Kisspeptin neuron projections to oxytocin neurons are not necessary for parturition in the mouse

Oxytocin is synthesized by hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) neurons and is released from the posterior pituitary gland to trigger uterine contractions during parturition. In rats, oxytocin neuron innervation by periventricular nucleus (PeN) kisspeptin neurons increases over pregnancy and intra-SON kisspeptin administration excites oxytocin neurons only in late pregnancy. To test the hypothesis that kisspeptin neurons excite oxytocin neurons to trigger uterine contractions during birth in C57/B6J mice, double-label immunohistochemistry for kisspeptin and oxytocin first confirmed that kisspeptin neurons project to the SON and PVN. Furthermore, kisspeptin fibers expressed synaptophysin and formed close appositions with oxytocin neurons in the mouse SON and PVN before and during pregnancy. Stereotaxic viral delivery of caspase-3 into the AVPV/PeN of Kiss-Cre mice before mating reduced kisspeptin expression in the AVPV, PeN, SON and PVN by > 90% but did not affect the duration of pregnancy or the timing of delivery of each pup during parturition. Therefore, it appears that AVPV/PeN kisspeptin neuron projections to oxytocin neurons are not necessary for parturition in the mouse.

Increased neuronal activation in sympathoregulatory regions of the brain and spinal cord in type 2 diabetic rats

Increased cardiac sympathetic nerve activity in type 2 diabetes mellitus (DM) suggests impaired autonomic control of the heart. However, the central regions that contribute to the autonomic cardiac pathologies in type 2 DM are unknown. Therefore, we tested the hypothesis that neuronal activation would be increased in central sympathoregulatory areas in a pre-clinical type 2 DM animal model. Immunohistochemistry in 20-week-old male Zucker diabetic fatty (ZDF) rats revealed an increased number of neurones expressing ΔFosB (a marker of chronic neuronal activation) in the intermediolateral column (IML) of the spinal cord in DM compared to non-diabetic (non-DM) rats (P < 0.05). Rostral ventrolateral medulla (RVLM) neurones activate IML neurones and receive inputs from the hypothalamic paraventricular nucleus (PVN), as well as the nucleus tractus solitarius (NTS) and area postrema (AP), in the brainstem. We observed more ΔFosB-positive noradrenergic RVLM neurones (P < 0.001) and corticotrophin-releasing hormone PVN neurones (P < 0.05) in DM compared to non-DM rats. More ΔFosB-positive neurones were also observed in the NTS (P < 0.05) and AP (P < 0.01) of DM rats compared to non-DM rats. Finally, because DM ZDF rats are obese, we also expected increased activation of pro-opiomelanocortin (POMC) arcuate nucleus (ARC) neurones in DM rats; however, fewer ΔFosB-positive POMC ARC neurones were observed in DM compared to non-DM rats (P < 0.01). In conclusion, increased neuronal activation in the IML of type 2 DM ZDF rats might be driven by RVLM neurones that are possibly activated by PVN, NTS and AP inputs. Elucidating the contribution of central sympathoexcitatory drive in type 2 DM might improve the effectiveness of pharmacotherapies for diabetic heart disease.

Plasticity in Intrinsic Excitability of Hypothalamic Magnocellular Neurosecretory Neurons in Late-Pregnant and Lactating Rats

Oxytocin and vasopressin secretion from the posterior pituitary gland are required for normal pregnancy and lactation. Oxytocin secretion is relatively low and constant under basal conditions but becomes pulsatile during birth and lactation to stimulate episodic contraction of the uterus for delivery of the fetus and milk ejection during suckling. Vasopressin secretion is maintained in pregnancy and lactation despite reduced osmolality (the principal stimulus for vasopressin secretion) to increase water retention to cope with the cardiovascular demands of pregnancy and lactation. Oxytocin and vasopressin secretion are determined by the action potential (spike) firing of magnocellular neurosecretory neurons of the hypothalamic supraoptic and paraventricular nuclei. In addition to synaptic input activity, spike firing depends on intrinsic excitability conferred by the suite of channels expressed by the neurons. Therefore, we analysed oxytocin and vasopressin neuron activity in anaesthetised non-pregnant, late-pregnant, and lactating rats to test the hypothesis that intrinsic excitability of oxytocin and vasopressin neurons is increased in late pregnancy and lactation to promote oxytocin and vasopressin secretion required for successful pregnancy and lactation. Hazard analysis of spike firing revealed a higher incidence of post-spike hyperexcitability immediately following each spike in oxytocin neurons, but not in vasopressin neurons, in late pregnancy and lactation, which is expected to facilitate high frequency firing during bursts. Despite lower osmolality in late-pregnant and lactating rats, vasopressin neuron activity was not different between non-pregnant, late-pregnant, and lactating rats, and blockade of osmosensitive ΔN-TRPV1 channels inhibited vasopressin neurons to a similar extent in non-pregnant, late-pregnant, and lactating rats. Furthermore, supraoptic nucleus ΔN-TRPV1 mRNA expression was not different between non-pregnant and late-pregnant rats, suggesting that sustained activity of ΔN-TRPV1 channels might maintain vasopressin neuron activity to increase water retention during pregnancy and lactation.

Estrous Cycle Plasticity in the Central Clock Output to Kisspeptin Neurons: Implications for the Preovulatory Surge

Coordination of ovulation and behavior is critical to reproductive success in many species. During the female estrous cycle, the preovulatory gonadotropin surge occurs when ovarian follicles reach maturity and, in rodents, it begins just before the daily onset of activity, ensuring that ovulation coincides with sex behavior. Timing of the surge relies on projections from the suprachiasmatic nucleus (SCN), the locus of the central circadian clock, to hypothalamic circuits that regulate gonadotropin secretion. The cellular mechanisms through which the SCN controls these circuits and gates the preovulatory surge to the appropriate estrous cycle stage, however, are poorly understood. We investigated in mice the functional impact of SCN arginine-vasopressin (AVP) neuron projections to kisspeptin (Kiss1) neurons in the rostral periventricular area of the third ventricle (RP3VKiss1), responsible for generating the preovulatory surge. Conditional anterograde tracing revealed that SCNAVP neurons innervate approximately half of the RP3VKiss1 neurons. Optogenetic activation of SCNAVP projections in brain slices caused an AVP-mediated stimulation of RP3VKiss1 action potential firing in proestrus, the cycle stage when the surge is generated. This effect was less prominent in diestrus, the preceding cycle stage, and absent in estrus, following ovulation. Remarkably, in estrus, activation of SCNAVP projections resulted in GABA-mediated inhibition of RP3VKiss1 neuron firing, an effect rarely encountered in other cycle stages. Together, these data reveal functional plasticity in SCNAVP neuron output that drives opposing effects on RP3VKiss1 neuron activity across the ovulatory cycle. This might contribute to gating activation of the preovulatory surge to the appropriate estrous cycle stage.

Impact of chronic variable stress on neuroendocrine hypothalamus and pituitary in male and female C57BL/6J mice

Chronic stress exerts multiple negative effects on the physiology and health of an individual. In the present study, we examined hypothalamic, pituitary and endocrine responses to 14 days of chronic variable stress (CVS) in male and female C57BL/6J mice. In both sexes, CVS induced a significant decrease in body weight and enhanced the acute corticosterone stress response, which was accompanied by a reduction in thymus weight only in females. However, single-point blood measurements of basal prolactin, thyroid-stimulating hormone, luteinising hormone, growth hormone and corticosterone levels taken at the end of the CVS were not different from those of controls. Similarly, pituitary mRNA expression of Fshb, Lhb, Prl and Gh was unchanged by CVS, although Pomc and Tsh were significantly elevated. Within the adrenal medulla, mRNA for Th, Vip and Gal were elevated following CVS. Avp transcript levels within the paraventricular nucleus of the hypothalamus were increased by CVS; however, levels of Gnrh1, Crh, Oxt, Sst, Trh, Ghrh, Th and Kiss1 remained unchanged. Oestrous cycles were lengthened slightly by CVS and ovarian histology revealed a reduction in the number of preovulatory follicles and corpora lutea. Taken together, these observations indicate that 14 days of CVS induces an up-regulation of the neuroendocrine stress axis and creates a mild disruption of female reproductive function. However, the lack of changes in other neuroendocrine axes controlling anterior and posterior pituitary secretion suggest that most neuroendocrine axes are relatively resilient to CVS.

Prolactin regulation of oxytocin neurone activity in pregnancy and lactation

KEY POINTS

During lactation, prolactin promotes milk synthesis and oxytocin stimulates milk ejection. In virgin rats, prolactin inhibits the activity of oxytocin-secreting neurones. We found that prolactin inhibition of oxytocin neurone activity is lost in lactation, and that some oxytocin neurones were excited by prolactin in lactating rats. The change in prolactin regulation of oxytocin neurone activity was not associated with a change in activation of intracellular signalling pathways known to couple to prolactin receptors. The change in prolactin regulation of oxytocin neurone activity in lactation might allow coordinated activation of both populations of neurones when required for successful lactation.

ABSTRACT

Secretion of prolactin for milk synthesis and oxytocin for milk secretion is required for successful lactation. In virgin rats, prolactin inhibits oxytocin neurones but this effect would be counterproductive during lactation when secretion of both hormones is required for synthesis and delivery of milk to the newborn. Hence, we determined the effects of intracerebroventricular (i.c.v.) prolactin on oxytocin neurones in urethane-anaesthetised virgin, pregnant and lactating rats. Prolactin (2 μg) consistently inhibited oxytocin neurones in virgin and pregnant rats (by 1.9 ± 0.4 and 1.8 ± 0.5 spikes s^-1 , respectively), but not in lactating rats; indeed, prolactin excited six of 27 oxytocin neurones by >1 spike s^-1 in lactating rats but excited none in virgin or pregnant rats (χ²₂  = 7.2, P = 0.03). Vasopressin neurones were unaffected by prolactin (2 μg) in virgin rats but were inhibited by 1.1 ± 0.2 spikes s^-1 in lactating rats. Immunohistochemistry showed that i.c.v. prolactin increased oxytocin expression in virgin and lactating rats and increased signal transducer and activator of transcription 5 phosphorylation to a similar extent in oxytocin neurones of virgin and lactating rats. Western blotting showed that i.c.v. prolactin did not affect phosphorylation of extracellular regulated kinase 1 or 2, or of Akt in the supraoptic or paraventricular nuclei of virgin or lactating rats. Hence, prolactin inhibition of oxytocin neurones is lost in lactation, which might allow concurrent elevation of prolactin secretion from the pituitary gland and activation of oxytocin neurones for synthesis and delivery of milk to the newborn.

Development of an excitatory kisspeptin projection to the oxytocin system in late pregnancy

KEY POINTS

Oxytocin release from the posterior pituitary gland stimulates uterine contraction during birth but the central mechanisms that activate oxytocin neurones for birth are not well characterized. We found that that kisspeptin fibre density around oxytocin neurones increases in late-pregnant rats. These kisspeptin fibres originated from hypothalamic periventricular nucleus neurones that upregulated kisspeptin expression in late pregnancy. Oxytocin neurones were excited by central kisspeptin administration in late-pregnant rats but not in non-pregnant rats or early- to mid-pregnant rats. Our results reveal the emergence of a new excitatory kisspeptin projection to the oxytocin system in late pregnancy that might contribute to oxytocin neurone activation for birth.

ABSTRACT

The hormone oxytocin promotes uterine contraction during parturition. Oxytocin is synthesized by magnocellular neurones in the hypothalamic supraoptic and paraventricular nuclei and is released into the circulation from the posterior pituitary gland in response to action potential firing. Systemic kisspeptin administration increases oxytocin neurone activity to elevate plasma oxytocin levels. Here, immunohistochemistry revealed that rats on the expected day of parturition (day 21 of gestation) had a higher density of kisspeptin-positive fibres in the perinuclear zone surrounding the supraoptic nucleus (which provides dense glutamatergic and GABAergic innervation to the supraoptic nucleus) than was evident in non-pregnant rats. Retrograde tracing showed the kisspeptin projections to the perinuclear zone originated from the hypothalamic periventricular nucleus. Quantitative RT-PCR showed that kisspeptin receptor mRNA, Kiss1R mRNA, was expressed in the perinuclear zone-supraoptic nucleus and that the relative Kiss1R mRNA expression does not change over the course of pregnancy. Finally, intracerebroventricular administration of kisspeptin increased the firing rate of oxytocin neurones in anaesthetized late-pregnant rats (days 18-21 of gestation) but not in non-pregnant rats, or in early- or mid-pregnant rats. Taken together, these results suggest that kisspeptin expression is upregulated in the periventricular nucleus projection to the perinuclear zone of the supraoptic nucleus towards the end of pregnancy. Hence, this input might activate oxytocin neurones during parturition.

Reproductive Regulation of Gene Expression in the Hypothalamic Supraoptic and Paraventricular Nuclei

Oxytocin secretion is required for successful reproduction. Oxytocin is synthesised by magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei and the physiological demand for oxytocin synthesis and secretion is increased for birth and lactation. Therefore, we used a polymerase chain reaction (PCR) array screen to determine whether genes that might be important for synthesis and/or secretion of oxytocin are up- or down-regulated in the supraoptic and paraventricular nuclei of late-pregnant and lactating rats, compared to virgin rats. We then validated the genes that were most highly regulated using real time-quantitative PCR. Among the most highly regulated genes were those that encode for suppressors of cytokine signalling, which are intracellular inhibitors of prolactin signalling. Prolactin receptor activation changes gene expression via phosphorylation of signal transducer and activator of transcription 5 (STAT5). Using double-label immunohistochemistry, we found that phosphorylated STAT5 was expressed in almost all oxytocin neurones of late-pregnant and lactating rats but was almost absent from oxytocin neurones of virgin rats. We conclude that increased prolactin activation of oxytocin neurones might contribute to the changes in gene expression by oxytocin neurones required for normal birth and lactation.

Induction of hypertension blunts baroreflex inhibition of vasopressin neurons in the rat

Vasopressin secretion from the posterior pituitary gland is determined by action potential discharge of hypothalamic magnocellular neurosecretory cells. Vasopressin is a potent vasoconstrictor, but vasopressin levels are paradoxically elevated in some patients with established hypertension. To determine whether vasopressin neurons are excited in hypertension, extracellular single-unit recordings of vasopressin neurons from urethane-anaesthetized Cyp1a1-Ren2 rats with inducible angiotensin-dependent hypertension were made. The basal firing rate of vasopressin neurons was higher in hypertensive Cyp1a1-Ren2 rats than in non-hypertensive Cyp1a1-Ren2 rats. The increase in firing rate was specific to vasopressin neurons because oxytocin neuron firing rate was unaffected by the induction of hypertension. Intravenous injection of the α1-adrenoreceptor agonist, phenylephrine (2.5 μg/kg), transiently increased mean arterial blood pressure to cause a baroreflex-induced inhibition of heart rate and vasopressin neuron firing rate (by 52 ± 9%) in non-hypertensive rats. By contrast, intravenous phenylephrine did not inhibit vasopressin neurons in hypertensive rats, despite a similar increase in mean arterial blood pressure and inhibition of heart rate. Circulating angiotensin II can excite vasopressin neurons via activation of afferent inputs from the subfornical organ. However, the increase in vasopressin neuron firing rate and the loss of inhibition by intravenous phenylephrine were not blocked by intra-subfornical organ infusion of the angiotensin AT1 receptor antagonist, losartan. It can be concluded that increased vasopressin neuron activity at the onset of hypertension is driven, at least in part, by reduced baroreflex inhibition of vasopressin neurons and that this might exacerbate the increase in blood pressure at the onset of hypertension.

Metal nanoshell assembly on a virus bioscaffold

Chilo iridescent virus is demonstrated as a useful core substrate in the fabrication of metallodielectric, plasmonic nanostructures. A gold shell is assembled around the wild-type viral core by attaching small, 2-5-nm gold nanoparticles to the virus surface by means of the chemical functionality found inherently on the surface of the proteinaceous viral capsid. The density of these nucleation sites was maximized by reducing the repulsive forces between the gold particles through electrolyte addition. These gold nanoparticles then act as nucleation sites for the electroless deposition of gold ions from solution around the biotemplate. The optical extinction spectra of the metalloviral complex is in quantitative agreement with Mie scattering theory. Overall, the utilization of a native virus and the inherent chemical functionality of the capsid afford the ability to grow and harvest biotemplates for metallodielectric nanoshells in large quantities, potentially providing cores with a narrower size distribution and smaller diameters (below 80 nm) than for currently used silica.