Delayed acquisition of behavioral and hippocampal responses during jaw movement conditioning in aging rabbits

Young (3-7 months; n = 7) and aging (40-49 months; n = 7) rabbits (Oryctolagus cuniculus) were classically conditioned in a trace jaw movement paradigm (300 ms tone, 450 ms trace, 200 ms intraoral water) after implantation of electrodes into area CAI of dorsal hippocampus. Aging rabbits took significantly more trials to reach a behavioral criterion of 8 conditioned responses in any 9 consecutive trials than young rabbits (p = 0.04), and their conditioned, but not unconditioned, jaw movement responses were of a lower frequency than those of young rabbits (p = 0.02). Early in training, aging rabbits' hippocampal responses were significantly smaller just before water onset than corresponding responses of young rabbits (p = .03). The magnitude of this response was negatively correlated with trials to criterion (r = -0.60, p = 0.03). These results are interpreted in terms of age-related differences in the hippocampal contribution to jaw movement learning.