Age-related changes in the serotonin 2A receptor in the hypoglossal nucleus of male and female rats

Abstract

Hypoglossal motoneuron output to the genioglossus muscle contributes to upper airway patency. Serotonin (5HT) plays an important role in regulating hypoglossal motoneuron excitability via serotonin 2A receptors (5HT(2A)). The purpose of this study was to investigate whether there are age-associated changes in 5HT(2A) receptor expression in the hypoglossal nucleus of male and female rats. The brains of young, middle-aged and old F344 rats were sectioned, reacted immunocytochemically for the presence of 5HT(2A) receptor, and the staining density quantified. The estrus stage of female rats was determined and circulating sex hormone levels measured and correlated with 5HT(2A) levels. The results show that there was significantly greater 5HT(2A) receptor immunoreactivity in the hypoglossal nucleus of female than of male rats. With increasing age, there was an increase in 5HT(2A) receptor immunoreactivity in the hypoglossal nucleus of female rats, whereas no age-associated changes were observed in male rats. Previous studies have shown a reduction in 5HT-dependent respiratory plasticity and an age-associated decrease in 5HT in the hypoglossal nucleus in male but not female rats. Data from the present study suggest that aging male rats fail to compensate adequately for reduced 5HT in the hypoglossal nucleus by upregulating the expression of the 5HT(2A) receptor.

Figure 1

Photomicrograph of a horizontal section through the medulla stained for 5HT_2A immunoreactivity. Rostral (1), middle (2) and caudal (3) sample areas are shown together with the location of the sample area analyzed for background label (B). Scale bar = 750 μm.

Figure 2

Quantitative analysis of 5HT_2A receptor immunoreactivity in the hypoglossal nucleus of young, middle-aged and old male and female rats. Receptor level was measured by optical density (OD), averaged within each rat and then across rats in each group. Middle-aged and old females has significantly more 5HT_2A receptor immunoreactivity than males. Old females also has significantly more 5HT_2A receptor immunoreactivity than young (P=0.001) and middle-aged (P=0.002) females. There were no significant differences between young, middle-aged and old male rats. YM, young male, n=4; MM, middle-aged male, n=4; OM, old male, n=4; YF, young female, n=6; MF, middle-aged female, n=6; OF, old female, n=3. Values are means ± S.E.

Figure 3

Photomicrographs of horizontal sections through the hypoglossal nucleus of young and old female rats stained for 5HT_2A immunoreactivity. A. Young female, dorsal section. B. Old female, dorsal section. C. Young female, ventral section. D. Old female, ventral section. Sections from young and old rats were reacted at the same time. Images were captured using identical illumination parameters; no further adjustments were made. Scale bar = 750 μm.

Figure 4

Quantitative analysis of 5HT_2A receptor immunoreactivity in the hypoglossal nucleus of female rats in estrus and diestrus. Receptor level was measured by optical density (OD), averaged within each rat and then across rats in each group. Although there is a trend towards an increase in diestrus, this was not significant. YF-E, young female in estrus, n=3; YF-D, young female in diestrus, n=3; MF-E, middle-aged female in estrus,, n=3; MF-D, middle-aged female in diestrus, n=3; O, old female, n=3. Values are means ± S.E.

Figure 5

Relationship between 5HT_2A immunoreactivity in the hypoglossal nucleus and sex hormone levels. A. The magnitude of 5HT_2A 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 5HT_2A 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 5HT_2A 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.