RESUMO
Sleep deprivation is accompanied by the progressive development of an irresistible need to sleep, a phenomenon whose mechanism has remained elusive. Here, we identified for the first time a reflection of that phenomenon in vitro by showing that, after a short 2 h period of total sleep deprivation, the action of noradrenaline on the wake-promoting hypocretin/orexin neurons changes from an excitation to an inhibition. We propose that such a conspicuous modification of responsiveness should contribute to the growing sleepiness that accompanies sleep deprivation.
Assuntos
Hipotálamo/patologia , Peptídeos e Proteínas de Sinalização Intracelular/metabolismo , Neurônios/efeitos dos fármacos , Neuropeptídeos/metabolismo , Norepinefrina/farmacologia , Privação do Sono/fisiopatologia , Vigília/efeitos dos fármacos , Animais , Northern Blotting/métodos , Estimulação Elétrica/métodos , Imuno-Histoquímica/métodos , Técnicas In Vitro , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Potenciais da Membrana/efeitos da radiação , Neurônios/fisiologia , Orexinas , Técnicas de Patch-Clamp/métodos , RNA Mensageiro/biossíntese , Ratos , Ratos Sprague-Dawley , Reação em Cadeia da Polimerase Via Transcriptase Reversa/métodos , Privação do Sono/patologia , Vigília/fisiologiaRESUMO
The hypocretin-orexin (hcrt-orx) neurons are thought to maintain wakefulness because their loss results in narcolepsy. This role may be fulfilled by the excitatory action that the hcrt-orx peptide exerts on multiple brainstem and forebrain systems that, in turn, promote cortical activation. Here, we examined whether hcrt-orx may also exert a postsynaptic excitatory action at the level of the cortex, where hcrt-orx fibers project. However, we found that neurons in layers 2-5 in the primary somatosensory cortex (SSp) were unresponsive to hcrt-orx. We then found that although all neurons tested in sublayer 6a were also unresponsive to hcrt-oxr, all those tested in sublayer 6b were highly sensitive to the peptide. The sublayer selectivity of hcrt-oxr was not restricted to the somatosensory cortex, because it was also found to be present in the primary visual cortex, the motor cortex, and the cingulate cortex. In the SSp, in which the hcrt-oxr effect was investigated further, it was demonstrated to be postsynaptic, to result from an interaction with Hcrtr2-OX2 receptors and to depend on the closure of a potassium conductance. Similar to the selectivity of action in the thalamus, where hcrt-oxr excites the nonspecific thalamocortical projection neurons and not the specific sensory relay neurons, here in the cortex, it excites a specific subset of cortical neurons which, through corticocortical projections, may also be involved in promoting widespread cortical activation.
Assuntos
Nível de Alerta/efeitos dos fármacos , Córtex Cerebral/efeitos dos fármacos , Peptídeos e Proteínas de Sinalização Intracelular/farmacologia , Neurônios/efeitos dos fármacos , Neuropeptídeos/farmacologia , Receptores de Neuropeptídeos/efeitos dos fármacos , Animais , Nível de Alerta/fisiologia , Córtex Cerebral/citologia , Córtex Cerebral/fisiologia , Giro do Cíngulo/citologia , Giro do Cíngulo/efeitos dos fármacos , Giro do Cíngulo/fisiologia , Técnicas In Vitro , Canais Iônicos/fisiologia , Córtex Motor/citologia , Córtex Motor/efeitos dos fármacos , Córtex Motor/fisiologia , Vias Neurais/efeitos dos fármacos , Neurônios/classificação , Neurônios/fisiologia , Receptores de Orexina , Orexinas , Técnicas de Patch-Clamp , Potássio/fisiologia , Ratos , Receptores Acoplados a Proteínas G , Receptores de Neuropeptídeos/fisiologia , Córtex Somatossensorial/citologia , Córtex Somatossensorial/efeitos dos fármacos , Córtex Somatossensorial/fisiologia , Transmissão Sináptica/efeitos dos fármacos , Córtex Visual/citologia , Córtex Visual/efeitos dos fármacos , Córtex Visual/fisiologia , Vigília/efeitos dos fármacos , Vigília/fisiologiaRESUMO
According to multiple lines of evidence, neurons in the ventrolateral preoptic area (VLPO) that contain GABA promote sleep by inhibiting neurons of the arousal systems. Reciprocally, transmitters used by these systems, including acetylcholine (ACh) and noradrenaline (NA), exert an inhibitory action on the VLPO neurons. Because nicotine, an agonist of ACh, acts as a potent stimulant, we queried whether it might participate in the cholinergic inhibition of these sleep-promoting cells. Indeed, we found that ACh inhibits the VLPO neurons through a nicotinic, as well as a muscarinic, action. As evident in the presence of atropine, the non-muscarinic component was mimicked by epibatidine, a nonselective nicotinic ACh receptor (nAChR) agonist and was blocked by dihydro-beta-erythroidine, a nonselective nAChR antagonist. It was not, however, blocked by methyllycaconitine, a selective antagonist of the alpha7 subtype, indicating that the action was mediated by non-alpha7 nAChRs. The nicotinic inhibition was attributed to a presynaptic facilitation of NA release because it persisted in the presence of tetrodotoxin and was blocked by yohimbine and RS 79948, which are both selective antagonists of alpha2 adrenergic receptors. Sleep-promoting VLPO neurons are thus dually inhibited by ACh through a muscarinic postsynaptic action and a nicotinic presynaptic action on noradrenergic terminals. Such dual complementary actions allow ACh and nicotine to enhance wakefulness by inhibiting sleep-promoting systems while facilitating other wake-promoting systems.
Assuntos
Acetilcolina/farmacologia , Inibição Neural , Neurônios/fisiologia , Agonistas Nicotínicos/farmacologia , Norepinefrina/metabolismo , Área Pré-Óptica/fisiologia , Animais , Células Cultivadas , Sinergismo Farmacológico , Condutividade Elétrica , Agonistas Muscarínicos/farmacologia , Antagonistas Muscarínicos/farmacologia , Neurônios/efeitos dos fármacos , Técnicas de Patch-Clamp , Canais de Potássio/metabolismo , Área Pré-Óptica/anatomia & histologia , Área Pré-Óptica/citologia , Terminações Pré-Sinápticas/metabolismo , Ratos , Receptores Muscarínicos/metabolismo , Receptores Nicotínicos/metabolismo , Sono/fisiologiaRESUMO
Wakefulness depends on the activity of hypocretin-orexin neurons because their lesion results in narcolepsy. How these neurons maintain their activity to promote wakefulness is not known. Here, by recording for the first time from hypocretin-orexin neurons and comparing their properties with those of neurons expressing melanin-concentrating hormone, we show that hypocretin-orexin neurons are in an intrinsic state of membrane depolarization that promotes their spontaneous activity. We propose that wakefulness and associated energy expenditure thus depend on that property, which allows the hypocretin-orexin neurons to maintain a tonic excitatory influence on the central arousal and peripheral sympathetic systems.
Assuntos
Proteínas de Transporte/metabolismo , Membrana Celular/fisiologia , Peptídeos e Proteínas de Sinalização Intracelular , Neurônios/metabolismo , Neuropeptídeos/metabolismo , Vigília/fisiologia , Potenciais de Ação/efeitos dos fármacos , Potenciais de Ação/fisiologia , Animais , Nível de Alerta/fisiologia , Quelantes/farmacologia , Colina/farmacologia , Estimulação Elétrica , Antagonistas de Aminoácidos Excitatórios/farmacologia , Hormônios Hipotalâmicos/metabolismo , Hipotálamo/citologia , Técnicas In Vitro , Melaninas/metabolismo , Potenciais da Membrana/efeitos dos fármacos , Potenciais da Membrana/fisiologia , Neurônios/citologia , Neurônios/efeitos dos fármacos , Níquel/farmacologia , Orexinas , Técnicas de Patch-Clamp , Hormônios Hipofisários/metabolismo , Ratos , Bloqueadores dos Canais de Sódio/farmacologia , Transmissão Sináptica/efeitos dos fármacos , Transmissão Sináptica/fisiologia , Tetrodotoxina/farmacologia , Ácido gama-Aminobutírico/farmacologiaRESUMO
As is evident from the pathological consequences of its absence in narcolepsy, orexin (hypocretin) appears to be critical for the maintenance of wakefulness. Via diffuse projections through the brain, orexin-containing neurons in the hypothalamus may act on a number of wake-promoting systems. Among these are the intralaminar and midline thalamic nuclei, which project in turn in a widespread manner to the cerebral cortex within the nonspecific thalamocortical projection system. Testing the effect of orexin in rat brain slices, in two nuclei of this system, centromedial (CM) nuclei and rhomboid nuclei, we found that it depolarized and excited all neurons tested through a direct postsynaptic action. An additional analysis of this effect in CM neurons indicates that it results from the decrease of a potassium conductance. By a detailed comparison of the effects of orexin A and B, we established that orexin B was more potent than orexin A, indicating the probable mediation by orexin type 2 receptors. In contrast to its effect on the nonspecific thalamocortical projection neurons, orexin had no effect on the specific sensory relay neurons of the somatic, ventral posterolateral, and visual dorsal lateral geniculate nuclei. Orexin differs in this regard from norepinephrine and acetylcholine, to which neurons in the specific and nonspecific systems are sensitive. Orexin may thus act in the thalamus to promote wakefulness by exciting neurons of the nonspecific thalamocortical projection system, which, through widespread projections to the cerebral cortex, stimulate and maintain cortical activation.