Physiology, pharmacology, and topography of cholinergic neocortical oscillations in vitro

Effects of ketamine and propofol on muscarinic plateau potentials in rat neocortical pyramidal cells

Propofol and ketamine are widely used general anaesthetics, but have different effects on consciousness: propofol gives a deeply unconscious state, with little or no dream reports, whereas vivid dreams are often reported after ketamine anaesthesia. Ketamine is an N-methyl-D-aspartate (NMDA) receptor antagonist, while propofol is a γ-aminobutyric-acid (GABAA) receptor positive allosteric modulator, but these mechanisms do not fully explain how these drugs alter consciousness. Most previous in vitro studies of cellular mechanisms of anaesthetics have used brain slices or neurons in a nearly "comatose" state, because no "arousing" neuromodulators were added. Here we tested mechanisms of anaesthetics in rat medial prefrontal cortex (mPFC) slices after bath-applying the cholinergic agonist muscarine to partly mimic an "aroused-like" state, using whole-cell patch-clamp recordings from layer 2/3 pyramidal cells (L2/3PCs). According to leading theories of access consciousness and working memory, L2/3PCs are particularly important for these cognitive functions. We found that muscarine induced long-lasting depolarising plateau potentials (PPs) and spiking following brief depolarising current injections in the L2/3PCs. After 2 hours of pre-incubation with ketamine or propofol, the muscarine-induced PPs were altered in seemingly different ways: 3 μM propofol reduced the PPs and (significantly) spiking, whereas 20 μM ketamine seemed to enhance PPs and spiking (non-significantly). Brief wash-in of these drug concentrations failed to induce such effects, probably due to insufficient equilibration by diffusion in the slices. In contrast, pre-incubation with a high dose (100 μM) of ketamine suppressed the PPs and spiking. We discuss whether the apparently different effects on PPs may possibly be related to contrasting clinical effects: ketamine causing atypical anaesthesia with vivid, "psychedelic" dreaming while propofol causes less dreaming.

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

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

The 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.

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

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

Protective role of rosmarinic acid on amyloid beta 42-induced echoic memory decline: Implication of oxidative stress and cholinergic impairment

In the present study, we examined whether rosmarinic acid (RA) reverses amyloid β (Aβ) induced reductions in antioxidant defense, lipid peroxidation, cholinergic damage as well as the central auditory deficits. For this purpose, Wistar rats were randomly divided into four groups; Sham(S), Sham + RA (SR), Aβ42 peptide (Aβ) and Aβ42 peptide + RA (AβR) groups. Rat model of Alzheimer was established by bilateral injection of Aβ42 peptide (2,2 nmol/10 μl) into the lateral ventricles. RA (50 mg/kg, daily) was administered orally by gavage for 14 days after intracerebroventricular injection. At the end of the experimental period, we recorded the auditory event related potentials (AERPs) and mismatch negativity (MMN) response to assess auditory functions followed by histological and biochemical analysis. Aβ42 injection led to a significant increase in the levels of thiobarbituric acid reactive substances (TBARS) and 4-Hydroxy-2-nonenal (4-HNE) but decreased the activity of antioxidant enzymes (SOD, CAT, GSH-Px) and glutathione levels. Moreover, Aβ42 injection resulted in a reduction in the acetylcholine content and acetylcholine esterase activity. RA treatment prevented the observed alterations in the AβR group. Furthermore, RA attenuated the increased Aβ staining and astrocyte activation. We also found that Aβ42 injection decreased the MMN response and theta power/coherence of AERPs, suggesting an impairing effect on auditory discrimination and echoic memory processes. RA treatment reversed the Aβ42 related alterations in AERP parameters. In conclusion, our study demonstrates that RA prevented Aβ-induced antioxidant-oxidant imbalance and cholinergic damage, which may contribute to the improvement of neural network dynamics of auditory processes in this rat model.

Phase-amplitude coupled persistent theta and gamma oscillations in rat primary motor cortex in vitro

In vivo, theta (4-7 Hz) and gamma (30-80 Hz) neuronal network oscillations are known to coexist and display phase-amplitude coupling (PAC). However, in vitro, these oscillations have for many years been studied in isolation. Using an improved brain slice preparation technique we have, using co-application of carbachol (10 μM) and kainic acid (150 nM), elicited simultaneous theta (6.6 ± 0.1 Hz) and gamma (36.6 ± 0.4 Hz) oscillations in rodent primary motor cortex (M1). Each oscillation showed greatest power in layer V. Using a variety of time series analyses we detected significant cross-frequency coupling in 74% of slice preparations. Differences were observed in the pharmacological profile of each oscillation. Thus, gamma oscillations were reduced by the GABA_A receptor antagonists, gabazine (250 nM and 2 μM), and picrotoxin (50 μM) and augmented by AMPA receptor antagonism with SYM2206 (20 μM). In contrast, theta oscillatory power was increased by gabazine, picrotoxin and SYM2206. GABA_B receptor blockade with CGP55845 (5 μM) increased both theta and gamma power, and similar effects were seen with diazepam, zolpidem, MK801 and a series of metabotropic glutamate receptor antagonists. Oscillatory activity at both frequencies was reduced by the gap junction blocker carbenoxolone (200 μM) and by atropine (5 μM). These data show theta and gamma oscillations in layer V of rat M1 in vitro are cross-frequency coupled, and are mechanistically distinct. The development of an in vitro model of phase-amplitude coupled oscillations will facilitate further mechanistic investigation of the generation and modulation of coupled activity in mammalian cortex.

Attention and the Cholinergic System: Relevance to Schizophrenia

Traditional methods of drug discovery often rely on a unidirectional, "bottom-up" approach: A search for molecular compounds that target a particular neurobiological substrate (e.g., a receptor type), the refinement of those compounds, testing in animal models using high-throughput behavioral screening methods, and then human testing for safety and effectiveness. Many attempts have found the "effectiveness" criterion to be a major stumbling block, and we and others have suggested that success may be improved by an alternative approach that considers the neural circuits mediating the effects of genetic and molecular manipulations on behavior and cognition. We describe our efforts to understand the cholinergic system's role in attention using parallel approaches to test main hypotheses in both rodents and humans as well as generating converging evidence using methods and levels of analysis tailored to each species. The close back-and-forth between these methods has enhanced our understanding of the cholinergic system's role in attention both "bottom-up" and "top-down"-that is, the basic neuroscience identifies potential neuronal circuit-based mechanisms of clinical symptoms, and the patient and genetic populations serve as natural experiments to test and refine hypotheses about its contribution to specific processes. Together, these studies have identified (at least) two major and potentially independent contributions of the cholinergic system to attention: a neuromodulatory component that influences cognitive control in response to challenges from distractors that either make detection more difficult or draw attention away from the distractor, and a phasic or transient cholinergic signal that instigates a shift from ongoing behavior and the activation of cue-associated response. Right prefrontal cortex appears to play a particularly important role in the neuromodulatory component integrating motivational and cognitive influences for top-down control across populations, whereas the transient cholinergic signal involves orbitofrontal regions associated with shifts between internal and external attention. Understanding how these two modes of cholinergic function interact and are perturbed in schizophrenia will be an important prerequisite for developing effective treatments.

Monitoring Population Membrane Potential Signals from Neocortex

Voltage-sensitive dyes (VSDs) and optical imaging are useful tools for studying spatiotemporal patterns of population neuronal activity in cortex. Because fast VSDs respond to membrane potential changes with microsecond temporal resolution, these are better suited than calcium indicators for recording rapid neural signals. Here we describe methods for using a 464 element photodiode array and fast VSDs to record signals ranging from large scale network activity in brain slices and in vivo mammalian preparations with sensitivity comparable to local field potential (LFP) recordings. With careful control of dye bleaching and phototoxicity, long recording times can be achieved. Absorption dyes have less photo-toxicity than fluorescent dyes. In brain slices, the total recording time in each slice can be 1,000-2,000 s, which can be divided into hundreds of short recording trials over several hours. In intact brains when fluorescent dyes are used, reduced light intensity can also increase recording time. In this chapter, we will discuss technical details for the methods to achieve reliable VSD imaging with high sensitivity and long recording time.

CNS Adverse Effects: From Functional Observation Battery/Irwin Tests to Electrophysiology

This chapter describes various approaches for the preclinical assessment of drug-induced central nervous system (CNS) adverse effects. Traditionally, methods to evaluate CNS effects have consisted of observing and scoring behavioral responses of animals after drug is administered. Among several behavioral testing paradigms, the Irwin and the functional observational battery (FOB) are the most commonly used assays for the assessment of CNS effects. The Irwin and FOB are considered good first-tier assays to satisfy the ICH S7A guidance for the preclinical evaluation of new chemical entities (NCE) intended for humans. However, experts have expressed concern about the subjectivity and lack of quantitation that is derived from behavioral testing. More importantly, it is difficult to gain insight into potential mechanisms of toxicity by assessing behavioral outcomes. As a complement to behavioral testing, we propose using electrophysiology-based assays, both in vivo and in vitro, such as electroencephalograms and brain slice field-potential recordings. To better illustrate these approaches, we discuss the implementation of electrophysiology-based techniques in drug-induced assessment of seizure risk, sleep disruption, and cognitive impairment.

Muscarinic cholinergic receptors modulate inhibitory synaptic rhythms in hippocampus and neocortex

Activation of muscarinic acetylcholine (ACh) receptors (mAChRs) powerfully affects many neuronal properties as well as numerous cognitive behaviors. Small neuronal circuits constitute an intermediate level of organization between neurons and behaviors, and mAChRs affect interactions among cells that compose these circuits. Circuit activity is often assessed by extracellular recordings of the local field potentials (LFPs), which are analogous to in vivo EEGs, generated by coordinated neuronal interactions. Coherent forms of physiologically relevant circuit activity manifest themselves as rhythmic oscillations in the LFPs. Frequencies of rhythmic oscillations that are most closely associated with animal behavior are in the range of 4–80 Hz, which is subdivided into theta (4–14 Hz), beta (15–29 Hz) and gamma (30–80 Hz) bands. Activation of mAChRs triggers rhythmic oscillations in these bands in the hippocampus and neocortex. Inhibitory responses mediated by GABAergic interneurons constitute a prominent feature of these oscillations, and indeed, appear to be their major underlying factor in many cases. An important issue is which interneurons are involved in rhythm generation. Besides affecting cellular and network properties directly, mAChRs can cause the mobilization of endogenous cannabinoids (endocannabinoids, eCBs) that, by acting on the principal cannabinoid receptor of the brain, CB1R, regulate the release of certain neurotransmitters, including GABA. CB1Rs are heavily expressed on only a subset of interneurons and, at lower density, on glutamatergic neurons. Exogenous cannabinoids typically disrupt oscillations in the theta (θ) and gamma (γ) ranges, which probably contributes to the behavioral effects of these drugs. It is important to understand how neuronal circuit activity is affected by mAChR-driven eCBs, as this information will provide deeper insight into the actions of ACh itself, as well as into the effects of eCBs and exogenous cannabinoids in animal behavior. After covering some basic aspects of the mAChR system, this review will focus on recent findings concerning the mechanisms and circuitry that generate θ and γ rhythms in hippocampus and neocortex. The ability of optogenetic methods to probe the many roles of ACh in rhythm generation is highlighted.

Low-intensity electric fields induce two distinct response components in neocortical neuronal populations

Low-intensity alternating electric fields applied to the scalp are capable of modulating cortical activity and brain functions, but the underlying mechanisms remain largely unknown. Here, we report two distinct components of voltage-sensitive dye signals induced by low-intensity, alternating electric fields in rodent cortical slices: a "passive component," which corresponds to membrane potential changes directly induced by the electric field; and an "active component," which is a widespread depolarization that is dependent on excitatory synaptic transmission. The passive component is stationary, with amplitude and phase accurately reflecting the cortical cytoarchitecture. In contrast, the active component is initiated from a local "hot spot" of activity and spreads to a large population as a propagating wave with rich local dynamics. The propagation of the active component may play a role in modulating large-scale cortical activity by spreading a low level of excitation from a small initiation point to a vast neuronal population.

In vitro recordings of human neocortical oscillations

Electrophysiological oscillations are thought to create temporal windows of communication between brain regions. We show here that human cortical slices maintained in vitro can generate oscillations similar to those observed in vivo. We have characterized these oscillations using local field potential and whole-cell recordings obtained from neocortical slices acquired during epilepsy surgery. We confirmed that such neocortical slices maintain the necessary cellular and circuitry components, and in particular inhibitory mechanisms, to manifest oscillatory activity when exposed to glutamatergic and cholinergic agonists. The generation of oscillations was dependent on intact synaptic activity and muscarinic receptors. Such oscillations differed in electrographic and pharmacological properties from epileptiform activity. Two types of activity, theta oscillations and high gamma activity, uniquely characterized this model-activity not typically observed in animal cortical slices. We observed theta oscillations to be synchronous across cortical laminae suggesting a novel role of theta as a substrate for interlaminar communication. As well, we observed cross-frequency coupling (CFC) between theta phase and high gamma amplitude similar to that observed in vivo. The high gamma "bursts" generated by such CFC varied in their frequency content, suggesting that this variability may underlie the broadband nature of high gamma activity.

Atropine-sensitive theta rhythm in the posterior hypothalamic area: in vivo and in vitro studies

Theta rhythm is the largest, most prominent, and well-documented electroencephalography activity present in a number of mammals, including humans. Spontaneous theta activity recorded locally in the posterior hypothalamic area (PHa) has never been the subject of detailed studies. The authors have shown that local theta field potentials could be generated in urethane-anesthetized rats in the supramammillary (SuM) nuclei and posterior hypothalamic (PH) nuclei. Theta recorded in the PHa was produced independently of simultaneously occurring hippocampal theta. These data were confirmed in the PHa maintained in vitro. Local theta field activity was recorded in the SuM and PH nuclei of PHa slice preparations perfused with cholinergic agonist carbachol. Both in vivo and in vitro recorded PHa theta rhythmicity had a cholinergic-muscarinic profile, that is, it was antagonized by muscarinic antagonist atropine sulfate.

Sevoflurane-induced loss of consciousness is paralleled by a prominent modification of neural activity during cortical down-states

Networks of neocortical neurons display a bistable activity pattern characterised by phases of high frequency action potential firing, so called up-states, and episodes of low discharge activity (down-states). We hypothesised that during down-states neocortical neurons are vulnerable to anaesthetic agents. To tackle this issue, it is necessary to identify analytical methods, which are sufficiently sensitive for resolving anaesthetic effects during phases of scarce neuronal activity. The local field potential was recorded in organotypic cultures (OTC) from rat neocortex under control conditions and in the presence of increasing concentrations of sevoflurane by extracellular electrodes. Epochs from down-states were cut from the local field potential and analysed using power spectrum density as well as non-linear parameters approximate entropy (ApEn) and order recurrence rate (ORR). ApEn and ORR proved to be suitable tools for analysing the actions of volatile anaesthetics on cortical down-states. During these phases of low neuronal activity, sevoflurane caused prominent changes in the local field potential. Time series analysis using ApEn showed a reduction of signal predictability in the presence of sevoflurane. Furthermore, the ORR displayed an abrupt decrease at sevoflurane concentrations corresponding to loss of consciousness in vivo, indicating a drug-induced decrease in the signal to noise ratio. The actions of volatile anaesthetics on cortical down-states have been neglected so far, perhaps due to the lack of suitable analysis tools. In the current in vitro study the non-linear parameters ApEn and ORR are introduced to characterise volatile anaesthetics actions. Sevoflurane alters cortical down-states as indicated by non-linear parameter analysis of local field potential recording from cultured neuronal networks. ORR even displays an abrupt change, i.e., a step-like behaviour indicating an increased signal complexity at concentrations of sevoflurane corresponding to loss of consciousness in humans.

Differential effects of cholinergic and noradrenergic neuromodulation on spontaneous cortical network dynamics

Cholinergic and noradrenergic neuromodulation play a key role in determining overall behavioral state by shaping the underlying cortical network dynamics. The effects of these systems on synaptic and intrinsic cellular targets are quite diverse and a comprehensive understanding of how these neuromodulators regulate (spontaneous) cortical network activity has remained elusive. Here, we used multielectrode electrophysiology in vitro to investigate the effect of these neuromodulators on spontaneous network dynamics in acute slices of mouse visual cortex. We found that application of Carbachol (CCh) and Norepinephrine (NE) both enhanced the spontaneous network dynamics by increasing (1) the activity levels, (2) the temporal complexity of the network activity, and (3) the spatial complexity by decorrelating the network activity over a wide range of neuromodulator concentrations (1 μM, 10 μM, 50 μM, and 100 μM). Interestingly, we found that cholinergic neuromodulation was limited to the presence of CCh in the bath whereas the effects of NE, in particular for higher concentrations, induced plasticity that caused outlasting effects most prominently in the deep cortical layers. Together, these results provide a comprehensive network-level understanding of the similarities and differences of cholinergic and noradrenergic modulation of spontaneous network dynamics.

The role of inhibition in oscillatory wave dynamics in the cortex

Cortical oscillations arise during behavioral and mental tasks, and all temporal oscillations have particular spatial patterns. Studying the mechanisms that generate and modulate the spatiotemporal characteristics of oscillations is important for understanding neural information processing and the signs and symptoms of dynamical diseases of the brain. Nevertheless, it remains unclear how GABAergic inhibition modulates these oscillation dynamics. Using voltage-sensitive dye imaging, pharmacological methods, and tangentially cut occipital neocortical brain slices (including layers 3-5) of Sprague-Dawley rat, we found that GABAa disinhibition with bicuculline can progressively simplify oscillation dynamics in the presence of carbachol in a concentration-dependent manner. Additionally, GABAb disinhibition can further simplify oscillation dynamics after GABAa receptors are blocked. Both GABAa and GABAb disinhibition increase the synchronization of the neural network. Theta frequency (5-15-Hz) oscillations are reliably generated by using a combination of GABAa and GABAb antagonists alone. These theta oscillations have basic spatiotemporal patterns similar to those generated by carbachol/bicuculline. These results are illustrative of how GABAergic inhibition increases the complexity of patterns of activity and contributes to the regulation of the cortex.

Isoflurane enhances both fast and slow synaptic inhibition in the hippocampus at amnestic concentrations

BACKGROUND

Inhibition mediated by γ-aminobutyric acid type A (GABA A) receptors has long been considered an important target for a variety of general anesthetics. In the hippocampus, two types of phasic GABA A receptor-mediated inhibition coexist: GABA A,fast, which is expressed primarily at peri-somatic sites, and GABAA,slow, which is expressed primarily in the dendrites. Their spatial segregation suggests distinct functions: GABA A,slow may control plasticity of dendritic synapses, whereas GABA A,fast controls action potential initiation at the soma. We examined modulation of GABA A,fast and GABA A,slow inhibition by isoflurane at amnesic concentrations, and compared it with modulation by behaviorally equivalent doses of the GABA A receptor-selective drug etomidate.

METHODS

Whole cell recordings were obtained from pyramidal cells in organotypic hippocampal cultures prepared from C57BL/6 × 129/SvJ F1 hybrid mice. GABA A receptor-mediated currents were isolated using glutamate receptor antagonists. GABAA,slow currents were evoked by electrical stimulation in the stratum lacunosum-moleculare. Miniature GABA A,fast currents were recorded in the presence of tetrodotoxin.

RESULTS

100 μM isoflurane (approximately EC50,amnesia) slowed fast- and slow-inhibitory postsynaptic current decay by approximately 25%. Higher concentrations, up to 400 μM, produced proportionally greater effects without altering current amplitudes. The effects on GABA A,slow were approximately one-half those produced by equi-amnesic concentrations of etomidate.

CONCLUSIONS

Isoflurane enhances both types of phasic GABA A receptor-mediated inhibition to similar degrees at amnesic concentrations. This pattern differs from etomidate, which at low concentrations selectively enhances slow inhibition. These effects of isoflurane are sufficiently large that they may contribute substantially to its suppression of hippocampal learning and memory.

The role of hyperpolarization-activated cationic current in spike-time precision and intrinsic resonance in cortical neurons in vitro

Hyperpolarization-activated cyclic nucleotide modulated current (I(h)) sets resonance frequency within the θ-range (5–12 Hz) in pyramidal neurons. However, its precise contribution to the temporal fidelity of spike generation in response to stimulation of excitatory or inhibitory synapses remains unclear. In conditions where pharmacological blockade of I(h) does not affect synaptic transmission, we show that postsynaptic h-channels improve spike time precision in CA1 pyramidal neurons through two main mechanisms. I(h) enhances precision of excitatory postsynaptic potential (EPSP)--spike coupling because I(h) reduces peak EPSP duration. I(h) improves the precision of rebound spiking following inhibitory postsynaptic potentials (IPSPs) in CA1 pyramidal neurons and sets pacemaker activity in stratum oriens interneurons because I(h) accelerates the decay of both IPSPs and after-hyperpolarizing potentials (AHPs). The contribution of h-channels to intrinsic resonance and EPSP waveform was comparatively much smaller in CA3 pyramidal neurons. Our results indicate that the elementary mechanisms by which postsynaptic h-channels control fidelity of spike timing at the scale of individual neurons may account for the decreased theta-activity observed in hippocampal and neocortical networks when h-channel activity is pharmacologically reduced.

Spatio-temporal dynamics of theta oscillations in hippocampal-entorhinal slices

Theta oscillations (4-12 Hz) are associated with learning and memory and are found in the hippocampus and the entorhinal cortex (EC). The spatio-temporal organization of rhythmic activity in the hippocampal-EC complex was investigated in vitro. The voltage sensitive absorption dye NK3630 was used to record the changes in aggregated membrane voltage simultaneously from the neuronal networks involved. Oscillatory activity at 7.0 Hz (range, 5.8-8.2) was induced in the slice with the muscarinic agonist carbachol (75-100 microM) in the presence of bicuculline (5 microM). Time relations between all recording sites were analyzed using cross-correlation functions which revealed systematic phase shifts in the theta oscillation recorded from the different entorhinal and hippocampal subregions. These phase shifts could be interpreted as propagation delays. The oscillation propagates over the slice in a characteristic spatio-temporal sequence, where the entorhinal cortex leads, followed by the subiculum and then the dentate gyrus (DG), to finally reach the CA3 and the CA1 area. The delay from dentate gyrus to the CA3 area was 12.4 +/- 1.1 ms (mean +/- s.e.m.) and from the CA3 to the CA1 region it was 10.9 +/- 1.9 ms. The propagation delays between the hippocampal subregions resemble the latencies of electrically evoked responses in the same subregions. Removing the entorhinal cortex from the slice changed the spatiotemporal pattern into a more clustered pattern with higher local synchrony. We conclude that in the slice, carbachol-induced theta oscillations are initiated in the entorhinal cortex. The EC could serve to control the information flow through the neuronal network in the subregions of the hippocampus by synchronizing and/or entraining their responses to external inputs.

Slow oscillation electrical brain stimulation during waking promotes EEG theta activity and memory encoding

The application of transcranial slow oscillation stimulation (tSOS; 0.75 Hz) was previously shown to enhance widespread endogenous EEG slow oscillatory activity when applied during a sleep period characterized by emerging endogenous slow oscillatory activity. Processes of memory consolidation typically occurring during this state of sleep were also enhanced. Here, we show that the same tSOS applied in the waking brain also induced an increase in endogenous EEG slow oscillations (0.4-1.2 Hz), although in a topographically restricted fashion. Applied during wakefulness tSOS, additionally, resulted in a marked and widespread increase in EEG theta (4-8 Hz) activity. During wake, tSOS did not enhance consolidation of memories when applied after learning, but improved encoding of hippocampus-dependent memories when applied during learning. We conclude that the EEG frequency and related memory processes induced by tSOS critically depend on brain state. In response to tSOS during wakefulness the brain transposes stimulation by responding preferentially with theta oscillations and facilitated encoding.

Association of single nucleotide polymorphisms in a glutamate receptor gene (GRM8) with theta power of event-related oscillations and alcohol dependence

Evidence suggests the P3 amplitude of the event-related potential and its underlying superimposed event-related oscillations (EROs), primarily in the theta (4-5 Hz) and delta (1-3 Hz) frequencies, as endophenotypes for the risk of alcoholism and other disinhibitory disorders. Major neurochemical substrates contributing to theta and delta rhythms and P3 involve strong GABAergic, cholinergic and glutamatergic system interactions. The aim of this study was to test the potential associations between single nucleotide polymorphisms (SNPs) in glutamate receptor genes and ERO quantitative traits. GRM8 was selected because it maps at chromosome 7q31.3-q32.1 under the peak region where we previously identified significant linkage (peak LOD = 3.5) using a genome-wide linkage scan of the same phenotype (event-related theta band for the target visual stimuli). Neural activities recorded from scalp electrodes during a visual oddball task in which rare target elicited P3s were analyzed in a subset of the Collaborative Study on the Genetics of Alcoholism (COGA) sample comprising 1,049 Caucasian subjects from 209 families (with 472 DSM-IV alcohol dependent individuals). The family-based association test (FBAT) detected significant association (P < 0.05) with multiple SNPs in the GRM8 gene and event-related theta power to target visual stimuli, and also with alcohol dependence, even after correction for multiple comparisons by false discovery rate (FDR). Our results suggest that variation in GRM8 may be involved in modulating event-related theta oscillations during information processing and also in vulnerability to alcoholism. These findings underscore the utility of electrophysiology and the endophenotype approach in the genetic study of psychiatric disorders.

Conditional bursting enhances resonant firing in neocortical layer 2-3 pyramidal neurons

The frequency response properties of neurons are critical for signal transmission and control of network oscillations. At subthreshold membrane potential, some neurons show resonance caused by voltage-gated channels. During action potential firing, resonance of the spike output may arise from subthreshold mechanisms and/or spike-dependent currents that cause afterhyperpolarizations (AHPs) and afterdepolarizations (ADPs). Layer 2-3 pyramidal neurons (L2-3 PNs) have a fast ADP that can trigger bursts. The present study investigated what stimuli elicit bursting in these cells and whether bursts transmit specific frequency components of the synaptic input, leading to resonance at particular frequencies. We found that two-spike bursts are triggered by step onsets, sine waves in two frequency bands, and noise. Using noise adjusted to elicit firing at approximately 10 Hz, we measured the gain for modulation of the time-varying firing rate as a function of stimulus frequency, finding a primary peak (7-16 Hz) and a high-frequency resonance (250-450 Hz). Gain was also measured separately for single and burst spikes. For a given spike rate, bursts provided higher gain at the primary peak and lower gain at intermediate frequencies, sharpening the high-frequency resonance. Suppression of bursting using automated current feedback weakened the primary and high-frequency resonances. The primary resonance was also influenced by the SK channel-mediated medium AHP (mAHP), because the SK blocker apamin reduced the sharpness of the primary peak. Our results suggest that resonance in L2-3 PNs depends on burst firing and the mAHP. Bursting enhances resonance in two distinct frequency bands.

Propagating waves of activity in the neocortex: what they are, what they do

The development of voltage-sensitive dyes (VSD) and fast optical imaging techniques have brought us a new tool for examining spatiotemporal patterns of population neuronal activity in the neocortex. Propagating waves have been observed during almost every type of cortical processing examined by VSD imaging or electrode arrays. These waves provide subthreshold depolarization to individual neurons and increase their spiking probability. Therefore, the propagation of the waves sets up a spatiotemporal framework for increased excitability in neuronal populations, which can help to determine when and where the neurons are likely to fire. In this review, first discussed is propagating waves observed in various systems and possible mechanisms for generating and sustaining these waves. Then discussed are wave dynamics as an emergent behavior of the population activity that can, in turn, influence the activity of individual neurons. The functions of spontaneous and sensory-evoked waves remain to be explored. An important next step will be to examine the interaction between dynamics of propagating waves and functions in the cortex, and to verify if cortical processing can be modified when these waves are altered.

Neuronal mechanisms of cortical alpha oscillations in awake-behaving macaques

Field potential oscillations at approximately 10 Hz (alpha rhythm) are widely noted in the visual cortices, but their physiological mechanisms and significance are poorly understood. In vitro studies have implicated pyramidal neurons in both infragranular and supragranular layers as pacemakers. The generality of these observations for the intact brain in the behaving subject is unknown. We analyzed laminar profiles of spontaneous local field potentials and multiunit activity (MUA) recorded with linear array multielectrodes from visual areas V2, V4, and inferotemporal (IT) cortex of two macaque monkeys during performance of a sensory discrimination task. Current source density (CSD) analysis was combined with CSD-MUA coherence to identify intracortical alpha current generators and their potential for alpha pacemaking. The role of each alpha current generator was further delineated by Granger causality analyses. In V2 and V4, alpha current generators were found in all layers, with the infragranular generator acting as primary local pacemaking generator. In contrast, in IT, alpha current generators were found only in supragranular and infragranular layers, with the supragranular generator acting as primary local pacemaking generator. The amplitude of alpha activity in V2 and V4 was negatively correlated with behavioral performance, whereas the opposite was true in IT. The alpha rhythm in IT thus appears to differ from that in the lower-order cortices, both in terms of its underlying physiological mechanism and its behavioral correlates. This work may help to reconcile some of the diverse findings and conclusions on the functional significance of alpha band oscillations in the visual system.

Selective, state-dependent activation of somatostatin-expressing inhibitory interneurons in mouse neocortex

The specific functions of subtypes of cortical inhibitory neurons are not well understood. This is due in part to a dearth of information about the behaviors of interneurons under conditions when the surrounding circuit is in an active state. We investigated the firing behavior of a subset of inhibitory interneurons, identified using mice that express green fluorescent protein (GFP) in a subset of somatostatin-expressing inhibitory cells ("GFP-expressing inhibitory neuron" [GIN] cells). The somata of the GIN cells were in layer 2/3 of somatosensory cortex and had dense, layer 1-projecting axons that are characteristic of Martinotti neurons. Interestingly, GIN cells fired similarly during a variety of diverse activating conditions: when bathed in fluids with low-divalent cation concentrations, when stimulated with brief trains of local synaptic inputs, when exposed to group I metabotropic glutamate receptor agonists, or when exposed to muscarinic cholinergic receptor agonists. During these manipulations, GIN cells fired rhythmically and persistently in the theta-frequency range (3-10 Hz). Synchronous firing was often observed and its strength was directly proportional to the magnitude of electrical coupling between GIN cells. These effects were cell type specific: the four manipulations that persistently activated GIN cells rarely caused spiking of regular-spiking (RS) pyramidal cells or fast-spiking (FS) inhibitory interneurons. Our results suggest that supragranular GIN interneurons form an electrically coupled network that exerts a coherent 3- to 10-Hz inhibitory influence on its targets. Because GIN cells are more readily activated than RS and FS cells, it is possible that they act as "first responders" when cortical excitatory activity increases.

Slow GABA(A) mediated synaptic transmission in rat visual cortex

BACKGROUND

Previous reports of inhibition in the neocortex suggest that inhibition is mediated predominantly through GABA(A) receptors exhibiting fast kinetics. Within the hippocampus, it has been shown that GABA(A) responses can take the form of either fast or slow response kinetics. Our findings indicate, for the first time, that the neocortex displays synaptic responses with slow GABA(A) receptor mediated inhibitory postsynaptic currents (IPSCs). These IPSCs are kinetically and pharmacologically similar to responses found in the hippocampus, although the anatomical specificity of evoked responses is unique from hippocampus. Spontaneous slow GABA(A) IPSCs were recorded from both pyramidal and inhibitory neurons in rat visual cortex.

RESULTS

GABA(A) slow IPSCs were significantly different from fast responses with respect to rise times and decay time constants, but not amplitudes. Spontaneously occurring GABA(A) slow IPSCs were nearly 100 times less frequent than fast sIPSCs and both were completely abolished by the chloride channel blocker, picrotoxin. The GABA(A) subunit-specific antagonist, furosemide, depressed spontaneous and evoked GABA(A) fast IPSCs, but not slow GABA(A)-mediated IPSCs. Anatomical specificity was evident using minimal stimulation: IPSCs with slow kinetics were evoked predominantly through stimulation of layer 1/2 apical dendritic zones of layer 4 pyramidal neurons and across their basal dendrites, while GABA(A) fast IPSCs were evoked through stimulation throughout the dendritic arborization. Many evoked IPSCs were also composed of a combination of fast and slow IPSC components.

CONCLUSION

GABA(A) slow IPSCs displayed durations that were approximately 4 fold longer than typical GABA(A)fast IPSCs, but shorter than GABA(B)-mediated inhibition. The anatomical and pharmacological specificity of evoked slow IPSCs suggests a unique origin of synaptic input. Incorporating GABA(A) slow IPSCs into computational models of cortical function will help improve our understanding of cortical information processing.

Physiological responses of 5-month-old infants to smiling and blank faces

Physiological responses (i.e., EEG, heart period, respiratory sinus arrhythmia (RSA)) were monitored in 5-month-old infants during the replacement of an adult's smiling (SF) with a blank face (BF) in a face-to-face setting. Affect, while the infant looked at and away from the adult's face during both conditions, was analyzed. Infants displayed neutral and some positive affect while looking at both SF and BF. RSA was quantified continuously during both conditions. RSA increased during BF relative to SF. EEG was quantified only while the infants were looking at the adult's face during both conditions. An increase in theta over multiple scalp areas (AF3,4; F7,8; FC3; T6) was observed during BF relative to SF. The data suggest that infant attention to BF and SF reflect different psychophysiological processes that can be indexed by RSA and scalp-recorded theta.

Increased frontal phase-locking of event-related theta oscillations in Alzheimer patients treated with cholinesterase inhibitors

This is a pilot study describing event-related oscillations in patients with Alzheimer-type dementia (AD). Theta responses of 22 mild probable AD subjects according to NINCDS-ADRDA criteria (11 non-treated, 11 treated by cholinesterase inhibitors), and 20 healthy elderly controls were analyzed by using the conventional visual oddball paradigm. We aimed to compare theta responses of the three groups in a range between 4-7 Hz at the frontal electrodes. At F(3) location, theta responses of healthy subjects were phase locked to stimulation and theta oscillatory responses of non-treated Alzheimer patients showed weaker phase-locking, i.e. average of Z-transformed means of correlation coefficients between single trials was closer to zero. In treated AD patients, phase-locking following target stimulation was two times higher in comparison to the responses of non-treated patients. The results indicate that the phase-locking of theta oscillations at F(3) in the treated patients is as strong as the control subjects. The F(4) theta responses were not statistically significant between the groups. Our findings imply that the theta responses at F(3) location are highly unstable in comparison to F(4) in non-treated mild AD patients and cholinergic agents may modulate event-related theta oscillatory activities in the frontal regions.

Spatiotemporal patterns of an evoked network oscillation in neocortical slices: coupled local oscillators

We have discovered an evoked network oscillation in rat neocortical slices and have examined its spatiotemporal patterns with voltage-sensitive dye imaging. The slices (visual and auditory cortices) were prepared in a medium of low calcium, high magnesium and with sodium replaced by choline to reduce the excito-toxicity and sodium loading. After slicing, the choline was washed out while normal calcium, magnesium, and sodium concentrations were restored. The oscillation was evoked by a single electrical shock to slices bathed in normal artificial cerebral spinal fluid (ACSF). The oscillation was organized as an all-or-none epoch containing 4-13 cycles at a central frequency approximately 25 Hz. The activity can be reversibly blocked by 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX). 2-amino-5-phosphonopentanoic acid (APV), and atropine but not by bicuculline, indicating polysynaptic excitatory mechanisms. Voltage-sensitive dye imaging showed high-amplitude oscillation signals in superficial and middle cortical layers. Spatiotemporally, the oscillations were organized as waves, propagating horizontally along cortical laminar. Each oscillation cycle was associated with one wave propagating in space. The waveforms were often different at different locations (e.g., extra cycles), suggesting the co-existence of multiple local oscillators. For different cycles, the waves often initiated at different locations, suggesting that local oscillators are competing to initiate each oscillation cycle. Overall our results suggest that this cortical network oscillation is organized at two levels: locally, oscillating neurons are tightly coupled to form local oscillators, and globally the coupling between local oscillators is weak, allowing abrupt spatial phase lags and propagating waves with multiple initiation sites.

A Cholinergic Receptor Gene (CHRM2) Affects Event-related Oscillations

We report genetic linkage and association findings which implicate the gene encoding the muscarinic acetylcholine receptor M2 (CHRM2) in the modulation of a scalp-recorded electrophysiological phenotype. The P3 (P300) response was evoked using a three-stimulus visual oddball paradigm and a phenotype that relates to the energy in the theta band (4-5 Hz) was analyzed. Studies have shown that similar electrophysiological measures represent cognitive correlates of attention, working memory, and response selection; a role has been suggested for the ascending cholinergic pathway in the same functions. The results of our genetic association tests, combined with knowledge regarding the presence of presynaptic cholinergic M2 autoreceptors in the basal forebrain, indicate that the cognitive processes required by the experiment may in part be mediated by inhibitory neural networks. These findings underscore the utility of electrophysiology and neurogenetics in the understanding of cognitive function and the study of brain-related disorders.

Dopamine D1/5 Receptor Modulation of Firing Rate and Bidirectional Theta Burst Firing in Medial Septal/Vertical Limb of Diagonal Band Neurons In Vivo

The medial septum/vertical limb of diagonal band complex (MS/vDB) consists of cholinergic, GABAergic, and glutamatergic neurons that project to the hippocampus and functionally regulate attention, memory, and cognitive processes. Using tyrosine hydroxlase (TH) immunocytochemistry and dark-field light microscopy, we found that the MS/vDB is innervated by a sparse network of TH-immunoreactive (putative catecholaminergic) terminals. MS/vDB neurons are known to fire in rhythmic theta burst frequency of 3–7 Hz to pace hippocampal theta rhythm. Extracellular single-unit recording in theta and non-theta firing MS/vDB neurons and antidromically identified MS/vDB-hippocampal neurons were made in urethan-anesthetized rats. Tail-pinch noxious stimuli and ventral tegmental area (VTA) stimulation (20 Hz) evoked spontaneous theta burst firing in MS/vDB neurons. Systemic D1/5 antagonists SCH23390 or SCH39166 (0.1 mg/kg iv) alone suppressed the spontaneous theta bursts, suggesting a tonic facilitatory endogenous dopamine D1 “tone” that modulates theta bursts in vivo. Activation of D1/5 receptor by dihydrexidine (10 mg/kg iv) led to an increase in mean firing rate in 60% of all theta and non-theta MS/vDB neurons with an increase in the number of theta bursts and spikes/burst in theta cells. In strong theta firing MS/vDB neurons, D1/5 receptor stimulation suppressed the occurrence of theta burst firing, whereas the overall increase in spontaneous mean firing rate remained. In low baseline theta MS/vDB neurons D1/5 receptor stimulation increases the occurrence of theta bursts along with a net increase in mean firing rate. Atropine injection consistently disrupts theta burst pattern and reduced the time spent in theta firing. Collectively, these data suggest that dopamine D1/5 stimulation enhances the mean firing rate of most MS/vDB neurons and also provides a state-dependent bidirectional modulation of theta burst occurrence. Some of these MS/vDB neurons may be cholinergic or GABAergic that may indirectly regulate theta rhythm in the hippocampus.

The properties of carbachol-induced beta oscillation in rat hippocampal slices

The rhythmical and pharmacological properties of carbachol-induced beta oscillation were studied using rat hippocampal slices. With the application of 30 microM carbachol, beta-range oscillations with frequencies of 13-20 Hz were recorded from the CA3 region. An AMPA receptor antagonist, CNQX, diminished the oscillations. An NMDA receptor antagonist, APV, significantly suppressed the pre-established beta oscillations. The pre-application of APV blocked the start of the carbachol-induced beta oscillations. When bicuculline (BIC), a GABAA receptor antagonist, was applied to the pre-established beta oscillations, the frequency decreased to the theta-range. When 5 microM BIC was applied with 30 microM carbachol, the beta oscillations did not start; instead, theta-like activities were induced. It has been reported that carbachol in hippocampal slices can induce theta-like activities, which are not modulated by BIC, while BIC's facilitating the start of the activities. The results of the present study suggest that the GABAA receptor-mediated inhibitory transmission modulates the beta oscillation and that the transmission is needed for the start process of the oscillations. Therefore, the start and generation mechanisms of carbachol-induced beta oscillation will be different from those of carbachol-induced theta-like activities.

Spatio-temporal cholinergic modulation in cultured networks of rat cortical neurons: spontaneous activity

Activation of the cholinergic innervation of the cortex has been implicated in sensory processing, learning, and memory. At the cellular level, acetylcholine both increases excitability and depresses synaptic transmission, and its effects on network firing are hard to predict. We studied the effects of carbachol, a cholinergic agonist, on network firing in cultures of rat cortical neurons, using electrode arrays to monitor the activity of large numbers of neurons simultaneously. These cultures show stable spontaneous synchronized burst firing which propagates through dense synaptic connections. Carbachol (10-50 microM), acting through muscarinic receptors, was found to induce a switch to asynchronous single-spike firing and to result in a loss of regularity and fragmentation of the burst structure. To obtain a quantitative measure of cholinergic actions on cortical networks, we applied a cluster Poisson-process model to sets of paralleled spike-trains in the presence and absence of carbachol. This revealed that the time series can be well-characterized by such a simple model, consistent with the observed 1/f(b)-like spectra (0.04<b<2.08). After applying higher concentrations of carbachol the property of the spectra shifted toward a Poisson-process (white) spectrum. These results indicate that cholinergic neurotransmitters have a strong and reliable desynchronizing action on cortical neural activity.

Spiral waves in disinhibited mammalian neocortex

Spiral waves are a basic feature of excitable systems. Although such waves have been observed in a variety of biological systems, they have not been observed in the mammalian cortex during neuronal activity. Here, we report stable rotating spiral waves in rat neocortical slices visualized by voltage-sensitive dye imaging. Tissue from the occipital cortex (visual) was sectioned parallel to cortical lamina to preserve horizontal connections in layers III-V (500-mum-thick, approximately 4 x 6 mm(2)). In such tangential slices, excitation waves propagated in two dimensions during cholinergic oscillations. Spiral waves occurred spontaneously and alternated with plane, ring, and irregular waves. The rotation rate of the spirals was approximately 10 turns per second, and the rotation was linked to the oscillations in a one-cycle- one-rotation manner. A small (<128 mum) phase singularity occurred at the center of the spirals, about which were observed oscillations of widely distributed phases. The phase singularity drifted slowly across the tissue ( approximately 1 mm/10 turns). We introduced a computational model of a cortical layer that predicted and replicated many of the features of our experimental findings. We speculate that rotating spiral waves may provide a spatial framework to organize cortical oscillations.

The role of the electrophysiological properties of neurons in the mechanisms grouping their discharges in the cerebral cortex

Studies using intracellular recording in living slices of rat sensorimotor cortex addressed the interaction between the properties of neuron spike activity (n = 80) and the membrane potentials of the neurons. Spike sequences containing discharges with regularly increasing and decreasing interspike intervals were analyzed. Parameters were identified which were closely associated with the mean neuron discharge frequency: the number of spikes in sequences (5-30% of the total number of spikes recorded), the amplitude of oscillations in the afterhyperpolarization potential (0-1.5 mV), etc. There was a biphasic relationship in changes in the number of spikes in sequences with a critical mean discharge frequency over the range 5-7 Hz. Groups of cells without and with a depolarization component in conditions of afterhyperpolarization had different morphological and electrophysiological properties, though the relationships between their parameter and mean discharge frequency were similar. The possible roles of spike sequences and these regular features in the formation of rhythmic processes in the neocortex are discussed.

Linkage and linkage disequilibrium of evoked EEG oscillations with CHRM2 receptor gene polymorphisms: implications for human brain dynamics and cognition

Event-related oscillations (ERO) offer an alternative theoretical and methodological approach to the analysis of event-related EEG responses. The P300 event-related potential (ERP) is elicited through the superposition of the delta (1-3 Hz) and theta (3-7 Hz) band oscillatory responses. The cholinergic neurotransmitter system has a key function in modulating excitatory post-synaptic potentials caused by glutamate, and therefore influences P300 generation and the underlying oscillatory responses. Here we report significant linkage and linkage disequilibrium between target case frontal theta band, visual evoked brain oscillations and a single nucleotide polymorphism (SNP) from the cholinergic muscarinic receptor gene (CHRM2) on chromosome 7. We also demonstrate significant linkage disequilibrium between CHRM2 SNPs and target case parietal delta band visual evoked oscillations (LD P<0.001). These findings were not observed for the equivalent non-target case data, suggesting a role for the CHRM2 gene in higher cognitive processing in humans.

Anesthetic-induced Burst Suppression EEG Activity Requires Glutamate-mediated Excitatory Synaptic Transmission

Many anesthetics evoke electroencephalogram (EEG) burst suppression activity in humans and animals during anesthesia, and the mechanisms underlying this activity remain unclear. The present study used a rat neocortical brain slice EEG preparation to investigate excitatory synaptic mechanisms underlying anesthetic-induced burst suppression activity. Excitatory synaptic mechanisms associated with burst suppression activity were probed using glutamate receptor antagonists (CNQX and APV), GABA receptor antagonists, and simultaneous whole cell patch clamp and microelectrode EEG recordings. Clinically relevant concentrations of thiopental (50--70 microM), propofol (5--10 microM) or isoflurane (0.7--2.1 vol%, 0.5--1.5 rat minimum aveolar concentration (MAC), 200--700 microM) evoked delta slow wave activity and burst suppression EEG patterns similar to in vivo responses. These effects on EEG signals were blocked by glutamate receptor antagonists CNQX (8.6 microM) or APV (50 microM). Depolarizing intracellular bursts (amplitude=34.7+/-4.5 mV; half width=132+/-60 ms) always accompanied EEG bursts, and hyperpolarization increased intracellular burst amplitudes. Barrages of glutamate-mediated excitatory events initiated EEG bursting activity. Glutamate-mediated excitatory postsynaptic currents were significantly depressed by higher anesthetic concentrations that depressed burst suppression EEG activity. A GABA(A) agonist produced a similar EEG effect to the anesthetics. It appears that anesthetic effects at both glutamate and GABA synapses contribute to EEG patterns seen during anesthesia.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14-P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10 microM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II-III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Repeated exposure to low-level extremely low frequency-modulated microwaves affects baseline and scopolamine-modified electroencephalograms in freely moving rats

PURPOSE

To compare in the electroencephalogram of rats the effects of scopolamine (an acetylcholine receptor antagonist) alone and after repeated exposure to low-level microwaves modulated at extremely low frequency.

MATERIALS AND METHODS

Averaged frequency spectra (0.5-30 Hz) of the electroencephalogram were studied in freely moving rats with carbon electrodes implanted into the somatosensory cortex. The rats were repeatedly (3 days, 30 min day(-1)) exposed to low-intensity (approximately = 0.3 mW cm(-2)) microwaves (915 MHz, 20-ms pulse duration), amplitude modulated (square-wave) at extremely low frequency (4 Hz).

RESULTS

The exposure to extremely low frequency microwaves alone significantly enhanced the fast electroencephalographic rhythms (18-30 Hz). This effect was observed neither in subsequent sham-exposure experiment nor in radiation-naïve animals. In the microwave-exposed rats, scopolamine (0.1 mg kg(-1), subcutaneously) did not cause a slowing in the electroencephalogram that was shown in non-exposed rats. A similarity between the scopolamine-induced electroencephalogram effect in the microwave-exposed rats and that of physostigmine (enhancing the acetylcholine level in the brain) in radiation-naïve animals was noted. This paradoxical phenomenon stimulates new experimentation for understanding its mechanism(s).

CONCLUSIONS

The data obtained provide additional evidence that repeated low-level exposure to extremely low frequency microwaves can modify an activity of cholinergic system in the brain.

Basal ganglia–hippocampal interactions support the role of the hippocampal formation in sensorimotor integration

Experiments were carried out to evaluate whether neural activity in the basal ganglia is functionally related to the neural activity underlying mechanisms of theta band oscillation and synchrony in the hippocampal formation. Experiment 1 demonstrated that electrical stimulation administered to the substantia nigra, globus pallidus (GP) and caudate-putamen (CPu) in urethane anesthetized rats elicited theta field activity in the hippocampal formation. Subsequent microinfusion of the local anesthetic procaine hydrochloride into the medial septum reversibly abolished this effect. In Experiment 2, single cell discharge profiles established for 152 cells recorded in nuclei of the basal ganglia resulted in 101 (66%) being classified as theta-related and 51 (34%) classified as nonrelated. Theta-related cells were further subclassified as tonic theta-ON cells (n = 79) and tonic theta-OFF (n = 22). Tonic theta-ON and tonic theta-OFF cells displayed irregular or regular (tonic) discharge patterns. Rhythmic discharge patterns did not occur in any theta-related cells in the nuclei of the basal ganglia. However, analyses using Kaneoke and Vitek's [J. Neurosci. Methods 68, (1996) 211] algorithms revealed that 51/101 (50%) theta-related cells displayed periodicity in their discharge patterns whereas 27/51 (53%) of the nonrelated cells displayed periodicity in their discharge patterns. The periodicities in the majority of cells were in frequency ranges above that of theta band oscillation and synchrony. The results support the following conclusions: (1) the cellular activity of the basal ganglia, composed of nuclei traditionally associated with motor functions, is functionally connected with the neural circuitry involved in the generation of theta band oscillation and synchrony in the hippocampal formation; (2) the observed functional connectivity provides support for the role of the hippocampal formation in sensorimotor integration.

Functional connectivity between the red nucleus and the hippocampus supports the role of hippocampal formation in sensorimotor integration

Experiments were carried out in urethane-anesthetized rats to evaluate the hypothesis that the red nucleus has functional connections with the hippocampal formation. Depth profiles of electrical stimulation in experiment 1 confirmed that stimulation administered to the red nucleus elicited theta field activity in the hippocampal formation with a linear relationship between stimulus intensity and theta frequency. Experiment 2 showed that microinfusion of local anesthetic procaine hydrochloride into the medial septum resulted in a reversible blockade of theta field activity elicited by electrical stimulation of the red nucleus. In experiment 3, the discharge activity of red nucleus cells was recorded during the field conditions of hippocampal synchrony (theta) and hippocampal asynchrony [large amplitude irregular activity (LIA)]. Analysis revealed that 26/46 (56%) of red nucleus cells were theta-related, whereas the remaining 20 (44%) were nonrelated. The majority of theta-related cells were classified as tonic theta-on. A brief increase above the basal discharge rate of tonic theta-on red nucleus cells during LIA predicted the transition from LIA to theta with 400- to 500-ms latency. Furthermore, higher frequency transitional discharges predicted higher theta frequencies, whereas higher discharge rates during theta predicted shifts to higher theta frequencies. The results supported the conclusion that the red nucleus, traditionally associated with motor functions, is functionally connected with the neural circuitry involved in the generation of theta band oscillation and synchrony in the hippocampal formation, in agreement with the predictions of the sensorimotor integration model of hippocampal function.

Skilled-learning-induced potentiation in rat sensorimotor cortex: a transient form of behavioural long-term potentiation

The relation between the acquisition of a skilled motor task and synaptic plasticity in the sensorimotor cortex of the awake, freely behaving rat was examined. Skilled-motor training was previously found to induce a functional reorganization of the caudal forelimb area, and to induce an increase in synaptic efficacy, measured in vitro, on the side contralateral to the reaching forelimb. Here, we repeatedly measured neocortical evoked potential recordings in awake, freely behaving rats to examine whether skilled training would induce changes in polysynaptic efficacy on the side contralateral to the reaching forelimb. We found that the increase in task proficiency, but not the acquisition of task requirements or the maintenance of task proficiency, induced an increase in synaptic efficacy on the side contralateral to the reaching forelimb. We also tested the hypothesis that skilled learning induced potentiation shares similar mechanisms to long-term potentiation (LTP) and long-term depression by artificially manipulating polysynaptic efficacy in skilled rats with high- and low-frequency stimulation. We observed that, compared with the ipsilateral side, less potentiation but more depression could be induced on the side contralateral to the reaching forelimb. We conclude that a transient, network-based LTP-like mechanism operates during the learning of a skilled motor task.

Effects of scopolamine on MEG spectral power and coherence in elderly subjects

OBJECTIVE

Scopolamine, a muscarinic receptor antagonist, can produce temporary cognitive impairments as well as electroencephalographic changes that partially resemble those observed in Alzheimer's disease. In order to test the sensitivity of spectral power and hemispheric coherence to changes in cholinergic transmission, we evaluated quantitative magnetoencephalogram (MEG) after intravenous injection of scopolamine.

METHODS

MEG of 8 elderly healthy subjects (59-80 years) were measured with a whole-head magnetometer after intravenous injection of scopolamine. An injection of glycopyrrolate, a peripheral muscarinic antagonist, was used as the placebo in a double-blind, randomized, cross-over design. Spectral power and coherence were computed over 7 brain regions in 3 frequency bands.

RESULTS

Scopolamine administration increased theta activity (4-8 Hz) and resulted in the abnormal pattern of MEG desynchronization in eyes-open vs. eyes-closed conditions in the alpha band (8-13 Hz). These effects were most prominent over the posterior regions. Interhemispheric and left intrahemispheric coherence was significantly decreased in the theta band (4-8 Hz).

CONCLUSIONS

Spontaneous cortical activity at the theta and alpha range and functional coupling in the theta band are modulated by the cholinergic system. MEG may provide a tool for monitoring brain dynamics in neurological disorders associated with cholinergic abnormalities.

Human theta oscillations related to sensorimotor integration and spatial learning

oscillations in the rat hippocampus have been implicated in sensorimotor integration (Bland, 1986), especially during exploratory and wayfinding behavior. We propose that human cortical activity coordinates sensory information with a motor plan to guide wayfinding behavior to known goal locations. To test this hypothesis, we analyzed invasive recordings from epileptic patients while they performed a spatially immersive, virtual taxi driver task. Consistent with this hypothesis, we found oscillations during both exploratory search and goal-seeking behavior and, in particular, during virtual movement, when sensory information and motor planning were both in flux, compared with periods of self-initiated stillness. oscillations had different topographic and spectral characteristics during searching than during goal-seeking, suggesting that different cortical networks exhibit depending on which cognitive functions are driving behavior (spatial learning during exploration vs orienting to a learned representation during goal-seeking). In contrast, oscillations in the beta band appeared to be related to simple motor planning, likely a variant of the Rolandic mu rhythm. These findings suggest that human cortical oscillations act to coordinate sensory and motor brain activity in various brain regions to facilitate exploratory learning and navigational planning.

Propagating wave and irregular dynamics: spatiotemporal patterns of cholinergic theta oscillations in neocortex in vitro

Neocortical "theta" oscillation (5-12 Hz) has been observed in animals and human subjects but little is known about how the oscillation is organized in the cortical intrinsic networks. Here we use voltage-sensitive dye and optical imaging to study a carbachol/bicuculline induced theta ( approximately 8 Hz) oscillation in rat neocortical slices. The imaging has large signal-to-noise ratio, allowing us to map the phase distribution over the neocortical tissue during the oscillation. The oscillation was organized as spontaneous epochs and each epoch was composed of a "first spike," a "regular" period (with relatively stable frequency and amplitude), and an "irregular" period (with variable frequency and amplitude) of oscillations. During each cycle of the regular oscillation, one wave of activation propagated horizontally (parallel to the cortical lamina) across the cortical section at a velocity of approximately 50 mm/s. Vertically the activity was synchronized through all cortical layers. This pattern of one propagating wave associated with one oscillation cycle was seen during all the regular cycles. The oscillation frequency varied noticeably at two neighboring horizontal locations (330 microm apart), suggesting that the oscillation is locally organized and each local oscillator is about </=300 microm wide horizontally. During irregular oscillations, the spatiotemporal patterns were complex and sometimes the vertical synchronization decomposed, suggesting a de-coupling among local oscillators. Our data suggested that neocortical theta oscillation is sustained by multiple local oscillators. The coupling regime among the oscillators may determine the spatiotemporal pattern and switching between propagating waves and irregular patterns.