Abstract:
Laboratory rodents provided chronic unlimited access to running wheels display increased neurogenesis in the hippocampal dentate gyrus. In addition, recent studies indicate that such an access to wheels stimulates dendritic arborization in newly formed neurons. However, (i) the presence of the running wheel in the housing environment might also bear intrinsic influences on the number and shape of new neurons and (ii) the dendritic arborization of new neurons might be insensitive to moderate daily running activity (i.e. several hours). In keeping with these uncertainties, we have examined neurogenesis and dendritic arborization in newly formed granular cells in adult C57Bl/6N male mice housed for 3 weeks under standard conditions, with a locked wheel, with a running wheel set free 3 h/day, or with a running wheel set permanently free. The results indicate that the presence of a blocked wheel in the home cage increased cell proliferation, but not the number of new neurons while running increased in a duration-dependent manner the number of newborn neurons, as assessed by DCX labeling. Morphological analyses of the dendritic tree of newborn neurons, as identified by BrdU-DCX co-staining, revealed that although the presence of the wheel stimulated their dendritic architecture, the amplitude of this effect was lower than that elicited by running activity, and was found to be running duration-dependent. This article is protected by copyright. All rights reserved.

Abstract:
Several studies have indicated that animal models of exercise, such as voluntary wheel running, might be endowed with anxiolytic properties. Using the light/dark test of unconditioned anxiety, we have reported that one confounding factor in the estimation of wheel running impacts on anxiety might be the housing condition of the sedentary controls. The present mouse study analyzed whether the aforementioned observation in the light/dark test (i) could be repeated in the elevated plus-maze and social interaction tests of unconditioned anxiety, (ii) extended to conditioned anxiety, as assessed during cued fear recall tests, and (iii) required unlimited daily access to the running wheel. Housing with a locked wheel or with a free wheel that allowed limited or unlimited running activity triggered anxiolysis in the light/dark test, but not in the elevated plus-maze test, compared to standard housing. In the social interaction test, the duration, but not the number, of social contacts was increased in mice provided unlimited (but not limited) access to a wheel, compared to standard housing or housing with a locked wheel. Lastly, freezing responses to a cue during fear recall tests indicated that the reduction in freezing observed in mice provided limited or unlimited access to the wheels was fully accounted for by housing with a wheel. Besides confirming that the housing condition of the sedentary controls might bias the estimation of the effects of wheel running on anxiety, this study further shows that this estimation is dependent on the test used to assess anxiety.

Abstract:
BACKGROUND: We have shown that the endogenous stimulation of cannabinoid type-1 (CB(1)) receptors is a prerequisite for voluntary running in mice, but the precise mechanisms through which the endocannabinoid system exerts a tonic control on running performance remain unknown. METHODS: We analyzed the respective impacts of constitutive/conditional CB(1) receptor mutations and of CB(1) receptor blockade on wheel-running performance. We then assessed the consequences of ventral tegmental area (VTA) CB(1) receptor blockade on the wheel-running performances of wildtype (gamma-aminobutyric acid [GABA]-CB(1)(+)/(+)) and mutant (GABA-CB(1)(-)/(-)) mice for CB(1) receptors in brain GABA neurons. Using in vivo electrophysiology, the consequences of wheel running on VTA dopamine (DA) neuronal activity were examined in GABA-CB(1)(+)/(+) and GABA-CB(1)(-)/(-) mice. RESULTS: Conditional deletion of CB(1) receptors from brain GABA neurons, but not from several other neuronal populations or from astrocytes, decreased wheel-running performance in mice. The inhibitory consequences of either the systemic or the intra-VTA administration of CB1 receptor antagonists on running behavior were abolished in GABA-CB(1)(-)/(-) mice. The absence of CB1 receptors from GABAergic neurons led to a depression of VTA DA neuronal activity after acute/repeated wheel running. CONCLUSIONS: This study provides evidence that CB(1) receptors on VTA GABAergic terminals exert a permissive control on rodent voluntary running performance. Furthermore, it is shown that CB(1) receptors located on GABAergic neurons impede negative consequences of voluntary exercise on VTA DA neuronal activity. These results position the endocannabinoid control of inhibitory transmission as a prerequisite for wheel-running performance in mice.

Abstract:
The endocannabinoid system (ECS) tightly controls emotional responses to acute aversive stimuli. Repeated stress alters ECS activity but the role played by the ECS in the emotional consequences of repeated stress has not been investigated in detail. This study used social defeat stress, together with pharmacology and genetics to examine the role of cannabinoid type-1 (CB(1)) receptors on repeated stress-induced emotional alterations. Seven daily social defeat sessions increased water (but not food) intake, sucrose preference, anxiety, cued fear expression, and adrenal weight in C57BL/6N mice. The first and the last social stress sessions triggered immediate brain region-dependent changes in the concentrations of the principal endocannabinoids anandamide and 2-arachidonoylglycerol. Pretreatment before each of the seven stress sessions with the CB(1) receptor antagonist rimonabant prolonged freezing responses of stressed mice during cued fear recall tests. Repeated social stress abolished the increased fear expression displayed by constitutive CB(1) receptor-deficient mice. The use of mutant mice lacking CB(1) receptors from cortical glutamatergic neurons or from GABAergic neurons indicated that it is the absence of the former CB(1) receptor population that is responsible for the fear responses in socially stressed CB(1) mutant mice. In addition, stress-induced hypolocomotor reactivity was amplified by the absence of CB(1) receptors from GABAergic neurons. Mutant mice lacking CB(1) receptors from serotonergic neurons displayed a higher anxiety but decreased cued fear expression than their wild-type controls. These mutant mice failed to show social stress-elicited increased sucrose preference. This study shows that (i) release of endocannabinoids during stress exposure impedes stress-elicited amplification of cued fear behavior, (ii) social stress opposes the increased fear expression and delayed between-session extinction because of the absence of CB(1) receptors from cortical glutamatergic neurons, and (iii) CB(1) receptors on central serotonergic neurons are involved in the sweet consumption response to repeated stress.

Abstract:
This study has investigated the role of hypothalamic and amygdalar type-1 cannabinoid (CB1) receptors in the emotional and neuroendocrine responses to stress. To do so, we used the Cre/loxP system to generate conditional mutant mice lacking the CB1 gene in neurons expressing the transcription factor single-minded 1 (Sim1). This choice was dictated by former evidence for Sim1-Cre transgenic mice bearing Cre activity in all areas expressing Sim1, which chiefly includes the hypothalamus (especially the paraventricular nucleus, the supraoptic nucleus, and the posterior hypothalamus) and the mediobasal amygdala. Genomic DNA analyses in Sim1-CB1(-/-) mice indicated that the CB1 allele was excised from the hypothalamus and the amygdala, but not from the cortex, the striatum, the thalamus, the nucleus accumbens, the brainstem, the hippocampus, the pituitary gland, and the spinal cord. Double-fluorescent in situ hybridization experiments further indicated that Sim1-CB1(-/-) mice displayed a weaker CB1 receptor mRNA expression in the paraventricular nucleus of the hypothalamus and the mediobasal part of the amygdala, compared to wild-type animals. Individually housed Sim1-CB1(-/-) mice and their Sim1-CB1(+/+) littermates were exposed to anxiety and fear memory tests under basal conditions as well as after acute/repeated social stress. A principal component analysis of the behaviors of Sim1-CB1(-/-) and Sim1-CB1(+/+) mice in anxiety tests (open field, elevated plus-maze, and light/dark box) revealed that CB1 receptors from Sim1-expressing neurons exert tonic, albeit opposite, controls of locomotor and anxiety reactivity to novel environments. No difference between genotypes was observed during the recall of contextual fear conditioning or during active avoidance learning. Sim1-CB1(-/-), but not Sim1-CB1(+/+), mice proved sensitive to an acute social stress as this procedure reverted the increased ambulation in the center of the open field. The stimulatory influence of repeated social stress on body and adrenal weights, water intake, and sucrose preference was similar in the two genotypes. On the other hand, repeated social stress abolished the decrease in cued-fear conditioned expression that was observed in Sim1-CB1(-/-) mice, compared to Sim1-CB1(+/+) mice. This study suggests that CB1 receptors located on Sim1-expressing neurons exert a tonic control on locomotor reactivity, unconditioned anxiety, and cued-fear expression under basal conditions as well as after acute or repeated stress.

Abstract:

Abstract:
Chronic voluntary wheel-running activity has been reported to hypersensitise central CB1 receptors in mice. On the other hand, pharmacological findings suggest that the CB1 receptor could be involved in wheel-running behaviour and in running-induced neurogenesis in the hippocampus. We analysed wheel-running behaviour for 6 weeks and measured its consequences on hippocampal neurogenesis in CB1 knockout (CB1(-/-)) animals, compared to wild-type (CB1(+/+)) littermates. Because wheel running has been shown to affect locomotor reactivity in novel environments, memory for aversive events and depression-like behaviours, we also assessed these behaviours in control and running CB1(+/+) and CB1(-/-) mice. When compared with running CB1(+/+) mice, the distance covered weekly by CB1(-/-) mice was decreased by 30-40%, an observation accounted for by decreased time spent and maximal velocity on the wheels. Analyses of running distances with respect to the light/dark cycle revealed that mutant covered less distance throughout both the inactive and the active phases of that cycle. Locomotion in an activity cage, exploration in an open field, and immobility time in the forced swim test proved insensitive to chronic wheel running in either genotype. Wheel running, per se, did not influence the expression and extinction of cued fear memory but counteracted in a time-dependent manner the deficiency of extinction measured in CB1(-/-) mice. Hippocampal neurogenesis, assessed by doublecortin labelling of immature neurons in the dentate gyrus, was lowered by 40% in control CB1(-/-) mice, compared to control CB1(+/+) mice. Although CB1(-/-) mice ran less than their wild-type littermates, the 6-week running protocol increased neurogenesis to similar extents (37-39%) in both genotypes. This study suggests that mouse CB1 receptors control wheel running but not its neurogenic consequences in the hippocampus.

Abstract:

Abstract:
BACKGROUND: Assessing the relative value of cocaine and how it changes with chronic drug use represents a long-standing goal in addiction research. Surprisingly, recent experiments in rats--by far the most frequently used animal model in this field--suggest that the value of cocaine is lower than previously thought. METHODOLOGY/PRINCIPAL FINDINGS: Here we report a series of choice experiments that better define the relative position of cocaine on the value ladder of rats (i.e., preference rank-ordering of different rewards). Rats were allowed to choose either taking cocaine or drinking water sweetened with saccharin--a nondrug alternative that is not biologically essential. By systematically varying the cost and concentration of sweet water, we found that cocaine is low on the value ladder of the large majority of rats, near the lowest concentrations of sweet water. In addition, a retrospective analysis of all experiments over the past 5 years revealed that no matter how heavy was past cocaine use most rats readily give up cocaine use in favor of the nondrug alternative. Only a minority, fewer than 15% at the heaviest level of past cocaine use, continued to take cocaine, even when hungry and offered a natural sugar that could relieve their need of calories. CONCLUSIONS/SIGNIFICANCE: This pattern of results (cocaine abstinence in most rats; cocaine preference in few rats) maps well onto the epidemiology of human cocaine addiction and suggests that only a minority of rats would be vulnerable to cocaine addiction while the large majority would be resilient despite extensive drug use. Resilience to drug addiction has long been suspected in humans but could not be firmly established, mostly because it is difficult to control retrospectively for differences in drug self-exposure and/or availability in human drug users. This conclusion has important implications for preclinical research on the neurobiology of cocaine addiction and for future medication development.