Activity Of Orexinergic Neurons Research Articles

Nicotine suppresses central post-stroke pain via facilitation of descending noradrenergic neuron through activation of orexinergic neuron

Central post-stroke pain (CPSP) is a type of central neuropathic pain, whose underlying mechanisms remain unknown. We previously reported that bilateral carotid artery occlusion (BCAO)-induced CPSP model mice showed mechanical hypersensitivity and decreased mRNA levels of preproorexin, an orexin precursor, in the hypothalamus. Recently, nicotine was shown to regulate the neuronal activity of orexin in the lateral hypothalamus (LH) and suppress inflammatory and neuropathic pain. In this study, we evaluated whether nicotine could suppress BCAO-induced mechanical allodynia through the activation of orexinergic neurons. Mice were subjected to BCAO for 30 min. Mechanical hypersensitivity was assessed by the von Frey test. BCAO mice showed hypersensitivity to mechanical stimuli three days after BCAO surgery. The intracerebroventricular injection of nicotine suppressed BCAO-induced mechanical hypersensitivity in a dose-dependent manner. These effects were inhibited by α7 or α4β2-nicotinic receptor antagonists. After nicotine injection, the level of c-fos, a neuronal activity marker, increased in the LH and locus coeruleus (LC) of Sham and BCAO mice. Increased number of c-Fos-positive cells partly colocalized with orexin A-positive cells in the LH, as well as tyrosine hydroxylase-positive cells in the LC.Orexinergic neurons project to the LC area. Nicotine-induced antinociception tended to cancel by the pretreatment of SB334867, an orexin receptor1 antagonist into the LC. Intra-LH microinjection of nicotine attenuated BCAO-induced mechanical hypersensitivity. Nicotine-induced antinociception was inhibited by intrathecal pre-treatment with yohimbine, an α2 adrenergic receptor antagonist. These results indicated that nicotine may suppress BCAO-induced mechanical hypersensitivity through the activation of the descending pain control system via orexin neurons.

Cholinergic and cortical activity is recovered by insulin-like growth factor I in aging mice brains.

It is known that IGF-I facilitates a waking state and modulates sleep architecture through the activation of orexinergic neurons. However, IGF-I may activate other neurons that may facilitate waking states, such as the locus coeruleus and the basal forebrain, orchestrating an intricate role in brain and cortical regulation. The BF is a structure that contains cholinergic neurons that project entirely to the brain cortex and support cerebral activity and cognitive function. Besides, cholinergic dysfunction through aging has been related to AD and cognitive decline. This work aims to determine if IGF-I has a modulatory role in cortical and BF activity, facilitating waking state by activating cholinergic neurons in young and old mice. We use transgenic mice models with channel-rhodopsin specifically in cholinergic neurons, and C57 (less than 6 months and over 22 months of age). We take advantage of immunofluorescence, electrophysiology and optogenetics experiments. We demonstrate that ChAT neurons have IGF-I receptors and respond to intraparenchymal IGF-I diffusion, facilitating an increased firing rate in young but not in old mice. This facilitation of cholinergic activity by IGF-I increases the EEG activation (faster activity increases in the power spectrum). The effect depends on muscarinic receptors because their activity is blocked by atropine. Also, IGF-I facilitates theta rhythm generation that is a hallmark of the waking state in young rodents. After IGF-I chronic treatment, the waking state's effects, ChAT neuronal response, and cortical activity was recovered in old mice. Consequently with our results, IGF-I can activate wakefulness by synchronously activating structures that favor it. This finding may suggest that IGF-I or mimetic drugs of this peptide would favor cholinergic activity during aging.

Localization of 5-HT2C and 5-HT1B Serotonin Receptors in Orexinergic Neurons of the Hypothlamic Perifornical Area of Rodents

Both the orexin peptides produced from prepro-orexin in orexinergic neurons of the lateral hypothalamus and hypothalamic perifornical area (PFA) and serotonin coming to these hypothalamic areas from synaptic endings of serotonergic neurons of the midbrain raphe nuclei play an important role in the regulation of feeding behavior and circadian rhythms. This indicates a close relationship between the serotoninergic and orexinergic systems in the lateral hypothalamus and PFA which, however, remains poorly studied. The aim of this study was to identify immunohistochemically and quantify the 2C-subtype (5-HT2CR) and the 1B-subtype (5-HT1BR) serotonin receptors in orexinergic neurons of the hypothalamic PFA in mice and rats. An immunohistochemical reaction for orexin-A was used to identify orexinergic neurons, and the double fluorescence immunolabeling was applied to assess the colocalization of serotonin receptors and orexin-A. Using double immunolabeling, the colocalization of orexin-A and 5-HT2CR and 5-HT1BR in the hypothalamic PFA of different rodent species, such as C57Bl/6J mice and Wistar rats, was demonstrated. It was shown that all orexinergic neurons expressed both types of serotonin receptors. At the same time, only 32–35% of 5-HT2CR and 5-HT1BR were localized in the orexinergic neurons, while the remaining part of them was expressed in the other types of PFA neurons. Thus, the immunohistochemical evidence was obtained for the serotonin-mediated regulation of functional activity of orexinergic neurons in the rodent hypothalamic PFA; also it was established that these effects of serotonin can be realized via 5-HT2CR and 5-HT1BR.

A common neuronal mechanism of hypertension and sleep disturbances in spontaneously hypertensive rats: Role of orexinergic neurons

Epidemiologic studies have shown that sleep disorders are associated with the development of hypertension. The present study investigated dynamic changes in sleep patterns during the development of hypertension across the lifespan in spontaneously hypertensive rats (SHRs) and the neural mechanism that underlies these comorbidities, with a focus on the orexinergic system. Blood pressure in rats was measured using a noninvasive blood pressure tail cuff. Sleep was monitored by electroencephalographic and electromyographic recordings. Immunohistochemistry was used to detect the density and activity of orexinergic neurons in the perifornical nucleus. Hcrt2-SAP (400 or 800 ng) was microinjected in the lateral hypothalamus to lesion orexinergic neurons. Compared with Wistar-Kyoto rats, SHRs exhibited various patterns of sleep disturbances. In SHRs, dynamic changes in hypersomnia in the rats' active phase was not synchronized with the development of hypertension, but hyperarousal in the inactive phase and difficulties in falling asleep were observed concurrently with the development of hypertension. Furthermore, the density and activity of orexinergic neurons in the perifornical nucleus were significantly higher in SHRs than in age-matched Wistar-Kyoto rats. The reduction of orexinergic neurons in the lateral hypothalamus partially ameliorated the development of hypertension and prevented difficulties in falling asleep in SHRs. These results indicate that although the correlation between sleep disturbances and hypertension is very complex, common mechanisms may underlie these comorbidities in SHRs. Overactivity of the orexin system may be one such common mechanism.

A Neurochemical Approach to the Search for Drugs for the Treatment of Symptoms of Alzheimer’s Disease

We performed an analysis of the reciprocal influences of neurons involved in the rapid-eye-movement phase of sleep (REM sleep) under the conditions of changes in the concentrations of various neuromodulators that are characteristic of Alzheimer’s disease. Several approaches to the search for drugs for the weakening of disease symptoms have been proposed taking the results of this analysis into account. The antagonists of orexin receptors were recently suggested for the treatment of Alzheimer’s disease because the anomalously high concentration of orexin, which is specific for this pathology, results in impairment of REM sleep. However, our analysis shows that the treatment with the antagonists of orexin receptors is not appropriate because it may lead to depression of the excitation of cholinergic cells, an additional decrease in the cholinergic deficit, and aggravation of symptoms of the disease. For the same reason it is unlikely that one should apply the antagonists of histamine H1 receptors and the agonists of adenosine A1 receptors. Substances that can lead to reduced activity of orexinergic neurons may be helpful instead. These substances include melatonin and the agonists of melatonin M1 receptors. Administration of these substances should improve REM sleep because it decreases the efficacy of excitation of orexinergic and histaminergic cells. Melanin-concentrating hormone also should decrease the efficacy of excitation of orexinergic cells. Additionally, the antagonists of histamine H3 receptors may be used because they promote the increased efficacy of excitation of neurons that secrete the melanin-concentrating hormone. Experimental evidence exists that indicates the benefits of these substances for improvement of REM sleep and attenuation of cognitive impairments in Alzheimer’s disease. These drugs must be applied before nightfall and their action should not be prolonged because orexinergic cells must be active in the daytime.

Injections of the of the α1-adrenoceptor antagonist prazosin into the median raphe nucleus increase food intake and Fos expression in orexin neurons of free-feeding rats

Previously, we showed that the blockade of α1-adrenoreceptors in the median raphe nucleus (MnR) increased food intake in free-feeding rats, indicating that adrenergic mechanisms in the MnR participate in the regulation of food intake. However, the impact of such a pharmacological manipulation on other neural circuits related to food intake remains unknown. In the current study, we sought to identify forebrain regions which are responsive to α1-adrenergic receptor blockade and presumably involved in the modulation of the feeding response. For this purpose, we examined the induction of c-Fos immunoreactivity in forebrain structures following injections of the α1-adrenoceptor antagonist prazosin into the MnR of free-feeding rats. To determine the chemical identity of hypothalamic c-Fos-positive cells, we then conducted double-label immunohistochemistry for Fos/orexin (OX) or Fos/melanin-concentrating hormone (MCH). Finally, we combined anterograde tracing from the MnR with immunohistochemical detection of orexin. Prazosin injections into the MnR significantly increased food intake. The ingestive response was accompanied by an increase in Fos expression in the basolateral amygdala (BLA) and lateral hypothalamic area (LHA). In the LHA, Fos expression occurred in neurons expressing OX, but not MCH. Combined anterograde tracing experiments revealed that LHA OX neurons are prominently targeted by MnR axons. These findings suggest that intra-MnR injection of prazosin, via activation of orexinergic neurons in the LHA and non-orexinergic neurons in the BLA, evoked a motivational response toward food intake.

Orexin-A Facilitates Emergence from Propofol Anesthesia in the Rat

Hypothalamic orexinergic neurons play a critical role in the promotion and maintenance of wakefulness in mammals. Previous studies have demonstrated that activities of orexinergic neurons were inhibited by isoflurane and sevoflurane, and microinjection of orexin facilitated the emergence from volatile anesthesia. In this study we first examined the hypothesis that the activity of orexin neurons is inhibited by propofol anesthesia. Moreover, the role of the orexinergic signals in basal forebrain in regulating the anesthesia-arousal cycle of propofol anesthesia is also elucidated. Rats were killed at 0, 30, 60, and 120 minutes of propofol infusion as well as at the time the righting reflex returned after the termination of anesthesia. Activated orexinergic neurons were detected by c-Fos expression. The plasma concentrations of orexin-A were measured by radioimmunoassay. Orexin-A (30 or 100 pmol) or the orexin-1 receptor antagonist, SB-334867A (5 or 20 μg), was microinjected into the basal forebrain 15 minutes before propofol infusion, or 15 minutes before the termination of propofol infusion. The loss and the return of the righting reflex time were recorded as the induction and the emergence time. Propofol anesthesia resulted in an inhibition of orexinergic neuron activity as demonstrated by the reduced numbers of c-Fos-immunoreactive orexinergic neurons. The activities of orexinergic neurons were restored when rats emerged from anesthesia. Propofol anesthesia decreased plasma orexin-A concentrations. Intrabasalis microinjection of orexin-A had no effect on the induction time but facilitated the emergence from propofol anesthesia. Inversely, intrabasalis microinjection of the orexin-1 receptor antagonist SB-334867A delayed the emergence from propofol anesthesia. Our findings indicate that activity of orexinergic neurons is inhibited by propofol anesthesia, and the orexin signals in basal forebrain are involved in anesthesia-arousal regulation from propofol anesthesia.

Respiratory regulation in narcolepsy

Narcolepsy is a debilitating sleep disorder characterized by excessive daytime sleepiness, cataplexy and intrusive REM sleep. Deficits in endogenous orexins are a major pathogenic component of the disease. This disorder is also associated with the gene marker, HLADQB1*0602. An increased prevalence of sleep apnea in narcolepsy suggested interactions among ventilatory chemosensitivity, narcolepsy-cataplexy, and sleep apnea. Evidence from animal studies using orexin knockout mice and focal microdialysis of an orexin receptor antagonist demostrated that orexins are also contributed to respiratory regulation in a vigilance state-dependent manner, as animals with orexins dysregulation have attenuated hypercapnic ventilatory responses predominately in wakefulness, which is consistent with the notion that the activity of orexinergic neurons is higher during wake than sleep periods. Human model of hypocretin deficiency is patients with narcolepsy-cataplexy. In contrast to findings suggested by animal studies, we found significant decrease in hypoxic responsiveness but not in hypercapnic responsiveness in narcoleptics, and further analysis indicated that decreased ventilatory responses to hypoxia in human narcolepsy-cataplexy is in relation to HLA-DQB1*0602 status, not hypocretin deficiency. Unlike in mouse, hypocretin-1 is not a major factor contributing to chemoresponsiveness in human. Species differences may exist.

Narcolepsy, orexins and respiratory regulation

Narcolepsy is a debilitating sleep disorder characterized by excessive daytime sleepiness, cataplexy and intrusive rapid–eye movement sleep. Deficits in endogenous orexins are a major pathogenic component of the disease. This disorder is also associated with the gene marker HLADQB1*0602. Orexins as hypothalamic neuropeptides have multiple physiological functions, and their primary functions are regulation of the sleep–wake cycle and feeding. Evidence from animal studies using orexin knockout mice and focal microdialysis of an orexin receptor antagonist at the retrotrapezoid nucleus and medullary raphe in rats demonstrated that orexins also contribute to respiratory regulation in a vigilance state–dependent manner, as animals with orexin dysregulation have attenuated hypercapnic ventilatory responses predominantly in wakefulness. These findings are consistent with the notion that the activity of orexinergic neurons is higher during wake than sleep periods. Orexin neurons seem to be a pivotal link between conscious and unconscious brain functions in animals. The human model of hypocretin deficiency is patients with narcolepsy–cataplexy. In contrast to the findings suggested by animal studies, we found significant decreases in hypoxic responsiveness, but not in hypercapnic responsiveness, in narcoleptics, and further analysis indicated that decreased ventilatory responses to hypoxia in human narcolepsy–cataplexy is in relation to HLA-DQB1*0602 status, not hypocretin deficiency. This is confirmed by the fact that the hypoxic responsiveness was lower in HLA positive versus negative controls. Unlike in mice, hypocretin-1 is not a major factor contributing to depressed hypoxic responses in humans. Species differences may exist.