Arcuate nucleus

The arcuate nucleus is a small part of the hypothalamus that plays an extremely important role in the regulation of hormone secretion from the pituitary gland, and in the regulation of appetite and body weight.

The arcuate nucleus is located in the mediobasal hypothalamus at the base of the brain, it lies on either side of the third ventricle and just above the median eminence. It includes several important populations of neurons, including:

• Neuroendocrine neurons with nerve endings in the median eminence, which release dopamine into the hypophysial portal blood to regulate the secretion of the hormone prolactin, which in turn controls the production of milk (lactogenesis). These are sometimes called the "tuberoinfundibular dopamine" (TIDA) neurons. In lactating females, TIDA neurons are inhibited by the stimulus of suckling. Dopamine released from their nerve endings at the median eminence is transported to the anterior pituitary gland, where it inhibits prolactin secretion, so, when the TIDA neurons are inhibited, prolactin secretion is increased. Dopamine neurons of the arcuate also inhibit the release of gonadotropin-releasing hormone, explaining in part why lactating (or otherwise hyperprolactinemic) women experience oligomenorrhea or amenorrhea (infrequency or absence of menses).

• Neuroendocrine neurons, mainly in the ventrolateral part of the nucleus, that make growth hormone-releasing hormone (GHRH). Like the TIDA neurons, these neurons have nerve endings in the median eminence. GHRH released into the hypophysial portal blood is transported to the anterior pituitary gland, where it stimulates the secretion of growth hormone. .
  
• Centrally-projecting neurons that contain neuropeptide Y (NPY), agouti-related protein (AGRP), and the inhibitory neurotransmitter GABA. These neurons, in the most ventromedial part of the nucleus, project strongly to the lateral hypothalamus and to the paraventricular nucleus of the hypothalamus, and are important in the regulation of appetite. The expression of NPY and AgRP is strongly increased after a period of fasting, and when activated, these neurons can produce ravenous eating when food is available. These neurons are regulated by circulating concentrations of leptin (a hormone secreted from fat cells (adipose tissue) and ghrelin (a hormone secretd from the stomach when it is empty). Destruction of these neurons leads to severe loss of appetite in experimental animals

• Centrally-projecting neurons that contain peptide products of pro-opiomelanocortin (POMC), and another appetite-inhibiting peptide, cocaine-and-amphetamine-regulating transcript (CART). These neurons project to many brain areas, including to all parts of the hypothalamus. They are also very important in the regulation of appetite, and, when activated, they inhibit feeding. These neurons are also regulated by circulating concentrations of leptin and ghrelin, and they are directly innervated by the NPY neurons. POMC neurons that project to the medial preoptic nucleus are also involved in the regulation of sexual behavior in both males and females.

• Centrally-projecting neurons that make somatostatin; the neurosecretory somatostatin neurons that regulate growth hormone secretion are a different population, located in the periventricular nucleus.

• A small population of neurons that make the potent appetite-stimulating peptide ghrelin. The role of this population is not known; many neurons in the arcuate nucleus have receptors for ghrelin, but these are thought to respond mainly to blood-borne ghrelin, secreted by the empty stomach.

• Neurons that synthesise kisspeptin, a peptide that is a major regulator of the secretion of luteinizing hormone releasing hormone (LHRH).

The arcuate nucleus also contains a population of specialized astrocytes, called tanycytes.