Neurotoxic lesions of the dorsal hippocampus disrupt auditory-cued trace heart rate (fear) conditioning in rabbits

Abstract

The first experiment in this study used the classical heart rate (HR) conditioning paradigm to determine if rabbits could associate an auditory conditioned stimulus (CS) and a fear‐producing shock‐unconditioned stimulus (US) separated by an empty 10‐s trace interval. Trace conditioned rabbits (n = 7) acquired significant bradycardiac conditioned HR responses on CS‐alone test trials during a single 35‐trial conditioning session. Control animals (n = 7) which received unpaired CSs and USs did not show HR conditioning. During a retention session of CS‐alone trials 24 h after the conditioning session, some trace‐conditioned animals showed conditioned HR responses immediately following CS onset (n = 3), while others showed responses appropriately timed to the US onset (n = 4) used in trace conditioning 24 h earlier. Thus, rabbits remember the duration of the long 10‐s trace interval 24 h after a single day of training. The second part of this study sought to determine if cells in the dorsal hippocampus play a role in trace HR conditioning. Rabbits were given bilateral ibotenic acid lesions in the neocortex (n = 7) or dorsal hippocampus (n = 8). During trace conditioning and retention, neocortical animals showed conditioned HR responses to the CS, whereas the hippocampal group showed no significant HR conditioning. One week after trace conditioning, the same animals received a delay HR conditioning session where no trace interval separated the CS and US. During delay conditioning, hippocampal animals showed significant conditioned HR responses to the CS that were similar to the neocortical group. Thus, the dorsal hippocampus plays a critical role in rabbit HR conditioning when the CS and US are separated by a 10‐s trace interval. This paradigm may be ideal for in vivo electrophysiological recording studies because rabbits are easily immobilized during the testing procedure, and learning occurs during a single day of training. Hippocampus 2000;10:739–751.