Canada

Clinical data proposes that the common street drug MDMA (ecstasy) increases water intake by inducing the sensation of thirst and promoting fluid retention by the kidney. These effects account for the high incidence of hyponatremia (low blood sodium and excel water in serum) in patients visiting hospital emergency departments as a result of experiencing headaches and dizziness (and other signs of hyponatremia) following ingestion of MDMA. Clinical as well as rodent studies also reveal an increase in serum vasopressin levels when MDMA is ingested or administered, which is likely a key factor in fluid retention. To investigate the neurophysiological mechanisms underlying these effects brain slices obtained from transgenic rat brains were prepared at a specific angle to preserve circuitry important for the regulation of thirst and vasopressin release. Whole cell current and voltage clamp recordings from identified vasopressin neurons revealed that a bath application of MDMA causes an excitatory response; mediated by membrane depolarization. Moreover, a proportion of tonically active vasopressin neurons were observed to transition to a phasic firing pattern known to facilitate peptide release. These results suggest that MDMA can act directly on neurons controlling vasopressin thereby increasing the likelihood of fluid retention.

CAN-ACN 2021

Mechanisms of MDMA induced vasopressin release in rats

mdma

electrophysiology

vasopressin

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

Dorsolateral prefrontal cortex (DLPFC) is associated with executive control and response selection, but the extent to which it is involved in effector-specific transformations is unclear. We addressed this question by recording single neurons from dorsolateral prefrontal cortex (DLPFC) while two trained monkeys performed a head-unrestrained reaching paradigm that allowed freely coordinated motion of gaze, head reaching in depth. Animals touched one of three central LEDs at waist level while maintaining gaze on a central fixation dot and were rewarded if they touched a target appearing at one of 15 locations in a 40° x 20° (visual angle) array. Preliminary analysis of 271 neurons in both monkeys showed an assortment of target/stimulus, gaze, pre-reach and reach-timed responses in DLPFC. We first tested for gaze, head, and hand gain fields during the different neuronal responses and found that 38% of the responses were gain modulated by initial hand position. A small fraction of neurons showed gain fields for initial eye position (4%), and for both initial eye and hand position (6%). After removing the gain field effects, we fitted the residual data against various spatial models and found that the visual response best encoded the target relative to space (Ts), whereas responses at gaze and hand onset showed a tendency towards coding hand displacement (dA). In addition, some (20%) neurons showed a preference for coding final head position or displacement. A more complete analysis will describe the complete coding and distribution of gaze, head, and reach signals in this region.

Effector-specific spatial codes in dorsolateral prefrontal cortex during a head-unrestrained reach task

The dI3 population of spinal cord interneurons (INs) is a group of excitatory INs that receive varied sensory afferent input and project to ipsilateral motoneurons. Prior experiments have discovered that the permanent silencing of dI3 INs has a minimal impact on locomotor function in intact animals, but substantially interferes with locomotor rehabilitation/recovery after spinal cord injury (SCI). To gain insight into how dI3 INs are involved with locomotor function and recovery, the inhibitory DREADD receptor (hM4Di) was expressed in dI3 INs using a hybrid line of transgenic mice (Isl1-Cre:Vglut2-Flp x FlexloxFlexFRT-hM4Di). Consistent with prior experiments, transient inhibition of hM4Di-expressing dI3 neurons with the DREADD agonist JHU37160 (0.5mg/kg) did not significantly impact locomotor function in intact animals. However, in T9-T10 SCI mice, transient silencing of dI3 INs resulted in a significant loss of flexor motor tone, demonstrated by a significant increase in the resting ankle joint angle (+43.9° ± 11.6°, n=7, t-test, p=0.0065). This coincided with decreased stepping and the qualitative appearance of hindlimb flaccidity during the treadmill locomotor task. Based on these results, dI3 INs appear to adopt a more significant role in hindlimb function after SCI, specifically the maintenance of basal motor tone in the absence of supraspinal input. Given that SCI patients often experience both hypotonia and spasticity (increased motor tone), these findings provide the impetus for targeting dI3 INs to modulate motor tone below the level of the lesion.

dI3 interneurons maintain hindlimb motor tone in mice following spinal cord injury

Multiple studies have shown that mouse higher visual areas are involved in discrimination task through intricate functional properties and complex networks. However, it remains enigmatic how this highly connected network define specialized perceptual roles in extrastriate areas and how these interact in mouse behavior. Here we sought to study the connectivity and perceptual roles of these extrastriate areas using patterned optogenetic inactivation combined with mesoscopic calcium imaging. (A) Our first goal will be to develop a novel system of photostimulation using a the digital micromirror device (DMD), which uses mirrors to precisely shape a light beam into a large surface. Such innovative technology opens new doors in optogenetics as brain areas often have complex shape and are interconnected with remote areas. Validation on Channelrhodopsin-expressing mice showed robust local and long-range activations using the red calcium indicator jrGECO1a. Our second goal (B) is to implement this novel method of stimulation to our inactivation paradigm and assess how the inactivation of a whole extrastriate area impact the functional properties of adjacent areas and on feedback connections to the primary visual cortex using calcium imaging. Our final goal (C) is to study how perceptual learning is affected during spatially integrated optogenetic inactivation. Overall, studying visual perception require rigorous knowledge on the function and the organization of visual areas and this research will have the potential to bring new light unto this interaction.

Exploring the interaction between function and organization in mouse visual cortex using a novel spatially integrated optogenetic system

Allocentric (landmark-centered) and egocentric (eye-centered) visual information are optimally integrated for goal-directed movements. This process has been observed within the supplementary and frontal eye fields, but the underlying processes for this combination remain a puzzle, mainly due to inadequacy of current theoretical models to explain data at different levels (i.e., behavior, single neuron, and distributed network). Here, we propose a physiologically inspired neural network with two major components: First, a Convolutional Neural Network (CNN) is used to extract the allocentric information (target and landmark): We used repeated (2 layers) convolution, rectifications, and normalization followed by a feature pooling layer to extract allocentric information (target and landmark). Second, a Multi-Layer Perceptron network (MLP, 3 fully connected layers) is used to incrementally transform allocentric information into an integrated motor response. We added an additional layer to transform motor responses into final gaze positions. The network was trained on both idealized and actual monkey gaze behavior. MLP output units accurately simulated prefrontal motor responses (including open-ended response fields that shifted partially with the landmark) and their decoded output achieved good correspondence (MLP: R2 = 0.80) with actual gaze behavior (Bharmauria et al. Cerebral Cortex 2020). These results suggest that our framework works and provides a suitable tool to study the underlying mechanisms of allocentric and egocentric integration. Supported by a VISTA Program fellowship.

Mechanisms for integrating allocentric and egocentric visual information for goal-directed movements: a neural network approach

Considerable regional evidence has accumulated for dorsal-ventral modularity in the visual system, but it is not clear how these modules function at the whole brain network level, or how these networks are influenced by naturally occurring saccades. We addressed these questions using graph theory analysis of fMRI data collected during a task where participants had to remember, then discriminate between two different object features. Seventeen participants judged whether a remembered object changed shape or orientation with or without an intervening saccade. BOLD activation from 50 cortical nodes was used to identify local and global network properties. A network modularity analysis revealed three sub-networks during fixation: a bilateral dorsal sub-network linking areas involved in visuospatial processing and two lateralized ventral sub-networks linking areas involved in object feature processing. Importantly, when horizontal saccades across the remembered object required visual comparisons between hemifields, the two lateralized ventral sub-networks became functionally integrated into a single bilateral sub-network. Comparisons of 'betweenness centrality' between conditions identified several significant hub regions in occipital, parietal, and frontal cortex involved in linking distant network nodes during saccades. These results provide support of a ventral and dorsal stream distinction in human perception and show how hemispheric sub-networks are modified to functionally integrate information across saccades.

Functional network topography reveals dorsal-ventral cortical modules for visual feature memory, and lateralized modules that combine for transsaccadic integration

The prevalence of many chronic pain syndromes differs across sex. Recent studies have identified differences in immune signalling within spinal nociceptive circuits as a potential mediator. Although it has been proposed that sex-specific pain mechanisms converge once they reach neurons within the superficial dorsal horn (SDH), direct investigations using rodent and human preclinical pain models have been lacking. Here, we discovered that in the Freund's Adjuvant in vivo model of inflammatory pain, where both male and female rats display tactile allodynia, a pathological coupling between loss of inhibition and NMDA receptor potentiation within SDH neurons was observed in male but not female rats. Unlike males, the neuroimmune mediator BDNF failed to downregulate inhibitory signalling elements and upregulate excitatory elements in female rats, resulting in no effect of ex vivo BDNF on synaptic NMDA receptor responses in female lamina I neurons. Importantly, this sex difference in spinal pain processing was conserved from rodents to humans. As in rodents, ex vivo spinal treatment with BDNF downregulated markers of disinhibition and upregulated markers of facilitated excitation in SDH neurons from male but not female human organ donors. Ovariectomy in female rats recapitulated the male pathological pain phenotype. The discovery of sexual dimorphism in a central neuronal mechanism of chronic pain across species provides a foundational step towards a better understanding and treatment of pain for both sexes.

Sexual dimorphism in a neuronal mechanism of spinal hyperexcitability across rodent and human models of pathological pain

Natural stimuli display complex spatiotemporal characteristics. In order to encode such stimuli efficiently, sensory systems must continuously adapt by changing their response properties. The computational role of such adaptation remains poorly understood because adaptation can increase coding ambiguity. We investigated how vestibular thalamocortical neurons (VTN) and their afferent input within the vestibular nuclei (VON) respond to simple artificial and complex natural selfmotion stimuli in rhesus macaques. We found that both groups displayed comparable response properties to artificial stimuli which led to ambiguity. While such ambiguity persisted for artificial stimuli for VON, VTN instead faithfully followed the timecourse of natural selfmotion stimuli. A model including gain adaptation successfully reproduced our experimental data. Our results challenge the common wisdom that adaptation leads to ambiguity by showing that such adaptation actually leads to unambiguous encoding of natural stimuli. Second, we investigated the role of these VTN in the perception of selfmotion. We tested whether their responses can account for violation of Weber's law, i.e. discrimination performance is enhanced at higher stimulus amplitudes. While neural gain decreased as a function of stimulus amplitude, neural variability saturated at high values. As a result, neural populations thresholds saturated and agreed with perception. Taken together, we provide novel insights as to how variability and gain control contribute to encoding of natural stimuli with continually varying statistics.

Gain adaptation and variability of vestibular corticothalamic neurons shape our perception of natural self motion stimuli

The transient sensations experienced by the vestibular system are critical for everyday activities such as gaze stabilization and the perception of self-motion. Accordingly, the investigation of vestibular neural coding would greatly benefit from the use of optogenetic methods; with which neuronal activity can be precisely and reliably modulated with high temporal precision. Due to our shared bipedality and close evolutionary origins, non-human primates (NHPs) are the ideal model animals. However, unlike application in invertebrates and rodents, optogenetic techniques using NHPs have not been well established, requiring researchers to infer the best approach. This project aims to provide robust results of NHP optogenetics in the vestibular system through in-vivo injection of an opsin adenovirus to key interconnected vestibular cortical and subcortical areas: the parietoinsular vestibular cortex (PIVC) and the vestibular nuclei in the brainstem of cynomolgus macaques. We can determine the efficacy and specificity of our vectors, chosen based on anatomical differences between the target regions, and evaluate opsin expression using histological methods such as immunohistochemistry and fluorescence microscopy. With the confirmation of histological expression, this project will open the door to confidently employing optogenetic methods to study vestibular electrophysiology and the behaviours it codes for, as well as to investigate feedback between our cortical and subcortical areas of interest.

Optogenetic expression in the cortical and subcortical areas of the non-human primate vestibular system

Previous studies have focused on the encoding of the first-order features of vestibular stimuli, but few studies, to date, have studied the encoding of the second-order attribute that consist of changes in vestibular stimuli amplitude or envelope. Because envelopes are critical for perception, we investigated if and how vestibular only neurons in the vestibular nuclei, known to mediate self-motion perception, encode the envelope attribute of the head motion stimulus. Specifically, we studied neural responses to stimuli consisting of a noisy waveform whose envelope varied sinusoidally at lower frequency. Importantly, stimulus amplitude was kept low such as not to elicit static nonlinearities from afferents (e.g., rectification, saturation). Our results show that the envelope attribute of the stimulus is encoded at single-neuron level in the vestibular nuclei. On average, we found that the gain of the neuron's response to the envelope remained constant across frequencies (p = 0.24). Moreover, the neuron's response shows a slight phase lead that increases as the frequency of the envelope increases (p=0.01). We found that static nonlinearities do not account for neural envelope response. Finally, we calculated the discrimination threshold and found higher values than that found in the Thalamus and for perception. Overall, our results will have important implication in our understanding of how early processing of vestibular signal can encode second-order attributes of head motion that will further be used to ensure stable perception of the world and accurate motor control.

Second-order attribute of head motion is encoded at single-neuron level in the vestibular nuclei

Self motion is composed of both rotational and translational motions which occur with characteristic statistics in everyday life. In the context of rotation, we have previously shown that neurons at the first central stage of vestibular processing (VN) are adapted to match their tuning curves to the statistics of natural rotation. This strategy effectively removes redundancy in the neural response, and produces responses which are independent of frequency (i.e. temporally whitened). However, unlike for rotation, translation sensitive neurons display significantly more heterogeneity in their tuning curves to artificial stimuli, and their responses to natural stimuli which cover the full physiological range of motion frequencies has not been explored. Here we investigated whether and, if so, how the responses of translation sensitive vestibular-only (otolith VO) cells in the VN of awake behaving macaques are adapted to naturalistic translation stimuli. Our results indicate that otolith VOs are not consistently matched to natural statistics in the same way as their rotation-sensitive counterparts, and instead fall roughly into 3 categories covering a range from strongly low-pass filtered to optimally whitened responses. This study is the first to examine the responses of otolith VO neurons in the context of natural self-motion, and suggests that low-pass filtered, faithful reconstruction, and optimally encoded representations of the stimulus may each be implemented in the translational vestibular system via parallel encoding strategies.

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Effector-specific spatial codes in dorsolateral prefrontal cortex during a head-unrestrained reach task

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