Hypothalamic-Pituitary
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The PVN Powerhouse: How the Paraventricular Nucleus Rules Hormone Secretion

Hold onto your hypothalamus, folks! We’re about to dive into the wild world of the Paraventricular Nucleus (PVN), where tiny cells pack a mighty hormonal punch!

Picture this: deep in the brain’s control center, the hypothalamus, sits the PVN – a cluster of cells that’s like the body’s own hormone factory. But we’re not talking about just any cells here. Oh no, we’re talking about the magnocellular neurosecretory cells, the big kahunas of the hormone world. Magnocellular neurosecretory cells are called “magnocellular” because of their relatively large size compared to other neurons in the brain. The term “magno” comes from the Latin word “magnus,” meaning “great” or “large,” and “cellular” refers to cells. These neurons are among the largest cells in the brain.

Specifically, magnocellular cells are characterized by their large size in comparison to parvocellular cells, which are smaller neurons found in the same regions. This size difference is particularly noticeable in the lateral geniculate nucleus of the thalamus, where magnocellular cells occupy distinct layers. The large size of these cells is not just a matter of appearance but also relates to their function. Their substantial cell bodies allow them to produce and store large amounts of hormone-containing vesicles, which is crucial for their role in secreting important hormones like oxytocin and vasopressin.

These magnocellular marvels are the brain’s very own brewmasters, crafting two of the body’s most important peptide hormones: oxytocin (the “love hormone”) and vasopressin (the “anti-diuretic hormone”). It’s like having a microbrew pub right in your brain, but instead of craft beers, they’re serving up custom-made hormones!

But wait, there’s more! These cells aren’t just content with making hormones. They’ve got to deliver them too. Each magnocellular cell sends out a single long axon, like a hormonal highway, stretching all the way to the posterior pituitary gland. And we’re not talking about a quiet country road here – each axon branches out into about 10,000 neurosecretory terminals, ready to unleash their hormonal cargo into the bloodstream.

And just when you thought these cells couldn’t get any cooler, they pull out another trick. Their dendrites (those branchy bits that usually receive signals) are also packed with hormone-filled vesicles. This means they can release oxytocin and vasopressin right into the brain too. It’s like having a local delivery service and a mail-order business all in one!

So there you have it, folks. The magnocellular cells of the PVN: tiny powerhouses pumping out love and water balance, one hormone at a time. Who knew the hypothalamus could be so hip?

Regulating the Hormone Factory: MNC Activity Control

Our magnocellular neurosecretory cells (MNCs) aren’t just hormone-producing powerhouses; they’re also finely tuned instruments responding to a symphony of signals:

Glial Cell Whispers: Local glial cells act like cellular DJs, modulating MNC activity through neurotransmitter management and metabolic support.

Self-Regulation: MNCs have their own internal rhythm, using intrinsic mechanisms to keep their hormone production on beat.

External Inputs: MNCs are always listening to the body’s needs:

Reproductive Rhythms: From the estrous cycle to pregnancy, lactation, and even mating, these cells adjust their tune to the reproductive melody.

Osmotic Orchestra: They’re the first to respond when your body’s water balance hits a sour note.

Cardiovascular Crescendos: Blood pressure and volume changes can cause these cells to change their tempo.

This multi-layered regulation ensures our MNCs are always playing the right hormonal tune, keeping our body’s symphony in perfect harmony. Whether it’s preparing for pregnancy, adjusting to dehydration, or responding to blood pressure changes, these cellular maestros are always ready to conduct the perfect hormonal performance!

Other Notes (Wikipedia)

CRH and TRH are secreted into the hypophyseal portal system and act on different targets neurons in the anterior pituitary. PVN is thought to mediate many diverse functions through these different hormones, including osmoregulation, appetite, and the response of the body to stress.

Magnocellular neurosecretory neurons

Magnocellular neurons of the PVN and SON project to the posterior pituitary

Magnocellular neurosecretory cells are large neuroendocrine cells within the supraoptic nucleus and paraventricular nucleus of the hypothalamus. They are also found in smaller numbers in accessory cell groups between these two nuclei, the largest one being the circular nucleus.

  • “BrainInfo”. braininfo.rprc.washington.edu. Retrieved 16 August 2022.

There are two types of magnocellular neurosecretory cells, oxytocin-producing cells and vasopressin-producing cells, but a small number can produce both hormones. These cells are neuroendocrine neurons, are electrically excitable, and generate action potentials in response to afferent stimulation.

Vasopressin is produced from the vasopressin-producing cells via the AVP gene, a molecular output of circadian pathways.[citation needed]

Magnocellular neurosecretory cells in rats (where these neurons have been most extensively studied) in general have a single long varicose axon, which projects to the posterior pituitary. Each axon gives rise to about 10,000 neurosecretory terminals and many axon swellings that store very large numbers of hormone-containing vesicles.

  • Ludwig, Mike; Leng, Gareth (31 January 2006). “Dendritic peptide release and peptide-dependent behaviours”. Nature Reviews Neuroscience. 7 (2): 126–136. doi:10.1038/nrn1845. PMID 16429122. S2CID 31018227.

These vesicles are released from the axon swellings and nerve terminals by exocytosis in response to calcium entry through voltage-gated ion channels, which occurs when action potentials are propagated down the axons.

The cells typically have two or three long dendrites, which also contain large dilations and a very high density of hormone-containing vesicles. Oxytocin and vasopressin can, thus, be released within the brain from these dendrites, as well as into the blood from the terminals in the posterior pituitary gland. However, the release of oxytocin and vasopressin from dendrites is not consistently accompanied by peripheral secretion, as dendritic release is regulated differently. Dendritic release can be triggered by depolarisation, but can also be triggered by the mobilisation of intracellular calcium stores. The dendrites receive most of the synaptic inputs from afferent neurons that regulate the magnocellular neurons; typically a magnocellular neuron receives about 10,000 synapses from afferent neurons.

The activity of magnocellular neurosecretory cells is regulated by local glial cells as well as through themselves (intrinsically). Their activity is also dependent on reproductive, osmotic, and cardiovascular inputs.

Parvocellular neurosecretory neurons

The axons of the parvocellular neurosecretory neurons of the PVN project to the median eminence, a neurohemal organ at the base of the brain, where their neurosecretory nerve terminals release their hormones at the primary capillary plexus of the hypophyseal portal system. The median eminence contains fiber terminals from many hypothalamic neuroendocrine neurons, secreting different neurotransmitters or neuropeptides, including vasopressin, corticotropin-releasing hormone (CRH), thyrotropin-releasing hormone (TRH), gonadotropin-releasing hormone (GnRH), growth hormone-releasing hormone (GHRH), dopamine (DA) and somatostatin (growth hormone release inhibiting hormone, GIH) into blood vessels in the hypophyseal portal system. The blood vessels carry the peptides to the anterior pituitary gland, where they regulate the secretion of hormones into the systemic circulation. The parvocellular neurosecretory cells include those that make:

Corticotropin-releasing hormone (CRH), which regulates ACTH secretion from the anterior pituitary gland

Vasopressin, which also regulates ACTH secretion (vasopressin and CRH act synergistically to stimulate ACTH secretion)

Thyrotropin-releasing hormone (TRH), which regulates TSH and prolactin secretion

Centrally-projecting neurons

As well as neuroendocrine neurons, the PVN contains interneurons and populations of neurons that project centrally (i.e., to other brain regions). The centrally-projecting neurons include

  • Parvocellular oxytocin cells, which project mainly to the brainstem and spinal cord. These neurons are thought to have a role in gastric reflexes and penile erection,
  • Parvocellular vasopressin cells, which project to many points in the hypothalamus and limbic system, as well as to the brainstem and spinal cord (these are involved in blood pressure and temperature regulation), and brown fat thermogenesis.
  • Parvocellular CRH neurons, which are thought to be involved in stress-related behaviors.

Afferent inputs

The PVN receives afferent inputs from many brain regions and different parts of the body, by hormonal control.

  • Fox SI (2011). Human Physiology (Twelfth ed.). McGraw Hill. p. 665.

Among these, inputs from neurons in structures adjacent to the anterior wall of the third ventricle (the “AV3V region”) carry information about the electrolyte composition of the blood, and about circulating concentrations of such hormones as angiotensin and relaxin, to regulate the magnocellular neurons.

Inputs from the brainstem (the nucleus of the solitary tract) and the ventrolateral medulla carry information from the heart and stomach. Inputs from the hippocampus to the CRH neurones are important regulators of stress responses.

Inputs from neuropeptide Y-containing neurons in the arcuate nucleus coordinate metabolic regulation (via TRH secretion) with regulation of energy intake.

Specifically, the projections from the arcuate nucleus seem to exert their effect on appetite via MC4R-expressing oxytocinergic cells of the PVN.

Inputs from suprachiasmatic nucleus about levels of lighting (circadian rhythms).

Inputs from glucose sensors within the brain stimulate release of vasopressin and corticotropin-releasing hormone from parvocellular neurosecretory cells.

Parvocellular neurosecretory cells are small neurons that produce hypothalamic releasing and inhibiting hormones. The cell bodies of these neurons are located in various nuclei of the hypothalamus or in closely related areas of the basal brain, mainly in the medial zone of the hypothalamus. All or most of the axons of the parvocellular neurosecretory cells project to the median eminence, at the base of the brain, where their nerve terminals release the hypothalamic hormones. These hormones are then immediately absorbed into the blood vessels of the hypothalamo-pituitary portal system, which carry them to the anterior pituitary gland, where they regulate the secretion of hormones into the systemic circulation.

Types

The parvocellular neurosecretory cells include those that make:

See also

References

  1. Ferguson AV, Latchford KJ, Samson WK (June 2008). “The paraventricular nucleus of the hypothalamus – a potential target for integrative treatment of autonomic dysfunction”. Expert Opinion on Therapeutic Targets. 12 (6): 717–27. doi:10.1517/14728222.12.6.717. PMC 2682920. PMID 18479218.
  2. Fox SI (2011). Human Physiology (Twelfth ed.). McGraw Hill. p. 665.
  3. Giuliano F, Allard J (August 2001). “Dopamine and sexual function”. International Journal of Impotence Research. 13 Suppl 3: S18-28. doi:10.1038/sj.ijir.3900719. PMID 11477488.
  4. Argiolas A, Melis MR (May 2005). “Central control of penile erection: role of the paraventricular nucleus of the hypothalamus”. Progress in Neurobiology. 76 (1): 1–21. doi:10.1016/j.pneurobio.2005.06.002. PMID 16043278. S2CID 24929538.
  5. Russell JA, Blackburn RE, Leng G (June 1988). “The role of the AV3V region in the control of magnocellular oxytocin neurons”. Brain Research Bulletin. 20 (6): 803–10. doi:10.1016/0361-9230(88)90095-0. PMID 3044525. S2CID 4762486.
  6. Beck B (July 2006). “Neuropeptide Y in normal eating and in genetic and dietary-induced obesity”. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 361 (1471): 1159–85. doi:10.1098/rstb.2006.1855. PMC 1642692. PMID 16874931.
  7. Konturek PC, Konturek JW, Cześnikiewicz-Guzik M, Brzozowski T, Sito E, Konturek SJ (December 2005). “Neuro-hormonal control of food intake: basic mechanisms and clinical implications” (PDF). Journal of Physiology and Pharmacology. 56 Suppl 6: 5–25. PMID 16340035.
  8. Nillni EA (April 2010). “Regulation of the hypothalamic thyrotropin releasing hormone (TRH) neuron by neuronal and peripheral inputs”. Frontiers in Neuroendocrinology. 31 (2): 134–56. doi:10.1016/j.yfrne.2010.01.001. PMC 2849853. PMID 20074584.
  9. Qin C, Li J, Tang K (September 2018). “The Paraventricular Nucleus of the Hypothalamus: Development, Function, and Human Diseases”. Endocrinology. 159 (9): 3458–3472. doi:10.1210/en.2018-00453. PMID 30052854.
  10. Hall, John E. (2021). Guyton and Hall Textbook of Medical Physiology. Michael E. Hall (14th ed.). Philadelphia, PA. pp. 931–932. ISBN 978-0-323-59712-8. OCLC 1129099861.
  11. Splittgerber, Ryan (2019). Snell’s Clinical Neuroanatomy. Richard S. Preceded by Snell (8th ed.). Philadelphia. pp. 379–380. ISBN 978-1-4963-4675-9. OCLC 1045082168.
  12. Sawchenko, PE (Dec 29, 1987). “Evidence for differential regulation of corticotropin-releasing factor and vasopressin immunoreactivities in parvocellular neurosecretory and autonomic-related projections of the paraventricular nucleus”. Brain Research. 437 (2): 253–63. doi:10.1016/0006-8993(87)91641-6. PMID 3325130. S2CID 38822848.
  13. Kovács, KJ; Sawchenko, PE (January 1996). “Sequence of stress-induced alterations in indices of synaptic and transcriptional activation in parvocellular neurosecretory neurons”. The Journal of Neuroscience. 16 (1): 262–73. doi:10.1523/JNEUROSCI.16-01-00262.1996. PMC 6578740. PMID 8613792.
  14. Ghamari-Langroudi, M.; Vella, K. R.; Srisai, D.; Sugrue, M. L.; Hollenberg, A. N.; Cone, R. D. (13 October 2010). “Regulation of Thyrotropin-Releasing Hormone-Expressing Neurons in Paraventricular Nucleus of the Hypothalamus by Signals of Adiposity”. Molecular Endocrinology. 24 (12): 2366–2381. doi:10.1210/me.2010-0203. PMC 2999480. PMID 20943814.
  15. Lennard, DE; Eckert, WA; Merchenthaler, I (April 1993). “Corticotropin-releasing hormone neurons in the paraventricular nucleus project to the external zone of the median eminence: a study combining retrograde labeling with immunocytochemistry”. Journal of Neuroendocrinology. 5 (2): 175–81. doi:10.1111/j.1365-2826.1993.tb00378.x. PMID 8485552. S2CID 9640772.
  16. Sawchenko, PE; Swanson, LW; Vale, WW (March 1984). “Co-expression of corticotropin-releasing factor and vasopressin immunoreactivity in parvocellular neurosecretory neurons of the adrenalectomized rat”. Proceedings of the National Academy of Sciences of the United States of America. 81 (6): 1883–7. Bibcode:1984PNAS…81.1883S. doi:10.1073/pnas.81.6.1883. PMC 345027. PMID 6369332.
  17. Horn, A. M.; Robinson, I. C. A. F.; Fink, G. (1 February 1985). “Oxytocin and vasopressin in rat hypophysial portal blood: experimental studies in normal and Brattleboro rats”. Journal of Endocrinology. 104 (2): 211–NP. doi:10.1677/joe.0.1040211. PMID 3968510.
  18. Freeman, ME; Kanyicska, B; Lerant, A; Nagy, G (October 2000). “Prolactin: structure, function, and regulation of secretion”. Physiological Reviews. 80 (4): 1523–631. doi:10.1152/physrev.2000.80.4.1523. PMID 11015620.
  19. Johnston, CA; Negro-Vilar, A (January 1988). “Role of oxytocin on prolactin secretion during proestrus and in different physiological or pharmacological paradigms”. Endocrinology. 122 (1): 341–50. doi:10.1210/endo-122-1-341. PMID 3335212.
  20. Watanobe, H; Takebe, K (April 1993). “In vivo release of neurotensin from the median eminence of ovariectomized estrogen-primed rats as estimated by push-pull perfusion: correlation with luteinizing hormone and prolactin surges”. Neuroendocrinology. 57 (4): 760–4. doi:10.1159/000126434. PMID 8367038.
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