Blockage of N-Methyl-<i>D</i>-Aspartate Receptors Decreases Testosterone Levels and Enhances Postnatal Neuronal Apoptosis in the Preoptic Area of Male Rats

Abstract

Sexual dimorphism has been found in the preoptic area of the hypothalamus (POA), a major site of glutamate actions via N-methyl-D-aspartate (NMDA) receptors. The sexually dimorphic nucleus of the preoptic area (SDN-POA) of male rats exhibits about seven-fold greater nuclear volume than that of females. A naturally occurring neonatal neuronal apoptosis, that can be prevented by testosterone, may contribute to this sexual difference in SDN-POA nuclear volume. Since activation of NMDA receptors in the POA induces GnRH secretion, it may be involved in both elevation of serum testosterone and prevention of neuronal death in the SDN-POA. In the present study, protein expression of NMDA receptors in the POA of male and female fetuses was quantified on the day preceding the fetal testosterone peak (embryonic day 16; ED 16). Rats were then distributed in four groups: (1) untreated males, (2) untreated females, (3) males pretreated with MK-801 (a noncompetitive NMDA receptor antagonist), and (4) females pretreated with MK-801. Serum levels of testosterone were estimated on the afternoon of ED 18. Expression of Bcl-2 and Bax, as well as neuronal apoptosis in SDN-POA, were observed on postnatal day 8. The results showed that (1) expression of NMDA receptors in the POA of male fetuses was higher than that of females on ED 16; (2) levels of testosterone were lower in MK-801 pretreated male fetuses than in intact males on ED 18; (3) expression of Bcl-2 in the POA of MK-801 pretreated male rats was significantly less than that of control males; (4) the apoptotic incidence in the SDN-POA of MK-801 pretreated male rats was significantly greater than in control males, while there was no significant difference in apoptotic incidence in the SDN-POA between MK-801 pretreated and intact females. These results suggest that the NMDA receptor is highly expressed in prenatal male fetuses, and that it might play an important role in the elevation of testosterone levels. Moreover, activation of NMDA receptors may protect SDN-POA neurons from naturally occurring neuronal death, by modulating testosterone and/or Bcl-2 expression.

Developmental programming and endocrine disruptor effects on reproductive neuroendocrine systems

“…There are sex differences in expression of pro-and anti-apoptotic genes in the POA. For example, bcl-2 (an anti-apoptotic protein) expression is higher in male than female rats on postnatal day 8, consistent with the overall larger POA size of male rats [78]. A subsequent study from the Hsu group also showed specificity of this phenomenon to the SDN-POA, for which bcl-2 expression was higher in neonatal (postnatal day 2) males than females [79].…”

Global Birth Hypoxia Increases the Neonatal Testosterone Surge in the Rat

“…N-Methyl- D -aspartate (NMDA) receptor activation in the hypothalamus is known to stimulate gonadotropin-releasing hormone (GnRH) release leading to secretion of luteinizing hormone (LH) and testosterone in the rat [22], while NMDA receptor blockade also decreases testosterone release prenatally in rats [23]. CNS hypoxia is well known to stimulate acute release of glutamate [24, 25], suggesting the possibility that increased NMDA receptor activation might be responsible for the increased neonatal testosterone levels induced by birth hypoxia.…”

Gene regulation by NMDA receptor activation in the SDN-POA neurons of male rats during sexual development

“…Our previous results showed that the protein content of Bcl-2 in the POA of male rats was higher than that of females at PND 8, and MK-801 pre-treatment diminished the expression of Bcl-2 in male rats (Hsu et al 2000). It suggests that the prominent expression of Bcl-2 in male rats might inhibit naturally occurring neuronal apoptosis in SDN-POA during normal sexual development.…”

“…It suggests that the prominent expression of Bcl-2 in male rats might inhibit naturally occurring neuronal apoptosis in SDN-POA during normal sexual development. Additionally, activation of NMDA receptors may protect against neuronal death in the SDN-POA by modulating the expression of Bcl-2 (Hsu et al 2000), which is transactivated by NF B (Mattson & Camandola 2001) and inhibits the release of cytochrome c and/or mitochondrial apoptogenic proteins . In the present study, the expressions of Bcl-2 mRNA and protein in the SDN-POA of male rat were significantly suppressed by MK-801 treatment.…”