Noradrenergic suppression of synaptic transmission may influence cortical signal-to-noise ratio

Toward a computational role for locus coeruleus/norepinephrine arousal systems

Brain and behavior undergo measurable changes in their underlying state and neuromodulators are thought to contribute to these fluctuations. Why do we undergo such changes, and what function could the underlying neuromodulatory systems perform? Here we examine theoretical answers to these questions with respect to the locus coeruleus/norepinephrine system focusing on peripheral markers for arousal, such as pupil diameter, that are thought to provide a window into brain wide noradrenergic signaling. We explore a computational role for arousal systems in facilitating internal state transitions that facilitate credit assignment and promote accurate perceptions in non-stationary environments. We summarize recent work that supports this idea and highlight open questions as well as alternative views of how arousal affects cognition.

Norepinephrine changes behavioral state via astroglial purinergic signaling

Both neurons and glia communicate via diffusible neuromodulatory substances, but the substrates of computation in such neuromodulatory networks are unclear. During behavioral transitions in the larval zebrafish, the neuromodulator norepinephrine drives fast excitation and delayed inhibition of behavior and circuit activity. We find that the inhibitory arm of this feedforward motif is implemented by astroglial purinergic signaling. Neuromodulator imaging, behavioral pharmacology, and perturbations of neurons and astroglia reveal that norepinephrine triggers astroglial release of adenosine triphosphate, extracellular conversion into adenosine, and behavioral suppression through activation of hindbrain neuronal adenosine receptors. This work, along with a companion piece by Lefton and colleagues demonstrating an analogous pathway mediating the effect of norepinephrine on synaptic connectivity in mice, identifies a computational and behavioral role for an evolutionarily conserved astroglial purinergic signaling axis in norepinephrine-mediated behavioral and brain state transitions.

Norepinephrine Signals Through Astrocytes To Modulate Synapses

Locus coeruleus (LC)-derived norepinephrine (NE) drives network and behavioral adaptations to environmental saliencies by reconfiguring circuit connectivity, but the underlying synapse-level mechanisms are elusive. Here, we show that NE remodeling of synaptic function is independent from its binding on neuronal receptors. Instead, astrocytic adrenergic receptors and Ca2+ dynamics fully gate the effect of NE on synapses as the astrocyte-specific deletion of adrenergic receptors and three independent astrocyte-silencing approaches all render synapses insensitive to NE. Additionally, we find that NE suppression of synaptic strength results from an ATP-derived and adenosine A1 receptor-mediated control of presynaptic efficacy. An accompanying study from Chen et al. reveals the existence of an analogous pathway in the larval zebrafish and highlights its importance to behavioral state transitions. Together, these findings fuel a new model wherein astrocytes are a core component of neuromodulatory systems and the circuit effector through which norepinephrine produces network and behavioral adaptations, challenging an 80-year-old status quo.

In Search of Dispersed Memories: Generative Diffusion Models Are Associative Memory Networks

Uncovering the mechanisms behind long-term memory is one of the most fascinating open problems in neuroscience and artificial intelligence. Artificial associative memory networks have been used to formalize important aspects of biological memory. Generative diffusion models are a type of generative machine learning techniques that have shown great performance in many tasks. Similar to associative memory systems, these networks define a dynamical system that converges to a set of target states. In this work, we show that generative diffusion models can be interpreted as energy-based models and that, when trained on discrete patterns, their energy function is (asymptotically) identical to that of modern Hopfield networks. This equivalence allows us to interpret the supervised training of diffusion models as a synaptic learning process that encodes the associative dynamics of a modern Hopfield network in the weight structure of a deep neural network. Leveraging this connection, we formulate a generalized framework for understanding the formation of long-term memory, where creative generation and memory recall can be seen as parts of a unified continuum.

Noradrenergic regulation of cue-guided decision making and impulsivity is doubly dissociable across frontal brain regions

RATIONALE

Win-paired stimuli can promote risk taking in experimental gambling paradigms in both rats and humans. We previously demonstrated that atomoxetine, a noradrenaline reuptake inhibitor, and guanfacine, a selective α2A adrenergic receptor agonist, reduced risk taking on the cued rat gambling task (crGT), a rodent assay of risky choice in which wins are accompanied by salient cues. Both compounds also decreased impulsive premature responding.

OBJECTIVE

The key neural loci mediating these effects were unknown. The lateral orbitofrontal cortex (lOFC) and the medial prefrontal cortex (mPFC), which are highly implicated in risk assessment, action selection, and impulse control, receive dense noradrenergic innervation. We therefore infused atomoxetine and guanfacine directly into either the lOFC or prelimbic (PrL) mPFC prior to task performance.

RESULTS

When infused into the lOFC, atomoxetine improved decision making score and adaptive lose-shift behaviour in males, but not in females, without altering motor impulsivity. Conversely, intra-PrL atomoxetine improved impulse control in risk preferring animals of both sexes, but did not alter decision making. Guanfacine administered into the PrL, but not lOFC, also altered motor impulsivity in all subjects, though in the opposite direction to atomoxetine.

CONCLUSIONS

These data highlight a double dissociation between the behavioural effects of noradrenergic signaling across frontal regions with respect to risky choice and impulsive action. Given that the influence of noradrenergic manipulations on motor impulsivity could depend on baseline risk preference, these data also suggest that the noradrenaline system may function differently in subjects that are susceptible to the risk-promoting lure of win-associated cues.

The HPA and SAM axis mediate the impairment of creativity under stress

With the ever-changing social environment, individual creativity is facing a severe challenge induced by stress. However, little is known regarding the underlying mechanisms by which acute stress affects creative cognitive processing. The current research explored the impacts of the neuroendocrine response on creativity under stress and its underlying cognitive flexibility mechanisms. The enzyme-linked immuno sorbent assay was employed to assess salivary cortisol, which acted as a marker of stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis. Eye blink rate (EBR) and pupil diameter were measured as respective indicators of dopamine and noradrenaline released by the activation of the sympathetic-adrenal-medullary (SAM) axis. The Wisconsin card task (WCST) measured cognitive flexibility, while the alternative uses task (AUT) and the remote association task (RAT) measured separately divergent and convergent thinking in creativity. Results showed higher cortisol increments following acute stress induction in the stress group than control group. Ocular results showed that the stress manipulation significantly increased EBR and pupil diameter compared to controls, reflecting increased SAM activity. Further analysis revealed that stress-released cortisol impaired the originality component of the AUT, reducing cognitive flexibility as measured by perseverative errors on the WCST task. Serial mediation analyses showed that both EBR and pupil diameter were also associated with increased perseverative errors leading to poor originality on the AUT. These findings confirm that physiological arousal under stress can impair divergent thinking through the regulation of different neuroendocrine pathways, in which the deterioration of flexible switching plays an important mediating role.

From fear of falling to choking under pressure: A predictive processing perspective of disrupted motor control under anxiety

Under the Predictive Processing Framework, perception is guided by internal models that map the probabilistic relationship between sensory states and their causes. Predictive processing has contributed to a new understanding of both emotional states and motor control but is yet to be fully applied to their interaction during the breakdown of motor movements under heightened anxiety or threat. We bring together literature on anxiety and motor control to propose that predictive processing provides a unifying principle for understanding motor breakdowns as a disruption to the neuromodulatory control mechanisms that regulate the interactions of top-down predictions and bottom-up sensory signals. We illustrate this account using examples from disrupted balance and gait in populations who are anxious/fearful of falling, as well as 'choking' in elite sport. This approach can explain both rigid and inflexible movement strategies, as well as highly variable and imprecise action and conscious movement processing, and may also unite the apparently opposing self-focus and distraction approaches to choking. We generate predictions to guide future work and propose practical recommendations.

Differential Serotonergic Modulation of Synaptic Inputs to the Olfactory Cortex

Serotonin (5-hydroxytriptamine, 5-HT) is an important monoaminergic neuromodulator involved in a variety of physiological and pathological functions. It has been implicated in the regulation of sensory functions at various stages of multiple modalities, but its mechanisms and functions in the olfactory system have remained elusive. Combining electrophysiology, optogenetics and pharmacology, here we show that afferent (feed-forward) pathway-evoked synaptic responses are boosted, whereas feedback responses are suppressed by presynaptic 5-HT_1B receptors in the anterior piriform cortex (aPC) in vitro. Blocking 5-HT_1B receptors also reduces the suppressive effects of serotonergic photostimulation of baseline firing in vivo. We suggest that by regulating the relative weights of synaptic inputs to aPC, 5-HT finely tunes sensory inputs in the olfactory cortex.

Causal effects of psychostimulants on neural connectivity: a mechanistic, randomized clinical trial

BACKGROUND

Psychostimulants are frequently used to treat attention-deficit/hyperactivity disorder (ADHD), but side effects are common leading to many patients discontinuing treatment. Identifying neural mechanisms by which psychostimulants attenuate symptoms may guide the development of more refined and tolerable therapeutics.

METHODS

We conducted a 12-week, randomized, placebo-controlled trial (RCT) of a long-acting amphetamine, lisdexamfetamine (LDEX), in patients with ADHD, ages 6-25 years old. Of the 58 participants who participated in the RCT, 49 completed pre- and post-RCT magnetic resonance imaging scanning with adequate data quality. Healthy controls (HCs; n = 46) were included for comparison. Treatment effects on striatal and thalamic functional connectivity (FC) were identified using static (time-averaged) and dynamic (time-varying) measures and then correlated with symptom improvement. Analyses were repeated in independent samples from the Adolescent Brain Cognitive Development study (n = 103) and the ADHD-200 Consortium (n = 213).

RESULTS

In 49 participants (25 LDEX; 24 Placebo), LDEX increased static and decreased dynamic FC (DFC). However, only DFC was associated with the therapeutic effects of LDEX. Additionally, at baseline, DFC was elevated in unmedicated-ADHD participants relative to HCs. Independent samples yielded similar findings - ADHD was associated with increased DFC, and psychostimulants with reduced DFC. Static FC findings were inconsistent across samples.

CONCLUSIONS

Changes in dynamic, but not static, FC were associated with the therapeutic effects of psychostimulants. While prior research has focused on static FC, DFC may offer a more reliable target for new ADHD interventions aimed at stabilizing network dynamics, though this needs confirmation with subsequent investigations.

Role of the locus coeruleus and basal forebrain in arousal and attention

Experimental evidence has implicated multiple neurotransmitter systems in either the direct or indirect modulation of cortical arousal and attention circuitry. In this review, we selectively focus on three such systems: 1) norepinephrine (NE)-containing neurons of the locus coeruleus (LC), 2) acetylcholine (ACh)-containing neurons of the basal forebrain (BF), and 3) parvalbumin (PV)-containing gamma-aminobutyric acid neurons of the BF. Whereas BF-PV neurons serve as a rapid and transient arousal system, LC-NE and BF-ACh neuromodulation are typically activated on slower but longer-lasting timescales. Recent findings suggest that the BF-PV system serves to rapidly respond to even subtle sensory stimuli with a microarousal. We posit that salient sensory stimuli, such as those that are threatening or predict the need for a response, will quickly activate the BF-PV system and subsequently activate both the BF-ACh and LC-NE systems if the circumstances require longer periods of arousal and vigilance. We suggest that NE and ACh have overlapping psychological functions with the main difference being the precise internal/environmental sensory situations/contexts that recruit each neurotransmitter system - a goal for future research to determine. Implications of dysfunction of each of these three attentional systems for our understanding of neuropsychiatric conditions are considered. Finally, the contemporary availability of research tools to selectively manipulate and measure the activity of these distinctive neuronal populations promises to answer longstanding questions, such as how various arousal systems influence downstream decision-making and motor responding.

Understanding Categorical Learning in Neural Circuits Through the Primary Olfactory Cortex

Knowing which elements in the environment are associated with various opportunities and dangers is advantageous. A major role of mammalian sensory systems is to provide information about the identity of such elements which can then be used for adaptive action planning by the animal. Identity-tuned sensory representations are categorical, invariant to nuances in the sensory stream and depend on associative learning. Although categorical representations are well documented across several sensory modalities, these tend to situate synaptically far from the sensory organs which reduces experimenter control over input-output transformations. The formation of such representations is a fundamental neural computation that remains poorly understood. Odor representations in the primary olfactory cortex have several characteristics that qualify them as categorical and identity-tuned, situated only two synapses away from the sensory epithelium. The formation of categorical representations is likely critically dependent on—and dynamically controlled by—recurrent circuitry within the primary olfactory cortex itself. Experiments suggest that the concerted activity of several neuromodulatory systems plays a decisive role in shaping categorical learning through complex interactions with recurrent activity and plasticity in primary olfactory cortex circuits. In this perspective we discuss missing pieces of the categorical learning puzzle, and why several features of olfaction make it an attractive model system for this challenge.

Noradrenergic Modulation of the Piriform Cortex: A Possible Avenue for Understanding Pre-Clinical Alzheimer’s Disease Pathogenesis

Olfactory dysfunction is one of the biomarkers for Alzheimer’s disease (AD) diagnosis and progression. Deficits with odor identification and discrimination are common symptoms of pre-clinical AD, preceding severe memory disorder observed in advanced stages. As a result, understanding mechanisms of olfactory impairment is a major focus in both human studies and animal models of AD. Pretangle tau, a precursor to tau tangles, is first observed in the locus coeruleus (LC). In a recent animal model, LC pretangle tau leads to LC fiber degeneration in the piriform cortex (PC), a cortical area associated with olfactory dysfunction in both human AD and rodent models. Here, we review the role of LC-sourced NE in modulation of PC activity and suggest mechanisms by which pretangle tau-mediated LC dysfunction may impact olfactory processing in preclinical stage of AD. Understanding mechanisms of early olfactory impairment in AD may provide a critical window for detection and intervention of disease progression.

The modulation of brain network integration and arousal during exploration

There is growing interest in how neuromodulators shape brain networks. Recent neuroimaging studies provide evidence that brainstem arousal systems, such as the locus coeruleus-norepinephrine system (LC-NE), influence functional connectivity and brain network topology, suggesting they have a role in flexibly reconfiguring brain networks in order to adapt behavior and cognition to environmental demands. To date, however, the relationship between brainstem arousal systems and functional connectivity has not been assessed within the context of a task with an established relationship between arousal and behavior, with most prior studies relying on incidental variations in arousal or pharmacological manipulation and static brain networks constructed over long periods of time. These factors have likely contributed to a heterogeneity of effects across studies. To address these issues, we took advantage of the association between LC-NE-linked arousal and exploration to probe the relationships between exploratory choice, arousal—as measured indirectly via pupil diameter—and brain network dynamics. Exploration in a bandit task was associated with a shift toward fewer, more weakly connected modules that were more segregated in terms of connectivity and topology but more integrated with respect to the diversity of cognitive systems represented in each module. Functional connectivity strength decreased, and changes in connectivity were correlated with changes in pupil diameter, in line with the hypothesis that brainstem arousal systems influence the dynamic reorganization of brain networks. More broadly, we argue that carefully aligning dynamic network analyses with task designs can increase the temporal resolution at which behaviorally- and cognitively-relevant modulations can be identified, and offer these results as a proof of concept of this approach.

The Computational, Pharmacological, and Physiological Determinants of Sensory Learning under Uncertainty

The ability to represent and respond to uncertainty is fundamental to human cognition and decision-making. Noradrenaline (NA) is hypothesized to play a key role in coordinating the sensory, learning, and physiological states necessary to adapt to a changing world, but direct evidence for this is lacking in humans. Here, we tested the effects of attenuating noradrenergic neurotransmission on learning under uncertainty. We probed the effects of the β-adrenergic receptor antagonist propranolol (40 mg) using a between-subjects, double-blind, placebo-controlled design. Participants performed a probabilistic associative learning task, and we employed a hierarchical learning model to formally quantify prediction errors about cue-outcome contingencies and changes in these associations over time (volatility). Both unexpectedness and noise slowed down reaction times, but propranolol augmented the interaction between these main effects such that behavior was influenced more by prior expectations when uncertainty was high. Computationally, this was driven by a reduction in learning rates, with people slower to update their beliefs in the face of new information. Attenuating the global effects of NA also eliminated the phasic effects of prediction error and volatility on pupil size, consistent with slower belief updating. Finally, estimates of environmental volatility were predicted by baseline cardiac measures in all participants. Our results demonstrate that NA underpins behavioral and computational responses to uncertainty. These findings have important implications for understanding the impact of uncertainty on human biology and cognition.

Time as the fourth dimension in the hippocampus

Experiences of animal and human beings are structured by the continuity of space and time coupled with the uni-directionality of time. In addition to its pivotal position in spatial processing and navigation, the hippocampal system also plays a central, multiform role in several types of temporal processing. These include timing and sequence learning, at scales ranging from meso-scales of seconds to macro-scales of minutes, hours, days and beyond, encompassing the classical functions of short term memory, working memory, long term memory, and episodic memories (comprised of information about when, what, and where). This review article highlights the principal findings and behavioral contexts of experiments in rats showing: 1) timing: tracking time during delays by hippocampal 'time cells' and during free behavior by hippocampal-afferent lateral entorhinal cortex ramping cells; 2) 'online' sequence processing: activity coding sequences of events during active behavior; 3) 'off-line' sequence replay: during quiescence or sleep, orderly reactivation of neuronal assemblies coding awake sequences. Studies in humans show neurophysiological correlates of episodic memory comparable to awake replay. Neural mechanisms are discussed, including ion channel properties, plateau and ramping potentials, oscillations of excitation and inhibition of population activity, bursts of high amplitude discharges (sharp wave ripples), as well as short and long term synaptic modifications among and within cell assemblies. Specifically conceived neural network models will suggest processes supporting the emergence of scalar properties (Weber's law), and include different classes of feedforward and recurrent network models, with intrinsic hippocampal coding for 'transitions' (sequencing of events or places).

The Role of the Noradrenergic System in Autism Spectrum Disorders, Implications for Treatment

Autism spectrum disorder (ASD) is frequently associated with anxiety and hyperarousal. While the pathological changes in the noradrenergic system in ASD are not entirely clear, a number of functional indices of the sympathetic/parasympathetic balance are altered in individuals with ASD, often with a high degree of inter-individual variability. The neuropsychopharmacological effects of α2 agonists and β-adrenergic antagonists make agents targeting these receptors of particular interest. α2 agonists have shown beneficial effects for attention deficit hyperactivity disorder (ADHD) and in other domains in individuals with ASD, but effects on core ASD symptoms are less clear. Case series and single dose psychopharmacological challenges suggest that treatment with β-adrenergic antagonists has beneficial effects on language and social domains. Additionally, psychophysiological markers and premorbid anxiety may predict response to these medications. As a result, β-adrenergic antagonists are currently being utilized in a clinical trial for improving core symptoms as well as anxiety in individuals with ASD.

Regulation of vocal precision by noradrenergic modulation of a motor nucleus

Norepinephrine (NE) function is often implicated in regulating arousal levels. Recent theory suggests that the noradrenergic system also regulates the optimization of behavior with respect to reward maximization by controlling a switch between exploration and exploitation of the specific actions that yield greatest utility. We show in the songbird that NE can act directly on a cortical motor area and cause a switch between exploratory and exploitative behavior.

Recurrent circuitry is required to stabilize piriform cortex odor representations across brain states

Pattern completion, or the ability to retrieve stable neural activity patterns from noisy or partial cues, is a fundamental feature of memory. Theoretical studies indicate that recurrently connected auto-associative or discrete attractor networks can perform this process. Although pattern completion and attractor dynamics have been observed in various recurrent neural circuits, the role recurrent circuitry plays in implementing these processes remains unclear. In recordings from head-fixed mice, we found that odor responses in olfactory bulb degrade under ketamine/xylazine anesthesia while responses immediately downstream, in piriform cortex, remain robust. Recurrent connections are required to stabilize cortical odor representations across states. Moreover, piriform odor representations exhibit attractor dynamics, both within and across trials, and these are also abolished when recurrent circuitry is eliminated. Here, we present converging evidence that recurrently-connected piriform populations stabilize sensory representations in response to degraded inputs, consistent with an auto-associative function for piriform cortex supported by recurrent circuitry.

Noradrenergic But Not Dopaminergic Neurons Signal Task State Changes and Predict Reengagement After a Failure

The two catecholamines, noradrenaline and dopamine, have been shown to play comparable roles in behavior. Both noradrenergic and dopaminergic neurons respond to cues predicting reward availability and novelty. However, even though both are thought to be involved in motivating actions, their roles in motivation have seldom been directly compared. We therefore examined the activity of putative noradrenergic neurons in the locus coeruleus and putative midbrain dopaminergic neurons in monkeys cued to perform effortful actions for rewards. The activity in both regions correlated with engagement with a presented option. By contrast, only noradrenaline neurons were also (i) predictive of engagement in a subsequent trial following a failure to engage and (ii) more strongly activated in nonrepeated trials, when cues indicated a new task condition. This suggests that while both catecholaminergic neurons are involved in promoting action, noradrenergic neurons are sensitive to task state changes, and their influence on behavior extends beyond the immediately rewarded action.

The Unexplored Territory of Neural Models: Potential Guides for Exploring the Function of Metabotropic Neuromodulation

The space of possible neural models is enormous and under-explored. Single cell computational neuroscience models account for a range of dynamical properties of membrane potential, but typically do not address network function. In contrast, most models focused on network function address the dimensions of excitatory weight matrices and firing thresholds without addressing the complexities of metabotropic receptor effects on intrinsic properties. There are many under-explored dimensions of neural parameter space, and the field needs a framework for representing what has been explored and what has not. Possible frameworks include maps of parameter spaces, or efforts to categorize the fundamental elements and molecules of neural circuit function. Here we review dimensions that are under-explored in network models that include the metabotropic modulation of synaptic plasticity and presynaptic inhibition, spike frequency adaptation due to calcium-dependent potassium currents, and afterdepolarization due to calcium-sensitive non-specific cation currents and hyperpolarization activated cation currents. Neuroscience research should more effectively explore possible functional models incorporating under-explored dimensions of neural function.

Muscarinic and Nicotinic Modulation of Memory but not Verbal Problem-solving

Aspects of cognitive flexibility are modulated by the noradrenergic system, which is important in arousal and attention. Acetylcholine also modulates arousal and attention, as well as working memory. Effects of muscarinic and nicotinic antagonism on memory are well established. Our purpose was to test whether muscarinic and nicotinic antagonism affect aspects of cognitive flexibility, specifically verbal problem-solving, as well as memory, given acetylcholine's role in attention and arousal. Eighteen participants attended three testing sessions. Two hours before testing, participants received either 0.6 mg scopolamine, 10 mg mecamylamine, or placebo. Then, participants were tested on three memory tasks (Buschke Selective Reminding Test [BSRT], California Verbal Learning Test [CVLT], Rey Complex Figure Test), two verbal problem-solving/cognitive flexibility tasks (Compound Remote Associates Test, a timed anagram test), and a spatial inductive reasoning task (Raven's Progressive Matrices). Task order and drug order were counterbalanced. Memory impairment was seen on one BSRT measure and multiple CVLT measures with scopolamine, and with one BSRT measure with mecamylamine. There were no effects of either drug on any of the tasks involving cognitive flexibility, including verbal problem-solving. Specific memory impairments were detected using muscarinic, and to a marginal extent, nicotinic antagonists, as expected, but no effect was seen on cognitive flexibility. Therefore, although both the noradrenergic and cholinergic systems play important roles in arousal and cortical signal-to-noise processing, the cholinergic system does not appear to have the same effect as the noradrenergic system on cognitive flexibility, including verbal problem-solving.

Depression-related disturbances in rat maternal behaviour are associated with altered monoamine levels within mesocorticolimbic structures

The ability of mothers to sensitively attune their maternal responses to the needs of their developing young is fundamental to a healthy mother-young relationship. The biological mechanisms that govern how mothers adjust caregiving to the dynamic changes in the demands of the young remain an open question. In the present study, we examined whether changes in monoamine levels, within discrete mesocorticolimbic structures involved in cognitive and motivational processes key to parenting, modulate this flexibility in caregiving across the postpartum period. The present study used a Wistar-Kyoto (WKY) animal model of depression and control Sprague-Dawley (SD) rats, which differ dramatically in their cognitive, motivational, and parenting performance. Levels of the monoamine neurotransmitters, dopamine, noradrenaline and serotonin, as well as their major metabolites, were measured within the medial prefrontal cortex, striatum, nucleus accumbens and medial preoptic area of SD and WKY mothers at early (postpartum day [PPD]7-8), late (PPD15-16) and weaning (PPD25) postpartum stages using high-performance liquid chromatography with electrochemical detection. Consistent with our prior work, we find that caregiving of SD mothers declined as the postpartum period progressed. Relative to nulliparous females, early postpartum mothers had lower intracellular concentrations of monoamines, as well as lower noradrenaline turnover, and an elevated serotonin turnover within most structures. Postpartum behavioural trajectories subsequently corresponded to a progressive increase in all three monoamine levels within multiple structures. Compared to SD mothers, WKY mothers were inconsistent and disorganised in caring for their offspring and exhibit profound deficits in maternal behaviour. Additionally, WKY mothers had generally lower levels of all three monoamines, as well as different patterns of change across the postpartum period, compared to SD mothers, suggesting dysfunctional central monoamine pathways in WKY mothers as they transition and experience motherhood. Taken together, the results of the present study suggest a role for monoamines at multiple mesocorticolimbic structures with repect to modulating caregiving behaviours attuned to the changing needs of the young.

A Computational Model of Oxytocin Modulation of Olfactory Recognition Memory

Social recognition in mammals depends on complex interactions between sensory and other brain areas as well as modulatory inputs by specific neuropeptides such as oxytocin (OXT). Social recognition memory specifically has been shown to depend among others on olfactory processing, and can be probed using methods similar to those used when probing non-social odor memory. We here use a computational model of two interconnected olfactory networks in the mouse, the olfactory bulb (OB) and anterior olfactory nucleus, to propose a mechanism for olfactory short-term recognition memory and its modulation in social situations. Based on previous experiments, we propose one early locus for memory to be the OB. During social encounters in mice, pyramidal cells in the anterior olfactory nucleus, themselves driven by olfactory input, are rendered more excitable by OXT release, resulting in stronger feedback to OB local interneurons. This additional input to the OB creates stronger dynamics and improves signal-to-noise ratio of odor responses in the OB proper. As a consequence, mouse social olfactory memories are more strongly encoded and their duration is modulated.

Visual hallucinations, thalamocortical physiology and Lewy body disease: A review

One of the core diagnostic criteria for Dementia with Lewy Bodies (DLB) is the presence of visual hallucinations. The presence of hallucinations, along with fluctuations in the level of arousal and sleep disturbance, point to potential pathological mechanisms at the level of the thalamus. However, the potential role of thalamic dysfunction in DLB, particularly as it relates to the presence of formed visual hallucinations is not known. Here, we review the literature on the pathophysiology of DLB with respect to modern theories of thalamocortical function and attempt to derive an understanding of how such hallucinations arise. Based on the available literature, we propose that combined thalamic-thalamic reticular nucleus and thalamocortical pathology may explain the phenomenology of visual hallucinations in DLB. In particular, diminished α7 cholinergic activity in the thalamic reticular nucleus may critically disinhibit thalamocortical activity. Further, concentrated pathological changes within the posterior regions of the thalamus may explain the predilection for the hallucinations to be visual in nature.

Neuropsychopharmacological regulation of performance on creativity-related tasks

A number of factors affect performance on tasks associated with creativity. Two pharmacological systems in particularly been identified as important for their impact on creativity, the noradrenergic system and the dopaminergic systems. Furthermore, stress is also established as an important factor impacting performance, most likely mediated by its effects on these neurotransmitter systems. Herein, we review the current literature on the relationships between stress, the noradrenergic system, the dopaminergic system, and other pharmacological factors and their effects on performance on tasks associated with creativity.

The Link Between Creativity, Cognition, and Creative Drives and Underlying Neural Mechanisms

Having a creative mind is one of the gateways for achieving fabulous success and remarkable progress in professional, personal and social life. Therefore, a better understanding of the neural correlates and the underlying neural mechanisms related to creative ideation is crucial and valuable. However, the current literature on neural systems and circuits underlying creative cognition, and on how creative drives such as motivation, mood states, and reward could shape our creative mind through the associated neuromodulatory systems [i.e., the dopaminergic (DA), the noradrenergic (NE) and the serotonergic (5-HT) system] seems to be insufficient to explain the creative ideation and production process. One reason might be that the mentioned systems and processes are usually investigated in isolation and independent of each other. Through this review, we aim at advancing the current state of knowledge by providing an integrative view on the interactions between neural systems underlying the creative cognition and the creative drive and associated neuromodulatory systems (see Figure 1).

The homotopic connectivity of the functional brain: a meta-analytic approach

Homotopic connectivity (HC) is the connectivity between mirror areas of the brain hemispheres. It can exhibit a marked and functionally relevant spatial variability, and can be perturbed by several pathological conditions. The voxel-mirrored homotopic connectivity (VMHC) is a technique devised to enquire this pattern of brain organization, based on resting state functional connectivity. Since functional connectivity can be revealed also in a meta-analytical fashion using co-activations, here we propose to calculate the meta-analytic homotopic connectivity (MHC) as the meta-analytic counterpart of the VMHC. The comparison between the two techniques reveals their general similarity, but also highlights regional differences associated with how HC varies from task to rest. Two main differences were found from rest to task: (i) regions known to be characterized by global hubness are more similar than regions displaying local hubness; and (ii) medial areas are characterized by a higher degree of homotopic connectivity, while lateral areas appear to decrease their degree of homotopic connectivity during task performance. These findings show that MHC can be an insightful tool to study how the hemispheres functionally interact during task and rest conditions.

A network model of behavioural performance in a rule learning task

Humans demonstrate differences in performance on cognitive rule learning tasks which could involve differences in properties of neural circuits. An example model is presented to show how gating of the spread of neural activity could underlie rule learning and the generalization of rules to previously unseen stimuli. This model uses the activity of gating units to regulate the pattern of connectivity between neurons responding to sensory input and subsequent gating units or output units. This model allows analysis of network parameters that could contribute to differences in cognitive rule learning. These network parameters include differences in the parameters of synaptic modification and presynaptic inhibition of synaptic transmission that could be regulated by neuromodulatory influences on neural circuits. Neuromodulatory receptors play an important role in cognitive function, as demonstrated by the fact that drugs that block cholinergic muscarinic receptors can cause cognitive impairments. In discussions of the links between neuromodulatory systems and biologically based traits, the issue of mechanisms through which these linkages are realized is often missing. This model demonstrates potential roles of neural circuit parameters regulated by acetylcholine in learning context-dependent rules, and demonstrates the potential contribution of variation in neural circuit properties and neuromodulatory function to individual differences in cognitive function.This article is part of the theme issue 'Diverse perspectives on diversity: multi-disciplinary approaches to taxonomies of individual differences'.

Locus Coeruleus tracking of prediction errors optimises cognitive flexibility: An Active Inference model

The locus coeruleus (LC) in the pons is the major source of noradrenaline (NA) in the brain. Two modes of LC firing have been associated with distinct cognitive states: changes in tonic rates of firing are correlated with global levels of arousal and behavioural flexibility, whilst phasic LC responses are evoked by salient stimuli. Here, we unify these two modes of firing by modelling the response of the LC as a correlate of a prediction error when inferring states for action planning under Active Inference (AI). We simulate a classic Go/No-go reward learning task and a three-arm ‘explore/exploit’ task and show that, if LC activity is considered to reflect the magnitude of high level ‘state-action’ prediction errors, then both tonic and phasic modes of firing are emergent features of belief updating. We also demonstrate that when contingencies change, AI agents can update their internal models more quickly by feeding back this state-action prediction error–reflected in LC firing and noradrenaline release–to optimise learning rate, enabling large adjustments over short timescales. We propose that such prediction errors are mediated by cortico-LC connections, whilst ascending input from LC to cortex modulates belief updating in anterior cingulate cortex (ACC). In short, we characterise the LC/ NA system within a general theory of brain function. In doing so, we show that contrasting, behaviour-dependent firing patterns are an emergent property of the LC that translates state-action prediction errors into an optimal balance between plasticity and stability.

The brain uses sensory information to build internal models and make predictions about the world. When errors of prediction occur, models must be updated to ensure desired outcomes are still achieved. Neuromodulator chemicals provide a possible pathway for triggering such changes in brain state. One such neuromodulator, noradrenaline, originates predominantly from a cluster of neurons in the brainstem—the locus coeruleus (LC)—and plays a key role in behaviour, for instance, in determining the balance between exploiting or exploring the environment. Here we use Active Inference (AI), a mathematical model of perception and action, to formally describe LC function. We propose that LC activity is triggered by errors in prediction and that the subsequent release of noradrenaline alters the rate of learning about the environment. Biologically, this describes an LC-cortex feedback loop promoting behavioural flexibility in times of uncertainty. We model LC output as a simulated animal performs two tasks known to elicit archetypal responses. We find that experimentally observed ‘phasic’ and ‘tonic’ patterns of LC activity emerge naturally, and that modulation of learning rates improves task performance. This provides a simple, unified computational account of noradrenergic computational function within a general model of behaviour.

Locus Coeruleus tracking of prediction errors optimises cognitive flexibility: An Active Inference model

The locus coeruleus (LC) in the pons is the major source of noradrenaline (NA) in the brain. Two modes of LC firing have been associated with distinct cognitive states: changes in tonic rates of firing are correlated with global levels of arousal and behavioural flexibility, whilst phasic LC responses are evoked by salient stimuli. Here, we unify these two modes of firing by modelling the response of the LC as a correlate of a prediction error when inferring states for action planning under Active Inference (AI). We simulate a classic Go/No-go reward learning task and a three-arm 'explore/exploit' task and show that, if LC activity is considered to reflect the magnitude of high level 'state-action' prediction errors, then both tonic and phasic modes of firing are emergent features of belief updating. We also demonstrate that when contingencies change, AI agents can update their internal models more quickly by feeding back this state-action prediction error-reflected in LC firing and noradrenaline release-to optimise learning rate, enabling large adjustments over short timescales. We propose that such prediction errors are mediated by cortico-LC connections, whilst ascending input from LC to cortex modulates belief updating in anterior cingulate cortex (ACC). In short, we characterise the LC/ NA system within a general theory of brain function. In doing so, we show that contrasting, behaviour-dependent firing patterns are an emergent property of the LC that translates state-action prediction errors into an optimal balance between plasticity and stability.

Structural Changes Observed in the Piriform Cortex in a Rat Model of Pre-motor Parkinson's Disease

Early diagnosis of Parkinson’s disease (PD) offers perhaps, the most promising route to a successful clinical intervention, and the use of an animal model exhibiting symptoms comparable to those observed in PD patients in the early stage of the disease, may facilitate screening of novel therapies for delaying the onset of more debilitating motor and behavioral abnormalities. In this study, a rat model of pre-motor PD was used to study the etiology of hyposmia, a non-motor symptom linked to the early stage of the disease when the motor symptoms have yet to be experienced. The study focussed on determining the effect of a partial reduction of both dopamine and noradrenaline levels on the olfactory cortex. Neuroinflammation and striking structural changes were observed in the model. These changes were prevented by treatment with a neuroprotective drug, a glucagon-like peptide-1 (GLP1) receptor agonist, exendin-4 (EX-4).

Specific Basal Forebrain-Cortical Cholinergic Circuits Coordinate Cognitive Operations

Based on recent molecular genetics, as well as functional and quantitative anatomical studies, the basal forebrain (BF) cholinergic projections, once viewed as a diffuse system, are emerging as being remarkably specific in connectivity. Acetylcholine (ACh) can rapidly and selectively modulate activity of specific circuits and ACh release can be coordinated in multiple areas that are related to particular aspects of cognitive processing. This review discusses how a combination of multiple new approaches with more established techniques are being used to finally reveal how cholinergic neurons, together with other BF neurons, provide temporal structure for behavior, contribute to local cortical state regulation, and coordinate activity between different functionally related cortical circuits. ACh selectively modulates dynamics for encoding and attention within individual cortical circuits, allows for important transitions during sleep, and shapes the fidelity of sensory processing by changing the correlation structure of neural firing. The importance of this system for integrated and fluid behavioral function is underscored by its disease-modifying role; the demise of BF cholinergic neurons has long been established in Alzheimer's disease and recent studies have revealed the involvement of the cholinergic system in modulation of anxiety-related circuits. Therefore, the BF cholinergic system plays a pivotal role in modulating the dynamics of the brain during sleep and behavior, as foretold by the intricacies of its anatomical map.

A Neuroeconomic Framework for Creative Cognition

Neuroeconomics is the study of the neurobiological bases of subjective preferences and choices. We present a novel framework that synthesizes findings from the literatures on neuroeconomics and creativity to provide a neurobiological description of creative cognition. We propose that value-based decision-making processes and activity in the locus ceruleus-norepinephrine (LC-NE) neuromodulatory system underlie creative cognition, as well as the large-scale brain network dynamics shown to be associated with creativity. This reconceptualization leads to several falsifiable hypotheses that can further understanding of creativity, decision making, and brain network dynamics.

Noradrenaline Increases mEPSC Frequency in Pyramidal Cells in Layer II of Rat Barrel Cortex via Calcium Release From Presynaptic Stores

Somatosensory cortex is innervated by afferents originating from the locus coeruleus which typically release noradrenaline. We tested if activation of presynaptic α₁-adrenoceptors (AR) coupled to a G_q-mediated signaling cascade resulted in calcium (Ca²⁺) release from stores and thereby increased spontaneous transmitter release in rat barrel cortex. Adding 1–100 μM noradrenaline (NA) or 5 μM cirazoline (CO), a α₁-AR specific agonist, to the standard artificial cerebrospinal fluid increased the frequency of miniature excitatory postsynaptic currents (mEPSC) by 64 ± 7% in 51% of pyramidal cells in layer II (responders) with no effect on the amplitude. In 42 responders, the mEPSC frequency during control was significantly smaller (39 ± 2 vs. 53 ± 4 Hz) and upon NA exposure, the input resistance (R_in) decreased (9 ± 7%) compared to non-responders. Experiments using CO and the antagonist prazosin revealed that NA acted via binding to α₁-ARs, which was further corroborated by simultaneously blocking β- and α₂-ARs with propranolol and yohimbine, which did not prevent the increase in mEPSC frequency. To verify elements in the signaling cascade, both the phospholipase C inhibitor edelfosine and the membrane permeable IP₃ receptor blocker 2-APB averted the increase in mEPSC frequency. Likewise, emptying Ca²⁺ stores with cyclopiazonic acid or the chelation of intracellular Ca²⁺ with BAPTA-AM prevented the frequency increase, suggesting that the frequency increase was caused by presynaptic store release. When group I metabotropic glutamate receptors were activated with DHPG, co-application of NA occluded a further frequency increase suggesting that the two receptor activations may not signal independently of each other. The increased mEPSC frequency in a subset of pyramidal cells results in enhanced synaptic noise, which, together with the reduction in R_in, will affect computation in the network.

Beta-adrenergic antagonism modulates functional connectivity in the default mode network of individuals with and without autism spectrum disorder

The beta-adrenergic antagonist propranolol benefits some social and communication domains affected in autism spectrum disorder (ASD), and these benefits appear to be associated with increased functional connectivity (FC) in the brain during task performance. FC is implicated in ASD, with the majority of studies suggesting long distance hypo-connectivity combined with regionally specific local hyper-connectivity. The objective in the current investigation was to examine the effect of propranolol on FC at rest and determine whether ASD-specific effects exist. Participants with and without ASD attended three sessions in which propranolol, nadolol (a beta-adrenergic antagonist that does not cross the blood-brain barrier), or placebo were administered. Resting-state fMRI data were acquired, and graph theory techniques were utilized to assess additional aspects of FC. Compared to placebo, propranolol administration was associated with decreased FC in the dorsal medial prefrontal cortex subnetwork of the default mode network and increased FC in the medial temporal lobe subnetwork, regardless of diagnosis. These effects were not seen with nadolol suggesting that the alterations in FC following propranolol administration were not exclusively due to peripheral cardiovascular effects. Thus, beta-adrenergic antagonism can up- or down- regulate FC, depending on the network, and alter coordinated functional activation in the brain. These changes in information processing, as demonstrated by FC, may mediate some of the clinical and behavioral effects of beta-adrenergic antagonism previously reported in patients with ASD.

Stimulation of the Locus Ceruleus Modulates Signal-to-Noise Ratio in the Olfactory Bulb

Norepinephrine (NE) has been shown to influence sensory, and specifically olfactory processing at the behavioral and physiological levels, potentially by regulating signal-to-noise ratio (S/N). The present study is the first to look at NE modulation of olfactory bulb (OB) in regards to S/N in vivo We show, in male rats, that locus ceruleus stimulation and pharmacological infusions of NE into the OB modulate both spontaneous and odor-evoked neural responses. NE in the OB generated a non-monotonic dose-response relationship, suppressing mitral cell activity at high and low, but not intermediate, NE levels. We propose that NE enhances odor responses not through direct potentiation of the afferent signal per se, but rather by reducing the intrinsic noise of the system. This has important implications for the ways in which an animal interacts with its olfactory environment, particularly as the animal shifts from a relaxed to an alert behavioral state.SIGNIFICANCE STATEMENT Sensory perception can be modulated by behavioral states such as hunger, fear, stress, or a change in environmental context. Behavioral state often affects neural processing via the release of circulating neurochemicals such as hormones or neuromodulators. We here show that the neuromodulator norepinephrine modulates olfactory bulb spontaneous activity and odor responses so as to generate an increased signal-to-noise ratio at the output of the olfactory bulb. Our results help interpret and improve existing ideas for neural network mechanisms underlying behaviorally observed improvements in near-threshold odor detection and discrimination.

Brain energetics during the sleep-wake cycle

Brain activity during wakefulness is associated with high metabolic rates that are believed to support information processing and memory encoding. In spite of loss of consciousness, sleep still carries a substantial energy cost. Experimental evidence supports a cerebral metabolic shift taking place during sleep that suppresses aerobic glycolysis, a hallmark of environment-oriented waking behavior and synaptic plasticity. Recent studies reveal that glial astrocytes respond to the reduction of wake-promoting neuromodulators by regulating volume, composition and glymphatic drainage of interstitial fluid. These events are accompanied by changes in neuronal discharge patterns, astrocyte-neuron interactions, synaptic transactions and underlying metabolic features. Internally-generated neuronal activity and network homeostasis are proposed to account for the high sleep-related energy demand.

An update on medications and noninvasive brain stimulation to augment language rehabilitation in post-stroke aphasia

INTRODUCTION

Aphasia is among the most debilitating outcomes of stroke. Aphasia is a language disorder occurring in 10-30% of stroke survivors. Speech and Language Therapy (SLT) is the gold standard, mainstay treatment for aphasia, but gains from SLT may be incomplete. Pharmaceutical and noninvasive brain stimulation (NIBS) techniques may augment the effectiveness of SLT. Areas covered: Herein reviewed are studies of the safety and efficacy of these adjunctive interventions for aphasia, including randomized placebo-controlled and open-label trials, as well as case series from Pubmed, using search terms 'pharmacological,' 'tDCS' or 'TMS' combined with 'aphasia' and 'stroke.' Expert commentary: Relatively small studies have included participants with a range of aphasia types and severities, using inconsistent interventions and outcome measures. Results to-date have provided promising, but weak to moderate evidence that medications and/or NIBS can augment the effects of SLT for improving language outcomes. We end with recommendations for future approaches to studying these interventions, with multicenter, double-blind, randomized controlled trials.

Adults with autism overestimate the volatility of the sensory environment

Insistence on sameness and intolerance of change are among the diagnostic criteria for autism spectrum disorder (ASD), but little research has addressed how people with ASD represent and respond to environmental change. Here, behavioral and pupillometric measurements indicated that adults with ASD are less surprised than neurotypical adults when their expectations are violated, and decreased surprise is predictive of greater symptom severity. A hierarchical Bayesian model of learning suggested that in ASD, a tendency to overlearn about volatility in the face of environmental change drives a corresponding reduction in learning about probabilistically aberrant events, thus putatively rendering these events less surprising. Participant-specific modeled estimates of surprise about environmental conditions were linked to pupil size in the ASD group, thus suggesting heightened noradrenergic responsivity in line with compromised neural gain. This study offers insights into the behavioral, algorithmic and physiological mechanisms underlying responses to environmental volatility in ASD.

Arousal amplifies biased competition between high and low priority memories more in women than in men: The role of elevated noradrenergic activity

Recent findings indicate that emotional arousal can enhance memory consolidation of goal-relevant stimuli while impairing it for irrelevant stimuli. According to one recent model, these goal-dependent memory tradeoffs are driven by arousal-induced release of norepinephrine (NE), which amplifies neural gain in target sensory and memory processing brain regions. Past work also shows that ovarian hormones modulate activity in the same regions thought to support NE's effects on memory, such as the amygdala, suggesting that men and women may be differentially susceptible to arousal's dual effects on episodic memory. Here, we aimed to determine the neurohormonal mechanisms that mediate arousal-biased competition processes in memory. In a competitive visuo-attention task, participants viewed images of a transparent object overlaid on a background scene and explicitly memorized one of these stimuli while ignoring the other. Participants then heard emotional or neutral audio-clips and provided a subjective arousal rating. Hierarchical generalized linear modeling (HGLM) analyses revealed that greater pre-to-post task increases in salivary alpha-amylase (sAA), a biomarker of noradrenergic activity, was associated with significantly greater arousal-enhanced memory tradeoffs in women than in men. These sex-dependent effects appeared to result from phasic and background noradrenergic activity interacting to suppress task-irrelevant representations in women but enhancing them in men. Additionally, in naturally cycling women, low ovarian hormone levels interacted with increased noradrenergic activity to amplify memory selectivity independently of emotion-induced arousal. Together these findings suggest that increased noradrenergic transmission enhances preferential consolidation of goal-relevant memory traces according to phasic arousal and ovarian hormone levels in women.

Effects of acute beta-adrenergic antagonism on verbal problem solving in autism spectrum disorder and exploration of treatment response markers

Autism spectrum disorder (ASD) is characterized by impairments in social communication as well as restricted, repetitive behaviors. Evidence suggests that some individuals with ASD have cognitive impairments related to weak central coherence and hyperrestricted processing. Reducing noradrenergic activity may improve aspects of network processing and thus improve cognitive abilities, such as verbal problem solving, in individuals with ASD. The present pilot study explores the effects of acute administration of the beta-adrenergic antagonist propranolol on verbal problem solving in adults and adolescents with ASD. In a within-subject crossover-design, 20 participants with ASD received a single dose of propranolol or placebo on one of two sessions in a double-blinded, counterbalanced manner. Verbal problem solving was assessed via an anagram task. Baseline measurements of autonomic nervous system functioning were obtained, and anxiety was assessed at baseline and following drug administration. Participants solved the anagrams more quickly in the propranolol condition, as compared to the placebo condition, suggesting a potential cognitive benefit of this agent. Additionally, we observed a negative linear relationship between response to propranolol on the anagram task and two measures of baseline autonomic activity, as well as a positive linear relationship between drug response and baseline anxiety. These relationships propose potential markers for treatment response, as propranolol influences both autonomic functioning and anxiety. Further investigation is needed to expand on the present single-dose psychopharmacological challenge and explore the observed effects of propranolol in a serial-dose setting.

The Olfactory Mosaic: Bringing an Olfactory Network Together for Odor Perception

Olfactory perception and its underlying neural mechanisms are not fixed, but rather vary over time, dependent on various parameters such as state, task, or learning experience. In olfaction, one of the primary sensory areas beyond the olfactory bulb is the piriform cortex. Due to an increasing number of functions attributed to the piriform cortex, it has been argued to be an associative cortex rather than a simple primary sensory cortex. In fact, the piriform cortex plays a key role in creating olfactory percepts, helping to form configural odor objects from the molecular features extracted in the nose. Moreover, its dynamic interactions with other olfactory and nonolfactory areas are also critical in shaping the olfactory percept and resulting behavioral responses. In this brief review, we will describe the key role of the piriform cortex in the larger olfactory perceptual network, some of the many actors of this network, and the importance of the dynamic interactions among the piriform-trans-thalamic and limbic pathways.

Monoaminergic control of brain states and sensory processing: Existing knowledge and recent insights obtained with optogenetics

Monoamines are key neuromodulators involved in a variety of physiological and pathological brain functions. Classical studies using physiological and pharmacological tools have revealed several essential aspects of monoaminergic involvement in regulating the sleep-wake cycle and influencing sensory responses but many features have remained elusive due to technical limitations. The application of optogenetic tools led to the ability of monitoring and controlling neuronal populations with unprecedented temporal precision and neurochemical specificity. Here, we focus on recent advances in revealing the roles of some monoamines in brain state control and sensory information processing. We summarize the central position of monoamines in integrating sensory processing across sleep-wake states with an emphasis on research conducted using optogenetic techniques. Finally, we discuss the limitations and perspectives of new integrated experimental approaches in understanding the modulatory mechanisms of monoaminergic systems in the mammalian brain.

Neuromodulation of olfactory transformations

The olfactory bulb and piriform cortex are the best studied structures of the mammalian olfactory system and are heavily innervated by extrinsic neuromodulatory inputs. The state-dependent release of acetylcholine, norepinephrine, serotonin, and other neuromodulators into these olfactory structures alters a constellation of physiological parameters in neurons and synapses that together modify the computations performed on sensory signals. These modifications affect the specificity, detectability, discriminability, and other properties of odor representations and thereby govern perceptual performance. Whereas different neuromodulators have distinct cellular effects, and tend to be associated with nominally different functions, it also is clear that these purported functions overlap substantially, and that ad hoc hypotheses regarding the roles of particular neuromodulators may have reached the limits of their usefulness.

Optogenetic Activation of Dorsal Raphe Serotonin Neurons Rapidly Inhibits Spontaneous But Not Odor-Evoked Activity in Olfactory Cortex

Serotonin (5-hydroxytriptamine; 5-HT) is implicated in a variety of brain functions including not only the regulation of mood and control of behavior but also the modulation of perception. 5-HT neurons in the dorsal raphe nucleus (DRN) often fire locked to sensory stimuli, but little is known about how 5-HT affects sensory processing, especially on this timescale. Here, we used an optogenetic approach to study the effect of 5-HT on single-unit activity in the mouse primary olfactory (anterior piriform) cortex. We show that activation of DRN 5-HT neurons rapidly inhibits the spontaneous firing of olfactory cortical neurons, acting in a divisive manner, but entirely spares sensory-driven firing. These results identify a new role for serotonergic modulation in dynamically regulating the balance between different sources of neural activity in sensory systems, suggesting a possible role for 5-HT in perceptual inference.

SIGNIFICANCE STATEMENT

Serotonin is implicated in a wide variety of (pato)physiological functions including perception, but its precise role has remained elusive. Here, using optogenetic tools in vivo, we show that serotonergic neuromodulation prominently inhibits the spontaneous electrical activity of neurons in the primary olfactory cortex on a rapid (<1 s) timescale but leaves sensory responses unaffected. These results identify a new role for serotonergic modulation in rapidly changing the balance between different sources of neural activity in sensory systems.

Activation of β-adrenergic receptors in rat visual cortex expands astrocytic processes and reduces extracellular space volume

Brain extracellular space (ECS) is an interconnected channel that allows diffusion-mediated transport of signaling molecules, metabolites, and drugs. We tested the hypothesis that β-adrenergic receptor (βAR) activation impacts extracellular diffusion-mediated transport of molecules through alterations in the morphology of astrocytes. Two structural parameters of ECS-volume fraction and tortuosity-govern extracellular diffusion. Volume fraction (α) is the volume of ECS relative to the total tissue volume. Tortuosity (λ) is a measure of the hindrance that molecules experience in the ECS, compared to a free medium. The real-time iontophoretic (RTI) method revealed that treatment of acutely prepared visual cortical slices of adult female rats with a βAR agonist, DL-isoproterenol (ISO), decreases α significantly, from 0.22 ± 0.03 (mean ± SD) for controls without agonist to 0.18 ± 0.03 with ISO, without altering λ (control: 1.64 ± 0.04; ISO: 1.63 ± 0.04). Electron microscopy revealed that the ISO treatment significantly increased the cytoplasmic area of astrocytic distal endings per unit area of neuropil by 54%. These findings show that norepinephrine decreases α, in part, through an increase in astrocytic volume following βAR activation. Norepinephrine is recognized to be released within the brain during the awake state and increase neurons' signal-to-noise ratio through modulation of neurons' biophysical properties. Our findings uncover a new mechanism for noradrenergic modulation of neuronal signals. Through astrocytic activation leading to a reduction of α, noradrenergic modulation increases extracellular concentration of neurotransmitters and neuromodulators, thereby facilitating neuronal interactions, especially during wakefulness. Synapse 70:307-316, 2016. © 2016 Wiley Periodicals, Inc.