Cholinergic theta rhythm in transected hippocampal slices: independent CA1 and dentate generators

Optogenetic Suppression of Lateral Septum Somatostatin Neurons Enhances Hippocampus Cholinergic Theta Oscillations and Local Synchrony

The septal complex regulates both motivated and innate behaviors, chiefly by the action of its diverse population of long-range projection neurons. A small population of somatostatin-expressing GABAergic cells in the lateral septum projects deep into subcortical regions, yet on its way it also targets neighboring medial septum neurons that profusely innervate cortical targets by ascending synaptic pathways. Here, we used optogenetic stimulation and extracellular recordings in acutely anesthetized transgenic mice to show that lateral septum somatostatin neurons can disinhibit the cholinergic septo-hippocampal pathway, thus enhancing the amplitude and synchrony of theta oscillations while depressing sharp-wave ripple episodes in the dorsal hippocampus. These results suggest that septal somatostatin cells can recruit ascending cholinergic pathways to promote hippocampal theta oscillations.

The Theta Rhythm of the Hippocampus: From Neuronal and Circuit Mechanisms to Behavior

This review focuses on the neuronal and circuit mechanisms involved in the generation of the theta (θ) rhythm and of its participation in behavior. Data have accumulated indicating that θ arises from interactions between medial septum-diagonal band of Broca (MS-DbB) and intra-hippocampal circuits. The intrinsic properties of MS-DbB and hippocampal neurons have also been shown to play a key role in θ generation. A growing number of studies suggest that θ may represent a timing mechanism to temporally organize movement sequences, memory encoding, or planned trajectories for spatial navigation. To accomplish those functions, θ and gamma (γ) oscillations interact during the awake state and REM sleep, which are considered to be critical for learning and memory processes. Further, we discuss that the loss of this interaction is at the base of various neurophatological conditions.

Molecular Mechanisms of REM Sleep

Rapid-eye movement (REM) sleep is a paradoxical sleep state characterized by brain activity similar to wakefulness, rapid-eye-movement, and lack of muscle tone. REM sleep is a fundamental brain function, evolutionary conserved across species, including human, mouse, bird, and even reptiles. The physiological importance of REM sleep is highlighted by severe sleep disorders incurred by a failure in REM sleep regulation. Despite the intense interest in the mechanism of REM sleep regulation, the molecular machinery is largely left to be investigated. In models of REM sleep regulation, acetylcholine has been a pivotal component. However, even newly emerged techniques such as pharmacogenetics and optogenetics have not fully clarified the function of acetylcholine either at the cellular level or neural-circuit level. Recently, we discovered that the G_q type muscarinic acetylcholine receptor genes, Chrm1 and Chrm3, are essential for REM sleep. In this review, we develop the perspective of current knowledge on REM sleep from a molecular viewpoint. This should be a starting point to clarify the molecular and cellular machinery underlying REM sleep regulation and will provide insights to explore physiological functions of REM sleep and its pathological roles in REM-sleep-related disorders such as depression, PTSD, and neurodegenerative diseases.

From adagio to allegretto: The changing tempo of theta frequencies in epilepsy and its relation to interneuron function

Despite decades of research, our understanding of epilepsy, including how seizures are generated and propagate, is incomplete. However, there is growing recognition that epilepsy is more than just the occurrence of seizures, with patients often experiencing comorbid deficits in cognition that are poorly understood. In addition, the available therapies for treatment of epilepsy, from pharmaceutical treatment to surgical resection and seizure prevention devices, often exacerbate deficits in cognitive function. In this review, we discuss the hypothesis that seizure generation and cognitive deficits have a similar pathological source characterized by, but not limited to, deficits in theta oscillations and their influence on interneurons. We present a new framework that describes oscillatory states in epilepsy as alternating between hyper- and hypo-synchrony rather than solely the spontaneous transition to hyper-excitability characterized by the seizures. This framework suggests that as neural oscillations, specifically in the theta range, vary their tempo from a slowed almost adagio tempo during interictal periods to faster, more rhythmic allegretto tempo preictally, they impact the function of interneurons, modulating their ability to control seizures and their role in cognitive processing. This slow wave oscillatory framework may help explain why current therapies that work to reduce hyper-excitability do not completely eliminate seizures and often lead to exacerbated cognitive deficits.

Carbachol-Induced theta-like oscillations in the rodent brain limbic system: Underlying mechanisms and significance

Theta oscillations (4-12 Hz) represent one of the most prominent physiological oscillatory activity in the mammalian EEG. They are observed in several areas of the hippocampus and in parahippocampal structures. Theta oscillations play important roles in modulating synaptic plasticity during memory and learning; moreover, they are dependent on septal cholinergic inputs. Theta oscillations can be reproduced in vitro in several regions of the temporal lobe in the absence of the septum by employing the cholinergic agonist carbachol (CCh). Here, we review the mechanisms underlying CCh-induced theta oscillations. We address: (i) the ability of temporal lobe neuronal networks to oscillate independently at theta frequency during CCh treatment; (ii) the contribution of intrinsic ionic currents; (iii) the participation of principal cells and interneurons; and (iv) their pharmacological profiles. We also discuss the similarities between CCh-induced theta oscillations and physiological type II theta activity, as well as their roles in synaptic plasticity. Finally, we consider experimental evidence pointing to the contribution of spontaneous and CCh-induced theta activity to epileptiform synchronization.

Cell Type-specific Intrinsic Perithreshold Oscillations in Hippocampal GABAergic Interneurons

The hippocampus plays a critical role in learning, memory, and spatial processing through coordinated network activity including theta and gamma oscillations. Recent evidence suggests that hippocampal subregions (e.g., CA1) can generate these oscillations at the network level, at least in part, through GABAergic interneurons. However, it is unclear whether specific GABAergic interneurons generate intrinsic theta and/or gamma oscillations at the single-cell level. Since major types of CA1 interneurons (i.e., parvalbumin-positive basket cells (PVBCs), cannabinoid type 1 receptor-positive basket cells (CB1BCs), Schaffer collateral-associated cells (SCAs), neurogliaform cells and ivy cells) are thought to play key roles in network theta and gamma oscillations in the hippocampus, we tested the hypothesis that these cells generate intrinsic perithreshold oscillations at the single-cell level. We performed whole-cell patch-clamp recordings from GABAergic interneurons in the CA1 region of the mouse hippocampus in the presence of synaptic blockers to identify intrinsic perithreshold membrane potential oscillations. The majority of PVBCs (83%), but not the other interneuron subtypes, produced intrinsic perithreshold gamma oscillations if the membrane potential remained above -45 mV. In contrast, CB1BCs, SCAs, neurogliaform cells, ivy cells, and the remaining PVBCs (17%) produced intrinsic theta, but not gamma, oscillations. These oscillations were prevented by blockers of persistent sodium current. These data demonstrate that the major types of hippocampal interneurons produce distinct frequency bands of intrinsic perithreshold membrane oscillations.

Relevance of excitable media theory and retinal spreading depression experiments in preclinical pharmacological research

In preclinical neuropharmacological research, molecular, cell-based, and systems using animals are well established. On the tissue level the situation is less comfortable, although during the last decades some effort went into establishing such systems, i.e. using slices of the vertebrate brain together with optical and electrophysiological techniques. However, these methods are neither fast, nor can they be automated or upscaled. By contrast, the chicken retina can be used as a suitable model. It is easy accessible and can be kept alive in vitro for hours up to days. Due to its structure, in addition the retina displays remarkable intrinsic optical signals, which can be easily used in experiments. Also to electrophysiological methods the retina is well accessible. In excitable tissue, to which the brain and the retina belong, propagating excitation waves can be expected, and the spreading depression is such a phenomenon. It has been first observed in the forties of the last century. Later, Martins-Ferreira established it in the chicken retina (retinal spreading depression or RSD). The electrophysiological characteristics of it are identical with those of the cortical SD. The metabolic differences are known and can be taken into account. The experimental advantage of the RSD compared to the cortical SD is the pronounced intrinsic optical signal (IOS) associated with the travelling wave. This is due to the maximum transparency of retinal tissue in the functional state; thus any physiological event will change it markedly and therefore can be easily seen even by naked eye. The theory can explain wave spread in one (action potentials), two (RSDs) and three dimensions (one heart beat). In this review we present the experimental and the excitable media context for the data interpretation using as example the cholinergic pharmacology in relation to functional syndromes. We also discuss the intrinsic optical signal and how to use it in pre-clinical research.

Neural circuits underlying the generation of theta oscillations

Theta oscillations represent the neural network configuration underlying active awake behavior and paradoxical sleep. This major EEG pattern has been extensively studied, from physiological to anatomical levels, for more than half a century. Nevertheless the cellular and network mechanisms accountable for the theta generation are still not fully understood. This review synthesizes the current knowledge on the circuitry involved in the generation of theta oscillations, from the hippocampus to extra hippocampal structures such as septal complex, entorhinal cortex and pedunculopontine tegmentum, a main trigger of theta state through direct and indirect projections to the septal complex. We conclude with a short overview of the perspectives offered by technical advances for deciphering more precisely the different neural components underlying the emergence of theta oscillations.

Entrainment of neocortical neurons and gamma oscillations by the hippocampal theta rhythm

Although it has been tacitly assumed that the hippocampus exerts an influence on neocortical networks, the mechanisms of this process are not well understood. We examined whether and how hippocampal theta oscillations affect neocortical assembly patterns by recording populations of single cells and transient gamma oscillations in multiple cortical regions, including the somatosensory area and prefrontal cortex in behaving rats and mice. Laminar analysis of neocortical gamma bursts revealed multiple gamma oscillators of varying frequency and location, which were spatially confined and synchronized local groups of neurons. A significant fraction of putative pyramidal cells and interneurons as well as localized gamma oscillations in all recorded neocortical areas were phase biased by the hippocampal theta rhythm. We hypothesize that temporal coordination of neocortical gamma oscillators by hippocampal theta is a mechanism by which information contained in spatially widespread neocortical assemblies can be synchronously transferred to the associative networks of the hippocampus.

Differential cholinergic modulation of synaptic encoding and gain control mechanisms in rat hippocampus

Recent studies have highlighted a variety of cognitive effects caused by cholinolytic drug injections into different cortical structures. These findings were largely interpreted as evidence for location-specific cholinergic modulation of synaptic encoding mechanisms. Here, using evoked field responses in anaesthetized rat dorsal hippocampus we show that in addition to reinforcement of synaptic connections (long-term potentiation, LTP), endogenous acetylcholine also regulates firing gain of CA1 pyramidal neurons (EPSP-spike potentiation). Gain augmentation upon increase in cholinergic drive involves evoked synchronous firing at both apical and basal afferent projections, unlike enhancement of activity-induced LTP constrained to the basal afferent system. These data indicate that acetylcholine can act as an effective input and gain controller in the hippocampus. Modulation of synaptic plasticity would determine the relative dominance of afferent inputs while the facilitation of synchronous firing is likely to promote a more generalized spread of excitation and long range communication within the limbic cortex.

Slow and Fast Inhibition and an H-Current Interact to Create a Theta Rhythm in a Model of CA1 Interneuron Network

The oriens-lacunosum moleculare (O-LM) subtype of interneuron is a key component in the formation of the theta rhythm (8–12 Hz) in the hippocampus. It is known that the CA1 region of the hippocampus can produce theta rhythms in vitro with all ionotropic excitation blocked, but the mechanisms by which this rhythmicity happens were previously unknown. Here we present a model suggesting that individual O-LM cells, by themselves, are capable of producing a single-cell theta-frequency firing, but coupled O-LM cells are not capable of producing a coherent population theta. By including in the model fast-spiking (FS) interneurons, which give rise to IPSPs that decay faster than those of the O-LM cells, coherent theta rhythms are produced. The inhibition to O-LM cells from the FS cells synchronizes the O-LM cells, but only when the FS cells themselves fire at a theta frequency. Reciprocal connections from the O-LM cells to the FS cells serve to parse the FS cell firing into theta bursts, which can then synchronize the O-LM cells. A component of the model O-LM cell critical to the synchronization mechanism is the hyperpolarization-activated h-current. The model can robustly reproduce relative phases of theta frequency activity in O-LM and FS cells.

Defined types of cortical interneurone structure space and spike timing in the hippocampus

The cerebral cortex encodes, stores and combines information about the internal and external environment in rhythmic activity of multiple frequency ranges. Neurones of the cortex can be defined, recognized and compared on the comprehensive application of the following measures: (i) brain area- and cell domain-specific distribution of input and output synapses, (ii) expression of molecules involved in cell signalling, (iii) membrane and synaptic properties reflecting the expression of membrane proteins, (iv) temporal structure of firing in vivo, resulting from (i)-(iii). Spatial and temporal measures of neurones in the network reflect an indivisible unity of evolutionary design, i.e. neurones do not have separate structure or function. The blueprint of this design is most easily accessible in the CA1 area of the hippocampus, where a relatively uniform population of pyramidal cells and their inputs follow an instantly recognizable laminated pattern and act within stereotyped network activity patterns. Reviewing the cell types and their spatio-temporal interactions, we suggest that CA1 pyramidal cells are supported by at least 16 distinct types of GABAergic neurone. During a given behaviour-contingent network oscillation, interneurones of a given type exhibit similar firing patterns. During different network oscillations representing two distinct brain states, interneurones of the same class show different firing patterns modulating their postsynaptic target-domain in a brain-state-dependent manner. These results suggest roles for specific interneurone types in structuring the activity of pyramidal cells via their respective target domains, and accurately timing and synchronizing pyramidal cell discharge, rather than providing generalized inhibition. Finally, interneurones belonging to different classes may fire preferentially at distinct time points during a given oscillation. As different interneurones innervate distinct domains of the pyramidal cells, the different compartments will receive GABAergic input differentiated in time. Such a dynamic, spatio-temporal, GABAergic control, which evolves distinct patterns during different brain states, is ideally suited to regulating the input integration of individual pyramidal cells contributing to the formation of cell assemblies and representations in the hippocampus and, probably, throughout the cerebral cortex.

Depth amplitude and phase profiles of carbachol-induced theta in hippocampal formation slices

Phase and amplitude depth profiles of EEG theta-like activity (TLA) induced by bath perfusion of 50 micro M of the cholinergic agonist carbachol were investigated using rat hippocampal formation slices. The data showed that the amplitude and phase of laminar profiles of TLA recorded in vitro were the same as the type III theta profiles recorded in in vivo rat preparations. These results supported the conclusion that the neural mechanisms involved in the generation of theta field activity in in vivo preparations and the generation of TLA in in vitro preparations were similar. The study provided further validation for the use of carbachol in vitro slice preparation as a model for studying the neural circuitry underlying theta generation in the hippocampal formation.

Changes in EEG power spectra and behavioral states in rats exposed to the acetylcholinesterase inhibitor chlorpyrifos and muscarinic agonist oxotremorine

Organophosphates (OPs) inhibit acetylcholinesterase (AChE) activity causing cholinergic stimulation in the central nervous system (CNS). Cholinergic systems are crucial in electroencephalogram (EEG) generation and regulation of behavior; however, little is known about how OP exposure affects the EEG and behavioral states. We recorded EEG, core temperature and motor activity before and after exposure to the OP pesticide chlorpyrifos (CHP) in adult female rats implanted with telemetric transmitters. The recording and reference electrodes were placed in the occipital and frontal bones, respectively. The animals received CHP, 25 mg/kg, p.o., or oxotremorine (OX), 0.2 mg/kg, s.c. CHP led to a significant increase in delta (0.1-3.5 Hz), slow theta (4-6.5 Hz), gamma 2 (35.5-50 Hz), reduction in fast theta (7-8.5 Hz), alpha/sigma (9-14 Hz), beta 1 (14.5-24 Hz), beta 2 (24.5-30 Hz) and gamma 1 (30.5-35 Hz) powers, slowing of peak frequencies in 1-9 Hz range, hypothermia and decrease in motor activity. The drop in 7-14 Hz was associated with cholinergic suppression of sleep spindles. Changes in behavioral state were characterized by dramatic diminution of sleep postures and exploring activity and prolongation of quiet waking. There was recovery in all bands in spite of continued inhibition of AChE activity [44,45] in rats exposed to CHP. OX-induced EEG and behavioral alterations were similar to CHP except there was no increase in delta and the onset and recovery were more rapid. We did not find a correlation between the EEG and core temperature alterations. Overall, changes in EEG (except in delta band) and behavior following CHP were attributable to muscarinic stimulation. Cortical arousal together with increased quiet waking and decreased sleep after CHP occurred independently from inhibition of motor activity and lowering of core temperature.

Mathematical model of the CA1 region of the rat hippocampus

A mathematical transcription of the intrinsic circuit of the CA1 region of the rat dorsal hippocampus was made and the model parameters adjusted according to experimental data from intracellular recordings and single channel kinetics. This model was able to simulate well the profile of the field potentials recorded extracellularly and the well known phenomenon of the paired-pulse depression. The results suggest that the depression of the second pulse, often interpreted in the literature as resulting from GABA(A) inhibition, can also be due to 'shunting' effects on the CA1 pyramids' membrane. The rhythmic oscillations of the field potential (EEG) was obtained as an emergent property of the network dynamics. The frequency of the field oscillation followed the main synaptic input in the region (Schaffer collaterals).

Properties of carbachol-induced oscillatory activity in rat hippocampus

Properties of carbachol-induced oscillatory activity in rat hippocampus. J. Neurophysiol. 78: 2631-2640, 1997. The recent resurgence of interest in carbachol oscillations as an in vitro model of theta rhythm in the hippocampus prompted us to evaluate the circuit mechanisms involved. In extracellular recordings, a regularly spaced bursting pattern of field potentials was observed in both CA3 and CA1 subfields in the presence of carbachol. Removal of the CA3 region abolished oscillatory activity observed in CA1, suggesting that the oscillatory generator is located in CA3. An alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), blocked carbachol oscillations, indicating that AMPA receptor-mediated synaptic currents are necessary for the population oscillation. Moreover, the spread of oscillatory activity into CA1 required intact N-methyl--aspartate receptors. These data are more consistent with epileptiform bursting than with theta rhythm described in vivo. In the presence of carbachol, individual CA3 pyramidal cells exhibited a slow, rhythmic intrinsic oscillation that was not blocked by DNQX and that was enhanced by membrane hyperpolarization. We hypothesize that this slower oscillation is the fundamental oscillator that participates in triggering the population oscillation by exciting multiple synaptically connected CA3 neurons. gamma-aminobutyric acid-A (GABAA) receptors are not necessary for carbachol to elicit synchronous CA3 field events but are essential to the bursting pattern observed. Neither GABAB nor metabotropic glutamate receptors appear to be necessary for carbachol oscillations. However, both nicotinic and M1 and M3 muscarinic cholinergic receptors contribute to the generation of this activity. These results establish the local circuit elements and neurotransmitter receptors that contribute to carbachol-induced oscillations and indicate that carbachol-induced oscillations are fundamentally distinct from theta rhythm in vivo.

Cholinergic-dependent plateau potential in hippocampal CA1 pyramidal neurons

Cholinergic stimulation of the hippocampal formation results in excitation and/or seizure. We report here, using whole-cell patch-clamp techniques in the hippocampal slice (34-35 degrees C), a cholinergic-dependent slow afterdepolarization (sADP) and long-lasting plateau potential (PP). In the presence of 20 microM carbachol, action potential firing evoked by weak intracellular current injection elicited an sADP that lasted several seconds. Increased spike firing evoked by stronger depolarizing stimuli resulted in long-duration PPs maintained close to -20 mV. Removal of either Na+ or Ca2+ from the external media, intracellular Ca2+ ([Ca2+]i) chelation with 10 mM bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid, or the addition of 100 microM Cd2+ to the perfusate abolished both the sADP and PP. The sADP was depressed and the PP was abolished by either 10 microM nimodipine or 1 microM omega-conotoxin, whereas 1.2 microM tetrodotoxin was ineffective. The involvement of a Na+/Ca2+ exchanger was minimal because both the sADP and PP persisted after equimolar substitution of 50 mM Li+ for Na+ in the external media or reduction of the bath temperature to 25 degrees C. Finally in the absence of carbachol the sADP and PP could not be evoked when K+ channels were suppressed, suggesting that depression of K+ conductances alone was not sufficient to unmask the conductance. Based on these data, we propose that a Ca2+-activated nonselective cation conductance was directly enhanced by muscarinic stimulation. The sADP, therefore, represents activation of this conductance by residual [Ca2+]i, whereas the PP represents a novel regenerative event involving the interplay between high-voltage-activated Ca2+ channels and the Ca2+-activated nonselective cation conductance. This latter mechanism may contribute significantly to ictal depolarizations observed during cholinergic-induced seizures.

Global and serial neurons form A hierarchically arranged interface proposed to underlie memory and cognition

It is hypothesized that the cholinergic and monoaminergic neurons of the brain from a global network. What is meant by a global network is that these neurons operate as a unified whole, generating widespread patterns of activity in concert with particular electroencephalographic states, moods and cognitive gestalts. Apart from cholinergic and monoaminergic global systems, most other mammalian neurons relay sensory information about the external and internal milieu to serially ordered loci. These "serial" neurons are neurochemically distinct from global neurons and commonly use small molecule amino acid neurotransmitters such as glutamate or aspartate. Viewing the circuitry of the mammalian brain within the global-serial dichotomy leads to a number of novel interpretations and predictions. Global systems seem to be capable of transforming incoming sensory data into cognitive-related activity patterns. A comparative examination of global and serial systems anatomy, development and physiology reveals how global systems might turn sensation into mentation. An important step in this process is the permanent encoding of memory. Global neurons are particularly plastic, as are the neurons receiving global inputs. Global afferents appear to be capable of reorganizing synapses on recipient serial cells, thus leading to enhanced responding to a signal, in a particular context and state of arousal.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Hippocampal EEG responses induced by carbachol and atropine infusions into the septum and the hippocampus in the urethane-anaesthetized rat

Infusion of 1 microgram of carbachol, a potent cholinergic agonist, into the lateral septum of the urethane-anaesthetized rat systematically caused the induction of clear-cut hippocampal theta (theta). However, infusion of an equivalent amount of the drug into the hippocampus, close to the recording electrode, failed to induce theta in 50% of the animals and produced a mixture of theta waves and desynchronized activity, resulting in atypical EEG patterns, in the remaining subjects. Both carbachol EEG effects were blocked by intraseptal infusion of the antimuscarinic agent, atropine. Our data demonstrate that muscarinic receptors in the septum are predominant sites for cholinergic agonist-antagonist action capable of generating or suppressing hippocampal theta in the rat. They also indicate that intraseptal cholinergic mechanisms play an important role in the initiation and generation of this rhythm.

The extrinsic modulation of hippocampal theta depends on the coactivation of cholinergic and GABA-ergic medial septal inputs

The long trains of theta field activity recorded from the hippocampal formation of urethane-anesthetized rats are thought to be primarily dependent on cholinergic afferents originating in the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB). Recent anatomical studies have revealed the existence of a septal GABA-ergic input to the hippocampal formation which synapses mainly on intrinsic GABA-ergic interneurons. The present work investigated the possibility that some form of interaction between cholinergic and GABA-ergic MS/vDBB inputs might be required for the generation of hippocampal theta field and cellular activities in urethane-anesthetized rats. Reversible inactivation of the MS/vDBB completely abolished theta field and theta-on cell activities, but "released" theta-off cells. The theta field and theta-on cell activities induced by direct intrahippocampal microinfusions of carbachol were also abolished by MS/vDBB inactivation. We speculated that septal suppression was producing two effects: 1) removing excitatory, cholinergic input; and 2) removing inhibitory control of hippocampal GABA-ergic interneurons, thereby increasing the overall level of hippocampal inhibition. Sequential administration of both carbachol and the GABA-A antagonist, bicuculline, resulted in theta-like oscillations similar to those seen in hippocampal slices bath perfused with carbachol alone. Thus, following MS/vDBB inactivation hippocampal GABA-ergic systems are overactive; this enhances intrinsic inhibition and blocks carbachol theta. By reducing the overall level of inhibition in the hippocampus with bicuculline, it is possible to reinstate its oscillatory properties. Conversely, increasing the level of inhibition in the hippocampus (with muscimol) results in the abolishment of theta field activity and the discharges of both theta-on and theta-off cells. Based on these findings we are proposing that cholinergic and GABA-ergic systems originating in the MS/vDBB act synergistically to modulate hippocampal theta. Cholinergic projections provide the afferent excitatory drive for hippocampal theta-on cells and septal GABA-ergic projections act to reduce the overall level of inhibition by inhibiting hippocampal GABA-ergic interneurons (hippocampal theta-off cells). Both activities must be present for the generation of hippocampal theta field and cellular activities. The balance between the cholinergic and GABA-ergic systems may determine whether hippocampal synchrony (theta) or asynchrony (LIA) occurs.

Theta rhythms in the rat medial entorhinal cortex in vitro: evidence for involvement of muscarinic receptors

Slice preparation obtained from the rat were used to study cholinergically induced field potentials in the medial entorhinal cortex. Perfusion of slices containing medial entorhinal cortex with acetylcholine, muscarine and eserine induced theta-like activity in a frequency range of 3-10 Hz and an amplitude of 200-300 microV. Nicotine, in contrast, did not produce any rhythmical slow waveforms. The cholinergically induced theta-like oscillations were abolished by perfusion of the muscarinic antagonists atropine sulphate and scopolamine but were unaffected by the nicotine blockers hexamethonium and mecamylamine.

Carbachol-induced rhythmic slow activity (theta) in cat hippocampal formation slices

Application of the cholinergic agonist, carbachol, produced theta-like rhythmical waveforms, recorded in the stratum moleculare of the dentate gyrus in the cat hippocampal formation slices. This effect of carbachol was antagonized by atropine but not D-tubocurarine. These results provide first direct evidence that the hippocampal formation neuronal network in the cat is capable of producing synchronized slow wave activity when isolated from pulsed rhythmic inputs of the medial septum.

Carbachol-induced theta-like activity in entorhinal cortex slices

The present study was conducted for two purposes: the first was to evaluate whether activation of cholinergic receptors of the entorhinal cortex in vitro (complete deafferentation) with carbachol (100 microM) was capable of producing theta (theta)-like slow activity. The second purpose was to determine whether carbachol-induced slow waveforms were mediated by muscarinic or nicotinic receptors. We demonstrated that carbachol was capable of producing theta-like slow activity. This activity was not altered by nicotinic antagonists, (+)-tubocurarine and hexametonium. Atropine and scopolamine, in contrast, completely blocked in vitro induced slow waves, indicating entorhinal muscarinic receptors to be actively involved in the mechanism generating cholinergic theta rhythm.

In vivo intrahippocampal microinfusion of carbachol and bicuculline induces theta-like oscillations in the septally deafferented hippocampus

In their laboratory the authors have previously demonstrated that hippocampal slices could be induced to generate trains of "theta-like" oscillations by whole-bath perfusions of carbachol. Until recently, it has not been possible to generate similar activity in the septally deafferented hippocampus of an otherwise intact brain by microinfusions of carbachol. This study presents a full report of the first demonstration of a theta-like oscillation in the in vivo, septally deafferented hippocampal formation. Rats were anesthetized with urethane and implanted with microinfusion cannulae in the region of the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB) and at single or multiple sites in the stratum moleculare of the fascia dentata. The MS/vDBB was microinfused with procaine hydrochloride to produce a reversible suppression lasting for approximately 20 minutes. Intrahippocampal microinfusions of carbachol or bicuculline alone (in the postprocaine condition of the MS/vDBB) failed to produce any theta-like oscillations. The combination of carbachol and bicuculline produced trains of theta-like oscillations during suppression of the MS/vDBB very similar to those seen in the slice preparations. The oscillations were blocked by intravenous administration of atropine sulfate, and they had the same depth profile as that of theta. Theta-on cells were shown to discharge in rhythmic bursts in synchrony with the oscillations. Thus, it would appear that the essential nature of the medial septal input to the hippocampal formation, for the generation of theta field activity in the intact brain, consists of a critical balance between cholinergic and GABAergic circuitry.

Effects of cholinergic agonists on immature rat hippocampal neurons

We have investigated the effects of acetylcholine (ACh) and the cholinergic agonist carbachol on several cell types in the developing rat hippocampus. Pyramidal cells were responsive to cholinergic applications on the first day examined (postnatal day 2), indicating that postsynaptic cholinoceptivity develops early, perhaps before functional cholinergic innervation is present. These drugs, which induce a membrane depolarization and a conductance decrease in mature pyramidal cells, had similar effects (both magnitude and pharmacology) on most immature neurons. However, a minority of cells in immature tissue exhibited decreased input resistance (Rin) during the cholinergic-induced depolarization. This response is likely a product of cholinergic action on local circuit neurons: non-pyramidal-type cells from animals as young as 8 days demonstrated excitatory responses to application of cholinergic agonists. The study revealed a number of other features of immature cells which may have functional significance. Lucifer yellow injections showed significant dye coupling among CA3 (but not CA1) pyramidal cells in immature tissue, suggesting close metabolic and/or electrotonic coupling between those cells during development. Mature CA3 cells showed less dye coupling, but increased anomalous rectification, and longer time constant. Developmental changes in intrinsic cell properties, coupled to alterations in local circuit interactions, may alter tissue responsiveness to neurotransmitters such as acetylcholine, even if the receptor-mediated drug action remains stable.

Topographical distribution of septohippocampal projections demonstrated by the PHA-L immunohistochemical method in rats

The anterograde tracer, Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected into the medial septum-diagonal band complex of the rat. When PHA-L was injected into the rostral part of the medial septum, labeled axon terminals were distributed largely in the dorsal hippocampus. After PHA-L injection into the caudal part of the medial septum or vertical limb of diagonal band of Broca, labeled terminals were observed predominantly in the ventral hippocampus.

Electrophysiological properties of rat septal region neurons during development in culture

We report on the development of membrane properties of septal region neurons from embryonic rats in serum-free culture during 1-25 days in vitro (DIV). Na(+)-dependent action potentials could be evoked within 1 day after plating and 3 different types of outward current were observed by means of the patch-clamp technique: IK, IA and IC. In some neurons the neurotransmitter GABA evoked a chloride current after 2 DIV. In addition a cationic current elicited by glutamate appeared after 4 DIV. Within 8-12 DIV virtually all neurons were sensitive to both GABA and glutamate. Spontaneous action potentials and postsynaptic potentials occurred after 7-10 DIV but cultured septal neurons did not generate any pacemaker-like activity.

Effects of transient cerebral ischemia on the hippocampal dentate theta (theta) profile in the acute rat: a study 4-5 months following recirculation

This study mainly describes the long-term effects of 20 min of cerebral ischemia on the profile of the presumed cholinergic theta rhythm in the rat dorsal hippocampal formation during ether anesthesia and injection of the muscarinic agonist agent arecoline. The experimental data were collected 4-5 months after ischemia. They show that ischemia results in a statistically significant reduction in both superficial and deep theta recorded from the CA1 area of the hippocampus and the dentate gyrus, respectively. Amplitude reduction is similar for both rhythms and co-varies positively with the extent of CA1 stratum pyramidale damage which, from light microscope observation, appeared to be the major neuroanatomical consequence of ischemic insult in the dorsal hippocampal formation. The medial septal nucleus-diagonal band of Broca complex involved in theta generation did not suffer visible anatomical damage. Moreover, no significant alteration in the spatial distribution and the density of hippocampal dentate acetylcholinesterase reaction product was seen in ischemic animals. These histological data were statistically confirmed by computerized image analysis. Finally, this is the first investigation to show that transient interruption of cerebral blood flow results in a long-lasting alteration of theta rhythm which is probably the major aspect of the basic activity of the hippocampal formation. Thus, the present findings obtained in the acute rat at 4-5 months postischemia confirm and extend, in most respects, our previous results collected in the chronic animal 2-29 days following 4-vessel occlusion. Possible significance of these findings for the hypothesis of the dependent generation sites of superficial and deep thetas in the hippocampus assumed to be crucial in learning and memory, is discussed.

The classification of medial septum-diagonal band cells as theta-on or theta-off in relation to hippocampal EEG states

The discharge patterns of cells located in the medial septum (MS)/vertical limb of the Diagonal Band of Broca (vDBB) were recorded simultaneously with hippocampal formation large amplitude irregular activity (LIA), or theta, in urethane-anesthetized rats. The main conclusion was that the majority of cells in the MS/vDBB were theta-related and could be classified according to the same scheme developed for theta-related cells in the hippocampal formation. That is, cells were classified as theta-on or theta-off, with subtypes defined as tonic or phasic, linear or non-linear. The discharge properties of hippocampal and MS/vDBB cell populations were compared. It was argued that this classification scheme encompassed all the crucial properties of theta-related cells in the hippocampal formation and the MS/vDBB. An alternative model to the septal pacemaker hypothesis, concerning the role of the medial septum in the generation of hippocampal formation theta activity, was presented and discussed.

Evidence that activation of in vitro hippocampal theta rhythm only involves muscarinic receptors

The present study was conducted for two purposes: the first was to evaluate whether activation of nicotinic receptors in the hippocampal formation in vitro (slice) preparation was capable of producing type 2 (atropine-sensitive) theta rhythm. The cholinergic nature and involvement of muscarinic receptors in this type of theta has been previously well documented. The second purpose was to determine whether perfusion of a number of (other) putative neurotransmitters shown to be present in the hippocampal formation could elicit type 1 (atropine-resistant) theta in the slice preparation. Further experiments were conducted to determine if these agents interacted in any manner with cholinergically-induced type 2 theta. Electroencephalic (EEG) theta activity was not induced by nicotine, providing evidence for an exclusive muscarinic receptor involvement in this cholinergically-induced type 2 theta. In addition, theta activity was not elicited by the application of gamma-aminobutyric acid (GABA), glutamate, norepinephrine, dopamine or serotonin. The application of any of these agents did not significantly alter the production of cholinergically-induced theta. These results suggest that type 1 theta originates in regions extrinsic to the hippocampus, or is the result of the interaction of several neurotransmitters on different receptors.

Carbachol-induced EEG 'theta' in hippocampal formation slices: evidence for a third generator of theta in CA3c area

The topography of carbachol-induced EEG theta activity was studied using the hippocampal formation slice preparation. Systematic tracking with electrodes exhibited two amplitude maxima of cholinergic-induced theta, one located in the stratum oriens of the CA1 pyramidal cells and the other in a region of CA3c pyramidal neurons. In addition, mapping experiments demonstrated EEG theta in the CA3a and CA3b cell body layers, but not in the subicular and parasubicular regions, or the ventral blade of the dentate gyrus. Furthermore, transected slice (trans-slice) preparations used in the present study revealed that the CA3c region could generate carbachol-induced theta independently of CA1 and dentate gyrus generator zones and conversely, CA1 and dentate gyrus areas were capable of generating cholinergic-induced theta rhythm independently of the CA3c region. These results provide strong evidence for 3 independent, anatomically separated generators of theta: one located in the stratum oriens of CA1 neurons, a second in the stratum moleculare of the dentate gyrus and a third one in the region of Ca3c cells. In addition, the results support previous in vivo suggestions that theta rhythm can be either elicited or blocked by cholinergic agents acting on sites within the hippocampal formation.

Intracellular records of carbachol-induced theta rhythm in hippocampal slices

Intracellular recordings were made in the CA1, CA3 and dentate cell layers prior to, during and after the bath perfusion of 50 microM carbachol on hippocampal slices. Fifty-six percent of the cells in this sample were termed theta (theta)-related, i.e., they exhibited membrane potential oscillations of 5-28 mV and rhythmic spike discharges related to the carbachol-induced extracellular theta-rhythm. The remaining 44% of the cells did not show the above relationships to the extracellular theta-rhythm. Carbachol produced an overall depolarization in all cells, in the range of 10-20 mV. These results demonstrated the cellular basis of carbachol-induced theta in hippocampal slices. This preparation will be a valuable model for studying cellular mechanisms and network properties underlying electroencephalographic activity.

Phase shifting of CA1 and dentate EEG theta rhythms in hippocampal formation slices

Carbachol-induced hippocampal EEG theta-rhythms were recorded simultaneously from the CA1 and dentate areas in rat hippocampal brain slices. Phase shifts ranging from 0 to 180 degrees between the CA1 and dentate theta generators were observed. The differences in slow wave theta phase relations between in vitro and in vivo preparations were interpreted as resulting from deafferentation of the hippocampal slices.