Sex differences in the expression of GnRH regulators in the preoptic area of guinea pigs

The preoptic area (POA) of the hypothalamus is a neuroendocrine centre regulating reproduction via gonadotropin-releasing hormone (GnRH) neurons. Although sexual dimorphism in reproductive behaviors is well described, sex differences in expression of GnRH modulators in guinea pigs are poorly characterized. We investigated the distribution and density of tyrosine hydroxylase (TH), neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), and enkephalin (ENK) in the POA of male and female guinea pigs using immunohistochemistry and quantitative densitometry. TH-, NPY-, and CART-immunoreactive fibers were abundant across POA subregions, with males showing significantly higher volume densities in all regions (p < 0.05). ENK-immunoreactive elements exhibited a sex-specific pattern: females had greater densities in the periventricular and median preoptic nuclei, whereas males displayed higher densities in the medial and lateral preoptic areas (p < 0.05). Protein-protein interaction network analysis revealed functional interconnections among these regulators, implicating dopaminergic modulation, metabolic signaling, and reproductive hormone control. These findings provide first evidence of sex-specific expression patterns of key GnRH modulators in the guinea pig POA, suggesting that differential innervation and peptide signaling may underlie sex-specific GnRH pulsatility and reproductive hormone secretion. Our results clarify neuroendocrine mechanisms of sexual dimorphism and support the guinea pig as a model for human reproductive physiology.