A hallmark of reproductive aging in rats is a delay in the initiation and peak, and a decrease in the amplitude, of both proestrous and steroid-induced surges of LH and a decrease in the number of GnRH neurons that express Fos during the surge. The altered timing of the LH surge and the decline in Fos expression in GnRH neurons may be due to changes in the rhythmic expression of vasoactive intestinal polypeptide (VIP), a neuropeptide that carries time-of-day information from the circadian pacemaker, located in the suprachiasmatic nuclei (SCN), to GnRH neurons. The goals of our study were to determine if aging alters 1) the innervation of GnRH neurons by VIP and 2) the ability of VIP to activate GnRH neurons by examining the effects of aging on the number of GnRH neurons apposed by VIP fibers and the number of GnRH neurons that receive VIP input that express Fos. Immunocytochemistry for GnRH and VIP; or GnRH, VIP, and Fos was performed on tissue sections collected from young (2–4 mo), regularly cycling females and middle-aged (10–12 mo) females in constant estrus. The number of GnRH neurons, GnRH neurons apposed by VIP fibers, and GnRH neurons that express Fos and apposed by VIP fibers were counted in both age groups. Our results clearly demonstrate that aging does not alter the number of GnRH neurons that receive VIP innervation. However, the number of GnRH neurons that receive VIP innervation and coexpress Fos decreases significantly. We conclude that the age-related delay in the timing of the LH surge is not due to a change in VIP innervation of GnRH neurons, but instead may result from a decreased sensitivity of GnRH neurons to VIP input.
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