Magnocellular Neurons

Abstract

Magnocellular neurons (MCN) are neuroendocrine cells located in the hypothalamus; they are among the largest cells in the brain, and synthesise the hormones arginine vasopressin (AVP) and oxytocin (OT). These neuropeptides are secreted from MCN terminals in the neurohypophysis (NH). It is this hypothalamic‐neurohypophysial system (HNS) which allowed the original formulation of the calcium hypothesis for stimulus–secretion coupling. Vasopressin is a vasoconstrictor and an antidiuretic and, thus, is involved in fluid homoeostasis. OT has recognised functions in parturition and lactation, and has an emerging role as a natriuretic agent. Both hormones may also be central neurotransmitters and have been implicated in stress, social behaviours, learning and memory processes, as well as the development and maintenance of tolerance to ethanol and other drugs of abuse. Furthermore, OT has possible therapeutic applications for social disorders.

Key Concepts:

  • MCNs in the hypothalamus synthesise the peptide hormones, vasopressin and oxytocin.

  • These neuropeptides are secreted from MCN nerve terminals in the NH.

  • It is this HNS system which allowed the original formulation of the calcium hypothesis for stimulus–secretion coupling.

  • AVP is a vasoconstrictor and an antidiuretic and, thus, is involved in fluid homoeostasis.

  • Oxytocin has recognised functions in parturition and lactation, and has an emerging role as a natriuretic agent.

  • Both hormones may also be central neurotransmitters and have been implicated in stress, learning and memory processes, social behaviours, as well as the development and maintenance of tolerance to ethanol and other drugs of abuse.

  • Oxytocin has possible therapeutic applications for social disorders.

Keywords: vasopressin; oxytocin; hypothalamus; neurohypophysis; posterior pituitary; osmoregulation; lactation

Figure 1.

Anatomy of magnocellular neurons. Schematic drawing of MCNs in the hypothalamic supraoptic nucleus and paraventricular nucleus projecting via hypothalamic and hypophyseal tracts to the neurohypophysis and adenohypophysis, the latter arising mainly from parvocellular neurons in the Paraventricular nucleus. Note axons projecting to other CNS targets and afferent projections from osmoreceptors in the subfornical organ and the organum vasculosum of the lamina terminalis.

Figure 2.

Model of stimulus–secretion coupling at neurohypophysial (NH) terminals. Action potentials invading the terminal lead to its depolarisation and the opening of voltage‐dependent calcium channels. These channels are differentially distributed in the NH terminals: the (L) type are found diffusely throughout, whereas the (N) type appears to be co‐localised with release sites in both types of terminal. The (Q) type appears to be found only in arginine vasopressin (AVP)‐releasing NH terminals, whereas the R‐type is only on oxytocin‐releasing NH terminals (not shown). Intracellular Ca2+ release from ryanodine receptor (RyR) channels could also contribute to intracellular Ca2+ [Ca]. Co‐released adenosine triphosphate (ATP) can act via P2X receptors to increase [Ca] and subsequent AVP release. After its breakdown by ectoATPases, Adenosine inhibits release via the A1 receptor's membrane‐delimited pathway (shown by arrow). DynA, an endogenous opioid, co‐release is another of many examples of such autoregulation in the NH.

Figure 3.

Model of hormonal feedback regulation of magnocellular neurons (MCNs). The hormones, oxytocin (OT) and arginine vasopressin (AVP), perhaps released locally from dendrites, increase Ca2+ levels in OT‐ and AVP‐synthesising MCNs, respectively. This autofeedback is mediated via specific receptors and involves different intracellular mechanisms. Although both types of MCN express T‐, L‐, N‐, P‐, Q‐ and R‐type Ca2+ channels, the AVP response is mediated mainly by T, L and N types whereas voltage‐gated channels are not involved in the OT response. This response is via IP3‐mediated release of intracellular Ca2+. Courtesy of Dr. Govindan Dayanithi, Montpellier, France.

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Further Reading

Armstrong WE and Hatton GI (2006) The puzzle of pulsatile oxytocin secretion during lactation: some new pieces. American Journal of Physiology ‐ Regulatory, Integrative and Comparative Physiology 291(1): R26–R28.

Bourque CW and Oliet SHR (1997) Osmoreceptors in the CNS. Annual Review of Physiology 59: 601–619.

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Theodosis DT, Poulain DA and Oliet SHR (2008) Activity‐dependent structural and functional plasticity of astrocyte‐neuron interactions. Physiological Reviews 88(3): 983–1008.

Ueta Y, Fujihara H, Dayanithi G, Kawata M and Murphy D (2008) Specific expression of optically active reporter gene in arginine vasopressin‐secreting neurosecretory cells in the hypothalamic‐neurohypophyseal system. Journal of Neuroendocrinology 20(6): 660–664.

Veenema AH and Neumann ID (2008) Central vasopressin and oxytocin release: regulation of complex social behaviours. Progress in Brain Research 170: 261–276.

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Lemos, José R(Jun 2012) Magnocellular Neurons. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000176.pub2]