Mechanism of ketanserin-induced sympatho-inhibition.
Experiments were undertaken to determine if sympatho-inhibition produced by ketanserin is due to antagonism of central nervous system alpha 1-adrenoceptors rather than central 5-HT2 receptors and if (like prazosin) it produces sympatho-inhibition indirectly via a central (presynaptic) alpha 2-adrenoceptor mechanism. Administration of ketanserin (0.03-3.0 mg/kg i.v.) caused a dose-related depression of sympathetic-cholinergic electrodermal responses evoked by electrical stimulation of the hypothalamus in pentobarbital anesthetized cats. No effect of ketanserin was observed on electrodermal responses evoked by preganglionic sympathetic nerve stimulation nor did the more specific 5-HT2 receptor antagonist, cinanserin, produce a central sympatholytic effect at dosages up to 3 mg/kg i.v. Pretreatment with alpha 2-adrenoceptor blockers yohimbine, idazoxan, or rauwolscine significantly antagonized ketanserin-induced sympatho-inhibition. Depletion of central nervous system (CNS) monoamines totally prevented ketanserin-induced sympatho-inhibition although clonidine (30 micrograms/kg i.v.) continued to be effective. These results suggest that ketanserin acts in the CNS to reduce sympathetic reactivity by blocking alpha 1-adrenoceptors and not 5-HT2 receptors. In this regard, ketanserin appears to act in a manner similar to other alpha 1-adrenoceptor antagonists (e.g. prazosin and indoramin) by an apparent presynaptic facilitation of alpha 2-adrenoceptor mediated tonic inhibition descending from the lower brainstem.
Department of Pharmacology, University of Oklahoma Health Sciences Center, Oklahoma City 73190.
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Koss
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D015306:Biogenic Monoamines / Q000502:physiology; D002415:Cats; D002490:Central Nervous System / Q000187:drug effects; D002928:Cinanserin / Q000494:pharmacology; D004558:Electric Stimulation; D005260:Female; D005712:Galvanic Skin Response / Q000187:drug effects; D007034:Hypothalamus, Posterior / Q000187:drug effects; D007650:Ketanserin / Q000494:pharmacology*; D008297:Male; D013564:Sympathetic Nervous System / Q000187:drug effects*; D013569:Synapses / Q000187:drug effects
D000317:Adrenergic alpha-Antagonists; D015306:Biogenic Monoamines; D007650:Ketanserin; D002928:Cinanserin