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Median Preoptic Nucleus

A. The median preoptic nucleus is located in the preoptic area of the hypothalamus

a. The Hypothalamus is located beneath the Thalamus, the sensory relay station of the brain

b. The Preoptic area is located in the anterior hypothalamus

B. This nucleus is involved in osmoregulation and thermoregulation as well as homeostatic sleep regulation.

a. The median preoptic nucleus is involved in regulation renal function in response to changes in blood sodium concentration

b. The median preoptic nucleus has regulatory function in body temperature and fever responses

c. The median preoptic nucleus is involved in the inhibition of wakefulness areas

Anatomical Position

A.The median preoptic nucleus is an integral part of the anteroventral third ventricle of the forebrain

a. Area critically involved in regulation of blood pressure.

b. Parrallel pathways in this region regulate body temperature

B. The median preoptic nucleus is involved in fluid and cardiovascular homeostasis, among other functions.

C. It is located dorsal to the other three preoptic nuclei .

Role in Theroregulation

A. The median preoptic nucleus has inhibitory effects on areas of the brain which raise body temperature

a. Projections from the median preoptic nucleus to the rostral medullary raphe express GABA

b. Inputs to the RMR express inhibitory receptors

B. Lesions on the Median preoptic nucleus result in changes in regulation of body temperature .

a. Elevated baseline body temperature

b. Loss of fever responses

Role in Osmoregulation

A. The median preoptic nucleus is involved in the regulation of renal sympathetic nerve activity

a. Release of various effectors such as oxytocin and atrial natriuretic peptide.

b. Following sudden spike in blood sodium concentration

B. Stimulation of adrenergic neurons in the median preoptic nucleus

a. Noradrenergic projections from the caudal ventrolateral medulla

b. Stimulation in response to changes in tonicity and circulating volume .

Role in Homeostatic Regulation of Sleep

Based on anatomical and electrophysiological experiments on adult rats an importnat role for the median preoptic nucleus in the production of sleep. The first evidence of this was an observation that damage to this area caused insommnia in human patients . Current experiments using c-Fos expression as a marker for activation of neurons during sleep shows a dichotomy of function in sleep promotion and maintenance between the ventrolateral preoptic nucleus and the median preoptic nucleus. Evidence suggests that GABAergic neurons in the median preoptic nucleus play a greater role in the promotion of the onset of sleep, while neurons in the ventrolateral preoptic nucleus play a greater role in the maintenance of sleep . While the idea of a complete separation of function between these two nuclei is an attractive one, it is more likely a question of degree of involvement of these two nuclei in the onset and maintenance of sleep, rather than playing completely separate roles.

The ventrolateral (VLPO) and median preoptic (MnPn) nuclei promote sleep through GABAergic neuronal projections to wake-active areas of the brain. Activation of the neurons in the VLPO and MnPn leads to increased concentrations of the primary inhibitory neurotransmitter, GABA, in wakefulness areas of the brain, such as the tuberomammillary nucleusand the locus coeruleus. This leads to the inhibition of cholinergic, noradrenergic and serotinergic activity in these areas. Noradrenergic projections from wake-promoting areas inhibit sleep-promoting areas, establishing a "reciprocal inhibitory interaction" between sleep and wakefulness areas which leads to the regulation of sleep patterns. The mechanism for the activation of the sleep-promoting neurons in the VLPO and MnPn has not been well defined, however, it has been suggested that the suprachiasmic nucleus may play a role, as well as simply decreased sensory input at the onset of sleepiness .

References

Gvilia, I., Suntsova, N., Angara, B., McGinty, D., & Szymusiak, R. (2011). Maturation of sleep homeostasis in developing rats: a role for preoptic area neurons. American Journal of Physiology-Regulatory Integrative and Comparative Physiology, 300(4), R885-R894

Luppi, P. H., & Fort, P. (2011). What are the mechanisms activating the sleep-active neurons located in the preoptic area? Sleep and Biological Rhythms, 9, 59-64.

Pedrino, G. R., Monaco, L. R., & Cravo, S. L. (2009). Renal vasodilation induced by hypernatraemia Role of alpha-adrenoceptors in the median preoptic nucleus. Clinical and Experimental Pharmacology and Physiology, 36(12)

Yoshida, K., Li, X. D., Cano, G., Lazarus, M., & Saper, C. B. (2009). Parallel Preoptic Pathways for Thermoregulation. Journal of Neuroscience, 29(38)