Hypocretins (orexins) regulate serotonin neurons in the dorsal raphe nucleus by excitatory direct and inhibitory indirect actions.
The hypocretins (hcrt1 and hcrt2) are expressed by a discrete population of hypothalamic neurons projecting to many regions of the CNS, including the dorsal raphe nucleus (DRN), where serotonin (5-HT) neurons are concentrated. In this study, we investigated responses to hcrts in 216 physiologically identified 5-HT and non-5-HT neurons of the DRN using intracellular and whole-cell recording in rat brain slices. Hcrt1 and hcrt2 induced similar amplitude and dose-dependent inward currents in most 5-HT neurons tested (EC50, approximately 250 nm). This inward current was not blocked by the fast Na+ channel blocker TTX or in a Ca2+-free solution, indicating a direct postsynaptic action. The hcrt-induced inward current reversed near -18 mV and was primarily dependent on external Na+ but not on external or internal Ca2+, features typical of Na+/K+ nonselective cation channels. At higher concentrations, hcrts also increased spontaneous postsynaptic currents in 5-HT neurons (EC50, approximately 450-600 nm), which were TTX-sensitive and mostly blocked by the GABA(A) antagonist bicuculline, indicating increased impulse flow in local GABA interneurons. Accordingly, hcrts were found to increase the basal firing of presumptive GABA interneurons. Immunolabeling showed that hcrt fibers projected to both 5-HT and GABA neurons in the DRN. We conclude that hcrts act directly to excite 5-HT neurons primarily via a TTX-insensitive, Na+/K+ nonselective cation current, and indirectly to activate local inhibitory GABA inputs to 5-HT cells. The greater potency of hcrts in direct excitation compared with indirect inhibition suggests a negative feedback function for the latter at higher levels of hcrt activity.
Liu
Rong-Jian
RJ
Department of Psychiatry, Yale School of Medicine, New Haven, Connecticut 06508, USA. rongjianl@hotmail.com
van den Pol
Anthony N
AN
Aghajanian
George K
GK
D000200:Action Potentials / Q000187:drug effects / Q000502:physiology; D000818:Animals; D001369:Axons / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects; D005456:Fluorescent Dyes; D018756:GABA Antagonists / Q000494:pharmacology; D058787:GABA-A Receptor Antagonists; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D007395:Interneurons / Q000187:drug effects / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins; D008297:Male; D008839:Microelectrodes; D009431:Neural Conduction / Q000187:drug effects / Q000502:physiology*; D009433:Neural Inhibition / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009479:Neuropeptides / Q000494:pharmacology*; D064446:Orexin Receptors; D000068797:Orexins; D018408:Patch-Clamp Techniques; D011903:Raphe Nuclei / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; D012701:Serotonin / Q000378:metabolism* / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D002352:Carrier Proteins; D005456:Fluorescent Dyes; D018756:GABA Antagonists; D058787:GABA-A Receptor Antagonists; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D064446:Orexin Receptors; D000068797:Orexins; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; D012701:Serotonin; D013779:Tetrodotoxin; D005680:gamma-Aminobutyric Acid; D002118:Calcium