Canada

Subiculum pyramidal neurons form the main output of the hippocampus, and are classically viewed as giving rise to memory by branched structures that receive proximal and distal input. Here, we identified a sparse pyramidal cell subpopulation within the subiculum that diverges from classical pyramidal neuron characteristics across connectivity, morphology, and intrinsic activity. Via transcriptomics, we identified a sparse excitatory neuron population that occupies the deepest layer of the subiculum, and generated a new transgenic mouse line that allows cre-mediated cell-type-specific access of these deep cells. Combining this transgenic line with cell-type-specific viral tracing tools, we discovered that this deep cell population forms dedicated projections to the anterior thalamic nuclei, a key brain region in spatial working memory. Moreover, these deep cells lack radial oblique dendrites typical of classical subiculum pyramidal cells. Motivated by this unique local and long-range structure of these cells, we investigated potential differences in intrinsic function via whole cell patch-clamp recordings. Recording results revealed that these deep cells exhibited an increased input resistance compared to classical pyramidal cells in the subiculum. Altogether, our data provides evidence for a previously unknown subiculum pyramidal cell subtype that exhibits unique structural and intrinsic characteristics, which could indicate a potentially specialized role in spatial memory. Future work will be integral to identifying the functional correlates of this atypical population.

CAN-ACN 2021

Atypical subiculum pyramidal neurons form dedicated projections and exhibit morphological and electrophysiological differences

cell types

transgenic mice

hippocampus

electrophysiology

memory

Colleagues and Friends:

I am very happy to welcome you to the 14th annual Canadian Neuroscience Meeting, and to the first virtual edition of our meeting. I want to highlight the important work the CAN meeting chair, **Roger Thompson** and co-chair, **Alyson Fournier**, did to put together the great program we are offering this year. Moving to a virtual format required a complete re-design of the program, and both Roger and Alyson did a splendid job in providing us with an appealing and well adapted program for our first virtual event. I wish to thank them for this very important effort.

Highlights of this year’s scientific meeting include invited speakers **Michael Hausser, Franck Polleux, Gwyneth Card, Magdalena Götz**and Brain Prize winner **Huda Zoghbi**. Plenary and parallel symposia, proposed by our members complete our diverse [scientific program.](https://can-acn.org/meeting-2021/program-2021/)

The 2021 Advocacy session, chaired and organized by CAN advocacy committee chair **Karun Singh**, will feature a special lecture and question period with Member of Parliament and past Science Minister **Kirsty Duncan**. This will be followed by a short overview of CAN’s advocacy activities over the last year. We invite you to participate in this session for insight from Dr. Duncan on the workings of Parliament Hill, and to learn more about what CAN has been doing to promote neuroscience research in Canada.

We are also excited to host **Lisa Willis** in the Equity Diversity and Inclusion session, organized by EDI committee chair **Jibran Khokhar**.

A special highlight of our meeting will be the lectures by award-winning young neuroscientists. We are please to host the CAN 2020 Young Investigator Award winner **Mihaela Iordanova**, and CAN 2021 Young Investigator Award winners **Aaron Phillips** and **Tabrez Siddiqui**. Highlights will also include presentation by the top three winners of the CAN-CIHR-INMHA Brain Star Award winners, **Claire Gizowski, Nuria Daviu Abant** and **Lukas Tamayo-Orrego**.

Independently organized satellite symposia complement and complete this exciting program, check them out here: https://can-acn.org/meeting-2021/2021-satellite-symposia/

Some of the most engaging sessions are always the poster sessions featuring the work of our trainees, so make sure to log in to Gather Town during the poster sessions to chat with the presenters.
We hope you enjoy this program, and look forward to welcoming you, virtually.

Shernaz Bamji

President of the Canadian Association for Neuroscience

![](https://assets.underline.io/uploads/markdown_image/1/image/b9aea25343a87d5aac3b9367dc780974.jpg)



Please head to https://can-acn.org/meeting-2021/registration-2021/ to register. 

14th Canadian Neuroscience Meeting from August 23rd-25th

poster

Nearly 50 different retinal ganglion cell (RGC) types sample the visual scene for distinct features such as motion direction or line orientation. RGC feature selectivity arises from its synapses with a specific subset of amacrine (AC) and bipolar cell (BC) types, but how their dendrites arborize and collect input from these specific subsets remains poorly understood. We developed a method that combines calcium imaging and post hoc histology that revealed a family of circuits that express the adhesion molecule Sidekick 1 (Sdk1) which include a novel RGC type (S1-RGC) that responds to local edges. Genetic and physiological studies revealed that Sdk1 loss selectively disrupts S1-RGC visual responses which result from a loss of excitatory and inhibitory inputs on this neuron. These phenotypes were specific to S1-RGCs and were accompanied by selective deficits in their dendritic growth. We conclude that Sdk1 shapes dendrite growth and wiring to help S1-RGCs become feature selective.

The cell adhesion molecule Sdk1 shapes assembly of a retinal circuit that detects localized edges

Rett Syndrome (RTT) is a neurodevelopmental disorder caused by a mutation in the gene MECP2. MeCP2 protein is found to be most abundant in neurons where it binds to DNA to act as an A-D global transcriptional regulator. Previous work has shown neurons with MECP2 mutations exhibited morphological and functional hypoconnectivity at a single neuron level. We hypothesized this hypoconnectivity between MECP2¬-mutant neurons will result in altered network development and function. Excitatory neurons derived from iPSCs from a RTT patient, two control patients, and isogenic pairs were generated. Neurons were plated on a multielectrode array and electrophysiology was measured over 6 weeks. Spatiotemporal analyses were used to quantify the electrophysiological features and computational network modeling was used to explore mechanisms driving the differences seen in vitro. iPSC-derived cultures exhibited bursting patterns that went from sparse firing to asynchronous bursting to synchronous bursting across development - quantified through burst frequency. As neuronal cultures developed, mutants grew increasingly different in their burst frequency compared to isogenic controls. Spiking network modelling revealed adaptation currents had the greatest influence on bursting. As adaptation currents increased, the burst frequency shifted to lower values. Simulated knockout experiments supported these results. This suggest that RTT networks may be mediated by adaptation currents, and various regulators for channels involved in adaptation may be implicated as downstream targets of MeCP2.

Electrophysiological characterization of Rett Syndrome in iPSC-derived neuronal networks using computational network modeling

Fragile-X syndrome (FXS), the leading genetic cause of intellectual disability, occurs when the Fmr1 gene on the X-chromosome is silenced, reducing the levels of Fragile-X mental retardation protein (FMRP). This loss of FMRP hinders neurodevelopment and leads to several neurological pathologies, one of which is learning and memory deficits that are attributed to dysregulated hippocampal neurogenesis. The purinergic signalling pathway, where cells use ATP and its metabolites as signalling molecules, is essential for neurogenesis but has yet to be considered in the FXS hippocampus. However, our lab has discovered abnormal purinergic signalling in the FXS cortex, suggesting FMRP regulates this pathway. We hypothesize that purinergic signalling abnormalities exist in the FXS hippocampus and contribute to the dysregulation of neurogenesis. To begin our investigation, we used the Fmr1-KO mouse model to characterize the expression of purinergic receptors known to be involved in neurogenesis. After performing Western Blots on hippocampal tissue at postnatal day 1 (P1), P7, P14, and P21, we found that the P2X7 receptor was upregulated at P7. In mice, P7 is when hippocampal neurogenesis peaks; therefore, overexpression of P2X7 at this critical time point may have increased consequences for the adult mouse. Because P2X7 can play several roles in neurogenesis, it is unclear what those long-term consequences may be. Our next step is to determine which cells overexpress the P2X7 receptor to determine which aspect(s) of neurogenesis this P2X7 overexpression may impact.

Purinergic signalling is altered in the Fmr1-KO mouse hippocampus

Based on the 2016 Survey on Drug use and Health in the USA, 4.9% of pregnant women 15-44 years old reported recent cannabis use. Studies on the long-term outcomes of gestational cannabis exposure (GCE) are limited. Although cannabis ingestion triggers short-term hyperphagia, the incidence of obesity is lower in frequent cannabis users. It is unknown whether GCE will influence the metabolic dysfunction associated with high-fat diet (HFD) consumption in adult offspring. Pregnant female mice received edible cannabis extract, equivalent to THC (5mg/kg/day) or vehicle from GD1.5 till PD10. At least 9 litters culled to 6 mice/litter from each group were examined. Daily body weight and food intake of the dams were recorded during pregnancy and lactation. Body weight and locomotor activity of the pups were monitored at several age points. Adult offspring (7 weeks old) received HFD or control diet (CD) for 12 weeks. Body weight, adiposity, glucose tolerance, insulin sensitivity, and gut microbiota composition are examined. While GCE did not alter the dams' weight gain, food intake, pregnancy duration, or litter size, it reduced the pups' body weight at PD6 and PD11, but not at later ages and decreased their locomotor activity at PD10. GCE female but not male offspring that received CD were more insulin resistant but not heavier or more glucose intolerant than their controls. HFD induced more insulin resistance independent of weight gain in GCE male but not female offspring. Perinatal cannabis exposure negatively impacts insulin sensitivity of adult offspring in a sex-specific manner.

Sex differences in the metabolic outcomes of high-fat diet in the offspring after gestational cannabis exposure

During the dynamic process of brain development, the neurite formation and elongation are essential to establish a complex neural network in the central nervous system. Somatostatin (SST), a growth hormone inhibitory peptide, is known to play a critical role in neurotransmission, neurite formation, migration and maturation. In the present study, we examined the time-dependent (at day 1, 3, 5 and 7) developmental changes in the expression of crucial marker that govern the functional activity of neurons, including SST, choline acetyltransferase, and tyrosine hydroxylase, brain nitric oxide synthase, GABA and synaptophysin in non-differentiated and all-trans retinoic acid (RA)-induced differentiated SH-SY5Y cells. We also determined SST-mediated modulation of subcellular distribution and expression of neurotransmitter and Synaptic markers using immunocytochemistry and western blot analysis. Our results suggest a distinct response of RA and SST alone or in combination in regulating different neurotransmitter markers during the transition of SH-SY5Y cells from non-neuronal entity to neuronal phenotype. Here we present markers specific and time-dependent changes in differentiated cells when compared to non-differentiated SH-SY5Y cells. These data suggest that SST in SH-SY5Y cells might be associated with the stabilization and migration of neurotransmitter markers to neurites. Taken together, results described here support the role of SST in neurogenesis with a possible implication in neurological disease exhibiting interrupted neuronal communication and loss of cognitive function.

Role of somatostatin in regulating developmental changes of neurotransmitters in retinoic acid-induced differentiated SH-SY5Y cells

Throughout life, the retina undergoes chemical and structural changes that ultimately result in altered functional properties. Electrophysiological testing is an objective method to functionally assess the state of the neuroretina. By using appropriate visual stimuli in the light-adapted eye, it is possible to isolate the contributions of specific cellular components to the elicited electrical response. In this study, we performed a light-adapted (30 cd m² background) flicker electroretinogram (ERG) in 36 adult vervet monkeys aged 9 to 28 years old, with no obvious signs of ocular pathology. A standard intensity light flash (2.57 cd s m²) was applied at eight different frequencies, ranging from 5 to 40 Hz. Changes in the stimulus-response characteristics were found in older individuals (21-28 years-old, N=16), when compared with recordings from younger monkeys (9-20 years-old, N=20). Specifically, a significant reduction in the averaged amplitude together with a prolonged implicit time of the b-wave was observed in older monkeys across all stimulus frequencies tested (p<0.05). Altogether, our findings corroborate the existent human literature, where ageing detrimentally affects photopic retinal responses. These results also highlight the potential use of non-human primates as a reliable model to study the early stages of visual processing over the lifespan. Overall, these age-dependent voltage and temporal changes might reflect ongoing pathological processes targeting the cone-bipolar subsystem.

Age-related changes in retinal function in the vervet monkey

Parvalbumin (PV)-GABAergic interneurons constitute the majority of interneurons in the cortex and play a key role in the function and synchronization of cortical networks. Alterations in PV-interneuron connectivity, in the medial prefrontal cortex (mPFC), have been related to psychiatric disorders. We have previously shown that the expression of the p75 neurotrophin receptor (p75NTR) regulates the time course of PV cell synapse maturation in a cell-autonomous fashion. Here, we study how p75NTR removal in postnatal PV cells affects the connectivity of PV interneurons and mPFC function in adult PV_Cre;p75NTRlox/lox mice. One important indication of PV cell maturation is the appearance of specialized extracellular matrix structures called perineural nets (PNNs) around the soma and primary dendrites of mature cortical PV cells. Conditional postnatal deletion of p75NTR in PV cells resulted in an increased perineural net (PNN) density and intensity around these cells, along with higher excitatory afferent and inhibitory efferent connectivity in PFC. These mice also showed deficits in attentional set-shifting task, a measure of attention and cognitive flexibility in mPFC, and in extinction of fear memories, both of which rely on mPFC function. Finally, we investigate whether the overexpression of P75NTR in mPFC of adult cKO can rescue PV cell structural deficit. Using a Cre-dependent adeno-associated virus, we observed a significant decrease in PV cell inhibitory efferent connectivity. Analysis on the PNN density and intensity around PV cell somata is currently ongoing. These findings uncover P75NTR as a critical regulator of PV circuit refinement and plasticity in adult PFC by simultaneously controlling PV cellular connectivity underlying cognitive function.

Role of p75 neurotrophin receptor in cortical parvalbumin-positive GABAergic interneurons and cognitive flexibility 

Topographic organization is commonly found in the sensory systems of vertebrates. Functional topographic maps are believed to follow a developmental progression from initially coarse to more precise through an activity-dependent refinement process. To examine the emergence of topographic organization in the retinotectal system, we performed longitudinal visual receptive field mapping by calcium imaging in the optic tectum of GCaMP6-expressing transgenic Xenopus laevis tadpoles. At stage 42, just one day after retinal axons arrive in the optic tectum, and the earliest time point when visually evoked responses could be detected, a crude retinotopic functional map was evident. Animals were imaged over the following week at stage 45 and stage 48, during which more precise retinotopic organization was observed. Next, by microinjecting GCaMP6s mRNA into one blastomere of two-cell stage embryos, we acquired bilateral mosaic tadpoles with GCaMP6s expression in postsynaptic tectal neurons on one side of the animal and in RGC axons crossing over to the tectum on the opposite side. This allowed independent observation of retinotopic maps in the pre- and post-synaptic elements in the tectum. To examine the contribution of N-methyl-D-aspartate (NMDA) receptors to map refinement, we reared tadpoles in the NMDA receptor antagonist MK-801. MK-801-rearing did not prevent the emergence of retinotopic maps, but produced changes in overall map organization compared to untreated animals, including more discontinuous topographic gradients and altered receptive field characteristics. These results provide evidence that NMDA receptor activity is dispensable for coarse topographic ordering of retinotectal inputs, but play a part in the fine-scale refinement of the retinotectal projection.

Topographic map formation and the effects of NMDA receptor blockade in the developing Xenopus retinotectal system

The formation and remodeling of synaptic contacts require the precise distribution and trafficking of proteins to specialized compartments. This dynamic trafficking of synaptic proteins is partly controlled by S-palmitoylation, which is the most common form of post-translational lipid modification in the brain. Notably, several studies have shown that synaptic proteins can be differentially palmitoylated in response to synaptic activity. However, it is unclear how changes in synaptic activity alters protein palmitoylation. Here we show that increasing synaptic activity in primary rat hippocampal cultures using a well-established chemical LTP (cLTP) paradigm leads to post-translational modifications in ZDHHC enzymes which in turn impact enzyme stability, protein interaction and function. Notably, we observe the same post-translational modifications following the acquisition of contextual fear memory in vivo. In contrast to that observed for palmitoylating enzymes, we observed no activity-induced changes in the activity of thioesterases nor post-translational modifications of the thioesterase, ABHD17. These findings suggest that the differential palmitoylation of synaptic proteins following synaptic stimulation is mediated through post-translational modifications of ZDHHC enzymes that in turn regulate enzyme stability and function, and not through changes in depalmitoylating thioesterases.

Dynamic regulation of palmitoyltransferases by synaptic activity

Mammalian spinal cord injury (SCI) occurs by a primary mechanical trauma that is exacerbated by a secondary cellular insult in the form of neuroinflammation, cell death, and reactive gliosis. The lack of functional recovery may be attributed to the inability of mammals to replace lost neurons. In zebrafish (Danio rerio), the secondary insult is attenuated and allows for the development of a spinal microenvironment that fosters regeneration. Radial glia lining the zebrafish central canal undergo injury-induced proliferation and neuronal differentiation, yet the molecular mechanisms that underlie these processes remain elusive. Previous reports have identified roles for purinergic P2Y2 and P2X7 receptors in mammalian neural progenitor cell proliferation and neuronal differentiation, respectively. Given that the purinergic system is evolutionarily conserved among vertebrates, we hypothesize that radial glia proliferation and neurogenesis following zebrafish SCI may be mediated by P2X7 and P2Y2 signaling. Preliminary immunohistochemical data suggests that radial glia express P2X7 and P2Y2 receptors. Preliminary Western blotting data shows significant upregulation of P2Y2 receptor expression at 1 day following zebrafish SCI, as well as significant downregulation of P2X7 receptor expression at 7 days. This contrasts the more gradual increase in P2Y2 receptor expression as well as the general increase in P2X7 receptor expression following mammalian SCI. Specific blockers for P2X7 and P2Y2 will be used to ascertain the roles of these receptors in the zebrafish SCI response. This work is well suited to advance our current understanding of successful central nervous system regeneration and elucidate the signaling pathways involved.

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The cell adhesion molecule Sdk1 shapes assembly of a retinal circuit that detects localized edges

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