β-Adrenergic Control of Hippocampal Function: Subserving the Choreography of Synaptic Information Storage and Memory

Interplay of hippocampal long-term potentiation and long-term depression in enabling memory representations

Hippocampal long-term potentiation (LTP) and long-term depression (LTD) are Hebbian forms of synaptic plasticity that are widely believed to comprise the physiological correlates of associative learning. They comprise a persistent, input-specific increase or decrease, respectively, in synaptic efficacy that, in rodents, can be followed for days and weeks in vivo. Persistent (>24 h) LTP and LTD exhibit distinct frequency-dependencies and molecular profiles in the hippocampal subfields. Moreover, causal and genetic studies in behaving rodents indicate that both LTP and LTD fulfil specific and complementary roles in the acquisition and retention of spatial memory. LTP is likely to be responsible for the generation of a record of spatial experience, which may serve as an associative schema that can be re-used to expedite or facilitate subsequent learning. In contrast, LTD may enable modification and dynamic updating of this representation, such that detailed spatial content information is included and the schema is rendered unique and distinguishable from other similar representations. Together, LTP and LTD engage in a dynamic interplay that supports the generation of complex associative memories that are resistant to generalization. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Microstructural integrity of the locus coeruleus and its tracts reflect noradrenergic degeneration in Alzheimer's disease and Parkinson's disease

BACKGROUND

Degeneration of the locus coeruleus (LC) noradrenergic system contributes to clinical symptoms in Alzheimer's disease (AD) and Parkinson's disease (PD). Diffusion magnetic resonance imaging (MRI) has the potential to evaluate the integrity of the LC noradrenergic system. The aim of the current study was to determine whether the diffusion MRI-measured integrity of the LC and its tracts are sensitive to noradrenergic degeneration in AD and PD.

METHODS

Post-mortem in situ T1-weighted and multi-shell diffusion MRI was performed for 9 AD, 14 PD, and 8 control brain donors. Fractional anisotropy (FA) and mean diffusivity were derived from the LC, and from tracts between the LC and the anterior cingulate cortex, the dorsolateral prefrontal cortex (DLPFC), the primary motor cortex (M1) or the hippocampus. Brain tissue sections of the LC and cortical regions were obtained and immunostained for dopamine-beta hydroxylase (DBH) to quantify noradrenergic cell density and fiber load. Group comparisons and correlations between outcome measures were performed using linear regression and partial correlations.

RESULTS

The AD and PD cases showed loss of LC noradrenergic cells and fibers. In the cortex, the AD cases showed increased DBH + immunoreactivity in the DLPFC compared to PD cases and controls, while PD cases showed reduced DBH + immunoreactivity in the M1 compared to controls. Higher FA within the LC was found for AD, which was correlated with loss of noradrenergic cells and fibers in the LC. Increased FA of the LC-DLPFC tract was correlated with LC noradrenergic fiber loss in the combined AD and control group, whereas the increased FA of the LC-M1 tract was correlated with LC noradrenergic neuronal loss in the combined PD and control group. The tract alterations were not correlated with cortical DBH + immunoreactivity.

CONCLUSIONS

In AD and PD, the diffusion MRI-detected alterations within the LC and its tracts to the DLPFC and the M1 were associated with local noradrenergic neuronal loss within the LC, rather than noradrenergic changes in the cortex.

Contextual memory engrams, and the neuromodulatory influence of the locus coeruleus

Here, we review the basis of contextual memory at a conceptual and cellular level. We begin with an overview of the philosophical foundations of traversing space, followed by theories covering the material bases of contextual representations in the hippocampus (engrams), exploring functional characteristics of the cells and subfields within. Next, we explore various methodological approaches for investigating contextual memory engrams, emphasizing plasticity mechanisms. This leads us to discuss the role of neuromodulatory inputs in governing these dynamic changes. We then outline a recent hypothesis involving noradrenergic and dopaminergic projections from the locus coeruleus (LC) to different subregions of the hippocampus, in sculpting contextual representations, giving a brief description of the neuroanatomical and physiological properties of the LC. Finally, we examine how activity in the LC influences contextual memory processes through synaptic plasticity mechanisms to alter hippocampal engrams. Overall, we find that phasic activation of the LC plays an important role in promoting new learning and altering mnemonic processes at the behavioral and cellular level through the neuromodulatory influence of NE/DA in the hippocampus. These findings may provide insight into mechanisms of hippocampal remapping and memory updating, memory processes that are potentially dysregulated in certain psychiatric and neurodegenerative disorders.

Epinephrine injected into the basolateral amygdala affects anxiety-like behavior and memory performance in stressed rats

The amygdala is known to mediate in moderating the impacts of emotional arousal and stress on memory. According to a growing body of research, the basolateral amygdala (BLA) is an important locus for integrating neuromodulator influences coordinating the retrieval of different types of memory and anxiety. This study aimed to investigate how the epinephrine in the BLA affects hippocampal fear memory, anxiety, and plasticity in control and stressed rats. For four days, male Wistar rats were exposed to electrical foot-shock stress. Animals received bilateral micro-injections of either vehicle or epinephrine (1 µg/side) into the BLA over four days (5 min before foot-shock stress). Behavioral characteristics (fear memory and anxiety-like behavior), histological features and electrophysiological parameters were investigated. Epinephrine injection into BLA resulted in a considerable impairment of fear memory in stressed rats. On the other hand, epinephrine effectively affected fear memory in control rats. Under stress conditions, epinephrine in the BLA is thought to increase anxiety-like behaviors. Treatment with epinephrine significantly increases the slope of fEPSP in the CA1 region of the hippocampus in the control and stress rats. In different groups, foot-shock stress had no effect on the apical and basal dendritic length in the CA1 region of the hippocampus. These results indicate that activating adrenergic receptors diminish fear memory and anxiety-like behaviors in the foot-shock stress, which this impact is independent of CA1 long-term potentiation induction.

Daily biofeedback to modulate heart rate oscillations affects structural volume in hippocampal subregions targeted by the locus coeruleus in older adults but not younger adults

Using data from a clinical trial, we tested the hypothesis that daily sessions modulating heart rate oscillations affect older adults' volume of a region-of-interest (ROI) comprised of adjacent hippocampal subregions with relatively strong locus coeruleus (LC) noradrenergic input. Younger and older adults were randomly assigned to one of two daily biofeedback practices for 5 weeks: (1) engage in slow-paced breathing to increase the amplitude of oscillations in heart rate at their breathing frequency (Osc+); (2) engage in self-selected strategies to decrease heart rate oscillations (Osc-). The interventions did not significantly affect younger adults' hippocampal volume. Among older adults, the two conditions affected volume in the LC-targeted hippocampal ROI differentially as reflected in a significant condition × time-point interaction on ROI volume. These condition differences were driven by opposing changes in the two conditions (increased volume in Osc+ and decreased volume in Osc-) and were mediated by the degree of heart rate oscillation during training sessions.

Shaping Memories Via Stress: A Synaptic Engram Perspective

Stress modulates the activity of various memory systems and can thereby guide behavioral interaction with the environment in an adaptive or maladaptive manner. At the cellular level, a large body of evidence indicates that (nor)adrenaline and glucocorticoid release induced by acute stress exposure affects synapse function and synaptic plasticity, which are critical substrates for learning and memory. Recent evidence suggests that memories are supported in the brain by sparsely distributed neurons within networks, termed engram cell ensembles. While the physiological and molecular effects of stress on the synapse are increasingly well characterized, how these synaptic modifications shape the multiscale dynamics of engram cell ensembles is still poorly understood. In this review, we discuss and integrate recent information on how acute stress affects synapse function and how this may alter engram cell ensembles and their synaptic connectivity to shape memory strength and memory precision. We provide a mechanistic framework of a synaptic engram under stress and put forward outstanding questions that address knowledge gaps in our understanding of the mechanisms that underlie stress-induced memory modulation.

Declining locus coeruleus-dopaminergic and noradrenergic modulation of long-term memory in aging and Alzheimer's disease

Memory is essential in defining our identity by guiding behavior based on past experiences. However, aging leads to declining memory, disrupting older adult's lives. Memories are encoded through experience-dependent modifications of synaptic strength, which are regulated by the catecholamines dopamine and noradrenaline. While cognitive aging research demonstrates how dopaminergic neuromodulation from the substantia nigra-ventral tegmental area regulates hippocampal synaptic plasticity and memory, recent findings indicate that the noradrenergic locus coeruleus sends denser inputs to the hippocampus. The locus coeruleus produces dopamine as biosynthetic precursor of noradrenaline, and releases both to modulate hippocampal plasticity and memory. Crucially, the locus coeruleus is also the first site to accumulate Alzheimer's-related abnormal tau and severely degenerates with disease development. New in-vivo assessments of locus coeruleus integrity reveal associations with Alzheimer's markers and late-life memory impairments, which likely stem from impaired dopaminergic and noradrenergic neurotransmission. Bridging research across species, the reviewed findings suggest that degeneration of the locus coeruleus results in deficient dopaminergic and noradrenergic modulation of hippocampal plasticity and thus memory decline.

Attention-dependent coupling with forebrain and brainstem neuromodulatory nuclei changes across the lifespan

Attentional states continuously reflect the predictability and uncertainty in one's environment having important consequences for learning and memory. Beyond well known cortical contributions, rapid shifts in attention are hypothesized to also originate from deep nuclei, such as the basal forebrain (BF) and locus coeruleus (LC) neuromodulatory systems. These systems are also the first to change with aging. Here we characterized the interplay between these systems and their regulation of afferent targets - the hippocampus (HPC) and posterior cingulate cortex (PCC) - across the lifespan. To examine the role of attentional salience on task-dependent functional connectivity, we used a target-distractor go/no go task presented during functional MRI. In younger adults, BF coupling with the HPC, and LC coupling with the PCC, increased with behavioral relevance (targets vs distractors). Although the strength and presence of significant regional coupling changed in middle age, the most striking change in network connectivity was in old age, such that in older adults BF and LC coupling with their cortical afferents was largely absent and replaced by stronger interconnectivity between LC-BF nuclei. Overall rapid changes in attention related to behavioral relevance revealed distinct roles of subcortical neuromodulatory systems. The pronounced changes in functional network architecture across the lifespan suggest a decrease in these distinct roles, with deafferentation of cholinergic and noradrenergic systems associated with a shift towards mutual support during attention guided to external stimuli.

Effects and safety of vagus nerve stimulation on upper limb function in patients with stroke: a systematic review and meta-analysis

Vagus nerve stimulation (VNS) is used to deliver electric current to stimulate the vagus nerve. The aim of this study is to carry out a systematic review and meta-analysis to determine its effects on motor function in patients with stroke. PubMED, Embase, Web of Science (WoS), and Scopus were searched. Data on time since stroke, and mean scores and standard deviation on outcomes such as level of impairment and motor function were extracted. The results showed that invasive (MD 2.66, 95% CI 1.19-4.13, P = 0.0004) and non-invasive (MD 24.16, 95% CI 23.56-24.75, P = 0.00001) VNS are superior at improving level of motor impairment than the control post intervention and at follow-up respectively. Similarly, VNS improved motor function post intervention (MD 0.28, 95% CI 0.15-0.41, P < 0.0001); and there was no significant difference in adverse events between invasive VNS and control (OR 2.15, 95% CI 0.97-4.74, P = 0.06), and between non-invasive VNS and control (OR 4.54, 95% CI 0.48-42.97, P = 0.19). VNS can be used to improve motor function in patients with stroke.

Daily biofeedback to modulate heart rate oscillations affects structural volume in hippocampal subregions targeted by the locus coeruleus in older adults but not younger adults

Using data from a clinical trial, we tested the hypothesis that daily sessions modulating heart rate oscillations affect older adults' volume of a region-of-interest (ROI) comprised of adjacent hippocampal subregions with relatively strong locus coeruleus (LC) noradrenergic input. Younger and older adults were randomly assigned to one of two daily biofeedback practices for 5 weeks: 1) engage in slow-paced breathing to increase the amplitude of oscillations in heart rate at their breathing frequency (Osc+); 2) engage in self-selected strategies to decrease heart rate oscillations (Osc-). The interventions did not significantly affect younger adults' hippocampal volume. Among older adults, the two conditions affected volume in the LC-targeted hippocampal ROI differentially as reflected in a significant condition x time-point interaction on ROI volume. These condition differences were driven by opposing changes in the two conditions (increased volume in Osc+ and decreased volume in Osc-) and were mediated by the degree of heart rate oscillation during training sessions.

The integrity of dopaminergic and noradrenergic brain regions is associated with different aspects of late-life memory performance

Changes in dopaminergic neuromodulation play a key role in adult memory decline. Recent research has also implicated noradrenaline in shaping late-life memory. However, it is unclear whether these two neuromodulators have distinct roles in age-related cognitive changes. Here, combining longitudinal MRI of the dopaminergic substantia nigra-ventral tegmental area (SN-VTA) and noradrenergic locus coeruleus (LC) in younger (n = 69) and older (n = 251) adults, we found that dopaminergic and noradrenergic integrity are differentially associated with memory performance. While LC integrity was related to better episodic memory across several tasks, SN-VTA integrity was linked to working memory. Longitudinally, we found that older age was associated with more negative change in SN-VTA and LC integrity. Notably, changes in LC integrity reliably predicted future episodic memory. These differential associations of dopaminergic and noradrenergic nuclei with late-life cognitive decline have potential clinical utility, given their degeneration in several age-associated diseases.

The psychosis risk factor RBM12 encodes a novel repressor of GPCR/cAMP signal transduction

RBM12 is a high-penetrance risk factor for familial schizophrenia and psychosis, yet its precise cellular functions and the pathways to which it belongs are not known. We utilize two complementary models, HEK293 cells and human iPSC-derived neurons, and delineate RBM12 as a novel repressor of the G protein-coupled receptor/cAMP/PKA (GPCR/cAMP/PKA) signaling axis. We establish that loss of RBM12 leads to hyperactive cAMP production and increased PKA activity as well as altered neuronal transcriptional responses to GPCR stimulation. Notably, the cAMP and transcriptional signaling steps are subject to discrete RBM12-dependent regulation. We further demonstrate that the two RBM12 truncating variants linked to familial psychosis impact this interplay, as the mutants fail to rescue GPCR/cAMP signaling hyperactivity in cells depleted of RBM12. Lastly, we present a mechanism underlying the impaired signaling phenotypes. In agreement with its activity as an RNA-binding protein, loss of RBM12 leads to altered gene expression, including that of multiple effectors of established significance within the receptor pathway. Specifically, the abundance of adenylyl cyclases, phosphodiesterase isoforms, and PKA regulatory and catalytic subunits is impacted by RBM12 depletion. We note that these expression changes are fully consistent with the entire gamut of hyperactive signaling outputs. In summary, the current study identifies a previously unappreciated role for RBM12 in the context of the GPCR-cAMP pathway that could be explored further as a tentative molecular mechanism underlying the functions of this factor in neuronal physiology and pathophysiology.

Fingerprinting heterocellular β-adrenoceptor functional expression in the brain using agonist activity profiles

Noradrenergic projections from the brainstem locus coeruleus drive arousal, attentiveness, mood, and memory, but specific adrenoceptor (AR) function across the varied brain cell types has not been extensively characterized, especially with agonists. This study reports a pharmacological analysis of brain AR function, offering insights for innovative therapeutic interventions that might serve to compensate for locus coeruleus decline, known to develop in the earliest phases of neurodegenerative diseases. First, β-AR agonist activities were measured in recombinant cell systems and compared with those of isoprenaline to generate Δlog(Emax/EC50) values, system-independent metrics of agonist activity, that, in turn, provide receptor subtype fingerprints. These fingerprints were then used to assess receptor subtype expression across human brain cell systems and compared with Δlog(Emax/EC50) values arising from β-arrestin activation or measurements of cAMP response desensitization to assess the possibility of ligand bias among β-AR agonists. Agonist activity profiles were confirmed to be system-independent and, in particular, revealed β2-AR functional expression across several human brain cell types. Broad β2-AR function observed is consistent with noradrenergic tone arising from the locus coeruleus exerting heterocellular neuroexcitatory and homeostatic influence. Notably, Δlog(Emax/EC50) measurements suggest that tested β-AR agonists do not show ligand bias as it pertains to homologous receptor desensitization in the system examined. Δlog(Emax/EC50) agonist fingerprinting is a powerful means of assessing receptor subtype expression regardless of receptor expression levels or assay readout, and the method may be applicable to future use for novel ligands and tissues expressing any receptor with available reference agonists.

Noradrenergic and cholinergic systems take centre stage in neuropsychiatric diseases of ageing

Noradrenergic and cholinergic systems are among the most vulnerable brain systems in neuropsychiatric diseases of ageing, including Alzheimer's disease, Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. As these systems fail, they contribute directly to many of the characteristic cognitive and psychiatric symptoms. However, their contribution to symptoms is not sufficiently understood, and pharmacological interventions targeting noradrenergic and cholinergic systems have met with mixed success. Part of the challenge is the complex neurobiology of these systems, operating across multiple timescales, and with non-linear changes across the adult lifespan and disease course. We address these challenges in a detailed review of the noradrenergic and cholinergic systems, outlining their roles in cognition and behaviour, and how they influence neuropsychiatric symptoms in disease. By bridging across levels of analysis, we highlight opportunities for improving drug therapies and for pursuing personalised medicine strategies.

Functional and phosphoproteomic analysis of β-adrenergic receptor signaling at excitatory synapses in the CA1 region of the ventral hippocampus

Activation of β-adrenergic receptors (β-ARs) not only enhances learning and memory but also facilitates the induction of long-term potentiation (LTP), a form of synaptic plasticity involved in memory formation. To identify the mechanisms underlying β-AR-dependent forms of LTP we examined the effects of the β-AR agonist isoproterenol on LTP induction at excitatory synapses onto CA1 pyramidal cells in the ventral hippocampus. LTP induction at these synapses is inhibited by activation of SK-type K⁺ channels, suggesting that β-AR activation might facilitate LTP induction by inhibiting SK channels. However, although the SK channel blocker apamin enhanced LTP induction, it did not fully mimic the effects of isoproterenol. We therefore searched for potential alternative mechanisms using liquid chromatography-tandem mass spectrometry to determine how β-AR activation regulates phosphorylation of postsynaptic density (PSD) proteins. Strikingly, β-AR activation regulated hundreds of phosphorylation sites in PSD proteins that have diverse roles in dendritic spine structure and function. Moreover, within the core scaffold machinery of the PSD, β-AR activation increased phosphorylation at several sites previously shown to be phosphorylated after LTP induction. Together, our results suggest that β-AR activation recruits a diverse set of signaling pathways that likely act in a concerted fashion to regulate LTP induction.

Compartmentalised cAMP signalling in the primary cilium

cAMP is a universal second messenger that relies on precise spatio-temporal regulation to control varied, and often opposing, cellular functions. This is achieved via selective activation of effectors embedded in multiprotein complexes, or signalosomes, that reside at distinct subcellular locations. cAMP is also one of many pathways known to operate within the primary cilium. Dysfunction of ciliary signaling leads to a class of diseases known as ciliopathies. In Autosomal Dominant Polycystic Kidney Disease (ADPKD), a ciliopathy characterized by the formation of fluid-filled kidney cysts, upregulation of cAMP signaling is known to drive cystogenesis. For decades it has been debated whether the primary cilium is an independent cAMP sub-compartment, or whether it shares a diffusible pool of cAMP with the cell body. Recent studies now suggest it is a specific pool of cAMP generated in the cilium that propels cyst formation in ADPKD, supporting the notion that this antenna-like organelle is a compartment within which cAMP signaling occurs independently from cAMP signaling in the bulk cytosol. Here we present examples of cAMP function in the cilium which suggest this mysterious organelle is home to more than one cAMP signalosome. We review evidence that ciliary membrane localization of G-Protein Coupled Receptors (GPCRs) determines their downstream function and discuss how optogenetic tools have contributed to establish that cAMP generated in the primary cilium can drive cystogenesis.

Neuronal activity in the human amygdala and hippocampus enhances emotional memory encoding

Emotional events comprise our strongest and most valuable memories. Here we examined how the brain prioritizes emotional information for storage using direct brain recording and deep brain stimulation. First, 148 participants undergoing intracranial electroencephalographic (iEEG) recording performed an episodic memory task. Participants were most successful at remembering emotionally arousing stimuli. High-frequency activity (HFA), a correlate of neuronal spiking activity, increased in both the hippocampus and the amygdala when participants successfully encoded emotional stimuli. Next, in a subset of participants (N = 19), we show that applying high-frequency electrical stimulation to the hippocampus selectively diminished memory for emotional stimuli and specifically decreased HFA. Finally, we show that individuals with depression (N = 19) also exhibit diminished emotion-mediated memory and HFA. By demonstrating how direct stimulation and symptoms of depression unlink HFA, emotion and memory, we show the causal and translational potential of neural activity in the amygdalohippocampal circuit for prioritizing emotionally arousing memories.

Whole-brain afferent input mapping to functionally distinct brainstem noradrenaline cell types

The locus coeruleus (LC) is a small region in the pons and the main source of noradrenaline (NA) to the forebrain. While traditional models suggested that all LC-NA neurons project indiscriminately throughout the brain, accumulating evidence indicates that these cells can be heterogeneous based on their anatomical connectivity and behavioral functionality and exhibit distinct coding modes. How LC-NA neuronal subpopulations are endowed with unique functional properties is unclear. Here, we used a viral-genetic approach for mapping anatomical connectivity at different levels of organization based on inputs and outputs of defined cell classes. Specifically, we studied the whole-brain afferent inputs onto two functionally distinct LC-NA neuronal subpopulations which project to amygdala or medial prefrontal cortex (mPFC). We found that the global input distribution is similar for both LC-NA neuronal subpopulations. However, finer analysis demonstrated important differences in inputs from specific brain regions. Moreover, sex related differences were apparent, but only in inputs to amygdala-projecting LC-NA neurons. These findings reveal a cell type and sex specific afferent input organization which could allow for context dependent and target specific control of NA outflow to forebrain structures involved in emotional control and decision making.

Exercise may alleviate age-related spatial memory impairment by rescuing β-adrenergic receptor dysregulation via both G protein-dependent and β-arrestin-dependent mechanisms in rat hippocampus

Hippocampal-dependent memory abilities including spatial memory decline with age. Exercise improves memory decline in aging brain, but, the precise mechanisms are still unknown. Learning and memory are recently hypothesized to be mediated by a β-arrestin (βArr)-dependent β-adrenergic pathway. Hence, we examined the effect of 8 weeks of treadmill exercise on hippocampal expression of β-adrenergic receptors (β-ARs; members of the G protein-coupled receptor family), and βArrs as well as spatial learning and memory in aged male rats to determine whether β-AR/βArr pathway could be involved in age-related memory decline. A total of 24 young (3-month-old) and aged (18-month-old) male Wistar rats were divided into young control, aged sedentary, and aged + exercise (n = 8 for each). Western blot for β1- and β2-ARs as well as βArr1 and βArr2 was performed. Spatial learning and memory were evaluated with the Morris water maze. The results showed significant up-regulation of β1-ARs as well as significant down-regulation of β2-AR and βArrs (βArr1 and βArr2) in the hippocampus of aged rats. Spatial memory, but not spatial learning, was impaired in aging, and treadmill exercise improved it. Notably, the improvement in spatial memory was accompanied by amelioration of β-ARs dysregulation and increase in βArr2 levels after exercise. There was a negative association between the expression of βArr2 and β1-AR, but not β2-AR, such that an increase in βArr2 by exercise was associated with reduced β1-AR expression, suggesting βArr2 may contribute to posttranslational down-regulation of β1-ARs. These data suggest that both G protein-dependent and β-arrestin-dependent β-AR pathways may regulate spatial learning and memory in aging brain.

Characterisation of the neural basis underlying appetitive extinction & renewal in Cacna1c rats

Recent studies have revealed impairments in Cacna1c ± heterozygous animals (a gene that encodes the Cav 1.2 L-type voltage-gated calcium channels and is implicated in risk for multiple neuropsychiatric disorders) in aversive forms of learning, such as latent inhibition, reversal learning or context discrimination. However, the role of Cav 1.2 L-type voltage-gated calcium channels in extinction of appetitive associations remains under-investigated. Here, we used an appetitive Pavlovian conditioning task and evaluated extinction learning (EL) with a change of context from that of training and test (ABA) and without such a change (AAA) in Cacna1c ± male rats versus their wild-type (WT) littermates. In addition, we used fluorescence in situ hybridization of somatic immediate early genes (IEGs) Arc and Homer1a expression to scrutinize associated changes in the medial prefrontal cortex and the amygdala. Cacna1c ± animals successfully adapt their responses by engaging in appetitive EL and renewal. However, the regional IEG expression profile changed. For the EL occurring in the same context, Cacna1c ± animals presented higher IEG expression in the infralimbic cortex and the central amygdala than controls. The prelimbic region presented a larger neural ensemble in Cacna1c ± than WT animals, co-labelled for the time window of EL in the original context and prolonged exposure to the unrewarded context. With a context change, the Cacna1c ± infralimbic region displayed higher IEG expression during renewal than controls. Taken together, our findings provide novel evidence of distinct brain activation patterns occurring in Cacna1c ± rats after appetitive extinction and renewal despite preserved behavioral responses. This article is part of the Special Issue on "L-type calcium channel mechanisms in neuropsychiatric disorders".

Arrestin-dependent nuclear export of phosphodiesterase 4D promotes GPCR-induced nuclear cAMP signaling required for learning and memory

G protein-coupled receptors (GPCRs) promote the expression of immediate early genes required for learning and memory. Here, we showed that β2-adrenergic receptor (β2AR) stimulation induced the nuclear export of phosphodiesterase 4D5 (PDE4D5), an enzyme that degrades the second messenger cAMP, to enable memory consolidation. We demonstrated that the endocytosis of β2AR phosphorylated by GPCR kinases (GRKs) mediated arrestin3-dependent nuclear export of PDE4D5, which was critical for promoting nuclear cAMP signaling and gene expression in hippocampal neurons for memory consolidation. Inhibition of the arrestin3-PDE4D5 association prevented β2AR-induced nuclear cAMP signaling without affecting receptor endocytosis. Direct PDE4 inhibition rescued β2AR-induced nuclear cAMP signaling and ameliorated memory deficits in mice expressing a form of the β2AR that could not be phosphorylated by GRKs. These data reveal how β2AR phosphorylated by endosomal GRK promotes the nuclear export of PDE4D5, leading to nuclear cAMP signaling, changes in gene expression, and memory consolidation. This study also highlights the translocation of PDEs as a mechanism to promote cAMP signaling in specific subcellular locations downstream of GPCR activation.

Altered norepinephrine transmission after spatial learning impairs sleep-mediated memory consolidation in rats

The therapeutic use of noradrenergic drugs makes the evaluation of their effects on cognition of high priority. Norepinephrine (NE) is an important neuromodulator for a variety of cognitive processes and may importantly contribute to sleep-mediated memory consolidation. The NE transmission fluctuates with the behavioral and/or brain state and influences associated neural activity. Here, we assessed the effects of altered NE transmission after learning of a hippocampal-dependent task on neural activity and spatial memory in adult male rats. We administered clonidine (0.05 mg/kg, i.p.; n = 12 rats) or propranolol (10 mg/kg, i.p.; n = 11) after each of seven daily learning sessions on an 8-arm radial maze. Compared to the saline group (n = 9), the drug-treated rats showed lower learning rates. To assess the effects of drugs on cortical and hippocampal activity, we recorded prefrontal EEG and local field potentials from the CA1 subfield of the dorsal hippocampus for 2 h after each learning session or drug administration. Both drugs significantly reduced the number of hippocampal ripples for at least 2 h. An EEG-based sleep scoring revealed that clonidine made the sleep onset faster while prolonging quiet wakefulness. Propranolol increased active wakefulness at the expense of non-rapid eye movement (NREM) sleep. Clonidine reduced the occurrence of slow oscillations (SO) and sleep spindles during NREM sleep and altered the temporal coupling between SO and sleep spindles. Thus, pharmacological alteration of NE transmission produced a suboptimal brain state for memory consolidation. Our results suggest that the post-learning NE contributes to the efficiency of hippocampal-cortical communication underlying memory consolidation.

The psychosis risk factor RBM12 encodes a novel repressor of GPCR/cAMP signal transduction

RBM12 is a high-penetrance risk factor for familial schizophrenia and psychosis, yet its precise cellular functions and the pathways to which it belongs are not known. We utilize two complementary models, HEK293 cells and human iPSC-derived neurons, and delineate RBM12 as a novel repressor of the G protein-coupled receptor/cyclic AMP/protein kinase A (GPCR/cAMP/PKA) signaling axis. We establish that loss of RBM12 leads to hyperactive cAMP production and increased PKA activity as well as altered neuronal transcriptional responses to GPCR stimulation. Notably, the cAMP and transcriptional signaling steps are subject to discrete RBM12-dependent regulation. We further demonstrate that the two RBM12 truncating variants linked to familial psychosis impact this interplay, as the mutants fail to rescue GPCR/cAMP signaling hyperactivity in cells depleted of RBM12. Lastly, we present a mechanism underlying the impaired signaling phenotypes. In agreement with its activity as an RNA-binding protein, loss of RBM12 leads to altered gene expression, including that of multiple effectors of established significance within the receptor pathway. Specifically, the abundance of adenylyl cyclases, phosphodiesterase isoforms, and PKA regulatory and catalytic subunits is impacted by RBM12 depletion. We note that these expression changes are fully consistent with the entire gamut of hyperactive signaling outputs. In summary, the current study identifies a previously unappreciated role for RBM12 in the context of the GPCR/cAMP pathway that could be explored further as a tentative molecular mechanism underlying the functions of this factor in neuronal physiology and pathophysiology.

Peripheral CB1 receptor blockade acts as a memory enhancer through a noradrenergic mechanism

Peripheral inputs continuously shape brain function and can influence memory acquisition, but the underlying mechanisms have not been fully understood. Cannabinoid type-1 receptor (CB1R) is a well-recognized player in memory performance, and its systemic modulation significantly influences memory function. By assessing low arousal/non-emotional recognition memory in mice, we found a relevant role of peripheral CB1R in memory persistence. Indeed, the peripherally-restricted CB1R specific antagonist AM6545 showed significant mnemonic effects that were occluded in adrenalectomized mice, and after peripheral adrenergic blockade. AM6545 also transiently impaired contextual fear memory extinction. Vagus nerve chemogenetic inhibition reduced AM6545-induced mnemonic effect. Genetic CB1R deletion in dopamine β-hydroxylase-expressing cells enhanced recognition memory persistence. These observations support a role of peripheral CB1R modulating adrenergic tone relevant for cognition. Furthermore, AM6545 acutely improved brain connectivity and enhanced extracellular hippocampal norepinephrine. In agreement, intra-hippocampal β-adrenergic blockade prevented AM6545 mnemonic effects. Altogether, we disclose a novel CB1R-dependent peripheral mechanism with implications relevant for lengthening the duration of non-emotional memory.

Aβ oligomers from human brain impair mossy fiber LTP in CA3 of hippocampus, but activating cAMP-PKA and cGMP-PKG prevents this

Early cognitive impairment in Alzheimer's disease may result in part from synaptic dysfunction caused by the accumulation oligomeric assemblies of amyloid β-protein (Aβ). Changes in hippocampal function seem critical for cognitive impairment in early Alzheimer's disease (AD). Diffusible oligomers of Aβ (oAβ) have been shown to block canonical long-term potentiation (LTP) in the CA1 area of hippocampus, but whether there is also a direct effect of oAβ on synaptic transmission and plasticity at synapses between mossy fibers (axons) from the dentate gyrus granule cells and CA3 pyramidal neurons (mf-CA3 synapses) is unknown. Studies in APP transgenic mice have suggested an age-dependent impairment of mossy fiber LTP. Here we report that although endogenous AD brain-derived soluble oAβ had no effect on mossy-fiber basal transmission, it strongly impaired paired-pulse facilitation in the mossy fiber pathway and presynaptic mossy fiber LTP (mf-LTP). Selective activation of both β1 and β2 adrenergic receptors and their downstream cAMP/PKA signaling pathway prevented oAβ-mediated inhibition of mf-LTP. Unexpectedly, activation of the cGMP/PKG signaling pathway also prevented oAβ-impaired mf-LTP. Our results reveal certain specific pharmacological targets to ameliorate human oAβ-mediated impairment at the mf-CA3 synapse.

Distribution and inter-regional relationship of amyloid-beta plaque deposition in a 5xFAD mouse model of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia. Although previous studies have selectively investigated the localization of amyloid-beta (Aβ) deposition in certain brain regions, a comprehensive characterization of the rostro-caudal distribution of Aβ plaques in the brain and their inter-regional correlation remain unexplored. Our results demonstrated remarkable working and spatial memory deficits in 9-month-old 5xFAD mice compared to wildtype mice. High Aβ plaque load was detected in the somatosensory cortex, piriform cortex, thalamus, and dorsal/ventral hippocampus; moderate levels of Aβ plaques were observed in the motor cortex, orbital cortex, visual cortex, and retrosplenial dysgranular cortex; and low levels of Aβ plaques were located in the amygdala, and the cerebellum; but no Aβ plaques were found in the hypothalamus, raphe nuclei, vestibular nucleus, and cuneate nucleus. Interestingly, the deposition of Aβ plaques was positively associated with brain inter-regions including the prefrontal cortex, somatosensory cortex, medial amygdala, thalamus, and the hippocampus. In conclusion, this study provides a comprehensive morphological profile of Aβ deposition in the brain and its inter-regional correlation. This suggests an association between Aβ plaque deposition and specific brain regions in AD pathogenesis.

Propranolol Administration Modulates Neural Activity in the Hippocampal Hilus During Fear Retrieval

Altered fear learning is a strong behavioral component of anxiety disorders such as post-traumatic stress disorder (PTSD). Recent efforts have attempted to combine exposure therapies with drugs that target fear memory retrieval and memory reconsolidation, in order to improve treatment efficacy. The noradrenergic (NA) signaling system is of particular interest, due to its role in regulating the stress response and its involvement in fear and learning processes. Importantly, propranolol (P), a non-selective β-adrenergic antagonist, has shown the potential in decreasing exaggerated fear in both humans and animal models. In a previous study, we utilized an activity-dependent tagging murine model to determine the neural mechanisms by which propranolol attenuates learned fear. We found that propranolol acutely decreased memory trace reactivation specifically in the dorsal dentate gyrus (dDG), but not in CA3 or CA1. Here, we extended our previous study by investigating whether propranolol additionally altered activity in the hilus, a polymorphic layer that consists of neurons, mossy cells, and GABAergic interneurons. We found that propranolol acutely reduced overall hilar activity in both the dorsal and ventral hilus. Moreover, we report that propranolol significantly altered the activity of parvalbumin (PV)⁺ cells in the ventral (vDG), but not dorsal DG (dDG). Together, these results suggest that a β-adrenergic blockade may affect the activity of excitatory and inhibitory cell types in the hilar layer of the DG, and that these alterations may contribute to manipulating fear memory traces.

The Neuromodulatory Role of the Noradrenergic and Cholinergic Systems and Their Interplay in Cognitive Functions: A Focused Review

The noradrenergic and cholinergic modulation of functionally distinct regions of the brain has become one of the primary organizational principles behind understanding the contribution of each system to the diversity of neural computation in the central nervous system. Decades of work has shown that a diverse family of receptors, stratified across different brain regions, and circuit-specific afferent and efferent projections play a critical role in helping such widespread neuromodulatory systems obtain substantial heterogeneity in neural information processing. This review briefly discusses the anatomical layout of both the noradrenergic and cholinergic systems, as well as the types and distributions of relevant receptors for each system. Previous work characterizing the direct and indirect interaction between these two systems is discussed, especially in the context of higher order cognitive functions such as attention, learning, and the decision-making process. Though a substantial amount of work has been done to characterize the role of each neuromodulator, a cohesive understanding of the region-specific cooperation of these two systems is not yet fully realized. For the field to progress, new experiments will need to be conducted that capitalize on the modular subdivisions of the brain and systematically explore the role of norepinephrine and acetylcholine in each of these subunits and across the full range of receptors expressed in different cell types in these regions.

Cognitive Impairment, Sleep Disturbance, and Depression in Women with Silicone Breast Implants: Association with Autoantibodies against Autonomic Nervous System Receptors

Background: Silicone breast implants (SBIs) has been shown to be associated with an increased risk of autoimmune diseases. In the current study, we aimed to explore the potential association between circulating autoantibodies against the autonomic nervous system and cognitive impairment, memory deficit, and depressive symptoms reported by women with SBIs. Methods: ELISA assays were used to quantify anti-adrenergic receptors (α1, α2, β1, β2), anti-muscarinic receptors (M1-M5), anti-endothelin receptor type A, and anti-angiotensin II type 1 receptor titers in the sera of 93 symptomatic female subjects with SBIs and 36 age-matched healthy female controls. Results: A significant difference was detected in the level of autoantibodies against the autonomic nervous system receptors in women with SBIs who reported memory impairment, cognitive impairment, and sleep disturbance as compared with both women with SBIs who did not complain of these symptoms or with healthy individuals without SBIs. Conclusions: Clinical symptoms such as depression, cognitive impairment, and sleep disturbances were found to be associated with dysregulation of the levels of circulating autoantibodies targeting the autonomous nervous system receptors in women with SBIs. These autoantibodies may have diagnostic significance in diseases associated with breast implants.

Frequency-dependency of the involvement of dopamine D1/D5 and beta-adrenergic receptors in hippocampal LTD triggered by locus coeruleus stimulation

Patterned stimulation of the locus coeruleus (LC, 100 Hz), in conjunction with test-pulse stimulation of hippocampal afferents, results in input-specific long-term depression (LTD) of synaptic plasticity in the hippocampus. Effects are long-lasting and have been described in Schaffer-collateral-CA1 and perforant path-dentate gyrus synapses in behaving rats. To what extent LC-mediated hippocampal LTD (LC-LTD) is frequency-dependent is unclear. Here, we report that LC-LTD can be triggered by LC stimulation with 2 and 5 Hz akin to tonic activity, 10 Hz equivalent to phasic activity, and 100 Hz akin to high-phasic activity in the dentate gyrus (DG) of freely behaving rats. LC-LTD at both 2 and 100 Hz can be significantly prevented by an NMDA receptor antagonist. The LC releases both noradrenaline (NA) and dopamine (DA) from its hippocampal terminals and may also trigger hippocampal DA release by activating the ventral tegmental area (VTA). Unclear is whether both neurotransmitters contribute equally to hippocampal LTD triggered by LC stimulation (LC-LTD). Both DA D1/D5 receptors (D1/D5R) and beta-adrenergic receptors (β-AR) are critically required for hippocampal LTD that is induced by patterned stimulation of hippocampal afferents, or is facilitated by spatial learning. We, therefore, explored to what extent these receptor subtypes mediate frequency-dependent hippocampal LC-LTD. LC-LTD elicited by 2, 5, and 10 Hz stimulation was unaffected by antagonism of β-AR with propranolol, whereas LC-LTD induced by these frequencies was prevented by D1/D5R-antagonism using SCH23390. By contrast, LC-LTD evoked at 100 Hz was prevented by β-AR-antagonism and only mildly affected by D1/D5R-antagonism. Taken together, these findings support that LC-LTD can be triggered by LC activity at a wide range of frequencies. Furthermore, the contribution of D1/D5R and β-AR to hippocampal LTD that is triggered by LC activity is frequency-dependent and suggests that D1/D5R may be involved in LC-mediated hippocampal tonus.

The Intriguing Contribution of Hippocampal Long-Term Depression to Spatial Learning and Long-Term Memory

Long-term potentiation (LTP) and long-term depression (LTD) comprise the principal cellular mechanisms that fulfill established criteria for the physiological correlates of learning and memory. Traditionally LTP, that increases synaptic weights, has been ascribed a prominent role in learning and memory whereas LTD, that decreases them, has often been relegated to the category of “counterpart to LTP” that serves to prevent saturation of synapses. In contradiction of these assumptions, studies over the last several years have provided functional evidence for distinct roles of LTD in specific aspects of hippocampus-dependent associative learning and information encoding. Furthermore, evidence of the experience-dependent “pruning” of excitatory synapses, the majority of which are located on dendritic spines, by means of LTD has been provided. In addition, reports exist of the temporal and physical restriction of LTP in dendritic compartments by means of LTD. Here, we discuss the role of LTD and LTP in experience-dependent information encoding based on empirical evidence derived from conjoint behavioral and electrophysiological studies conducted in behaving rodents. We pinpoint the close interrelation between structural modifications of dendritic spines and the occurrence of LTP and LTD. We report on findings that support that whereas LTP serves to acquire the general scheme of a spatial representation, LTD enables retention of content details. We argue that LTD contributes to learning by engaging in a functional interplay with LTP, rather than serving as its simple counterpart, or negator. We propose that similar spatial experiences that share elements of neuronal representations can be modified by means of LTD to enable pattern separation. Therewith, LTD plays a crucial role in the disambiguation of similar spatial representations and the prevention of generalization.

Sex specific effects of the fatty acid amide hydrolase inhibitor URB597 on memory and brain β2-adrenergic and D1-dopamine receptors

An increasing body of evidence shows significant sex differences in the mammalian brain in multiple behaviours and psychiatric and neurological diseases and as well as that the endocannabinoid system may differ between males and females. In this study we investigated sex differences in working, short-term and long-term memory and the expression of β₂-adrenergic and D1- and D2-receptors in the mPFC and hippocampus, brain regions that are involved in stress response and memory modulation in rats exposed to the chronic unpredictable stress (CUS) and the potential beneficial effects of the chronic fatty acid amide hydrolase inhibitor URB597 treatment. Chronically stressed male rats had an improvement of working memory, while stressed females showed very low object-recognition abilities. On the other hand, animals of both sexes exhibited long-term memory impairment. Our results showed that CUS decreased the expression of β₂-adrenoceptors in the mPFC and D1 receptors in the mPFC and hippocampus of male rats and decreased β₂-adrenoceptors and D1- receptors in the hippocampus of female. URB597 treatment had a positive effect on the short-term memory of stressed animals of both sexes whereas failed to restore long-term memory and did not affect the protein levels β2-adrenoceptors and D1 receptors in the hippocampus of CUS female rats. The present results support that endocannabinoids induced long-term memory and neurochemical alternations which are sex dependent, suggesting sex specific treatment strategies of mental disorders.

Bidirectional Regulation of Hippocampal Synaptic Plasticity and Modulation of Cumulative Spatial Memory by Dopamine D2-Like Receptors

Dopamine is a key factor in the enablement of cognition and hippocampal information processing. Its action in the hippocampus is mediated by D1/D5 and D2-like (D2, D3, D4) receptors. While D1/D5-receptors are well recognized as strong modulators of hippocampal synaptic plasticity and information storage, much less is known about the role of D2-like receptors (D2R) in these processes. Here, we explored to what extent D2R contribute to synaptic plasticity and cumulative spatial memory derived from semantic and episodic-like information storage. In freely behaving adult rats, we also assessed to what extent short and long-term forms of synaptic plasticity are influenced by pharmacological activation or blockade of D2R. Antagonism of D2R by means of intracerebral treatment with remoxipride, completely prevented the expression of both short-term (<1 h) and long-term potentiation (>4 h), as well as the expression of short-term depression (STD, <1 h) in the hippocampal CA1 region. Scrutiny of involvement of D2R in spatial learning revealed that D2R-antagonism prevented retention of a semantic spatial memory task, and also significantly impaired retention of recent spatiotemporal aspects of an episodic-like memory task. Taken together, these findings indicate that D2R are required for bidirectional synaptic plasticity in the hippocampal CA1 region. Furthermore, they are critically involved in enabling cumulative and episodic-like forms of spatial learning.

Consequences of Hyperphosphorylated Tau in the Locus Coeruleus on Behavior and Cognition in a Rat Model of Alzheimer's Disease

BACKGROUND

The locus coeruleus (LC) is one of the earliest brain regions to accumulate hyperphosphorylated tau, but a lack of animal models that recapitulate this pathology has hampered our understanding of its contributions to Alzheimer's disease (AD) pathophysiology.

OBJECTIVE

We previously reported that TgF344-AD rats, which overexpress mutant human amyloid precursor protein and presenilin-1, accumulate early endogenous hyperphosphorylated tau in the LC. Here, we used TgF344-AD rats and a wild-type (WT) human tau virus to interrogate the effects of endogenous hyperphosphorylated rat tau and human tau in the LC on AD-related neuropathology and behavior.

METHODS

Two-month-old TgF344-AD and WT rats received bilateral LC infusions of full-length WT human tau or mCherry control virus driven by the noradrenergic-specific PRSx8 promoter. Rats were subsequently assessed at 6 and 12 months for arousal (sleep latency), anxiety-like behavior (open field, elevated plus maze, novelty-suppressed feeding), passive coping (forced swim task), and learning and memory (Morris water maze and fear conditioning). Hippocampal microglia, astrocyte, and AD pathology were evaluated using immunohistochemistry.

RESULTS

In general, the effects of age were more pronounced than genotype or treatment; older rats displayed greater hippocampal pathology, took longer to fall asleep, had reduced locomotor activity, floated more, and had impaired cognition compared to younger animals. TgF344-AD rats showed increased anxiety-like behavior and impaired learning and memory. The tau virus had negligible influence on most measures.

CONCLUSION

Effects of hyperphosphorylated tau on AD-like neuropathology and behavioral symptoms were subtle. Further investigation of different forms of tau is warranted.

Vagus Nerve Stimulation as a Treatment for Fear and Anxiety in Individuals with Autism Spectrum Disorder

Anxiety disorders affect a large percentage of individuals who have an autism spectrum disorder (ASD). In children with ASD, excessive anxiety is also linked to gastrointestinal problems, self-injurious behaviors, and depressive symptoms. Exposure-based cognitive behavioral therapies are effective treatments for anxiety disorders in children with ASD, but high relapse rates indicate the need for additional treatment strategies. This perspective discusses evidence from preclinical research, which indicates that vagus nerve stimulation (VNS) paired with exposure to fear-provoking stimuli and situations could offer benefits as an adjuvant treatment for anxiety disorders that coexist with ASD. Vagus nerve stimulation is approved for use in the treatment of epilepsy, depression, and more recently as an adjuvant in rehabilitative training following stroke. In preclinical models, VNS shows promise in simultaneously enhancing consolidation of extinction memories and reducing anxiety. In this review, we will present potential mechanisms by which VNS could treat fear and anxiety in ASD. We also discuss potential uses of VNS to treat depression and epilepsy in the context of ASD, and noninvasive methods to stimulate the vagus nerve.

Blockade of beta adrenergic receptors protects the blood brain barrier and reduces systemic pathology caused by HIV-1 Nef protein

Combination antiretroviral therapy (cART) targets viral replication, but early viral protein production by astrocytes may still occur and contribute to the progression of HIV-1 associated neurocognitive disorders and secondary complications seen in patients receiving cART. In prior work with our model, astrocytic HIV-1 Nef expression exhibits neurotoxic effects leading to neurological damage, learning impairment, and immune upregulation that induces inflammation in the lungs and small intestine (SI). In this follow-up study, we focus on the sympathetic nervous system (SNS) as the important branch for peripheral inflammation resulting from astrocytic Nef expression. Male and female Sprague Dawley rats were infused with transfected astrocytes to produce Nef. The rats were divided in four groups: Nef, Nef + propranolol, propranolol and naïve. The beta-adrenergic blocker, propranolol, was administered for 3 consecutive days, starting one day prior to surgery. Two days after the surgery, the rats were sacrificed, and then blood, brain, small intestine (SI), and lung tissues were collected. Levels of IL-1β were higher in both male and female rats, and treatment with propranolol restored IL-1β to basal levels. We observed that Nef expression decreased staining of the tight junction protein claudin-5 in brain tissue while animals co-treated with propranolol restored claudin-5 expression. Lungs and SI of rats in the Nef group showed histological signs of damage including larger Peyer's Patches, increased tissue thickness, and infiltration of immune cells; these findings were abrogated by propranolol co-treatment. Results suggest that interruption of the beta adrenergic signaling reduces the peripheral organ inflammation caused after Nef expression in astrocytes of the brain.

Noradrenergic Profile of Hippocampal Formation Theta Rhythm in Anaesthetized Rats

The present study was undertaken to identify the noradrenergic receptors underlying the production of hippocampal formation (HPC) type 2 theta rhythm. The experiments were performed on urethanized rats wherein type 2 theta is the only rhythm present. In three independent stages of experiments, the effects of noradrenaline (NE) and selective noradrenergic α and β agonists and antagonists were tested. We indicate that the selective activation of three HPC noradrenergic receptors, α1, α2 and β1, induced a similar effect (i.e., inhibition) on type 2 theta rhythm. The remaining HPC β2 and β3 noradrenergic receptors do not seem to be directly involved in the pharmacological mechanism responsible for the suppression of theta rhythm in anaesthetized rats. Obtained results provide evidence for the suppressant effect of exogenous NE on HPC type 2 theta rhythm and show the crucial role of α1, α2 and β1 noradrenergic receptors in the modulation of HPC mechanisms of oscillations and synchrony. This finding is in contrast to the effects of endogenous NE produced by electrical stimulation of the locus coeruleus (LC) and procaine injection into the LC (Broncel et al., 2020).

Norepinephrine as a spatial memory reset signal

Contextual information is represented in the hippocampus (HPC) partially through the recruitment of distinct neuronal ensembles. It is believed that reactivation of these ensembles underlies memory retrieval processes. Recently, we showed that norepinephrine input from phasic locus coeruleus activation induces hippocampal plasticity resulting in the recruitment of new neurons and disengagement from previously established representations. We hypothesize that norepinephrine may provide a neuromodulatory mnemonic switch signaling the HPC to move from a state of retrieval to encoding in the presence of novelty, and therefore, plays a role in memory updating. Here, we tested whether bilateral dorsal dentate gyrus (dDG) infusions of the β-adrenergic receptor (BAR) agonist isoproterenol (ISO), administered prior to encoding or retrieval, would impair spatial working and reference memory by reverting, the system to encoding (thereby recruiting new neurons) potentially interfering with the retrieval of the previously established spatial ensemble. We also investigated whether dDG infusions of ISO could promote cognitive flexibility by switching the system to encoding when it is adaptive (ie, when new information is presented, eg, reversal learning). We found that intra-dDG infusions of ISO given prior to retrieval caused deficits in working and reference memory which was blocked by pretreatment with the BAR-antagonist, propranolol (PRO). In contrast, ISO administered prior to reversal learning led to improved performance. These data support our hypothesis that norepinephrine serves as a novelty signal to update HPC contextual representations via BAR activation-facilitated recruitment of new neurons. This can be both maladaptive and adaptive depending on the situation.

Inhibition of Astrocytic Histamine N-Methyltransferase as a Possible Target for the Treatment of Alzheimer's Disease

Alzheimer’s disease (AD) represents the principal cause of dementia among the elderly. Great efforts have been established to understand the physiopathology of AD. Changes in neurotransmitter systems in patients with AD, including cholinergic, GABAergic, serotoninergic, noradrenergic, and histaminergic changes have been reported. Interestingly, changes in the histaminergic system have been related to cognitive impairment in AD patients. The principal pathological changes in the brains of AD patients, related to the histaminergic system, are neurofibrillary degeneration of the tuberomammillary nucleus, the main source of histamine in the brain, low histamine levels, and altered signaling of its receptors. The increase of histamine levels can be achieved by inhibiting its degrading enzyme, histamine N-methyltransferase (HNMT), a cytoplasmatic enzyme located in astrocytes. Thus, increasing histamine levels could be employed in AD patients as co-therapy due to their effects on cognitive functions, neuroplasticity, neuronal survival, neurogenesis, and the degradation of amyloid beta (Aβ) peptides. In this sense, the evaluation of the impact of HNMT inhibitors on animal models of AD would be interesting, consequently highlighting its relevance.

Micro-injection of propranolol within basolateral amygdala impaired fear and spatial memory and dysregulated evoked responses of CA1 neurons following foot shock stress in rats

The basolateral nucleus of the amygdala (BLA) is responsible for memory retrieval after stress. It regulates hippocampal long-term potentiation (LTP) during stress. Although β-adrenoceptors of the BLA have a critical role in memory, few studies have addressed this question in the BLA, and the results still have been contradictory. The present study was designed to investigate the involvement of β-adrenoceptors of the BLA on hippocampus memory, anxiety, and plasticity in intact and stressed rats. Male Wistar rats were submitted to the electrical foot-shock stress for four consecutive days. Over four consecutive days, animals received bilateral micro-injections of either vehicle or propranolol (4 µg in 1 µl/side) into the BLA (5 min before foot-shock stress). Behavioral (memory, as well as anxiety-like behaviors), electrophysiological, and histological (neural arborization in the hippocampal CA1 pyramidal neurons) studies were performed. Results showed that inhibition of β-adrenoceptors of BLA by propranolol significantly further impaired fear and spatial memory in stressed rats. Similarly, propranolol effectively impaired both memories in the intact animals. Propranolol significantly amplified the slope and amplitude of fEPSP in the CA1 area of the hippocampus only in stressed rats. Foot-shock stress significantly increased the number of dendritic branches in the hippocampus, and propranolol suppressed this effect of stress. It is suggested that β-adrenoceptors in the BLA promote memory and reduce anxiety-like behaviors under tonic and stress conditions. Propranolol dysregulated LTP parameters and reduced dendritic branches, resulting in memory impairment. Probably β-adrenoceptors of BLA regulate evoked responses of CA1 neurons only in stress- and not the tonic condition.

Septotemporal variation in beta-adrenergic modulation of short-term dynamics in the hippocampus

Recent evidence shows a greater facilitating effect of beta-adrenergic receptors (β-ARs) on long-term synaptic plasticity in the ventral versus the dorsal hippocampus. Here, using field potentials from the CA1 area and a ten-pulse stimulation train of varying frequency we show that activation of β-ARs by isoproterenol preferentially facilitates the output from the dorsal hippocampus at the frequency range of 3–40 Hz without affecting short-term synaptic plasticity. Furthermore, isoproterenol increases basal synaptic transmission in the dorsal hippocampus only and enhances basal neuronal excitation more in the dorsal than the ventral hippocampus. These results suggest that β-AR-modulation of short-term neuronal dynamics differs along the longitudinal axis of the hippocampus, thereby contributing to functional specialization along the same axis.

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We studied the effects of isoproterenol (ISO) in dorsal (DH) and ventral (VH) hippocampus.

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ISO increased synaptic transmission and population spike more in DH than VH.

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ISO modulated short-term changes of population spike in the dorsal hippocampus only.

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ISO did not affect short-term changes of synaptic transmission in DH or VH.

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β adrenergic receptors modulate short-term changes in excitation in DH only.

The genie in the bottle-magnified calcium signaling in dorsolateral prefrontal cortex

Neurons in the association cortices are particularly vulnerable in cognitive disorders such as schizophrenia and Alzheimer's disease, while those in primary visual cortex remain relatively resilient. This review proposes that the special molecular mechanisms needed for higher cognitive operations confer vulnerability to dysfunction, atrophy, and neurodegeneration when regulation is lost due to genetic and/or environmental insults. Accumulating data suggest that higher cortical circuits rely on magnified levels of calcium (from NMDAR, calcium channels, and/or internal release from the smooth endoplasmic reticulum) near the postsynaptic density to promote the persistent firing needed to maintain, manipulate, and store information without "bottom-up" sensory stimulation. For example, dendritic spines in the primate dorsolateral prefrontal cortex (dlPFC) express the molecular machinery for feedforward, cAMP-PKA-calcium signaling. PKA can drive internal calcium release and promote calcium flow through NMDAR and calcium channels, while in turn, calcium activates adenylyl cyclases to produce more cAMP-PKA signaling. Excessive levels of cAMP-calcium signaling can have a number of detrimental effects: for example, opening nearby K+ channels to weaken synaptic efficacy and reduce neuronal firing, and over a longer timeframe, driving calcium overload of mitochondria to induce inflammation and dendritic atrophy. Thus, calcium-cAMP signaling must be tightly regulated, e.g., by agents that catabolize cAMP or inhibit its production (PDE4, mGluR3), and by proteins that bind calcium in the cytosol (calbindin). Many genetic or inflammatory insults early in life weaken the regulation of calcium-cAMP signaling and are associated with increased risk of schizophrenia (e.g., GRM3). Age-related loss of regulatory proteins which result in elevated calcium-cAMP signaling over a long lifespan can additionally drive tau phosphorylation, amyloid pathology, and neurodegeneration, especially when protective calcium binding proteins are lost from the cytosol. Thus, the "genie" we need for our remarkable cognitive abilities may make us vulnerable to cognitive disorders when we lose essential regulation.

Injection of Anti-proBDNF Attenuates Hippocampal-Dependent Learning and Memory Dysfunction in Mice With Sepsis-Associated Encephalopathy

Sepsis-associated encephalopathy (SAE) is a risk factor for cognitive and memory dysfunction; however, the mechanism remains unclear. Brain-derived neurotrophic factor (BDNF) was reported to have a positive effect on cognition and emotion regulation, but the study of its precursor, proBDNF, has been limited. This study aimed to elucidate the effects and associated mechanisms of hippocampal proBDNF in a lipopolysaccharide (LPS)-induced SAE mouse model. In this study, we found that the mice exhibited cognitive dysfunction on day 7 after LPS injection. The expression of proBDNF and its receptor, p75^NTR, was also increased in the hippocampus, while the levels of BDNF and its receptor, TrkB, were decreased. A co-localization study showed that proBDNF and p75^NTR were mainly co-localized with neurons. Furthermore, LPS treatment reduced the expression of NeuN, Nissl bodies, GluR4, NR1, NR2A, and NR2B in the hippocampus of SAE mice. Furthermore, an intrahippocampal or intraperitoneal injection of anti-proBDNF antibody was able to ameliorate LPS-induced cognitive dysfunction and restore the expression of NeuN, Nissl bodies, GluR4, NR1, NR2A, NR2B, and PSD95. These results indicated that treatment with brain delivery by an intrahippocampal and systemic injection of mAb-proBDNF may represent a potential therapeutic strategy for treating patients with SAE.

A locus coeruleus to dentate gyrus noradrenergic circuit modulates aversive contextual processing

Dysregulation in contextual processing is believed to affect several forms of psychopathology, such as post-traumatic stress disorder (PTSD). The dentate gyrus (DG), a subregion of the hippocampus, is thought to be an important brain region for disambiguating new experiences from prior experiences. Noradrenergic (NE) neurons in the locus coeruleus (LC) are more tonically active during stressful events and send dense projections to the DG, yet an understanding of their function in DG-dependent contextual discrimination has not been established. Here, we isolate a key function of the LC-NE-DG circuit in contextual aversive generalization using selective manipulations and in vivo single-cell calcium imaging. We report that activation of LC-NE neurons and terminal activity results in contextual generalization. We found that these effects required β-adrenergic-mediated modulation of hilar interneurons to ultimately promote aversive generalization, suggesting that disruption of noradrenergic tone may serve as an important avenue for treating stress-induced disorders.

Heightened Hippocampal β-Adrenergic Receptor Function Drives Synaptic Potentiation and Supports Learning and Memory in the TgF344-AD Rat Model during Prodromal Alzheimer's Disease

The central noradrenergic (NA) system is critical for the maintenance of attention, behavioral flexibility, spatial navigation, and learning and memory, those cognitive functions lost first in early Alzheimer's disease (AD). In fact, the locus coeruleus (LC), the sole source of norepinephrine (NE) for >90% of the brain, is the first site of pathologic tau accumulation in human AD with axon loss throughout forebrain, including hippocampus. The dentate gyrus is heavily innervated by LC-NA axons, where released NE acts on β-adrenergic receptors (ARs) at excitatory synapses from entorhinal cortex to facilitate long-term synaptic plasticity and memory formation. These synapses experience dysfunction in early AD before cognitive impairment. In the TgF344-AD rat model of AD, degeneration of LC-NA axons in hippocampus recapitulates human AD, providing a preclinical model to investigate synaptic and behavioral consequences. Using immunohistochemistry, Western blot analysis, and brain slice electrophysiology in 6- to 9-month-old wild-type and TgF344-AD rats, we discovered that the loss of LC-NA axons coincides with the heightened β-AR function at medial perforant path-dentate granule cell synapses that is responsible for the increase in LTP magnitude at these synapses. Furthermore, novel object recognition is facilitated in TgF344-AD rats that requires β-ARs, and pharmacological blockade of β-ARs unmasks a deficit in extinction learning only in TgF344-AD rats, indicating a greater reliance on β-ARs in both behaviors. Thus, a compensatory increase in β-AR function during prodromal AD in TgF344-AD rats heightens synaptic plasticity and preserves some forms of learning and memory.SIGNIFICANCE STATEMENT The locus coeruleus (LC), a brain region located in the brainstem which is responsible for attention and arousal, is damaged first by Alzheimer's disease (AD) pathology. The LC sends axons to hippocampus where released norepinephrine (NE) modulates synaptic function required for learning and memory. How degeneration of LC axons and loss of NE in hippocampus in early AD impacts synaptic function and learning and memory is not well understood despite the importance of LC in cognitive function. We used a transgenic AD rat model with LC axon degeneration mimicking human AD and found that heightened function of β-adrenergic receptors in the dentate gyrus increased synaptic plasticity and preserved learning and memory in early stages of the disease.

Propranolol Decreases Fear Expression by Modulating Fear Memory Traces

BACKGROUND

Posttraumatic stress disorder can develop after a traumatic event and results in heightened, inappropriate fear and anxiety. Although approximately 8% of the U.S. population is affected by posttraumatic stress disorder, only two drugs have been approved by the Food and Drug Administration to treat it, both with limited efficacy. Propranolol, a nonselective β-adrenergic antagonist, has shown efficacy in decreasing exaggerated fear, and there has been renewed interest in using it to treat fear disorders.

METHODS

Here, we sought to determine the mechanisms by which propranolol attenuates fear by utilizing an activity-dependent tagging system, ArcCreERT2 x eYFP mice. 129S6/SvEv mice were administered a 4-shock contextual fear conditioning paradigm followed by immediate or delayed context reexposures. Saline or propranolol was administered either before or after the first context reexposure. To quantify hippocampal, prefrontal, and amygdalar memory traces, ArcCreERT2 x eYFP mice were administered a delayed context reexposure with either a saline or propranolol injection before context reexposure.

RESULTS

Propranolol decreased fear expression only when administered before a delayed context reexposure. Fear memory traces were affected in the dorsal dentate gyrus and basolateral amygdala after propranolol administration in the ArcCreERT2 x eYFP mice. Propranolol acutely altered functional connectivity between the hippocampal, cortical, and amygdalar regions.

CONCLUSIONS

These data indicate that propranolol may decrease fear expression by altering network-correlated activity and by weakening the reactivation of the initial traumatic memory trace. This work contributes to the understanding of noradrenergic drugs as therapeutic aids for patients with posttraumatic stress disorder.

Hippocampal glucocorticoid target genes associated with enhancement of memory consolidation

Glucocorticoids enhance memory consolidation of emotionally arousing events via largely unknown molecular mechanisms. This glucocorticoid effect on the consolidation process also requires central noradrenergic neurotransmission. The intracellular pathways of these two stress mediators converge on two transcription factors: the glucocorticoid receptor (GR) and phosphorylated cAMP response element‐binding protein (pCREB). We therefore investigated, in male rats, whether glucocorticoid effects on memory are associated with genomic interactions between the GR and pCREB in the hippocampus. In a two‐by‐two design, object exploration training or no training was combined with post‐training administration of a memory‐enhancing dose of corticosterone or vehicle. Genomic effects were studied by chromatin immunoprecipitation followed by sequencing (ChIP‐seq) of GR and pCREB 45 min after training and transcriptome analysis after 3 hr. Corticosterone administration induced differential GR DNA‐binding and regulation of target genes within the hippocampus, largely independent of training. Training alone did not result in long‐term memory nor did it affect GR or pCREB DNA‐binding and gene expression. No strong evidence was found for an interaction between GR and pCREB. Combination of the GR DNA‐binding and transcriptome data identified a set of novel, likely direct, GR target genes that are candidate mediators of corticosterone effects on memory consolidation. Cell‐specific expression of the identified target genes using single‐cell expression data suggests that the effects of corticosterone reflect in part non‐neuronal cells. Together, our data identified new GR targets associated with memory consolidation that reflect effects in both neuronal and non‐neuronal cells.

Context-dependent extinction learning emerging from raw sensory inputs: a reinforcement learning approach

The context-dependence of extinction learning has been well studied and requires the hippocampus. However, the underlying neural mechanisms are still poorly understood. Using memory-driven reinforcement learning and deep neural networks, we developed a model that learns to navigate autonomously in biologically realistic virtual reality environments based on raw camera inputs alone. Neither is context represented explicitly in our model, nor is context change signaled. We find that memory-intact agents learn distinct context representations, and develop ABA renewal, whereas memory-impaired agents do not. These findings reproduce the behavior of control and hippocampal animals, respectively. We therefore propose that the role of the hippocampus in the context-dependence of extinction learning might stem from its function in episodic-like memory and not in context-representation per se. We conclude that context-dependence can emerge from raw visual inputs.