Orexin A activates locus coeruleus cell firing and increases arousal in the rat

Abstract

The localization of orexin neuropeptides in the lateral hypothalamus has focused interest on their role in ingestion. The orexigenic neurones in the lateral hypothalamus, however, project widely in the brain, and thus the physiological role of orexins is likely to be complex. Here we describe an investigation of the action of orexin A in modulating the arousal state of rats by using a combination of tissue localization and electrophysiological and behavioral techniques. We show that the brain region receiving the densest innervation from orexinergic nerves is the locus coeruleus, a key modulator of attentional state, where application of orexin A increases cell firing of intrinsic noradrenergic neurones. Orexin A increases arousal and locomotor activity and modulates neuroendocrine function. The data suggest that orexin A plays an important role in orchestrating the sleep-wake cycle.Since the discovery of the orexins (1) investigations of their functions have been guided by evidence for their hypothalamic distribution (1, 2), focusing on feeding, energy homeostasis (1, 3), and neurocrine functions (3). Our studies now show the presence of orexin A immunoreactive fibers and varicosities in extrahypothalamic areas, particularly the locus coeruleus, and demonstrate that the functions of orexin A extend beyond the hypothalamus.Orexin A and B are derived from a 130-aa precursor, prepro-orexin, which is encoded by a gene localized to human chromosome 17q21 (1). Prepro-orexin, or preprohypocretin (2), was identified in the rat hypothalamus by directional tag PCR subtractive hybridization (2) and has been shown by Northern blot analysis to be abundant in the brain and detectable at low levels in testes but not in a variety of other tissues (1, 2). Hypocretins had been identified as hypothalamic neuropeptides, but their biological role was not described (2). Nucleotide sequence alignment shows that hypocretins 1 and 2 have sequence in common with orexins A and B, respectively, but additional amino acids are present in both hypocretins. In situ hybridization maps confirm dense prepro-orexin mRNA expression in the hypothalamus (1, 2). Immunocytochemical mapping of orexin A has identified a population of mediumsized neurones within the hypothalamus, median eminence (3), and ventral thalamic nuclei of rat brain (1, 3). This distribution has been confirmed in human tissue (4).Orexin A binds with high affinity to the novel G proteincoupled receptors orexin 1 (OX 1 ) (IC 50 20 nM) and orexin 2 (OX 2 ) (IC 50 38 nM). Calcium mobilization assays in transfected HEK293 cells confirm that orexin A is a potent agonist at both OX 1 (EC 50 30 nM) and OX 2 (EC 50 34 nM) (1). Emerging evidence suggests the existence of an extensive extrahypothalamic projection of orexin-immunoreactive neurones. Peyron et al. (5), in addition to confirming the presence of immunoreactive cell somata within the hypothalamus, reported immunolabeled fibers throughout extrahypothalamic regions, including septal nuclei, substantia nigra, and raphe nucle...

The Orexin-1 Receptor Antagonist SB-334867 Blocks the Effects of Antipsychotics on the Activity of A9 and A10 Dopamine Neurons: Implications for Antipsychotic Therapy

“…Orexin-A has been previously reported to increase the activity of LC cells following direct application to brainstem slices and iontophoretic application directly onto LC cells (Hagan et al, 1999;Bourgin et al, 2000;Murai and Akaike, 2005) and SB-334867 has been shown to block these effects (Soffin et al, 2002). The present study confirms and extends these findings by showing that i.c.v.…”

A Commentary on the Neurobiology of the Hypocretin/Orexin System

“…Obesity is generally believed to reflect decreased metabolic activity and energy expenditure in hypocretin deficient animals, an effect that would be quantitatively more important than hypophagia. Interestingly, intracerebroventricular injection of hypocretin has potent stimulatory effects on sympathetic outflow (Samson et al, 1999;Shirasaka et al 1999) and cortisol release but decreases prolactin and growth hormone release (Hagan et al 1999). Central administration of hypocretin-1 was also shown to increase metabolic rate (Lubkin and Stricker-Krongrad 1998).…”

“…This finding was followed by the observation that hypocretin knockout mice have sleep and behavioral abnormalities reminiscent of narcolepsy (Chemelli et al 1999). Other studies have indicated dense projections to all monoaminergic cell groups and wake-promoting effects of hypocretins when administered centrally (Hagan et al 1999). More recently, clinical studies have shown that most patients with narcolepsy have undetectable hypocretins in the CSF and a striking decrease in hypocretin immunoreactivity and transcript levels in the perifornical hypothalamus Thannickal et al 2000).…”

Distribution of orexinergic neurons and their terminal networks in the brains of two species of African mole rats

“…Orexin (also known as hypocretin) is a neuropeptide that has been reported to play a role in the regulation of feeding, drinking, body temperature, general activity (Lubkin and Stricker-Krongrad, 1998;Edwards et al, 1999;Hagan et al, 1999;Kunii et al, 1999;Mondal et al, 1999;Piper et al, 2000;Estabrooke et al, 2001;Hungs et al, 2001;Yoshimichi et al, 2001;Kotz et al, 2002;Berthoud et al, 2005), energy homeostasis (Mintz et al, 2001), stimulation of gastric secretion in rats (Takahashi et al, 1999), increasing metabolic rate in rats (Lubkin and Stricker-Krongard, 1998), altering luteinising hormone release in rats (Pu et al, 1998) and in the regulation of the sleepwake cycle specifically associated with increased wakefulness and inhibition of REM sleep (Sakurai et al, 1998;Chemelli et al, 1999;Siegel, 1999;Bourgin et al, 2000;Kilduff and Peyron, 2000;Thannickal et al, 2000;van den Pol, 2000).…”

“…The distribution of orexin-immunopositive (Orx +) cell bodies and terminal networks within the brain has been reported in a range of mammals including: humans (Homo sapiens, Moore et al, 2001); domestic cat (Felis catus, Zhang et al, 2001Zhang et al, , 2002; domestic sheep (Ovis aries, Iqbal et al, 2001); six species of rodent (laboratory rat, Rattus norvegicus -Broberger et al, 1998;Peyron et al, 1998;Chen et al, 1999;Cutler et al, 1999;Date et al, 1999;Hagan et al, 1999;Nambu et al, 1999, Risold et al, 1999Baldo et al, 2003;Chou et al, 2004;Espana et al, 2005;Kirouac et al, 2005;Nixon and Smale, 2007; Nile grass rat, Arvicanthus niloticus - Novak and Albers, 2002;Nixon and Smale, 2007;golden or Syrian hamster, Mesocricetus auratus -McGranaghan and Piggens, 2001;Mintz et al, 2001;Vidal et al, 2005;Nixon and Smale, 2007; laboratory mouse, Mus musculus, C57B1 strain, Broberger et al, 1998;Siberian or Djungarian hamster, Phodopus sungorus -McGranaghan and Piggins, 2001;Khorooshi and Klingenspor, 2005;degu, Octodon degus -Nixon and Smale, 2007); five microchiropteran species (Kruger et al, 2010); and the Eastern grey kangaroo (Macropus giganteus, Yamamoto et al, 2006). The Orx + neuronal cell bodies were invariably localized within the hypothalamus and while for most mammals they were represented as a rather homogenous loosely packed large cluster of neurons located in the perifornical and lateral hypothalamus (see above references), in certain rodents there may be up to four clusters, or nuclei, of orexinergic neurons -the two described above, plus one cluster located in the anterior hypothalamic paraventricular subnucleus and one in the lateral ventral hypothalamic supraoptic area (LVHA) (Nixon and Smale, 2007).…”