Aged rats show dominant modulation of lower frequency hippocampal theta rhythm during running

Older rats show slow modulation of hippocampal theta rhythm during voluntary running

Aging causes brain function degeneration and slows many motor and behavioural responses. The hippocampal theta rhythm (4-12 Hz) is related to cognition and locomotion. However, the findings on aging-related changes in the frequency and amplitude of hippocampal theta oscillations have been inconsistent. We hypothesized that older rats have slower responses in terms of hippocampal theta rhythm during voluntary wheel running than do young adult rats. By simultaneously recording electroencephalography and physical activity (PA), we evaluated theta oscillations in 8-week-old (young adult) and 60-week-old (middle-aged) rats before and during wheel running, which was conducted only during the rats' 12-h dark period. To test the alterations of hippocampal theta rhythm in voluntary wheel running, we analyzed the signals without (8-s) or with (2-s) chronological order. No significant difference was observed in total frequency (TP, 4-12 Hz), low-frequency (LT, 4-6.5 Hz), or high-frequency (9.5-12 Hz) theta activity between active waking and overall running in either group. The theta oscillations were slower in the middle-aged rats than in the young adult rats during wheel running but increased during running for both age groups. During wheel running, the middle-aged rats exhibited an increased LT, which was related to PA. On the basis of the chronological order of running, the young adult rats exhibited increased TP, and the middle-aged rats exhibited significant increases in middle-frequency (MT, 6.5-9.5 Hz) theta activity. The dominant modulations of MT in the middle-aged rats may have caused nonsignificant changes in total activity. These between-group differences in theta rhythm characteristics during voluntary running provide insights into age-related brain function decline.

Voluntary exercise enhances hippocampal theta rhythm and cognition in the rat

Regular exercise promotes learning and memory functions. Theta activity is known to relate to various cognitive functions. An increase in theta power may be related to higher cognitive functioning and learning functions. However, evidence is lacking to directly confirm that exercise training can increase the theta activity and promote various cognitive functions simultaneously. We hypothesize that long-term voluntary exercise increases the activity of hippocampal theta rhythm and enhances memory behavior. We used the voluntary wheel running model and a training period of 8 weeks. We started the training when the rats were 12 weeks old. Before and after intervention, we performed a 24 -h electrophysiological recording and 8-arm radial maze test to analyze the hippocampal theta rhythm in awake stage, and spatial memory functions. We discovered that middle to high range frequency (6.5-12 Hz) of theta power was increased after exercise intervention. In addition, the working memory error of 8-arm radial maze test in the exercise group decreased significantly after the 8 weeks of treatment, and these reductions were negatively correlated with hippocampal theta activity. Our results demonstrate that 8-weeks voluntary exercise increases both hippocampal theta amplitude and spatial memory in the rats.

Aging Alters Olfactory Bulb Network Oscillations and Connectivity: Relevance for Aging-Related Neurodegeneration Studies

The aging process eventually cause a breakdown in critical synaptic plasticity and connectivity leading to deficits in memory function. The olfactory bulb (OB) and the hippocampus, both regions of the brain considered critical for the processing of odors and spatial memory, are commonly affected by aging. Using an aged wild-type C57B/6 mouse model, we sought to define the effects of aging on hippocampal plasticity and the integrity of cortical circuits. Specifically, we measured the long-term potentiation of high-frequency stimulation (HFS-LTP) at the Shaffer-Collateral CA1 pyramidal synapses. Next, local field potential (LFP) spectra, phase-amplitude theta-gamma coupling (PAC), and connectivity through coherence were assessed in the olfactory bulb, frontal and entorhinal cortices, CA1, and amygdala circuits. The OB of aged mice showed a significant increase in the number of histone H2AX-positive neurons, a marker of DNA damage. While the input-output relationship measure of basal synaptic activity was found not to differ between young and aged mice, a pronounced decline in the slope of field excitatory postsynaptic potential (fEPSP) and the population spike amplitude (PSA) were found in aged mice. Furthermore, aging was accompanied by deficits in gamma network oscillations, a shift to slow oscillations, reduced coherence and theta-gamma PAC in the OB circuit. Thus, while the basal synaptic activity was unaltered in older mice, impairment in hippocampal synaptic transmission was observed only in response to HFS. However, age-dependent alterations in neural network appeared spontaneously in the OB circuit, suggesting the neurophysiological basis of synaptic deficits underlying olfactory processing. Taken together, the results highlight the sensitivity and therefore potential use of LFP quantitative network oscillations and connectivity at the OB level as objective electrophysiological markers that will help reveal specific dysfunctional circuits in aging-related neurodegeneration studies.

Behaviour consistency is a sensitive tool for distinguishing the effects of aging on physical activity

We attempted to establish a novel parameter of behaviour consistency to help determine the effect of age on physical activity. Using the speed of movement to quantify behaviour might not be sufficient to determine this effect. The slowing of motor activities that occurs with aging is related to the decline of the aging brain. Previous studies have found different running-related hippocampal theta rhythm responses in the aging and exercise model. Therefore, we hypothesized that a familiarity with the environment and physical strength affect behavioural consistency in rats during running exercises. For this study, we used a treadmill and 30-minute running test at constant speeds and compared changes in the triaxial accelerometer and hippocampal theta rhythm between adult and middle-aged rats. No significant differences in RR intervals, mean cross-correlations (MCCs), or the proportion of good correlation coefficient (PGCC) were observed between adult and middle-aged rats in awake states before running on the treadmill. The root mean square (RMS) of the triaxial acceleration vectors in middle-aged rats was higher than that in adult rats. In the treadmill running tests, the RMS observed in middle-aged rats was significantly lower than that observed in adult rats. MCC and PGCC, which indicate movement consistencies, were significantly higher in middle-aged rats than they were in adult rats during the entire running test. However, only the RMS of the adult rats showed a negative correlation with exercise duration. Both MCC and PGCC were positively correlated with exercise duration. By contrast, a similar phenomenon was not found in the changes or differences in hippocampal theta rhythms between these two groups. Therefore, we consider that the MCC and PGCC could distinguish age-related movement differences and indicate coordination/adaptation during exercise. Changes in physical activity and alterations in the hippocampal theta rhythm were not different between the groups.

Exercise Prevents Hypertension and Disrupts the Correlation Between Vascular Sympathetic Activity and Age-Related Increase in Blood Pressure in SHRs

BACKGROUND

Hypertension usually accompanies the elevated sympathetic activity and sleep interruption. Few researches explored the dynamic changes and possible correlations in cardiovascular functions and sleep patterns during the development of hypertension. In contrast, exercise training provides several benefits on cardiovascular and sleep function in hypertensive subjects. However, controlling various factors during a long period of exercise training is difficult in hypertensive subjects, an animal model may be essential. This study aimed to explore dynamic changes in cardiovascular functions and sleep patterns during the development period of hypertension (10-20 weeks old) in spontaneously hypertensive rats (SHRs) and effects of exercise intervention.

METHODS

We used the treadmill exercise model for 8 weeks and started when SHRs were 12 weeks old. Electroencephalogram, electromyogram, electrocardiogram, and blood pressure (BP) were recorded simultaneously for 24 hours once a week over 11 weeks.

RESULTS

Untrained SHRs revealed the age-related increments in BP, and the significant increasing slopes of differences on BP and vascular sympathetic activity were observed during the development period of hypertension. Compared with untrained rats, age-related increases in BP and vascular sympathetic activity were significantly suppressed in trained SHRs. Nevertheless, trained SHRs showed more quiet sleep time at partial weeks. The positive correlation between the differences from 10 weeks of vascular sympathetic activity and BP was disappeared in trained SHRs.

CONCLUSIONS

There existed the significant correlation between the dynamic changes of vascular sympathetic activity and age-related elevation of BP during the development period of hypertension; however, exercise prevented hypertension and disrupted this correlation.