Peri-pubertal emergence of UNC-5 homologue expression by dopamine neurons in rodents

Disproportionate neuroanatomical effects of DCC haploinsufficiency in adolescence compared with adulthood: links to dopamine, connectivity, covariance, and gene expression brain maps in mice

BACKGROUND

Critical adolescent neural refinement is controlled by the DCC (deleted in colorectal cancer) protein, a receptor for the netrin-1 guidance cue. We sought to describe the effects of reduced DCC on neuroanatomy in the adolescent and adult mouse brain.

METHODS

We examined neuronal connectivity, structural covariance, and molecular processes in a DCC-haploinsufficient mouse model, compared with wild-type mice, using new, custom analytical tools designed to leverage publicly available databases from the Allen Institute.

RESULTS

We included 11 DCC-haploinsufficient mice and 16 wild-type littermates. Neuroanatomical effects of DCC haploinsufficiency were more severe in adolescence than adulthood and were largely restricted to the mesocorticolimbic dopamine system. The latter finding was consistent whether we identified the regions of the mesocorticolimbic dopamine system a priori or used connectivity data from the Allen Brain Atlas to determine de novo where these dopamine axons terminated. Covariance analyses found that DCC haploinsufficiency disrupted the coordinated development of the brain regions that make up the mesocorticolimbic dopamine system. Gene expression maps pointed to molecular processes involving the expression of DCC, UNC5C (encoding DCC's co-receptor), and NTN1 (encoding its ligand, netrin-1) as underlying our structural findings.

LIMITATIONS

Our study involved a single sex (males) at only 2 ages.

CONCLUSION

The neuroanatomical phenotype of DCC haploinsufficiency described in mice parallels that observed in DCC-haploinsufficient humans. It is critical to understand the DCC-haploinsufficient mouse as a clinically relevant model system.

The scheduling of adolescence with Netrin-1 and UNC5C

Dopamine axons are the only axons known to grow during adolescence. Here, using rodent models, we examined how two proteins, Netrin-1 and its receptor, UNC5C, guide dopamine axons towards the prefrontal cortex and shape behaviour. We demonstrate in mice ( Mus musculus ) that dopamine axons reach the cortex through a transient gradient of Netrin-1 expressing cells - disrupting this gradient reroutes axons away from their target. Using a seasonal model (Siberian hamsters; Phodopus sungorus ) we find that mesocortical dopamine development can be regulated by a natural environmental cue (daylength) in a sexually dimorphic manner - delayed in males, but advanced in females. The timings of dopamine axon growth and UNC5C expression are always phase-locked. Adolescence is an ill-defined, transitional period; we pinpoint neurodevelopmental markers underlying this period.

Relationship between insulin and Netrin-1/DCC guidance cue pathway regulation in the prefrontal cortex of rodents exposed to prenatal dietary restriction

Fetal restriction (FR) alters insulin sensitivity, but it is unknown how the metabolic profile associated with restriction affects development of the dopamine (DA) system and DA-related behaviors. The Netrin-1/DCC guidance cue system participates in maturation of the mesocorticolimbic DA circuitry. Therefore, our objective was to identify if FR modifies Netrin-1/DCC receptor protein expression in the prefrontal cortex (PFC) at birth and mRNA in adulthood in rodent males. We used cultured HEK293 cells to assess if levels of miR-218, microRNA regulator of DCC, are sensitive to insulin. To assess this, pregnant dams were subjected to a 50% FR diet from gestational day 10 until birth. Medial PFC (mPFC) DCC/Netrin-1 protein expression was measured at P0 at baseline and Dcc/Netrin-1 mRNA levels were quantified in adults 15 min after a saline/insulin injection. miR-218 levels in HEK-293 cells were measured in response to insulin exposure. At P0, Netrin-1 levels are downregulated in FR animals in comparison to controls. In adult rodents, insulin administration results in an increase in Dcc mRNA levels in control but not FR rats. In HEK293 cells, there is a positive correlation between insulin concentration and miR-218 levels. Since miR-218 is a Dcc gene expression regulator and our in vitro results show that insulin regulates miR-218 levels, we suggest that FR-induced changes in insulin sensitivity could be affecting Dcc expression via miR-218, impacting DA system maturation and organization. As fetal adversity is linked to nonadaptive behaviors later in life, this may contribute to early identification of vulnerability to chronic diseases associated with fetal adversity.

Amphetamine disrupts dopamine axon growth in adolescence by a sex-specific mechanism in mice

Initiating drug use during adolescence increases the risk of developing addiction or other psychopathologies later in life, with long-term outcomes varying according to sex and exact timing of use. The cellular and molecular underpinnings explaining this differential sensitivity to detrimental drug effects remain unexplained. The Netrin-1/DCC guidance cue system segregates cortical and limbic dopamine pathways in adolescence. Here we show that amphetamine, by dysregulating Netrin-1/DCC signaling, triggers ectopic growth of mesolimbic dopamine axons to the prefrontal cortex, only in early-adolescent male mice, underlying a male-specific vulnerability to enduring cognitive deficits. In adolescent females, compensatory changes in Netrin-1 protect against the deleterious consequences of amphetamine on dopamine connectivity and cognitive outcomes. Netrin-1/DCC signaling functions as a molecular switch which can be differentially regulated by the same drug experience as function of an individual's sex and adolescent age, and lead to divergent long-term outcomes associated with vulnerable or resilient phenotypes.

Netrin-1 regulates the balance of synaptic glutamate signaling in the adult ventral tegmental area

The axonal guidance cue netrin-1 serves a critical role in neural circuit development by promoting growth cone motility, axonal branching, and synaptogenesis. Within the adult mouse brain, expression of the gene encoding (Ntn1) is highly enriched in the ventral midbrain where it is expressed in both GABAergic and dopaminergic neurons, but its function in these cell types in the adult system remains largely unknown. To address this, we performed viral-mediated, cell-type specific CRISPR-Cas9 mutagenesis of Ntn1 in the ventral tegmental area (VTA) of adult mice. Ntn1 loss-of-function in either cell type resulted in a significant reduction in excitatory postsynaptic connectivity. In dopamine neurons, the reduced excitatory tone had a minimal phenotypic behavioral outcome; however, reduced glutamatergic tone on VTA GABA neurons induced behaviors associated with a hyperdopaminergic phenotype. Simultaneous loss of Ntn1 function in both cell types largely rescued the phenotype observed in the GABA-only mutagenesis. These findings demonstrate an important role for Ntn1 in maintaining excitatory connectivity in the adult midbrain and that a balance in this connectivity within two of the major cell types of the VTA is critical for the proper functioning of the mesolimbic system.

Reduced dopamine release in Dcc haploinsufficiency male mice abolishes the rewarding effects of cocaine but not those of morphine and ethanol

RATIONALE

The Netrin-1/DCC guidance cue pathway is critically involved in the adolescent organization of the mesocorticolimbic dopamine circuitry. Adult mice heterozygous for Dcc show reduced dopamine release in the nucleus accumbens in response to amphetamine and, in turn, blunted sensitivity to the rewarding effects of this drug.

OBJECTIVE

Here, we tested whether the protective effects of Dcc haploinsufficiency are specific to stimulant drugs of abuse or instead extrapolate to opioids and ethanol.

METHODS

We used the place preference paradigm to measure the rewarding effects of cocaine (20 mg/kg), morphine (5 or 10 mg/Kg), or ethanol (20%) in adult (PND 75) male Dcc haploinsufficient mice or their wild-type litter mates. In a second experiment, we compared in these two genotypes, in vivo dopamine release in the nucleus accumbens after a single i.p. injection of morphine (10 mg/kg).

RESULTS

We found reduced morphine-induced dopamine release in the nucleus accumbens of Dcc haploinsufficient male mice, but, contrary to the effects of stimulant drugs, there is no effect of genotype on morphine-induced conditioned preference.

CONCLUSION

These findings show that reduced drug-induced mesolimbic dopamine in Dcc haploinsufficient male mice protects specifically against the rewarding effects of stimulant drugs, but not against the rewarding properties of morphine and ethanol. These results suggest that these drugs exert their rewarding effect via different brain circuits.

The role of dopamine and endocannabinoid systems in prefrontal cortex development: Adolescence as a critical period

The prefrontal cortex plays a central role in the control of complex cognitive processes including action control and decision making. It also shows a specific pattern of delayed maturation related to unique behavioral changes during adolescence and allows the development of adult cognitive processes. The adolescent brain is extremely plastic and critically vulnerable to external insults. Related to this vulnerability, adolescence is also associated with the emergence of numerous neuropsychiatric disorders involving alterations of prefrontal functions. Within prefrontal microcircuits, the dopamine and the endocannabinoid systems have widespread effects on adolescent-specific ontogenetic processes. In this review, we highlight recent advances in our understanding of the maturation of the dopamine system and the endocannabinoid system in the prefrontal cortex during adolescence. We discuss how they interact with GABA and glutamate neurons to modulate prefrontal circuits and how they can be altered by different environmental events leading to long-term neurobiological and behavioral changes at adulthood. Finally, we aim to identify several future research directions to help highlight gaps in our current knowledge on the maturation of these microcircuits.

Adolescent self-administration of the synthetic cannabinoid receptor agonist JWH-018 induces neurobiological and behavioral alterations in adult male mice

RATIONALE

The use of synthetic cannabinoid receptor agonists (SCRAs) is growing among adolescents, posing major medical and psychiatric risks. JWH-018 represents the reference compound of SCRA-containing products.

OBJECTIVES

This study was performed to evaluate the enduring consequences of adolescent voluntary consumption of JWH-018.

METHODS

The reinforcing properties of JWH-018 were characterized in male CD1 adolescent mice by intravenous self-administration (IVSA). Afterwards, behavioral, neurochemical, and molecular evaluations were performed at adulthood.

RESULTS

Adolescent mice acquired operant behavior (lever pressing, Fixed Ratio 1-3; 7.5 µg/kg/inf); this behavior was specifically directed at obtaining JWH-018 since it increased under Progressive Ratio schedule of reinforcement, and was absent in vehicle mice. JWH-018 IVSA was reduced by pretreatment of the CB1-antagonist/inverse agonist AM251. Adolescent exposure to JWH-018 by IVSA increased, at adulthood, both nestlet shredding and marble burying phenotypes, suggesting long-lasting repetitive/compulsive-like behavioral effects. JWH-018 did not affect risk proclivity in the wire-beam bridge task. In adult brains, there was an increase of ionized calcium binding adaptor molecule 1 (IBA-1) positive cells in the caudate-putamen (CPu) and nucleus accumbens (NAc), along with a decrease of glial fibrillary acidic protein (GFAP) immunoreactivity in the CPu. These glial alterations in adult brains were coupled with an increase of the chemokine RANTES and a decrease of the cytokines IL2 and IL13 in the cortex, and an increase of the chemokine MPC1 in the striatum.

CONCLUSIONS

This study suggests for the first time that male mice self-administer the prototypical SCRA JWH-018 during adolescence. The adolescent voluntary consumption of JWH-018 leads to long-lasting behavioral and neurochemical aberrations along with glia-mediated inflammatory responses in adult brains.

UNC5C Receptor Proteolytic Cleavage by Active AEP Promotes Dopaminergic Neuronal Degeneration in Parkinson's Disease

Netrin-1 is a chemotropic cue mediating axon growth and neural migration in neuronal development, and its receptors deletion in colorectal cancer and UNC5s act as dependence receptors regulating neuronal apoptosis. Asparagine endopeptidase (AEP) is an age-dependent protease that cuts human alpha-synuclein (α-Syn) at N103 and triggers its aggregation and neurotoxicity. In the current study, it is reported that UNC5C receptor is cleaved by AEP in Parkinson's disease (PD) and facilitates dopaminergic neuronal loss. UNC5C is truncated by active AEP in human α-SNCA transgenic mice in an age-dependent manner or induced by neurotoxin rotenone. Moreover, UNC5C is fragmented by AEP in PD brains, inversely correlated with reduced netrin-1 levels. Netrin-1 deprivation in primary cultures induces AEP and caspase-3 activation, triggering UNC5C proteolytic fragmentation and enhancing neuronal loss. Noticeably, blocking UNC5C cleavage by AEP attenuates netrin-1 deprivation-elicited neuronal death and motor disorders in netrin flox/flox mice. Overexpression of AEP-truncated UNC5C intracellular fragment strongly elicits α-Syn aggregation and dopaminergic loss, locomotor deficits in α-SNCA transgenic mice. Hence, the findings demonstrate that netrin-1 reduction and UNC5C truncation by AEP contribute to PD pathogenesis.

Mesocorticolimbic Dopamine Pathways Across Adolescence: Diversity in Development

Mesocorticolimbic dopamine circuity undergoes a protracted maturation during adolescent life. Stable adult levels of behavioral functioning in reward, motivational, and cognitive domains are established as these pathways are refined, however, their extended developmental window also leaves them vulnerable to perturbation by environmental factors. In this review, we highlight recent advances in understanding the mechanisms underlying dopamine pathway development in the adolescent brain, and how the environment influences these processes to establish or disrupt neurocircuit diversity. We further integrate these recent studies into the larger historical framework of anatomical and neurochemical changes occurring during adolescence in the mesocorticolimbic dopamine system. While dopamine neuron heterogeneity is increasingly appreciated at molecular, physiological, and anatomical levels, we suggest that a developmental facet may play a key role in establishing vulnerability or resilience to environmental stimuli and experience in distinct dopamine circuits, shifting the balance between healthy brain development and susceptibility to psychiatric disease.

The Netrin-1/DCC Guidance Cue Pathway as a Molecular Target in Depression: Translational Evidence

The Netrin-1/DCC guidance cue pathway plays a critical role in guiding growing axons toward the prefrontal cortex during adolescence and in the maturational organization and adult plasticity of prefrontal cortex connectivity. In this review, we put forward the idea that alterations in prefrontal cortex architecture and function, which are intrinsically linked to the development of major depressive disorder, originate in part from the dysregulation of the Netrin-1/DCC pathway by a mechanism that involves microRNA-218. We discuss evidence derived from mouse models of stress and from human postmortem brain and genome-wide association studies indicating an association between the Netrin-1/DCC pathway and major depressive disorder. We propose a potential role of circulating microRNA-218 as a biomarker of stress vulnerability and major depressive disorder.

DCC-related developmental effects of abused- versus therapeutic-like amphetamine doses in adolescence

The guidance cue receptor DCC controls mesocortical dopamine development in adolescence. Repeated exposure to an amphetamine regimen of 4 mg/kg during early adolescence induces, in male mice, downregulation of DCC expression in dopamine neurons by recruiting the Dcc microRNA repressor, microRNA-218 (miR-218). This adolescent amphetamine regimen also disrupts mesocortical dopamine connectivity and behavioral control in adulthood. Whether low doses of amphetamine in adolescence induce similar molecular and developmental effects needs to be established. Here, we quantified plasma amphetamine concentrations in early adolescent mice following a 4 or 0.5 mg/kg dose and found peak levels corresponding to those seen in humans following recreational and therapeutic settings, respectively. In contrast to the high doses, the low amphetamine regimen does not alter Dcc mRNA or miR-218 expression; instead, it upregulates DCC protein levels. Furthermore, high, but not low, drug doses downregulate the expression of the DCC receptor ligand, Netrin-1, in the nucleus accumbens and prefrontal cortex. Exposure to the low-dose regimen did not alter the expanse of mesocortical dopamine axons or their number/density of presynaptic sites in adulthood. Strikingly, adolescent exposure to the low-dose drug regimen does not impair behavioral inhibition in adulthood; instead, it induces an overall increase in performance in a go/no-go task. These results show that developmental consequences of exposure to therapeutic- versus abused-like doses of amphetamine in adolescence have dissimilar molecular signatures and opposite behavioral effects. These findings have important clinical relevance since amphetamines are widely used for therapeutic purposes in youth.

Dopamine Axon Targeting in the Nucleus Accumbens in Adolescence Requires Netrin-1

The fine arrangement of neuronal connectivity during development involves the coordinated action of guidance cues and their receptors. In adolescence, the dopamine circuitry is still developing, with mesolimbic dopamine axons undergoing target-recognition events in the nucleus accumbens (NAcc), while mesocortical projections continue to grow toward the prefrontal cortex (PFC) until adulthood. This segregation of mesolimbic versus mesocortical dopamine pathways is mediated by the guidance cue receptor DCC, which signals dopamine axons intended to innervate the NAcc to recognize this region as their final target. Whether DCC-dependent mesolimbic dopamine axon targeting in adolescence requires the action of its ligand, Netrin-1, is unknown. Here we combined shRNA strategies, quantitative analysis of pre- and post-synaptic markers of neuronal connectivity, and pharmacological manipulations to address this question. Similar to DCC levels in the ventral tegmental area, Netrin-1 expression in the NAcc is dynamic across postnatal life, transitioning from high to low expression across adolescence. Silencing Netrin-1 in the NAcc in adolescence results in an increase in the expanse of the dopamine input to the PFC in adulthood, with a corresponding increase in the number of presynaptic dopamine sites. This manipulation also results in altered dendritic spine density and morphology of medium spiny neurons in the NAcc in adulthood and in reduced sensitivity to the behavioral activating effects of the stimulant drug of abuse, amphetamine. These cellular and behavioral effects mirror those induced by Dcc haploinsufficiency within dopamine neurons in adolescence. Dopamine targeting in adolescence requires the complementary interaction between DCC receptors in mesolimbic dopamine axons and Netrin-1 in the NAcc. Factors regulating either DCC or Netrin-1 in adolescence can disrupt mesocorticolimbic dopamine development, rendering vulnerability or protection to phenotypes associated with psychiatric disorders.

The Netrin-1/DCC guidance system: dopamine pathway maturation and psychiatric disorders emerging in adolescence

Axon guidance molecules direct growing axons toward their targets, assembling the intricate wiring of the nervous system. One of these molecules, Netrin-1, and its receptor, DCC (deleted in colorectal cancer), has profound effects, in laboratory animals, on the adolescent expansion of mesocorticolimbic pathways, particularly dopamine. Now, a rapidly growing literature suggests that (1) these same alterations could occur in humans, and (2) genetic variants in Netrin-1 and DCC are associated with depression, schizophrenia, and substance use. Together, these findings provide compelling evidence that Netrin-1 and DCC influence mesocorticolimbic-related psychopathological states that emerge during adolescence.

Partial Reconstruction of the Nigrostriatal Circuit along a Preformed Molecular Guidance Pathway

The overall goal of our research is to establish a preformed molecular guidance pathway to direct the growth of dopaminergic axons from embryonic ventral mesencephalon (VM), tissue placed within the substantia nigra (SN), into the striatum to reconstruct the nigrostriatal pathway in a hemi-Parkinson's disease rat model. Guidance pathways were prepared by injecting lentivirus encoding either GFP or a combination of glial-cell-line-derived neurotrophic factor (GDNF) with either GDNF family receptor α1 (GFRα1) or netrin1. In another cohort of animals, adeno-associated virus (AAV) encoding brain-derived neurotrophic factor (BDNF) was injected within the striatum after guidance pathway formation. GDNF combined with either GFRα1 or netrin significantly increased growth of dopaminergic axons out of transplants and along the pathway, resulting in a significant reduction in the number of amphetamine-induced rotations. Retrograde tract tracing showed that the dopaminergic axons innervating the striatum were from A9 neurons within the transplant. Increased dopaminergic innervation of the striatum and improved behavioral recovery were observed with the addition of BDNF. Preformed guidance pathways using a combination of GDNF and netrin1 can be used to reconstruct the nigrostriatal pathway and improve motor recovery.

Mesocorticolimbic Connectivity and Volumetric Alterations in DCC Mutation Carriers

The axon guidance cue receptor DCC (deleted in colorectal cancer) plays a critical role in the organization of mesocorticolimbic pathways in rodents. To investigate whether this occurs in humans, we measured (1) anatomical connectivity between the substantia nigra/ventral tegmental area (SN/VTA) and forebrain targets, (2) striatal and cortical volumes, and (3) putatively associated traits and behaviors. To assess translatability, morphometric data were also collected in Dcc-haploinsufficient mice. The human volunteers were 20 DCC^+/- mutation carriers, 16 DCC^+/+ relatives, and 20 DCC^+/+ unrelated healthy volunteers (UHVs; 28 females). The mice were 11 Dcc^+/- and 16 wild-type C57BL/6J animals assessed during adolescence and adulthood. Compared with both control groups, the human DCC^+/- carriers exhibited the following: (1) reduced anatomical connectivity from the SN/VTA to the ventral striatum [DCC^+/+: p = 0.0005, r(effect size) = 0.60; UHV: p = 0.0029, r = 0.48] and ventral medial prefrontal cortex (DCC^+/+: p = 0.0031, r = 0.53; UHV: p = 0.034, r = 0.35); (2) lower novelty-seeking scores (DCC^+/+: p = 0.034, d = 0.82; UHV: p = 0.019, d = 0.84); and (3) reduced striatal volume (DCC^+/+: p = 0.0009, d = 1.37; UHV: p = 0.0054, d = 0.93). Striatal volumetric reductions were also present in Dcc^+/- mice, and these were seen during adolescence (p = 0.0058, d = 1.09) and adulthood (p = 0.003, d = 1.26). Together these findings provide the first evidence in humans that an axon guidance gene is involved in the formation of mesocorticolimbic circuitry and related behavioral traits, providing mechanisms through which DCC mutations might affect susceptibility to diverse neuropsychiatric disorders.SIGNIFICANCE STATEMENT Opportunities to study the effects of axon guidance molecules on human brain development have been rare. Here, the identification of a large four-generational family that carries a mutation to the axon guidance molecule receptor gene, DCC, enabled us to demonstrate effects on mesocorticolimbic anatomical connectivity, striatal volumes, and personality traits. Reductions in striatal volumes were replicated in DCC-haploinsufficient mice. Together, these processes might influence mesocorticolimbic function and susceptibility to diverse neuropsychiatric disorders.

Non-Contingent Exposure to Amphetamine in Adolescence Recruits miR-218 to Regulate Dcc Expression in the VTA

The development of the dopamine input to the medial prefrontal cortex occurs during adolescence and is a process that is vulnerable to disruption by stimulant drugs such as amphetamine. We have previously linked the amphetamine-induced disruption of dopamine connectivity and prefrontal cortex maturation during adolescence to the downregulation of the Netrin-1 receptor, DCC, in dopamine neurons. However, how DCC expression in dopamine neurons is itself regulated is completely unknown. MicroRNA (miRNA) regulation of mRNA translation and stability is a prominent mechanism linking environmental events to changes in protein expression. Here, using male mice, we show that miR-218 is expressed in dopamine neurons and is a repressor of DCC. Whereas Dcc mRNA levels increase from early adolescence to adulthood, miR-218 exhibits the exact opposite switch, most likely maintaining postnatal Dcc expression. This dynamic regulation appears to be selective to Dcc since the expression of Robo 1, the other guidance cue receptor target of miR-218, does not vary with age. Amphetamine in adolescence, but not in adulthood, increases miR-218 in the VTA and this event is required for drug-induced downregulation of Dcc mRNA and protein expression. This effect seems to be specific to Dcc because amphetamine does not alter Robo1. Furthermore, the upregulation of miR-218 by amphetamine requires dopamine D2 receptor activation. These findings identify miR-218 as regulator of DCC in the VTA both in normal development and after drug exposure in adolescence.

DCC Receptors Drive Prefrontal Cortex Maturation by Determining Dopamine Axon Targeting in Adolescence

BACKGROUND

Dopaminergic input to the prefrontal cortex (PFC) increases throughout adolescence and, by establishing precisely localized synapses, calibrates cognitive function. However, why and how mesocortical dopamine axon density increases across adolescence remains unknown.

METHODS

We used a developmental application of axon-initiated recombination to label and track the growth of dopamine axons across adolescence in mice. We then paired this recombination with cell-specific knockdown of the netrin-1 receptor DCC to determine its role in adolescent dopamine axon growth. We then assessed how altering adolescent PFC dopamine axon growth changes the structural and functional development of the PFC by quantifying pyramidal neuron morphology and cognitive performance.

RESULTS

We show, for the first time, that dopamine axons continue to grow from the striatum to the PFC during adolescence. Importantly, we discover that DCC, a guidance cue receptor, controls the extent of this protracted growth by determining where and when dopamine axons recognize their final target. When DCC-dependent adolescent targeting events are disrupted, dopamine axons continue to grow ectopically from the nucleus accumbens to the PFC and profoundly change PFC structural and functional development. This leads to alterations in cognitive processes known to be impaired across psychiatric conditions.

CONCLUSIONS

The prolonged growth of dopamine axons represents an extraordinary period for experience to influence their adolescent trajectory and predispose to or protect against psychopathology. DCC receptor signaling in dopamine neurons is a molecular link where genetic and environmental factors may interact in adolescence to influence the development and function of the prefrontal cortex.

Making Dopamine Connections in Adolescence

A dramatic maturational process ongoing in adolescence is prefrontal cortex development, including its dopamine innervation. Dopamine axons grow from the striatum to the prefrontal cortex, the only known case of long-distance axon growth during adolescence. This is coordinated by the Netrin-1 guidance cue receptor DCC (deleted in colorectal cancer), which in turn controls the intrinsic development of the prefrontal cortex itself. Stimulant drugs in adolescence alter DCC in dopamine neurons and, in turn prefrontal cortex maturation, impacting cognitive abilities. Variations in DCC expression are linked to psychiatric conditions of prefrontal cortex dysfunction, and microRNA regulation of DCC may be key to determining adolescent vulnerability or resilience. Since early interventions are proving to effectively ameliorate disease outcome, the Netrin-1 system is a promising therapeutic target.

DCC Confers Susceptibility to Depression-like Behaviors in Humans and Mice and Is Regulated by miR-218

BACKGROUD

Variations in the expression of the Netrin-1 guidance cue receptor DCC (deleted in colorectal cancer) appear to confer resilience or susceptibility to psychopathologies involving prefrontal cortex (PFC) dysfunction.

METHODS

With the use of postmortem brain tissue, mouse models of defeat stress, and in vitro analysis, we assessed microRNA (miRNA) regulation of DCC and whether changes in DCC levels in the PFC lead to vulnerability to depression-like behaviors.

RESULTS

We identified miR-218 as a posttranscriptional repressor of DCC and detected coexpression of DCC and miR-218 in pyramidal neurons of human and mouse PFC. We found that exaggerated expression of DCC and reduced levels of miR-218 in the PFC are consistent traits of mice susceptible to chronic stress and of major depressive disorder in humans. Remarkably, upregulation of Dcc in mouse PFC pyramidal neurons causes vulnerability to stress-induced social avoidance and anhedonia.

CONCLUSIONS

These data are the first demonstration of microRNA regulation of DCC and suggest that, by regulating DCC, miR-218 may be a switch of susceptibility versus resilience to stress-related disorders.

Dcc haploinsufficiency regulates dopamine receptor expression across postnatal lifespan

Adolescence is a period during which the medial prefrontal cortex (mPFC) undergoes significant remodeling. The netrin-1 receptor, deleted in colorectal cancer (DCC), controls the extent and organization of mPFC dopamine connectivity during adolescence and in turn directs mPFC functional and structural maturation. Dcc haploinsufficiency leads to increased mPFC dopamine input, which causes improved cognitive processing and resilience to behavioral effects of stimulant drugs of abuse. Here we examine the effects of Dcc haploinsufficiency on the dynamic expression of dopamine receptors in forebrain targets of C57BL6 mice. We conducted quantitative receptor autoradiography experiments with [³H]SCH-23390 or [³H]raclopride to characterize D1 and D2 receptor expression in mPFC and striatal regions in male Dcc haploinsufficient and wild-type mice. We generated autoradiograms at early adolescence (PND21±1), mid-adolescence (PND35±2), and adulthood (PND75±15). C57BL6 mice exhibit overexpression and pruning of D1, but not D2, receptors in striatal regions, and a lack of dopamine receptor pruning in the mPFC. We observed age- and region-specific differences in D1 and D2 receptor density between Dcc haploinsufficient and wild-type mice. Notably, neither group shows the typical pattern of mPFC dopamine receptor pruning in adolescence, but adult haploinsufficient mice show increased D2 receptor density in the mPFC. These results show that DCC receptors contribute to the dynamic refinement of D1 and D2 receptor expression in striatal regions across adolescence. The age-dependent expression of dopamine receptor in C57BL6 mice shows marked differences from previous characterizations in rats.

Mesocortical Dopamine Phenotypes in Mice Lacking the Sonic Hedgehog Receptor Cdon

Motivated behaviors and many psychopathologies typically involve changes in dopamine release from the projections of the ventral tegmental area (VTA) and/or the substantia nigra pars compacta (SNc). The morphogen Sonic Hedgehog (Shh) specifies fates of midbrain dopamine neurons, but VTA-specific effects of Shh signaling are also being uncovered. In this study, we assessed the role of the Shh receptor Cdon in the development of VTA and SNc dopamine neurons. We find that Cdon is expressed in the proliferating progenitor zone of the embryonic ventral midbrain and that the number of proliferating cells in this region is increased in mouse Cdon(-/-) embryos. Consistent with a role of Shh in the regulation of neuronal proliferation in this region, we find that the number of tyrosine hydroxylase (TH)-positive neurons is increased in the VTA of Cdon(-/-) mice at birth and that this effect endures into adulthood. In contrast, the number of TH-positive neurons in the SNc is not altered in Cdon(-/-) mice at either age. Moreover, adult Cdon(-/-) mice have a greater number of medial prefrontal cortex (mPFC) dopamine presynaptic sites, and increased baseline concentrations of dopamine and dopamine metabolites selectively in this region. Finally, consistent with increased dopamine function in the mPFC, we find that adult Cdon(-/-) mice fail to exhibit behavioral plasticity upon repeated amphetamine treatment. Based on these data, we suggest that Cdon plays an important role encoding the diversity of dopamine neurons in the midbrain, influencing both the development of the mesocortical dopamine pathway and behavioral outputs that involve this neural circuitry.

GENOME-WIDE ASSOCIATION STUDY (GWAS) AND GENOME-WIDE BY ENVIRONMENT INTERACTION STUDY (GWEIS) OF DEPRESSIVE SYMPTOMS IN AFRICAN AMERICAN AND HISPANIC/LATINA WOMEN

BACKGROUND

Genome-wide association studies (GWAS) have made little progress in identifying variants linked to depression. We hypothesized that examining depressive symptoms and considering gene-environment interaction (GxE) might improve efficiency for gene discovery. We therefore conducted a GWAS and genome-wide by environment interaction study (GWEIS) of depressive symptoms.

METHODS

Using data from the SHARe cohort of the Women's Health Initiative, comprising African Americans (n = 7,179) and Hispanics/Latinas (n = 3,138), we examined genetic main effects and GxE with stressful life events and social support. We also conducted a heritability analysis using genome-wide complex trait analysis (GCTA). Replication was attempted in four independent cohorts.

RESULTS

No SNPs achieved genome-wide significance for main effects in either discovery sample. The top signals in African Americans were rs73531535 (located 20 kb from GPR139, P = 5.75 × 10(-8) ) and rs75407252 (intronic to CACNA2D3, P = 6.99 × 10(-7) ). In Hispanics/Latinas, the top signals were rs2532087 (located 27 kb from CD38, P = 2.44 × 10(-7) ) and rs4542757 (intronic to DCC, P = 7.31 × 10(-7) ). In the GEWIS with stressful life events, one interaction signal was genome-wide significant in African Americans (rs4652467; P = 4.10 × 10(-10) ; located 14 kb from CEP350). This interaction was not observed in a smaller replication cohort. Although heritability estimates for depressive symptoms and stressful life events were each less than 10%, they were strongly genetically correlated (rG = 0.95), suggesting that common variation underlying self-reported depressive symptoms and stressful life event exposure, though modest on their own, were highly overlapping in this sample.

CONCLUSIONS

Our results underscore the need for larger samples, more GEWIS, and greater investigation into genetic and environmental determinants of depressive symptoms in minorities.

Molecular heterogeneity of midbrain dopaminergic neurons--Moving toward single cell resolution

Since their discovery, midbrain dopamine (DA) neurons have been researched extensively, in part because of their diverse functions and involvement in various neuropsychiatric disorders. Over the last few decades, reports have emerged that midbrain DA neurons were not a homogeneous group, but that DA neurons located in distinct anatomical locations within the midbrain had distinctive properties in terms of physiology, function, and vulnerability. Accordingly, several studies focused on identifying heterogeneous gene expression across DA neuron clusters. Here we review the importance of understanding DA neuron heterogeneity at the molecular level, previous studies detailing heterogeneous gene expression in DA neurons, and finally recent work which brings together previous heterogeneous gene expression profiles in a coordinated manner, at single cell resolution.

Resilience to amphetamine in mouse models of netrin-1 haploinsufficiency: role of mesocortical dopamine

RATIONALE

Signaling through the netrin-1 receptor, deleted in colorectal cancer (DCC), in dopamine neurons controls the extent of their innervation to the medial prefrontal cortex (mPFC) during adolescence. In mice, dcc haploinsufficiency results in increased mPFC dopamine innervation and concentrations in adulthood. In turn, dcc haploinsufficiency leads to resilience to the effects of stimulant drugs of abuse on dopamine release in the nucleus accumbens and behavior.

OBJECTIVES

First, we set out to determine whether increased mPFC dopamine innervation causes blunted behavioral responses to amphetamine in adult dcc haploinsufficient mice. Second, we investigated whether unc5c, another netrin-1 receptor expressed by dopamine neurons, is involved in these effects. Third, we assessed whether haploinsufficiency of netrin-1 itself leads to blunted behavioral responding to amphetamine, whether this phenotype emerges before or after adolescence and whether increased mPFC dopamine input is the underlying mechanism.

RESULTS

Adult, but not adolescent, dcc, unc5c and netrin-1 haploinsufficient mice exhibit blunted behavioral responses to amphetamine. Furthermore, adult dcc, unc5c, and netrin-1 haploinsufficient mice have exaggerated mPFC dopamine concentrations in comparison to their wild-type littermates. Importantly, resilience to amphetamine-induced behavioral activation in all the three mouse models is abolished by selective dopamine depletion in the medial prefrontal cortex.

CONCLUSIONS

dcc, unc5c, or netrin-1 haploinsufficiency leads to increased dopamine content in the mPFC and to resilience against amphetamine-induced behavioral activation. Our findings raise the hypothesis that DCC, UNC5C, and netrin-1 act in concert to organize the adolescent development of mesocortical dopamine innervation and, in turn, determine behavioral responses to drugs of abuse.

Establishing diversity in the dopaminergic system

Midbrain dopaminergic neurons (MbDNs) modulate cognitive processes, regulate voluntary movement, and encode reward prediction errors and aversive stimuli. While the degeneration of MbDNs underlies the motor defects in Parkinson's disease, imbalances in dopamine levels are associated with neuropsychiatric disorders such as depression, schizophrenia and substance abuse. In recent years, progress has been made in understanding how MbDNs, which constitute a relatively small neuronal population in the brain, can contribute to such diverse functions and dysfunctions. In particular, important insights have been gained regarding the distinct molecular, neurochemical and network properties of MbDNs. How this diversity of MbDNs is established during brain development is only starting to be unraveled. In this review, we summarize the current knowledge on the diversity in MbDN progenitors and differentiated MbDNs in the developing rodent brain. We discuss the signaling pathways, transcription factors and transmembrane receptors that contribute to setting up these diverse MbDN subpopulations. A better insight into the processes that establish diversity in MbDNs will ultimately improve the understanding of the architecture and function of the dopaminergic system in the adult brain.

Nicotine and the adolescent brain

Adolescence encompasses a sensitive developmental period of enhanced clinical vulnerability to nicotine, tobacco, and e-cigarettes. While there are sociocultural influences, data at preclinical and clinical levels indicate that this adolescent sensitivity has strong neurobiological underpinnings. Although definitions of adolescence vary, the hallmark of this period is a profound reorganization of brain regions necessary for mature cognitive and executive function, working memory, reward processing, emotional regulation, and motivated behavior. Regulating critical facets of brain maturation are nicotinic acetylcholine receptors (nAChRs). However, perturbations of cholinergic systems during this time with nicotine, via tobacco or e-cigarettes, have unique consequences on adolescent development. In this review, we highlight recent clinical and preclinical data examining the adolescent brain's distinct neurobiology and unique sensitivity to nicotine. First, we discuss what defines adolescence before reviewing normative structural and neurochemical alterations that persist until early adulthood, with an emphasis on dopaminergic systems. We review how acute exposure to nicotine impacts brain development and how drug responses differ from those seen in adults. Finally, we discuss the persistent alterations in neuronal signaling and cognitive function that result from chronic nicotine exposure, while highlighting a low dose, semi-chronic exposure paradigm that may better model adolescent tobacco use. We argue that nicotine exposure, increasingly occurring as a result of e-cigarette use, may induce epigenetic changes that sensitize the brain to other drugs and prime it for future substance abuse.

Amphetamine in adolescence disrupts the development of medial prefrontal cortex dopamine connectivity in a DCC-dependent manner

Initiation of drug use during adolescence is a strong predictor of both the incidence and severity of addiction throughout the lifetime. Intriguingly, adolescence is a period of dynamic refinement in the organization of neuronal connectivity, in particular medial prefrontal cortex (mPFC) dopamine circuitry. The guidance cue receptor, DCC (deleted in colorectal cancer), is highly expressed by dopamine neurons and orchestrates their innervation to the mPFC during adolescence. Furthermore, we have shown that amphetamine in adolescence regulates DCC expression in dopamine neurons. Drugs in adolescence may therefore induce their enduring behavioral effects via DCC-mediated disruption in mPFC dopamine development. In this study, we investigated the impact of repeated exposure to amphetamine during adolescence on both the development of mPFC dopamine connectivity and on salience attribution to drug context in adulthood. We compare these effects to those induced by adult exposure to an identical amphetamine regimen. Finally, we determine whether DCC signaling within dopamine neurons is necessary for these events. Exposure to amphetamine in adolescence, but not in adulthood, leads to an increase in the span of dopamine innervation to the mPFC, but a reduction of presynaptic sites present on these axons. Amphetamine treatment in adolescence, but not in adulthood, also produces an increase in salience attribution to a previously drug-paired context in adulthood. Remarkably, DCC signaling within dopamine neurons is required for both of these effects. Drugs of abuse in adolescence may therefore induce their detrimental behavioral consequences by disrupting mesocortical dopamine development through alterations in the DCC signaling cascade.

Haloperidol treatment downregulates DCC expression in the ventral tegmental area

A core feature in the pathophysiology of schizophrenia is abnormal development and function of mesocorticolimbic dopamine (DA) circuitry. We have previously shown that variations in the function of the netrin-1 receptor, deleted in colorectal cancer (DCC), result in changes to the development, organization and ongoing plasticity of DA circuitry. In rodents, repeated exposure to the indirect DA-agonist, amphetamine upregulates DCC expression in the ventral tegmental area (VTA), but not in DA terminal regions. This elevation in DCC expression is associated with increased vulnerability to developing and maintaining sensitized mesolimbic DA function. Antipsychotic medications remain the best treatment option for managing the symptoms in schizophrenia. The peak effects of these medications are gradual, suggesting that a therapeutic component of antipsychotic treatment involves structural reorganization. Here we assessed whether repeated exposure to typical and atypical antipsychotics could also regulate DCC. Adult mice were orally administered haloperidol, clozapine, or risperidone via their drinking water for 4 weeks. Levels of DCC were measured by Western blot analysis of tissue punches of the VTA, medial prefrontal cortex, nucleus accumbens, and dorsal striatum. Haloperidol decreased DCC levels by approximately 50% in the VTA, but not in DA targets. Furthermore, haloperidol did not alter UNC-5 homologue levels, another family of netrin-1 receptors, confirming that its effects target DCC-mediated netrin-1 signaling specifically. The atypical antipsychotics did not alter DCC expression. These results suggest that typical antipsychotics induce selective functional reorganization in the VTA via DCC-mediated netrin-1 signaling.

The Netrin-1 receptor DCC is a regulator of maladaptive responses to chronic morphine administration

BACKGROUND

Opioids are the cornerstone of treatment for moderate to severe pain, but chronic use leads to maladaptations that include: tolerance, dependence and opioid-induced hyperalgesia (OIH). These responses limit the utility of opioids, as well as our ability to control chronic pain. Despite decades of research, we have no therapies or proven strategies to overcome this problem. However, murine haplotype based computational genetic mapping and a SNP data base generated from analysis of whole-genome sequence data (whole-genome HBCGM), provides a hypothesis-free method for discovering novel genes affecting opioid maladaptive responses.

RESULTS

Whole genome-HBCGM was used to analyze phenotypic data on morphine-induced tolerance, dependence and hyperalgesia obtained from 23 inbred strains. The robustness of the genetic mapping results was analyzed using strain subsets. In addition, the results of analyzing all of the opioid-related traits together were examined. To characterize the functional role of the leading candidate gene, we analyzed transgenic animals, mRNA and protein expression in behaviorally divergent mouse strains, and immunohistochemistry in spinal cord tissue. Our mapping procedure identified the allelic pattern within the netrin-1 receptor gene (Dcc) as most robustly associated with OIH, and it was also strongly associated with the combination of the other maladaptive opioid traits analyzed. Adult mice heterozygous for the Dcc gene had significantly less tendency to develop OIH, become tolerant or show evidence of dependence after chronic exposure to morphine. The difference in opiate responses was shown not to be due to basal or morphine-stimulated differences in the level of Dcc expression in spinal cord tissue, and was not associated with nociceptive neurochemical or anatomical alterations in the spinal cord or dorsal root ganglia in adult animals.

CONCLUSIONS

Whole-genome HBCGM is a powerful tool for identifying genes affecting biomedical traits such as opioid maladaptations. We demonstrate that Dcc affects tolerance, dependence and OIH after chronic opioid exposure, though not through simple differences in expression in the adult spinal cord.

Adolescence: a time of transition for the phenotype of dcc heterozygous mice

RATIONALE

Stark differences exist between adult (>PND 70) and juvenile (∼PND 21-34) rodents in how DCC (deleted in colorectal cancer) receptors and sensitization to amphetamine interact. In adults, repeated amphetamine upregulates DCC receptor expression selectively in the ventral tegmental area (VTA), an effect that is critical for sensitization. In contrast, amphetamine administered to juveniles downregulates VTA DCC expression. Moreover, whereas adult dcc heterozygous mice fail to sensitize when repeatedly treated with amphetamine, drug treatment during the juvenile period actually abolishes this adult "protective" phenotype.

OBJECTIVES

We set out to determine whether adolescence (PND ∼35-55) is a period during which: (1) amphetamine-induced alterations in VTA DCC expression switch from downregulation to upregulation; (2) the "protective" phenotype of adult dcc heterozygotes against sensitization becomes evident; and (3) the adult "protective" phenotype of dcc heterozygotes can still be abolished by repeated amphetamine treatment.

RESULTS

Repeated amphetamine did not significantly alter VTA DCC expression in adolescent rodents when assessed 1 week later. Both wild-type and dcc heterozygous mice exhibited sensitization at this time. Remarkably, wild-type mice, but not dcc heterozygotes, exhibited sensitization when tested during adulthood.

CONCLUSIONS

Adolescence is a time of transition for dcc heterozygotes as related to sensitization. Our results support the hypothesis that DCC may be a key factor in determining age-dependent individual differences in vulnerability to sensitization. Given that exposure to drugs of abuse during adolescence can have profound consequences for adulthood, the resilience of adult dcc heterozygous mice against adolescent exposure to amphetamine is particularly salient.

Netrin 1 and Dcc signalling are required for confinement of central axons within the central nervous system

The establishment of anatomically stereotyped axonal projections is fundamental to neuronal function. While most neurons project their axons within the central nervous system (CNS), only axons of centrally born motoneurons and peripherally born sensory neurons link the CNS and peripheral nervous system (PNS) together by navigating through specialized CNS/PNS transition zones. Such selective restriction is of importance because inappropriate CNS axonal exit could lead to loss of correct connectivity and also to gain of erroneous functions. However, to date, surprisingly little is known about the molecular-genetic mechanisms that regulate how central axons are confined within the CNS during development. Here, we show that netrin 1/Dcc/Unc5 chemotropism contributes to axonal confinement within the CNS. In both Ntn1 and Dcc mutant mouse embryos, some spinal interneuronal axons exit the CNS by traversing the CNS/PNS transition zones normally reserved for motor and sensory axons. We provide evidence that netrin 1 signalling preserves CNS/PNS axonal integrity in three ways: (1) netrin 1/Dcc ventral attraction diverts axons away from potential exit points; (2) a Dcc/Unc5c-dependent netrin 1 chemoinhibitory barrier in the dorsolateral spinal cord prevents interneurons from being close to the dorsal CNS/PNS transition zone; and (3) a netrin 1/Dcc-dependent, Unc5c-independent mechanism that actively prevents exit from the CNS. Together, these findings provide insights into the molecular mechanisms that maintain CNS/PNS integrity and, to the best of our knowledge, present the first evidence that chemotropic signalling regulates interneuronal CNS axonal confinement in vertebrates.

Target-dependent expression of the netrin-1 receptor, UNC5C, in projection neurons of the ventral tegmental area

We have shown previously that the netrin-1 receptor, unc-5 homologue C (UNC5C), is expressed by ventral tegmental area (VTA) dopamine (DA) neurons of rodents, but only from adolescence onwards (Manitt et al., 2010; Auger et al., 2013). The goal of this study was to characterize the expression of UNC5C by these neurons. Specifically, we assessed whether UNC5C expression is selective to DA neurons that project to the medial prefrontal cortex (mPFC), which undergo significant maturation during the adolescent period. To this end, we injected fluorescent retrograde tracer beads into the mPFC, nucleus accumbens (NAcc) core, or NAcc lateral shell of adult male wild-type C57Bl/6J mice and processed their brains for tyrosine hydroxylase (TH) and UNC5C immunofluorescence 2-3weeks later. VTA neurons with any combination of these immunolabels were visualized and counted using optical fractionator stereology. Our analysis revealed two main findings: (1) there are no differences in the proportions of UNC5C-positive DA neurons projecting to the mPFC, NAcc core, or NAcc lateral shell, and (2) the proportion of non-DA UNC5C-positive neurons targeting the mPFC is greater than the proportions of non-DA UNC5C-positive neurons targeting the NAcc core or lateral shell. These findings show that, contrary to our hypothesis, DA neurons projecting to the mPFC do not express UNC5C selectively. However, UNC5C expression by non-DA VTA neurons is predominantly found in those projecting to the mPFC and, as such, may play a role in their function.

dcc orchestrates the development of the prefrontal cortex during adolescence and is altered in psychiatric patients

Adolescence is a period of heightened susceptibility to psychiatric disorders of medial prefrontal cortex (mPFC) dysfunction and cognitive impairment. mPFC dopamine (DA) projections reach maturity only in early adulthood, when their control over cognition becomes fully functional. The mechanisms governing this protracted and unique development are unknown. Here we identify dcc as the first DA neuron gene to regulate mPFC connectivity during adolescence and dissect the mechanisms involved. Reduction or loss of dcc from DA neurons by Cre-lox recombination increased mPFC DA innervation. Underlying this was the presence of ectopic DA fibers that normally innervate non-cortical targets. Altered DA input changed the anatomy and electrophysiology of mPFC circuits, leading to enhanced cognitive flexibility. All phenotypes only emerged in adulthood. Using viral Cre, we demonstrated that dcc organizes mPFC wiring specifically during adolescence. Variations in DCC may determine differential predisposition to mPFC disorders in humans. Indeed, DCC expression is elevated in brains of antidepressant-free subjects who committed suicide.

Netrin-1 receptor-deficient mice show age-specific impairment in drug-induced locomotor hyperactivity but still self-administer methamphetamine

RATIONALE

The mesocorticolimbic dopamine system undergoes significant reorganization of neuronal connectivity and functional refinement during adolescence. Deleted in colorectal cancer (DCC), a receptor for the guidance cue netrin-1, is involved in this reorganization. Previous studies have shown that adult mice with a heterozygous (het) loss-of-function mutation in DCC exhibit impairments in sensitization and conditioned place preference (CPP) to psychostimulants. However, the commonly abused psychostimulant methamphetamine (METH) has not been assessed, and the role of DCC in drug self-administration remains to be established.

OBJECTIVES

Using dcc het mice and wildtype (WT) littermates, we extended previous findings on dcc haplodeficiency by examining self-administration of METH in adult mice, including cue-induced drug seeking following abstinence. We also examined hyperactivity, sensitization, and CPP to a METH-paired context in adult and adolescent mice.

RESULTS

While adult dcc het mice expressed largely similar METH self-administration and cue-induced drug seeking as WT littermates, they failed to modulate responding according to dose of METH. Compared to WT, both adult and adolescent dcc het mice expressed impaired locomotor hyperactivity to acute METH but nevertheless showed comparable behavioral sensitization. Conditioned hyperactivity increased with age in WT but not in dcc het mice.

CONCLUSIONS

Impaired METH-induced hyperactivity and dose-related responding in adult dcc het mice suggest that reduced DCC alters METH-related behaviors. Adolescence is identified as a vulnerable period during which impairment in hyperactivity due to reduced DCC can be overcome with repeated METH injections. Nevertheless, DCC appears to have a somewhat limited role in METH-consumption and seeking following abstinence.

Long distance directional growth of dopaminergic axons along pathways of netrin-1 and GDNF

Different experimental and clinical strategies have been used to promote survival of transplanted embryonic ventral mesencephalic (VM) neurons. However, few studies have focused on the long-distance growth of dopaminergic axons from VM transplants. The aim of this study is to identify some of the growth and guidance factors that support directed long-distance growth of dopaminergic axons from VM transplants. Lentivirus encoding either glial cell line-derived neurotrophic factor (GDNF) or netrin-1, or a combination of lenti-GDNF with either lenti-GDNF family receptor α1 (GFRα-1) or lenti-netrin-1 was injected to form a gradient along the corpus callosum. Two weeks later, a piece of embryonic day 14 VM tissue was transplanted into the corpus callosum adjacent to the low end of the gradient. Results showed that tyrosine hydroxylase (TH(+)) axons grew a very short distance from the VM transplants in control groups, with few axons reaching the midline. In GDNF or netrin-1 expressing groups, more TH(+) axons grew out of transplants and reached the midline. Pathways co-expressing GDNF with either GFRα-1 or netrin-1 showed significantly increased axonal outgrowth. Interestingly, only the GDNF/netrin-1 combination resulted in the majority of axons reaching the distal target (80%), whereas along the GDNF/GFRα-1 pathway only 20% of the axons leaving the transplant reached the distal target. This technique of long-distance axon guidance may prove to be a useful strategy in reconstructing damaged neuronal circuits, such as the nigrostriatal pathway in Parkinson's disease.

unc5c haploinsufficient phenotype: striking similarities with the dcc haploinsufficiency model

DCC and UNC5 homologs (UNC5H) are guidance cue receptors highly expressed by mesocorticolimbic dopamine neurons. We have shown that dcc heterozygous mice exhibit increased dopamine, but not norepinephrine, innervation and function in medial prefrontal cortex. Concomitantly, dcc heterozygotes show blunted mesolimbic dopamine release and behavioral responses to stimulant drugs. These changes appear only in adulthood. Recently, we found an adolescent emergence of UNC5H expression by dopamine neurons and co-expression of DCC and UNC5H by single dopamine cells. Here, we demonstrate selective expression of unc5 homolog c mRNA by dopamine neurons in adulthood. We show that unc5c haploinsufficiency results in diminished amphetamine-induced locomotion in male and female mice. This phenotype is identical to that produced by dcc haploinsufficiency and is observed after adolescence. Notably, and similar to dcc haploinsufficiency, unc5c haploinsufficiency leads to dramatic increases in tyrosine hydroxylase expression in the medial prefrontal cortex, but not in the nucleus accumbens. In contrast, medial prefrontal cortex dopamine-β-hydroxylase expression is not altered. We confirmed that UNC5C protein is reduced in the ventral tegmental area of unc5c heterozygous mice, but that DCC expression in this region remains unchanged. UNC5C receptors may also play a role in dopamine function and influence sensitivity to behavioral effects of stimulant drugs of abuse, at least upon first exposure. The striking similarities between the dcc and the unc5c haploinsufficient phenotypes raise the possibility that functions mediated by DCC/UNC5C complexes may be at play.

The sea lamprey UNC5 receptors: cDNA cloning, phylogenetic analysis and expression in reticulospinal neurons at larval and adult stages of development

UNC5 receptors mediate repulsive signaling of netrin on neurons. Although only one UNC5 receptor has been identified in invertebrates, four members of the UNC5 family have been identified in gnathostomes. Lampreys, together with mixynes, belong to the oldest branch of extant vertebrates, and their phylogenetic position near to the vertebrate root makes them an interesting model for understanding molecular evolution. Here, we cloned three sea lamprey UNC5 (UNC5L) receptors, and phylogenetic analyses indicated that the first two duplications of the ancestral UNC5 gene occurred before the separation of jawless and jawed vertebrates. UNC5 receptors play important roles during early development, but expression studies have also suggested that UNC5 receptors play roles in the mature nervous system. Here, we report the expression of the different UNC5L receptor transcripts in identified reticulospinal neurons of mature larval or adult sea lampreys detected by in situ hybridization in wholemounted brain preparations. In addition, an extensive expression of the UNC5 receptors was also observed in most brain regions of the adult lamprey. An increase in the types of identifiable reticulospinal neurons expressing the UNC5L receptors was observed in adults compared with larvae. Expression of UNC5 receptors at late developmental stages appears to be a shared characteristic of lampreys and mammals. In larvae, expression of UNC5L receptors was observed in reticulospinal neurons that when axotomized are known to be "bad regenerators." Results in lampreys and mammals suggest that the UNC5-Netrin axonal guidance system may play a role in limiting axonal regeneration after spinal cord injury.

Spatiotemporal expression of repulsive guidance molecules (RGMs) and their receptor neogenin in the mouse brain

Neogenin has been implicated in a variety of developmental processes such as neurogenesis, neuronal differentiation, apoptosis, migration and axon guidance. Binding of repulsive guidance molecules (RGMs) to Neogenin inhibits axon outgrowth of different neuronal populations. This effect requires Neogenin to interact with co-receptors of the uncoordinated locomotion-5 (Unc5) family to activate downstream Rho signaling. Although previous studies have reported RGM, Neogenin, and/or Unc5 expression, a systematic comparison of RGM and Neogenin expression in the developing nervous system is lacking, especially at later developmental stages. Furthermore, information on RGM and Neogenin expression at the protein level is limited. To fill this void and to gain further insight into the role of RGM-Neogenin signaling during mouse neural development, we studied the expression of RGMa, RGMb, Neogenin and Unc5A-D using in situ hybridization, immunohistochemistry and RGMa section binding. Expression patterns in the primary olfactory system, cortex, hippocampus, habenula, and cerebellum were studied in more detail. Characteristic cell layer-specific expression patterns were detected for RGMa, RGMb, Neogenin and Unc5A-D. Furthermore, strong expression of RGMa, RGMb and Neogenin protein was found on several major axon tracts such as the primary olfactory projections, anterior commissure and fasciculus retroflexus. These data not only hint at a role for RGM-Neogenin signaling during the development of different neuronal systems, but also suggest that Neogenin partners with different Unc5 family members in different systems. Overall, the results presented here will serve as a framework for further dissection of the role of RGM-Neogenin signaling during neural development.

Dopaminergic axon guidance: which makes what?

Mesotelencephalic pathways in the adult central nervous system have been studied in great detail because of their implication in major physiological functions as well as in psychiatric, neurological, and neurodegenerative diseases. However, the ontogeny of these pathways and the molecular mechanisms that guide dopaminergic axons during embryogenesis have been only recently studied. This line of research is of crucial interest for the repair of lesioned circuits in adulthood following neurodegenerative diseases or common traumatic injuries. For instance, in the adult, the anatomic and functional repair of the nigrostriatal pathway following dopaminergic embryonic neuron transplantation suggests that specific guidance cues exist which govern embryonic fibers outgrowth, and suggests that axons from transplanted embryonic cells are able to respond to theses cues, which then guide them to their final targets. In this review, we first synthesize the work that has been performed in the last few years on developing mesotelencephalic pathways, and summarize the current knowledge on the identity of cellular and molecular signals thought to be involved in establishing mesotelencephalic dopaminergic neuronal connectivity during embryogenesis in the central nervous system of rodents. Then, we review the modulation of expression of these molecular signals in the lesioned adult brain and discuss their potential role in remodeling the mesotelencephalic dopaminergic circuitry, with a particular focus on Parkinson's disease (PD). Identifying guidance molecules involved in the connection of grafted cells may be useful for cellular therapy in Parkinsonian patients, as these molecules may help direct axons from grafted cells along the long distance they have to travel from the substantia nigra to the striatum.

Juvenile exposure to methylphenidate reduces cocaine reward and alters netrin-1 receptor expression in adulthood

The guidance cue netrin-1 acting on mesocorticolimbic dopamine (DA) neurons through its receptor DCC (deleted in colorectal cancer) has been implicated in the neuronal plasticity induced by psychostimulant drugs. We examined in C57/BL6 mice the effects of repeated juvenile methylphenidate (MPH) exposure on cocaine-reward sensitivity in adulthood and determined whether early MPH treatment alters adult expression of DCC in mesocorticolimbic DA regions. By using place conditioning, we show that adult mice exposed to MPH during the juvenile period are less sensitive to cocaine-reward compared to saline-controls. Furthermore, by means of immunoblotting, we demonstrate that early MPH treatment attenuates adult DCC expression in the ventral tegmental area (VTA) selectively. These results support previous evidence that developmental MPH treatment diminishes cocaine-reward in adulthood and are the first to suggest that DCC in the VTA may participate in this enduring effect.

Abolition of the behavioral phenotype of adult netrin-1 receptor deficient mice by exposure to amphetamine during the juvenile period

RATIONALE

Netrin-1 guidance cues contribute to amphetamine-induced plasticity of the adult mesocorticolimbic dopamine system in rodents. The netrin-1 receptor, deleted in colorectal cancer (DCC), is upregulated by repeated amphetamine treatment selectively in the ventral tegmental area (VTA) of adult rats and wild-type mice. Furthermore, adult dcc heterozygous mice fail to show amphetamine-induced increases in VTA DCC expression and do not develop sensitization to this drug.

OBJECTIVES

The effects of netrin-1 receptor signaling on mesocorticolimbic dopamine system function change across development. However, the effects of AMPH on DCC receptor regulation and behavioral sensitization before puberty have not been determined. Here we examined whether (1) repeated amphetamine treatment would also alter DCC expression in juvenile rats and wild-type mice, and (2) dcc heterozygotes treated with amphetamine during the juvenile period (PND 22-32) would develop behavioral sensitization to this drug.

RESULTS

Repeated amphetamine downregulates DCC expression selectively in the VTA of juvenile rodents. Moreover, the behavioral phenotype of adult dcc heterozygous mice is not present before puberty and is abolished by amphetamine treatment during the juvenile period. Remarkably, adult dcc heterozygotes pretreated with amphetamine as juveniles no longer exhibit reduced DCC expression in the VTA compared to wild-type controls.

CONCLUSIONS

Our results indicate that netrin-1 receptor signaling may be a key factor in determining individual differences in vulnerability to the behaviorally sensitizing effects of amphetamine at different ages. Moreover, they suggest that the juvenile period marks a window of vulnerability during which exposure to stimulant drugs can reverse the behavioral phenotype of adult dcc heterozygous mice.

Role of netrin-1 in the organization and function of the mesocorticolimbic dopamine system

Changes in mesocorticolimbic dopamine (DA) neurons and their target cells can be induced throughout life and are important determinants of individual differences in susceptibility to psychopathology. The goal of my research is to gain insight into the nature of the cellularand molecular mechanism underlying the selective plasticity of mesocorticolimbic DA neurons. Here, I review work showing that the guidance cue netrin-1 is implicated in the organization, plasticity and function of mesocorticolimbic DA neurons in rodents. Developmental variations in netrin-1 receptor function result in selective reorganization of medial prefrontal DA circuitry during adolescence and in an adult phenotype protected against schizophrenia-like dopaminergic and behavioural abnormalities. Furthermore, in adulthood, expression of netrin-1 receptors is upregulated by repeated exposure to stimulant drugs of abuse in DA somatodendritic regions and is necessary for drug-induced behavioural plasticity. I propose that risk factors associated with DA-related adult psychiatric disorders alter netrin-1 function.

The netrin receptor DCC is required in the pubertal organization of mesocortical dopamine circuitry

Netrins are guidance cues involved in neural connectivity. We have shown that the netrin-1 receptor DCC (deleted in colorectal cancer) is involved in the functional organization of the mesocorticolimbic dopamine (DA) system. Adult mice with a heterozygous loss-of-function mutation in dcc exhibit changes in indexes of DA function, including DA-related behaviors. These phenotypes are only observed after puberty, a critical period in the maturation of the mesocortical DA projection. Here, we examined whether dcc heterozygous mice exhibit structural changes in medial prefrontal cortex (mPFC) DA synaptic connectivity, before and after puberty. Stereological counts of tyrosine-hydroxylase (TH)-positive varicosities were increased in the cingulate 1 and prelimbic regions of the pregenual mPFC. dcc heterozygous mice also exhibited alterations in the size, complexity, and dendritic spine density of mPFC layer V pyramidal neuron basilar dendritic arbors. Remarkably, these presynaptic and postsynaptic partner phenotypes were not observed in juvenile mice, suggesting that DCC selectively influences the extensive branching and synaptic differentiation that occurs in the maturing mPFC DA circuit at puberty. Immunolabeling experiments in wild-type mice demonstrated that DCC is segregated to TH-positive fibers innervating the nucleus accumbens, with only scarce DCC labeling in mPFC TH-positive fibers. Netrin had an inverted target expression pattern. Thus, DCC-mediated netrin-1 signaling may influence the formation/maintenance of mesocorticolimbic DA topography. In support of this, we report that dcc heterozygous mice exhibit a twofold increase in the density of mPFC DCC/TH-positive varicosities. Our results implicate DCC-mediated netrin-1 signaling in the establishment of mPFC DA circuitry during puberty.