Vasopressin (VP) is an antidiuretic hormone that is synthesized and released by osmosensitive magnocellular neurosecretory cells (MNCs) to regulate water homeostasis in the body. The rate and firing pattern of MNCs determines the amount of VP release, which is elevated during physiological stress particularly dehydration. During acute osmotic changes the MNCs shrink and swell due to hypertonic and hypotonic stimuli, respectively. In contrast to hippocampal neurons, which display regulatory volume increases (RVI) and regulatory volume decreases (RVD) in response to hypertonic and hypotonic stimuli, MNCs do not have compensatory mechanisms. The MNCs undergo hypertrophy as a part of their physiological structural and functional plasticity during chronic dehydration. These changes are thought to be important during long term osmotic changes for the sustained and high level releases of hormone. However, the mechanism of hypertrophy is still unclear and it is difficult to address this issue in vivo. We therefore undertook studies on acutely isolated MNCs to test hypertrophy in MNCs. We observed that acutely isolated MNCs treated with hyperosmolar solution (325 mOsmol kg-1) for 150 minutes in vitro showed hypertrophy (a 9% increase in CSA) and recovered their original size when returned to isotonic solution (295 mOsmol kg-1) for another 60 minutes. Whole cell patch clamp experiments showed a 34% increase in cell membrane capacitance following treatment with hypertonic solution for 90-150 minutes. The osmotically-evoked hypertrophic response was blocked by using a TAT (human immunodeficiency virus transactivator of transcription) peptide (TAT-NSF700) that prevents SNARE-mediated exocytotic fusion by blocking the function of NSF (N-ethylmaleimide-sensitive factor). The hypertrophic response did not appear to be altered by a scrambled version of the peptide, showing that osmotically-evoked hypertrophy depends on SNARE-mediated exocytotic fusion. The VP and OT-MNCs exposed to hyperosmolar solution for two hours showed increased immunofluorescence for L-type Ca²⁺ channels (both Cav1.2 and Cav1.3). Our data suggest that the osmotically-evoked hypertrophy is associated with an increase in the total membrane surface area due to the exocytotic fusion of intracellular granules with the plasma membrane and with increased expression of L-type Ca2+ channels. This study will be helpful in understanding of the adaptation that MNCs undergo during long term dehydration and pathological conditions that lead to increased plasma osmolality.

Vasopressin (VP) is an antidiuretic hormone that is synthesized and released by osmosensitive magnocellular neurosecretory cells (MNCs) to regulate water homeostasis in the body. The rate and firing pattern of MNCs determines the amount of VP release, which is elevated during physiological stress particularly dehydration. During acute osmotic changes the MNCs shrink and swell due to hypertonic and hypotonic stimuli, respectively. In contrast to hippocampal neurons, which display regulatory volume increases (RVI) and regulatory volume decreases (RVD) in response to hypertonic and hypotonic stimuli, MNCs do not have compensatory mechanisms. The MNCs undergo hypertrophy as a part of their physiological structural and functional plasticity during chronic dehydration. These changes are thought to be important during long term osmotic changes for the sustained and high level releases of hormone. However, the mechanism of hypertrophy is still unclear and it is difficult to address this issue in vivo. We therefore undertook studies on acutely isolated MNCs to test hypertrophy in MNCs. We observed that acutely isolated MNCs treated with hyperosmolar solution (325 mOsmol kg-1) for 150 minutes in vitro showed hypertrophy (a 9% increase in CSA) and recovered their original size when returned to isotonic solution (295 mOsmol kg-1) for another 60 minutes. Whole cell patch clamp experiments showed a 34% increase in cell membrane capacitance following treatment with hypertonic solution for 90-150 minutes. The osmotically-evoked hypertrophic response was blocked by using a TAT (human immunodeficiency virus transactivator of transcription) peptide (TAT-NSF700) that prevents SNARE-mediated exocytotic fusion by blocking the function of NSF (N-ethylmaleimide-sensitive factor). The hypertrophic response did not appear to be altered by a scrambled version of the peptide, showing that osmotically-evoked hypertrophy depends on SNARE-mediated exocytotic fusion. The VP and OT-MNCs exposed to hyperosmolar solution for two hours showed increased immunofluorescence for L-type Ca²⁺ channels (both Cav1.2 and Cav1.3). Our data suggest that the osmotically-evoked hypertrophy is associated with an increase in the total membrane surface area due to the exocytotic fusion of intracellular granules with the plasma membrane and with increased expression of L-type Ca2+ channels. This study will be helpful in understanding of the adaptation that MNCs undergo during long term dehydration and pathological conditions that lead to increased plasma osmolality.

The magnocellular neurosecretory cells (MNCs) of the hypothalamus are responsible for the synthesis and secretion of vasopressin (VP), which is important for fluid homeostasis, and oxytocin (OT), which is responsible for uterine contraction during parturition and milk let-down during lactation. VP-ergic MNCs undergo a number of structural and functional changes during dehydration, including the adoption of a bursting pattern of firing, the retraction of glial processes from MNC somata and terminals, the translocation of kappa-opioid receptors from internal stores to the plasma membrane, and the somatodendritic release of VP and OT. Since voltage-gated Ca2+ channels have been found on intracellular granules, and since an increase in Ca2+ current could regulate firing patterns and neuropeptide release, the surface expression of Ca2+ channel subtypes in MNCs was tested to determine if it would be altered by 16-24 hours of water deprivation. Using radioligand binding of antagonists of N-type and L-type Ca2+ channels, channel density was measured in the supraoptic nucleus (SON), which is largely composed of MNC somata, and in the neurohypophysis (NH), which is largely composed of MNC terminals. Dehydration caused an increase in the density of L-type channels in the SON, while causing no significant change in the N-type density. No change in density of either channel type was observed in the NH. Electrophysiological measurements in isolated MNC somata showed no change in total Ca2+ current, but a significant increase in the nifedipine-sensitive current following dehydration. Reverse transcription-polymerase chain reaction (RT-PCR) demonstrated no increase in messenger RNA levels for L-type channels, suggesting that the increase in channel density is not a consequence of de novo synthesis. These results suggest that L-type Ca2+ channels may be translocated from internal stores to the plasma membrane of MNCs in response to dehydration. Such a process may be important in maximizing secretion of VP when the physiological need is high.

The magnocellular neurosecretory cells (MNCs) of the hypothalamus are responsible for the synthesis and secretion of vasopressin (VP), which is important for fluid homeostasis, and oxytocin (OT), which is responsible for uterine contraction during parturition and milk let-down during lactation. VP-ergic MNCs undergo a number of structural and functional changes during dehydration, including the adoption of a bursting pattern of firing, the retraction of glial processes from MNC somata and terminals, the translocation of kappa-opioid receptors from internal stores to the plasma membrane, and the somatodendritic release of VP and OT. Since voltage-gated Ca2+ channels have been found on intracellular granules, and since an increase in Ca2+ current could regulate firing patterns and neuropeptide release, the surface expression of Ca2+ channel subtypes in MNCs was tested to determine if it would be altered by 16-24 hours of water deprivation. Using radioligand binding of antagonists of N-type and L-type Ca2+ channels, channel density was measured in the supraoptic nucleus (SON), which is largely composed of MNC somata, and in the neurohypophysis (NH), which is largely composed of MNC terminals. Dehydration caused an increase in the density of L-type channels in the SON, while causing no significant change in the N-type density. No change in density of either channel type was observed in the NH. Electrophysiological measurements in isolated MNC somata showed no change in total Ca2+ current, but a significant increase in the nifedipine-sensitive current following dehydration. Reverse transcription-polymerase chain reaction (RT-PCR) demonstrated no increase in messenger RNA levels for L-type channels, suggesting that the increase in channel density is not a consequence of de novo synthesis. These results suggest that L-type Ca2+ channels may be translocated from internal stores to the plasma membrane of MNCs in response to dehydration. Such a process may be important in maximizing secretion of VP when the physiological need is high.

Magnocellular neurosecretory cells (MNCs) are neuroendocrine cells with somata located in the hypothalamus and nerve terminals in the posterior pituitary. They receive neuronal inputs from the brain and release vasopressin and oxytocin into the blood to regulate many important functions such as water balance, lactation, and parturition. The process of hormone release depends on Ca2+ influx mediated by voltage-gated Ca2+ channels (VGCCs) on the plasma membranes of neuroendocrine cells. To better understand the cellular and molecular components that are involved in regulating secretory vesicle exocytosis, this thesis work was conducted to investigate the expression and function of different subtypes of VGCCs in MNCs and pituicytes (the glial cells surrounding MNC nerve terminals).<p> Molecular biology, immunohistochemistry and cellular biology were used to detect expression and alternative splicing of different VGCC subtypes in MNCs, neurons, and pituicytes. First, the presence of CaV2.2 and CaV2.3 channels were detected on the pituicytes in situ. When the pituicytes were isolated and cultured for 14 days, more VGCC subtypes were expressed including CaV1.2 channels. Regulation of VGCC expression was measured in normal and dehydrated rats, and CaV1.2 channels were found to be selectively up-regulated in pituicytes after 24 hours of dehydration.<p> Second, two splice variants of CaV2.1 channels (CaV2.1Ä1 and Ä2) that lack a large portion of the synprint (synaptic protein interaction) site were detected in the rat brain. To determine whether the splice variants were expressed in MNCs, we did immunocytochemistry using two antibodies (the selective and the inclusive antibody) that recognized the carboxyl-terminus of channels and the synprint site, respectively, in different cell types. We found that vasopressin MNCs, but not the oxytocin MNCs, and one type of endocrine cell (the melanotropes of the pituitary gland) expressed the synprint site deleted variants, whereas the hippocampal neurons mainly expressed the full-length isoform. The splice variants were properly distributed on the plasma membrane of the somata and nerve terminals of the MNCs, suggesting the synprint site is not essential for CaV2.1 channel targeting into the nerve terminals of neuroendocrine cells.<p> Third, expression and distribution of CaV2.2 channels were studied in the MNCs. All CaV2.2 isoforms we detected contained the full-length synprint site. To test the importance of the CASK/Mint1 binding site for CaV2.2 channel targeting, we over-expressed a peptide that inhibits the interaction between CaV2.2 channels and CASK/Mint1 in differentiated PC12 cells (a neuroendocrine cell line). We found that the distribution of CaV2.2 channels in the growth cones of PC12 cells were significantly decreased, suggesting that the CASK/Mint1 interaction is important for CaV2.2 channel targeting into the neuroendocrine terminals.<p> In conclusion, these results provide insights of VGCC expression in neuroendocrine cells, and also give rise to a better understanding of the molecular components that are involved in forming the exocytotic machinery in these cells.

Magnocellular neurosecretory cells (MNCs) are neuroendocrine cells with somata located in the hypothalamus and nerve terminals in the posterior pituitary. They receive neuronal inputs from the brain and release vasopressin and oxytocin into the blood to regulate many important functions such as water balance, lactation, and parturition. The process of hormone release depends on Ca2+ influx mediated by voltage-gated Ca2+ channels (VGCCs) on the plasma membranes of neuroendocrine cells. To better understand the cellular and molecular components that are involved in regulating secretory vesicle exocytosis, this thesis work was conducted to investigate the expression and function of different subtypes of VGCCs in MNCs and pituicytes (the glial cells surrounding MNC nerve terminals).<p> Molecular biology, immunohistochemistry and cellular biology were used to detect expression and alternative splicing of different VGCC subtypes in MNCs, neurons, and pituicytes. First, the presence of CaV2.2 and CaV2.3 channels were detected on the pituicytes in situ. When the pituicytes were isolated and cultured for 14 days, more VGCC subtypes were expressed including CaV1.2 channels. Regulation of VGCC expression was measured in normal and dehydrated rats, and CaV1.2 channels were found to be selectively up-regulated in pituicytes after 24 hours of dehydration.<p> Second, two splice variants of CaV2.1 channels (CaV2.1Ä1 and Ä2) that lack a large portion of the synprint (synaptic protein interaction) site were detected in the rat brain. To determine whether the splice variants were expressed in MNCs, we did immunocytochemistry using two antibodies (the selective and the inclusive antibody) that recognized the carboxyl-terminus of channels and the synprint site, respectively, in different cell types. We found that vasopressin MNCs, but not the oxytocin MNCs, and one type of endocrine cell (the melanotropes of the pituitary gland) expressed the synprint site deleted variants, whereas the hippocampal neurons mainly expressed the full-length isoform. The splice variants were properly distributed on the plasma membrane of the somata and nerve terminals of the MNCs, suggesting the synprint site is not essential for CaV2.1 channel targeting into the nerve terminals of neuroendocrine cells.<p> Third, expression and distribution of CaV2.2 channels were studied in the MNCs. All CaV2.2 isoforms we detected contained the full-length synprint site. To test the importance of the CASK/Mint1 binding site for CaV2.2 channel targeting, we over-expressed a peptide that inhibits the interaction between CaV2.2 channels and CASK/Mint1 in differentiated PC12 cells (a neuroendocrine cell line). We found that the distribution of CaV2.2 channels in the growth cones of PC12 cells were significantly decreased, suggesting that the CASK/Mint1 interaction is important for CaV2.2 channel targeting into the neuroendocrine terminals.<p> In conclusion, these results provide insights of VGCC expression in neuroendocrine cells, and also give rise to a better understanding of the molecular components that are involved in forming the exocytotic machinery in these cells.