Orexin neurons project to diverse sympathetic outflow systems

Both orexin receptors are expressed in rat ovaries and fluctuate with the estrous cycle: effects of orexin receptor antagonists on gonadotropins and ovulation

Orexins are peptides controlling feeding, sleep, and neuroendocrine functions. They are synthesized by the hypothalamus with projections throughout the brain. Orexins and their orexin 1 (OX(1)) and orexin 2 receptors (OX(2)) are present outside the central nervous system. Here the expression of preproorexin (PPO), OX(1), and OX(2) was studied in rat ovaries. PPO, OX(1), and OX(2) were determined by quantitative real-time RT-PCR in ovaries of cycling Sprague-Dawley rats on all days of the cycle. Serum hormones and food consumption were determined. Ovarian OX(1) and OX(2) expression was then studied after ovulation blockade with Cetrorelix or Nembutal. Finally, proestrous rats were treated at 1400 and 1900 with a selective OX(1) antagonist (SB-334867-A) and/or a selective OX(2) antagonist (JNJ-10397049), and hormone levels, ovulation, and ovarian histology were studied. Both receptors' expression increased in the ovary between 1700 and 2300 of proestrus exclusively, in coincidence with hormone peaks, but not with the dark-light cycle or food intake. PPO was not detected. Cetrorelix or Nembutal prevented the increases of OX(1) and OX(2) while blunting gonadotropin peaks. SB-334867-A and JNJ-10397049, alone or combined, decreased serum gonadotropins and reduced ova number the following morning; ovaries showed a bloody (hyperemic and/or hemorrhagic) reaction with more preovulatory follicles and less corpora lutea. Here we demonstrate for the first time an increased ovarian expression of both OX(1) and OX(2), only during proestrous afternoon, and its hormone dependence but not dependence on the dark-light cycle. Two new receptor antagonists reduced proestrous gonadotropins and/or ova number while producing ovarian structural changes.

Responses of hypothalamic orexin-containing neurons to cyclophosphamide, EHF-irradiation of the skin, and their combination in rats

Orexins are neuromediators that participate in the regulation of feeding behavior, energy metabolism, circadian rhythms and perception of pain. The aim of the present study was to clarify the responses of the hypothalamic orexin-containing neurons to an intraperitoneal injection of cyclophosphamide (CPA), extremely high frequency (EHF)-electromagnetic stimulation of skin, which is used to modulate side effects of cytostatics and their combination. The activation of orexin-containing neurons was determined by recording of the intensity of c-Fos protein expression. Injection of cyclophosphamide (40mg/kg) or EHF-irradiation of the skin decreased the staining of orexin-containing neurons, which was most pronounced in the subfornical region of the lateral hypothalamic area (LHAs). A redistribution of orexin from the perinuclear space to the processes of these cells took place, which occurs after the activation and the expression of the c-fos-gene. c-Fos protein was expressed in most neurons with minimum content of orexin, i.e. activation of these neurons correlated with the redistribution of orexins caused by skin EHF-irradiation and injection of cyclophosphamide (CPA). EHF-irradiation of the skin before and after injection of CPA increased the staining of orexin-containing neurons, i.e. it prevented the redistribution of orexin.

Glycoprotein D-independent spread of pseudorabies virus infection in cultured peripheral nervous system neurons in a compartmented system

The molecular mechanisms underlying the directional neuron-to-epithelial cell transport of herpesvirus particles during infection are poorly understood. To study the role of the viral glycoprotein D (gD) in the directional spread of herpes simplex virus (HSV) and pseudorabies virus (PRV) infection, a culture system consisting of sympathetic neurons or epithelial cells in different compartments was employed. We discovered that PRV infection could spread efficiently from neurons to cells and back to neurons in the absence of gD, the viral ligand required for entry of extracellular particles. Unexpectedly, PRV infection can also spread transneuronally via axo-axonal contacts. We show that this form of interaxonal spread between neurons is gD independent and is not mediated by extracellular virions. We also found that unlike PRV gD, HSV-1 gD is required for neuron-to-cell spread of infection. Neither of the host cell gD receptors (HVEM and nectin-1) is required in target primary fibroblasts for neuron-to-cell spread of HSV-1 or PRV infection.

Impact of proestrous milieu on expression of orexin receptors and prepro-orexin in rat hypothalamus and hypophysis: actions of Cetrorelix and Nembutal

Orexins and their receptors OX1 and OX2 regulate energy balance and the sleep-wake cycle. We studied the expression of prepro-orexin (PPO), OX1, and OX2 in brain and pituitary under the influence of the hormonal status in adult rats. Primarily, PPO, OX1, and OX2 expression was determined in Sprague-Dawley female cycling rats during proestrus and in males. Animals were killed at 2-h intervals. Anterior (AH) and mediobasal (MBH) hypothalamus, anterior pituitary (P), and frontoparietal cortex (CC) were homogenized in TRIzol, and mRNAs were obtained for screening of PPO, OX1, OX2 expression by semiquantitative RT-PCR. Main findings were confirmed and extended to all days of the cycle by quantitative real-time RT-PCR. Hormones and food consumption were determined. Finally, OX1, OX2, and PPO were measured by real-time RT-PCR in tissues collected at 1900 of proestrus after treatments at 1400 with ovulation-blocking agents Cetrorelix or pentobarbital. OX1 and OX2 expression increased at least threefold in AH, MBH, and P, but not in CC, between 1700 and 2300 of proestrus, without variations in estrus, diestrus, or in males. PPO in AH and MBH showed a fourfold or higher increase only during proestrus afternoon. Cetrorelix or pentobarbital prevented increases of OX1 and OX2 only in the pituitary and blunted gonadotropin surges, but left OX1, OX2, and PPO brain expression unchanged. Reproduction, energy balance, and sleep-wake cycle are integrated. Here, we demonstrate that, in the physiological neuroendocrine condition leading to ovulation, information to the orexinergic system acts in hypothalamus and pituitary by different mechanisms.

Naturally occurring hypertension in New World nonhuman primates: potential role of the perifornical hypothalamus

Hypertension is a prominent underlying factor in the genesis of cardiovascular-related morbidity and mortality. A major impediment to the investigation into the causes of the disease is the paucity of naturally occurring animal models of the disease. There is evidence that some species of New World primates spontaneously become hypertensive. We used chronically implanted pressure transducers to assess normally occurring blood pressure and heart rate levels at rest and during routine laboratory procedures in a group of one of these New World primates ( Aotus sp.). Resting mean arterial pressure ranged from 72 to 130 mmHg. Three animals were judged to have resting mean arterial pressure levels in the hypertensive range (≥110 mmHg). In all of the animals, pressor responses to routine laboratory events were exaggerated (average highest mean pressure during 1 min from any session was 97–196 mmHg). Subsequently, the region of the perifornical/lateral hypothalamus known to produce elevated blood pressure and heart rate responses to electrical stimulation was removed, and the blood pressure responses to the laboratory routines were significantly decreased and, in some cases, eliminated. Control lesions in nearby tissue had no effect on these responses. This region may play a critical role in initiating or exacerbating cardiovascular responses that contribute to the development of essential hypertension.

Integration of feeding and spontaneous physical activity: role for orexin

Spontaneous physical activity is activity that is non-volitional, or subconscious, such as fidgeting and shifting in one's seat, and time spent moving (standing and ambulating). Recent evidence indicates that spontaneous physical activity, and the resulting thermogenesis (non-exercise activity thermogenesis) may be regulated by brain systems. A large number of brain areas, with their associated neurotransmitter populations and connectivity, participate in the regulation of feeding behavior by acting as energy sensing and modulating centers. Although less well characterized, it is likely that a multitude of neurotransmitters and brain areas act to mediate spontaneous physical activity. These two behaviors, feeding and spontaneous physical activity, affect energy intake and expenditure and thus are important to body weight. Interestingly, often the two behaviors are affected simultaneously; when feeding is affected, so too is spontaneous physical activity, and both food intake and physical activity (whether spontaneous or volitional) influence activity of brain areas important to both. Several brain areas and neuropeptides are important to feeding and spontaneous physical activity. The lateral hypothalamus is one area that appears important to both behaviors, as stimulation or lesion of this region produces alterations in feeding behavior and spontaneous physical activity. Orexin neurons, with their central location in the lateral hypothalamus, widespread projections and connectivity to other brain areas important to energy homeostasis, are well situated to perform an integrative function. This review focuses on how hypothalamic orexins participate in both feeding and spontaneous physical activity, and provides potential models for the integration of signals important to both.

Multiple components of the defense response depend on orexin: Evidence from orexin knockout mice and orexin neuron-ablated mice

Stressor induces not only cognitive, emotional and behavioral changes but also autonomic changes. Although research on the neural circuits underlying such autonomic changes has implicated the hypothalamus in the defense response against stressors, neurotransmitters in this multifaceted and coordinated response have not been revealed. In this brief review, here we summarize our recent discovery using orexin knockout mice and orexin neuron-ablated mice of possible contribution of orexin in the defense response and discuss future directions.

Orexin neuron-mediated skeletal muscle vasodilation and shift of baroreflex during defense response in mice

We have previously shown that some features of the defense response, such as increases in arterial blood pressure (AP), heart rate (HR), and ventilation were attenuated in prepro-orexin knockout (ORX-KO) mice. Here, we examined whether the same was true in orexin neuron-ablated [orexin/ataxin-3 transgenic mice (ORX/ATX-Tg)] mice. In addition, we examined other features of the defense response: skeletal muscular vasodilation and shift of baroreceptor reflex. In both anesthetized and conscious conditions, basal AP in ORX/ATX-Tg mice was significantly lower by ∼20 mmHg than in wild-type (WT) controls, as was the case in ORX-KO mice. The difference in AP disappeared after treatment with an α-blocker but not with a β-blocker, indicating lower sympathetic vasoconstrictor outflow. Stimulation of the perifornical area (PFA) in urethane-anesthetized ORX/ATX-Tg mice elicited smaller and shorter-lasting increases in AP, HR, and ventilation, and skeletal muscle vasodilation than in WT controls. In addition, air jet stress-induced elevations of AP and HR were attenuated in conscious ORX/ATX-Tg mice. After pretreatment with a β-blocker, atenolol, stimulation of PFA suppressed phenylephrine (50 μg/kg iv)-induced bradycardia (ΔHR = −360 ± 29 beats/min without PFA stimulation vs. −166 ± 26 during stimulation) in WT. This demonstrated the resetting of the baroreflex. In ORX/ATX-Tg mice, however, no significant suppression was observed (−355 ± 16 without stimulation vs. −300 ± 30 during stimulation). The present study provided further support for our hypothesis that orexin-containing neurons in PFA play a role as a master switch to activate multiple efferent pathways of the defense response and also operate as a regulator of basal AP.

Aldosterone-sensitive neurons in the nucleus of the solitary tract: Efferent projections

The nucleus of the solitary tract (NTS) contains a subpopulation of neurons that express the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), which makes them uniquely sensitive to aldosterone. These neurons may drive sodium appetite, which is enhanced by aldosterone. Anterograde and retrograde neural tracing techniques were used to reveal the efferent projections of the HSD2 neurons in the rat. First, the anterograde tracer Phaseolus vulgaris leucoagglutinin was used to label axonal projections from the medial NTS. Then, NTS-innervated brain regions were injected with a retrograde tracer, cholera toxin beta subunit, to determine which sites are innervated by the HSD2 neurons. The HSD2 neurons project mainly to the ventrolateral bed nucleus of the stria terminalis (BSTvl), the pre-locus coeruleus (pre-LC), and the inner division of the external lateral parabrachial nucleus (PBel). They also send minor axonal projections to the midbrain ventral tegmental area, lateral and paraventricular hypothalamic nuclei, central nucleus of the amygdala, and periaqueductal gray matter. The HSD2 neurons do not innervate the ventrolateral medulla, a key brainstem autonomic site. Additionally, our tracing experiments confirmed that the BSTvl receives direct axonal projections from the neighboring A2 noradrenergic neurons in the NTS, and from the same pontine sites that receive major inputs from the HSD2 neurons (PBel and pre-LC). The efferent projections of the HSD2 neurons may provide new insights into the brain circuitry responsible for sodium appetite.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

Central autonomic control of the bone marrow: multisynaptic tract tracing by recombinant pseudorabies virus

Bone marrow is the primary place of hematopoiesis, where the development, survival and release of multipotent stem cells, progenitors, precursors and mature cells are under continuous humoral and neural control. Dense network of nerve fibers, containing various neurotransmitters is found in the bone marrow, however, the central neuronal circuit that regulates the activities of the bone marrow through these fibers remained unexplored. Transsynaptically connected neurons were mapped by virus-based transneuronal tracing technique using two isogenic, genetically engineered pseudorabies viruses, Bartha-DupGreen and Ba-DupLac expressing green fluorescent protein and beta-galactosidase, respectively. Bartha-DupGreen was injected into the femoral bone marrow of male rats and the progression of infection was followed 4-7 days post-inoculation. Virus-labeled cells were revealed in ganglia of the paravertebral chain and in the intermediolateral cell column of the lower thoracic spinal cord. Neurons were retrogradely labeled in the C1, A5, A7 catecholaminergic cell groups and several other nuclei of the ventrolateral and ventromedial medulla, the periaqueductal gray matter, the paraventricular and other hypothalamic nuclei, and in the insular and piriform cortex. Nerve transections and double-virus tracing from the bone marrow and the surrounding muscles were used to confirm the specific spreading of the virus. These results provide anatomical evidence for the CNS control of the bone marrow and identify putative brain areas, which are involved in autonomic regulation of the hematopoiesis, the release of progenitor cells, the blood supply and the immune cell function in the bone marrow.

Comparison of melanin-concentrating hormone and hypocretin/orexin mRNA expression patterns in a new parceling scheme of the lateral hypothalamic zone

A high-resolution spatial distribution analysis of hypothalamic neurons expressing melanin-concentrating hormone or hypocretin/orexin was performed in adult male rats with in situ hybridization cytochemistry. For the analysis, a new parcellation of the lateral zone with some two-dozen regions was used, and distributions were plotted on 15 transverse reference levels through the hypothalamus. Qualitatively the results confirm earlier, much lower resolution mapping studies, although some discrepancies are clarified. Previous work indicates that each of these cell populations is far from homogeneous, and the present results should help establish a framework for clarifying more precisely how they are differentiated and organized in terms of axonal input-output relationships and gene expression patterns, and for defining precise relationships with other hypothalamic neuron populations.

CNS neurons with links to both mood-related cortex and sympathetic nervous system

Cardiovascular changes occur during mental stress and in certain types of mood disorders. The neural basis for this phenomenon is unknown but it may be dependent on CNS neurons that provide branched projections to affective processing regions of the brain, such as the medial prefrontal cortex, and to the sympathetic outflow system. Because these putative neurons may be connected to these two target sites by chains of neurons, we performed double virus transneuronal tracing experiments and show here that a select subset of neurons in the medial preoptic nucleus (MPN), lateral hypothalamic area (LHA), and nucleus tractus solitarius (NTS) are co-linked to these two sites. Neurotensin MPN, orexin-containing LHA, and catecholamine NTS neurons were the major phenotypes involved in these projections. This novel class of neurons may coordinate cardiovascular changes seen in different emotional states.

Hypocretins (orexins): clinical impact of the discovery of a neurotransmitter

Hypothalamic excitatory hypocretin (orexin) neurons have been discovered in 1998 and found to have widespread projections to basal forebrain, monoaminergic and cholinergic brainstem, and spinal cord regions. The hypocretin system is influenced both neuronally (e.g. suprachiasmatic nucleus, GABAergic, cholinergic and aminergic brainstem nuclei) as well as metabolically (e.g. glucose, ghrelin, and leptin). Physiologically the hypocretin system has been implicated in the regulation of behaviours that are associated with wakefulness, locomotion, and feeding. A role in REM sleep, neuroendocrine, autonomic and metabolic functions has also been suggested. Pathophysiologically a deficient hypocretin neurotransmission has been found in human narcolepsy and (engineered) animal models of the disorder. Different mechanisms are involved including (1) degeneration of hypocretin neurons (mice), (2) hypocretin ligand deficiency (humans, mice, dogs), (3) hypocretin receptor deficiency (mice, dogs). Reports of low hypocretin-1 cerebrospinal fluid levels in neurologic conditions (e.g. Guillain-Barré syndrome, traumatic brain injury, hypothalamic lesions) with and without sleep-wake disturbances and, on the other hand, observations of normal levels in about 11% of narcoleptics raise questions about the exact nature and pathophysiological base of the link between hypocretin deficiency and clinical manifestations in human narcolepsy.

GABAergic and glycinergic presympathetic neurons of rat medulla oblongata identified by retrograde transport of pseudorabies virus and in situ hybridization

Electron microscopy suggests that up to half the synaptic input to sympathetic preganglionic neurons (SPGNs) is GABAergic or glycinergic. A proportion of this input is suspected to originate from neurons located within the medulla oblongata. The present study provides definitive evidence for the existence of these supraspinal presympathetic (PS) neurons with inhibitory phenotypes. PS neurons were identified by retrograde trans-synaptic migration of pseudorabies virus (PRV) injected into the adrenal gland. GABAergic or glycinergic cell bodies were identified by the presence of glutamate decarboxylase (GAD)-67 mRNA or glycine transporter (GlyT)-2 mRNA detected with in situ hybridization (ISH). Neither GABAergic nor glycinergic PS neurons were tyrosine hydroxylase (TH)-immunoreactive (ir). GABAergic PS neurons were located within the ventral gigantocellular nucleus, gigantocellular nucleus alpha, and medial reticular formation, mostly medial to the TH-ir PS neurons. About 30% of GABAergic PS neurons were serotonergic cells located in the raphe pallidus (RPa) and parapyramidal region (PPyr). Glycinergic PS neurons had the same general distribution as the GABAergic cells, except that no glycinergic neurons were located in the RPa or PPyr and none were serotonergic. PRV immunohistochemistry combined with ISH for both GlyT2 and GAD-67 mRNAs showed that at least 63% of midline medulla GABAergic PS neurons were also glycinergic and 76% of glycinergic PS neurons were GABAergic. In conclusion, the rostral ventromedial medulla contains large numbers of GABAergic and glycinergic neurons that innervate adrenal gland SPGNs. Over half of these PS neurons may release both transmitters. The physiological role of this medullary inhibitory input remains to be explored.

Novel tracing paradigms--genetically engineered herpesviruses as tools for mapping functional circuits within the CNS: present status and future prospects

The mammalian CNS is composed of an extremely complex meshwork of highly ordered interconnections among billions of neurons. To understand the diverse functions of this neuronal network we need to differentiate between functionally related and nonrelated elements. A powerful labeling method for defining intricate neural circuits is based on the utilization of neurotropic herpesviruses, including pseudorabies virus and herpes simplex virus type 1. The recent development of genetically engineered tracing viruses can open the way toward the conception of novel tract-tracing paradigms. These new-generation tracing viruses may facilitate the clarification of problems, which were inaccessible to earlier approaches. This article first presents a concise review of the general aspects of neuroanatomical tracing protocols. Subsequently, it discusses the molecular biology of alpha-herpesviruses, and the genetic manipulation and gene expression techniques that are utilized for the construction of virus-based tracers. Finally, it describes the current utilization of genetically modified herpesviruses for circuit analysis, and the future directions in their potential applications.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.