Single Cell Analysis of Hippocampal Neural Ensembles during Theta-Triggered Eyeblink Classical Conditioning in the Rabbit

SINGLE CELL ANALYSIS OF HIPPOCAMPAL NEURAL ENSEMBLES DURING THETATRIGGERED EYEBLINK CLASSICAL CONDITIONING IN THE RABBIT By Ryan Daniel Darling Rabbit eyeblink classical conditioning (EBCC) is a task widely used to understand the neurobiological correlates of associative learning and memory. The hippocampus has proven to be an important structure in acquiring the association between the conditioning stimuli and the development of conditioned responses. This study used a brain-computer interface to trigger conditioning trials in the presence (T+) or absence (T-) of a frequency component of the hippocampal field potential historically related to sensory processing and attention, termed theta. The presence of theta has been shown to facilitate learning in this task as well as accelerate learning related unit responses in the hippocampus, but the precise nature of its beneficial effect in hippocampal neurophysiology has yet to be determined. In this study, tetrodes were lowered into the dorsal hippocampus of four groups of rabbits including those who received paired or unpaired conditioning stimuli in both T+ and Ttheta conditions. Specialized signal processing software compared the extracted data streams from each wire of the tetrode to separate the waveforms into single neuron responses. Each sorted neuron was then categorized according to its firing properties as pyramidal cells or one of the known types of interneurons that exist in the hippocampus. The individual units were analyzed for their relation to the pretrial period as well as for how they responded to the conditioning stimuli. Interneurons were highly correlated with hippocampal state used to trigger the trials, supporting the existence of interneurons that systematically vary with the ongoing theta activity. Interneuron responses to the conditioning stimuli were generally dependent on hippocampal state, demonstrating excitatory responses in T+ groups and suppression in Tgroups. Pyramidal cells that demonstrated suppression to the conditioning stimuli were also more common in Tgroups, while excitatory pyramidal cells were more related to associative training, being more prevalent in T+ paired animals. The results are discussed in terms of how hippocampal state enhances hippocampal neurophysiology during EBCC and likely produces accelerated learning. This is an important step in understanding the complex relationship between locally recorded neural oscillations, single neuron responses, and memory acquisition in an awake and behaving animal. SINGLE CELL ANALYSIS OF HIPPOCAMPAL NEURAL ENSEMBLES DURING THETATRIGGERED EYEBLINK CLASSICAL CONDITIONING IN THE RABBIT