Introduction Childhood absence epilepsy (CAE) is associated with abnormal thalamocortical oscillations and enhanced GABAergic function in the ventrobasal (VB) thalamus, including increased extrasynaptic GABAA receptor-mediated tonic currents in thalamocortical (TC) neurons. Serotonin signaling modulates seizure activity in several epilepsy models, and activation of 5-HT2C receptors (5-HT2CRs) has been reported to exert anti-absence seizure effects, although the underlying cellular mechanisms remain unclear. Here, we examined the thalamic distribution of 5-HT2CRs and their functional impact on tonic GABAA inhibition and absence seizures.Methods 5-HT2CR expression in the nucleus reticularis thalami (NRT) and VB was assessed by immunohistochemistry in adult Wistar rats, Genetic Absence Epilepsy Rats from Strasbourg (GAERS), and their non-epileptic control strain (NEC). Whole-cell patch-clamp recordings were used to measure tonic GABAA currents in VB TC neurons in thalamic slices. In vivo EEG recordings in freely moving GAERS rats were performed to evaluate the effects of systemic administration of the 5-HT2CR agonist Ro 60-0175 on absence seizures.Results No differences in 5-HT2CR expression were observed in the NRT across strains. In the VB, receptor expression was lowest in GAERS and highest in Wistar rats compared with NEC. Tonic GABAA currents in TC neurons were larger in GAERS than in Wistar or NEC rats. Activation of 5-HT2CRs with Ro 60-0175 reduced tonic GABAA currents in TC neurons in all strains. Systemic administration of Ro 60-0175 in adult GAERS produced a clear reduction in absence seizures.Discussion These findings indicate that 5-HT2CRs regulate thalamic extrasynaptic GABAA inhibition and that their activation reduces tonic inhibitory drive in TC neurons while exerting anti-absence effects in vivo. The lower expression of 5-HT2CRs in the GAERS VB suggests altered serotonergic control of thalamic inhibition in absence epilepsy. By reducing tonic GABAergic currents, 5-HT2CR activation may rebalance thalamocortical activity and suppress pathological oscillations, supporting these receptors as potential therapeutic targets for CAE.

Summary The prefrontal cortex (PFC), a brain region critical for executive and cognitive functions, is characterized by its protracted maturation extending through adolescence until early adulthood. During adolescence, the PFC undergoes substantial rearrangements, creating a window of heightened plasticity allowing experience-dependent refinement of neural networks. While this extended plasticity supports the development of higher-order cognitive functions, it also confers increased vulnerability to environmental and biological perturbations that can disrupt circuit development and contribute to cognitive and behavioral impairments relevant to psychiatric disorders. Astrocytes are central regulators of brain homeostasis and actively participate in developmental processes that shape postnatal brain maturation. Although astrocyte dysfunction has been increasingly linked to psychiatric pathophysiology, it remains unknown whether aberrant astrocyte activity can directly influence PFC development and cognitive maturation. Here, using selective modulation of astrocyte activity during defined developmental windows in the PFC, we show that abnormal astrocyte activity during adolescence induces transient synaptic loss through enhanced microglial phagocytosis, produces long-lasting alterations in fast-spiking parvalbumin (PV) interneurons, and results in persistent deficits in PFC-dependent behaviors. Together, these findings provide causal evidence that disrupted astrocyte function during adolescent PFC maturation can lead to persistent neuronal and cognitive deficits with relevance to major psychiatric disorders.

Synaptic plasticity is a fundamental substrate of behavioral adaptation, yet the underlying molecular dynamics remain poorly defined. We tested the hypothesis that, within striatal circuits, flexibility relies on nanoscale remodeling of synaptic machinery coupling anterograde glutamatergic transmission to retrograde endocannabinoid signaling, a process disrupted in states of rigidity and aging. In the dorsolateral striatum, we found cell-type-specific facilitation of metabotropic glutamate receptor 5 (mGlu5)-dependent, endocannabinoid-mediated long-term depression at cortico-striatal synapses of indirect pathway neurons in flexible goal-directed behavior, but not after training promoting inflexibility. Stochastic Optical Reconstruction Microscopy (STORM) super-resolution imaging revealed that behavioral adaptation, but not rigidity, is accompanied by increased postsynaptic abundance of mGlu5 and diacylglycerol lipase-α (DAGLα), an endocannabinoid-synthesizing enzyme, and presynaptic CB1 cannabinoid receptors. In parallel, the nanoscale distance between mGlu5 and DAGLα is reduced in postsynaptic spine heads. These nanoscale changes emerged within the time window required for behavioral updating. Intriguingly, the molecular densities of mGlu5, DAGLα, and CB1 receptors predict the strength of behavioral adaptation. In aging mice, these nanoscale changes were absent in association with behavioral rigidity. These findings identify a nanoscale synaptic remodeling mechanism that enables behavioral flexibility and reveal how its failure contributes to rigidity, including that observed in aging.

The control of instrumental actions engages distinct behavioral strategies whose contributions are regulated with experience. Instrumental performance, which depends on the causal relationship between actions and their outcomes (A-O), relies on flexible, goal-directed control of behavior. Actions can become less sensitive to changes in action-outcome (A-O) contingencies with repetition, resulting in more inflexible, habitual behaviors. The loss of flexibility with repetition requires plasticity at corticostriatal circuits. However, the underlying molecular mechanisms are not yet established, and how these mechanisms specifically relate to the inability to adapt to new contingencies is unknown. In mice, we find that inflexible behavioral performance following overtraining of an appetitive instrumental task is associated with a reduced capacity of mGluR5 receptors in the dorsolateral striatum (DLS) to engage intracellular signaling in response to changes in action-outcome contingency. We also observed dichotomous modulation of timing-dependent synaptic depression (tLTD) at striatal projection neurons of the indirect (iSPNs) and direct (dSPNs) pathways. Preventing overstimulation of mGluR5 signaling through a homotypic process preserved behavioral sensitivity to changes in A-O contingencies despite overtraining, and averted the related biochemical and synaptic changes. Furthermore, mGluR5 couples to different signaling pathways to regulate tLTD in iSPNs and dSPNs. Our findings demonstrate that decreased signaling capacity of mGluR1/5, accompanied by cell-type-specific modulation of corticostriatal synapses in the DLS, represents a key molecular mechanism underlying overtraining-induced behavioral inflexibility.

Gephyrin is the main scaffolding protein at inhibitory postsynaptic sites, and its clusters are the signaling hubs where several molecular pathways converge. Post-translational modifications (PTMs) of gephyrin alter GABAA receptor clustering at the synapse, but it is unclear how this affects neuronal activity at the circuit level. We assessed the contribution of gephyrin PTMs to microcircuit activity in the mouse barrel cortex by slice electrophysiology and in vivo two-photon calcium imaging of layer 2/3 (L2/3) pyramidal cells during single-whisker stimulation. Our results suggest that, depending on the type of gephyrin PTM, the neuronal activities of L2/3 pyramidal neurons can be differentially modulated, leading to changes in the size of the neuronal population responding to the single-whisker stimulation. Furthermore, we show that gephyrin PTMs have their preference for selecting synaptic GABAA receptor subunits. Our results identify an important role of gephyrin and GABAergic postsynaptic sites for cortical microcircuit function during sensory stimulation.

Aptamer-functionalized biosensors exhibit high selectivity for monitoring neurotransmitters in complex environments. We translated nanoscale aptamer-modified nanopipette sensors to detect endogenous dopamine release in vitro and ex vivo. These sensors employ quartz nanopipettes with nanoscale pores (ca. 10 nm diameter) that are functionalized with aptamers that enable the selective capture of dopamine through target-specific conformational changes. The dynamic behavior of aptamer structures upon dopamine binding leads to the rearrangement of surface charge within the nanopore, resulting in measurable changes in ionic current. To assess sensor performance in real time, we designed a fluidic platform to characterize the temporal dynamics of nanopipette sensors. We then conducted differential biosensing by deploying control sensors modified with nonspecific DNA alongside dopamine-specific sensors in biological milieu. Our results confirm the functionality of aptamer-modified nanopipettes for direct measurements in undiluted complex fluids, specifically in the culture media of human-induced pluripotent stem cell-derived dopaminergic neurons. Moreover, sensor implantation and repeated measurements in acute brain slices was possible, likely owing to the protected sensing area inside nanoscale DNA-filled orifices, minimizing exposure to nonspecific interferents and preventing clogging. Further, differential recordings of endogenous dopamine released through electrical stimulation in the dorsolateral striatum demonstrate the potential of aptamer-modified nanopipettes for ex vivo recordings with unprecedented spatial resolution and reduced tissue damage.

The molecular code that controls synapse formation and maintenance in vivo has remained quite sparse. Here, we identify that the secreted protein Adamtsl3 functions as critical hippocampal synapse organizer acting through the transmembrane receptor DCC (deleted in colorectal cancer). Traditionally, DCC function has been associated with glutamatergic synaptogenesis and plasticity in response to Netrin-1 signaling. We demonstrate that early post-natal deletion of Adamtsl3 in neurons impairs DCC protein expression, causing reduced density of both glutamatergic and GABAergic synapses. Adult deletion of Adamtsl3 in either GABAergic or glutamatergic neurons does not interfere with DCC-Netrin-1 function at glutamatergic synapses but controls DCC signaling at GABAergic synapses. The Adamtsl3-DCC signaling unit is further essential for activity-dependent adaptations at GABAergic synapses, involving DCC phosphorylation and Src kinase activation. These findings might be particularly relevant for schizophrenia because genetic variants in Adamtsl3 and DCC have been independently linked with schizophrenia in patients.

Cell signaling is central to neuronal activity and its dysregulation may lead to neurodegeneration and cognitive decline. Here, we show that selective genetic potentiation of neuronal ERK signaling prevents cell death in vitro and in vivo in the mouse brain, while attenuation of ERK signaling does the opposite. This neuroprotective effect mediated by an enhanced nuclear ERK activity can also be induced by the novel cell penetrating peptide RB5. In vitro administration of RB5 disrupts the preferential interaction of ERK1 MAP kinase with importinα1/KPNA2 over ERK2, facilitates ERK1/2 nuclear translocation, and enhances global ERK activity. Importantly, RB5 treatment in vivo promotes neuroprotection in mouse models of Huntington's (HD), Alzheimer's (AD), and Parkinson's (PD) disease, and enhances ERK signaling in a human cellular model of HD. Additionally, RB5‐mediated potentiation of ERK nuclear signaling facilitates synaptic plasticity, enhances cognition in healthy rodents, and rescues cognitive impairments in AD and HD models. The reported molecular mechanism shared across multiple neurodegenerative disorders reveals a potential new therapeutic target approach based on the modulation of KPNA2‐ERK1/2 interactions. Shared mechanisms governing both neuronal cell survival and neuroplasticity have not yet been explored therapeutically. This study demonstrates that modulation of ERK1 and KPNA2 interactions strengthens nuclear ERK signaling in the brain, promotes cognitive enhancement, and delays neurodegeneration. Shared mechanisms governing both neuronal cell survival and neuroplasticity have not yet been explored therapeutically. This study demonstrates that modulation of ERK1 and KPNA2 interactions strengthens nuclear ERK signaling in the brain, promotes cognitive enhancement, and delays neurodegeneration.

Microglia play a key role in shaping the formation and refinement of the excitatory network of the brain. However, less is known about whether and how they organize the development of distinct inhibitory networks. We find that microglia are essential for the proper development of somatostatin-positive (SST+) cell synapses during the second postnatal week. We further identify a pair of molecules that act antagonistically to one another in the organization of SST+ cell axonal elaboration. Whereas CX3CL1 acts to suppress axonal growth and complexity, CXCL12 promotes it. Assessing the functional importance of microglia in the development of cortical activity, we find that a whisker stimulation paradigm that drives SST+ cell activation leads to reduced cortical spiking in brains depleted of microglia. Collectively, our data demonstrate an important role of microglia in regulating the development of SST+ cell output early in life.

ABSTRACT Bilateral sensory information is indispensable for navigating the world. In most mammals, signals sensed by either side of the midline will ultimately reach the cortex where they will be integrated for perception and appropriate action selection. Even though information transferred across the hemispheres is routed through the corpus callosum, how and which microcircuits are key in integrating it is not well understood. Here we identify an essential role for layer 1 NDNF + inhibitory cells of mice in integrating bilateral whisker-evoked information in an NMDA receptor-dependent manner. Direct connections from the contralateral cortex and the ipsilateral side activate NDNF + neurons, which subsequently inhibit the late spiking activity of underlying layer 2/3 neurons, but not layer 5. Our results identify a feed-forward regulatory pathway for bilateral cortical sensory processing of upper layer cortical neurons actuated via layer 1 NDNF + interneurons.

Despite extensive research into understanding synaptic mechanisms of striatal plasticity, the functional role played by astrocytes in this region remains to be fully elucidated. It was recently demonstrated that high-frequency stimulation (HFS) of cortical inputs induced long-term depression (LTD) mediated by adenosine A1 receptor (A1R) activation at corticostriatal synapses of the direct pathway [cortico-striatal projection neuron (dSPN)] in the dorsolateral striatum (DLS). Because astrocyte-derived adenosine has been shown to regulate synaptic transmission in several brain areas, we investigated whether this form of neuron-astrocyte signaling contributes to synaptic plasticity in the DLS of male and female mice. We found that cortical HFS increases calcium (Ca2+) levels in striatal astrocytes through activation of metabotropic glutamate receptor type 5 (mGluR5) signaling and that this astrocyte-mediated response is necessary for A1R-mediated LTD. Consistent with this, astrocyte activation with Gq designer receptors exclusively activated by designer drugs (DREADDs) induced A1R-mediated synaptic depression at cortico-dSPN synapses. Together, these results indicate that astrocytes are integral elements of striatal A1R-mediated LTD.SIGNIFICANCE STATEMENT Abnormal striatal circuit function is implicated in several disorders such as Parkinson's disease and Huntington's disease. Thus, there is a need to better understand the mechanisms supporting proper striatal activity. While extensive work has revealed the many important contributions from neurons in striatal function, far less is known about the role of astrocytes in this brain area. We show that long-term depression (LTD) at corticostriatal synapses of the direct pathway is not strictly a neuronal phenomenon; astrocytes respond to corticostriatal stimulation and this astrocyte response is necessary for LTD. This research adds to the accumulating evidence that astrocytes are active and integral players in synaptic communication, and that neuron-astrocyte interactions are key cellular processes involved in brain function.

A diverse set of GABA A receptors (GABA A Rs) enable synaptic plasticity adaptations at inhibitory postsynaptic sites in collaboration with the scaffolding protein gephyrin. Early studies helped to identify distinctions between GABA A R subtypes allocated within specific functional circuits, but their contribution to the changing dynamics of a microcircuit remains unclear. Here, using the whisker-barrel system in mouse, we assessed the contribution of specific synaptic GABA A R subtypes and gephyrin scaffolding changes to sensory processing in vivo . We monitored spontaneous and evoked Ca 2+ transients in layer 2/3 pyramidal cells with the genetically encoded Ca 2+ sensor RCaMP1.07. Using Gabra1 or Gabra2 global and conditional knockout mice, we uncovered that α1- and α2-GABA A Rs determine the sparseness of L2/3 pyramidal neuron encoding. In a cell-type dependent manner, α1-GABA A Rs and α2-GABA A Rs affected neuronal excitability and the reliability of neuronal responses after whisker stimulation. We also discerned that gephyrin with its diverse post-translational modifications (PTMs) shows preference for specific GABA A R subtype to facilitate microcircuit activity. Our results underscore the relevance of the diversity of GABA A Rs within a cortical microcircuit. Key points While GABAergic inhibition from interneuron subtypes regulates cortical microcircuit activity the molecular determinants have remain unclear. We demonstrate that specific-GABA A receptor subtypes contribute differentially to layer 2/3 neuronal activities in mouse barrel cortex. Importantly, we link the GABAAR contributions to the scaffolding properties of its important postsynaptic density protein gephyrin. We show that different PTMs on gephyrin determines neuronal excitability via GABAAR recruitment and modulation of inhibition within layer 2/3 neurons. Specifically, α1 and α2 subunits containing GABA A receptors, along with their scaffolding protein gephyrin determine the distribution of high, medium and low activity pyramidal neurons during sensory encoding, whereby controlling the total activity of cortical microcircuit.

Cell signalling mechanisms are central to neuronal activity and their dysregulation may lead to neurodegenerative processes and associated cognitive decline. So far, a major effort has been directed toward the dissection of disease specific pathways with the still unmet promise to develop precision medicine strategies. With a different approach, here we show that a selective genetic potentiation of neuronal ERK signalling prevents cell death in vitro and in vivo in the mouse brain while ERK attenuation does the opposite. This neuroprotective effect can also be induced pharmacologically by a cell permeable peptide mimicking the loss of ERK1 MAP kinase, leading to a selective enhancement of ERK2 mediated nuclear cell signalling. The drug treatment prevents neurodegeneration in mouse models of Huntington's (HD), Alzheimer's (AD), and Parkinson's disease (PD). Importantly, the selective potentiation of ERK2 signalling facilitates both structural and synaptic plasticity, enhances cognition in healthy mice and rescues mild cognitive impairments in both models of AD and HD. Altogether, our observation truly represents a remarkable example of a shared molecular mechanism across multiple neurodegenerative disorders and a potentially valuable therapeutic target for neuro-enhancement.

Monoaminergic modulation of cortical and thalamic inputs to the dorsal striatum (DS) is crucial for reward-based learning and action control. While dopamine has been extensively investigated in this context, the synaptic effects of serotonin (5-HT) have been largely unexplored. Here, we investigated how serotonergic signaling affects associative plasticity at glutamatergic synapses on the striatal projection neurons of the direct pathway (dSPNs). Combining chemogenetic and optogenetic approaches reveals that impeding serotonergic signaling preferentially gates spike-timing-dependent long-term depression (t-LTD) at thalamostriatal synapses. This t-LTD requires dampened activity of the 5-HT4 receptor subtype, which we demonstrate controls dendritic Ca2+ signals by regulating BK channel activity, and which preferentially localizes at the dendritic shaft. The synaptic effects of 5-HT signaling at thalamostriatal inputs provide insights into how changes in serotonergic levels associated with behavioral states or pathology affect striatal-dependent processes.

Abnormal hippocampal neural plasticity has been implicated in behavioural abnormalities and complex neuropsychiatric conditions, including bipolar disorder (BD). However, the determinants of this neural alteration remain unknown. This work tests the hypothesis that the neurotransmitter serotonin (5-HT) is a key determinant of hippocampal neuroplasticity, and its absence leads to maladaptive behaviour relevant for BD. Depletion of brain 5-HT in Tph2 mutant mice resulted in reduced behavioural despair, reduced anxiety, marked aggression and lower habituation in novel environments, reminiscent of bipolar-associated manic behaviour. Treatment with valproate produced a substantial improvement of the mania-like behavioural phenotypes displayed by Tph2 mutants. Brain-wide fMRI mapping in mutants revealed functional hippocampal hyperactivity in which we also observed dramatically increased neuroplasticity. Importantly, remarkable correspondence between the transcriptomic profile of the Tph2 mutant hippocampus and neurons from bipolar disorder patients was observed. Chronic stress reversed the emotional phenotype and the hippocampal transcriptional landscape of Tph2 mutants. These changes were associated with inappropriate activation of transcriptional adaptive response to stress as assessed by gene set enrichment analyses in the hippocampus of Tph2 mutant mice. These findings delineate 5-HT as a critical determinant in BD associated maladaptive emotional responses and aberrant hippocampal neuroplasticity, and support the use of Tph2−/− mice as a new research tool for mechanistic and therapeutic research in bipolar disorder.

The cJun N-terminal kinase (JNK) signaling pathway has been extensively studied with regard to its involvement in neurodegenerative processes, but little is known about its functions in neurotransmission. In a mouse model of Parkinson's disease (PD), we show that the pharmacological activation of dopamine D1 receptors (D1R) produces a large increase in JNK phosphorylation. This effect is secondary to dopamine depletion, and is restricted to the striatal projection neurons that innervate directly the output structures of the basal ganglia (dSPN). Activation of JNK in dSPN relies on cAMP-induced phosphorylation of the dopamine- and cAMP-regulated phosphoprotein of 32kDa (DARPP-32), but does not require N-methyl-d-aspartate (NMDA) receptor transmission. Electrophysiological experiments on acute brain slices from PD mice show that inhibition of JNK signaling in dSPN prevents the increase in synaptic strength caused by activation of D1Rs. Together, our findings show that dopamine depletion confers to JNK the ability to mediate dopamine transmission, informing the future development of therapies for PD.

Ambrisentan is a propanoic acid antagonist of endothelin-A receptors. It is highly specific for endothelin receptors as in binding studies it showed no affinity for more than 100 other receptors. Ambrisentan is currently approved in the US and EU for the treatment of Pulmonary Arterial Hypertension (PAH), a disease characterized by a progressive increase in pulmonary vascular resistance leading to right ventricular heart failure and premature death. Blocking endothelin receptors ambrisentan counteracts many of the detrimental effects of endothelin, a vasoactive peptide that has a role in the pathogenesis of PAH. They include vasoconstriction, intimal fibrosis, decrease in nitric oxide synthesis and inflammation.

Absence seizures (ASs) are the hallmark of childhood/juvenile absence epilepsy. Monotherapy with first-line anti-absence drugs only controls ASs in 50% of patients, indicating the need for novel therapeutic targets. Since serotonin family-2 receptors (5-HT2Rs) are known to modulate neuronal activity in the cortico-thalamo-cortical loop, the main network involved in AS generation, we investigated the effect of selective 5-HT2AR and 5-HT2CR ligands on ASs in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a well established polygenic rat model of these non-convulsive seizures. GAERS rats were implanted with fronto-parietal EEG electrodes under general anesthesia, and their ASs were later recorded under freely moving conditions before and after intraperitoneal administration of various 5-HT2AR and 5-HT2CR ligands. The 5-HT2A agonist TCB-2 dose-dependently decreased the total time spent in ASs, an effect that was blocked by the selective 5-HT2A antagonist MDL11,939. Both MDL11,939 and another selective 5-HT2A antagonist (M100,907) increased the length of individual seizures when injected alone. The 5-HT2C agonists lorcaserin and CP-809,101 dose-dependently suppressed ASs, an effect blocked by the selective 5-HT2C antagonist SB 242984. In summary, 5-HT2ARs and 5-HT2CRs negatively control the expression of experimental ASs, indicating that selective agonists at these 5-HT2R subtypes might be potential novel anti-absence drugs.

Absence seizures (ASs) are the hallmark of childhood/juvenile absence epilepsy. Monotherapy with firstline anti-absence drugs only controls ASs in 50% of patients, indicating the need for novel therapeutic targets. Since serotonin family-2 receptors (5-HT2Rs) are known to modulate neuronal activity in the cortico-thalamo-cortical loop, the main network involved in AS generation, we investigated the effect of selective 5-HT2AR and 5-HT2CR ligands on ASs in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), a well established polygenic rat model of these non-convulsive seizures. GAERS rats were implanted with fronto-parietal EEG electrodes under general anesthesia, and their ASs were later recorded under freely moving conditions before and after intraperitoneal administration of various 5HT2AR and 5-HT2CR ligands. The 5-HT2A agonist TCB-2 dose-dependently decreased the total time spent in ASs, an effect that was blocked by the selective 5-HT2A antagonist MDL11,939. Both MDL11,939 and another selective 5-HT2A antagonist (M100,907) increased the length of individual seizures when injected alone. The 5-HT2C agonists lorcaserin and CP-809,101 dose-dependently suppressed ASs, an effect blocked by the selective 5-HT2C antagonist SB 242984. In summary, 5-HT2ARs and 5-HT2CRs negatively control the expression of experimental ASs, indicating that selective agonists at these 5-HT2R subtypes might be potential novel anti-absence drugs. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). -HTR); Knockout, (KO); Metagyrus activation, (MDA); t from Strasbourg, (GAERS); y, (i.p.); Two-way analysis of ); Thalamic reticular nucleus,