Selective plasticity of layer 2/3 inputs onto distal forelimb controlling layer 5 corticospinal neurons with skilled grasp motor training.

Layer 5 neurons of the neocortex receive their principal inputs from layer 2/3 neurons. We seek to identify the nature and extent of the plasticity of these projections with motor learning. Using optogenetic and viral intersectional tools to selectively stimulate distinct neuronal subsets in rat primary motor cortex, we simultaneously record from pairs of corticospinal neurons associated with distinct features of motor output control: distal forelimb vs. proximal forelimb. Activation of Channelrhodopsin2-expressing layer 2/3 afferents onto layer 5 in untrained animals produces greater monosynaptic excitation of neurons controlling the proximal forelimb. Following skilled grasp training, layer 2/3 inputs onto corticospinal neurons controlling the distal forelimb associated with skilled grasping become significantly stronger. Moreover, peak excitatory response amplitude nearly doubles while latency shortens, and excitatory-to-inhibitory latencies become significantly prolonged. These findings demonstrate distinct, highly segregated, and cell-specific plasticity of layer 2/3 projections during skilled grasp motor learning.

Neural dynamics underlying associative learning in the dorsal and ventral hippocampus.

Animals associate cues with outcomes and update these associations as new information is presented. This requires the hippocampus, yet how hippocampal neurons track changes in cue-outcome associations remains unclear. Using two-photon calcium imaging, we tracked the same dCA1 and vCA1 neurons across days to determine how responses evolve across phases of odor-outcome learning. Initially, odors elicited robust responses in dCA1, whereas, in vCA1, odor responses primarily emerged after learning and embedded information about the paired outcome. Population activity in both regions rapidly reorganized with learning and then stabilized, storing learned odor representations for days, even after extinction or pairing with a different outcome. Additionally, we found stable, robust signals across CA1 when mice anticipated outcomes under behavioral control but not when mice anticipated an inescapable aversive outcome. These results show how the hippocampus encodes, stores and updates learned associations and illuminates the unique contributions of dorsal and ventral hippocampus.

Rhesus macaque versus rat divergence in the corticospinal projectome.

We used viral intersectional tools to map the entire projectome of corticospinal neurons associated with fine distal forelimb control in Fischer 344 rats and rhesus macaques. In rats, we found an extraordinarily diverse set of collateral projections from corticospinal neurons to 23 different brain and spinal regions. Remarkably, the vast weighting of this "motor" projection was to sensory systems in both the brain and spinal cord, confirmed by optogenetic and transsynaptic viral intersectional tools. In contrast, rhesus macaques exhibited far heavier and narrower weighting of corticospinal outputs toward spinal and brainstem motor systems. Thus, corticospinal systems in macaques primarily constitute a final output system for fine motor control, whereas this projection in rats exerts a multi-modal integrative role that accesses far broader CNS regions. Unique structural-functional correlations can be achieved by mapping and quantifying a single neuronal system's total axonal output and its relative weighting across CNS targets.

The Dentate Gyrus Classifies Cortical Representations of Learned Stimuli.

Animals must discern important stimuli and place them onto their cognitive map of their environment. The neocortex conveys general representations of sensory events to the hippocampus, and the hippocampus is thought to classify and sharpen the distinctions between these events. We recorded populations of dentate gyrus granule cells (DG GCs) and lateral entorhinal cortex (LEC) neurons across days to understand how sensory representations are modified by experience. We found representations of odors in DG GCs that required synaptic input from the LEC. Odor classification accuracy in DG GCs correlated with future behavioral discrimination. In associative learning, DG GCs, more so than LEC neurons, changed their responses to odor stimuli, increasing the distance in neural representations between stimuli, responding more to the conditioned and less to the unconditioned odorant. Thus, with learning, DG GCs amplify the decodability of cortical representations of important stimuli, which may facilitate information storage to guide behavior.

Reorganization of Recurrent Layer 5 Corticospinal Networks Following Adult Motor Training.

Recurrent synaptic connections between neighboring neurons are a key feature of mammalian cortex, accounting for the vast majority of cortical inputs. Although computational models indicate that reorganization of recurrent connectivity is a primary driver of experience-dependent cortical tuning, the true biological features of recurrent network plasticity are not well identified. Indeed, whether rewiring of connections between cortical neurons occurs during behavioral training, as is widely predicted, remains unknown. Here, we probe M1 recurrent circuits following motor training in adult male rats and find robust synaptic reorganization among functionally related layer 5 neurons, resulting in a 2.5-fold increase in recurrent connection probability. This reorganization is specific to the neuronal subpopulation most relevant for executing the trained motor skill, and behavioral performance was impaired following targeted molecular inhibition of this subpopulation. In contrast, recurrent connectivity is unaffected among neighboring layer 5 neurons largely unrelated to the trained behavior. Training-related corticospinal cells also express increased excitability following training. These findings establish the presence of selective modifications in recurrent cortical networks in adulthood following training.SIGNIFICANCE STATEMENT Recurrent synaptic connections between neighboring neurons are characteristic of cortical architecture, and modifications to these circuits are thought to underlie in part learning in the adult brain. We now show that there are robust changes in recurrent connections in the rat motor cortex upon training on a novel motor task. Motor training results in a 2.5-fold increase in recurrent connectivity, but only within the neuronal subpopulation most relevant for executing the new motor behavior; recurrent connectivity is unaffected among adjoining neurons that do not execute the trained behavior. These findings demonstrate selective reorganization of recurrent synaptic connections in the adult neocortex following novel motor experience, and illuminate fundamental properties of cortical function and plasticity.

Anxiety Cells in a Hippocampal-Hypothalamic Circuit.

The hippocampus is traditionally thought to transmit contextual information to limbic structures where it acquires valence. Using freely moving calcium imaging and optogenetics, we show that while the dorsal CA1 subregion of the hippocampus is enriched in place cells, ventral CA1 (vCA1) is enriched in anxiety cells that are activated by anxiogenic environments and required for avoidance behavior. Imaging cells defined by their projection target revealed that anxiety cells were enriched in the vCA1 population projecting to the lateral hypothalamic area (LHA) but not to the basal amygdala (BA). Consistent with this selectivity, optogenetic activation of vCA1 terminals in LHA but not BA increased anxiety and avoidance, while activation of terminals in BA but not LHA impaired contextual fear memory. Thus, the hippocampus encodes not only neutral but also valence-related contextual information, and the vCA1-LHA pathway is a direct route by which the hippocampus can rapidly influence innate anxiety behavior.

A shared neural ensemble links distinct contextual memories encoded close in time.

Recent studies suggest that a shared neural ensemble may link distinct memories encoded close in time. According to the memory allocation hypothesis, learning triggers a temporary increase in neuronal excitability that biases the representation of a subsequent memory to the neuronal ensemble encoding the first memory, such that recall of one memory increases the likelihood of recalling the other memory. Here we show in mice that the overlap between the hippocampal CA1 ensembles activated by two distinct contexts acquired within a day is higher than when they are separated by a week. Several findings indicate that this overlap of neuronal ensembles links two contextual memories. First, fear paired with one context is transferred to a neutral context when the two contexts are acquired within a day but not across a week. Second, the first memory strengthens the second memory within a day but not across a week. Older mice, known to have lower CA1 excitability, do not show the overlap between ensembles, the transfer of fear between contexts, or the strengthening of the second memory. Finally, in aged mice, increasing cellular excitability and activating a common ensemble of CA1 neurons during two distinct context exposures rescued the deficit in linking memories. Taken together, these findings demonstrate that contextual memories encoded close in time are linked by directing storage into overlapping ensembles. Alteration of these processes by ageing could affect the temporal structure of memories, thus impairing efficient recall of related information.

Spinal cord reconstitution with homologous neural grafts enables robust corticospinal regeneration.

The corticospinal tract (CST) is the most important motor system in humans, yet robust regeneration of this projection after spinal cord injury (SCI) has not been accomplished. In murine models of SCI, we report robust corticospinal axon regeneration, functional synapse formation and improved skilled forelimb function after grafting multipotent neural progenitor cells into sites of SCI. Corticospinal regeneration requires grafts to be driven toward caudalized (spinal cord), rather than rostralized, fates. Fully mature caudalized neural grafts also support corticospinal regeneration. Moreover, corticospinal axons can emerge from neural grafts and regenerate beyond the lesion, a process that is potentially related to the attenuation of the glial scar. Rat corticospinal axons also regenerate into human donor grafts of caudal spinal cord identity. Collectively, these findings indicate that spinal cord 'replacement' with homologous neural stem cells enables robust regeneration of the corticospinal projection within and beyond spinal cord lesion sites, achieving a major unmet goal of SCI research and offering new possibilities for clinical translation.

Thalamocortical Projections onto Behaviorally Relevant Neurons Exhibit Plasticity during Adult Motor Learning.

Layer 5 neurons of the neocortex receive direct and relatively strong input from the thalamus. However, the intralaminar distribution of these inputs and their capacity for plasticity in adult animals are largely unknown. In slices of the primary motor cortex (M1), we simultaneously recorded from pairs of corticospinal neurons associated with control of distinct motor outputs: distal forelimb versus proximal forelimb. Activation of ChR2-expressing thalamocortical afferents in M1 before motor learning produced equivalent responses in monosynaptic excitation of neurons controlling the distal and proximal forelimb, suggesting balanced thalamic input at baseline. Following skilled grasp training, however, thalamocortical input shifted to bias activation of corticospinal neurons associated with control of the distal forelimb. This increase was associated with a cell-specific increase in mEPSC amplitude but not presynaptic release probability. These findings demonstrate distinct and highly segregated plasticity of thalamocortical projections during adult learning.

Motor cortex maturation is associated with reductions in recurrent connectivity among functional subpopulations and increases in intrinsic excitability.

Behavior is derived from the configuration of synaptic connectivity among functionally diverse neurons. Fine motor behavior is absent at birth in most mammals but gradually emerges during subsequent postnatal corticospinal system maturation; the nature of circuit development and reorganization during this period has been largely unexplored. We investigated connectivity and synaptic signaling among functionally distinct corticospinal populations in Fischer 344 rats from postnatal day 18 through 75 using retrograde tracer injections into specific spinal cord segments associated with distinct aspects of forelimb function. Primary motor cortex slices were prepared enabling simultaneous patch-clamp recordings of up to four labeled corticospinal neurons and testing of 3489 potential synaptic connections. We find that, in immature animals, local connectivity is biased toward corticospinal neurons projecting to the same spinal cord segment; this within-population connectivity significantly decreases through maturation until connection frequency is similar between neurons projecting to the same (within-population) or different (across-population) spinal segments. Concomitantly, postnatal maturation is associated with a significant reduction in synaptic efficacy over time and an increase in intrinsic neuronal excitability, altering how excitation is effectively transmitted across recurrent corticospinal networks. Collectively, the postnatal emergence of fine motor control is associated with a relative broadening of connectivity between functionally diverse cortical motor neurons and changes in synaptic properties that could enable the emergence of smaller independent networks, enabling fine motor movement. These changes in synaptic patterning and physiological function provide a basis for the increased capabilities of the mature versus developing brain.

Nerve growth factor is primarily produced by GABAergic neurons of the adult rat cortex.

Within the cortex, nerve growth factor (NGF) mediates the innervation of cholinergic neurons during development, maintains cholinergic corticopetal projections during adulthood and modulates cholinergic function through phenotypic control of the cholinergic gene locus. Recent studies suggest NGF may also play an important role in cortical plasticity in adulthood. Previously, NGF-producing cells have been shown to colocalize with GABAergic cell markers within the hippocampus, striatum, and basal forebrain. Classification of cells producing NGF in the cortex is lacking, however, and cholinergic corticopetal projections have been shown to innervate both pyramidal and GABAergic neurons in the cortex. In order to clarify potential trophic interactions between cortical neurons and cholinergic projections, we used double-fluorescent immunohistochemistry to classify NGF-expressing cells in several cortical regions, including the prefrontal cortex, primary motor cortex, parietal cortex and temporal cortex. Our results show that NGF colocalizes extensively with GABAergic cell markers in all cortical regions examined, with >91% of NGF-labeled cells coexpressing GAD65/67. Conversely, NGF-labeled cells exhibit very little co-localization with the excitatory cell marker CaMKIIα (<5% of cells expressing NGF). NGF expression was present in 56% of GAD-labeled cells, suggesting that production is confined to a specific subset of GABAergic neurons. These findings demonstrate that GABAergic cells are the primary source of NGF production in the cortex, and likely support the maintenance and function of basal forebrain cholinergic projections in adulthood.

Genomic resources for songbird research and their use in characterizing gene expression during brain development.

Vocal learning and neuronal replacement have been studied extensively in songbirds, but until recently, few molecular and genomic tools for songbird research existed. Here we describe new molecular/genomic resources developed in our laboratory. We made cDNA libraries from zebra finch (Taeniopygia guttata) brains at different developmental stages. A total of 11,000 cDNA clones from these libraries, representing 5,866 unique gene transcripts, were randomly picked and sequenced from the 3' ends. A web-based database was established for clone tracking, sequence analysis, and functional annotations. Our cDNA libraries were not normalized. Sequencing ESTs without normalization produced many developmental stage-specific sequences, yielding insights into patterns of gene expression at different stages of brain development. In particular, the cDNA library made from brains at posthatching day 30-50, corresponding to the period of rapid song system development and song learning, has the most diverse and richest set of genes expressed. We also identified five microRNAs whose sequences are highly conserved between zebra finch and other species. We printed cDNA microarrays and profiled gene expression in the high vocal center of both adult male zebra finches and canaries (Serinus canaria). Genes differentially expressed in the high vocal center were identified from the microarray hybridization results. Selected genes were validated by in situ hybridization. Networks among the regulated genes were also identified. These resources provide songbird biologists with tools for genome annotation, comparative genomics, and microarray gene expression analysis.

Choline supplementation following third-trimester-equivalent alcohol exposure attenuates behavioral alterations in rats.

Despite the known adverse consequences of prenatal alcohol exposure, some pregnant women continue to drink alcohol, making it imperative to identify treatments for children with fetal alcohol spectrum disorders. The authors recently reported that perinatal choline supplementation can reduce some fetal alcohol effects (J. D. Thomas, M. Garrison, & T. M. O'Neill, 2004), and the present study examined whether choline supplementation is effective when administered after third-trimester-equivalent ethanol treatment. Rat pups were exposed to 6.0 g/kg/day ethanol during the neonatal brain growth spurt (Postnatal Days [PD] 4-9) and treated with choline chloride (0, 10, 50, or 100 mg/kg) from PD 10-30. Behavioral testing occurred after choline treatment had ceased. Female subjects exposed to ethanol were overactive and exhibited spatial learning deficits, effects that were attenuated with all doses of choline supplementation. These data indicate that choline supplementation can alter brain development following a developmental insult. Moreover, the data suggest that early dietary interventions may reduce the severity of some fetal alcohol effects, even when administered after birth.