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 two 80-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 M omega 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) Acquisition group based on the number of blink CRs shown on the second training day (P less than 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 = 26). The mean maximal AHP amplitudes after 4 spikes were -2.9 +/- 0.34 mV and -4.0 +/- 0.31 mV, respectively, in the High and Low Acquisition groups (P less than 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 less than 0.02) and for each of the individual time points (at least P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)