The amygdala modulates prepulse inhibition of the auditory startle reflex through excitatory inputs to the caudal pontine reticular nucleus

PDE4B Missense Variant Increases Susceptibility to Post-traumatic Stress Disorder-Relevant Phenotypes in Mice

Large-scale genome-wide association studies (GWASs) have associated intronic variants in PDE4B, encoding cAMP-specific phosphodiesterase-4B (PDE4B), with increased risk for post-traumatic stress disorder (PTSD), as well as schizophrenia and substance use disorders that are often comorbid with it. However, the pathophysiological mechanisms of genetic risk involving PDE4B are poorly understood. To examine the effects of PDE4B variation on phenotypes with translational relevance to psychiatric disorders, we focused on PDE4B missense variant M220T, which is present in the human genome as rare coding variant rs775201287. When expressed in HEK-293 cells, PDE4B1-M220T exhibited an attenuated response to a forskolin-elicited increase in the intracellular cAMP concentration. In behavioral tests, homozygous Pde4b M220T male mice with a C57BL/6JJcl background exhibited increased reactivity to novel environments, startle hyperreactivity, prepulse inhibition deficits, altered cued fear conditioning, and enhanced spatial memory, accompanied by an increase in cAMP signaling pathway-regulated expression of BDNF in the hippocampus. In response to a traumatic event (10 tone-shock pairings), neuronal activity was decreased in the cortex but enhanced in the amygdala and hippocampus of Pde4b M220T mice. At 24 h post-trauma, Pde4b M220T mice exhibited increased startle hyperreactivity and decreased plasma corticosterone levels, similar to phenotypes exhibited by PTSD patients. Trauma-exposed Pde4b M220T mice also exhibited a slower decay in freezing at 15 and 30 d post-trauma, demonstrating enhanced persistence of traumatic memories, similar to that exhibited by PTSD patients. These findings provide substantive mouse model evidence linking PDE4B variation to PTSD-relevant phenotypes and thus highlight how genetic variation of PDE4B may contribute to PTSD risk.

The role of the Ventral Nucleus of the Trapezoid Body in the auditory prepulse inhibition of the acoustic startle reflex

Cholinergic signaling is essential to mediate the auditory prepulse inhibition (PPI), an operational measure of sensorimotor gating, that refers to the reduction of the acoustic startle reflex (ASR) when a low-intensity, non-startling acoustic stimulus (the prepulse) is presented just before the onset of the acoustic startle stimulus. The cochlear root neurons (CRNs) are the first cells of the ASR circuit to receive cholinergic inputs from non-olivocochlear neurons of the ventral nucleus of the trapezoid body (VNTB) and subsequently decrease their neuronal activity in response to auditory prepulses. Yet, the contribution of the VNTB-CRNs pathway to the mediation of PPI has not been fully elucidated. In this study, we used the immunotoxin anti-choline acetyltransferase (ChAT)-saporin as well as electrolytic lesions of the medial olivocochlear bundle to selectively eliminate cholinergic VNTB neurons, and then assessed the ASR and PPI paradigms. Retrograde track-tracing experiments were conducted to precisely determine the site of lesioning VNTB neurons projecting to the CRNs. Additionally, the effects of VNTB lesions and the integrity of the auditory pathway were evaluated via auditory brain responses tests, ChAT- and FOS-immunohistochemistry. Consequently, we established three experimental groups: 1) intact control rats (non-lesioned), 2) rats with bilateral lesions of the olivocochlear bundle (OCB-lesioned), and 3) rats with bilateral immunolesions affecting both the olivocochlear bundle and the VNTB (OCB/VNTB-lesioned). All experimental groups underwent ASR and PPI tests at several interstimulus intervals before the lesion and 7, 14, and 21 days after it. Our results show that the ASR amplitude remained unaffected both before and after the lesion across all experimental groups, suggesting that the VNTB does not contribute to the ASR. The%PPI increased across the time points of evaluation in the control and OCB-lesioned groups but not in the OCB/VNTB-lesioned group. At the ISI of 50 ms, the OCB-lesioned group exhibited a significant increase in%PPI (p < 0.01), which did not occur in the OCB/VNTB-lesioned group. Therefore, the ablation of cholinergic non-olivocochlear neurons in the OCB/VNTB-lesioned group suggests that these neurons contribute to the mediation of auditory PPI at the 50 ms ISI through their cholinergic projections to CRNs. Our study strongly reinforces the notion that auditory PPI encompasses a complex mechanism of top-down cholinergic modulation, effectively attenuating the ASR across different interstimulus intervals within multiple pathways.

Peripubertal antagonism of corticotropin-releasing factor receptor 1 results in sustained, sex-specific changes in behavioral plasticity and the transcriptomic profile of the amygdala

Peripuberty is a significant period of neurodevelopment with long-lasting effects on the brain and behavior. Blocking type 1 corticotropin-releasing factor receptors (CRFR1) in neonatal and peripubertal rats attenuates detrimental effects of early-life stress on neural plasticity, behavior, and stress hormone action, long after exposure to the drug has ended. CRFR1 antagonism can also impact neural and behavioral development in the absence of stressful stimuli, suggesting sustained alterations under baseline conditions. To investigate this further, we administered a CRFR1 antagonist (CRFR1a), R121919, to young adolescent male and female rats across 4 days. Following each treatment, rats were tested for locomotion, social behavior, mechanical allodynia, or PPI of the acoustic startle reflex. Acute CRFR1 blockade immediately reduced PPI in peripubertal males, but not females. In adulthood, each assay was repeated without CRFR1a exposure to test for long-term effects of the adolescent treatment, with males continuing to experience deficits in PPI, while females displayed altered locomotion, PPI, and social behavior. The amygdala was collected to measure long-term effects on gene expression in pathways related to neural plasticity and neurodevelopmental disorders. Relative expression of cannabinoid type 1 receptors (CB1R), which mediate sensorimotor and HPA function, was also measured. In the adult amygdala, peripubertal CRFR1a induced alterations in pathways related to neural plasticity and stress in males and lower expression of CB1R protein in females. Understanding how acute exposure to neuropharmacological agents can have sustained impacts on brain and behavior, in the absence of further exposures, has important clinical implications for adolescent psychiatric treatment protocols.

Deciphering the role of brainstem glycinergic neurons during startle and prepulse inhibition

Prepulse inhibition (PPI) of the auditory startle response, a key measure of sensorimotor gating, diminishes with age and is impaired in various neurological conditions. While PPI deficits are often associated with cognitive impairments, their reversal is routinely used in experimental systems for antipsychotic drug screening. Yet, the cellular and circuit-level mechanisms of PPI remain unclear, even under non-pathological conditions. We recently showed that brainstem neurons located in the caudal pontine reticular nucleus (PnC) expressing the glycine transporter type 2 (GlyT2±) receive inputs from the central nucleus of the amygdala (CeA) and contribute to PPI but via an uncharted pathway. Here, using tract-tracing, immunohistochemistry and in vitro optogenetic manipulations coupled to field electrophysiological recordings, we reveal the neuroanatomical distribution of GlyT2± PnC neurons and PnC-projecting CeA glutamatergic neurons and we provide mechanistic insights on how these glutamatergic inputs suppress auditory neurotransmission in PnC sections. Additionally, in vivo experiments using GlyT2-Cre mice confirm that optogenetic activation of GlyT2± PnC neurons enhances PPI and is sufficient to induce PPI in young mice, emphasizing their role. However, in older mice, PPI decline is not further influenced by inhibiting GlyT2± neurons. This study highlights the importance of GlyT2± PnC neurons in PPI and underscores their diminished activity in age-related PPI decline.

Mapping of afferent and efferent connections of phenylethanolamine N-methyltransferase-expressing neurons in the nucleus tractus solitarii

OBJECTIVE

Phenylethanolamine N-methyltransferase (PNMT)-expressing neurons in the nucleus tractus solitarii (NTS) contribute to the regulation of autonomic functions. However, the neural circuits linking these neurons to other brain regions remain unclear. This study aims to investigate the connectivity mechanisms of the PNMT-expressing neurons in the NTS (NTSPNMT neurons).

METHODS

The methodologies employed in this study included a modified rabies virus-based retrograde neural tracing technique, conventional viral anterograde tracing, and immunohistochemical staining procedures.

RESULTS

A total of 43 upstream nuclei projecting to NTSPNMT neurons were identified, spanning several key brain regions including the medulla oblongata, pons, midbrain, cerebellum, diencephalon, and telencephalon. Notably, dense projections to the NTSPNMT neurons were observed from the central amygdaloid nucleus, paraventricular nucleus of the hypothalamus, area postrema, and the gigantocellular reticular nucleus. In contrast, the ventrolateral medulla, lateral parabrachial nucleus, and lateral hypothalamic area were identified as the primary destinations for axon terminals originating from NTSPNMT neurons. Additionally, reciprocal projections were evident among 21 nuclei, primarily situated within the medulla oblongata.

CONCLUSION

Our research findings demonstrate that NTSPNMT neurons form extensive connections with numerous nuclei, emphasizing their essential role in the homeostatic regulation of vital autonomic functions.

A Computational Model for the Simulation of Prepulse Inhibition and Its Modulation by Cortical and Subcortical Units

The sensorimotor gating is a nervous system function that modulates the acoustic startle response (ASR). Prepulse inhibition (PPI) phenomenon is an operational measure of sensorimotor gating, defined as the reduction of ASR when a high intensity sound (pulse) is preceded in milliseconds by a weaker stimulus (prepulse). Brainstem nuclei are associated with the mediation of ASR and PPI, whereas cortical and subcortical regions are associated with their modulation. However, it is still unclear how the modulatory units can influence PPI. In the present work, we developed a computational model of a neural circuit involved in the mediation (brainstem units) and modulation (cortical and subcortical units) of ASR and PPI. The activities of all units were modeled by the leaky-integrator formalism for neural population. The model reproduces basic features of PPI observed in experiments, such as the effects of changes in interstimulus interval, prepulse intensity, and habituation of ASR. The simulation of GABAergic and dopaminergic drugs impaired PPI by their effects over subcortical units activity. The results show that subcortical units constitute a central hub for PPI modulation. The presented computational model offers a valuable tool to investigate the neurobiology associated with disorder-related impairments in PPI.

Characterization of early markers of disease in the mouse model of mucopolysaccharidosis IIIB

BACKGROUND

Mucopolysaccharidosis (MPS) IIIB, also known as Sanfilippo Syndrome B, is a devastating childhood disease. Unfortunately, there are currently no available treatments for MPS IIIB patients. Yet, animal models of lysosomal storage diseases have been valuable tools in identifying promising avenues of treatment. Enzyme replacement therapy, gene therapy, and bone marrow transplant have all shown efficacy in the MPS IIIB model systems. A ubiquitous finding across rodent models of lysosomal storage diseases is that the best treatment outcomes resulted from intervention prior to symptom onset. Therefore, the aim of the current study was to identify early markers of disease in the MPS IIIB mouse model as well as examine clinically-relevant behavioral domains not yet explored in this model.

METHODS

Using the MPS IIIB mouse model, we explored early developmental trajectories of communication and gait, and later social behavior, fear-related startle and conditioning, and visual capabilities. In addition, we examined brain structure and function via magnetic resonance imaging and diffusion tensor imaging.

RESULTS

We observed reduced maternal isolation-induced ultrasonic vocalizations in MPS IIIB mice relative to controls, as well as disruption in a number of the spectrotemporal features. MPS IIIB also exhibited disrupted thermoregulation during the first two postnatal weeks without any differences in body weight. The developmental trajectories of gait were largely normal. In early adulthood, we observed intact visual acuity and sociability yet a more submissive phenotype, increased aggressive behavior, and decreased social sniffing relative to controls. MPS IIIB mice showed greater inhibition of startle in response to a pretone with a decrease in overall startle response and reduced cued fear memory. MPS IIIB also weighed significantly more than controls throughout adulthood and showed larger whole brain volumes and normalized regional volumes with intact tissue integrity as measured with magnetic resonance and diffusion tensor imaging, respectively.

CONCLUSIONS

Together, these results indicate disease markers are present as early as the first two weeks postnatal in this model. Further, this model recapitulates social, sensory and fear-related clinical features. Our study using a mouse model of MPS IIIB provides essential baseline information that will be useful in future evaluations of potential treatments.

Effects of 700 and 3500 MHz 5G radiofrequency exposure on developing zebrafish embryos

Telecommunications industries are rapidly deploying the fifth generation (5G) spectrum and there is public concern about the safety and health impacts of this type of Radio Frequency Radiation (RFR), in part because of the lack of comparable scientific evidence. In this study we have used a validated commercially available setting producing a uniform field to expose zebrafish embryos (ZFe) to unmodulated 700 and 3500 MHz frequencies. We have combined a battery of toxicity, developmental and behavioral assays to further explore potential RFR effects. Our neurobehavioral profiles include a tail coiling assay, a light/dark activity assay, two thigmotaxis anxiety assays (auditory and visual stimuli), and a startle response - habituation assay in response to auditory stimuli. ZFe were exposed for 1 and 4 h during the blastula period of development and endpoints evaluated up to 120 hours post fertilization (hpf). Our results show no effects on mortality, hatching or body length. However, we have demonstrated specific organ morphological effects, and behavioral effects in activity, anxiety-like behavior, and habituation that lasted in larvae exposed during the early embryonic period. A decrease in acetylcholinesterase activity was also observed and could explain some of the observed behavioral alterations. Interestingly, effects were more pronounced in ZFe exposed to the 700 MHz frequency, and especially for the 4 h exposure period. In addition, we have demonstrated that our exposure setup is robust, flexible with regard to frequency and power testing, and highly comparable. Future work will include exposure of ZFe to 5G modulated signals for different time periods to better understand the potential health effects of novel 5G RFR.

Inversed Effects of Nav1.2 Deficiency at Medial Prefrontal Cortex and Ventral Tegmental Area for Prepulse Inhibition in Acoustic Startle Response

Numerous pathogenic variants of SCN2A gene, encoding voltage-gated sodium channel α2 subunit Nav1.2 protein, have been identified in a wide spectrum of neuropsychiatric disorders including schizophrenia. However, pathological mechanisms for the schizophrenia-relevant behavioral abnormalities caused by the variants remain poorly understood. Here in this study, we characterized mouse lines with selective Scn2a deletion at schizophrenia-related brain regions, medial prefrontal cortex (mPFC) or ventral tegmental area (VTA), obtained by injecting adeno-associated viruses (AAV) expressing Cre recombinase into homozygous Scn2a-floxed (Scn2afl/fl) mice, in which expression of the Scn2a was locally deleted in the presence of Cre recombinase. The mice lacking Scn2a in the mPFC exhibited a tendency for a reduction in prepulse inhibition (PPI) in acoustic startle response. Conversely, the mice lacking Scn2a in the VTA showed a significant increase in PPI. We also found that the mice lacking Scn2a in the mPFC displayed increased sociability, decreased locomotor activity, and increased anxiety-like behavior, while the mice lacking Scn2a in the VTA did not show any other abnormalities in these parameters except for vertical activity which is one of locomotor activities. These results suggest that Scn2a-deficiencies in mPFC and VTA are inversely relevant for the schizophrenic phenotypes in patients with SCN2A variants.

Assessing the Cntnap2 knockout rat prepulse inhibition deficit through prepulse scaling of the baseline startle response curve

Many neurodevelopmental disorders, including autism spectrum disorder (ASD), are associated with changes in sensory processing and sensorimotor gating. The acoustic startle response and prepulse inhibition (PPI) of startle are widely used translational measures for assessing sensory processing and sensorimotor gating, respectively. The Cntnap2 knockout (KO) rat has proven to be a valid model for ASD, displaying core symptoms, including sensory processing perturbations. Here, we used a novel method to assess startle and PPI in Cntnap2 KO rats that allows for the identification of separate scaling components: startle scaling, which is a change in startle amplitude to a given sound, and sound scaling, which reflects a change in sound processing. Cntnap2 KO rats show increased startle due to both an increased overall response (startle scaling) and a left shift of the sound/response curve (sound scaling). In the presence of a prepulse, wildtype rats show a reduction of startle due to both startle scaling and sound scaling, whereas Cntnap2 KO rats show normal startle scaling, but disrupted sound scaling, resulting in the reported PPI deficit. These results validate that startle and sound scaling by a prepulse are indeed two independent processes, with only the latter being impaired in Cntnap2 KO rats. As startle scaling is likely related to motor output and sound scaling to sound processing, this novel approach reveals additional information on the possible cause of PPI disruptions in preclinical models.

A review of the auditory-gut-brain axis

Hearing loss places a substantial burden on medical resources across the world and impacts quality of life for those affected. Further, it can occur peripherally and/or centrally. With many possible causes of hearing loss, there is scope for investigating the underlying mechanisms involved. Various signaling pathways connecting gut microbes and the brain (the gut-brain axis) have been identified and well established in a variety of diseases and disorders. However, the role of these pathways in providing links to other parts of the body has not been explored in much depth. Therefore, the aim of this review is to explore potential underlying mechanisms that connect the auditory system to the gut-brain axis. Using select keywords in PubMed, and additional hand-searching in google scholar, relevant studies were identified. In this review we summarize the key players in the auditory-gut-brain axis under four subheadings: anatomical, extracellular, immune and dietary. Firstly, we identify important anatomical structures in the auditory-gut-brain axis, particularly highlighting a direct connection provided by the vagus nerve. Leading on from this we discuss several extracellular signaling pathways which might connect the ear, gut and brain. A link is established between inflammatory responses in the ear and gut microbiome-altering interventions, highlighting a contribution of the immune system. Finally, we discuss the contribution of diet to the auditory-gut-brain axis. Based on the reviewed literature, we propose numerous possible key players connecting the auditory system to the gut-brain axis. In the future, a more thorough investigation of these key players in animal models and human research may provide insight and assist in developing effective interventions for treating hearing loss.

Aberrant Dopamine System Function in the Ferrous Amyloid Buthionine (FAB) Rat Model of Alzheimer's Disease

Antipsychotics increase the risk of death in elderly patients with Alzheimer's disease (AD). Thus, there is an immediate need for novel therapies to treat comorbid psychosis in AD. Psychosis has been attributed to a dysregulation of the dopamine system and is associated with aberrant regulation by the hippocampus. Given that the hippocampus is a key site of pathology in AD, we posit that aberrant regulation of the dopamine system may contribute to comorbid psychosis in AD. A ferrous amyloid buthionine (FAB) rodent model was used to model a sporadic form of AD. FAB rats displayed functional hippocampal alterations, which were accompanied by decreases in spontaneous, low-frequency oscillations and increases in the firing rates of putative pyramidal neurons. Additionally, FAB rats exhibited increases in dopamine neuron population activity and augmented responses to the locomotor-inducing effects of MK-801, as is consistent with rodent models of psychosis-like symptomatology. Further, working memory deficits in the Y-maze, consistent with an AD-like phenotype, were observed in FAB rats. These data suggest that the aberrant hippocampal activity observed in AD may contribute to dopamine-dependent psychosis, and that the FAB model may be useful for the investigation of comorbid psychosis related to AD. Understanding the pathophysiology that leads to comorbid psychosis in AD will ultimately lead to the discovery of novel targets for the treatment of this disease.

L-carnitine attenuates acoustic startle reflex dysfunction in adult male rats exposed to mancozeb

The fungicide mancozeb increases oxygen-free radicals in the central nervous system. As an antioxidant, L-carnitine protects DNA and cell membranes from damage caused by oxygen-free radicals. The present study investigated how L-carnitine affected the acoustic startle response (ASR) in rats exposed to mancozeb. In this experimental study, male Wistar rats were gavaged orally with mancozeb (500, 1000, and 2000 mg/kg), L-carnitine (100, 200, and 400 mg/kg), or L-carnitine (200 mg/kg) + mancozeb (500 mg/kg) three times in 1 week. In the sham group, saline (0.9%, 10 mL/kg) was gavaged at a volume equivalent to that of the drugs. The control group did not receive any treatment. The results showed that locomotor activity and the percentage of prepulse inhibition in the mancozeb groups decreased compared to the sham group while these parameters increased in the L-carnitine group (200 mg/kg) compared to sham rats. In conclusion, mancozeb may increase the risk factor for cognitive diseases such as schizophrenia in people exposed to it while pretreatment with L-carnitine can attenuate the toxic effect.

Steady electrocorticogram characteristics predict specific stress-induced behavioral phenotypes

Introduction

Depending on the individual, exposure to an intense stressor may, or may not, lead to a stress-induced pathology. Predicting the physiopathological evolution in an individual is therefore an important challenge, at least for prevention. In this context, we developed an ethological model of simulated predator exposure in rats: we call this the multisensorial stress model (MSS). We hypothesized that: (i) MSS exposure can induce stress-induced phenotypes, and (ii) an electrocorticogram (ECoG) recorded before stress exposure can predict phenotypes observed after stress.

Methods

Forty-five Sprague Dawley rats were equipped with ECoG telemetry and divided into two groups. The Stress group (n = 23) was exposed to an MSS that combined synthetic fox feces odor deposited on filter paper, synthetic blood odor, and 22 kHz rodent distress calls; the Sham group (n = 22) was not exposed to any sensorial stimulus. Fifteen days after initial exposure, the two groups were re-exposed to a context that included a filter paper soaked with water as a traumatic object (TO) reminder. During this re-exposure, freezing behavior and avoidance of the filter paper were measured.

Results

Three behaviors were observed in the Stress group: 39% developed a fear memory phenotype (freezing, avoidance, and hyperreactivity); 26% developed avoidance and anhedonia; and 35% made a full recovery. We also identified pre-stress ECoG biomarkers that accurately predicted cluster membership. Decreased chronic 24 h frontal Low θ relative power was associated with resilience; increased frontal Low θ relative power was associated with fear memory; and decreased parietal β2 frequency was associated with the avoidant-anhedonic phenotype.

Discussion

These predictive biomarkers open the way to preventive medicine for stress-induced diseases.

Altered frontoparietal activity in acoustic startle priming tasks during reticulospinal tract facilitation: An fNIRS study

Background

Because it is one of the important pathways for promoting motor recovery after cortical injury, the function of the reticulospinal tract (RST) has received increasing attention in recent years. However, the central regulatory mechanism of RST facilitation and reduction of apparent response time is not well understood.

Objectives

To explore the potential role of RST facilitation in the acoustic startle priming (ASP) paradigm and observe the cortical changes induced by ASP reaching tasks.

Methods

Twenty healthy participants were included in this study. The reaching tasks were performed with their left and right hands. Participants were instructed to get ready after the warning cue and complete the reach as soon as they heard the Go cue. Half of the testing trials were set as control trials with an 80-dB Go cue. The other half of the trials had the Go cue replaced with 114-dB white noise to evoke the StartleReact effect, inducing reticulospinal tract facilitation. The response of the bilateral sternocleidomastoid muscle (SCM) and the anterior deltoid was recorded via surface electromyography. Startle trials were labeled as exhibiting a positive or negative StartleReact effect, according to whether the SCM was activated early (30-130 ms after the Go cue) or late, respectively. Functional near-infrared spectroscopy was used to synchronously record the oxyhemoglobin and deoxyhemoglobin fluctuations in bilateral motor-related cortical regions. The β values representing cortical responses were estimated via the statistical parametric mapping technique and included in the final analyses.

Results

Separate analyses of data from movements of the left or right side revealed significant activation of the right dorsolateral prefrontal cortex during RST facilitation. Moreover, left frontopolar cortex activation was greater in positive startle trials than in control or negative startle trials during left-side movements. Furthermore, decreased activity of the ipsilateral primary motor cortex in positive startle trials during ASP reaching tasks was observed.

Conclusion

The right dorsolateral prefrontal cortex and the frontoparietal network to which it belongs may be the regulatory center for the StartleReact effect and RST facilitation. In addition, the ascending reticular activating system may be involved. The decreased activity of the ipsilateral primary motor cortex suggests enhanced inhibition of the non-moving side during the ASP reaching task. These findings provide further insight into the SE and into RST facilitation.

Conditioned medium from BV2 microglial cells having polyleucine specifically alters startle response in mice

Repeat-associated non-AUG translation (RAN translation) is observed in transcripts that are causative for polyglutamine (polyQ) diseases and generates proteins with mono amino acid tracts such as polyalanine (polyA), polyleucine (polyL) and polyserine (polyS) in neurons, astrocytes and microglia. We have previously shown that microglia with aggregated polyQ led to defective differentiation and degeneration of neuron-like cells. However, it has not been determined whether only microglia containing a specific RAN product, but not other RAN products, is harmful in vitro and in vivo. Here we show that polyL-incorporating microglia specifically led to altered startle response in mice. Aggregated polyA, polyS and polyL induced aberrant differentiation of microglia-like BV2 cells. Differentiated PC12 cells treated with conditioned medium (CM) of polyS- and polyL- but not polyA-incorporating microglia-like BV2 cells showed retraction of neurites and loss of branch of neurites. Injection of the polyL-CM, but not polyA-CM and polyS-CM, into the lateral ventricle lowered startle response in mice. Consistently, polyL induced the highest expression of CD68 in BV2 cells. The lowered startle response was replicated in mice given the polyL-CM in the caudal pontine reticular nucleus (PnC), the key region of startle response. Thus, endogenous RAN proteins having polyL derived from polyQ diseases-causative genes in microglia might specifically impair startle response.

Neurodegeneration Within the Amygdala Is Differentially Induced by Opioid and HIV-1 Tat Exposure

Opioid use disorder (OUD) is a critical problem that contributes to the spread of HIV and may intrinsically worsen neuroHIV. Despite the advent of combined antiretroviral therapies (cART), about half of persons infected with HIV (PWH) experience cognitive and emotional deficits that can be exacerbated by opioid abuse. HIV-1 Tat is expressed in the central nervous system (CNS) of PWH on cART and is thought to contribute to neuroHIV. The amygdala regulates emotion and memories associated with fear and stress and is important in addiction behavior. Notwithstanding its importance in emotional saliency, the effects of HIV and opioids in the amygdala are underexplored. To assess Tat- and morphine-induced neuropathology within the amygdala, male Tat transgenic mice were exposed to Tat for 8 weeks and administered saline and/or escalating doses of morphine twice daily (s.c.) during the last 2 weeks of Tat exposure. Eight weeks of Tat exposure decreased the acoustic startle response and the dendritic spine density in the basolateral amygdala, but not the central nucleus of the amygdala. In contrast, repeated exposure to morphine alone, but not Tat, increased the acoustic startle response and whole amygdalar levels of amyloid-β (Aβ) monomers and oligomers and tau phosphorylation at Ser396, but not neurofilament light chain levels. Co-exposure to Tat and morphine decreased habituation and prepulse inhibition to the acoustic startle response and potentiated the morphine-induced increase in Aβ monomers. Together, our findings indicate that sustained Tat and morphine exposure differentially promote synaptodendritic degeneration within the amygdala and alter sensorimotor processing.

Research Progress in the Study of Startle Reflex to Disease States

The startle reflex is considered a primitive physiological reflex, a defense response that occurs in the organism when the body feels sudden danger and uneasiness, characterized by habituation and sensitization effects, and studies on the startle reflex often deal with pre-pulse inhibition (PPI) and sensorimotor gating. Under physiological conditions, the startle reflex is stable at a certain level, and when the organism is in a pathological state, such as stroke, spinal cord injury, schizophrenia, and other diseases, the reflex undergoes a series of changes, making it closely related to the progress of disease. This paper summarizes the startle reflex in physiological and pathological states by reviewing the databases of PubMed, Web of Science, Cochrane Library, EMBASE, China Biology Medicine, China National Knowledge Infrastructure, VIP Database for Chinese Technical Periodical, Wanfang Data, and identifies and analyzes the startle reflex and excessive startle reaction disorder.

Differential Expression Patterns of Lynx Proteins and Involvement of Lynx1 in Prepulse Inhibition

Negative allosteric modulators, such as lynx1 and lynx2, directly interact with nicotinic acetylcholine receptors (nAChRs). The nAChRs are integral to cholinergic signaling in the brain and have been shown to mediate different aspects of cognitive function. Given the interaction between lynx proteins and these receptors, we examined whether these endogenous negative allosteric modulators are involved in cognitive behaviors associated with cholinergic function. We found both cell-specific and overlapping expression patterns of lynx1 and lynx2 mRNA in brain regions associated with cognition, learning, memory, and sensorimotor processing, including the prefrontal cortex (PFC), cingulate cortex, septum, hippocampus, amygdala, striatum, and pontine nuclei. Since lynx proteins are thought to play a role in conditioned associations and given the expression patterns across brain regions, we first assessed whether lynx knockout mice would differ in a cognitive flexibility task. We found no deficits in reversal learning in either the lynx1^–/– or lynx2^–/– knockout mice. Thereafter, sensorimotor gating was examined with the prepulse inhibition (PPI) assessment. Interestingly, we found that both male and female lynx1^–/– mice exhibited a deficit in the PPI behavioral response. Given the comparable expression of lynx2 in regions involved in sensorimotor gating, we then examined whether removal of the lynx2 protein would lead to similar behavioral effects. Unexpectedly, we found that while male lynx2^–/– mice exhibited a decrease in the baseline startle response, no differences were found in sensorimotor gating for either male or female lynx2^–/– mice. Taken together, these studies provide insight into the expression patterns of lynx1 and lynx2 across multiple brain regions and illustrate the modulatory effects of the lynx1 protein in sensorimotor gating.