Search

The physiological demands of parturition and lactation lead to the increased pulsatile release of oxytocin (OT) into the circulation from the neurohypophysial axons of OT neurones in the supraoptic (SON) and paraventricular (PVN) nuclei. These states of increased OT release are accompanied by a significant plasticity in magnocellular OT neurones and their synaptic connections, and many of these changes require activation of a central OT receptor. The mitogen-activated protein kinase/extracellular signal-regulated kinase pathway (MAPK/ERK) is assumed to be up-regulated in the PVN during lactation, and many of the effects of OT in peripheral and brain tissue are mediated through a MAPK/ERK pathway. The present study investigated whether this pathway is altered in the SON and PVN during late pregnancy [embryonic day (E)20-21], which is a critical period for OT plasticity induction, and for lactation, when plastic changes are sustained. Based on immunoreactivity for phosphorylated ERK1/2 (pERK1/2), the results suggest an enhanced activation of MAPK/ERK pathway in OT neurones specifically during late pregnancy in both the SON and PVN. Although immunoblots from the SON confirm this pregnancy-associated up-regulation in late pregnancy, they also suggest enhancement into lactation as well. Together, the results suggest an important role for the MAPK/ERK pathway during reproductive changes in the SON and PVN., (© 2016 The Authors. Journal of Neuroendocrinology published by John Wiley & Sons Ltd on behalf of British Society for Neuroendocrinology.)

The epithelial Na⁺ channels (ENaCs) are present in kidney and contribute to Na⁺ and water homeostasis. All three ENaC subunits (α, β, and γ) were demonstrated in the cardiovascular regulatory centers of the rat brain, including the magnocellular neurons (MNCs) in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN). However, the functional significance of ENaCs in vasopressin (VP) and oxytocin (OT) synthesizing MNCs is completely unknown. In this study, we show with immunocytochemical double-labeling that the α-ENaC is colocalized with either VP or OT in MNCs in the SON and PVN. In addition, parvocellular neurons in the dorsal, ventrolateral, and posterior subregions of the PVN (not immunoreactive to VP or OT) are also immunoreactive for α-ENaC. In contrast, immunoreactivity to β- and γ-ENaC is colocalized with VP alone within the MNCs. Furthermore, immunoreactivity for a known target for ENaC expression, the mineralcorticoid receptor (MR), is colocalized with both VP and OT in MNCs. Using single-cell RT-PCR, we detected mRNA for all three ENaC subunits and MR in cDNA libraries derived from single MNCs. In whole cell voltage clamp recordings, application of the ENaC blocker benzamil reversibly reduced a steady-state inward current and decreased cell membrane conductance approximately twofold. Finally, benzamil caused membrane hyperpolarization in a majority of VP and about one-half of OT neurons in both spontaneously firing and quiet cells. These results strongly suggest the presence of functional ENaCs that may affect the firing patterns of MNCs, which ultimately control the secretion of VP and OT.

The neurohypophysial hormones, vasopressin (VP) and oxytocin (OT), are synthesised by magnocellular cells in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of the hypothalamus. The release of VP into the general circulation from the neurohypophysis increases during hyperosmolality, hypotension and hypovolaemia. VP neurones increase hormone release by increasing their firing rate as a result of adopting a phasic bursting. Depolarising after potentials (DAPs) following a series of action potentials are considered to be involved in the generation of the phasic bursts by summating to plateau potentials. We recently discovered a fast DAP (fDAP) in addition to the slower DAP characterised previously. Almost all VP neurones expressed the fDAP, whereas only 16% of OT neurones had this property, which implicates the involvement of fDAP in the generation of the firing patterns in VP neurones. Our findings obtained from electrophysiological experiments suggested that the ionic current underlying the fDAP is mediated by those of two closely-related Ca(2+) -activated cation channels: the melastatin-related subfamily of transient receptor potential channels, TRPM4 and TRPM5. In the present study, double/triple immunofluorescence microscopy and reverse transcriptase-polymerase chain reaction techniques were employed to evaluate whether TRPM4 and TRPM5 are specifically located in VP neurones. Using specific antibodies against these channels, TRPM5 immunoreactivity was found almost exclusively in VP neurones, but not in OT neurones in both the SON and PVN. The most prominent TRPM5 immunoreactivity was in the dendrites of VP neurones. By contrast, most TRPM4 immunoreactivity occurred in cell bodies of both VP and OT neurones. TRPM4 and TRPM5 mRNA were both found in a cDNA library derived from SON punches. These results indictate the possible involvement of TRPM5 in the generation of the fDAP, and these channels may play an important role in determining the distinct firing properties of VP neurones in the SON., (© 2011 The Authors. Journal of Neuroendocrinology © 2011 Blackwell Publishing Ltd.)

When released from dendrites within the supraoptic (SON) and paraventricular (PVN) nuclei (intranuclear release) during suckling, oxytocin exerts autocrine and paracrine effects on oxytocin neurons that are necessary for the unique timing and episodic pattern of oxytocin release into the systemic circulation that is characteristic of lactation. Recent reports have shown that stimulation of central noradrenergic and histaminergic receptors are both necessary for intranuclear release of oxytocin in response to suckling. In addition, in vitro studies indicate that excitatory amino acids may also be critical for central oxytocin secretion, although in vivo experiments have not provided direct support for this hypothesis. In addition to a critical role in intranuclear oxytocin release during lactation, norepinephrine has also been shown to stimulate central oxytocin during gestation. Stimulation of central oxytocin receptors during gestation appears critical for normal systemic oxytocin secretion in responses to suckling during the subsequent period of lactation. Oxytocin receptor blockade during pregnancy alters normal timing of systemic oxytocin release during suckling and reduces milk delivery. Several adaptations occur in the central oxytocin system that are necessary for determining the unique response characteristic observed during parturition and gestation. Central oxytocin receptor stimulation during gestation has been implicated in pregnancy-related morphological changes in magnocellular oxytocin neurons, disinhibition of oxytocin neurons to GABA, and adaptations in membrane response characteristics of oxytocin neurons. In conclusion, intranuclear oxytocin release during gestation and lactation are critical for establishing, and then evoking the unique pattern of systemic oxytocin secretion in response to the suckling offspring necessary for adequate milk delivery. Furthermore, activation of central noradrenergic receptors appears to be critical for release of central oxytocin in both of these reproductive states.

Cell area, percentage of cells with more than one nucleolus, and percentage of cells with the nucleolus apposed to the nuclear membrane (marginated) were compared in the medial and lateral magnocellular portions of the paraventricular nucleus (PVN) during water deprivation and voluntary rehydration. Lateral cells were larger and had a greater percentage of cells with multiple nucleoli in all conditions of deprivation and rehydration. In addition laterals were found to have more cells with marginated nucleoli in these cells with only one nucleoli. Medial and lateral cells showed similar responses to deprivation and rehydration.

Projection neurons in the rat hypothalamic paraventricular nucleus (pv) were identified using the retrograde transport of horseradish peroxidase (HRP) following injections of this tracer into the neurohypophysis or jugular vein. The objective was to determine if parvocellular pv neurons might be labelled by either injection in addition to the well known magnocellular contingent. Injections confined to the neurohypophysis resulted in the labelling of only magnocellular pv neurons, while intrajugular injections which produced spread of the HRP to the arcuate nucleus-median eminence region resulted in the labelling of both parvocellular and magnocellular pv elements. Thus parvocellular pv neurons may project to the arcuate nucleus and/or the external layer of the median eminance but not to the neurohypophysis.