Reorganization of dopamine circuitry in the anterior corpus callosum between early adolescence and adulthood in the mouse.

The maturation of forebrain dopamine circuitry occurs over multiple developmental periods, extending from early postnatal life until adulthood, with the precise timing of maturation defined by the target region. We recently demonstrated in the adult mouse brain that axon terminals arising from midbrain dopamine neurons innervate the anterior corpus callosum and that oligodendrocyte lineage cells in this white matter tract express dopamine receptor transcripts. Whether corpus callosal dopamine circuitry undergoes maturational changes between early adolescence and adulthood is unknown but may be relevant to understanding the dramatic micro- and macro-anatomical changes that occur in the corpus callosum of multiple species during early adolescence, including in the degree of myelination. Using quantitative neuroanatomy, we show that dopamine innervation in the forceps minor, but not the rostral genu, of the corpus callosum, is greater during early adolescence (P21) compared to adulthood (>P90) in wild-type mice. We further demonstrate with RNAscope that, as in the adult, Drd1 and Drd2 transcripts are expressed at higher levels in oligodendrocyte precursor cells (OPCs) and decline as these cells differentiate into oligodendrocytes. In addition, the number of OPCs that express Drd1 transcripts during early adolescence is double the number of those expressing the transcript during early adulthood. These data further implicate dopamine in axon myelination and myelin regulation. Moreover, because developmental (activity-independent) myelination peaks during early adolescence, with experience-dependent (activity-dependent) myelination greatest during early adulthood, our data suggest that potential roles of dopamine on callosal myelination shift between early adolescence and adulthood, from a developmental role to an experience-dependent role.

Early Adolescence is a Critical Period for the Maturation of Inhibitory Behavior.

Psychiatric conditions marked by impairments in cognitive control often emerge during adolescence, when the prefrontal cortex (PFC) and its inputs undergo structural and functional maturation and are vulnerable to disruption by external events. It is not known, however, whether there exists a specific temporal window within the broad range of adolescence when the development of PFC circuitry and its related behaviors are sensitive to disruption. Here we show, in male mice, that repeated exposure to amphetamine during early adolescence leads to impaired behavioral inhibition, aberrant PFC dopamine connectivity, and reduced PFC dopamine function in adulthood. Remarkably, these deficits are not observed following exposure to the exact same amphetamine regimen at later times. These findings demonstrate that there is a critical period for the disruption of the adolescent maturation of cognitive control and PFC dopamine function and suggest that early adolescence is particularly relevant to the emergence of psychopathology in humans.

Differential sensitivity to the acute and sensitizing behavioral effects of methylphenidate as a function of strain in adolescent and young adult rats.

BACKGROUND: Behavioral effects of stimulant drugs are influenced by non-pharmacological factors, including genetic variability and age. We examined acute and sensitized locomotor effects of methylphenidate in adolescent and early adult male Sprague Dawley (SD), spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats using a drug regimen that differentiates clearly between initial and enduring differences in drug responsiveness. We probed for strain and age differences in the sensitizing effects of methylphenidate using a cocaine challenge. Methylphenidate was administered to the rats in a non-home environment. FINDINGS: Strain differences in sensitivity to single methylphenidate injections depend on age and change with continuing drug pretreatment. While SHR rats are more sensitive to methylphenidate relative to WKY regardless of age and pretreatment day, SHR rats become more sensitive to methylphenidate than SD rats towards the end of pretreatment during early adulthood. SD rats exhibit greater sensitivity to methylphenidate relative to the WKY group during adolescence, an effect that dissipates with continued drug pretreatment during adulthood. Remarkably, only SHR rats, regardless of age, exhibit methylphenidate-induced cross-sensitization to the behavioral effects of cocaine. CONCLUSIONS: Our findings suggest that SHR rats are more vulnerable than other strains to methylphenidate-induced cross-sensitization to cocaine, at least when methylphenidate is administered in a non-home environment. Given that SHR rats are typically used to model features of attention deficit hyperactivity disorder, these findings may have important implications for the treatment of this disorder with methylphenidate.

Axo-glial interactions between midbrain dopamine neurons and oligodendrocyte lineage cells in the anterior corpus callosum.

Oligodendrocyte progenitor cells (OPCs) receive synaptic innervation from glutamatergic and GABAergic axons and can be dynamically regulated by neural activity, resulting in activity-dependent changes in patterns of axon myelination. However, it remains unclear to what extent other types of neurons may innervate OPCs. Here, we provide evidence implicating midbrain dopamine neurons in the innervation of oligodendrocyte lineage cells in the anterior corpus callosum and nearby white matter tracts of male and female adult mice. Dopaminergic axon terminals were identified in the corpus callosum of DAT-Cre mice after injection of an eYFP reporter virus into the midbrain. Furthermore, fast-scan cyclic voltammetry revealed monoaminergic transients in the anterior corpus callosum, consistent with the anatomical findings. Using RNAscope, we further demonstrate that ~ 40% of Olig2 + /Pdfgra + cells and ~ 20% of Olig2 + /Pdgfra- cells in the anterior corpus callosum express Drd1 and Drd2 transcripts. These results suggest that oligodendrocyte lineage cells may respond to dopamine released from midbrain dopamine axons, which could affect myelination. Together, this work broadens our understanding of neuron-glia interactions with important implications for myelin plasticity by identifying midbrain dopamine axons as a potential regulator of corpus callosal oligodendrocyte lineage cells.

Dopamine Neurons That Cotransmit Glutamate, From Synapses to Circuits to Behavior.

Discovered just over 20 years ago, dopamine neurons have the ability to cotransmit both dopamine and glutamate. Yet, the functional roles of dopamine neuron glutamate cotransmission and their implications for therapeutic use are just emerging. This review article encompasses the current body of evidence investigating the functions of dopamine neurons of the ventral midbrain that cotransmit glutamate. Since its discovery in dopamine neuron cultures, further work in vivo confirmed dopamine neuron glutamate cotransmission across species. From there, growing interest has led to research related to neural functioning including roles in synaptic signaling, development, and behavior. Functional connectome mapping reveals robust connections in multiple forebrain regions to various cell types, most notably to cholinergic interneurons in both the medial shell of the nucleus accumbens and the lateral dorsal striatum. Glutamate markers in dopamine neurons reach peak levels during embryonic development and increase in response to various toxins, suggesting dopamine neuron glutamate cotransmission may serve neuroprotective roles. Findings from behavioral analyses reveal prominent roles for dopamine neuron glutamate cotransmission in responses to psychostimulants, in positive valence and cognitive systems and for subtle roles in negative valence systems. Insight into dopamine neuron glutamate cotransmission informs the pathophysiology of neuropsychiatric disorders such as addiction, schizophrenia and Parkinson Disease, with therapeutic implications.

Netrin-1 receptor in the ventral tegmental area is required for sensitization to amphetamine.

Fundamental to neural organization during development, the netrin-1 guidance cue and its receptor, deleted in colorectal cancer (DCC), continue to be expressed in the adult brain. We have shown recently that adult dcc heterozygous mice do not develop sensitization to the stimulant drug of abuse amphetamine (AMPH) and that repeated exposure to AMPH upregulates DCC expression in adult rats. This upregulation is selective to the ventral tegmental area (VTA), a site critical for the initiation of behavioral plasticity induced by stimulant drugs, and is glutamate-dependent. Here we demonstrate that the lack of AMPH-induced sensitization in dcc heterozygotes is associated with a failure of AMPH to upregulate DCC receptor expression in the VTA. Further, we show that, in wild-type mice, repeated AMPH induces increases in VTA expression of the dendritic spine-associated protein, spinophilin. Significantly, however, this effect is not observed in dcc heterozygotes. In parallel experiments conducted in adult rats, we show that VTA DCC receptor activation, at the time of AMPH pretreatment, is critical for sensitization to AMPH. Together, these results demonstrate that the DCC netrin-1 receptor, a protein traditionally known for its role in organizing brain development, plays a critical function in adult brain plasticity, possibly via orchestration of neuronal circuitry reorganization. We propose VTA DCC receptor signaling as a novel mechanism in the series of glutamate-dependent cellular processes that lead to enduring plasticity by drugs of abuse.

Dopamine neuron glutamate cotransmission evokes a delayed excitation in lateral dorsal striatal cholinergic interneurons.

Dopamine neurons have different synaptic actions in the ventral and dorsal striatum (dStr), but whether this heterogeneity extends to dStr subregions has not been addressed. We have found that optogenetic activation of dStr dopamine neuron terminals in mouse brain slices pauses the firing of cholinergic interneurons in both the medial and lateral subregions, while in the lateral subregion the pause is shorter due to a subsequent excitation. This excitation is mediated mainly by metabotropic glutamate receptor 1 (mGluR1) and partially by dopamine D1-like receptors coupled to transient receptor potential channel 3 and 7. DA neurons do not signal to spiny projection neurons in the medial dStr, while they elicit ionotropic glutamate responses in the lateral dStr. The DA neurons mediating these excitatory signals are in the substantia nigra (SN). Thus, SN dopamine neurons engage different receptors in different postsynaptic neurons in different dStr subregions to convey strikingly different signals. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Heterogeneity in Dopamine Neuron Synaptic Actions Across the Striatum and Its Relevance for Schizophrenia.

Brain imaging has revealed alterations in dopamine uptake, release, and receptor levels in patients with schizophrenia that have been resolved on the scale of striatal subregions. However, the underlying synaptic mechanisms are on a finer scale. Dopamine neuron synaptic actions vary across the striatum, involving variations not only in dopamine release but also in dopamine neuron connectivity, cotransmission, modulation, and activity. Optogenetic studies have revealed that dopamine neurons release dopamine in a synaptic signal mode, and that the neurons also release glutamate and gamma-aminobutyric acid as cotransmitters, with striking regional variation. Fast glutamate and gamma-aminobutyric acid cotransmission convey discrete patterns of dopamine neuron activity to striatal neurons. Glutamate may function not only in a signaling role at a subset of dopamine neuron synapses, but also in mediating vesicular synergy, contributing to regional differences in loading of dopamine into synaptic vesicles. Regional differences in dopamine neuron signaling are likely to be differentially involved in the schizophrenia disease process and likely determine the subregional specificity of the action of psychostimulants that exacerbate the disorder, and antipsychotics that ameliorate the disorder. Elucidating dopamine neuron synaptic signaling offers the potential for achieving greater pharmacological specificity through intersectional pharmacological actions targeting subsets of dopamine neuron synapses.

A role for affect in context-dependent sensitization to amphetamine.

Repeated exposure to stimulant drugs, such as amphetamine, induces sensitization to their behavioral activating effects. It is commonly assumed that behavioral sensitization is expressed in the environment explicitly paired with the drug but not in a different environment explicitly unpaired with the drug. The experiments reported here show that this assumption is incorrect. It was found that sensitization was expressed in an environment explicitly unpaired with amphetamine, but imbued with positive affective valence by its association with a natural reward, oral sucrose. These results suggest that the affective valence of the environment in which the drug is administered plays a decisive role in the expression of drug effects, regardless of any previous association of that environment with the drug.

Sources of input to the rostromedial tegmental nucleus, ventral tegmental area, and lateral habenula compared: A study in rat.

Profound inhibitory control exerted on midbrain dopaminergic neurons by the lateral habenula (LHb), which has mainly excitatory outputs, is mediated by the GABAergic rostromedial tegmental nucleus (RMTg), which strongly innervates dopaminergic neurons in the ventral midbrain. Early reports indicated that the afferent connections of the RMTg, excepting its very strong LHb inputs, do not differ appreciably from those of the ventral tegmental area (VTA). Presumably, however, the RMTg contributes more to behavioral synthesis than to simply invert the valence of the excitatory signal coming from the LHb. Therefore, the present study was done to directly compare the inputs to the RMTg and VTA and, in deference to its substantial involvement with this circuitry, the LHb was also included in the comparison. Data indicated that, while the afferents of the RMTg, VTA, and LHb do originate within the same large pool of central nervous system (CNS) structures, each is also related to structures that project more strongly to it than to the others. The VTA gets robust input from ventral striatopallidum and extended amygdala, whereas RMTg biased inputs arise in structures with a more direct impact on motor function, such as deep layers of the contralateral superior colliculus, deep cerebellar and several brainstem nuclei, and, via a relay in the LHb, the entopeduncular nucleus. Input from the ventral pallidal-lateral preoptic-lateral hypothalamus continuum is strong in the RMTg and VTA and dominant in the LHb. Axon collateralization was also investigated, providing additional insights into the organization of the circuitry of this important triad of structures.

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.

Comparison of the locomotor-activating effects of bicuculline infusions into the preoptic area and ventral pallidum.

Ambulatory locomotion in the rodent is robustly activated by unilateral infusions into the basal forebrain of type A gamma-aminobutyric acid receptor antagonists, such as bicuculline and picrotoxin. The present study was carried out to better localize the neuroanatomical substrate(s) underlying this effect. To accomplish this, differences in total locomotion accumulated during a 20-min test period following bicuculline versus saline infusions in male Sprague-Dawley rats were calculated, rank ordered and mapped on a diagram of basal forebrain transposed from immunoprocessed sections. The most robust locomotor activation was elicited by bicuculline infusions clustered in rostral parts of the preoptic area. Unilateral infusions of bicuculline into the ventral pallidum produced an unanticipatedly diminutive activation of locomotion, which led us to evaluate bilateral ventral pallidal infusions, and these also produced only a small activation of locomotion, and, interestingly, a non-significant trend toward suppression of rearing. Subjects with bicuculline infused bilaterally into the ventral pallidum also exhibited persistent bouts of abnormal movements. Bicuculline infused unilaterally into other forebrain structures, including the bed nucleus of stria terminalis, caudate-putamen, globus pallidus, sublenticular extended amygdala and sublenticular substantia innominata, did not produce significant locomotor activation. Our data identify the rostral preoptic area as the main substrate for the locomotor-activating effects of basal forebrain bicuculline infusions. In contrast, slight activation of locomotion and no effect on rearing accompanied unilateral and bilateral ventral pallidal infusions. Implications of these findings for forebrain processing of reward are discussed.

Protracted maturation of forebrain afferent connections of the ventral tegmental area in the rat.

The mesocorticolimbic dopamine system has long attracted the interest of researchers concerned with the unique gamut of behavioral and mental health vulnerabilities associated with adolescence. Accordingly, the development of the mesocorticolimbic system has been studied extensively, but almost exclusively with regard to dopaminergic output, particularly in the nucleus accumbens and medial prefrontal cortex. To the contrary, the ontogeny of inputs to the ventral tegmental area (VTA), the source of mesocorticolimbic dopamine, has been neglected. This is not a trivial oversight, as the activity of VTA neurons, which reflects their capacity to transmit information about salient events, is sensitively modulated by inputs. Here, we assessed the development of VTA afferent connections using the ß subunit of cholera toxin (Ctß) as a retrograde axonal tracer in adolescent (postnatal day 39) and early adult (8-9-week-old) rats. After intra-VTA injections of Ctß, adolescent and early adult animals exhibited qualitatively similar distributions of retrogradely labeled neurons in the sense that VTA-projecting neurons were present at all of the same rostrocaudal levels in all of the same structures in both age groups. However, quantitation of retrogradely labeled neurons revealed that adolescent brains, compared with early adult brains, had significantly fewer VTA-projecting neurons preferentially within an interconnected network of cortical and striatopallidal forebrain structures. These findings provide a novel perspective on the development of the mesocorticolimbic dopamine system and may have important implications for age-dependent specificity in the function of this system, particularly with regard to adolescent impulsivity and mental health vulnerabilities.

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.