Flexible control of motor timing is crucial for behaviour. Before volitional movement begins, the frontal cortex and striatum exhibit ramping spiking activity, with variable ramp slopes anticipating movement onsets. This activity in the cortico-basal ganglia loop may function as an adjustable 'timer,' triggering actions at the desired timing. However, because the frontal cortex and striatum share similar ramping dynamics and are both necessary for timing behaviours, distinguishing their individual roles in this timer function remains challenging. Here, to address this, we conducted perturbation experiments combined with multi-regional electrophysiology in mice performing a flexible lick-timing task. Following transient silencing of the frontal cortex, cortical and striatal activity swiftly returned to pre-silencing levels and resumed ramping, leading to a shift in lick timing close to the silencing duration. Conversely, briefly inhibiting the striatum caused a gradual decrease in ramping activity in both regions, with ramping resuming from post-inhibition levels, shifting lick timing beyond the inhibition duration. Thus, inhibiting the frontal cortex and striatum effectively paused and rewound the timer, respectively. These findings are consistent with a model in which the striatum is part of a network that temporally integrates input from the frontal cortex and generates ramping activity that regulates motor timing.

All mammals exhibit flexible decision policies that depend, at least in part, on the cortico-basal ganglia-thalamic (CBGT) pathways. Yet understanding how the complex connectivity, dynamics, and plasticity of CBGT circuits translate into experience-dependent shifts of decision policies represents a longstanding challenge in neuroscience. Here we present the results of a computational approach to address this problem. Specifically, we simulated decisions during the early learning process driven by CBGT circuits under baseline, unrewarded conditions using a spiking neural network, and fit an evidence accumulation model to the resulting behavior. Using canonical correlation analysis, we then replicated the identification of three control ensembles (responsiveness, pliancy and choice) within CBGT circuits, with each of these subnetworks mapping to a specific configuration of the evidence accumulation process. We subsequently simulated learning in a simple two-choice task with one optimal (i.e., rewarded) target and found that, during early stages of learning, feedback-driven dopaminergic plasticity on cortico-striatal synapses effectively increases reward rate over time. The learning-related changes in the decision policy can be decomposed in terms of the contributions of each control ensemble, whose influence is driven by sequential reward prediction errors on individual trials. Our results provide a clear and simple mechanism for how dopaminergic plasticity shifts subnetworks within CBGT circuits so as to increase reward rate by strategically modulating how evidence is used to drive decisions.

Dopamine (DA) is essential for the production of vigorous actions, but how DA modifies the gain of motor commands remains unclear. Here we show that subsecond DA transients in the striatum of mice are neither required nor sufficient for specifying the vigor of ongoing forelimb movements. Our findings have important implications for our understanding of how DA contributes to motor control under physiological conditions and in Parkinson's disease.

Dopaminergic transmission within the ventral striatum is broadly implicated in risk/reward decision making and impulse control, and the rat gambling task (rGT) measures both behaviours concurrently. While the resulting indices of risky choice and impulsivity correlate at the population level, dopaminergic manipulations rarely impact both behaviours uniformly, with changes in choice more likely when dopaminergic transmission is altered during task acquisition. Although the task structure of the rGT remains constant, the relative importance of ventral striatal dopamine signals relevant for reward prediction versus impulse control may vary as learning progresses; the former should dominate while rats learn the probabilistic contingencies of the task, whereas suppression of premature responses becomes more valuable once a decision-making strategy is established and exploited. Striatal cholinergic interneurons (CINs) critically influence reinforcement learning by modulating dopamine release and gating periods of dopamine-facilitated neuroplasticity. We therefore hypothesised that ventral striatal CINs (vsCINs) could influence reward learning or impulse control during task acquisition or stable performance, respectively. Using chemogenetics in Sprague Dawley rats (Rattus norvegicus), we found support for this hypothesis: activation and inhibition of vsCINs once behaviour was stable increased and decreased motor impulsivity in both sexes but had no effect on choice patterns. In contrast, activating and inhibiting vsCINs during task acquisition did not alter motor impulsivity but instead decreased and increased risky choice, respectively. Notably, the former effect was only observed in males, and the latter in females. We conclude by proposing testable predictions regarding acetylcholine-dopamine interactions that may explain sex differences. Impairments in decision making and impulsivity are central to psychiatric conditions such as addiction, ADHD, and impulse control disorders. Understanding how these behaviours are regulated in the brain, and why they differ across individuals and sexes, is critical for developing targeted treatments. This study identifies ventral striatal cholinergic interneurons as important modulators of both impulsivity and risk-based decision making, with their influence depending on learning stage and biological sex. These results show how acetylcholine and dopamine systems interact to shape behaviour in flexible and individualized ways. By revealing circuit-level mechanisms that may underlie sex-specific vulnerabilities and stage-specific treatment outcomes, this work lays the groundwork for more personalized approaches to treating disorders involving poor impulse control and risky decision making.

The substantia nigra pars reticulata (SNr), a key basal ganglia output nucleus, is modulated by dopamine (DA) believed to be released locally from midbrain DA neurons. Although DA has been proposed to regulate γ-aminobutyric acid (GABA) release from medium spiny neuron (MSN) terminals via presynaptic D1 receptors, the precise mechanisms remain unclear. Using presynaptic optical recordings of synaptic vesicle fusion, calcium influx in D1-MSN synapses together with postsynaptic patch-clamp recordings from SNr neurons, we found that DA inhibits D1-MSN GABA release in a frequency-dependent manner. Unexpectedly, this effect was independent of DA receptors and instead required 5-HT1B receptor activation. Using two-photon serotonin biosensor imaging in slices and fiber photometry in vivo, we demonstrate that DA enhances extracellular serotonin in the SNr via inhibition of serotonin reuptake. Our results suggest that serotonin mediates DAergic control of basal ganglia output and contributes to the therapeutic actions of dopaminergic medications for Parkinson's disease and psychostimulant-related disorders.

Research indicates that midbrain dopaminergic neurons encode reward prediction error (RPE) signals involved in positive reinforcement learning. However, studies on dopamine's role in negative reinforcement learning (NRL) are scarce. Learning to escape aversive stimuli is vital for survival and may differ significantly from positive reinforcement in behavior and neural mechanisms. This study employs footshocks as aversive stimuli to investigate neural activity, synaptic transmission, and intrinsic excitability in a NRL paradigm using fiber photometry and ex vivo electrophysiology. Results show that inescapable footshocks initially increase activity in substantia nigra pars compacta (SNc) dopaminergic neurons, which later shifts to reflect shock termination as exposure increases. Electrophysiological observations reveal increased intrinsic excitability and excitatory synaptic transmission in SNc neurons, with decreased inhibitory transmission. After mice learn to escape the shock by nose-poking, dopaminergic activity shifts from shock termination to shock onset. Furthermore, inhibitory input increases, while excitatory input decreases after learning, with intrinsic excitability returning to baseline levels. This indicates that SNc dopaminergic neurons exhibit RPE-like signals in response to aversive stimuli, with their intrinsic excitability adjusting according to expectations of shock termination. These findings enhance our understanding of RPE encoding in negative reinforcement learning and may inform therapeutic strategies for disorders caused by environmental factors such as aversive stimuli.

Active ensembles of neurons form an engram during learning. However, engrams are not immutable, and their organization may change with time via systems consolidation. Here, we labeled engram ensembles in the prelimbic (PrL) cortex during contextual fear conditioning. We found that distinct engram subpopulations ("sub-engrams") contribute to memory recall at recent versus remote delays, with sub-engram contribution determined by their projection profile. At recent delays, sub-engrams projecting to the basal amygdala (BA) and lateral entorhinal cortex (LEC) are activated, and their activity is necessary and sufficient for memory retrieval. At remote delays, sub-engrams projecting to the nucleus reuniens (NRe) and nucleus accumbens (NAc) are additionally recruited, and their activity is necessary and sufficient for memory retrieval. Recruitment of NRe- and NAc-projecting sub-engrams to remote recall is an active process, depending on post-training activation of PrL parvalbumin-expressing interneurons. Post-training chemogenetic inhibition of PrL parvalbumin-expressing interneurons prevented sub-engram recruitment and impaired remote memory.

Previous studies revealed critical involvement of the striatum in adapting to the environment by actions that anticipate rewards from experiences as a policy. However, it remains unclear how current policy is evaluated to explore more advantageous alternatives. Here, we show that during policy-based sequential actions in a rat reversal task, the dorsomedial striatum plays an essential role in pathway-specific manner. Recording and optical manipulation of the indirect pathway showed that late-onset activity following unrewarded suboptimal action represents a lowered valuation of the current action policy and a heightened bias to try the suboptimal action. The early-onset activity complementarily mediated policy-based suppression of unrewarded action. These results demonstrate the indirect pathway's role in monitoring unreliability of current action policy and probing alternative one. This study extends conventional understanding of consequence-guided persistence with reward-oriented action policy and provides key insights regarding how the dorsomedial striatum enables proactive and flexible adaptation to environmental changes.