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

Behavioral sensitization to amphetamine: an emerging role for netrin-1 guidance cue proteins

The netrin-1 guidance cue and its receptors, DCC and UNC5H, contribute to the organization and function of mesocorticolimbic dopamine (mDA) circuitry during development. Interestingly, netrin-1 and its receptors continue to be expressed in mDA cell body and terminal regions throughout the lifespan, although there is a dramatic shift in the ratio of DCC: UNC5H receptor expression by mDA neurons in the ventral tegmental area (VTA), the mDA cell body region, during the adolescent period. This suggests that netrin-1 signaling may also play a role in the plasticity of MDA circuitry across the lifespan. Indeed, sensitizing regimens of the stimulant drug, amphetamine (AMPH), significantly upregulate DCC expression in adult rats. This upregulation, which is glutamate-dependent, is selective to the VTA, a site critical for the initiation of behavioral plasticity induced by stimulant drugs of abuse. Furthermore, adult dcc heterozygous (+/-) mice are "protected" against the sensitizing behavioral effects of repeated AMPH treatment. The aims of this Thesis were twofold: In the first instance, experiments were designed to determine whether netrin-1 signaling in the VTA plays a functional role in AMPH-induced sensitization during adulthood. Using a combination of behavioral and molecular techniques, it is demonstrated that the lack of AMPH-induced sensitization in adult dcc +/- mice is associated with a failure of AMPH to both upregulate DCC receptor expression and induce dendritic plasticity in the VTA. In parallel experiments conducted in adult rats, it is shown that VTA DCC receptor activation, at the time of AMPH pretreatment, is critical for sensitization to AMPH. These data implicate VTA DCC receptor signaling as a novel mechanism in the series of glutamate-dependent cellular processes that lead to enduring plasticity by stimulant drugs of abuse during adulthood. However, the effects of DCC/netrin-1 signaling on mDA system function have been shown to change across development, with the adolescent period being a crucial time in this netrin-1 /DA system interaction. Thus, the second aim of this Thesis was to begin to explore the role for netrin-1 receptor signaling in AMPH-induced sensitization during juvenile and adolescent periods. It is demonstrated that repeated exposure to AMPH during these early periods differentially regulates DCC expression in the VTA, in comparison to during adulthood. Significantly, it is also shown that the "protective" phenotype of adult dcc +/- mice is not present before adolescence, and in fact, is abolished when these mice are treated with AMPH during the juvenile, but not adolescent, period. Because DCC heterozygosity has recently been identified in the human population, these results may be relevant to the debate about stimulant use in children. Collectively, the findings reported in this Thesis are the first to demonstrate a function of DCC/netrin-1 signaling in the adult brain. DCC/netrin-1 receptor signaling may be a key factor in determining individual differences in vulnerability to drug abuse at different ages.La nétrine-1, une molécule de guidage axonale, et ses récepteurs, DCC et UNC5H, contribuent à l'organisation et à la fonction des circuit dopaminergiques mésocorticolimbiques pendant le développement cérébral. Fait intéressant, la nétrine-1 et ses récepteurs continuent à s'exprimer dans les régions du corps cellulaire et des terminaisons nerveuses de la dopamine mésocorticolimbique tout au long de la vie, malgré un changement radical du ratio de l'expression des récepteurs DCC:UNC5H par les neurones de la dopamine mésocorticolimbique dans l'aire tegmentale ventrale (ATV), la région somato-dendritique de la dopamine mésocorticolimbique, à l'adolescence. Ceci suggère que la signalisation induite par la nétrine-1 est impliquée dans la plasticité du circuit dopaminergique mésocorticolimbique tout au long de la vie. En effet, la sensibilisation à l'amphétamine (AMPH), une drogue stimulante, provoque une augmentation considérable du niveau d'expression de DCC chez les rats d'âge adulte. Cette augmentation, qui depend de la « transmission » glutaminergique, est propre à l'ATV, une zone cruciale pour l'initiation de la plasticité comportementale en réponse à l'abus de drogues stimulantes. En outre, les souris adultes hétérozygotes (+/-) pour le gène encodant DCC sont « prémunies » contre les effets de la sensibilisation comportementale en réponse à l'administration répétée d'AMPH. Les objectifs de cette thèse comportaient deux volets : en premier lieu, le but était de déterminer si la signalisation induite par la nétrine-1 dans l'ATV joue un rôle fonctionnel dans la sensibilisation en réponse à l'AMPH à l'âge adulte. À l'aide d'une combinaison de techniques comportementales et moléculaires, il est démontré que le manque de sensibilisation en réponse à l'AMPH chez les souris adultes +/- pour le gène encodant dcc est lié à l'incapacité de l'AMPH d'augmenter le niveau d'expression du récepteur DCC et de provoquer la plasticité dendritique dans l'ATV. Dans des expériences parallèles menées chez des rats adultes, il est montré que l'activation du récepteur DCC dans l'ATV au moment du prétraitement à l'AMPH est cruciale pour la sensibilisation à l'AMPH. Ces données décrivent la signalisation par le récepteur DCC de l'AVT comme nouveau mécanisme dans les séries de processus cellulaires nécessitant la présence de glutamate qui mènent à une plasticité durable avec l'usage de drogues stimulantes à l'âge adulte. Toutefois, il a été démontré que les effets de la signalisation par DCC/nétrine-1 sur la fonction du système dopaminergique mésocorticolimbique changent durant le développement cérébral, et que l'adolescence est une période cruciale dans cette interaction entre la nétrine-1 et le système dopaminergique mésocorticolimbique. Par conséquent, le deuxième volet de cette thèse visait à commencer l'étude du rôle de la signalisation des récepteurs pour la nétrine-1 dans la sensibilisation en réponse à l'AMPH pendant les périodes juvéniles et adolescentes. Il est démontré que l'exposition répétée à l'AMPH lors de ces périodes d'âge précoce régule de manière différente l'expression du DCC dans l'ATV, comparativement à la période de l'âge adulte. De manière significative, il est également démontré que le phénotype « protecteur » des souris adultes +/- pour le gène encodant dcc est absent avant l'adolescence, et en fait, disparaît lorsque ces souris sont soumises à des traitements d'AMPH lors de la période juvénile, contrairement à la période adolescente. Étant donné que l'hétérozygosité DCC a été récemment identifiée chez l'être humain, ces résultats peuvent être pertinents au débat sur les implications de l'usage de stimulants chez les enfants. Globalement, les résultats présentés dans cette thèse démontrent pour la première fois que la signalisation par DCC/nétrine-1 joue un rôle dans le cerveau adulte

The development of sensitization to amphetamine : a possible involvement of netrin-1 receptors

Repeated exposure to amphetamine (AMPH) induces sensitization to its behavioral-activating effects. The development of sensitization depends on (1) the direct actions of AMPH in the ventral tegmental area (VTA), the cell body region of the mesocorticolimbic DA system, and (2) AMPH-induced glutamatergic neurotransmission in this region. Moreover, sensitization is accompanied by morphological changes in mesocorticolimbic DA circuitry. During development, the DA system is organized, at least in part, by the netrin-1 family of guidance cues. Both netrin-1 and its DCC and UNC-5 receptors continue to be expressed in the mesocorticolimbic DA system of the adult brain. Importantly, netrin-1 receptor deficient mice do not develop sensitization to AMPH, implicating an involvement of netrin-1 signaling in AMPH-induced plasticity of the DA system. To explore this possibility, adult rats were pretreated with repeated AMPH or saline, and DCC and UNC-5 receptor expression was examined in DA cell body and terminal regions using western blot. Striking AMPH-induced increases in the expression of DCC and UNC-5 were observed in the VTA only. Remarkably, these changes depended on NMDA-mediated glutamatergic neurotransmission. This is the first demonstration that repeated AMPH pretreatment regulates netrin-1 receptor expression in the adult brain and suggests that netrin-1 receptor regulation may be involved in the development of AMPH-induced sensitization