Septo-hippocampal pathway necessary for dentate theta production

Basal Forebrain Impairment: Understanding the Mnemonic Function of the Septal Region Translates in Therapeutic Advances

The basal forebrain is one of the three major brain circuits involved in episodic memory formation together with the hippocampus and the diencephalon. The dysfunction of each of these regions is known to cause anterograde amnesia. While the hippocampal pyramidal neurons are known to encode episodic information and the diencephalic structures are known to provide idiothetic information, the contribution of the basal forebrain to memory formation has been exclusively associated with septo-hippocampal cholinergic signaling. Research data from the last decade broadened our understanding about the role of septal region in memory formation. Animal studies revealed that septal neurons process locomotor, rewarding and attentional stimuli. The integration of these signals results in a systems model for the mnemonic function of the medial septum that could guide new therapeutic strategies for basal forebrain impairment (BFI). BFI includes the disorders characterized with basal forebrain amnesia and neurodegenerative disorders that affect the basal forebrain. Here, we demonstrate how the updated model of septal mnemonic function can lead to innovative translational treatment approaches that include pharmacological, instrumental and behavioral techniques.

The Role of the Medial Septum-Associated Networks in Controlling Locomotion and Motivation to Move

The Medial Septum and diagonal Band of Broca (MSDB) was initially studied for its role in locomotion. However, the last several decades were focussed on its intriguing function in theta rhythm generation. Early studies relied on electrical stimulation, lesions and pharmacological manipulation, and reported an inconclusive picture regarding the role of the MSDB circuits. Recent studies using more specific methodologies have started to elucidate the differential role of the MSDB's specific cell populations in controlling both theta rhythm and behaviour. In particular, a novel theory is emerging showing that different MSDB's cell populations project to different brain regions and control distinct aspects of behaviour. While the majority of these behaviours involve movement, increasing evidence suggests that MSDB-related networks govern the motivational aspect of actions, rather than locomotion per se. Here, we review the literature that links MSDB, theta activity, and locomotion and propose open questions, future directions, and methods that could be employed to elucidate the diverse roles of the MSDB-associated networks.

Inferring the direction of rhythmic neural transmission via inter-regional phase-amplitude coupling (ir-PAC)

Phase-amplitude coupling (PAC) estimates the statistical dependence between the phase of a low-frequency component and the amplitude of a high-frequency component of local field potentials (LFP). To date PAC has been mainly applied to one signal. In this work, we introduce a new application of PAC to two LFPs and suggest that it can be used to infer the direction and strength of rhythmic neural transmission between distinct brain networks. This hypothesis is based on the accumulating evidence that transmembrane currents related to action potentials contribute a broad-band component to LFP in the high-gamma band, and PAC calculated between the amplitude of high-gamma (>60 Hz) in one LFP and the phase of a low-frequency oscillation (e.g., theta) in another would therefore relate the output (spiking) of one area to the input (somatic/dendritic postsynaptic potentials) of the other. We tested the hypothesis on theta-band long range communications between hippocampus and prefrontal cortex (PFC) and theta-band short range communications between dentate gyrus (DG) and the Ammon’s horn (CA1) within the hippocampus. The ground truth was provided by the known anatomical connections predicting hippocampus → PFC and DG → CA1, i.e., theta transmission is unidirectional in both cases: from hippocampus to PFC and from DG to CA1 along the tri-synaptic pathway within hippocampus. We found that (1) hippocampal high-gamma amplitude was significantly coupled to PFC theta phase, but not vice versa; (2) similarly, DG high-gamma amplitude was significantly coupled to CA1 theta phase, but not vice versa, and (3) the DG high-gamma-CA1 theta PAC was significantly correlated with DG → CA1 Granger causality, a well-established analytical measure of directional neural transmission. These results support the hypothesis that inter-regional PAC (ir-PAC) can be used to relate the output of a rhythmic “driver” network (i.e., high gamma) to the input of a rhythmic “receiver” network (i.e., theta) and thereby establish the direction and strength of rhythmic neural transmission.

Assessing Granger Causality in Electrophysiological Data: Removing the Adverse Effects of Common Signals via Bipolar Derivations

Multielectrode voltage data are usually recorded against a common reference. Such data are frequently used without further treatment to assess patterns of functional connectivity between neuronal populations and between brain areas. It is important to note from the outset that such an approach is valid only when the reference electrode is nearly electrically silent. In practice, however, the reference electrode is generally not electrically silent, thereby adding a common signal to the recorded data. Volume conduction further complicates the problem. In this study we demonstrate the adverse effects of common signals on the estimation of Granger causality, which is a statistical measure used to infer synaptic transmission and information flow in neural circuits from multielectrode data. We further test the hypothesis that the problem can be overcome by utilizing bipolar derivations where the difference between two nearby electrodes is taken and treated as a representation of local neural activity. Simulated data generated by a neuronal network model where the connectivity pattern is known were considered first. This was followed by analyzing data from three experimental preparations where a priori predictions regarding the patterns of causal interactions can be made: (1) laminar recordings from the hippocampus of an anesthetized rat during theta rhythm, (2) laminar recordings from V4 of an awake-behaving macaque monkey during alpha rhythm, and (3) ECoG recordings from electrode arrays implanted in the middle temporal lobe and prefrontal cortex of an epilepsy patient during fixation. For both simulation and experimental analysis the results show that bipolar derivations yield the expected connectivity patterns whereas the untreated data (referred to as unipolar signals) do not. In addition, current source density signals, where applicable, yield results that are close to the expected connectivity patterns, whereas the commonly practiced average re-reference method leads to erroneous results.

Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze

Hippocampal theta activity is related to spatial information processing, and high-frequency theta activity, in particular, has been linked to efficient spatial memory performance. Theta activity is regulated by the synchronizing ascending system (SAS), which includes mesencephalic and diencephalic relays. The supramamillary nucleus (SUMn) is located between the reticularis pontis oralis and the medial septum (MS), in close relation with the posterior hypothalamic nucleus (PHn), all of which are part of this ascending system. It has been proposed that the SUMn plays a role in the modulation of hippocampal theta-frequency; this could occur through direct connections between the SUMn and the hippocampus or through the influence of the SUMn on the MS. Serotonergic raphe neurons prominently innervate the hippocampus and several components of the SAS, including the SUMn. Serotonin desynchronizes hippocampal theta activity, and it has been proposed that serotonin may regulate learning through the modulation of hippocampal synchrony. In agreement with this hypothesis, serotonin depletion in the SUMn/PHn results in deficient spatial learning and alterations in CA1 theta activity-related learning in a Morris water maze. Because it has been reported that SUMn inactivation with lidocaine impairs the consolidation of reference memory, we asked whether changes in hippocampal theta activity related to learning would occur through serotonin depletion in the SUMn, together with deficiencies in memory. We infused 5,7-DHT bilaterally into the SUMn in rats and evaluated place learning in the standard Morris water maze task. Hippocampal (CA1 and dentate gyrus), septal and SUMn EEG were recorded during training of the test. The EEG power in each region and the coherence between the different regions were evaluated. Serotonin depletion in the SUMn induced deficient spatial learning and altered the expression of hippocampal high-frequency theta activity. These results provide evidence in support of a role for serotonin as a modulator of hippocampal learning, acting through changes in the synchronicity evoked in several relays of the SAS.

Synergy of direct and indirect cholinergic septo-hippocampal pathways coordinates firing in hippocampal networks

The medial septum/diagonal band of Broca complex (MSDB) is a key structure that modulates hippocampal rhythmogenesis. Cholinergic neurons of the MSDB play a central role in generating and pacing theta-band oscillations in the hippocampal formation during exploration, novelty detection, and memory encoding. How precisely cholinergic neurons affect hippocampal network dynamics in vivo, however, has remained elusive. In this study, we show that stimulation of cholinergic MSDB neurons in urethane-anesthetized mice acts on hippocampal networks via two distinct pathways. A direct septo-hippocampal cholinergic projection causes increased firing of hippocampal inhibitory interneurons with concomitantly decreased firing of principal cells. In addition, cholinergic neurons recruit noncholinergic neurons within the MSDB. This indirect pathway is required for hippocampal theta synchronization. Activation of both pathways causes a reduction in pyramidal neuron firing and a more precise coupling to the theta oscillatory phase. These two anatomically and functionally distinct pathways are likely relevant for cholinergic control of encoding versus retrieval modes in the hippocampus.

Harnessing the power of theta: natural manipulations of cognitive performance during hippocampal theta-contingent eyeblink conditioning

Neurobiological oscillations are regarded as essential to normal information processing, including coordination and timing of cells and assemblies within structures as well as in long feedback loops of distributed neural systems. The hippocampal theta rhythm is a 3–12 Hz oscillatory potential observed during cognitive processes ranging from spatial navigation to associative learning. The lower range, 3–7 Hz, can occur during immobility and depends upon the integrity of cholinergic forebrain systems. Several studies have shown that the amount of pre-training theta in the rabbit strongly predicts the acquisition rate of classical eyeblink conditioning and that impairment of this system substantially slows the rate of learning. Our lab has used a brain-computer interface (BCI) that delivers eyeblink conditioning trials contingent upon the explicit presence or absence of hippocampal theta. A behavioral benefit of theta-contingent training has been demonstrated in both delay and trace forms of the paradigm with a two- to four-fold increase in learning speed. This behavioral effect is accompanied by enhanced amplitude and synchrony of hippocampal local field potential (LFP)s, multi-unit excitation, and single-unit response patterns that depend on theta state. Additionally, training in the presence of hippocampal theta has led to increases in the salience of tone-induced unit firing patterns in the medial prefrontal cortex, followed by persistent multi-unit activity during the trace interval. In cerebellum, rhythmicity and precise synchrony of stimulus time-locked LFPs with those of hippocampus occur preferentially under the theta condition. Here we review these findings, integrate them into current models of hippocampal-dependent learning and suggest how improvement in our understanding of neurobiological oscillations is critical for theories of medial temporal lobe processes underlying intact and pathological learning.

Interconnection and synchronization of neuronal populations in the mouse medial septum/diagonal band of Broca

The medial septum/diagonal band of Broca (MS/DBB) is crucial for hippocampal theta rhythm generation (4-12 Hz). However, the mechanisms behind theta rhythmogenesis are still under debate. The MS/DBB consists, in its majority, of three neuronal populations that use acetylcholine, GABA, or glutamate as neurotransmitter. While the firing patterns of septal neurons enable the MS/DBB to generate rhythmic output critical for the generation of the hippocampal theta rhythm, the ability to synchronize these action potentials is dependent on the interconnectivity between the three major MS/DBB neuronal populations, yet little is known about intraseptal connections. Here we assessed the connectivity between pairs of MS/DBB neurons with paired patch-clamp recordings. We found that glutamatergic and GABAergic neurons provide intraseptal connections and produce sizable currents in MS/DBB postsynaptic cells. We also analyzed linear and nonlinear relationships between the action potentials fired by pairs of neurons belonging to various MS/DBB neuronal populations. Our results show that while the synchrony index for action potential firing was significantly higher in pairs of GABAergic neurons, coherence of action potential firing in the theta range was similarly low in all pairs analyzed. Recurrence analysis demonstrated that individual action potentials were more recurrent in cholinergic neurons than in other cell types. Implementing sparse connectivity in a computer model of the MS/DBB network reproduced our experimental data. We conclude that the interplay between the intrinsic membrane properties of different MS/DBB neuronal populations and the connectivity among these populations underlie the ability of the MS/DBB network to critically contribute to hippocampal theta rhythmogenesis.

Distribution and targets of the relaxin-3 innervation of the septal area in the rat

Neural tracing studies have revealed that the rat medial and lateral septum are targeted by ascending projections from the nucleus incertus, a population of tegmental GABA neurons. These neurons express the relaxin-family peptide, relaxin-3, and pharmacological modulation of relaxin-3 receptors in medial septum alters hippocampal theta rhythm and spatial memory. In an effort to better understand the basis of these interactions, we have characterized the distribution of relaxin-3 fibers/terminals in relation to different septal neuron populations identified using established protein markers. Dense relaxin-3 fiber plexuses were observed in regions of medial septum containing hippocampal-projecting choline acetyltransferase (ChAT)-, neuronal nitric oxide synthase (nNOS)-, and parvalbumin (PV)-positive neurons. In lateral septum (LS), relaxin-3 fibers were concentrated in the ventrolateral nucleus of rostral LS and the ventral nucleus of caudal LS, with sparse labeling in the dorsolateral and medial nuclei of rostral LS, dorsal nucleus of caudal LS, and ventral portion nuclei. Relaxin-3 fibers were also observed in the septofimbrial and triangular septal nuclei. In the medial septum, we observed relaxin-3-immunoreactive contacts with ChAT-, PV-, and glutamate decarboxylase-67-positive neurons that projected to hippocampus, and contacts between relaxin-3 terminals and calbindin- and calretinin-positive neurons. Relaxin-3 colocalized with synaptophysin in nerve terminals in all septal areas, and ultrastructural analysis revealed these terminals were symmetrical and contacted spines, somata, dendritic shafts, and occasionally other axonal terminals. These data predict that this GABA/peptidergic projection modulates septohippocampal activity and hippocampal theta rhythm related to exploratory navigation, defensive and ingestive behaviors, and responses to neurogenic stressors.

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.

Differential expression of voltage-gated K+ currents in medial septum/diagonal band complex neurons exhibiting distinct firing phenotypes

The medial septum/diagonal band complex (MSDB) controls hippocampal excitability, rhythms and plastic processes. Medial septal neuronal populations display heterogeneous firing patterns. In addition, some of these populations degenerate during age-related disorders (e.g. cholinergic neurons). Thus, it is particularly important to examine the intrinsic properties of theses neurons in order to create new agents that effectively modulate hippocampal excitability and enhance memory processes. Here, we have examined the properties of voltage-gated, K(+) currents in electrophysiologically-identified neurons. These neurons were taken from young rat brain slices containing the MS/DB complex. Whole-cell, patch recordings of outward currents were obtained from slow firing, fast-spiking, regular-firing and burst-firing neurons. Slow firing neurons showed depolarization-activated K(+) current peaks and densities larger than in other neuronal subtypes. Slow firing total current exhibited an inactivating A-type current component that activates at subthreshold depolarization and was reliably blocked by high concentrations of 4-AP. In addition, slow firing neurons expressed a low-threshold delayed rectifier K(+) current component with slow inactivation and intermediate sensitivity to tetraethylammonium. Fast-spiking neurons exhibited the smaller I(K) and I(A) current densities. Burst and regular firing neurons displayed an intermediate firing phenotype with I(K) and I(A) current densities that were larger than the ones observed in fast-spiking neurons but smaller than the ones observed in slow-firing neurons. In addition, the prevalence of each current differed among electrophysiological groups with slow firing and regular firing neurons expressing mostly I(A) and fast spiking and bursting neurons exhibiting mostly delayer rectifier K(+) currents with only minimal contributions of the I(A). The pharmacological or genetic modulations of these currents constitute an important target for the treatment of age-related disorders.

Neurophysiological and computational principles of cortical rhythms in cognition

Synchronous rhythms represent a core mechanism for sculpting temporal coordination of neural activity in the brain-wide network. This review focuses on oscillations in the cerebral cortex that occur during cognition, in alert behaving conditions. Over the last two decades, experimental and modeling work has made great strides in elucidating the detailed cellular and circuit basis of these rhythms, particularly gamma and theta rhythms. The underlying physiological mechanisms are diverse (ranging from resonance and pacemaker properties of single cells to multiple scenarios for population synchronization and wave propagation), but also exhibit unifying principles. A major conceptual advance was the realization that synaptic inhibition plays a fundamental role in rhythmogenesis, either in an interneuronal network or in a reciprocal excitatory-inhibitory loop. Computational functions of synchronous oscillations in cognition are still a matter of debate among systems neuroscientists, in part because the notion of regular oscillation seems to contradict the common observation that spiking discharges of individual neurons in the cortex are highly stochastic and far from being clocklike. However, recent findings have led to a framework that goes beyond the conventional theory of coupled oscillators and reconciles the apparent dichotomy between irregular single neuron activity and field potential oscillations. From this perspective, a plethora of studies will be reviewed on the involvement of long-distance neuronal coherence in cognitive functions such as multisensory integration, working memory, and selective attention. Finally, implications of abnormal neural synchronization are discussed as they relate to mental disorders like schizophrenia and autism.

Medial septal beta-amyloid 1-40 injections alter septo-hippocampal anatomy and function

Degeneration of septal neurons in Alzheimer's disease (AD) results in abnormal information processing at cortical circuits and consequent brain dysfunction. The septum modulates the activity of hippocampal and cortical circuits and is crucial to the initiation and occurrence of oscillatory activities such as the hippocampal theta rhythm. Previous studies suggest that amyloid beta peptide (Abeta) accumulation may trigger degeneration in AD. This study evaluates the effects of single injections of Abeta 1-40 into the medial septum. Immunohistochemistry revealed a decrease in septal cholinergic (57%) and glutamatergic (53%) neurons in Abeta 1-40 treated tissue. Additionally, glutamatergic terminals were significantly less in Abeta treated tissue. In contrast, septal GABAergic neurons were spared. Unitary recordings from septal neurons and hippocampal field potentials revealed an approximately 50% increase in firing rates of slow firing septal neurons during theta rhythm and large irregular amplitude (LIA) hippocampal activities and a significantly reduced hippocampal theta rhythm power (49%) in Abeta 1-40 treated tissue. Abeta also markedly reduced the proportion of slow firing septal neurons correlated to the hippocampal theta rhythm by 96%. These results confirm that Abeta alters the anatomy and physiology of the medial septum contributing to septo-hippocampal dysfunction. The Abeta induced injury of septal cholinergic and glutamatergic networks may contribute to an altered hippocampal theta rhythm which may underlie the memory loss typically observed in AD patients.

Coupling of mesoscopic brain oscillations: recent advances in analytical and theoretical perspectives

Oscillatory brain activities have been traditionally studied in the context of how oscillations at a single frequency recorded from a single area could reveal functional insights. Recent advances in methodology used in signal analysis have revealed that cross-frequency coupling, within or between functional related areas, is more informative in determining the possible roles played by brain oscillations. In this review, we begin by describing the cellular basis of oscillatory field potentials and its theorized as well as demonstrated role in brain function. The recent development of mathematical tools that allow the investigation of cross-frequency and cross-area oscillation coupling will be presented and discussed in the context of recent advances in oscillation research based on animal data. Particularly, some pitfalls and caveats of methods currently available are discussed. Data generated from the application of examined techniques are integrated back into the theoretical framework regarding the functional role of brain oscillations. We suggest that the coupling of oscillatory activities at different frequencies between brain regions is crucial for understanding the brain from a functional ensemble perspective. Effort should be directed to elucidate how cross-frequency and area coupling are modulated and controlled. To achieve this, only the correct application of analytical tools may shed light on the intricacies of information representation, generation, binding, encoding, storage and retrieval in the brain.

Hippocampal theta rhythm and its coupling with gamma oscillations require fast inhibition onto parvalbumin-positive interneurons

Hippocampal theta (5-10 Hz) and gamma (35-85 Hz) oscillations depend on an inhibitory network of GABAergic interneurons. However, the lack of methods for direct and cell-type-specific interference with inhibition has prevented better insights that help link synaptic and cellular properties with network function. Here, we generated genetically modified mice (PV-Deltagamma(2)) in which synaptic inhibition was ablated in parvalbumin-positive (PV+) interneurons. Hippocampal local field potential and unit recordings in the CA1 area of freely behaving mice revealed that theta rhythm was strongly reduced in these mice. The characteristic coupling of theta and gamma oscillations was strongly altered in PV-Deltagamma(2) mice more than could be accounted for by the reduction in theta rhythm only. Surprisingly, gamma oscillations were not altered. These data indicate that synaptic inhibition onto PV+ interneurons is indispensable for theta- and its coupling to gamma oscillations but not for rhythmic gamma-activity in the hippocampus. Similar alterations in rhythmic activity were obtained in a computational hippocampal network model mimicking the genetic modification, suggesting that intrahippocampal networks might contribute to these effects.

Nerve growth factor rapidly increases muscarinic tone in mouse medial septum/diagonal band of Broca

Nerve growth factor (NGF) has been implicated in maintaining and regulating normal functioning of the septohippocampal pathway. However, many aspects of its physiological actions and the underlying mechanisms await elucidation. In this study, we investigated the effect of acute NGF exposure on neurons in the mouse medial septum/diagonal band of Broca (MS/DB), focusing on the cholinergic neurons and the subpopulation of noncholinergic neurons that were identified to be putatively GABAergic. We report that MS/DB neurons in a thin slice preparation, when exposed to NGF via bath perfusion, rapidly and indiscriminately increased the rate of spontaneous firing in all MS/DB neurons. However, focal application of NGF to individual MS/DB neurons increased spontaneous firing in cholinergic, but not in the noncholinergic, subpopulation. The NGF-induced effect on cholinergic neurons was direct, requiring activation and signaling via TrkA receptors, which were immunohistochemically localized to the cholinergic neurons in the MS/DB. TrkA receptors were absent in putative GABAergic MS/DB neurons, and blockade of TrkA signaling in these and other noncholinergic neurons had no effect on their firing activity after exposure to NGF. Conversely, methyl scopolamine, blocked the increased firing activity of noncholinergic neurons during bath perfusion of NGF. We propose a cell type-specific mode of action for NGF in the MS/DB. The neurotrophin directly enhances cholinergic neuronal activity in the MS/DB through TrkA-mediated signaling, increasing acetylcholine release and, thus, muscarinic tone. This increase in muscarinic tone, in turn, results in heightened firing activity in noncholinergic MS/DB neurons.

Characterization of medial septal glutamatergic neurons and their projection to the hippocampus

The two neuronal populations that have been typically investigated in the septum use acetylcholine and GABA as neurotransmitters. The existence of noncholinergic, non-GABAergic, most likely glutamatergic septal neurons has recently been reported. However, their morphological characteristics, numbers, distribution, and connectivity have not been determined. Furthermore, the projection of septal glutamatergic neurons to the hippocampus has not been characterized. To address these issues, subpopulations of cholinergic and GABAergic neurons were identified by immunohistochemistry. In addition, the retrograde tracer fluorogold was injected into the hippocampus to determine the characteristics of a glutamatergic septo-hippocampal projection. Our work revealed that although glutamatergic neurons are found throughout the septum, they concentrate in medial septal regions. Using stereological probes, approximately 16,000 glutamatergic neurons were estimated in the medial septal region. Triple immunostaining showed that most glutamatergic neurons do not immunoreact with cholinergic or GABAergic neuronal markers (anti-ChAT or anti-GAD67 antibodies, respectively). Fluorogold injections into CA1, CA3, and dentate gyrus of the hippocampus showed that septal glutamatergic neurons project to each of these hippocampal regions, forming approximately 23% of the septo-hippocampal projection. Most cell bodies of septo-hippocampal glutamatergic neurons were located in the medial septum. The remaining cell bodies were found in the diagonal band. This data shows that glutamatergic neurons constitute a significant neuronal population in the septum and that a subpopulation of these neurons projects to hippocampal regions. Thus, the septo-hippocampal projection needs to be reconsidered as a three neurotransmitter pathway.

Septal networks: relevance to theta rhythm, epilepsy and Alzheimer's disease

Information processing and storing by brain networks requires a highly coordinated operation of multiple neuronal groups. The function of septal neurons is to modulate the activity of archicortical (e.g. hippocampal) and neocortical circuits. This modulation is necessary for the development and normal occurrence of rhythmical cortical activities that control the processing of sensory information and memory functions. Damage or degeneration of septal neurons results in abnormal information processing in cortical circuits and consequent brain dysfunction. Septal neurons not only provide the optimal levels of excitatory background to cortical structures, but they may also inhibit the occurrence of abnormal excitability states.

Fornix, medial prefrontal cortex, nucleus accumbens, and mediodorsal thalamic nucleus: Roles in a fear-based context discrimination task

The goal of the present study was to evaluate the contributions of various brain structures anatomically and functionally linked to the hippocampus and amygdala in a fear-based context discrimination task. The brain areas of interest included the fornix, medial prefrontal cortex, mediodorsal (MD) thalamic nucleus, and nucleus accumbens. Damage to the MD thalamic nucleus and medial prefrontal cortex produced the largest impairment in context-specific fear responses. Damage to the fornix impaired some fear responses (freezing, ultrasonic vocalizations, defecation, and approach/avoidance) while leaving conditioned fear expression of heart rate and urination unaltered. Damage to the nucleus accumbens was also coupled with deficits in the discriminative expression of some (heart rate, urination, and ultrasonic vocalizations) but sparing of context-appropriate freezing, defecation, and approach/avoidance behaviors.

The hippocampal intrinsic network oscillator

Oscillatory activity characterizes the activity of the hippocampus in vivo; however, the underlying mechanism remains unknown. It is also known that during oscillations the number of action potentials provided by the principal cells is surprisingly low, and it is still an open question how oscillations can emerge under such constraints. One suggestion is that the discharge activity of inhibitory cells takes this function; however, this has been found, in my previous studies, not to be the case for cholinergically mediated and intrinsically generated hippocampal oscillations. This study identifies the hippocampal intrinsic network oscillator and the interactions which underlie the concurrent expression of cholinergically mediated theta (4-15 Hz) and gamma (20-80 Hz) oscillations. A particular axonal network that involves the hippocampal associative pathway, shown to consist of axonal collaterals of CA2 and some CA3 pyramidal cells, forms the oscillator core element. It is functionally activated via two cholinergically mediated reactions. First, direct activation of CA2 and CA3 pyramidal cells to discharge. Second, enhancement of gap junction-mediated axo-axonic interactions among axons of the core element and associated axons of interneurones, which together form the full oscillator. With these two reactions it is possible to explain the rhythmicities and patterns of activity, under the condition of a low number of action potentials. The discharge of CA3 pyramidals serves mainly as a trigger, while firing by CA2 pyramidals, and to a lesser degree by CA3 pyramidals, maintains the oscillatory activity. The cholinergically mediated 2-fold increase in axonal gap junction communication between cells serves two functions: (a) creation of specific activation pathways to produce the rhythmicities and patterns, and (b) formation of a reverberatory system that extends the time during which the sparsely generated action potentials can interact in the network, thereby providing a new source of action potentials, critical for the expression of oscillatory activity.

Inhibitory control of intrinsic hippocampal oscillations?

An oscillatory mode of activity is a basic operational mode of the hippocampus. Such activity involves the concurrent expression of several rhythmic processes, of which theta (4-15 Hz) and gamma (20-80 Hz) oscillations are prominent and considered to be important for cognitive processing. In an experimental model that preserves the intrinsic network oscillator, exhibiting the dependency on cholinergic inputs and consequent expression of concurrent theta and gamma oscillations, we investigate the intrinsic mechanisms underlying such integrated hippocampal network responses. This experimental framework is used here to examine the currently prevailing dogma, that interneurons control hippocampal oscillations. The spontaneous response of individual pyramidal cells (in areas CA3 and CA1) and interneurons (area CA3), during oscillatory activity, was monitored intracellularly. Particular attention was given to the initiation of interneuron discharge during oscillations, to the impact of the synaptic output of discharging interneurons on the oscillatory activity, and to the time at which interneurons discharge in relation to the oscillatory cycles. Analysis of the spontaneous patterns of activity in individual interneurons and their outcome, during the oscillatory activity, revealed that interneuron activity is incompatible with initiating, pacing or determining the oscillatory frequencies, although contributing to the apparent rhythmic patterns. Moreover, our results show that non-interneuronal members of the network control interneuron activity. We therefore suggest that the activity of the excitatory cells, i.e., principle cells, is critical toward the initiation, pacing and synchronization of intrinsic hippocampal network oscillations.

Neuromodulation, theta rhythm and rat spatial navigation

Cholinergic and GABAergic innervation of the hippocampus plays an important role in human memory function and rat spatial navigation. Drugs which block acetylcholine receptors or enhance GABA receptor activation cause striking impairments in the encoding of new information. Lesions of the cholinergic innervation of the hippocampus reduce the amplitude of hippocampal theta rhythm and cause impairments in spatial navigation tasks, including the Morris water maze, eight-arm radial maze, spatial reversal and delayed alternation. Here, we review previous work on the role of cholinergic modulation in memory function, and we present a new model of the hippocampus and entorhinal cortex describing the interaction of these regions for goal-directed spatial navigation in behavioral tasks. These mechanisms require separate functional phases for: (1) encoding of pathways without interference from retrieval, and (2) retrieval of pathways for guiding selection of the next movement. We present analysis exploring how phasic changes in physiological variables during hippocampal theta rhythm could provide these different phases and enhance spatial navigation function.

Simultaneous activation of gamma and theta network oscillations in rat hippocampal slice cultures

Hippocampal activity in vivo is characterized by concurrent oscillations at theta (4-15 Hz) and gamma (20-80 Hz) frequencies. Here we show that cholinergic receptor activation (methacholine 10-20 nm) in hippocampal slice cultures induces an oscillatory mode of activity, in which the intrinsic network oscillator (located in the CA3 area) expresses simultaneous theta and gamma network oscillations. Pyramidal cells display synaptic theta oscillations, characterized by cycles consisting of population EPSP-IPSP sequences that are dominated by population IPSPs. These rhythmic IPSPs most probably result from theta-modulated spiking activity of several interneurons. At the same time, the majority of interneurons consistently display synaptic gamma oscillations. These oscillatory cycles consist of fast depolarizing rhythmic events that are likely to reflect excitatory input from CA3 pyramidal cells. Interneurons comprising this functional group were identified morphologically. They include four known types of interneurons (basket, O-LM, bistratified and str. lucidum-specific cells) and one new type of CA3 interneuron (multi-subfield cell). The oscillatory activity of these interneurons is only weakly correlated between neighbouring cells, and in about half of these (44 %) is modulated by depolarizing theta rhythmicity. The overall characteristics of acetylcholine-induced oscillations in slice cultures closely resemble the rhythmicity observed in hippocampal field and single cell recordings in vivo. Both rhythmicities depend on intrinsic synaptic interactions, and are expressed by different cell types. The fact that these oscillations persist in a network lacking extra-hippocampal connections emphasizes the importance of intrinsic mechanisms in determining this form of hippocampal activity.

Pacemaker neurons for the theta rhythm and their synchronization in the septohippocampal reciprocal loop

Hippocampal theta (4-10 Hz) oscillation represents a well-known brain rhythm implicated in spatial cognition and memory processes. Its cellular mechanisms remain a matter of debate, and previous computational work has focused mostly on mechanisms intrinsic to the hippocampus. On the other hand, experimental data indicate that GABAergic cells in the medial septum play a pacemaker role for the theta rhythm. We have used biophysical modeling to address two major questions raised by the septal pacemaker hypothesis: what is the ion channel mechanism for the single-cell pacemaker behavior and how do these cells become synchronized? Our model predicts that theta oscillations of septal GABAergic cells depend critically on a low-threshold, slowly inactivating potassium current. Network simulations show that theta oscillations are not coherent in an isolated population of pacemaker cells. Robust synchronization emerges with the addition of a second GABAergic cell population. Such a reciprocally inhibitory circuit can be realized by the hippocampo-septal feedback loop.

Effects of metrifonate on the hippocampal theta rhythm of freely moving intact and MS-lesioned mice

Changes in hippocampal electroencephalogram (EEG) have been suggested to be closely associated with spatial learning ability. Spatial learning can be improved in medial septal (MS)-lesioned mice by metrifonate, a cholinesterase inhibitor. We designed this study to investigate the effects of metrifonate on the hippocampal theta oscillation of intact and MS-lesioned mice. Intact and MS-lesioned C57BL mice were treated with acute injections of metrifonate (doses: 15, 50 and 100 mg/kg ip). These included a dose that considerably improved spatial memory of MS-lesioned mice in our earlier study. In addition, subtype selective muscarinic agents, BIBN-99, AF267B and AF150(S) were used. Recordings of hippocampal theta during movement and awake immobility revealed a dramatic reduction of theta in the lesioned animals. Metrifonate induced prominent changes in the EEG of intact mice, but not of MS-lesioned mice. The effect of metrifonate was not mimicked by two selective M(1)-agonists and was augmented by a combined injection of a selective M(2)-antagonist. These data suggest that improved spatial learning by the cholinesterase inhibitor metrifonate is unrelated to its effects on the hippocampal EEG. These two effects may be mediated through different muscarinic receptor subtypes.

Acetylcholine release in the hippocampus of the urethane anaesthetised rat positively correlates with both peak theta frequency and relative power in the theta band

The need to achieve a clearer understanding of relations between hippocampal theta characteristics and cholinergic septohippocampal neuron activity, prompted us to re-examine, in the urethane-anaesthetised rat, the statistical relationships between the electrophysiological and neurochemical variables using a procedure which is believed to enhance significantly the degree of confidence with which parameters of theta recorded with classic macroelectrodes can be related to concomitant acetylcholine output measured by high-performance liquid chromatography with electrochemical detection. Firstly, the theta rhythm and the acetylcholine content were derived from the same hippocampus. Secondly, the hippocampal electroencephalogram was quantified using spectral analysis which permits the more objective quantitative evaluation of selected electroencephalogram samples. Thirdly, a larger number of rats than in our previous study was used here, thus enhancing the validity of statistical results. This procedure yielded, in our time-course determination, two main findings. The first finding is that acetylcholine release was positively correlated with frequency at the peak power of the theta band which reflects the frequency of the theta signal. This finding had not been reported yet. The second finding is that hippocampal acetylcholine outflow also covaried with relative power of the theta band which reflects the amplitude of the theta signal. This finding is consistent with our previous study in which EEG was quantified by means of a traditional method. These findings suggest that the cholinergic component of the septohippocampal system, which is the main source of hippocampal acetylcholine, and neurophysiological mechanisms involved in the modulation of both the amplitude and the frequency of theta are functionally related. The possibility that, at least in the urethane-anaesthetised rat, hippocampal acetylcholine is involved in these modulator mechanisms is discussed.

Neuronal activity of the septal pacemaker of theta rhythm under the influence of stimulation and blockade of the median raphe nucleus in the awake rabbit

The control of theta rhythm in neuronal activity of the medial septal area and hippocampal electroencephalogram by the brainstem structures was investigated in waking rabbits. In the first series of experiments stimulating electrodes were implanted into the midbrain reticular formation and median raphe nucleus. The standard frequency of theta-bursts in medial septal area neurons and in the electroencephalogram was uniformly and chronically decreased in all rabbits with electrodes implanted into the median raphe nucleus (4.7 +/- 0.5 Hz versus 5.2 +/- 0.19 Hz in animals without electrodes in median raphe nucleus). Weak electrical stimulation of the median raphe nucleus resulted in additional decrease of theta expression in the medial septal area neurons and its disappearance from the hippocampal electroencephalogram, where it was substituted by delta-waves and spindles. Stimulation of the reticular formation had the opposite effect, with an increase in theta frequency, regularity and expression in medial septal area neuronal activity and hippocampal electroencephalogram. In the second series of experiments reversible functional blockade of the median raphe nucleus by local microinjection of lidocaine was performed. This resulted in expression of theta-bursts in an additional group of medial septal area neurons, an increase in theta-burst frequency (by 0.5-2 Hz) and regularity with concomitant changes in the electroencephalogram. The effects of sensory stimuli on the background of increased theta activity were suppressed or significantly decreased. It is concluded that, in accordance with the data of other authors, the median raphe nucleus can be regarded as a functional antagonist of the reticular formation, powerfully suppressing theta-bursts of the medial septal area neurons and hippocampal theta rhythm. It is suggested that, in combination with the theta-enhancing influences of reticular formation, the median raphe nucleus may participate in termination of attention, its switching to other stimuli and stabilization of the effects of learning.

Activation of intrinsic hippocampal theta oscillations by acetylcholine in rat septo-hippocampal cocultures

1. Oscillatory electro-encephalographic activity at theta frequencies (4-15 Hz) can be recorded from the hippocampus in vivo and depends on intact septal projections. The hypothesis that these oscillations are imposed on the hippocampus by rhythmically active septal inputs was tested using dual intracellular recordings from CA1 and CA3 pyramidal cells in septo-hippocampal cocultures. 2. Septo-hippocampal cocultures displayed spontaneous oscillatory synaptic activity at theta frequencies. In CA3 cells, EPSP/IPSP sequences predominated, whereas only EPSPs were apparent in CA1 cells. Synaptic potentials in CA3 cells preceded those in CA1 cells by 5-10 ms. 3. Oscillatory synaptic activity was blocked in cocultures by the muscarinic antagonist atropine (0.1 microM), facilitated but unchanged in frequency upon application of the acetylcholinesterase inhibitor neostigmine (1 microM), and not seen in hippocampal monocultures. 4. The muscarinic agonist methacholine (5-20 nM) induced oscillatory synaptic activity at 4-15 Hz in hippocampal monocultures, which was identical to that occurring spontaneously in septo-hippocampal cocultures. 5. Synaptic theta activity was observed in cocultures of septal tissue with subdissected hippocampal slices containing area CA3 alone, but not in septo-CA1 cocultures. 6. We conclude that oscillatory synaptic activity at theta frequencies, with similar characteristics to theta activity in vivo, can be generated by the hippocampal network in response to activation of muscarinic receptors by synaptically released acetylcholine from septal afferents. Furthermore, the oscillatory activity is determined by mechanisms intrinsic to the hippocampal circuitry, particularly area CA3. Rhythmic septal input is not required.

Interactions between hippocampus and medial septum during sharp waves and theta oscillation in the behaving rat

The medial septal region and the hippocampus are connected reciprocally via GABAergic neurons, but the physiological role of this loop is still not well understood. In an attempt to reveal the physiological effects of the hippocamposeptal GABAergic projection, we cross-correlated hippocampal sharp wave (SPW) ripples or theta activity and extracellular units recorded in the medial septum and diagonal band of Broca (MSDB) in freely moving rats. The majority of single MSDB cells (60%) were significantly suppressed during SPWs. Most cells inhibited during SPW (80%) fired rhythmically and phase-locked to the negative peak of the CA1 pyramidal layer theta waves. Because both SPW and the negative peak of local theta waves correspond to the maximum discharge probability of CA1 pyramidal cells and interneuron classes, the findings indicate that the activity of medial septal neurons can be negatively (during SPW) or positively (during theta waves) correlated with the activity of hippocampal interneurons. We hypothesize that the functional coupling between medial septal neurons and hippocampal interneurons varies in a state-dependent manner.

Effects of combined medial septal area, fimbria-fornix and entorhinal cortex tetrodotoxin inactivations on passive avoidance response consolidation in the rat

On the basis of previous experimental evidence, it has been concluded that the entorhinal cortex (EC), the fimbria-fornix (FF) complex and medial septal area (MSA) do not take part in the consolidation phase of passive avoidance response (PAR) memorization. On the other hand, a mnemonic role during consolidation of at least two of these structures has been argued, based on several considerations. In order to ascertain whether the EC and FF are still involved in PAR memorization during consolidation, the coupled fully reversible functional tetrodotoxin (TTX) inactivation of MSA, FF and EC was performed in rats having undergone a PAR training. In Experiment 1 MSA, FF and EC were inactivated pair-wise (FF and EC always bilaterally). Permanently cannulated animals were injected stereotaxically with TTX (5 ng in 0.5 microliter saline) or saline (0.5 microliter) immediately following PAR acquisition. It was shown that combined FF-EC inactivation induced PAR retention impairment, whereas FF-MSA and EC-MSA inactivation was not followed by amnesic effects. Having obtained a positive result, in Experiment 2 the combined FF-EC inactivation was performed at different post-acquisition delays (0.25 h, 1.5 h, 6 h), so as to assess the duration of their involvement in PAR consolidation. It was shown that only the coupled inactivation performed at the shortest post-acquisition delay was followed by amnesic effects. Thus EC and FF play a definite role during early consolidation. The results are discussed in relation to EC, FF, MSA, and hippocampal involvement in PAR memorization, as reported in previous studies, and to their connectivity.

Entorhinal cortex and fimbria-fornix role in rat's passive avoidance response memorization

The stereotaxic administration of tetrodotoxin (TTX) was employed to induce the fully reversible inactivation of the fimbria-fornix complex (FF) and of the entorhinal cortex (EC), in order to ascertain the role of these structures in the memorization of a passive avoidance response (PAR). On permanently cannulated rats TTX (5 ng in 0.5 microliter saline) or saline (0.5 microliter) was injected uni- or bilaterally, respectively, in the FF and in the EC, 60 min before PAR acquisition, immediately after PAR acquisition and 60 min before PAR retrieval, always performed 48 h after the acquisition trial. It was shown that EC unilateral or bilateral pre-acquisition inactivation was followed by amnesia, while TTX inactivation in post-acquisition and pre-retrieval had no effects. Identical results were obtained by TTX administration in FF. The experimental evidence indicates that both EC and FF play a role during acquisition of PAR engram. The results are discussed in comparison with previous ones concerning dorsal and ventral hippocampus TTX inactivation effects on rat's PAR, and in relation to hippocampal and medial septal area connectivity.

Septal lesions impair the acquisition of a cued place navigation task: attentional or memory deficit?

These experiments were designed to analyze how medial septal lesions reducing the cholinergic innervation in the hippocampus might affect place learning. Rats with quisqualic lesions of the medial septal area (MS) were trained in a water maze and on a homing table where the escape position was located at a spatially fixed position and further indicated by a salient cue suspended above it. The lesioned rats were significantly impaired in reaching the cued escape platform during training. In addition rats, did not show any discrimination of the training sector during a probe trial in which no platform or cue was present. This impairment remained significant during further training in the absence of the cue. When the cued escape platform was located at an unpredictable spatial location, the MS-lesioned rats showed no deficit and spent more time under the cue than control rats during the probe trial. On the homing board, with a salient object in close proximity to the escape hole, the MS rats showed no deficit in escape latencies, although a significant reduction in spatial memory was observed. However, this was overcome by additional training in the absence of the cue. Under these conditions, rats with septal lesions were prone to develop a pure guidance strategy, whereas normal rats combined a guidance strategy with a memory of the escape position relative to more distant landmarks. The presence of a salient cue appeared to decrease attention to environmental landmarks, thus reducing spatial memory. These data confirm the general hypothesis that MS lesions reduce the capacity to rely on a representation of the relation between several landmarks with different salience.

Hippocampal formation is involved in movement selection: evidence from medial septal cholinergic modulation and concurrent slow-wave (theta rhythm) recording

Hippocampal rhythmical slow-wave field activity which occurs in response to sensory stimulation is predominantly cholinergic (atropine-sensitive theta rhythm), can precede movement initiation, and co-occurs during non-cholinergic theta rhythm associated with ongoing movement (atropine-resistant). This relationship suggests that theta rhythm plays some role in movement control. The present naturalistic experiments tested the idea that atropine-sensitive theta rhythm plays a role in sensory integration and planning required for initiating appropriate movements. One of a pair of hungry rats, the victim, implanted with hippocampal field recording electrodes, a septal injection cannula, and a posterior hypothalamic stimulating electrode, was given food which the other, the robber, tries to steal. Since the victim dodges from the robber with a latency, distance, and velocity dependent upon the size of the food, elapsed eating time, and proximity of the robber, the movement requires sensory integration and planning. Although eating behavior seemed normal, atropine-sensitive theta rhythm and dodging were disrupted by an infusion of a cholinergic antagonist into the medial septum. When the victim in turn attempted to steal the food back, Type 1 theta rhythm was present and robbery attempts seemed normal. Prior to cholinergic blockade, posterior hypothalamic stimulation produced theta rhythm and dodges, even in the absence of the robber, but following injections, atropine-sensitive theta rhythm and dodging were absent as the animals dropped the food and ran. The results provide the first evidence to link atropine-sensitive theta rhythm and hippocampal structures to a role in sensory integration and planning for the initiation of movement.

Electrolytic lesions of the fimbria/fornix, dorsal hippocampus, or entorhinal cortex produce anterograde deficits in contextual fear conditioning in rats

Recent data indicate that dorsal hippocampal (DH) lesions disrupt Pavlovian fear conditioning to contextual cues in rats. In the present study, we examined the effects of electrolytic lesions of the fimbria/fornix (FX) or entorhinal cortex (EC), the primary afferent projection systems to the DH, on contextual fear conditioning in rats. Conditioning consisted of the delivery of unsignaled footshocks in a novel observation chamber, and freezing served as the measure of conditional fear. Electrolytic lesions of the FX, DH, or EC made 1 week before training produced anterograde impairments in both immediate postshock freezing on the conditioning day and freezing during the context extinction test 24 h following training. The deficits in conditional freezing produced by FX, DH, and EC lesions were not statistically different, although the deficits in rats with FX or EC lesions tended to be more severe than those in rats with DH lesions. In addition to producing deficits in conditional freezing, FX, DH, or EC lesions produced a pronounced locomotor hyperactivity. Within the lesion and sham groups, however, locomotor activity was not significantly correlated with conditional freezing. These results indicate that contextual fear deficits in rats with hippocampal formation damage are equivalent following either FX, DH, or EC lesions. The relationship of freezing deficits and locomotor hyperactivity in rats with hippocampal formation lesions is discussed.

Relations between acetylcholine release and electrophysiological characteristics of theta rhythm: a microdialysis study in the urethane-anesthetized rat hippocampus

In urethane-anesthetized rats, recording electrodes were implanted in the left dorsal hippocampus and a dialysis probe was placed in the contralateral dorsal or ventral hippocampus. Samples of extracellular acetylcholine (ACh) levels were assessed at 10-min intervals over a period of 30 min using microdialysis with high-performance liquid chromatography with electrochemical detection. EEG was recorded during the same period and amplitude, frequency, and duration of theta rhythm were calculated for each of the three 10-min intervals. Data were analyzed using the two-tailed Spearman rank-order correlation test. A positive and high rank correlation coefficient (rho = 0.90, p < 0.01, n = 8) was seen between the average ACh outflow in the dorsal hippocampus and the average theta amplitude, both being calculated for the entire collection period. A lower but statistically significant positive correlation (rho = 0.59, p < 0.01) between dorsal hippocampus ACh output and theta amplitude was also found when the couples of values collected for the 30-min period were pooled (n = 20). In contrast, frequency and duration of theta were not significantly correlated with dorsal hippocampus ACh release. Also, no statistically significant correlation (p > 0.05) was found between ACh output in the ventral hippocampus and theta parameters. Because changes in hippocampal ACh outflow are believed to be the reflection of changes in number and/or level of activity of cholinergic afferents to the dorsal hippocampus, our present findings support the view that, at least in the dorsal hippocampus of the urethane-anesthetized rat, the septohippocampal cholinergic projection regulates the theta amplitude but not frequency. Finally, the possibility that ACh outflow increase and tonic release in the hippocampus is not a sufficient condition to induce and maintain theta in the urethane-anesthetized rat is discussed.

Regulation of the septal pacemaker theta rhythm by the cervical nuclei of the midbrain

Neuronal activity in the medial septal region (the medial nucleus and the diagonal band nucleus, MN-DBN) was recorded along with hippocampal EEG traces in conscious rabbits with stimulatory electrodes implanted in the median cervical nucleus (MCN) and the reticular formation (RF) of the midbrain and pons. In all animals with electrodes in the MCN, the background theta activity frequency was low (4.6 +/- 0.15 Hz) as compared with intact rabbits or those with electrodes implanted only in the RF (5.2 +/- 0.19 Hz, p < 0.5). Stimulation of the MCN with weak low-frequency impulses reduced theta volleys from MN-DBN cells, reducing their frequency and regularity and inducing the appearance or strengthening of low-frequency delta modulation. The number of spikes in a volley decreased, and the duration of inter-volley intervals increased. Stimulation of the MCN led to a gradual decrease in the frequency and amplitude of theta waves, induced irregular delta waves and spindles of 12 Hz in the hippocampal EEG. Stimulation of the RF produced the opposite changes in volley activity in the MN-DBN and hippocampal EEG, with increases in theta and decreases in delta components. These results support a role for the midbrain cervical nuclei as structures limiting the generation of theta activity by the reticular-septal system, but do not support the existence of an MN-DBN-independent high-frequency serotoninergic theta rhythm. It is proposed that the effect of the MCN may be important for suppression and switching of attention.

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 theta activity following selective lesion of the septal cholinergic system

The characteristic electroencephalographic patterns within the hippocampus are theta and sharp waves. Septal neurons are believed to play an essential role in the rhythm generation of the theta pattern. The present study examined the physiological consequences of complete and selective damage of septohippocampal cholinergic neurons on hippocampal theta activity in rats. A selective immunotoxin against nerve growth factor receptor bearing cholinergic neurons (192 immunoglobulin G-saporin), [Wiley R. G. et al. (1991) Brain Res. 562, 149-153] was infused into the medial septal area (0.11-0.42 microgram). Hippocampal electrical activity was monitored during trained wheel running, drinking and the paradoxical phase of sleep, as well as following cholinomimetic treatment. A moderate dose of toxin (0.21 microgram) eliminated the septohippocampal cholinergic projection, as evidenced by a near total absence of choline acetyltransferase-immunoreactive neurons in the medial septum and the vertical limb of the diagonal band, and by the absence of acetylcholinesterase-positive fibers in the dorsal hippocampus. In the same rats, parvalbumin immunoreactivity, a reliable marker for septohippocampal GABAergic neurons, [Freund T. F. (1989) Brain Res. 478, 375-381], remained unaltered. In addition, retrograde transport of the tracer fluorogold demonstrated that the parvalbumin cell population preserved its axonal projection to the hippocampus. Following toxin treatment, the power of hippocampal theta, but not its frequency, decreased in a dose-dependent manner. Reduction of theta power occurred between three and seven days after the toxin treatment and remained unaltered thereafter up to eight weeks. A dose which eliminated all septohippocampal cholinergic neurons (0.21 microgram) left a small but significant theta peak in the power spectra during wheel running, paradoxical phase of sleep and intraseptal infusion of carbachol (5 micrograms). Peripheral administration of physostigmine (1 mg/kg) induced only slow (1.5-2.0 Hz) rhythmic waves. No changes were observed in the gamma (50-100 Hz) band. These findings indicate that the integrity of the septohippocampal GABAergic projection is sufficient to maintain some hippocampal theta activity. We hypothesize that cholinergic neurons serve to increase the population phase-locking of septal cells and thereby regulate the magnitude of hippocampal theta.

Site-specific hippocampal pathophysiology due to cerebral overexpression of interleukin-6 in transgenic mice

Transgenic mice expressing the cytokine interleukin-6 exhibit distinctive hippocampal interneuron pathology and behavioral seizures. Electroencephalographic recordings from these mice revealed anomalous hippocampal paroxysmal discharges and suppressed theta rhythm. Analysis of hippocampal field responses evoked by monosynaptic afferent stimulation revealed a site-specific increase in recurrent inhibition in the dentate gyrus. In addition, the cholinergic component of septohippocampal conditioning of dentate-evoked activity was absent in the transgenic mice. These results indicate that overexpression of interleukin-6 selectively disrupts cholinergic transmission by inducing a functional pathophysiology of hippocampal cholinoceptive target neurons.

Effects of kainic acid microinfusions on hippocampal type 2 RSA (theta)

This study investigated the effects of bilateral, selective lesions of subfield CA3, produced by intrahippocampal administration of kainic acid, on the generation of hippocampal type 2 RSA. Within 4 weeks of lesioning, animals were anesthetized with urethane and microelectrode depth profiles were performed throughout the dorsal-ventral extent of the hippocampus. In control animals, spontaneous and stimulation-induced RSA was present at the amplitude maxima in stratum oriens of the CA1 and at the level of the hippocampal fissure. Animals that received intrahippocampal microinfusions of kainic acid showed a significant reduction of RSA amplitude at both the stratum oriens and fissure regions. These results suggest that the CA3 subfield may play an important role in the production of type 2 RSA.

Stimulation-induced RSA-like field activity in region CA1 of the hippocampal slice: amplitude maxima and topography

In the present experiment RSA-patterned stimulation was applied to afferents in the CA1 region of the hippocampal slice preparation and depth profiles of the resultant field activity were performed. RSA-patterned stimulation applied to the stratum lacunosum-moleculare region resulted in field activity through the CA1 region with similar morphology, phase, and amplitude profiles to the type 1 RSA profile found in vivo. In contrast, RSA-patterned stimulation applied to the stratum oriens and midstratum radiatum regions did not result in field profiles characteristic of the RSA recorded in vivo. The results of the present study confirm predictions made by previous modelling experiments and CSD analyses performed in the freely moving animal, which suggest that the type 1 RSA profile is primarily the result of distal excitation onto CA1 pyramidal cells.

Effects of atropine on hippocampal theta cells and complex-spike cells

The medial septal nuclei are essential for the naturally occurring hippocampal theta rhythm. Evidence that the rhythmic activity of the septum is carried via cholinergic afferents to the hippocampus has been: (a) the existence of a cholinergic septo-hippocampal projection, and (b) the sensitivity of one type of theta rhythm to antimuscarinic agents or cholinergic depletion. The muscarinic action of acetylcholine on pyramidal cells, however, is too slow to carry even a 4 Hz signal. Recent in vitro studies have confirmed a fast excitatory response by some hippocampal interneurons to muscarinic agonists. In urethane anesthetized rats, iontophoretic application of atropine to 17 hippocampal theta cells (presumed interneurons) during the theta rhythm, reduced their firing rates to an average of 24% of control rates. The effect of iontophoretic atropine application to 4 CA1 complex-spike cells (presumed pyramidal cells) was a selective elimination of their bursting activity with no significant effect on overall firing rate. The data suggest that: (1) interneuronal firing, during the hippocampal theta rhythm, is dominated by an excitatory cholinergic input and not by excitatory collaterals of pyramidal cells; and (2) somatic burst firing by CA1 pyramidal cells requires the presence of acetylcholine.