EFFECTS OF CONCOMITANT CHOLINERGIC AND ADRENERGIC-STIMULATION ON LEARNING AND MEMORY PERFORMANCE BY YOUNG AND AGED MONKEYS

The beneficial effects of physostigmine and other centrally acting AChE inhibitors on learning and memory processes have been documented in several studies. Some of these compounds are currently being examined for their potential in the treatment of Alzheimer's disease. However, the ability to employ this class of agents alone is limited by the potential for debilitating and dangerous side effects. In addition, a growing body of evidence also suggests a role for a number of other transmitter systems in this disorder. Agents such as clonidine that address adrenergic deficits have recently been demonstrated to enhance memory performance in monkeys. Clonidine also inhibits the function of cholinergic neurons in specific brain regions and reduces certain side effects of physostigmine. This evidence provided the impetus to evaluate a combination of physostigmine and clonidine in a learning and memory paradigm in monkeys. Seven young adult and three aged macaque monkeys performing a delayed matching-to-sample task received regimens of increasing doses of clonidine and physostigmine on separate occasions to determine the optimal dose of each agent in terms of enhanced memory performance. The optimal dose of clonidine was combined with a series of physostigmine doses and performance was compared to that using the two drugs alone. The best combination regimen of clonidine and physostigmine was more effective than either drug alone in enhancing memory performance. Part of the benefit may have been due to the ability of the animals to tolerate significantly higher doses of physostigmine in the combination regimen. A single injection of the combination resulted in enhanced performance both on the day of administration as well as on the following day. These results are consistent with the ability of clonidine to limit the expression or intensity of certain physostigmine-induced autonomic side effects, while allowing the beneficial cognitive effects of the cholinesterase inhibitor.