Aging-induced decrements in neuropeptide Y: The retention of ejaculatory behavior is associated with site-selective differences

Abstract

Neuropeptide Y (NPY) has been implicated in the control of reproductive and cardiovascular function. We observed an age-related decrease in the number of males copulating to ejaculation and a moderate systolic hypertension in middle-aged (16- to 17-month-old) rats. NPY content was examined in microdissected brain nuclei in 5 groups of rats: 2 groups of young rats, 1 heterosexually naive and the other ejaculating in 3 successive mating test; 3 groups of middle-aged rats, 1 heterosexually naive, 1 group that had extensive sexual experience but failed to ejaculate in tests at 16.5 months of age, and the third continuing to ejaculate at 16.5 months of age. NPY levels were found to vary depending on the brain area, the age of the animals, and the maintenance of ejaculatory behavior. In sexually naive middle-aged males, NPY levels were uniformly lower than in younger males. There were no differences in NPY levels of young animals, regardless of sexual experience. In the medial preoptic area, the group that retained ejaculatory behavior through 16.5 months of age, had higher levels of NPY than those observed in young sexually experienced rats. In sexually experienced rats that were no longer ejaculating at 16.5 months of age levels were lower than all other groups except the sexually naive middle-aged group. In the hypothalamic arcuate nucleus, levels were equivalent in the young groups and in the middle-aged rats that retained ejaculatory behavior, being greater than in the middle-aged rats that were no longer ejaculating or were sexually naive. In the hypothalamic paraventricular nucleus, levels were highest in young rats, lowest in middle-aged rats that were sexually naive or no longer ejaculating and intermediate in the group of rats retaining ejaculatory behavior. In contrast, in the hypothalamic dorsomedial and ventromedial nuclei, there were no significant differences between the three middle aged groups. We suggest that as the sexually active rat ages there is a neural site-specific hypersecretion of NPY in an attempt to maintain pituitary-gonadal hormone production and copulatory behavior. Following this, there is a decrease in activity of NPY neurons which results in ejaculatory failure. The suggested sexually relevant source of NPY to the medial preoptic area and the hypothalamic paraventricular nucleus is the NPY-synthesizing cells in the hypothalamic arcuate nucleus.