van Wijk DC et al. (2010), Ultrastructural and neurochemical architecture ...

XB-ART-40924

Gen Comp Endocrinol 2010 Sep 01;1682:293-301. doi: 10.1016/j.ygcen.2010.01.008.

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Ultrastructural and neurochemical architecture of the pituitary neural lobe of Xenopus laevis.

van Wijk DC , Meijer KH , Roubos EW .

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The melanotrope cell in the amphibian pituitary pars intermedia is a model to study fundamental aspects of neuroendocrine integration. They release alpha-melanophore-stimulating hormone (alphaMSH), under the control of a large number of neurochemical signals derived from various brain centers. In Xenopus laevis, most of these signals are produced in the hypothalamic magnocellular nucleus (Mg) and are probably released from neurohemal axon terminals in the pituitary neural lobe, to stimulate alphaMSH-release, causing skin darkening. The presence in the neural lobe of at least eight stimulatory factors implicated in melanotrope cell control has led us to investigate the ultrastructural architecture of this neurohemal organ, with particular attention to the diversity of neurohemal axon terminals and their neurochemical contents. Using regular electron microscopy, we here distinguish six types of neurohemal axon terminal, on the basis of the size, shape and electron-density of their secretory granule contents. Subsequently, we have identified the neurochemical contents of these terminal types by immuno-electron microscopy and antisera raised against not only the 'classical' neurohormones vasotocin and mesotocin but also brain-derived neurotrophic factor, cocaine- and amphetamine-regulated transcript peptide, corticotropin-releasing factor, metenkephalin, pituitary adenylyl cyclase-activating polypeptide, thyrotropin-releasing hormone and urocortin-1. This has revealed that each terminal type possesses a unique set of neurochemical messengers, containing at least four, but in some cases up to eight messengers. These results reveal the potential of the Mg/neural lobe system to release a wide variety of neurochemical messengers in a partly co-ordinated and partly differential way to control melanotrope cell activity as well as ion and water balance regulatory organs, in response to various, continuously changing, environmental stimuli.

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Species referenced: Xenopus laevis
Genes referenced: adcyap1 avp bdnf cartpt crh pomc trh ucn1

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	Fig. 1. Ultrastructure of part of the neural lobe of the pituitary gland of Xenopus laevis, with numerous neurohemal axon terminals (T) filled with secretory granules, near capillary (C) with erythrocyte (E). P, pituicyte. Bar = 2 μm.
	Fig. 2. Quantization of six axon terminal types in the neural pituitary lobe of X. laevis. (a) Relative frequency of each type of terminal, expressed as percentage of all (n = 1000) terminals. (b) Mean diameter of secretory granule profiles present in a particular type of terminal, averaged over all profiles of that terminal type, +standard error of the mean. Numbers above bars indicate the experimental group with which no statistically significant difference exists (P < 0.05).
	Fig. 3. Ultrastructure of the six types of axon terminal (1–6) in the X. laevis pituitary neural lobe. The types have been distinguished on the basis of the size, shape and electron-density of their secretory granules. Types 1–3 (a–c) terminals contain round secretory granules, which differ in size and electron-density. Type 4 (d) has flat as well as pleomorph granules, type 5 (e) shows granules with either a round or a pleomorph shape, and granules in type 6 (f) are rather small and pleomorph. Scale bar in panel a holds for the whole figure and =200 nm.
	Fig. 4. Immunogold electron microscopy of the six axon terminal types distinguished in the X. laevis pituitary neural lobe. (a) Type 1 with anti-BDNF; (b) type 1 with anti-AVT; (c) type 2 with anti-MT; (d) type 3 with anti-TRH; (e) type 3 with anti-CRF (silver-enhanced); (f) type 4 with anti-CARTp; (g) type 5 with anti-PACAP; (h) type 5 with anti-ME; (i) type 6 with anti-Ucn1. Scale bar in panel 1 holds for the whole figure, and =200 μm.