The effects of clonidine on discrete-trial delayed spatial alternation in two rat models of memory loss

Spatial memory impairments observed in Alzheimer's disease and schizophrenia have been attributed to many factors, including glutamate hypofunction and reduced hippocampal volume. Clonidine, a non-specific alpha(2) adrenergic receptor agonist, improves spatial memory in animals treated with the N-methyl-D-aspartate (NMDA) receptor antagonist, phencyclidine; however, its effects on memory deficits produced by other NMDA antagonists or hippocampal damage have not been fully characterized. The purpose of this study was to determine if clonidine could alleviate memory deficits produced by the NMDA antagonist, MK-801, or by excitotoxic hippocampal damage. In the first phase of the study, male rats were pretreated with clonidine (0.01 or 0.05 mg/kg) or saline, and treated with MK-801 (0.1 mg/kg) or saline prior to discrete-trial delayed alternation or radial-arm maze testing. MK-801 impaired delayed alternation performance and increased the number of arm revisits in the radial-arm maze. Clonidine pretreatment significantly alleviated these drug-induced deficits. In the second phase of the study, excitotoxic damage was produced in the dorsal hippocampus with NMDA. Hippocampal damage produced a significant impairment in the delayed alternation task, yet pretreatment with clonidine did not alleviate this damage-induced deficit. Taken together, the data indicate that clonidine alleviates memory impairments produced by glutamate hypofunction, but not by hippocampal damage. This caveat may be important in designing treatments for memory disorders not linked to a single pathophysiological mechanism.