The contribution of the amygdala to conditioned thalamic arousal

Regulation of Dark Period Sleep by the Amygdala: A microinjection and optogenetics study

The central nucleus of the amygdala (CNA) projects to brainstem regions that generate and regulate rapid eye movement sleep (REM). We used optogenetics to assess the influence of CNA inputs into reticularis pontis oralis (RPO), pedunculopontine tegmentum (PPT) and nucleus subcoeruleus (SubC) on dark period sleep. We compared these results to effects of microinjections into CNA of the GABA_A agonist, muscimol (MUS, inhibition of cell bodies) and tetrodotoxin (TTX, inhibition of cell bodies and fibers of passage). For optogenetics, male Wistar rats received excitatory (AAV5-EF1a-DIO -hChR2(H134R)-EYFP) or inhibitory (AAV-EF1a-DIO-eNpHR3.0-EYFP; DIO-eNpHR3.0) opsins into CNA and AAV5-EF1a-mCherry-IRES-WGA-Cre into RPO, PPT, or SubC. This enabled only CNA neurons synaptically connected to each region to express opsin. Optic cannulae for light delivery into CNA and electrodes for determining sleep were implanted. Sleep was recorded with and without blue or amber light stimulation of CNA. Separate rats received MUS or TTX into CNA prior to recording sleep. Optogenetic activation of CNA neurons projecting to RPO enhanced REM and did not alter non-REM (NREM) whereas activation of CNA neurons projecting to PPT or SubC did not significantly affect sleep. Inhibition of CNA neurons projecting to any region did not significantly alter sleep. TTX inactivation of CNA decreased REM and increased NREM whereas muscimol inactivation did not significantly alter sleep. Thus, the amygdala can regulate decreases and increases in REM, and RPO is important for CNA promotion of REM. Fibers passing through CNA, likely from the basolateral nucleus of the amygdala, also play a role in regulating sleep.

EEG revealed improved vigilance regulation after stress exposure under Nx4 - A randomized, placebo-controlled, double-blind, cross-over trial

Objectives

Vigilance is characterized by alertness and sustained attention. The hyper-vigilance states are indicators of stress experience in the resting brain. Neurexan (Nx4) has been shown to modulate the neuroendocrine stress response. Here, we hypothesized that the intake of Nx4 would alter brain vigilance states at rest.

Method

In this post-hoc analysis of the NEURIM study, EEG recordings of three, 12 min resting-state conditions in 39 healthy male volunteers were examined in a randomized, placebo-controlled, double-blind, cross-over clinical trial. EEG was recorded at three resting-state sessions: at baseline (RS0), after single-dose treatment with Nx4 or placebo (RS1), and subsequently after a psychosocial stress task (RS2). During each resting-state session, each 2-s segment of the consecutive EEG epochs was classified into one of seven different brain states along a wake-sleep continuum using the VIGALL 2.1 algorithm.

Results

In the post-stress resting-state, subjects exhibited a hyper-stable vigilance regulation characterized by an increase in the mean vigilance level and by more rigidity in the higher vigilance states for a longer period of time. Importantly, Nx4-treated participants exhibited significantly lower mean vigilance level compared to placebo-treated ones. Also, Nx4- compared to placebo-treated participants spent comparably less time in higher vigilance states and more time in lower vigilance states in the post-stress resting-state.

Conclusion

Study participants showed a significantly lower mean vigilance level in the post-stress resting-state condition and tended to stay longer in lower vigilance states after treatment with Nx4. These findings support the known stress attenuation effect of Nx4.

The orchestration of conscious experience by subcortical structures

It is argued that conscious emotional feelings can not be adequately explained by just particular circuits or coherent activations within the brain, as is conventionally believed; nor by activations representing environmental stimuli going to the brain. According to the model suggested herein, the limbic system responds to sensory and other inputs according to how closely they are associated with built-in rewards or punishments. It does this by (a) activating the autonomic nervous system so that it prepares the body to acquire a reward or avoid a punishment, and (b) also activating the prefrontal cortex (PFC). The PFC activations are temporally correlated with the autonomic activations and the feedback to them, so that they become identified with the autonomic attempts to acquire (a reward) or avoid (a punishment). The PFC circuit thus acquires a valence. The valence, along with arousal in a given context, underlies conscious emotional feelings. The model is related to: (a) how attention progresses along networks within working memory; (b) how a single, unified percept is formed; (c) how both value-based and cognitive-based responses are formulated; and (d) how the stream of consciousness is put together and driven forward. These concepts are integrated into a scenario of the orchestration of conscious experience and behaviour by subcortical-limbic system structures interacting with the cortex, and are shown to be consistent with much of the literature.

Modulation of anticipatory emotion and perception processing by cognitive control

Strategies of cognitive control are helpful in reducing anxiety experienced during anticipation of unpleasant or potentially unpleasant events. We investigated the associated cerebral information processing underlying the use of a specific cognitive control strategy during the anticipation of affect-laden events. Using functional magnetic resonance imaging, we examined differential brain activity during anticipation of events of unknown and negative emotional valence in a group of eighteen healthy subjects that used a cognitive control strategy, similar to "reality checking" as used in psychotherapy, compared with a group of sixteen subjects that did not exert cognitive control. While expecting unpleasant stimuli, the "cognitive control" group showed higher activity in left medial and dorsolateral prefrontal cortex areas but reduced activity in the left extended amygdala, pulvinar/lateral geniculate nucleus and fusiform gyrus. Cognitive control during the "unknown" expectation was associated with reduced amygdalar activity as well and further with reduced insular and thalamic activity. The amygdala activations associated with cognitive control correlated negatively with the reappraisal scores of an emotion regulation questionnaire. The results indicate that cognitive control of particularly unpleasant emotions is associated with elevated prefrontal cortex activity that may serve to attenuate emotion processing in for instance amygdala, and, notably, in perception related brain areas.

Neural pathways associated with loss of consciousness caused by intracerebral microinjection of GABAA-active anesthetics

Anesthesia, slow-wave sleep, syncope, concussion and reversible coma are behavioral states characterized by loss of consciousness, slow-wave cortical electroencephalogram, and motor and sensory suppression. We identified a focal area in the rat brainstem, the mesopontine tegmental anesthesia area (MPTA), at which microinjection of pentobarbital and other GABA(A) receptor (GABA(A)-R) agonists reversibly induced an anesthesia-like state. This effect was attenuated by local pre-treatment with the GABA(A)-R antagonist bicuculline. Using neuroanatomical tracing we identified four pathways ascending from the MPTA that are positioned to mediate electroencephalographic synchronization and loss of consciousness: (i) projections to the intralaminar thalamic nuclei that, in turn, project to the cortex; (ii) projections to several pontomesencephalic, diencephalic and basal forebrain nuclei that project cortically and are considered parts of an ascending "arousal system"; (iii) a projection to other parts of the subcortical forebrain, including the septal area, hypothalamus, zona incerta and striato-pallidal system, that may indirectly affect cortical arousal and hippocampal theta rhythm; and (iv) modest projections directly to the frontal cortex. Several of these areas have prominent reciprocal projections back to the MPTA, notably the zona incerta, lateral hypothalamus and frontal cortex. We hypothesize that barbiturate anesthetics and related agents microinjected into the MPTA enhance the inhibitory response of local GABA(A)-R-bearing neurons to endogenous GABA released at baseline during wakefulness. This modulates activity in one or more of the identified ascending neural pathways, ultimately leading to loss of consciousness.

Amygdalar unit activity during three learning tasks: eyeblink classical conditioning, Pavlovian fear conditioning, and signaled avoidance conditioning

Neural activity in central and basolateral amygdala nuclei (CeA and BLA, respectively) was recorded during delay eyeblink conditioning, Pavlovian fear conditioning, and signaled barpress avoidance. During paired training, the CeA exhibited robust learning-related excitatory activity during all 3 tasks. By contrast, the BLA exhibited minimal activity during eyeblink conditioning, while demonstrating pronounced increases in learning-related excitatory responsiveness during fear conditioning and barpress avoidance. In addition, the relative amount of amygdalar activation observed appeared to be related to the relative intensity of the unconditioned stimulus and somatic requirements of the task. Results suggest the CeA mediates the Pavlovian association between sensory stimuli and the BLA mediates the modulation of instrumental responding through the assignment of motivational value to the unconditioned stimulus.

Extinction deficit and fear reinstatement after electrical stimulation of the amygdala: implications for kindling-associated fear and anxiety

Generalized seizures produced by electrical kindling of the amygdala in laboratory rats are a widely used animal model of temporal lobe epilepsy. In addition to seizure evolution amygdala kindling enhances emotionality. The relative roles of electrical stimulation and seizure induction in fear responding are unclear. Here we investigate this issue using extinction and reinstatement of fear-potentiated startle. After classical conditioning (light+footshock pairings) laboratory rats were fear extinguished with each light presentation followed by nonepileptogenic amygdala stimulation. In contrast to the normal extinction learning of control subjects, amygdala stimulated animals exhibited conditioned fear after 120 presentations of the nonreinforced conditioned stimulus (CS). In a second experiment electrical stimulation of the amygdala restored extinguished fear responding and the fear reinstatement was specific to extinction context. The reinstatement effect did not involve sensitized fear to the CS produced by amygdala stimulation. The possibility that electrical activation of the amygdala produces unconditioned fear was considered. Animals uniformly failed to demonstrate fear-potentiated startle using electrical stimulation of the amygdala as the unconditioned stimulus. This was the case with a subthreshold afterdischarge stimulus and a stimulation schedule that produced kindled seizures. The extinction deficit and fear reinstatement results were interpreted to suggest that amygdala stimulation activates acquired excitatory stimulus-affect neural connections formed during Pavlovian fear conditioning. Our data supports a model in which excitation of an amygdala-based memory-retrieval system reinforces the expression of learned fear behaviors.

Evidence for tinnitus-related plasticity in the auditory and limbic system, demonstrated by arg3.1 and c-fos immunocytochemistry

Distributions of arg3.1 and c-fos immunoreactive neurons (IRN) in gerbil auditory cortex (AC) and amygdala showed characteristic differences when comparing systemic application of the tinnitus-eliciting drug salicylate with acoustic stimulation or saline injections. In AC, arg3.1 IRN induced by stimulation focused in regions corresponding to the frequency content of the stimulus. Injections of salicylate (350 mg/kg body weight) led to accumulation of arg3.1 IRN in the high frequency domain, while saline injections produced a diffuse distribution. After all treatments, c-fos IRN outnumbered arg3.1 IRN in AC and showed a broad distribution. In subcortical auditory structures arg3.1 IRN were absent in all but one brain. In ventral cochlear nucleus, c-fos IRN were always found after stimulation and often also after saline injections, whereas none were present when injecting salicylate. Similarly, in inferior colliculus, numbers of c-fos IRN were lowest after salicylate injections. In the amygdala, c-fos and arg3.1 IRN were increased substantially after salicylate injections compared to auditory stimulation or saline injections. In particular in its central nucleus, c-fos and arg3.1 IRN were found exclusively after the tinnitus-inducing treatment, suggesting that coactivation of the AC and the amygdala may by an essential feature of tinnitus-related activation.

Early amygdala reaction to fear spreading in occipital, temporal, and frontal cortex: a depth electrode ERP study in human

The amygdala involvement in fear processing has been reported in behavioral, electrophysiological, and functional imaging studies. However, the literature does not provide precise data on the temporal course of facial emotional processing. Intracranial event-related potentials to facial expressions were recorded in epileptic patients implanted with depth electrodes during a presurgical evaluation. Specific potentials to fear beginning 200 ms poststimulus were observed in amygdala, both individually in two patients and in a ten patient population study. These potentials occurred 100 ms earlier than potentials to disgust recorded in insula in a previous study. Potentials to fear were confined in amygdala during a first transient period and then, during a second period of sustained activity, spread to occipito-temporal, anterior temporal, and orbitofrontal cortex in two patients. This study clarifies the temporal course of the involvement of these structures known to be part of a neural network recruited to process emotional information.

Stabilization of thalamo-cortical long-term potentiation by the amygdala: cholinergic and transcription-dependent mechanisms

Synaptic potentiation allows neurons to enhance excitability and store information for extended time periods. We examined the role of the amygdaloid complex, known to facilitate long-term memory encoding, to influence synaptic strength at thalamo-cortical synapses. In urethane-anaesthetized rats, theta-burst stimulation of the dorsal lateral geniculate nucleus of the thalamus induced early phase (1-2 h) long-term potentiation (LTP) of the field postsynaptic potential (fPSP) recorded in the ipsilateral primary visual cortex. Electrical stimulation (100 Hz) of the amygdala 5 min after thalamic stimulation converted early phase LTP to stable late-phase (> 4 h) LTP. This effect was not correlated with the degree of electrocorticographic activation of V1 induced by amygdala stimulation. Amygdala stimulation without thalamic theta-burst stimulation did not change thalamo-cortical fPSPs. The centrally acting cholinergic-muscarinic receptor antagonist scopolamine (1 mg/kg, i.p.), but not peripherally acting methyl-scopolamine, completely blocked the amygdala-induced conversion of early to late-phase thalamo-cortical LTP. Further, ventricular application of the transcription inhibitor anisomycin (250 micro g) reduced amygdala-induced late-phase LTP induction. These results demonstrate that the amygdaloid complex transforms time-limited synaptic enhancement of thalamo-cortical transmission into long lasting increases in synaptic strength. These processes are mediated, at least in part, by cholinergic and transcription-dependent mechanisms. These amygdaloid-induced effects provide a potential mechanism underlying long-term enhancement of sensory transmission and information encoding in thalamo-cortical networks.

Two routes to emotional memory: distinct neural processes for valence and arousal

Prior investigations have demonstrated that emotional information is often better remembered than neutral information, but they have not directly contrasted effects attributable to valence and those attributable to arousal. By using functional MRI and behavioral studies, we found that distinct cognitive and neural processes contribute to emotional memory enhancement for arousing information versus valenced, nonarousing information. The former depended on an amygdalar-hippocampal network, whereas the latter was supported by a prefrontal cortex-hippocampal network implicated in controlled encoding processes. A behavioral companion study, with a divided-attention paradigm, confirmed that memory enhancement for valenced, nonarousing words relied on controlled encoding processes: concurrent task performance reduced the enhancement effect. Enhancement for arousing words occurred automatically, even when encoding resources were diverted to the secondary task.

Anxiolytic activity of the MGLU2/3 receptor agonist LY354740 on the elevated plus maze is associated with the suppression of stress-induced c-Fos in the hippocampus and increases in c-Fos induction in several other stress-sensitive brain regions

LY354740 is a potent and selective agonist for group II metabotropic glutamate (mGlu) receptors, mGlu2 and mGlu3 receptors, with anxiolytic activity in several animal models of anxiety, including the elevated plus maze (EPM) test. Here, we studied neuronal activation in mouse brain after EPM exposure in saline- and LY354740-treated mice using c-Fos immunoreactivity as a marker. The effect of LY354740 on c-Fos expression was also studied in cage control (no EPM) mice. Pretreatment with LY354740 (20 mg/kg, s.c.) produced robust anxiolytic behavior on the EPM. LY354740 administration decreased EPM-induced increases in c-Fos expression in the CA3 of the hippocampus, while having no significant effects on basal c-Fos expression in the hippocampus. LY354740 administration significantly increased c-Fos expression in specific limbic regions, including the lateral division of the central nucleus of the amygdala (CeL), lateral parabrachial nucleus, locus coeruleus, and Edinger-Westphal nucleus, whether or not animals were exposed to the EPM. Moreover, LY354740 administration per se significantly increased c-Fos expression in regions processing sensory information, including the paraventricular and lateral geniculate nucleus of the thalamus as well as the nucleus of the optic tract and superior colliculus. In particular, the suppression of fear-evoked neuronal activity in the hippocampus and drug-induced increases in neuronal activation in the CeL have been previously linked to the anxiolytic effects of clinically effective drugs such as benzodiazepines, and thus may contribute to anxiolytic actions of LY354740 in animal models and human anxiety patients.