Cytoarchitectonic analysis of the SDN‐POA of the intact and gonadectomized rat

Abstract

The densely staining group of cells referred to as the sexually dimorphic nucleus of the preoptic area (SDN-POA) is greater in volume in the male than in the female rat. Because we and others have reported absolute volumes that have been consistent within individual studies but that vary considerably, we characterized the SDN-POA by describing its morphology with respect to the cytoarchitectonic divisions of the medial preoptic nucleus (MPN) in intact and gonadectomized rats. We report three major findings: the SDN-POA is heterogeneous and is composed of cells belonging to three distinct cytoarchitectonic divisions; the cytoarchitecture of the MPN and its medial and lateral divisions (MPNm and MPNl, respectively) in male rats appear to be influenced by the hormonal status in adulthood; and a small anteroventral division of the MPN (MPNav) is present in males but virtually absent in females. Specifically, the SDN-POA is located within the MPNm, but consists of subcomponents located within the central division of the MPN (MPNc), the MPNav, and part of the MPNm-exclusive of the MPNc and MPNav. The percentage of the total SDN-POA located within the MPNc and MPNav. The percentage of the total SDN-POA located within the MPNc and MPNav was greater in males, and that in the MPNm-exclusive of the MPNc and MPNav was greater in females, indicating that the SDN-POA has a different cytoarchitectonic composition in the two sexes. Gonadectomy produced no significant differences in SDN-POA volume, but the MPN, MPNl, and MPNm were significantly reduced in gonadectomized versus intact males, suggesting an activational effect of testicular hormones on these structures.

Topography in the preoptic region: Differential regulation of appetitive and consummatory male sexual behaviors

“…Theses lateral cells therefore tend to be larger in males than in females [102] since males on average have higher circulating concentrations of testosterone in the blood [18]. The volume of the entire POM and the size of neurons in the lateral part of this nucleus thus provide a clear morphological signature of testosterone action in the brain contrary to what is observed for sexually dimorphic brain areas in the preoptic region in other species such as the SDN-POA of rats [11,73,143] but similar to observations reported concerning the gerbil SDA [52,53,55] or the rat MPN [41,42]. Interestingly, the effects of testosterone on the neuronal size are more pronounced in the lateral part of the POM than in the medial part of the nucleus adjacent to the third ventricle [102].…”

“…However, these studies did not determine whether neurons in the preoptic region involved in copulatory behavior are limited to the MPN. Neurochemical studies and analysis of connectivity have revealed clearly defined sub-regions within this nucleus (medial, central and lateral parts; e.g., [41,128,129]) but the specific implication of these sub-regions in the control of different aspects of male sexual behavior is still not well understood. A central subdivision of the MPN in rats that is larger in volume in males than in females was shown to be sexually differentiated by the action of steroids during embryonic life (the sexually dimorphic nucleus of the mPOA or SDN-POA; [41,72]).…”

“…Neurochemical studies and analysis of connectivity have revealed clearly defined sub-regions within this nucleus (medial, central and lateral parts; e.g., [41,128,129]) but the specific implication of these sub-regions in the control of different aspects of male sexual behavior is still not well understood. A central subdivision of the MPN in rats that is larger in volume in males than in females was shown to be sexually differentiated by the action of steroids during embryonic life (the sexually dimorphic nucleus of the mPOA or SDN-POA; [41,72]). Several experiments were therefore designed to investigate whether this cell group plays a role in the control of copulatory behavior but, despite the fact that large mPOA/MPN lesions severely disrupt copulatory behavior (as reviewed previously), more discrete lesions specifically destroying the SDN-POA do not seem to affect copulatory behavioral expression in the male [9] (see [143] for review).…”

Deletion of Bax eliminates sex differences in the mouse forebrain

“…One possible limitation of these volumetric findings is that we examined adult, gonadally intact animals. Circulating levels of gonadal steroid hormones in adulthood affect neuronal size and͞or the volume of cell groups in several neural regions (33)(34)(35) and could have affected volume measures here. However, we are not aware of any reports of circulating hormones influencing AVPV volume, and a sex difference in BNSTp volume of guinea pigs is not altered by adult gonadectomy (36).…”

Medial preoptic area afferents to periaqueductal gray medullo-output neurons: a combined Fos and tract tracing study

“…Taken together, these results suggest that the MPO + PAG + RVM projection constitutes a functional pathway. This circuit may coordinately regulate neuroendocrine, motor, and autonomic adjustments necessary for the elaboration of sexual behaviors.Key words: reproduction; antinociception; cardiovascular regulation; brainstem; sexual behavior; immunohistochemistryThe medial preoptic area (h/[PO) is a sexually dimorphic structure Simerly et al, 1984;Bloch and Gorski, 1988) that plays a pivotal role in sexual behavior and neuroendocrine function (Lisk, 1966;Powers and Valenstein, 1972;Pfaff and Sakuma, 1979;Hansen et al, 1982;Arendash and Gorski, 1983;Kalra and Kalra, 1983;Docke et al, 1984;Sachs and Meisel, 1988;Simerly et al, 1990;DonCarlos et al, 1991;Takeo et al, 1993;Hoshina et al, 1994). We recently reported that MPO robustly innervates the midbrain periaqueductal gray (PAG) (Rizvi et al, 1992) and terminates in discrete, longitudinally organized columns running through the rostrocaudal axis of PAG.…”