Ming-Yuan Min

The locus coeruleus (LC) is the brain's primary source of norepinephrine. It modulates the brain's overall state of arousal. Recent evidence suggests that the LC also regulates immediate attentional responses by resetting related cortical networks to optimize behavioral outcomes. Cortical regions associated with high cognitive function, such as the medial prefrontal cortex (mPFC), are theorized to influence LC output directly to regulate behavior. However, the available evidence is insufficient for a comprehensive understanding of the underlying mechanisms and properties. To address this gap in knowledge, we employed a combination of ex vivo whole‐cell recording and optogenetic techniques to investigate the synaptic transmission of inputs from the mPFC to noradrenergic (NA) neurons in the LC, as well as to inhibitory interneurons (iINs) that synapse onto NA neurons. Our findings suggest that the mPFC forms monosynaptic connections with both LC‐NA neurons and LC‐iINs. These synaptic connections demonstrate disparities in glutamate release properties specific to cell type. Compared to LC‐iINs, mPFC fibers synapsing onto LC‐NA neurons exhibit a higher paired‐pulse ratio, indicating lower release probability, and demonstrate presynaptic enhancement of glutamate release efficacy when stimulated by a paradigm mimicking mPFC unit activity during decision‐making behavior. We propose that the simultaneous connections onto LC‐NA neurons and LC‐iINs, along with cell type–specific differences in transmitter release plasticity, may enable the mPFC to effectively multiplex information to the LC for adaptive behavioral regulation.

The locus coeruleus (LC) is the primary source of norepinephrine in the brain and is known to modulate brain-wide arousal state. Recent evidence suggests that it also regulates immediate attentional responses by resetting related cortical networks to optimize behavioral outcomes. Cortical regions of high cognitive function, such as the medial prefrontal cortex (mPFC), are theorized to directly influence LC output for the purpose of behavioral regulation. However, the available evidence is insufficient to provide a comprehensive understanding of the underlying mechanisms and properties. To provide further comprehensive data on this issue, we combined ex vivo whole-cell recording with an optogenetic approach to study the synaptic transmission of mPFC inputs to LC neurons, including noradrenergic (NA) neurons and GABAergic neurons presynaptic to them (preLC neurons). Our findings indicate that the mPFC exhibits monosynaptic connections with both NA and GABAergic preLC neurons. These synaptic connections demonstrate cell-type-specific disparities in glutamate release properties. In comparison to those on GABAergic preLC neurons, the mPFC fibers synapsing on LC-NA exhibit a lower release probability (higher paired-pulse ratio) and demonstrate a presynaptic enhancement of glutamate release efficacy during behavior. The features of simultaneous connections onto LC-NA and GABAergic preLC neurons, which exhibit cell-type-specific differences in plastic function of the transmitter release, enable the mPFC to effectively multiplex information to the LC for the adaptive regulation of behavior.

Summary: The subthalamic nucleus (STN) plays a critical role in modulating motor and cognitive functions within the basal ganglia, with its dysfunction being implicated in movement disorders such as Parkinson’s disease. However, the behavioral representations of individual STN neurons remain incompletely understood. Using in vivo two-photon calcium imaging in behaving mice, we systematically mapped the activity of single STN neurons across diverse behavioral contexts, including locomotion, licking, and reward-driven actions. Our findings reveal that STN neurons exhibit mixed selectivity, encoding multiple behaviors with distinct temporal dynamics and excitatory or inhibitory response patterns. This mixed selectivity allows the STN to robustly encode motor parameters such as locomotion speed and licking intensity while integrating contextual information from different behavioral states. Comparisons with the adjacent zona incerta (ZI) revealed distinct encoding properties: while both regions represent locomotion, STN neurons more faithfully track motor states, whereas ZI neurons exhibit prolonged calcium events with weaker movement correlations. Population-level analysis showed STN activity in a low-dimensional neural manifold, with components linked to movement velocity and licking intensity. Notably, locomotion encoding in STN was context-dependent, diverging when movements were internally generated versus reward-modulated. Together, these findings highlight the specialized yet flexible role of the STN in integrating motor and reward-related signals, supporting a framework in which STN neurons contribute to motor control through multiplexed and context-dependent encoding. This work provides new insights into the functional organization of basal ganglia circuits and has implications for understanding STN’s role in both physiological and pathological conditions.

The locus coeruleus (LC) contains predominantly norepinephrine (NE) neurons that project widely throughout the brain. The LC plays a critical role in controlling behavior, particularly arousal. Historically, it was thought that the LC-NE system performed its behavioral control function by uniformly releasing NE throughout most brain regions. However, recent evidence suggests that the LC’s cortical projections are organized into modules, which allows for the coordination of diverse, and sometimes opposing, functions such as fear memory formation and extinction. Nevertheless, many details remain unclear and require data from the axon collaterals of sparse neurons. We modified a viral tracing protocol using a dual-recombinase system to trace the axonal collaterals of sparse LC neurons projecting to the cingulate cortex (CgC). Our results show that even a small number of LC neurons have broad cortical projections, though the pattern is not uniform. Centered-log ratio transformation of NE fiber distribution across the cortex and hippocampus reveals a few preferential target areas (PTAs) of the labeled LC-NE neurons’ axonal projections. The summed NE fiber length in these defined PTs is enriched relative to the geometric mean of all other cortical and hippocampal regions where NE fibers were detected. Notably, the defined PTAs—including the rostral splenial cortex, dorsal hippocampus, somatosensory cortex, and CgC (the retrograde viral labeling injection site)—are functionally related to navigation. These results demonstrate that LC-NE neurons are organized into distinct projection modules, each comprising a small number of neurons with functionally correlated major cortical targets.

The brainstem cell group, locus coeruleus (LC), is present across vertebrates and influences cardiorespiratory, metabolic, immune, and cognitive functions by activating in two putatively distinct firing patterns. Yet, the degree to which the LC firing rates and patterns are homogenous across species has never been assessed due to inherently limited sample sizes. To remedy this, we pooled cross-species data from 20 laboratories to show that firing rates differ across species and are modulated by sex, age, and type of in vitro or in vivo preparation. Contrary to the prevailing dual-mode firing pattern schema, we observed patterns spread across a low-dimensional manifold, with subregions enriched for specific biological factors and neurodegenerative disease models. Our findings show considerable diversity in an ancestral vertebrate neuromodulatory system.

Proprioceptors are low-threshold mechanoreceptors involved in perceiving body position and strain bearing. However, the physiological response of proprioceptors to fatigue- and muscle-acidosis-related disturbances remains unknown. Here, we employed whole-cell patch-clamp recordings to probe the effect of mild acidosis on the mechanosensitivity of the proprioceptive neurons of dorsal root ganglia (DRG) in mice. We cultured neurite-bearing parvalbumin-positive (Pv+) DRG neurons on a laminin-coated elastic substrate and examined mechanically activated currents induced through substrate deformation-driven neurite stretch (SDNS). The SDNS-induced inward currents (I_SDNS) were indentation depth-dependent and significantly inhibited by mild acidification (pH 7.2~6.8). The acid-inhibiting effect occurred in neurons with an I_SDNS sensitive to APETx2 (an ASIC3-selective antagonist) inhibition, but not in those with an I_SNDS resistant to APETx2. Detailed subgroup analyses revealed I_SDNS was expressed in 59% (25/42) of Parvalbumin-positive (Pv+) DRG neurons, 90% of which were inhibited by APETx2. In contrast, an acid (pH 6.8)-induced current (I_Acid) was expressed in 76% (32/42) of Pv+ DRG neurons, 59% (21/32) of which were inhibited by APETx2. Together, ASIC3-containing channels are highly heterogenous and differentially contribute to the I_SNDS and I_Acid among Pv+ proprioceptors. In conclusion, our findings highlight the importance of ASIC3-containing ion channels in the physiological response of proprioceptors to acidic environments.

The locus coeruleus (LC) consists of noradrenergic (NA) neurons and plays an important role in controlling behaviours. Although much of the knowledge regarding LC functions comes from studying behavioural outcomes upon administration of muscarinic acetylcholine receptor (mAChR) agonists into the nucleus, the exact mechanisms remain unclear. Here, we report that the application of carbachol (CCh), an mAChR agonist, increased the spontaneous action potentials (sAPs) of both LC‐NA neurons and local inhibitory interneurons (LC I‐INs) in acute brain slices by activating M1/M3 mAChRs (m_1/3AChRs). Optogenetic activation of LC I‐INs evoked inhibitory postsynaptic currents (IPSCs) in LC‐NA neurons that were mediated by γ‐aminobutyric acid type A (GABA_A) and glycine receptors, and CCh application decreased the IPSC amplitude through a presynaptic mechanism by activating M4 mAChRs (m₄AChRs). LC‐NA neurons also exhibited spontaneous phasic‐like activity (sPLA); CCh application increased the incidence of this activity. This effect of CCh application was not observed with blockade of GABA_A and glycine receptors, suggesting that the sPLA enhancement occurred likely because of the decreased synaptic transmission of LC I‐INs onto LC‐NA neurons by the m₄AChR activation and/or increased spiking rate of LC I‐INs by the m_1/3AChR activation, which could lead to fatigue of the synaptic transmission. In conclusion, we report that CCh application, while inhibiting their synaptic transmission, increases sAP rates of LC‐NA neurons and LC I‐INs. Collectively, these effects provide insight into the cellular mechanisms underlying the behaviour modulations following the administration of muscarinic receptor agonists into the LC reported by the previous studies.

Key points: The locus coeruleus (LC) contains noradrenergic (NA) neurons that respond to novel stimuli in the environment with phasic activation to initiate an orienting response; phasic LC activation is also triggered by stimuli, representing the outcome of task‐related decision processes, to facilitate ensuing behaviours and help optimize task performance. Here, we report that LC‐NA neurons exhibit bursts of action potentials in vitro resembling phasic LC activation in vivo, and the activity is gated by inhibitory interneurons (I‐INs) located in the peri‐LC. We also observe that inhibition of peri‐LC I‐INs enhances prepulse inhibition and axons from cortical areas that play important roles in evaluating the cost/reward of a stimulus synapse on both peri‐LC I‐INs and LC‐NA neurons. The results help us understand the cellular mechanisms underlying the generation and regulation of phasic LC activation with a focus on the role of peri‐LC I‐INs. Noradrenergic (NA) neurons in the locus coeruleus (LC) have global axonal projection to the brain. These neurons discharge action potentials phasically in response to either novel stimuli in the environment to initiate an orienting behaviour or stimuli representing the outcome of task‐related decision processes to facilitate ensuing behaviours and help optimize task performance. Nevertheless, the cellular mechanisms underlying the generation and regulation of phasic LC activation remain unknown. We report here that LC‐NA neurons recorded in brain slices exhibit bursts of action potentials that resembled the phasic activation‐pause profile observed in animals. The activity was referred to as phasic‐like activity (PLA) and was suppressed and enhanced by blocking excitatory and inhibitory synaptic transmissions, respectively. These results suggest the existence of a local circuit to drive PLA, and the activity could be regulated by the excitatory–inhibitory balance of the circuit. In support of this notion, we located a population of inhibitory interneurons (I‐INs) in the medial part of the peri‐LC that exerted feedforward inhibition of LC‐NA neurons through GABAergic and glycinergic transmissions. Selective inhibition of peri‐LC I‐INs with chemogenetic methods could enhance PLA in brain slices and increase prepulse inhibition in animals. Moreover, axons from the orbitofrontal and prelimbic cortices, which play important roles in evaluating the cost/reward of a stimulus, synapse on both peri‐LC I‐INs and LC‐NA neurons. These observations demonstrate functional roles of peri‐LC I‐INs in integrating inputs of the frontal cortex onto LC‐NA neurons and gating the phasic LC output.