Subtypes of receptors for serotonin.

Interest in the physiological functions of 5-hydroxytryptamine (5-HT; seroto­ nin) has increased steadily since its discovery in the intestine (1) and in serum (2). Its identification in brain in the early 1950s (3), followed a decade later by the initial studies of its distribution in brain by histofluorescence (4), stimulated investigations of the possible behavioral functions of this in­ dolealkylamine. In addition to studies of its function in brain, a steady stream of investigations have described its possible functions in the periphery, particularly in the gastrointestinal tract (see 5) and, more recently, in the cardiovascular system (see 6). Indeed, the initial suggestion that there might be more than one type of receptor for 5-HT came from experiments on the isolated guinea pig ileum. Gaddum & Picarelli (7) demonstrated that only a portion of its contractile response to 5-HT could be blocked by high con­ centrations of morphine but that the remainder of the response could be