Vol 22, No 9 (2024)
- Year: 2024
- Articles: 13
- URL: https://rjsocmed.com/1570-159X/issue/view/10068
Neurology
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Subcortical Contribution to the Role of the Basal Ganglia in Action Selection



The Basal Ganglia Downstream Control of Action An Evolutionarily Conserved Strategy
Abstract
The motor areas of the cortex and the basal ganglia both contribute to determining which motor actions will be recruited at any moment in time, and their functions are intertwined. Here, we review the basal ganglia mechanisms underlying the selection of behavior of the downstream control of motor centers in the midbrain and brainstem and show that the basic organization of the forebrain motor system is evolutionarily conserved throughout vertebrate phylogeny. The output level of the basal ganglia (e.g. substantia nigra pars reticulata) has GABAergic neurons that are spontaneously active at rest and inhibit a number of specific motor centers, each of which can be relieved from inhibition if the inhibitory output neurons themselves become inhibited. The motor areas of the cortex act partially via the dorsolateral striatum (putamen), which has specific modules for the forelimb, hindlimb, trunk, etc. Each module operates in turn through the two types of striatal projection neurons that control the output modules of the basal ganglia and thereby the downstream motor centers. The mechanisms for lateral inhibition in the striatum are reviewed as well as other striatal mechanisms contributing to action selection. The motor cortex also exerts a direct excitatory action on specific motor centers. An overview is given of the basal ganglia control exerted on the different midbrain/brainstem motor centers, and the efference copy information fed back via the thalamus to the striatum and cortex, which is of importance for the planning of future movements.



Pathways from the Superior Colliculus to the Basal Ganglia
Abstract
The present work aims to review the structural organization of the mammalian superior colliculus (SC), the putative pathways connecting the SC and the basal ganglia, and their role in organizing complex behavioral output. First, we review how the complex intrinsic connections between the SCs laminae projections allow for the construction of spatially aligned, visual-multisensory maps of the surrounding environment. Moreover, we present a summary of the sensory-motor inputs of the SC, including a description of the integration of multi-sensory inputs relevant to behavioral control. We further examine the major descending outputs toward the brainstem and spinal cord. As the central piece of this review, we provide a thorough analysis covering the putative interactions between the SC and the basal ganglia. To this end, we explore the diverse thalamic routes by which information from the SC may reach the striatum, including the pathways through the lateral posterior, parafascicular, and rostral intralaminar thalamic nuclei. We also examine the interactions between the SC and subthalamic nucleus, representing an additional pathway for the tectal modulation of the basal ganglia. Moreover, we discuss how information from the SC might also be relayed to the basal ganglia through midbrain tectonigral and tectotegmental projections directed at the substantia nigra compacta and ventrotegmental area, respectively, influencing the dopaminergic outflow to the dorsal and ventral striatum. We highlight the vast interplay between the SC and the basal ganglia and raise several missing points that warrant being addressed in future studies.



The Basal Ganglia and Mesencephalic Locomotor Region Connectivity Matrix
Abstract
Although classically considered a relay station for basal ganglia (BG) output, the anatomy, connectivity, and function of the mesencephalic locomotor region (MLR) were redefined during the last two decades. In striking opposition to what was initially thought, MLR and BG are actually reciprocally and intimately interconnected. New viral-based, optogenetic, and mapping technologies revealed that cholinergic, glutamatergic, and GABAergic neurons coexist in this structure, which, in addition to extending descending projections, send long-range ascending fibers to the BG. These MLR projections to the BG convey motor and non-motor information to specific synaptic targets throughout different nuclei. Moreover, MLR efferent fibers originate from precise neuronal subpopulations located in particular MLR subregions, defining independent anatomo-functional subcircuits involved in particular aspects of animal behavior such as fast locomotion, explorative locomotion, posture, forelimb- related movements, speed, reinforcement, among others. In this review, we revised the literature produced during the last decade linking MLR and BG. We conclude that the classic framework considering the MLR as a homogeneous output structure passively receiving input from the BG needs to be revisited. We propose instead that the multiple subcircuits embedded in this region should be taken as independent entities that convey relevant and specific ascending information to the BG and, thus, actively participate in the execution and tuning of behavior.



Separation of Channels Subserving Approach and Avoidance/Escape at the Level of the Basal Ganglia and Related Brainstem Structures
Abstract
The basal ganglia have the key function of directing our behavior in the context of events from our environment and/or our internal state. This function relies on afferents targeting the main input structures of the basal ganglia, entering bids for action selection at the level of the striatum or signals for behavioral interruption at the level of the subthalamic nucleus, with behavioral reselection facilitated by dopamine signaling. Numerous experiments have studied action selection in relation to inputs from the cerebral cortex. However, less is known about the anatomical and functional link between the basal ganglia and the brainstem. In this review, we describe how brainstem structures also project to the main input structures of the basal ganglia, namely the striatum, the subthalamic nucleus and midbrain dopaminergic neurons, in the context of approach and avoidance (including escape from threat), two fundamental, mutually exclusive behavioral choices in an animals repertoire in which the brainstem is strongly involved. We focus on three particularly well-described loci involved in approach and avoidance, namely the superior colliculus, the parabrachial nucleus and the periaqueductal grey nucleus. We consider what is known about how these structures are related to the basal ganglia, focusing on their projections toward the striatum, dopaminergic neurons and subthalamic nucleus, and explore the functional consequences of those interactions.



Striatal Acetylcholine and Dopamine Interactions Produce Situationappropriate Action Selection
Abstract
Individuals often learn how to perform new actions for particular outcomes against a complex background of existing action-outcome associations. As such, this new knowledge can interfere or even compete with existing knowledge, such that individuals must use internal and external cues to determine which action is appropriate to the current situation. The question thus remains as to how this problem is solved at a neural level. Research over the last decade or so has begun to determine how the brain achieves situation-appropriate action selection. Several converging lines of evidence suggest that it is achieved through the complex interactions of acetylcholine and dopamine within the striatum in a manner that relies on glutamatergic inputs from the cortex and thalamus. Here we briefly review this evidence, then relate it to several very recent findings to provide new, speculative insights regarding the precise nature of striatal acetylcholine/dopamine interaction dynamics and their relation to situation-appropriate action selection.



Implicit Selective Attention: The Role of the Mesencephalic-basal Ganglia System
Abstract
The ability of the brain to recognize and orient attention to relevant stimuli appearing in the visual field is highlighted by a tuning process, which involves modulating the early visual system by both cortical and subcortical brain areas. Selective attention is coordinated not only by the output of stimulus-based saliency maps but is also influenced by top-down cognitive factors, such as internal states, goals, or previous experiences. The basal ganglia system plays a key role in implicitly modulating the underlying mechanisms of selective attention, favouring the formation and maintenance of implicit sensory-motor memories that are capable of automatically modifying the output of priority maps in sensory-motor structures of the midbrain, such as the superior colliculus. The article presents an overview of the recent literature outlining the crucial contribution of several subcortical structures to the processing of different sources of salient stimuli. In detail, we will focus on how the mesencephalic- basal ganglia closed loops contribute to implicitly addressing and modulating selective attention to prioritized stimuli. We conclude by discussing implicit behavioural responses observed in clinical populations in which awareness is compromised at some level. Implicit (emergent) awareness in clinical conditions that can be accompanied by manifest anosognosic symptomatology (i.e., hemiplegia) or involving abnormal conscious processing of visual information (i.e., unilateral spatial neglect and blindsight) represents interesting neurocognitive "test cases" for inferences about mesencephalicbasal ganglia closed-loops involvement in the formation of implicit sensory-motor memories.



Sensory Reinforced Corticostriatal Plasticity
Abstract
Background:Regional changes in corticostriatal transmission induced by phasic dopaminergic signals are an essential feature of the neural network responsible for instrumental reinforcement during discovery of an action. However, the timing of signals that are thought to contribute to the induction of corticostriatal plasticity is difficult to reconcile within the framework of behavioural reinforcement learning, because the reinforcer is normally delayed relative to the selection and execution of causally-related actions.
Objective:While recent studies have started to address the relevance of delayed reinforcement signals and their impact on corticostriatal processing, our objective was to establish a model in which a sensory reinforcer triggers appropriately delayed reinforcement signals relayed to the striatum via intact neuronal pathways and to investigate the effects on corticostriatal plasticity.
Methods:We measured corticostriatal plasticity with electrophysiological recordings using a light flash as a natural sensory reinforcer, and pharmacological manipulations were applied in an in vivo anesthetized rat model preparation.
Results:We demonstrate that the spiking of striatal neurons evoked by single-pulse stimulation of the motor cortex can be potentiated by a natural sensory reinforcer, operating through intact afferent pathways, with signal timing approximating that required for behavioural reinforcement. The pharmacological blockade of dopamine receptors attenuated the observed potentiation of corticostriatal neurotransmission.
Conclusion:This novel in vivo model of corticostriatal plasticity offers a behaviourally relevant framework to address the physiological, anatomical, cellular, and molecular bases of instrumental reinforcement learning.



Spatiomolecular Characterization of Dopamine D2 Receptors Cells in the Mouse External Globus Pallidus
Abstract
The external globus pallidus (GPe) is part of the basal ganglia circuit and plays a key role in controlling the actions. Although, many evidence indicate that dopamine through its activation of dopamine D2 receptors (D2Rs) modulates the GPe neuronal activity, the precise spatiomolecular characterization of cell populations expressing D2Rs in the mouse GPe is still lacking. By combining single molecule in situ hybridization, cell type-specific imaging analyses, and electrophysiology slice recordings, we found that GPe D2R cells are neurons preferentially localized in the caudal portion of GPe. These neurons comprising pallido-striatal, pallido-nigral, and pallido-cortical neurons segregate into two distinct populations displaying molecular and electrophysiological features of GPe GABAergic PV/NKX2.1 and cholinergic neurons respectively. By clarifying the spatial molecular identity of GPe D2R neurons in the mouse, this work provides the basis for future studies aiming at disentangling the action of dopamine within the GPe.



Inhibitory Pedunculopontine Neurons Gate Dopamine-Mediated Motor Actions of Unsigned Valence
Abstract
Background:The pedunculopontine nucleus (PPN) maintains a bidirectional connectivity with the basal ganglia that supports their shared roles in the selection and execution of motor actions. Previous studies identified a role for PPN neurons in goal-directed behavior, but the cellular substrates underlying this function have not been elucidated. We recently revealed the existence of a monosynaptic GABAergic input from the PPN that inhibits dopamine neurons of the substantia nigra. Activation of this pathway interferes with the execution of learned motor sequences when the actions are rewarded, even though the inhibition of dopamine neurons did not shift the value of the action, hence suggesting executive control over the gating of behavior.
Objective:To test the attributes of the inhibition of dopamine neurons by the PPN in the context of goal-directed behavior regardless of whether the outcome is positive or negative.
Methods:We delivered optogenetic stimulation to PPN GABAergic axon terminals in the substantia nigra during a battery of behavioral tasks with positive and negative valence.
Results:Inhibition of dopamine neurons by PPN optogenetic activation during an appetitive task impaired the initiation and overall execution of the behavioral sequence without affecting the consumption of reward. During an active avoidance task, the same activation impaired the ability of mice to avoid a foot shock, but their escape response was unaffected. In addition, responses to potential threats were significantly attenuated.
Conclusion:Our results show that PPN GABAergic neurons modulate learned, goal-directed behavior of unsigned valence without affecting overall motor behavior.



Two Distinct Neuronal Populations in the Rat Parafascicular Nucleus Oppositely Encode the Engagement in Stimulus-driven Reward-seeking
Abstract
Background::The thalamus is a phylogenetically well-preserved structure. Known to densely contact cortical regions, its role in the transmission of sensory information to the striatal complex has been widely reconsidered in recent years.
Methods::The parafascicular nucleus of the thalamus (Pf) has been implicated in the orientation of attention toward salient sensory stimuli. In a stimulus-driven reward-seeking task, we sought to characterize the electrophysiological activity of Pf neurons in rats.
Results::We observed a predominance of excitatory over inhibitory responses for all events in the task. Neurons responded more strongly to the stimulus compared to lever-pressing and reward collecting, confirming the strong involvement of the Pf in sensory information processing. The use of long sessions allowed us to compare neuronal responses to stimuli between trials when animals were engaged in action and those when they were not. We distinguished two populations of neurons with opposite responses: MOTIV+ neurons responded more intensely to stimuli followed by a behavioral response than those that were not. Conversely, MOTIV- neurons responded more strongly when the animal did not respond to the stimulus. In addition, the latency of excitation of MOTIV- neurons was shorter than that of MOTIV+ neurons.
Conclusion::Through this encoding, the Pf could perform an early selection of environmental stimuli transmitted to the striatum according to motivational level



The Integration of Top-down and Bottom-up Inputs to the Striatal Cholinergic Interneurons
Abstract
Background:Cholinergic interneurons (ChIs) are important for learning and memory. They exhibit a multiphasic excitation-pause-rebound response to reward or sensory cues indicating a reward, believed to gate dopamine-dependent learning. Although ChIs receive extensive top-down inputs from the cortex and bottom-up inputs from the thalamus and midbrain, it is unclear which inputs are involved in the development of ChI multiphasic activity.
Methods:We used a single-unit recording of putative ChIs (pChIs) in response to cortical and visual stimulation to investigate how top-down and bottom-up inputs regulate the firing pattern of ChIs.
Results:We demonstrated that cortical stimulation strongly regulates pChIs, with the maximum firing rate occurring at the peak of the inverted local field potential (iLFP), reflecting maximum cortical stimulation. Pauses in pChIs occurred during the descending phase of iLFP, indicating withdrawal of excitatory cortical input. Visual stimulation induced long pauses in pChIs, but it is unlikely that bottom- up inputs alone induce pauses in behaving animals. Also, the firing pattern of ChIs triggered by visual stimulation did not correlate with the iLFP as it did after cortical stimulation. Top-down and bottom-up inputs independently regulate the firing pattern of ChIs with similar efficacy but notably produce a well-defined pause in ChI firing.
Conclusion:This study provides in vivo evidence that the multiphasic ChI response may require both top-down and bottom-up inputs. The findings suggest that the firing pattern of ChIs correlated to the iLFP might be a useful tool for estimating the degree of contribution of top-down and bottom-up inputs in regulating the firing activity of ChIs.


