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In 1929, Harvey Cushing said of the hypothalamus, ‘Here in this well-concealed spot, almost to be covered with a thumb nail, lies the very main spring of primitive existence – vegetative, emotional and reproductive – on which with more or less success, man has come to superimpose a cortex of inhibitions’. The hypothalamus is the single most important integrator of vegetative and endocrine regulation of the body. It controls vital bodily processes including cardiovascular regulation, sleep, metabolism, stress, thermoregulation, water and electrolyte balance, appetite regulation, sexual behavior and, together with the master gland (the anterior pituitary), endocrine and immune responses.

Within the hypothalamus, neuroendocrine neurons have proven to be fascinating model systems for studying hormone- and activity-dependent plasticity both during development (Simerly, 2005) and in adulthood (Hatton, 1997; Theodosis et al., 2008; Prevot et al., 2010). Neurons of the hypothalamo–neurohypophyseal complex release oxytocin and vasopressin directly into the general circulation. In contrast, neurons of the hypothalamo–adenohypophyseal complex project to the median eminence of the hypothalamus where they make contact with the basal lamina and open into the pericapillary space of the primary hypophyseal portal plexus. Upon reaching the pituitary portal system, neurohormones travel to the adenohypophysis to stimulate the synthesis and secretion of pituitary hormones. Blood-borne pituitary hormones then act on target glands to direct their function and promote peripheral hormone secretion, which in turn feeds back into the brain to modulate its activity (Fig. 1).

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Figure 1.  Neuroendocrine systems are the final common pathway for the central control of neurohormone secretion. As such, they are subject to a complex array of excitatory and inhibitory trans-synaptic inputs that modulate their activity. Whereas neurons of the hypothalamo–neurohypophyseal complex directly release oxytocin (OX) and vasopressin (VP) into the general circulation, neurons of the hypothalamo–adenohypophyseal complex project to the median eminence of the hypothalamus, where they make contact with the basal lamina and open into the pericapillary space of the primary hypophyseal portal plexus. Upon reaching the pituitary portal system, neurohormones travel to the pituitary to stimulate the synthesis and secretion of pituitary hormones. Blood-borne pituitary hormones act on target glands (the gonads, thyroid, adrenal, etc.) to regulate their function. Within the brain and pituitary, hormones produced by these glands influence the activity of the endocrine axes via feedback loops. Daily and seasonal rhythms in physiology and behavior normally synchronize to environmental cycles of light–dark availability. EAA, excitatory amino acids; NA, noradrenalin; GABA, gamma-aminobutyric acid; GnRH, gonadotropin-releasing hormone; CRH, corticotropin-releasing hormone; TRH, thyrotropin-releasing hormone; LH, luteinizing hormone; FSH, follicle-stimulating hormone; ACTH, adrenocorticotropic hormone; TSH, thyroid-stimulating hormone; T4, thyroxin. Adapted from Prevot et al. (2010) with permission.

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Current models of neuronal plasticity stress the importance of functional and biochemical adaptations of synaptic transmission (Pozo & Goda, 2010). However, in the hypothalamus as in most areas of the central nervous system, the classic notion of one-way communication at the chemical synapse, the functional unit for the transmission of information between the nerve terminal and its target, is being re-evaluated on the strength of evidence indicating that astrocytes, the presumed electrically silent cohabitants of the nervous system, could be a critical third element of the synapse (Araque et al., 1999; Haydon, 2001; Volterra & Meldolesi, 2005; Martineau et al., 2006; Bains & Oliet, 2007; Barres, 2008; Pfrieger, 2010). In the present special issue of the European Journal of Neuroscience, six review articles address these different aspects of neural plasticity (Azcoitia et al., 2010; Wamsteeker & Bains, 2010; Franceschini et al., 2010; Ojeda et al., 2010; Oliet & Bonfardin, 2010; Sudbury et al., 2010). In addition, emerging concepts regarding adult neurogenesis (Kriegstein & Alvarez-Buylla, 2009), suggesting that pre-existing neural and endocrine networks can morphologically and functionally integrate newborn cells to adapt to physiological demand and/or environmental constraints, add a further level of complexity. Three review articles in the present issue address the topic of cellular plasticity in both the hypothalamus (Migaud et al., 2010) and pituitary (Rizzoti, 2010; Vankelecom, 2010). Two additional articles discuss the mechanisms by which multiple external stimuli converge on a single cell in the pituitary to effect plastic changes in its secretory activity (Roubos et al., 2010), and those by which endocrine systems coordinate cell activity and adapt to evolving needs in terms of hormone release, under the control of the hypothalamus (Schaeffer et al., 2010).

Intriguingly, the very same hormonal cues (e.g. gonadal steroids) that regulate hypothalamic circuits via neuroendocrine feedback loops in the mature brain also specify brain architecture by regulating key developmental events during critical windows in mammals, or by regulating environment-mediated changes throughout life in birds and fish. Three reviews in the present issue focusing on sexual differentiation will exemplify this in different vertebrate species (Balthazart et al., 2010; Lenz & McCarthy, 2010; Page et al., 2010).

This issue also addresses the key role that the suprachiasmatic nucleus plays as a circadian pacemaker, primarily entraining the organism to light/dark information and controlling rhythmicity of melatonin synthesis from the pineal gland (Challet, 2007), for the daily and seasonal regulation of neuroendocrine systems (Girardet et al., 2010; Meijer et al., 2010). Finally, the last paper of this issue especially focuses on neuronal plasticity in seasonal reproduction (Lehman et al., 2010).

In all, it is my hope that this special issue will become a useful and handy reference for scientists working in the exciting field of neuroendocrinology, and that it will also help to attract the interest of other neuroscientists not as yet involved in this fascinating area of research.

Acknowledgements

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  2. Acknowledgements
  3. References

I would like to thank the neuroendocrinologists who submitted their work for publication in this special issue of the European Journal of Neuroscience. The issue has been prepared in association with the satellite symposium on ‘Plasticity of Neuroendocrine Systems’ of the 7th International Congress of Neuroendocrinology (Rouen, France, 2010). The symposium was held in Tours, France, on July 9 and 10, 2010, under the direction of the chairs of the local organizing committee, Yves Tillet and Isabelle Franceschini (INRA, Nouzilly), who made the meeting a tremendous success. Finally, I gratefully acknowledge the support of the Editors-in-Chief, Jean-Marc Fritschy and Martin Sarter, and their Editorial Assistants, Sue Fromant and Angela Cole, for assembling this issue.

References

  1. Top of page
  2. Acknowledgements
  3. References