Adrenergic mechanisms in cerebral circulation of the goat.

The effects of electrical stimulation of the cervical sympathetic nerves and tyramine on cerebral blood flow (CBF) were investigated in 13 unanesthetized goats in which electromagnetic flow probes had been previously implanted on the internal maxillary artery. Nerve stimulation (1.5-12 cycles/s) produced frequency-dependent reductions in CBF, a decrease of 50 percent occurring with the highest frequency. Injections of tyramine (50-500 mug) into the internal maxillary artery produced dose-dependent reductions in CBF, a decrease of 36 percent occurring with the highest dose. Both reserpine and phentolamine diminished the vasoconstriction induced by nerve stimulation and tyramine. Cocaine partially abolished the effects of tyramine and did not alter those of nerve stimulation. In addition, phentolamine produced cerebral vasodilatation, which was greatly reduced by previous treatment with reserpine. These results show that sympathetic stimulation and tyramine produce cerebral vasoconstriction by the release of the transmitter from the perivascular nerve endings, and they suggest the presence of a tonic sympathetic activity in the cerebral vessels.