Survival and regeneration of neurons of the supraoptic nucleus following surgical transection of neurohypophysial axons depend on the existence of collateral projections of these neurons to the dorsolateral hypothalamus

Structure and function in the conceptual development of mammalian neuroendocrinology between 1920 and 1965

With the growing realization in the 1930s that the brain played a crucial role in regulating the secretions of the pituitary gland, neuroendocrinology as we now know it developed from two rather separate directions. One approach relied heavily on morphological techniques to define neurosecretion; a novel, but for many years flawed model that was originally developed to explain the presence of gland-like cells in the diencephalon. During its first 20 years neurosecretion, as a concept, made no significant contribution to our understanding of how the pituitary was controlled. Then, following the identification by Sanford Palay and Wolfgang Bargmann of a continuous neurosecretory pathway from the hypothalamus to the neural lobe, neurosecretion became incorporated into a more broadly based concept of pituitary function, particularly regarding the neural lobe. The second approach integrated structural and functional methods to investigate neural regulation of the pituitary. This work eventually explained how the pituitary was controlled by the brain. It led directly to our understanding of the control of vasopressin and oxytocin release by neuroendocrine terminals in the neural lobe, the neurohumoral control of the pars distalis, and eventually to a detailed description of the neural networks that control pituitary function. As increasingly sophisticated morphological, neurophysiological, and eventually molecular biological techniques were applied to the problem, the original notion of the diencephalic gland and neurosecretion became unsustainable. The gland-nerve cells of the 1930s became the neurosecretory cells of the 1940s and 1950s, and then finally neuroendocrine neurons in the 1960s. From then on neuroendocrinology developed into the more unified discipline we know today. The chronology of these two approaches will be examined here using examples from research that occurred approximately between 1920 and 1965. The goal is not to give a comprehensive history of pituitary function or neuroendocrinology. Instead, the focus will be to compare the rationales and effectiveness of two contrasting experimental approaches: predominantly structural analyses as opposed to more integrated approaches.

Effects of ciliary neurotrophic factor and leukemia inhibiting factor on oxytocin and vasopressin magnocellular neuron survival in rat and mouse hypothalamic organotypic cultures

Organotypic cultures of mouse and rat magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS) have served as important experimental models for the molecular and physiological study of this neuronal phenotype. However, it has been difficult to maintain significant numbers of the MCNs, particularly vasopressin MCNs, in these cultures for long periods. In this paper, we describe the use of the neurotrophic factors, leukemia inhibiting factor (LIF) and ciliary neurotrophic factor (CNTF) to rescue rat vasopressin (Avp)- and oxytocin (Oxt)-MCNs from axotomy-induced, programmed cell death in vitro. Quantitative data are presented for the efficacy of the LIF family of neurotrophic factors on the survival of MCNs in three nuclei, the paraventricular (PVN), supraoptic (SON), and accessory (ACC) nuclei in the mouse and rat hypothalamus.

Apoptosis of supraoptic AVP neurons is involved in the development of central diabetes insipidus after hypophysectomy in rats

Background

It has been reported that various types of axonal injury of hypothalamo-neurohypophyseal tract can result in degeneration of the magnocellular neurons (MCNs) in hypothalamus and development of central diabetes insipidus (CDI). However, the mechanism of the degeneration and death of MCNs after hypophysectomy in vivo is still unclear. This present study was aimed to disclose it and to figure out the dynamic change of central diabetes insipidus after hypophysectomy.

Results

The analysis on the dynamic change of daily water consumption (DWC), daily urine volume(DUV), specific gravity of urine(USG) and plasma vasopressin concentration showed that the change pattern of them was triphasic and neuron counting showed that the degeneration of vasopressin neurons began at 10 d, aggravated at 20 d and then stabilized at 30 d after hypophysectomy. There was marked upregulation of cleaved Caspase-3 expression of vasopressin neurons in hypophysectomy rats. A "ladder" pattern of migration of DNA internucleosomal fragments was detected and apoptotic ultrastructure was found in these neurons. There was time correlation among the occurrence of diabetes insipidus, the changes of plasma vasopressin concentration and the degeneration of vasopressin neurons after hypophysectomy.

Conclusion

This study firstly demonstrated that apoptosis was involved in degeneration of supraoptic vasopressin neurons after hypophysectomy in vivo and development of CDI. Our study on time course and correlations among water metabolism, degeneration and apoptosis of vasopressin neurons suggested that there should be an efficient therapeutic window in which irreversible CDI might be prevented by anti-apoptosis.

Bcl-xL and caspase inhibition increase the survival of rat oxytocin and vasopressin magnocellular neurons in organotypic culture

Hypothalamic magnocellular neurons (MCNs) are highly vulnerable to axotomy-induced cell death in vivo and in vitro. In this study, we determined whether the anti-apoptotic agent Bcl-xL, a member of the Bcl-2 family which prevents programmed cell death in the central nervous system, can rescue oxytocin (OT) and vasopressin (VP) MCNs in the supraoptic nucleus (SON) in organotypic culture. We found that the novel, membrane permeant form of Bcl-xL that we employed in these studies protected both OT and VP MCNs from degeneration as long as the Bcl-xL was present in the medium. In contrast, z-VAD-fmk, an inhibitor of caspases that are involved in apoptosis, was less effective in that it significantly rescued OT MCNs (P < 0.01) but not VP MCNs (P > 0.09). Unlike the Bcl-xL, Z-VAD-fmk's effectiveness in reducing MCN cell death was not sustained for the full 15 days in vitro.

Neural activity protects hypothalamic magnocellular neurons against axotomy-induced programmed cell death

Axotomy typically leads to retrograde neuronal degeneration in the CNS. Studies in the hypothalamo-neurohypophysial system (HNS) have suggested that neural activity is supportive of magnocellular neuronal (MCN) survival after axotomy. In this study, we directly test this hypothesis by inhibiting neural activity in the HNS, both in vivo and in vitro, by the use of tetrodotoxin (TTX). After median eminence compression to produce axonal injury, unilateral superfusion of 3 microM TTX into the rat supraoptic nucleus (SON), delivered with the use of a miniature osmotic pump for 2 weeks in vivo, produced a decrease in the number of surviving MCNs in the TTX-treated SON, compared with the contralateral untreated side of the SON. In vitro application of 2.5 microM TTX for 2 weeks to the SON in organotypic culture produced a 73% decrease in the surviving MCNs, compared with untreated control cultures. Raising the extracellular KCl in the culture medium to 25 mM rescued the MCNs from the axotomy- and TTX-induced cell death. These data support the proposal that after axotomy, neural activity is neuroprotective in the HNS.

Synchronization of oxytocin neurons in suckled rats: possible role of bilateral innervation of hypothalamic supraoptic nuclei by single medullary neurons

We have previously shown that oxytocin neurons located in the four hypothalamic magnocellular nuclei display synchronous bursts of action potentials before each milk ejection. The mechanisms involved in such a synchronization have, however, not yet been elucidated. In this study, we test the hypothesis of an extranuclear synchronization arising from a common extrahypothalamic input innervating bilateral magnocellular nuclei. First, two different retrograde tracers were injected into the right and left supraoptic nuclei of rats that were fixed 5-7 days later. Each tracer labelled numerous neurons in various brain regions ipsilateral or contralateral to the injection site, but colocalization of the two tracers within the same cell body could only be detected bilaterally in neurons in the ventromedial regions of the medulla oblongata. The axonal projections of these medullary neurons were then visualized by the unilateral microinjection of an anterograde tracer (BDA) within the ventromedial medulla oblongata. BDA-labelled axons afferent to the hypothalamus were found to branch towards both supraoptic nuclei through medial portions of the optic chiasma. Finally, in anaesthetized lactating rats, surgical lesions were placed medially through the optic chiasma and the electrical activity of oxytocin neurons in bilateral supraoptic nuclei was pair-recorded during suckling. The incidence of synchronous bursts in oxytocin neurons located within bilateral supraoptic nuclei were dramatically altered only when the medial portions of the optic chiasma were totally lesioned. Taken together, these data suggest that medullary neurons afferent to bilateral supraoptic nuclei are involved in the recruitment and synchronization of bursting in oxytocin neurons during suckling.