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Environment and experience influence defensive behaviors, but the neural circuits mediating such effects are not well understood. We describe a new experimental model in which either flight or freezing reactions can be elicited from mice by innately aversive ultrasound. Flight and freezing are negatively correlated, suggesting a competition between fear motor systems. An unfamiliar environment or a previous aversive event, moreover, can alter the balance between these behaviors. To identify potential circuits controlling this competition, global activity patterns in the whole brain were surveyed in an unbiased manner by c-fos in situ hybridization, using novel experimental and analytical methods. Mice predominantly displaying freezing behavior had preferential neural activity in the lateral septum ventral and several medial and periventricular hypothalamic nuclei, whereas mice predominantly displaying flight had more activity in cortical, amygdalar, and striatal motor areas, the dorsolateral posterior zone of the hypothalamus, and the vertical limb of the diagonal band. These complementary patterns of c-fos induction, taken together with known connections between these structures, suggest ways in which the brain may mediate the balance between these opponent defensive behaviors.

International audience; Stress-induced alterations in neuronal plasticity and in hippocampal functions have been suggested to be involved in the development of mood disorders. In this context, we investigated in the hippocampus the activation of intracellular signaling cascades, the expression of epigenetic markers and plasticity-related genes in a mouse model of stress-induced hyperactivity and of mixed affective disorders. We also determined whether the antidepressant drug agomelatine, a MT1/MT2 melatonergic receptor agonist/5-HT2C receptor antagonist, could prevent some neurobiological and behavioral alterations produced by stress. C57BL/6J mice, exposed for 3 weeks to daily unpredictable socio-environmental stressors of mild intensity, were treated during the whole procedure with agomelatine (50 mg kg−1 per day, intraperitoneal). Stressed mice displayed robust increases in emotional arousal, vigilance and motor activity, together with a reward deficit and a reduction in anxiety-like behavior. Neurobiological investigations showed an increased phosphorylation of intracellular signaling proteins, including Atf1, Creb and p38, in the hippocampus of stressed mice. Decreased hippocampal level of the repressive epigenetic marks HDAC2 and H3K9me2, as well as increased level of the permissive mark H3K9/14ac suggested that chronic mild stress was associated with increased gene transcription, and clear-cut evidence was further indicated by changes in neuroplasticity-related genes, including Arc, Bcl2, Bdnf, Gdnf, Igf1 and Neurod1. Together with other findings, the present data suggest that chronic ultra-mild stress can model the hyperactivity or psychomotor agitation, as well as the mixed affective behaviors often observed during the manic state of bipolar disorder patients. Interestingly, agomelatine could normalize both the behavioral and the molecular alterations induced by stress, providing further insights into the mechanism of action of this new generation antidepressant drug.

Purpose of the study: Traumatic brain injury (TBI) results in white matter injury (WMI), evidenced by demyelination. WMI may originate from neuroinflammation and is associated with late emotional and cognitive disorders in patients. Experimental research focuses on brain lesions and neurological deficits occurring days after injury, occulting long-term consequences of TBI. The present study aims to evaluate the time-course of neuroinflammation, WMI and behavioral disorders related to anxiety, compulsiveness, depression, behavioral flexibility and spatial memory after TBI. Methods used: Three groups of male Swiss mice were studied: non-operated, sham-operated and traumatic brain injured mice. TBI was induced by a controlled cortical impact. Neuroinflammation was determined by microglia activation using CD11b immunostaining. The expression of myelin basic protein (MBP), a marker of myelination, was measured in the corpus callosum using Western blot (for all biomarkers evaluations n = 5−6/group). At 15 days, 1, 2 and 3 months, sensorimotor performances were determined with pole test and spontaneous locomotion with actimeter. Spatial learning and memory, and behavioral flexibility were assessed using Barnes maze: two weeks after TBI, mice were trained to find a target hole 3 trials/day during 4 days, followed by a probe test (i.e. in absence of target box) on the last day. At 1, 2 and 3 months, mice were trained to find a new target hole location as previously described. Compulsiveand anxiety-like behaviors using the marble-burying, the open field and the elevated plus maze tests and immobility/escape behavior in the forced swim test, more relevant to depression, were assessed 3 months postTBI (for all behavioral measurements: n = 9−12/group). Data were analyzed with one-way ANOVA. Results: Number of CD11b+ cells was increased from 24 h (P< 0.001) up to 7 days post-TBI (P< 0.001), showing neuroinflammation. MBP expression was decreased at 7 days and 3 months (P< 0.01) in ipsiand contralateral corpus callosum demonstrating acute and long-term bilateral WMI. Spontaneous locomotor activity, motor coordination performances, anxiety-like and despair-like behavioral measurements were undisturbed by TBI. Although, spatial learning performances were not disturbed at 2 weeks post-TBI, deficit was apparent when moving the target hole location 1, 2 and 3 months after TBI. While all mice discriminated the target zone correctly during the probe tests, latencies to find the new target hole, distances travelled and number of errors, were all significantly higher in injured mice during the first training day (P< 0.05), suggesting that TBI induces an early deficit in learning processes, behavioral adaptability and flexibility disorder. Conclusions: These data clearly demonstrate that TBI induces acute microglial activation that precedes acute and long term demyelination, suggesting the involvement of neuroinflammation in WMI. Moreover, the latter is associated with a reduction of flexibility and a spatial learning deficit, in the absence of sensorimotor and emotional states impairments, suggesting that changes in the escape behavior to the target hole, were related to behavioral flexibility rather than defense behavior. Behavioral evaluation is still ongoing until 12 months after TBI. This model will provide an overview of various late behavioral disorders and demyelination that could help to validate pharmacological strategies.

Objectives5-HT2C receptors are well known to be involved in anxiety, but their implication in stress-induced changes of 5-HT transmission remained to be investigated. We thus assess the behavioral and neurochemical effects of 5-HT2C receptor activation in naïve and stressed mice, and after chronic paroxetine known to exert anxiolytic effects in humans.Methods and resultsThe effects of the preferential 5-HT2C agonists m-chlorophenylpiperazine (mCPP) and RO60-0175, the selective 5-HT2C receptor antagonist SB242,084 and restraint-stress on anxiety-like behavior in mice were assessed using the social interaction test, while the neurochemical effects of these treatments on 5-HT turnover (5-HIAA/5-HT ratio) and extracellular 5-HT were determined using HPLC and microdialysis. Both mCPP and restraint-stress increased anxiety-like behavior in the social interaction test, and these effects were blocked by pretreatment with SB242,084. Restraint-stress increased 5-HT turnover in various brain areas, and this effect could be prevented by the 5-HT2C receptor agonist RO60-0175. Acute administration of SB242,084 potentiated the stress-induced increase in 5-HT turnover and blocked the inhibitory effect of RO60-0175. Microdialysis studies in frontal cortex revealed that RO60-0175 has an inhibitory effect on the stress-induced increase in extracellular 5-HT levels, but not on basal 5-HT levels. Chronic paroxetine prevented the anxiogenic effect of mCPP and prevented the inhibitory effect of RO60-0175 on restraint-stress-induced increase in 5-HT turnover.ConclusionsThese data strongly suggest that 5-HT2C receptor activation mediates the anxiogenic effect of stress. In addition, the anxiolytic action of long term treatment with SSRIs might be causally related to a clear-cut 5-HT2C receptor desensitization.

[3H]U69,593 and [3H]ethylketazocine (μ + δ suppressed) binding was measured in homogenates of guinea-pig brain. Both ligands bind with high affinity to a single class of opioid sites. The relative equilibrium dissociation constant (KD) for [3H]U69,593 is 1.15 nM, while [3H]ethylketazocine has a KD value of 0.33 nM. Their respective maximum binding capacities are 4.49 and 4.48 pmol/g of wet tissue. Various μ-selective, δ-selective, κ-selective, and nonselective opioids were tested in competition studies against the binding of [3H]U69,593 or [3H]ethylketazocine (in the presence of μ- and δ-blockers) to measure their relative affinity. [D-Ala2, MePhe4,Gly5-ol]enkephalin (μ-selective) has low affinity (600–3000 nM) and [D-Pen2,D-Pen5]enkephalin and [D-Ser2, Leu5, Thr6]enkephalin (δ-selective) have very low affinities (> 20 000 nM) at the sites labelled with [3H]U69,593 or [3H]ethylketazocine. On the other hand, unlabelled U69,593, U50,488H, and tifluadom (all three κ-selective substances) display high affinity (1–5 nM) at those sites. Nonselective opioids, such as bremazocine, levorphanol, and ethylketazocine show similar affinities at the sites labelled with [3H]U69,593 and at the sites labelled with [3H]ethylketazocine. These data indicate that [3H]U69,593 is a selective high-affinity ligand for the same sites that are labelled with [3H]ethylketazocine (in the presence of μ- and δ-blockers) and that these are κ-sites.