AHP reductions in rabbit hippocampal neurons during conditioning correlate with acquisition of the learned response

Young adult male albino rabbits were conditioned using a free field auditory conditioned stimulus (CS) and periorbital shock unconditioned stimulus (US) in a short delay eye blink paradigm. All rabbits received two80-trial training sessions. Intracellular recordings were made from hippocampal CA1 pyramidal neurons within brain slices prepared 24 h following the second training session. All 46 CA1 neurons included in the analysis had stable penetration, at least 70 mV impulse amplitudes and at least 40 MΩ input resistance. Recording and initial data analysis were done 'blind' regarding behavioral training performance of the rabbit from which the slices were prepared. The animals were separated into a High (86 ± 6% CRs, n = 12) and Low (12 ± 4% CRs, n = 10) Acquisiton group based on the number of blink CRs shown on the second training day (P < 0.001). CA1 pyramidal neurons from the High Acquisition group (n = 20) showed a significant reduction in the afterhypolarization (AHP) response following 4 impulses elicited by intracellular current injection as compared to neurons from the Low Acquisition group (n = 4). The mean maximal AHP amplitudes after 4 spikes were -2.9 ± 0.34 mV and -0.4.0 ± 0.31 mV, respectively, in the High and Low Acquisition groups (P < 0.01). The size of the AHP examined at 100 ms intervals during the first 1.7 s after the current pulse proved to be reduced in the High group both when evaluated for all points (F = 5.88, df = 1.44, P < 0.02) and for each of the individual time points (at least P < 0.05). The mean AHP areas were 1.6 ± 0.26 and 2.6 ± 0.32 mV·s, respectively (P < 0.02). These AHP differences were present in the absence of differences in mean resting potential (-69 ± 2.2 mV and -70 ± 2.0 mV), spike height (87 ± 1.9 mV and 87 ± 1.6 mV), input assistance 72 ± 4.8 MΩ, and 68 ± 3.5 MΩ), and current required to elicit 4 action potentials (0.45 ± 0.05 nA and 0.43 ± 0.04 nA), respectively, between the High and Low Acquisition groups. We have previously demonstrated conditioning-specific reductions in the AHP of CA1 pyramidal neurons in hippocampal slices taken from well-trained rabbits¹¹. In the present experiments, all rabbits received the same number of CS-US pairings. But those which learned had CA1 pyramidal cells with significantly reduced AHPs. These data link the AHP reduction observed in vitro to conditioned response acquisition observed behaviorally. They also provide evidence that the AHP is not reduced merely as a non-specific consequences of training trial presentation. This ionic alteration, intrinsic to hippocampus for its expression, indeed intrinsic to the CA1 cells themselves, is stored in CA1 pyramidal cells after a symptotic learning occurs.