Orexins are highly involved in regulating the circadian rhythm, the brain's reward mechanism, and the neuroendocrine response to stress. The disruption of orexin regulation is known to be associated with depression. Preclinical studies in rodents have identified the dorsomedial/perifornical and lateral areas of the hypothalamus as the population of orexinergic neurons that are primarily responsible for mediating depression-induced neuroanatomical changes in the brain. There is still no consensus regarding whether hyperactivity or hypoactivity of orexin signaling is responsible for producing depressive-like behaviour. Likewise, clinical studies indicated a general disruption in orexin signaling in depressive patients, but did not report definitive evidence of either hyperactivity or hypoactivity. Nevertheless, given the various reciprocal connections between orexin neurons and multiple brain regions, it is plausible that this involves a differential signaling network with orexin neurons as the coordination center. Here, an overview of preclinical and clinical evidence is provided as a basis for understanding the consequences of altered orexin signaling on neural circuitries modulating different aspects of the physiopathology of depression.
Adequate pain management remains an unmet medical need. We previously revealed an opioid-independent analgesic mechanism mediated by orexin 1 receptor (OX1R)-initiated 2-arachidonoylglycerol (2-AG) signaling in the ventrolateral periaqueductal gray (vlPAG). Here, we found that low-frequency median nerve stimulation (MNS) through acupuncture needles at the PC6 (Neiguan) acupoint (MNS-PC6) induced an antinociceptive effect that engaged this mechanism. In mice, MNS-PC6 reduced acute thermal nociceptive responses and neuropathy-induced mechanical allodynia, increased the number of c-Fos-immunoreactive hypothalamic orexin neurons, and led to higher orexin A and lower GABA levels in the vlPAG. Such responses were not seen in mice with PC6 needle insertion only or electrical stimulation of the lateral deltoid, a nonmedian nerve-innervated location. Directly stimulating the surgically exposed median nerve also increased vlPAG orexin A levels. MNS-PC6-induced antinociception (MNS-PC6-IA) was prevented by proximal block of the median nerve with lidocaine as well as by systemic or intravlPAG injection of an antagonist of OX1Rs or cannabinoid 1 receptors (CB1Rs) but not by opioid receptor antagonists. Systemic blockade of OX1Rs or CB1Rs also restored vlPAG GABA levels after MNS-PC6. A cannabinoid (2-AG)-dependent mechanism was also implicated by the observations that MNS-PC6-IA was prevented by intravlPAG inhibition of 2-AG synthesis and was attenuated in Cnr1-/- mice. These findings suggest that PC6-targeting low-frequency MNS activates hypothalamic orexin neurons, releasing orexins to induce analgesia through a CB1R-dependent cascade mediated by OX1R-initiated 2-AG retrograde disinhibition in the vlPAG. The opioid-independent characteristic of MNS-PC6-induced analgesia may provide a strategy for pain management in opioid-tolerant patients.
NEW FINDINGS: What is the topic of this review? We describe roles of crucial signalling molecules in the paraventricular nucleus of the hypothalamus and highlight recent data suggesting sex-specific changes in the expression of crucial signalling molecules and their receptors, which may underlie sex differences in both cardiovascular and metabolic function. What advances does it highlight? This review highlights the integrative capacity of the paraventricular nucleus in mediating cardiovascular and metabolic effects by integrating information from multiple signalling molecules. It also proposes that these signalling molecules have sex-specific differential gene expression, indicating the importance of considering these differences in our ongoing search to understand the female-male differences in the regulation of crucial autonomic systems. Many traditional cardiovascular hormones have been implicated in metabolic function. Conversely, many hormones traditionally involved in metabolic regulation have an effect on cardiovascular function. Many of these signalling molecules exert such effects through specific actions in the paraventricular nucleus, an integrative autonomic control centre located in the hypothalamus. Here, we focus on four cardiovascular/metabolic peptide hormones that signal within the paraventricular nucleus, namely angiotensin II, orexin, adiponectin and nesfatin-1. Each of these hormones has specific electrophysiological effects on paraventricular nucleus neurons that can be related to its physiological actions. In addition, we introduce preliminary transcriptomic data indicating that the genes for some of these hormones and their receptors have sex-specific differential expression.
Energy balance plays an important role in the control of reproduction. However, the cellular and molecular mechanisms connecting the two systems are not well understood especially in teleosts. The hypothalamus plays a crucial role in the regulation of both energy balance and reproduction, and contains a number of neuropeptides, including gonadotropin-releasing hormone (GnRH), orexin, neuropeptide-Y, ghrelin, pituitary adenylate cyclase-activating polypeptide, α-melanocyte stimulating hormone, melanin-concentrating hormone, cholecystokinin, 26RFamide, nesfatin, kisspeptin, and gonadotropin-inhibitory hormone. These neuropeptides are involved in the control of energy balance and reproduction either directly or indirectly. On the other hand, synthesis and release of these hypothalamic neuropeptides are regulated by metabolic signals from the gut and the adipose tissue. Furthermore, neurons producing these neuropeptides interact with each other, providing neuronal basis of the link between energy balance and reproduction. This review summarizes the advances made in our understanding of the physiological roles of the hypothalamic neuropeptides in energy balance and reproduction in teleosts, and discusses how they interact with GnRH, kisspeptin, and pituitary gonadotropins to control reproduction in teleosts.
Neurons containing neuropeptide S (NPS) and orexins are activated during stress. Previously, we reported that orexins released during stress, via orexin OX1 receptors (OX1 Rs), contribute to the reinstatement of cocaine seeking through endocannabinoid/CB1 receptor (CB1 R)-mediated dopaminergic disinhibition in the ventral tegmental area (VTA). Here, we further demonstrated that NPS released during stress is an up-stream activator of this orexin-endocannabinoid cascade in the VTA, leading to the reinstatement of cocaine seeking. Mice were trained to acquire cocaine conditioned place preference (CPP) by context-pairing cocaine injections followed by the extinction training with context-pairing saline injections. Interestingly, the extinguished cocaine CPP in mice was significantly reinstated by intracerebroventricular injection (i.c.v.) of NPS (1 nmol) in a manner prevented by intraperitoneal injection (i.p.) of SHA68 (50 mg/kg), an NPS receptor antagonist. This NPS-induced cocaine reinstatement was prevented by either i.p. or intra-VTA microinjection (i.vta.) of SB-334867 (15 mg/kg, i.p. or 15 nmol, i.vta.) and AM 251 (1.1 mg/kg, i.p. or 30 nmol, i.vta.), antagonists of OX1 Rs and CB1 Rs, respectively. Besides, NPS (1 nmol, i.c.v.) increased the number of c-Fos-containing orexin neurons in the lateral hypothalamus (LH) and increased orexin-A level in the VTA. The latter effect was blocked by SHA68. Furthermore, a 30-min restraint stress in mice reinstated extinguished cocaine CPP and was prevented by SHA68. These results suggest that NPS is released upon stress and subsequently activates LH orexin neurons to release orexins in the VTA. The released orexins then reinstate extinguished cocaine CPP via an OX1 R- and endocannabinoid-CB1 R-mediated signaling in the VTA.
The locus coeruleus (LC) as a target of addictive drugs receives a dense projection of orexinergic fibres from the lateral hypothalamus (LH) and is accordingly a candidate site for the expression of the somatic aspects of morphine withdrawal. Recently it has been shown that the inhibitory synaptic currents of LC neurons decrease partly through orexin type 1 receptors in the context of naloxone-induced morphine withdrawal; however, its cellular mechanism remains unclear. In this study, whole-cell patch clamp recordings of LC neurons in brainstem slices were used to investigate the impact of protein kinase C (PKC) on GABAergic inhibitory post-synaptic currents (IPSCs) in the context of naloxone-induced morphine withdrawal. Male Wistar rats (P14-P21) received morphine (20 mg/kg, i.p.) daily for 7 consecutive days to induce morphine dependency. Our results showed that the application of PKC inhibitor (Go 6983; 1 µM) alone did not decrease the probability of GABA release in the LC neurons of the morphine-treated rats in the presence of naloxone. Although, Go 6983 reversed the reduction of the amplitude of evoked IPSCs (eIPSCs) and spontaneous IPSCs (sIPSCs) frequency induced by orexin-A but did not change the sIPSCs amplitude. These results indicate that the suppressive effect of orexin-A on IPSCs is probably reversed by PKC inhibitor in the LC neurons of morphine-treated rats in the context of naloxone withdrawal.
Orexins (also called hypocretins) are implicated in reward and addiction, but little is known about their role(s) in the association between hippocampal synaptic plasticity and drug preference. Previously, we found that exogenous orexin via OX1 and OX2 receptors can impair low frequency stimulation-induced depotentiation, i.e. restoring potentiation of excitatory synaptic transmission (re-potentiation) in mouse hippocampal slices. Here, we found this re-potentiation in hippocampal slices from mice that had acquired conditioned place preference (CPP) to cocaine. Both 10 and 20 mg/kg of cocaine induced similar magnitudes of CPP in mice and re-potentiation in their hippocampal slices, but differed in their susceptibility to TCS1102, a dual (OX1 and OX2 ) orexin receptor antagonist. TCS1102 significantly attenuated CPP and hippocampal re-potentiation induced by cocaine at 10 mg/kg but not at 20 mg/kg. Nonetheless, SCH23390, an antagonist of dopamine D1-like receptors (D1-likeRs), inhibited the effects induced by both doses of cocaine. SKF38393, a D1-likeR-selective agonist, also induced hippocampal re-potentiation in vitro. Interestingly, this effect was attenuated by TCS1102. Conversely, SCH23390 prevented orexin A-induced hippocampal re-potentiation. These results suggest that endogenous orexins are released in mice during cocaine-CPP acquisition, which sustains potentiated hippocampal transmission via OX1 /OX2 receptors and may contribute to the addiction memory of cocaine. This effect of endogenous orexins, however, may be substituted by dopamine that may dominate hippocampal re-potentiation and CPP via D1-likeRs when the reinforcing effect of cocaine is high.
Restless legs syndrome (RLS) is a common sensory motor neurological disorder that is characterised by an irresistible urge to move the legs that significantly affects the quality of life of the patient. Prevalence in the general population is 5-25% and it is twice as prevalent in women as in men. RLS is the most common movement disorder in pregnancy with a fourfold increased risk of developing this disorder later in life. The pathophysiology of RLS is centred on dopaminergic dysfunction, reduced central nervous system iron, genetic linkages, or alteration in neurotransmitters such as hypocretins, endorphins levels and immune dysfunction and inflammatory mechanisms. With the emergence of new evidence, there are changes to the previous treatment recommendations for RLS. There is sufficient evidence to conclude that dopamine agonists such as rotigotine transdermal patch, pramipexole, ropinirole, gabapentin enacarbil, pregabalin and gabapentin are effective in the short-term treatment of RLS and rotigotine, followed by gabapentin enacarbil, ropinirole, pramipexole and gabapentin for long-term treatment. Based on expert consensus, the recommendation for daily RLS is dopamine agonists or gabapentin or low-potency opioids. Levodopa is less preferred for treating daily RLS due to its high risk of augmentation. For intermittent RLS, it is levodopa or dopamine agonists or low-potency opioids or benzodiazepines. For refractory RLS, the choice is to change to gabapentin or a different dopamine agonist, addition of a second agent like gabapentin or benzodiazepine to the existing drug or changing to a high-potency opioid or tramadol. Medications with safety record in pregnancy include opioids and antiepileptics such as carbamazepine and gabapentin. There are concerns that patients with RLS are at risk for metabolic deregulation, autonomic dysfunction and cardiovascular morbidity. However, a recent study concluded that RLS is not associated with increased risk of cardiovascular complications.