Serotonin depresses the after-hyperpolarization through the inhibition of the Na+/K+ electrogenic pump in T sensory neurones of the leech.
In T sensory neurones of the leech, a train of impulses elicited by intracellular electrical stimulation leads to an after-hyperpolarization of up to 30 mV, mainly due to the activation of the electrogenic Na+/K(+)-ATPase but partly to a Ca2(+)-activated K+ conductance. It was found that serotonin reversibly reduced the amplitude of this after-hyperpolarization. We investigated the mechanism of action of serotonin and found: (1) after inhibition of the Ca2(+)-activated K+ conductance with BaCl2 or CdCl2, serotonin was still able to reduce the after-hyperpolarization; (2) when penetration of T cells with microelectrodes leaking sodium was preceded by serotonin perfusion of the ganglia, the normal hyperpolarization due to the activation of the electrogenic pump was converted to a depolarization; (3) after long-lasting perfusion with K(+)-free saline solution (which inhibits the Na+/K+ pump), the application of CsCl caused repolarization by reactivating the electrogenic ATPase; serotonin slowed and reduced this repolarization; (4) serotonin potentiated the depolarization of T neurones caused by the inhibition of the Na+/K+ pump following cooling of ganglia and depressed the hyperpolarization after rewarming to room temperature. These data taken together suggest that serotonin directly inhibits the Na+/K+ electrogenic pump.
Catarsi
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Dipartimento di Fisiologia e Biochimica, Università di Pisa, Italy.
Brunelli
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M
D000200:Action Potentials; D000818:Animals; D001464:Barium; D017609:Barium Compounds; D001693:Biological Transport, Active / Q000502:physiology; D002104:Cadmium; D019256:Cadmium Chloride; D002118:Calcium / Q000378:metabolism; D002586:Cesium; D002712:Chlorides; D003080:Cold Temperature; D007865:Leeches; D009475:Neurons, Afferent / Q000201:enzymology / Q000502:physiology*; D010042:Ouabain; D011188:Potassium / Q000378:metabolism; D012701:Serotonin / Q000502:physiology*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D017609:Barium Compounds; D002712:Chlorides; D002104:Cadmium; C024986:barium chloride; D002586:Cesium; D001464:Barium; D012701:Serotonin; D010042:Ouabain; D000254:Sodium-Potassium-Exchanging ATPase; C028019:cesium chloride; D019256:Cadmium Chloride; D011188:Potassium; D002118:Calcium