Reichl_1980_Eur.J.Pharmacol_68_349

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.