CURRENT & ONGOING PROJECTS
*denotes undergraduate co-authors
🦠 Steifman C*, Lagasse O*, Chapon C*, Martinez N*, Tong MT. Microglia-Mediated Loss of Hippocampal Perineuronal Nets in Mild Respiratory COVID.
Post-acute sequelae of SARS-CoV-2 infection, or “long COVID,” is characterized by ongoing respiratory and cognitive symptoms. “Brain fog” is common amongst long COVID sufferers who report many cognitive deficits, including memory impairments, that impact their daily lives. Previous research using COVID mouse models have found cellular and molecular differences in critical memory regions, like the hippocampus. In this study, we investigate the effects of mild COVID on perineuronal nets (PNNs) in the hippocampus. PNNs are condensed extracellular matrix structures that have been shown to control plasticity and thus may play a role in COVID-related memory symptoms. We used a mouse (CD-1 males) model of mild SARS-CoV-2 infection limited to the respiratory system where infection clears within 1 week. Seven weeks post-infection, brain tissue from infected and control mice were harvested for extensive histological analysis to assess microglia activation (CD68+/Iba1+) and PNNs (WFA+) across hippocampal subregions dentate gyrus (DG), CA1, and CA2/3. Our preliminary findings showed that mice with mild COVID have a higher density of activated microglia that were closely associated with and engulfing PNNs in the DG and CA1, but not CA2/3. This interestingly corresponds to higher PNN density in CA2/3 of COVID mice, but no differences in DG and CA1. These findings suggest that microglia-mediated regulation of PNNs in the hippocampus occurs in respiratory SARS-CoV-2 infection and may provide an explanation for the memory impairments experienced by long COVID sufferers.
🦵🏽 Pineda J*, Allodi I, Tong MT. Novel Changes to Perineuronal Nets in Amyotrophic Lateral Sclerosis Mouse Model.
Amyotrophic lateral sclerosis (ALS) is a devastating neurodegenerative disease characterized by the progressive loss of motor function through the degeneration of motor neurons in the spinal cord. Previous research has indicated a pivotal role of glycinergic V1 inhibitory interneurons in modulating both motor neuron activity and locomotion (Allodi 2021). The canonical loss of motor neurons in ALS is due to the loss of synaptic connections from the early loss of these Gly+ interneurons (Allodi 2021; Mora 2022). Perineuronal nets (PNNs), primarily composed of chondroitin sulfate proteoglycans, are known for their involvement in synapse stabilization and have been proposed to offer neuroprotective effects elsewhere in the nervous system. In this study, we look at PNN expression throughout pre-symptomatic (P45, P60, P84) and symptomatic (P112) stages of ALS disease in male and female SOD-1-GlyT2GFP mice, a recognized ALS model. We used immunohistochemistry to investigate PNN number and distribution within the cervical spinal cord, specifically their co-localization with glycinergic interneurons and motor neurons. Analyses using two-way ANOVAs (genotype: wild-type or SOD-1 by age: P45, P60, P84, P112) revealed a significant interaction between age and genotype for the number of motor neurons, number of PNNs, and number of Gly+ interneurons. Post-hoc tests confirmed both motor neuron and Gly+ interneuron loss with age in the SOD-1 mouse. Interestingly, we found that while wild-type mice showed an increase in the proportion of Gly+ interneurons surrounded by PNNs with age, the SOD-1 mice showed a decrease. We also observed PNNs in the dorsal horn of the spinal cord, a known sensory input region, around gly- cells. The number of PNNs in this region also decreased with age in the SOD-1, but not wild-type mice. Taken together, our novel findings suggest a significant role of PNNs in ALS-associated neuron loss and offer a critical new avenue for exploration.
💊 Ruffino S, Papasergia D, Perez E, Ortiz-Fishman E*, Willis L*, Xu J*, ML Shawn Bates, & Tong MT. Sex differences in adolescent diazepam exposure on reward-related memory and perineuronal nets.
Benzodiazepines (BZD) are used to treat anxiety disorders. However, misuse of BZDs has increased in adolescents, with females reporting more misuse than males. Additionally, users engage in polydrug use with opioids. Despite the prevalence of BZD use in adolescence, few preclinical studies have examined its ramifications for this age group. Even fewer have looked at the role of extracellular matrix in sex differences in drug-related memory consolidation. Adolescent male and female mice were group-housed and placed in one of three drug conditions, diazepam (DZP) low dose (1mg/kg), DZP high dose (2 mg/kg), and saline. Anxiety levels were measured lusing the elevated plus maze (EPM) after drug or vehicle exposure. A conditioned place preference (CPP) paradigm was used to examine the rewarding effects of DZP. Also, we measured somatic withdrawal symptoms after exposure to naloxone. We used immunohistochemistry (IHC) to investigate perineuronal net (PNN) number and distribution in the hippocampus (HPC) and nucleus accumbens (NAc). We examine the co-localization of PNNs with medium spiny neurons in the NAc as well as neurons expressing estrogen receptor alpha (ERa) which are critical for LTP at the HPC-NAc synapse in females, but not males. Our results suggest that DZP at both doses (1 & 2 mg/kg) produces a significant place preference in males, but only the higher dose (2 mg/kg) produces a preference in females. This represents a rightward shift in females. Chronic DZP exposure only reduced anxiety-like behaviors in males. Furthermore, withdrawal from DZP increased anxiety-like behaviors in both sexes. However, DZP exposure reduces naloxone-precipitated withdrawal symptoms in both males and females. These results suggest that adolescents may be vulnerable to DZP abuse, with a heightened vulnerability in males. DZP induced reward-related place memory and attenuated opioid-withdrawal symptoms in adolescent male and female mice. However, female mice required a higher dose of DZP to form reward-related place memories, and chronic DZP only reduced anxiety-like behaviors in males. The findings suggest that the HPC-NAc-VTA circuit in females is less plastic overall, which strongly implicates the role of PNNs as a main mechanism for sex differences observed. These findings should be of interest to clinicians who prescribe benzodiazepines, as well as the scientific community interested in the role of the extracellular matrix in reward-related plasticity.
🪀 Werth J, Lohmeier C*, Baman S, Guruprasad R*, Cleland TA, Tong MT. Removal of Olfactory Bulb Extracellular Matrix Reduces Spike Timing Regularity and Attenuates Gamma Power.
In the main olfactory bulb (MOB), synchronized spiking in the gamma band arises from dynamical interactions between inhibitory granule cells and mitral/tufted cells (MTCs). Improvements in odor discrimination correspond to increases in MOB gamma power, suggesting that synchrony plays a critical role in olfactory behavior. While gamma oscillations are known to be influenced by intrinsic circuit dynamics, the role of extracellular mechanisms is far less understood. In this study, we describe multiple forms of Wisteria floribunda agglutinin (WFA)-positive extracellular matrix (ECM) in the MOB, anterior olfactory nucleus, and the piriform cortex. In the MOB, we observed filament-like, condensed extracellular structures along the superficial boundary of the internal plexiform layer, corresponding to the axon initial segments of MTCs, suggesting an effect of these ECM structures on MTC function. Using a planar microelectrode array to record from MOB slices, we found that enzymatic removal of the ECM led to reduced spike timing regularity towards Poisson process expectations. We also saw a reduction of gamma power, with no overall effect on spontaneous firing rates. These findings suggest that the ECM in the MOB plays a role in regulating mitral cell spike synchronization, generalizing a role for ECM structures in neuron physiology.
🤯 Yang L*, Hughes S, Tong MT. Decolonizing Neuroscience by Reading Sociology in Undergraduate Neuroscience Courses. [Talk Slides]
While ideas of “decolonizing” curriculum and pedagogy in higher education have been gaining traction in recent years, efforts to implement meaningful curricular changes have been primarily in the humanities and social science. A recent global, comparative review of 207 articles and book chapters on decolonizing curriculum found a notable absence of contributions from the STEM (Science, Technology, Engineering, and Mathematics) disciplines. As enrollment in STEM disciplines and national investment in the STEM workforce continue to rise, the STEM classroom is an important site of meaningful decolonizing interventions. Thus, the development of decolonizing materials and its implementation in STEM courses deserve critical examination. In this study, we seek to evaluate the effectiveness of an experimental curriculum intentionally designed for neuroscience students to engage with decolonial ideas of their field. Using interviews, a focus group, and pre-/post-course surveys, we examine (a) the effect of the curriculum on students’ sense of self-efficacy in their field and their sense of social belonging in the science classroom; (b) the efficacy of the implementation across 4 previously described implementation categories (probing the positionality of knowledge, constructing an inclusive curriculum, relational teaching and learning, and bridging higher education institutions with community and/or sociopolitical movements.)
PUBLISHED PROJECTS
🍫 Mudra Rakshasa, A*, & Tong, MT. (2020). Making Good Choices: Social Interaction in Mice Mitigates Chronic Stress-Induced Adaptive Changes in Decision Making. Frontiers in Behavioral Neuroscience, 14:81. [Article Link] [bioRxiv][Data Repository][Arish’s amazing Twitter paper summary thread]
Chronic stress can impact decision-making and lead to a preference for immediate rewards rather than long-term payoffs. Factors that may influence these effects of chronic stress on decision-making are under-explored. Here we used a mouse model to investigate the changes in decision-making caused by the experience of chronic stress and the role of social isolation in exaggerating these changes. To test decision-making, mice were trained to perform a Cost-Benefit Conflict (CBC) task on a T-maze, in which they could choose between a high-reward, high-risk alternative and a low-reward, low-risk alternative. Mice were either housed in groups or alone throughout the experiment. Both groups of mice underwent a seven-day period of repeated immobilization to induce chronic stress. Stress levels were confirmed using behavioral (open field test) and physiological (urine corticosterone ELISA) measures. We found a significant increase in frequency of high-risk decisions after exposure to chronic stress among both socially- and individually-housed mice. Crucially, socially-housed mice showed a significantly smaller increase in high-risk decision-making compared to singly-housed mice. These findings suggest that chronic stress leads to an increase in high-risk decision-making in mice, and that lack of social interaction may exacerbate this stress effect.
🐭 Awol, AK*, & Tong MT. (2020). A low-cost, odor-reward association task for tests of learning and memory. Journal of Visualized Experiments. (159), e59756. [Article Link]
Robust and simple behavioral paradigms of appetitive, associative memory are crucial for researchers interested in cellular and molecular mechanisms of memory. In this paper, an effective and low-cost mouse behavioral protocol is described for examining the effects of physiological manipulation (such as the infusion of pharmacological agents) on the learning rate and duration of odor-reward memory. Representative results are provided from a study examining the differential role of tyrosine kinase receptor activity in short-term (STM) and long-term memory (LTM). Male mice were conditioned to associate a reward (sugar pellet) with one of the two odors, and their memory for the association was tested 2 or 48 h later. Immediately prior to the training, a tyrosine kinase (Trk) receptor inhibitor or vehicle infusions were delivered into the olfactory bulb (OB). Although there was no effect of the infusion on the learning rate, blockade of the Trk receptors in the OB selectively impaired LTM (48 h), and not short-term memory (STM; 2 h). The LTM impairment was attributed to the diminished odor selectivity as measured by the length of the digging time. The culmination of the results of this experiment showed that Trk receptor activation in the OB is the key in olfactory memory consolidation.
🐟 Schroeder MK*, Tong MT, & Rosenberg RL. (2019). The effects of Curcumin on glial morphology following spinal cord injury in lampreys. IMPULSE. [Article Link ]
Reactive and consequently scar-forming glial cells, particularly astrocytes, are implicated in the inability of mammalian spinal cords to regenerate following spinal cord injury (SCI). Thus, it is relevant to study pharmacological methods of manipulating these cells, which could result in efficacious treatments for SCI in humans. The current study used larval sea lampreys (Petromyzon marinus), because lampreys can functionally recover from a spinal cord transection, making them a relevant model organism for studying how the process of physiological recovery may be sped up. Lampreys were given either a single injection of 27 μM curcumin, an established anti-inflammatory compound, or a vehicle not containing curcumin immediately following spinal cord transection and allowed to recover for 24 hours or one week. To assess the effects of curcumin on glial cells, spinal cord slices were labeled with an anti-cytokeratin antibody, LCM29, that labels lamprey glial cytoskeleton. There were no significant differences in immunoreactivity of the LCM29 antibody between vehicle- and curcumin-injected groups as measured by mean pixel value, or as measured by the standard deviations of these means. Despite the overall lack of significant main effects of curcumin according to these quantitative measures, there were distinct visual differences between spinal cord slices from curcumin- and vehicle-injected animals. While curcumin did not affect the overall levels of glial filaments, it may affect the uniformity of location of these filaments by attenuating the development of darker patches (decreased expression of glial filaments) and brighter patches (increased expression of glial filaments) seen in slices from vehicle-injected lampreys. Future studies should continue investigating curcumin for its ability or inability to attenuate reactive glial cells and explore new methods for quantifying glial reactivity.
⚡Tong, MT, Kim, PT-Y.*, & Cleland, TA. (2018). Kinase activity in the olfactory bulb is required for odor memory consolidation. Learning and Memory, 25, 198-205. [Article Link]
Long-term fear memory formation in the hippocampus and neocortex depends upon brain-derived neurotrophic factor (BDNF) signaling after acquisition. Incremental, appetitive odor discrimination learning is thought to depend substantially on the differentiation of adult-born neurons within the olfactory bulb (OB)—a process that is closely associated with BDNF signaling. We sought to elucidate the role of neurotrophin signaling within the OB on odor memory consolidation. Male mice were trained on odor–reward associative discriminations after bilateral infusion of the kinase inhibitor K252a, or vehicle control, into the OB. K252a is a partially selective inhibitor of tyrosine kinase (Trk) receptors, including the TrkB receptor for BDNF, though it also inhibits other plasticity-related kinases such as PKC and CaMKII/IV. K252a infusion into the OB did not impair odor acquisition or short-term (2 h) memory for the learned discriminations, but significantly impaired long-term (48 h) odor memory (LTM). This LTM deficit also was associated with reduced selectivity for the conditioned odorant in a reward-seeking digging task. Infusions of K252a immediately prior to testing did not impair LTM recall. These results indicate that kinase activation in the OB is required for the consolidation of odor memory of incrementally acquired information.
📜 Tong, MT, Peace, ST, and Cleland, TA (2014). Molecular and structural mechanisms of olfactory bulb memory. Frontiers in Behavioral Neuroscience, 8, 238. [Article Link]
Memories are dynamic physical phenomena with psychometric forms as well as characteristic timescales. Most of our understanding of the cellular mechanisms underlying the neurophysiology of memory, however, derives from one-trial learning paradigms that, while powerful, do not fully embody the gradual, representational, and statistical aspects of cumulative learning. The early olfactory system—particularly olfactory bulb—comprises a reasonably well-understood and experimentally accessible neuronal network with intrinsic plasticity that underlies both one-trial (adult aversive, neonatal) and cumulative (adult appetitive) odor learning. These olfactory circuits employ many of the same molecular and structural mechanisms of memory as, for example, hippocampal circuits following inhibitory avoidance conditioning, but the temporal sequences of post-conditioning molecular events are likely to differ owing to the need to incorporate new information from ongoing learning events into the evolving memory trace. Moreover, the shapes of acquired odor representations, and their gradual transformation over the course of cumulative learning, also can be directly measured, adding an additional representational dimension to the traditional metrics of memory strength and persistence. In this review, we describe some established molecular and structural mechanisms of memory with a focus on the timecourses of post-conditioning molecular processes. We describe the properties of odor learning intrinsic to the olfactory bulb and review the utility of the olfactory system of adult rodents as a memory system in which to study the cellular mechanisms of cumulative learning.