Pharmacological targeting of striatal indirect pathway neurons improves subthalamic nucleus dysfunction and reduces repetitive behaviors in C58 mice.

Repetitive behaviors (e.g., stereotypic movements, compulsions, rituals) are common features of a number of neurodevelopmental disorders. Clinical and animal model studies point to the importance of cortical-basal ganglia circuitry in the mediation of repetitive behaviors. In the current study, we tested whether a drug cocktail (dopamine D2 receptor antagonist + adenosine A2A receptor agonist + glutamate mGlu5 positive allosteric modulator) designed to activate the indirect basal ganglia pathway would reduce repetitive behavior in C58 mice after both acute and sub-chronic administration. In addition, we hypothesized that sub-chronic administration (i.e. 7 days of twice-daily injections) would increase the functional activation of the subthalamic nucleus (STN), a key node of the indirect pathway. Functional activation of STN was indexed by dendritic spine density, analysis of GABA, glutamate, and synaptic plasticity genes, and cytochrome oxidase activity. The drug cocktail used significantly reduced repetitive motor behavior in C58 mice after one night as well as seven nights of twice-nightly injections. These effects did not reflect generalized motor behavior suppression as non-repetitive motor behaviors such as grooming, digging and eating were not reduced relative to vehicle. Sub-chronic drug treatment targeting striatopallidal neurons resulted in significant changes in the STN, including a four-fold increase in brain-derived neurotrophic factor (BDNF) mRNA expression as well as a significant increase in dendritic spine density. The present findings are consistent with, and extend, our prior work linking decreased functioning of the indirect basal ganglia pathway to expression of repetitive motor behavior in C58 mice and suggest novel therapeutic targets.

Early exposure to a methyl donor supplemented diet and the development of repetitive motor behavior in a mouse model.

Motor behaviors that are repetitive and exhibit little variability in form are common in neurodevelopmental disorders (e.g., autism spectrum disorder). C58 mice exhibit persistent, high levels of repetitive motor behavior when reared in restricted, but not enriched, environments implicating epigenetic mechanisms (e.g., DNA methylation). We sought to determine if alteration of DNA methylation played a role in the development of repetitive behavior in C58 mice. Thus, we tested the hypothesis that early exposure (in utero and preweaning) to a methyl donor supplemented diet would alter the developmental trajectory of repetitive behavior. Such dietary exposure resulted in significant attenuation of repetitive motor behavior development, persisting through early adulthood. This was despite mice being housed in standard cages and maintained on a standard diet, postweaning. Early exposure to methyl donor supplementation not only affected the frequency of repetitive behavior but also its temporal structure, resulting in more variable patterns of repetitive behavior. Early exposure to the diet was also shown to induce long-lasting increases in DNA methylation in brain tissue of female mice. The role for alterations in DNA methylation in this model may be one mechanism accounting for the robust effects of the environment on the development of repetitive behavior.

Reduction of repetitive behavior by co-administration of adenosine receptor agonists in C58 mice.

Repetitive behaviors are diagnostic for autism spectrum disorder (ASD) and commonly observed in other neurodevelopmental disorders. Currently, there are no effective pharmacological treatments for repetitive behavior in these clinical conditions. This is due to the lack of information about the specific neural circuitry that mediates the development and expression of repetitive behavior. Our previous work in mouse models has linked repetitive behavior to decreased activation of the subthalamic nucleus, a brain region in the indirect and hyperdirect pathways in the basal ganglia circuitry. The present experiments were designed to further test our hypothesis that pharmacological activation of the indirect pathway would reduce repetitive behavior. We used a combination of adenosine A1 and A2A receptor agonists that have been shown to alter the firing frequency of dorsal striatal neurons within the indirect pathway of the basal ganglia. This drug combination markedly and selectively reduced repetitive behavior in both male and female C58 mice over a six-hour period, an effect that required both A1 and A2A agonists as neither alone reduced repetitive behavior. The adenosine A1 and A2A receptor agonist combination also significantly increased the number of Fos transcripts and Fos positive cells in dorsal striatum. Fos induction was found in both direct and indirect pathway neurons suggesting that the drug combination restored the balance of activation across these complementary basal ganglia pathways. The adenosine A1 and A2A receptor agonist combination also maintained its effectiveness in reducing repetitive behavior over a 7-day period. These findings point to novel potential therapeutic targets for development of drug therapies for repetitive behavior in clinical disorders.

Targeting Dopamine D2, Adenosine A2A, and Glutamate mGlu5 Receptors to Reduce Repetitive Behaviors in Deer Mice.

Repetitive behaviors are seemingly purposeless patterns of behavior that vary little in form and are characteristic of many neurodevelopmental, psychiatric, and neurologic disorders. Our work has identified an association between hypofunctioning of the indirect basal ganglia pathway and the expression of repetitive behavior in the deer mouse model. In this study, we targeted indirect pathway cells of the striatum with single drugs and drug combinations that bind to dopamine D2, adenosine A2A, and glutamate mGlu5 receptors. These receptors function both individually and as receptor heteromers. We found that only the triple drug cocktail (L-741,626+CGS21680+CDPPB) that was designed to increase striatal indirect basal ganglia pathway cell function reduced repetitive behavior in adult male deer mice. No single drug or double drug combinations were effective at selectively reducing repetitive behavior. We found this triple drug cocktail reduced repetitive behavior in both short-acting and long-acting formulations and was effective throughout 7 days of daily administration. Conversely, another triple drug cocktail (quinpirole+SCH58261+MTEP) that was designed to further reduce striatal indirect basal ganglia pathway cell function caused a significant increase in repetitive behavior. Significant and behaviorally selective effects on repetitive behavior were only achieved with the triple drug cocktails that included doses of L-741,626 and quinpirole that have off-target effects (e.g., dopamine D3 receptors). These data further a role for decreased indirect basal ganglia pathway activation in repetitive behavior and suggest that targeting these receptors and/or heteromeric complexes on the indirect pathway neurons of the striatum may offer pharmacotherapeutic benefit for individuals with repetitive behavior disorders.

Repetitive motor behavior: further characterization of development and temporal dynamics.

Repetitive behaviors are diagnostic for autism spectrum disorders, common in related neurodevelopmental disorders, and normative in typical development. In order to identify factors that mediate repetitive behavior development, it is necessary to characterize the expression of these behaviors from an early age. Extending previous findings, we characterized further the ontogeny of stereotyped motor behavior both in terms of frequency and temporal organization in deer mice. A three group trajectory model provided a good fit to the frequencies of stereotyped behavior across eight developmental time points. Group based trajectory analysis using a measure of temporal organization of stereotyped behavior also resulted in a three group solution. Additionally, as the frequency of stereotyped behavior increased with age, the temporal distribution of stereotyped responses became increasingly regular or organized indicating a strong association between these measures. Classification tree and principal components analysis showed that accurate classification of trajectory group could be done with fewer observations. This ability to identify trajectory group membership earlier in development allows for examination of a wide range of variables, both experiential and biological, to determine their impact on altering the expected trajectory of repetitive behavior across development. Such studies would have important implications for treatment efforts in neurodevelopmental disorders such as autism.

Elucidating the role of neurotensin in the pathophysiology and management of major mental disorders.

Neurotensin (NT) is a neuropeptide that is closely associated with, and is thought to modulate, dopaminergic and other neurotransmitter systems involved in the pathophysiology of various mental disorders. This review outlines data implicating NT in the pathophysiology and management of major mental disorders such as schizophrenia, drug addiction, and autism. The data suggest that NT receptor analogs have the potential to be used as novel therapeutic agents acting through modulation of neurotransmitter systems dys-regulated in these disorders.

Individual differences in vulnerability for self-injurious behavior: studies using an animal model.

Self-injurious behavior (SIB) is a debilitating characteristic that is prevalent across a broad spectrum of neurodevelopmental disorders. In most of these disorders, some individuals exhibit SIB, whereas others do not. However, the neurobiological mechanisms that confer vulnerability are virtually unexplored. We examined innate characteristics that contribute to vulnerability or resistance for SIB in an animal model of the behavioral pathology. Eighteen outbred Long-Evans rats were screened for behavioral responsiveness to the mild stress of a novel environment. The rats were then categorized as high responders (HR; those rats that had the highest locomotor counts) or low responders (LR; those rats that had lower locomotor counts) by median split. All the rats were then given daily injections of the indirect monoamine agonist pemoline (150 mg/kg/day) for 10 days, and self-injury was evaluated. All 9 HR rats and 5 of the 9 LR rats exhibited self-injury. The HR rats spent more time self-injuring, injured more body sites, and caused larger areas of tissue damage than the LR rats did. Furthermore, the behavioral responsiveness to novelty stress was significantly correlated with each of these measures of self-injury. The HR rats did not exhibit substantially enhanced responses on other measures of psychostimulant action (stereotypy, grooming, locomotion, rearing). Accordingly, vulnerability to develop pemoline-induced SIB is positively correlated with, and can be predicted based upon, a behavioral measure of innate stress responsiveness. These findings suggest that characteristics that are common in developmental disorders may help predispose afflicted individuals to self-injure. The findings also extend the variety of behavioral pathologies (e.g. drug addiction) for which the HR/LR model predicts vulnerability.

Glutamate-mediated neuroplasticity in an animal model of self-injurious behaviour.

Self-injurious behaviour (SIB) is exhibited by individuals with a broad variety of developmental disorders and genetic abnormalities, including autism and Lesch-Nyhan, Prader-Willi and Rett syndromes. Most research has focused on environmental factors that reinforce SIB, and less is known about the biological basis of this behaviour disorder. However, animal models have been developed to study the neurochemical pathology that underlies SIB. In one model, rats exhibit self-biting after repeated daily administration of moderately high doses of pemoline (100-200mg/kg). Dopaminergic and glutamatergic neurotransmission have been implicated in this model. Accordingly, we investigated the role of glutamatergic neurotransmission in pemoline-induced SIB, using the N-methyl-d-aspartate (NMDA) receptor antagonists MK-801 and memantine. MK-801 is a high affinity antagonist which blocks glutamate-mediated neuroplasticity and behavioural sensitization to other psychostimulants. It lessened the incidence of SIB, the time spent self-injuring, and the area of tissue damage in the pemoline model. Memantine, on the other hand, is a low affinity antagonist which does not disrupt glutamate-mediated neuroplasticity, and it had little if any effect on any measure of pemoline-induced SIB. These results suggest that repeated pemoline administration induces glutamate-mediated neuroplastic changes that lead to the eventual expression of SIB. Further investigation of these changes may reveal specific neurochemical factors that contribute to SIB in this animal model of self-injury.

Pemoline (2-amino-5-phenyl-1,3-oxazol-4-one)-induced self-injurious behavior: a rodent model of pharmacotherapeutic efficacy.

Self-injury is a devastating, maladaptive behavior disorder that is common in developmental disabilities and is comorbid with numerous psychiatric disorders. Examples of self-injurious behavior (SIB) include head-banging, self-biting, and self-punching. The neurochemical basis of SIB is unknown; however, many different classes of drugs are prescribed (e.g., neuroleptics, atypical neuroleptics, anti-epileptics, opioid antagonists) to reduce these behaviors. These drugs have all shown clinically significant but limited efficacy in patient populations, and no class of drug is effective for all patients. The development and characterization of a valid animal model could provide important information regarding the neurochemical basis of SIB and could be used to screen potential new pharmacotherapies. In one model of SIB, high doses of pemoline (2-amino-5-phenyl-1,3-oxazol-4-one) are administered to rats. Using this model, we evaluated the effectiveness of three drugs (risperidone, valproate, and topiramate) that reduce SIB in humans. We also screened the potential effectiveness of tramadol, a drug that decreases stereotyped and compulsive behaviors but has not been assessed in human self-injurers. We found that risperidone, valproate, and topiramate each significantly attenuate pemoline-induced SIB, whereas tramadol does not. These findings suggest that the pemoline model of SIB has predictive validity across a range of drug classes and implicate important potential neurochemical mechanisms that may contribute to the behavior disorder. The findings also indicate that tramadol may not be an effective pharmacotherapy for SIB.

Nifedipine suppresses self-injurious behaviors in animals.

Self-injurious behavior is a common problem in many developmental disorders. The neurobiology of this behavior is not well understood, but the differing behavioral manifestations and associations with different disorders suggest that the underlying biological mechanisms are heterogeneous. The behavioral and biological heterogeneity is also evident in several animal models, where different manifestations can be provoked under different experimental conditions. Identifying commonalities among the different mechanisms is likely to be helpful in the design of treatments useful for the broadest populations of patients. The current studies reveal that nifedipine suppresses self-injurious behavior in 4 unrelated animal models: acute administration of high doses of +/-BayK 8644 or methamphetamine in mice, dopamine agonist treatment in rats with lesions of dopamine pathways during early development and repeated administration of pemoline in rats. The effect of nifedipine does not appear to be due to nonspecific mechanisms, such as sedation, since other classes of behaviors are unaffected or exaggerated. These results suggest that nifedipine may target a common biological mechanism in the expression of self-injurious behavior, and they suggest it should be considered in the treatment of self-injury in humans.