New tricks by an old dogma: mechanisms of the Organizational/Activational Hypothesis of steroid-mediated sexual differentiation of brain and behavior

Abstract

The hormonal regulation of sexual behavior has been the topic of study for over 50 years and yet controversies persist regarding the importance of early versus late events and the identity of the critical neural and cellular substrates. We have taken a mechanistic approach toward the masculinizing actions of the gonadal steroid estradiol, as a means to understand how organization of the neuroarchitechture during a perinatal sensitive period exerts enduring influences on adult behavior. We have identified important roles for prostaglandins, FAK and paxillin, PI3 kinase and glutamate, and determined that cell-to-cell signaling is a critical component of the early organizational events. We have further determined that the mechanisms mediating different components of sexual behavior are distinct and regionally specific. The multitude of mechanisms by which the steroid estradiol, exerts divergent effects on the developing nervous system provides for a multitude of phenotypes which can vary significantly both within and between the sexes.

Fig. 1

Estradiol induces prostaglandin E2 (PGE2) synthesis to promote dendritic spine formation and sexual differentiation in the POA. Estradiol aromatized from testicular androgens binds to ER alpha and induces transcription of the COX-1 and COX-2 genes, the rate limiting enzymes in PGE2 synthesis. PGE2 activates multiple receptors, the most important being EP2 and EP4, both of which are linked to activation of Protein Kinase A (PKA). Through mechanisms that remain poorly understood, PKA enhances the actions of glutamate at AMPA, but not NMDA receptors, and the formation of new dendritic spines and the organization of a higher density of spine synapses on male POA neurons compared to female.

Fig. 2

Feminization versus defeminization. We propose three theoretical conceptualizations of feminization versus defeminization. The first option is that the two processes are inseparable and estradiol-induced defeminization erases feminization. A prediction of this option would be the inability to recover the capacity for female sex behavior in adulthood. The second option proposes distinct mechanisms for defeminization and feminization, both of which occur but the former is induced by estradiol and suppresses the latter which is independent of estrogen action during development. An example would be the formation of a larger SDN in order to suppress the female sexual behavior neural template. The third option involves two separate mechanisms induced by estradiol, one to suppress the process of feminization and the other to initiate defeminization. The suppression of FAK and paxillin and the induction of glutamate release by estradiol in the developing VMN would fit this model.

Fig. 3

Estradiol enhances glutamate release to promote cell-to-cell communication and sexual differentiation in the VMN. In the developing male VMN dendrites are longer and branch more frequently than in the females, resulting in a greater number of dendritic spines. This permanent organizational effect begins with the nongenomic activation of PI3 kinase by ligand-activated ER alpha. The increased released glutamate binds to receptors on the post-synaptic neuron, inducing activation of MAP kinase, protein synthesis and the formation of new dendritic spines (Schwarz et al., 2008).

Fig. 4

Multiple phenotypes due to multiple mechanisms. Estradiol simultaneously activates multiple cellular mechanisms in discrete brain regions during the sensitive period of sexual differentiation for feminization, defeminization and masculinization. The ultimate impact is a far greater number of potential phenotypes than would be achieved by a single unitary mechanism that mediated sex differences in physiology and behavior.

Fig. 5

Synaptic profiles predict male behavior. Increases in spinophilin protein correlated with the density of dendritic spines on POA neurons and measures of adult male sexual behavior correlated with adult POA spinophilin protein. A) Stylized representation of spinophilin in the necks of dendritic spines. B) Image of POA cultured neuron visualized by immunoctyochemistry for spinophillin which is concentrated in spine-like protrusions (Amateau and McCarthy, 2004). C) Effect of neonatal treatment with EP receptor agonists on adult POA spinophilin protein assayed by western blot. Each lane represents the POA of one animal in a typical western immunoblot for spinophilin (above) and GAPDH loading control (below). POA spinophilin protein levels incrased 2–4 fold above vehicle (VEH) treated animals and equivalent to PGE2 treated animals (Wright et al., 2008). D) Regression of adult POA spinophilin protein levels and number of mounts in individual animals shows a significant correlation (p<0.001) between the two endpoints.

Fig. 6

Neural circuit(s) controlling sex behavior. The question of whether there are two distinct neural circuits that separately control expression of male versus female sexual behavior, or a single neural circuit that is preferentially weighted towards one versus the other, remains unresolved. The observation of increased density of dendritic spines on POA neurons correlating with male sexual behavior is consistent with a single circuit that is weighted in favor of males in response to salient olfactory and somatosensory input. Similarly, the observation of decreased dendritic branching and fewer dendritic spine synapses in the VMN suggests a weighting in favor of female sexual behavior. But in both instances, the potential for synaptic plasticity and expression of opposite sex behavior still exists.