Eyeblink conditioning contingent on hippocampal theta enhances hippocampal and medial prefrontal responses

Abstract

Trace eyeblink classical conditioning (tEBCC) can be accelerated by making training trials contingent on the naturally generated hippocampal 3- to 7-Hz theta rhythm. However, it is not well-understood how the presence (or absence) of theta affects stimulus-driven changes within the hippocampus and how it correlates with patterns of neural activity in other essential trace conditioning structures, such as the medial prefrontal cortex (mPFC). In the present study, a brain-computer interface delivered paired or unpaired conditioning trials to rabbits during the explicit presence (T⁺) or absence (T⁻) of theta, yielding significantly faster behavioral learning in the T⁺-paired group. The stimulus-elicited hippocampal unit responses were larger and more rhythmic in the T⁺-paired group. This facilitation of unit responses was complemented by differences in the hippocampal local field potentials (LFP), with the T⁺-paired group demonstrating more coherent stimulus-evoked theta than T⁻-paired animals and both unpaired groups. mPFC unit responses in the rapid learning T⁺-paired group displayed a clear inhibitory/excitatory sequential pattern of response to the tone that was not seen in any other group. Furthermore, sustained mPFC unit excitation continued through the trace interval in T⁺animals but not in T⁻animals. Thus theta-contingent training is accompanied by 1) acceleration in behavioral learning, 2) enhancement of the hippocampal unit and LFP responses, and 3) enhancement of mPFC unit responses. Together, these data provide evidence that pretrial hippocampal state is related to enhanced neural activity in critical structures of the distributed network supporting the acquisition of tEBCC.