Changes in Palatability Processing across the Estrous Cycle Are Modulated by Hypothalamic Estradiol Signaling

Consumption varies across the stages (metestrus, diestrus, proestrus, estrus) of a rat's estrous cycle, changing in ways that might be expected to reflect, in part, a direct impact of hormones on taste palatability. Evidence regarding this hypothesis has been mixed, however, and critical within-subject experiments comparing consumption of multiple tastes with distinct valences across all estrous phases have been few. Here, we assayed female rats' licking of palatable (saccharin, sucrose, NaCl) and aversive (quinine-HCl, citric acid) tastes in brief-access trials, while tracking their estrous cycles through vaginal cytology. We observed sucrose palatability to be high at metestrus, the same phase at which the palatability of the aversive citric acid was low. These patterns were consistent across tastes of similar palatability, despite vast differences between the substances' receptor mechanisms and central impacts. Together, these results reveal a general (i.e., independent of particular tastant identity) magnification of palatability-higher than average for palatable tastes and lower for aversive tastes-centered largely on metestrus. We tested whether this phenomenon reflects lateral hypothalamic (LH) estradiol processing, by infusing LH with an estrogen receptor blocker (ICI182, 780) across five consecutive tasting sessions. Control infusions replicated the metestrus magnification of palatability; as predicted, ICI infusions blocked this effect, but estrogen receptor inhibition failed to render preferences "cycle free," instead delaying the palatability magnification until diestrus. In summary, the estrous cycle directly mediates taste palatability in a manner involving hypothalamic actions of estradiol, but this effect is only one of several impacting consumption across the estrous cycle.