Glutamatergic synaptic transmission participates in generating the hippocampal EEG

Using EEG to study sensorimotor adaptation

Sensorimotor adaptation, or the capacity to flexibly adapt movements to changes in the body or the environment, is crucial to our ability to move efficiently in a dynamic world. The field of sensorimotor adaptation is replete with rigorous behavioural and computational methods, which support strong conceptual frameworks. An increasing number of studies have combined these methods with electroencephalography (EEG) to unveil insights into the neural mechanisms of adaptation. We review these studies: discussing EEG markers of adaptation in the frequency and the temporal domain, EEG predictors for successful adaptation and how EEG can be used to unmask latent processes resulting from adaptation, such as the modulation of spatial attention. With its high temporal resolution, EEG can be further exploited to deepen our understanding of sensorimotor adaptation.

Anticataleptic activity of nicotine in rats: involvement of the lateral entorhinal cortex

RATIONALE

Recently, it was found that cyclosomatostatin-induced catalepsy in middle-aged rats is accompanied by neuronal hypoactivation in the lateral entorhinal cortex (LEntCx); this hypoactivation was reversed by systemic administration of nicotine combined with diphenhydramine. These findings suggest the ability of nicotine to regulate catalepsy and the involvement of the LEntCx in this nicotine effect.

OBJECTIVES

The study was aimed to assess whether nicotine alone influences catalepsy when injected into the LEntCx and some other neuroanatomical structures.

METHODS

Experiments were conducted with male Wistar rats of 540-560 days of age. Catalepsy was induced by intracerebroventricular injection of cyclosomatostatin and assessed by the standard bar test. Nicotine was injected into the LEntCx, prelimbic cortex (PrCx), or basolateral amygdala (BLA). The tissue levels of tyrosine hydroxylase, dopamine, and DOPAC in the substantia nigra pars compacta and dorsal striatum were measured with use of HPLC and ELISA.

RESULTS

Injections of nicotine into the LEntCx but not into the PrCx and BLA produced anticataleptic effect; the nicotine effect was significantly reversed by intra-LEntCx administration of NMDA and non-NMDA glutamate receptor antagonists. Nicotine also attenuated cataleptogen-induced changes in nigrostriatal dopamine metabolism.

CONCLUSIONS

This may be the first demonstration of anticataleptic activity of nicotine. The results show that the effect is mediated by nicotine receptors in the LEntCx, via a glutamatergic mechanism. These findings may help advance the development of novel treatments for extrapyramidal disorders, including parkinsonism.

The Role of the Posterior Hypothalamus in the Modulation and Production of Rhythmic Theta Oscillations

Theta rhythm recorded as an extracellular synchronous field potential is generated in a number of brain sites including the hippocampus. The physiological occurrence of hippocampal theta rhythm is associated with the activation of a number of structures forming the ascending brainstem-hippocampal synchronizing pathway. Experimental evidence indicates that the supramammillary nucleus and posterior hypothalamic nuclei, considered as the posterior hypothalamic area, comprise a critical node of this ascending pathway. The posterior hypothalamic area plays an important role in movement control, place-learning, memory processing, emotion and arousal. In the light of multiplicity of functions of the posterior hypothalamic area and the influence of theta field oscillations on a number of neural processes, it is the authors' intent to summarize the data concerning the involvement of the supramammillary nucleus and posterior hypothalamic nuclei in the modulation of limbic theta rhythmicity as well as the ability of these brain structures to independently generate theta rhythmicity.

Modulation of Septo-Hippocampal Neural Responses in Anesthetized and Behaving Rats by Septal AMPA Receptor Mechanisms

This study explored the effects of septal glutamatergic transmission on septal-hippocampal theta activity via intraseptal microinjection of antagonist at AMPA receptors (AMPAR). The current results showed that microinjection of AMPAR antagonist, NBQX (2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione, 20 μg/μl, 0.5 μl), evoked a decrease in the frequency of theta activity evoked by various means in anesthetized and behaving rat. Theta wave activity was induced on: (a) intraseptal microinjection of carbachol, an agonist at cholinergic receptors, (b) reticular stimulation, (c) exploration in novel open field (OF), and (d) hind paw (HP) injection of the algogen, formalin. The effect on frequency in the formalin test was observed in an early period on injection of formalin, which was novel to the animal, but not in the later more sustained phase of the formalin test. The effect of NBQX, being seen in both anesthetized and behaving animals, suggests that the modulation of theta wave frequency, including in novelty, is a function of AMPAR in MS. The effect of the antagonist on theta power was less apparent, being observed only in anesthetized animals. In addition to theta power and frequency, intraseptal NBQX also attenuated suppression of CA1 population spike (PS) induced by intraseptal carbachol, thus suggesting that septal glutamate neurotransmission is involved in the spectrum of MS-mediated network responses. Indeed, in the context of behavior, formalin injection induced an increase in the level of septal glutamate, while NBQX attenuated nociceptive behaviors. Notably, MS is involved in the modulation of formalin nociception. These findings suggest that AMPA receptors are a key modulator of septal physiological function.

Hippocampal Interneuronal α7 nAChRs Modulate Theta Oscillations in Freely Moving Mice

Muscarinic acetylcholine receptors (mAChRs) are critically involved in hippocampal theta generation, but much less is known about the role of nicotinic AChRs (nAChRs). Here we provide evidence that α7 nAChRs expressed on interneurons, particularly those in oriens lacunosum moleculare (OLM), also regulate hippocampal theta generation. Local hippocampal infusion of a selective α7 nAChR antagonist significantly reduces hippocampal theta power and impairs Y-maze spontaneous alternation performance in freely moving mice. By knocking out receptors in different neuronal subpopulations, we find that α7 nAChRs expressed in OLM interneurons regulate theta generation. Our in vitro slice studies indicate that α7 nAChR activation increases OLM neuron activity that, in turn, enhances pyramidal cell excitatory postsynaptic currents (EPSCs). Our study also suggests that mAChR activation promotes transient theta generation, while α7 nAChR activation facilitates future theta generation by similar stimulations, revealing a complex mechanism whereby cholinergic signaling modulates different aspects of hippocampal theta oscillations through different receptor subtypes.

Forebrain Acetylcholine Modulates Isoflurane and Ketamine Anesthesia in Adult Mice

BACKGROUND

Cholinergic drugs are known to modulate general anesthesia, but anesthesia responses in acetylcholine-deficient mice have not been studied. It was hypothesized that mice with genetic deficiency of forebrain acetylcholine show increased anesthetic sensitivity to isoflurane and ketamine and decreased gamma-frequency brain activity.

METHODS

Male adult mice with heterozygous knockdown of vesicular acetylcholine transporter in the brain or homozygous knockout of the transporter in the basal forebrain were compared with wild-type mice. Hippocampal and frontal cortical electrographic activity and righting reflex were studied in response to isoflurane and ketamine doses.

RESULTS

The loss-of-righting-reflex dose for isoflurane was lower in knockout (mean ± SD, 0.76 ± 0.08%, n = 18, P = 0.005) but not knockdown (0.78 ± 0.07%, n = 24, P = 0.021), as compared to wild-type mice (0.83 ± 0.07%, n = 23), using a significance criterion of P = 0.017 for three planned comparisons. Loss-of-righting-reflex dose for ketamine was lower in knockout (144 ± 39 mg/kg, n = 14, P = 0.006) but not knockdown (162 ± 32 mg/kg, n = 20, P = 0.602) as compared to wild-type mice (168 ± 24 mg/kg, n = 21). Hippocampal high-gamma (63 to 100 Hz) power after isoflurane was significantly lower in knockout and knockdown mice compared to wild-type mice (isoflurane-dose and mouse-group interaction effect, F[8,56] = 2.87, P = 0.010; n = 5 to 6 mice per group). Hippocampal high-gamma power after ketamine was significantly lower in both knockout and knockdown mice when compared to wild-type mice (interaction effect F[2,13] = 6.06, P = 0.014). The change in frontal cortical gamma power with isoflurane or ketamine was not statistically different among knockout, knockdown, and wild-type mice.

CONCLUSIONS

These findings suggest that forebrain cholinergic neurons modulate behavioral sensitivity and hippocampal gamma activity during isoflurane and ketamine anesthesia.

EDITOR’S PERSPECTIVE

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.

Voltage-independent GluN2A-type NMDA receptor Ca2+ signaling promotes audiogenic seizures, attentional and cognitive deficits in mice

The NMDA receptor-mediated Ca²⁺ signaling during simultaneous pre- and postsynaptic activity is critically involved in synaptic plasticity and thus has a key role in the nervous system. In GRIN2-variant patients alterations of this coincidence detection provoked complex clinical phenotypes, ranging from reduced muscle strength to epileptic seizures and intellectual disability. By using our gene-targeted mouse line (Grin2a^N615S), we show that voltage-independent glutamate-gated signaling of GluN2A-containing NMDA receptors is associated with NMDAR-dependent audiogenic seizures due to hyperexcitable midbrain circuits. In contrast, the NMDAR antagonist MK-801-induced c-Fos expression is reduced in the hippocampus. Likewise, the synchronization of theta- and gamma oscillatory activity is lowered during exploration, demonstrating reduced hippocampal activity. This is associated with exploratory hyperactivity and aberrantly increased and dysregulated levels of attention that can interfere with associative learning, in particular when relevant cues and reward outcomes are disconnected in space and time. Together, our findings provide (i) experimental evidence that the inherent voltage-dependent Ca²⁺ signaling of NMDA receptors is essential for maintaining appropriate responses to sensory stimuli and (ii) a mechanistic explanation for the neurological manifestations seen in the NMDAR-related human disorders with GRIN2 variant-meidiated intellectual disability and focal epilepsy.

Ilaria Bertocchi et al. use a mouse model of Grin2a dysfunction to show that activity-independent NMDA receptors are involved in audiogenic seizure generation. Their results suggest a role for NMDA receptors in maintaining an appropriate response to sensory stimuli and a potential mechanism for disease phenotypes in epilepsy patients with GRIN2A mutations.

Levetiracetam induction of theta frequency oscillations in rodent hippocampus in vitro

Levetiracetam (LEV) has been demonstrated to improve cognitive function. Hippocampal theta rhythm (4-12 Hz) is associated with a variety of cognitively related behaviors, such as exploration in both humans and animal models. We investigated the effects of LEV on the theta rhythm in the rat hippocampal CA3 in hippocampal slices in vitro. We found that LEV increased the theta power in a dose-dependent manner. The increase in theta power can be blocked by GABA_A receptor (GABA_AR) or NMDA receptor (NMDAR) antagonists but not by AMPA receptor antagonist, indicating the involvement of GABA_AR and NMDAR in the induction of theta activity. Interestingly, LEV enhancement of theta power can be also blocked by taurine or GABA-A agonist THIP, indicating that LEV induction of theta may be related to the indirect boosting of GABA action via reduction of extrasynaptic GABA_AR activation. Furthermore, the increased theta power can be partially reduced by the mACh receptor (mAChR) antagonist atropine but not by nACh receptor antagonists, suggesting that mAChR activation provides excitatory input into local network responsible for LEV-induced theta. Our study demonstrated that LEV induced a novel theta oscillation in vitro, which may have implications in the treatment of the neuronal disorders with impaired theta oscillation and cognitive function.

Long-Term Potentiation and Excitability in the Hippocampus Are Modulated Differently by θ Rhythm

Oscillations in the brain facilitate neural processing and cognitive functions. This study investigated the dependence of long-term potentiation (LTP), a neural correlate of memory, on the phase of the hippocampal θ rhythm, a prominent brain oscillation. Multichannel field potentials and current source-sinks were analyzed in hippocampal CA1 of adult male rats under urethane anesthesia. A single burst (five pulses at 200 Hz) stimulation of stratum oriens (OR) induced LTP of the basal dendritic excitatory sink (ES), which was maximal when the burst was delivered at ∼340° and ∼160° of the distal dendritic θ rhythm. Apical dendritic sink evoked by stratum radiatum (RAD) stimulation also showed biphasic maxima at ∼30° and ∼210° of the distal dendritic θ rhythm, about 50° phase delay to basal dendritic LTP. By contrast, maximal population spike (PS) excitability, following single-pulse excitation of the basal or mid-apical dendrites, occurred at a θ phase of ∼140°, and maximal basal dendritic ES occurred at ∼20°; γ (30–57 Hz) activity recorded in CA1 RAD had maximal power at ∼300° of the distal dendritic θ rhythm, different from the phases of maximal LTP. LTP induced during the rising θ phase was NMDA receptor sensitive. It is suggested that the θ phase modulation of CA1 PS excitability is mainly provided by θ-rhythmic proximal inhibition, while dendritic LTP is also modulated by dendritic inhibition and excitation, specific to basal and apical dendrites. In summary, basal and apical dendritic synaptic plasticity and spike excitability are facilitated at different θ phases in a compartmental fashion.

Forebrain medial septum sustains experimental neuropathic pain

The present study explored the role of the medial septal region (MS) in experimental neuropathic pain. For the first time, we found that the MS sustains nociceptive behaviors in rodent models of neuropathic pain, especially in the chronic constriction injury (CCI) model and the paclitaxel model of chemotherapy-induced neuropathic pain. For example, inactivation of the MS with intraseptal muscimol (2 μg/μl, 0.5 μl), a GABA mimetic, reversed peripheral hypersensitivity (PH) in the CCI model and induced place preference in a conditioned place preference task, a surrogate measure of spontaneous nociception. The effect of intraseptal muscimol on PH was comparable to that seen with microinjection of the local anesthetic, lidocaine, into rostral ventromedial medulla which is implicated in facilitating experimental chronic nociception. Cellular analysis in the CCI model showed that the MS region sustains nociceptive gain with CCI by facilitating basal nociceptive processing and the amplification of stimulus-evoked neural processing. Indeed, consistent with the idea that excitatory transmission through MS facilitates chronic experimental pain, intraseptal microinjection of antagonists acting at AMPA and NMDA glutamate receptors attenuated CCI-induced PH. We propose that the MS is a central monitor of bodily nociception which sustains molecular plasticity triggered by persistent noxious insult.

Hippocampus and Entorhinal Cortex Recruit Cholinergic and NMDA Receptors Separately to Generate Hippocampal Theta Oscillations

Although much progress has been made in understanding type II theta rhythm generation under urethane anesthesia, less is known about the mechanisms underlying type I theta generation during active exploration. To better understand the contributions of cholinergic and NMDA receptor activation to type I theta generation, we recorded hippocampal theta oscillations from freely moving mice with local infusion of cholinergic or NMDA receptor antagonists to either the hippocampus or the entorhinal cortex (EC). We found that cholinergic receptors in the hippocampus, but not the EC, and NMDA receptors in the EC, but not the hippocampus, are critical for open-field theta generation and Y-maze performance. We further found that muscarinic M1 receptors located on pyramidal neurons, but not interneurons, are critical for cholinergic modulation of hippocampal synapses, theta generation, and Y-maze performance. These results suggest that hippocampus and EC neurons recruit cholinergic-dependent and NMDA-receptor-dependent mechanisms, respectively, to generate theta oscillations to support behavioral performance.

Fullerenol C60(OH)36 at relatively high concentrations impairs hippocampal theta oscillations (in vivo and in vitro) and triggers epilepsy (in vitro) - A dose response study

Since the first identification of fullerenes (C₆₀) and their synthesis in 1985, those compounds have been extensively studied in the biomedical field. In particular, their water-soluble derivatives, fullerenols (C₆₀(OH)_n, n = 2-48), have recently been the subject of numerous investigations concerning their antioxidant and prooxidant properties in biological systems. A small fraction of that research has focused on the possible use of C₆₀ and C₆₀(OH)_n in neuroscience and the therapy of pathologies such as dementia, amyloid-β (Aβ) formation, and Parkinson's disease. However, only a few studies have focused on their direct effects on neuronal network viability and excitability, especially with the use of electrophysiological and electrochemical approaches. Therefore, we addressed the issue of the direct effect of hydroxylated fullerene nanoparticles C₆₀(OH)₃₆ on local field potentials at the hippocampal formation (HPC) level. With the use of in vitro hippocampal formation slices as a stable model of inducing theta oscillations, and an in vivo model of an anesthetized rat, herein we provide the first convergent electropharmacological evidence that C₆₀(OH)₃₆ at relatively high concentrations (60 μM and 80 μM in vitro; 0.2 μg/μl in vivo) is capable of attenuating the amplitude, power, and frequency of theta oscillations in the HPC neuronal network. At the same time, lower concentrations did not induce any apparent changes. Theta band oscillations constitute a key physiological phenotypic property, which served here as a sensitive assay enabling the study of neural network excitability. Moreover, we report that C₆₀(OH)₃₆ at the concentrations of 60 μM and 80 μM is capable of producing epilepsy in the HPC in vitro, which suggests that C₆₀(OH)_n, when applied at higher doses, may have a deleterious effect on the functioning of neuronal networks.

Neural mechanisms of navigation involving interactions of cortical and subcortical structures

Animals must perform spatial navigation for a range of different behaviors, including selection of trajectories toward goal locations and foraging for food sources. To serve this function, a number of different brain regions play a role in coding different dimensions of sensory input important for spatial behavior, including the entorhinal cortex, the retrosplenial cortex, the hippocampus, and the medial septum. This article will review data concerning the coding of the spatial aspects of animal behavior, including location of the animal within an environment, the speed of movement, the trajectory of movement, the direction of the head in the environment, and the position of barriers and objects both relative to the animal's head direction (egocentric) and relative to the layout of the environment (allocentric). The mechanisms for coding these important spatial representations are not yet fully understood but could involve mechanisms including integration of self-motion information or coding of location based on the angle of sensory features in the environment. We will review available data and theories about the mechanisms for coding of spatial representations. The computation of different aspects of spatial representation from available sensory input requires complex cortical processing mechanisms for transformation from egocentric to allocentric coordinates that will only be understood through a combination of neurophysiological studies and computational modeling.

Effects of chronic ketamine on hippocampal cross-frequency coupling: implications for schizophrenia pathophysiology

Disrupted neuronal oscillations have been identified as a potentially important biomarker for the perceptual and cognitive symptoms of schizophrenia. Emerging evidences suggest that interactions between different frequency bands, cross-frequency coupling (CFC), serve an important role in integrating sensory and cognitive information and may contribute to disease pathophysiology. In this study, we investigated the effects of 14-day consecutive administration of ketamine (30 mg/kg i.p.) vs. saline on alterations in amplitude and changes in the coupling of low-frequency (0-30 Hz) phase and high-frequency (30-115 Hz) amplitude in the CA1 hippocampus of Long Evans rats. Intracranial electrode recordings were conducted pre- and post-injection while the animals performed a foraging task on a four-arm rectangular maze. Permutation analysis of frequency band-specific change in amplitudes revealed between-group differences in theta (6-12 Hz) and slow gamma (25-50 Hz) but not fast gamma (65-100 Hz) bands at both slow and fast speeds. Chronic ketamine challenge resulted in decreased coupling (pre to post) at slow speeds but increased coupling at faster speeds, compared to either no or modest increased coupling in the saline group. These results demonstrate that chronic ketamine administration alters the interaction of low-frequency phase and high-frequency oscillations chronically and that such coupling varies as a function of locomotive speed. These findings provide evidence for the potential relevance of CFC to the pathophysiology of schizophrenia.

Disruption of Hippocampal Multisynaptic Networks by General Anesthetics

Background

Previous studies showed that synaptic transmission is affected by general anesthetics, but an anesthetic dose response in freely moving animals has not been done. The hippocampus provides a neural network for the evaluation of isoflurane and pentobarbital on multisynaptic transmission that is relevant to memory function.

Methods

Male Long-Evans rats were implanted with multichannel and single electrodes in the hippocampus. Spontaneous local field potentials and evoked field potentials were recorded in freely behaving rats before (baseline) and after various doses of isoflurane (0.25 to 1.5%) and sodium pentobarbital (10 mg/kg intraperitoneal).

Results

Monosynaptic population excitatory postsynaptic potentials at the basal and apical dendrites of CA1 were significantly decreased at greater than or equal to 0.25% (n = 4) and greater than or equal to 1.0% (n = 6) isoflurane, respectively. The perforant path evoked multisynaptic response at CA1 was decreased by ~50% at greater than or equal to 0.25% isoflurane (n = 5). A decreased population excitatory postsynaptic potential was accompanied by increased paired-pulse facilitation. Population spike amplitude in relation to apical dendritic population excitatory postsynaptic potential was not significantly altered by isoflurane. Spontaneous hippocampal local field potential at 0.8 to 300 Hz was dose-dependently suppressed by isoflurane (n = 6), with local field potential power in the 50- to 150-Hz band showing the highest decrease with isoflurane dose, commensurate with the decrease in trisynaptic CA1 response. Low-dose pentobarbital (n = 7) administration decreased the perforant path evoked trisynaptic CA1 response and hippocampal local field potentials at 78 to 125 Hz.

Conclusions

Hippocampal networks are sensitive to low doses of isoflurane and pentobarbital, possibly through both glutamatergic and γ-aminobutyric acid–mediated transmission. Network disruption could help explain the impairment of hippocampal-dependent cognitive functions with low-dose anesthetic.

An integrative model of the intrinsic hippocampal theta rhythm

Hippocampal theta oscillations (4–12 Hz) are consistently recorded during memory tasks and spatial navigation. Despite several known circuits and structures that generate hippocampal theta locally in vitro, none of them were found to be critical in vivo, and the hippocampal theta rhythm is severely attenuated by disruption of external input from medial septum or entorhinal cortex. We investigated these discrepancies that question the sufficiency and robustness of hippocampal theta generation using a biophysical spiking network model of the CA3 region of the hippocampus that included an interconnected network of pyramidal cells, inhibitory basket cells (BC) and oriens-lacunosum moleculare (OLM) cells. The model was developed by matching biological data characterizing neuronal firing patterns, synaptic dynamics, short-term synaptic plasticity, neuromodulatory inputs, and the three-dimensional organization of the hippocampus. The model generated theta power robustly through five cooperating generators: spiking oscillations of pyramidal cells, recurrent connections between them, slow-firing interneurons and pyramidal cells subnetwork, the fast-spiking interneurons and pyramidal cells subnetwork, and non-rhythmic structured external input from entorhinal cortex to CA3. We used the modeling framework to quantify the relative contributions of each of these generators to theta power, across different cholinergic states. The largest contribution to theta power was that of the divergent input from the entorhinal cortex to CA3, despite being constrained to random Poisson activity. We found that the low cholinergic states engaged the recurrent connections in generating theta activity, whereas high cholinergic states utilized the OLM-pyramidal subnetwork. These findings revealed that theta might be generated differently across cholinergic states, and demonstrated a direct link between specific theta generators and neuromodulatory states.

The forebrain medial septal region and nociception

The forebrain medial septum, which is an integral part of the septo-hippocampal network, is implicated in sensorimotor integration, fear and anxiety, and spatial learning and memory. A body of evidence also suggests that the septal region affects experimental pain. Indeed, some explorations in humans have raised the possibility that the region may modulate clinical pain as well. This review explores the evidence that implicates the medial septum in nociception and suggests that non-overlapping circuits in the region facilitate acute nociceptive behaviors and defensive behaviors that reflect affect and cognitive appraisal, especially in relation to persistent nociception. In line with a role in nociception, the region modulates nociceptive responses in the neuraxis, including the hippocampus and the anterior cingulate cortex. The aforementioned forebrain regions have also been implicated in persistent/long-lasting nociception. The review also weighs the effects of the medial septum on nociception vis-à-vis the known roles of the region and emphasizes the fact that the region is a part of network of forebrain structures which have been long associated with reward, cognition and affect-motivation and are now implicated in persistent/long-lasting nociception.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

GABAergic neurotransmission and new strategies of neuromodulation to compensate synaptic dysfunction in early stages of Alzheimer's disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disease characterized by cognitive decline, brain atrophy due to neuronal and synapse loss, and formation of two pathological lesions: extracellular amyloid plaques, composed largely of amyloid-beta peptide (Aβ), and neurofibrillary tangles formed by intracellular aggregates of hyperphosphorylated tau protein. Lesions mainly accumulate in brain regions that modulate cognitive functions such as the hippocampus, septum or amygdala. These brain structures have dense reciprocal glutamatergic, cholinergic, and GABAergic connections and their relationships directly affect learning and memory processes, so they have been proposed as highly susceptible regions to suffer damage by Aβ during AD course. Last findings support the emerging concept that soluble Aβ peptides, inducing an initial stage of synaptic dysfunction which probably starts 20–30 years before the clinical onset of AD, can perturb the excitatory–inhibitory balance of neural circuitries. In turn, neurotransmission imbalance will result in altered network activity that might be responsible of cognitive deficits in AD. Therefore, Aβ interactions on neurotransmission systems in memory-related brain regions such as amygdaloid complex, medial septum or hippocampus are critical in cognitive functions and appear as a pivotal target for drug design to improve learning and dysfunctions that manifest with age. Since treatments based on glutamatergic and cholinergic pharmacology in AD have shown limited success, therapies combining modulators of different neurotransmission systems including recent findings regarding the GABAergic system, emerge as a more useful tool for the treatment, and overall prevention, of this dementia. In this review, focused on inhibitory systems, we will analyze pharmacological strategies to compensate neurotransmission imbalance that might be considered as potential therapeutic interventions in AD.

A systematic review of the effects of NMDA receptor antagonists on oscillatory activity recorded in vivo

Distinct frequency bands can be differentiated from neuronal ensemble recordings, such as local field potentials or electrocorticogram recordings. Recent years have witnessed a rapid acceleration of research examining how N-methyl-D-aspartate receptor (NMDAR) antagonists influence fundamental frequency bands in cortical and subcortical brain regions. Herein, we systematically review findings from in vivo studies with a focus on delta, theta, gamma and more recently identified high-frequency oscillations. We also discuss some of the current hypotheses that are considered to account for the actions of NMDAR antagonists on these frequency bands. The data emphasize a close relationship between altered oscillatory activity and NMDAR blockade, with both local and large-scale networks accounting for their effects. These findings may have fundamental implications for the psychotomimetic effects produced by NMDAR antagonists.

Hippocampal excitability is increased in aged mice

Aging is known to be associated with a high risk of developing seizure disorders. Currently, the mechanisms underlying this increased seizure susceptibility are not fully understood. Several previous studies have shown a loss of subgroups of GABAergic inhibitory interneurons in the hippocampus of aged rodents, yet the network excitability intrinsic to the aged hippocampus remains to be elucidated. The aim of this study is to examine age-dependent changes of hippocampal network activities in young adult (3-5 months), aging (16-18 months), and aged (24-28 months) mice. We conducted intracranial electroencephalographic (EEG) recordings in free-moving animals and extracellular recordings in hippocampal slices in vitro. EEG recordings revealed frequent spikes in aging and aged mice but only occasionally in young adults. These EEG spikes were suppressed following diazepam administration. Spontaneous field potentials with large amplitudes were frequently observed in hippocampal slices of aged mice but rarely in slices from young adults. These spontaneous field potentials originated from the CA3 area and their generation was dependent upon the excitatory glutamatergic activity. We therefore postulate that hippocampal network excitability is increased in aged mice and that such hyperactivity may be relevant to the increased seizure susceptibility observed in aged subjects.

NMDA preconditioning attenuates cortical and hippocampal seizures induced by intracerebroventricular quinolinic acid infusion

Searching for new therapeutic strategies through modulation of glutamatergic transmission using effective neuroprotective agents is essential. Glutamatergic excitotoxicity is a common factor to neurodegenerative diseases and acute events such as cerebral ischemia, traumatic brain injury, and epilepsy. This study aimed to evaluate behavioral and electroencephalographic (EEG) responses of mice cerebral cortex and hippocampus to subconvulsant and convulsant application of NMDA and quinolinic acid (QA), respectively. Moreover, it aimed to evaluate if EEG responses may be related to the neuroprotective effects of NMDA. Mice were preconditioned with NMDA (75 mg/kg, i.p.) and EEG recordings were performed for 30 min. One day later, QA was injected (36.8 nmol/site) and EEG recordings were performed during 10 min. EEG analysis demonstrated NMDA preconditioning promotes spike-wave discharges (SWDs), but it does not display behavioral manifestation of seizures. Animals that were protected by NMDA preconditioning against QA-induced behavioral seizures, presented higher number of SWD after NMDA administration, in comparison to animals preconditioned with NMDA that did display behavioral seizures after QA infusion. No differences were observed in latency for the first seizure or duration of seizures. EEG recordings after QA infusion demonstrated there were no differences in the number of SWD, latency for the first seizure or duration of seizures in animals pretreated with saline or in animals preconditioned by NMDA that received QA. A negative correlation was identified between the number of NMDA-induced SWD and QA-induced seizures severity. These results suggest a higher activation during NMDA preconditioning diminishes mice probability to display behavioral seizures after QA infusion.

Ketamine disrupts theta synchrony across the septotemporal axis of the CA1 region of hippocampus

The hippocampal theta signal reflects moment-to-moment variation in the synchrony of synaptic input to hippocampal neurons. Consistent with the topography of hippocampal afferents, the synchrony (coherence) of the theta signal varies across the septotemporal axis. Septotemporal variation in the theta signal can also be observed in relation to ongoing and past experience. Thus there is a systematic decrease in the relationship between locomotor speed and theta power across the septotemporal axis, septal hippocampus exhibiting the strongest relationship. Conversely, theta in temporal hippocampus decrements over repeated behavioral experience (running episodes), while theta in the septal hippocampus does not. Ketamine is an N-methyl-D-aspartate (NMDA) antagonist that can decrease theta power. The present study examined whether ketamine treatment could alter theta coherence across the long axis independent of changes in locomotor behavior. Rats were well trained to navigate a linear runway and outfitted with electrodes at different septotemporal positions within CA1. Locomotor behavior and theta coherence and power were examined after administration of 2.5 and 10 mg/kg ketamine. Ketamine (2.5 mg/kg) decreased theta coherence between distant CA1 electrode sites without altering running speed or theta power. Both doses of ketamine also blunted and reversed the decrement in theta power observed at midseptotemporal and temporal electrodes over repeated run sessions. The results demonstrate the sensitivity of global network synchronization to relatively low doses of ketamine and septotemporal differences in the influence of ketamine on hippocampal dynamics in relation to past experience.

Septohippocampal GABAergic neurons mediate the altered behaviors induced by n-methyl-D-aspartate receptor antagonists

We hypothesize that selective lesion of the septohippocampal GABAergic neurons suppresses the altered behaviors induced by an N-methyl-D-aspartate (NMDA) receptor antagonist, ketamine or MK-801. In addition, we hypothesize that septohippocampal GABAergic neurons generate an atropine-resistant theta rhythm that coexists with an atropine-sensitive theta rhythm in the hippocampus. Infusion of orexin-saporin (ore-SAP) into the medial septal area decreased parvalbumin-immunoreactive (GABAergic) neurons by ~80%, without significantly affecting choline-acetyltransferase-immunoreactive (cholinergic) neurons. The theta rhythm during walking, or the immobility-associated theta induced by pilocarpine, was not different between ore-SAP and sham-lesion rats. Walking theta was, however, more disrupted by atropine sulfate in ore-SAP than in sham-lesion rats. MK-801 (0.5 mg/kg i.p.) induced hyperlocomotion associated with an increase in frequency, but not power, of the hippocampal theta in both ore-SAP and sham-lesion rats. However, MK-801 induced an increase in 71-100 Hz gamma waves in sham-lesion but not ore-SAP lesion rats. In sham-lesion rats, MK-801 induced an increase in locomotion and an impairment of prepulse inhibition (PPI), and ketamine (3 mg/kg s.c.) induced a loss of gating of hippocampal auditory evoked potentials. MK-801-induced behavioral hyperlocomotion and PPI impairment, and ketamine-induced auditory gating deficit were reduced in ore-SAP rats as compared to sham-lesion rats. During baseline without drugs, locomotion and auditory gating were not different between ore-SAP and sham-lesion rats, and PPI was slightly but significantly increased in ore-SAP as compared with sham lesion rats. It is concluded that septohippocampal GABAergic neurons are important for the expression of hyperactive and psychotic symptoms an enhanced hippocampal gamma activity induced by ketamine and MK-801, and for generating an atropine-resistant theta. Selective suppression of septohippocampal GABAergic activity is suggested to be an effective treatment of some symptoms of schizophrenia.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Neural activation in arousal and reward areas of the brain in day-active and night-active grass rats

In the diurnal unstriped Nile grass rat (Arvicanthis niloticus) access to a running wheel can trigger a shift in active phase preference, with some individuals becoming night-active (NA), while others continue to be day-active (DA). To investigate the contributions of different neural systems to the support of this shift in locomotor activity, we investigated the association between chronotype and Fos expression during the day and night in three major nuclei in the basal forebrain (BF) cholinergic (ACh) arousal system - medial septum (MS), vertical and horizontal diagonal band of Broca (VDB and HDB respectively) -, and whether neural activation in these areas was related to neural activity in the orexinergic system. We also measured Fos expression in dopaminergic and non-dopaminergic cells of two components of the reward system that also participate in arousal - the ventral tegmental area (VTA) and supramammillary nucleus (SUM). NAs and DAs were compared to animals with no wheels. NAs had elevated Fos expression at night in ACh cells, but only in the HDB. In the non-cholinergic cells of the BF of NAs, enhanced nocturnal Fos expression was almost universally seen, but only associated with activation of the orexinergic system for the MS/VDB region. For some of the areas and cell types of the BF, the patterns of Fos expression of DAs appeared similar to those of NAs, but were never associated with activation of the orexinergic system. Also common to DAs and NAs was a general increase in Fos expression in non-dopaminergic cells of the SUM and anterior VTA. Thus, in this diurnal species, voluntary exercise and a shift to a nocturnal chronotype changes neural activity in arousal and reward areas of the brain known to regulate a broad range of neural functions and behaviors, which may be also affected in human shift workers.

Adenosine as an endogenous regulating factor of hippocampal sharp waves

The rodent hippocampus exhibits population activities called sharp waves (SPWs) during slow wave sleep and wake immobility. SPWs are important for hippocampal-cortical communication and memory consolidation, and abnormal sharp wave-ripple complexes are closely related to epileptic seizures. Although the SPWs are known to arise from the CA3 circuit, the local mechanisms underlying their generation are not fully understood. We hypothesize that endogenous adenosine is a local regulator of hippocampal SPWs. We tested this hypothesis in thick mouse hippocampal slices that encompass a relatively large hippocampal circuit and have a high propensity of generating spontaneous in vitro SPWs. We found that application of adenosine A1 receptor antagonists induced in vitro SPWs and that such induction was sensitive to blockade by NMDA receptor antagonists. By contrast, an increase in endogenous adenosine via pharmacological inhibition of adenosine transporters or adenosine degrading enzymes suppressed spontaneous in vitro SPWs. We thus suggest that the initiation and incidence of sharp wave-like population events are under tight control by the activity of endogenously stimulated A1 receptors.

Cellular mechanisms of cobalt-induced hippocampal epileptiform discharges

PURPOSE

To explore the cellular mechanisms of cobalt-induced epileptiform discharges in mouse hippocampal slices.

METHODS

Hippocampal slices were prepared from adult mice and briefly exposed to a CoCl(2)-containing external solution. Population and single cell activities were examined via extracellular and whole-cell patch recordings.

RESULTS

Brief cobalt exposure induced spontaneous, ictal-like discharges originating from the CA3 area. These discharges were suppressed by anticonvulsants, gap junction blockers, or by raising extracellular Ca(2+), but their generation was not associated with overall hyperexcitability or impairment in GABAergic inhibition in the CA3 circuit. Electroencephalographic ictal discharges of similar waveforms were observed in behaving rats following intrahippocampal cobalt infusion.

DISCUSSION

Mechanisms involving activity-dependent facilitation of gap junctional communication may play a major role in cobalt-induced epileptiform discharges.

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.

Modulation of normal and altered hippocampal excitability states by septal networks

The septal region of the basal forebrain plays a dual role: 1) It modulates hippocampal excitability, facilitating synaptic plasticity within hippocampal circuits. Through this mechanism, the septum facilitates diverse cognitive processes that involve hippocampal circuits. 2) Additionally, the septum maintains the hippocampal networks working within normal ranges, decreasing the probability of abnormal excitability states. Through this second mechanism, the septum prevents the occurrence of epileptic discharges. Thus, septal alterations may lead to both decreased cognitive functions and epilepsy, as observed in elderly patients affected with Alzheimer's disease.

Neurophysiological biomarkers for drug development in schizophrenia

Schizophrenia represents a pervasive deficit in brain function, leading to hallucinations and delusions, social withdrawal and a decline in cognitive performance. As the underlying genetic and neuronal abnormalities in schizophrenia are largely unknown, it is challenging to measure the severity of its symptoms objectively, or to design and evaluate psychotherapeutic interventions. Recent advances in neurophysiological techniques provide new opportunities to measure abnormal brain functions in patients with schizophrenia and to compare these with drug-induced alterations. Moreover, many of these neurophysiological processes are phylogenetically conserved and can be modelled in preclinical studies, offering unique opportunities for use as translational biomarkers in schizophrenia drug discovery.

Modulation of CA3 afferent inputs by novelty and theta rhythm

Models of hippocampal function suggest that the modulation of CA3 afferent input during theta rhythm allows for a rapid alternation between encoding and retrieval states, with each phase enhancing either extrinsic or intrinsic CA3 afferents, favoring either encoding or retrieval, respectively. Here, we show that during the initial exploration of a novel environment by rats, intrinsic CA3-CA3 synaptic inputs are attenuated on CA3 theta peaks, favoring extrinsic CA3 inputs, whereas extrinsic perforant path-CA3 synaptic inputs are attenuated on CA3 theta troughs, favoring intrinsic CA3 inputs. This modulation is absent when animals are re-exposed to the same environment 2 or 48 h later and thus habituates with familiarity, suggesting a process involved in learning. Modulation of CA3 synaptic inputs during novelty was blocked by atropine at a dose that blocks type 2 theta rhythm. Re-exposure to the same novel environment 48 h later in the absence of atropine did not result in habituation, but instead modulated CA3 synaptic responses as though the environment were novel and explored for the first time. The NMDA receptor antagonist (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), administered in a dose that blocks long-term potentiation induction, did not alter CA3 synaptic modulation during initial exploration. However, like atropine, CPP blocked the habituation of synaptic modulation normally observed with re-exposure, as though the environment were novel and explored for the first time. Thus, as predicted theoretically, recurrent and cortical CA3 afferents are differentially modulated during phases of theta rhythm. This modulation is atropine sensitive and habituates in an NMDA receptor-dependent manner, suggesting an NMDA receptor-dependent process that, in conjunction with theta rhythm, contributes to encoding of novel information in the hippocampus.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

The effect of atropine administered in the medial septum or hippocampus on high- and low-frequency theta rhythms in the hippocampus of urethane anesthetized rats

Cholinergic mechanisms are critical for the generation of hippocampal theta rhythm. Cholinergic innervation of the hippocampus originates from the medial septum (MS) and cholinergic receptors are expressed in both the MS and hippocampus. In this study, we compared the effects of the muscarinic receptor antagonist atropine in the MS and the hippocampus on theta generation. Hippocampal theta rhythm was elicited by electrical stimulation of the pontine reticular formation using series of stimuli with varying intensities. Atropine was administered either systemically (50 mg/kg i.p.) or locally in the MS (microdialysis; 25 and 75 mM for 30 or 90 min) or in the hippocampus on one side (microinjection; 20 or 40 ug). The relative power at the peak theta frequency was calculated and averaged over episodes of low-intensity and high-intensity stimulations. We found that atropine drastically reduced theta rhythmic synchronization when injected in either location. After MS administration of atropine, however, high-frequency theta elicited by high-intensity stimuli was more resistant (58% and 67% decrease after 25 mM and 75 mM atropine, respectively) than slow theta elicited by low-intensity stimuli (86% and 91% decrease). There was no significant difference between the powers of the two oscillations after hippocampal injections (70-75% decrease). We conclude that the theta suppressing effect of atropine involves both hippocampal and septal mechanisms and that low-frequency theta as compared with fast theta rhythm is more sensitive to muscarinic acetylcholine receptor antagonism in the MS but not in the hippocampus.

The supramammillo-septal-hippocampal pathway mediates sensorimotor gating impairment and hyperlocomotion induced by MK-801 and ketamine in rats

RATIONALE

Ketamine or MK-801 induced sensorimotor gating deficit, but the underlying neural mechanisms are not completely known. We have previously demonstrated that the medial septum (MS) mediated the phencyclidine-induced deficit in prepulse inhibition of the acoustic startle (PPI) in rats.

OBJECTIVES

We investigated the involvement of the supramammillary area (SUM) to MS pathway in PPI impairment and behavioral hyperlocomotion induced by MK-801 or ketamine in rats and correlated the behavioral deficits with hippocampal gamma wave increase.

MATERIALS AND METHODS

Ketamine (6 mg/kg, s.c.) or MK-801 (0.5 mg/kg, i.p.) was administered after infusion of saline or the GABA(A) receptor agonist, muscimol (0.25 microg), into the MS or SUM. Locomotion, PPI, and hippocampal electroencephalogram (EEG) were recorded.

RESULTS

MK-801 or ketamine induced PPI impairment and behavioral hyperlocomotion, accompanied by an increase in hippocampal gamma waves (30-100 Hz). The changes in behavior and gamma waves induced by ketamine or MK-801 were antagonized by pre-infusion of muscimol, but not saline, into the SUM or MS. Infusion of muscimol into the SUM alone did not significantly affect PPI, but it suppressed spontaneous locomotor behavior and hippocampal EEG. Infusion of ionotropic glutamate receptor antagonists into the MS did not affect the PPI deficit or the gamma wave increase after MK-801.

CONCLUSIONS

A non-glutamatergic component of the supramammillo-septal pathway mediates the hyperlocomotion and the deficits in PPI induced by MK-801 or ketamine. Inactivation of the MS or SUM normalized both the hippocampal gamma waves and the behavioral deficits (PPI impairment and hyperlocomotion).

Local generation of theta-frequency EEG activity in the parasubiculum

The parasubiculum is a major component of the hippocampal formation that receives inputs from the CA1 region, anterior thalamus, and medial septum and that projects primarily to layer II of the entorhinal cortex. Hippocampal theta-frequency (4-12 Hz) electroencephalographic (EEG) activity has been correlated with sensorimotor integration, spatial navigation, and memory functions. The present study was aimed at determining if theta is also generated locally within the parasubiculum versus volume conducted from adjacent structures. In urethan-anesthetized rats, the phase-reversal of theta activity between superficial and deep layers of the parasubiculum was demonstrated using differential recordings from movable bipolar electrodes that eliminate the influence of volume-conducted activity. Parasubicular theta was abolished by atropine, and was in phase with theta in stratum radiatum/lacunosum-moleculare of the CA1 region. Whole cell current-clamp recordings in brain slices were then used to determine if parasubicular theta may be generated in part by membrane potential oscillations in layer II neurons. Membrane potential oscillations occurred in most layer II neurons, including four putative interneurons, when cells were held at near-threshold voltages using current injection. The frequency of oscillations increased from 3.2 to 6.1 Hz when bath temperature was raised from 22 to 32 degrees C, and oscillations persisted in the presence of blockers of fast ionotropic glutamatergic and GABAergic synaptic transmission. Oscillations are therefore likely generated by intrinsic, voltage-dependent ionic conductances. These results indicate that theta field activity is generated locally within the parasubiculum and that intrinsic membrane potential oscillations, synchronized by local inhibitory inputs, may contribute to the generation of this activity.

Inhibition of kainate receptors reduces the frequency of hippocampal theta oscillations

We investigated the role of kainate receptors in the generation of theta oscillations using (S)-1-(2-amino-2-carboxyethyl)-3-(2-carboxythiophene-3-yl-methyl)pyrimidine-2,4-dione (UBP304), a novel, potent and highly selective antagonist of GLU(K5)-containing kainate receptors. EEG and single-unit recordings were made from the dorsal hippocampus of awake, freely moving rats trained to forage for food. Bilateral intracerebroventricular injections of UBP304 (2.0 microl, two times; 2.08 mM) caused a clear (approximately 25%) reduction in theta frequency that was dissociable from behavioral effects of the drug. The locations of firing fields of principal cells in the hippocampal formation were generally preserved, but both field firing rates and the precision of field organization decreased. UBP304 lowered the frequency of the theta modulation of hippocampal interneuron discharge, accurately matching the reduced frequency of the theta field oscillation. UBP308 [(R)-1-(2-amino-2-carboxyethyl)-3-(2-carboxythiophene-3-yl-methyl)pyrimidine-2,4-dione], the inactive enantiomer of UBP304, caused none of these effects. Our results suggest that GLU(K5) receptors have an important role in modulating theta activity. In addition, the effects on cellular responses provide both insight into the mechanisms of theta pacing, and useful information for models of temporal coding.

GABAB receptor blockade enhances theta and gamma rhythms in the hippocampus of behaving rats

The participation of GABA(B) receptors in hippocampal EEG generation was studied by intracerebroventricular (icv) and intracerebral infusions of GABA(B) receptor antagonist p-(3-aminopropyl)-p-diethoxymethyl-phosphinic acid (CGP35348) in freely behaving rats. During awake-immobility, icv CGP35348 induced a theta rhythm and increased gamma waves (30-100 Hz) in the hippocampus. The immobility theta peaked at 6-7 Hz and had a theta phase in CA1 stratum radiatum of approximately 160 degrees with reference to the theta at the alveus, when compared with approximately 130 degrees during walking. Immobility theta power peaks at 6-7 Hz was also found in normal rats, and it was detected in 27% of the EEG segments during immobility. Incidence of immobility theta increased to 87.5% after 480 nmol of CGP35348 icv. Muscarinic antagonist scopolamine (5 mg/kg, ip) suppressed the induction of immobility theta and the gamma power increase after icv CGP35348. CGP35348 icv did not significantly change the hippocampal theta power at 7-8 Hz during walking (theta fundamental), but it increased power at 12-15 Hz, at the second harmonic of theta. CGP35348 icv also increased 30-50 Hz gamma power during walking. Medial septal infusion of CGP35348 (12 nmol in 0.4 microl) increased the power and the frequency of the hippocampal theta second harmonic during walking, but did not increase gamma activity. Infusion of CGP35348 (8 nmol in 0.4 microl) in the hippocampus increased the local gamma activity at 30-100 Hz, but did not induce immobility theta or affect the walking theta rhythm. In conclusion, icv GABA(B) receptor blockade increased an atropine-sensitive input that generated an immobility theta rhythm, while GABA(B) receptor blockade of the medial septum increased atropine-resistant theta harmonics possibly generated by apical dendritic spikes. GABA(B) receptor blockade may enhance cognitive task performance by activating hippocampal theta and gamma rhythms in behaving rats.

Septohippocampal properties ofN-methyl-D-aspartate-induced theta-band oscillation and synchrony

Microinfusion of N-methyl-D-aspartate (NMDA) into apical dendrites of hippocampal CA1 pyramidal cells of urethane-anesthetized rats resulted in long lasting (20-30 min) induction of hippocampal synchrony at the field and cellular level. Power but not frequency of NMDA-induced theta was significantly greater than tail pinch-induced theta activity. This effect was antagonized by intrahippocampal infusion of AP5, but unaffected by i.v. atropine sulfate. During AP5 blockade tail pinch theta frequency and power were significantly reduced. Microinfusion of NMDA into the medial septum also resulted in long lasting induction of hippocampal theta field activity. Contrary to the results of hippocampal NMDA microinfusions, frequency but not power of NMDA-induced theta was significantly greater than tail pinch- induced theta activity. Microinfusion of AP5 into the medial septum significantly lowered power of tail pinch-induced theta but did not affect frequency. Wheel running behavior of rats induced by low levels of electrical stimulation of the posterior hypothalamic nucleus (PH) was completely abolished by microinfusion of AP5 into the medial septum, accompanied by a significant reduction in theta power and frequency. Wheel running and theta were maintained at control levels with high intensity PH stimulation. We propose that: (1) the glutamatergic septohippocampal projection represents a third pathway capable of generating hippocampal field and cellular synchrony, independent of that generated by the septohippocampal cholinergic and GABAergic projections, and (2) the septohippocampal glutamatergic projection serves to function as an interface between cholinergic and GABAergic modulated sensory processing Type 2 theta and movement related Type 1 theta.

N-methyl-d-aspartate receptors in the medial septal area have a role in spatial and emotional learning in the rat

Cholinergic and GABAergic neurons in the medial septal/vertical limb of the diagonal band of Broca (MS/vDB) area project to the hippocampus and constitute the septohippocampal pathway, which has been implicated in learning and memory. There is also evidence for extrinsic and intrinsic glutamatergic neurons in the MS/vDB, which by regulating septohippocampal neurons can influence hippocampal functions. The potential role of glutamatergic N-methyl-D-aspartate (NMDA) receptors within the MS/vDB for spatial and emotional learning was studied using the water maze and step-through passive avoidance (PA) tasks, which are both hippocampal-dependent. Blockade of septal NMDA receptors by infusion of the competitive NMDA receptor antagonist D-(-)-2-amino-5-phosphonopentanoic acid (D-AP5) (0.3-5 microg/rat), infused 15 min prior to training, impaired spatial learning and memory at the 5 microg dose of D-AP5, while doses of 0.3 and 1 microg per rat had no effect. The impairment in spatial learning appears not to be caused by sensorimotor or motivational disturbances, or anxiogenic-like behavior. Thus, d-AP5-treated rats were not impaired in swim performance or visuospatial abilities and spent more time in the open arms of the elevated plus-maze. In the PA task, intraseptal D-AP5 infused 15 min before training impaired retention as examined 24 h after training. This impairment was observed already at the 0.3 microg dose, suggesting that NMDA receptors within the MS/vDB may be more important for emotional than spatial memory. In summary, the present data indicate that changes in septal glutamate transmission and NMDA receptor activity can influence activity-dependent synaptic plasticity in the hippocampus and thereby learning and memory.

How do glutamatergic and GABAergic cells contribute to synchronization in the medial septum?

The medial septum-diagonal band (MSDB) complex is considered as a pacemaker for the hippocampal theta rhythm. Identification of the different cell types, their electro-physiological properties and their possible function in the generation of a synchronized activity in the MSDB is a hot topic. A recent electro-physiological study showed the presence of two antiphasically firing populations of parvalbumin containing GABAergic neurons in the MSDB. Other papers described a network of cluster-firing glutamatergic neurons, which is able to generate synchronized activity in the MSDB. We propose two different computer models for the generation of synchronized population theta oscillation in the MSDB and compare their properties. In the first model GABAergic neurons are intrinsically theta periodic cluster-firing cells; while in the second model GABAergic cells are fast-firing cells and receive periodic input from local glutamatergic neurons simulated as cluster-firing cells. Using computer simulations we show that the GABAergic neurons in both models are capable of generating antiphasic theta periodic population oscillation relying on local, septal mechanisms. In the first model antiphasic theta synchrony could emerge if GABAergic neurons form two populations preferentially innervate each other. In the second model in-phase synchronization of glutamatergic neurons does not require specific network structure, and the network of these cells are able to act as a theta pacemaker for the local fast-firing GABAergic circuit. Our simulations also suggest that neurons being non-cluster-firing in vitro might exhibit clustering properties when connected into a network in vivo.