Abstract
The present study attempted to identify an alpha-2 agonist that could improve working memory in aged nonhuman primates without the marked hypotensive and sedative side effects produced by clonidine. Toward this end, the hypotensive, sedative, and memory-altering properties of the alpha-2 adrenergic agonists, B-HT920 and guanfacine, were compared with clonidine's effects in 9 aged rhesus monkeys. Memory capacity was assessed by a variable delay, spatial delayed response paradigm that requires the animal to remember information over short temporal intervals and to update this information on every trial. B-HT920 was found to produce a dose-response profile qualitatively similar to, but weaker than, clonidine: low doses impaired memory and began to lower blood pressure and produce sedation, while high doses improved memory. In contrast, guanfacine produced a dose-response profile opposite to that seen with clonidine: low doses improved memory without inducing hypotension or sedation, while the memory-impairing, hypotensive, and sedating properties of the drug were observed at higher doses. The potency of the 3 agonists to lower blood pressure was clonidine = B- HT920 greater than guanfacine; sedation was affected in the order clonidine greater than B-HT920 greater than guanfacine; for memory impairment, as measured by performance on the delayed response task, the rank order potency was clonidine greater than B-HT920 greater than guanfacine, while for memory improvement it was guanfacine greater than clonidine greater than B-HT920. These differences in rank order potency are consistent with the recent proposal of alpha-2 receptor subtypes, a rauwolscine-sensitive site (Rs) that binds clonidine greater than B- HT920 greater than guanfacine and a rauwolscine-insensitive site (Ri) that binds guanfacine greater than clonidine greater than B-HT920 (Boyajian and Leslie, 1987). The data suggest that the hypotensive, sedating, and memory-impairing effects of alpha-2 agonists may be due to actions at one subtype of receptor (Rs), while the memory-enhancing effects of these drugs may result from actions at another alpha-2 receptor subtype, the Ri site. The ability of low doses of guanfacine to improve memory without inducing hypotension or sedation indicates that this agonist may be an excellent candidate for treating memory disorders in man.