Discharge Of Locus Coeruleus Neurons Research Articles

Impaired Phasic Discharge of Locus Coeruleus Neurons Based on Persistent High Tonic Discharge-A New Hypothesis With Potential Implications for Neurodegenerative Diseases.

The locus coeruleus (LC) is a small brainstem nucleus with widely distributed noradrenergic projections to the whole brain, and loss of LC neurons is a prominent feature of age-related neurodegenerative diseases, such as Alzheimer's disease (AD) and Parkinson's disease (PD). This article discusses the hypothesis that in early stages of neurodegenerative diseases, the discharge mode of LC neurons could be changed to a persistent high tonic discharge, which in turn might impair phasic discharge. Since phasic discharge of LC neurons is required for the release of high amounts of norepinephrine (NE) in the brain to promote anti-inflammatory and neuroprotective effects, persistent high tonic discharge of LC neurons could be a key factor in the progression of neurodegenerative diseases. Transcutaneous vagal stimulation (t-VNS), a non-invasive technique that potentially increases phasic discharge of LC neurons, could therefore provide a non-pharmacological treatment approach in specific disease stages. This article focuses on LC vulnerability in neurodegenerative diseases, discusses the hypothesis that a persistent high tonic discharge of LC neurons might affect neurodegenerative processes, and finally reflects on t-VNS as a potentially useful clinical tool in specific stages of AD and PD.

Low-Dose Methylphenidate Actions on Tonic and Phasic Locus Coeruleus Discharge

Methylphenidate (MPH) and other psychostimulants are highly effective in the treatment of attention deficit hyperactivity disorder (ADHD). Evidence indicates the therapeutic actions of stimulants in ADHD probably involve the locus coeruleus (LC)-norepinephrine system. LC neurons display different firing modes (tonic and phasic), each associated with distinct behavioral and cognitive processes. To date, the impact of low, clinically relevant doses of psychostimulants on LC discharge is unknown. The present study examined the effects of low-dose MPH on LC tonic and phasic discharge in the halothane-anesthetized rat. In these studies, MPH produced a dose-dependent suppression of tonic and phasic discharge that was relatively modest at the lower doses. Nonetheless, these lower doses of MPH suppressed the signal-to-noise ratio of excitatory phasic discharge and increased the signal-to-noise ratio of the inhibitory component of the phasic response. Largely comparable effects were observed with oral and intraperitoneal administration of MPH. Combined, these observations indicate relatively modest suppression of LC neuronal discharge activity by low-dose MPH and that evoked discharge may be more sensitive than tonic activity to the lowest doses of MPH. It is posited that the behavioral-calming and cognition-enhancing effects of low-dose psychostimulants probably involve modest alterations in LC discharge combined with increased catecholamine efflux within select forebrain regions (i.e., the prefrontal cortex).

Opposing regulation of the locus coeruleus by corticotropin-releasing factor and opioids

Substantial clinical and preclinical findings support an association between stress and opiate abuse. To understand the mechanisms underlying this association, it is important to identify substrates of the stress response and endogenous opioid systems that interact and specific points at which stress circuits and endogenous opioid systems intersect. This review focuses on corticotropin-releasing factor (CRF), a critical substrate of the stress response, and its potential for interactions with endogenous opioid systems within the pontine nucleus, locus coeruleus (LC), a brain region that has been implicated as a target in response to stress and opiates. Evidence is reviewed supporting the hypothesis that CRF and endogenous opioids interact to co-regulate the LC. Thus, CRF- and enkephalin-immunoreactive fibers innervating LC dendritic fields overlap, and some axon terminals in this region co-localize CRF and enkephalin. CRF and opioids have opposing effects on LC neuronal discharge and on intracellular signaling mechanisms within LC neurons. Finally, a history of stress or opiate use induces plasticity in CRF-LC or opiate-LC interactions, respectively. Disruptions in the CRF/opioid balance as a result of this plasticity are proposed to result in hyperactivity or hyperresponsiveness of the LC-norepinephrine (NE) system. Co-regulation of the LC-NE system by CRF and opioids may be important in acute adaptation to stress. Potential clinical consequences of an imbalance in this regulation as a result of prior stress include increased risk of opiate self administration and decreased sensitivity to opiates used in clinical settings. Conversely, chronic exposure to opiates may predispose individuals to stress-related psychiatric disorders.

Relationship between locus coeruleus discharge rates and rates of norepinephrine release within neocortex as assessed by in vivo microdialysis

The relationship between discharge rates of locus coeruleus noradrenergic neurons and rates of norepinephrine release was examined in the anesthetized rat. Neuronal discharge rates of locus coeruleus neurons were altered and quantified using a combined recording-infusion probe. Peri-locus coeruleus infusions of either the cholinergic agonist, bethanechol, or the α 2-agonist, clonidine, were used to enhance or suppress neuronal discharge activity, respectively. Alterations in concentrations of extracellular norepinephrine within the prefrontal cortex were determined using in vivo microdialysis and high-pressure liquid chromatography with electrochemical detection. A linear relationship between locus coeruleus activity and norepinephrine dialysate concentration was observed between complete suppression of locus coeruleus discharge activity and approximately 300–400% of basal discharge levels (1.58±0.29 Hz). Above these levels, increases in locus coeruleus discharge rates were not accompanied by similar increases in dialysate norepinephrine concentrations. In general, neither activation nor suppression of locus coeruleus neuronal discharge rates appeared to alter the relationship between discharge activity and norepinephrine efflux during subsequent epochs. The one exception to this was observed during recovery from relatively high-magnitude locus coeruleus activation. In two out of three cases in which locus coeruleus discharge rates were increased greater than 450%, a recovery of norepinephrine concentrations to basal levels occurred more quickly than the recovery of locus coeruleus neuronal discharge rates to basal levels. Although limited, these latter observations suggest that dysregulation of norepinephrine release may occur following sustained activation of locus coeruleus at the highest rates examined, which may mimic those associated with intense arousal or stress.

Activation of Locus Coeruleus Enhances the Responses of Olfactory Bulb Mitral Cells to Weak Olfactory Nerve Input

The main olfactory bulb (MOB) receives a dense projection from the pontine nucleus locus coeruleus (LC), the largest collection of norepinephrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously applied NE on mitral cells, the principal output neurons of MOB, are contradictory. The effect of synaptically released NE on mitral cell activity is not known, nor is the influence of NE on responses of mitral cells to olfactory nerve inputs. The goal of the present study was to assess the influence of LC activation on spontaneous and olfactory nerve-evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetyicholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharge. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/18 cells (mean Increase = 67%). The discharge rate of mitral cells at the time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activity in LC-NE projections to MOB may enhance detection of relatively weak odors.

GABAergic inhibitory response of locus coeruleus neurons to caloric vestibular stimulation in rats

We examined the effects of caloric vestibular stimulation on the neuronal activity of the locus coeruleus (LC) in urethane-anesthetized rats. The middle ear cavity was irrigated with hot (44°C) or cold (30°C) water through a polyethylene tube. Most neurons (hot water: 76%, 55 72 ; cold water: 90%, 19 21 ) exhibited suppression of neuronal discharge in response to caloric stimulation. The suppression of LC neuronal discharge following caloric stimulation occurred with a long latency (approximately 80 s), and lasted a long period of time (approximately 3 min). Neither caloric stimulation of the auricle, nor irrigation of the middle ear with water at 37°C, nor caloric stimulation of the middle ear after labyrinthectomy inhibited LC neuronal discharge. The caloric stimulation-induced LC neuronal inhibition was significantly attenuated by the intravenous injection of picrotoxin and by the iontophoretic application of bicuculline methiodide. These findings indicate that the predominant effect of caloric vestibular stimulation on LC neuronal discharge is inhibitory, and that the caloric stimulation-induced LC neuronal inhibition is mediated by GABAA receptors located on the membrane of LC neurons. It is suggested that the suppressed activity of noradrenergic LC neurons is involved in the vestibulo-autonomic reflex.

Previous stress alters corticotropin-releasing factor neurotransmission in the locus coeruleus.

Spontaneous and stress-evoked discharge of locus coeruleus neurons were characterized in rats with a history of stress. Rats exposed to one or five daily 30-min sessions of footshock were anesthetized with halothane and surgically prepared for locus coeruleus single-unit recording immediately following the last session. Locus coeruleus spontaneous discharge rate and discharge evoked by sciatic nerve stimulation were comparable between acutely and repeatedly stressed rats and controls. In contrast, locus coeruleus activation produced by intracerebroventricular administration of corticotropin-releasing factor (3 micrograms) or by hypotensive challenge (which requires endogenous corticotropin-releasing factor release in the locus coeruleus) was greatly attenuated in acutely stressed rats. The corticotropin-releasing factor dose-response curve was shifted to the right in acutely stressed rats compared with controls. In repeatedly stressed rats, the effects of 3 micrograms corticotropin-releasing factor on locus coeruleus discharge were similarly diminished. Although the maximum effect produced by corticotropin-releasing factor was decreased in these rats, the dose-response curve was shifted to the left, indicative of sensitization. Hypotensive challenge, which was ineffective in acutely stressed rats, increased locus coeruleus discharge of repeatedly stressed rats by a similar magnitude as in matched controls. The return of locus coeruleus responsiveness to hypotension in repeatedly stressed rats may be related to the sensitization to corticotropin-releasing factor. Finally, the protocol of repeated stress did not alter the affinity or density of corticotropin-releasing factor receptors in either the frontal cortex or brainstem. Taken together, the results suggest that a history of stress alters corticotropin-releasing factor neurotransmission in the locus coeruleus at the postsynaptic level. However, these effects are not reflected by corticotropin-releasing factor binding kinetics in brainstem. Stress-induced changes in corticotropin-releasing factor neurotransmitter function in the locus coeruleus may play a role in certain symptoms of stress-related psychiatric disorders.

Effects of locus coeruleus inactivation on electroencephalographic activity in neocortex and hippocampus

The effects of inhibition of locus coeruleus neuronal discharge activity on cortical and hippocampal electroencephalographic activity were examined in halothane-anesthetized rats. A combined recording/infusion probe was used to place 35–150-nl infusions of theα 2-noradrenergic agonist, clonidine (1 ng/nl) which inhibits locus coeruleus neuronal discharge activity, immediately adjacent to the locus coeruleus. The recording electrode allowed verification and quantification of the electrophysiological effects of these infusions. Simultaneously, electroencephalographic activity was recorded from sites in frontal neocortex and dorsal hippocampus and subjected to power spectrum analyses. Neither cortical nor hippocampal electroencephalographic activity was substantially affected following unilateral locus coeruleus inactivation. In contrast, bilateral clonidine infusions that completely suppressed locus coeruleus neuronal discharge activity in both hemispheres altered cortical and hippocampal electroencephalographic status. The cortical response to bilateral LC inhibition was characterized by a shift from low-amplitude, high-frequency to large-amplitude, slow-wave activity. Additionally, theta-dominated activity in the hippocampus was replaced with mixed frequency activity. The onset of these changes in forebrain electroencephalographic activity was coincident with the complete bilateral inhibition of locus coeruleus neuronal discharge activity. The resumption of pre-infusion electroencephalographic patterns closely followed recovery of locus coeruleus neuronal activity or could be induced with systemic administration of theα 2-noradrenergic antagonist, idazoxan. Clonidine infusions placed 800–1200 μm from the locus coeruleus were less effective at inducing a complete suppression of locus coeruleus activity. These infusions either did not completely inhibit locus coeruleus discharge (35 nl infusions), or did so with a longer latency to complete locus coeruleus inhibition and a shorter duration of inhibition (150 nl infusions). Changes in forebrain electroencephalographic activity occurred only following the complete bilateral suppression of locus coeruleus neuronal discharge activity. These electroencephalographic responses closely followed or coincided with the onset of complete bilateral locus coeruleus inhibition and persisted throughout the period during which bilateral LC neuronal discharge activity was completely absent (60–240 min). Recovery of electroencephalographic patterns was coincident with the reappearance of locus coeruleus discharge activity. These results suggest that the clonidine-induced changes in forebrain electroencephalographic activity were dependent on the complete bilateral suppression of locus coeruleus discharge activity, and that under the present experimental conditions the locus coeruleus/noradrenergic system exerts a potent and tonic activating influence on forebrain electroencephalographic state. These results support the hypothesis that this system may be an important modulator of behavioral state and/or state-dependent processes.

A 5-hydroxytryptamine 2 agonist augments γ-aminobutyric acid and excitatory amino acid inputs to noradrenergic locus coeruleus neurons

We examined the effects of the 5-hydroxytryptamine 2 receptor agonist, (+)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, on spontaneous and evoked discharge of locus coeruleus neurons in the rat. Extracellular recordings were obtained from single locus coeruleus neurons while (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane was injected systemically or locally into the locus coeruleus. Systemic, but not local, administration of (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane decreased spontaneous discharge of locus coeruleus neurons in a dose-dependent manner while simultaneously increasing responses evoked by somatosensory stimulation, consistent with previous studies using 5-hydroxytrypta-mine 2 agonists. Increased responsiveness was observed after both low- and high-intensity stimulation and, in the latter, resulted from the addition of a second, longer latency response after (∓)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane administration, when cells typically responded to each stimulation with two driven spikes instead of one. Both of these effects could be completely reversed by systemic administration of the 5-hydroxytryptamine 2 receptor antagonist, ketanserin. Furthermore, we report that: (i) the (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane-induced decrease in spontaneous firing was blocked by local infusion of the GABA antagonists bicuculline or picrotoxin into the locus coeruleus, but not by local infusion of the alpha-2 adrenoceptor antagonist, idazoxan; (ii) the enhancement of locus coeruleus sensory responses after high-intensity stimulation was blocked by local application of the selective antagonist of N-methyl- d-aspartate receptors, 2-amino-5-phosphonopentanoic acid, but not by local infusion of the preferential antagonist of Non-N-methyl- d-aspartate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione. Together, these results lead us to propose that systemic 5-hydroxytryptamine 2 agonists influence locus coeruleus indirectly, causing tonic activation of a GABAergic input to the locus coeruleus, and facilitating sensory inputs that act via excitatory amino acid receptors within locus coeruleus.

Tonic activation of locus coeruleus neurons by systemic or intracoerulear microinjection of an irreversible acetylcholinesterase inhibitor: Increased discharge rate and induction of c-fos

Recent studies in this laboratory have demonstrated that intramuscular injection of the irreversible acetylcholinesterase (AChE) inhibitor, soman (pinacolylmethylphosphonofluoridate), produces a rapid (1–2 h) and profound depletion (70% of control) of norepinephrine (NE) in the olfactory bulb and forebrain. NE is decreased only in convulsing animals. As NE-containing locus coeruleus (LC) neurons provide the only NE input to the olfactory bulb and the major NE innervation of the forebrain, the reduction of NE suggests that soman may cause tonic activation of LC neurons leading to rapid depletion of NE. Activation of LC may result from: (i) facilitation of cholinergic transmission in LC; (ii) soman-induced activation of excitatory inputs to LC; or (iii) generalized activation of LC neurons due to seizures. The present experiments were designed to assess these alternatives. We examined whether LC neuronal activity, c-fos expression, and AChE staining are altered after peripheral (systemic) or direct intracoerulear injection of soman in anesthetized rats. Both modes of soman administration rapidly and potently increase the spontaneous discharge rate of LC neurons. This activation was associated with a desynchronization of the electroencephalogram, but not with seizures. The discharge of LC neurons remained elevated at all postsoman intervals examined (up to 2 h) and was rapidly and completely reversed by systemic injection of the muscarinic receptor antagonist scopolamine hydrochloride, but not by the nicotinic receptor antagonist mecamylamine. Both systemic and intracoerulear soman administration completely inhibited AChE staining in LC and rapidly induced the expression of c-fos in LC neurons. These results demonstrate that soman potently and tonically activates LC neurons. This effect appears to be mediated by direct inhibition of AChE in LC leading to a rapid accumulation of ACh. Unhydrolyzed ACh tonically activates LC neurons via muscarinic receptors. Soman-induced activation of LC neurons does not require seizures. We conclude that depletion of forebrain and olfactory bulb NE after systemic administration of soman results from tonic hypercholinergic stimulation of LC.

A neuroanatomical and biochemical basis for attention deficit disorder with hyperactivity in children: A defect in tonic adrenaline mediated inhibition of locus coeruleus stimulation

Attention deficit disorder with hyperactivity (ADDH) is characterized by a high level of inappropriate activity, distractability and impulsivity as well as learning deficits. In mammals, the protective mechanism for acquisition of sensory information, processed for its survival value, can be termed “vigilance”. In ADDH, a low threshold for novel or sensory stimuli, results in a “hard wired” mammalian response, orienting, followed by exploratory behavior, the orienting reaction. This behavior is mediated in the brainstem through sensory neurons in the reticular formation regulating the discharge of locus coeruleus noradrenergic neurons. It is proposed that adrenaline acts as a tonic inhibitor of these neurons, modulating the level of sensory stimulation necessary to elicit locus coeruleus excitation. An imbalance in adrenaline formation or alpha-2 adrenergic receptor number (or affinity) leads to the inability to maintain the appropriate threshold for discharge of locus coeruleus neurons. The consequences are inability to maintain focused attention, difficulty in falling asleep or light levels of sleep, inattention to consumptive behavior, and probable inappropriate response for reward as all these behaviors require disruption of processing of sensory stimuli. Cognitive deficits may occur secondarily.

Influence of the alpha-2 Agonist Oxaminozoline (S3341) on Firing Rate of Central Noradrenergic and Serotonergic Neurons in the Rat. Comparison with Clonidine

The firing rate of central locus coeruleus (LC) noradrenergic neurons and dorsal raphe (DR) serotonergic neurons was recorded in rats anaesthetized with chloral hydrate. The iontophoretic application or the i.v. perfusion of S3341, a new antihypertensive drug or clonidine decreased the frequency of discharge of LC neurons. Depending on the mode of administration clonidine was 54-63 times more potent than S3341. The selectivity of action of both drugs on alpha-2 vs. alpha-1 adrenoceptors was confirmed using yohimbine and prazosin: yohimbine completely blocked the inhibitory effect of S3341 or clonidine while prazosin did not prevent this effect. S3341 and clonidine regularly reduced the firing rate of DR neurons during i.v. perfusion but not during iontophoretic application. From these experiments is it concluded that S3341 and clonidine have a direct inhibitory effect on LC neurons via stimulation of alpha-2 autoreceptors and that both drugs have an indirect inhibitory effect on DR neurons, probably via impairment of noradrenergic transmission. Clinical studies show that S3341 induces much less sedative side effects than clonidine. In view of the great difference in the potency of these drugs to inhibit the firing rate of monoaminergic neurons which are known to be involved in sleep mechanisms, it is possible that the electrophysiological effects reported here relate to the sedative effects of these drugs.

Comparative Electrophysiological Investigations on Oxiracetam and Piracetam

Acute effects of the two psychogeriatric agents, oxiracetam (ISF 2522) and piracetam, were studied in rat brain tissue in vivo and in vitro. In vivo, both compounds elicited a moderate activation of locus coeruleus neuronal discharge rate with doses of 1,000 mg/kg ip. Neither of the compounds however affected spontaneous firing rates of neurons of the medial septal nucleus in vivo if administered intravenously. In the hippocampal slice preparation, the two compounds elicited weak but opposite effects on extracellularly recorded field potentials if administered in high concentrations. Whereas oxiracetam elicited a moderate depressant effect on pyramidal cell excitability, piracetam increased it in a reversible fashion. Piracetam had no effect on the excitatory postsynaptic potential (epsp) evoked by stimulation of the Schaffer commissural fiber pathway but oxiracetam reversibly attenuated it at concentrations of 100 microM to 1 mM. In intracellular recordings from CA1 pyramidal neurons in vitro, piracetam had no effect on resting membrane potential, conductance or afterhyperpolarizations. Oxiracetam, however, slightly hyperpolarized six out of twelve CA1 neurons. Membrane conductance was not affected. In conclusion, electrophysiological studies with the psychogeriatrics oxiracetam and piracetam revealed that the drugs exert weak effects on rat brain tissue in acute experiments, and demonstrated that the two compounds differ in their pharmacological profile.