The endogenous somnogen adenosine excites a subset of sleep-promoting neurons via A2A receptors in the ventrolateral preoptic nucleus.
Recent research has shown that neurons in the ventrolateral preoptic nucleus are crucial for sleep by inhibiting wake-promoting systems, but the process that triggers their activation at sleep onset remains to be established. Since evidence indicates that sleep induced by adenosine, an endogenous sleep-promoting substance, requires activation of brain A(2A) receptors, we examined the hypothesis that adenosine could activate ventrolateral preoptic nucleus sleep neurons via A(2A) adenosine receptors in rat brain slices. Following on from our initial in vitro identification of these neurons as uniformly inhibited by noradrenaline and acetylcholine arousal transmitters, we established that the ventrolateral preoptic nucleus comprises two intermingled subtypes of sleep neurons, differing in their firing responses to serotonin, inducing either an inhibition (Type-1 cells) or an excitation (Type-2 cells). Since both cell types contained galanin and expressed glutamic acid decarboxylase-65/67 mRNAs, they potentially correspond to the sleep promoting neurons inhibiting arousal systems. Our pharmacological investigations using A(1) and A(2A) adenosine receptors agonists and antagonists further revealed that only Type-2 neurons were excited by adenosine via a postsynaptic activation of A(2A) adenosine receptors. Hence, the present study is the first demonstration of a direct activation of the sleep neurons by adenosine. Our results further support the cellular and functional heterogeneity of the sleep neurons, which could enable their differential contribution to the regulation of sleep. Adenosine and serotonin progressively accumulate during arousal. We propose that Type-2 neurons, which respond to these homeostatic signals by increasing their firing are involved in sleep induction. In contrast, Type-1 neurons would likely play a role in the consolidation of sleep.
Gallopin
T
T
UMR 5167 CNRS, Physiopathologie des Réseaux Neuronaux du Cycle Veille-Sommeil, Faculté de Médecine RTH Laënnec, Institut Fédératif des Neurosciences de Lyon (IFR19), Université Claude Bernard Lyon I, France.
Luppi
P-H
PH
Cauli
B
B
Urade
Y
Y
Rossier
J
J
Hayaishi
O
O
Lambolez
B
B
Fort
P
P
D000241:Adenosine / Q000378:metabolism*; D058908:Adenosine A2 Receptor Agonists; D058917:Adenosine A2 Receptor Antagonists; D000818:Animals; D009474:Neurons / Q000166:cytology* / Q000378:metabolism; D009924:Organ Culture Techniques; D018408:Patch-Clamp Techniques; D011301:Preoptic Area / Q000166:cytology* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D043705:Receptor, Adenosine A2A / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012701:Serotonin / Q000378:metabolism; D012890:Sleep / Q000502:physiology*
D058908:Adenosine A2 Receptor Agonists; D058917:Adenosine A2 Receptor Antagonists; D043705:Receptor, Adenosine A2A; D012701:Serotonin; D000241:Adenosine