Evidence that serotonin mediates non-cholinergic neocortical low voltage fast activity, non-cholinergic hippocampal rhythmical slow activity and contributes to intelligent behavior

Stimulation of spinal cord according to recorded theta hippocampal rhythm during rat move on treadmill

OBJECTIVES

Several studies have revealed that after spinal cord injury (SCI), in acute and sub-acute phase the spinal cord neurons below the injury are alive and could stimulate by use of electrical pulses. Spinal cord electrical stimulation could generate movement for paralyzed limbs and is a rehabilitation strategy for paralyzed patients. An innovative idea for controlling spinal cord electrical stimulation onset time is presented in current study.

METHODS

In our method, the time of applying electrical pulse on the spinal cord is according to rat behavioral movement and two movements behaviors are recognized only based on rat EEG theta rhythm on the treadmill line. Briefly, 5 rats were placed on the treadmill and the animals experienced zero or 12 m/min speeds.

RESULTS

These speeds were recognized based on EEG signals and off-line periodogram analysis. Finally, the electrical stimulation pulses had been applied to the spinal cord if the results of the EEG analysis had detected running behavior.

CONCLUSIONS

These findings may guide future research in utilizing theta rhythms for the recognition of animal motor behavior and designing electrical stimulation systems based on it.

Older rats show slow modulation of hippocampal theta rhythm during voluntary running

Aging causes brain function degeneration and slows many motor and behavioural responses. The hippocampal theta rhythm (4-12 Hz) is related to cognition and locomotion. However, the findings on aging-related changes in the frequency and amplitude of hippocampal theta oscillations have been inconsistent. We hypothesized that older rats have slower responses in terms of hippocampal theta rhythm during voluntary wheel running than do young adult rats. By simultaneously recording electroencephalography and physical activity (PA), we evaluated theta oscillations in 8-week-old (young adult) and 60-week-old (middle-aged) rats before and during wheel running, which was conducted only during the rats' 12-h dark period. To test the alterations of hippocampal theta rhythm in voluntary wheel running, we analyzed the signals without (8-s) or with (2-s) chronological order. No significant difference was observed in total frequency (TP, 4-12 Hz), low-frequency (LT, 4-6.5 Hz), or high-frequency (9.5-12 Hz) theta activity between active waking and overall running in either group. The theta oscillations were slower in the middle-aged rats than in the young adult rats during wheel running but increased during running for both age groups. During wheel running, the middle-aged rats exhibited an increased LT, which was related to PA. On the basis of the chronological order of running, the young adult rats exhibited increased TP, and the middle-aged rats exhibited significant increases in middle-frequency (MT, 6.5-9.5 Hz) theta activity. The dominant modulations of MT in the middle-aged rats may have caused nonsignificant changes in total activity. These between-group differences in theta rhythm characteristics during voluntary running provide insights into age-related brain function decline.

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.

The role of the posterior hypothalamic area in the generation of theta rhythm

Theta rhythm is one of the best synchronized patterns of the oscillatory activity recorded in the mammalian brain. In humans, this rhythm is associated with REM sleep, spatial navigation, memory functions, analytical and language processes. On the other hand, it can be treated as a non-specific marker of such pathological states of the central nervous system as Alzheimer’s disease or epilepsy. The hippocampal formation is the key structure involved in the generation of this bioelectric phenomenon, both in humans and rodents (the most commonly studied laboratory animals). Theta rhythm appearance in the hippocampus is dependent on the interaction of multiple different structures of the nervous system. One of them is the posterior hypothalamic area (PHa), which constitutes a crucial part of the neuronal system modulating the ability of the hippocampal formation to generate theta rhythm. Although the research results encompassed in this paper emphasize the essential role of the PHa as a modulator of the hippocampal theta rhythm, it was the authors’ intent to indicate that this area is also capable of generating local rhythmical theta oscillations, independently of the influence of other brain structures.

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.

Somatostatin interneurons activated by 5-HT2A receptor suppress slow oscillations in medial entorhinal cortex

Serotonin (5-HT) is one of the major neuromodulators present in the mammalian brain and has been shown to play a role in multiple physiological processes. The mechanisms by which 5-HT modulates cortical network activity, however, are not yet fully understood. We investigated the effects of 5-HT on slow oscillations (SOs), a synchronized cortical network activity universally present across species. SOs are observed during anesthesia and are considered to be the default cortical activity pattern. We discovered that (±)3,4-methylenedioxymethamphetamine (MDMA) and fenfluramine, two potent 5-HT releasers, inhibit SOs within the entorhinal cortex (EC) in anesthetized mice. Combining opto- and pharmacogenetic manipulations with in vitro electrophysiological recordings, we uncovered that somatostatin-expressing (Sst) interneurons activated by the 5-HT_2A receptor (5-HT_2AR) play an important role in the suppression of SOs. Since 5-HT_2AR signaling is involved in the etiology of different psychiatric disorders and mediates the psychological effects of many psychoactive serotonergic drugs, we propose that the newly discovered link between Sst interneurons and 5-HT will contribute to our understanding of these complex topics.

Unexpected awakenings in severe dementia from case reports to laboratory

OBJECTIVE

Case report notions of unexpected memory retrieval in patients with severe dementia near to death are starting to alter the central "irreversible" paradigm of dementia and locate dementia as a problem of memory retrieval, not consolidation. We suggest that the most likely central tenet of this paradoxical memory retrieval is the fluctuation of neuromodulators projecting from the brain stem to the medial prefrontal cortex and the hippocampus. The neuromodulation-centric explanation of this phenomenon aims to open the "irreversible" paradigm of dementia up for discussion and suggest a plausible treatment strategy by questioning how the devastating process of death fluctuates memory performance in severe dementia.

BACKGROUND

Supporting demented patients, who are mostly unresponsive, without making demands or asking a question and regarding them as valuable human beings unexpectedly improve their memory performance around the time of death.

NEW LUCIDITY HYPOTHESIS

Around the time of death, neurological signs (hyper-arousal and -attention) of demented people point out that neurotransmitter discharges are dramatically changed. Relatively resistant neuromodulator circuits to neurodegeneration can maintain optimal levels of arousal and attention for memory processing. In this way, unexpected episodes of lucidity can be triggered. Also, corticotropin-releasing peptides might increase mental clarity by increasing the excitability of the neuromodulator circuits. The science of memory retrieval is more complicated and nuanced than retrieval observations in case reports, but the rapid development of new techniques holds promise for future understanding of lucidity in severe dementia.

MAJOR CHALLENGE FOR THE MODEL

There is no an animal or human model to test this hypothesis; however, the similarities between neurological signs (instantaneous cognitive fluctuations) of delirium and paradoxical lucidity could provide a unique window to understand neural events of terminal lucidity on a modified animal model of delirium. Likewise, similarities between unexpected consciousness signs of terminal lucidity and lucid dreaming suggest that lucid dreaming episodes might be considered a human model for terminal lucidity research.

Voluntary exercise enhances hippocampal theta rhythm and cognition in the rat

Regular exercise promotes learning and memory functions. Theta activity is known to relate to various cognitive functions. An increase in theta power may be related to higher cognitive functioning and learning functions. However, evidence is lacking to directly confirm that exercise training can increase the theta activity and promote various cognitive functions simultaneously. We hypothesize that long-term voluntary exercise increases the activity of hippocampal theta rhythm and enhances memory behavior. We used the voluntary wheel running model and a training period of 8 weeks. We started the training when the rats were 12 weeks old. Before and after intervention, we performed a 24 -h electrophysiological recording and 8-arm radial maze test to analyze the hippocampal theta rhythm in awake stage, and spatial memory functions. We discovered that middle to high range frequency (6.5-12 Hz) of theta power was increased after exercise intervention. In addition, the working memory error of 8-arm radial maze test in the exercise group decreased significantly after the 8 weeks of treatment, and these reductions were negatively correlated with hippocampal theta activity. Our results demonstrate that 8-weeks voluntary exercise increases both hippocampal theta amplitude and spatial memory in the rats.

Behaviour consistency is a sensitive tool for distinguishing the effects of aging on physical activity

We attempted to establish a novel parameter of behaviour consistency to help determine the effect of age on physical activity. Using the speed of movement to quantify behaviour might not be sufficient to determine this effect. The slowing of motor activities that occurs with aging is related to the decline of the aging brain. Previous studies have found different running-related hippocampal theta rhythm responses in the aging and exercise model. Therefore, we hypothesized that a familiarity with the environment and physical strength affect behavioural consistency in rats during running exercises. For this study, we used a treadmill and 30-minute running test at constant speeds and compared changes in the triaxial accelerometer and hippocampal theta rhythm between adult and middle-aged rats. No significant differences in RR intervals, mean cross-correlations (MCCs), or the proportion of good correlation coefficient (PGCC) were observed between adult and middle-aged rats in awake states before running on the treadmill. The root mean square (RMS) of the triaxial acceleration vectors in middle-aged rats was higher than that in adult rats. In the treadmill running tests, the RMS observed in middle-aged rats was significantly lower than that observed in adult rats. MCC and PGCC, which indicate movement consistencies, were significantly higher in middle-aged rats than they were in adult rats during the entire running test. However, only the RMS of the adult rats showed a negative correlation with exercise duration. Both MCC and PGCC were positively correlated with exercise duration. By contrast, a similar phenomenon was not found in the changes or differences in hippocampal theta rhythms between these two groups. Therefore, we consider that the MCC and PGCC could distinguish age-related movement differences and indicate coordination/adaptation during exercise. Changes in physical activity and alterations in the hippocampal theta rhythm were not different between the groups.

Neural Circuits for Sleep-Wake Regulation

The neural mechanisms of sleep, a fundamental biological behavior from invertebrates to humans, have been a long-standing mystery and present an enormous challenge. Gradually, perspectives on the neurobiology of sleep have been more various with the technical innovations over the recent decades, and studies have now identified many specific neural circuits that selectively regulate the initiation and maintenance of wake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep. The cholinergic system in basal forebrain (BF) that fire maximally during waking and REM sleep is one of the key neuromodulation systems related to waking and REM sleep. Here we outline the recent progress of the BF cholinergic system in sleep-wake cycle. The intricate local connectivity and multiple projections to other cortical and subcortical regions of the BF cholinergic system elaborately presented here form a conceptual framework for understanding the coordinating effects with the dissecting regions. This framework also provides evidences regarding the relationships between the general anesthesia and wakefulness/sleep cycle focusing on the neural circuitry of unconsciousness induced by anesthetic drugs.

Medial septal cholinergic mediation of hippocampal theta rhythm induced by vagal nerve stimulation

BACKGROUND

Electrical vagal nerve stimulation (VNS) has been used for years to treat patients with drug-resistant epilepsy. This technique also remains under investigation as a specific treatment of patients with Alzheimer's disease. Recently we discovered that VNS induced hippocampal formation (HPC) type II theta rhythm, which is involved in memory consolidation. In the present study, we have extended our previous observation and addressed the neuronal substrate and pharmacological profile of HPC type II theta rhythm induced by VNS in anesthetized rats.

METHODS

Male Wistar rats were implanted with a VNS cuff electrode around the left vagus nerve, a tungsten microelectrode for recording the HPC field activity, and a medial septal (MS) cannula for the injection of a local anesthetic, procaine, and muscarinic agents. A direct, brief effect of VNS on the HPC field potential was evaluated before and after medial-septal drug injection.

RESULTS

Medial septal injection of local anesthetic, procaine, reversibly abolished VNS-induced HPC theta rhythm. With the use of cholinergic muscarinic agonist and antagonists, we demonstrated that medial septal M1 receptors are involved in the mediation of the VNS effect on HPC theta field potential.

CONCLUSION

The MS cholinergic M1 receptor mechanism integrates not only central inputs from the brainstem synchronizing pathway, which underlies the production of HPC type II theta rhythm, but also the input from the vagal afferents in the brain stem.

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.

Transient inhibition and long-term facilitation of locomotion by phasic optogenetic activation of serotonin neurons

Serotonin (5-HT) is associated with mood and motivation but the function of endogenous 5-HT remains controversial. Here, we studied the impact of phasic optogenetic activation of 5-HT neurons in mice over time scales from seconds to weeks. We found that activating dorsal raphe nucleus (DRN) 5-HT neurons induced a strong suppression of spontaneous locomotor behavior in the open field with rapid kinetics (onset ≤1 s). Inhibition of locomotion was independent of measures of anxiety or motor impairment and could be overcome by strong motivational drive. Repetitive place-contingent pairing of activation caused neither place preference nor aversion. However, repeated 15 min daily stimulation caused a persistent increase in spontaneous locomotion to emerge over three weeks. These results show that 5-HT transients have strong and opposing short and long-term effects on motor behavior that appear to arise from effects on the underlying factors that motivate actions.

DOI:http://dx.doi.org/10.7554/eLife.20975.001

The brain controls sleep, movement and the other behaviors that an animal needs to survive. A chemical called serotonin plays an important role in controlling these behaviors as it regulates the activity of nerve cells (known as neurons) throughout the brain. Serotonin is produced by a specific group of neurons found in an area at the base of the brain called the raphe nuclei. From there, serotonin is released into other parts of the brain to influence different behaviors. Although drugs that target serotonin are widely used as antidepressants, how this chemical signal acts in the brain remains a mystery. This is due, in part, to it being technically challenging to carry out experiments on the serotonin-producing neurons.

A technique called optogenetics uses light to selectively activate or inhibit individual cells in live animals. Here, Correia, Lottem et al. use optogenetics to activate serotonin-producing neurons in the dorsal raphe nucleus of mice. The experiments show that triggering serotonin production for a few seconds causes the mice to move around more slowly as they explore their surroundings. This short-term release of serotonin only slows the mice down if they are not already occupied with other activities, such as finding water or balancing on a moving object. These experiments suggest that serotonin decreases an individual’s motivation to move but that this can be overcome by sufficiently powerful goals. In contrast, repeatedly activating the serotonin neurons over a period of several weeks led to long-term changes of the opposite kind – the mice begin to move around more quickly.

The findings of Correia, Lottem et al. have possible implications for the use of drugs that target serotonin to treat mental disorders as it suggests important links between serotonin, movement, and the ability of the brain to change how it responds to certain situations. The next steps will be to investigate how the two different effects of serotonin are connected, which areas in the brain are involved and how best to apply these findings to clinical studies.

DOI:http://dx.doi.org/10.7554/eLife.20975.002

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long-lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross-correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem-VTA-hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe).

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

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

Cortical neuronal activity does not regulate sleep homeostasis

The neural substrate of sleep homeostasis is unclear, but both cortical and subcortical structures are thought to be involved in sleep regulation. To test whether prior neuronal activity in the cortex or in subcortical regions drives sleep rebound, we systemically administered atropine (100mg/kg) to rats, producing a dissociated state with slow-wave cortical electroencephalogram (EEG) but waking behavior (e.g. locomotion). Atropine injections during the light period produced 6h of slow-wave cortical EEG but also subcortical arousal. Afterward, rats showed a significant increase in non-rapid eye movement (NREM) sleep, compared to the same period on a baseline day. Consistent with the behavioral and cortical EEG state produced by systemic atropine, c-Fos expression was low in the cortex but high in multiple subcortical arousal systems. These data suggest that subcortical arousal and behavior are sufficient to drive sleep homeostasis, while a sleep-like pattern of cortical activity is not sufficient to satisfy sleep homeostasis.

NMDA-glutamatergic activation of the ventral tegmental area induces hippocampal theta rhythm in anesthetized rats

Glutamate afferents reaching the ventral tegmental area (VTA) affect dopamine (DA) cells in this structure probably mainly via NMDA receptors. VTA appears to be one of the structures involved in regulation of hippocampal theta rhythm, and this work aimed at assessing the role of glutamatergic activation of the VTA in the theta regulation. Male Wistar rats (n=17) were divided into groups, each receiving intra-VTA microinjection (0.5 μl) of either solvent (water), glutamatergic NMDA agonist (0.2 μg) or antagonist (MK-801, 3.0 μg). Changes in local field potential were assessed on the basis of peak power (Pmax) and corresponding peak frequency (Fmax) for the delta (0.5-3 Hz) and theta (3-6 Hz) bands. NMDA microinjection evoked long-lasting hippocampal theta. The rhythm appeared with a latency of ca. 12 min post-injection and lasted for over 30 min; Pmax in this band was significantly increased for 50 min, while simultaneously Pmax in the delta band remained lower than in control conditions. Theta Fmax and delta Fmax were increased in almost entire post-injection period (by 0.3-0.5 Hz and 0.3-0.7 Hz, respectively). MK-801 depressed the sensory-evoked theta: tail pinch could not induce theta for 30 min after the injection; Pmax significantly decreased in the theta band and at the same time it increased in the delta band. Theta Fmax decreased 10 and 20 min post injection (by 0.4-0.5 Hz) and delta Fmax decreased in almost entire post injection period (by 0.3-0.7 Hz). NMDA injection generates theta rhythm probably through stimulation of dopaminergic activity within the VTA.

The electrical coupling and the hippocampal formation theta rhythm in rats

Gap junctions (GJs) were discovered more than five decades ago, and since that time enormous strides have been made in understanding their structure and function. Despite the voluminous literature concerning the function of GJs, the involvement of these membrane structures in the central mechanisms underlying oscillations and synchrony in the neuronal network is still a matter of intensive debate. This review summarizes what is known concerning the involvement of GJs as electrical synapses in mechanisms underlying the generation of theta band oscillations. The first part of the chapter discusses the role of GJs in mechanisms of oscillations and synchrony. Following this, in vitro, ex vivo, and in vivo experiments concerning the involvement of GJs in the generation of hippocampal formation theta in rats are reviewed.

Voluntary and involuntary running in the rat show different patterns of theta rhythm, physical activity, and heart rate

Involuntarily exercising rats undergo more physical and mental stress than voluntarily exercising rats; however, these findings still lack electrophysiological evidence. Many studies have reported that theta rhythm appears when there is mental stress and that it is affected by emotional status. Thus we hypothesized that the differences between voluntary and involuntary movement should also exist in the hippocampal theta rhythm. Using the wheel and treadmill exercise models as voluntary and involuntary exercise models, respectively, this study wirelessly recorded the hippocampal electroencephalogram, electrocardiogram, and three-dimensional accelerations of young male rats. Treadmill and wheel exercise produced different theta patterns in the rats before and during running. Even though the waking baselines for the two exercise types were recorded in different environments, there did not exist any significant difference after distinguishing the rats' sleep/wake status. When the same movement-related parameters are considered, the treadmill running group showed more changes in their theta frequency (4-12 Hz), in their theta power between 9.5-12 Hz, and in their heart rate than the wheel running group. A positive correlation between the changes in high-frequency (9.5-12 Hz) theta power and heart rate was identified. Our results reveal various voluntary and involuntary changes in hippocampal theta rhythm as well as divergences in heart rate and high-frequency theta activity that may represent the effects of an additional emotional state or the sensory interaction during involuntary running by rats.

Attention-deficit/hyperactivity disorder: neurophysiology, information processing, arousal and drug development

In this review, we draw on literature from both animal and human neurophysiological studies to consider the neurochemical mechanisms underlying attention-deficit/ hyperactivity disorder (ADHD). Psychophysiological and neuropsychological research is used to propose possible etiological endophenotypes of ADHD. These are conceptualized as patients with distinct cortical-arousal, information-processing or maturational abnormalities, or a combination thereof, and how the endophenotypes can be used to help drug development and optimize treatment and management. To illustrate, the paper focuses on neuro- and psychophysiological evidence that suggests cholinergic mechanisms may underlie specific information-processing abnormalities that occur in ADHD. The clinical implications for a cholinergic hypothesis of ADHD are considered, along with its possible implications for treatment and pharmacological development.

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.

Oscillatory serotonin function in depression

Oscillations in brain activities with periods of minutes to hours may be critical for normal mood behaviors. Ultradian (faster than circadian) rhythms of mood behaviors and associated central nervous system activities are altered in depression. Recent data suggest that ultradian rhythms in serotonin (5HT) function also change in depression. In two separate studies, 5HT metabolites in cerebrospinal fluid (CSF) were measured every 10 min for 24 h before and after chronic antidepressant treatment. Antidepressant treatments were associated with enhanced ultradian amplitudes of CSF metabolite levels. Another study used resting-state functional magnetic resonance imaging (fMRI) to measure amplitudes of dorsal raphé activation cycles following sham or active dietary depletions of the 5HT precursor (tryptophan). During depletion, amplitudes of dorsal raphé activation cycles increased with rapid 6 s periods (about 0.18 Hz) while functional connectivity weakened between dorsal raphé and thalamus at slower periods of 20 s (0.05 Hz). A third approach studied MDMA (ecstasy, 3,4-methylenedioxy-N-methylamphetamine) users because of their chronically diminished 5HT function compared with non-MDMA polysubstance users (Karageorgiou et al., 2009). Compared with a non-MDMA using cohort, MDMA users showed diminished fMRI intra-regional coherence in motor regions along with altered functional connectivity, again suggesting effects of altered 5HT oscillatory function. These data support a hypothesis that qualities of ultradian oscillations in 5HT function may critically influence moods and behaviors. Dysfunctional 5HT rhythms in depression may be a common endpoint and biomarker for depression, linking dysfunction of slow brain network oscillators to 5HT mechanisms affected by commonly available treatments. 5HT oscillatory dysfunction may define illness subtypes and predict responses to serotonergic agents. Further studies of 5HT oscillations in depression are indicated.

Cortical state and attention

The brain continuously adapts its processing machinery to behavioural demands. To achieve this, it rapidly modulates the operating mode of cortical circuits, controlling the way that information is transformed and routed. This article will focus on two experimental approaches by which the control of cortical information processing has been investigated: the study of state-dependent cortical processing in rodents and attention in the primate visual system. Both processes involve a modulation of low-frequency activity fluctuations and spiking correlation, and are mediated by common receptor systems. We suggest that selective attention involves processes that are similar to state change, and that operate at a local columnar level to enhance the representation of otherwise non-salient features while suppressing internally generated activity patterns.

Potential Mechanisms of Failure in the Sudden Infant Death Syndrome

Current evidence suggests that multiple neural mechanisms contribute to the fatal lethal event in SIDS. The processes may develop from a range of otherwise seemingly-innocuous circumstances, such as unintended external airway obstruction or accidental extreme flexion of the head of an already-compromised structure of the infant upper airway. The fatal event may occur in a sleep state which can suppress muscle tone essential to restore airway patency or exert muscle action to overcome a profound loss of blood pressure. Neural processes that could overcome those transient events with reflexive compensation appear to be impaired in SIDS infants. The evidence ranges from subtle physiological signs that appear very early in life, to autopsy findings of altered neurotransmitter, including serotonergic, systems that have extensive roles in breathing, cardiovascular regulation, and thermal control. Determination of the fundamental basis of SIDS is critical to provide biologic plausibility to SIDS risk reduction messages and to develop specific prevention strategies.

Persistent hyperdopaminergia decreases the peak frequency of hippocampal theta oscillations during quiet waking and REM sleep

Long-term changes in dopaminergic signaling are thought to underlie the pathophysiology of a number of psychiatric disorders. Several conditions are associated with cognitive deficits such as disturbances in attention processes and learning and memory, suggesting that persistent changes in dopaminergic signaling may alter neural mechanisms underlying these processes. Dopamine transporter knockout (DAT-KO) mice exhibit a persistent five-fold increase in extracellular dopamine levels. Here, we demonstrate that DAT-KO mice display lower hippocampal theta oscillation frequencies during baseline periods of waking and rapid-eye movement sleep. These altered theta oscillations are not reversed via treatment with the antidopaminergic agent haloperidol. Thus, we propose that persistent hyperdopaminergia, together with secondary alterations in other neuromodulatory systems, results in lower frequency activity in neural systems responsible for various cognitive processes.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

To move or not: Previous experience in a runway avoidance task determines the appearance of hippocampal Type 2 sensory processing theta

Rats in a runway avoidance task responded to a test shock probe with a period of immobility lasting from 2 to 6s. The shock avoidance-trained group displayed hippocampal theta during the immobility response. The inescapable shock group, in contrast, displayed large amplitude irregular activity (LIA). Following reversal training to escapable shock, all shock avoidance-trained rats responded with LIA and inescapable shock trained rats, reversed to shock avoidance, displayed theta.

Medial septum lesions disrupt exploratory trip organization: Evidence for septohippocampal involvement in dead reckoning

Rats organize their open field behavior into a series of exploratory trips focused around a central location or home base. In addition, differences in movement kinematics have been used to fractionate the exploratory trip into tour (i.e., sequences of linear movement or progressions punctuated by stops) and homeward (i.e., single progression direct to the home base) segments. The observation of these characteristics independent of environmental familiarity and visual cue availability has suggested a role for self-movement information or dead reckoning in organizing exploratory behavior. Although previous work has implicated a role for the septohippocampal system in dead reckoning based navigation, as of yet, no studies have investigated the contribution of the medial septum to dead reckoning. First, the present study examined the organization of exploratory behavior under dark and light conditions in control rats and rats receiving either electrolytic or sham medial septum lesions. Medial septum lesions produced a significant increase in homeward segment path circuity and variability of temporal pacing of linear speeds. Second, as an independent assessment of the effectiveness of the medial septum lesions, rats were trained to locate a hidden platform in the standard water maze procedure. Consistent with previous research, medial septum lesions attenuated learning the location of the hidden platform. These results demonstrate a role for the medial septum in organizing exploratory behavior and provide further support for the role of the septohippocampal system in dead reckoning based navigation.

Dopaminergic control of sleep-wake states

Dopamine depletion is involved in the pathophysiology of Parkinson's disease, whereas hyperdopaminergia may play a fundamental role in generating endophenotypes associated with schizophrenia. Sleep disturbances are known to occur in both schizophrenia and Parkinson's disease, suggesting that dopamine plays a role in regulating the sleep-wake cycle. Here, we show that novelty-exposed hyperdopaminergic mice enter a novel awake state characterized by spectral patterns of hippocampal local field potentials that resemble electrophysiological activity observed during rapid-eye-movement (REM) sleep. Treatment with haloperidol, a D2 dopamine receptor antagonist, reduces this abnormal intrusion of REM-like activity during wakefulness. Conversely, mice acutely depleted of dopamine enter a different novel awake state characterized by spectral patterns of hippocampal local field potentials that resemble electrophysiological activity observed during slow-wave sleep (SWS). This dopamine-depleted state is marked by an apparent suppression of SWS and a complete suppression of REM sleep. Treatment with D2 (but not D1) dopamine receptor agonists recovers REM sleep in these mice. Altogether, these results indicate that dopamine regulates the generation of sleep-wake states. We propose that psychosis and the sleep disturbances experienced by Parkinsonian patients result from dopamine-mediated disturbances of REM sleep.

Resting state oscillatory brain dynamics in Parkinson’s disease: An MEG study

OBJECTIVE

The pathophysiological mechanisms of cognitive dysfunction and dementia in Parkinson's disease (PD) are still poorly understood. Altered resting state oscillatory brain activity may reflect underlying neuropathological changes. The present study using magneto encephalography (MEG) was set up to study differences in the pattern of resting state oscillatory brain activity in groups of demented and non-demented PD patients and healthy, elderly controls.

METHODS

The pattern of MEG background oscillatory activity was studied in 13 demented PD patients, 13 non-demented PD patients and 13 healthy controls. Whole head MEG recordings were obtained in the morning in an eyes closed and an eyes open, resting state condition. Relative spectral power was calculated using Fast Fourier Transformation in delta, theta, alpha, beta and gamma frequency bands.

RESULTS

In the non-demented PD patients, relative theta power was diffusely increased and beta power concomitantly decreased relative to controls. gamma Power was decreased in central and parietal channels. In the demented PD patients, a diffuse increase in relative delta and to lesser extent theta power and a decrease in relative alpha, beta and to lesser extent gamma power were found in comparison to the non-demented PD group. In addition, reactivity to eye opening was much reduced in the demented PD group.

CONCLUSIONS

Parkinson's disease is characterized by a slowing of resting state brain activity involving theta, beta and gamma frequency bands. Dementia in PD is associated with a further slowing of resting state brain activity, additionally involving delta and alpha bands, as well as a reduction in reactivity to eye-opening.

SIGNIFICANCE

The differential patterns of slowing of resting state brain activity in demented and non-demented PD patients suggests that, in conjunction with a progression of the pathological changes already present in non-demented patients, additional mechanisms are involved in the development of dementia in PD.

State-dependent alterations in hippocampal oscillations in serotonin 1A receptor-deficient mice

Mice lacking the serotonin 1A receptor (5-HT(1A)R) show increased levels of anxiety-related behavior across multiple tests and background strains. Tissue-specific rescue experiments, lesion studies, and neurophysiological findings all point toward the hippocampus as a potential mediator of the phenotype. Serotonin, acting through 5-HT(1A)Rs, can suppress hippocampal theta-frequency oscillations, suggesting that theta oscillations might be increased in the knock-outs. To test this hypothesis, local field potential recordings were obtained from the hippocampus of awake, behaving knock-outs and wild-type littermates. The magnitude of theta oscillations was increased in the knock-outs, specifically in the anxiety-provoking elevated plus maze and not in a familiar environment or during rapid eye movement sleep. Theta power correlated with the fraction of time spent in the open arms, an anxiety-related behavioral variable. These results suggest a possible role for the hippocampus, and theta oscillations in particular, in the expression of anxiety in 5-HT(1A)R-deficient mice.

Serotonergic neuron diversity: identification of raphe neurons with discharges time-locked to the hippocampal theta rhythm

The serotonergic system plays a key role in the regulation of brain states, and many of the known features of serotonergic neurons appear to match this function. Midbrain raphe nuclei provide a diffuse projection to all regions of the forebrain, and raphe neurons exhibit a slow metronome-like activity that sets the ambient levels of serotonin across the sleep-wake cycle. Serotonergic cells have also been implicated, however, in a variety of more specific functions that can hardly be related to their low-rate monotonous patterns of discharges. The amazing variety of serotonergic receptors and their type-specific distribution on cortical neurons also raise the possibility of a more intimate coordination between the activity of serotonergic neurons and their target cortical circuits. Here we report an unexpected diversity in the behavior of immunohistochemically identified serotonergic neurons. Two outstanding subpopulations were identified by using the in vivo juxtacellular recording and labeling technique. The first subpopulation of serotonergic cells exhibited the classic clock-like activity with no apparent short timescale interaction with the hippocampal electroencephalogram. The other subpopulation discharged action potentials that were phase-locked to the hippocampal theta rhythm, the oscillatory pattern associated with acquisition of information and memory formation. These results indicate that the ascending serotonergic system comprises cells involved in complex information processing beyond the regulation of state transitions. The heterogeneity of serotonergic neuron behavior can also help to explain the complexity of symptoms associated with serotonergic dysfunction.

Microinjection of procaine and electrolytic lesion in the ventral tegmental area suppresses hippocampal theta rhythm in urethane-anesthetized rats

The midbrain ventral tegmental area (VTA), a key structure of the mesocorticolimbic system is anatomically connected with the hippocampal formation. In addition mesocortical dopamine was found to influence hippocampus-related memory and hippocampal synaptic plasticity, both being linked to the theta rhythm. Therefore, the aim of the present study was to evaluate the possible role of the VTA in the regulation of the hippocampal theta activity. The study was performed on urethane-anesthetized male Wistar rats in which theta rhythm was evoked by tail pinch. It was found that unilateral, temporal inactivation of the VTA by means of direct procaine injection resulted in bilateral suppression of the hippocampal theta which manifested as a loss of synchronization of hippocampal EEG and respective reduction of the power and also the frequency of the 3-6 Hz theta band. Depression of the power of the 3-6 Hz component of the EEG signal was also seen in spontaneous hippocampal EEG after procaine. The permanent destruction of the VTA by means of unilateral electrocoagulation evoked a long-lasting, mainly ipsilateral depression of the power of the theta with some influence on its frequency. Simultaneously, there was a substantial increase of the power in higher frequency bands indicating decrease of a synchrony of the hippocampal EEG activity. On the basis of these results indicating impairment of synchronization of the hippocampal activity the VTA may be considered as another part of the brainstem theta synchroning system.

Clues to the functions of mammalian sleep

The functions of mammalian sleep remain unclear. Most theories suggest a role for non-rapid eye movement (NREM) sleep in energy conservation and in nervous system recuperation. Theories of REM sleep have suggested a role for this state in periodic brain activation during sleep, in localized recuperative processes and in emotional regulation. Across mammals, the amount and nature of sleep are correlated with age, body size and ecological variables, such as whether the animals live in a terrestrial or an aquatic environment, their diet and the safety of their sleeping site. Sleep may be an efficient time for the completion of a number of functions, but variations in sleep expression indicate that these functions may differ across species.

Stress adversely affects efficacy of physostigmine–scopolamine pretreatment against soman in guinea pigs

During military operations, soldiers may be exposed to mixtures of chemicals and to physical, emotional and psychological stress factors, which all may influence efficacy of any treatment, including the nerve agent pretreatment regimen. The purpose of this study was therefore to investigate the influence of chronic intermittent, variable, unpredictable and uncontrollable stress conditions on the side effects and therapeutic efficacy of the combination of physostigmine (0.025 mg/kg/h) and scopolamine (0.018 mg/kg/h) as a pretreatment against 2 x LD50 soman intoxication in guinea pigs. Stress during pretreatment led to an increase of motor activity and an increase of power in the EEG delta2 frequency band. After chronic stress, exposure of pretreated animals to 2 x LD50 soman resulted in more severe intoxication symptoms, a more persistent effect on the startle response, and considerable more severe and persistent effects on the EEG power-spectrum, indicating irreversible brain damage.