The action of 5-hydroxytryptamine on chemoreceptor discharges of the cat's carotid body.
1 Chemoreceptor discharges were recorded in vivo from fine filaments of the carotid sinus nerve containing a single or several active units; their frequency was used as an index of receptor activity. The effects of 5-hydroxytryptamine (5-HT) on chemoreceptors were studied in 26 adult cats. At times, sinus baroreceptor discharges were recorded from the carotid nerve and the effect of 5-HT on the discharges was examined. 2 Intra-carotid injections of 5-HT (2-20 mug) induced a sharp and brief increase in chemoreceptor discharges, followed by depression or block which lasted for several seconds. Repeated injections at short intervals, and a small dose after a large dose of 5-HT resulted in depressed or blocked response to 5-HT. 3 5-HT in high doses (10-20 mug, i.a.) slightly depressed the chemoreceptor discharges induced by either acetylcholine (ACh) or NaCN, when these substances were applied within 20 s after 5-HT. 5-HT (5-20 mug, i.a.) applied during asphyxia induced a further increase in chemoreceptor discharges, soon followed by block of the discharges lasting for several seconds. 4 Atropine or hexamethonium in high doses did not change the chemoreceptor response to 5-HT, while that to ACh was markedly depressed. 5 (+)-Lysergic diethylamide (LSD), methysergide or gramine did not alter the response to 5-HT, while LSD in low doses produced a marked increase in chemoreceptor discharges. 6 Acute and chronic treatment with reserpine (5-10 mg/kg, i.v.) of the animals did not change the sensitivity and the reactivity of the chemoreceptor to ACh and NaCN, while the chemoreceptor response to 5-HT was augmented, indicating an increase in the sensitivity of chemoreceptors to 5-HT. 7 5-HT in small doses (2-10 mug, i.a.) induced a marked increase in sinus baroreceptor discharges; subsequently discharges were depressed or blocked for several seconds. 8 The results are discussed in relation to possible mechanism of action of 5-HT on the chemoreceptors. It is concluded that the exogenous 5-HT probably acts directly on the chemosensory nerve endings and depolarizes them, but 5-HT contained in the carotid body does not play a significant role in the generation of chemoreceptor discharges.
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001794:Blood Pressure / Q000187:drug effects; D002344:Carotid Body / Q000502:physiology*; D002415:Cats; D002628:Chemoreceptor Cells / Q000187:drug effects*; D003404:Creatinine / Q000494:pharmacology; D003486:Cyanides / Q000494:pharmacology; D005727:Ganglia, Spinal / Q000502:physiology; D006584:Hexamethonium Compounds / Q000494:pharmacology; D007269:Injections, Intra-Arterial; D008238:Lysergic Acid Diethylamide / Q000494:pharmacology; D008784:Methysergide / Q000494:pharmacology; D010100:Oxygen / Q000494:pharmacology; D011311:Pressoreceptors / Q000187:drug effects; D012110:Reserpine / Q000494:pharmacology; D012701:Serotonin / Q000008:administration & dosage / Q000494:pharmacology*; D012702:Serotonin Antagonists
D003486:Cyanides; D006584:Hexamethonium Compounds; D012702:Serotonin Antagonists; D012701:Serotonin; D001285:Atropine; D012110:Reserpine; D008238:Lysergic Acid Diethylamide; D003404:Creatinine; D000109:Acetylcholine; D010100:Oxygen; D008784:Methysergide