Orexin-A, an hypothalamic peptide with analgesic properties

Effects of anesthetics on the function of orexin-1 receptors expressed in Xenopus oocytes

Neurons in the hypothalamus containing the neuropeptide orexin have been implicated in the control of sleep and wakefulness and in the pathology of narcolepsy. In this study, we investigated the effects of volatile anesthetics, ethanol and intravenous anesthetics on orexin-A-induced Ca2+-activated Cl- currents using Xenopus oocytes expressing orexin-1 receptors (OX1Rs). The volatile anesthetics isoflurane, enflurane and halothane inhibited Cl- currents elicited by 1-micromol/l orexin-A. Ethanol and the intravenous anesthetics pentobarbital and ketamine also inhibited the action of orexin-A. The inhibitory effects of all of the compounds tested were shown to be caused by the inhibition of OX1R function. These results may, at least in part, explain their hypnotic effects.

Normal cerebrospinal fluid levels of hypocretin-1 (orexin A) in patients with fibromyalgia syndrome

BACKGROUND

The hypothalamic neuropeptide hypocretin (orexin) modulates sleep-wake, feeding and endocrine functions. Cerebrospinal fluid (CSF) hypocretin-1 (Hcrt-1) concentrations are low in patients with narcolepsy-cataplexy, a sleep disorder characterized by hypersomnolence and rapid eye movement (REM) sleep abnormalities.

METHODS

We determined CSF Hcrt-1 concentrations of patients with the fibromyalgia syndrome (FMS), a condition characterized by fatigue, insomnia and in some cases daytime hypersomnolence.

RESULTS

Basal CSF levels of Hcrt-1 in FMS did not differ from those in healthy normal controls.

CONCLUSIONS

These findings suggest that abnormally low Hcrt-1 is not a likely cause of fatigue in FMS.

The selective orexin 1 receptor antagonist SB-334867-A impairs acquisition and consolidation but not retrieval of spatial memory in Morris water maze

The novel neuropeptides orexin-A and orexin-B derive from a common 130-amino acid precursor molecule (prepro-orexin), are mainly localized to neurons within and around the lateral hypothalamus, and exhibit high affinity to the closely related G-Protein-coupled receptors orexin 1 and 2 receptor (OX1R, OX2R). Orexinergic neurons send their axons to the hippocampal formation (CA1, CA2 and dentate gyrus), which expresses OX1Rs. Recent studies have shown that central administration of orexin-A and orexin-B have effects on learning and memory but literature concerning the role of orexinergic system in cognition remains controversial. More recently, antagonists have been described. The most potent and selective is SB-334867-A, which has an affinity of 40 nM at OX1R which is at least 50-fold selective over OX2R. It is likely that the intracerebroventricular (i.c.v.) administration may block OX1Rs in many brain regions. Previously we have shown that intra-CA1 injection of SB-334867-A impairs acquisition, consolidation and retrieval of spatial memory in MWM task. In the present study, the effect of pre-training, post-training and pre-probe of trial intra-DG (dentate gyrus) administration of SB-334867-A (1.5, 3, 6 microg/0.5 microl) on acquisition, consolidation and retrieval in a single-day testing version of MWM (Morris water maze) task was examined. Our results show impaired acquisition and consolidation of MWM task for SB-334867-A as compared with the control group. However, SB-334867-A had no effect on retrieval in spatial memory. Also, this antagonist had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous orexin-A and orexin-B, through DG OX1Rs, play an important role in spatial learning and memory in the rat.

Modulation of nociceptive dural input to the trigeminal nucleus caudalis via activation of the orexin 1 receptor in the rat

Migraine pathophysiology is thought to involve the trigeminal innervation of the dura mater and intracranial blood vessels. Electrical stimulation of dural blood vessels is painful in humans and causes activation of neurons in the caudal-most portion of the trigeminal nucleus in experimental animals. The hypothalamic neuropeptides orexin A and B are selectively synthesized in the lateral and posterior hypothalamus, and recent findings have implicated their involvement in nociceptive processing. To evaluate the potential for orexin receptor modulation of trigeminovascular nociceptive afferents, we examined the effects of intravenous orexin A and B on responses of neurons in the trigeminal nucleus caudalis. To dissect the receptor pharmacology of responses to stimulation we utilized the novel orexin 1 receptor (OX(1)R) antagonist N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea (SB-334867). Orexin A 30 microg/kg (F(1.9,9.8) = 21.93, P < 0.001) and 50 microg/kg (F(3.2,16.4) = 3.28, P < 0.045) inhibited the A-fibre responses to dural electrical stimulation over 60 min. Maximum inhibition was achieved at 25 min for both 30 microg/kg (t(5) = 19.83, n = 6, P < 0.001) and 50 microg/kg (t(5) = 7.74, n = 6, P < 0.001). The response with orexin A 30 microg/kg was reversed by pretreatment with the OX(1)R antagonist SB-334867 (F(3.5,17.5) = 0.49, P = 0.73), which had no effect when given alone. Orexin B and control vehicle administration had no significant effect on trigeminal neuronal firing. The current study demonstrates that orexin A is able to inhibit A-fibre responses to dural electrical stimulation via activation of the OX(1)R.

Expression of orexin-A and orexin receptors in the kidney and the presence of orexin-A-like immunoreactivity in human urine

Orexin-A (hypocretin-1), a neuropeptide with stimulatory actions on arousal and appetite, was originally shown to be specifically expressed in the hypothalamus. We studied expression of orexin-A and orexin receptors in the kidney and the presence of orexin-A-like immunoreactivity in human urine. Immunocytochemistry showed that orexin-A-like immunoreactivity and two types of orexin receptors (types 1 and 2) were localized in the tubules of the human kidney obtained at autopsy. Orexin-A-like immunoreactivity was detected in human kidneys (21.3 +/- 6.2 fmol/g wet weight, mean +/- S.E.M., n = 4) and rat kidneys (16.2 +/- 1.6 fmol/g wet weight, n = 5) by radioimmunoassay, although the levels were much lower than the levels in the brain. Orexin-A-like immunoreactivity was present in the urine obtained from male healthy volunteers (67.8 +/- 4.5 pmol/l, n = 5). Reverse phase high-performance liquid chromatography showed that most of orexin-A-like immunoreactivity of the urine extract was eluted earlier than authentic orexin-A, suggesting that orexin-A-like immunoreactivity in urine was modified to hydrophilic forms. Reverse transcriptase polymerase chain reaction showed expression of orexin receptors 1 and 2 mRNAs in the human kidney. These findings suggest that orexin-A is produced by the renal tubular cells and secreted into urine. Orexin-A may act on the kidney in the autocrine or paracrine fashion, or via the urine (urocrine fashion).

Orexin 1 receptor activation attenuates neurogenic dural vasodilation in an animal model of trigeminovascular nociception

The pathophysiology underlying the pulsating quality of the pain of a migraine attack is not fully understood, although trigeminal vascular afferents containing the sensory neuropeptide calcitonin gene-related peptide (CGRP) must have a role. Antimigraine drugs, such as triptans, serotonin 5-hydroxytryptamine(1B/1D) receptor agonists, reproducibly block neurogenic vasodilation associated with CGRP release. We examined the effects of the hypothalamic neuropeptides orexin A and orexin B on neurogenic dural vasodilation, dissecting out the receptor pharmacology with the novel orexin 1 (OX1) receptor antagonist N-(2-methyl-6-benzoxazolyl)-N''-1,5-naphthyridin-4-yl urea (SB-334867). Electrical stimulation of dural afferents (50-300 microA) resulted in reproducible dural vasodilation of 136 +/- 9%. Orexin A 30 microg kg(-1), but not 3 and 10 microg kg(-1), inhibited the dilation brought about by electrical stimulation over 60 min and maximally after 15 min by 60% (t7= 7.138; P < 0.001; n = 8). This response was reversed by pretreatment with the OX1 receptor antagonist SB-334867. Addition of CGRP(8-37) at the point of maximal effect of orexin A produced a further significant decrease in neurogenic dural vasodilation compared with orexin A only. CGRP administration (1 microg kg(-1)) produced a reproducible dural blood vessel dilation of 145 +/- 7% that was not inhibited by intravenous administration of orexin A (30 microg kg(-1)). Orexin B had no significant effect even at the highest dose. The current study demonstrates that orexin A is able to inhibit neurogenic dural vasodilation via activation of the OX1 receptor, resulting in inhibition of prejunctional release of CGRP from trigeminal neurons.

Enhanced antinociception by intracerebroventricularly administered orexin A in histamine H1 or H2 receptor gene knockout mice

Orexins are neuropeptides that are mostly expressed in the posterior and lateral hypothalamus, and related to the central control of appetite, arousal, and antinociception. Orexin neurons projected to the tuberomammillary nucleus and orexins may release histamine from the histamine neurons in this nucleus. Histamine is known to cause hypernociception. The roles of histamine H1 and H2 receptors in the orexin A-induced antinociception, however, have not been clarified yet. Here we studied the effects of histamine H1 and H2 receptors on orexin A-produced antinociception using histamine receptor knockout mice in four assays of nociception; the hot-plate, the tail-flick, the tail-pressure and the capsaicin tests. Furthermore we studied effects of histamine H1 and H2 receptor antagonists on orexin A-produced antinociception in C57BL/6 mice. The antinociceptive effects of i.c.v. orexin A were greater in histamine H1 receptor or H2 receptor knockout mice than in the wild-type mice in all four assays of pain. Furthermore, treatment of C57BL/6 mice with a combination of i.c.v. orexin A and d-chlorpheniramine (a histamine H1 receptor antagonist) or cimetidine (a histamine H2 receptor antagonist) showed a greater antinociception than i.c.v. orexin A alone in all four assays. These findings suggest the possibility that orexin A may activate H1 and H2 receptors in the supraspinal levels through the release of histamine from neurons, which might attenuate the antinociceptive effects of orexin A. Thus, the blocking of the histamine H1 or H2 receptor may produce antinociception and enhance the orexin A-induced antinociception.

Orexin expression and function: glucocorticoid manipulation, stress, and feeding studies

We investigated the effects of glucocorticoid manipulation on orexin-A-induced feeding and prepro-orexin mRNA levels in the lateral hypothalamic area (LHA) of the rat brain. Adrenalectomy (ADX) reduced orexin-A-induced feeding over 4 h by about 60%, compared with shams, an effect that was reversed by corticosterone (CORT) replacement. ADX had no effect on prepro-orexin mRNA levels in the LHA in either the morning or the evening; however, message was up-regulated by CORT in the morning but not the evening. An increased number of emulsion grains per cell in the LHA suggests that this is a specific increase in prepro-orexin mRNA and is not due to an increased number of cells expressing message. Prepro-orexin mRNA levels in the LHA were elevated 4 h after injection of lipopolysaccharide, compared with saline-injected controls. Partial but not complete abolition of orexin-A-induced feeding by ADX suggests that orexin-A-induced feeding may be mediated through glucocorticoid-dependent and glucocorticoid-independent pathways. In the morning increased prepro-orexin mRNA after CORT replacement demonstrates that orexin expression is sensitive to increased concentrations of glucocorticoids. However, the lack of effect of ADX on prepro-orexin mRNA levels suggests that endogenous glucocorticoids are not involved in tonic regulation of basal prepro-orexin expression. Overall our data constitute a body of evidence for an integrated relationship between central orexin expression, stress, glucocorticoid manipulation, and feeding patterns in the rat.

Can we Develop Neurally Acting Drugs for the Treatment of Migraine?

Serotonin (5-HT)(1B/1D) receptor agonists, which are also known as triptans, represent the most important advance in migraine therapeutics in the four millennia that the condition has been recognized. The vasoconstrictive activity of triptans produced a small clinical penalty in terms of coronary vasoconstriction but also raised an enormous intellectual question: to what extent is migraine a vascular problem? Functional neuroimaging and neurophysiological studies have consistently developed the theme of migraine as a brain disorder and, therefore, demanded that the search for neurally acting antimigraine drugs should be undertaken. The prospect of non-vasoconstrictor acute migraine therapies, potential targets for which are discussed here, offers a real opportunity to patients and provides a therapeutic rationale that places migraine firmly in the brain as a neurological problem, where it undoubtedly belongs.

Spinal orexin-1 receptors mediate anti-hyperalgesic effects of intrathecally-administered orexins in diabetic neuropathic pain model rats

Orexin-A and orexin-B are endogenous ligands of orexin receptors that contain orexin-1 and orexin-2. Activation of the orexinergic system can produce antinociceptive effects in acute inflammatory, mono-neuropathic, and postoperative pain animal models, though the effects of orexins on diabetic neuropathic pain have not been previously investigated. In this study, we studied the anti-hyperalgesic effects of intrathecally administered orexins in a streptozotocin-induced diabetic rat. First, dose-dependent effects were investigated by measuring hind paw withdrawal thresholds in response to noxious-heat and punctate stimuli, after which orexin levels in the cerebrospinal fluid of diabetic rats were measured and compared with those of normal rats using a radioimmunoassay method. The functional role of spinal orexin-1 receptors with the anti-hyperalgesic effects of orexins was also investigated using intrathecal pretreatment with SB-334867, a selective orexin-1 receptor antagonist. Intrathecally administered orexins produced an antinociceptive effect in diabetic rats, however, not in normal rats, though the orexin levels in the cerebrospinal fluid of diabetic rats were similar to those in normal rats. In addition, the anti-hyperalgesic effects of orexins were significantly inhibited by pretreatment with SB-334867. These findings demonstrate that the anti-hyperalgesic effects of orexins in diabetic rats are unlikely due to any direct effect by the supplement on decreased endogenous orexins in the cerebrospinal fluid and that orexin-1 receptors in the spinal cord may be involved in the modulation of nociceptive transmission in diabetic neuropathy. We conclude that the spinal orexinergic system may be a possible target for elucidating the mechanisms of diabetes-induced hyperalgesia.

Enhanced antinociception by intracerebroventricularly and intrathecally-administered orexin A and B (hypocretin-1 and -2) in mice

Orexins are neuropeptides located exclusively in neurons of the lateral hypothalamic area, which send projections to most monoaminergic nuclei, such as noradrenergic locus coeruleus, dopaminergic ventral tegmental areas, and histaminergic tuberomammillary nuclei. The present work was carried out to examine the role of orexins in nociception in mice. C57BL/6 mice were administered with orexin A and B intracerebroventricularly (i.c.v.), intrathecally (i.t.) and subcutaneously (s.c.) to reveal the sites of action of these peptides and to examine the pain thresholds using four kinds of nociceptive tasks. Orexins showed antinociceptive effects in all four types of assays for thermal (hot-plate, tail-flick, paw-withdrawal), mechanical (tail-pressure), chemical (formalin, capsaicin and abdominal stretch) nociceptions and nociceptin-induced behavioral responses, when administered i.c.v. or i.t., whereas the s.c. administration was ineffective. The antinociceptive effects of orexin A were more remarkable than those of orexin B. The i.c.v. administration of orexin A was as effective as, or more potent than the i.t. administration. The effects of orexin A were completely blocked by adenosine type 1 receptor antagonists, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) and theophylline, but not by naloxone, suggesting a possible involvement of the adenosine-containing neurons and/or the adenosine pathway in these orexin actions. The i.c.v. administration of nociceptin had no significant effects on orexin expression in the brain and spinal cord. The present findings suggest that orexins have an antinociceptive role in at least four different types of pains, probably acting on both the brain and spinal cord.

Persistent pain and stress activate pain-inhibitory orexin pathways

Orexins are synthesized by neurons in the hypothalamus and contribute to multiple physiological functions. Orexin fibers innervate many regions of the CNS, which include areas involved in descending control of pain. We examined the role orexins may play in endogenous modulation of pain transmission using prepro-orexin (precursor of orexin A and B) knockout mice. Baseline pain thresholds of knockout and wild type mice were not different. Knockout mice presented greater degree of hyperalgesia induced by peripheral inflammation and less stress-induced analgesia than wild type mice. Double staining of orexin and c-Fos in wild type mice revealed activation of orexin neurons under both conditions. These results suggest that persistent pain and stress activate orexin neurons, which act to inhibit pain transmission.

Satiety enhancement by selective orexin-1 receptor antagonist SB-334867: influence of test context and profile comparison with CCK-8S

Acute systemic treatment with the selective orexin-1 (OX1R) antagonist SB-334867 reduces food intake in rats, an effect associated with an acceleration in behavioural satiety and unrelated to gross behavioural disruption, alterations in palatability, or toxicity. However, as enhanced satiety is behaviourally indexed by an earlier-than-normal transition from eating to resting, and since orexin-A has been implicated in mechanisms of arousal, it remains possible that sedation contributes to the anorectic effect of acute OX1R blockade. Previous work has shown that, when treated with SB-334867 (30 mg/kg, i.p.) 30 min before a 1h test with palatable food, rats begin to show appreciable levels of resting 10-15 min earlier than under control conditions (i.e. around 20 min versus 30-35 min into the session). The present results demonstrate that a 20 min increase in the injection-test interval (i.e. 50 min) had no significant impact on the anorectic, behavioural or weight gain effects of SB-334867 in non-deprived male rats. Most importantly, this altered treatment regimen led to a temporal profile of resting virtually identical to that previously observed with the more conventional 30 min injection-test interval. Although parallel studies indicated that the OX1R antagonist accelerated the onset of resting (and suppressed most active behaviours) even in the absence of food, an equianorectic dose of the natural satiety-related signal cholescystokinin octapeptide (CCK-8S; 5 microg/kg, i.p.) also produced very similar behavioural effects regardless of the presence of food. Together with evidence that SB-334867 preserves the structural integrity of natural feeding behaviour, does not induce nausea/illness or alter taste/palatability and fails to influence EEG measures of arousal/sleep, the present findings are consistent with the view that acute OX1R antagonism selectively enhances satiety. However, unlike the immediate short-circuiting of the satiety sequence induced by CCK-8S, the slower response to SB-334867 implies a more indirect mechanism of action.

Activation of spinal orexin-1 receptor produces anti-allodynic effect in the rat carrageenan test

Orexin-A and orexin-1 receptors are found in the dorsal root ganglion cells and the spinal dorsal horn and this suggests that orexin-A is involved in the spinal nociceptive transmission. The authors examined the effect of intrathecally administered orexin-A on the level of mechanical allodynia and thermal hyperalgesia induced by paw carrageenan injection in the rat. Intrathecal injection of 0.3 and 3 nmol of orexin-A suppressed the level of mechanical allodynia, but not that of thermal hyperalgesia, and the effect of orexin-A on mechanical allodynia was antagonized by the pretreatment of 1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthyridin-4-yl urea hydrochloride, SB-334867, a selective orexin-1 receptor antagonist. These data suggest that the activation of spinal orexin-1 receptor modulates the mechanical information transmission, but not thermal information transmission, in the spinal cord during carrageenan test.

Ultrastructure of orexin-1 receptor immunoreactivities in the spinal cord dorsal horn

The ultrastructural properties of orexin 1-receptor-like immunoreactive (OX1R-LI) neurons in the dorsal horn of the rat spinal cord were examined using light and electron microscopy techniques. At the light microscopy level, the most heavily immunostained OX1R-LI neurons were found in the ventral horn of the spinal cord, while some immunostained profiles, including nerve fibers and small neurons, were also found in the dorsal horn. At the electron microscopy level, OX1R-LI perikarya were identified containing numerous dense-cored vesicles which were more heavily immunostained than any other organelles. Similar vesicles were also found within the axon terminals of the OX1R-LI neurons. The perikarya and dendrites of some of the OX1R-LI neurons could be seen receiving synapses from immunonegative axon terminals. These synapses were found mostly asymmetric in shape. Occasionally, some OX1R-LI axon terminals were found making synapses on dendrites that were OX1R-LI in some cases and immunonegative in others. The synapses made by OX1R-LI axon terminals were found both asymmetric and symmetric in appearance. The results provide solid morphological evidence that OX1R is transported in the dense-cored vesicles from the perikarya to axon terminals and that OX1R-LI neurons in the dorsal horn of the spinal cord have complex synaptic relationships both with other OX1R-LI neurons as well as other neuron types.

Pharmacological characterisation of the orexin receptor subtype mediating postsynaptic excitation in the rat dorsal raphe nucleus

Electrophysiological recordings from dorsal raphe nucleus (DRN) neurones in rat brain slices have revealed that the orexins can cause direct and reversible depolarisation of the postsynaptic membrane. Whilst it is known that the membrane depolarisation produced by orexin-A can dramatically increase the firing rate of DRN neurones, quantitative pharmacological analysis that determines the receptor subtype mediating the orexinergic response has not yet been performed. Here, we demonstrate that the rank order of potencies of orexin receptor agonists to excite serotonergic DRN neurones is orexin-A = orexin-B > SB-668875-DM. In contrast, the rank order of potency of these agonists to excite noradrenergic locus coreleus (LC) neurones is orexin-A > orexin-B > SB-668875-DM. We show further that the orexin receptor antagonist, SB-334867-A, inhibits the effects of orexin-A in the LC and DRN with pKB values of 6.93 and 5.84, respectively, values similar to those calculated for human OX1 (7.27) and OX2 (5.60) receptors expressed in CHO cells. These data suggest a differential role for OX1 and OX2 receptors in stimulating distinct populations of monoaminergic neurones in the rat CNS with OX2 receptors exhibiting a more pronounced functional significance in serotonergic neurones and OX1 in noradrenergic neurones.

Differential modulation of nociceptive dural input to [hypocretin] orexin A and B receptor activation in the posterior hypothalamic area

The novel neuropeptides orexin A and B are selectively synthesised in the lateral and posterior hypothalamus and are involved in hypothalamic regulation of autonomic and neuroendocrine functions. Recent findings point also to a role in nociception. As the posterior hypothalamus is involved in the central modulation of nociception we studied the effects of hypocretin/orexin receptor activation in the posterior hypothalamic area (PH) of the rat on dural nociceptive input. Orexins were microinjected into the PH and the effects on responses of neurones in the caudal trigeminal nucleus studied. Injection of orexin A decreased the A- and C-fibre responses to dural electrical stimulation as well as spontaneous activity. Responses to noxious thermal stimulation of the facial skin were also decreased by orexin A. Injection of orexin B into the PH, however, elicited increased responses to dural stimulation in A- and C-fibre responses and resulted in increased spontaneous activity. Responses to facial thermal stimulation were also increased by orexin B. Control injection of saline into the PH had no significant effect. The results show a differential modulation of dural nociceptive input by orexin A and B receptor activation in the PH. The results support the role of the PH in the nociceptive processing of meningeal input. As both peptides are also involved in hypothalamic regulation of neuroendocrine and autonomic functions, orexinergic mechanisms in the PH may provide a link for endocrine and autonomic changes as well as nociceptive phenomena seen in primary headache disorders.

Orexinergic neurons and barbiturate anesthesia

Orexins (OXs) regulate sleep with possible interactions with brain noradrenergic neurons. In addition, noradrenergic activity affects barbiturate anesthesia. As we have also recently reported that OXs selectively evoke norepinephrine release from rat cerebrocortical slices we hypothesized that barbiturate anesthesia may result from of an interaction with central orexinergic systems. To test this hypothesis, we performed a series of in vivo and in vitro studies in rats. In vivo, the effects of i.c.v. OX A, B and SB-334867-A (OX1 receptor antagonist) on pentobarbital, thiopental or phenobarbital-induced anesthesia times (loss of righting reflex) was assessed. In vitro effects of barbiturates and SB-334867-A on OX-evoked norepinephrine release from cerebrocortical slice was examined. In Chinese hamster ovary cells expressing human OX1/OX2 receptors OX A- and B-evoked increases in intracellular Ca2+ were measured with and without barbiturates. OX A and B significantly decreased pentobarbital, thiopental and phenobarbital anesthesia times by 15-40%. SB-334867-A increased thiopental-induced anesthesia time by approximately by 40%, and reversed the decrease produced by OX A. In vitro, all anesthetic barbiturates inhibited OX-evoked norepinephrine release with clinically relevant IC50 values. A GABAA antagonist, bicuculline, did not modify the inhibitory effects of thiopental and the GABAA agonist, muscimol, did not inhibit norepinephrine release. In addition there was no interaction of barbiturates with either OX1 or OX2 receptors. Collectively our data suggest that orexinergic neurons may be an important target for barbiturates, and GABAA, OX1 and OX2 receptors may not be involved in this interaction.

Collateral axonal projections from hypothalamic hypocretin neurons to cardiovascular sites in nucleus ambiguus and nucleus tractus solitarius

Hypocretin-1 (hcrt-1)-containing axons have been shown to have an extensive distribution within the central nervous system, although the total number of hypothalamic hcrt-1 neurons has been shown to be small. This suggests that hcrt-1 neurons may innervate central structures with similar function through collateral axonal projections. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether hypothalamic hcrt-1-containing neurons send collateral axonal projections to cardiovascular sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb) in the rat. Fluorogold- (FG) and/or rhodamine (Rd)-labeled latex microspheres were microinjected into either the NTS or Amb at sites that elicited bardycardia responses (L-glutamate; 0.25 M; 10 nl). After a survival period of 10-15 days, the rats were sacrificed and tissue sections of the hypothalamus were processed immunohistochemically for the identification of hcrt-1-containing cell bodies. After injection of the tract-tracers into the NTS or Amb, retrogradely labeled neurons were observed within several hypothalamic regions; the paraventricular hypothalamic nucleus, lateral hypothalamic area, perifornical hypothalamic area, and posterior hypothalamus, bilaterally, but with an ipsilateral predominance. In addition, after NTS injections, retrogradely labeled neurons were found within the ipsilateral caudal arcuate nucleus. Of the total number (1107+/-97) of hcrt-1-immunoreactive neurons found bilaterally within the lateral and perifornical hypothalamic nuclei, 7.9+/-1.4% were found to be retrogradely labeled from the NTS, 16.4+/-1.8% from the Amb, and 3.1+/-0.5% from both medullary sites. Hcrt-1 neurons projecting to the NTS were found mainly in and around the perifornical hypothalamic region, with a smaller number in the caudal lateral hypothalamic area. On the other hand, those innervating the Amb were primarily observed within the caudal lateral hypothalamic area, with a smaller number in the perifornical hypothalamic area. Neurons with collateral axonal projections to NTS and Amb were observed within two specific hypothalamic areas: one group of neurons was found in the perifornical hypothalamic area, and the other was observed in the lateral hypothalamic region just dorsal to the retrochiasmatic component of the supraoptic nucleus. These data indicate that axons from hcrt-1 neurons bifurcate to innervate functionally similar cardiovascular-responsive sites in the NTS and Amb. Although the function of these hcrt-1-containing hypothalamic-medullary pathways is not known, they likely represent the anatomical substrate by which the lateral hypothalamic hcrt-1 neurons simultaneously coordinate autonomic-cardiovascular responses to different behaviors.

Antiallodynic effects of intrathecal orexins in a rat model of postoperative pain

Orexin A and B (hypocretin 1 and 2) are the endogenous ligands of orexin receptors, a G-protein-coupled orphan receptor family containing orexin 1 (OX1) and orexin 2 (OX2) types. Orexin A induces analgesia in acute and inflammatory pain models. We further elucidated the possible antiallodynic effect of intrathecal orexins in a rat model of postoperative pain. Mechanical allodynia was induced by incising the rat hind paw and evaluated with the withdrawal threshold to von Frey filament stimulation. Intrathecal orexin A (0.03-1 nmol) and orexin B (0.1-3 nmol) dose dependently attenuated the incision-induced allodynia. Orexin A (ED50 = 0.06 nmol) is more potent than orexin B. The effects of orexin A and B were abolished by their respective antibodies, but not by naloxone, and were attenuated by suramin and strychnine, the P2X purinergic and glycine receptor antagonists, respectively. SB-334867, an OX1 receptor antagonist, at 30 nmol completely blocked the effect of orexin A but, even at 100 nmol, only partially antagonized the effect of orexin B. Orexin A antibody, SB-334867, suramin, strychnine, or naloxone enhanced the incision-induced allodynic response. It is concluded that intrathecal orexins reduce incision-induced allodynia through OX1 receptors. Glycine and P2X purinergic receptors, but not opioid receptors, might be involved in the antiallodynic effects of orexins. Endogenous orexin might be released after incision injury to activate the spinal OX1 receptors as an endogenous analgesic protector.

Cardioacceleratory responses to hypocretin-1 injections into rostral ventromedial medulla

Intracisternal injections of hypocretin-1 (hcrt-1) have been shown to elicit sympathoexciatory responses. However, the location of central sites that may mediate these cardiovascular effects have not been clearly elucidated. This study was done in male Wistar rats to investigate the effects of microinjections of hcrt-1 into the rostral ventromedial medulla (RVMM) on mean arterial pressure (MAP), heart rate (HR) and the arterial baroreflex. An initial series of experiments was done to provide a detailed mapping of the location of hcrt-1- and hcrt-1 receptors (hcrtR-1)-like immunoreactivity (i.r.) in the RVMM region. Hcrt-1 and hcrtR-1 ir were found throughout the RVMM region, but primarily within the magnocellular reticular nucleus and the adjacent nucleus paragigantocellularis lateralis. In the second series, this region containing hcrt-1 and hcrtR-1 ir was explored for sites that elicited changes in MAP and HR in the anaesthetized rat. Microinjection of hcrt-1 (0.5-2.5 pmol) into the region of magnocellular reticular nucleus elicited a dose-dependent increase in HR, with little or no change in MAP. Administration (i.v.) of the muscarinic receptor antagonist atropine methyl bromide significantly attenuated ( approximately 62%) the HR response whereas, the total autonomic blockade abolished the HR response. Finally, unilateral or bilateral microinjection of hcrt-1 into the magnocellular reticular nucleus significantly attenuated the reflex bradycardia resulting from the activation of the baroreflex following the increase in MAP from an iv injection of phenylephrine. These data suggest that hcrt-1 in the RVMM region activates neuronal circuits that both inhibit vagal activity and increase sympathetic activity to the heart, and that it alters the excitability of central circuits that reflexly control the circulation.

Cardiovascular responses to hypocretin-1 in nucleus ambiguus of the ovariectomized female rat

Experiments were done to investigate the effect of chronic estrogen (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus ambiguus (Amb). Microinjections of hcrt-1 (0.5-2.5 pmol) into the external formation of Amb (Ambe) in the urethane anaesthetized, E treated OVX animal or OVX only animal, elicited a dose-related decrease in heart rate (HR). On the other hand, hcrt-1 injections into Ambe did not elicit consistent changes in mean arterial pressure (MAP). The HR response was mediated by vagal excitation as ipsilateral vagotomy abolished the bradycardia response. The bradycardia responses were consistently of greater magnitude and longer duration in the OVX+E animals compared to the OVX only female animals. Finally, it was found that the reflex bradycardia to activation of arterial baroreceptors, as a result of increasing systemic arterial pressure with phenylephrine, was only significantly potentiated in the OVX+E animals. These data suggest that hcrt-1 in the Ambe of the female elicits an increase in vagal cardiomotor neuronal activity to the heart, and that the circulating level of E alters not only the sensitivity of Ambe neurons to hcrt-1 but also the sensitivity of these neurons during activation of baroreceptor afferent inputs.

Immunoelectron microscopic examination of orexin-like immunoreactive fibers in the dorsal horn of the rat spinal cord

The ultrastructure and synaptic relationships of orexin A-like immunoreactive neuronal fibers in the dorsal horn of the rat cervical spinal cord were examined at both the light and electron microscopic levels. At the light microscopic level, many intensely immunostained orexin A-like fibers were found, while at the electron microscopic level, immunoreactivity in these fibers was mostly confined to axon terminals. Most of the axon terminals contained dense-cored vesicles. Immunoreactive and immunonegative dense-cored vesicles were occasionally found within the same orexin A-like immunoreactive axon terminals, which were often found making synapses with immunonegative dendrites. These synapses were both asymmetric and symmetric, with the asymmetric ones predominant. Orexin A-like immunoreactive processes that contained no synaptic vesicles were also found with less frequency. These processes were also observed receiving synaptic inputs from immunonegative axon terminals, but the synapses were mostly asymmetric. Sometimes, such processes were found to receive multiple synaptic inputs for which the presynaptic immunonegative axon terminals could make synapses on other immunonegative dendrites simultaneously. Occasionally, synapses between the orexin A-like immunoreactive axon terminals and orexin A-like immunoreactive processes containing no synaptic vesicles were also found. The present results provide solid morphological evidence that orexin A may be involved in pain-inhibition mechanisms in the spinal cord and suggest that this function may be complex and occur in conjunction with the regulatory effects of other neurotransmitters.

Anti-mechanical allodynic effect of intrathecal and intracerebroventricular injection of orexin-A in the rat neuropathic pain model

Orexin-A has been reported to produce an analgesic effect in the hot plate test and in the inflammatory pain models. In the present study, the authors examined the effect of orexin-A on the mechanical allodynia induced by partial sciatic nerve ligation (a model of neuropathic pain) in the rat. Partial sciatic nerve ligation is created by tight ligation of one-third or one-half of the right sciatic nerve. Orexin-A was administered intrathecally or intracerebroventricularly 7 days after a partial sciatic nerve injury. Either intrathecal or intracerebroventricular injection of orexin-A attenuated the level of mechanical allodynia induced by partial sciatic nerve ligation. These data suggest that either intrathecal or intracerebroventricular injection of orexin-A is a new therapeutic approach to treating mechanical allodynia caused by nerve injury.

Effects of central hypocretin-1 administration on hemodynamic responses in young-adult and middle-aged rats

The prevalence of hypertension in middle age correlates with impaired autonomic regulation and as norepinephrinergic neurons decline with increasing age, and this reduction may contribute to this impairment. Central hypocretin-activated norepinephrinergic neurons contribute to sympathetic regulation. In the present study we compared sympathoadrenal effects of intracerebroventricular (i.c.v.) hypocretin-1(5 nmol) between young-adult (12-14 weeks) and middle-aged (12-14 months) rats. Arterial blood pressure, heart rate and plasma catecholamines were assessed under pentobarbital anesthesia. In addition, we compared hypocretin-1 and K(+)-evoked norepinephrine release from the cerebrocortical slices prepared from young-adult and middle-aged rats. We also examined whether the novel hypocretin receptor-1 antagonist (SB-334867) could reverse these hypocretin-1 effects both in vivo and in vitro. I.c.v. hypocretin-1 significantly increased blood pressure by some 7%, heart rate by 9% and plasma norepinephrine concentrations by 100% in young-adult rats. In middle-aged rats these parameters did not change. Plasma epinephrine did not increase in either group. There was a significant correlation between changes in mean arterial pressure and plasma norepinephrine. Similarly, hypocretin-1 evoked norepinephrine release from cerebrocortical slices prepared from young-adult rats was significantly higher than that of middle-aged rats whilst K(+)-evoked release did not differ between the groups. SB-334867 significantly attenuated hypocretin-1-increased blood pressure and both in vivo and in vitro norepinephrine release. The present data suggest that hypocretinergic neurons may contribute to the regulation of central but not adrenal sympathetic activity. Moreover, sympathetic regulation by hypocretinergic neurones may disappear in middle-age in rats.

Estrogen alters the bradycardia response to hypocretin-1 in the nucleus tractus solitarius of the ovariectomized female

Experiments were performed to investigate the effect of 17beta-estradiol (E; 30 pg/ml plasma) treatment (15-25 days) in the ovariectomized (OVX) female Wistar rat on the cardiovascular responses to hypocretin-1 (hcrt-1) in the nucleus tractus solitarius (NTS). In an initial series of experiments, the distribution of hcrt-1-like immunoreactivity within the region of the NTS was mapped in both OVX only and OVX+E animals. Hcrt-1 immunoreactivity was found throughout the NTS region in both groups of females, predominantly within the caudal interstitial, commissural, medial and lateral subnuclei of the NTS. The relative density of hcrt-1 immunoreactivity in all NTS subnuclei was similar in both female groups. Microinjections of hcrt-1 (0.5-10 pmol) into the caudal lateral and medial subnuclei of the NTS complex of the alpha-chloralose of the urethane-anaesthetized E-treated OVX rat elicited a dose-related decrease in heart rate (HR). On the other hand, although a dose-response effect on arterial pressure was evident, significant arterial pressure responses were observed only at the higher dose of hcrt-1 (>2.5 pmol). In the OVX only female rat, microinjection of hcrt-1 into similar NTS sites elicited a bradycardia and depressor response only at the highest dose of hcrt-1, and these responses were significantly smaller in magnitude than those elicited in the OVX+E animal. In addition, in the OVX only animals, a few sites within the caudal commissural subnucleus of the NTS complex were found at which hcrt-1 elicited tachycardia and pressor responses. Finally, it was found that the reflex bradycardia to the activation of arterial baroreceptors as a result of increasing systemic arterial pressure with phenylephrine (2-4 microg/kg) was significantly potentiated in the OVX+E animals only. These data suggest that hcrt-1 in the NTS of the female activates a neuronal circuit that controls the circulation and that the circulating level of E alters the sensitivity of these cardiovascular circuits to hcrt-1.

Cardiac effects of hypocretin-1 in nucleus ambiguus

Although recent studies have reported hypocretin 1 (hcrt-1)-like-immunoreactivity (ir) within the region of the nucleus ambiguus (Amb) in the caudal brain stem, the function of hcrt-1 in the Amb on cardiovascular function is not known. Three series of experiments were done in male Wistar rats to investigate the effects of microinjections of hcrt-1 into Amb on heart rate (HR), mean arterial pressure (MAP), and the arterial baroreceptor reflex. In the first series, a detailed mapping of the distribution of hcrt-1- and hcrt-1 receptor (hcrtR-1)-like-ir was obtained of the Amb region. Although hcrt-1-like- and hcrtR-1-like-ir were found throughout the rostrocaudal extent of the Amb and adjacent ventrolateral medullary reticular formation, most of the hcrtR-1-like-ir was observed in the area just ventral to the compact formation of Amb, in the region of the external formation of the nucleus (Ambe). In the second series, the Amb region that contained hcrt-1 and hcrtR-1-ir was explored for sites that elicited changes in HR and MAP in urethane and alpha-chloralose-anesthetized rats. Microinjections of hcrt-1 (0.5-2.5 pmol) into the Ambe elicited a dose-related decrease in HR, with little or no direct change in MAP. The small decreases in MAP were found to be secondary to the HR changes. The largest bradycardia responses were elicited from sites in the Ambe. Administration (iv) of the muscarinic receptor antagonist atropine methyl bromide or ipsilateral vagotomy abolished the HR response, indicating that the HR response was due to activation of vagal cardiomotor neurons. In the final series, microinjections of hcrt-1 into the Ambe significantly potentiated the reflex bradycardia elicited by activation of the baroreflex as a result of the increased MAP after the intravenous injection of phenylephrine. These data suggest that hcrt-1 in the Ambe activates neuronal systems that alter the excitability of central circuits that reflexly control the circulation through the activation of vagal preganglionic cardioinhibitory neurons.

Electron microscopic examination of the orexin immunoreactivity in the dorsal raphe nucleus

The ultrastructure and the synaptic relationships of the orexin-A-like immunoreactive fibers in the dorsal raphe nucleus were examined with an immunoelectron microscopic method. At the electron microscopic level, most of the immunoreactive fibers, a varicosity appearance at the light microscopic level, were found as axon terminals. The large dense-cored vesicles contained in the immunoreactive axon terminals were the most intensely immunostained organellae. These axon terminals were often found to make synapses. While the axo-dendritic synapses were usually asymmetric in appearance, the axo-somatic synapses were symmetric. Orexin-A-like immunoreactive processes with no synaptic vesicles were also found. These processes often received asymmetric synapses. With less frequency, the synapses were found between the orexin-like immunoreactive processes. The results suggest that the orexin peptides are stored in the large dense-cored vesicles; the orexin-containing fibers may have influences on the physiological activities of the dorsal raphe nucleus through direct synaptic relationships.

Lack of an interaction between orexinergic and opioid/nociceptinergic systems in rat cerebrocortical slices

We have recently reported that orexins (OXs) selectively evoke norepinephrine release from rat cerebrocortical slices. In the present study, we have examined orexin-opioid interactions in OXA (100 nM) and K(+) (40 mM)-evoked norepinephrine release. OXA-evoked norepinephrine release was reversed approximately 90% by SB-334867 (OX(1)-receptor antagonist) (10 microM) but not naloxone (10 microM). [D-Pen(2),D-Pen(5)]-enkephalin (DPDPE) (DOP-agonist) and nociceptin/orphanin-FQ (N/OFQ) also failed to affect OXA-evoked release. [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) (MOP-agonist) and spiradoline (KOP-agonist) significantly reduced OXA-evoked release with the concentration producing 50% of the maximal inhibition (EC(50)) [maximal inhibition (E(max))] of 3.2 microM [41.8%] and 4.3 microM [54.9%] respectively. The effects of DAMGO and spiradoline were naloxone (10 microM)-insensitive. In contrast, naloxone significantly antagonized the inhibitory effects of DAMGO and spiradoline on K(+)-evoked release. We conclude that opioid receptors (DOP and KOP) are involved in K(+) but not OXA-evoked release. Moreover, we have failed to demonstrate an interaction between orexinergic and opioid/N/OFQ-ergic systems in this system.

Cardiovascular effects of hypocretin-1 in nucleus of the solitary tract

Experiments were done in male Wistar rats to investigate the effects of microinjection of hypocretin-1 (Hcrt-1) into the nucleus of the solitary tract (NTS) on mean arterial pressure (MAP), heart rate (HR), and the baroreflex. In the first series, the distribution of Hcrt-1-like immunoreactivity (Ir) was mapped within the region of NTS. Hcrt-1 Ir was found throughout the NTS region, predominantly within the caudal dorsolateral (Slt), medial (Sm), and interstitial subnuclei of the NTS. In the second series, in alpha-chloralose or urethane-anesthetized rats, microinjection of Hcrt-1 (0.5-5 pmol) into the caudal NTS elicited a dose-dependent decrease in MAP and HR. A mapping of the caudal NTS region showed that the largest depressor and bradycardia responses elicited by Hcrt-1 were from sites in the Slt and Sm. In addition, doses >2.5 pmol at a small number of sites localized to the caudal commissural nucleus of NTS elicited pressor and tachycardia responses. Intravenous administration of the muscarinic receptor blocker atropine methyl bromide abolished the bradycardia response and attenuated the depressor response, whereas subsequent administration of the nicotinic receptor blocker hexamethonium bromide abolished the remaining MAP response. Finally, microinjection of Hcrt-1 into the NTS significantly potentiated the reflex bradycardia to activation of arterial baroreceptors as a result of increasing MAP by systemic injections of phenylephrine (2-4 microg/kg). These results suggest that Hcrt-1 in the NTS activates neuronal circuits that increases vagal activity to the heart, inhibits sympathetic activity to the heart and vasculature, and alters the excitability of NTS neuronal circuits that reflexly control the circulation.

Identification of neurons containing orexin-B (hypocretin-2) immunoreactivity in limbic structures

Orexins (hypocretins) are neuropeptides which have recently been identified exclusively within lateral hypothalamic and perifornical neurons, and these orexin (ox) containing neurons appear to have extensive projections to all levels of the neuraxis. In this study, we report the identification of two distinct clusters of neurons containing ox-B-like immunoreactivity within the amygdaloid complex of the rat. A cluster of small to medium size ovoid shaped neurons containing ox-B-like immunoreactivity was found predominantly within the lateral division of the central nucleus of the amygdala (ACe). A second distinct, but smaller group of ox-B labelled neurons with similar shapes and sizes to those in ACe was also identified in the anterior lateral subnucleus of the bed nucleus of the stria terminalis (BST) immediately adjacent the internal capsule, and in an area just ventral to the lateral ventricle. Neurons containing ox-A-like immunoreactivity were not observed in either structure. However, both structures contained ox-A- and ox-B labelled varicose fibers. Unilateral electrolytic lesions of the lateral hypothalamic area that contained ox-A and ox-B neurons did not alter the labelling of either ACe or BST ox-B pericarya. As both the ACe and BST are known to be involved in integrating complex homeostatic mechanisms associated with behaviours, these data suggest that a specific subset of ox-B neurons within the amygdaloid complex may serve as a component of neuronal circuits coordinating these responses.

Cardiovascular effects of leptin and orexins

Leptin, the product of the ob gene, is a satiety factor secreted mainly in adipose tissue and is part of a signaling mechanism regulating the content of body fat. It acts on leptin receptors, most of which are located in the hypothalamus, a region of the brain known to control body homeostasis. The fastest and strongest hypothalamic response to leptin in ob/ob mice occurs in the paraventricular nucleus, which is involved in neuroendocrine and autonomic functions. On the other hand, orexins (orexin-A and -B) or hypocretins (hypocretin-1 and -2) were recently discovered in the hypothalamus, in which a number of neuropeptides are known to stimulate or suppress food intake. These substances are considered important for the regulation of appetite and energy homeostasis. Orexins were initially thought to function in the hypothalamic regulation of feeding behavior, but orexin-containing fibers and their receptors are also distributed in parts of the brain closely associated with the regulation of cardiovascular and autonomic functions. Functional studies have shown that these peptides are involved in cardiovascular and sympathetic regulation. The objective of this article is to summarize evidence on the effects of leptin and orexins on cardiovascular function in vivo and in vitro and to discuss the pathophysiological relevance of these peptides and possible interactions.

Pain in the brain: are hormones to blame?

Pain is a multi-dimensional process involving the physical, emotional and perceptual integration of noxious information. The physical component is relayed via the spinal cord to several brain areas to initiate the detection of pain. The emotional aspect is encoded by the limbic system and encapsulates the relationship between pain and mood. Within the limbic system, the hypothalamus undertakes a diversity of separate and interrelated functions. Dysfunction of the hypothalamo-pituitary-adrenal axis has been implicated in a variety of chronic pain conditions and might also be associated with increased risk of developing mood disorders. Experimental and clinical evidence also exists to implicate the effects of other hormonal modulators in the manifestation of chronic pain. Specific targeting of hormonal cascade and effector mechanisms could provide an alternative strategy for the treatment of various chronic pain conditions.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Reciprocal synaptic relationships between orexin- and melanin-concentrating hormone-containing neurons in the rat lateral hypothalamus: a novel circuit implicated in feeding regulation

OBJECTIVE

Both orexin (ORX)- and melanin-concentrating hormone (MCH) are expressed in different neurons in the lateral hypothalamic area (LH), and are considered to have common effects on stimulating food intake. There are no reports to demonstrate neural interactions between them at the ultrastructural level. We observed these neurons in the LH to evaluate the relationships between them.

DESIGN

We used two different types of double immunostaining to reveal the ultrastructure of both the ORX- and MCH-containing neurons. A preembedding double immunostaining technique was used to study the synaptic relationships between the two kinds of neuron.

RESULTS

The main new findings are as follows: 1) Both ORX- and MCH-containing neurons received other synaptic input and made synaptic input to other neurons; 2) Reciprocal synaptic relationships were observed between the ORX- and MCH-containing neurons.

CONCLUSION

The ORX- and MCH-containing neurons in the lateral hypothalamic area may influence food intake through synapse with each other.

Functions of the orexinergic/hypocretinergic system

Orexin A and orexin B are hypothalamic peptides that act on their targets via two G protein-coupled receptors (OX1 and OX2 receptors). In the central nervous system, the cell bodies producing orexins are localized in a narrow region within the lateral hypothalamus and project mainly to regions involved in feeding, sleep, and autonomic functions. Via putative pre- and postsynaptic effects, orexins increase synaptic activity in these regions. In isolated neurons and cells expressing recombinant receptors orexins cause Ca2+ elevation, which is mainly dependent on influx. The activity of orexinergic cells appears to be controlled by feeding- and sleep-related signals via a variety of neurotransmitters/hormones from the brain and other tissues. Orexins and orexin receptors are also found outside the central nervous system, particularly in organs involved in feeding and energy metabolism, e.g., gastrointestinal tract, pancreas, and adrenal gland. In the present review we focus on the physiological properties of the cells that secrete or respond to orexins.

Stimulation of amylase release by Orexin is mediated by Orexin 2 receptor in AR42J cells

INTRODUCTION AND AIMS

Orexins have been demonstrated to have mainly central physiological functions, including regulation of food and water intake, sleep, and arousal. However, little is known about their direct peripheral effects, if any. As a first step toward understanding the role of Orexin in non-neuronal tissues or cells, we initiated studies to examine expression of Orexin receptors (OXR) in an established pancreatic tumor cell line AR42J. Secondly, we wanted to determine whether Orexins, in various molecular forms, are active to stimulate any known pancreatic cell functions in AR42J cells.

METHODOLOGY

Reverse transcription-PCR analysis was performed to identify the presence of specific Orexin receptor subtypes. Intracellular calcium mobilization and cAMP levels were measured following stimulation by Orexin A and B peptides, their respective C-terminal decapeptide fragments, and hypocretin-2-gly (glycine-extended Orexin B). Release of alpha-amylase was measured in conditioned media after acute stimulation with the set of Orexin peptides for 30 minutes. Cell proliferation was determined by H-thymidine incorporation after 24 hours following treatment with Orexins under serum-free condition.

RESULTS

RT-PCR and sequencing results showed that Orexin receptor subtype 2 (OX2R) was the main form expressed in AR42J cells. Orexins stimulated dose-dependent increases in intracellular calcium mobilization with EC50 0.05 nM for Orexin A and 0.1 nM for Orexin B but were unable to stimulate any significant cAMP accumulation or DNA synthesis even at micromolar concentrations. Both Orexin-A and -B, but not hypocretin-2-gly, also stimulated dose-dependent increases in amylase release in the AR42J cells. Orexin-A and -B carboxyl-terminal decapeptides elicited significant but much lower calcium and amylase responses.

CONCLUSION

Our data demonstrate that OX2R mediates Ca -dependent amylase release in AR42J cells, suggesting that Orexins may have secretory functions in pancreatic tumor cells.

The role of hypocretins (orexins) in sleep regulation and narcolepsy

The hypocretins (orexins) are two novel neuropeptides (Hcrt-1 and Hcrt-2), derived from the same precursor gene, that are synthesized by neurons located exclusively in the lateral, posterior, and perifornical hypothalamus. Hypocretin-containing neurons have widespread projections throughout the CNS with particularly dense excitatory projections to monoaminergic centers such as the noradrenergic locus coeruleus, histaminergic tuberomammillary nucleus, serotoninergic raphe nucleus, and dopaminergic ventral tegmental area. The hypocretins were originally believed to be primarily important in the regulation of appetite; however, a major function emerging from research on these neuropeptides is the regulation of sleep and wakefulness. Deficiency in hypocretin neurotransmission results in the sleep disorder narcolepsy in mice, dogs, and humans. The hypocretins are also uniquely positioned to link sleep, appetite, and neuroendocrine control. The aim of this review is to describe and discuss the current knowledge regarding the hypocretin neurotransmitter system in narcolepsy and normal sleep.

Orexins and appetite regulation

Initial research on the functional significance of two novel hypothalamic neuropeptides, orexin-A and orexin-B, suggested an important role in appetite regulation. Since then, however, these peptides have also been shown to influence a wide range of other physiological and behavioural processes. In this paper, we review the now quite extensive literature on orexins and appetite control, and consider their additional effects within this context. Although the evidence for orexin (particularly orexin-A and the orexin-1 receptor) involvement in many aspects of ingestive physiology and behaviour is incontrovertible, central administration of orexins is also associated with increased EEG arousal and wakefulness, locomotor activity and grooming, sympathetic and HPA activity, and pain thresholds. Since the orexin system is selectively activated by signals indicating severe nutritional depletion, it would be highly adaptive for a hungry animal not only to seek sustenance but also to remain fully alert to dangers in the environment. Crucial evidence indicates that orexin-A increases food intake by delaying the onset of a behaviourally normal satiety sequence. In contrast, a selective orexin-1 receptor antagonist (SB-334867) suppresses food intake and advances the onset of a normal satiety sequence. These data suggest that orexin-1 receptors mediate the episodic signalling of satiety and appear to bridge the transition from eating to resting in the rats' feeding-sleep cycle. The argument is developed that the diverse physiological and behavioural effects of orexins can best be understood in terms of an integrated set of reactions which function to rectify nutritional status without compromising personal survival. Indeed, many of the non-ingestive effects of orexin administration are identical to the cluster of active defences mediated via the lateral and dorsolateral columns of the midbrain periaqueductal gray matter, i.e., somatomotor activation, vigilance, tachycardia, hypertension and non-opioid analgesia. In our view, therefore, the LH orexin system is very well placed to orchestrate the diverse subsystems involved in foraging under potentially dangerous circumstances, i.e., finding and ingesting food without oneself becoming a meal for someone else.

Analgesic effect of intrathecally administered orexin-A in the rat formalin test and in the rat hot plate test

1. Orexin-A and orexin-B (also known as hypocretin-1 and hypocretin-2) are hypothalamic peptides and regulate feeding behaviour, energy metabolism and the sleep-wake cycle. Orexin-A binds equally to both orexin-1 and orexin-2 receptors, while orexin-B has a preferential affinity for orexin-2 receptors. 2. Orexins are also known to be concentrated in superficial laminae of the spinal dorsal horn, and orexin-A and orexin-1 receptors are found in the dorsal root ganglion cells. 3. In the present study, the authors examined the effect of intrathecal injection of either orexin-A or orexin-B in the rat formalin test (a model of inflammatory pain) and in the rat hot plate test. The paw formalin injection induces biphasic flinching (phase 1: 0-6 min; phase 2: 10-60 min) of the injected paw. 4. Intrathecal injection of orexin-A, but not orexin-B, decreased the sum of flinches in phases 1 and 2 in the formalin test and increased the hot plate latency. These effects of orexin-A were completely antagonized by pre-treatment with SB-334867, a selective orexin-1 receptor antagonist. Intrathecal injection of SB-334867 alone had no effect in the formalin test or in the hot plate test. 5. Intrathecal injection of orexin-A suppressed the expression of Fos-like immunoreactivity (Fos-LI), induced by paw formalin injection, in laminae I-II of L4-5 of the spinal cord. 6. These data suggest that the spinal orexin-1 receptor is involved in the nociceptive transmission and that the activation of the spinal orexin-1 receptor produces analgesic effects in the rat formalin test and in the rat hot plate test.

Age-related changes in plasma orexin-A concentrations

To study the effect of aging on orexin-A, we measured plasma orexin-A concentrations in 82 healthy volunteers (55 men and 27 women) over a wide range of ages (mean 50.2 +/- 13.9 years, ranging from 23 to 79 years). Correlation analyses revealed that plasma orexin-A concentrations correlated with age (r = 0.50, P < 0.01). When comparing three age groups, it appeared that the concentrations in the group of more than 60 years were significantly higher than those in the group of less than 39 years in both genders (P < 0.05). These findings suggest that orexin-A could be involved in aging in a healthy population.

Orexins in rat dorsal motor nucleus of the vagus potently stimulate gastric motor function

Orexins regulate food intake, arousal, and the sleep-wake cycle. They are synthesized by neurons in the lateral hypothalamus and project to autonomic areas in the hindbrain. Orexin A applied to the dorsal surface of the medulla stimulates gastric acid secretion via a vagally mediated pathway. We tested the hypothesis that orexins in the dorsal motor nucleus (DMN) of the vagus regulate gastric motor function. Multibarelled micropipette assemblies were used to administer vehicle, L-glutamate, orexins A (1 and 10 pmol) and B (10 pmol), and a dye marker into this site in anesthetized rats. When the pipette was positioned in the DMN rostral to the obex (where excitation of neurons by L-glutamate evoked an increase in contractility), orexins A and B increased intragastric pressure and antral motility. In contrast, 10 pmol orexin A microinjected into the DMN caudal to the obex (where L-glutamate evokes gastric relaxation through a vagal inhibitory pathway) did not significantly alter gastric motor function. In separate immunocytochemical studies, orexin receptor 1 was highly expressed in neurons in the DMN. Specifically, it was present in retrogradely labeled preganglionic neurons in the DMN that innervate the stomach. These data are consistent with the idea that orexin A stimulates vagal excitatory motor neurons. These are the first data to suggest that orexins in the DMN have potent and long-lasting effects to increase gastric contractility.

Activation of orexin neurones after noxious but not conditioned fear stimuli in rats

In rats, noxious stimuli increase food intake while conditioned fear stimuli decrease it. Orexin neurones play a facilitative role in food intake. Here, we examined expression of Fos protein in orexin neurones after noxious or conditioned fear stimuli in rats. Noxious stimuli significantly induced Fos protein in orexin neurones. On the other hand, conditioned fear stimuli did not significantly change expression of Fos protein in orexin neurones. The results demonstrate selective activation of orexin neurones by noxious stimuli, suggesting that effects of stressful stimuli upon orexin neurones are dependent upon the stimuli used. This finding is consistent with an idea that orexin neurones are involved in stress-induced food intake.

Increased cortical expression of the orexin-1 receptor following permanent middle cerebral artery occlusion in the rat

The orexins (hypocretins) have recently been implicated in neurodegeneration associated with narcolepsy. Therefore, the current study was designed to investigate changes in the expression of prepro-orexin and the orexin receptors, OX1R and OX2R following permanent middle cerebral artery occlusion (MCAO) in the rat. Six and twenty-four hours following MCAO, increased OX1R mRNA and protein expression (as assessed by Western blotting and immunohistochemistry) was detected in the ischaemic cortex compared with control tissue. In contrast, however, no increase in OX2R mRNA was detected at any time-point and prepro-orexin levels in the cortex were below assay detection levels. This study shows that orexin receptor localization is altered following cerebral ischaemia. The development of selective orexin receptor antagonists will be crucial in establishing a role for this family of novel peptides in the mechanisms underlying ischaemic cell death.

The hypocretins: setting the arousal threshold

Over a short period in the late 1990s, three groups converged on the discovery of a neuropeptide system, centred in the dorsolateral hypothalamus, that regulates arousal states, influences feeding and is implicated in the sleep disorder narcolepsy. Subsequent studies have illuminated many aspects of the circuitry of the hypocretin (also called orexin) system, which also influences hormone secretion and autonomic homeostasis, and have led to the hypothesis that most human narcolepsies result from an autoimmune attack against the hypocretin-producing neurons. The biochemical, physiological and anatomical components that regulate the switch between waking and sleeping are becoming clear. The rapidity with which the hypocretin story has emerged is a testament to both the conceptual and the technical evolution of genomic science in the past two decades.

Orexins and the treatment of obesity

Orexin-A and -B are two peptides derived by proteolytic cleavage from a 130-amino acid precursor, prepro-orexin, which were recently isolated from the rat hypothalamus. Orexin-A is fully conserved across mammalian species, whilst rat and human orexin-B differ by two amino acids. These peptides bind to two Gq-coupled receptors, termed orexin-1 and orexin-2. The receptors are 64% homologous and highly conserved across species. Orexin-A is equipotent at orexin-1 and orexin-2 receptors, whilst orexin-B displays moderate (approximately 10 fold) selectivity for orexin-2 receptors. The distribution and pharmacology of the orexin peptides and their receptors indicate that they play a role in various regulatory systems including energy homeostasis and the regulation of feeding, the evidence for which is reviewed here.

Hypocretins/orexins as integrators of physiological information: lessons from mutant animals

The hypocretins/orexins (hcrts) are two recently described neuropeptides derived from the same precursor and expressed in a few thousand neurons in the perifornical area of the lateral hypothalamus, which project throughout the brain. The hypocretins bind to two G-protein coupled receptors with different selective affinities. Positional cloning of the gene responsible for a canine model of narcolepsy revealed that this disease is caused by mutations in hypocretin receptor type 2. Parallel studies with hypocretin/orexin knockout mice showed behavioral arrests reminiscent of narcolepsy-like attacks. Narcoleptic patients have decreased hypocretin-containing neurons suggesting that narcolepsy in humans is caused by selective neurodegeneration of hypocretinergic neurons. Additional functions for the hypocretins on regulation of energy balance neuroendocrine release and sympathetic outflow have been described. Here we review studies in humans and mutant animals that have provided clues about the functions of the hypocretinergic system, which appear to involve the coherent regulation of networks that dictate the states of arousal.

The physiology and pharmacology of the orexins

Orexin-A and orexin-B are two peptides derived by proteolytic cleavage from a 130 amino acid precursor prepro-orexin, which recently were isolated from the rat hypothalamus. Orexin-A is fully conserved across mammalian species, whilst rat and human orexin-B differ by 2 amino acids. These peptides bind to two G(q)-coupled receptors, termed OX(1) and OX(2). The receptors are 64% homologous and highly conserved across species. Orexin-A is equipotent at OX(1) and OX(2), whilst orexin-B displays moderate ( approximately 10-fold) selectivity for OX(2). Prepro-orexin is found in the hypothalamus and, to a markedly lesser extent, the testes, adrenals, and myenteric plexus. However, orexin-A and orexin-B are found throughout the CNS, due to extrahypothalamic projections, as well as in the adrenals and small intestine. OX(1) is expressed mainly in the hypothalamus and locus coeruleus, as well as other brain regions and the spinal cord. OX(2) is expressed in the hypothalamus, cortex, spinal cord, and a few discrete brain nuclei. Both receptors are also expressed in the gut. The orexins modulate feeding behaviour and energy homeostasis, as well as associated drinking behaviours, and also regulate the sleep-wake cycle. Moreover, disruption of prepro-peptide expression or mutations in the gene encoding OX(2) result in a narcoleptic phenotye in various animal models, whilst several clinical studies have linked disruption of the orexin system to narcolepsy in humans. The orexins also have cardiovascular and neuroendocrine effects. This review further details the pharmacology and localisation of these peptides and summarises the evidence for their role in the physiology outlined above.