Cell assemblies of the basal forebrain

Hippocampal beta rhythms as a bridge between sensory learning and memory-guided decision-making

A pillar of systems neuroscience has been the study of neural oscillations. Research into these oscillations spans brain areas, species, and disciplines, giving us common ground for discussing typically disparate fields of neuroscience. In this review, we aim to strengthen the dialog between sensory systems research and learning and memory systems research by examining a 15-40 Hz oscillation known as the beta rhythm. Starting with foundational observations based largely in olfactory systems neuroscience, we review evidence suggesting beta-based activity may extend across sensory systems generally, as well as into the hippocampus and areas well known for coordinating decisions and memory-guided behaviors. After evaluating this work, we propose a framework wherein the hippocampal beta oscillation and its diverse coupling with other brain areas can support both sensory learning and memory-guided decision-making. Using this framework, we also propose circuitries that may support these processes, and experiments to test our hypothesis.

Basal Forebrain Chemogenetic Inhibition Converts the Attentional Control Mode of Goal-Trackers to That of Sign-Trackers

Sign tracking versus goal tracking in rats indicate vulnerability and resistance, respectively, to Pavlovian cue-evoked addictive drug taking and relapse. Here, we tested hypotheses predicting that the opponent cognitive-behavioral styles indexed by sign tracking versus goal tracking include variations in attentional performance which differentially depend on basal forebrain projection systems. Pavlovian Conditioned Approach (PCA) testing was used to identify male and female sign-trackers (STs) and goal-trackers (GTs), as well as rats with an intermediate phenotype (INTs). Upon reaching asymptotic performance in an operant task requiring the detection of visual signals (hits) as well as the reporting of signal absence for 40 min per session, GTs scored more hits than STs, and hit rates across all phenotypes correlated with PCA scores. STs missed relatively more signals than GTs specifically during the last 15 min of a session. Chemogenetic inhibition of the basal forebrain decreased hit rates in GTs but was without effect in STs. Moreover, the decrease in hits in GTs manifested solely during the last 15 min of a session. Transfection efficacy in the horizontal limb of the diagonal band (HDB), but not substantia innominate (SI) or nucleus basalis of Meynert (nbM), predicted the behavioral efficacy of chemogenetic inhibition in GTs. Furthermore, the total subregional transfection space, not transfection of just cholinergic neurons, correlated with performance effects. These results indicate that the cognitive-behavioral phenotype indexed by goal tracking, but not sign tracking, depends on activation of the basal forebrain-frontal cortical projection system and associated biases toward top-down or model-based performance.

The Regulation Effect of α7nAChRs and M1AChRs on Inflammation and Immunity in Sepsis

The inflammatory storm in the early stage and immunosuppression in the late stage are responsible for the high mortality rates and multiple organ dysfunction in sepsis. In recent years, studies have found that the body's cholinergic system can spontaneously and dynamically regulate inflammation and immunity in sepsis according to the needs of the body. Firstly, the vagus nerve senses and regulates local or systemic inflammation by means of the Cholinergic Anti-inflammatory Pathway (CAP) and activation of α7-nicotinic acetylcholine receptors (α7nAChRs); thus, α7nAChRs play important roles for the central nervous system (CNS) to modulate peripheral inflammation; secondly, the activation of muscarinic acetylcholine receptors 1 (M1AChRs) in the forebrain can affect the neurons of the Medullary Visceral Zone (MVZ), the core of CAP, to regulate systemic inflammation and immunity. Based on the critical role of these two cholinergic receptor systems in sepsis, it is necessary to collect and analyze the related findings in recent years to provide ideas for further research studies and clinical applications. By consulting the related literature, we draw some conclusions: MVZ is the primary center for the nervous system to regulate inflammation and immunity. It coordinates not only the sympathetic system and vagus system but also the autonomic nervous system and neuroendocrine system to regulate inflammation and immunity; α7nAChRs are widely expressed in immune cells, neurons, and muscle cells; the activation of α7nAChRs can suppress local and systemic inflammation; the expression of α7nAChRs represents the acute or chronic inflammatory state to a certain extent; M1AChRs are mainly expressed in the advanced centers of the brain and regulate systemic inflammation; neuroinflammation of the MVZ, hypothalamus, and forebrain induced by sepsis not only leads to their dysfunctions but also underlies the regulatory dysfunction on systemic inflammation and immunity. Correcting the neuroinflammation of these regulatory centers and adjusting the function of α7nAChRs and M1AChRs may be two key strategies for the treatment of sepsis in the future.

Contribution of the basal forebrain to corticocortical network interactions

Basal forebrain (BF) cholinergic neurons provide the cerebral cortex with acetylcholine. Despite the long-established involvement of these cells in sensory processing, attention, and memory, the mechanisms by which cholinergic signaling regulates cognitive processes remain elusive. In this study, we recorded spiking and local field potential data simultaneously from several locations in the BF, and sites in the orbitofrontal and visual cortex in transgenic ChAT-Cre rats performing a visual discrimination task. We observed distinct differences in the fine spatial distributions of gamma coherence values between specific basalo-cortical and cortico-cortical sites that shifted across task phases. Additionally, cholinergic firing induced spatial changes in cortical gamma power, and optogenetic activation of BF increased coherence between specific cortico-cortical sites, suggesting that the cholinergic system contributes to selective modulation of cortico-cortical circuits. Furthermore, the results suggest that cells in specific BF locations are dynamically recruited across behavioral epochs to coordinate interregional cortical processes underlying cognition.

Phase precession in the human hippocampus and entorhinal cortex

Knowing where we are, where we have been, and where we are going is critical to many behaviors, including navigation and memory. One potential neuronal mechanism underlying this ability is phase precession, in which spatially tuned neurons represent sequences of positions by activating at progressively earlier phases of local network theta oscillations. Based on studies in rodents, researchers have hypothesized that phase precession may be a general neural pattern for representing sequential events for learning and memory. By recording human single-neuron activity during spatial navigation, we show that spatially tuned neurons in the human hippocampus and entorhinal cortex exhibit phase precession. Furthermore, beyond the neural representation of locations, we show evidence for phase precession related to specific goal states. Our findings thus extend theta phase precession to humans and suggest that this phenomenon has a broad functional role for the neural representation of both spatial and non-spatial information.

Differential recruitment of ventral pallidal e-types by behaviorally salient stimuli during Pavlovian conditioning

The ventral pallidum (VP) is interfacing striatopallidal and limbic circuits, conveying information about salience and valence crucial to adjusting behavior. However, how VP neuron populations with distinct electrophysiological properties (e-types) represent these variables is not fully understood. Therefore, we trained mice on probabilistic Pavlovian conditioning while recording the activity of VP neurons. Many VP neurons responded to punishment (54%), reward (48%), and outcome-predicting auditory stimuli (32%), increasingly differentiating distinct outcome probabilities through learning. We identified e-types based on the presence of bursts or fast rhythmic discharges and found that non-bursting, non-rhythmic neurons were the most sensitive to reward and punishment. Some neurons exhibited distinct responses of their bursts and single spikes, suggesting a multiplexed coding scheme in the VP. Finally, we demonstrate synchronously firing neuron assemblies, particularly responsive to reinforcing stimuli. These results suggest that electrophysiologically defined e-types of the VP differentially participate in transmitting reinforcement signals during learning.

Distinct synchronization, cortical coupling and behavioral function of two basal forebrain cholinergic neuron types

Basal forebrain cholinergic neurons (BFCNs) modulate synaptic plasticity, cortical processing, brain states and oscillations. However, whether distinct types of BFCNs support different functions remains unclear. Therefore, we recorded BFCNs in vivo, to examine their behavioral functions, and in vitro, to study their intrinsic properties. We identified two distinct types of BFCNs that differ in their firing modes, synchronization properties and behavioral correlates. Bursting cholinergic neurons (Burst-BFCNs) fired synchronously, phase-locked to cortical theta activity and fired precisely timed bursts after reward and punishment. Regular-firing cholinergic neurons (Reg-BFCNs) were found predominantly in the posterior basal forebrain, displayed strong theta rhythmicity and responded with precise single spikes after behavioral outcomes. In an auditory detection task, synchronization of Burst-BFCNs to the auditory cortex predicted the timing of behavioral responses, whereas tone-evoked cortical coupling of Reg-BFCNs predicted correct detections. We propose that differential recruitment of two basal forebrain cholinergic neuron types generates behavior-specific cortical activation.

Forebrain Cholinergic Signaling: Wired and Phasic, Not Tonic, and Causing Behavior

Conceptualizations of cholinergic signaling as primarily spatially diffuse and slow-acting are based largely on measures of extracellular brain ACh levels that require several minutes to generate a single data point. In addition, most such studies inhibited the highly potent catalytic enzyme for ACh, AChE, to facilitate measurement of ACh. Absent such inhibition, AChE limits the presence of ambient ACh and thus renders it unlikely that ACh influences target regions via slow changes in extracellular ACh concentrations. We describe an alternative view by which forebrain signaling in cortex driving cognition is largely phasic (milliseconds to perhaps seconds), and unlikely to be volume-transmitted. This alternative is supported by new evidence from real-time amperometric recordings of cholinergic signaling indicating a specific function of rapid, phasic, transient cholinergic signaling in attentional contexts. Previous neurochemical evidence may be reinterpreted in terms of integrated phasic cholinergic activity that mediates specific behavioral and cognitive operations; this reinterpretation fits well with recent computational models. Optogenetic studies support a causal relationship between cholinergic transients and behavior. This occurs in part via transient-evoked muscarinic receptor-mediated high-frequency oscillations in cortical regions. Such oscillations outlast cholinergic transients and thus link transient ACh signaling with more sustained postsynaptic activity patterns to support relatively persistent attentional biases. Reconceptualizing cholinergic function as spatially specific, phasic, and modulating specific cognitive operations is theoretically powerful and may lead to pharmacologic treatments more effective than those based on traditional views.Dual Perspectives Companion Paper: Diverse Spatiotemporal Scales of Cholinergic Signaling in the Neocortex, by Anita A. Disney and Michael J. Higley.

Altered motor, anxiety-related and attentional task performance at baseline associate with multiple gene copies of the vesicular acetylcholine transporter and related protein overexpression in ChAT::Cre+ rats

Transgenic rodents expressing Cre recombinase cell specifically are used for exploring mechanisms regulating behavior, including those mediated by cholinergic signaling. However, it was recently reported that transgenic mice overexpressing a bacterial artificial chromosome containing choline acetyltransferase (ChAT) gene, for synthesizing the neurotransmitter acetylcholine, present with multiple vesicular acetylcholine transporter (VAChT) gene copies, resulting in altered cholinergic tone and accompanying behavioral abnormalities. Since ChAT::Cre+ rats, used increasingly for understanding the biological basis of CNS disorders, utilize the mouse ChAT promotor to control Cre recombinase expression, we assessed for similar genotypical and phenotypical differences in such rats compared to wild-type siblings. The rats were assessed for mouse VAChT copy number, VAChT protein expression levels and for sustained attention, response control and anxiety. Rats were also subjected to a contextual fear conditioning paradigm using an unconditional fear-inducing stimulus (electrical foot shocks), with blood samples taken at baseline, the fear acquisition phase and retention testing, for measuring blood plasma markers of hypothalamic–pituitary–adrenal gland (HPA)-axis activity. ChAT::Cre+ rats expressed multiple mouse VAChT gene copies, resulting in significantly higher VAChT protein expression, revealed anxiolytic behavior, hyperlocomotion and deficits in tasks requiring sustained attention. The HPA-axis was intact, with unaltered circulatory levels of acute stress-induced corticosterone, leptin and glucose. Our findings, therefore, reveal that in ChAT::Cre+ rats, VAChT overexpression associates with significant alterations of certain cognitive, motor and affective functions. Although highly useful as an experimental tool, it is essential to consider the potential effects of altered cholinergic transmission on baseline behavior in ChAT::Cre rats.

Basal forebrain chemogenetic inhibition disrupts the superior complex movement control of goal-tracking rats

Sign- and goal-tracking behavior signifies the influence of opposed cognitive-motivational styles, with the former being characterized by a tendency for approaching and contacting reward cues, including a readiness for attending, bottom-up, to salient cues, and a relatively greater vulnerability for developing and maintaining addiction-like behaviors. We previously demonstrated that these styles also impact the cognitive-motor interactions that are taxed during traversal of dynamic surfaces, with goal-trackers (GTs) making less movement errors and falling less frequently than sign-trackers (STs). The present experiment tested the hypothesis that complex movement control in GTs, but not STs, depends on activation of the basal forebrain projection system to telencephalic regions. Chemogenetic inhibition of the basal forebrain increased movement errors and falls in GTs during traversal of a rotating zigzag rod but had no significant effect on the relatively lower performance of STs. Neurochemical evidence confirmed the efficacy of the inhibitory designer receptor exclusively activated by designer drug (DREADD). Administration of clozapine-N-oxide (CNO) had no significant effect in GTs not expressing the DREADD. These results indicate that GTs, but not STs, activate the basal forebrain projection system to mediate their relatively superior ability for complex movement control. STs may also serve as an animal model in research on the role of basal forebrain systems in aging- and Parkinson's disease-associated falls. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Regulation of cholinergic basal forebrain development, connectivity, and function by neurotrophin receptors

Cholinergic basal forebrain (cBF) neurons are defined by their expression of the p75 neurotrophin receptor (p75^NTR) and tropomyosin-related kinase (Trk) neurotrophin receptors in addition to cholinergic markers. It is known that the neurotrophins, particularly nerve growth factor (NGF), mediate cholinergic neuronal development and maintenance. However, the role of neurotrophin signalling in regulating adult cBF function is less clear, although in dementia, trophic signalling is reduced and p75^NTR mediates neurodegeneration of cBF neurons. Here we review the current understanding of how cBF neurons are regulated by neurotrophins which activate p75^NTR and TrkA, B or C to influence the critical role that these neurons play in normal cortical function, particularly higher order cognition. Specifically, we describe the current evidence that neurotrophins regulate the development of basal forebrain neurons and their role in maintaining and modifying mature basal forebrain synaptic and cortical microcircuit connectivity. Understanding the role neurotrophin signalling plays in regulating the precision of cholinergic connectivity will contribute to the understanding of normal cognitive processes and will likely provide additional ideas for designing improved therapies for the treatment of neurological disease in which cholinergic dysfunction has been demonstrated.

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.

Space and Time: The Hippocampus as a Sequence Generator

Neural computations are often compared to instrument-measured distance or duration, and such relationships are interpreted by a human observer. However, neural circuits do not depend on human-made instruments but perform computations relative to an internally defined rate-of-change. While neuronal correlations with external measures, such as distance or duration, can be observed in spike rates or other measures of neuronal activity, what matters for the brain is how such activity patterns are utilized by downstream neural observers. We suggest that hippocampal operations can be described by the sequential activity of neuronal assemblies and their internally defined rate of change without resorting to the concept of space or time.

Multiplexed oscillations and phase rate coding in the basal forebrain

Complex behaviors demand temporal coordination among functionally distinct brain regions. The basal forebrain's afferent and efferent structure suggests a capacity for mediating this coordination at a large scale. During performance of a spatial orientation task, synaptic activity in this region was dominated by four amplitude-independent oscillations temporally organized by the phase of the slowest, a theta-frequency rhythm. Oscillation amplitudes were also organized by task epoch and positively correlated to the task-related modulation of individual neuron firing rates. For many neurons, spiking was temporally organized through phase precession against theta band field potential oscillations. Theta phase precession advanced in parallel to task progression, rather than absolute spatial location or time. Together, the findings reveal a process by which associative brain regions can integrate independent oscillatory inputs and transform them into sequence-specific, rate-coded outputs that are adaptive to the pace with which organisms interact with their environment.

Transformation of a Spatial Map across the Hippocampal-Lateral Septal Circuit

The hippocampus constructs a map of the environment. How this "cognitive map" is utilized by other brain regions to guide behavior remains unexplored. To examine how neuronal firing patterns in the hippocampus are transmitted and transformed, we recorded neurons in its principal subcortical target, the lateral septum (LS). We observed that LS neurons carry reliable spatial information in the phase of action potentials, relative to hippocampal theta oscillations, while the firing rates of LS neurons remained uninformative. Furthermore, this spatial phase code had an anatomical microstructure within the LS and was bound to the hippocampal spatial code by synchronous gamma frequency cell assemblies. Using a data-driven model, we show that rate-independent spatial tuning arises through the dynamic weighting of CA1 and CA3 cell assemblies. Our findings demonstrate that transformation of the hippocampal spatial map depends on higher-order theta-dependent neuronal sequences. VIDEO ABSTRACT.

Selective Activation of Basal Forebrain Cholinergic Neurons Attenuates Polymicrobial Sepsis-Induced Inflammation via the Cholinergic Anti-Inflammatory Pathway

Supplemental Digital Content is available in the text.

Objectives:

Basal forebrain cholinergic neurons are proposed as a major neuromodulatory system in inflammatory modulation. However, the function of basal forebrain cholinergic neurons in sepsis is unknown, and the neural pathways underlying cholinergic anti-inflammation remain unexplored.

Design:

Animal research.

Setting:

University research laboratory.

Subjects:

Male wild-type C57BL/6 mice and ChAT-ChR2-EYFP (ChAT) transgenic mice.

Interventions:

The cholinergic neuronal activity of the basal forebrain was manipulated optogenetically. Cecal ligation and puncture was produced to induce sepsis. Left cervical vagotomy and 6-hydroxydopamine injection to the spleen were used.

Measurements and Main Results:

Photostimulation of basal forebrain cholinergic neurons induced a significant decrease in the levels of tumor necrosis factor-α and interleukin-6 in the serum and spleen. When cecal ligation and puncture was combined with left cervical vagotomy in photostimulated ChAT mice, these reductions in tumor necrosis factor-α and interleukin-6 were partly reversed. Furthermore, photostimulating basal forebrain cholinergic neurons induced a large increase in c-Fos expression in the basal forebrain, the dorsal motor nucleus of the vagus, and the ventral part of the solitary nucleus. Among them, 35.2% were tyrosine hydroxylase positive neurons. Furthermore, chemical denervation showed that dopaminergic neurotransmission to the spleen is indispensable for the anti-inflammation.

Conclusions:

These results are the first to demonstrate that selectively activating basal forebrain cholinergic neurons is sufficient to attenuate systemic inflammation in sepsis. Specifically, photostimulation of basal forebrain cholinergic neurons activated dopaminergic neurons in dorsal motor nucleus of the vagus/ventral part of the solitary nucleus, and this dopaminergic efferent signal was further transmitted by the vagus nerve to the spleen. This cholinergic-to-dopaminergic neural circuitry, connecting central cholinergic neurons to the peripheral organ, might have mediated the anti-inflammatory effect in sepsis.

Distinct Temporal Coordination of Spontaneous Population Activity between Basal Forebrain and Auditory Cortex

The basal forebrain (BF) has long been implicated in attention, learning and memory, and recent studies have established a causal relationship between artificial BF activation and arousal. However, neural ensemble dynamics in the BF still remains unclear. Here, recording neural population activity in the BF and comparing it with simultaneously recorded cortical population under both anesthetized and unanesthetized conditions, we investigate the difference in the structure of spontaneous population activity between the BF and the auditory cortex (AC) in mice. The AC neuronal population show a skewed spike rate distribution, a higher proportion of short (≤80 ms) inter-spike intervals (ISIs) and a rich repertoire of rhythmic firing across frequencies. Although the distribution of spontaneous firing rate in the BF is also skewed, a proportion of short ISIs can be explained by a Poisson model at short time scales (≤20 ms) and spike count correlations are lower compared to AC cells, with optogenetically identified cholinergic cell pairs showing exceptionally higher correlations. Furthermore, a smaller fraction of BF neurons shows spike-field entrainment across frequencies: a subset of BF neurons fire rhythmically at slow (≤6 Hz) frequencies, with varied phase preferences to ongoing field potentials, in contrast to a consistent phase preference of AC populations. Firing of these slow rhythmic BF cells is correlated to a greater degree than other rhythmic BF cell pairs. Overall, the fundamental difference in the structure of population activity between the AC and BF is their temporal coordination, in particular their operational timescales. These results suggest that BF neurons slowly modulate downstream populations whereas cortical circuits transmit signals on multiple timescales. Thus, the characterization of the neural ensemble dynamics in the BF provides further insight into the neural mechanisms, by which brain states are regulated.

The menagerie of the basal forebrain: how many (neural) species are there, what do they look like, how do they behave and who talks to whom?

The diverse cell-types of the basal forebrain control sleep-wake states, cortical activity and reward processing. Large, slow-firing, cholinergic neurons suppress cortical delta activity and promote cortical plasticity in response to reinforcers. Large, fast-firing, cortically-projecting GABAergic neurons promote wakefulness and fast cortical activity. In particular, parvalbumin/GABAergic neurons promote neocortical gamma band activity. Conversely, excitation of slower-firing somatostatin/GABAergic neurons promotes sleep through inhibition of cortically-projecting neurons. Activation of glutamatergic neurons promotes wakefulness, likely by exciting other cortically-projecting neurons. Similarly, cholinergic neurons indirectly promote wakefulness by excitation of wake-promoting, cortically-projecting GABAergic neurons and/or inhibition of sleep-promoting somatostatin/GABAergic neurons. Both glia and neurons increase the levels of adenosine during prolonged wakefulness. Adenosine presynaptically inhibits glutamatergic inputs to wake-promoting cholinergic and GABAergic/parvalbumin neurons, promoting sleep.

Gamma band directional interactions between basal forebrain and visual cortex during wake and sleep states

The basal forebrain (BF) is an important regulator of cortical excitability and responsivity to sensory stimuli, and plays a major role in wake-sleep regulation. While the impact of BF on cortical EEG or LFP signals has been extensively documented, surprisingly little is known about LFP activity within BF. Based on bilateral recordings from rats in their home cage, we describe endogenous LFP oscillations in the BF during quiet wakefulness, rapid eye movement (REM) and slow wave sleep (SWS) states. Using coherence and Granger causality methods, we characterize directional influences between BF and visual cortex (VC) during each of these states. We observed pronounced BF gamma activity particularly during wakefulness, as well as to a lesser extent during SWS and REM. During wakefulness, this BF gamma activity exerted a directional influence on VC that was associated with cortical excitation. During SWS but not REM, there was also a robust directional gamma band influence of BF on VC. In all three states, directional influence in the gamma band was only present in BF to VC direction and tended to be regulated specifically within each brain hemisphere. Locality of gamma band LFPs to the BF was confirmed by demonstration of phase locking of local spiking activity to the gamma cycle. We report novel aspects of endogenous BF LFP oscillations and their relationship to cortical LFP signals during sleep and wakefulness. We link our findings to known aspects of GABAergic BF networks that likely underlie gamma band LFP activations, and show that the Granger causality analyses can faithfully recapitulate many known attributes of these networks.

Rhythmic coordination of hippocampal neurons during associative memory processing

Hippocampal oscillations are dynamic, with unique oscillatory frequencies present during different behavioral states. To examine the extent to which these oscillations reflect neuron engagement in distinct local circuit processes that are important for memory, we recorded single cell and local field potential activity from the CA1 region of the hippocampus as rats performed a context-guided odor-reward association task. We found that theta (4–12 Hz), beta (15–35 Hz), low gamma (35–55 Hz), and high gamma (65–90 Hz) frequencies exhibited dynamic amplitude profiles as rats sampled odor cues. Interneurons and principal cells exhibited unique engagement in each of the four rhythmic circuits in a manner that related to successful performance of the task. Moreover, principal cells coherent to each rhythm differentially represented task dimensions. These results demonstrate that distinct processing states arise from the engagement of rhythmically identifiable circuits, which have unique roles in organizing task-relevant processing in the hippocampus.

DOI:http://dx.doi.org/10.7554/eLife.09849.001

Electrodes placed on the surface of the scalp can reveal rhythmic patterns of electrical activity within the brain. These rhythms reflect the coordinated firing of large numbers of neurons that are connected together within a network in order to process information. A single network can show rhythms with various different frequencies depending on its local connections and the pattern of input that it receives at any given time.

One region that exhibits striking changes in these rhythmic patterns is the hippocampus: a brain area that plays a key role in memory. The hippocampus contains many cell types, including interneurons (which form connections with nearby cells) and principal cells (which connect with cells outside of this region). Though both participate in rhythmic circuits, little is known about the different extents to which these distinct cell types are engaged in rhythmic processing, or how rhythmic processing might support memory.

Rangel, Rueckemann, Rivière et al. have now addressed these questions by using electrodes to record from the hippocampus as rats learned to associate specific odors in different environments with a reward. As the rats sniffed the odors, their brains showed four different hippocampal rhythms: from a low frequency called “theta”, through “beta” and “low gamma” up to “high gamma” frequencies. Each of these hippocampal rhythms varied in strength over time, indicating that rhythmic processing is dynamic during the task.

Rangel, Rueckemann, Rivière et al. found that neurons fired rhythmically during trials in which the rat chose the correct odor-environment combination. In these correct trials, individual principal cells were more likely to fire in synchrony with only one of the rhythms. In contrast, interneurons were more likely to fire in synchrony to each of the four rhythms at some point during a correct choice. Among the four rhythms, coordinated principal cell and interneuron firing with respect to the beta rhythm was most tightly linked with a correct choice. These findings reveal that investigation of rhythmic dynamics in the hippocampus can provide insight into how the timing of cell activity is coordinated to support memory.

DOI:http://dx.doi.org/10.7554/eLife.09849.002

Endocannabinoids Mediate Muscarinic Acetylcholine Receptor-Dependent Long-Term Depression in the Adult Medial Prefrontal Cortex

Cholinergic inputs into the prefrontal cortex (PFC) are associated with attention and cognition; however there is evidence that acetylcholine also has a role in PFC dependent learning and memory. Muscarinic acetylcholine receptors (mAChR) in the PFC can induce synaptic plasticity, but the underlying mechanisms remain either opaque or unresolved. We have characterized a form of mAChR mediated long-term depression (LTD) at glutamatergic synapses of layer 5 principal neurons in the adult medial PFC. This mAChR LTD is induced with the mAChR agonist carbachol and inhibited by selective M₁ mAChR antagonists. In contrast to other cortical regions, we find that this M₁ mAChR mediated LTD is coupled to endogenous cannabinoid (eCB) signaling. Inhibition of the principal eCB CB₁ receptor blocked carbachol induced LTD in both rats and mice. Furthermore, when challenged with a sub-threshold carbachol application, LTD was induced in slices pretreated with the monoacylglycerol lipase (MAGL) inhibitor JZL184, suggesting that the eCB 2-arachidonylglyerol (2-AG) mediates M₁ mAChR LTD. Yet, when endogenous acetylcholine was released from local cholinergic afferents in the PFC using optogenetics, it failed to trigger eCB-LTD. However coupling patterned optical and electrical stimulation to generate local synaptic signaling allowed the reliable induction of LTD. The light—electrical pairing induced LTD was M₁ mAChR and CB₁ receptor mediated. This shows for the first time that connecting excitatory synaptic activity with coincident endogenously released acetylcholine controls synaptic gain via eCB signaling. Together these results shed new light on the mechanisms of synaptic plasticity in the adult PFC and expand on the actions of endogenous cholinergic signaling.

Basal forebrain dynamics during nonassociative and associative olfactory learning

Cholinergic and GABAergic projections from the horizontal diagonal band (HDB) and medial preoptic area (MCPO) of the basal forebrain to the olfactory system are associated with odor discrimination and odor learning, as well as modulation of neural responses in olfactory structures. Whereas pharmacological and lesion studies give insights into the functional role of these modulatory inputs on a slow timescale, the response dynamics of neurons in the HDB/MCPO during olfactory behaviors have not been investigated. In this study we examined how these neurons respond during two olfactory behaviors: spontaneous investigation of odorants and odor-reward association learning. We observe rich heterogeneity in the response dynamics of individual HDB/MCPO neurons, with a substantial fraction of neurons exhibiting task-related modulation. HDB/MCPO neurons show both rapid and transient responses during bouts of odor investigation and slow, long-lasting modulation of overall response rate based on behavioral demands. Specifically, baseline rates were higher during the acquisition phase of an odor-reward association than during spontaneous investigation or the recall phase of an odor reward association. Our results suggest that modulatory projections from the HDB/MCPO are poised to influence olfactory processing on multiple timescales, from hundreds of milliseconds to minutes, and are therefore capable of rapidly setting olfactory network dynamics during odor processing and learning.