Power spectral analysis of electroencephalographic desynchronization induced by cocaine in rats: correlation with evaluation of noradrenergic neurotransmission at the medial prefrontal cortex

Neural Oscillations: Understanding a Neural Code of Pain

Neural oscillations play an important role in the integration and segregation of brain regions that are important for brain functions, including pain. Disturbances in oscillatory activity are associated with several disease states, including chronic pain. Studies of neural oscillations related to pain have identified several functional bands, especially alpha, beta, and gamma bands, implicated in nociceptive processing. In this review, we introduce several properties of neural oscillations that are important to understand the role of brain oscillations in nociceptive processing. We also discuss the role of neural oscillations in the maintenance of efficient communication in the brain. Finally, we discuss the role of neural oscillations in healthy and chronic pain nociceptive processing. These data and concepts illustrate the key role of regional and interregional neural oscillations in nociceptive processing underlying acute and chronic pains.

Reorganization of theta phase-locking in the orbitofrontal cortex drives cocaine choice under the influence

Cortical theta oscillations of neuronal activity are a fundamental mechanism driving goal-directed behavior. We previously identified in the rat orbitofrontal cortex (OFC) a neuronal correlate of individual preferences between cocaine use and an alternative nondrug reward (i.e. saccharin). Whether theta oscillations are also associated with choice behavior between a drug and a nondrug reward remains unknown. Here we investigated the temporal structure between single unit activity and theta band oscillations (4-12 Hz) in the OFC of rats choosing between cocaine and saccharin. First, we found that the relative amplitude of theta oscillations is associated with subjective value and preference between two rewards. Second, OFC phase-locked neurons fired on opposite phase of the theta oscillation during saccharin and cocaine rewards, suggesting the existence of two separable neuronal assemblies. Finally, the pharmacological influence of cocaine at the moment of choice altered both theta band power and theta phase-locking in the OFC. That is, this drug influence shifted spike-phase relative to theta cycle and decreased the synchronization of OFC neurons relative to the theta oscillation. Overall, this study indicates that the reorganization of theta phase-locking under the influence of cocaine biases OFC neuronal assemblies in favor of cocaine choice and at the expense of a normally preferred alternative, a neuronal change that may contribute to drug preference in cocaine addiction.

Task-positive and task-negative networks in major depressive disorder: A combined fMRI and EEG study

BACKGROUND

The study of intrinsic connectivity networks, i.e., sets of brain regions that show a high degree of interconnectedness even in the absence of a task, showed that major depressive disorder (MDD) patients demonstrate an increased connectivity within the default mode network (DMN), which is active in a resting state and is implicated in self-referential processing, and a decreased connectivity in task-positive networks (TPNs), which increase their activity in attention tasks. Cortical localization of this 'dominance' of the DMN over the TPN in MDD patients is not fully understood. Besides, this effect has been investigated using fMRI and its electrophysiological underpinning is not known.

METHOD

In this study, we tested the dominance hypothesis using seed-based connectivity analysis of resting-state fMRI and EEG data obtained in 41 MDD patients and 23 controls.

RESULTS

In MDD patients, as compared to controls, insula, pallidum/putamen, amygdala, and left dorso- and ventrolateral prefrontal cortex are more strongly connected with DMN than with TPN seeds. In EEG, all significant effects were obtained in the delta frequency band.

LIMITATIONS

fMRI and EEG data were not obtained simultaneously during the same session.

CONCLUSIONS

In MDD patients, major emotion and attention regulation circuits are more strongly connected with DMN than with TPN implying they are more prepared to respond to internally generated self-related thoughts than to environmental challenges.

Pleasure: The missing link in the regulation of sleep

Although largely unrecognized by sleep scholars, sleeping is a pleasure. This report aims first, to fill the gap: sleep, like food, water and sex, is a primary reinforcer. The levels of extracellular mesolimbic dopamine show circadian oscillations and mark the "wanting" for pro-homeostatic stimuli. Further, the dopamine levels decrease during waking and are replenished during sleep, in opposition to sleep propensity. The wanting of sleep, therefore, may explain the homeostatic and circadian regulation of sleep. Accordingly, sleep onset occurs when the displeasure of excessive waking is maximal, coinciding with the minimal levels of mesolimbic dopamine. Reciprocally, sleep ends after having replenished the limbic dopamine levels. Given the direct relation between waking and mesolimbic dopamine, sleep must serve primarily to gain an efficient waking. Pleasant sleep (i.e. emotional sleep), can only exist in animals capable of feeling emotions. Therefore, although sleep-like states have been described in invertebrates and primitive vertebrates, the association sleep-pleasure clearly marks a difference between the sleep of homeothermic vertebrates and cool blooded animals.

Psychoacoustic tinnitus loudness and tinnitus-related distress show different associations with oscillatory brain activity

Background

The phantom auditory perception of subjective tinnitus is associated with aberrant brain activity as evidenced by magneto- and electroencephalographic studies. We tested the hypotheses (1) that psychoacoustically measured tinnitus loudness is related to gamma oscillatory band power, and (2) that tinnitus loudness and tinnitus-related distress are related to distinct brain activity patterns as suggested by the distinction between loudness and distress experienced by tinnitus patients. Furthermore, we explored (3) how hearing impairment, minimum masking level, and (4) psychological comorbidities are related to spontaneous oscillatory brain activity in tinnitus patients.

Methods and Findings

Resting state oscillatory brain activity recorded electroencephalographically from 46 male tinnitus patients showed a positive correlation between gamma band oscillations and psychoacoustic tinnitus loudness determined with the reconstructed tinnitus sound, but not with the other psychoacoustic loudness measures that were used. Tinnitus-related distress did also correlate with delta band activity, but at electrode positions different from those associated with tinnitus loudness. Furthermore, highly distressed tinnitus patients exhibited a higher level of theta band activity. Moreover, mean hearing loss between 0.125 kHz and 16 kHz was associated with a decrease in gamma activity, whereas minimum masking levels correlated positively with delta band power. In contrast, psychological comorbidities did not express significant correlations with oscillatory brain activity.

Conclusion

Different clinically relevant tinnitus characteristics show distinctive associations with spontaneous brain oscillatory power. Results support hypothesis (1), but exclusively for the tinnitus loudness derived from matching to the reconstructed tinnitus sound. This suggests to preferably use the reconstructed tinnitus spectrum to determine psychoacoustic tinnitus loudness. Results also support hypothesis (2). Moreover, hearing loss and minimum masking level correlate with oscillatory power in distinctive frequency bands. The lack of an association between psychological comorbidities and oscillatory power may be attributed to the overall low level of mental health problems in the present sample.

EEG delta oscillations as a correlate of basic homeostatic and motivational processes

Functional significance of delta oscillations is not fully understood. One way to approach this question would be from an evolutionary perspective. Delta oscillations dominate the EEG of waking reptiles. In humans, they are prominent only in early developmental stages and during slow-wave sleep. Increase of delta power has been documented in a wide array of developmental disorders and pathological conditions. Considerable evidence on the association between delta waves and autonomic and metabolic processes hints that they may be involved in integration of cerebral activity with homeostatic processes. Much evidence suggests the involvement of delta oscillations in motivation. They increase during hunger, sexual arousal, and in substance users. They also increase during panic attacks and sustained pain. In cognitive domain, they are implicated in attention, salience detection, and subliminal perception. This evidence shows that delta oscillations are associated with evolutionary old basic processes, which in waking adults are overshadowed by more advanced processes associated with higher frequency oscillations. The former processes rise in activity, however, when the latter are dysfunctional.

The role of the nucleus accumbens and rostral anterior cingulate cortex in anhedonia: integration of resting EEG, fMRI, and volumetric techniques

Anhedonia, the reduced propensity to experience pleasure, is a promising endophenotype and vulnerability factor for several psychiatric disorders, including depression and schizophrenia. In the present study, we used resting electroencephalography, functional magnetic resonance imaging, and volumetric analyses to probe putative associations between anhedonia and individual differences in key nodes of the brain's reward system in a non-clinical sample. We found that anhedonia, but not other symptoms of depression or anxiety, was correlated with reduced nucleus accumbens (NAcc) responses to rewards (gains in a monetary incentive delay task), reduced NAcc volume, and increased resting delta current density (i.e., decreased resting activity) in the rostral anterior cingulate cortex (rACC), an area previously implicated in positive subjective experience. In addition, NAcc reward responses were inversely associated with rACC resting delta activity, supporting the hypothesis that delta might be lawfully related to activity within the brain's reward circuit. Taken together, these results help elucidate the neural basis of anhedonia and strengthen the argument for anhedonia as an endophenotype for depression.

Motivation, emotion, and their inhibitory control mirrored in brain oscillations

Recent studies suggest brain oscillations as a mechanism for cerebral integration. Such integration can exist across a number of functional domains, with different frequency rhythms associated with each domain. Here, evidence is summarized which shows that delta oscillations depend on activity of motivational systems and participate in salience detection. Theta oscillations are involved in memory and emotional regulation. Alpha oscillations participate in inhibitory processes which contribute to a variety of cognitive operations such as attention and memory. The importance of inhibitory functions associated with alpha oscillations increases during the course of evolution. In ontogenesis, these functions develop later and may be more sensitive to a variety of detrimental environmental influences. In a number of developmental stages and pathological conditions, a deficient alpha and/or increased slow-wave activity are associated with cognitive deficits and a lack of inhibitory control. It is shown that slow-wave and alpha oscillations are reciprocally related to each other. This reciprocal relationship may reflect an inhibitory control over motivational and emotional drives which is implemented by the prefrontal cortex.

Locus ceruleus control of slow-wave homeostasis

Sleep intensity is regulated by the duration of previous wakefulness, suggesting that waking results in the progressive accumulation of sleep need (Borbely and Achermann, 2000). In mammals, sleep intensity is reflected by slow-wave activity (SWA) in the nonrapid eye movement (NREM) sleep electroencephalogram, which increases in proportion to the time spent awake. However, the mechanisms responsible for the increase of NREM SWA after wakefulness remain unclear. According to a recent hypothesis (Tononi and Cirelli, 2003), the increase in SWA occurs because during wakefulness, many cortical circuits undergo synaptic potentiation, as evidenced by the widespread induction of long-term potentiation (LTP)-related genes in the brain of awake animals. A direct prediction of this hypothesis is that manipulations interfering with the induction of LTP-related genes should result in a blunted SWA response. Here, we examined SWA response in rats in which cortical norepinephrine (NA) was depleted, a manipulation that greatly reduces the induction of LTP-related genes during wakefulness (Cirelli and Tononi, 2004). We found that the homeostatic response of the lower-range SWA was markedly and specifically reduced after NA depletion. These data suggest that the wake-dependent accumulation of sleep need is causally related to cellular changes dependent on NA release, such as the induction of LTP-related genes, and support the hypothesis that sleep SWA homeostasis may be related to synaptic potentiation during wakefulness.

Cocaine administration produces a protracted decoupling of neural and haemodynamic responses to intense sensory stimuli

Evidence suggests that for relatively weak sensory stimuli, cocaine elevates background haemodynamic parameters but still allows enhanced neural responses to be reflected in enhanced haemodynamic responses. The current study investigated the possibility that for more intense stimuli, the raised background may produce a protracted attenuation of the haemodynamic response. Three experiments were performed to measure effects of i.v. cocaine administration (0.5 mg/kg) or saline on responses in rat barrel cortex to electrical stimulation of the whisker pad. The first experiment used optical imaging spectroscopy (OIS) and laser Doppler flowmetry (LDF) to measure haemodynamic changes. Cocaine caused an increase in baseline blood flow (peak approximately 90%), which lasted for the duration of the test period (25 min). Haemodynamic responses to whisker stimulation were substantially reduced throughout. The second experiment used a 16-channel multi-electrode to measure evoked potentials at 100 mum intervals through the barrel cortex. Summed neural responses (collapsed across the spatial dimension) after cocaine administration were similar to those after saline. The third experiment extended experiment 1 by examining the effects of cocaine on whisker sensory responses using functional magnetic resonance imaging (and concurrent OIS or LDF). Cocaine caused a similar increase in baseline and reduction in the evoked response to that seen in experiment 1. Together, the results of these three experiments show that cocaine produces a protracted decoupling of neural activity and haemodynamic responses to intense sensory stimulation, which suggests that imaging techniques based on changes in haemodynamic parameters may be unsuitable for studying the effects of cocaine on sensory processing in humans.

Behavioral correlates of activity in identified hypocretin/orexin neurons

Micropipette recording with juxtacellular Neurobiotin ejection, linked micropipette-microwire recording, and antidromic and orthodromic activation from the ventral tegmental area and locus coeruleus were used to identify hypocretin (Hcrt) cells in anesthetized rats and develop criteria for identification of these cells in unanesthetized, unrestrained animals. We found that Hcrt cells have broad action potentials with elongated later positive deflections that distinguish them from adjacent antidromically identified cells. They are relatively inactive in quiet waking but are transiently activated during sensory stimulation. Hcrt cells are silent in slow wave sleep and tonic periods of REM sleep, with occasional burst discharge in phasic REM. Hcrt cells discharge in active waking and have moderate and approximately equal levels of activity during grooming and eating and maximal activity during exploratory behavior. Our findings suggest that these cells are activated during emotional and sensorimotor conditions similar to those that trigger cataplexy in narcoleptic animals.

The role of norepinephrine in epilepsy: from the bench to the bedside

This article provides a brief review of the role of norepinephrine (NE) in epilepsy, starting from early studies reproducing the kindling model in NE-lesioned rats, through the use of specific ligands for adrenergic receptors in experimental models of epilepsy, up to recent advances obtained by using transgenic and knock-out mice for specific genes expressed in the NE system. Data obtained from multiple experimental models converge to demonstrate the antiepileptic role of endogenous NE. This effect predominantly consists in counteracting the development of an epileptic circuit (such as in the kindling model) rather than increasing the epileptic threshold. This suggests that NE activity is critical in modifying epilepsy-induced neuronal changes especially on the limbic system. These data encompass from experimental models to clinical applications as recently evidenced by the need of an intact NE innervation for the antiepileptic mechanisms of vagal nerve stimulation (VNS) in patients suffering from refractory epilepsy. Finally, recent data demonstrate that NE loss increases neuronal damage following focally induced limbic status epilepticus, confirming a protective effect of brain NE, which has already been shown in other neurological disorders.

Effect of noradrenergic denervation of medial prefrontal cortex and dentate gyrus on recovery after sleep deprivation in the rat

The noradrenergic-locus coeruleus (LC) system has a regulatory influence on forebrain neuronal networks. We have previously shown that the amygdala is strongly implicated in the mechanism of rebound seen after a 10 h sleep deprivation (SD). In the present study, our objective was to determine whether the medial prefrontal cortex and dentate gyrus (DG) which receive an important innervation from the LC, play a role in the rebound mechanisms. We found that microinjection of the specific noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, into these regions had no effect on the increase in paradoxical sleep duration seen after SD, suggesting that noradrenergic (NA) innervation of the prefrontal cortex and DG are not involved in sleep rebound regulation.

Effects of Huperzine used as pre-treatment against soman-induced seizures

Huperzine A (HUP), an alkaloid isolated from the Chinese club moss, Huperzia serrata is a reversible inhibitor of cholinesterases which crosses the blood-brain barrier and shows high specificity for acetylcholinesterase (AChE) and a prolonged biological half-life. We tested, in vivo, its efficiency in protecting cortical AChE from soman inhibition and preventing subsequent seizures. The release of acetylcholine (ACh) was also followed in the cortex of freely moving rats using microdialysis techniques. We previously found that soman-induced seizures occurred in rodents only when the cortical AChE inhibition was over 65% and when the increase of ACh level was over 200 times the baseline level. This was verified in the present study in control animals intoxicated by 1 LD50 of soman (90 microg/kg). Using the same dose of soman in rats pre-treated with 500 microg/kg of HUP, we observed that 93% of the animals survived and none of them had seizures. This dose of HUP reduced AChE inhibition to 54% and increase of ACh level to 230 times baseline value. HUP thus appears as a promising compound to protect subjects against organophosphorus intoxication.

Hippocampal noradrenergic neurotransmission in concurrent EEG desynchronization and inhibition of penile erection induced by cocaine in the rat

We previously reported that cocaine may induce activation of cortical (cEEG) and hippocampal (hEEG) electroencephalographic signals, concurrent with inhibition of penile erection, via an action on the hippocampal formation. The present study further evaluates the role of noradrenergic neurotransmission at the hippocampal formation in this process, using adult, male Sprague-Dawley rats anaesthetized and maintained by chloral hydrate. Unilateral microinjection of cocaine (100 nmoles) into the hippocampal CA1 or CA3 subfield or dentate gyrus elicited significant activation of both cEEG and hEEG activity. At the same time, the intracavernous pressure (ICP), our experimental index for penile erection, underwent a discernible reduction. Co-administration of equimolar doses (250 pmoles) of prazosin, naftopidil, yohimbine or rauwolscine significantly reversed those effects elicited by cocaine on cEEG, hEEG and ICP. Microinjection unilaterally of equimolar doses (5 nmoles) of norepinephrine, phenylephrine or BHT 933 into the hippocampal formation, similar to cocaine, also induced appreciable cEEG and hEEG excitation, with a simultaneous decrease in ICP. We conclude that cocaine may activate cEEG and hEEG and decrease ICP via noradrenergic neurotransmission, possibly engaging at least alpha(1A/D)-, alpha(2B)- and alpha(2C)-adrenoceptors at the hippocampal formation.

Relationship between cortical electrical and cardiac autonomic activities in the awake lizard, Gallotia galloti

ECG and EEG signals were simultaneously recorded in lizards, Gallotia galloti, both in control conditions and under autonomic nervous system (ANS) blockade, in order to evaluate possible relationships between the ANS control of heart rate and the integrated central nervous system activity in reptiles. The ANS blockers used were prazosin, propranolol, and atropine. Time-domain summary statistics were derived from the series of consecutive R-R intervals (RRI) of the ECG to measure beat-to-beat heart rate variability (HRV), and spectral analysis techniques were applied to the EEG activity to assess its frequency content. Both prazosin and atropine did not alter the power spectral density (PSD) of the EEG low frequency (LF: 0.5-7.5 Hz) and high frequency (HF: 7.6-30 Hz) bands, whereas propranolol decreased the PSD in these bands. These findings suggest that central beta-adrenergic receptor mechanisms could mediate the reptilian waking EEG activity without taking part any alpha(1)-adrenergic and/or cholinergic receptor systems. In 55% of the lizards in control conditions, and in approximately 43% of the lizards under prazosin and atropine, a negative correlation between the coefficient of variation of the series of RRI value (CV(RRI)) and the mean power frequency (MPF) of the EEG spectra was found, but not under propranolol. Consequently, the lizards' HRV-EEG-activity relationship appears to be independent of alpha(1)-adrenergic and cholinergic receptor systems and mediated by beta-adrenergic receptor mechanisms.

Differential participation of hippocampal formation in cocaine-induced cortical electroencephalographic desynchronization and penile erection in the rat

We evaluated the role of the hippocampal formation in cocaine-induced cortical electroencephalographic (EEG) desynchronization and penile erection. Adult, male Sprague-Dawley rats anesthetized and maintained by chloral hydrate were used. Intravenous (1.5 or 3.0 mg/kg) administration of cocaine dose-dependently increased intracavernous pressure (ICP), our experimental index for penile erection. This was accompanied by desynchronization of EEG activity recorded from the somatosensory cortex (cEEG), as represented by a decrease in root mean square (RMS) and an increase in mean power frequency (MPF) values. There was a simultaneous increase in the RMS values, without significant changes in the MPF values of EEG signals recorded from the hippocampal formation (hEEG). In animals that received prior application of 10% xylocaine either intrathecally (i.t.) at the L6-S1 spinal levels or directly into the bilateral hippocampal formation, the RMS values of both cEEG and hEEG signals induced by cocaine (1.5 or 3.0 mg/kg, i.v.) were appreciably reduced, along with a further increase in ICP. Unilateral microinjection of cocaine (15 or 30 microg) into the hippocampal formation elicited discernible excitation of both cEEG and hEEG signals. Intriguingly, the ICP underwent a significant and dose-dependent reduction, which was discernibly antagonized by i.t. application of xylocaine. We conclude that cocaine may effect cortical EEG desynchronization but cause a reduction in ICP via an action on the hippocampal formation.

Participation of hippocampal formation in negative feedback inhibition of penile erection in the rat

Detailed information on how the central nervous system regulates penile erection, particularly the inhibitory aspect, is sparse. We observed in Sprague-Dawley rats anesthetized and maintained with chloral hydrate that administration of papaverine (400 microg) directly into the corpora cavernosum of the penis produced an increase in intracavernous pressure (ICP). This elicited experimental index for penile erection was accompanied by a transient increase in the root mean square values, concurrent with a shift in the contribution of Theta (increase) and delta (decrease) power to the hippocampal electroencephalographic (hEEG) activity. Reversal blockade of these hEEG responses with xylocaine, given either intrathecally at the L6-S1 spinal levels or unilaterally to the hippocampal formation, significantly heightened and prolonged the ICP response. Pretreatment with xylocaine by itself, however, did not alter appreciably the baseline ICP or hEEG activity. These results suggest the presence of a novel negative feedback inhibitory mechanism in the hippocampal formation, which is triggered by ascending sensory inputs initiated by tumescence of the penis during normal erectile processes.

Nitric oxide as a mediator of cocaine-induced penile erection in the rat

1. The effect of local application of cocaine to the corpus cavernosum on intracavernous pressure (ICP), an experimental index for penile erection, was examined in Sprague-Dawley rats anaesthetized with chloral hydrate. The potential involvement of dopamine, noradrenaline or nitric oxide as the chemical mediator in this process, and the pharmacological action of cocaine as a local anaesthetic in the induced increase in ICP, were also investigated. 2. Intracavernous (i.c.) administration of cocaine (40, 80 or 160 micrograms) to the corpus cavernosum resulted in a dose-related increase in both amplitude and duration of ICP. 3. The elevation of ICP induced by cocaine (160 micrograms, i.c.) was not significantly influenced by prior injection into the corpus cavernosum of either the D1 or D2 dopamine receptor antagonist, R-(+)-SCH 22390 (250 pmol) or (-)-sulpiride (250 pmol). 4. Similarly, penile erection promoted by cocaine (160 micrograms, i.c.) was not appreciably affected by i.c. pretreatment with the alpha 1-, alpha 2-, or beta-adrenoceptor antagonist, prazosin (50 pmol), yohimbine (50 pmol) or propranolol (5 nmol). 5. Whereas lignocaine (4 mumol, i.c.) depressed penile erection induced by papaverine (400 micrograms, i.c.), local application of cocaine (160 micrograms) into the corpus cavernosum still elicited significant elevation in ICP in the presence of lignocaine or papaverine. 6. The increase in ICP induced by cocaine (160 micrograms, i.c.) was attenuated dose-dependently by prior cavernosal administration of the NO synthase inhibitor, N omega-nitro-L -arginine methyl ester (L-NAME, 0.5, 1 or 5 pmol) or NG-monomethyl-L-arginine (L-NMMA, 2.5, 5 or 10 pmol). The blunting effect of L-NAME or L-NMMA was reversed by co-administration of the NO precursor, L-arginine (1 nmol, i.c.). 7. Pretreatment by local application into the corpus cavernosum of methylene blue (2.5 mumol), an inhibitor of cytosolic guanylyl cyclase, antagonized cocaine-induced penile erection. 8. Direct i.c. administration of a NO donor, nitroglycerin (10 or 20 nmol), mimicked the local action of cocaine by promoting a significant increase in ICP. 9. It is concluded that cocaine may induce penile erection by increasing ICP via a local action on the corpus cavernosum. This process did not appear to involve either dopamine or noradrenaline as the chemical mediator, nor the pharmacological action of cocaine as a local anaesthetic. On the other hand, it is likely that initiation and maintenance of penile erection elicited by cavernosal application of cocaine engaged an active participation of NO and subsequent activation of guanylyl cyclase in the corpus cavernosum.