Individual differences in aging: behavioral and neurobiological correlates.

The goal of this experiment was to determine the correlations among different behavioral and neurobiological measures in aged rats. Aged Sprague-Dawley rats were given a battery of cognitive and sensorimotor tests, followed by electrophysiological assessment of sleep and biochemical measurements of various neurotransmitter systems. The behavioral tests included the following: Activity level in an open field; short-term and long-term memory of a spatial environment as assessed by habituation: spatial navigation, discrimination reversal, and cue learning in the Morris water pool; spatial memory in a T-maze motivated by escape from water; spatial memory and reversal on the Barnes circular platform task; passive avoidance; motor skills. Sleep was assessed by electrographic cortical records. The following neurotransmitter markers were examined: Choline acetyltransferase; the density of nicotinic, benzodiazepine and glutamine receptors in the cortex and caudate nucleus; endogenous levels of norepinephrine, dopamine, and serotonin in the cortex and hippocampus. The duration of bouts of paradoxical sleep was strongly correlated with several cognitive measures and selected serotonergic markers. This finding suggests that changes in sleep patterns and brain biochemistry contribute directly to deficits in learning and memory, or that the same neurobiological defect contributes to age-related impairments in sleep and in learning and memory.