INAH 3

Interstitial nucleus of the anterior hypothalamus
Anatomical terms of neuroanatomy []

INAH-3 is the short form for the third nucleus of the anterior hypothalamus, and is the sexually dimorphic nucleus of humans. The INAH-3 is significantly larger in males than females regardless of age and larger in heterosexual males than in homosexual males and heterosexual females. Homologues of the INAH-3 have been observed taking a direct role in sexual behavior in rhesus monkeys,sheep, and rats.

The term INAH (interstitial nuclei of the anterior hypothalamus), first proposed in 1989 by a group of the University of California at Los Angeles, refers to 4 previously undescribed cell groups of the preoptic-anterior hypothalamic area (PO-AHA) of the human brain, which is a structure that influences gonadotropin secretion, maternal behaviour, and sexual behavior in several mammalian species. There are four nuclei in the PO-AHA (INAH1-4). One of these nuclei, INAH-3, was found to be 2.8 times larger in the male brain than in the female brain regardless of age.

A study authored by Simon LeVay and published in the journal Science suggests that the region is an important biological substrate with regards to sexual orientation. This article reported the INAH-3 to be smaller on average in homosexual men than in heterosexual men, and in fact has approximately the same size in homosexual men as in heterosexual women. Further research has found that the INAH3 is smaller in volume in homosexual men than in heterosexual men because homosexual men have a higher neuronal packing density (the number of neurons per cubic millimeter) in the INAH3 than heterosexual men; there is no difference in the number or cross-sectional area of neurons in the INAH3 of homosexual versus heterosexual men. It has also been found that there is no effect of HIV infection on the size of INAH3, that is, HIV infection cannot account for the observed difference in INAH3 volume between homosexual and heterosexual men.

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