On the annual meeting of Society for Neuroscience, Leann Kinnunen (2003) presented results of investigation where fluoxetine was given to 8 heterosexual and 8 homosexual men and then glucose metabolism is observed by PET. It is discovered that in homosexual men reduction of glucose metabolism was less than in control group of heterosexual men in anterior hypothalamus.

I have one theoretical explanation for such a phenomena and I would like to get your feedback on that. First of all, fluoxetine blocks serotonin uptake receptors on synapse to elongate effect of serotonin in synaptic cave. If you apply fluoxetine in healthy brain you will affect normal serotonin neurotransmission and as a result you will get reduction of glucose metabolism. If reduction in homosexual men is weaker it might be possible that there are locally backup systems in the brain to prevent disruptions maden by external influence. In the hypothalamus, serotonin has primary role in regulation of circadian and circaanual rythms governing SCN function. Swaab et al. (1990) found that AVP subdivision of SCN is as twice as larger in homosexual than in heterosexual men. If there is greater AVP cell number in SCN it is possible that vassopresinergic neurotransmission would be more intense. In his later research, Swaab (1999) discovered that AVP subdivision was as twice as larger in depressive men, although there was no correlation between depression and sexual orientation of his homosexual subjects from 1990.
If causality of depression relies on reduction of serotoninergic transmission, then some backup mechanisms locally might restore some vital functions. And function of SCN is vital. It might be possible that vasopressin acts as substitution for serotonin in depression, so that SCN function could be maintaned. Although is unclear why there is more AVP cells in SCN of homosexual men, in the case of fluoxetine application, if you disrupt serotonine transmission vasopressin could act as substitution. In heterosexual subjects, where locally (I mean on hypothalamus) levels of vasopressine are average, there would not be substitution for serotonin and that distruption would affect glucose metabolism.