Activation of fos in mouse amygdala by local infusion of norepinephrine or atipamezole

Dexmedetomidine attenuates sepsis-associated inflammation and encephalopathy via central α2A adrenoceptor

Sepsis-associated encephalopathy (SAE) is a significant clinical issue that is associated with increased mortality and cost of health care. Dexmedetomidine, an α2 adrenoceptor agonist that is used to provide sedation, has been shown to induce neuroprotection under various conditions. This study was designed to determine whether dexmedetomidine protects against SAE and whether α2 adrenoceptor plays a role in this protection. Six- to eight-week old CD-1 male mice were subjected to cecal ligation and puncture (CLP). They were treated with intraperitoneal injection of dexmedetomidine in the presence or absence of α2 adrenoceptor antagonists, atipamezole or yohimbine, or an α2A adrenoceptor antagonist, BRL-44408. Hippocampus and blood were harvested for measuring cytokines. Mice were subjected to Barnes maze and fear conditioning 14 days after CLP to evaluate their learning and memory. CLP significantly increased the proinflammatory cytokines including tumor necrosis factor α, interleukin (IL)-6 and IL-1β in the blood and hippocampus. CLP also increased the permeability of blood-brain barrier (BBB) and impaired learning and memory. These CLP detrimental effects were attenuated by dexmedetomidine. Intracerebroventricular application of atipamezole, yohimbine or BRL-44408 blocked the protection of dexmedetomidine on the brain but not on the systemic inflammation. Astrocytes but not microglia expressed α2A adrenoceptors. Microglial depletion did not abolish the protective effects of dexmedetomidine. These results suggest that dexmedetomidine reduces systemic inflammation, neuroinflammation, injury of BBB and cognitive dysfunction in septic mice. The protective effects of dexmedetomidine on the brain may be mediated by α2A adrenoceptors in the astrocytes.

Localization of the delta opioid receptor and corticotropin-releasing factor in the amygdalar complex: role in anxiety

It is well established that central nervous system norepinephrine (NE) and corticotropin-releasing factor (CRF) systems are important mediators of behavioral responses to stressors. More recent studies have defined a role for delta opioid receptors (DOPR) in maintaining emotional valence including anxiety. The amygdala plays an important role in processing emotional stimuli, and has been implicated in the development of anxiety disorders. Activation of DOPR or inhibition of CRF in the amygdala reduces baseline and stress-induced anxiety-like responses. It is not known whether CRF- and DOPR-containing amygdalar neurons interact or whether they are regulated by NE afferents. Therefore, this study sought to better define interactions between the CRF, DOPR and NE systems in the basolateral (BLA) and central nucleus of the amygdala (CeA) of the male rat using anatomical and functional approaches. Irrespective of the amygdalar subregion, dual immunofluorescence microscopy showed that DOPR was present in CRF-containing neurons. Immunoelectron microscopy confirmed that DOPR was localized to both dendritic processes and axon terminals in the BLA and CeA. Semi-quantitative dual immunoelectron microscopy analysis of gold-silver labeling for DOPR and immunoperoxidase labeling for CRF revealed that 55 % of the CRF neurons analyzed contained DOPR in the BLA while 67 % of the CRF neurons analyzed contained DOPR in the CeA. Furthermore, approximately 41 % of DOPR-labeled axon terminals targeted BLA neurons that expressed CRF while 29 % of DOPR-labeled axon terminals targeted CeA neurons that expressed CRF. Triple label immunofluorescence microscopy revealed that DOPR and CRF were co-localized in common cellular profiles that were in close proximity to NE-containing fibers in both subregions. These anatomical results indicate significant interactions between DOPR and CRF in this critical limbic region and reveal that NE is poised to regulate these peptidergic systems in the amygdala. Functional studies were performed to determine if activation of DOPR could inhibit the anxiety produced by elevation of NE in the amygdala using the pharmacological stressor yohimbine. Administration of the DOPR agonist, SNC80, significantly attenuated elevated anxiogenic behaviors produced by yohimbine as measured in the rat on the elevated zero maze. Taken together, results from this study demonstrate the convergence of three important systems, NE, CRF, and DOPR, in the amygdala and provide insight into their functional role in modulating stress and anxiety responses.

Chemical stimulation or glutamate injections in the nucleus of solitary tract enhance conditioned taste aversion

Taste memory depends on motivational and post-ingestional consequences after a single taste-illness pairing. During conditioned taste aversion (CTA), the taste and visceral pathways reach the nucleus of the solitary tract (NTS), which is the first relay in the CNS and has a vital function in receiving vagal chemical stimuli and humoral signals from the area postrema that receives peripheral inputs also via vagal afferent fibers. The specific aim of the present set of experiments was to determine if the NTS is involved in the noradrenergic and glutamatergic activation of the basolateral amygdala (BLA) during CTA. Using in vivo microdialysis, we examined whether chemical NTS stimulation induces norepinephrine (NE) and/or glutamate changes in the BLA during visceral stimulation with intraperitoneal (i.p.) injections of low (0.08 M) and high (0.3 M) concentrations of lithium chloride (LiCl) during CTA training. The results showed that strength of CTA can be elicited by chemical NTS stimulation (Ringer's high potassium solution; 110 mM KCl) and by intra-NTS microinjections of glutamate, immediately after, but not before, low LiCl i.p. injections that only induce a week aversive memory. However visceral stimulation (with low or high i.p. LiCl) did not induce significantly more NE release in the amygdala compared with the NE increment induced by NTS potassium depolarization. In contrast, high i.p. concentrations of LiCl and chemical NTS stimulation induced a modest glutamate sustained release, that it is not observed with low LiCl i.p. injections. These results indicate that the NTS mainly mediates the visceral stimulus processing by sustained releasing glutamate in the BLA, but not by directly modulating NE release in the BLA during CTA acquisition, providing new evidence that the NTS has an important function in the transmission of signals from the periphery to brain systems that process aversive memory formation.

Direct targeting of peptidergic amygdalar neurons by noradrenergic afferents: linking stress-integrative circuitry

Amygdalar norepinephrine (NE) plays a key role in regulating neural responses to emotionally arousing stimuli and is involved in memory consolidation of emotionally charged events. Corticotropin-releasing factor (CRF) and dynorphin (DYN), two neuropeptides that mediate the physiological and behavioral responses to stress, are abundant in the central nucleus of the amygdala (CeA), and directly innervate brainstem noradrenergic locus coeruleus (LC) neurons. Whether the CRF- and DYN-containing amygdalar neurons receive direct noradrenergic innervation has not yet been elucidated. The present study sought to define cellular substrates underlying noradrenergic modulation of CRF- and DYN-containing neurons in the CeA using immunohistochemistry and electron microscopy. Ultrastructural analysis revealed that NE-labeled axon terminals form synapses with CRF- and DYN-containing neurons in the CeA. Semi-quantitative analysis showed that approximately 31 % of NET-labeled axon terminals targeted CeA neurons that co-expressed DYN and CRF. As a major source of CRF innervation to the LC, it is also not known whether CRF-containing CeA neurons are directly targeted by noradrenergic afferents. To test this, retrograde tract tracing using FluoroGold from the LC was combined with immunocytochemical detection of CRF and NET in the CeA. Our results revealed a population of LC-projecting CRF-containing CeA neurons that are directly innervated by NE afferents. Analysis showed that approximately 34 % of NET-labeled axon terminals targeted LC-projecting CeA neurons that contain CRF. Taken together, these results indicate significant interactions between NE, CRF and DYN in this critical limbic region and reveal direct synaptic interactions of NE with amygdalar CRF that influence the LC-NE arousal system.

Noradrenergic modulation of arousal

Through a highly divergent efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. State-dependent neuronal discharge activity of locus coeruleus neurons has long-suggested a role of this system in the induction of an alert waking state. More recent work supports this hypothesis, demonstrating robust wake-promoting actions of the locus coeruleus-noradrenergic system. Norepinephrine enhances arousal, in part, via actions of beta- and alpha1-receptors located within multiple subcortical structures, including the general regions of the medial septal area and the medial preoptic areas. Recent anatomical studies suggest that arousal-enhancing actions of norepinephrine are not limited to the locus coeruleus system and likely include the A1 and A2 noradrenergic cell groups. Thus, noradrenergic modulation of arousal state involves multiple noradrenergic systems acting within multiple subcortical regions. Pharmacological studies indicate that the combined actions of these systems are necessary for the sustained maintenance of arousal levels associated with spontaneous waking. Enhanced arousal state is a prominent aspect of both stress and psychostimulant drug action and evidence indicates that noradrenergic systems likely play an important role in both stress-related and psychostimulant-induced arousal. These and other observations suggest that the dysregulation of noradrenergic neurotransmission could well contribute to the dysregulation of arousal associated with a variety of behavioral disorders including insomnia and stress-related disorders.

Neuroanatomical and pharmacological assessment of Fos expression induced in the rat brain by the phosphodiesterase-4 inhibitor 6-(4-pyridylmethyl)-8-(3-nitrophenyl) quinoline

A major obstacle in the therapeutic development of phosphodiesterase-4 (PDE4) inhibitors is the production of adverse side effects such as nausea and vomiting. Immunohistochemical detection of Fos-like immunoreactivity (FLI) was used to address the neuroanatomical basis for the pharmacological actions of PDE4 inhibitors. The potent and selective PDE4 inhibitors 6-(4-pyridylmethyl)-8-(3-nitrophenyl) quinoline (PMNPQ) and rolipram elevated FLI in brain regions potentially relevant to the anti-depressant and emetic effects of PDE4 inhibition. PMNPQ and rolipram elevated FLI in the locus coeruleus, habenula, paraventricular nucleus of the thalamus, amygdala and nucleus accumbens, all structures with strong limbic connectivity implicated in arousal, memory and affective aspects of behaviour. Consistent with the emetic effects of PDE4 inhibitors such as PMNPQ and rolipram, these compounds elevated FLI in caudal brainstem nuclei such as the area postrema and nucleus of the solitary tract. Administration of the NK(1) antagonist RP 67580 prior to PMNPQ reversed increases in FLI produced by PMNPQ in these regions. RP 67580 did not, however, reduce PMNPQ-induced FLI in limbic structures. These findings suggest that PDE4 inhibitors produce emesis by increasing NK(1) receptor activation in the AP/NTS and implicate brain regions associated with reward and mood such as the amygdala, paraventricular nucleus of the thalamus, habenula and nucleus accumbens in the anti-depressant activity of such compounds.

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.

Activation of alpha2 adrenergic receptors suppresses fear conditioning: expression of c-Fos and phosphorylated CREB in mouse amygdala

alpha(2) adrenergic agonists such as dexmedetomidine generally suppress noradrenergic transmission and have sedative, analgesic, and antihypertensive properties. Considering the importance of the neurotransmitter norepinephrine in forming memories for fearful events, we have investigated the acute and chronic effects of dexmedetomidine on discrete cue and contextual fear conditioning in mice. When administered before training, dexmedetomidine (10-20 microg/kg, i.p.) selectively suppressed discrete cue fear conditioning without affecting contextual memory. This behavioral change was associated with a decrease in memory retrieval-induced expression of c-Fos and P-CREB in the lateral, basolateral, and central nuclei of the amygdala. Dexmedetomidine's action on discrete cue memory did not occur in alpha(2A) adrenoceptor knockout (KO) mice. When dexmedetomidine was administered after training, it suppressed contextual memory, an effect that did not occur in alpha(2A) adrenoceptor KO mice. We conclude that dexmedetomidine, acting at alpha(2A) adrenoceptors, must be present during the encoding process to decrease discrete cue fear memory; however, its ability to suppress contextual memory is likely the result of blocking the consolidation process. The ability of alpha(2) agonists to suppress fear memory may be a valuable property clinically in order to suppress the formation of memories during stressful situations.

The locus coeruleus-noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

Through a widespread efferent projection system, the locus coeruleus-noradrenergic system supplies norepinephrine throughout the central nervous system. Initial studies provided critical insight into the basic organization and properties of this system. More recent work identifies a complicated array of behavioral and electrophysiological actions that have in common the facilitation of processing of relevant, or salient, information. This involves two basic levels of action. First, the system contributes to the initiation and maintenance of behavioral and forebrain neuronal activity states appropriate for the collection of sensory information (e.g. waking). Second, within the waking state, this system modulates the collection and processing of salient sensory information through a diversity of concentration-dependent actions within cortical and subcortical sensory, attention, and memory circuits. Norepinephrine-dependent modulation of long-term alterations in synaptic strength, gene transcription and other processes suggest a potentially critical role of this neurotransmitter system in experience-dependent alterations in neural function and behavior. The ability of a given stimulus to increase locus coeruleus discharge activity appears independent of affective valence (appetitive vs. aversive). Combined, these observations suggest that the locus coeruleus-noradrenergic system is a critical component of the neural architecture supporting interaction with, and navigation through, a complex world. These observations further suggest that dysregulation of locus coeruleus-noradrenergic neurotransmission may contribute to cognitive and/or arousal dysfunction associated with a variety of psychiatric disorders, including attention-deficit hyperactivity disorder, sleep and arousal disorders, as well as certain affective disorders, including post-traumatic stress disorder. Independent of an etiological role in these disorders, the locus coeruleus-noradrenergic system represents an appropriate target for pharmacological treatment of specific attention, memory and/or arousal dysfunction associated with a variety of behavioral/cognitive disorders.

Noradrenergic receptor blockade of the NTS attenuates the mnemonic effects of epinephrine in an appetitive light-dark discrimination learning task

These experiments examined the contribution of noradrenergic neurons in the nucleus of the solitary tract (NTS) in mediating the memory-facilitating effects of epinephrine. In Experiment 1, saline or 0.05 or 0.1 mg/kg of epinephrine was given intraperitoneally (ip) to rats after the second day of training in a light-dark Y-maze discrimination task. On a 20-trial retention test given 2 and 7 days later, the 0.1 mg/kg epinephrine group made significantly more correct responses than controls and required fewer trials to reach criterion. In Experiment 2, phosphate-buffered saline or the noradrenergic antagonist dl-propranolol (0.3 or 1.0 microg/0.5 microl) was infused into the NTS prior to an ip injection of saline or 0.1 mg/kg of epinephrine. The memory-enhancing effects of epinephrine were attenuated by the infusion of 0.3 microg/0.5 microl of dl-propranolol into the NTS. These findings indicate an involvement of NTS noradrenergic neurons in mediating the effects of peripheral epinephrine on memory storage processes.