Noradrenergic component in the vasoconstriction induced by 5-hydroxytryptamine in goat cerebral arteries

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Abstract
5‐Hydroxytryptamine (5‐HT) elicited dose‐dependent increases in tension in the middle cerebral artery of the goat, which were significantly antagonized by lysergic acid diethylamide (LSD, 10−8 M) and methysergide (10−7 M). In the presence of phentolamine (10−6 M), the dose‐response curve to 5‐HT was shifted to the right, the pA2 value for this antagonism was 6·52. Pretreatment of goats with reserpine (0·02 mg kg−1 day−1 for three days) or removal of both superior cervical ganglia 15 days before the experiment brought about a significant decrease in the vasoconstriction induced by doses of 5‐HT higher than 10−7 M. The remaining contraction produced by 5‐HT in arterial segments from reserpinized or gangliectomized goats was further reduced in the presence of LSD. In addition, high concentrations of 5‐HT induced tritium release from goat pial arteries preloaded with (−)‐[3H]noradrenaline, 2 × 10−7 M) which was significantly decreased in vessels from gangliectomized or reserpinized goats. These results in goat cerebral arteries indicate that in the contraction evoked by 5‐HT there are two components. The first appears with low concentrations (up to 10−7 M) in which 5‐HT acts directly on 5‐HT receptors. The second occurs at high doses (>10−7 M) in which 5‐HT also acts indirectly on α‐adrenoceptors by release of noradrenaline from noradrenergic nerve endings.