A role for dopamine-mediated learning in the pathophysiology and treatment of Parkinson's disease

Review: electrophysiology of basal ganglia and cortex in models of Parkinson disease

Incomplete understanding of the systems-level pathophysiology of Parkinson Disease (PD) remains a significant barrier to improving its treatment. Substantial progress has been made, however, due to the availability of neurotoxins that selectively target monoaminergic (in particular, dopaminergic) neurons. This review discusses the in vivo electrophysiology of basal ganglia (BG), thalamic, and cortical regions after dopamine-depleting lesions. These include firing rate changes, neuronal burst-firing, neuronal oscillations, and neuronal synchrony that result from a combination of local microanatomic changes and network-level interactions. While much is known of the clinical and electrophysiological phenomenology of dopamine loss, a critical gap in our conception of PD pathophysiology is the link between them. We discuss potential mechanisms by which these systems-level electrophysiological changes may emerge, as well as how they may relate to clinical parkinsonism. Proposals for an updated understanding of BG function are reviewed, with an emphasis on how emerging frameworks will guide future research into the pathophysiology and treatment of PD.

Cyclic AMP and afferent activity govern bidirectional synaptic plasticity in striatopallidal neurons

Recent experimental evidence suggests that the low dopamine conditions in Parkinson's disease (PD) cause motor impairment through aberrant motor learning. Those data, along with computational models, suggest that this aberrant learning results from maladaptive corticostriatal plasticity and learned motor inhibition. Dopaminergic modulation of both corticostriatal long-term depression (LTD) and long-term potentiation (LTP) is proposed to be critical for these processes; however, the regulatory mechanisms underlying bidirectional corticostriatal plasticity are not fully understood. Previously, we demonstrated a key role for cAMP signaling in corticostriatal LTD. In this study, mouse brain slices were used to perform a parametric experiment that tested the impact of varying both intracellular cAMP levels and the strength of excitatory inputs on corticostriatal plasticity. Using slice electrophysiology in the dorsolateral striatum, we demonstrate that both LTP and LTD can be sequentially induced in the same D2-expressing neuron and that LTP was strongest with high intracellular cAMP and LFS, whereas LTD required low intracellular cAMP and high-frequency stimulation. Our results provide a molecular and cellular basis for regulating bidirectional corticostriatal synaptic plasticity and may help to identify novel therapeutic targets for blocking or reversing the aberrant synaptic plasticity that likely contributes to motor deficits in PD.

Dissociable effects of dopamine on learning and performance within sensorimotor striatum

Striatal dopamine is an important modulator of current behavior, as seen in the rapid and dramatic effects of dopamine replacement therapy in Parkinson Disease (PD). Yet there is also extensive evidence that dopamine acts as a learning signal, modulating synaptic plasticity within striatum to affect future behavior. Disentangling these "performance" and "learning" functions is important for designing effective, long-term PD treatments. We conducted a series of unilateral drug manipulations and dopamine terminal lesions in the dorsolateral striatum of rats highly-trained to perform brief instructed head/neck movements (two-alternative forced choice task). Reaction times and accuracy were measured longitudinally to determine if task behavior changed immediately, progressed over time, and/or persisted after drug withdrawal. Enhanced dopamine signaling with amphetamine caused an immediate, nonprogressive, and bilateral decrease in reaction times (RT). The altered RT distributions were consistent with reduced distance to threshold in the linear approach to threshold with ergodic rate (LATER) model of decision-making. Conversely, the dopamine antagonist flupenthixol caused experience-dependent, persistent changes in RT and accuracy indicative of a "learning" effect. These RT distributions were consistent with a slowed rate of approach to decision threshold. Our results show that dopaminergic signaling makes dissociable contributions to current and future behavior even within a single striatal subregion, and provide important clues for both models of normal decision-making and the design of novel drug therapies in PD.

Mouse models of neurodevelopmental disease of the basal ganglia and associated circuits

This chapter focuses on neurodevelopmental diseases that are tightly linked to abnormal function of the striatum and connected structures. We begin with an overview of three representative diseases in which striatal dysfunction plays a key role--Tourette syndrome and obsessive-compulsive disorder, Rett's syndrome, and primary dystonia. These diseases highlight distinct etiologies that disrupt striatal integrity and function during development, and showcase the varied clinical manifestations of striatal dysfunction. We then review striatal organization and function, including evidence for striatal roles in online motor control/action selection, reinforcement learning, habit formation, and action sequencing. A key barrier to progress has been the relative lack of animal models of these diseases, though recently there has been considerable progress. We review these efforts, including their relative merits providing insight into disease pathogenesis, disease symptomatology, and basal ganglia function.

The use and abuse of large-scale brain models

We provide an overview and comparison of several recent large-scale brain models. In addition to discussing challenges involved with building large neural models, we identify several expected benefits of pursuing such a research program. We argue that these benefits are only likely to be realized if two basic guidelines are made central to the pursuit. The first is that such models need to be intimately tied to behavior. The second is that models, and more importantly their underlying methods, should provide mechanisms for varying the level of simulated detail. Consequently, we express concerns with models that insist on a 'correct' amount of detail while expecting interesting behavior to simply emerge.

Nicotinic receptors regulate the dynamic range of dopamine release in vivo

Nicotinic acetylcholine receptors (nAChRs) are expressed presynaptically on dopamine axon terminals, and their activation by endogenous acetylcholine from striatal cholinergic interneurons enhances dopamine release both independently of and in concert with dopamine neuron activity. Acute nAChR inactivation is believed to enhance the contrast between low- and high-frequency dopamine cell activity. Although these studies reveal a key role for acute activation and inactivation of nAChRs in striatal microcircuitry, it remains unknown if chronic inactivation/desensitization of nAChRs can alter dopamine release dynamics. Using in vivo cyclic voltammetry in anaesthetized mice, we examined whether chronic inactivation of nAChRs modulates dopamine release across a parametric range of stimulation, varying both frequency and pulse number. Deletion of β2*nAChRs and chronic nicotine exposure greatly diminished dopamine release across the entire range of stimulation parameters. In addition, we observed a facilitation of dopamine release at low frequency and pulse number in wild-type mice that is absent in the β2* knockout and chronic nicotine mice. These data suggest that deletion or chronic desensitization of nAChRs reduces the dynamic range of dopamine release in response to dopamine cell activity, decreasing rather than increasing contrast between high and low dopamine activity.

The Enemy within: Propagation of Aberrant Corticostriatal Learning to Cortical Function in Parkinson's Disease

Motor dysfunction in Parkinson’s disease is believed to arise primarily from pathophysiology in the dorsal striatum and its related corticostriatal and thalamostriatal circuits during progressive dopamine denervation. One function of these circuits is to provide a filter that selectively facilitates or inhibits cortical activity to optimize cortical processing, making motor responses rapid and efficient. Corticostriatal synaptic plasticity mediates the learning that underlies this performance-optimizing filter. Under dopamine denervation, corticostriatal plasticity is altered, resulting in aberrant learning that induces inappropriate basal ganglia filtering that impedes rather than optimizes cortical processing. Human imaging suggests that increased cortical activity may compensate for striatal dysfunction in PD patients. In this Perspective article, we consider how aberrant learning at corticostriatal synapses may impair cortical processing and learning and undermine potential cortical compensatory mechanisms. Blocking or remediating aberrant corticostriatal plasticity may protect cortical function and support cortical compensatory mechanisms mitigating the functional decline associated with progressive dopamine denervation.

Frontal theta overrides pavlovian learning biases

Pavlovian biases influence learning and decision making by intricately coupling reward seeking with action invigoration and punishment avoidance with action suppression. This bias is not always adaptive-it can often interfere with instrumental requirements. The prefrontal cortex is thought to help resolve such conflict between motivational systems, but the nature of this control process remains unknown. EEG recordings of midfrontal theta band power are sensitive to conflict and predictive of adaptive control over behavior, but it is not clear whether this signal reflects control over conflict between motivational systems. Here we used a task that orthogonalized action requirements and outcome valence while recording concurrent EEG in human participants. By applying a computational model of task performance, we derived parameters reflective of the latent influence of Pavlovian bias and how it was modulated by midfrontal theta power during motivational conflict. Between subjects, those who performed better under Pavlovian conflict exhibited higher midfrontal theta power. Within subjects, trial-to-trial variance in theta power was predictive of ability to overcome the influence of the Pavlovian bias, and this effect was most pronounced in subjects with higher midfrontal theta to conflict. These findings demonstrate that midfrontal theta is not only a sensitive index of prefrontal control, but it can also reflect the application of top-down control over instrumental processes.

The role of neuroplasticity in dopaminergic therapy for Parkinson disease

Dopamine replacement is a mainstay of therapeutic strategies for Parkinson disease (PD). The motor response to therapy involves an immediate improvement in motor function, known as the short-duration response (SDR), followed by a long-duration response (LDR) that develops more slowly, over weeks. Here, we review evidence in patients and animal models suggesting that dopamine-dependent corticostriatal plasticity, and retention of such plasticity in the absence of dopamine, are the mechanisms underlying the LDR. Conversely, experience-dependent aberrant plasticity that develops slowly under reduced dopamine levels could contribute substantially to PD motor symptoms before initiation of dopamine replacement therapy. We place these findings in the context of the role of dopamine in basal ganglia function and corticostriatal plasticity, and provide a new framework suggesting that therapies that enhance the LDR could be more effective than those targeting the SDR. We further propose that changes in neuroplasticity constitute a form of disease modification that is distinct from prevention of degeneration, and could be responsible for some of the unexplained disease-modifying effects of certain therapies. Understanding such plasticity could provide novel therapeutic approaches that combine rehabilitation and pharmacotherapy for treatment of neurological and psychiatric disorders involving basal ganglia dysfunction.