3-(1,2,5,6-Tetrahydropyrid-4-yl)pyrrolo(3,2-b)pyrid-5-one: A Potent and Selective Serotonin (5-HT1B) Agonist and Rotationally Restricted Phenolic Analogue of 5-Methoxy-3-(1,2,5,6-tetrahydropyrid-4-yl)indole.