine solution, and a sucrose-quinine solution), was examined at each stage of the estrous cycle. The degree to which CCK-8 was able to modify the palatability of the infused tastants, was related to the quality of the taste stimulus, that is, sweet, bitter or bittersweet. Notably, this study failed to identify reliable differences in the sensitivity of female rats to the effects of CCK-8 during the estrous cycle. To further elucidate the relationship between CCK and ovarian hormones the fourth study determined the effect of CCK-8 on the palatability of a sucrose solution in ovariectomized rats with and without hormonal replacement. CCK-8 produced a robust decrease in ingestive responses, a robust increase in passive drips, as well as an increase in aversive responses in ovariectomized rats receiving hormonal replacement. In contrast, CCK-8 produced a small, immediate enhancement of ingestive responses, and a modest increase in passive drips in ovariectomized rats receiving vehicle injections only. Thus, this last study revealed that CCK-8 had a more pronounced effect on the taste reactivity responses elicited by the hormonal replacement group relative to the control group. These data provide evidence of a synergistic relationship between CCK and ovarian hormones, like estradiol. Collectively, these studies provide preliminary evidence of the impact of, and the relationship between, ovarian hormones and CCK on the regulation of ingestive behaviour, in terms of satiety and palatability.