A direct neurokinin B projection from the arcuate nucleus regulates magnocellular vasopressin cells of the supraoptic nucleus

Central administration of neurokinin B (NKB) agonists stimulates immediate early gene expression in the hypothalamus and increases secretion of vasopressin from the posterior pituitary through a mechanism that depends on the activation of neurokinin receptor 3 receptors (NK3R). Here we report that, in the rat, immunoreactivity for NK3R is expressed in magnocellular vasopressin and oxytocin neurones in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of the hypothalamus, and that NKB immunoreactivity is expressed in fibres in close juxtaposition with vasopressin neurones at both of these sites. Retrograde tracing in the rat showed that some NKB-expressing neurones in the arcuate nucleus project to the SON, and in mice, using an anterograde tracing approach, we found that kisspeptin-expressing neurones of the arcuate nucleus, which are known to co-express NKB, project to the SON and PVN. Finally, we show that i.c.v. injection of the NK3R agonist senktide potently increases the electrical activity of vasopressin neurones in the SON in vivo with no significant effect detected on oxytocin neurons. The results suggest that NKBcontaining neurones in the arcuate nucleus regulate the secretion of vasopressin from magnocellular neurones in rodents, and we discuss the possible significance of this.This work was supported by the Newton International Fellowship program awarded to RPR (Ref. NF130516), co-funded by the Royal Society and the British Academy, and the British Society for Neuroendocrinology (Project Support Grant), and supported in part by funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no. 245009 (Neurofast).http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-28262017-04-30hb2016Zoology and Entomolog

Effects of Estrogen on Morphological and Electrophysiological Properties of Arcuate NKB Neurons

Infundibular (arcuate) neurokinin B (NKB) neurons play a critical role in neuroendocrine control of reproduction. Specifically, a local network of arcuate neurons that co-express kisspeptin, neurokinin B, and dynorphin (so-called, KNDy neurons), has emerged as a potential pacemaker driving the pulsatile secretion of gonadotropin-releasing hormone (GnRH) that is required for normal reproduction. These neurons are the target of estrogen and may be an important link in estrogen negative feedback on GnRH functioning. KNDy neurons respond to estrogen withdrawal with dramatic changes in gene expression and somatic hypertrophy, an effect that is reversible by estradiol replacement. Studies addressing the effects of estrogen withdrawal and replacement on morphological and electrophysiological features of KNDy neurons have been hindered by the inability to target this subpopulation of neurons in the live tissue. This dissertation examines estrogen-induced changes in arcuate NKB circuitry and excitability and discusses its implications in reproductive axis. First, the novel Tac2-EGFP transgenic mouse model was characterized. The reproductive function, EGFP-ir distribution in the brain, and co-localization of EGFP with proNKB in the arcuate nucleus were examined and compared to littermate controls. Indices of reproductive function (puberty onset, estrous cyclicity, and LH pulsatility) were comparable between Tac2 and wildtype mice, suggesting that the transgenic animals have preserved estrogen negative feedback. The long-term estrogen withdrawal via ovariectomy and estradiol replacement model was used to examine electrophysiological and morphological changes in arcuate NKB neurons. We found that low-dose chronic estradiol replacement results in decreased excitability of arcuate NKB neurons, a finding that is consistent with the proposed role of this neuronal population in estrogen negative feedback on reproductive axis. Changes in excitability were seen despite the overall similarity in intrinsic properties of estradiol-treated and untreated ovariectomized mice. We also demonstrated for the first time that single arcuate NKB neurons form a local network by way of recurrent collaterals. Axonal targets of single NKB neurons included the internal zone of the median eminence, ependymal layer of the 3rd ventricle, and sites lateral and dorsal to the borders of the arcuate nucleus. Long-term treatment with estradiol resulted in decreased somatic volume and decreased dendritic spine density. Together, these data demonstrate that low-dose chronic estradiol replacement in ovariectomized mice resulted in morphological plasticity of arcuate NKB neurons that was accompanied by changes in excitability of this neuronal population, supporting the role of these neurons in estrogen negative feedback on GnRH secretion

Influence of developmental nicotine exposure on glutamatergic neurotransmission in rhythmically active hypoglossal motoneurons

Developmental nicotine exposure (DNE) is associated with increased risk of cardiorespiratory, intellectual, and behavioral abnormalities in neonates, and is a risk factor for apnea of prematurity, altered arousal responses and Sudden Infant Death Syndrome. Alterations in nicotinic acetylcholine receptor signaling (nAChRs) after DNE lead to changes in excitatory neurotransmission in neural networks that control breathing, including a heightened excitatory response to AMPA microinjection into the hypoglossal motor nucleus. Here, we report on experiments designed to probe possible postsynaptic and presynaptic mechanisms that may underlie this plasticity. Pregnant dams were exposed to nicotine or saline via an osmotic mini-pump implanted on the 5th day of gestation. We used whole-cell patch clamp electrophysiology to record from hypoglossal motoneurons (XIIMNs) in thick medullary slices from neonatal rat pups (N = 26 control and 24 DNE cells). To enable the translation of our findings to breathing-related consequences of DNE, we only studied XIIMNs that were receiving rhythmic excitatory drive from the respiratory central pattern generator. Tetrodotoxin was used to isolate XIIMNs from presynaptic input, and their postsynaptic responses to bath application of L-glutamic acid (glutamate) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were studied under voltage clamp. DNE had no influence on inward current magnitude evoked by either glutamate or AMPA. However, in cells from DNE animals, bath application of AMPA was associated with a right shift in the amplitude distribution (P = 0.0004), but no change in the inter-event interval distribution of miniature excitatory postsynaptic currents (mEPSCs). DNE had no influence on mEPSC amplitude or frequency evoked by glutamate application, or under (unstimulated) baseline conditions. Thus, in the presence of AMPA, DNE is associated with a small but significant increase in quantal size, but no change in the probability of glutamate release.National Institutes of Health [R01 HD071302]12 month embargo; Available online 29 July 2016This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Diverse physiological properties of hypoglossal motoneurons innervating intrinsic and extrinsic tongue muscles

The mammalian tongue contains eight muscles that collaborate to ensure that suckling, swallowing, and other critical functions are robust and reliable. Seven of the eight tongue muscles are innervated by hypoglossal motoneurons (XIIMNs). A somatotopic organization of the XII motor nucleus, defined in part by the mechanical action of a neuron’s target muscle, has been described, but whether or not XIIMNs within a compartment are functionally specialized is unsettled. We hypothesize that developing XIIMNs are assigned unique functional properties that reflect the challenges that their target muscle faces upon the transition from in utero to terrestrial life. To address this, we studied XIIMNs that innervate intrinsic and extrinsic tongue muscles, because intrinsic muscles play a more prominent role in suckling than the extrinsic muscles. We injected dextran-rhodamine into the intrinsic longitudinal muscles (IL) and the extrinsic genioglossus, and physiologically characterized the labeled XIIMNs. Consistent with earlier work, IL XIIMNs ( n = 150) were located more dorsally within the nucleus, and GG XIIMNs ( n = 55) more ventrally. Whole cell recordings showed that resting membrane potential was, on average, 9 mV more depolarized in IL than in GG XIIMNs ( P = 0.0019), and the firing threshold in response to current injection was lower in IL (−31 ± 23 pA) than in GG XIIMNs (225 ± 39 pA; P &lt; 0.0001). We also found that the appearance of net outward currents in GG XIIMNs occurred at more hyperpolarized membrane potentials than IL XIIMNs, consistent with lower excitability in GG XIIMNs. These observations document muscle-specific functional specializations among XIIMNs. NEW &amp; NOTEWORTHY The hypoglossal motor nucleus contains motoneurons responsible for innervating one of seven different muscles with notably different biomechanics and patterns of use. Whether or not motoneurons innervating the different muscles also have unique functional properties (e.g., spiking behavior, synaptic physiology) is poorly understood. In this work we show that neonatal hypoglossal motoneurons innervating muscles that shape the tongue (intrinsic longitudinal muscles) have different electrical properties than those innervating the genioglossus, which controls tongue position. </jats:p