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Ventromedial Nucleus

The ventromedial nucleus (sometimes referred to as the ventromedial hypothalamus or VMH) is a nucleus of the hypothalamus. The ventromedial hypothalamus (VMH) is a distinct morphological nucleus involved in feeding, fear, thermoregulation, and sexual activity. Division It has four subdivisions: anterior (VMHa) dorsomedial (VMHdm) ventrolateral (VMHvl) central (VMHc). These subdivisions differ anatomically, neurochemically, and behaviorally. Lateral Hypothalamus: This region of the brain is associated with hunger recognition. Ventromedial Hypothalamus: This nuclear region is involved with the recognition of the feeling of fullness. Function The ventromedial nucleus (VMN) is most commonly associated with satiety. Early studies showed that VMN lesions caused over-eating and obesity in rats. However, the interpretation of these experiments was summarily discredited when Gold's research demonstrated that precision lesioning of the VMN did not result in hyperphagia. Nevertheless, numerous studies have shown that the immediacy of hyperphagia and obesity syndrome are a consequence of VMN lesions or procaine injections, and point to the VMN's role in satiety. A major review of the subject in 2006 concluded that, "anatomical studies done both before and after Gold's study did not replicate his results with lesions, and in nearly every published direct comparison of VMH lesions vs. PVN or VNAB lesions, the group with VMH lesions ate substantially more food and gained twice as much weight." This strongly substantiates the classification of VMN as the primary satiety center in the hypothalamus. It has also been found that lesions to the VMH in rats caused increased plasma insulin levels. Rats with a VMH lesion compared to normal rats overproduce a circulating satiety factor, to which the control rats can respond and rats with a VMH lesion cannot respond. A lesion to the VMH makes rats overproduce leptin, which they cannot respond to causing them to over eat, leading to obesity. Two researchers, Heterington and Ranson looked at series of twenty-one animals of various degrees of adiposity, with respect to growth appearance, fat distribution, general physical condition, and the correlation between the level of adiposity attained and the correlation of the hypothalamic lesion. Lesions in the hypothalamic area, particularly the region of the ventromedial hypothalamus interrupts a large number of the descending fibers from the hypothalamic cell groups that were found to contribute to obesity in rats. Taylor and Jamshi found that there seems to be a higher concentration of cannabinoid receptor mRNA within the ventromedial hypothalamus in comparison to other nuclei within the hypothalamus. The cannabinoid ingestion has been linked to rewarding processes, and also with the release of dopamine in the brain. VMH is also important in 'playing' behaviour seen in mammals. Lesions to VMH along with the hippocampus, amygdala, the cerebellum and the lateral hypothalamus will all reduce play behaviour. The VMHdm has a role in the male vocalizations and scent marking behaviors. The VMHvl plays a role in sexual behaviors in females (lordosis), thus stimulating their sexual arousal.