Circuit mechanism for suppression of frontal cortical ignition during NREM sleep.

Conscious perception is greatly diminished during sleep, but the underlying circuit mechanism is poorly understood. We show that cortical ignition-a brain process shown to be associated with conscious awareness in humans and non-human primates-is strongly suppressed during non-rapid-eye-movement (NREM) sleep in mice due to reduced cholinergic modulation and rapid inhibition of cortical responses. Brain-wide functional ultrasound imaging and cell-type-specific calcium imaging combined with optogenetics showed that activity propagation from visual to frontal cortex is markedly reduced during NREM sleep due to strong inhibition of frontal pyramidal neurons. Chemogenetic activation and inactivation of basal forebrain cholinergic neurons powerfully increased and decreased visual-to-frontal activity propagation, respectively. Furthermore, although multiple subtypes of dendrite-targeting GABAergic interneurons in the frontal cortex are more active during wakefulness, soma-targeting parvalbumin-expressing interneurons are more active during sleep. Chemogenetic manipulation of parvalbumin interneurons showed that sleep/wake-dependent cortical ignition is strongly modulated by perisomatic inhibition of pyramidal neurons.

The retrotrapezoid nucleus and the neuromodulation of breathing.

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO2/H+ and regulate several aspects of breathing, including inspiration and active expiration.

The Functional Organization of Cortical and Thalamic Inputs onto Five Types of Striatal Neurons Is Determined by Source and Target Cell Identities.

To understand striatal function, it is essential to know the functional organization of the numerous inputs targeting the diverse population of striatal neurons. Using optogenetics, we activated terminals from ipsi- or contralateral primary somatosensory cortex (S1) or primary motor cortex (M1), or thalamus while obtaining simultaneous whole-cell recordings from pairs or triplets of striatal medium spiny neurons (MSNs) and adjacent interneurons. Ipsilateral corticostriatal projections provided stronger excitation to fast-spiking interneurons (FSIs) than to MSNs and only sparse and weak excitation to low threshold-spiking interneurons (LTSIs) and cholinergic interneurons (ChINs). Projections from contralateral M1 evoked the strongest responses in LTSIs but none in ChINs, whereas thalamus provided the strongest excitation to ChINs but none to LTSIs. In addition, inputs varied in their glutamate receptor composition and their short-term plasticity. Our data revealed a highly selective organization of excitatory striatal afferents, which is determined by both pre- and postsynaptic neuronal identity.

Cholinergic Transmission at Muscarinic Synapses in the Striatum Is Driven Equally by Cortical and Thalamic Inputs.

The release of acetylcholine from cholinergic interneurons (ChIs) directly modulates striatal output via muscarinic receptors on medium spiny neurons (MSNs). While thalamic inputs provide strong excitatory input to ChIs, cortical inputs primarily regulate MSN firing. Here, we found that, while thalamic inputs do drive ChI firing, a subset of ChIs responds robustly to stimulation of cortical inputs as well. To examine how input-evoked changes in ChI firing patterns drive acetylcholine release at cholinergic synapses onto MSNs, muscarinic M4-receptor-mediated synaptic events were measured in MSNs overexpressing G-protein gated potassium channels (GIRK2). Stimulation of both cortical and thalamic inputs was sufficient to equally drive muscarinic synaptic events in MSNs, resulting from the broad synaptic innervation of the stimulus-activated ChI population across many MSNs. Taken together, this indicates an underappreciated role for the extensive cholinergic network, in which small populations of ChIs can drive substantial changes in post-synaptic receptor activity across the striatum.

Nucleus basalis of Meynert neuronal activity in Parkinson's disease.

OBJECTIVE: Neuronal loss within the cholinergic nucleus basalis of Meynert (nbM) correlates with cognitive decline in dementing disorders such as Alzheimer's disease and Parkinson's disease (PD). In nonhuman primates, the nbM firing pattern (5-40 Hz) has also been correlated with working memory and sustained attention. In this study, authors performed microelectrode recordings of the globus pallidus pars interna (GPi) and the nbM immediately prior to the implantation of bilateral deep brain stimulation (DBS) electrodes in PD patients to treat motor symptoms and cognitive impairment, respectively. Here, the authors evaluate the electrophysiological properties of the nbM in patients with PD. METHODS: Five patients (4 male, mean age 66 ± 4 years) with PD and mild cognitive impairment underwent bilateral GPi and nbM DBS lead implantation. Microelectrode recordings were performed through the GPi and nbM along a single trajectory. Firing rates and burst indices were characterized for each neuronal population with the patient at rest and performing a sustained-attention auditory oddball task. Action potential (AP) depolarization and repolarization widths were measured for each neuronal population at rest. RESULTS: In PD patients off medication, the authors identified neuronal discharge rates that were specific to each area populated by GPi cells (92.6 ± 46.1 Hz), border cells (34 ± 21 Hz), and nbM cells (13 ± 10 Hz). During the oddball task, firing rates of nbM cells decreased (2.9 ± 0.9 to 2.0 ± 1.1 Hz, p < 0.05). During baseline recordings, the burst index for nbM cells (1.7 ± 0.6) was significantly greater than those for GPi cells (1.2 ± 0.2, p < 0.05) and border cells (1.1 ± 0.1, p < 0.05). There was no significant difference in the nbM burst index during the oddball task relative to baseline (3.4 ± 1.7, p = 0.20). With the patient at rest, the width of the depolarization phase of APs did not differ among the GPi cells, border cells, and nbM cells (p = 0.60); however, during the repolarization phase, the nbM spikes were significantly longer than those for GPi high-frequency discharge cells (p < 0.05) but not the border cells (p = 0.20). CONCLUSIONS: Neurons along the trajectory through the GPi and nbM have distinct firing patterns. The profile of nbM activity is similar to that observed in nonhuman primates and is altered during a cognitive task associated with cholinergic activation. These findings will serve to identify these targets intraoperatively and form the basis for further research to characterize the role of the nbM in cognition.

Cholinergic Interneurons Amplify Thalamostriatal Excitation of Striatal Indirect Pathway Neurons in Parkinson's Disease Models.

The motor symptoms of Parkinson's disease (PD) are thought to stem from an imbalance in the activity of striatal direct- and indirect-pathway spiny projection neurons (SPNs). Disease-induced alterations in the activity of networks controlling SPNs could contribute to this imbalance. One of these networks is anchored by the parafascicular nucleus (PFn) of the thalamus. To determine the role of the PFn in striatal PD pathophysiology, optogenetic, chemogenetic, and electrophysiological tools were used in ex vivo slices from transgenic mice with region-specific Cre recombinase expression. These studies revealed that in parkinsonian mice, the functional connectivity of PFn neurons with indirect pathway SPNs (iSPNs) was selectively enhanced by cholinergic interneurons acting through presynaptic nicotinic acetylcholine receptors (nAChRs) on PFn terminals. Attenuating this network adaptation by chemogenetic or genetic strategies alleviated motor-learning deficits in parkinsonian mice, pointing to a potential new therapeutic strategy for PD patients.

Excitatory extrinsic afferents to striatal interneurons and interactions with striatal microcircuitry.

The striatum constitutes the main input structure of the basal ganglia and receives two major excitatory glutamatergic inputs, from the cortex and the thalamus. Excitatory cortico- and thalamostriatal connections innervate the principal neurons of the striatum, the spiny projection neurons (SPNs), which constitute the main cellular input as well as the only output of the striatum. In addition, corticostriatal and thalamostriatal inputs also innervate striatal interneurons. Some of these inputs have been very well studied, for example the thalamic innervation of cholinergic interneurons and the cortical innervation of striatal fast-spiking interneurons, but inputs to most other GABAergic interneurons remain largely unstudied, due in part to the relatively recent identification and characterization of many of these interneurons. In this review, we will discuss and reconcile some older as well as more recent data on the extrinsic excitatory inputs to striatal interneurons. We propose that the traditional feed-forward inhibitory model of the cortical input to the fast-spiking interneuron then inhibiting the SPN, often assumed to be the prototype of the main functional organization of striatal interneurons, is incomplete. We provide evidence that the extrinsic innervation of striatal interneurons is not uniform but shows great cell-type specificity. In addition, we will review data showing that striatal interneurons are themselves interconnected in a highly cell-type-specific manner. These data suggest that the impact of the extrinsic inputs on striatal activity critically depends on synaptic interactions within interneuronal circuitry.

Remodeling of cholinergic input to the hippocampus after noise exposure and tinnitus induction in Guinea pigs.

Here, we investigate remodeling of hippocampal cholinergic inputs after noise exposure and determine the relevance of these changes to tinnitus. To assess the effects of noise exposure on the hippocampus, guinea pigs were exposed to unilateral noise for 2 hr and 2 weeks later, immunohistochemistry was performed on hippocampal sections to examine vesicular acetylcholine transporter (VAChT) expression. To evaluate whether the changes in VAChT were relevant to tinnitus, another group of animals was exposed to the same noise band twice to induce tinnitus, which was assessed using gap-prepulse Inhibition of the acoustic startle (GPIAS) 12 weeks after the first noise exposure, followed by immunohistochemistry. Acoustic Brainstem Response (ABR) thresholds were elevated immediately after noise exposure for all experimental animals but returned to baseline levels several days after noise exposure. ABR wave I amplitude-intensity functions did not show any changes after 2 or 12 weeks of recovery compared to baseline levels. In animals assessed 2-weeks following noise-exposure, hippocampal VAChT puncta density decreased on both sides of the brain by 20-60% in exposed animals. By 12 weeks following the initial noise exposure, changes in VAChT puncta density largely recovered to baseline levels in exposed animals that did not develop tinnitus, but remained diminished in animals that developed tinnitus. These tinnitus-specific changes were particularly prominent in hippocampal synapse-rich layers of the dentate gyrus and areas CA3 and CA1, and VAChT density in these regions negatively correlated with tinnitus severity. The robust changes in VAChT labeling in the hippocampus 2 weeks after noise exposure suggest involvement of this circuitry in auditory processing. After chronic tinnitus induction, tinnitus-specific changes occurred in synapse-rich layers of the hippocampus, suggesting that synaptic processing in the hippocampus may play an important role in the pathophysiology of tinnitus.

The Role of Cholinergic Midbrain Neurons in Startle and Prepulse Inhibition.

One of the two major cholinergic centers of the mammalian brain is located in the midbrain, i.e., the pedunculopontine tegmentum (PPTg) and the adjacent laterodorsal tegmentum. These cholinergic neurons have been shown to be important for e.g., arousal, reward associations, and sleep. They also have been suggested to mediate sensorimotor gating, measured as prepulse inhibition of startle (PPI). PPI disruptions are a hallmark of schizophrenia and are observed in various other psychiatric disorders, where they are associated with, and often predictive of, other cognitive symptoms. PPI has been proposed to be mediated by a short midbrain circuitry including inhibitory cholinergic projections from PPTg to the startle pathway. Although the data indicating the involvement of the PPTg is very robust, some more recent evidence challenges that there is a cholinergic contribution to PPI. We here use transient optogenetic activation of specifically the cholinergic PPTg neurons in male and female rats to address their role in startle modulation in general, and in PPI specifically. Although we could confirm the crucial role of PPTg cholinergic neurons in associative reward learning, validating our experimental approach, we found that activation of cholinergic PPTg neurons did not inhibit startle responses. In contrast, activation of cholinergic PPTg neurons enhanced startle, which is in accordance with their general role in arousal and indicate a potential involvement in sensitization of startle. We conclude that noncholinergic PPTg neurons mediate PPI in contrast to the longstanding hypothetical view that PPI is mediated by cholinergic PPTg neurons.SIGNIFICANCE STATEMENT Activation of cholinergic neurons in the midbrain has been assumed to mediate prepulse inhibition of startle (PPI), a common measure of sensorimotor gating that is disrupted in schizophrenia and other psychiatric disorders. We here revisit this long-standing hypothesis using optogenetic activation of these specific neurons combined with startle testing in rats. In contrast to the hypothetical role of these neurons in startle modulation, we show that their activation leads to an increase of baseline startle and to prepulse facilitation. This supports recent data by others that have started to cast some doubt on the cholinergic hypothesis of PPI, and calls for a revision of the theoretical construct of PPI mechanisms.

Cholinergic neurons in the dorsomedial hypothalamus regulate food intake.

OBJECTIVE: Central cholinergic neural circuits play a role in the regulation of feeding behavior. The dorsomedial hypothalamus (DMH) is considered the appetite-stimulating center and contains cholinergic neurons. Here, we study the role of DMH cholinergic neurons in the control of food intake. METHODS: To selectively stimulate DMH cholinergic neurons, we expressed stimulatory designer receptors exclusively activated by designer drugs (DREADDs) and channelrhodopsins in DMH cholinergic neurons by injection of adeno-associated virus (AAV) vectors into the DMH of choline acetyltransferase (ChAT)-IRES-Cre mice. We also generated transgenic mice expressing channelrhodopsins in cholinergic neurons with the Cre-LoxP technique. To delete the Chat gene exclusively in the DMH, we injected an AAV carrying a Cre recombinase transgene into the DMH of floxed ChAT mice. Food intake was measured with and without selective stimulation of DMH cholinergic neurons. RESULTS: Mice lacking the Chat gene in the DMH show reduced body weight as compared to control. Chemogenetic activation of DMH cholinergic neurons promotes food intake. This orexigenic effect is further supported by experiments of optogenetic stimulation of DMH cholinergic neurons. DMH cholinergic neurons innervate pro-opiomelanocortin neurons in the arcuate nucleus of the hypothalamus (ARC). Treatment with acetylcholine (ACh) enhances GABAergic inhibitory transmission to ARC POMC neurons that is blocked by the muscarinic receptor antagonist. Direct activation of cholinergic fibers in the ARC readily stimulates food intake that is also abolished by the muscarinic receptor antagonist. CONCLUSION: ACh released from DMH cholinergic neurons regulates food intake and body weight. This effect is mediated in part through regulation of ARC POMC neurons. Activation of muscarinic receptors on GABAergic axon terminals enhances inhibitory tone to ARC POMC neurons. Hence, this novel DMHACh â†’ ARCPOMC pathway plays an important role in the control of food intake and body weight.

Metabotropic glutamate receptor 2 inhibits thalamically-driven glutamate and dopamine release in the dorsal striatum.

The striatum plays critical roles in action control and cognition, and activity of striatal neurons is driven by glutamatergic input. Inhibition of glutamatergic inputs to projection neurons and interneurons of the striatum by presynaptic G protein-coupled receptors (GPCRs) stands to modulate striatal output and striatum-dependent behaviors. Despite knowledge that a substantial number of glutamatergic inputs to striatal neurons originate in the thalamus, most electrophysiological studies assessing GPCR modulation do not differentiate between effects on corticostriatal and thalamostriatal transmission, and synaptic inhibition is frequently assumed to be mediated by activation of GPCRs on corticostriatal terminals. We used optogenetic techniques and recently-discovered pharmacological tools to dissect the effects of a prominent presynaptic GPCR, metabotropic glutamate receptor 2 (mGlu2), on corticostriatal vs. thalamostriatal transmission. We found that an agonist of mGlu2 and mGlu3 induces long-term depression (LTD) at synapses onto MSNs from both the cortex and the thalamus. Thalamostriatal LTD is selectively blocked by an mGlu2-selective negative allosteric modulator and reversed by application of an antagonist following LTD induction. Activation of mGlu2/3 also induces LTD of thalamostriatal transmission in striatal cholinergic interneurons (CINs), and pharmacological activation of mGlu2/3 or selective activation of mGlu2 inhibits CIN-mediated dopamine release evoked by selective stimulation of thalamostriatal inputs. Thus, mGlu2 activation exerts effects on striatal physiology that extend beyond modulation of corticostriatal synapses, and has the potential to influence cognition and striatum-related disorders via inhibition of thalamus-derived glutamate and dopamine release.

Aging-Related Dysfunction of Striatal Cholinergic Interneurons Produces Conflict in Action Selection.

For goal-directed action to remain adaptive, new strategies are required to accommodate environmental changes, a process for which parafascicular thalamic modulation of cholinergic interneurons in the striatum (PF-to-CIN) appears critical. In the elderly, however, previously acquired experience frequently interferes with new learning, yet the source of this effect has remained unexplored. Here, combining sophisticated behavioral designs, cell-specific manipulation, and extensive neuronal imaging, we investigated the involvement of the PF-to-CIN pathway in this process. We found functional alterations of this circuit in aged mice that were consistent with their incapacity to update initial goal-directed learning, resulting in faulty activation of projection neurons in the striatum. Toxicogenetic ablation of CINs in young mice reproduced these behavioral and neuronal defects, suggesting that age-related deficits in PF-to-CIN function reduce the ability of older individuals to resolve conflict between actions, likely contributing to impairments in adaptive goal-directed action and executive control in aging. VIDEO ABSTRACT.

Functional differentiation of cholinergic and noradrenergic modulation in a biophysical model of olfactory bulb granule cells.

Olfactory bulb granule cells are modulated by both acetylcholine (ACh) and norepinephrine (NE), but the effects of these neuromodulators have not been clearly distinguished. We used detailed biophysical simulations of granule cells, both alone and embedded in a microcircuit with mitral cells, to measure and distinguish the effects of ACh and NE on cellular and microcircuit function. Cholinergic and noradrenergic modulatory effects on granule cells were based on data obtained from slice experiments; specifically, ACh reduced the conductance densities of the potassium M current and the calcium-dependent potassium current, whereas NE nonmonotonically regulated the conductance density of an ohmic potassium current. We report that the effects of ACh and NE on granule cell physiology are distinct and functionally complementary to one another. ACh strongly regulates granule cell firing rates and afterpotentials, whereas NE bidirectionally regulates subthreshold membrane potentials. When combined, NE can regulate the ACh-induced expression of afterdepolarizing potentials and persistent firing. In a microcircuit simulation developed to investigate the effects of granule cell neuromodulation on mitral cell firing properties, ACh increased spike synchronization among mitral cells, whereas NE modulated the signal-to-noise ratio. Coapplication of ACh and NE both functionally improved the signal-to-noise ratio and enhanced spike synchronization among mitral cells. In summary, our computational results support distinct and complementary roles for ACh and NE in modulating olfactory bulb circuitry and suggest that NE may play a role in the regulation of cholinergic function.

Oral Administration of Gintonin Attenuates Cholinergic Impairments by Scopolamine, Amyloid-ß Protein, and Mouse Model of Alzheimer's Disease.

Gintonin is a novel ginseng-derived lysophosphatidic acid (LPA) receptor ligand. Oral administration of gintonin ameliorates learning and memory dysfunctions in Alzheimer's disease (AD) animal models. The brain cholinergic system plays a key role in cognitive functions. The brains of AD patients show a reduction in acetylcholine concentration caused by cholinergic system impairments. However, little is known about the role of LPA in the cholinergic system. In this study, we used gintonin to investigate the effect of LPA receptor activation on the cholinergic system in vitro and in vivo using wild-type and AD animal models. Gintonin induced [Ca(2+)]i transient in cultured mouse hippocampal neural progenitor cells (NPCs). Gintonin-mediated [Ca(2+)]i transients were linked to stimulation of acetylcholine release through LPA receptor activation. Oral administration of gintonin-enriched fraction (25, 50, or 100 mg/kg, 3 weeks) significantly attenuated scopolamine-induced memory impairment. Oral administration of gintonin (25 or 50 mg/kg, 2 weeks) also significantly attenuated amyloid-ß protein (Aß)-induced cholinergic dysfunctions, such as decreased acetylcholine concentration, decreased choline acetyltransferase (ChAT) activity and immunoreactivity, and increased acetylcholine esterase (AChE) activity. In a transgenic AD mouse model, long-term oral administration of gintonin (25 or 50 mg/kg, 3 months) also attenuated AD-related cholinergic impairments. In this study, we showed that activation of G protein-coupled LPA receptors by gintonin is coupled to the regulation of cholinergic functions. Furthermore, this study showed that gintonin could be a novel agent for the restoration of cholinergic system damages due to Aß and could be utilized for AD prevention or therapy.

Cortical network switching: possible role of the lateral septum and cholinergic arousal.

BACKGROUND: Cortical networks undergo large-scale switching between states of increased or decreased activity in normal sleep and cognition as well as in pathological conditions such as epilepsy. We previously found that focal hippocampal seizures in rats induce increased neuronal firing and cerebral blood flow in subcortical structures including the lateral septal area, along with frontal cortical slow oscillations resembling slow wave sleep. In addition, stimulation of the lateral septum in the absence of a seizure resulted in cortical deactivation with slow oscillations. HYPOTHESIS: We hypothesized that lateral septal activation might cause neocortical deactivation indirectly, possibly through impaired subcortical arousal. But how does subcortical stimulation cause slow wave activity in frontal cortex? How do arousal neurotransmitter levels (e.g. acetylcholine) change in cortex during the excitation of inhibitory projection nuclei? METHODS AND RESULTS: In the current study, we used simultaneous electrophysiology and enzyme-based amperometry in a rat model, and found a decrease in choline, along with slow wave activity in orbital frontal cortex during lateral septal stimulation in the absence of seizures. In contrast, the choline signal and local field potential in frontal cortex had no significant changes when stimulating the hippocampus, but showed increased choline and decreased slow wave activity with an arousal stimulus produced by toe pinch. CONCLUSIONS: These findings indicate that the activation of subcortical inhibitory structures (such as lateral septum) can depress subcortical cholinergic arousal. This mechanism may play an important role in large-scale transitions of cortical activity in focal seizures, as well as in normal cortical function.

Wake-sleep, thermoregulatory, and autonomic effects of cholinergic activation of the lateral hypothalamus in the rat: a pilot study.

A major role in the wake-promoting effects of the activation of the lateral hypothalamus (LH) has been ascribed to a population of orexin (ORX)-containing neurons that send projections to central areas which regulate Wake-Sleep and autonomic function. Since, in the rat, a substantial amount of ORX neurons receive cholinergic projections from cells involved in Wake-Sleep regulation, the aim of this study was to assess the role played by LH cholinoceptive cells in Wake-Sleep and autonomic regulations. To this end, the effects of a microinjection of the cholinergic agonist Carbachol (CBL) into the LH were compared to those obtained through the activation of a wider cell population by the microinjection of the GABAA antagonist GABAzine (GBZ). The results of this pilot study showed that both drugs elicited the same behavioral and autonomic effects, those caused by GBZ being larger and longer-lasting than those following administration of CBL. Briefly, wakefulness was enhanced and sleep was depressed, and brain temperature and heart rate consistently increased, while mean arterial pressure showed only a mild increment. Surprisingly, the administration of the drug vehicle (SAL) elicited a similar pattern of Wake-Sleep effects which, although much smaller, were sufficient to mask any statistical significance between treatment and control data. In conclusion, the results of this work show that the arousal elicited by LH disinhibition by GABAzine is concomitant with autonomic responses set by the intervention of cold-defense mechanisms. Since the same response is elicited at a lower level by CBL administration, the hypothesis of an involvement of cholinoceptive ORX neurons in its generation is discussed.

Specific multi-nutrient enriched diet enhances hippocampal cholinergic transmission in aged rats.

Fortasyn Connect (FC) is a specific nutrient combination designed to target synaptic dysfunction in Alzheimer's disease by providing neuronal membrane precursors and other supportive nutrients. The aim of the present study was to investigate the effects of FC on hippocampal cholinergic neurotransmission in association with its effects on synaptic membrane formation in aged rats. Eighteen-month-old male Wistar rats were randomized to receive a control diet for 4 weeks or an FC-enriched diet for 4 or 6 weeks. At the end of the dietary treatments, acetylcholine (ACh) release was investigated by in vivo microdialysis in the right hippocampi. On completion of microdialysis studies, the rats were sacrificed, and the left hippocampi were obtained to determine the levels of choline, ACh, membrane phospholipids, synaptic proteins, and choline acetyltransferase. Our results revealed that supplementation with FC diet for 4 or 6 weeks, significantly enhanced basal and stimulated hippocampal ACh release and ACh tissue levels, along with levels of phospholipids. Feeding rats the FC diet for 6 weeks significantly increased the levels of choline acetyltransferase, the presynaptic marker Synapsin-1, and the postsynaptic marker PSD-95, but decreased levels of Nogo-A, a neurite outgrowth inhibitor. These data show that the FC diet enhances hippocampal cholinergic neurotransmission in aged rats and suggest that this effect is mediated by enhanced synaptic membrane formation. These data provide further insight into cellular and molecular mechanisms by which FC may support memory processes in Alzheimer's disease.

Mechanisms underlying desynchronization of cholinergic-evoked thalamic network activity.

Synchronous neuronal activity in the thalamocortical system is critical for a number of behaviorally relevant computations, but hypersynchrony can limit information coding and lead to epileptiform responses. In the somatosensory thalamus, afferent inputs are transformed by networks of reciprocally connected thalamocortical neurons in the ventrobasal nucleus (VB) and GABAergic neurons in the thalamic reticular nucleus (TRN). These networks can generate oscillatory activity, and studies in vivo and in vitro have suggested that thalamic oscillations are often accompanied by synchronous neuronal activity, in part mediated by widespread divergence and convergence of both reticulothalamic and thalamoreticular pathways, as well as by electrical synapses interconnecting TRN neurons. However, the functional organization of thalamic circuits and its role in shaping input-evoked activity patterns remain poorly understood. Here we show that optogenetic activation of cholinergic synaptic afferents evokes near-synchronous firing in mouse TRN neurons that is rapidly desynchronized in thalamic networks. We identify several mechanisms responsible for desynchronization: (1) shared inhibitory inputs in local VB neurons leading to asynchronous and imprecise rebound bursting; (2) TRN-mediated lateral inhibition that further desynchronizes firing in the VB; and (3) powerful yet sparse thalamoreticular connectivity that mediates re-excitation of the TRN but preserves asynchronous firing. Our findings reveal how distinct local circuit features interact to desynchronize thalamic network activity.

Neuromodulators produce distinct activated states in neocortex.

Neocortical population activity varies between deactivated and activated states marked by the presence and absence of slow oscillations, respectively. Neocortex activation occurs during waking and vigilance and is readily induced in anesthetized animals by stimulating the brainstem reticular formation, basal forebrain, or thalamus. Neuromodulators are thought to be responsible for these changes in cortical activity, but their selective cortical effects (i.e., without actions in other brain areas) on neocortical population activity in vivo are not well defined. We found that selective cholinergic and noradrenergic stimulation of the barrel cortex produces well differentiated activated states in rats. Cholinergic cortical stimulation activates the cortex by abolishing synchronous slow oscillations and shifting firing to a tonic mode, which increases in rate at high doses. This shift causes the sensory thalamus itself to become activated. In contrast, noradrenergic cortical stimulation activates the cortex by abolishing synchronous slow oscillations but suppresses overall cortical firing rate, which deactivates the thalamus. Cortical activation produced by either of these neuromodulators leads to suppressed sensory responses and more focused receptive fields. High-frequency sensory stimuli are best relayed to barrel cortex during cortical cholinergic activation because this also activates the thalamus. Cortical neuromodulation sets different cortical and thalamic states that may serve to control sensory information processing according to behavioral contingencies.

Assessment of sensorimotor gating following selective lesions of cholinergic pedunculopontine neurons.

Sensorimotor gating is the state-dependent transfer of sensory information into a motor system. When this occurs at an early stage of the processing stream it enables stimuli to be filtered out or partially ignored, thereby reducing the demands placed on advanced systems. Prepulse inhibition (PPI) of the acoustic startle reflex (ASR) is the standard measure of sensorimotor gating. A brainstem-midbrain circuitry is widely viewed as mediating both PPI and ASR. In this circuitry, the pedunculopontine tegmental nucleus (PPTg) integrates sensory input and cortico-basal ganglia output and, via presumed cholinergic signaling, inhibits ASR-generating neurons within the reticular formation. Non-selective damage to all neuronal types within PPTg reduces PPI. We assessed whether this effect originates in the loss of cholinergic signaling by examining ASR and PPI in rats bearing non-selective (excitotoxic) or selective cholinergic (Dtx-UII) lesions of PPTg. Excitotoxic lesions had no effect on ASR but reduced PPI at all prepulse levels tested. In contrast, selective depletion of cholinergic neurons reduced ASR to the extent that PPI was not measurable with standard (10-20 s) inter-trial intervals. Subsequent testing revealed appreciable ASRs could be generated when the inter-trial interval was increased (180 s). Under these conditions, PPI was assessed and no deficits were found after lesions of cholinergic PPTg neurons. These results show that cholinergic output from PPTg is essential for rapidly regenerating the ASR, but has no influence on PPI. Results are discussed in terms of sensorimotor integration circuitry and psychiatric disorders that feature disrupted ASR and PPI.