Relationship between 5HT2A immunoreactivity in the hypoglossal nucleus and sex hormone levels. A. The magnitude of 5HT2A immunoreactivity (optical density) in the hypoglossal nucleus is plotted against progesterone/estradiol for all15 female rats. The age and estrus stage of each rat is indicated. B. Relationship between the magnitude of 5HT2A immunoreactivity in the ventral half of the hypoglossal nucleus and progesterone/estradiol in rats in young and middle-aged rats in diestrus (n = 6; R2 = 0.647; P = 0.054). C. Relationship between the magnitude of 5HT2A immunoreactivity in the ventral half of the hypoglossal nucleus and progesterone/estradiol in young and middle-aged rats in estrus (n = 6; R2 = 0.603; P = 0.070). D, diestrus; E, estrus; P, proestrus; Y, young; M, middle aged; O, old.