title: Inhibition of neurosympathetic cerebroarterial constriction by clonidine in cats.
abstract: In cats anaesthetized with chloralose the cervical sympathetic nerves were separated from the vagosympathetic trunk and severed. Bilateral preganglionic electrical stimulation of the efferent sympathetic stumps induced rate-dependent decrements in cerebrospinal fluid pressure measured in the cerebral lateral ventricle. Intravenous injection of 3, 10, and 30 micrograms/kg clonidine caused a dose-related shift in threshold and inhibition of the pressure decrease particularly at lower rates of stimulation. Intravenous infusion of 3.75 micrograms/kg . min yohimbine for 30 min reduced the inhibitory effect of 30 micrograms/kg clonidine i.v. Intravenous injection of 5 mg/kg phentolamine reduced the depressor effect at all rates of electrical sympathetic stimulation. In another group of cats craniotomy was performed and the diameter of pial arteries (0.1-0.2 mm) was measured with an eyepiece micrometer under the microscope. Bilateral electrical stimulation of the sympathetic nerves decreased the artery diameter by 16%. This effect was diminished by 30 micrograms/kg clonidine i.v. Because of the characteristic shift of the frequency-response curve caused by clonidine and the antagonistic effect of yohimbine it is assumed that clonidine inhibits sympathetic neurotransmission in pial arteries by activation of prejunctional alpha-adrenoceptors. The inhibitory effect of clonidine in neurosympathetic transmission might explain the efficacy of the drug in the treatment of migraine.
mesh_terms: D000818:Animals; D002415:Cats; D002536:Cerebral Arteries / Q000187:drug effects* / Q000294:innervation; D003000:Clonidine / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D007427:Intracranial Pressure / Q000187:drug effects; D008297:Male; D013564:Sympathetic Nervous System / Q000187:drug effects*; D014661:Vasoconstriction / Q000187:drug effects*
chemical_list: D003000:Clonidine