Clonidine, but not guanfacine, impairs choice reaction time performance inyoung healthy volunteers

The present study compares the effects of two alpha(2)-adrenoceptor agonist s, clonidine (0.5, 2, and 5 mu g/kg, p.o.), and guanfacine (7 and 29 mu g/k g, p.o.), in young healthy volunteers on attentional performance. A placebo -controlled double-blind cross-over design (one drug dose/group) was employ ed. Neither of the drugs affected measures of mot or performance or perform ance at easy levels in an attentional test. However, at the most difficult level in the attentional test, the highest dose of clonidine (5 mu g/kg), b at not guanfacine, decreased the number of correct responses and increased reaction latency. Clonidine 5 and guanfacine 29 mu g/kg equally increased s ubjective feelings of sedation and reduced systolic and diastolic blood pre ssures. Thus, the effects of the drugs on attentional performance could be dissociated from their sedative effects. The results demonstrate that cloni dine, but not guanfacine, disrupts performance in an attentional task requi ring effortful processing, while leaving performance intact in tests requir ing more automatic processing. The lower alpha(2A)-vs. alpha(2C)-adrenocept or selectivity ratio of clonidine and the affinity for alpha(1)-adrenocepto rs of clonidine may be responsible for the different action of these drugs on attention. (C) 1999 American College of Neuropsychopharmacology. Publish ed by Elsevier Science Inc.