The Journal of Physiology

Persistent inward currents (PICs) govern motoneuron output and are influenced by diffuse neuromodulation and local inhibition. When large diameter afferent feedback is lost, as in some neurological conditions, PICs might additionally amplify and prolong synaptic inputs. Here, we examined whether reducing Ia afferent transmission via ischaemic nerve block alters PIC contribution to tibialis anterior (TA) motor unit (MU) discharge. Across two experiments 12 adults (5 female) performed triangular-shaped isometric dorsiflexion to 30% (Experiments 1 and 2) and 50% (Experiment 2) maximum voluntary force (MVF) at baseline, after a 20-minute rest (control), and during occlusion after inducing an ischaemic nerve block, confirmed by abolition of the soleus H-reflex. TA myoelectrical activity measured during contractions was decomposed into MU spike trains, and from smoothed MU discharges, discharge rate hysteresis ({Delta}F) and ascending non-linearity (brace height) were quantified. Results from Experiment 1 involving contractions matched to absolute force levels revealed increased peak discharge rate, {Delta}F, and brace height post-occlusion. However, {Delta}F normalised to maximal theoretical hysteresis did not change across time points. In Experiment 2, where MVF was reassessed at each timepoint and contractions were matched to relative force, peak discharge rate, normalised {Delta}F and brace height increased post-occlusion compared to pre-, across both contraction intensities. {Delta}F only increased post-occlusion at 50% MVF, with no changes at 30% MVF. These results show that ischaemic block of large-diameter axons, likely reducing reciprocal inhibition, increases PIC contribution to discharge rate modulation, highlighting the role of Ia afferent input in shaping motoneuron output in humans.

Motoneuron subtypes exhibit distinct firing properties that are critical for the graded control of muscle force. A key determinant of these differences is the medium afterhyperpolarization (mAHP), which shapes discharge rate and firing gain. While subtype-specific variation in mAHP properties has traditionally been attributed to differences in small-conductance calcium-activated potassium (SK) channel expression, emerging evidence suggests that additional conductances may contribute. Here, we investigated the role of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in regulating the mAHP and excitability of mouse spinal motoneurons during postnatal development. Using whole-cell patch-clamp recordings, we show that, by the onset of the third postnatal week, an h current (Ih) is active at resting potential in fast motoneurons and is correlated with the amplitude of the mAHP. Pharmacological blockade of HCN channels with ZD7288 increased mAHP amplitude in fast but not slow motoneurons, without affecting mAHP duration, indicating a subtype-specific contribution to mAHP amplitude. In line with the mAHP regulating firing gain, ZD7288 also reduced firing gain in fast but not slow motoneurons. These findings support a contribution of HCN channel activity to the regulation of mAHP amplitude and firing gain in fast motoneurons, highlighting a potential interaction between Ih and SK channel-dependent mechanisms in shaping motoneuron excitability. Key PointsO_LIThe amplitude of the medium afterhyperpolarization (mAHP) is negatively correlated with h-current (Ih) amplitude measured near resting potential in mouse lumbar motoneurons. C_LIO_LIPharmacological blockade of HCN channels selectively increases mAHP amplitude in fast, delayed firing alpha motoneurons, with no effect observed in slow, immediate firing alpha motoneurons. C_LIO_LIInhibition of HCN channels reduces firing gain in fast motoneurons, while slow motoneurons remain unaffected. C_LIO_LIHCN channels regulate firing gain in fast motoneurons, at least in part, through modulation of mAHP amplitude. C_LI

Mass is a fundamental aspect of muscle contractile function, yet the inertial effects of inactive muscle mass is generally neglected in modeling and not quantified in studies on small muscles or isolated fibers. However, during submaximal contractions, inactive muscle tissue may take longer to be accelerated by active fibers, and may be subject to prolonged deceleration, both of which may potentially reduce force development and work output. We sought to test if inactive tissue mass imposes an inertial penalty on muscle performance, using in situ sinusoidal work-loop experiments on rat plantaris muscles. Regional fascicle dynamics, measured across supramaximal and submaximal levels of activation, showed that decreasing activation significantly reduced fascicle strain and increased both shortening and lengthening latency. Contrary to our predictions, however, reductions in work, beyond those explained by decreased fascicle strain, were negligible. Normalized work did not decline disproportionately relative to force, suggesting no clear inertial penalty on work at this muscle size. Our findings suggest that while inactive muscle mass influences the dynamics of submaximal contractions, its impact on work during submaximal contractions at small muscle sizes is limited.

Sympathetic preganglionic neurons (SPNs) provide the final pathway through which the central nervous system regulates autonomic function. SPN axons projecting to paravertebral sympathetic chain ganglia branch extensively and diverge across multiple segments, enabling amplification of central sympathetic commands through extensive postganglionic neuronal populations. Spike propagation along these projections has generally been assumed to occur reliably. However, most SPN axons are extremely small unmyelinated fibers, a structural feature predicted to reduce the safety factor for spike propagation. Using an isolated mouse thoracic sympathetic chain preparation, we combined anatomical tracing with multi-site compound action potential recordings to assess conduction across SPN axons. Neurobiotin labeling revealed widespread rostrocaudal divergence through interganglionic nerves, while axon measurements confirmed that most SPN axons are small unmyelinated fibers. Across preparations, supramaximal recruitment of SPNs revealed substantial intertrial variability in compound responses, indicating frequent conduction failures. Failures were most prominent in slow-conducting axons and occurred in both branching interganglionic pathways and the unbranching axons within the splanchnic nerve. During repetitive activation, frequency dependent depression was observed at 1, 5 and 10Hz, but only slow-conducting branching axons exhibited pronounced depression. Overall, these findings indicate that spike propagation in SPN axons may operate probabilistically rather than deterministically, with reliability strongly dependent on axonal subtype and recent activity history. We conclude that axonal conduction variability constitutes an intrinsic and dynamically regulated mechanism that shapes sympathetic output. By varying the recruitment of postganglionic populations, unreliable spike propagation in SPN axons introduces a previously unrecognized presynaptic gain-control mechanism, operating independently of central spike generation to modulate sympathetic output. SIGNIFICANCESympathetic preganglionic neurons provide the final pathway through which the central nervous system controls end-organs. These neurons project through the sympathetic chain where their axons branch extensively to recruit more numerous paravertebral postganglionic neurons. Spike propagation along these projections has generally been assumed to occur reliably. Here we show that this assumption is incorrect. Using anatomical tracing and electrophysiological recordings in mouse sympathetic chain preparations, we demonstrate that spike conduction in sympathetic preganglionic axons is frequently variable and prone to failure, particularly in the slowest-conducting unmyelinated fibers. Conduction variability was preferentially enhanced in branching axonal pathways during repetitive activation. These findings reveal that axonal conduction reliability represents an important presynaptic mechanism regulating the magnitude and variability of sympathetic output.

Strikingly, highly trained athletes engaged in vertiginous activities (e.g., dance and slacklining) and patients with bilateral vestibular loss show a similar pattern of neural plasticity, likely resulting from reduced vestibular sensory processes. However, unlike patients, these athletes show no balance impairments, quite the opposite. This suggests that the attenuation of vestibular processing represents an adaptive recalibration to excessive vestibular stimulation rather than a sign of dysfunction. Concurrently, tactile processing increases as vestibular processing attenuates. Our findings indicate that effective adaptation extends beyond simple tactile compensation: it involves a strengthened tactile-brain pathway. Indeed, following unexpected base-of-support translations, the coupling between plantar shear forces (i.e., a proxy of plantar sole tactile afferents) and cortical responses over the somatosensory areas was markedly enhanced in Athletes. Cross-correlation analysis revealed stronger (r = 0.71) and faster (36 ms) tactile-brain coupling in Athletes (n = 25) compared with age- and gender-matched Controls (n = 18). This enhancement occurred within the first 180 ms following translation, that is, during the critical early phase of skin-surface interaction. Notably, artistic swimmers, who undergo intense vestibular stimulation in a weightless underwater environment without balance equilibrium constraints, also exhibit enhanced tactile-brain coupling. This suggests that strengthening the tactile-brain coupling is not merely a byproduct of balance expertise, but rather a broader adaptive response to sustained vestibular stimulation. Multimodal neurons integrating vestibular and somatosensory inputs, such as those in the somatosensory cortex and thalamus, may increase their responsiveness to foot tactile afferents when vestibular inputs become excessive. In such contexts, the somatosensory system may assume a dominant role in providing gravity-related information for balance control.

Drug-induced inhibition of the delayed rectifier potassium (IKr) current predisposes to early afterdepolarisations (EADs) and cardiac arrhythmias. Here, we sought to determine the contribution of action potential duration (APD), APD variability and spontaneous calcium release from the sarcoplasmic reticulum (SR) in the formation of EADs. In isolated sheep ventricular myocytes, EADs were induced by combined inhibition of IKr with dofetilide and {beta}-adrenergic stimulation. The onset of EADs was preceded by increased beat-to-beat variability of APD. To isolate the role of APD in EAD initiation, the sarcoplasmic reticulum (SR) was depleted of calcium with caffeine. The first beat post-caffeine was associated with prolonged APD but not an EAD. During {beta}-AR stimulation, increasing ryanodine receptor open probability had no effect on APD but increased APD variability and induced both EADs and delayed afterdepolarisations (DADs). Targeting RyR open probability with K201 reversibly abolished afterdepolarisations. APD variability was a better predictor of EADs than APD alone. During an EAD, changes in [Ca2+]i preceded those of membrane depolarisation and the changes in [Ca2+]i were in the form of calcium sparks. In silico modelling demonstrated that membrane time constant effects account for the delay between changes in [Ca2+]i and membrane potential. In summary, using a drug-induced model of action potential prolongation with {beta}-AR stimulation, EADs are preceded by increased APD variability and an increase in Ca2+ sparks. Targeting SR function abolishes EADs. These results suggest a key role for SR Ca2+ overload in the formation of EADs and indicate that EADs and DADs share common mechanisms. Key PointsO_LIDrugs that prolong the cardiac action potential and ECG QT interval are a major cause of early afterdepolarisations and dangerous ventricular arrhythmias initiated by early afterdepolarisations. C_LIO_LIProlongation of the action potential is widely assumed to be the primary driver of these events. C_LIO_LIWe show that early afterdepolarisations are instead preceded by increased beat-to-beat variability of action potential duration and that this variability has better sensitivity and specificity for early afterdepolarisations than action potential duration. C_LIO_LISmall, spontaneous calcium release events known as calcium sparks occur before membrane depolarisation driving early afterdepolarisations. C_LIO_LISuppressing calcium release from the sarcoplasmic reticulum abolishes early afterdepolarisations, identifying calcium handling instability as potentially a key mechanism of drug-induced arrhythmia. C_LI

Sympathetic neuronal (SN) activity critically regulates the development and function of peripheral organs and tissues. Activity-dependent plasticity has been shown to modulate SN output, suggesting that compensatory forms of plasticity could contribute to maintaining stability of sympathetic circuits. Early SN hyperactivity drives the development of hypertension in humans and in the spontaneously hypertensive rat (SHR). In this study we used chemogenetic and pharmacological approaches, and took advantage of the enhanced activity of SHR SNs, to examine how long-term changes in activity impact synaptic properties in neonatal SN cultures. We showed that bidirectional changes in SN activity result in compensatory shifts in synaptic density that counteract long-term activity manipulations. These changes were mediated by satellite glial cells (SGCs), a non-neuronal cell in the sympathetic ganglia that has been shown to influence cholinergic synaptic sites during development. In the absence of SGCs there was no induction of homeostatic plasticity. Further, direct chemogenetic activation of SGCs was sufficient to drive compensatory plasticity, while glial inhibition blocked SN plasticity. We found that SGCs respond to cholinergic signaling by downregulating the expression of the synaptic regulators NGF and TNF, suggesting that neurons and glia interact to stabilize sympathetic output during long-term changes in circuit activity. Finally, we investigated whether these plasticity mechanisms are present in neonatal SHR SNs. We demonstrated that SHR SNs have an attenuated response to glia, both during synapse formation and activity-dependent plasticity. Taken together, this work outlines a novel homeostatic activity-dependent plasticity mechanism in the peripheral nervous system.

Optogenetic defibrillation uses light-gated ion channels to terminate cardiac arrhythmias through targeted illumination. Previous studies assessed the feasibility of using either cation (e.g. ChR2) or anion (e.g. GtACR1) non-selective channels, both of which depolarise resting cardiomyocytes upon photoactivation. In contrast, recently identified light-gated K+-channels (e.g. WiChR) suppress cardiomyocyte activity while maintaining the membrane potential near its resting state. Here, we use biophysically detailed simulations to compare the defibrillation potential of ChR2, GtACR1, and WiChR. Single-cell simulations show that activation of ChR2 and GtACR1 markedly increase diastolic intracellular Ca2+ concentration (by 42.6% and 52.6%, respectively), whereas WiChR induces only minimal changes (4.0% increase), suggesting a lower pro-arrhythmogenic risk. WiChR activation, however, slightly increases intracellular Na+ levels (by 15.1% compared to 0.1% and 3.4% for ChR2 and GtACR), consistent with the residual Na+ permeability of all currently available K+-selective channelrhodopsins. Simulations of human ventricles and atria demonstrate that GtACR1 most effectively terminates re-entrant arrhythmias at low light intensities, while WiChR achieves comparable efficacy at light levels [≥]5 mW/mm2. Complementary tissue-scale simulations reveal that defibrillation is either based on depolarisation within the excitable gap, followed by fast Na+ channel inactivation (depolarising variants ChR2 and GtACR1), or based on a reduction in membrane resistance supporting arrhythmia termination at sufficiently high light levels (large-conductance ion channels GtACR1 and WiChR). Overall, our findings identify channelrhodopsin ion selectivity as a key determinant of both arrhythmia termination success and mechanisms underlying defibrillation. Key points summaryO_LIWe use computational simulations to compare non-selective cation (ChR2), anion (GtACR1), and K+-selective channelrhodopsins (WiChR) for optogenetic termination of re-entrant arrhythmia. C_LIO_LISingle-cardiomyocyte simulations suggest that ChR2 and GtACR1 activation can cause progressive accumulation of intracellular Ca2+, which is minimised when using WiChR. C_LIO_LISimulations of human left ventricles and atria indicate that GtACR1 is most effective in terminating re-entrant arrhythmia at low light intensities, while WiChR becomes similarly effective at higher intensities. C_LIO_LITissue-scale simulations indicate distinct defibrillation mechanisms: Excitable gap extinction by de-novo action potential initiation followed by inactivation of fast Na+ channels for depolarising channelrhodopsins (ChR2, GtACR1), and reduction in membrane resistance for the large-conductance channels (GtACR1, WiChR), effectively clamping the membrane potential at each channels reversal potential at high light levels. C_LI

Sympathetic preganglionic neurons (SPNs) distribute signals widely across paravertebral ganglia, yet the reliability of spike propagation along their predominantly unmyelinated axons remains poorly defined. We examined temperature- and activity-dependent modulation of SPN axonal conduction using an ex vivo adult mouse thoracic sympathetic chain preparation. Population compound action potentials (CAPs) were evoked by supramaximal stimulation of T10 ventral roots and recorded from branching axons in interganglionic compared to unbranching axons in the splanchnic nerve. At physiological temperature (36{degrees}C), scaled CAP magnitude was reduced by [~]50% relative to 22{degrees}C, with preferential loss of slower-conducting axonal components. These reductions are consistent with substantial temperature-dependent decreases in effective axonal recruitment, likely reflecting conduction failure in a large fraction of SPNs. Losses were more pronounced in interganglionic pathways, suggesting increased vulnerability in branching projections. To assess activity-dependent effects, stimuli were delivered at 1, 5, and 20 Hz with focus on 5 and 20 Hz stimulus trains (20s duration). The overall time-course of train-evoked depression was similar across temperatures; however, the underlying axonal populations differed. At 22{degrees}C, slower-conducting axons exhibited marked frequency-dependent depression, whereas at 36{degrees}C the remaining faster-conducting axons displayed facilitation, particularly at 20 Hz. Slower-conducting responses also showed post-train potentiation at physiological temperature. These findings indicate that SPN axonal conduction is not uniformly reliable and is strongly modulated by temperature and activation history. Preferential vulnerability of slow-conducting, likely small-diameter and branching axons identifies axonal conduction as a physiologically regulated site of gain control in sympathetic output.

Ankle clonus is a sustained, involuntary, rhythmic muscle contraction frequently observed in humans with spinal cord injury (SCI). Although its pathophysiology remains incompletely understood, converging evidence suggests a role for brainstem systems in its generation. Following SCI, brainstem neuromodulatory inputs partially compensate for the loss of descending motor pathways by regulating motoneuron excitability during involuntary contractions, suggesting their involvement in the generation of clonus. To test this hypothesis, motoneuron excitability in response to Ia synaptic input was quantified using the soleus H reflex and maximal motor response (H/M ratio), and brainstem involvement was probed using the long lasting component of the cutaneous reflex (LLR) in the tibialis anterior and soleus muscles, as well as the StartReact response-an involuntary release of a movement triggered by a startling stimulus thought to engage the reticulospinal tract. We studied individuals with chronic SCI, both with and without ankle clonus, using standardized clinical tests across two days. Participants with clonus showed elevated H/M ratios, indicating increased motoneuron excitability, whereas those without clonus exhibited lower values than controls. Additionally, individuals with clonus exhibited longer LLR duration and greater LLR magnitude in both muscles, along with shorter reaction times to startle stimuli, consistent with enhanced monoaminergic and reticulospinal contributions. Notably, LLR duration was positively correlated with both StartReact response and H/M ratio. Together, these findings support a role for descending brainstem systems-particularly monoaminergic and reticulospinal pathways-in the maintenance of clonus in chronic SCI.

Voluntary contraction in one limb can facilitate motor output in a distant limb, a phenomenon commonly referred to as the remote effect. However, the neural mechanisms underlying this remote interlimb facilitation remain unclear. This study investigated cortical and spinal contributions to the remote effect in able-bodied participants. Transcranial magnetic stimulation (TMS) was applied over the hand area of the primary motor cortex using posterior-anterior (PA) and anterior-posterior (AP) current directions, which are sensitive to different cortical inputs. Cortical excitability was assessed using single- and paired-pulse paradigms to measure short-interval intracortical inhibition (SICI), short-interval intracortical facilitation (SICF), and short-latency afferent inhibition (SAI). Spinal motoneuron excitability was assessed from F-waves elicited by peripheral nerve stimulation. During voluntary lower-limb contractions, single-pulse TMS elicited larger motor evoked potentials in hand muscles across current directions, indicating a broad increase in net corticospinal output. However, only AP-sensitive paired-pulse measures showed reduced SICI and enhanced SICF during contraction, whereas PA-sensitive SICI and SICF were not significantly altered, suggesting that cortical modulation during the remote effect is expressed more clearly in AP-sensitive measures. SAI with PA stimulation was less consistently expressed during contraction, suggesting that afferent-related inhibitory modulation may also be influenced during the remote effect. In parallel, F-wave amplitude and persistence increased, consistent with enhanced spinal motoneuron excitability. Together, these results provide converging evidence that the remote effect in humans involves broad corticospinal and spinal facilitation, accompanied by current direction-dependent modulation of cortical excitability measures. KEY POINTS SUMMARYO_LIVoluntary contraction in one limb can facilitate motor output in a distant limb, but the mechanisms underlying this remote interlimb facilitation remain unclear. C_LIO_LIWe tested whether remote lower-limb contraction modulates corticospinal output, intracortical excitability, and spinal motoneuron excitability in a resting hand muscle. C_LIO_LISingle-pulse transcranial magnetic stimulation showed that motor evoked potentials in the hand were facilitated during remote lower-limb contraction across multiple current directions, indicating a broad increase in net corticospinal output. C_LIO_LIPaired-pulse measures were modulated preferentially with anterior-posterior stimulation, with reduced short-interval intracortical inhibition and increased short-interval intracortical facilitation, suggesting current direction-dependent modulation of cortical excitability measures. C_LIO_LIF-wave amplitude and persistence were also enhanced during remote lower-limb contraction, indicating increased spinal motoneuron excitability. These findings provide converging evidence that the remote effect involves both cortical and spinal contributions. C_LI

Increased mechanical loading induces skeletal muscle growth and, at the ultrastructural level, promotes myofibrillogenesis and the radial growth of myofibrils. However, the mechanisms regulating these ultrastructural adaptations are not known. Here, we sought to determine whether the mechanistic target of rapamycin complex 1 (mTORC1) regulates these processes. To accomplish this, muscle-specific, tamoxifen-inducible raptor knockout (iRAmKO) mice were used to inhibit signaling through mTORC1, and growth was induced with a model of chronic mechanical overload (MOV). Using a next-generation fluorescence imaging pipeline for ultrastructural analyses, we found that mTORC1 is a critical regulator of the myofibrillogenesis and radial growth of myofibrils that occur in response to MOV. Together with other recent advances in the field, we propose a model in which mTORC1 acts as a gatekeeper that permits the retention, rather than the synthesis, of proteins that drive the ultrastructural adaptations.

Humans adapt walking to different speeds and compliant surfaces, but whether gait speed and surface stiffness shape joint kinematics and muscle activity independently or interactively remains unclear. We reanalyzed an open variable-stiffness treadmill dataset collected at three speeds and four stiffness levels. Tensor decomposition extracted joint and muscle synergies and condition-specific recruitment coefficients. Stride length and stride time were modulated by speed and stiffness without interactions. Joint-synergy recruitment showed speed effects in all three components and stiffness effects in two components, but no speed-by-stiffness interactions. Among five muscle synergies, three were modulated without interaction, whereas two related to weight acceptance and forward propulsion showed significant speed-by-stiffness interactions. Their recruitment increased with speed, but stiffness-dependent differences decreased at higher speeds. These findings suggest that speed and stiffness largely modulate stride- and joint-level control independently, while interactively shaping selected muscle-synergy recruitment.

Hearts change their wall thickness (concentric growth) and chamber size (eccentric growth) as they adapt to circulatory demands and the intrinsic function of their contractile cells. Factors associated with wall thickening include variants of sarcomeric proteins that enhance contractility, mitochondrial dysfunction, and hypertension. Chambers can dilate due to many factors including sarcomeric variants that depress contractility and aortic and / or mitral valve insufficiency. Despite intensive study, the mechanisms that regulate cardiac growth remain unclear. It is also uncertain whether inherited variants induce growth via the same mechanisms as more common clinical pathologies, such as hypertension. Here we show that computer simulations of a beating left ventricle reproduce both variant and non-variant-related growth patterns when myocytes grow concentrically to regulate intracellular ATP concentration and eccentrically to maintain titin-based intracellular stress. The simulations support the hypothesis that cardiac growth reflects homeostatic feedback through three interacting systems whereby myocytes add or remove mitochondria and sarcomeres (1) in parallel to match ATP generation to myocardial energy demand, and (2) in series to regulate passive tension, while (3) the autonomic nervous system regulates cardiac power, and thus myocardial ATPase, via baroreflex control. The new framework provides a mechanistic basis for the patterns of eccentric and concentric growth induced by a wide range of clinically-relevant conditions and could facilitate in silico testing of potential therapies for cardiac disease. Significance statementHearts grow in response to both physiological and pathological stimuli. The patterns of concentric (wall thickening / thinning) and eccentric (chamber dilation / constriction) induced by different challenges are well recognized but the underlying mechanisms remain unclear. This work presents simulations of a beating left ventricle where (1) concentric growth is regulated by myocytes attempting to stabilize the intracellular ATP concentration and (2) eccentric growth is regulated by titin-mediated stress. The calculations reproduce the growth associated with inherited variants of sarcomeric proteins, mitochondrial dysfunction, hypertension, and both mitral and aortic valve insufficiency. The new ability to predict cardiac growth and its potential modification by treatments, including myotropes, brings the field closer to in silico optimization of therapy for cardiovascular disease.

Motor skills such as bicycle riding are considered robust and transferable across bicycle types. However, when the steering direction is inverted (reversed bicycle) control is disrupted to the extent that the bicycle cannot be ridden. With sufficient practice, the reversed bicycle can be learned, but this learning appears to produce impairment of normal bicycle riding suggesting modification of this long-established motor memory. Here we investigate the learning process of riding a reversed bicycle over four days of practice, while repeatedly assessing normal bicycle performance to measure any potential interference. Introduction of the reversed bicycle disrupted predictive control, reflected in a consistently increased time lag in the steering-roll coupling during reversed bicycle trials. This increase in delay suggests that predictive behavior in normal bicycle riding cannot be transferred to the reversed bicycle. With training, some participants successfully learned to ride the reversed bicycle by gradually reorganizing this coupling, whereas others failed to acquire this inverted coupling. Notably, even short-term exposure to the reversed bicycle interfered with normal bicycle riding, reducing distance ridden and increasing variability in steering rate. Together, we show that even a highly practiced whole-body motor skill is susceptible to rapid interference when control dynamics are altered.

Circadian rhythms regulate diverse physiological processes, including metabolism, and their disruption has been implicated in metabolic disorders such as obesity. However, the tissue-specific effects of obesity on peripheral circadian clocks remain incompletely understood. Here, we investigated the impact of high-fat diet (HFD)-induced obesity on circadian gene expression in skeletal muscle, liver, and white adipose tissue (WAT). Mice were fed either a regular diet (RD) or HFD for 6 weeks, followed by tissue collection at 4-hour intervals over a 24-hour period. Under RD conditions, key circadian regulators and their downstream targets exhibited robust 24-hour oscillations across all tissues. In contrast, HFD feeding induced distinct, tissue-specific alterations. In the liver, Per2, Dbp, and Rev-erb showed phase-advanced expression patterns, whereas in WAT, rhythmic expression was markedly attenuated. Notably, skeletal muscle largely preserved circadian gene expression patterns, indicating relative resistance to HFD-induced circadian disruption. In addition, HFD feeding altered metabolic gene expression in adipose tissue, characterized by reduced Pgc1 expression and increased Leptin expression. Together, these findings demonstrate that HFD-induced obesity differentially disrupts peripheral circadian clocks in a tissue-specific manner and highlight skeletal muscle as a relatively resilient tissue. These results provide insight into how circadian dysregulation contributes to metabolic abnormalities in obesity.

The heart maintains systemic perfusion through the coordinated function of its four chambers: the left and right atria and ventricles. Each chamber has distinct structural, functional, and molecular properties tailored to its role in circulation, which may result in chamber-specific differences in myofilament structure and regulation between atria and ventricles. To test this hypothesis, we employed muscle mechanics and X-ray diffraction to investigate functional and structural differences in porcine left atrial (LA) and left ventricular (LV) tissue. Here, we report the first X-ray diffraction study of atrial tissue, demonstrating that under resting conditions, myosin filaments in LA adopted a more ON-like, structurally distinct configuration compared with those in LV. Under contracting conditions, LV generated greater force and exhibited higher sinusoidal stiffness than LA across multiple calcium concentrations. LA showed faster kTR than in LV, with no calcium-dependence, in contrast to the calcium-dependence of kTR seen in LV. Structurally, the distinct myosin head configuration seen in the relaxed LA persisted during contraction. Furthermore, using the troponin inhibitor MYK-7660 to inhibit active contraction, we showed that, unlike LV, LA showed no direct calcium-dependent thick filament activation, reconciling discrepancies between fast rat and slow porcine ventricular myocardium regarding calciums role in thick filament regulation. Altogether, our study reveals that LA myosin filaments adopt a molecular architecture and regulatory mechanism distinct from their LV counterparts, suggesting that myosin filament structure and regulation have evolved differently to meet the unique functional demands of each cardiac chamber. Moreover, atrial disease is often associated with cardiomyopathy-related genetic variants, highlighting the atrial myocardium as an important therapeutic target and understanding atrial-specific regulatory mechanisms provides new insights into therapeutic strategies for atrial diseases.

Electrical transmission is mediated by intercellular channels that cluster into structures known as gap junctions (GJ). In vertebrates, GJ channels are encoded by the gene family of connexin (Cx) proteins that assemble as hexamers, termed hemichannels, in the pre- and postsynaptic membranes, and that subsequently dock to form GJ channels. Auditory contacts on the fish Mauthner cells serve as model to study the properties and organization of vertebrate electrical synapses. Electrical transmission at these synapses is mediated by multiple co-existing GJs at which the presence of intercellular channels is regulated by a molecular scaffold. Zebrafish contain four homologs of the neuronal Cx36: Cx35.5 and Cx35.1 (gjd2a and b, respectively), and Cx34.1 and Cx34.7 (gjd1a and b). Cx mutations suggested that GJs are formed by heterotypic channels made of presynaptic Cx35.5 and postsynaptic Cx34.1. Using transgenic fish in which Cxs were tagged, we found that a second Cx, Cx34.7, is present together with Cx34.1 on the postsynaptic side at some but not all GJs at these terminals. When exogenously expressed, both Cx34.1 and Cx34.7 formed heterotypic functional channels with Cx35.5, each with substantially different voltage-dependent properties, indicating they can serve differential functions. However, we previously demonstrated that electrical transmission is lost in Cx34.1 but not Cx34.7 null mutants, suggesting that Cx34.7 cannot compensate for the loss of Cx34, despite the intrinsic ability of Cx34.1 and Cx34.7 to create functional channels. The findings reveal an unanticipated functional organization in the electrical synapse, where Cx34.1 is obligatory and Cx34.7 accessory, roles that appear to be defined by the postsynaptic molecular scaffold, with two postsynaptic Cxs possibly assembling under specific functional contexts. Thus, our results indicate that electrical synapses share an organizational motif with chemical synapses, akin to how they combine postsynaptic receptor types to modify synaptic function.

The adult skeletal muscle regenerates robustly upon injury, but this regenerative capacity rapidly declines with age. In this study, we identify the lanthionine synthetase C-Like (LanCL) proteins, mammalian homologs of the bacterial peptide cyclase LanC, as positive regulators of muscle regeneration in middle-aged mice. In a barium chloride-induced injury model, we found the protein levels of LanCL1 and LanCL2 to increase during an early phase of regeneration in middle-aged (12-month-old) but not young adult (4-month-old) mice. Utilizing a mouse line lacking all three LanCL proteins (LanCL triple KO or LTKO), we examined a potential role of LanCL in injury-induced muscle regeneration. Consistent with an age-dependent function of LanCL, we observed a delayed regeneration of the tibialis anterior (TA) muscle after injury, as reflected by reduced sizes of regenerating myofibers in middle-aged (but not young) LTKO compared to age-matched WT mice. Although the pool size of quiescent satellite cells (Pax7+) was comparable between 12-month-old LTKO and WT muscles without injury, the number of Pax7+ cells was significantly higher in regenerating LTKO muscles at day 5 after injury, accompanied by drastically decreased numbers of MyoD+ and MyoG+ cells, as well as increased numbers of proliferating cells. In addition, we detected elevated expression of pro-inflammatory cytokines in regenerating LTKO muscles, while the number of macrophages was similar comparing LTKO and WT muscles. Taken together, our observations suggest that in aging muscles LanCLs are important for proper timing of inflammation resolution and regeneration upon injury. New & NoteworthyPhysiological roles of the mammalian homologs of bacterial LanC, LanCLs, are poorly understood. Our work uncovers a function of LanCLs in post-injury regeneration of aging skeletal muscles. Middle-aged LanCL triple KO mice displayed a delay in satellite cell differentiation and regenerative myofiber formation, as well as persistent inflammatory cytokine expression, suggesting that LanCLs may have an age-dependent role in modulating inflammation in the injured muscles to facilitate regeneration.

Perinatal opioid exposure is a prevalent clinical concern linked to respiratory instability and adverse infant outcomes. The opioid buprenorphine is prescribed as a medication for opioid use disorder during pregnancy and used to treat neonatal opioid withdrawal syndrome, yet its direct effects on neonatal control of breathing have not been examined. Here, we asked how acute buprenorphine exposure affects breathing at rest, and during chemoreceptor stimulation. Using dual-chamber head-out plethysmography, we measured pulmonary ventilation rate ([V]I) and metabolic rate in awake male and female Sprague-Dawley neonatal rats on postnatal days 4-5 (P4-5) during eupnea and a hypoxic-hypercapnic (HH) challenge. The effects of buprenorphine and two opioid receptor antagonists, naloxone hydrochloride, or peripherally restricted naloxone methiodide, were assessed using a repeated measures design. [V]I during eupnea and HH were markedly depressed following buprenorphine administration. Buprenorphine reduced [V]O2 and [V]CO2 and produced ventilatory equivalents for O2 and CO2 consistent with frank hypoventilation, driven by reduced breathing frequency and tidal volume (VT). When administered after buprenorphine, neither naloxone hydrochloride nor naloxone methiodide could rescue the buprenorphine-mediated hypoventilation in eupnea or during HH. In contrast, pre-treatment with either naloxone hydrochloride or naloxone methiodide attenuated buprenorphine-induced hypoventilation by preserving VT. These findings demonstrate that neonatal protective chemoreceptor reflexes are depressed by buprenorphine and suggest that pre-treatment with a peripheral opioid receptor antagonist could mitigate buprenorphine-induced hypoventilation without inducing opioid withdrawal. Key PointsO_LIAcute buprenorphine exposure significantly depressed pulmonary ventilation rate ([V]I) during eupnea and hypoxic hypercapnia (HH) in awake neonatal rats. C_LIO_LIBuprenorphine-induced hypoventilation was driven by reduced tidal volume (VT) and breathing frequency. C_LIO_LIBuprenorphine also reduced oxygen consumption ([V]O2) and carbon dioxide production ([V]CO2). C_LIO_LINaloxone given after buprenorphine failed to reverse hypoventilation. C_LIO_LIIn contrast, pre-treatment with either naloxone hydrochloride or peripherally restricted naloxone methiodide mitigated buprenorphine-induced hypoventilation by preserving VT. C_LI