Neuroscience

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.

Homeostatic plasticity of intrinsic excitability (IE) in the visual system has been essentially shown at the cortical level but whether thalamic nuclei also express homeostatic plasticity of IE is unknown. We show here that 4 days of monocular deprivation (MD) at eye opening induces a homeostatic change in IE in dorsal lateral geniculate nucleus (dLGN) neurons. Neurons recorded in the dLGN region activated by the deprived eye are more excitable than neurons recorded in the dLGN region activated by the open eye. No significant changes were observed following 7 days of MD, however. Enhanced excitability in neurons from the deprived side after 4 days of MD was associated with a reduced Kv1-dependent LTP-IE, a smaller voltage ramp, and a reduced inter-spike interval, suggesting that Kv1 channels are down-regulated in deprived dLGN neurons. Furthermore, the ankyrin G signal of the axon initial segment was larger in deprived dLGN neurons compared with open ones, indicating that Nav1 channel number also undergoes homeostatic regulation, and Kv1.1 channel signals were lower in deprived neurons compared to open ones. In addition, electrical coupling was found to be strengthened in neurons displaying enhanced IE following either brief (4 days) or long (10 days) MD. These results suggest that homeostatic and Hebbian plasticity in the dLGN share common expression mechanisms involving the regulation of Kv1 channels, Nav1 channels and electrical coupling between relay neurons.

The addictive properties of opioids are due in part to these drugs ability to alter ventral tegmental area (VTA) activity via activation of mu opioid receptors (MORs) on local and distal inputs. Prior studies have identified numerous opioid-modulated afferents to the VTA, some of which show differing levels of functional modulation by opioids, but the degree to which this parallels differences in receptor expression is not known. Hence, we used retrograde labeling combined with RNAscope to examine oprm1 mRNA expression in VTA-projecting afferents arising from a variety of distal brain regions. Because opioids are thought to be particularly influential on GABAergic afferents to the VTA, we also examined colocalization of oprm1 with GABAergic markers in VTA-projecting neurons. Interestingly, we found that oprm1 mRNA is present in both GABAergic and non-GABAergic VTA-projecting neurons. However, many (though not all) GABAergic afferents expressed higher levels of oprm1 compared to most non-GABAergic afferents (especially those arising from the cortex). These results complement previous anatomical studies that had examined oprm1 expression in these regions but in a non-quantitative way and without regard to their efferent targets. Our findings encourage future work to examine the functional implications of MOR sensitivity within these afferent pathways.

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.

Interlimb skill generalization, defined as the transfer of a newly learned skill from the trained to the untrained limb, represents a fundamental aspect of human motor behavior with significant implications for rehabilitation and athletic training. Skill generalization is influenced by processes that drive learning and interact with the newly acquired memory. For instance, in our recent work, we reported that performing a secondary, cognitively demanding task immediately after a short skill-training session impaired skill generalization when the untrained arm was tested 24-hour later. This suggests that working memory (WM) interacts with the early stage of skill memory consolidation processes and thereby impacts skill generalization. Motivated by this finding, in the current study, we investigate how WM interacts with reactivated skill memory and its subsequent impact on skill generalization, tested 24 or 48-hour post skill training. We recruited right-handed young participants (n=95) who performed a fast, accurate reaching task with their dominant right arm during a short training session (50 trials) on Day-1. After 24-hour on Day-2, depending on the group type, participants had a brief skill reactivation session (10 trials or no reactivation) and then performed the WM task (or a control task) with their right arm. Interlimb generalization to the untrained left arm was assessed either immediately after the WM/control task on Day-2 or after a 24-hour gap on Day-3. We found that, engaging in the WM task (compared to the control task) after skill reactivation on Day-2 enhanced immediate generalization. Conversely, when generalization was tested 24-hour later on Day-3, the same WM engagement impaired skill generalization. These findings demonstrate that WM engagement during the post-reactivation phase has a time-dependent influence on interlimb generalization. WM can facilitate immediate generalization, possibly by sustaining neural processes that promote skill memory generalization across effectors. However, when a 24-hour time gap is introduced, generalization is disrupted following WM engagement, possibly because of interference between underlying neural processes involved in WM and reactivation-induced (re)consolidation of the skill memory. This study highlights the delicate interplay among WM, motor memory reactivation dynamics, and skill generalization and suggests a time-dependent interplay of neural processes critical for optimizing outcomes in motor learning and clinical rehabilitation protocols.

Lack of social interaction results in various behavioral abnormalities in rodents, including increased anxiety levels, altered sociability, and impaired cognitive ability. Epigenetic factors regulate gene expression, however, how they contribute to juvenile social isolation (jSI)-induced behavioral alterations remains largely unknown. Here, we focused on the nucleus accumbens (NAc), a critical brain region of the reward system that regulates motivation-related behaviors. We first performed RNA-seq on neuronal nuclei and found alterations in genes related to neuronal function, as well as in transcriptional and epigenetic regulation. Protein-protein interaction (PPI) analysis of differentially expressed genes (DEGs) showed that top key nodes among down-regulated genes include membrane receptors (Ntrk2, Grin3a, and Grik1) and an apoptosis regulator (Bcl2). To further investigate whether jSI-induced gene expression alterations are mediated by histone modifications, we next performed CUT&Tag for four histone modifications (H3K4me1, H3K4me3, H3K27ac, and H3K27me3), and the results implied that epigenetic alterations may also play a role in neuronal function as well as transcriptional regulation. Reanalysis of previously published RNA-seq data on the manipulation of histone modification-associated factors (including Kdm6b, Brd4, and Setd1a) suggested that these enzymes were probably involved in jSI-induced gene expression alterations. Taken together, our comprehensive analysis implies the involvement of histone modification regulation in jSI-related alterations of gene expression in NAc.

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

Subthreshold depression is associated with significant functional impairment and elevated risk of major depressive disorder. A negative self-concept may disrupt the implicit positive association evoked by ones own face, impairing incidental encoding of self-relevant information. Whether subthreshold depression involves a selective deficit in encoding self-face identity remains unclear. The attribute amnesia paradigm is well suited to address this question because it can dissociate attentional selection from working memory encoding. Using this paradigm, we examined the issue across two experiments. Experiment 1 employed nonsocial stimuli (animal drawings) and confirmed an intact attribute amnesia effect in subthreshold depression (n = 30) comparable to healthy controls (n = 30), ruling out a generalized encoding deficit. Experiment 2 replaced targets with faces (self or other) and revealed a selective enhancement of the attribute amnesia effect for self-face identity in subthreshold depression. Specifically, on the surprise trial, accuracy for self-face identity dropped to near-chance levels in the subthreshold depression group, whereas no such deficit emerged for other-faces or in controls. Encoding recovered rapidly once explicit memory expectations were introduced, indicating intact basic encoding capacity. These findings suggest that subthreshold depression is associated with a specific impairment in incidentally encoding self-face identity. This impairment likely stems from a negative self-concept that weakens self-face salience under incidental encoding conditions. By capturing this selective encoding failure, the present study reveals that the self-processing deficit in subthreshold depression can arise at the gating stage between attention and working memory consolidation.

Vocal communication involves a series of cognitive processes, which can be broadly categorized into three components: perceiving communicative signals; deciding whether and how to respond; and generating vocal motor output. These processes must work harmoniously, with integration and bridging between components being crucial for effective communication. Previous research on vocal communication has typically focused on specific brain regions or isolated cognitive functions, often lacking a holistic perspective of macro-scale, whole-cortical dynamics and their role in the complete communication process. Therefore, although the cortical areas associated with each cognitive component have been localized in humans, the macro-scale cortical dynamics underlying the integration of these cognitive processes remain unknown. Building on recent findings linking macro-scale cortical dynamics to behavioral performance, we hypothesized that traveling wave like cross-areal interactions play a role in integrating the three communicative components. To test this hypothesis, we recorded whole-cortical activity using epidural electrocorticography (ECoG) while subject marmosets vocally interacted with partners. We found theta-band activation in several cortical areas, including the parietal and auditory cortices, while listening to partners calls. This activity was further modulated depending on whether the subjects engaged in vocal interactions, potentially representing the transformation of sensory processing into decision-making and vocal motor preparation. Given the widespread nature of this modulation, we next characterized whole-brain activity patterns by employing a novel analytical method, Weakly Orthogonal Conjugate Contrast Analysis (WOCCA). This analysis revealed that cortical activity could be decomposed into two distinct traveling wave like propagation patterns, a rotational and a translational wave, and both waves discriminated communicative conditions consistent with localized activity. The rotational wave further represented vocal motor preparation through trigger-like temporal pattern. In addition, the magnitude of the translational wave immediately before subjects vocal production correlated with the vocal production-induced suppression of high-gamma-band activity, particularly in the prefrontal and auditory cortices. As vocalization-induced suppression is believed to reflect sensory prediction, the translational wave may propagate specific decision-related or acoustic information necessary for subsequent vocal production to local cortical areas. These findings suggest that the brain orchestrates the sequential cognitive processes underlying vocal communication through macro-scale traveling waves.

Dendritic arbor morphology is shaped in part by interactions with neighboring dendrites, and its geometry strongly influences the spatial distribution and strength of synapses. These observations raise the possibility that local dendritic contacts help determine where synapses accumulate and strengthen. Previous work in cultured hippocampal neurons showed that dendrite-dendrite contact sites are non-random and associated with local synaptic clustering. Here we asked whether a different type of dendritic contact, formed between a dendrite and the soma of a neighboring neuron, behaves similarly. Using dissociated hippocampal cultures, immunofluorescence imaging, time-lapse microscopy, quantitative image analysis, stochastic spatial simulations, and minimal quantitative modeling, we identified three recurrent classes of dendrite-soma interactions (DSIs): dendrites crossing directly over a neighboring soma, growing tangentially along the soma perimeter, or contacting the proximal region where a neighboring dendrite emerges from the soma. These interactions were abundant, occurred exclusively between different neurons, and showed substantial structural persistence over several days. Their overall frequency exceeded stochastic predictions across culture densities, and two configurations - proximal and tangential contacts - were selectively enriched above random expectation, whereas soma-crossing contacts were largely consistent with stochastic overlap. DSI composition also changed over development, with proximal contacts becoming progressively more prevalent. At DSI sites, synaptophysin-positive puncta were significantly denser and more intense than on non-interacting dendritic segments, consistent with local enrichment and strengthening of presynaptic specializations. Minimal modeling further indicated that biased formation together with developmental stabilization explains the observed organization better than stochastic geometry alone. These findings identify DSIs as non-random structural motifs in cultured hippocampal networks and suggest that dendrite contact geometry can contribute to synaptic distribution and strengthening.

The phenomenon of splitting was originally observed in hamsters which, after prolonged exposure to constant light, exhibit two rest/wake cycles within a subjective day. Splitting is a consequence of the left and right suprachiasmatic nuclei (SCN) falling out of synchrony. While it is known that split activity is characterized by an antiphase relationship between the left and right SCN and between the core and shell within each hemisphere, the role of the commissural projections that connect the right and left SCN is not known. In the present study, we investigate the impact of the inter-hemispheric connections on the split and unsplit dynamics of a computational model of the bilateral SCN. Our model has 4 nodes corresponding to each right and left core and shell. We simulated our bilateral model under different lighting conditions and measured its period and the phase relationships among the 4 nodes. To further characterize the dynamics of the system, we performed a bifurcation analysis. We found that the bilateral model automatically splits unless entrained by bright light/dark cycles, or unless it has excitatory inter-hemispheric connections. This suggests that excitatory cross-connections may be important for freerunning behavior. We found that constant light of varying intensities transitions the model between split and unsplit activity only in very limited conditions, but the strength and polarity of the contralateral connections play a much greater role in this dynamical transition. These findings suggest that splitting may involve plasticity of the inter-hemispheric connections of the SCN.

Pregnancy requires tightly regulated immune adaptation, which may be altered by maternal adiposity and HIV infection. In high HIV prevalence settings with rising obesity rates, overlapping metabolic and infectious inflammation may shape gestational immune trajectories. We examined patterns of C-reactive (CRP), serum amyloid A (SAA), and interferon-gamma inducible protein-10 (IP-10) among 527 pregnant women living with HIV in Cape Town, South Africa. Immune markers were measured at up to four antenatal visits. Maternal body mass index (BMI) was categorised as normal, overweight or obese. Antiretroviral therapy (ART) exposure was classified as initiation before conception or during pregnancy. Mixed-effects models assessed associations, adjusting for baseline CD4 cell count and viral load. CRP concentrations remained elevated across pregnancy and were significantly higher among obese women independent of ART timing. In contrast, IP-10 concentrations were strongly associated with ART timing, with higher levels observed prior to ART initiation and declining thereafter; no independent association with BMI was observed. SAA concentrations were modestly lower among obese women and among those on preconception ART. These divergent patterns persisted after adjustment for HIV disease severity. These findings suggest that metabolic and HIV-related inflammatory pathways operate in parallel during pregnancy, with adiposity predominantly influencing acute-phase responses and HIV-related immune activation shaping interferon-associated chemokines trajectories.

Elite athletes exhibit sport-specific neural adaptations, yet it remains unclear whether such changes reflect general effects of training or the unique demands of individual sports. Skiing requires postural control and whole-body coordination under dynamically unstable environments, placing high demands on somatosensory processing and sensorimotor integration. The present study aimed to identify structural brain characteristics specific to elite skiers by comparing them with athletes from other sports disciplines and non-athletes. T1-weighted MRI data were analyzed using voxel-based morphometry in 13 skiers, 23 non-ski control athletes and 25 non-athletes. Whole-brain analysis comparing skiers with non-ski athletes revealed a significant cluster showing greater gray matter volume in skiers compared with non-ski athletes in the left postcentral gyrus, extending into the superior parietal lobule. The identified cluster primarily encompassed cytoarchitectonic Areas 2 and 5L. These regions are involved in higher-order somatosensory processing and multisensory integration. Importantly, region-of-interest analysis demonstrated that gray matter volume within this cluster was greater in skiers compared with non-ski athletes and non-athletes, with no difference between non-ski athletes and non-athletes. These findings highlight the relative prominence of structural adaptations within somatosensory-parietal networks, reflecting the unique integration of proprioceptive and other sensory information required for elite skiing. Overall, these findings provide evidence for sport-specific structural brain differences in elite athletes and highlight the importance of somatosensory and parietal regions in sensorimotor integration relevant to skiing. These findings may have implications for understanding neural markers of expertise and may inform future approaches to training and performance evaluation in skiing.

Phagocytic and immune-like cells have been observed in the satellite envelope of neuronal somata in peripheral sensory ganglia of many species for several decades. These cells likely play an important role in normal function of sensory neurons and they may also play an important role in neuronal dysfunction and neurodegeneration seen with neuropathy. Recent findings have described a satellite macrophage population transcriptomically similar to microglia in peripheral ganglia of some mammalian species. The function of these cells, and the mechanisms by which they may influence neurons in neuropathy are unclear. We sought to understand the phenotype and localization of these cells in the human dorsal root ganglion (hDRG) using large-scale single nucleus and spatial transcriptomic datasets from individuals with and without a history of peripheral diabetic neuropathy. We observed a large population of macrophages that express classical microglia makers such as TMEM119 and P2RY12 in the hDRG, as previously described. Our findings confirm that these microglia-like cells (MLCs) localize to the satellite envelope around neuronal somata, yet are transcriptomically distinct from all glial cell types characterized in the hDRG. These MLCs exhibit changes in abundance and localization with diabetic painful neuropathy (DPN) in both the hDRG and sural nerves suggesting that they are not exclusively localized to the DRG. We conclude that microglia-like cells are likely the resident tissue macrophage (RTM) of the hDRG, and perhaps the peripheral nervous system (PNS) given their localization to the sural nerve and other ganglia, where they are predicted to regulate homeostatic neuronal functions and response to injury. HighlightsO_LIMLCs are likely the RTM of hDRGs C_LIO_LIMLCs localize to the satellite envelope and recede with Nageotte nodule formation C_LIO_LIMLC activation state and signaling shift with diabetic neuropathy C_LIO_LIMLCs are also present in other ganglia and sural nerve C_LI

Motoneurons are under strong pressure to maintain stable motor output throughout an individual life, through homeostatic regulation of their electrical properties. Dysregulated spinal motoneuron excitability has long been implicated in the pathogenesis of amyotrophic lateral sclerosis (ALS). Recent work in SOD1G93A mice suggests that the homeostatic response of motoneurons becomes dysregulated as cellular processes are disrupted by the disease, causing fluctuations in motoneuron electrical properties. Yet, few studies directly test whether ALS motoneurons respond differently than wild type motoneurons to a common chronic perturbation. Here, we used in vivo electrophysiology to test whether motoneurons from pre-symptomatic SOD1G93A mice modulate excitability differently than wild type motoneurons in response to the same homeostatic perturbation: chronic inhibition exerted by the benzodiazepine diazepam. Using linear mixed-effects statistical models, we assessed whether diazepam treatment differentially modulated passive properties, firing behavior, spike properties, and/or synaptic inputs in SOD1G93A versus wild type motoneurons. We identified a significant genotype x treatment interaction effect selectively for properties related to passive membrane integration and spike initiation, including membrane time constant, peak input resistance, and recruitment current. In contrast, firing gain, spike waveform characteristics, and synaptic inputs were largely unaffected. These findings indicate that sustained inhibitory perturbation selectively triggered overactive intrinsic compensatory mechanisms in SOD1G93A motoneurons rather than inducing widespread changes in firing or synaptic transmission. Together, our results provide direct evidence for over-active homeostatic control of motoneuron excitability and support a view of motoneuron dysfunction in ALS as a problem of altered feedback regulation rather than simply hyper- or hypo-excitability. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=52 SRC="FIGDIR/small/725609v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@25f125org.highwire.dtl.DTLVardef@faf2c9org.highwire.dtl.DTLVardef@15993a8org.highwire.dtl.DTLVardef@1ed006a_HPS_FORMAT_FIGEXP M_FIG C_FIG

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

Tourette syndrome (TS) is a neurodevelopmental disorder of childhood onset characterised by vocal and motor tics and is associated with cortical-striatal-thalamic-cortical circuit [CSTC] dysfunction. TS often follows a developmental time course in which tics become increasingly more controlled during adolescence. However, many individuals continue to have debilitating tics into adulthood. This indicates that there may be important differences between adults with TS for whom the clinical phenotype is more stable, and children and adolescents with the disorder who may be undergoing developmental neuroplastic changes linked to the reduction of their tics. Previous studies have used transcranial magnetic stimulation (TMS) to investigate changes in cortical motor excitability in individuals with TS, including measurement of resting motor threshold (RMT). However, the findings from these studies have been mixed, have varied between adult and child samples, and have often been based on small sample sizes. Here we report a multi-centre, mega-analytic, study in which RMT data collected from children and adults with TS at multiple research centres was pooled for analysis. Results confirmed that mean RMT was significantly increased in individuals with TS compared to neurotypical controls. However, this result can be explained by the more important findings that: (a) RMT for adults with TS did not differ from that of neurotypical adults; and (b) the rate that RMT decreases with age during childhood and adolescence is reduced in individuals with TS compared to controls. Thus, while neurotypical individuals reach an adult RMT level by ~12-13 years of age, individuals with TS are substantially delayed in doing so, and do not reach an adult RMT level until much later, at ~24 years of age. We conclude therefore that differences in measures of cortical excitability between children and adolescents with TS and chronologically age-matched neurotypical controls may likely reflect a developmental delay in the maturation of functional brain networks in individuals with TS, which may normalise with age.

The thalamus is essential for learning, dynamically engaging with other subcortical and cerebral cortex regions throughout the learning process. Here, the thalamus serves as a critical connector hub and synchroniser within the thalamocortical system of the brain. However, whilst higher order thalamic nuclei are known to be particularly important for this process, the exact contributions of individual higher order and first order thalamic nuclei, alongside their individual involvement with cortical networks and subcortical regions, remains unexplored within the initial phase of learning. In light of this, we analysed fMRI data obtained within a paradigm which is designed to examine initial learning processes within feedback-driven stimulus-response learning, in order to explore thalamic contributions. We investigated dynamic learning-related functional connectivity alterations between various thalamic nuclei with other subcortical regions and cortical networks. Our results show that the initial phase of learning was associated with: (1) decreasing functional connectivity between thalamic nuclei and frontoparietal and cingulo-opercular networks, (2) increasing functional connectivity between thalamic nuclei with default mode and salience networks, (3) decreasing functional connectivity between thalamic nuclei and the putamen, and (4) decreasing functional connectivity amongst higher order thalamic nuclei. Furthermore (5) these dynamic alterations were associated primarily by mediodorsal thalamus. Altogether, these results indicate that higher order thalamic nuclei play a crucial role within initial learning and in the generation of novel goal-directed behaviour. This was demonstrated through enhanced functional connectivity with selected cortical networks which drive goal-directed behaviour, alongside decreased functional connectivity with striatal regions which drive motor selectivity.

The endogenous peptide dynorphin (Dyn) and its target the kappa opioid receptor (KOR) play a crucial role in regulating factors related to stress and reward. The KOR is expressed in multiple cell types in the nucleus accumbens (NAc), including presynaptic dopamine (DA) terminals, where it inhibits DA release modulates the function of the DA transporter (DAT). The Dyn/KOR system is upregulated by exposure to drugs of abuse including the DAT inhibitor, cocaine, and their activity is integrally involved in negative affective states associated with withdrawal from substance abuse. We aimed to better understand the impact of the Dyn/KOR system on presynaptic DA terminals and potential effects on DAT interactions with cocaine by measuring the impact of the KOR agonist U50,488 on electrically-evoked DA release and subsequent reuptake in NAc slices from C57BL6/J mice. We showed that superfusion of U50,488 inhibited DA release and markedly reduced cocaine-induced inhibition of DA reuptake, indicating tolerance to cocaine effects. We replicated this finding in the NAc of rhesus macaques using the DAT/NET inhibitor nomifensine, demonstrating that these mechanisms are conserved across DAT inhibitors and in non-human primates. KOR activation results in phosphorylation of the Threonine-53 site on the DAT, a process thought to mediate its impact on DAT function. We tested whether this phosphorylation site is required for the KOR-mediated reduction cocaine effects. To tackle this question, we employed a knock-in mouse line with an Alanine-53 on the DAT (DAT-T53A), rendering that residue insensitive to phosphorylation. We show that DAT-T53A mice have enhanced DA release and uptake, and U50,488 has a reduced inhibitory effect on peak DA release. Remarkably, U50,488 no longer modified the effect of cocaine on uptake in these mice, demonstrating the dependence of this effect on phosphorylated Threonine-53 and highlighting a potential mechanism underlying cocaine tolerance.

Microglia are the brains resident immune cells that exhibit complex signaling behavior, including phagocytic activity in response to threats and prolonged neuronal activity. Adenosine triphosphate (ATP) is a chemoattractant for microglia. In the nucleus accumbens (NAc), ATP is co-packaged and released with DA, and microglia express dopamine (DA) receptors and ATP receptors. The present work examines microglia chemotactic motility for these transmitters using iontophoresis and multiphoton microscopy approaches in NAc brain slices from GFP-monocyte labeled transgenic mice. ATP chemoattraction was more regularly observed than DA chemoattraction, and DA chemoattraction occurred in only a small subset of microglia. The DA chemoattraction of this subset was blocked by DA D1 antagonism. Microglia are reactive oxygen species (ROS) scavengers. Application of glucose oxidase produces mild but consistent increases in ROS and induced inflammatory-related changes in microglial morphology and motility. Glucose oxidase application decreased DA release but had variable effects on ATP release. The toll-like receptor 4 (TLR4) agonist lipopolysaccharide (LPS) transitioned microglia from ramified to amoeboid morphology over a period of 4 hours, and increased DA and ATP release across this same period. These studies highlight the complex relationship between local immune activation and DA terminal functionality.