The facts presently available support the hypothesis that leptin plays an important role in relaying energetic status to the reproductive axis; however, the mechanisms subserving leptin’s effects in this context remain elusive. While the presence of Ob-R in the ventral forebrain is tantalizing, there is as yet no conclusive evidence that leptin directly influences the activity of GnRH-containing neurons. If one proposes that leptin acts indirectly to affect GnRH secretion, several possible modes of action are conceivable. Lep-tin may positively or negatively influence the synthesis and/ or secretion of other hypothalamic peptides (e.g., POMC gene products, NPY, corticotropin-releasing hormone, gal-anin), which then in turn modulate GnRH release (Fig. 3). Buy Advair Diskus Online
This hypothesis has yet to be verified by pharmacological experiments that are designed to identify the signaling pathways purported to be involved. Promising new insights have come from experiments that have examined the possible interaction between leptin and glucose availability. Syrian hamsters show disruption of sex behavior, es-trous cyclicity, and ovulation with fasting.
FIG. 3. Proposed model of leptin’s actions on the neuroendocrine reproductive axis. Leptin acts on hypothalamic neurons expressing its receptor (Ob-R). GnRH mRNA-containing neurons are not thought to be direct targets for leptin’s actions, as they do not express easily detectable amounts of Ob-R mRNA. Neurons containing POMC and NPY mRNA also express Ob-R mRNA, and both POMC and NPY mRNA are regulated by leptin. Changes in the synthesis and/or release of these or other hypothalamic peptides could then act at GnRH neurons to affect GnRH release.