Effects of forced exercise on spatial memory and cytochrome c oxidase activity in aged rats

Learning and metabolic brain differences between juvenile male and female rats in the execution of different training regimes of a spatial memory task

Spatial memory is responsible for encoding spatial information to form a path, storing this mental representation, and evaluating and recovering spatial configurations to find a target location in the environment. It is mainly supported by the hippocampus and its interaction with other structures, such as the prefrontal cortex, and emerges in rodents around postnatal day (PND) 20. Sex differences in spatial tasks have been found in adults, with a supposedly better performance in males. However, few studies have examined sex differences in orientation throughout postnatal development. This study aimed to analyse the performance of juvenile (PND 23) male (n=18) and female (n=21) Wistar rats in a spatial reference memory task in the Morris water maze (MWM) with two different training regimes in the acquisition phase, and their subjacent metabolic brain activity. Based on sex, subjects were assigned to two different groups: one that performed four learning trials per day (n=9 males and n=8 females) and the other that was submitted to two trials per day (n=9 males and n=13 females). After the behavioural protocols, metabolic activity was evaluated using cytochrome c oxidase histochemistry. Results showed no metabolic brain or behavioural differences in the four-trial protocol performance, in which both sexes reached the learning criterion on the fourth day. By contrast, the two-trial protocol revealed an advantage for females, who reached the learning criterion on day four, whereas males needed more training and succeeded on day six. The female group showed lower metabolic activity than the male group in the cingulate and prelimbic cortex. These results suggest a faster consolidation process in the female group than the male group. Further research is needed to understand sex differences in spatial memory at early stages.

Repetitive transcranial magnetic stimulation during a spatial memory task leads to a decrease in brain metabolic activity

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive neuromodulation technique that is able to generate causal-based interferences between brain networks and cognitive or behavioral responses. It has been used to improve cognition in several disease models. However, although its exploration in healthy animals is essential to attribute its pure effect in learning and memory processes, studies in this regard are scarce. We aimed to evaluate whether rTMS leads to memory facilitation in healthy rats, and to explore the brain-related oxidative metabolism. We stimulated healthy Wistar rats with a high-frequency (100 Hz) and low-intensity (0.33 T) protocol during three consecutive days and evaluated the effect on the performance of an allocentric spatial reference learning and memory task. Following the last day of learning, we assessed oxidative brain metabolism through quantitative cytochrome c oxidase (CCO) histochemistry. The results showed that rTMS did not improve spatial memory in healthy rats, but the behavioral outcome was accompanied by a CCO reduction in the prefrontal, retrosplenial, parietal, and rhinal cortices, as well as in the striatum, amygdala, septum, mammillary bodies, and the hippocampus, reflecting a lower metabolic activity. In conclusion, rTMS induces a highly efficient use of brain regions associated with spatial memory.

Changes in COX histochemistry in the brain of mice and rats exposed to chronic subcutaneous rotenone

Exposure of experimental animals to the mitochondrial toxin rotenone is considered to be a model of environmental progression of Parkinson's disease (PD). We investigated the differential vulnerability of various brain regions to generalized inhibition of complex I, induced by subcutaneous rotenone injections for the duration of 1, 3 and 7 days in both rats (2 mg/kg dosage) and mice (4 mg/kg dosage). To examine patterns of metabolic activity changes in the brain, histochemical evaluation of cytochrome C oxidase (COX) activity was performed in post mortem brain sections. Animals displayed a similar time course of neuronal loss in substantia nigra pars compacta (SNpc), reaching 44 % in mice and 42 % in rats by the 7th day. The pattern of COX activity changes, however, was different for the two species. In both experiments, metabolic changes were evident not only in the substantia nigra, but also in non-specific structures (cortex and hippocampus). In mice, a decrease in COX activity was shown mostly for the non-specific areas (V1 cortex and ventral hippocampus) after the single exposure to rotenone. Data from the experiment conducted on rats demonstrated both an acute metabolic decrease in mesencephalic structures (SNpc and nucleus ruber) after a single injection of rotenone and secondary changes in cortical structures (S1 cortex and dorsal hippocampus) after chronic 7 day exposure. These changes reflect the general effect of rotenone on neuronal metabolic rate.

Adult social isolation leads to anxiety and spatial memory impairment: Brain activity pattern of COx and c-Fos

Social isolation during adulthood is a frequent problem that leads to a large variety of adverse emotional and cognitive effects. However, most of the social isolation rodent procedures begin the separation early post-weaning. This work explores locomotor activity, anxiety-like behaviour, and spatial working memory after twelve weeks of adult social isolation. In order to study the functional contribution of selected brain areas following a working memory task, we assessed neuronal metabolic activity through quantitative cytochrome oxidase histochemistry and c-Fos immunohistochemistry. Behaviourally, we found that isolated animals (IS) showed anxiety-like behaviour and worse working memory than controls, whereas motor functions were preserved. Moreover, IS rats showed lower levels of learning-related c-Fos immunoreactivity, compared to controls, in the medial prefrontal cortex (mPFC), ventral tegmental area (VTA), and nucleus accumbens shell. In addition, the IS group showed lower neuronal metabolic activity in the mPFC, VTA, and CA1 subfield of the hippocampus. These results indicate that twelve weeks of social isolation in adult rats leads to different behavioural and brain alterations, and they highlight the importance of social support, not only in development, but also in adulthood.

Sex differences in aerobic exercise efficacy to improve cognition: A systematic review and meta-analysis of studies in older rodents

Research in humans indicates that women may show greater cognitive benefits from aerobic training (AT) than men. To determine whether this sex difference extends to rodents, we conducted a systematic review and meta-analysis of studies in healthy, older rodents. Results indicate that compared to controls, AT improved hippocampus-dependent and -independent learning and memory. A sex difference was found with males showing larger benefits from AT on conditioned-avoidance and non-spatial memory tasks. AT also increased brain-derived neurotrophic factor compared to controls, with larger effects in females. As an exploratory analysis, sex differences in voluntary AT were examined separately from forced AT. Voluntary AT enhanced non-spatial memory to a greater extent in males. Forced AT enhanced hippocampus-dependent learning and memory more so in females. These findings suggest that sex is an important factor to consider, and studies directly assessing sex differences in the ability of exercise to improve brain function are needed.

Effects of voluntary exercise on structure and function of cortical microvasculature

Aerobic activity has been shown highly beneficial to brain health, yet much uncertainty still surrounds the effects of exercise on the functioning of cerebral microvasculature. This study used two-photon fluorescence microscopy to examine cerebral hemodynamic alterations as well as accompanying geometric changes in the cortical microvascular network following five weeks of voluntary exercise in transgenic mice endogenously expressing tdTomato in vascular endothelial cells to allow visualization of microvessels irrespective of their perfusion levels. We found a diminished microvascular response to a hypercapnic challenge (10% FiCO₂) in running mice when compared to that in nonrunning controls despite commensurate increases in transcutaneous CO₂ tension. The flow increase to hypercapnia in runners was 70% lower than that in nonrunners (p = 0.0070) and the runners' arteriolar red blood cell speed changed by only half the amount seen in nonrunners (p = 0.0085). No changes were seen in resting hemodynamics or in the systemic physiological parameters measured. Although a few unperfused new vessels were observed on visual inspection, running did not produce significant morphological differences in the microvascular morphometric parameters, quantified following semiautomated tracking of the microvascular networks. We propose that voluntary running led to increased cortical microvascular efficiency and desensitization to CO₂ elevation.

Functional brain networks underlying latent inhibition of conditioned disgust in rats

The present experiment examined the neuronal networks involved in the latent inhibition of conditioned disgust by measuring brain oxidative metabolism. Rats were given nonreinforced intraoral (IO) exposure to saccharin (exposed groups) or water (non-exposed groups) followed by a conditioning trial in which the animals received an infusion of saccharin paired (or unpaired) with LiCl. On testing, taste reactivity responses displayed by the rats during the infusion of the saccharin were examined. Behavioral data showed that preexposure to saccharin attenuated the development of LiCl-induced conditioned disgust reactions, indicating that the effects of taste aversion on hedonic taste reactivity had been reduced. With respect to cumulative oxidative metabolic activity across the whole study period, the parabrachial nucleus was the only single region examined which showed differential activity between groups which received saccharin-LiCl pairings with and without prior non-reinforced saccharin exposure, suggesting a key role in the effects of latent inhibition of taste aversion learning. In addition, many functional connections between brain regions were revealed through correlational analysis of metabolic activity, in particular an accumbens-amygdala interaction that may be involved in both positive and negative hedonic responses.

Altered metabolic activity in the developing brain of rats predisposed to high versus low depression-like behavior

Individual differences in human temperament can increase the risk of psychiatric disorders like depression and anxiety. Our laboratory utilized a rat model of temperamental differences to assess neurodevelopmental factors underlying emotional behavior differences. Rats selectively bred for low novelty exploration (Low Responders, LR) display high levels of anxiety- and depression-like behavior compared to High Novelty Responder (HR) rats. Using transcriptome profiling, the present study uncovered vast gene expression differences in the early postnatal HR versus LR limbic brain, including changes in genes involved in cellular metabolism. These data led us to hypothesize that rats prone to high (versus low) anxiety/depression-like behavior exhibit distinct patterns of brain metabolism during the first weeks of life, which may reflect disparate patterns of synaptogenesis and brain circuit development. Thus, in a second experiment we examined activity of cytochrome C oxidase (COX), an enzyme responsible for ATP production and a correlate of metabolic activity, to explore functional energetic differences in the HR/LR early postnatal brain. We found that HR rats display higher COX activity in the amygdala and specific hippocampal subregions compared to LRs during the first 2 weeks of life. Correlational analysis examining COX levels across several brain regions and multiple early postnatal time points suggested desynchronization in the developmental timeline of the limbic HR versus LR brain during the first two postnatal weeks. These early divergent COX activity levels may reflect altered circuitry or synaptic activity in the early postnatal HR/LR brain, which could contribute to the emergence of their distinct behavioral phenotypes.

A perspective on the future role of brain pet imaging in exercise science

Positron Emission Tomography (PET) bears a unique potential for examining the effects of physical exercise (acute or chronic) within the central nervous system in vivo, including cerebral metabolism, neuroreceptor occupancy, and neurotransmission. However, application of Neuro-PET in human exercise science is as yet surprisingly sparse. To date the field has been dominated by non-invasive neuroelectrical techniques (EEG, MEG) and structural/functional magnetic resonance imaging (sMRI/fMRI). Despite PET having certain inherent disadvantages, in particular radiation exposure and high costs limiting applicability at large scale, certain research questions in human exercise science can exclusively be addressed with PET: The "metabolic trapping" properties of (18)F-FDG PET as the most commonly used PET-tracer allow examining the neuronal mechanisms underlying various forms of acute exercise in a rather unconstrained manner, i.e. under realistic training scenarios outside the scanner environment. Beyond acute effects, (18)F-FDG PET measurements under resting conditions have a strong prospective for unraveling the influence of regular physical activity on neuronal integrity and potentially neuroprotective mechanisms in vivo, which is of special interest for aging and dementia research. Quantification of cerebral glucose metabolism may allow determining the metabolic effects of exercise interventions in the entire human brain and relating the regional cerebral rate of glucose metabolism (rCMRglc) with behavioral, neuropsychological, and physiological measures. Apart from FDG-PET, particularly interesting applications comprise PET ligand studies that focus on dopaminergic and opioidergic neurotransmission, both key transmitter systems for exercise-related psychophysiological effects, including mood changes, reward processing, antinociception, and in its most extreme form 'exercise dependence'. PET ligand displacement approaches even allow quantifying specific endogenous neurotransmitter release under acute exercise interventions, to which modern PET/MR hybrid technology will be additionally fruitful. Experimental studies exploiting the unprecedented multimodal imaging capacities of PET/MR in human exercise sciences are as yet pending.

Housing condition-related changes involved in reversal learning and its c-Fos associated activity in the prefrontal cortex

Our study examined how different housing conditions modulated the acquisition of a spatial reference memory task and also, a reversal task in the 4-radial arm water maze (4-RAWM). The animals were randomly assigned to standard or enriched cages, and, as a type of complementary stimulation along with the environmental enrichment (EE), a group of rats also ran 15 min/day in a Rotarod. Elevated-zero maze results allowed us to discard that our exercise training increased anxiety-related behaviors. 4-RAWM results revealed that the non-enriched group had a worse performance during the acquisition and also, during the first trial of each session with respect to the enriched groups. Regarding the reversal task, this group made more perseverative errors in the previous platform position. Interestingly, we hardly found differences between the two enriched groups (with and without exercise). We also analyzed how the reversal learning, depending on the previous housing condition, modulated the expression of c-Fos-positive nuclei in different subdivisions of the medial prefrontal cortex (cingulate (Cg), prelimbic (PL) and infralimbic (IL) cortices) and in the orbitofrontal (OF) cortex. The enriched groups had higher c-Fos expression in the Cg and OF cortices and lower in the IL cortex respect to the non-enriched animals. In the PL cortex, we did not find significant differences between the groups that performed the reversal task. Therefore, our short EE protocol improved the performance in a spatial memory and a reversal task, whereas the exercise training, combined with the EE, did not produce a greater benefit. This better performance seemed to be related with the specific pattern of c-Fos expression in brain regions involved in cognitive flexibility.

Caffeine suppresses exercise-enhanced long-term and location memory in middle-aged rats: Involvement of hippocampal Akt and CREB signaling

The cognitive function decline is closely related with brain changes generated by age. The ability of caffeine and exercise to prevent memory impairment has been reported in animal models and humans. The purpose of the present study was to investigate whether swimming exercise and caffeine administration enhance memory in middle-aged Wistar rats. Male Wistar rats (18months) received caffeine at a dose of 30mg/kg, 5days per week by a period of 4weeks. Animals were subjected to swimming training with a workload (3% of body weight, 20min per day for 4weeks). After 4weeks, the object recognition test (ORT) and the object location test (OLT) were performed. The results of this study demonstrated that caffeine suppressed exercise-enhanced long-term (ORT) and spatial (OLT) memory in middle-aged and this effect may be related to a decrease in hippocampal p-CREB signaling. This study also provided evidence that the effects of this protocol on memory were not accompanied by alterations in the levels of activated Akt. The [(3)H] glutamate uptake was reduced in hippocampus of rats administered with caffeine and submitted to swimming protocol.

Functional reorganization of motor and limbic circuits after exercise training in a rat model of bilateral parkinsonism

Exercise training is widely used for neurorehabilitation of Parkinson's disease (PD). However, little is known about the functional reorganization of the injured brain after long-term aerobic exercise. We examined the effects of 4 weeks of forced running wheel exercise in a rat model of dopaminergic deafferentation (bilateral, dorsal striatal 6-hydroxydopamine lesions). One week after training, cerebral perfusion was mapped during treadmill walking or at rest using [(14)C]-iodoantipyrine autoradiography. Regional cerebral blood flow-related tissue radioactivity (rCBF) was analyzed in three-dimensionally reconstructed brains by statistical parametric mapping. In non-exercised rats, lesions resulted in persistent motor deficits. Compared to sham-lesioned rats, lesioned rats showed altered functional brain activation during walking, including: 1. hypoactivation of the striatum and motor cortex; 2. hyperactivation of non-lesioned areas in the basal ganglia-thalamocortical circuit; 3. functional recruitment of the red nucleus, superior colliculus and somatosensory cortex; 4. hyperactivation of the ventrolateral thalamus, cerebellar vermis and deep nuclei, suggesting recruitment of the cerebellar-thalamocortical circuit; 5. hyperactivation of limbic areas (amygdala, hippocampus, ventral striatum, septum, raphe, insula). These findings show remarkable similarities to imaging findings reported in PD patients. Exercise progressively improved motor deficits in lesioned rats, while increasing activation in dorsal striatum and rostral secondary motor cortex, attenuating a hyperemia of the zona incerta and eliciting a functional reorganization of regions participating in the cerebellar-thalamocortical circuit. Both lesions and exercise increased activation in mesolimbic areas (amygdala, hippocampus, ventral striatum, laterodorsal tegmental n., ventral pallidum), as well as in related paralimbic regions (septum, raphe, insula). Exercise, but not lesioning, resulted in decreases in rCBF in the medial prefrontal cortex (cingulate, prelimbic, infralimbic). Our results in this PD rat model uniquely highlight the breadth of functional reorganizations in motor and limbic circuits following lesion and long-term, aerobic exercise, and provide a framework for understanding the neural substrates underlying exercise-based neurorehabilitation.

Age-dependent effects of environmental enrichment on brain networks and spatial memory in Wistar rats

We assessed the effect of 3h of environmental enrichment (EE) exposure per day started at different ages (3 and 18months old) on the performance in a spatial memory task and on brain regions involved in the spatial learning (SPL) process using the principal component analysis (PCA). The animals were tested in the four-arm radial water maze (4-RAWM) for 4days, with six daily trials. We used cytochrome c oxidase (COx) histochemistry to determine the brain oxidative metabolic changes related to age, SPL and EE. Behavioural results showed that the enriched groups, regardless of their age, achieved better performance in the spatial task. Interestingly, in the case of the distance travelled in the 4-RAWM, the effect of the EE was dependent on the age, so the young enriched group travelled a shorter distance compared to the aged enriched group. Respect to COx histochemistry results, we found that different brain mechanisms are triggered in aged rats to solve the spatial task, compared to young rats. PCA revealed the same brain functional network in both age groups, but the contribution of the brain regions involved in this network was slightly different depending on the age of the rats. Thus, in the aged group, brain regions involved in anxiety-like behaviour, such as the amygdala or the bed nucleus of the stria terminalis had more relevance; whereas in the young enriched group the frontal and the hippocampal subregions had more contribution.