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Microglia are the resident immune cells in the central nervous system (CNS). Originally thought to be primarily responsible for disposing of cellular debris and responding to neural insults, emerging research now shows that microglia are highly dynamic cells involved in a variety of neural processes, including development. The hypothalamus is a brain region critical for maintaining homeostatic processes such as energy balance, thirst, food intake, reproduction, and circadian rhythms. Given that microglia colonize the embryonic brain alongside key steps of hypothalamic development and that the loss of microglia causes obesity in newborn pups, consistent with a defect in neuroendocrine signaling, we explored how microglia influence these nearby hypothalamic progenitors. We found that embryonic microglia directly and indirectly interact with hypothalamic radial glial cells by both touching and clipping of their projections and also by releasing signaling cues, respectively. These interactions were increased following maternal stress, suggesting that fetal microglia might be sensors to the external environment. Combined, these data demonstrate that microglia can influence overall development of this important brain region, which might contribute to diseases later in life.

CAN-ACN 2021

Embryonic hypothalamic microglia: a sensor for maternal stressors and potential mediator of neurodevelopmental diseases

maternal-fetal environment

fetal programming

hypothalamus

neurodevelopment

microglia

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

technical paper

Melanin-concentrating hormone (MCH) activates MCH receptor 1 (MCHR1) to promote positive energy balance. MCH or MCHR1 deletion, in part at GABAergic cells, leads to a hyperdopaminergic state and hyperactivity, as MCH can inhibit dopamine release. Here we assessed if MCH can directly regulate dopaminergic or GABAergic neurons in the ventral tegmental area (VTA). We labelled mouse Mchr1 mRNA by RNAscope in situ hybridization throughout the VTA and quantified its expression on putative dopaminergic or GABAergic cells by colocalization with tyrosine hydroxylase (TH) immunoreactivity or Slc32a1 (Vgat) mRNA hybridization, respectively. Half of all Mchr1 cells expressed TH, which comprised 25% of TH VTA cells, while only 13% of Mchr1 cells expressed Vgat, which represented 13% of Vgat VTA cells. We then mapped the expression of Mchr1 in TH and Vgat VTA neurons onto Allen Reference Atlas brain templates to assess their spatial distribution. Mchr1 expression was most prevalent in the posterior VTA (Bregma -3.28 mm to -3.68 mm) and concentrated within the medial aspect of the VTA. Vgat expression and its colocalization with Mchr1 increased in more posterior levels but TH and Mchr1 colocalization was relatively higher posteriorly. Interestingly, 36% of Mchr1 cells were neither TH nor Vgat positive, and future experiments will determine if they may be glutamatergic VTA cells. Importantly, we provide evidence for Mchr1 expression in the VTA and provide the basis for electrophysiological recordings to assess MCH-mediated action at TH and Vgat VTA cells.

Spatial distribution of melanin-concentrating hormone receptor 1 in the ventral tegmental area

Object category recognition and discrimination may rely on the use of generalized category representations formed through exposure to various category exemplars. Presumably, to refine category representations, extraneous object features must be inhibited. Parvalbumin-containing GABAergic interneurons (PVINs) have been implicated in visual stimulus learning, including the inhibition of irrelevant stimuli. To determine the role of GABAergic transmission in the refinement of object category representations, C57/BL6 mice were administered a GABAA receptor antagonist (bicuculline) prior to exposure to various category exemplars. When tested on a rodent object category recognition (OCR) task, mice administered bicuculline failed to perform a 1-h retention delay which requires pre-exposure to category objects. Then, to specifically examine the role of GABAergic PVINs in the perirhinal cortex (PRh), a region necessary for object recognition, we infused male PVcre mice intracranially with adeno-associated virus containing inhibitory designer receptors exclusively activated by designer drugs (DREADDs) to control the activity of PVINs in the PRh. The DREADD agonist 21 (11-(1-piperazinyl)-5H-dibenzo[b,e][1,4]diazepine; C21) was administered systemically prior to exposure to object category exemplars. Mice administered C21 prior to pre-exposure sessions were not able to perform the OCR task with the 1-h retention delay. Thus, GABAergic transmission, and specifically PVIN activity in the PRh, may play an essential role in the refinement of object category representations.

The role of GABAergic inhibition in object category recognition

Perineuronal nets are highly organized components of the extracellular matrix which inhibit cellular plasticity and support cellular function. Depletion of perineuronal nets in rodents via developmental models of disease or via direct pharmacological interventions (e.g. chondroitinase) can impact cognitive function. In our current study, we utilized injections of an immune-evasive dual vector system, where the chondroitinase gene is under a doxycycline regulatory control, to induce bilateral secretion of chondroitinase, with temporal control, in the medial prefrontal cortex and retrosplenial cortex. These two brain regions are known to be involved in the function of working memory. We subjected mice to 30 days of activated chondroitinase-expression to deplete their perineuronal nets, followed by 30 days without to allow for a window of recovery. Prior to treatment, after treatment, and after 30 days without doxycycline-induced chondroitinase expression, animals were tested in a cross-modal object recognition task and an oddity task, which have been shown to be sensitive to manipulations of the medial prefrontal and retrosplenial cortex. Preliminary data indicates that controlled depletion of perineuronal nets in the medial prefrontal and retrosplenial cortex impacts performance of working memory. While there is some recovery in their working memory performance after 30 days without continuing enzymatic depletion, they do not recover to baseline behavioural performance. This data suggests that perineuronal net degradation in the medial prefrontal and retrosplenial cortex impacts working memory performance, and that these changes are long-lasting.

Cognitive impairments in mice after targeted perineuronal net depletion in the medial prefrontal or retrosplenial cortex.

Vicarious stress occurs when witnessing others undergo trauma and can be modeled in rodents. Prior stressor experience in witnesses may be required to enhance vicarious stress. Most studies utilize adult animals however less is known about vicarious stress in juveniles. We explored and compared the behavioural effects of vicarious stress and prior stressor experience in juvenile and adult female rats. Four witness groups were included: Control (no experience, no vicarious stress), Context (experience, no vicarious stress), Naive (no experience, vicarious stress), and Experienced (experience, vicarious stress). Experienced and Context groups were subjected to ten 1.0 mA footshocks over 12 minutes. On the next day, Naive and Experienced groups watched their cagemate endure footshocks from a perforated transparent barrier. Control and Context groups watched the cagemate roam freely. Fear expression was measured 24hrs after and social interactions and acoustic startle response were recorded. There were no age differences in Experienced groups which had greater fear expression during the vicarious stressor. Adult Experienced groups had greater fear expression after 24 hours compared to juveniles. There were no effects of vicarious stress and experience on the social interactions and acoustic startle response, yet juvenile rats spent more time interacting and had a larger startle response. In sum, prior stressor exposure was required for vicarious stress to increase fear expression acutely in adult and juvenile female rats yet led to unchanged behavioural responses.

Exploring the effects of prior stressor experience and vicarious stress in juvenile and adult female rats

Management of stress and anxiety is often listed as the primary motivation behind cannabis use, yet the understanding of how acute exposure to cannabis modulates the neurobehavioral and endocrine response to stress is not well characterized. As inhalation is the primary form of consumption in humans, modelling this approach in rodents is important given the robust impact of route of administration has on the pharmacokinetics of THC and its metabolites. With advancements in drug delivery for rodent studies, basic science approaches can now be employed to better establish the impact of cannabis use on stress reactivity in a highly translational manner. The current study aims to examine the impact of acute, controlled passive delivery of THC on male and female rats to determine how this impacts neural and endocrine responses to acute stress. Thirty-two male and 32 female adult rats were exposed to vehicle vapour for 9 days. On day ten, rats were randomly assigned to one of two vapour conditions: 1) control; and 2) THC and one of two stress conditions: 1) naïve; and 2) stress (30 min restraint stress). The initial exposure to vehicle vapour increases cort levels in male and female rats; however, only males habituated to exposure by day 9. Acute exposure to THC immediately elevated cort levels in males but not females. The impact of THC vapour on neuronal activity using c-fos is currently underway. These data indicate that acute exposure to THC vapour is sufficient to produce elevations in the stress hormone cort in males but does not impact stress-induced changes in cort.

Investigating the effect of THC vapour exposure on stress reactivity in rats following an acute stressor

To successfully navigate uncertain environments, we need an algorithm for exploration: a way to ensure that we sample policies whose value is uncertain. We also need a mechanism to reliably follow rewarding policies we discover: a system for exploitation. Understanding how exploration and exploitation are implemented in the brain is a major topic of research in decision-making neuroscience, with several studies pointing to pupil-linked mechanisms. However, these studies relied on ad hoc applications of reinforcement learning models that were not designed to identify exploration. As a result, it is still not clear whether pupil-linked mechanisms predict exploratory behavior per se, much less its neural signatures. Here we applied a novel approach that models exploration and exploitation as latent goal states generating decisions. This allowed us to contrast model-labelled explore and exploit decisions in terms of both (1) pupil size and (2) neural activity recorded in the frontal eye fields (FEF), a part of the prefrontal cortex implicated in sensorimotor decision-making. We found that pupil size was larger during exploration and ramped up in anticipation of exploration. Then, it abruptly decreased below baseline levels at the start of exploitation. Concurrently, pupil size predicted neural signatures of exploration in FEF neurons and neuronal populations. In sum, we found that pupil-linked processes anticipate exploration via disruption of neuronal tuning. Further research will be necessary to identify how exactly pupil-linked mechanism influence exploration.

Pupil size anticipates exploration and predicts disorganization in prefrontal cortex

Objective: The purpose of our study was to determine whether indirect social stress could affect brain areas important for spatial learning and memory in adult rats. Methods: Each week for 10 consecutive weeks, 4 male and 4 female Sprague-Dawley rats were randomly assigned to same-sex pairs. Next, twice daily for 5 consecutive days, one male and one female rat were placed on an elevated platform for 30 min (Platform Stress, PS). The cage-mates of PS animals were considered members of the Bystander Stress (ByS) group (i.e., those receiving indirect stress). As well, Platform Control (PC) animals were simply moved to another room twice daily for 30 min, and the cage-mate of each PC animal was considered to be a Bystander Control (ByC). Spatial learning and memory were then assessed over 5 days using standard training and probe trials in the Morris water maze. The animals were then sacrificed and brain tissue harvested to allow for the assessment of differences in synaptic proteins. Results: Regardless of sex, there were no statistically significant differences among the ByC and ByS animals in either spatial learning [training day four: F (3, 72) = 1.4; p = .24], or memory [F (3, 72) = 1; p = .17]. Conclusion: Our results suggest that ByS may not dramatically affect spatial learning and memory in either male, or female rats. As our next step, we will analyse the synaptic expression and phosphorylation level of several plasticity-related proteins (e.g., GluN1, GluA2, PSD-95) in the hippocampus across the treatment groups.

Would short-term, indirect social stress affect spatial learning and memory in either male, or female rats?

Theta rhythms (4-12 Hz) are crucial for hippocampal network coordination and functions, including memory and anxiety. Lesion and pharmacological studies have shown that the median raphe nucleus (MR) exerts a strong modulation over theta rhythms. However, the MR is composed of mixed neuronal populations, including serotonergic neurons (5-HT) and glutamatergic neurons expressing the vesicular glutamate transporter type 3 (VGLUT3). In this study, we aim to evaluate the contribution of these two largely distinct median raphe neuronal populations to theta rhythm properties. We used optogenetics to activate either 5HT or VGLUT3 neurons in the median raphe (MR) of freely behaving mice. To do so, we injected CRE-dependent AAV vectors in the MR of either SERT-CRE or VGLUT3-CRE mice, to selectively transfect 5-HT or VGLUT3 MR neurons with the excitatory opsin ChETAET/TC. An optic fiber implanted above the MR allowed light delivery and activation of 5-HT or VGLUT3 neurons, while simultaneously recording theta rhythms in the hippocampus during REM sleep and awake locomotion. Our results show that 5HT and VGLUT3 neurons participate in the modulation of hippocampal theta frequency and power. These results suggest a potential role for MR 5-HT and VGLUT3 neurons in the modulation of hippocampal functions such as memory and anxiety.

3Hippocampal theta rhythm modulation by the glutamatergic and serotonergic raphe

Variations in gene expression patterns and cortico-striatal circuit morphology have been linked to sex differences in depression. We investigated gene-by-environment (G*E) interactions between dopamine gene expression in these brain regions and the social environment on depressive symptomatology (CESD-10 scale) in 45-75 y.o. CLSA participants. We computed two biologically-informed genetic scores related to the D4 dopamine receptor (DRD4): 1. predicted prefrontal (PFC) expression using PrediXcan machine learning; 2. expression-based polygenic risk scores for the co-expression gene network in the striatum (STR-ePRS). Using latent profile analysis, we classified individuals based on social network size, support, cohesion, and participation in 3 distinct profiles: low-, medium-, and high-social network support (18/40/42%). In the full sample, G*E interactions were significant for STR-ePRS--with effects driven by men--but not for PFC expression. Analyses by sex revealed significant G*E effects for both DRD4 scores in men (n=7217) but not in women (n=6733). Lower STR-ePRS scores were associated with worse depressive symptomatology for men in the low-support profile. Higher PFC DRD4 expression was associated with worse depressive symptoms for men in the low-support profile but milder symptoms in the medium- and high-support groups. The DRD4 gene network overlapped with gene sets related to cardiometabolic health (blood pressure, BMI, diabetes), suggesting common pathways for physical and mental well-being. Our findings implicate DRD4 and related gene networks as potential moderators of social environmental influences in depression, particularly for male seniors with low social support, consistent with previously reported sex differences in reward sensitivity and decision-making.

Tracing molecular and social pathways to sex differences in depression: An empirical analysis of the Canadian Longitudinal Study on Aging (CLSA)

BACKGROUND AND AIM: Microglia are the principal immune cells of the brain. The consensus today is that once activated microglia/macrophages can acquire a wide repertoire of profiles ranging from the classical pro-inflammatory to alternative and more protective phenotypes. Over the past decade, we and others have shown that optimal and timely activation of microglial cells and innate immunity is instrumental in the control of the inflammation-induced damage to CNS. To add to complexity, increasing evidence suggests a marked sexual dimorphism in the processes associated with microglial activation following different types of brain injuries. At present, the in vivo molecular mechanisms involved in the control of microglia and/or macrophage immune profiles remain elusive METHODS: To decipher the molecular mechanisms underlying microglial activation in vivo, we created a transgenic model in which Flag/EGFP was fused to the N-terminus of the large subunit ribosomal protein L10a and expressed under the transcriptional control of a myeloid specific gene promoter Thus, a simple high affinity immunoprecipitation assay from brain homogenates allows a pull down of the transcriptome and/or proteome from the targeted cell type. By isolating both ribosome-attached mRNAs and peptides, we obtained a snapshot of the dynamic translational state of microglial ribosomes with mRNAs as input and newly synthesized peptides as output. Using this strategy, we identified mRNA and protein signatures associated with microglial activation. RESULTS: By using in vivo model-system for analysis of dynamic translational state of microglia ribosomes with mRNAs as input and newly synthesized peptides as an output, we discovered a marked dissociation of microglia mRNA and protein networks following innate immune challenge. We found that the highly up-regulated and polysome- associated mRNAs are not translated resulting in two distinct and rather divergent microglia molecular signatures: i) a highly specialized immune and pro-inflammatory mRNA signature and ii) a more immunomodulatory and homeostatic protein signature. As molecular mechanism, we discovered a selective 3'UTR-mediated translational suppression of highly expressed immune mRNAs. Furthermore, we identified a novel and previously unknown role for RNA binding protein Serine/Arginine-Rich Splicing Factor 3 SRSF3 as a master suppressor/regulator of innate immune genes translation in activated microglia. CONCLUSIONS: Our recent work suggests involvement of SRSF3 -mediated mechanisms in the modulation of immune profiles of activated microglia in different type of brain injuries and/or pathologies. Better understanding of SRFS3- mediated immunomodulation may open new avenues for therapeutic modulation of innate immune response following brain injuries.

Embryonic hypothalamic microglia: a sensor for maternal stressors and potential mediator of neurodevelopmental diseases

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Embryonic hypothalamic microglia: a sensor for maternal stressors and potential mediator of neurodevelopmental diseases

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Next from CAN-ACN 2021

Spatial distribution of melanin-concentrating hormone receptor 1 in the ventral tegmental area

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Microglial cells after brain injuries: sexually dimorphic and lost in translation

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RESOURCES

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