INAH-3 is the short form for the third
interstitial nucleus
Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
*Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucleu ...
of the
anterior
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
hypothalamus
The hypothalamus (: hypothalami; ) is a small part of the vertebrate brain that contains a number of nucleus (neuroanatomy), nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrin ...
, and is the
sexually dimorphic nucleus
The sexually dimorphic nucleus (SDN) is an ovoid, densely packed cluster of large cells located in the medial preoptic area (POA) of the hypothalamus which is believed to be related to sexual behavior in animals. Thus far, for all species of m ...
of humans. The INAH-3 is significantly larger in males than in females regardless of age and larger in heterosexual males than in homosexual males and heterosexual females.
Homologs of the INAH-3 have been found to play a direct role in sexual behavior in
quails,
rhesus macaques
The rhesus macaque (''Macaca mulatta''), colloquially rhesus monkey, is a species of Old World monkey. There are between six and nine recognised subspecies split between two groups, the Chinese-derived and the Indian-derived. Generally brown or g ...
,
sheep
Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to d ...
,
rats
Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' (pack rats), '' Bandicota'' (bandicoot ...
,
mice
A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
, and
ferrets.
Research
The term INAH (interstitial nuclei of the anterior hypothalamus), first proposed in 1989 by a group of the
University of California at Los Angeles
The University of California, Los Angeles (UCLA) is a public land-grant research university in Los Angeles, California, United States. Its academic roots were established in 1881 as a normal school then known as the southern branch of the Ca ...
, 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 behavior, 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
Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
'' suggests that the region is an important biological
substrate
Substrate may refer to:
Physical layers
*Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached
** Substrate (aquatic environment), the earthy material that exi ...
with regard to
sexual orientation
Sexual orientation is an enduring personal pattern of romantic attraction or sexual attraction (or a combination of these) to persons of the opposite sex or gender, the same sex or gender, or to both sexes or more than one gender. Patterns ar ...
. This article reported the INAH-3 to be smaller on average in
homosexual
Homosexuality is romantic attraction, sexual attraction, or sexual behavior between people of the same sex or gender. As a sexual orientation, homosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions" exc ...
men than in
heterosexual
Heterosexuality is romantic attraction, sexual attraction, or sexual behavior between people of the opposite sex or gender. As a sexual orientation, heterosexuality is "an enduring pattern of emotional, romantic, and/or sexual attractions ...
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
neuron
A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...
s 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.
[
LeVay noted three possibilities that could account for his findings: 1. The structural differences in INAH3 between homosexual and heterosexual males were present prenatally or in early life and aided in establishment of the men’s sexual orientation; 2. The differences appeared postnatally as a result of the men’s sexual feelings or behavior and; 3. Both the differences in INAH3 and sexual orientation are linked to some third ]confounding variable
In causal inference, a confounder is a variable that influences both the dependent variable and independent variable, causing a spurious association. Confounding is a causal concept, and as such, cannot be described in terms of correlati ...
(such as a developmental event in prenatal or early life). LeVay found the first possibility most probable and noted that the second possibility was unlikely in light of various homologous studies in other species.transgender
A transgender (often shortened to trans) person has a gender identity different from that typically associated with the sex they were sex assignment, assigned at birth.
The opposite of ''transgender'' is ''cisgender'', which describes perso ...
individuals by neuroanatomist Dick Swaab found male-to-female transgender people to have a size and number of neurons of INAH-3 closer to a normal female range, and that female-to-male transgender people have a size and number of INAH-3 neurons closer to a normal male range. This finding that the size of the INAH-3 more closely corresponded to the gender the subject identified with rather than their biological or chromosomal gender has since been repeated, but is still controversial due to potential confounds of hormone replacement therapy.
Kay Brown (2010), "the Incredible Shrinking Brain"
See also
* ''
The Sexual Brain''
Notes
References
{{reflist, 30em
Sexual orientation and science
Brain
Human biology
Neuroanatomy