Vasopressin and oxytocin action in the brain: cellular neurophysiological studies

An oxytocinergic neural pathway that stimulates thermogenic and cardiac sympathetic outflow

Oxytocin alters autonomic functions besides social behaviors. However, the central neuronal links between hypothalamic oxytocinergic neurons and the autonomic nervous system remain unclear. Here we show that oxytocinergic neurons in the rat paraventricular hypothalamic nucleus (PVH), a pivotal site for energy homeostasis, innervate sympathetic premotor neurons in the rostral medullary raphe region (rMR) to stimulate brown adipose tissue (BAT) thermogenesis and cardiovascular functions. Oxytocin receptor stimulation in the rMR evokes BAT thermogenesis and tachycardia. In vivo optogenetic stimulation of the PVH→rMR long-range oxytocinergic pathway, using a virus-mediated system for amplified gene expression in oxytocinergic neurons, not only elicits BAT thermogenic and cardiac responses but also potentiates sympathetic responses evoked by glutamatergic transmission in the rMR. The PVH→rMR oxytocinergic pathway connects the hypothalamic circuit for energy homeostasis to thermogenic and cardiac sympathetic outflow, and, therefore, its defects may cause obesity and impaired thermoregulation, as seen in Prader-Willi syndrome.

Clinical evaluation of labor: an evidence- and experience-based approach

During labor mother and fetus are evaluated at intervals to assess their well-being and determine how the labor is progressing. These assessments require skillful physical diagnosis and the ability to translate the acquired information into meaningful prognostic decision-making. We describe a coordinated approach to the assessment of labor. Graphing of serial measurements of cervical dilatation and fetal station creates "labor curves," which provide diagnostic and prognostic information. Based on these curves we recognize nine discrete labor abnormalities. Many may be related to insufficient or disordered contractile mechanisms. Several factors are strongly associated with development of labor disorders, including cephalopelvic disproportion, excess analgesia, fetal malpositions, intrauterine infection, and maternal obesity. Clinical cephalopelvimetry involves assessing pelvic traits and predicting their effects on labor. These observations must be integrated with information derived from the labor curves. Exogenous oxytocin is widely used. It has a high therapeutic index, but is easily misused. Oxytocin treatment should be restricted to situations in which its potential benefits clearly outweigh its risks. This requires there be a documented labor dysfunction or a legitimate medical reason to shorten the labor. Normal labor and delivery pose little risk to a healthy fetus; but dysfunctional labors, especially if stimulated excessively by oxytocin or terminated by complex operative vaginal delivery, have the potential for considerable harm. Conscientiously implemented, the approach to the evaluation of labor outlined in this review will result in a reasonable cesarean rate and minimize risks that may accrue from the labor and delivery process.

Endogenous oxytocin inhibits hypothalamic corticotrophin-releasing hormone neurones following acute hypernatraemia

Significant prior evidence indicates that centrally acting oxytocin robustly modulates stress responsiveness and anxiety-like behaviour, although the neural mechanisms behind these effects are not entirely understood. A plausible neural basis for oxytocin-mediated stress reduction is via inhibition of corticotrophin-releasing hormone (CRH) neurones in the paraventricular nucleus of the hypothalamus (PVN) that regulate activation of the hypothalamic-pituitary-adrenal axis. Previously, we have shown that, following s.c. injection of 2.0 mol L^-1 NaCl, oxytocin synthesising neurones are activated in the rat PVN, an oxytocin receptor (Oxtr)-dependent inhibitory tone develops on a subset of parvocellular neurones and stress-mediated increases in plasma corticosterone levels are blunted. In the present study, we utilised transgenic male CRH-reporter mice to selectively target PVN CRH neurones for whole-cell recordings. These experiments reveal that acute salt loading produces tonic inhibition of PVN CRH neurones through a mechanism that is largely independent of synaptic activity. Further studies reveal that a subset of CRH neurones within the PVN synthesise mRNA for Oxtr(s). Salt induced Oxtr-dependent inhibitory tone was eliminated in individual PVN CRH neurones filled with GDP-β-S. Additional electrophysiological studies suggest that reduced excitability of PVN CRH neurones in salt-loaded animals is associated with increased activation of inwardly rectifying potassium channels. Nevertheless, substantial effort to recapitulate the core effects of salt loading by activating Oxtr(s) with an exogenous agonist produced mixed results. Collectively, these results enhance our understanding of how oxytocin receptor-mediated signalling modulates the function of CRH neurones in the PVN.

Oxytocin Transforms Firing Mode of CA2 Hippocampal Neurons

Oxytocin is an important neuromodulator in the mammalian brain that increases information salience and circuit plasticity, but its signaling mechanisms and circuit effect are not fully understood. Here we report robust oxytocinergic modulation of intrinsic properties and circuit operations in hippocampal area CA2, a region of emerging importance for hippocampal function and social behavior. Upon oxytocin receptor activation, CA2 pyramidal cells depolarize and fire bursts of action potentials, a consequence of phospholipase C signaling to modify two separate voltage-dependent ionic processes. A reduction of potassium current carried by KCNQ-based M channels depolarizes the cell; protein kinase C activity attenuates spike rate of rise and overshoot, dampening after-hyperpolarizations. These actions, in concert with activation of fast-spiking interneurons, promote repetitive firing and CA2 bursting; bursting then governs short-term plasticity of CA2 synaptic transmission onto CA1 and, thus, efficacy of information transfer in the hippocampal network.

Late Pregnancy is a Critical Period for Changes in Phosphorylated Mitogen-Activated Protein Kinase/Extracellular Signal-Regulated Kinase 1/2 in Oxytocin Neurones

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.

Role of vasopressin V(1a) and V2 receptors for the development of secondary brain damage after traumatic brain injury in mice

Brain edema is still one of the most deleterious sequels of traumatic brain injury (TBI), and its pathophysiology is not sufficiently understood. The goal of the current study was to investigate the role of arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), an important regulator of tissue water homeostasis, for the formation of post-traumatic brain edema, intracranial pressure (ICP), brain damage, and functional deficits following brain trauma. C57/B16 mice (n=112) were subjected to controlled cortical impact (CCI; 8m/s, 1 mm). At 3 min after trauma, animals received 500 ng of the AVP V(1a)-receptor antogonist (deamino-Pen(1), O-Me-Tyr(2), Arg(8)]-Vasopressin) or 500 ng of the AVP V2-receptor antagonist (adamantaneacetyl(1), O-Et-D-Tyr(2),Val(4), Abu(6),Arg(8,9)]-Vasopressin) by intracerebroventricular injection. After trauma, cerebral water content (24 h), ICP (24 h), contusion volume (24 h and 7 days), and functional outcome (1-7 days) were assessed (n=8 per experimental group). Post-traumatic inhibition of AVP V(1A) receptors reduced ICP by 29% (p < 0.05), brain water content by 45% (p < 0.05), and secondary contusion expansion by 37% (p < 0.05), and it significantly improved motor function 6 and 7 days after trauma (p < 0.05). Inhibition of AVP V2 receptors had no significant effect. The current results demonstrate that vasopressin V(1A) receptors are involved in the pathogenesis of brain edema formation and the subsequent development of secondary brain damage after traumatic brain injury. Accordingly, our study suggests that vasopressin V(1A) receptors may represent a novel therapeutic target for the treatment of post-traumatic brain edema and secondary brain damage.

A neuromodulatory role for oxytocin within the supramammillary nucleus

Oxytocin functions as both a neurohypophysial hormone and central neuromodulatory peptide, and has been implicated in reproductive behaviours, anxiety and reward, as well as facilitation of the neuroendocrine milk-ejection reflex. A potential substrate for oxytocin is the supramammillary nucleus (SuM), a structure that contains oxytocin binding sites and serves as an important relay within the limbic system. Hence, this study investigated the neuromodulatory role of oxytocin within the SuM. Firstly, the effect of oxytocin on neuronal firing within the SuM was studied, using in vitro brain slices from virgin female rats. Oxytocin (10(-6)M) excited approximately 50% of SuM neurones, and similar results were obtained with the selective oxytocin agonist, Thr(4) Gly(7) oxytocin (TGOT) (10(-6) and 10(-7)M). The remaining neurones were unaffected. The TGOT response was blocked by application of the oxytocin antagonist, [d(CH(2))51,Tyr(Me)(2),Thr(4),Orn(8),Tyr-NH29]-vasotocin. Repeat doses of TGOT caused diminution of the response, indicative of desensitisation. In the second series of experiments, immunocytochemical techniques were used to study the oxytocinergic innervation of the SuM. The supramammillary decussation was found to contain numerous oxytocinergic fibres, and some could be seen coursing ventrally to enter the SuM. Whereas, some were clearly "en passant" fibres innervating the neurohypophysis, others followed a more convoluted and branching course, and appeared to terminate within the nucleus. Finally, in vivo microinfusion studies investigated whether oxytocin injected into the SuM facilitated the milk-ejection reflex, a well known action of central oxytocin. Oxytocin microinfusion in the region of the SuM caused a pronounced facilitation of the reflex, contrasting with the much smaller effects of microinfusions made rostral or caudal to the nucleus. Collectively, these results strongly support a neuromodulatory role for oxytocin within the SuM. This could have important implications for understanding the diverse neuroendocrine and behavioural functions of central oxytocin, including its role in reward.

The influence of gonadal hormones on neuronal excitability, seizures, and epilepsy in the female

It is clear from both clinical observations of women, and research in laboratory animals, that gonadal hormones exert a profound influence on neuronal excitability, seizures, and epilepsy. These studies have led to a focus on two of the primary ovarian steroid hormones, estrogen and progesterone, to clarify how gonadal hormones influence seizures in women with epilepsy. The prevailing view is that estrogen is proconvulsant, whereas progesterone is anticonvulsant. However, estrogen and progesterone may not be the only reproductive hormones to consider in evaluating excitability, seizures, or epilepsy in the female. It seems unlikely that estrogen and progesterone would exert single, uniform actions given our current understanding of their complex pharmacological and physiological relationships. Their modulatory effects are likely to depend on endocrine state, relative concentration, metabolism, and many other factors. Despite the challenges these issues raise to future research, some recent advances have helped clarify past confusion in the literature. In addition, testable hypotheses have developed for complex clinical problems such as "catamenial epilepsy." Clinical and animal research, designed with the relevant endocrinological and neurobiological issues in mind, will help advance this field in the future.

Functional organization of preoptic vasotocin and isotocin neurons in the brain of rainbow trout: central and neurohypophysial projections of single neurons

Preoptic magnocellular neurosecretory cells (NSCs) in the brain of rainbow trout show synchronization of periodic Ca(2+) pulses, patterns of which differ between vasotocin (VT) and isotocin (IT) neurons. To provide neuroanatomical bases of the synchronized periodic Ca(2+) pulses and their biological implications, we examined the organization of preoptic VT and IT neurons in the brain of rainbow trout. The cytoarchitecture of the preoptic neurosecretory system was characterized by a confocal double-color immunofluorescence. Two to five VT neurons, and also IT neurons, aggregate to form cell-type specific clusters. VT clusters tend to localize medially, while IT clusters laterally. VT neurons are closely apposed at the proximal neuronal processes. A Golgi-like immunohistochemistry demonstrated that VT and IT fibers distribute widely in the brain, such as ventral telencephalon, diencephalon, and various mesencephalic structures, in addition to the neurohypophysial projections. Projections from single VT and IT neurons were examined by an intracellular staining with biocytin injection in a sagittally hemisected brain preparation, which contains the entire forebrain region. Single VT and IT neurons project toward the pituitary and the extrahypothalamic regions. Some IT neurons, but not VT neurons, were dye-coupled. These results support the idea that the same types of NSCs are connected to form cell-type-specific networks responsible for the synchronization of periodic Ca(2+) pulses. The organization of the preoptic neurosecretory system shown in the present study is suitable for the simultaneous control of neurohypophysial and extrahypothalamic outputs through the synchronization of electrical activity.

Alterations in central neuropeptide expression, release, and receptor binding in rats bred for high anxiety: critical role of vasopressin

To model aspects of trait anxiety/depression, Wistar rats were bred for extremes in either hyper (HAB)- or hypo(LAB)-anxiety as measured on the elevated plus-maze and in a variety of additional behavioral tests. Similar to psychiatric patients, HAB rats prefer passive stress-coping strategies, indicative of depression-like behavior, show hyper-reactivity of the hypothalamo-pituitary-adrenal axis, and a pathological response to the dexamethasone/corticotropin-releasing hormone (CRH) challenge test. Here we tested central mRNA expression, release patterns, and receptor binding of neuropeptides critically involved in the regulation of both anxiety-related behavior and the HPA axis. Thus, CRH, arginine-8-vasopressin (AVP), and oxytocin (OXT) were studied in brains of HAB and LAB males both under basal conditions and after exposure to a mild emotional stressor. In HAB rats, CRH mRNA was decreased in the bed nucleus of the stria terminalis only. While no significant difference in CRH1-receptor binding was found in any brain area, CRH2-receptor binding was elevated in the hypothalamic paraventricular nucleus (PVN), the ventromedial hypothalamus, and the central amygdala of HABs compared to LABs. AVP, but not OXT, mRNA expression as well as release of the neuropeptide, were higher in the PVN of HABs, whereas AVP V1a-receptor binding failed to show significant differences in any brain region studied. Remarkably, intra-PVN treatment of HABs with the AVP V1-receptor antagonist d (CH(2))(5) Tyr (Me) AVP resulted in a decrease in anxiety/depression-related behavior. The elevated expression and release of AVP within the PVN of HAB rats together with the behavioral effects of the AVP V1-receptor antagonist suggest a critical involvement of this neuropeptide in neuroendocrine and behavioral phenomena associated with trait anxiety/depression.

Presence of functional vasopressin receptors in spinal ventral horn neurons of young rats: a morphological and electrophysiological study

The objective of the present work was double. (i) Light microscopic autoradiography was used to determine the distribution of vasopressin and oxytocin binding sites in the spinal cord of rats. (ii) Whole-cell recordings were performed in lumbar spinal cord slices in order to assess whether these receptors are functional, whether they are located pre- or postsynaptically and whether they are present in motoneurons. In newborns, vasopressin binding sites of the V1a type were present in all laminae of the central gray at all segmental levels, whereas oxytocin binding sites were found only in the superficial layers of the dorsal horn. In adults, binding sites for both neuropeptides were also present, but were less dense. The dissociation constants for vasopressin were similar in newborns and adults. Whole-cell recordings showed that in identified motoneurons vasopressin exerted a direct effect, by inducing a membrane depolarization or by generating a sustained inward current, and an indirect effect, by enhancing glycinergic and GABAergic inhibitory transmission. Vasopressin-induced facilitation of inhibitory transmission could also be demonstrated in unidentified ventral horn neurons. All these effects were mediated by V1a but not V1b receptors. In some neurons, glycinergic transmission was also facilitated by a selective oxytocin receptor agonist. Our data, together with data obtained previously in brainstem motor nuclei, suggest that vasopressin of hypothalamic origin could play a role in motricity. The neuropeptide could act as a neuromodulator, because it would not directly activate motoneurons, but rather render them more responsive to incoming excitatory inputs. Vasopressin may thus act as a regulator of muscular force.

Prevention of cannabinoid withdrawal syndrome by lithium: involvement of oxytocinergic neuronal activation

Cannabis (i.e., marijuana and cannabinoids) is the most commonly used illicit drug in developed countries, and the lifetime prevalence of marijuana dependence is the highest of all illicit drugs in the United States. To provide clues for finding effective pharmacological treatment for cannabis-dependent patients, we examined the effects and possible mechanism of lithium administration on the cannabinoid withdrawal syndrome in rats. A systemic injection of the mood stabilizer lithium, at serum levels that were clinically relevant, prevented the cannabinoid withdrawal syndrome. The effects of lithium were accompanied by expression of the cellular activation marker Fos proteins within most oxytocin-immunoreactive neurons and a significant increase in oxytocin mRNA expression in the hypothalamic paraventricular and supraoptic nuclei. Lithium also produced a significant elevation of oxytocin levels in the peripheral blood. We suggest that the effects of lithium against the cannabinoid withdrawal syndrome are mediated by oxytocinergic neuronal activation and subsequent release and action of oxytocin within the CNS. In support of our hypothesis, we found that the effects of lithium against the cannabinoid withdrawal syndrome were antagonized by systemic preapplication of an oxytocin antagonist and mimicked by systemic or intracerebroventricular injection of oxytocin. These results demonstrate that oxytocinergic neuronal activation plays a critical role in the action of lithium against the cannabinoid withdrawal syndrome in rats, thus providing a potentially novel strategy for the treatment of cannabis dependence in humans.

CO(2)-induced c-Fos expression in hypothalamic vasopressin containing neurons

Following exposure of anesthetized and unanesthetized rats to hypercapnic stress, arginine vasopressin (AVP)-containing neurons of the hypothalamic paraventricular (PVN) and supraoptic (SON) nuclei were examined for expression of the c-fos gene encoded protein (c-Fos). In addition, we determined whether AVP-containing PVN neurons activated by hypercapnia project to phrenic nuclei. In adult control rats, only scant c-Fos-like immunoreactive neurons were observed within the hypothalamic nuclei. A marked increase in c-Fos positive cells was induced after 2 h of breathing a gas mixture with elevated CO(2) (5% CO(2), 21% O(2) and 74% N(2), or 1 h following breathing of 12% CO(2,) 21% O(2,) and 67% N(2)). Colocalization studies of AVP and c-Fos protein revealed that in the PVN, 75% of AVP-containing cells expressed c-Fos immunoreactivity. c-Fos and AVP were coexpressed in 60% of SON neurons in anesthetized rats. In addition, retrograde labeling studies with cholera toxin b subunit (CTb) revealed that a subpopulation of PVN cells (15%) that project to phrenic nuclei are activated by hypercapnia, as indicated by c-Fos expression. These results indicate that (i) PVN and SON AVP-containing neurons are part of the neuronal networks that react to hypercapnic exposure; and (ii) a subset of CO(2) reactive PVN cells innervate phrenic nuclei.

TNF-alpha-induced c-Fos generation in the nucleus of the solitary tract is blocked by NBQX and MK-801

Previous studies have shown that identified neurons of the nucleus of the solitary tract (NST) are excited by the cytokine tumor necrosis factor-alpha (TNF-alpha). Vagal afferent connections with the NST are predominantly glutaminergic. Therefore, we hypothesized that TNF-alpha effects on NST neurons may be via modulation of glutamate neurotransmission. The present study used activation of the immediate early gene product c-Fos as a marker for neuronal activation in the NST. c-Fos expression was evaluated after microinjections of TNF-alpha in the presence or absence of either the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX) or the N-methyl-D- aspartate (NMDA) antagonist MK-801. To assess the specificity of the interaction between TNF-alpha and glutamate, c-Fos expression was also evaluated after injection of oxytocin (OT) (which has a direct excitatory effect in this area of the brain stem) in the presence and absence of NBQX or MK-801. c-Fos labeling was significantly increased in the NST after TNF-alpha exposure. Coinjection of either NBQX or MK-801 with TNF-alpha prevented significant c-Fos induction in the NST. Microinjections of OT also induced significant NST c-Fos elevation, but this expression was unaffected by coinjection of either antagonist with OT. These data lead us to conclude that TNF-alpha activation of NST neurons depends on glutamate and such an interaction is not generalized to all agonists that act on the NST.

Nicotinic acetylcholine receptors in neonatal motoneurons are regulated by axotomy: an electrophysiological and immunohistochemical study in human bcl-2 transgenic mice

Motoneuron axotomy was exploited as a model system for studying functional and morphological changes caused in motoneuron cell bodies by peripheral axon injury. Rodent facial motoneurons express functional nicotinic acetylcholine receptors. We have determined the effect of neonatal unilateral facial nerve transection on these receptors by using electrophysiological and immunohistochemical techniques. To avoid rapid apoptotic cell death of axotomized motoneurons, the study was done in mice overexpressing the human bcl-2 transgene. Intact motoneurons responded to acetylcholine by generating a rapidly rising inward current, which was insensitive to methyllycaconitine, a selective antagonist of alpha7-containing nicotinic receptors, but was suppressed by dihydro-beta-erythroidine, a broad-spectrum antagonist. This indicates that mouse facial motoneurons possess nicotinic receptors which are probably devoid of the alpha7 subunit. In striking contrast, axotomized motoneurons displayed little or no sensitivity to acetylcholine. Axotomy did not affect the sensitivity of facial motoneurons to the selective glutamate receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxaxolepropionic acid. Immunohistochemical studies revealed that the alpha4 nicotinic receptor subunit was present in intact motoneurons but was undetectable in axotomized motoneurons. By contrast, the beta2 subunit was comparable in intact and axotomized motoneurons. alpha3 immunoreactivity was undetectable, both in intact and in axotomized motoneurons.Thus, mouse facial nicotinic receptors are possibly of the alpha4beta2 type and axotomy interferes negatively with the expression of the alpha4 subunit. By down-regulating nicotinic receptors, peripheral nerve injury may facilitate motoneuron degeneration. Alternatively, nicotinic receptor downregulation and motoneuron degeneration may be independent consequences of peripheral axotomy.

Oxytocin receptor agonists enhance inhibitory synaptic transmission in the rat hippocampus by activating interneurons in stratum pyramidale

Oxytocin probably plays a role as a neurotransmitter/neuromodulator in the hippocampus of the rat. Oxytocin binding sites are present in the subiculum and CA1 region and oxytocin can excite a class of CA1 nonpyramidal neurons. In the present work we characterized the effect of oxytocin on hippocampal synaptic transmission. Whole-cell recordings were obtained from pyramidal neurons, in conditions of nearly symmetrical chloride concentrations. The selective oxytocin receptor agonist, [Thr4,Gly7]-oxytocin (TGOT), caused an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (IPSCs) in virtually all neurons. These peptide-enhanced IPSCs were blocked by bicuculline, but not by strychnine, and reversed near 0 mV, indicating that they were mediated by gamma-aminobutyric acid (GABA)A receptors. On average, TGOT caused a nearly threefold increase in the frequency and almost a doubling in the amplitude of spontaneous IPSCs. TGOT did not influence the frequency and the amplitude of miniature IPSCs or spontaneous excitatory postsynaptic currents (EPSCs), and had no effect on evoked IPSCs. The peptide did not affect the basic membrane properties of pyramidal neurons or their GABA sensitivity. Thus, TGOT facilitated inhibitory transmission by exerting an excitatory action on the soma and/or dendrites of GABAergic interneurons. Extracellular recordings were performed in interneurons located in various hippocampal strata. Their sensitivity to TGOT was compared to that of substance P (SP). Interneurons in stratum pyramidale were excited both by TGOT and by SP. By contrast, stratum radiatum interneurons responded to SP but not to TGOT. In stratum oriens, half of the interneurons responded to SP, but only a minority to TGOT. Thus, oxytocin-responsive interneurons appear to be preferentially located in close vicinity of pyramidal neurons.