Biased signaling by endogenous opioid peptides.

Opioids, such as morphine and fentanyl, are widely used for the treatment of severe pain; however, prolonged treatment with these drugs leads to the development of tolerance and can lead to opioid use disorder. The "Opioid Epidemic" has generated a drive for a deeper understanding of the fundamental signaling mechanisms of opioid receptors. It is generally thought that the three types of opioid receptors (µ, δ, κ) are activated by endogenous peptides derived from three different precursors: Proopiomelanocortin, proenkephalin, and prodynorphin. Posttranslational processing of these precursors generates >20 peptides with opioid receptor activity, leading to a long-standing question of the significance of this repertoire of peptides. Here, we address some aspects of this question using a technical tour de force approach to systematically evaluate ligand binding and signaling properties ([35S]GTPγS binding and ß-arrestin recruitment) of 22 peptides at each of the three opioid receptors. We show that nearly all tested peptides are able to activate the three opioid receptors, and many of them exhibit agonist-directed receptor signaling (functional selectivity). Our data also challenge the dogma that shorter forms of ß-endorphin do not exhibit receptor activity; we show that they exhibit robust signaling in cultured cells and in an acute brain slice preparation. Collectively, this information lays the groundwork for improved understanding of the endogenous opioid system that will help in developing more effective treatments for pain and addiction.

Potentiation of glutamatergic synaptic transmission onto lateral habenula neurons following early life stress and intravenous morphine self-administration in rats.

Early life stress presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine pathways. Using an early life stress model for child neglect (a single 24 h episode of maternal deprivation, MD) in rats, recent published works from our lab show that MD induces dysfunction in the ventral tegmental area and its negative controller, the lateral habenula (LHb). MD-induced potentiation of glutamatergic synaptic transmission onto LHb neurons shifts the coordination of excitation/inhibition (E/I) balance towards excitation, resulting in an increase in the overall spontaneous neuronal activity with elevation in bursting and tonic firing, and in the intrinsic excitability of LHb neurons in early adolescent male rats. Here, we explored how MD affects intravenous morphine self-administration (MSA) acquisition and sucrose preference as well as glutamatergic synaptic function in LHb neurons of adult male rats self-administering morphine. We found that MD-induced increases in LHb neuronal and glutamatergic synaptic activity and E/I ratio persisted into adulthood. Moreover, MD significantly reduced morphine intake, triggered anhedonia-like behaviour in the sucrose preference test and was associated with persistent glutamatergic potentiation 24 h after the last MSA session. MSA also altered the decay time kinetics of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR) currents in LHb neurons of control rats during this time period. Our data highlight that early life stress-induced glutamatergic plasticity in LHb may dampen the positive reinforcing and motivational properties of both natural rewards and opioids, and may contribute to the development of anhedonia and dysphoric states associated with opioids.

Aversive Stress Reduces Mu Opioid Receptor Expression in the Intercalated Nuclei of the Rat Amygdala.

The amygdala plays an important role in the integration of responses to noxious and fearful stimuli. Sensory information from many systems is integrated in the lateral and basolateral amygdala and transmitted to the central amygdala, the major output nucleus of the amygdala regulating both motor and emotional responses. The network of intercalated cells (ITC) which surrounds the lateral and basolateral amygdala and serves to modulate information flow from the lateral amygdala to the central nucleus, express a very high local concentration of mu-type opioid receptors. Loss of the ITC neurons impairs fear extinction. We demonstrate here that exposure of rats to a severe stress experience resulted in a marked downregulation of the level of expression of mu opioid receptors in the ITC nuclei over a period of at least 24 h after the end of the stress exposure. The endogenous opioid dynorphin is also expressed in the central and ITC nuclei of the amygdala. Following stress exposure, we also observed an increase in the expression in the more lateral regions of the central amygdala of pro-dynorphin mRNA and a peptide product of pro-dynorphin with known affinity for mu opioid receptors. It is possible that the downregulation of mu receptors in ITC neurons after stress may result from sustained activation and internalization of mu receptors following a stress-induced increase in the release of endogenous opioid peptides.

Development of a systems-based in situ multiplex biomarker screening approach for the assessment of immunopathology and neural tissue plasticity in male rats after traumatic brain injury.

Traumatic brain injuries (TBIs) pose a massive burden of disease and continue to be a leading cause of morbidity and mortality throughout the world. A major obstacle in developing effective treatments is the lack of comprehensive understanding of the underlying mechanisms that mediate tissue damage and recovery after TBI. As such, our work aims to highlight the development of a novel experimental platform capable of fully characterizing the underlying pathobiology that unfolds after TBI. This platform encompasses an empirically optimized multiplex immunohistochemistry staining and imaging system customized to screen for a myriad of biomarkers required to comprehensively evaluate the extent of neuroinflammation, neural tissue damage, and repair in response to TBI. Herein, we demonstrate that our multiplex biomarker screening platform is capable of evaluating changes in both the topographical location and functional states of resident and infiltrating cell types that play a role in neuropathology after controlled cortical impact injury to the brain in male Sprague-Dawley rats. Our results demonstrate that our multiplex biomarker screening platform lays the groundwork for the comprehensive characterization of changes that occur within the brain after TBI. Such work may ultimately lead to the understanding of the governing pathobiology of TBI, thereby fostering the development of novel therapeutic interventions tailored to produce optimal tissue protection, repair, and/or regeneration with minimal side effects, and may ultimately find utility in a wide variety of other neurological injuries, diseases, and disorders that share components of TBI pathobiology.

Morphine-induced synaptic plasticity in the VTA is reversed by HDAC inhibition.

Dopamine (DA) dysfunction originating from the ventral tegmental area (VTA) occurs as a result of synaptic abnormalities following consumption of drugs of abuse and underlies behavioral plasticity associated with drug abuse. Drugs of abuse can cause changes in gene expression through epigenetic mechanisms in the brain that underlie some of the lasting neuroplasticity and behavior associated with addiction. Here we investigated the function of histone acetylation and histone deacetylase (HDAC)2 in the VTA in recovery of morphine-induced synaptic modifications following a single in vivo exposure to morphine. Using a combination of immunohistochemistry, Western blot, and whole cell patch-clamp recording in rat midbrain slices, we show that morphine increased HDAC2 activity in VTA DA neurons and reduced histone H3 acetylation at lysine 9 (Ac-H3K9) in the VTA 24 h after the injection. Morphine-induced synaptic changes at glutamatergic synapses involved endocannabinoid signaling to reduce GABAergic synaptic strength onto VTA DA neurons. Both plasticities were recovered by in vitro incubation of midbrain slices with a class I-specific HDAC inhibitor (HDACi), CI-994, through an increase in acetylation of histone H3K9. Interestingly, HDACi incubation also increased levels of Ac-H3K9 and triggered GABAergic and glutamatergic plasticities in DA neurons of saline-treated rats. Our results suggest that acute morphine-induced changes in VTA DA activity and synaptic transmission engage HDAC2 activity locally in the VTA to maintain synaptic modifications through histone hypoacetylation.

Mu opioid receptor activation enhances regulator of G protein signaling 4 association with the mu opioid receptor/G protein complex in a GTP-dependent manner.

The interaction of Regulator of G protein Signaling 4 (RGS4) with the rat mu opioid receptor (MOR)/G protein complex was investigated. Solubilized MOR from rat brain membranes was immunoprecipitated in the presence of RGS4 with antibodies against the N-terminus of MOR (anti-MOR10-70 ). Activation of MOR with [D-Ala(2) , N-Me-Phe(4) , Gly(5) -ol] enkephalin (DAMGO) during immunoprecipitation caused a 150% increase in Goα and a 50% increase in RGS4 in the pellet. When 10 µM GTP was included with DAMGO, there was an additional 72% increase in RGS4 co-immunoprecipitating with MOR (p = 0.003). Guanosine 5'-O-(3-thiotriphosphate) (GTPγS) increased the amount of co-precipitating RGS4 by 93% (compared to DAMGO alone, p = 0.008), and the inclusion of GTPγS caused the ratio of MOR to RGS4 to be 1 : 1 (31 fmoles : 28 fmoles, respectively). GTPγS also increased the association of endogenous RGS4 with MOR. In His6 RGS4/Ni(2+) -NTA agarose pull down experiments, 0.3 µM GTPγS tripled the binding of Goα to His6 RGS4, whereas the addition of 100 µM GDP blocked this effect. Importantly, activation of solubilized MOR with DAMGO in the presence of 100 µM GDP and 0.3 µM GTPγS increased Goα binding to His6 RGS4/Ni(2+) -NTA agarose (p = 0.001). Regulators of G protein Signaling (RGS) shorten the time that G proteins are active. Activation of the mu opioid receptor (MOR) causes GTP to bind to and to activate Go (αoßγ). RGS4 then binds to the activated αo-GTP/MOR complex and accelerates the intrinsic GTPase of αo. After αo dissociates from MOR, RGS4 remains bound to the C-terminal region of MOR.

A-kinase anchoring protein-calcineurin signaling in long-term depression of GABAergic synapses.

The postsynaptic scaffolding A-kinase anchoring protein 79/150 (AKAP79/150) signaling complex regulates excitatory synaptic transmission and strength through tethering protein kinase A (PKA), PKC, and calcineurin (CaN) to the postsynaptic densities of neurons (Sanderson and Dell'Acqua, 2011), but its role in inhibitory synaptic transmission and plasticity is unknown. Using immunofluorescence and whole-cell patch-clamp recording in rat midbrain slices, we show that activation of postsynaptic D(2)-like family of dopamine (DA) receptor in the ventral tegmental area (VTA) induces long-term depression (LTD) of GABAergic synapses on DA neurons through an inositol triphosphate receptor-mediated local rise in postsynaptic Ca(2+) and CaN activation accompanied by PKA inhibition, which requires AKAP150 as a bridging signaling molecule. Our data also illuminate a requirement for a clathrin-mediated internalization of GABA(A) receptors in expression of LTD(GABA). Moreover, disruption of AKAP-PKA anchoring does not affect glutamatergic synapses onto DA neurons, suggesting that the PKA-AKAP-CaN complex is uniquely situated at GABA(A) receptor synapses in VTA DA neurons to regulate plasticity associated with GABA(A) receptors. Drug-induced modulation of GABAergic plasticity in the VTA through such novel signaling mechanisms has the potential to persistently alter the output of individual DA neurons and of the VTA, which may contribute to the reinforcing or addictive properties of drugs of abuse.

Dysregulation of kappa opioid receptor neuromodulation of lateral habenula synaptic function following a repetitive mild traumatic brain injury.

Mild traumatic brain injury (mTBI) increases the risk of affective disorders, anxiety and substance use disorder. The lateral habenula (LHb) plays an important role in pathophysiology of psychiatric disorders. Recently, we demonstrated a causal link between mTBI-induced LHb hyperactivity due to excitation/inhibition (E/I) imbalance and motivational deficits in male mice using a repetitive closed head injury mTBI model. A major neuromodulatory system that is responsive to traumatic brain injuries, influences affective states and also modulates LHb activity is the dynorphin/kappa opioid receptor (Dyn/KOR) system. However, the effects of mTBI on KOR neuromodulation of LHb function are unknown. Here, we first used retrograde tracing in male and female Cre mouse lines and identified several major KOR-expressing and two prominent Dyn-expressing inputs projecting to the mouse LHb, highlighting the medial prefrontal cortex (mPFC) and the ventromedial nucleus of the hypothalamus (VMH) as the main LHb-projecting Dyn inputs that regulate KOR signaling to the LHb. We then functionally evaluated the effects of in vitro KOR modulation of spontaneous synaptic activity within the LHb of male and female sham and mTBI mice at 4 week post-injury. We observed sex-specific differences in spontaneous release of glutamate and GABA from presynaptic terminals onto LHb neurons with higher levels of presynaptic glutamate and GABA release in females compared to male mice. However, KOR effects on the spontaneous E/I ratios and synaptic drive ratio within the LHb did not differ between male and female sham and mTBI mice. KOR activation generally suppressed spontaneous glutamatergic transmission without altering GABAergic transmission, resulting in a significant but sex-similar reduction in net spontaneous E/I and synaptic drive ratios in LHb neurons of sham mice. Following mTBI, while responses to KOR activation at LHb glutamatergic synapses remained intact, LHb GABAergic synapses acquired an additional sensitivity to KOR-mediated inhibition where we observed a reduction in GABA release probability in response to KOR stimulation in LHb neurons of mTBI mice. Further analysis of percent change in spontaneous synaptic ratios induced by KOR activation revealed that independent of sex mTBI switches KOR-driven synaptic inhibition of LHb neurons (normally observed in sham mice) in a subset of mTBI mice toward synaptic excitation resulting in mTBI-induced divergence of KOR actions within the LHb. Overall, we uncovered the sources of major Dyn/KOR-expressing synaptic inputs projecting to the mouse LHb. We demonstrate that an engagement of intra-LHb Dyn/KOR signaling provides a global KOR-driven synaptic inhibition within the mouse LHb independent of sex. The additional engagement of KOR-mediated action on LHb GABAergic transmission by mTBI could contribute to the E/I imbalance after mTBI, with Dyn/KOR signaling serving as a disinhibitory mechanism for LHb neurons of a subset of mTBI mice.

Dysregulation of Kappa Opioid Receptor Neuromodulation of Lateral Habenula Synaptic Function following a Repetitive Mild Traumatic Brain Injury.

Mild traumatic brain injury (mTBI) increases the risk of cognitive deficits, affective disorders, anxiety and substance use disorder in affected individuals. Substantial evidence suggests a critical role for the lateral habenula (LHb) in pathophysiology of psychiatric disorders. Recently, we demonstrated a causal link between persistent mTBI-induced LHb hyperactivity due to synaptic excitation/inhibition (E/I) imbalance and motivational deficits in self-care grooming behavior in young adult male mice using a repetitive closed head injury mTBI model. One of the major neuromodulatory systems that is responsive to traumatic brain and spinal cord injuries, influences affective states and also modulates LHb activity is the dynorphin/kappa opioid receptor (Dyn/KOR) system. However, the effects of mTBI on KOR neuromodulation of LHb function is unknown. To address this, we first used retrograde tracing to anatomically verify that the mouse LHb indeed receives Dyn/KOR expressing projections. We identified several major KOR-expressing and Dyn-expressing synaptic inputs projecting to the mouse LHb. We then functionally evaluated the effects of in vitro KOR modulation of spontaneous synaptic activity within the LHb of male and female sham and mTBI mice at 4week post-injury using the repetitive closed head injury mTBI model. Similar to what we previously reported in the LHb of male mTBI mice, mTBI presynaptically diminished spontaneous synaptic activity onto LHb neurons, while shifting synaptic E/I toward excitation in female mouse LHb. Furthermore, KOR activation in either mouse male/female LHb generally suppressed spontaneous glutamatergic transmission without altering GABAergic transmission, resulting in a significant reduction in E/I ratios and decreased excitatory synaptic drive to LHb neurons of male and female sham mice. Interestingly following mTBI, while responses to KOR activation at LHb glutamatergic synapses were observed comparable to those of sham, LHb GABAergic synapses acquired an additional sensitivity to KOR-mediated inhibition. Thus, in contrast to sham LHb, we observed a reduction in GABA release probability in response to KOR stimulation in mTBI LHb, resulting in a chronic loss of KOR-mediated net synaptic inhibition within the LHb. Overall, our findings uncovered the previously unknown sources of major Dyn/KOR-expressing synaptic inputs projecting to the mouse LHb. Further, we demonstrate that an engagement of intra-LHb Dyn/KOR signaling provides a global suppression of excitatory synaptic drive to the mouse LHb which could act as an inhibitory braking mechanism to prevent LHb hyperexcitability. The additional engagement of KOR-mediated modulatory action on LHb GABAergic transmission by mTBI could contribute to the E/I imbalance after mTBI, with Dyn/KOR signaling serving as a disinhibitory mechanism for LHb neurons in male and female mTBI mice.

Effects of Repetitive Mild Traumatic Brain Injury on Corticotropin-Releasing Factor Modulation of Lateral Habenula Excitability and Motivated Behavior.

Mild traumatic brain injury (mTBI) is a significant health burden due to mTBI-related chronic debilitating cognitive and psychiatric morbidities. Recent evidence from our laboratory suggests a possible dysregulation within reward/motivational circuit function at the level of a subcortical structure, the lateral habenula (LHb), where we demonstrated a causal role for hyperactive LHb in mTBI-induced motivational deficits in self-care grooming behavior in young adult male mice when exposed to mTBI during late adolescence (at ∼8 weeks old). In this study, we extended this observation by further characterizing neurobehavioral effects of this repetitive closed head injury model of mTBI in both young adult male and female mice on LHb excitability, corticotropin releasing factor (CRF) modulation of LHb activity, and behavioral responses of motivation to self-care behavior and approach versus avoidance behavior in the presence of a social- or threat-related stimulus. We show that mTBI increases LHb spontaneous tonic activity in female mice similar to what we previously observed in male mice, as well as promoting LHb neuronal hyperexcitability and hyperpolarization-induced LHb bursting in both male and female mice. Interestingly, mTBI only increases LHb intrinsic excitability in male mice coincident with higher levels of the hyperpolarization-activated cation currents (HCN/Ih) and reduces levels of the M-type potassium currents while potentiating M-currents without altering intrinsic excitability in LHb neurons of female mice. Because persistent dysregulation of brain CRF systems is suggested to contribute to chronic psychiatric morbidities and that LHb neurons are highly responsive to CRF, we tested whether the LHb CRF subsystem becomes engaged following mTBI. We found that in vitro inhibition of CRF receptor type 1 (CRFR1) within the LHb reverses mTBI-induced enhancement of LHb tonic activity and hyperexcitability in both sexes, suggesting that an augmented intra-LHb CRF-CRFR1-mediated signaling contributes to the overall LHb hyperactivity following mTBI. Behaviorally, mTBI diminishes motivation for self-care grooming in female mice as in male mice. mTBI also alters defensive behaviors in the looming shadow task by shifting the innate defensive behaviors toward more passive action locking rather than escape behaviors in response to an aerial threat in both male and female mice, as well as prolonging the latency to escape responses in female mice. While this model of mTBI reduces social preference in male mice, it induces higher social novelty seeking during the novel social encounters in both male and female mice. Overall, our study provides further translational validity for the use of this pre-clinical model of mTBI for investigation of mTBI-related reward circuit dysfunction and mood/motivation-related behavioral deficits in both sexes while uncovering a few sexually dimorphic neurobehavioral effects of this model that may differentially affect young males and females when exposed to this type of mTBI during late adolescence.

AKAP150-anchored PKA regulates synaptic transmission and plasticity, neuronal excitability and CRF neuromodulation in the mouse lateral habenula.

The scaffolding A-kinase anchoring protein 150 (AKAP150) is critically involved in kinase and phosphatase regulation of synaptic transmission/plasticity, and neuronal excitability. Emerging evidence also suggests that AKAP150 signaling may play a key role in brain's processing of rewarding/aversive experiences, however its role in the lateral habenula (LHb, as an important brain reward circuitry) is completely unknown. Using whole cell patch clamp recordings in LHb of male wildtype and ΔPKA knockin mice (with deficiency in AKAP-anchoring of PKA), here we show that the genetic disruption of PKA anchoring to AKAP150 significantly reduces AMPA receptor-mediated glutamatergic transmission and prevents the induction of presynaptic endocannabinoid-mediated long-term depression in LHb neurons. Moreover, ΔPKA mutation potentiates GABAA receptor-mediated inhibitory transmission while increasing LHb intrinsic excitability through suppression of medium afterhyperpolarizations. ΔPKA mutation-induced suppression of medium afterhyperpolarizations also blunts the synaptic and neuroexcitatory actions of the stress neuromodulator, corticotropin releasing factor (CRF), in mouse LHb. Altogether, our data suggest that AKAP150 complex signaling plays a critical role in regulation of AMPA and GABAA receptor synaptic strength, glutamatergic plasticity and CRF neuromodulation possibly through AMPA receptor and potassium channel trafficking and endocannabinoid signaling within the LHb.

Effects of Repetitive Mild Traumatic Brain Injury on Corticotropin-Releasing Factor Modulation of Lateral Habenula Excitability and Motivated Behavior.

Mild traumatic brain injury (mTBI) is a significant health burden due to mTBI-related chronic debilitating cognitive and psychiatric morbidities. Recent evidence from our laboratory suggests a possible dysregulation within reward/motivational circuit function at the level of a subcortical structure, the lateral habenula (LHb), where we demonstrated a causal role for hyperactive LHb in mTBI-induced motivational deficits in self-care grooming behavior in young adult male mice when exposed to mTBI injury during late adolescence (at ~8 weeks old). Here we extended this observation by further characterizing neurobehavioral effects of this repetitive closed head injury model of mTBI in both young adult male and female mice on LHb excitability, corticotropin releasing factor (CRF) modulation of LHb activity, and behavioral responses of motivation to self-care behavior, and approach versus avoidance behavior in the presence of a social- or threat-related stimulus. We show that mTBI increases LHb spontaneous tonic activity in female mice similar to what we previously observed in male mice as well as promoting LHb neuronal hyperexcitability and hyperpolarization-induced LHb bursting in both male and female mice. Interestingly, mTBI only increases LHb intrinsic excitability in male mice coincident with higher levels of the hyperpolarization-activated cation currents (HCN/Ih) and reduces levels of the M-type potassium currents while potentiating M-currents without altering intrinsic excitability in LHb neurons of female mice. Since persistent dysregulation of brain CRF systems is suggested to contribute to chronic psychiatric morbidities and that LHb neurons are highly responsive to CRF, we then tested whether LHb CRF subsystem becomes engaged following mTBI. We found that in vitro inhibition of CRF receptor type 1 (CRFR1) within the LHb normalizes mTBI-induced enhancement of LHb tonic activity and hyperexcitability in both sexes, suggesting that an augmented intra-LHb CRF-CRFR1-mediated signaling contributes to the overall LHb hyperactivity following mTBI. Behaviorally, mTBI diminishes motivation for self-care grooming in female mice as in male mice. mTBI also alters defensive behaviors in the looming shadow task by shifting the innate defensive behaviors towards more passive action-locking rather than escape behaviors in response to an aerial threat in both male and female mice as well as prolonging the latency to escape responses in female mice. While, this model of mTBI reduces social preference in male mice, it induces higher social novelty seeking during the novel social encounters in both male and female mice. Overall, our study provides further translational validity for the use of this preclinical model of mTBI for investigation of mTBI-related reward circuit dysfunction and mood/motivation-related behavioral deficits in both sexes while uncovering a few sexually dimorphic neurobehavioral effects of this model that may differentially affect young males and females when exposed to this type of mTBI injury during late adolescence.

Involvement of Lateral Habenula Dysfunction in Repetitive Mild Traumatic Brain Injury-Induced Motivational Deficits.

Affective disorders including depression (characterized by reduced motivation, social withdrawal, and anhedonia), anxiety, and irritability are frequently reported as long-term consequences of mild traumatic brain injury (mTBI) in addition to cognitive deficits, suggesting a possible dysregulation within mood/motivational neural circuits. One of the important brain regions that control motivation and mood is the lateral habenula (LHb), whose hyperactivity is associated with depression. Here, we used a repetitive closed-head injury mTBI model that is associated with social deficits in adult male mice and explored the possible long-term alterations in LHb activity and motivated behavior 10-18 days post-injury. We found that mTBI increased the proportion of spontaneous tonically active LHb neurons yet decreased the proportion of LHb neurons displaying bursting activity. Additionally, mTBI diminished spontaneous glutamatergic and GABAergic synaptic activity onto LHb neurons, while synaptic excitation and inhibition (E/I) balance was shifted toward excitation through a greater suppression of GABAergic transmission. Behaviorally, mTBI increased the latency in grooming behavior in the sucrose splash test suggesting reduced self-care motivated behavior following mTBI. To show whether limiting LHb hyperactivity could restore motivational deficits in grooming behavior, we then tested the effects of Gi (hM4Di)-DREADD-mediated inhibition of LHb activity in the sucrose splash test. We found that chemogenetic inhibition of LHb glutamatergic neurons was sufficient to reverse mTBI-induced delays in grooming behavior. Overall, our study provides the first evidence for persistent LHb neuronal dysfunction due to an altered synaptic integration as causal neural correlates of dysregulated motivational states by mTBI.

Repetitive mild traumatic brain injury induces persistent alterations in spontaneous synaptic activity of hippocampal CA1 pyramidal neurons.

Mild traumatic brain injury (mTBI) or concussion is the most common form of TBI which frequently results in persistent cognitive impairments and memory deficits in affected individuals [1]. Although most studies have investigated the role of hippocampal synaptic dysfunction in earlier time points following a single injury, the long-lasting effects of mTBI on hippocampal synaptic transmission following multiple brain concussions have not been well-elucidated. Using a repetitive closed head injury (3XCHI) mouse model of mTBI, we examined the alteration of spontaneous synaptic transmission onto hippocampal CA1 pyramidal neurons by recording spontaneous excitatory AMPA receptor (AMPAR)- and inhibitory GABAAR-mediated postsynaptic currents (sEPSCs and sIPSCs, respectively) in adult male mice 2-weeks following the injury. We found that mTBI potentiated postsynaptic excitatory AMPAR synaptic function while depressed postsynaptic inhibitory GABAAR synaptic function in CA1 pyramidal neurons. Additionally, mTBI slowed the decay time of AMPAR currents while shortened the decay time of GABAAR currents suggesting changes in AMPAR and GABAAR subunit composition by mTBI. On the other hand, mTBI reduced the frequency of sEPSCs while enhanced the frequency of sIPSCs resulting in a lower ratio of sEPSC/sIPSC frequency in CA1 pyramidal neurons of mTBI animals compared to sham animals. Altogether, our results suggest that mTBI induces persistent postsynaptic modifications in AMPAR and GABAAR function and their synaptic composition in CA1 neurons while triggering a compensatory shift in excitation/inhibition (E/I) balance of presynaptic drives towards more inhibitory synaptic drive to hippocampal CA1 cells. The persistent mTBI-induced CA1 synaptic dysfunction and E/I imbalance could contribute to deficits in hippocampal plasticity that underlies long-term hippocampal-dependent learning and memory deficits in mTBI patients long after the initial injury.

Blast-Induced Mild Traumatic Brain Injury Alterations of Corticotropin-Releasing Factor Neuronal Activity in the Mouse Hypothalamic Paraventricular Nucleus.

Blast-induced mild traumatic brain injury (mbTBI) is the most common cause of TBI in US service members and veterans. Those exposed to TBI are at greater risk of developing neuropsychiatric disorders such as posttraumatic stress disorder, anxiety and depressive disorders, and substance use disorders following TBI. Previously, we have demonstrated that mbTBI increases anxiety-like behaviors in mice and dysregulates stress at the level of corticotropin-releasing factor (CRF) neurons in the paraventricular nucleus (PVN). To expand on how mTBI may dysregulate the stress axis centrally, here PVN CRF neuronal activity was evaluated using whole cell-patch clamp recordings in hypothalamic slices from sham and mbTBI adult male CRF:tdTomato mice 7 days post-injury. We found that mbTBI generally did not affect the neuronal excitability and intrinsic membrane properties of PVN CRF neurons; this injury selectively increased the frequency of spontaneous neuronal firing of PVN CRF neurons localized to the dorsal PVN (dPVN) but not ventral PVN (vPVN). Consistently, mbTBI-induced dPVN CRF hyperactivity was associated with pre- and post-synaptic depression of spontaneous GABAergic transmission onto dPVN CRF neurons suggesting that mbTBI-induced GABAergic synaptic dysfunction may underlie dPVN CRF neuronal hyperactivity and increases in dPVN CRF signaling. The present results provide the first evidence for mbTBI-induced alterations in PVN CRF neuronal activity and GABAergic synaptic function that could mediate hypothalamic CRF dysregulation following mbTBI contributing to stress psychopathology associated with blast injury.

Early life stress dysregulates kappa opioid receptor signaling within the lateral habenula.

The lateral habenula (LHb) is an epithalamic brain region associated with value-based decision making and stress evasion through its modulation of dopamine (DA)-mediated reward circuitry. Specifically, increased activity of the LHb is associated with drug addiction, schizophrenia and stress-related disorders such as depression, anxiety and posttraumatic stress disorder. Dynorphin (Dyn)/Kappa opioid receptor (KOR) signaling is a mediator of stress response in reward circuitry. Previously, we have shown that maternal deprivation (MD), a severe early life stress, increases LHb spontaneous neuronal activity and intrinsic excitability while blunting the response of LHb neurons to extrahypothalamic corticotropin-releasing factor (CRF) signaling, another stress mediator. CRF pathways also interact with Dyn/KOR signaling. Surprisingly, there has been little study of direct KOR regulation of the LHb despite its distinct role in stress, reward and aversion processing. To test the functional role of Dyn/KOR signaling in the LHb, we utilized ex-vivo electrophysiology combined with pharmacological tools in rat LHb slices. We show that activation of KORs by a KOR agonist (U50,488) exerted differential effects on the excitability of two distinct sub-populations of LHb neurons that differed in their expression of hyperpolarization-activated cation currents (HCN, Ih). Specifically, KOR stimulation increased neuronal excitability in LHb neurons with large Ih currents (Ih+) while decreasing neuronal excitability in small/negative Ih (Ih-) neurons. We found that an intact fast-synaptic transmission was required for the effects of U50,488 on the excitability of both Ih- and Ih+ LHb neuronal subpopulations. While AMPAR-, GABAAR-, or NMDAR-mediated synaptic transmission alone was sufficient to mediate the effects of U50,488 on excitability of Ih- neurons, either GABAAR- or NMDAR-mediated synaptic transmission could mediate these effects in Ih+ neurons. Consistently, KOR activation also altered both glutamatergic and GABAergic synaptic transmission where stimulation of presynaptic KORs uniformly suppressed glutamate release onto LHb neurons while primarily decreased or in some cases increased GABA release. We also found that MD significantly increased immunolabeled Dyn (the endogenous KOR agonist) labeling in neuronal fibers in LHb while significantly decreasing mRNA levels of KORs in LHb tissues compared to those from non-maternally deprived (non-MD) control rats. Moreover, the U50,488-mediated increase in LHb neuronal firing observed in non-MD rats was absent following MD. Altogether, this is the first demonstration of the existence of functional Dyn/KOR signaling in the LHb that can be modulated in response to severe early life stressors such as MD.

Targeting histone deacetylation for recovery of maternal deprivation-induced changes in BDNF and AKAP150 expression in the VTA.

Severe early life stressors increase the probability of developing psychiatric disorders later in life through modifications in neuronal circuits controlling brain monoaminergic signaling. Our previous work demonstrated that 24 h maternal deprivation (MD) in male Sprague Dawley rats modifies dopamine (DA) signaling from the ventral tegmental area (VTA) through changes at GABAergic synapses that were reversible by in vitro histone deacetylase (HDAC) inhibition which led to restoration of the scaffold A-kinase anchoring protein (AKAP150) signaling and subsequently recovered GABAergic plasticity (Authement et al., 2015). Using a combination of in situ hybridization, Western blots and immunohistochemistry, we confirmed that MD-induced epigenetic modifications at the level of histone acetylation were associated with an upregulation of HDAC2. MD also increased Akap5 mRNA levels in the VTA. Western blot analysis of AKAP150 protein expression showed an increase in synaptic levels of AKAP150 protein in the VTA with an accompanying decrease in synaptic levels of protein kinase A (PKA). Moreover, the abundance of mature brain-derived neurotrophic factor (BDNF) protein of VTA tissues from MD rats was significantly lower than in control groups. In vivo systemic injection with a selective class I HDAC inhibitor (CI-994) was sufficient to reverse MD-induced histone hypoacetylation in the VTA for 24 h after the injection. Furthermore, HDAC inhibition normalized the levels of mBDNF and AKAP150 proteins at 24 h. Our data suggest that HDAC-mediated targeting of BDNF and AKAP-dependent local signaling within VTA could provide novel therapeutics for prevention of later-life psychopathology.

Mass Spectrometric Imaging of Ceramide Biomarkers Tracks Therapeutic Response in Traumatic Brain Injury.

Traumatic brain injury (TBI) is a serious public health problem and the leading cause of death in children and young adults. It also contributes to a substantial number of cases of permanent disability. As lipids make up over 50% of the brain mass and play a key role in both membrane structure and cell signaling, their profile is of particular interest. In this study, we show that advanced mass spectrometry imaging (MSI) has sufficient technical accuracy and reproducibility to demonstrate the anatomical distribution of 50 µm diameter microdomains that show changes in brain ceramide levels in a rat model of controlled cortical impact (CCI) 3 days post injury with and without treatment. Adult male Sprague-Dawley rats received one strike and were euthanized 3 days post trauma. Brain MS images showed increase in ceramides in CCI animals compared to control as well as significant reduction in ceramides in CCI treated animals, demonstrating therapeutic effect of a peptide agonist. The data also suggests the presence of diffuse changes outside of the injured area. These results shed light on the extent of biochemical and structural changes in the brain after traumatic brain injury and could help to evaluate the efficacy of treatments.

Mass spectrometry imaging of rat brain lipid profile changes over time following traumatic brain injury.

BACKGROUND: Mild traumatic brain injury (TBI) is a common public health issue that may contribute to chronic degenerative disorders. Membrane lipids play a key role in tissue responses to injury, both as cell signals and as components of membrane structure and cell signaling. This study demonstrates the ability of high resolution mass spectrometry imaging (MSI) to assess sequences of responses of lipid species in a rat controlled cortical impact model for concussion. NEW METHOD: A matrix of implanted silver nanoparticles was implanted superficially in brain sections for matrix-assisted laser desorption (MALDI) imaging of 50µm diameter microdomains across unfixed cryostat sections of rat brain. Ion-mobility time-of-flight MS was used to analyze and map changes over time in brain lipid composition in a rats after Controlled Cortical Impact (CCI) TBI. RESULTS: Brain MS images showed changes in sphingolipids near the CCI site, including increased ceramides and decreased sphingomyelins, accompanied by changes in glycerophospholipids and cholesterol derivatives. The kinetics differed for each lipid class; for example ceramides increased as early as 1 day after the injury whereas other lipids changes occurred between 3 and 7 days post injury. COMPARISON WITH EXISTING METHOD(S): Silver nanoparticles MALDI matrix is a sensitive new tool for revealing previously undetectable cellular injury response and remodeling in neural, glial and vascular structure of the brain. CONCLUSIONS: Lipid biochemical and structural changes after TBI could help highlighting molecules that can be used to determine the severity of such injuries as well as to evaluate the efficacy of potential treatments.

Histone Deacetylase Inhibition Rescues Maternal Deprivation-Induced GABAergic Metaplasticity through Restoration of AKAP Signaling.

Adverse early-life experiences such as child neglect and abuse increase the risk of developing addiction and stress-related disorders through alterations in motivational systems including the mesolimbic dopamine (DA) pathway. Here we investigated whether a severe early-life stress (i.e., maternal deprivation, MD) promotes DA dysregulation through an epigenetic impairment of synaptic plasticity within ventral tegmental area (VTA) DA neurons. Using a single 24-hr episode of MD and whole-cell patch clamp recording in rat midbrain slices, we show that MD selectively induces long-term depression (LTD) and shifts spike timing-dependent plasticity (STDP) toward LTD at GABAergic synapses onto VTA DA neurons through epigenetic modifications of postsynaptic scaffolding A-kinase anchoring protein 79/150 (AKAP79/150) signaling. Histone deacetylase (HDAC) inhibition rescues GABAergic metaplasticity and normalizes AKAP signaling in MD animals. MD-induced reversible HDAC-mediated GABAergic dysfunction within the VTA may be a mechanistic link for increased propensity to mental health disorders following MD.