Caracterização eletrofisiológica das oscilações delta e teta do hipocampo dorsal de ratos durante locomoção em esteira

Abstract

The hippocampal theta rhythm (6-10 Hz) has been extensively studied since the 1950’s. These researches have shown that the theta rhythm is involved with cognitive functions, sensorymotor integration, paradoxical sleep and locomotion. As reported by many works, power and frequency variations of hippocampal theta oscillations are associated with the speed of locomotion and with the intensity of execution of a motor program. At the same time, the delta rhythm (0.5-4 Hz) is commonly observed on the slow-wave sleep, inactivity behaviors and anesthesia. These slow frequency oscillations allow the exchange of information between neuronal assemblies across distant brain areas, and both the theta and delta rhythms are largely present in multiple cortical and subcortical regions. In the rat hippocampus, these oscillations occur in an antagonistic way and are classically used to define the active and inactive hippocampus. Being the delta rhythm characteristic of the inactive estate, during the slw wave sleep and awareness immobility; And the theta rhythm characteristic of the active hippocampus, occurring on REM sleep e mainly, during locomotion. In this way, the delta and theta coexistence on the rat hippocampus was not studied along the literature, and the theta/delta ratio is commonly used to define active and inactive periods on especial tasks. However, from an indication found in the literature, in the present work, we studied the dynamics of theta and delta oscillations in the CA1 area of the dorsal hippocampus of rodents through two distinct locomotion protocols on treadmill. In the first manuscript, we took data from the electrophysiological recordings from 3 long-evans male rats submitted to 11 sessions (n=11) of 35 runnings of 15 s at the constant speed of 30 cm/s on a treadmill. The analysis showed, respectively, an increase and a decrease of the delta and theta power across the 35 runnings. Beyond that, an increase of the interhemispheric phase coherence occurred in the delta band, without significant coherence variations in the theta band. Thus, we suggest that variations in power and frequency of the theta and delta oscillations may be associated with dynamic changes in the hippocampal network necessary to maintenance of the running exercise, not simply associated to the speed. Next, on the second manuscript, we recorded electrophysiological signalsfrom the CA1 area of the dorsal hippocampus of 6 male wistarrats on 22 sessions (n=22) of 48 bouts of 20 s long, escalated on a sequence of 8 blocks of 6 runs of which: 3 runs on constant acceleration ratios (1, 1.5 and 2 cm/s²) and 3 runs on constant speeds (20, 30, 40 cm/s). On the runnings with the highest acceleration ratios (1.5 and 2 cm/s²) and on the highest constant speed runnings (30 and 40 cm/s²) the power and frequency of the delta oscillations were higher than the runnings at the slower acceleration ratio (1 cm/s²) and the slower constant speed running (20 cm/s). As the delta oscillations, theta oscillations showed an increase in power on the highest acceleration ratios and constant speed runnings. Meanwhile, the theta frequency did not show significant differences according to the speed and acceleration increase. Through these observations, we concluded that higher locomotion speeds promote an increase in the power of the delta and theta oscillations, as well as an increase in the frequency of delta oscillations. We thus found that theta and delta oscillations can coexist in the hippocampus of rats during stationary running. Together, our results corroborate previous evidence of the association between the power of the hippocampal theta rhythm and the speed of locomotion and further demonstrate for the first time that the hippocampal delta oscillations are modulated by the speed of locomotion. However, further studies are needed to clarify the role of the hippocampal delta rhythm during the execution of locomotion protocols.