Integration of Multiple Determinants in the Neuronal Computation of Economic Values

Economic goods may vary on multiple dimensions (determinants). A central conjecture in decision neuroscience is that choices between goods are made by comparing subjective values computed through the integration of all relevant determinants. Previous work identified three groups of neurons in the orbitofrontal cortex (OFC) of monkeys engaged in economic choices: (1) offer value cells, which encode the value of individual offers; (2) chosen value cells, which encode the value of the chosen good; and (3) chosen juice cells, which encode the identity of the chosen good. In principle, these populations could be sufficient to generate a decision. Critically, previous work did not assess whether offer value cells (the putative input to the decision) indeed encode subjective values as opposed to physical properties of the goods, and/or whether offer value cells integrate multiple determinants. To address these issues, we recorded from the OFC while monkeys chose between risky outcomes. Confirming previous observations, three populations of neurons encoded the value of individual offers, the value of the chosen option, and the value-independent choice outcome. The activity of both offer value cells and chosen value cells encoded values defined by the integration of juice quantity and probability. Furthermore, both populations reflected the subjective risk attitude of the animals. We also found additional groups of neurons encoding the risk associated with a particular option, the risky nature of the chosen option, and whether the trial outcome was positive or negative. These results provide substantial support for the conjecture described above and for the involvement of OFC in good-based decisions.

Dynamic Fidelity Control to the Central Auditory System: Synergistic Glycine/GABAergic Inhibition in the Cochlear Nucleus

GABA and glycine are the major inhibitory transmitters that attune neuronal activity in the CNS of mammals. The respective transmitters are mostly spatially separated, that is, synaptic inhibition in the forebrain areas is mediated by GABA, whereas glycine is predominantly used in the brainstem. Accordingly, inhibition in auditory brainstem circuits is largely mediated by glycine, but there are few auditory synapses using both transmitters in maturity. Little is known about physiological advantages of such a two-transmitter inhibitory mechanism. We explored the benefit of engaging both glycine and GABA with inhibition at the endbulb of Held-spherical bushy cell synapse in the auditory brainstem of juvenile Mongolian gerbils. This model synapse enables selective in vivo activation of excitatory and inhibitory neuronal inputs through systemic sound stimulation and precise analysis of the input (endbulb of Held) output (spherical bushy cell) function. The combination of in vivo and slice electrophysiology revealed that the dynamic AP inhibition in spherical bushy cells closely matches the inhibitory conductance profile determined by the glycine-R and GABA_A-R. The slow and potent glycinergic component dominates the inhibitory conductance, thereby primarily accounting for its high-pass filter properties. GABAergic transmission enhances the inhibitory strength and shapes its duration in an activity-dependent manner, thus increasing the inhibitory potency to suppress the excitation through the endbulb of Held. Finally, in silico modeling provides a strong link between in vivo and slice data by simulating the interactions between the endbulb- and the synergistic glycine-GABA-conductances during in vivo-like spontaneous and sound evoked activities.

Deletion of Prostaglandin E2 Synthesizing Enzymes in Brain Endothelial Cells Attenuates Inflammatory Fever

Fever is a hallmark of inflammatory and infectious diseases. The febrile response is triggered by prostaglandin E₂ synthesis mediated by induced expression of the enzymes cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase 1 (mPGES-1). The cellular source for pyrogenic PGE₂ remains a subject of debate; several hypotheses have been forwarded, including immune cells in the periphery and in the brain, as well as the brain endothelium. Here we generated mice with selective deletion of COX-2 and mPGES1 in brain endothelial cells. These mice displayed strongly attenuated febrile responses to peripheral immune challenge. In contrast, inflammation-induced hypoactivity was unaffected, demonstrating the physiological selectivity of the response to the targeted gene deletions. These findings demonstrate that PGE₂ synthesis in brain endothelial cells is critical for inflammation-induced fever.

Engrailed Alters the Specificity of Synaptic Connections of Drosophila Auditory Neurons with the Giant Fiber

We show that a subset of sound-detecting Johnston's Organ neurons (JONs) in Drosophila melanogaster, which express the transcription factors Engrailed (En) and Invected (Inv), form mixed electrical and chemical synaptic inputs onto the giant fiber (GF) dendrite. These synaptic connections are detected by trans-synaptic Neurobiotin (NB) transfer and by colocalization of Bruchpilot-short puncta. We then show that misexpressing En postmitotically in a second subset of sound-responsive JONs causes them to form ectopic electrical and chemical synapses with the GF, in turn causing that postsynaptic neuron to redistribute its dendritic branches into the vicinity of these afferents. We also introduce a simple electrophysiological recording paradigm for quantifying the presynaptic and postsynaptic electrical activity at this synapse, by measuring the extracellular sound-evoked potentials (SEPs) from the antennal nerve while monitoring the likelihood of the GF firing an action potential in response to simultaneous subthreshold sound and voltage stimuli. Ectopic presynaptic expression of En strengthens the synaptic connection, consistent with there being more synaptic contacts formed. Finally, RNAi-mediated knockdown of En and Inv in postmitotic neurons reduces SEP amplitude but also reduces synaptic strength at the JON–GF synapse. Overall, these results suggest that En and Inv in JONs regulate both neuronal excitability and synaptic connectivity.

A Model of the Medial Superior Olive Explains Spatiotemporal Features of Local Field Potentials

Local field potentials are important indicators of in vivo neural activity. Sustained, phase-locked, sound-evoked extracellular fields in the mammalian auditory brainstem, known as the auditory neurophonic, reflect the activity of neurons in the medial superior olive (MSO). We develop a biophysically based model of the neurophonic that accounts for features of in vivo extracellular recordings in the cat auditory brainstem. By making plausible idealizations regarding the spatial symmetry of MSO neurons and the temporal synchrony of their afferent inputs, we reduce the challenging problem of computing extracellular potentials in a 3D volume conductor to a one-dimensional problem. We find that postsynaptic currents in bipolar MSO neuron models generate extracellular voltage responses that strikingly resemble in vivo recordings. Simulations reproduce distinctive spatiotemporal features of the in vivo neurophonic response to monaural pure tones: large oscillations (hundreds of microvolts to millivolts), broad spatial reach (millimeter scale), and a dipole-like spatial profile. We also explain how somatic inhibition and the relative timing of bilateral excitation may shape the spatial profile of the neurophonic. We observe in simulations, and find supporting evidence in in vivo data, that coincident excitatory inputs on both dendrites lead to a drastically reduced spatial reach of the neurophonic. This outcome surprises because coincident inputs are thought to evoke maximal firing rates in MSO neurons, and it reconciles previously puzzling evoked potential results in humans and animals. The success of our model, which has no axon or spike-generating sodium currents, suggests that MSO spikes do not contribute appreciably to the neurophonic.

Frequency-Dependent, Cell Type-Divergent Signaling in the Hippocamposeptal Projection

Hippocampal oscillations are critical for information processing, and are strongly influenced by inputs from the medial septum. Hippocamposeptal neurons provide direct inhibitory feedback from the hippocampus onto septal cells, and are therefore likely to also play an important role in the circuit; these neurons fire at either low or high frequency, reflecting hippocampal network activity during theta oscillations or ripple events, respectively. Here, we optogenetically target the long-range GABAergic projection from the hippocampus to the medial septum in rats, and thereby simulate hippocampal input onto downstream septal cells in an acute slice preparation. In response to optogenetic activation of hippocamposeptal fibers at theta and ripple frequencies, we elicit postsynaptic GABAergic responses in a subset (24%) of septal cells, most predominantly in fast-spiking cells. In addition, in another subset of septal cells (19%) corresponding primarily to cholinergic cells, we observe a slow hyperpolarization of the resting membrane potential and a decrease in input resistance, particularly in response to prolonged high-frequency (ripple range) stimulation. This slow response is partially sensitive to GIRK channel and D2 dopamine receptor block. Our results suggest that two independent populations of septal cells distinctly encode hippocampal feedback, enabling the septum to monitor ongoing patterns of activity in the hippocampus.

Combined Treatment with a BACE Inhibitor and Anti-A{beta} Antibody Gantenerumab Enhances Amyloid Reduction in APPLondon Mice

Therapeutic approaches for prevention or reduction of amyloidosis are currently a main objective in basic and clinical research on Alzheimer‘s disease. Among the agents explored in clinical trials are anti-Aβ peptide antibodies and secretase inhibitors. Most anti-Aβ antibodies are considered to act via inhibition of amyloidosis and enhanced clearance of existing amyloid, although secretase inhibitors reduce the de novo production of Aβ. Limited information is currently available on the efficacy and potential advantages of combinatorial antiamyloid treatment. We performed a chronic study in APP_London transgenic mice that received treatment with anti-Aβ antibody gantenerumab and BACE inhibitor RO5508887, either as mono- or combination treatment. Treatment aimed to evaluate efficacy on amyloid progression, similar to preexisting amyloidosis as present in Alzheimer's disease patients. Mono-treatments with either compound caused a dose-dependent reduction of total brain Aβ and amyloid burden. Combination treatment with both compounds significantly enhanced the antiamyloid effect. The observed combination effect was most pronounced for lowering of amyloid plaque load and plaque number, which suggests effective inhibition of de novo plaque formation. Moreover, significantly enhanced clearance of pre-existing amyloid plaques was observed when gantenerumab was coadministered with RO5508887. BACE inhibition led to a significant time- and dose-dependent decrease in CSF Aβ, which was not observed for gantenerumab treatment. Our results demonstrate that combining these two antiamyloid agents enhances overall efficacy and suggests that combination treatments may be of clinical relevance.

Abnormal High-Frequency Burst Firing of Cerebellar Neurons in Rapid-Onset Dystonia-Parkinsonism

Loss-of-function mutations in the α3 isoform of the Na⁺/K⁺ ATPase (sodium pump) are responsible for rapid-onset dystonia parkinsonism (DYT12). Recently, a pharmacological model of DYT12 was generated implicating both the cerebellum and basal ganglia in the disorder. Notably, partially blocking sodium pumps in the cerebellum was necessary and sufficient for induction of dystonia. Thus, a key question that remains is how partially blocking sodium pumps in the cerebellum induces dystonia. In vivo recordings from dystonic mice revealed abnormal high-frequency bursting activity in neurons of the deep cerebellar nuclei (DCN), which comprise the bulk of cerebellar output. In the same mice, Purkinje cells, which provide strong inhibitory drive to DCN cells, also fired in a similarly erratic manner. In vitro studies demonstrated that Purkinje cells are highly sensitive to sodium pump dysfunction that alters the intrinsic pacemaking of these neurons, resulting in erratic burst firing similar to that identified in vivo. This abnormal firing abates when sodium pump function is restored and dystonia caused by partial block of sodium pumps can be similarly alleviated. These findings suggest that persistent high-frequency burst firing of cerebellar neurons caused by sodium pump dysfunction underlies dystonia in this model of DYT12.

Unification of Neuronal Spikes, Seizures, and Spreading Depression

The pathological phenomena of seizures and spreading depression have long been considered separate physiological events in the brain. By incorporating conservation of particles and charge, and accounting for the energy required to restore ionic gradients, we extend the classic Hodgkin–Huxley formalism to uncover a unification of neuronal membrane dynamics. By examining the dynamics as a function of potassium and oxygen, we now account for a wide range of neuronal activities, from spikes to seizures, spreading depression (whether high potassium or hypoxia induced), mixed seizure and spreading depression states, and the terminal anoxic "wave of death." Such a unified framework demonstrates that all of these dynamics lie along a continuum of the repertoire of the neuron membrane. Our results demonstrate that unified frameworks for neuronal dynamics are feasible, can be achieved using existing biological structures and universal physical conservation principles, and may be of substantial importance in enabling our understanding of brain activity and in the control of pathological states.

Tat 101-Mediated Enhancement of Brain Pericyte Migration Involves Platelet-Derived Growth Factor Subunit B Homodimer: Implications for Human Immunodeficiency Virus-Associated Neurocognitive Disorders

In the era of antiretroviral therapy, although the human immunodeficiency virus (HIV) replication can be successfully controlled, complications of the CNS continue to affect infected individuals. Viral Tat protein is not only neurotoxic but has also been shown to disrupt the integrity of the blood–brain barrier (BBB). Although the role of brain microvascular endothelial cells and astrocytes in Tat-mediated impairment has been well documented, pericytes, which are important constituents of the BBB and play a key role in maintaining the integrity of the barrier, remain poorly studied in the context of HIV-associated neurocognitive disorders (HAND). In the present study, we demonstrated that exposure of human brain microvascular pericytes and C3H/10T1/2 cells to HIV-1 Tat101 resulted in increased expression of platelet-derived growth factor subunit B homodimer (PDGF-BB) and increased migration of the treated cells. Furthermore, we also demonstrated that this effect of Tat was mediated via activation of mitogen-activated protein kinases and nuclear factor-B pathways. Secreted PDGF-BB resulted in autocrine activation of the PDGF-BB/PDGF β receptor signaling pathway, culminating ultimately into increased pericyte migration. Ex vivo relevance of these findings was further corroborated in isolated microvessels of HIV Tg26 mice that demonstrated significantly increased expression of PDGF-BB in isolated brain microvessels with a concomitant loss of pericytes. Intriguingly, loss of pericyte coverage was also detected in sections of frontal cortex from humans with HIV-encephalitis compared with the uninfected controls. These findings thus implicate a novel role of PDGF-BB in the migration of pericytes, resulting in loss of pericyte coverage from the endothelium with a subsequent breach of the BBB.

Opposing Role of NMDA Receptor GluN2B and GluN2D in Somatosensory Development and Maturation

Development of correct topographical connections between peripheral receptors and central somatosensory stations requires activity-dependent synapse refinement, in which the NMDA type of glutamate receptors plays a key role. Here we compared functional roles of GluN2B (GluR2 or NR2B) and GluN2D (GluR4 or NR2D), two major regulatory subunits of neonatal NMDA receptors, in development of whisker-related patterning at trigeminal relay stations. Compared with control littermates, both the appearance of whisker-related patterning and the termination of the critical period, as assessed by unilateral infraorbital nerve transection, were delayed by nearly a day in the somatosensory cortex of GluN2B^+/– mice but advanced by nearly a day in GluN2D^–/– mice. Similar temporal shifts were found at subcortical relay stations in the thalamus and brainstem of GluN2B^+/– and GluN2D^–/– mice. In comparison, the magnitude of lesion-induced critical period plasticity in the somatosensory cortex, as assessed following row-C whisker removal, was normal in both mutants. Thus, GluN2B and GluN2D play counteractive roles in temporal development and maturation of somatosensory maps without affecting the magnitude of critical period plasticity. To understand the opposing action, we then examined neuronal and synaptic expressions of the two subunits along the trigeminal pathway. At each trigeminal station, GluN2B was predominant at asymmetrical synapses of non-GABAergic neurons, whereas GluN2D was selective to asymmetrical synapses of GABAergic neurons. Together, our findings suggest that GluN2B expressed at glutamatergic synapses on glutamatergic projection neurons facilitates refinement of ascending pathway synapses directly, whereas GluN2D expressed at glutamatergic synapses on GABAergic interneurons delays it indirectly.

Change of Fate Commitment in Adult Neural Progenitor Cells Subjected to Chronic Inflammation

Neural progenitor cells (NPCs) have regenerative capabilities that are activated during inflammation. We aimed at elucidating how NPCs, with special focus on the spinal cord-derived NPCs (SC-NPCs), are affected by chronic inflammation modeled by experimental autoimmune encephalomyelitis (EAE). NPCs derived from the subventricular zone (SVZ-NPCs) were also included in the study as a reference from a distant inflammatory site. We also investigated the transcriptional and functional difference between the SC-NPCs and SVZ-NPCs during homeostatic conditions. NPCs were isolated and propagated from the SVZ and cervical, thoracic, and caudal regions of the SC from naive rats and rats subjected to EAE. Using Affymetrix microarray analyses, the global transcriptome was measured in the different NPC populations. These analyses were paralleled by NPC differentiation studies. Assessment of basal transcriptional and functional differences between NPC populations in naive rat revealed a higher neurogenic potential in SVZ-NPCs compared with SC-NPCs. Conversely, during EAE, the neurogenicity of the SC-NPCs was increased while their gliogenicity was decreased. We detected an overall increase of inflammation and neurodegeneration-related genes while the developmentally related profile was decreased. Among the decreased functions, we isolated a gliogenic signature that was confirmed by differentiation assays where the SC-NPCs from EAE generated fewer oligodendrocytes and astrocytes but more neurons than control cultures. In summary, NPCs displayed differences in fate-regulating genes and differentiation potential depending on their rostrocaudal origin. Inflammatory conditions downregulated gliogenicity in SC-NPCs, promoting neurogenicity. These findings give important insight into neuroinflammatory diseases and the mechanisms influencing NPC plasticity during these conditions.

Plasticity of Binocularity and Visual Acuity Are Differentially Limited by Nogo Receptor

The closure of developmental critical periods consolidates neural circuitry but also limits recovery from early abnormal sensory experience. Degrading vision by one eye throughout a critical period both perturbs ocular dominance (OD) in primary visual cortex and impairs visual acuity permanently. Yet understanding how binocularity and visual acuity interrelate has proven elusive. Here we demonstrate the plasticity of binocularity and acuity are separable and differentially regulated by the neuronal nogo receptor 1 (NgR1). Mice lacking NgR1 display developmental OD plasticity as adults and their visual acuity spontaneously improves after prolonged monocular deprivation. Restricting deletion of NgR1 to either cortical interneurons or a subclass of parvalbumin (PV)-positive interneurons alters intralaminar synaptic connectivity in visual cortex and prevents closure of the critical period for OD plasticity. However, loss of NgR1 in PV neurons does not rescue deficits in acuity induced by chronic visual deprivation. Thus, NgR1 functions with PV interneurons to limit plasticity of binocularity, but its expression is required more extensively within brain circuitry to limit improvement of visual acuity following chronic deprivation.

Within-Hemifield Competition in Early Visual Areas Limits the Ability to Track Multiple Objects with Attention

It is much easier to divide attention across the left and right visual hemifields than within the same visual hemifield. Here we investigate whether this benefit of dividing attention across separate visual fields is evident at early cortical processing stages. We measured the steady-state visual evoked potential, an oscillatory response of the visual cortex elicited by flickering stimuli, of moving targets and distractors while human observers performed a tracking task. The amplitude of responses at the target frequencies was larger than that of the distractor frequencies when participants tracked two targets in separate hemifields, indicating that attention can modulate early visual processing when it is divided across hemifields. However, these attentional modulations disappeared when both targets were tracked within the same hemifield. These effects were not due to differences in task performance, because accuracy was matched across the tracking conditions by adjusting target speed (with control conditions ruling out effects due to speed alone). To investigate later processing stages, we examined the P3 component over central-parietal scalp sites that was elicited by the test probe at the end of the trial. The P3 amplitude was larger for probes on targets than on distractors, regardless of whether attention was divided across or within a hemifield, indicating that these higher-level processes were not constrained by visual hemifield. These results suggest that modulating early processing stages enables more efficient target tracking, and that within-hemifield competition limits the ability to modulate multiple target representations within the hemifield maps of the early visual cortex.

Knockdown of CRF1 Receptors in the Ventral Tegmental Area Attenuates Cue- and Acute Food Deprivation Stress-Induced Cocaine Seeking in Mice

Corticotrophin-releasing factor (CRF) modulates the influence of stress on cocaine reward and reward seeking acting at multiple sites, including the ventral tegmental area (VTA). There is controversy, however, concerning the contribution of CRF receptor type 1 (CRFR1) to this effect and whether CRF within the VTA is involved in other aspects of reward seeking independent of acute stress. Here we examine the role of CRFR1 within the VTA in relation to cocaine and natural reward using viral delivery of short hairpin RNAs (lenti-shCRFR1) and investigate the effect on operant self-administration and motivation to self-administer, as well as stress- and cue-induced reward seeking in mice. While knockdown of CRFR1 in the VTA had no effect on self-administration behavior for either cocaine or sucrose, it effectively blocked acute food deprivation stress-induced reinstatement of cocaine seeking. We also observed reduced cue-induced cocaine seeking assessed in a single extinction session after extended abstinence, but cue-induced sucrose seeking was unaffected, suggesting dissociation between the contribution of CRFR1 in the VTA in cocaine reward and sucrose and cocaine seeking. Further, our data indicate a role for VTA CRFR1 signaling in cocaine seeking associated with, and independent of, stress potentially involving conditioning and/or salience attribution of cocaine reward-related cues. CRFR1 signaling in the VTA therefore presents a target for convergent effects of both cue- and stress-induced cocaine-seeking pathways.

Beyond Blindsight: Properties of Visual Relearning in Cortically Blind Fields

Damage to the primary visual cortex (V1) or its immediate afferents results in a dense scotoma, termed cortical blindness (CB). CB subjects have residual visual abilities, or blindsight, which allow them to detect and sometimes discriminate stimuli with high temporal and low spatial frequency content. Recent work showed that with training, discriminations in the blind field can become more reliable, and even reach consciousness. However, the narrow spatiotemporal bandwidth of blindsight limits its functional usefulness in everyday vision. Here, we asked whether visual training can induce recovery outside the spatiotemporal bandwidth of blindsight. Specifically, could human CB subjects learn to discriminate static, nonflickering stimuli? Can such learning transfer to untrained stimuli and tasks, and does double training with moving and static stimuli provide additional advantages relative to static training alone? We found CB subjects capable of relearning static orientation discriminations following single as well as double training. However, double training with complex, moving stimuli in a separate location was necessary to recover complex motion thresholds at locations trained with static stimuli. Subjects trained on static stimuli alone could only discriminate simple motion. Finally, both groups had approximately equivalent, incomplete recovery of fine orientation and direction discrimination thresholds, as well as contrast sensitivity. These results support two conclusions: (1) from a practical perspective, complex moving stimuli and double training may be superior training tools for inducing visual recovery in CB, and (2) the cortically blind visual system can relearn to perform a wider range of visual discriminations than predicted by blindsight alone.

Phasic Dopaminergic Activity Exerts Fast Control of Cholinergic Interneuron Firing via Sequential NMDA, D2, and D1 Receptor Activation

Phasic increases in dopamine (DA) are involved in the detection and selection of relevant sensory stimuli. The DAergic and cholinergic system dynamically interact to gate and potentiate sensory inputs to striatum. Striatal cholinergic interneurons (CINs) respond to relevant sensory stimuli with an initial burst, a firing pause, or a late burst, or a combination of these three components. CIN responses coincide with phasic firing of DAergic neurons in vivo. In particular, the late burst of CINs codes for the anticipated reward. To examine whether DAergic midbrain afferents can evoke the different CIN responses, we recorded from adult olfactory tubercle slices in the mouse ventral striatum. Olfactory inputs to striatal projection neurons were gated by the cholinergic tone. Phasic optogenetic activation of DAergic terminals evoked combinations of initial bursts, pauses, and late bursts in subsets of CINs by distinct receptor pathways. Glutamate release from midbrain afferents evoked an NMDAR-dependent initial burst followed by an afterhyperpolarization-induced pause. Phasic release of DA itself evoked acute changes in CIN firing. In particular, in CINs without an initial burst, phasic DA release evoked a pause through D2-type DA receptor activation. Independently, phasic DA activated a slow depolarizing conductance and the late burst through a D1-type DA receptor pathway. In summary, DAergic neurons elicit transient subsecond firing responses in CINs by sequential activation of NMDA, D2-type, and D1-type receptors. This fast control of striatal cholinergic tone by phasic DA provides a novel dynamic link of two transmitter systems central to the detection and selection of relevant stimuli.

Fine-Scale Plasticity of Microscopic Saccades

When asked to maintain their gaze steady on a given location, humans continually perform microscopic eye movements, including fast gaze shifts known as microsaccades. It has long been speculated that these movements may contribute to the maintenance of fixation, but evidence has remained contradictory. We used a miniaturized version of saccadic adaptation, an experimental procedure by which motor control of saccades is modified through intrasaccadic displacements of the target. We found that the statistical distribution of microsaccade amplitudes changes after brief exposure to systematic shifts of the fixation point during microsaccade occurrence. Shifts in the same directions as microsaccades produce movements with larger amplitudes, whereas shifts against microsaccade directions result in smaller movements. Our findings show that microsaccades are precisely monitored during fixation and that their motor program is modified if the postsaccadic target position is not at the expected retinal location. These results demonstrate that saccadic adaptation occurs even when the stimulus is already close to the foveal center and precise execution of the movement may not be critical. They support the proposal that adaptation is necessary to maintain a consistent relationship between motor control and its visual consequences and that the representation of space is intrinsically multimodal, even during fixation.

Synaptopodin Regulates Spine Plasticity: Mediation by Calcium Stores

The role of synaptopodin (SP), an actin-binding protein residing in dendritic spines, in synaptic plasticity was studied in dissociated cultures of hippocampus taken from control and SP knock-out (SPKO) mice. Unlike controls, SPKO cultures were unable to express changes in network activity or morphological plasticity after intense activation of their NMDA receptors. SPKO neurons were transfected with SP-GFP, such that the only SP resident in these neurons is the fluorescent species. The localization and intensity of the transfected SP were similar to that of the native one. Because less than half of the spines in the transfected neurons contained SP, comparisons were made between SP-containing (SP(+)) and SP lacking (SP(–)) spines in the same dendritic segments. Synaptic plasticity was induced either in the entire network by facilitation of the activation of the NMDA receptor, or specifically by local flash photolysis of caged glutamate. After activation, spines that were endowed with SP puncta were much more likely to expand than SP(–) spines. The spine expansion was suppressed by thapsigargin, which disables calcium stores. The mechanism through which SP may promote plasticity is indicated by the observations that STIM-1, the sensor of calcium concentration in stores, and Orai-1, the calcium-induced calcium entry channel, are colocalized with SP, in the same dendritic spines. The structural basis of SP is likely to be the spine apparatus, found in control but not in SPKO cells. These results indicate that SP has an essential, calcium store-related role in regulating synaptic plasticity in cultured hippocampal neurons.

Fear Conditioning, Safety Learning, and Sleep in Humans

Fear conditioning is considered an animal model of post-traumatic stress disorder. Such models have shown fear conditioning disrupts subsequent rapid eye movement sleep (REM). Here, we provide a translation of these models into humans. Using the fear potentiated startle (FPS) procedure, we examined the effects of fear conditioning and safety signal learning on subsequent REM sleep in healthy adults. We also examined the effects of changes in REM sleep on retention of fear and safety learning. Participants (n = 42 normal controls) spent 3 consecutive nights in the laboratory. The first was an adaptation night. Following the second night, we administered a FPS procedure that included pairing a wrist shock with a threat signal and a safety signal never paired with a shock. The next day, we administered the FPS procedure again, with no wrist shocks to any stimulus, to measure retention of fear and safety. Canonical correlations assessed the relationship between FPS response and REM sleep. Results demonstrated that increased safety signal learning during the initial acquisition phase was associated with increased REM sleep consolidation that night, with 28.4% of the variance in increased REM sleep consolidation from baseline accounted for by safety signal learning. Overnight REM sleep was, in turn, related to overnight retention of fear and safety learning, with 22.5% of the variance in startle retention accounted for by REM sleep. These data suggest that sleep difficulties, specifically REM sleep fragmentation, may play a mechanistic role in post-traumatic stress disorder via an influence on safety signal learning and/or threat-safety discrimination.

Molecular Mechanisms Contributing to TARP Regulation of Channel Conductance and Polyamine Block of Calcium-Permeable AMPA Receptors

Many properties of fast synaptic transmission in the brain are influenced by transmembrane AMPAR regulatory proteins (TARPs) that modulate the pharmacology and gating of AMPA-type glutamate receptors (AMPARs). Although much is known about TARP influence on AMPAR pharmacology and kinetics through their modulation of the extracellular ligand-binding domain (LBD), less is known about their regulation of the ion channel region. TARP-induced modifications in AMPAR channel behavior include increased single-channel conductance and weakened block of calcium-permeable AMPARs (CP-AMPARs) by endogenous intracellular polyamines. To investigate how TARPs modify ion flux and channel block, we examined the action of -2 (stargazin) on GluA1 and GluA4 CP-AMPARs. First, we compared the permeation of organic cations of different sizes. We found that -2 increased the permeability of several cations but not the estimated AMPAR pore size, suggesting that TARP-induced relief of polyamine block does not reflect altered pore diameter. Second, to determine whether residues in the TARP intracellular C-tail regulate polyamine block and channel conductance, we examined various -2 C-tail mutants. We identified the membrane proximal region of the C terminus as crucial for full TARP-attenuation of polyamine block, whereas complete deletion of the C-tail markedly enhanced the TARP-induced increase in channel conductance; thus, the TARP C-tail influences ion permeation. Third, we identified a site in the pore-lining region of the AMPAR, close to its Q/R site, that is crucial in determining the TARP-induced changes in single-channel conductance. This conserved residue represents a site of TARP action, independent of the AMPAR LBD.

Prismatic Adaptation Changes Visuospatial Representation in the Inferior Parietal Lobule

Prismatic adaptation has been shown to induce a realignment of visuoproprioceptive representations and to involve parietocerebellar networks. We have investigated in humans how far other types of functions known to involve the parietal cortex are influenced by a brief exposure to prismatic adaptation. Normal subjects underwent an fMRI evaluation before and after a brief session of prismatic adaptation using rightward deviating prisms for one group or after an equivalent session using plain glasses for the other group. Activation patterns to three tasks were analyzed: (1) visual detection; (2) visuospatial short-term memory; and (3) verbal short-term memory. The prismatic adaptation-related changes were found bilaterally in the inferior parietal lobule when prisms, but not plain glasses, were used. This effect was driven by selective changes during the visual detection task: an increase in neural activity was induced on the left and a decrease on the right parietal side after prismatic adaptation. Comparison of activation patterns after prismatic adaptation on the visual detection task demonstrated a significant increase of the ipsilateral field representation in the left inferior parietal lobule and a significant decrease in the right inferior parietal lobule. In conclusion, a brief exposure to prismatic adaptation modulates differently left and right parietal activation during visual detection but not during short-term memory. Furthermore, the visuospatial representation within the inferior parietal lobule changes, with a decrease of the ipsilateral hemifield representation on the right and increase on the left side, suggesting thus a left hemispheric dominance.

Vesicular Glutamate Transporter Expression Level Affects Synaptic Vesicle Release Probability at Hippocampal Synapses in Culture

The vesicular glutamate transporter (VGLUT) plays an essential role in synaptic transmission by filling vesicles with glutamate. At mammalian synapses, VGLUT expression level determines the amount of glutamate packaged into vesicles, and the specific paralog of VGLUT expressed affects the release probability. In this study, we investigate whether there is a link between the number of VGLUTs on vesicles and release probability. We used a combination of electrophysiology and imaging techniques in cultured mouse hippocampal neurons where the VGLUT expression level has been severely altered. We found that vesicles with drastically reduced VGLUT expression were released with a lower probability. This deficit in release could only be rescued by a functional transporter, suggesting that the transport function, and not the molecular interactions, of the protein affects vesicle release. Based on these data, we propose a novel means of presynaptic vesicle release regulation—the intravesicular glutamate fill state of the vesicle.

Fast Phasic Release Properties of Dopamine Studied with a Channel Biosensor

Few other neurotransmitters are of as intense interest to neuropsychiatry and neurology as dopamine, yet existing techniques to monitor dopamine release leave an important spatiotemporal gap in our understanding. Electrochemistry and fluorescence imaging tools have been developed to fill the gap, but these methods have important limitations. We circumvent these limitations by introducing a dopamine-gated chloride channel into rat dorsal striatal medium spiny neurons, targets of strong dopamine innervation, thereby transforming dopamine from a slow transmitter into a fast transmitter and revealing new opportunities for studying moment-to-moment regulation of dopamine release. We demonstrate pharmacological and biophysical properties of the channel that make it suitable for fast, local dopamine measurements, and we demonstrate for the first time spontaneous and evoked responses to vesicular dopamine release in the dorsal striatum. Evoked dopamine currents were separated into a fast, monosynaptic component and a slower-rising and decaying disynaptic component mediated by nicotinic receptor activation. In summary, LGC-53 represents a dopamine biosensor with properties suitable for temporal separation of distinct dopamine signals in targets of dopamine innervation.

Differential Regulation of Cone Calcium Signals by Different Horizontal Cell Feedback Mechanisms in the Mouse Retina

Controlling neurotransmitter release by modulating the presynaptic calcium level is a key mechanism to ensure reliable signal transmission from one neuron to the next. In this study, we investigated how the glutamatergic output of cone photoreceptors (cones) in the mouse retina is shaped by different feedback mechanisms from postsynaptic GABAergic horizontal cells (HCs) using a combination of two-photon calcium imaging and pharmacology at the level of individual cone axon terminals. We provide evidence that hemichannel-mediated (putative ephaptic) feedback sets the cone output gain by defining the basal calcium level, a mechanism that may be crucial for adapting cones to the ambient light level. In contrast, pH-mediated feedback did not modulate the cone basal calcium level but affected the size and shape of light-evoked cone calcium signals in a contrast-dependent way: low-contrast light responses were amplified, whereas high-contrast light responses were reduced. Finally, we provide functional evidence that GABA shapes light-evoked calcium signals in cones. Because we could not localize ionotropic GABA receptors on cone axon terminals using electron microscopy, we suggest that GABA may act through GABA autoreceptors on HCs, thereby possibly modulating hemichannel- and/or pH-mediated feedback. Together, our results suggest that at the cone synapse, hemichannel-mediated (ephaptic) and pH-mediated feedback fulfill distinct functions to adjust the output of cones to changing ambient light levels and stimulus contrasts and that the efficacy of these feedback mechanisms is likely modulated by GABA release in the outer retina.

A Class of GABAergic Neurons in the Prefrontal Cortex Sends Long-Range Projections to the Nucleus Accumbens and Elicits Acute Avoidance Behavior

GABAergic projections from the neocortex to subcortical structures have been poorly characterized. Using Dlxi12b–Cre mice, we found anatomical evidence for GABAergic neurons that project from the mouse medial prefrontal cortex (mPFC) to multiple subcortical targets. We used a combination of patch-clamp electrophysiology, optogenetics, and pharmacology to confirm that Dlxi12b-labeled projections from the mPFC to the nucleus accumbens (NAcc) release GABA and do not corelease glutamate. Furthermore, optogenetic stimulation of these GABAergic projections from mPFC to NAcc induces avoidance behavior in a real-time place preference task, suggesting that these long-range projecting GABAergic neurons can transmit aversive signals. Finally, we found evidence for heterogeneous histochemical and/or electrophysiological properties of long-range projecting GABAergic neurons in the mPFC. Some of these neurons were labeled in parvalbumin–Cre and vasoactive intestinal peptide–Cre mice. We also used a novel intersectional targeting strategy to label GABAergic neurons in the mPFC that project to NAcc and found that these neurons have fast-spiking properties and express parvalbumin. These results define possible functions and properties for a class of long-range projecting GABAergic neurons in the neocortex.