Activity Of Locus Coeruleus Noradrenergic Neurons Research Articles

Circuit mechanism underlying fragmented sleep and memory deficits in 16p11.2 deletion mouse model of autism.

Sleep disturbances are prevalent in children with autism spectrum disorder (ASD) and have a major impact on the quality of life. Strikingly, sleep problems are positively correlated with the severity of ASD symptoms, such as memory impairment. However, the neural mechanisms underlying sleep disturbances and cognitive deficits in ASD are largely unexplored. Here, we show that non-rapid eye movement sleep (NREMs) is highly fragmented in the 16p11.2 deletion mouse model of ASD. The degree of sleep fragmentation is reflected in an increased number of calcium transients in the activity of locus coeruleus noradrenergic (LC-NE) neurons during NREMs. Exposure to a novel environment further exacerbates sleep disturbances in 16p11.2 deletion mice by fragmenting NREMs and decreasing rapid eye movement sleep (REMs). In contrast, optogenetic inhibition of LC-NE neurons and pharmacological blockade of noradrenergic transmission using clonidine reverse sleep fragmentation. Furthermore, inhibiting LC-NE neurons restores memory. Rabies-mediated unbiased screening of presynaptic neurons reveals altered connectivity of LC-NE neurons with sleep- and memory regulatory brain regions in 16p11.2 deletion mice. Our findings demonstrate that heightened activity of LC-NE neurons and altered brain-wide connectivity underlies sleep fragmentation in 16p11.2 deletion mice and identify a crucial role of the LC-NE system in regulating sleep stability and memory in ASD.

Chemogenetic activation of locus coeruleus neurons ameliorates the severity of multiple sclerosis

BackgroundMost current disease-modifying therapies approved for multiple sclerosis (MS) are immunomodulatory drugs that counteract the aberrant activity of the immune system. Hence, new pharmacological interventions that drive anti-inflammatory activity and neuroprotection would represent interesting alternative therapeutic approaches or complementary strategies to treat progressive forms of MS. There is evidence of reduced noradrenaline levels and alterations to locus coeruleus (LC) noradrenergic neurons in MS patients, as well as in animal models of this disease, potentially factors contributing to the pathophysiology. Drugs that enhance noradrenaline appear to have some beneficial effects in MS, suggesting their potential to dampen the underlying pathology and disease progression.MethodsTherefore, we explored the consequences of chronic LC noradrenergic neurons activation by chemogenetics in experimental autoimmune encephalomyelitis (EAE) mice, the most widely used experimental model of MS. LC activation from the onset or the peak of motor symptoms was explored as two different therapeutic approaches, assessing the motor and non-motor behavioral changes as EAE progresses, and studying demyelination, inflammation and glial activation in the spinal cord and cerebral cortex during the chronic phase of EAE.ResultsLC activation from the onset of motor symptoms markedly alleviated the motor deficits in EAE mice, as well as their anxiety-like behavior and sickness, in conjunction with reduced demyelination and perivascular infiltration in the spinal cord and glial activation in the spinal cord and prefrontal cortex (PFC). When animals exhibited severe paralysis, LC activation produced a modest alleviation of EAE motor symptoms and it enhanced animal well-being, in association with an improvement of the EAE pathology at the spinal cord and PFC level. Interestingly, the reduced dopamine beta-hydroxylase expression associated with EAE in the spinal cord and PFC was reversed through chemogenetic LC activation.ConclusionTherefore, clear anti-inflammatory and neuroprotective effects were produced by the selective activation of LC noradrenergic neurons in EAE mice, having greater benefits when LC activation commenced earlier. Overall, these data suggest noradrenergic LC neurons may be targets to potentially alleviate some of the motor and non-motor symptoms in MS.

Ultrastructural evidence for mu and delta opioid receptors at noradrenergic dendrites and glial profiles in the cat locus coeruleus

The Locus Coeruleus (LC) is a pontine nucleus involved in many physiological processes, including the control of the sleep/wake cycle (SWC). At cellular level, the LC displays a high density of opioid receptors whose activation decreases the activity of LC noradrenergic neurons. Also, microinjections of morphine administered locally in the LC of the cat produce sleep associated with synchronized brain activity in the electroencephalogram (EEG). Even though much of the research on sleep has been done in the cat, the subcellular location of opioid receptors in the LC and their relationship with LC noradrenergic neurons is not known yet in this species. Therefore, we conducted a study to describe the ultrastructural localization of mu-opioid receptors (MOR), delta-opioid receptors (DOR) and tyrosine hydroxylase (TH) in the cat LC using high resolution electron microscopy double-immunocytochemical detection. MOR and DOR were localized mainly in dendrites (45% and 46% of the total number of profiles respectively), many of which were noradrenergic (35% and 53% for MOR and DOR, respectively). TH immunoreactivity was more frequent in dendrites (65% of the total number of profiles), which mostly also expressed opioid receptors (58% and 73% for MOR and DOR, respectively). Because the distribution of MORs and DORs are similar, it is possible that a substantial sub-population of neurons co-express both receptors, which may facilitate the formation of MOR-DOR heterodimers. Moreover, we found differences in the cat subcellular DOR distribution compared with the rat. This opens the possibility to the existence of diverse mechanisms for opioid modulation of LC activity.

Central noradrenergic activity affects analgesic effect of Neuropeptide S.

Neuropeptide S (NPS) is an endogenous neuropeptide controlling anxiolysis, wakefulness, and analgesia. NPS containing neurons exist near to the locus coeruleus (LC) involved in the descending anti-nociceptive system. NPS interacts with central noradrenergic neurons; thus brain noradrenergic signaling may be involved in NPS-induced analgesia. We tested NPS analgesia in noradrenergic neuron-lesioned rats using a selective LC noradrenergic neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4). A total 66 male Sprague-Dawley rats weighing 350-450g were used. Analgesic effects of NPS were evaluated using hot-plate and tail-flick test with or without DSP-4. The animal allocated into 3 groups; hot-plate with NPS alone intracerebroventricular (icv) (0.0, 1.0, 3.3, and 10.0nmol), tail-flick NPS alone icv (0.0 and 10.0nmol), and hot-plate with NPS and DSP-4 (0 or 50mg/kg ip). In hot-plate with NPS and DSP-4 group, noradrenaline content in the cerebral cortex, pons, hypothalamus, were measured. NPS 10nmol icv prolonged hot plate (%MPE) but not tail flick latency at 30 and 40min after administration. DSP-4 50mg/kg decreased noradrenaline content in the all 3 regions. The NA depletion inhibited NPS analgesic effect in the hot plate test but not tail flick test. There was a significant correlation between hot plate latency (percentage of maximum possible effect: %MPE) with NPS 10nmol and NA content in the cerebral cortex (p=0.017, r 2=0.346) which noradrenergic innervation arisen mainly from the LC. No other regions had the correlation. NPS analgesia interacts with LC noradrenergic neuronal activity.

GABAergic ventrolateral pre‑optic nucleus neurons are involved in the mediation of the anesthetic hypnosis induced by propofol.

Intravenous anesthetics have been used clinically to induce unconsciousness for seventeen decades, however the mechanism of anesthetic-induced unconsciousness remains to be fully elucidated. It has previously been demonstrated that anesthetics exert sedative effects by acting on endogenous sleep-arousal circuits. However, few studies focus on the ventrolateral pre-optic (VLPO) to locus coeruleus (LC) sleep-arousal pathway. The present study aimed to investigate if VLPO is involved in unconsciousness induced by propofol. The present study additionally investigated if the inhibitory effect of propofol on LC neurons was mediated by activating VLPO neurons. Microinjection, target lesion and extracellular single-unit recordings were used to study the role of the VLPO-LC pathway in propofol anesthesia. The results demonstrated that GABAA agonist (THIP) or GABAA antagonist (gabazine) microinjections into VLPO altered the time of loss of righting reflex and the time of recovery of righting reflex. Furthermore, propofol suppressed the spontaneous firing activity of LC noradrenergic neurons. There was no significant difference observed in firing activity between VLPO sham lesion and VLPO lesion rats. The findings indicate that VLPO neurons are important in propofol-induced unconsciousness, however are unlikely to contribute to the inhibitory effect of propofol on LC spontaneous firing activity.

Chronic citalopram administration desensitizes prefrontal cortex but not somatodendritic α2-adrenoceptors in rat brain

Selective serotonin reuptake inhibitors (SSRIs) regulate brain noradrenergic neurotransmission both at somatodendritic and nerve terminal areas. Previous studies have demonstrated that noradrenaline (NA) reuptake inhibitors are able to desensitize α2-adrenoceptor-mediated responses. The present study was undertaken to elucidate the effects of repeated treatment with the SSRI citalopram on the α2-adrenoceptor sensitivity in locus coeruleus (LC) and prefrontal cortex (PFC), by using in vivo microdialysis and electrophysiological techniques, and in vitro stimulation of [35S]GTPγS binding autoradiography. Repeated, but not acute, treatment with citalopram (5 mg/kg, i.p., 14 days) increased extracellular NA concentration selectively in PFC. The α2-adrenoceptor agonist clonidine (0.3 mg/kg, i.p.), administered to saline-treated animals (1 ml/kg i.p., 14 days) induced NA decrease in LC (Emax = −44 ± 4%; p < 0.001) and in PFC (Emax = −61 ± 5%, p < 0.001). In citalopram chronically-treated rats, clonidine administration exerted a lower decrease of NA (Emax = −25 ± 7%; p < 0.001) in PFC whereas the effect in LC was not different to controls (Emax = −36 ± 4%). Clonidine administration (0.625–20 μg/kg, i.v.) evoked a dose-dependent decrease of the firing activity of LC noradrenergic neurons in both citalopram- (ED50 = 3.2 ± 0.4 μg/kg) and saline-treated groups (ED50 = 2.6 ± 0.5 μg/kg). No significant differences between groups were found in ED50 values. The α2-adrenoceptor agonist UK14304 stimulated specific [35S]GTPγS binding in brain sections containing LC (144 ± 14%) and PFC (194 ± 32%) of saline-treated animals. In citalopram-treated animals, this increase did not differ from controls in LC (146 ± 22%) but was lower in PFC (141 ± 8%; p < 0.05). Taken together, long-term citalopram treatment induces a desensitization of α2-adrenoceptors acting as axon terminal autoreceptors in PFC without changes in somatodendritic α2-adrenoceptor sensitivity.

Role of sex hormones in hypercapnia-induced activation of the locus coeruleus in female and male rats

The locus coeruleus (LC) has been suggested as a CO2 chemoreceptor site in mammals. Most of the studies involving the role of the LC in hypercapnic ventilatory responses have been performed in males. Since ovarian steroids modulate the activity of LC neurons and females have a different respiratory response to CO2 than males, we evaluated the activity of LC noradrenergic neurons during normocapnia and hypercapnia in female and male rats with distinct sex hormone levels. Ovariectomized (OVX), estradiol (E2)-treated ovariectomized (OVX+E2) and female rats on the diestrous day of the estrous cycle were evaluated. Concurrently, males were investigated as gonad-intact, orchidectomized (ORX), testosterone (T)-treated ORX (ORX+T), and E2-treated ORX (ORX+E2). Activation of LC neurons was determined by double-label immunohistochemistry to c-Fos and tyrosine hydroxylase (TH). Hypercapnia induced by 7% CO2 increased the number of c-Fos/TH-immunoreactive (ir) neurons in the LC of all groups when compared to air exposure. Hypercapnia-induced c-Fos expression did not differ between diestrous females and intact male rats. In the OVX+E2 group, there was attenuation in the c-Fos expression during normocapnia compared with OVX rats, but CO2 responsiveness was not altered. Moreover, in ORX rats, neither T nor E2 treatments changed c-Fos expression in LC noradrenergic neurons. Thus, in female rats, E2 reduces activation of LC noradrenergic neurons, whereas in males, sex hormones do not influence the LC activity.

CRH Engagement of the Locus Coeruleus Noradrenergic System Mediates Stress-Induced Anxiety

The locus coeruleus noradrenergic (LC-NE) system is one of the first systems engaged following a stressful event. While numerous groups have demonstrated that LC-NE neurons are activated by many different stressors, the underlying neural circuitry and the role of this activity in generating stress-induced anxiety has not been elucidated. Using a combination of invivo chemogenetics, optogenetics, and retrograde tracing, we determine that increased tonic activity ofthe LC-NE system is necessary and sufficient forstress-induced anxiety and aversion. Selective inhibition of LC-NE neurons during stress prevents subsequent anxiety-like behavior. Exogenously increasing tonic, but not phasic, activity of LC-NE neurons is alone sufficient for anxiety-like and aversive behavior. Furthermore, endogenous corticotropin-releasing hormone(+) (CRH(+)) LC inputs from the amygdala increase tonic LC activity, inducing anxiety-like behaviors. These studies position the LC-NE system as a critical mediator of acute stress-induced anxiety and offer a potential intervention for preventing stress-related affective disorders.

Possible involvement of activated locus coeruleus–noradrenergic neurons in pain-related sleep disorders

The locus coeruleus (LC) is a noradrenergic brainstem structure that is considered to play a role in promoting arousal. To further clarify the role of LC noradrenergic neurons, we performed an optogenetic assay by injecting AAV-channelrhodopsin-2 (ChR2) into the LC of cre-tyrosine hydrolase (TH) mice. We found here that the specific activation of LC noradrenergic neurons produced a significant increase in wakefulness and a significant decrease in non-rapid eye movement (NREM) sleep during photostimulation. On the other hand, neuropathic pain is believed to significantly interfere with sleep, and inadequate sleep may contribute to the stressful negative consequences of living with pain. In the present study, sciatic nerve ligation, which produced significant thermal hyperalgesia, significantly increased the levels of noradrenaline released in the prefrontal cortex (PFC) by the weak electrical stimulation of neurons in the LC. Under these conditions, the systemic administration of adrenaline α and β inhibitor cocktail at 7 days after sciatic nerve ligation restored the increased wakefulness and decreased NREM sleep to normal levels. These results suggest that neuropathic pain may accelerate neurons in the LC, and its overactivation may be, at least in part, associated with sleep disturbance under neuropathic pain.

Repetitive Treatment With Diluted Bee Venom Reduces Neuropathic Pain Via Potentiation of Locus Coeruleus Noradrenergic Neuronal Activity and Modulation of Spinal NR1 Phosphorylation in Rats

We previously demonstrated that a single injection of diluted bee venom (DBV) temporarily alleviates thermal hyperalgesia, but not mechanical allodynia, in neuropathic rats. The present study was designed to determine whether repetitive injection of DBV produces more potent analgesic effects on neuropathy-induced nociception and whether those effects are associated with increased neuronal activity in the locus coeruleus (LC) and with the suppression of spinal NMDA receptor NR1 subunit phosphorylation (pNR1). DBV (.25 mg/kg) was administered subcutaneously twice a day for 2 weeks beginning on day 15 post-chronic constrictive injury surgery. Pain responses were examined and potential changes in LC Fos expression and spinal pNR1 expression were determined. Repetitive DBV administration significantly reduced mechanical allodynia, as well as thermal hyperalgesia. The activity of LC noradrenergic neurons was increased and spinal pNR1 expression was significantly suppressed by repetitive DBV as compared with those of vehicle or single DBV injection. These suppressive effects of repetitive DBV on neuropathic pain and spinal pNR1 were prevented by intrathecal pretreatment of idazoxan, an alpha-2 adrenoceptor antagonist. These results indicate that repetitive DBV produces potent analgesic effects on neuropathic pain and this is associated with the activation of the LC noradrenergic system and with a reduction in spinal pNR1. PerspectiveThe results of current study demonstrate that repetitive administration of DBV significantly suppresses neuropathic pain. Furthermore, this study provides mechanistic information that repetitive treatment of DBV can produce more potent analgesic effect than single DBV treatment, indicating a potential novel strategy for the management of chronic pain.

Gulf War Syndrome: A role for organophosphate induced plasticity of locus coeruleus neurons

AbstractGulf War syndrome is a chronic multi-symptom illness that has affected about a quarter of the deployed veterans of the 1991 Gulf War. Exposure to prolonged low-level organophosphate insecticides and other toxic chemicals is now thought to be responsible. Chlorpyrifos was one commonly used insecticide. The metabolite of chlorpyrifos, chlorpyrifos oxon, is a potent irreversible inhibitor of acetylcholinesterase, much like the nerve agent Sarin. To date, the target brain region(s) most susceptible to the neuroactive effects of chlorpyrifos oxon have yet to be identified. To address this we tested ability of chlorpyrifos oxon to influence neuronal excitability and induce lasting changes in the locus coeruleus, a brain region implicated in anxiety, substance use, attention and emotional response to stress. Here we used an ex vivo rodent model to identify a dramatic effect of chlorpyrifos oxon on locus coeruleus noradrenergic neuronal activity. Prolonged exposure to chlorpyrifos oxon caused acute inhibition and a lasting rebound excitatory state expressed after days of exposure and subsequent withdrawal. Our findings indicate that the locus coeruleus is a brain region vulnerable to chlorpyrifos oxon-induced neuroplastic changes possibly leading to the neurological symptoms affecting veterans of the Gulf War.

Gulf War Syndrome: A role for organophosphate induced plasticity of locus coeruleus neurons

AbstractGulf War syndrome is a chronic multi-symptom illness that has affected about a quarter of the deployed veterans of the 1991 Gulf War. Exposure to prolonged low-level organophosphate insecticides and other toxic chemicals is now thought to be responsible. Chlorpyrifos was one commonly used insecticide. The metabolite of chlorpyrifos, chlorpyrifos oxon, is a potent irreversible inhibitor of acetylcholinesterase, much like the nerve agent Sarin. To date, the target brain region(s) most susceptible to the neuroactive effects of chlorpyrifos oxon have yet to be identified. To address this we tested ability of chlorpyrifos oxon to influence neuronal excitability and induce lasting changes in the locus coeruleus, a brain region implicated in anxiety, substance use, attention and emotional response to stress. Here we used an ex vivo rodent model to identify a dramatic effect of chlorpyrifos oxon on locus coeruleus noradrenergic neuronal activity. Prolonged exposure to chlorpyrifos oxon caused acute inhibition and a lasting rebound excitatory state expressed after days of exposure and subsequent withdrawal. Our findings indicate that the locus coeruleus is a brain region vulnerable to chlorpyrifos oxon-induced neuroplastic changes possibly leading to the neurological symptoms affecting veterans of the Gulf War.

Excitation of locus coeruleus noradrenergic neurons by thyrotropin‐releasing hormone

Locus coeruleus (LC) noradrenergic neurons are implicated in a variety of functions including the regulation of vigilance and the modulation of sensory processing. Thyrotropin-releasing hormone (TRH) is an endogenous neuropeptide that induces a variety of behavioural changes including arousal and antinociception. In the present study, we explored whether the activity of LC noradrenergic neurons is modulated by TRH. Using current-clamp recording from isolated rat LC neurons, we found that TRH increased the firing rate of spontaneous action potentials. The TRH action was mimicked by TRH analogues including taltirelin and TRH-gly. In voltage-clamp recording at a holding potential of 50 mV, TRH produced an inward current associated with a decrease in the membrane K+ conductance. This current was inhibited by the TRH receptor antagonist chlordiazepoxide. Following inhibition of the pH-sensitive K+ conductance by extracellular acidification, the TRH response was fully inhibited. The TRH-induced current was also inhibited by the phospholipase C (PLC) inhibitor U-73122, but not by the protein kinase C inhibitor chelerythrine nor by chelation of intracellular Ca2+ by BAPTA. The recovery from the facilitatory action of TRH on the spike frequency was markedly inhibited by a high concentration of wortmannin. These results suggest that TRH activates LC noradrenergic neurons by decreasing an acid-sensitive K+ conductance via PLC-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. The present findings demonstrate that TRH activates LC neurons and characterize the underlying signalling mechanisms. The action of TRH on LC neurons may influence a variety of CNS functions related to the noradrenergic system which include arousal and analgesia.

Acupuncture stimulation to the sacral segment affects state of vigilance in rats

The effects of acupuncture stimulation to the sacral segment on electroencephalograms (EEGs) and activity of locus coeruleus (LC) neurons were examined in urethane-anesthetized rats. In 71 of 112 trials, when EEGs displayed small amplitude and high frequency, stimulation to the sacral segment-induced large amplitude and slow EEGs with a latency of <450 s and duration ranged from 32 s to >42 min. Stimulus-induced EEGs comprised significant increases in delta power and significant decreases in theta and beta powers. After intraperitoneal administration of bicuculline, stimulation to the sacral segment failed to induce changes in EEG pattern. Firing rate of noradrenergic LC neurons decreased significantly from 2.9 ± 1.5 to 1.1 ± 0.8 Hz ( n = 11, p < 0.001). Decreased neuronal activity exhibited close relationships with increased EEG amplitude. These results suggest that acupuncture stimulation to the sacral segment changes the state of animals from light anesthesia to deep anesthesia, and that this change is mediated by GABAergic systems suppressing the activity of noradrenergic LC neurons.

Short-term (FOS) and long-term (FRA) protein expression in rat locus coeruleus neurons during the neurolab mission: contribution of altered gravitational fields, stress, and other factors

Changes in immediate-early gene (IEG) expression during and after space flight were studied in the rat locus coeruleus (LC) during the NASA Neurolab mission. The LC sends widespread projections throughout the brain and releases the neuromodulator norepinephrine. LC neurons respond to natural vestibular stimulation; their firing rate also increases during waking and decreases or ceases during sleep. LC neurons express IEGs such as c-fos after activation. Adult male albino Fisher 344 rats were killed at four mission time points, and the number of Fos- and Fos-related antigen (FRA)-positive LC cells were counted in flight and ground-based control rats. Half of the subjects at each time point were exposed to light for 60 min prior to killing to standardize their sleep–waking state. FRA-expressing cells were more numerous than Fos-expressing cells in both flight- and ground-based subjects. The difference between FRA- and Fos-expressing cells within individuals was significantly larger 24 h after landing in subjects exposed to both space flight and light pulse than in all other subjects at any mission time point. Fos and FRA responses scaled in proportion to the maximum response observed in any single individual showed similar patterns of variation. Analysis of the scaled and combined responses showed that LC IEG levels responded to both gravity changes and light pulses. Subjects exposed to either single stimulus had equivalent responses, significantly greater than those of control subjects maintained in dim light. The combination of gravity change and light pulse gave significantly higher LC responses than either stimulus alone 24 h after takeoff, and to a lesser extent after 12 days in space; the highest responses were obtained 24 h after landing. By 14 days after landing, animals exposed to space flight and light pulse responded no differently than ground-based subjects. No difference in LC IEG expression was clearly attributable to changes in the sleep–waking state of subjects. Activity of noradrenergic LC neurons has been previously shown to modulate IEG expression in target structures. The increased IEG LC activity (seen most especially 24 h after landing) may reflect large-scale activation of noradrenergic neurons that may serve as a trigger for molecular changes in target structures, and be critical for adaptation to gravity changes.

Pineal gland hormone and idiopathic scoliosis: possible effect of melatonin on sleep-related postural mechanisms.

Experimental and clinical evidences indicate that endocrine mechanisms, particularly involving the pineal gland, exert a role in the development of postural deficits leading to the occurrence of idiopatic scoliosis (IS). In particular, experiments performed in bipedal animals have shown that removal of the pineal gland, which secretes melatonin (M), induced a scoliosis, and that in such preparations, administration of this hormone prevented the development of this deformity (cf. 131). It appears also that adolescents with IS showed a reduced level of serum M with respect to age-related control subjects. The possible mechanisms involved in the M regulation of the tonic contraction of the axial musculature have been discussed. It is known that the pineal gland is implicated in the control of circadian rhythms, including the sleep-waking cycle, and that during this cycle there are prominent changes in postural activity, which affect not only the limbs, but also the axial musculature. These changes are characterized by a decrease followed by a suppression of postural activity, which occur particularly during transition from wakefulness to synchronized sleep and, more prominently, to rapid eye movement (REM) sleep. Episodes of postural atonia may also occur during the cataplectic episodes, which are typical of narcolepsy. Cholinergic and/or cholinoceptive neurons located in the dorsal pontine reticular formation (pRF) and the related medullary inhibitory reticulospinal (RS) system, intervene in the suppression of posture during REM sleep, as well as during the cataplectic episodes which occur in narcolepsy. These structures are under the modulatory (inhibitory) influence of the dorsomedial and the dorsolateral pontine tegmentum, where serotoninergic raphe nuclei (RN) neurons and noradrenergic locus coeruleus (LC) neurons are located. We postulated that M may act not only on the circadian pacemaker, but also directly on the pontine tegmental structures involved in the regulation of posture during the animal states indicated above. This hypothesis is supported by the facts that: 1) the dorsal pRF may contain specific binding sites for M; 2) this structure is particularly sensitive to M in adolescents, as well as in adult subjects affected by narcoleptic disturbances leading to cataplexy; 3) M increases the release of serotonin (5-HT), a neurotransmitter which enhances the postural tone by acting on the dorsal pRF: on the other hand, deficits in M levels may lower the activity of the serotoninergic raphe system, thus leading to a decrease or suppression of postural activity similar to that occurring either during REM sleep or during the cataplectic episodes typical of narcoleptic patients; 4) IS patients may show episodes of sleep apnea, a phenomenon which has been attibuted to a reduced tonic contraction of primary and accessory respiratory muscles during REM, resulting from a reduced release of 5-HT at dorsal pontine level. It has been postulated that, if the reduced M and 5-HT levels are subliminal to produce a complete suppression of posture under the conditions reported above, the reduced postural tone, which results from this condition may lead to the development of IS, due to hypotonia which affects the axial musculature. M secretion could be regulated not only by the activity of the serotoninergic raphe neurons projecting to the pineal gland, but probably also by the activity of noradrenergic LC neurons. It is likely that the development of IS, which results from a reduced level of M and 5-HT, may occur provided that the noradrenergic LC inhibition of the pontine structures is impaired. Such impairment could depend upon genetic factors, similar to those postulated to play a role in narcolepsy. In conclusion, the possibility exists that an impaired activity of brain monoaminergic systems may lead to disfunction in the production of M, which is apparently an important factor in the etiopathogenesis of IS.

Hippocampal norepinephrine-like voltammetric responses following infusion of corticotropin-releasing factor into the locus coeruleus

Intracerebroventricular (i.c.v.) administration of corticotropin-releasing factor (CRF) increases the activity of noradrenergic neurons in the locus coeruleus (LC) assessed by electrophysiological and neurochemical studies. It has been suggested that this effect of i.c.v. CRF is exerted directly on LC noradrenergic (LC-NE) neurons. Infusion of CRF directly into the LC increases cortical and hippocampal release of norepinephrine (NE) as indicated by in vivo microdialysis studies, but the electrophysiological studies have shown both increases and decreases. The present study used in vivo voltammetry to study changes in the extracellular concentrations of NE in the rat hippocampus in response to infusion of CRF (100 ng) into the LC. When the infusion cannula was located in or very close to the LC, the immediate response to CRF was a small decrease in the NE-like oxidation current, followed by a robust increase after about 6–7 min. The oxidation current reached a peak around 13 min and returned to baseline by about 30 min after CRF infusion. By contrast with CRF, infusion of glutamate into the LC increased the oxidation current with a delay of around 30 s and a peak within 90 s. The responses to LC infusion of CRF in rats treated with DSP-4 to deplete hippocampal NE were substantially smaller than those in untreated rats, suggesting that the oxidation signals in untreated rats reflected changes in concentrations of NE. The response to glutamate was markedly augmented by pretreatment with the NE reuptake inhibitor, desmethylimipramine, suggesting that the observed responses reflected changes in NE. Infusion of the same dose of CRF into brain structures outside the LC did not elicit consistent changes in oxidation current in the hippocampus. The time course of the responses to CRF is compatible with previously reported electrophysiological responses of LC-NE neurons to CRF and with neurochemical evidence indicating that CRF can affect the activity of LC-NE neurons. The results indicate that CRF may act in or close to the LC to induce release of hippocampal NE, but the delayed response to CRF compared with that to glutamate, suggests that CRF does not directly activate LC-NE neurons.

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.

Antagonizing effects of VA-045 on reduced activity of rat locus coeruleus neurons following head injury or intravenous injection of clonidine

Based on the finding that VA-045, a novel apovincaminic acid derivative, had improved disturbance in consciousness, we examined the effects of the drug on the electrical activity of locus coeruleus (LC) neurons in animal models of consciousness disturbance. The animal models of consciousness disturbance used in this experiment were closed head injury (CHI) and intravenous injection of clonidine. CHI as well as clonidine injection reduced the spontaneous activity of LC neurons. The reduction of the spontaneus activity of LC neurons following CHI or clonidine injection was restored by intravenous injection of VA-045. The change of LC neuronal activity induced by VA-045 preceded desynchronization of EEG. These results suggest that VA-045 exerts its ameliorating effect on consciousness disturbance, at least in part, by augmenting the spontaneous activity of noradrenergic LC neurons.

Effects of chronic desipramine administration on the locus coeruleus neuronal activity in the learned helplessness paradigm

One week after inescapable shock (IS) exposure, animals showed an increased number of escape failures in the shuttlebox and a decreased noradrenergic (NA) transmission. The latter was assessed by electric activity of locus coeruleus (LC) neurons. Seven daily injections of desipramine (DMI) were effective in reducing the number of escapes failures in the shuttlebox, but such treatment did not modify the decreased electric activity of LC NA neurons observed 7 days after IS. These results are discussed in terms of the action of DMI on NA terminals in reversing the behavioral deficit induced by IS.