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Nishi K. 
“The Action Of 5-hydroxytryptamine On Chemoreceptor Discharges Of The Cat's Carotid Body”. 
Brit. J. Pharmac.. 1975;55:27-40.
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. Intracarotid injections of 5-HT (2-20 mcg) 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 mcg, 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 mcg, 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 mcg, 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.
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