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In humans and rodents, the protracted development of the prefrontal cortex (PFC) throughout adolescence represents a time for marked vulnerability towards environmental adversities, such as stress or drug exposure. We previously showed that the extracellular matrix protein reelin is an instrumental synaptic modulator that shapes medial PFC’s (mPFC) circuitry during maturation and is a critical mediator of the vulnerability to environmental stress. Emerging evidence highlight the role of the endocannabinoid system in the postnatal maturation of the PFC and reelin deficiency influences behavioral abnormalities caused by heavy consumption of THC during adolescence. Could the reelin-dependent maturation of prefrontal networks may be vulnerable to cannabinoid exposure during adolescence? To explore this hypothesis, we studied the effects of a single in-vivo exposure to a synthetic cannabinoid on reelin expression and mPFC functions in adolescent male mice. The results show that a single cannabinoid exposure mimics reelin haploinsufficiency by decreasing prefrontal reelin expression in a layer-specific pattern without changing its transcriptional levels. Furthermore, this treatment impeded synaptic plasticity: adolescent cannabinoid lowered long-term potentiation to the magnitude observed in age-matched reelin haploinsufficient males. Quantitative PCR analysis showed that changes in the mRNA levels of NMDARs does not account for the reduction of TBS-LTP. Together, the data show that exposure to cannabinoid during adolescence phenocopies reelin haploinsufficiency and further identifies reelin as a key component of the vulnerability of PFC to environmental insults.

Throughout early adulthood, the anatomical and functional maturation of PFC circuitry continues under the influence of multiple extrinsic and intrinsic factors, most notably electrical activity, and molecular cues. We previously showed that the extracellular matrix protein reelin orchestrates the structural and functional maturation of deep layers medial PFC (mPFC) pyramidal neurons. Additionally, we reported that reelin haploinsufficiency is associated to prefrontal disruptions of long-term memory retention thereby illustrating the eminent role of reelin in cognitive maturation of the PFC. Prefrontal maturation follows a sex-specific developmental pattern, supporting the existence of sexual differences in the morphology-functional properties PFC. Here, we interrogated the role of reelin in the functional maturation of excitatory networks in the mPFC. The developmental trajectory of reelin’s expression and deep layer pyramidal neurons synaptic plasticity was tracked in the mPFC of male and female mice, from the juvenile period to adulthood. To assess the role of reelin in both sexes, wild-type and heterozygous reeler mice (HRM) were compared. The results show that the maturational profile of reelin expression in the mPFC is sex-dependent and that the developmental trajectory of long-term potentiation is different between wild-type males and females. These data demonstrate reelin’s influence on prefrontal synapses is sex and period specific.

The basolateral amygdala (BLA), the part of the amygdala complex involved in the transduction of perceptual stimuli into emotion, undergoes profound reorganization at adolescence in rodents and humans. How cellular and synaptic plasticity evolve throughout postnatal development in both sexes is only partially understood. We used a cross-sectional approach to compare the morphology, neuronal, and synaptic properties of BLA neurons in rats of both sexes at adolescence and adulthood. While BLA pyramidal neurons from rats of both sexes displayed similar current-voltage relationships, rheobases, and resting potentials during pubescence, differences in these parameters emerged between sexes at adulthood: BLA neurons were more excitable in males than females. During pubescence, BLA neuron excitability was highest in females and unchanged in males; male action potentials were smaller and shorter than females and fast afterhyperpolarizations were larger in males. During post-natal maturation, no difference in spine density was observed between groups or sexes but spine length increased and decreased in females and males, respectively. A reduction in spine head diameter and volume was observed exclusively in females. Basic synaptic properties also displayed sex-specific maturational differences. Stimulus-response relationships and maximal fEPSP amplitudes where higher in male adolescents compared with adults but were similar in females of both ages. Spontaneous excitatory postsynaptic currents mediated by AMPA receptors were smaller in BLA neurons from adolescent female compared with their adult counterparts but were unchanged in males. These differences did not directly convert into changes in overall synaptic strength estimated from the AMPA/NMDA ratio, which was smaller in adolescent females. Finally, the developmental courses of long-term potentiation and depression (LTP, LTD) were sexually dimorphic. LTP was similarly present during the adolescent period in males and females but was not apparent at adulthood in females. In contrast, LTD followed an opposite development: present in adolescent females and expressed in both sexes at adulthood. These data reveal divergent maturational trajectories in the BLA of male and female rats and suggest cellular substrates to the BLA linked sex-specific behaviors at adolescence and adulthood.

The prefrontal cortex (PFC) develops until early adulthood in rodents and humans, but how synaptic plasticity evolves throughout postnatal development is not known. Here, we used a cross-sectional approach to establish the postnatal maturational trajectories of intrinsic properties and synaptic plasticity in the PFC of rats of both sexes. We found that while layer 5 PFC pyramidal neurons from rats of both sexes displayed similar current-voltage relationships, rheobases and resting potentials across all age groups, excitability was lower in female adults compared to the other developmental stages. NMDAR-dependent long-term potentiation and mGluR2/3-mediated long-term depression (LTD) were equally expressed at the juvenile, pubescent and adult developmental stages in animals of both sexes. However, the developmental course of endocannabinoid (eCB)-mediated LTD was sexually dimorphic. First, eCB-LTD emerged during the juvenile period in females. However, although CB1Rs were functional in both sexes at all developmental stages, eCB-LTD’s first emerged during pubescence in male. Second, eCB-LTD engaged distinct receptors in males and females depending on their developmental stages. Female rats employ both CB1R and TRPV1R to produce eCB-LTD at the juvenile stage but solely CB1R at pubescence followed by only TRPV1R at adulthood. In contrast, in pubescent and adult males eCB-LTD always and exclusively depended on CB1R. Pharmacological blockade of 2AG’s principal degrading enzyme allowed incompetent male juvenile synapses to express eCB-LTD. The data reveal different maturational trajectories in the PFC of male and female rats and provide new cellular substrates to the sex-specific behavioral and synaptic abnormalities caused by adolescent exposure to cannabinoids.

The nucleus accumbens (NAc) is a mesocorticolimbic structure that integrates cognitive, emotional and motor functions. Although its role in psychiatric disorders is widely acknowledged, the understanding of its circuitry is not complete. Here we combined optogenetic and whole-cell recordings to draw a functional portrait of excitatory disambiguated synapses onto D1 and D2 medium spiny neurons (MSNs) in the adult mouse NAc core. Comparing synaptic properties of ventral hippocampus (vHipp), basolateral amygdala (BLA) and prefrontal cortex (PFC) inputs revealed a hierarchy of synaptic inputs and feedforward inhibition that depends on the identity of the postsynaptic target MSN. Thus, the BLA is the dominant excitatory pathway onto D1 MSNs (BLA > PFC = vHipp) while PFC inputs dominate D2 MSNs (PFC > vHipp > BLA). Feedforward inhibition of MSN firing too, was input and cell-type specific: while minimal at vHipp-D1 and vHipp-D2 inputs; it inhibited with similar efficacy BLA-D1 or BLA-D2 inputs, was minimal at PFC-D1 but maximal at PFC-D2 inputs. We also tested the hypothesis that endocannabinoids endow excitatory circuits with pathway- and cell-specific plasticity. Thus, while CB1 receptors (CB1R) uniformly depress excitatory pathways irrespective of MSNs identity, TRPV1 receptors (TRPV1R) bidirectionally control inputs onto the NAc core in a pathway-specific manner. Finally, we show how the interplay of TRPV1R/CB1R shapes plasticity at identified BLA-NAc synapses. Together these data shed new light on synapse and circuit specificity in the adult NAc core and illustrate how endocannabinoids contribute to pathway-specific synaptic plasticity.SIGNIFICANCE STATEMENTWe examined the impact of connections from the ventral hippocampus (vHipp,) basolateral amygdala (BLA) and prefrontal cortex (PFC) onto identified medium spiny neurons (MSN) in the adult accumbens core. We found BLA inputs were strongest at D1 MSNs while PFC inputs dominate D2 MSNs. We evaluated the role of the endocannabinoid system in pathway- and cell-specific plasticity and found that CB1 receptors (CB1R) and TRPV1 receptors (TRPV1R) bidirectionally control synaptic transmission and plasticity onto accumbens’ MSNs in a pathway- and cell-specific manner. Finally, we clarify how the interplay of TRPV1R/CB1R shapes plasticity at identified BLA-NAc synapses.

The nucleus accumbens (NAc) is a mesocorticolimbic structure that integrates cognitive, emotional and motor functions. Although its role in psychiatric disorders is widely acknowledged, the understanding of its circuitry is not complete. Here, we combined optogenetic and whole-cell recordings to draw a functional portrait of excitatory disambiguated synapses onto D1 and D2 medium spiny neurons (MSNs) in the adult male mouse NAc core. Comparing synaptic properties of ventral hippocampus (vHipp), basolateral amygdala (BLA) and prefrontal cortex (PFC) inputs revealed a hierarchy of synaptic inputs that depends on the identity of the postsynaptic target MSN. Thus, the BLA is the dominant excitatory pathway onto D1 MSNs (BLA > PFC = vHipp) while PFC inputs dominate D2 MSNs (PFC > vHipp > BLA). We also tested the hypothesis that endocannabinoids endow excitatory circuits with pathway- and cell-specific plasticity. Thus, whereas CB1 receptors (CB1R) uniformly depress excitatory pathways regardless of MSNs identity, TRPV1 receptors (TRPV1R) bidirectionally control inputs onto the NAc core in a pathway-specific manner. Finally, we show that the interplay of TRPV1R/CB1R shapes plasticity at BLA-NAc synapses. Together these data shed new light on synapse and circuit specificity in the adult NAc core and illustrate how endocannabinoids contribute to pathway-specific synaptic plasticity.SIGNIFICANCE STATEMENTWe examined the impact of connections from the ventral hippocampus (vHipp,) basolateral amygdala (BLA) and prefrontal cortex (PFC) onto identified medium spiny neurons (MSNs) in the adult accumbens core. We found BLA inputs were strongest at D1 MSNs while PFC inputs dominate D2 MSNs. Pathway- and cell-specific circuit control was also facilitated by endocannabinoids that endow bidirectional synaptic plasticity at identified BLA-NAc synapses. These data provide mechanistic insights on synapse and circuit specificity in the adult NAc core.

Overconsumption of high-fat diets (HFDs) can critically affect synaptic and cognitive functions within telencephalic structures such as the medial prefrontal cortex (mPFC). The underlying mechanisms, however, remain largely unknown. Here we show that adolescence is a sensitive period for the emergence of prefrontal cognitive deficits in response to HFD. We establish that the synaptic modulator reelin (RELN) is a critical mediator of this vulnerability because (1) periadolescent HFD (pHFD) selectively downregulates prefrontal RELN+ cells and (2) augmenting mPFC RELN levels using transgenesis or prefrontal pharmacology prevents the pHFD-induced prefrontal cognitive deficits. We further identify N-methyl-d-aspartate-dependent long-term depression (NMDA-LTD) at prefrontal excitatory synapses as a synaptic signature of this association because pHFD abolishes NMDA-LTD, a function that is restored by RELN overexpression. We believe this study provides the first mechanistic insight into the vulnerability of the adolescent mPFC towards nutritional stress, such as HFDs. Our findings have primary relevance to obese individuals who are at an increased risk of developing neurological cognitive comorbidities, and may extend to multiple neuropsychiatric and neurological disorders in which RELN deficiency is a common feature.

Summary The nucleus accumbens (NAc) plays a key role in drug-related behavior and natural reward learning. Synaptic plasticity in dopamine D1 and D2 receptor medium spiny neurons (MSNs) of the NAc and the endogenous cannabinoid (eCB) system have been implicated in reward seeking. However, the precise molecular and physiological basis of reward-seeking behavior remains unknown. We found that the specific deletion of metabotropic glutamate receptor 5 (mGluR5) in D1-expressing MSNs (D1miRmGluR5 mice) abolishes eCB-mediated long-term depression (LTD) and prevents the expression of drug (cocaine and ethanol), natural reward (saccharin), and brain-stimulation-seeking behavior. In vivo enhancement of 2-arachidonoylglycerol (2-AG) eCB signaling within the NAc core restores both eCB-LTD and reward-seeking behavior in D1miRmGluR5 mice. The data suggest a model where the eCB and glutamatergic systems of the NAc act in concert to mediate reward-seeking responses., Graphical Abstract, Highlights • mGluR5-D1-CB1-induced eCB-LTD mediates drugs of abuse and natural reward seeking • eCB-LTD in D2-MSNs plays no important role in processing of reward-seeking responses • Loss of eCB-LTD is a consequence of higher MAGL activity and lower CB1R expression • Acute drug administration stops craving for alternative rewards on following days, Neuroscience; Behavioral Neuroscience; Molecular Neuroscience

Heavy cannabis consumption among adolescents is associated with significant and lasting neurobiological, psychological and health consequences that depend on the age of first use. Chronic exposure to cannabinoid (CB) agonists during adolescence alters social behavior and prefrontal cortex (PFC) activity in adult rats. However, sex differences on social behavior as well as PFC synaptic plasticity after acute CB activation remain poorly explored. Here, we determined the consequences of a single CB activation differently affects PFC in males and females by assessing social behavior and PFC neuronal and synaptic functions in rats during pubertal or adulthood periods, 24h after a single in-vivo cannabinoid exposure (SCE). During puberty, SCE reduced play behavior in females but not males. In contrast, SCE impaired sociability in both sexes at adulthood. General exploration and memory recognition remained normal at both ages and both sexes. At the synaptic level, SCE ablated endocannabinoid-mediated long-term depression (eCB-LTD) in the PFC of females of both ages and heightened excitability of PFC pyramidal neurons at adulthood, while males were spared. In contrast, SCE was associated to impaired long-term potentiation in adult males. Together, the data indicate behavioral and synaptic sex differences in response to a single in-vivo exposure to cannabinoid at puberty and adulthood.

International audience; Cannabinoids can cross the placenta, thus may interfere with fetal endocannabinoid signaling during neurodevelopment, causing long-lasting deficits. Despite increasing reports of cannabis consumption during pregnancy, the protracted consequences of prenatal cannabinoid exposure (PCE) remain incompletely understood. Here, we report sex-specific differences in behavioral and neuronal deficits in the adult progeny of rat dams exposed to low doses of cannabinoids during gestation. In males, PCE reduced social interaction, ablated endocannabinoid long-term depression (LTD) and heightened excitability of prefrontal cortex pyramidal neurons, while females were spared. Group 1 mGluR and endocannabinoid signaling regulate emotional behavior and synaptic plasticity. Notably, sex-differences following PCE included levels of mGluR1/ 5 and TRPV1R mRNA. Finally, positive allosteric modulation of mGlu5 and enhancement of anandamide levels restored LTD and social interaction in PCE adult males. Together, these results highlight marked sexual differences in the effects of PCE and introduce strategies for reversing detrimental effects of PCE.

We investigated how metabotropic Acetylcholine receptors control excitatory synaptic plasticity in the mouse nucleus accumbens core. Pharmacological and genetic approaches revealed that M1 mAChRs trigger multiple and interacting forms of synaptic plasticity. As previously described in the dorsal striatum, moderate pharmacological activation of M1 mAChR potentiated postsynaptic NMDARs. The M1-potentiation of NMDAR masked a previously unknown coincident TRPV1-mediated long-term depression (LTD). In addition, strong pharmacological activation of M1 mAChR induced canonical retrograde LTD, mediated by presynaptic CB1R. In the fmr1-/y mouse model of Fragile X, we found that CB1R but not TRPV1 M1-LTD was impaired. Finally, pharmacological blockade of the degradation of anandamide and 2-arachidonylglycerol, the two principal eCBs restored fmr1-/y LTD to wild type levels. These findings shed new lights on the complex influence of Acetylcholine on excitatory synapses in the nucleus accumbens core and identify new substrates of the synaptic deficits of Fragile X.

Cannabinoids can cross the placenta, thus may interfere with fetal endocannabinoid signaling during neurodevelopment, causing long-lasting deficits. Despite increasing cannabis consumption during pregnancy, the protracted consequences of prenatal cannabinoid exposure (PCE) remain incompletely understood. Here we report sex-specific differences in behavioral and neuronal deficits in the adult progeny of rat dams exposed to low doses of cannabinoids during gestation. In males, PCE reduced social interaction, ablated endocannabinoid long-term depression (LTD) and heightened excitability of prefrontal cortex pyramidal neurons, while females were spared. Group 1 mGluR and endocannabinoid signaling regulate emotional behavior and synaptic plasticity. Notably, sex-differences following PCE included levels of mGluR1/5 and TRPV1R mRNA. Finally, positive allosteric modulation of mGlu5 and enhancement of anandamide levels restored LTD and social interaction in PCE adult males. Together, these results highlight marked sexual differences in the effects of PCE and introduce strategies for reversing detrimental effects of PCE.

The final maturation of the prefrontal cortex (PFC) continues into early adulthood and is delayed compared with other forebrain structures. However, how these late onset changes in the PFC relate to neurodevelopment disorders is poorly understood. Fragile X syndrome (FXS) is a prevalent neurogenetic disorder linked to deficits in PFC function. mGlu5 is an important molecular hub in the etiology of FXS. Thus we have examined changes in mGlu5 function in the PFC in a mouse model of FXS (Fmr1 knockout) during early adulthood and subsequent maturity. An unusual endophenotype was identified; during early adulthood (2-month-old) Fmr1 knockout mice show a severe deficit in mGlu5 dependent eCB synaptic plasticity; however, in 1-year-old this deficit self rectifies. This adulthood onset correction in mGlu5 function is linked to an engagement of TRPV1 receptors in 1-year-old mice. In 2-month-old Fmr1 knockout mice, mGlu5 mediated synaptic plasticity could be recovered with eCB system targeted drugs, but also by direct enhancement of mGlu5 function with a positive allosteric modulator. These results point to further refinements to the role of mGlu5 in FXS. Furthermore our findings suggest when studying neurodevelopmental disorders with a significant PFC phenotype consideration of late onset changes may be important.

The most common inherited monogenetic cause of intellectual disability is Fragile X syndrome (FXS). The clinical symptoms of FXS evolve with age during adulthood; however, neurophysiological data exploring this phenomenon are limited. The Fmr1 knockout (Fmr1KO) mouse models FXS, but studies in these mice of prefrontal cortex (PFC) function are underrepresented, and aging linked data are absent. We studied synaptic physiology and activity-dependent synaptic plasticity in the medial PFC of Fmr1KO mice from 2 to 12 months. In young adult Fmr1KO mice, NMDA receptor (NMDAR)-mediated long-term potentiation (LTP) is intact; however, in 12-month-old mice this LTP is impaired. In parallel, there was an increase in the AMPAR/NMDAR ratio and a concomitant decrease of synaptic NMDAR currents in 12-month-old Fmr1KO mice. We found that acute pharmacological blockade of mGlu5 receptor in 12-month-old Fmr1KO mice restored a normal AMPAR/NMDAR ratio and LTP. Taken together, the data reveal an age-dependent deficit in LTP in Fmr1KO mice, which may correlate to some of the complex age-related deficits in FXS.

Energy-dense, yet nutritionally poor food is a high-risk factor for mental health disorders. This is of particular concern during adolescence, a period often associated with increased consumption of low nutritional content food and higher prevalence of mental health disorders. Indeed, there is an urgent need to understand the mechanisms linking unhealthy diet and mental disorders. Deficiency in n-3 polyunsaturated fatty acids (PUFAs) is a hallmark of poor nutrition and mood disorders. Here, we developed a mouse model of n-3 PUFA deficiency lasting from adolescence into adulthood. Starting nutritional deficits in dietary n-3 PUFAs during adolescence decreased n-3 PUFAs in both medial prefrontal cortex (mPFC) and nucleus accumbens, increased anxiety-like behavior, and decreased cognitive function in adulthood. Importantly, we discovered that endocannabinoid/mGlu5-mediated LTD in the mPFC and accumbens was abolished in adult n-3-deficient mice. Additionally, mPFC NMDAR-dependent LTP was also lacking in the n-3-deficient group. Pharmacological enhancement of the mGlu5/eCB signaling complex, by positive allosteric modulation of mGlu5 or inhibition of endocannabinoid 2-arachidonylglycerol degradation, fully restored synaptic plasticity and normalized emotional and cognitive behaviors in malnourished adult mice. Our data support a model where nutrition is a key environmental factor influencing the working synaptic range into adulthood, long after the end of the perinatal period. These findings have important implications for the identification of nutritional risk factors for disease and design of new treatments for the behavioral deficits associated with nutritional n-3 PUFA deficiency.SIGNIFICANCE STATEMENT In a mouse model mimicking n-3 PUFA dietary deficiency during adolescence and adulthood, we found strong increases in anxiety and anhedonia which lead to decreases in specific cognitive functions in adulthood. We found that endocannabinoid/mGlu5-mediated LTD and NMDAR-dependent LTP were lacking in adult n-3-deficient mice. Acute positive allosteric modulation of mGlu5 or inhibition of endocannabinoid degradation normalized behaviors and synaptic functions in n-3 PUFA-deficient adult mice. These findings have important implications for the identification of nutritional risk for disease and the design of new treatments for the behavioral deficits associated with nutritional n-3 PUFAs' imbalance.

Defective brain extracellular matrix (ECM) is a factor of vulnerability in various psychiatric diseases such as schizophrenia, depression and autism. The glycoprotein reelin is an essential building block of the brain ECM that modulates neuronal development and participates to the functions of adult central synapses. The reelin gene (RELN) is a strong candidate in psychiatric diseases of early onset, but its synaptic and behavioral functions in juvenile brain circuits remain unresolved. Here, we found that in juvenile reelin-haploinsufficient heterozygous reeler mice (HRM), abnormal fear memory erasure is concomitant to reduced dendritic spine density and anomalous long-term potentiation in the prefrontal cortex. In juvenile HRM, a single in vivo injection with ketamine or Ro25-6981 to inhibit GluN2B-N-methyl-𝒟-aspartate receptors (NMDARs) restored normal spine density, synaptic plasticity and converted fear memory to an erasure-resilient state typical of adult rodents. The functional and behavioral rescue by ketamine was prevented by rapamycin, an inhibitor of the mammalian target of rapamycin pathway. Finally, we show that fear memory erasure persists until adolescence in HRM and that a single exposure to ketamine during the juvenile period reinstates normal fear memory in adolescent mice. Our results show that reelin is essential for successful structural, functional and behavioral development of juvenile prefrontal circuits and that this developmental period provides a critical window for therapeutic rehabilitation with GluN2B-NMDAR antagonists.

Social play behavior is a highly rewarding, developmentally important form of social interaction in young mammals. However, its neurobiological underpinnings remain incompletely understood. Previous work has suggested that opioid and endocannabinoid neurotransmission interact in the modulation of social play. Therefore, we combined behavioral, pharmacological, electrophysiological and genetic approaches to elucidate the role of the endocannabinoid 2-arachidonoylglycerol (2-AG) in social play, and how cannabinoid and opioid neurotransmission interact to control social behavior in adolescent rodents. Systemic administration of the 2-AG hydrolysis inhibitor JZL184 or the opioid receptor agonist morphine increased social play behavior in adolescent rats. These effects were blocked by systemic pretreatment with either CB1 cannabinoid receptor (CB1R) or mu-opioid receptor (MOR) antagonists. The social play-enhancing effects of systemic morphine or JZL184 treatment were also prevented by direct infusion of the CB1R antagonist SR141716 and the MOR antagonist naloxone into the nucleus accumbens core (NAcC). Searching for synaptic correlates of these effects in adolescent NAcC excitatory synapses, we observed that CB1R antagonism blocked the effect of the MOR agonist DAMGO and, conversely, that naloxone reduced the effect of a cannabinoid agonist. These results were recapitulated in mice, and completely abolished in CB1R and MOR knockout mice, suggesting that the functional interaction between CB1R and MOR in the NAcC in the modulation of mediates social behavior is widespread in rodents. The data shed new light on the mechanism by which endocannabinoid lipids and opioid peptides interact to orchestrate rodent socioemotional behaviors.

Energy-dense, yet nutritionally poor food is a high-risk factor for mental health disorders. This is of particular concern during adolescence, a period often associated with increased consumption of low nutritional content food and higher prevalence of mental health disorders. Indeed, there is an urgent need to understand the mechanisms linking unhealthy diet and mental disorders. Deficiency in n-3 polyunsaturated fatty acids (PUFAs) is a hallmark of poor nutrition and mood disorders. Here, we developed a mouse model of n-3 PUFA deficiency lasting from adolescence into adulthood. Starting nutritional deficits in dietary n-3 PUFAs during adolescence decreased n-3 PUFAs in both medial prefrontal cortex (mPFC) and nucleus accumbens, increased anxiety-like behavior, and decreased cognitive function in adulthood. Importantly, we discovered that endocannabinoid/mGlu(5)-mediated LTD in the mPFC and accumbens was abolished in adult n-3-deficient mice. Additionally, mPFC NMDAR-dependent LTP was also lacking in the n-3-deficient group. Pharmacological enhancement of the mGlu(5)/eCB signaling complex, by positive allosteric modulation of mGlu(5) or inhibition of endocannabinoid 2-arachidonylglycerol degradation, fully restored synaptic plasticity and normalized emotional and cognitive behaviors in malnourished adult mice. Our data support a model where nutrition is a key environmental factor influencing the working synaptic range into adulthood, long after the end of the perinatal period. These findings have important implications for the identification of nutritional risk factors for disease and design of new treatments for the behavioral deficits associated with nutritional n-3 PUFA deficiency. SIGNIFICANCE STATEMENT In a mouse model mimicking n-3 PUFA dietary deficiency during adolescence and adulthood, we found strong increases in anxiety and anhedonia which lead to decreases in specific cognitive functions in adulthood. We found that endocannabinoid/mGlu(5)-mediated LTD and NMDAR-dependent LTP were lacking in adult n-3-deficient mice. Acute positive allosteric modulation of mGlu(5) or inhibition of endocannabinoid degradation normalized behaviors and synaptic functions in n-3 PUFA-deficient adult mice. These findings have important implications for the identification of nutritional risk for disease and the design of new treatments for the behavioral deficits associated with nutritional n-3 PUFAs' imbalance.

The postnatal maturation of the prefrontal cortex (PFC) represents a period of increased vulnerability to risk factors and emergence of neuropsychiatric disorders. To disambiguate the pathophysiological mechanisms contributing to these disorders, we revisited the endophenotype approach from a developmental viewpoint. The extracellular matrix protein reelin which contributes to cellular and network plasticity, is a risk factor for several psychiatric diseases. We mapped the aggregate effect of the RELN risk allele on postnatal development of PFC functions by cross-sectional synaptic and behavioral analysis of reelin-haploinsufficient mice. Multivariate analysis of bootstrapped datasets revealed subgroups of phenotypic traits specific to each maturational epoch. The preeminence of synaptic AMPA/NMDA receptor content to pre-weaning and juvenile endophenotypes shifts to long-term potentiation and memory renewal during adolescence followed by NMDA-GluN2B synaptic content in adulthood. Strikingly, multivariate analysis shows that pharmacological rehabilitation of reelin haploinsufficient dysfunctions is mediated through induction of new endophenotypes rather than reversion to wild-type traits. By delineating previously unknown developmental endophenotypic sequences, we conceived a promising general strategy to disambiguate the molecular underpinnings of complex psychiatric disorders and for the rational design of pharmacotherapies in these disorders.

Adolescence is characterized by drastic behavioral adaptations and comprises a particularly vulnerable period for the emergence of various psychiatric disorders. Growing evidence reveals that the pathophysiology of these disorders might derive from aberrations of normal neurodevelopmental changes in the adolescent brain. Understanding the molecular underpinnings of adolescent behavior is therefore critical for understanding the origin of psychopathology, but the molecular mechanisms that trigger adolescent behavior are unknown. Here, we hypothesize that the cannabinoid type-1 receptor (CB1R) may play a critical role in mediating adolescent behavior because enhanced endocannabinoid (eCB) signaling has been suggested to occur transiently during adolescence. To study enhanced CB1R signaling, we introduced a missense mutation (F238L) into the ratCnr1gene that encodes for the CB1R. According to our hypothesis, rats with the F238L mutation (Cnr1F238L) should sustain features of adolescent behavior into adulthood. Gain of function of the mutated receptor was demonstrated byin silicomodeling and was verified functionally in a series of biochemical and electrophysiological experiments. Mutant rats exhibit an adolescent-like phenotype during adulthood compared with wild-type littermates, with typical high risk/novelty seeking, increased peer interaction, enhanced impulsivity, and augmented reward sensitivity for drug and nondrug reward. Partial inhibition of CB1R activity inCnr1F238Lmutant rats normalized behavior and led to a wild-type phenotype. We conclude that the activity state and functionality of the CB1R is critical for mediating adolescent behavior. These findings implicate the eCB system as an important research target for the neuropathology of adolescent-onset mental health disorders.SIGNIFICANCE STATEMENTWe present the first rodent model with a gain-of-function mutation in the cannabinoid type-1 receptor (CB1R). Adult mutant rats exhibit an adolescent-like phenotype with typical high risk seeking, impulsivity, and augmented drug and nondrug reward sensitivity. Adolescence is a critical period for suboptimal behavioral choices and the emergence of neuropsychiatric disorders. Understanding the basis of these disorders therefore requires a comprehensive knowledge of how adolescent neurodevelopment triggers behavioral reactions. Our behavioral observations in adult mutant rats, together with reports on enhanced adolescent CB1R signaling, suggest a pivotal role for the CB1R in an adolescent brain as an important molecular mediator of adolescent behavior. These findings implicate the endocannabinoid system as a notable research target for adolescent-onset mental health disorders.

Fragile X is the most common cause of inherited intellectual disability and a leading cause of autism. The disease is caused by mutation of a single X-linked gene called fmr1 that codes for the Fragile X mental retardation protein (FMRP), a 71 kDa protein, which acts mainly as a translation inhibitor. Fragile X patients suffer from cognitive and emotional deficits that coincide with abnormalities in dendritic spines. Changes in spine morphology are often associated with altered excitatory transmission and long-term plasticity, the most prominent deficit in fmr1-/y mice. The nucleus accumbens, a central part of the mesocortico-limbic reward pathway, is now considered as a core structure in the control of social behaviors. Although the socio-affective impairments observed in Fragile X suggest dysfunctions in the accumbens, the impact of the lack of FMRP on accumbal synapses has scarcely been studied. Here we report for the first time a new spike timing-dependent plasticity paradigm that reliably triggers NMDAR-dependent long-term potentiation (LTP) of excitatory afferent inputs of medium spiny neurons (MSN) in the nucleus accumbens core region. Notably, we discovered that this LTP was completely absent in fmr1-/y mice. In the fmr1-/y accumbens intrinsic membrane properties of MSNs and basal excitatory neurotransmission remained intact in the fmr1-/y accumbens but the deficit in LTP was accompanied by an increase in evoked AMPA/NMDA ratio and a concomitant reduction of spontaneous NMDAR-mediated currents. In agreement with these physiological findings, we found significantly more filopodial spines in fmr1-/y mice by using an ultrastructural electron microscopic analysis of accumbens core medium spiny neuron spines. Surprisingly, spine elongation was specifically due to the longer longitudinal axis and larger area of spine necks, whereas spine head morphology and postsynaptic density size on spine heads remained unaffected in the fmr1-/y accumbens. These findings together reveal new structural and functional synaptic deficits in Fragile X.

The geneDyrk1ais the mammalian ortholog of Drosophilaminibrain. Dyrk1alocalizes in the Down syndrome (DS) critical region of chromosome 21q22.2 and is a major candidate for the behavioral and neuronal abnormalities associated with DS. PFC malfunctions are a common denominator in several neuropsychiatric diseases, including DS, but the contribution of DYRK1A in PFC dysfunctions, in particular the synaptic basis for impairments of executive functions reported in DS patients, remains obscure. We quantified synaptic plasticity, biochemical synaptic markers, and dendritic morphology of deep layer pyramidal PFC neurons in adult mBACtgDyrk1a transgenic mice that overexpressDyrk1aunder the control of its own regulatory sequences. We found that overexpression of Dyrk1a largely increased the number of spines on oblique dendrites of pyramidal neurons, as evidenced by augmented spine density, higher PSD95 protein levels, and larger miniature EPSCs. The dendritic alterations were associated with anomalous NMDAR-mediated long-term potentiation and accompanied by a marked reduction in the pCaMKII/CaMKII ratio in mBACtgDyrk1a mice. Retrograde endocannabinoid-mediated long-term depression (eCB-LTD) was ablated in mBACtgDyrk1a mice. Administration of green tea extracts containing epigallocatechin 3-gallate, a potent DYRK1A inhibitor, to adult mBACtgDyrk1a mice normalized long-term potentiation and spine anomalies but not eCB-LTD. However, inhibition of the eCB deactivating enzyme monoacylglycerol lipase normalized eCB-LTD in mBACtgDyrk1a mice. These data shed light on previously undisclosed participation of DYRK1A in adult PFC dendritic structures and synaptic plasticity. Furthermore, they suggest its involvement in DS-related endophenotypes and identify new potential therapeutic strategies.

Fragile X syndrome, the most commonly known genetic cause of autism, is due to loss of the fragile X mental retardation protein, which regulates signal transduction at metabotropic glutamate receptor-5 in the brain. Fragile X mental retardation protein deletion in mice enhances metabotropic glutamate receptor-5-dependent long-term depression in the hippocampus and cerebellum. Here we show that a distinct type of metabotropic glutamate receptor-5-dependent long-term depression at excitatory synapses of the ventral striatum and prefrontal cortex, which is mediated by the endocannabinoid 2-arachidonoyl-sn-glycerol, is absent in fragile X mental retardation protein-null mice. In these mutants, the macromolecular complex that links metabotropic glutamate receptor-5 to the 2-arachidonoyl-sn-glycerol-producing enzyme, diacylglycerol lipase-α (endocannabinoid signalosome), is disrupted and metabotropic glutamate receptor-5-dependent 2-arachidonoyl-sn-glycerol formation is compromised. These changes are accompanied by impaired endocannabinoid-dependent long-term depression. Pharmacological enhancement of 2-arachidonoyl-sn-glycerol signalling normalizes this synaptic defect and corrects behavioural abnormalities in fragile X mental retardation protein-deficient mice. The results identify the endocannabinoid signalosome as a molecular substrate for fragile X syndrome, which might be targeted by therapy., Fragile X syndrome is a major genetic cause of autism and is caused by loss of the fragile X mental retardation protein. In a mouse model of fragile X syndrome, Jung et al. show that an absence of neuronal endocannabinoid signalling is responsible for the neurophysiological and behavioural defects.

International audience; The reason why neurons synthesize more than one endocannabinoid (eCB) and how this is involved in the regulation of synaptic plasticity in a single neuron is not known. We found that 2-arachidonoylglycerol (2-AG) and anandamide mediate different forms of plasticity in the extended amygdala of rats. Dendritic L-type Ca(2+) channels and the subsequent release of 2-AG acting on presynaptic CB1 receptors triggered retrograde short-term depression. Long-term depression was mediated by postsynaptic mGluR5-dependent release of anandamide acting on postsynaptic TRPV1 receptors. In contrast, 2-AG/CB1R-mediated retrograde signaling mediated both forms of plasticity in the striatum. These data illustrate how the eCB system can function as a polymodal signal integrator to allow the diversification of synaptic plasticity in a single neuron.