Serotonin plays a permissive role in conditioned olfactory learning induced by norepinephrine in the neonate rat

This study examined the role of serotonin (5-hydroxytryptamine; 5-HT) in co nditioned olfactory learning in the rat pup. By injecting various combinati ons of the 5-HT2A/2C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-ami nopropane (DOI), and the beta adrenoceptor agonist, isoproterenol, we obser ved that isoproterenol alone acted as an unconditioned stimulus (US) to ind uce learning, as reported previously, whereas DOI alone did not induce lear ning. DOI combined with isoproterenol produced a leftward shift in the effe ctive US dose such that suboptimal isoproterenol was effective, and previou sly optimal levels of isoproterenol were ineffective. Thus, 5-HT plays a fa cilitative but not an obligatory role in conditioned olfactory learning in the neonate rat.