Biology subjects

Shepard, R. D.

Publications and source records attributed to Shepard, R. D..

5 recordsLinked to original sources

bioRxiv2023

AKAP150-anchored PKA regulation of synaptic transmission and plasticity, neuronal excitability and CRF neuromodulation in the lateral habenula

Numerous studies of hippocampal synaptic function in learning and memory have established the functional significance of the scaffolding A-kinase anchoring protein 150 (AKAP150) in kinase and phosphatase regulation of synaptic receptor and ion channel trafficking/function and hence synaptic transmission/plasticity, and neuronal excitability. Emerging evidence also suggests that AKAP150 signaling may play a critical role in brains processing of rewarding/aversive experiences. Here we focused on an unexplored role of AKAP150 in the lateral habenula (LHb), a diencephalic brain region that integrates and relays negative reward signals from forebrain striatal and limbic structures to midbrain monoaminergic centers. LHb aberrant activity (specifically hyperactivity) is also linked to depression. Using whole cell patch clamp recordings in LHb of male wildtype (WT) and {Delta}PKA knockin mice (with deficiency in AKAP-anchoring of PKA), we found that the genetic disruption of PKA anchoring to AKAP150 significantly reduced AMPA receptor (AMPAR)-mediated glutamatergic transmission and prevented the induction of presynaptic endocannabinoid (eCB)-mediated long-term depression (LTD) in LHb neurons. Moreover, {Delta}PKA mutation potentiated GABAA receptor (GABAAR)-mediated inhibitory transmission postsynaptically while increasing LHb intrinsic neuronal excitability through suppression of medium afterhyperpolarizations (mAHPs). Given that LHb is a highly stress-responsive brain region, we further tested the effects of corticotropin releasing factor (CRF) stress neuromodulator on synaptic transmission and intrinsic excitability of LHb neurons in WT and {Delta}PKA mice. As in our earlier study in rat LHb, CRF significantly suppressed GABAergic transmission onto LHb neurons and increased intrinsic excitability by diminishing small-conductance potassium (SK) channel-mediated mAHPs. {Delta}PKA mutation-induced suppression of mAHPs also blunted the synaptic and neuroexcitatory actions of CRF in mouse LHb. Altogether, our data suggest that AKAP150 complex signaling plays a critical role in regulation of AMPAR and GABAAR synaptic strength, glutamatergic plasticity and CRF neuromodulation possibly through AMPAR and potassium channel trafficking and eCB signaling within the LHb.

neuroscience↗

bioRxiv2021

Paradoxical excitation of lateral habenula neurons by propofol involves enhanced presynaptic release of glutamate

Sleep is a fundamental physiological process conserved across most species. As such, deficits in sleep can result in a myriad of psychological and physical health issues. However, the mechanisms underlying the induction of sleep are relatively unknown. Interestingly, general anesthetics cause unconsciousness by positively modulating GABA-A receptors (GABAARs). Based on this observation, it is hypothesized that GABAARs play a critical role in modulating circuits involved in sleep to promote unconsciousness. Recently, the lateral habenula (LHb) has been demonstrated to play a role in sleep physiology and sedation. Specifically, propofol has been shown to excite LHb neurons to promote sedation. However, the mechanism by which this occurs is unknown. Here, we utilize whole-cell voltage and current clamp recordings from LHb neurons obtained from 8-10 week old male mice to determine the physiological mechanisms for this phenomenon. We show that bath application of 1.5M propofol is sufficient to increase LHb neuronal excitability involving synaptic transmission, but not through modulation of intrinsic properties. Additionally, although there is increased LHb neuronal excitability, GABAARs localized postsynaptically on LHb neurons are still responsive to propofol, as indicated by an increase in the decay time. Lastly, we find that propofol increases the synaptic drive onto LHb neurons involving enhanced presynaptic release of both glutamate and GABA. However, the greatest contributor to the potentiated synaptic drive is the increased release of glutamate which shifts the balance of synaptic transmission towards greater excitation. Taken together, this study is the first to identify the physiological basis for why LHb neurons are excited by propofol, rather than inhibited, and as a result promote sedation.

neuroscience↗

bioRxiv2021

Shisa7 phosphorylation regulates GABAergic transmission and neurodevelopmental behaviors

GABA-A receptors (GABAARs) are crucial for development and regulation of the central nervous system. Altered GABAergic signaling is hypothesized to be involved in the pathophysiology of neurodevelopmental disorders. Nevertheless, how aberrant cellular and molecular mechanisms affect GABAARs in these diseases remain elusive. Recently, we identified Shisa7 as a GABAAR auxiliary subunit that modulates GABAAR trafficking, kinetics, and pharmacology, and discovered a phosphorylation site in Shisa7 (S405) critical for extrasynaptic 5-GABAAR trafficking and tonic inhibition. However, the role of S405 phosphorylation in the regulation of synaptic inhibition, plasticity, and behavior remains unknown. Here, we found that expression of a phospho-null mutant (Shisa7 S405A) in heterologous cells and neurons diminishes 2-GABAAR trafficking. Subsequently, we generate a Shisa7 S405A knock-in (KI) mouse line that displays reduced surface expression of GABAARs in hippocampal neurons. Importantly, both synaptic and tonic inhibition are decreased in KI mice. Moreover, chemically induced inhibitory long-term potentiation is impaired, highlighting a critical role of Shisa7 S405 in GABAergic plasticity. Lastly, KI mice exhibit enhanced locomotor activity and grooming associated with neurodevelopmental disorders. Collectively, our study reveals a phosphorylation site critical for Shisa7-dependent trafficking of synaptic and extrasynaptic GABAARs which contributes to behavioral endophenotypes displayed in neurodevelopmental disorders.

neuroscience↗

bioRxiv2020

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

Early life stress (ELS) presents an important risk factor for drug addiction and comorbid depression and anxiety through persistent effects on the mesolimbic dopamine (DA) pathways1. Using an ELS 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 ventral tegmental area (VTA) DA neurons 2-4 and its negative controller, the lateral habenula (LHb) 5-7. In regard to 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 intrinsic excitability of LHb neurons in early adolescent male rats 5-7. 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 behavior in the sucrose preference test (SPT), and was associated with persistent glutamatergic potentiation 24h after the last MSA session. MSA also triggered postsynaptic glutamatergic potentiation in LHb neurons of control rats during this time period. Our data highlights that ELS-induced glutamatergic plasticity in LHb may dampen the positive reinforcing properties of natural rewards and opioids, and contribute to the development of anhedonic and dysphoric states associated with opioids.

neuroscience↗

bioRxiv2020

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 intrinsic excitability while blunting the response of LHb neurons to extra hypothalamic 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) exerts differential effects on the excitability of two distinct subpopulations of LHb neurons that differ in their expression of hyperpolarization-activated cation currents (HCN, Ih). Specifically, KOR stimulation increases neuronal excitability in LHb neurons with large Ih currents (Ih+) while decreases neuronal excitability in small/negative Ih (Ih-) neurons. Additionally, we found that an intact fast-synaptic transmission is required for the effects of U50,488 on the excitability of both Ih- and Ih+ LHb neuronal subpopulations. Consistently, KOR activation also altered both glutamatergic and GABAergic synaptic transmission. While stimulation of presynaptic KORs uniformly suppressed glutamate release onto LHb neurons, we found that U50, 488 either increased or decreased GABA release. We also found that MD significantly increased immunolabeled Dyn (the endogenous KOR agonist) labeling in neuronal fibers in LHb while significantly decreased mRNA levels of KORs in LHb tissues compared to those from non-maternally deprived (non-MD) control rats. While total p38 MAPK (a downstream signaling pathway driven by KOR activation) expression was elevated in the LHb of MD rats compared to non-MD controls, we found that application of KOR-specific agonist, U50,488, onto LHb slices was still able to alter phosphorylated p38 MAPK (ph-p38) expression in MD rats similar to non-MD controls. Moreover, we found that 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 the functional Dyn/KOR signaling in the LHb that can be modulated in response to severe early life stressors such as MD.

neuroscience↗