Kisspeptin-10 potentiates miniature excitatory postsynaptic currents in the rat supraoptic nucleus

Local kisspeptin excitation of rat oxytocin neurones in late pregnancy

Abstract

The hormone, oxytocin, is synthesised by magnocellular neurones of the supraoptic and paraventricular nuclei and is released from the posterior pituitary gland into the circulation to trigger uterine contractions during parturition. Kisspeptin fibre density increases around the supraoptic nucleus over pregnancy and intracerebroventricular kisspeptin excites oxytocin neurones only in late pregnancy. However, the mechanism of this excitation is unknown. Here, we found that microdialysis administration of kisspeptin into the supraoptic nucleus consistently increased the action potential (spike) firing rate of oxytocin neurones in urethane‐anaesthetised late‐pregnant rats (gestation day 18–21) but not in non‐pregnant rats. Hazard analysis of action potential firing showed that kisspeptin specifically increased the probability of another action potential firing immediately after each action potential (post‐spike excitability) in late‐pregnant rats. Patch‐clamp electrophysiology in hypothalamic slices showed that bath application of kisspeptin did not affect action potential frequency or baseline membrane potential in supraoptic nucleus neurones. Moreover, kisspeptin superfusion did not affect the frequency or amplitude of excitatory postsynaptic currents or inhibitory postsynaptic currents in supraoptic nucleus neurones. Taken together, these studies suggest that kisspeptin directly activates oxytocin neurones in late pregnancy, at least in part, via increased post‐spike excitability.

Key points

Oxytocin secretion is triggered by action potential firing in magnocellular neurones of the hypothalamic supraoptic and paraventricular nuclei to induce uterine contractions during birth.

In late pregnancy, kisspeptin expression increases in rat periventricular nucleus neurones that project to the oxytocin system.

Here, we show that intra‐supraoptic nucleus administration of kisspeptin increases the action potential firing rate of oxytocin neurones in anaesthetised late‐pregnant rats, and that the increased firing rate is associated with increased oxytocin neurone excitability immediately after each action potential.

By contrast, kisspeptin superfusion of hypothalamic slices did not affect the activity of supraoptic nucleus neurones or the strength of local synaptic inputs to supraoptic nucleus neurones.

Hence, kisspeptin might activate oxytocin neurons in late pregnancy by transiently increasing oxytocin neuron excitability after each action potential.

Modulation of synaptic inputs in magnocellular neurones in a rat model of cancer cachexia

In cancer cachexia, abnormal metabolism and neuroendocrine dysfunction cause anorexia, tissue damage and atrophy, which can in turn alter body fluid balance. Arginine vasopressin, which regulates fluid homeostasis, is secreted by magnocellular neurosecretory cells (MNCs) of the hypothalamic supraoptic nucleus. Arginine vasopressin secretion by MNCs is regulated by both excitatory and inhibitory synaptic activity, alterations in plasma osmolarity and various peptides, including angiotensin II. In the present study, we used whole-cell patch-clamp recordings of brain slices to determine whether hyperosmotic stimulation and/or angiotensin II potentiate excitatory synaptic input in a rat model of cancer cachexia, similar to their effects in normal (control) rats. Hyperosmotic (15 and 60 mmol L^-1   mannitol) stimulation and angiotensin II (0.1 μmol L^-1 ) increased the frequency, but not the amplitude, of miniature excitatory postsynaptic currents in normal rats; in model rats, both effects were significantly attenuated. These results suggest that cancer cachexia alters supraoptic MNC sensitivity to osmotic and angiotensin II stimulation.

Role of the Kiss1/Kiss1r system in the regulation of pituitary cell function

Kisspeptin (Kiss1) is an amidated neurohormone that belongs to the RF-amide peptide family, which has a key role in the control of reproduction. Specifically, kisspeptin regulates reproductive events, including puberty and ovulation, primarily by activating the surface receptor Kiss1r (aka GPR54), at hypothalamic gonadotropin-releasing hormone (GnRH) neurons. More recently, it has been found that kisspeptin peptide is present in the hypophyseal portal circulation and that the Kiss1/Kiss1r system is expressed in pituitary cells, which suggest that kisspeptin could exert an endocrine, paracrine or even autocrine role at the pituitary gland level. Indeed, mounting evidence is pointing towards a direct role of kisspeptin in the control of not only gonadotropins but also other pituitary secretions such as growth hormone or prolactin. In this review, we summarize the most recent advances in the study of the role that the Kiss/Kiss1r system plays in the control of pituitary gland function, paying special attention to the direct role of this neuropeptide on pituitary cells and its interactions with other relevant regulators.

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.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Kisspeptin Regulation of Neuronal Activity throughout the Central Nervous System

Kisspeptin signaling at the gonadotropin-releasing hormone (GnRH) neuron is now relatively well characterized and established as being critical for the neural control of fertility. However, kisspeptin fibers and the kisspeptin receptor (KISS1R) are detected throughout the brain suggesting that kisspeptin is involved in regulating the activity of multiple neuronal circuits. We provide here a review of kisspeptin actions on neuronal populations throughout the brain including the magnocellular oxytocin and vasopressin neurons, and cells within the arcuate nucleus, hippocampus, and amygdala. The actions of kisspeptin in these brain regions are compared to its effects upon GnRH neurons. Two major themes arise from this analysis. First, it is apparent that kisspeptin signaling through KISS1R at the GnRH neuron is a unique, extremely potent form or neurotransmission whereas kisspeptin actions through KISS1R in other brain regions exhibit neuromodulatory actions typical of other neuropeptides. Second, it is becoming increasingly likely that kisspeptin acts as a neuromodulator not only through KISS1R but also through other RFamide receptors such as the neuropeptide FF receptors (NPFFRs). We suggest likely locations of kisspeptin signaling through NPFFRs but note that only limited tools are presently available for examining kisspeptin cross-signaling within the RFamide family of neuropeptides.