A GABAergic nigrotectal pathway for coordination of drinking behavior

A collicular map for touch-guided tongue control

Accurate goal-directed behaviour requires the sense of touch to be integrated with information about body position and ongoing motion1,2. Behaviours such as chewing, swallowing and speech critically depend on precise tactile events on a rapidly moving tongue3, but neural circuits for dynamic touch-guided tongue control are unknown. Here, using high-speed videography, we examined three-dimensional lingual kinematics as mice drank from a water spout that unexpectedly changed position during licking, requiring re-aiming in response to subtle contact events on the left, centre or right surface of the tongue. Mice integrated information about both precise touch events and tongue position to re-aim ensuing licks. Touch-guided re-aiming was unaffected by photoinactivation of tongue sensory, premotor and motor cortices, but was impaired by photoinactivation of the lateral superior colliculus (latSC). Electrophysiological recordings identified latSC neurons with mechanosensory receptive fields for precise touch events that were anchored in tongue-centred, head-centred or conjunctive reference frames. Notably, latSC neurons also encoded tongue position before contact, information that is important for tongue-to-head-based coordinate transformations underlying accurate touch-guided aiming. Viral tracing revealed tongue sensory inputs to the latSC from the lingual trigeminal nucleus, and optical microstimulation in the latSC revealed a topographic map for aiming licks. These findings demonstrate that touch-guided tongue control relies on a collicular mechanosensorimotor map, analogous to collicular visuomotor maps associated with visually guided orienting across many species.

Functional segregation and dynamic integration of the corticotectal descending signal in rat

The superior colliculus (SC) receives inputs from various brain regions in a layer- and radial subregion-specific manner, but whether the SC exhibits subregion-specific dynamics remains unclear. To address this issue, we recorded the spiking activity of single SC neurons while photoactivating cortical areas in awake head-fixed Thy1-ChR2 rats. We classified 309 neurons that responded significantly into 8 clusters according to the response dynamics. Among them, neurons with monophasic excitatory responses (7-12 ms latency) that returned to baseline within 20 ms were commonly observed in the optic and intermediate gray layers of centromedial and centrolateral SC. In contrast, neurons with complex polyphasic responses were commonly observed in the deep layers of the anterolateral SC. Cross-correlation analysis suggested that the complex pattern could be only partly explained by an internal circuit of the deep gray layer. Our results indicate that medial to centrolateral SC neurons simply relay cortical activity, whereas neurons in the deep layers of the anterolateral SC dynamically integrate inputs from the cortex, SNr, CN, and local circuits. These findings suggest a spatial gradient in SC integration, with a division of labor between simple relay circuits and those integrating complex dynamics.

The superior colliculus directs goal-oriented forelimb movements

Skilled forelimb control is essential for daily living, yet our understanding of its neural mechanisms, although extensive, remains incomplete. Here, we present evidence that the superior colliculus (SC), a major midbrain structure, is necessary for accurate forelimb reaching in mice. We found that neurons in the lateral SC are active during goal-directed reaching, and by employing chemogenetic and phase-specific optogenetic silencing of these neurons, we show that the SC causally facilitates reach accuracy. Anatomical studies identified the deep cerebellar nuclei and substantia nigra pars reticulata as sources of inputs to the SC, while functional studies revealed a role for nigrotectal, but not cerebellotectal, neurons in controlling reach endpoints. Silencing the nigrotectal pathway caused paw deviations opposite to those seen with SC silencing, emphasizing the coordinated role of the substantia nigra and SC in regulating optimal reaching. Together, these findings establish the SC as a crucial regulator of skilled forelimb control.

Oral Transport, Penetration, and Aspiration in PD: Insights from a RCT on STN + SNr Stimulation

Dysphagia is frequent and detrimental in advanced Parkinson's disease (PD) and does not respond to standard treatments. Experimental models suggested that pathological overactivity of the substantia nigra pars reticulata (SNr) may hinder oral contributions to swallowing. Here, we hypothesized that the combined stimulation of subthalamic nucleus (STN) and SNr improves measures of dysphagia after eight weeks of active treatment. We enrolled 20 PD patients with dysphagia and deep brain stimulation (DBS). Patients were assessed in 'medication on' and 'STN' stimulation at baseline (V1) and then were randomized 1:1 to 'STN' or 'STN + SNr' stimulation. In addition, patients of both groups received swallowing therapy as a standard of care. The primary endpoint was the change in Penetration-Aspiration Scale (PAS) at eight-week follow-up (V2) with respect to the baseline (V1) under the hypothesis, that 'STN + SNr' was superior to 'STN'. We obtained further secondary endpoints on oral preparation, transport, pharyngeal phase, penetration, and aspiration. PAS change from V1 to V2 was not significantly different between groups (p = 0.221). When considering all patients for secondary analyses, we found that the entire study cohort showed better PAS scores at V2 compared to V1 irrespective from DBS treatment allocation (p = 0.0156). Both STN and STN + SNr treatments were safe. 'STN + SNr' stimulation was not superior compared to standard 'STN' stimulation both on PAS and the secondary endpoints. We found that the entire study cohort improved dysphagia after eight weeks, which presumably mirrors the effect of continued swallowing therapy and the increased patient attention on swallowing.

Brainstem inhibitory neurons enhance behavioral feature selectivity by sharpening the tuning of excitatory neurons

The brainstem is a hub for sensorimotor integration, which mediates crucial innate behaviors. This brain region is characterized by a rich population of GABAergic inhibitory neurons, required for the proper expression of these innate behaviors. However, what roles these inhibitory neurons play in innate behaviors and how they function are still not fully understood. Here, we show that inhibitory neurons in the nucleus of the optic tract and dorsal-terminal nuclei (NOT-DTN) of the mouse can modulate the innate eye movement optokinetic reflex (OKR) by shaping the tuning properties of excitatory NOT-DTN neurons. Specifically, we demonstrate that although these inhibitory neurons do not directly induce OKR, they enhance the visual feature selectivity of OKR behavior, which is mediated by the activity of excitatory NOT-DTN neurons. Moreover, consistent with the sharpening role of inhibitory neurons in OKR behavior, they have broader tuning relative to excitatory neurons. Last, we demonstrate that inhibitory NOT-DTN neurons directly provide synaptic inhibition to nearby excitatory neurons and sharpen their tuning in a sustained manner, accounting for the enhanced feature selectivity of OKR behavior. In summary, our findings uncover a fundamental principle underlying the computational role of inhibitory neurons in brainstem sensorimotor circuits.

The direct and indirect pathways of the basal ganglia antagonistically influence cortical activity and perceptual decisions

The striatum, the main input nucleus of the basal ganglia, receives topographically organized input from the cortex and gives rise to the direct and indirect output pathways, which have antagonistic effects on basal ganglia output directed to the cortex. We optogenetically stimulated the direct and indirect pathways in a visual and a working memory task in mice that responded by licking. Unilateral direct pathway stimulation increased the probability of lick responses toward the contralateral, non-stimulated side and increased cortical activity globally. In contrast, indirect pathway stimulation increased the probability of responses toward the stimulated side and decreased activity in the stimulated hemisphere. Moreover, direct pathway stimulation enhanced the neural representation of a contralateral visual stimulus during the delay of the working memory task, whereas indirect pathway stimulation had the opposite effect. Our results demonstrate how these two pathways influence perceptual decisions and working memory and modify activity in the dorsal cortex.

Basal ganglia-spinal cord pathway that commands locomotor gait asymmetries in mice

The basal ganglia are essential for executing motor actions. How the basal ganglia engage spinal motor networks has remained elusive. Medullary Chx10 gigantocellular (Gi) neurons are required for turning gait programs, suggesting that turning gaits organized by the basal ganglia are executed via this descending pathway. Performing deep brainstem recordings of Chx10 Gi Ca2+ activity in adult mice, we show that striatal projection neurons initiate turning gaits via a dominant crossed pathway to Chx10 Gi neurons on the contralateral side. Using intersectional viral tracing and cell-type-specific modulation, we uncover the principal basal ganglia-spinal cord pathway for locomotor asymmetries in mice: basal ganglia → pontine reticular nucleus, oral part (PnO) → Chx10 Gi → spinal cord. Modulating the restricted PnO → Chx10 Gi pathway restores turning competence upon striatal damage, suggesting that dysfunction of this pathway may contribute to debilitating turning deficits observed in Parkinson's disease. Our results reveal the stratified circuit architecture underlying a critical motor program.

Simulated Dopamine Modulation of a Neurorobotic Model of the Basal Ganglia

The vertebrate basal ganglia play an important role in action selection-the resolution of conflicts between alternative motor programs. The effective operation of basal ganglia circuitry is also known to rely on appropriate levels of the neurotransmitter dopamine. We investigated reducing or increasing the tonic level of simulated dopamine in a prior model of the basal ganglia integrated into a robot control architecture engaged in a foraging task inspired by animal behaviour. The main findings were that progressive reductions in the levels of simulated dopamine caused slowed behaviour and, at low levels, an inability to initiate movement. These states were partially relieved by increased salience levels (stronger sensory/motivational input). Conversely, increased simulated dopamine caused distortion of the robot's motor acts through partially expressed motor activity relating to losing actions. This could also lead to an increased frequency of behaviour switching. Levels of simulated dopamine that were either significantly lower or higher than baseline could cause a loss of behavioural integration, sometimes leaving the robot in a 'behavioral trap'. That some analogous traits are observed in animals and humans affected by dopamine dysregulation suggests that robotic models could prove useful in understanding the role of dopamine neurotransmission in basal ganglia function and dysfunction.

Brain-wide neural activity underlying memory-guided movement

Behavior relies on activity in structured neural circuits that are distributed across the brain, but most experiments probe neurons in a single area at a time. Using multiple Neuropixels probes, we recorded from multi-regional loops connected to the anterior lateral motor cortex (ALM), a circuit node mediating memory-guided directional licking. Neurons encoding sensory stimuli, choices, and actions were distributed across the brain. However, choice coding was concentrated in the ALM and subcortical areas receiving input from the ALM in an ALM-dependent manner. Diverse orofacial movements were encoded in the hindbrain; midbrain; and, to a lesser extent, forebrain. Choice signals were first detected in the ALM and the midbrain, followed by the thalamus and other brain areas. At movement initiation, choice-selective activity collapsed across the brain, followed by new activity patterns driving specific actions. Our experiments provide the foundation for neural circuit models of decision-making and movement initiation.

Pathways from the Superior Colliculus to the Basal Ganglia

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 SC's 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.

Low- and high-level coordination of orofacial motor actions

Orofacial motor actions are movements that, in rodents, involve whisking of the vibrissa, deflection of the nose, licking and lapping with the tongue, and consumption through chewing. These actions, along with bobbing and turning of the head, coordinate to subserve exploration while not conflicting with life-supporting actions such as breathing and swallowing. Orofacial and head movements are comprised of two additive components: a rhythm that can be entrained by the breathing oscillator and a broadband component that directs the actuator to the region of interest. We focus on coordinating the rhythmic component of actions into a behavior. We hypothesize that the precise timing of each constituent action is continually adjusted through the merging of low-level oscillator input with sensory-derived, high-level rhythmic feedback. Supporting evidence is discussed.

Superior colliculus bidirectionally modulates choice activity in frontal cortex

Action selection occurs through competition between potential choice options. Neural correlates of choice competition are observed across frontal cortex and downstream superior colliculus (SC) during decision-making, yet how these regions interact to mediate choice competition remains unresolved. Here we report that SC can bidirectionally modulate choice competition and drive choice activity in frontal cortex. In the mouse, topographically matched regions of frontal cortex and SC formed a descending motor pathway for directional licking and a re-entrant loop via the thalamus. During decision-making, distinct neuronal populations in both frontal cortex and SC encoded opposing lick directions and exhibited competitive interactions. SC GABAergic neurons encoded ipsilateral choice and locally inhibited glutamatergic neurons that encoded contralateral choice. Activating or suppressing these cell types could bidirectionally drive choice activity in frontal cortex. These results thus identify SC as a major locus to modulate choice competition within the broader action selection network.

Activity map of a cortico-cerebellar loop underlying motor planning

The neocortex and cerebellum interact to mediate cognitive functions. It remains unknown how the two structures organize into functional networks to mediate specific behaviors. Here we delineate activity supporting motor planning in relation to the mesoscale cortico-cerebellar connectome. In mice planning directional licking based on short-term memory, preparatory activity instructing future movement depends on the anterior lateral motor cortex (ALM) and the cerebellum. Transneuronal tracing revealed divergent and largely open-loop connectivity between the ALM and distributed regions of the cerebellum. A cerebellum-wide survey of neuronal activity revealed enriched preparatory activity in hotspot regions with conjunctive input-output connectivity to the ALM. Perturbation experiments show that the conjunction regions were required for maintaining preparatory activity and correct subsequent movement. Other cerebellar regions contributed little to motor planning despite input or output connectivity to the ALM. These results identify a functional cortico-cerebellar loop and suggest the cerebellar cortex selectively establishes reciprocal cortico-cerebellar communications to orchestrate motor planning.

Not everything, not everywhere, not all at once: a study of brain-wide encoding of movement

Activity related to movement is found throughout sensory and motor regions of the brain. However, it remains unclear how movement-related activity is distributed across the brain and whether systematic differences exist between brain areas. Here, we analyzed movement related activity in brain-wide recordings containing more than 50,000 neurons in mice performing a decision-making task. Using multiple techniques, from markers to deep neural networks, we find that movement-related signals were pervasive across the brain, but systematically differed across areas. Movement-related activity was stronger in areas closer to the motor or sensory periphery. Delineating activity in terms of sensory- and motor-related components revealed finer scale structures of their encodings within brain areas. We further identified activity modulation that correlates with decision-making and uninstructed movement. Our work charts out a largescale map of movement encoding and provides a roadmap for dissecting different forms of movement and decision-making related encoding across multi-regional neural circuits.

Superior colliculus cell types bidirectionally modulate choice activity in frontal cortex

Action selection occurs through competition between potential choice options. Neural correlates of choice competition are observed across frontal cortex and downstream superior colliculus (SC) during decision-making, yet how these regions interact to mediate choice competition remains unresolved. Here we report that cell types within SC can bidirectionally modulate choice competition and drive choice activity in frontal cortex. In the mouse, topographically matched regions of frontal cortex and SC formed a descending motor pathway for directional licking and a re-entrant loop via the thalamus. During decision-making, distinct neuronal populations in both frontal cortex and SC encoded opposing lick directions and exhibited push-pull dynamics. SC GABAergic neurons encoded ipsilateral choice and glutamatergic neurons encoded contralateral choice, and activating or suppressing these cell types could bidirectionally drive push-pull choice activity in frontal cortex. These results thus identify SC as a major locus to modulate choice competition within the broader action selection network.

Influence of Recent Trial History on Interval Timing

Interval timing is involved in a variety of cognitive behaviors such as associative learning and decision-making. While it has been shown that time estimation is adaptive to the temporal context, it remains unclear how interval timing behavior is influenced by recent trial history. Here we found that, in mice trained to perform a licking-based interval timing task, a decrease of inter-reinforcement interval in the previous trial rapidly shifted the time of anticipatory licking earlier. Optogenetic inactivation of the anterior lateral motor cortex (ALM), but not the medial prefrontal cortex, for a short time before reward delivery caused a decrease in the peak time of anticipatory licking in the next trial. Electrophysiological recordings from the ALM showed that the response profiles preceded by short and long inter-reinforcement intervals exhibited task-engagement-dependent temporal scaling. Thus, interval timing is adaptive to recent experience of the temporal interval, and ALM activity during time estimation reflects recent experience of interval.

Oscillation/Coincidence-Detection Models of Reward-Related Timing in Corticostriatal Circuits

The major tenets of beat-frequency/coincidence-detection models of reward-related timing are reviewed in light of recent behavioral and neurobiological findings. This includes the emphasis on a core timing network embedded in the motor system that is comprised of a corticothalamic-basal ganglia circuit. Therein, a central hub provides timing pulses (i.e., predictive signals) to the entire brain, including a set of distributed satellite regions in the cerebellum, cortex, amygdala, and hippocampus that are selectively engaged in timing in a manner that is more dependent upon the specific sensory, behavioral, and contextual requirements of the task. Oscillation/coincidence-detection models also emphasize the importance of a tuned ‘perception’ learning and memory system whereby target durations are detected by striatal networks of medium spiny neurons (MSNs) through the coincidental activation of different neural populations, typically utilizing patterns of oscillatory input from the cortex and thalamus or derivations thereof (e.g., population coding) as a time base. The measure of success of beat-frequency/coincidence-detection accounts, such as the Striatal Beat-Frequency model of reward-related timing (SBF), is their ability to accommodate new experimental findings while maintaining their original framework, thereby making testable experimental predictions concerning diagnosis and treatment of issues related to a variety of dopamine-dependent basal ganglia disorders, including Huntington’s and Parkinson’s disease.

Development of eight wireless automated cages system with two lick-o-meters each for rodents

Drinking behavior has been used in basic research to study metabolism, motivation, decision-making and different aspects of health problems, such as anhedonia and alcohol use disorders. In the majority of studies, liquid intake is measured by weighing the bottles before and after the experiment. This method does not tell much about the drinking microstructure, e.g., licking bouts and periods of preference for each liquid, which could be valuable to understand drinking behavior. To improve the data acquisition of drinking microstructure, companies have developed lick-o-meters devices that acquire timestamps when animals approach or drink from a specific sipper. Nevertheless, commercially available devices have elevated costs. Here, we present a low-cost alternative for a lick-o-meter system that allows wireless data acquisition of licking from eight cages with two sippers each. We run a three-phase validation protocol to ensure 1) proper choice of the sensor to detect licks; 2) adaptation of the device to a wireless transmission and realistic in silico tests; and 3) in vivo tests to correlate the amount of licks measured by the prototype and the bottle weight. The capacitive sensor presented appropriate recall and precision for our device. After adaptation to wireless transmission, the in silico validation demonstrated low reading and transmission errors for the device even when tested in extreme simultaneous licking conditions. Finally, a positive correlation between volume consumption and lick's count in the in vivo test was observed, showing that the prototype can be used for in vivo studies interested in rodent drinking microstructure.Significant StatementThis study presents an innovative and low-cost solution for drinking behavioral studies: a lick-o-meter system based on an open-source hardware platform with a user-friendly interface software, capable of simultaneously receiving data from eight automated cages with two drinking bottles each. The lick-o-meter brings an accessible device to acquire high-quality and detailed data. This device also has the possibility to be adaptable to new types of sensors or other neuroscience tools capable of measuring brain activity simultaneously to the behavior.

Neural Algorithms and Circuits for Motor Planning

The brain plans and executes volitional movements. The underlying patterns of neural population activity have been explored in the context of movements of the eyes, limbs, tongue, and head in nonhuman primates and rodents. How do networks of neurons produce the slow neural dynamics that prepare specific movements and the fast dynamics that ultimately initiate these movements? Recent work exploits rapid and calibrated perturbations of neural activity to test specific dynamical systems models that are capable of producing the observed neural activity. These joint experimental and computational studies show that cortical dynamics during motor planning reflect fixed points of neural activity (attractors). Subcortical control signals reshape and move attractors over multiple timescales, causing commitment to specific actions and rapid transitions to movement execution. Experiments in rodents are beginning to reveal how these algorithms are implemented at the level of brain-wide neural circuits.

The Superior Colliculus: Cell Types, Connectivity, and Behavior

The superior colliculus (SC), one of the most well-characterized midbrain sensorimotor structures where visual, auditory, and somatosensory information are integrated to initiate motor commands, is highly conserved across vertebrate evolution. Moreover, cell-type-specific SC neurons integrate afferent signals within local networks to generate defined output related to innate and cognitive behaviors. This review focuses on the recent progress in understanding of phenotypic diversity amongst SC neurons and their intrinsic circuits and long-projection targets. We further describe relevant neural circuits and specific cell types in relation to behavioral outputs and cognitive functions. The systematic delineation of SC organization, cell types, and neural connections is further put into context across species as these depend upon laminar architecture. Moreover, we focus on SC neural circuitry involving saccadic eye movement, and cognitive and innate behaviors. Overall, the review provides insight into SC functioning and represents a basis for further understanding of the pathology associated with SC dysfunction.

Cerebellum encodes and influences the initiation, performance, and termination of discontinuous movements in mice

The cerebellum is hypothesized to represent timing information important for organizing salient motor events during periodically performed discontinuous movements. To provide functional evidence validating this idea, we measured and manipulated Purkinje cell (PC) activity in the lateral cerebellum of mice trained to volitionally perform periodic bouts of licking for regularly allocated water rewards. Overall, PC simple spiking modulated during task performance, mapping phasic tongue protrusions and retractions, as well as ramping prior to both lick-bout initiation and termination, two important motor events delimiting movement cycles. The ramping onset occurred earlier for the initiation of uncued exploratory licking that anticipated water availability relative to licking that was reactive to water allocation, suggesting that the cerebellum is engaged differently depending on the movement context. In a subpopulation of PCs, climbing-fiber-evoked responses also increased during lick-bout initiation, but not termination, highlighting differences in how cerebellar input pathways represent task-related information. Optogenetic perturbation of PC activity disrupted the behavior by degrading lick-bout rhythmicity in addition to initiating and terminating licking bouts confirming a causative role in movement organization. Together, these results substantiate that the cerebellum contributes to the initiation and timing of repeated motor actions.

A behavioural correlate of the synaptic eligibility trace in the nucleus accumbens

Reward reinforces the association between a preceding sensorimotor event and its outcome. Reinforcement learning (RL) theory and recent brain slice studies explain the delayed reward action such that synaptic activities triggered by sensorimotor events leave a synaptic eligibility trace for 1 s. The trace produces a sensitive period for reward-related dopamine to induce synaptic plasticity in the nucleus accumbens (NAc). However, the contribution of the synaptic eligibility trace to behaviour remains unclear. Here we examined a reward-sensitive period to brief pure tones with an accurate measurement of an effective timing of water reward in head-fixed Pavlovian conditioning, which depended on the plasticity-related signaling in the NAc. We found that the reward-sensitive period was within 1 s after the pure tone presentation and optogenetically-induced presynaptic activities at the NAc, showing that the short reward-sensitive period was in conformity with the synaptic eligibility trace in the NAc. These findings support the application of the synaptic eligibility trace to construct biologically plausible RL models.

A Deep Mesencephalic Nucleus Circuit Regulates Licking Behavior

Licking behavior is important for water intake. The deep mesencephalic nucleus (DpMe) has been implicated in instinctive behaviors. However, whether the DpMe is involved in licking behavior and the precise neural circuit behind this behavior remains unknown. Here, we found that the activity of the DpMe decreased during water intake. Inhibition of vesicular glutamate transporter 2-positive (VGLUT2⁺) neurons in the DpMe resulted in increased water intake. Somatostatin-expressing (SST⁺), but not protein kinase C-δ-expressing (PKC-δ⁺), GABAergic neurons in the central amygdala (CeA) preferentially innervated DpMe VGLUT2⁺ neurons. The SST⁺ neurons in the CeA projecting to the DpMe were activated at the onset of licking behavior. Activation of these CeA SST⁺ GABAergic neurons, but not PKC-δ⁺ GABAergic neurons, projecting to the DpMe was sufficient to induce licking behavior and promote water intake. These findings redefine the roles of the DpMe and reveal a novel CeA^SST-DpMe^VGLUT2 circuit that regulates licking behavior and promotes water intake.

A nigra-subthalamic circuit is involved in acute and chronic pain states

ABSTRACT

The basal ganglia modulate somatosensory pain pathways but it is unclear whether a common circuit exists to mitigate hyperalgesia in pain states induced by peripheral nociceptive stimuli. As a key output nucleus of the basal ganglia, the substantia nigra pars reticulata (SNr) may be a candidate for this role. To test this possibility, we optogenetically modulated SNr GABAergic neurons and examined pain thresholds in freely behaving male mice in inflammatory and neuropathic pain states as well as comorbid depression in chronic pain. We observed that stimulation of either SNr GABAergic neurons or their projections to the subthalamic nucleus (STN) significantly alleviated nociceptive responses in all pain states on the contralateral side and comorbid depression in chronic pain, and that this analgesic effect was eliminated when SNr-STN GABAergic projection was blocked. However, SNr modulation did not affect baseline pain thresholds. We also found that SNr-STN GABAergic projection was attenuated in pain states, resulting in disinhibition of STN neurons. Thus, impairment of the SNr-STN GABAergic circuit may be a common pathophysiology for the maintenance of hyperalgesia in both inflammatory and neuropathic pain states and the comorbid depression in chronic pain; compensating this circuit has potential to effectively treat related pain conditions.

A medullary centre for lapping in mice

It has long been known that orofacial movements for feeding can be triggered, coordinated, and often rhythmically organized at the level of the brainstem, without input from higher centers. We uncover two nuclei that can organize the movements for ingesting fluids in mice. These neuronal groups, IRt^Phox2b and Peri5^Atoh1, are marked by expression of the pan-autonomic homeobox gene Phox2b and are located, respectively, in the intermediate reticular formation of the medulla and around the motor nucleus of the trigeminal nerve. They are premotor to all jaw-opening and tongue muscles. Stimulation of either, in awake animals, opens the jaw, while IRt^Phox2b alone also protracts the tongue. Moreover, stationary stimulation of IRt^Phox2b entrains a rhythmic alternation of tongue protraction and retraction, synchronized with jaw opening and closing, that mimics lapping. Finally, fiber photometric recordings show that IRt^Phox2b is active during volitional lapping. Our study identifies one of the subcortical nuclei underpinning a stereotyped feeding behavior.

Striatal indirect pathway mediates exploration via collicular competition

The ability to suppress actions that lead to a negative outcome and explore alternative actions is necessary for optimal decision making. Although the basal ganglia have been implicated in these processes^1-5, the circuit mechanisms underlying action selection and exploration remain unclear. Here, using a simple lateralized licking task, we show that indirect striatal projection neurons (iSPN) in the basal ganglia contribute to these processes through modulation of the superior colliculus (SC). Optogenetic activation of iSPNs suppresses contraversive licking and promotes ipsiversive licking. Activity in lateral superior colliculus (lSC), a region downstream of the basal ganglia, is necessary for task performance and predicts lick direction. Furthermore, iSPN activation suppresses ipsilateral lSC, but surprisingly excites contralateral lSC, explaining the emergence of ipsiversive licking. Optogenetic inactivation reveals inter-collicular competition whereby each hemisphere of the superior colliculus inhibits the other, thus allowing the indirect pathway to disinhibit the contralateral lSC and trigger licking. Finally, inactivating iSPNs impairs suppression of devalued but previously rewarded licking and reduces exploratory licking. Our results reveal that iSPNs engage the competitive interaction between lSC hemispheres to trigger a motor action and suggest a general circuit mechanism for exploration during action selection.

Functional coupling between target selection and acquisition in the superior colliculus

Survival in unpredictable environments requires that animals continuously evaluate their surroundings for behavioral targets, direct their movements toward those targets, and terminate movements once a target is reached. The ability to select, move toward, and acquire spatial targets depends on a network of brain regions, but it remains unknown how these goal-directed processes are linked by neural circuits. Within this network, common circuits in the midbrain superior colliculus (SC) mediate the selection and initiation of movements to spatial targets. However, SC activity often persists throughout movement, suggesting that the same SC circuits underlying target selection and movement initiation may also contribute to "target acquisition": stopping the movement at the selected target. Here, we examine the hypothesis that SC functional circuitry couples target selection and acquisition using a "default motor plan" generated by selection-related neuronal activity. Recordings from intermediate and deep layer SC neurons in mice performing a spatial choice task demonstrate that choice-predictive neurons, including optogenetically identified GABAergic neurons whose activity mediates target selection, exhibit increased activity during movement to the target. By recording from rostral and caudal SC in separate groups of mice, we also revealed higher activity in rostral than caudal neurons during target acquisition. Finally, we used an attractor model to examine how-invoking only SC circuitry-caudal SC activity related to selecting an eccentric target could generate higher rostral than caudal acquisition-related activity. Overall, our results suggest a functional coupling between SC circuits for target selection and acquisition, elucidating a key mechanism for goal-directed behavior.NEW & NOTEWORTHY How do neural circuits ensure that selected targets are successfully acquired? Here, we examine whether choice-related activity in the superior colliculus (SC) promotes a motor plan for target acquisition. By demonstrating that choice-predictive SC neurons-including GABAergic neurons-remain active throughout movement, while the activity of rostral SC neurons increases during acquisition, and by recapitulating these dynamics with an attractor model, our results support a role for SC circuits in coupling target selection and acquisition.

Direct and indirect pathway neurons in ventrolateral striatum differentially regulate licking movement and nigral responses

Drinking behavior in rodents is characterized by stereotyped, rhythmic licking movement, which is regulated by the basal ganglia. It is unclear how direct and indirect pathways control the lick bout and individual spout contact. We find that inactivating D1 and D2 receptor-expressing medium spiny neurons (MSNs) in the ventrolateral striatum (VLS) oppositely alters the number of licks in a bout. D1- and D2-MSNs exhibit different patterns of lick-sequence-related activity and different phases of oscillation time-locked to the lick cycle. On the timescale of a lick cycle, transient inactivation of D1-MSNs during tongue protrusion reduces spout contact probability, whereas transiently inactivating D2-MSNs has no effect. On the timescale of a lick bout, inactivation of D1-MSNs (D2-MSNs) causes rate increase (decrease) in a subset of basal ganglia output neurons that decrease firing during licking. Our results reveal the distinct roles of D1- and D2-MSNs in regulating licking at both coarse and fine timescales.

Pain modulates dopamine neurons via a spinal-parabrachial-mesencephalic circuit

Pain decreases the activity of many ventral tegmental area (VTA) dopamine (DA) neurons, yet the underlying neural circuitry connecting nociception and the DA system is not understood. Here we show that a subpopulation of lateral parabrachial (LPB) neurons is critical for relaying nociceptive signals from the spinal cord to the substantia nigra pars reticulata (SNR). SNR-projecting LPB neurons are activated by noxious stimuli and silencing them blocks pain responses in two different models of pain. LPB-targeted and nociception-recipient SNR neurons regulate VTA DA activity directly through feed-forward inhibition and indirectly by inhibiting a distinct subpopulation of VTA-projecting LPB neurons thereby reducing excitatory drive onto VTA DA neurons. Correspondingly, ablation of SNR-projecting LPB neurons is sufficient to reduce pain-mediated inhibition of DA release in vivo. The identification of a neural circuit conveying nociceptive input to DA neurons is critical to our understanding of how pain influences learning and behavior.

Inhibitory neurons in the superior colliculus mediate selection of spatially-directed movements

Decision making is a cognitive process that mediates behaviors critical for survival. Choosing spatial targets is an experimentally-tractable form of decision making that depends on the midbrain superior colliculus (SC). While physiological and computational studies have uncovered the functional topographic organization of the SC, the role of specific SC cell types in spatial choice is unknown. Here, we leveraged behavior, optogenetics, neural recordings and modeling to directly examine the contribution of GABAergic SC neurons to the selection of opposing spatial targets. Although GABAergic SC neurons comprise a heterogeneous population with local and long-range projections, our results demonstrate that GABAergic SC neurons do not locally suppress premotor output, suggesting that functional long-range inhibition instead plays a dominant role in spatial choice. An attractor model requiring only intrinsic SC circuitry was sufficient to account for our experimental observations. Overall, our study elucidates the role of GABAergic SC neurons in spatial choice.

Specific populations of basal ganglia output neurons target distinct brain stem areas while collateralizing throughout the diencephalon

Basal ganglia play a central role in regulating behavior, but the organization of their outputs to other brain areas is incompletely understood. We investigate the largest output nucleus, the substantia nigra pars reticulata (SNr), and delineate the organization and physiology of its projection populations in mice. Using genetically targeted viral tracing and whole-brain anatomical analysis, we identify over 40 SNr targets that encompass a roughly 50-fold range of axonal densities. Retrograde tracing from the volumetrically largest targets indicates that the SNr contains segregated subpopulations that differentially project to functionally distinct brain stem regions. These subpopulations are electrophysiologically specialized and topographically organized and collateralize to common diencephalon targets, including the motor and intralaminar thalamus as well as the pedunculopontine nucleus and the midbrain reticular formation. These findings establish that SNr signaling is organized as dense, parallel outputs to specific brain stem targets concurrent with extensive collateral branches that encompass the majority of SNr axonal boutons.

A cortico-collicular pathway for motor planning in a memory-dependent perceptual decision task

Survival in a dynamic environment requires animals to plan future actions based on past sensory evidence, known as motor planning. However, the neuronal circuits underlying this crucial brain function remain elusive. Here, we employ projection-specific imaging and perturbation methods to investigate the direct pathway linking two key nodes in the motor planning network, the secondary motor cortex (M2) and the midbrain superior colliculus (SC), in mice performing a memory-dependent perceptual decision task. We find dynamic coding of choice information in SC-projecting M2 neurons during motor planning and execution, and disruption of this information by inhibiting M2 terminals in SC selectively impaired decision maintenance. Furthermore, we show that while both excitatory and inhibitory SC neurons receive synaptic inputs from M2, these SC subpopulations display differential temporal patterns in choice coding during behavior. Our results reveal the dynamic recruitment of the premotor-collicular pathway as a circuit mechanism for motor planning.

Duan, Pan et al. find that the premotor cortex cooperates with the midbrain superior colliculus via direct projections to implement decision maintenance. These results reveal mechanisms of cortico-collicular interaction during cognition and action in a pathway- and cell-type-specific manner.

Fully autonomous mouse behavioral and optogenetic experiments in home-cage

Goal-directed behaviors involve distributed brain networks. The small size of the mouse brain makes it amenable to manipulations of neural activity dispersed across brain areas, but existing optogenetic methods serially test a few brain regions at a time, which slows comprehensive mapping of distributed networks. Laborious operant conditioning training required for most experimental paradigms exacerbates this bottleneck. We present an autonomous workflow to survey the involvement of brain regions at scale during operant behaviors in mice. Naive mice living in a home-cage system learned voluntary head-fixation (>1 hr/day) and performed difficult decision-making tasks, including contingency reversals, for 2 months without human supervision. We incorporated an optogenetic approach to manipulate activity in deep brain regions through intact skull during home-cage behavior. To demonstrate the utility of this approach, we tested dozens of mice in parallel unsupervised optogenetic experiments, revealing multiple regions in cortex, striatum, and superior colliculus involved in tactile decision-making.

A Disynaptic Circuit in the Globus Pallidus Controls Locomotion Inhibition

The basal ganglia (BG) inhibit movements through two independent circuits: the striatal neuron-indirect and the subthalamic nucleus-hyperdirect pathways. These pathways exert opposite effects onto external globus pallidus (GPe) neurons, whose functional importance as a relay has changed drastically with the discovery of two distinct cell types, namely the prototypic and the arkypallidal neurons. However, little is known about the synaptic connectivity scheme of different GPe neurons toward both motor-suppressing pathways, as well as how opposite changes in GPe neuronal activity relate to locomotion inhibition. Here, we optogenetically dissect the input organizations of prototypic and arkypallidal neurons and further define the circuit mechanism and behavioral outcome associated with activation of the indirect or hyperdirect pathways. This work reveals that arkypallidal neurons are part of a novel disynaptic feedback loop differentially recruited by the indirect or hyperdirect pathways and that broadcasts inhibitory control onto locomotion only when arkypallidal neurons increase their activity.

Learning speed and detection sensitivity controlled by distinct cortico-fugal neurons in visual cortex

Vertebrates can change their behavior upon detection of visual stimuli according to the outcome their actions produce. Such goal-directed behavior involves evolutionary conserved brain structures like the striatum and optic tectum, which receive ascending visual input from the periphery. In mammals, however, these structures also receive descending visual input from visual cortex (VC), via neurons that give rise to cortico-fugal projections. The function of cortico-fugal neurons in visually guided, goal-directed behavior remains unclear. Here, we address the impact of two populations of cortico-fugal neurons in mouse VC in the learning and performance of a visual detection task. We show that the ablation of striatal projecting neurons reduces learning speed, whereas the ablation of superior colliculus projecting neurons does not impact learning but reduces detection sensitivity. This functional dissociation between distinct cortico-fugal neurons in controlling learning speed and detection sensitivity suggests an adaptive contribution of cortico-fugal pathways even in simple goal-directed behavior.

Anatomically segregated basal ganglia pathways allow parallel behavioral modulation

In the basal ganglia (BG), anatomically segregated and topographically organized feedforward circuits are thought to modulate multiple behaviors in parallel. Although topographically arranged BG circuits have been described, the extent to which these relationships are maintained across the BG output nuclei and in downstream targets is unclear. Here, using focal trans-synaptic anterograde tracing, we show that the motor-action-related topographical organization of the striatum is preserved in all BG output nuclei. The topography is also maintained downstream of the BG and in multiple parallel closed loops that provide striatal input. Furthermore, focal activation of two distinct striatal regions induces either licking or turning, consistent with their respective anatomical targets of projection outside of the BG. Our results confirm the parallel model of BG function and suggest that the integration and competition of information relating to different behavior occur largely outside of the BG.

Brainstem Circuits Controlling Action Diversification

Neuronal circuits that regulate movement are distributed throughout the nervous system. The brainstem is an important interface between upper motor centers involved in action planning and circuits in the spinal cord ultimately leading to execution of body movements. Here we focus on recent work using genetic and viral entry points to reveal the identity of functionally dedicated and frequently spatially intermingled brainstem populations essential for action diversification, a general principle conserved throughout evolution. Brainstem circuits with distinct organization and function control skilled forelimb behavior, orofacial movements, and locomotion. They convey regulatory parameters to motor output structures and collaborate in the construction of complex natural motor behaviors. Functionally tuned brainstem neurons for different actions serve as important integrators of synaptic inputs from upstream centers, including the basal ganglia and cortex, to regulate and modulate behavioral function in different contexts.

Ventral Tegmental Dopamine Neurons Control the Impulse Vector during Motivated Behavior

The ventral tegmental area (VTA) is a major source of dopamine, especially to the limbic brain regions. Despite decades of research, the function of VTA dopamine neurons remains controversial. Here, using a novel head-fixed behavioral system with five orthogonal force sensors, we show for the first time that the activity of dopamine neurons precisely represents the impulse vector (force exerted over time) generated by the animal. Distinct populations of VTA dopamine neurons contribute to components of the impulse vector in different directions. Optogenetic excitation of these neurons shows a linear relationship between signal injected and impulse generated. Optogenetic inhibition paused force generation or produced force in the backward direction. At the same time, these neurons also regulate the initiation and execution of anticipatory licking. Our results indicate that VTA dopamine controls the magnitude, direction, and duration of force used to move toward or away from any motivationally relevant stimuli.

Mediodorsal Thalamus Contributes to the Timing of Instrumental Actions

The perception of time is critical to adaptive behavior. While prefrontal cortex and basal ganglia have been implicated in interval timing in the seconds to minutes range, little is known about the role of the mediodorsal thalamus (MD), which is a key component of the limbic cortico-basal ganglia-thalamocortical loop. In this study, we tested the role of the MD in timing, using an operant temporal production task in male mice. In this task, that the expected timing of available rewards is indicated by lever pressing. Inactivation of the MD with muscimol produced rightward shifts in peak pressing on probe trials as well as increases in peak spread, thus significantly altering both temporal accuracy and precision. Optogenetic inhibition of glutamatergic projection neurons in the MD also resulted in similar changes in timing. The observed effects were found to be independent of significant changes in movement. Our findings suggest that the MD is a critical component of the neural circuit for interval timing, without playing a direct role in regulating ongoing performance.SIGNIFICANCE STATEMENT The mediodorsal nucleus (MD) of the thalamus is strongly connected with the prefrontal cortex and basal ganglia, areas which have been implicated in interval timing. Previous work has shown that the MD contributes to working memory and learning of action-outcome contingencies, but its role in behavioral timing is poorly understood. Using an operant temporal production task, we showed that inactivation of the MD significantly impaired timing behavior.

[Structure and efferences of the substantia nigra pars compacta in Parkinson's disease]

There is consensus that the neuropathological characteristic of Parkinson's disease (PD) is the neuronal cell loss of the substantia nigra pars compacta (SNc) in connection with a Lewy pathology. The transsynaptic spread of Lewy pathology is considered essential in PD pathogenesis. Therefore, the knowledge of pre-existing neuroanatomical connections of the SNc is essential. We describe recent animal experiments on the afferent and efferent projections of the SNc and discuss the evidence for and against the sequential transsynaptic spread of Lewy pathology in the pathogenesis of PD.

A Causal Role for Mouse Superior Colliculus in Visual Perceptual Decision-Making

The superior colliculus (SC) is arguably the most important visual structure in the mouse brain and is well known for its involvement in innate responses to visual threats and prey items. In other species, the SC plays a central role in voluntary as well as innate visual functions, including crucial contributions to selective attention and perceptual decision-making. In the mouse, the possible role of the SC in voluntary visual choice behaviors has not been established. Here, we demonstrate that the mouse SC of both sexes plays a causal role in visual perceptual decision-making by transiently inhibiting SC activity during an orientation change detection task. First, unilateral SC inhibition-induced spatially specific deficits in detection. Hit rates were reduced, and reaction times increased for orientation changes in the contralateral but not ipsilateral visual field. Second, the deficits caused by SC inhibition were specific to a temporal epoch coincident with early visual burst responses in the SC. Inhibiting SC during this 100-ms period caused a contralateral detection deficit, whereas inhibition immediately before or after did not. Third, SC inhibition reduced visual detection sensitivity. Psychometric analysis revealed that inhibiting SC visual activity significantly increased detection thresholds for contralateral orientation changes. In addition, effects on detection thresholds and lapse rates caused by SC inhibition were larger in the presence of a competing visual stimulus, indicating a role for the mouse SC in visual target selection. Together, our results demonstrate that the mouse SC is necessary for the normal performance of voluntary visual choice behaviors.SIGNIFICANCE STATEMENT The mouse superior colliculus (SC) has become a popular model for studying the circuit organization and development of the visual system. Although the SC is a fundamental component of the visual pathways in mice, its role in visual perceptual decision-making is not clear. By investigating how temporally precise SC inhibition influenced behavioral performance during a visually guided orientation change detection task, we identified a 100-ms temporal epoch of SC visual activity that is crucial for the ability of mice to detect behaviorally relevant visual changes. In addition, we found that SC inhibition also caused deficits in visual target selection. Thus, our findings highlight the importance of the SC for visual perceptual choice behavior in the mouse.

Opponent regulation of action performance and timing by striatonigral and striatopallidal pathways

The basal ganglia have been implicated in action selection and timing, but the relative contributions of the striatonigral (direct) and striatopallidal (indirect) pathways to these functions remain unclear. We investigated the effects of optogenetic stimulation of D1+ (direct) and A2A+ (indirect) neurons in the ventrolateral striatum in head-fixed mice on a fixed time reinforcement schedule. Direct pathway stimulation initiates licking, whereas indirect pathway stimulation suppresses licking and results in rebound licking after stimulation. Moreover, direct and indirect pathways also play distinct roles in timing. Direct pathway stimulation produced a resetting of the internal timing process, whereas indirect pathway stimulation transiently paused timing, and proportionally delayed the next bout of licking. Our results provide evidence for the continuous and opposing contributions of the direct and indirect pathways in the production and timing of reward-guided behavior.

Basal Ganglia Circuits for Action Specification

Behavior is readily classified into patterns of movements with inferred common goals-actions. Goals may be discrete; movements are continuous. Through the careful study of isolated movements in laboratory settings, or via introspection, it has become clear that animals can exhibit exquisite graded specification to their movements. Moreover, graded control can be as fundamental to success as the selection of which action to perform under many naturalistic scenarios: a predator adjusting its speed to intercept moving prey, or a tool-user exerting the perfect amount of force to complete a delicate task. The basal ganglia are a collection of nuclei in vertebrates that extend from the forebrain (telencephalon) to the midbrain (mesencephalon), constituting a major descending extrapyramidal pathway for control over midbrain and brainstem premotor structures. Here we discuss how this pathway contributes to the continuous specification of movements that endows our voluntary actions with vigor and grace.

Impact of simultaneous subthalamic and nigral stimulation on dysphagia in Parkinson's disease

OBJECTIVES

Dysphagia is a frequent and highly relevant symptom in Parkinson's disease (PD) due to high associated morbidity and mortality. To compare the effect of simultaneous stimulation of the subthalamic nucleus (STN) and substantia nigra (SNr) with conventional STN-stimulation on swallowing function in Parkinson's disease.

METHODS

In this controlled, randomized, double-blind, cross-over clinical trial, 15 PD patients were assessed with DBS switched off (STIM OFF), STN-DBS, STN + SNr-DBS. Patients and 32 age-matched healthy controls were examined clinically and by flexible-endoscopic evaluation of swallowing (FEES) to evaluate the swallowing function. The primary endpoint was the assessment of residues, secondary endpoints were penetration/aspiration, leakage, retained pharyngeal secretions, drooling, and assessments of the patient's self-perception of swallowing on a visual analog scale.

RESULTS

Compared with healthy controls PD patients showed significantly more pharyngeal residues in STIM OFF and both DBS modes. Residues or aspiration events were found in 80% of the patients under STN-stimulation. Simultaneous STN + SNr-stimulation had no additional positive effect on objective dysphagia and self-reported swallowing function compared to STN-DBS.

INTERPRETATION

Simultaneous STN + SNr-stimulation seems to have no additional beneficial effects on dysphagia when compared with conventional STN-stimulation, but did not deteriorate the swallowing function. If STN + SNr-stimulation is planned to be applied for the improvement of axial symptoms and gait disorders in PD patients, it can be considered safe in terms of dysphagia.

A Head-Fixation System for Continuous Monitoring of Force Generated During Behavior

Many studies in neuroscience use head-fixed behavioral preparations, which confer a number of advantages, including the ability to limit the behavioral repertoire and use techniques for large-scale monitoring of neural activity. But traditional studies using this approach use extremely limited behavioral measures, in part because it is difficult to detect the subtle movements and postural adjustments that animals naturally exhibit during head fixation. Here we report a new head-fixed setup with analog load cells capable of precisely monitoring the continuous forces exerted by mice. The load cells reveal the dynamic nature of movements generated not only around the time of task-relevant events, such as presentation of stimuli and rewards, but also during periods in between these events, when there is no apparent overt behavior. It generates a new and rich set of behavioral measures that have been neglected in previous experiments. We detail the construction of the system, which can be 3D-printed and assembled at low cost, show behavioral results collected from head-fixed mice, and demonstrate that neural activity can be highly correlated with the subtle, whole-body movements continuously produced during head restraint.

Unilateral Optogenetic Inhibition and Excitation of Basal Ganglia Output Affect Directional Lick Choices and Movement Initiation in Mice

Models of basal ganglia (BG) function predict that tonic inhibitory output to motor thalamus (MT) suppresses unwanted movements, and that a decrease in such activity leads to action selection. Further, for unilateral activity changes in the BG, a lateralized effect on contralateral movements can be expected due to ipsilateral thalamocortical connectivity. However, a direct test of these outcomes of thalamic inhibition has not been performed. To conduct such a direct test, we utilized rapid optogenetic activation and inactivation of the GABAergic output of the substantia nigra pars reticulata (SNr) to MT in male and female mice that were trained in a sensory cued left/right licking task. Directional licking tasks have previously been shown to depend on a thalamocortical feedback loop between ventromedial MT and antero-lateral premotor cortex. In confirmation of model predictions, we found that unilateral optogenetic inhibition of GABAergic output from the SNr, during ipsilaterally cued trials, biased decision making towards a contralateral lick without affecting motor performance. In contrast, optogenetic excitation of SNr terminals in MT resulted in an opposite bias towards the ipsilateral direction confirming a bidirectional effect of tonic nigral output on directional decision making. However, direct optogenetic excitation of neurons in the SNr resulted in bilateral movement suppression, which is in agreement with previous results that show such suppression for nigral terminals in the superior colliculus (SC), which receives a bilateral projection from SNr.

The Neurocircuitry of fluid satiation

Fluid satiation, or quenching of thirst, is a critical homeostatic signal to stop drinking; however, its underlying neurocircuitry is not well characterized. Cutting-edge genetically encoded tools and techniques are now enabling researchers to pinpoint discrete neuronal populations that control fluid satiation, revealing that hindbrain regions, such as the nucleus of the solitary tract, area postrema, and parabrachial nucleus, primarily inhibit fluid intake. By contrast, forebrain regions such as the lamina terminalis, primarily stimulate thirst and fluid intake. One intriguing aspect of fluid satiation is that thirst is quenched tens of minutes before water reaches the circulation, and the amount of water ingested is accurately calibrated to match physiological needs. This suggests that 'preabsorptive' inputs from the oropharyngeal regions, esophagus or upper gastrointestinal tract anticipate the amount of fluid required to restore fluid homeostasis, and provide rapid signals to terminate drinking once this amount has been consumed. It is likely that preabsorptive signals are carried via the vagal nerve to the hindbrain. In this review, we explore our current understanding of the fluid satiation neurocircuitry, its inputs and outputs, and its interconnections within the brain, with a focus on recent studies of the hindbrain, particularly the parabrachial nucleus.

An open cortico-basal ganglia loop allows limbic control over motor output via the nigrothalamic pathway

Cortico-basal ganglia-thalamocortical loops are largely conceived as parallel circuits that process limbic, associative, and sensorimotor information separately. Whether and how these functionally distinct loops interact remains unclear. Combining genetic and viral approaches, we systemically mapped the limbic and motor cortico-basal ganglia-thalamocortical loops in rodents. Despite largely closed loops within each functional domain, we discovered a unidirectional influence of the limbic over the motor loop via ventral striatum-substantia nigra (SNr)-motor thalamus circuitry. Slice electrophysiology verifies that the projection from ventral striatum functionally inhibits nigro-thalamic SNr neurons. In vivo optogenetic stimulation of ventral or dorsolateral striatum to SNr pathway modulates activity in medial prefrontal cortex (mPFC) and motor cortex (M1), respectively. However, whereas the dorsolateral striatum-SNr pathway exerts little impact on mPFC, activation of the ventral striatum-SNr pathway effectively alters M1 activity. These results demonstrate an open cortico-basal ganglia loop whereby limbic information could modulate motor output through ventral striatum control of M1.

State transitions in the substantia nigra reticulata predict the onset of motor deficits in models of progressive dopamine depletion in mice

Parkinson's disease (PD) is a progressive neurodegenerative disorder whose cardinal motor symptoms are attributed to dysfunction of basal ganglia circuits under conditions of low dopamine. Despite well-established physiological criteria to define basal ganglia dysfunction, correlations between individual parameters and motor symptoms are often weak, challenging their predictive validity and causal contributions to behavior. One limitation is that basal ganglia pathophysiology is studied only at end-stages of depletion, leaving an impoverished understanding of when deficits emerge and how they evolve over the course of depletion. In this study, we use toxin- and neurodegeneration-induced mouse models of dopamine depletion to establish the physiological trajectory by which the substantia nigra reticulata (SNr) transitions from the healthy to the diseased state. We find that physiological progression in the SNr proceeds in discrete state transitions that are highly stereotyped across models and correlate well with the prodromal and symptomatic stages of behavior.

A cortico-cerebellar loop for motor planning

Persistent and ramping neural activity in the frontal cortex anticipates specific movements1-6. Preparatory activity is distributed across several brain regions7,8, but it is unclear which brain areas are involved and how this activity is mediated by multi-regional interactions. The cerebellum is thought to be primarily involved in the short-timescale control of movement9-12; however, roles for this structure in cognitive processes have also been proposed13-16. In humans, cerebellar damage can cause defects in planning and working memory13. Here we show that persistent representation of information in the frontal cortex during motor planning is dependent on the cerebellum. Mice performed a sensory discrimination task in which they used short-term memory to plan a future directional movement. A transient perturbation in the medial deep cerebellar nucleus (fastigial nucleus) disrupted subsequent correct responses without hampering movement execution. Preparatory activity was observed in both the frontal cortex and the cerebellar nuclei, seconds before the onset of movement. The silencing of frontal cortex activity abolished preparatory activity in the cerebellar nuclei, and fastigial activity was necessary to maintain cortical preparatory activity. Fastigial output selectively targeted the behaviourally relevant part of the frontal cortex through the thalamus, thus closing a cortico-cerebellar loop. Our results support the view that persistent neural dynamics during motor planning is maintained by neural circuits that span multiple brain regions17, and that cerebellar computations extend beyond online motor control13-15,18.