Importance of forebrain cholinergic and GABAergic systems to the age-related deficits in water maze performance of rats

The present study investigated the performance of rats at 3–4 months and 21 months of age in the Morris water maze and correlated age-related cognitive deficits with changes in both cholinergic and GABAergic systems in the frontal cortex. The older rats were divided into two groups, unimpaired old and impaired old according to their ability to find a hidden submerged platform in the water maze, for electrophysiological, neurochemical, and morphological studies. The firing rate of frontal cortical neurones was recorded from the motor area of the frontal cortex under urethane anaesthesia and was found to be significantly slower in the two aged groups of rats compared to the young rats, but there were no differences between the two aged groups. The sensitivity of frontal cortex neurones of the impaired and unimpaired old age groups to ACh and to carbachol was significantly lower than that of the young group, but there were no differences between the two old age groups. In contrast, sensitivity of frontal cortex neurones to bicuculline was significantly higher in the aged rats compared with the young rats and was significantly greater in the impaired old rats than in the unimpaired old rats. The sensitivity of cortical neurones to glutamate was unaffected by age. There were also significant correlations between the percentages of cortical neurones responding to ACh and bicuculline and different parameters of water maze acquisition during days 7–8, but not during days 2–3, when spatial learning had not begun, and days 13–14, when spatial learning was complete. Biochemical and morphological analyses did not show any significant differences in ChAT activity and AChE-positive fibre density in the frontoparietal cortices of the three groups of rats. The results demonstrate that the learning deficit observed in old age rats cannot be adequately explained solely by a reduction in cholinergic receptor sensitivity and that an age-related increase in GABAergic tone may be a more important determinant of cognitive impairment.