Age-, Sex-, and Gonadal Steroid-Related Changes in Immunoreactive Substance P in the Rat Anterior Pituitary Gland

Abstract

The interrelationship of anterior pituitary lobe (AP) immunoreactive substance P (I-SP) concentrations with age, sex, gonadal steroids, and estrous cyclicity in rats was examined. There was no difference between male and female AP I-SP levels at 0.5 month of age. At 2.0 and 5.0 months of age, a sex-linked difference in AP I-SP concentrations was evident, inasmuch as male APs contained approximately 3 and 8 times greater concentrations of I-SP, respectively, than APs from age-matched females. Long term (6 weeks) gonadectomy of adult rats resulted in an increase in I-SP concentrations in female APs and a decrease in the concentrations of the peptide in male APs compared to values in their respective sham-operated controls. Treatment of gonadectomized animals for the same length of time with gonadal steroid-filled Silastic capsule implants resulted in qualitatively identical responses in males and females; that is, estradiol benzoate decreased and 5 alpha-dihydrotestosterone propionate increased AP I-SP levels compared to the respective control values in castrates. Testosterone propionate treatment had no effect on AP I-SP levels compared with the respective gonadectomy control values. Short term (8 days) gonadectomy of adult males did not affect the AP concentration of I-SP. Likewise, gonadectomy of adult females was ineffective in altering the AP I-SP concentration compared with concentrations in females on diestrous day 1, diestrous day 2, proestrous, or estrous stages of the vaginal cycle. These data suggest that gonadal steroids are physiologically important in the regulation of I-SP concentrations in the AP. We hypothesize that I-SP is indigenous to the AP and that gonadal steroids act directly at the level of the AP to affect the synthesis and/or release of the peptide.