Interaction between picrotoxin and 5‐hydroxytryptamine in the superior cervical ganglion of the cat

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Abstract
1 . Electrophysiological techniques were utilized to study the actions of 5-hydroxytryptamine (5-HT) and picrotoxin on the superior cervical ganglion of the cat. 2 . The intra-arterial administration of 5-HT to the ganglion elicited both depressant and excitatory actions. In low doses (0·01–0·5 μg) the amine produced a depression of ganglionic transmission. In larger doses (2–50 μg) it produced an excitation of ganglion cells (early discharge) and an initial enhancement of transmission, which was followed by depression. Picrotoxin (25–500 μg, i.a.) blocked the initial excitatory effects of 5-HT but did not block the depression. Picrotoxin did not antagonize the excitatory actions of injected cholinomimetic agents or potassium chloride. 3 . In ganglia conditioned by repetitive stimulation of the preganglionic nerve (30 Hz for 30 s) 5-HT also elicited a late-occurring and very prolonged discharge on certain postganglionic nerves (‘spinal’) but not on others (external carotid). The late discharge was only partially depressed by picrotoxin. 4 . Recordings from the surface of the superior cervical ganglion revealed that 5-HT produced three types of ganglionic potentials: (1) an initial transient depolarization which coincided with the early discharge, (2) a late-occurring, prolonged depolarization which coincided with the late discharge, and (3) a hyperpolarization which in some experiments accompanied the depression of transmission. The late depolarization and hyperpolarization were not observed in every experiment. Picrotoxin (25–500 μg) blocked the initial depolarization, but did not block the late depolarization or the hyperpolarization. 5 . It is concluded the 5-HT can produce three distinct responses in the superior cervical ganglion: a depressant effect and two types of excitation. It seems likely that depression and excitation occur via the activation of different receptors, since picrotoxin selectively blocks the latter. The finding that picrotoxin is a 5-HT antagonist in peripheral ganglia raises the possibility that picrotoxin might also influence tryptaminergic mechanisms in the central nervous system.