INCREASED RESPONSIVENESS OF HIPPOCAMPAL PYRAMIDAL NEURONS TO NICOTINEIN AGED, LEARNING-IMPAIRED RATS

The effect of aging upon the responsiveness of hippocampal CA1 pyramid al neurons to nicotine was investigated using electrophysiological tec hniques in male Fischer 344 rats. Prior to electrophysiological record ing, animals were behaviorally tested using the Morris water maze. All 3-6 and 18-21 month rats displayed rapid place learning in this task, while none of the 27 30 month animals learned within the 5-day test p eriod. By contrast, rats of all age groups were able to learn a cue ve rsion of the water maze task. Following behavioral testing, the animal s were anesthetized with sodium pentobarbital for acute recording. Nic otine was locally applied to electrophysiologically identified CA] pyr amidal neurons using pressure microejection from two-barreled glass mi croelectrodes. For each neuron, a dose of nicotine was found which eli cited a 300-400% increase in basal firing rate. These data were used t o construct cumulative dose response curves for populations of neurons tested in 3-6-, 18-21 -, and 27-30-month-old animals. An age-related increase in the responsiveness of CA1 pyramidal neurons to locally app lied nicotine was observed. The results of this study suggest that an increase in hippocampal CA1 pyramidal cell responsiveness to nicotine could be related to the impaired place learning ability seen with agin g.