Hippocampal theta rhythm is reduced by suppression of the H-current in septohippocampal GABAergic neurons

Basal forebrain innervation of the amygdala: an anatomical and computational exploration

Theta oscillations of the mammalian amygdala are associated with processing, encoding and retrieval of aversive memories. In the hippocampus, the power of the network theta oscillation is modulated by basal forebrain (BF) GABAergic projections. Here, we combine anatomical and computational approaches to investigate if similar BF projections to the amygdaloid complex provide an analogous modulation of local network activity. We used retrograde tracing with fluorescent immunohistochemistry to identify cholinergic and non-cholinergic parvalbumin- or calbindin-immunoreactive BF neuronal subgroups targeting the input (lateral and basolateral nuclei) and output (central nucleus and the central bed nucleus of the stria terminalis) regions of the amygdaloid complex. We observed a dense non-cholinergic, putative GABAergic projection from the ventral pallidum (VP) and the substantia innominata (SI) to the basolateral amygdala (BLA). The VP/SI axonal projections to the BLA were confirmed using viral anterograde tracing and transsynaptic labeling. We tested the potential function of this VP/SI-BLA pathway in a 1000-cell biophysically realistic network model, which incorporated principal neurons and three major interneuron groups of the BLA, together with extrinsic glutamatergic, cholinergic, and VP/SI GABAergic inputs. We observed in silico that theta-modulation of VP/SI GABAergic projections enhanced theta oscillations in the BLA via their selective innervation of the parvalbumin-expressing local interneurons. Ablation of parvalbumin-, but not somatostatin- or calretinin-expressing, interneurons reduced theta power in the BLA model. These results suggest that long-range BF GABAergic projections may modulate network activity at their target regions through the formation of a common interneuron-type and oscillatory phase-specific disinhibitory motif.

Optogenetic silencing of medial septal GABAergic neurons disrupts grid cell spatial and temporal coding in the medial entorhinal cortex

The hippocampus and medial entorhinal cortex (MEC) form a cognitive map that facilitates spatial navigation. As part of this map, MEC grid cells fire in a repeating hexagonal pattern across an environment. This grid pattern relies on inputs from the medial septum (MS). The MS, and specifically GABAergic neurons, are essential for theta rhythm oscillations in the entorhinal-hippocampal network; however, the role of this population in grid cell function is unclear. To investigate this, we use optogenetics to inhibit MS-GABAergic neurons and observe that MS-GABAergic inhibition disrupts grid cell spatial periodicity. Grid cell spatial periodicity is disrupted during both optogenetic inhibition periods and short inter-stimulus intervals. In contrast, longer inter-stimulus intervals allow for the recovery of grid cell spatial firing. In addition, grid cell phase precession is also disrupted. These findings highlight the critical role of MS-GABAergic neurons in maintaining grid cell spatial and temporal coding in the MEC.

Missing Puzzle Pieces in Dementia Research: HCN Channels and Theta Oscillations

Increasing evidence indicates a role of hyperpolarization activated cation (HCN) channels in controlling the resting membrane potential, pacemaker activity, memory formation, sleep, and arousal. Their disfunction may be associated with the development of epilepsy and age-related memory decline. Neuronal hyperexcitability involved in epileptogenesis and EEG desynchronization occur in the course of dementia in human Alzheimer's Disease (AD) and animal models, nevertheless the underlying ionic and cellular mechanisms of these effects are not well understood. Some suggest that theta rhythms involved in memory formation could be used as a marker of memory disturbances in the course of neurogenerative diseases, including AD. This review focusses on the interplay between hyperpolarization HCN channels, theta oscillations, memory formation and their role(s) in dementias, including AD. While individually, each of these factors have been linked to each other with strong supportive evidence, we hope here to expand this linkage to a more inclusive picture. Thus, HCN channels could provide a molecular target for developing new therapeutic agents for preventing and/or treating dementia.

Huygens synchronization of medial septal pacemaker neurons generates hippocampal theta oscillation

Episodic learning and memory retrieval are dependent on hippocampal theta oscillation, thought to rely on the GABAergic network of the medial septum (MS). To test how this network achieves theta synchrony, we recorded MS neurons and hippocampal local field potential simultaneously in anesthetized and awake mice and rats. We show that MS pacemakers synchronize their individual rhythmicity frequencies, akin to coupled pendulum clocks as observed by Huygens. We optogenetically identified them as parvalbumin-expressing GABAergic neurons, while MS glutamatergic neurons provide tonic excitation sufficient to induce theta. In accordance, waxing and waning tonic excitation is sufficient to toggle between theta and non-theta states in a network model of single-compartment inhibitory pacemaker neurons. These results provide experimental and theoretical support to a frequency-synchronization mechanism for pacing hippocampal theta, which may serve as an inspirational prototype for synchronization processes in the central nervous system from Nematoda to Arthropoda to Chordate and Vertebrate phyla.

Lamotrigine Attenuates Neuronal Excitability, Depresses GABA Synaptic Inhibition, and Modulates Theta Rhythms in Rat Hippocampus

Theta oscillations generated in hippocampal (HPC) and cortical neuronal networks are involved in various aspects of brain function, including sensorimotor integration, movement planning, memory formation and attention. Disruptions of theta rhythms are present in individuals with brain disorders, including epilepsy and Alzheimer’s disease. Theta rhythm generation involves a specific interplay between cellular (ion channel) and network (synaptic) mechanisms. HCN channels are theta modulators, and several medications are known to enhance their activity. We investigated how different doses of lamotrigine (LTG), an HCN channel modulator, and antiepileptic and neuroprotective agent, would affect HPC theta rhythms in acute HPC slices (in vitro) and anaesthetized rats (in vivo). Whole-cell patch clamp recordings revealed that LTG decreased GABA_A-fast transmission in CA3 cells, in vitro. In addition, LTG directly depressed CA3 and CA1 pyramidal neuron excitability. These effects were partially blocked by ZD 7288, a selective HCN blocker, and are consistent with decreased excitability associated with antiepileptic actions. Lamotrigine depressed HPC theta oscillations in vitro, also consistent with its neuronal depressant effects. In contrast, it exerted an opposite, enhancing effect, on theta recorded in vivo. The contradictory in vivo and in vitro results indicate that LTG increases ascending theta activating medial septum/entorhinal synaptic inputs that over-power the depressant effects seen in HPC neurons. These results provide new insights into LTG actions and indicate an opportunity to develop more precise therapeutics for the treatment of dementias, memory disorders and epilepsy.

The Medial Septum as a Potential Target for Treating Brain Disorders Associated With Oscillopathies

The medial septum (MS), as part of the basal forebrain, supports many physiological functions, from sensorimotor integration to cognition. With often reciprocal connections with a broad set of peers at all major divisions of the brain, the MS orchestrates oscillatory neuronal activities throughout the brain. These oscillations are critical in generating sensory and emotional salience, locomotion, maintaining mood, supporting innate anxiety, and governing learning and memory. Accumulating evidence points out that the physiological oscillations under septal influence are frequently disrupted or altered in pathological conditions. Therefore, the MS may be a potential target for treating neurological and psychiatric disorders with abnormal oscillations (oscillopathies) to restore healthy patterns or erase undesired ones. Recent studies have revealed that the patterned stimulation of the MS alleviates symptoms of epilepsy. We discuss here that stimulus timing is a critical determinant of treatment efficacy on multiple time scales. On-demand stimulation may dramatically reduce side effects by not interfering with normal physiological functions. A precise pattern-matched stimulation through adaptive timing governed by the ongoing oscillations is essential to effectively terminate pathological oscillations. The time-targeted strategy for the MS stimulation may provide an effective way of treating multiple disorders including Alzheimer's disease, anxiety/fear, schizophrenia, and depression, as well as pain.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

Effects of Several Classes of Voltage-Gated Ion Channel Conductances on Gamma and Theta Oscillations in a Hippocampal Microcircuit Model

Gamma and theta oscillations have been functionally associated with cognitive processes, such as learning and memory. Synaptic conductances play an important role in the generation of intrinsic network rhythmicity, but few studies have examined the effects of voltage-gated ion channels (VGICs) on these rhythms. In this report, we have used a pyramidal-interneuron-gamma (PING) network consisting of excitatory pyramidal cells and two types of inhibitory interneurons. We have constructed a conductance-based neural network incorporating a persistent sodium current (I_NaP), a delayed rectifier potassium current (I_KDR), a inactivating potassium current (I_A) and a hyperpolarization-activated current (I_H). We have investigated the effects of several conductances on network theta and gamma frequency oscillations. Variation of all conductances of interest changed network rhythmicity. Theta power was altered by all conductances tested. Gamma rhythmogenesis was dependent on I_A and I_H. The I_KDR currents in excitatory pyramidal cells as well as both types of inhibitory interneurons were essential for theta rhythmogenesis and altered gamma rhythm properties. Increasing I_NaP suppressed both gamma and theta rhythms. Addition of noise did not alter these patterns. Our findings suggest that VGICs strongly affect brain network rhythms. Further investigations in vivo will be of great interest, including potential effects on neural function and cognition.

Entorhinal velocity signals reflect environmental geometry

The entorhinal cortex contains neurons that represent self-location, including grid cells that fire in periodic locations and velocity signals that encode running speed and head direction. Although the size and shape of the environment influence grid patterns, whether entorhinal velocity signals are equally influenced or provide a universal metric for self-motion across environments remains unknown. Here we report that speed cells rescale after changes to the size and shape of the environment. Moreover, head direction cells reorganize in an experience-dependent manner to align with the axis of environmental change. A knockout mouse model allows dissociation of the coordination between cell types, with grid and speed cells, but not head direction cells, responding in concert to environmental change. These results point to malleability in the coding features of multiple entorhinal cell types and have implications for which cell types contribute to the velocity signal used by computational models of grid cells.

Long-term high-intensity sound stimulation inhibits h current (Ih ) in CA1 pyramidal neurons

Afferent neurotransmission to hippocampal pyramidal cells can lead to long-term changes to their intrinsic membrane properties and affect many ion currents. One of the most plastic neuronal currents is the hyperpolarization-activated cationic current (I_h ), which changes in CA1 pyramidal cells in response to many types of physiological and pathological processes, including auditory stimulation. Recently, we demonstrated that long-term potentiation (LTP) in rat hippocampal Schaffer-CA1 synapses is depressed by high-intensity sound stimulation. Here, we investigated whether a long-term high-intensity sound stimulation could affect intrinsic membrane properties of rat CA1 pyramidal neurons. Our results showed that I_h is depressed by long-term high-intensity sound exposure (1 min of 110 dB sound, applied two times per day for 10 days). This resulted in a decreased resting membrane potential, increased membrane input resistance and time constant, and decreased action potential threshold. In addition, CA1 pyramidal neurons from sound-exposed animals fired more action potentials than neurons from control animals; however, this effect was not caused by a decreased I_h . On the other hand, a single episode (1 min) of 110 dB sound stimulation which also inhibits hippocampal LTP did not affect I_h and firing in pyramidal neurons, suggesting that effects on I_h are long-term responses to high-intensity sound exposure. Our results show that prolonged exposure to high-intensity sound affects intrinsic membrane properties of hippocampal pyramidal neurons, mainly by decreasing the amplitude of I_h .

Basal forebrain control of wakefulness and cortical rhythms

Wakefulness, along with fast cortical rhythms and associated cognition, depend on the basal forebrain (BF). BF cholinergic cell loss in dementia and the sedative effect of anti-cholinergic drugs have long implicated these neurons as important for cognition and wakefulness. The BF also contains intermingled inhibitory GABAergic and excitatory glutamatergic cell groups whose exact neurobiological roles are unclear. Here we show that genetically targeted chemogenetic activation of BF cholinergic or glutamatergic neurons in behaving mice produced significant effects on state consolidation and/or the electroencephalogram but had no effect on total wake. Similar activation of BF GABAergic neurons produced sustained wakefulness and high-frequency cortical rhythms, whereas chemogenetic inhibition increased sleep. Our findings reveal a major contribution of BF GABAergic neurons to wakefulness and the fast cortical rhythms associated with cognition. These findings may be clinically applicable to manipulations aimed at increasing forebrain activation in dementia and the minimally conscious state.

The mammalian basal forebrain controls cortical rhythm and wake-sleep. Anaclet et al. use genetically-targeted chemogenetic systems to activate or inhibit cholinergic, glutamatergic or GABAergic neurons in this region, and reveal their contributions to behavioral and electrocortical arousal in behaving mice.

Turning a Negative into a Positive: Ascending GABAergic Control of Cortical Activation and Arousal

Gamma-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the brain. Recent technological advances have illuminated the role of GABAergic neurons in control of cortical arousal and sleep. Sleep-promoting GABAergic neurons in the preoptic hypothalamus are well-known. Less well-appreciated are GABAergic projection neurons in the brainstem, midbrain, hypothalamus, and basal forebrain, which paradoxically promote arousal and fast electroencephalographic (EEG) rhythms. Thus, GABA is not purely a sleep-promoting neurotransmitter. GABAergic projection neurons in the brainstem nucleus incertus and ventral tegmental nucleus of Gudden promote theta (4-8 Hz) rhythms. Ventral tegmental area GABAergic neurons, neighboring midbrain dopamine neurons, project to the frontal cortex and nucleus accumbens. They discharge faster during cortical arousal and regulate reward. Thalamic reticular nucleus GABAergic neurons initiate sleep spindles in non-REM sleep. In addition, however, during wakefulness, they tonically regulate the activity of thalamocortical neurons. Other GABAergic inputs to the thalamus arising in the globus pallidus pars interna, substantia nigra pars reticulata, zona incerta, and basal forebrain regulate motor activity, arousal, attention, and sensory transmission. Several subpopulations of cortically projecting GABAergic neurons in the basal forebrain project to the thalamus and neocortex and preferentially promote cortical gamma-band (30-80 Hz) activity and wakefulness. Unlike sleep-active GABAergic neurons, these ascending GABAergic neurons are fast-firing neurons which disinhibit and synchronize the activity of their forebrain targets, promoting the fast EEG rhythms typical of conscious states. They are prominent targets of GABAergic hypnotic agents. Understanding the properties of ascending GABAergic neurons may lead to novel treatments for diseases involving disorders of cortical activation and wakefulness.

Cholinergic modulation of hippocampal network function

Cholinergic septohippocampal projections from the medial septal area to the hippocampus are proposed to have important roles in cognition by modulating properties of the hippocampal network. However, the precise spatial and temporal profile of acetylcholine release in the hippocampus remains unclear making it difficult to define specific roles for cholinergic transmission in hippocampal dependent behaviors. This is partly due to a lack of tools enabling specific intervention in, and recording of, cholinergic transmission. Here, we review the organization of septohippocampal cholinergic projections and hippocampal acetylcholine receptors as well as the role of cholinergic transmission in modulating cellular excitability, synaptic plasticity, and rhythmic network oscillations. We point to a number of open questions that remain unanswered and discuss the potential for recently developed techniques to provide a radical reappraisal of the function of cholinergic inputs to the hippocampus.

Inter-tissue networks between the basal forebrain, hippocampus, and prefrontal cortex in a model for depression caused by disturbed sleep

Disturbances in sleep are encountered in the majority of patients with depressive disorder. To elucidate the molecular mechanisms behind this relationship, we examined gene expression changes in a rodent model for disturbed sleep and depression. The animals were treated with daily injections of clomipramine to affect their sleep during early infancy. This early interference with sleep is known to induce depression-like behavior in adult animals. After 2 weeks of treatment, the change in gene expression was examined using the Affymetrix Rat 230.2 chip. We studied the gene expression in the basal forebrain, hippocampus, and frontal cortex and combined the results to reveal the otherwise indissectible networks between and around the tissues. The major disrupted pathways between the three brain areas were related to synaptic transmission, regulation of translation, and ubiquitinylation. The involved pathways were within the cellular components of the axons, growth cones, melanosomes, and pigment granules. A network analysis allowing for additional interactors, in the form of chemicals or gene products, revealed a disturbed communicational network between the different brain areas. This disturbed network is centered around serotonin, Mn(II), and Rhoa. The findings elucidate inter-tissue pathways and networks in the brain that are involved in sleep and mood regulation. The findings are of uttermost interest, some are quite predictable and obvious, but some are novel or have only been proposed by rare theoretical speculations (such as the melanosome and Mn(II) involvement). Equally important as the findings are the methods described in this article. In this study, we present two novel simple ways to perform system biological analysis based on gene expression array data. We used two already existing tools in a new way, and by careful planning of the input data, managed to extrapolate intricate hidden inter-tissue networks to build a molecular picture of disease.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Adenosine modulates the excitability of layer II stellate neurons in entorhinal cortex through A1 receptors

Stellate neurons in layer II entorhinal cortex (EC) provide the main output from the EC to the hippocampus. It is believed that adenosine plays a crucial role in neuronal excitability and synaptic transmission in the CNS, however, the function of adenosine in the EC is still elusive. Here, the data reported showed that adenosine hyperpolarized stellate neurons in a concentration-dependent manner, accompanied by a decrease in firing frequency. This effect corresponded to the inhibition of the hyperpolarization-activated, cation nonselective (HCN) channels. Surprisingly, the adenosine-induced inhibition was blocked by 3 μM 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A(1) receptor antagonists, but not by 10 μM 3,7-dimethyl-1-propargylxanthine (DMPX), a selective A(2) receptor antagonists, indicating that activation of adenosine A(1) receptors were responsible for the direct inhibition. In addition, adenosine reduced the frequency but not the amplitude of miniature EPSCs and IPSCs, suggesting that the global depression of glutamatergic and GABAergic transmission is mediated by a decrease in glutamate and GABA release, respectively. Again the presynaptic site of action was mediated by adenosine A(1) receptors. Furthermore, inhibition of spontaneous glutamate and GABA release by adenosine A(1) receptor activation was mediated by voltage-dependent Ca(2+) channels and extracellular Ca(2+) . Therefore, these findings revealed direct and indirect mechanisms by which activation of adenosine A(1) receptors on the cell bodies of stellate neurons and on the presynaptic terminals could regulate the excitability of these neurons.

The nestin-expressing and non-expressing neurons in rat basal forebrain display different electrophysiological properties and project to hippocampus

Background

Nestin-immunoreactive (nestin-ir) neurons have been identified in the medial septal/diagonal band complex (MS/DBB) of adult rat and human, but the significance of nestin expression in functional neurons is not clear. This study investigated electrophysiological properties and neurochemical phenotypes of nestin-expressing (nestin+) neurons using whole-cell recording combined with single-cell RT-PCR to explore the significance of nestin expression in functional MS/DBB neurons. The retrograde labelling and immunofluorescence were used to investigate the nestin+ neuron related circuit in the septo-hippocampal pathway.

Results

The results of single-cell RT-PCR showed that 87.5% (35/40) of nestin+ cells expressed choline acetyltransferase mRNA (ChAT+), only 44.3% (35/79) of ChAT+ cells expressed nestin mRNA. Furthermore, none of the nestin+ cells expressed glutamic acid decarboxylases 67 (GAD₆₇) or vesicular glutamate transporters (VGLUT) mRNA. All of the recorded nestin+ cells were excitable and demonstrated slow-firing properties, which were distinctive from those of GAD₆₇ or VGLUT mRNA-positive neurons. These results show that the MS/DBB cholinergic neurons could be divided into nestin-expressing cholinergic neurons (NEChs) and nestin non-expressing cholinergic neurons (NNChs). Interestingly, NEChs had higher excitability and received stronger spontaneous excitatory synaptic inputs than NNChs. Retrograde labelling combined with choline acetyltransferase and nestin immunofluorescence showed that both of the NEChs and NNChs projected to hippocampus.

Conclusions

These results suggest that there are two parallel cholinergic septo-hippocampal pathways that may have different functions. The significance of nestin expressing in functional neurons has been discussed.

Decreased hyperpolarization-activated currents in layer 5 pyramidal neurons enhances excitability in focal cortical dysplasia

Focal cortical dysplasia is associated with the development of seizures in children and is present in up to 40% of intractable childhood epilepsies. Transcortical freeze lesions in newborn rats reproduce many of the anatomical and physiological characteristics of human cortical dysplasia. Rats with freeze lesions have increased seizure susceptibility and a region of hyperexcitable cortex adjacent to the lesion. Since alterations in hyperpolarization-activated nonspecific cation (HCN) channels are often associated with epilepsy, we used whole cell patch-clamp recording and voltage-sensitive dye imaging to examine alterations in HCN channels and inwardly rectifying hyperpolarization-activated currents (I(h)) in cortical dysplasia. (L5) pyramidal neurons in lesioned animals had hyperpolarized resting membrane potentials, increased input resistances and reduced voltage "sag" associated with I(h) activation. These differences became nonsignificant after application of the I(h) blocker ZD7288. Temporal excitatory postsynaptic potential (EPSP) summation and intrinsic excitability were increased in neurons near the freeze lesion. Using voltage-sensitive dye imaging of neocortical slices, we found that inhibiting I(h) with ZD7288 increased the half-width of dye signals. The anticonvulsant lamotrigine produced a significant decrease in spread of activity. The ability of lamotrigine to decrease network activity was reduced in the hyperexcitable cortex near the freeze lesion. These results suggest that I(h) serves to constrain network activity in addition to its role in regulating cellular excitability. Reduced I(h) may contribute to increased network excitability in cortical dysplasia.

Reassessment of the structural basis of the ascending arousal system

The "ascending reticular activating system" theory proposed that neurons in the upper brainstem reticular formation projected to forebrain targets that promoted wakefulness. More recent formulations have emphasized that most neurons at the pontomesencephalic junction that participate in these pathways are actually in monoaminergic and cholinergic cell groups. However, cell-specific lesions of these cell groups have never been able to reproduce the deep coma seen after acute paramedian midbrain lesions that transect ascending axons at the caudal midbrain level. To determine whether the cortical afferents from the thalamus or the basal forebrain were more important in maintaining arousal, we first placed large cell-body-specific lesions in these targets. Surprisingly, extensive thalamic lesions had little effect on electroencephalographic (EEG) or behavioral measures of wakefulness or on c-Fos expression by cortical neurons during wakefulness. In contrast, animals with large basal forebrain lesions were behaviorally unresponsive and had a monotonous sub-1-Hz EEG, and little cortical c-Fos expression during continuous gentle handling. We then retrogradely labeled inputs to the basal forebrain from the upper brainstem, and found a substantial input from glutamatergic neurons in the parabrachial nucleus and adjacent precoeruleus area. Cell-specific lesions of the parabrachial-precoeruleus complex produced behavioral unresponsiveness, a monotonous sub-1-Hz cortical EEG, and loss of cortical c-Fos expression during gentle handling. These experiments indicate that in rats the reticulo-thalamo-cortical pathway may play a very limited role in behavioral or electrocortical arousal, whereas the projection from the parabrachial nucleus and precoeruleus region, relayed by the basal forebrain to the cerebral cortex, may be critical for this process.

Spatial memory alterations by activation of septal 5HT 1A receptors: no implication of cholinergic septohippocampal neurons

INTRODUCTION

In rats, activation of medial septum (MS) 5-HT(1A) receptors with the 5-HT(1A)/5-HT(7) receptor agonist 8-OH-DPAT disrupts encoding and consolidation, but not retrieval of a spatial memory in the water maze task. These findings might be explained by an action of 8-OH-DPAT on 5-HT(1A) receptors located on cholinergic neurons which the drug could transiently hyperpolarise. If so, selective damage of these neurons should mimic the effects of 8-OH-DPAT, or, at least, synergistically interfere with them.

METHODS

To test this hypothesis, rats were subjected to intraseptal infusions of 8-OH-DPAT (or phosphate-buffered saline) during acquisition of a water maze task before and/or after 192 IgG-saporin-induced MS cholinergic lesion (vs. sham-operated).

RESULTS

We confirmed that only pre-acquisition intraseptal 8-OH-DPAT infusions prevented learning and subsequent drug-free retrieval of the platform location in intact rats and found that (1) the cholinergic lesion did not prevent recall of the platform location, and (2) the impairing effects of 8-OH-DPAT were similar in sham-operated and lesioned rats, whether naïve or not, to the task before lesion surgery.

CONCLUSIONS

An action of 8-OH-DPAT on only MS cholinergic neurons is not sufficient to account for the drug-induced memory impairments. A concomitant 8-OH-DPAT-induced hyperpolarisation of cholinergic and/or GABAergic and/or glutamatergic neurons (intact rats), or of only GABAergic and/or glutamatergic ones after cholinergic lesion, might be necessary to obliterate task acquisition, confirming that, in the MS, (1) the three neuronal populations could cooperate to process hippocampal-dependent information, and (2) non-cholinergic septohippocampal neurons might be more important than cholinergic ones in serotonin-induced modulation of hippocampus-dependent memory processing.

Impact of ethanol on the developing GABAergic system

Alcohol intake during pregnancy has a tremendous impact on the developing brain. Embryonic and early postnatal alcohol exposures have been investigated experimentally to elucidate the fetal alcohol spectrum disorders' (FASD) milieu, and new data have emerged to support a devastating effect on the GABAergic system in the adult and developing nervous system. GABA is a predominantly inhibitory neurotransmitter that during development excites neurons and orchestrates several developmental processes such as proliferation, migration, differentiation, and synaptogenesis. This review summarizes and brings new data on neurodevelopmental aspects of the GABAergic system with FASD in experimental telencephalic models.

h channel-dependent deficit of theta oscillation resonance and phase shift in temporal lobe epilepsy

I(h) tunes hippocampal CA1 pyramidal cell dendrites to optimally respond to theta inputs (4-12 Hz), and provides a negative time delay to theta inputs. Decreased I(h) activity, as seen in experimental temporal lobe epilepsy (TLE), could significantly alter the response of dendrites to theta inputs. Here we report a progressive erosion of theta resonance and phase lead in pyramidal cell dendrites during epileptogenesis in a rat model of TLE. These alterations were due to decreased I(h) availability, via a decline in HCN1/HCN2 subunit expression resulting in decreased h currents, and altered kinetics of the residual channels. This acquired HCN channelopathy thus compromises temporal coding and tuning to theta inputs in pyramidal cell dendrites. Decreased theta resonance in vitro also correlated with a reduction in theta frequency and power in vivo. We suggest that the neuronal/circuitry changes associated with TLE, including altered I(h)-dependent inductive mechanisms, can disrupt hippocampal theta function.

The hippocamposeptal pathway generates rhythmic firing of GABAergic neurons in the medial septum and diagonal bands: an investigation using a complete septohippocampal preparation in vitro

The medial septum diagonal band area (MS/DB) projects to the hippocampus through the fornix/fimbria pathway and is implicated in generating hippocampal theta oscillations. The hippocampus also projects back to the MS/DB, but very little is known functionally about this input. Here, we investigated the physiological role of hippocamposeptal feedback to the MS/DB in a complete in vitro septohippocampal preparation containing the intact interconnecting fornix/fimbria pathway. We demonstrated that carbachol-induced rhythmic theta-like hippocampal oscillations recorded extracellularly were synchronized with powerful rhythmic IPSPs in whole-cell recorded MS/DB neurons. Interestingly, we found that these IPSPs evoked rebound spiking in GABAergic MS/DB neurons. In contrast, putative cholinergic and glutamatergic MS/DB neurons responded only weakly with rebound spiking and, as a result, were mostly silent during theta-like oscillations. We next determined the mechanism underlying the rebound spiking that followed the IPSPs in MS/DB GABAergic neurons using phasic electrical stimulation of the fornix/fimbria pathway. We demonstrate that the increased rebound spiking was attributable to the activation of I(h) current, because it was significantly reduced by low concentrations of the I(h) antagonist ZD7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyridinium chloride]. Together, these results suggest that rhythmical activity in hippocampus is transferred to the MS/DB and can preferentially phase the spiking of GABAergic MS/DB neurons because of their significant expression of I(h) currents. Our data demonstrate that hippocamposeptal inhibition facilitates theta rhythmic discharges in MS/DB GABAergic neurons while favoring the inhibition of most ACh and glutamate neurons.

Action potential-independent and nicotinic receptor-mediated concerted release of multiple quanta at hippocampal CA3-mossy fiber synapses

Presynaptic action potential-independent transmitter release is a potential means of information transfer across synapses. We show that in the hippocampal mossy fiber boutons, activation of the alpha7-subtype of nicotinic acetylcholine receptors (alpha7-nAChRs) results in a large increase in the amplitude of spontaneous events, resulting from concerted release of multiple quanta from the mossy fiber boutons. This amplitude increase is abolished at low temperatures. Activation of alpha7-nAChRs causes a rise in intraterminal calcium at mossy fiber boutons, involving ryanodine receptors. Regulation of concerted release requires the subsequent activation of presynaptic calcium/calmodulin-dependent protein kinase II (CaMKII). Activation of CaMKII is required to drive presynaptic action potential-independent transmission at the mossy fiber-CA3 pyramidal cell synapse. The effects of alpha7-nAChR activation are mediated by biologically relevant doses of nicotine. Our results demonstrate a novel form of synaptic plasticity mediated by presynaptic alpha7-nAChRs and store calcium that is temporally different and might respond to a different history of synaptic activity than that mediated by incoming action potentials.

Neurokinins robustly activate the majority of septohippocampal cholinergic neurons

In the brain, tachykinins acting via the three cloned neurokinin (NK) receptors are implicated in stress-related affective disorders. Hemokinin-1 is a novel tachykinin that reportedly prefers NK1 to NK2 or NK3 receptors. Although NK1 and NK3 receptors are abundantly expressed in the brain, NK2-receptor-mediated electrophysiological effects have rarely been described as NK2 receptors are expressed only in a few brain regions such as the nucleus of the medial septum/diagonal band. Medial septal/diagonal band neurons that control hippocampal mnemonic functions also colocalize NK1 and NK3 receptors. Functionally, intraseptal activation of all three NK receptors increases hippocampal acetylcholine release and NK2 receptors have specifically been implicated in stress-induced hippocampal acetylcholine release. Electrophysiological studies on the effects of NKs on septohippocampal cholinergic neurons are lacking and electrophysiological effects of hemokinin-1 have thus far not been reported in brain neurons. In the present study we examined the electrophysiological and pharmacological effects of multiple NKs on fluorescently tagged septohippocampal cholinergic neurons using whole-cell patch-clamp recordings in a rat brain slice preparation. We demonstrate that a vast majority of septohippocampal cholinergic cells are activated by NK1, NK2 and NK3 receptor agonists as well as by hemokinin-1 via direct post-synaptic mechanisms. Pharmacologically, hemokinin-1 recruits not only NK1 but also NK2 and NK3 receptors to activate septohippocampal cholinergic neurons that are the primary source of acetylcholine for the hippocampus.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

Muscarine activates the sodium-calcium exchanger via M3receptors in basal forebrain neurons

Neurons of the medial septum/diagonal band of Broca (MSDB) project to the hippocampus. Muscarinic cholinergic mechanisms within the MSDB are potent modulators of hippocampal functions; intraseptal scopolamine disrupts and intraseptal carbachol facilitates hippocampus-dependent learning and memory tasks, and the associated hippocampal theta rhythm. In earlier work, we demonstrated that, within the MSDB, the septohippocampal GABAergic but not cholinergic neurons are the primary target of muscarinic manipulations and that muscarinic activation of septohippocampal GABAergic neurons is mediated directly via M(3) receptors. In the present study, we examined the ionic mechanism(s) underlying the excitatory actions of muscarine in these neurons. Using whole-cell patch-clamp recording techniques in rat brain slices, we demonstrated that M(3) receptor-mediated muscarinic activation of MSDB neurons is dependent on external Na(+) and is also reduced by bath-applied Ni(2+) and KB-R7943 as well as by replacing external Na(+) with Li(+), suggesting a primary involvement of the Na(+)-Ca(2+) exchanger. We conclude that the M(3) receptor-mediated muscarinic activation of MSDB septohippocampal GABA-type neurons, that is important for cognitive functioning, is mediated via activation of the Na(+)-Ca(2+) exchanger.

Expression of a functional hyperpolarization-activated current (Ih) in the mouse nucleus reticularis thalami

The GABAergic neurons of the nucleus reticularis thalami (nRT) express the type 2 hyperpolarization-activated cAMP-sensitive (HCN2) subunit mRNA, but surprisingly, they were reported to lack the hyperpolarization-activated (Ih) current carried by this subunit. Using the voltage-clamp recordings in the thalamocortical slice preparation of the newborn and juvenile mice (P6-P23), we demonstrate that, in the presence of 1 mM barium (Ba2+), the nRT neurons express a slow hyperpolarization-activated inward current, suggesting that the Ih is present but masked in control conditions by K+ leak currents. We investigate the identity of the hyperpolarization-activated current in the nRT by studying its physiological and pharmacological profile in presence of Ba2+. We show that it has voltage- and time-dependent properties typical of the Ih, that it is blocked by cesium and ZD7288, two blockers of the Ih, and that it is carried both by the K+ and Na+ ions. We could also alter the gating characteristics of the hyperpolarization-activated current in the nRT by adding a nonhydrolysable analogue of cAMP to the pipette solution. Finally, using the current-clamp recording, we showed that blocking the hyperpolarization-activated current induced an hyperpolarization correlated with an increase of the R(in) of the nRT neurons. In conclusion, our results demonstrate that the nRT neurons express the Ih with slow kinetics similar to those described for the homomeric HCN2 channels, and we show that the Ih of the nRT contributes to the excitability of the nRT neurons in normal conditions.

Adenosine inhibits basal forebrain cholinergic and noncholinergic neurons in vitro

Adenosine has been proposed as a homeostatic "sleep factor" that promotes the transition from waking to sleep by affecting several sleep-wake regulatory systems. In the basal forebrain, adenosine accumulates during wakefulness and, when locally applied, suppresses neuronal activity and promotes sleep. However, the neuronal phenotype mediating these effects is unknown. We used whole-cell patch-clamp recordings in in vitro rat brain slices to investigate the effect of adenosine on identified cholinergic and noncholinergic neurons of the magnocellular preoptic nucleus and substantia innominata. Adenosine (0.5-100 microM) reduced the magnocellular preoptic nucleus and substantia innominata cholinergic neuronal firing rate by activating an inwardly rectifying potassium current that reversed at -82 mV and was blocked by barium (100 microM). Application of the A1 receptor antagonist 8-cyclo-pentyl-theophylline (200 nM) blocked the effects of adenosine. Adenosine was also tested on two groups of electrophysiologically distinct noncholinergic magnocellular preoptic nucleus and substantia innominata neurons. In the first group adenosine, via activation of postsynaptic A1 receptors, reduced spontaneous firing via inhibition of the hyperpolarization-activated cation current. Blocking the H-current with ZD7288 (20 microM) abolished adenosine effects on these neurons. The second group was not affected by adenosine. These results demonstrate that, in the magnocellular preoptic nucleus and substantia innominata region of the basal forebrain, adenosine inhibits both cholinergic neurons and a subset of noncholinergic neurons. Both of these effects occur via postsynaptic A1 receptors, but are mediated downstream by two separate mechanisms.

Genetic dissection of theta rhythm heterogeneity in mice

Rhythmic oscillatory activities at the theta frequency (4-12 Hz) in the hippocampus have long-attracted attention because they have been implicated in diverse brain functions, including spatial cognition. Although studies based on pharmacology and lesion experiments suggested heterogeneity of these rhythms and their behavioral correlates, controversies are abundant on these issues. Here we show that mice harboring a phospholipase C (PLC)-beta1(-/-) mutation (PLC-beta1(-/-) mice) lack one subset of theta rhythms normally observed during urethane anesthesia, alert immobility, and passive whole-body rotation. In contrast, the other subset of theta rhythms observed during walking or running was intact in these mutant mice. PLC-beta1(-/-) mice also have somewhat disrupted theta activity during paradoxical sleep but do have an atropine-resistant component of theta rhythm. In addition, carbachol-induced oscillations were obliterated in hippocampal slices of PLC-beta1(-/-) mice. Interestingly, PLC-beta1(-/-) mice showed deficits in a hidden platform version of the Morris water maze yet performed well in motor coordination tests and a visual platform version of the Morris water maze. The results genetically define the existence of at least two subtypes of theta rhythms and reveal their association with different behaviors.

Medial septal GABAergic neurons express the somatostatin sst2A receptor: functional consequences on unit firing and hippocampal theta

GABAergic septohippocampal neurons play a major role in the generation of hippocampal theta rhythm, but modulatory factors intervening in this function are poorly documented. The neuropeptide somatostatin (SST) may be one of these factors, because nearly all hippocampal GABAergic neurons projecting to the medial septum/diagonal band of Broca (MS-DB) express SST. In this study, we took advantage of the high and selective expression of the SST receptor sst2A in MS-DB to examine its possible role on theta-related activity. Immunohistochemical experiments demonstrated that sst2A receptors were selectively targeted to the somatodendritic domain of neurons expressing the GABAergic marker GAD67 but were not expressed by cholinergic neurons. In addition, a subpopulation of GABAergic septohippocampal projecting neurons expressing parvalbumin (PV) also displayed sst2A receptors. Using in vivo juxtacellular recording and labeling with neurobiotin, we showed that a number of bursting and nonbursting neurons exhibiting high discharge rates and brief spikes were immunoreactive for PV or GAD67 and expressed the sst2A receptor. Microiontophoresis applications of SST and the sst2A agonist octreotide (OCT) showed that sst2A receptor activation decreased the discharge rate of both nonbursting and bursting MS-DB neurons and lessened the rhythmic activity of the latter. Finally, intraseptal injections of OCT and SST in freely moving rats reduced the power of hippocampal EEG in the theta band. Together, these in vivo experiments suggest that SST action on MS-DB GABAergic neurons, through sst2A receptors, represents an important modulatory mechanism in the control of theta activity.

A behavioral role for dendritic integration: HCN1 channels constrain spatial memory and plasticity at inputs to distal dendrites of CA1 pyramidal neurons

The importance of long-term synaptic plasticity as a cellular substrate for learning and memory is well established. By contrast, little is known about how learning and memory are regulated by voltage-gated ion channels that integrate synaptic information. We investigated this question using mice with general or forebrain-restricted knockout of the HCN1 gene, which we find encodes a major component of the hyperpolarization-activated inward current (Ih) and is an important determinant of dendritic integration in hippocampal CA1 pyramidal cells. Deletion of HCN1 from forebrain neurons enhances hippocampal-dependent learning and memory, augments the power of theta oscillations, and enhances long-term potentiation (LTP) at the direct perforant path input to the distal dendrites of CA1 pyramidal neurons, but has little effect on LTP at the more proximal Schaffer collateral inputs. We suggest that HCN1 channels constrain learning and memory by regulating dendritic integration of distal synaptic inputs to pyramidal cells.

In vivo contribution of h-channels in the septal pacemaker to theta rhythm generation

One of the most intriguing network-level inferences made on the basis of in vitro and modelling data regarding the role of Ih current was that they participate in rhythmogenesis in different parts of the brain. The nature of Ih contribution to various neuronal oscillations is far from uniform however, and the proper evaluation of the role of Ih in each particular structure requires in situ investigations in the intact brain. In this study we tested the effect of Ih blockade in the medial septum on hippocampal theta rhythm in anaesthetized and freely behaving rats. We could not confirm the recent report of elimination of theta by septal injection of ZD7288 [C. Xu et al. (2004) Eur. J. Neurosci., 19, 2299-2309]; the observed effects were more subtle and more specific. We found that Ih blockade in the medial septum substantially decreased the frequency of hippocampal oscillations without changing the context in which theta occurred, i.e. specific behaviours in freely moving rats and spontaneous switching and brainstem stimulation under anaesthesia. Septal injection of ZD7288 eliminated atropine-resistant theta elicited by high intensity electrical stimulation of the reticular formation in anaesthetized rats but was ineffective in combination with the muscarinic agonist, carbachol. Thus, functional Ih was necessary for the septum to generate or transmit high frequency theta rhythm elicited by strong ascending activation, whereas low frequency theta persisted after Ih blockade. These results suggest that Ih plays a specific role in septal theta generation by promoting fast oscillations during exploratory behaviour and rapid eye movement sleep.

Histamine innervation and activation of septohippocampal GABAergic neurones: involvement of local ACh release

Recent studies indicate that the histaminergic system, which is critical for wakefulness, also influences learning and memory by interacting with cholinergic systems in the brain. Histamine-containing neurones of the tuberomammillary nucleus densely innervate the cholinergic and GABAergic nucleus of the medial septum/diagonal band of Broca (MSDB) which projects to the hippocampus and sustains hippocampal theta rhythm and associated learning and memory functions. Here we demonstrate that histamine, acting via H(1) and/or H(2) receptor subtypes, utilizes direct and indirect mechanisms to excite septohippocampal GABA-type neurones in a reversible, reproducible and concentration-dependent manner. The indirect mechanism involves local ACh release, is potentiated by acetylcholinesterase inhibitors and blocked by atropine methylbromide and 4-DAMP mustard, an M(3) muscarinic receptor selective antagonist. This indirect effect, presumably, results from a direct histamine-induced activation of septohippocampal cholinergic neurones and a subsequent indirect activation of the septohippocampal GABAergic neurones. In double-immunolabelling studies, histamine fibres were found in the vicinity of both septohippocampal cholinergic and GABAergic cell types. These findings have significance for Alzheimer's disease and other neurodegenerative disorders involving a loss of septohippocampal cholinergic neurones as such a loss would also obtund histamine effects on septohippocampal cholinergic and GABAergic functions and further compromise hippocampal arousal and associated cognitive functions.