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I came across the phrase intrinsic hypothalamic fibers which I cannot understand. What is meant by intrinsic - that these fibers (long myelinated axons?) start in the hypothalamus? Or that they start and end in hypothalamus? Or something else?

The article is "Pathways of Sexual Desire" by James Pfaus (2009).

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Figure 8 Brain opioid systems overlaid on opioid binding regions (high density of receptors indicated by red to yellow, lower density in green to blue). Three systems exist: the pro-opiomelanocortin (POMC)-derived opioids such as р-endorphin, which arise from the same population of projection neurons that produce the melanocortins; the interneurons in the cortex, limbic system, hypothalamus, and brain stem, which produce enkephalins and dynorphins; and the intrinsic hypothalamic fibers, which contain endo- morphins. These systems are activated at orgasm in humans, at ejaculation in male rats, and during paced copulation in female rats. They shut down hypothalamic regions associated with sexual arousal and desire, thereby inducing refractoriness. However, opioids are also released in the VTA during phases of sexual arousal and desire and help to disinhibit DA neurons and, thereby increasing DA release in mesolimbic terminals. This is an important mechanism by which experience with sexual rewards sensitizes sexual excitement once the inhibitory effects of refractoriness and satiety wear off. VTA = ventral tegmental area; DA = dopamine; mPOA = medial preoptic area; mPFC = medial prefrontal cortex; NAcc = nucleus accumbens; PIR = piriform cortex; MeA = medial amygdala.

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"Intrinsic hypothalamic fibers" are fibers instrinsic to the hypothalamus; so yes, these are fibers either projecting from somewhere in the hypothalamus to somewhere else in the hypothalamus, or locally in the hypothalamus (the hypothalamus itself is a pretty complex structure containing many distinct nuclei). The paper you reference specifically mentions the medial preoptic area, mPOA, as a target:

Infusions of mu opioid receptor agonists to the mPOA inhibit sexual behavior in male rats

and

infusions of a mu antagonist to the mPOA, but not NAcc, block sexually conditioned place preference

"Fibers" does not necessarily refer to myelinated axons, just axons of any sort.

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