To study the interaction of these brain areas, a novel paradigm was developed to be compatible with multiple acute electrophysiological recordings from the same animal with high counts of behavioral trials and isolated single neurons. To this end, the paradigm was developed in head-fixed rodents allowing well controlled exposures to stimuli that carry varying valence. The paradigm includes a treadmill on which the animal can run. The speed of running, in turn, controls a circular rotating platform (the carousel) that is placed in front of the animal. With each full rotation, the animal experiences different stimuli at particular locations of this platform. This experimental setup allows for up to 100 trials in one recording session thus providing sufficient statistical power for analysis of cell sequences and reactivations thereof, while at the same time allowing for exposures of physical stimuli such as conspecifics. Detail behavioral quantification is also incorporated in this setup through the monitoring of the carousel position in front of the animal together with the speed of running, video of the face that covers whisker, nose, eye, licking movements, as well as pupil size. Armed with these behavioral measures the degree of influence that the different stimuli have on the recorded animal could be explored. Accordingly, behavioral responses were observed to inanimate objects situated at various places on the carousel as well as strong responses to the presence of a conspecific (including strong pupil dilation, frequent stopping in the vicinity of the conspecific as well as increasing sniffing and changes to the ear position). Subsequently, the electrophysiological technique was implemented, first starting with recordings from a single brain structure, the dorsal hippocampus, during the behavioral task. These recordings confirmed the presence of electrophysiological signatures that are ubiquitous in naturally behaving mice such as single neurons whose activity is tuned to specific locations of the environment as well as sharp wave ripple oscillations which are highly synchronous events that excite large portions of the hippocampal network and are important for memory consolidation processes during sleep. Next, the necessary technical upgrades were performed allowing for simultaneous dual site recordings of single neuron resolution activity in the ventral hippocampus and basolateral amygdala. Importantly, the technique allows for simultaneous recordings from the same brain hemisphere. The recordings revealed novel findings showing that some cells in the basolateral amygdala are preferentially recruited during the hippocampal sharp wave ripple events. These observations confirmed the the interaction between the two brain structures during periods of rest and quiescence thus providing a basis for further investigations. Finally, the dual site recordings were combined with the previously behavioral task in a cohort of animals. Data has been collected and the analysis is ongoing. Preliminary results suggest that the coupling between these two structures goes beyond simple brain state dynamics with cells specifically recruited in the behavior experiencing a greater level of reactivation during subsequent rest. Analysis of cell sequences and their reactivation is currently ongoing and when finished will be disseminated through a scientific journal publication.
The developed behavioral paradigm has also been successfully put to use in testing novel technologies for recording from hundreds of cells at the same time from 1024 channels. The results were communicated in scientific meetings and a manuscript has been published in the biorxiv preprint server.
