The present study compares the effects of two alpha2agonists, clonidine
(0.5, 2, and 5 μg/kg, PO) and guanfacine (7 and 29 μg/kg, PO) in young
healthy volunteers on their performance in visual paired associates
learning (PAL) and delayed matching to sample (DMTS) visual short-term
recognition memory tests. In the PAL test, clonidine 2 and guanfacine 29
μg/kg improved the subjects' performance. In the DMTS test, clonidine at
5 μg/ kg delay-dependently impaired performance accuracy, and at 2 and 5
μg/kg it also slowed responses. Guanfacine had no effect on DMTS test
performance. Clonidine 5 and guanfacine 29 μg/kg equally increased
subjective feelings of sedation and reduced blood pressure. The results
suggest that both clonidine and guanfacine facilitated PAL learning by
improving frontal strategies, but only clonidine disrupted mnemonic
processing decreasing DMTS accuracy. The greater selectivity of
guanfacine for alpha2A-adrenoceptor subtype may explain the different
profile of action of the drugs.
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