LC-NE Neurons Research Articles

Electrophysiological Effects of Repeated Administration of Agomelatine on the Dopamine, Norepinephrine, and Serotonin Systems in the Rat Brain

Agomelatine is a melatonergic MT1/MT2 agonist and a serotonin (5-HT) 5-HT(2C) antagonist. The effects of 2-day and 14-day administration of agomelatine were investigated on the activity of ventral tegmental area (VTA) dopamine (DA), locus coeruleus (LC) norepinephrine (NE), and dorsal raphe nucleus (DRN) 5-HT neurons using in vivo electrophysiology in rats. The 5-HT(1A) transmission was assessed at hippocampus CA3 pyramidal neurons. After a 2-day regimen of agomelatine (40 mg/kg/day, i.p.), an increase in the number of spontaneously active VTA-DA neurons (p<0.001) and in the firing rate of LC-NE neurons (p<0.001) was observed. After 14 days, the administration of agomelatine induced an increase in: (1) the number of spontaneously active DA neurons (p<0.05), (2) the bursting activity of DA neurons (bursts/min, p<0.01 and percentage of spikes occurring in bursts, p<0.05), (3) the firing rate of DRN-5-HT neurons (p<0.05), and (4) the tonic activation of postsynaptic 5-HT(1A) receptors located in the hippocampus. The increase in 5-HT firing rate was D2 dependent, as it was antagonized by the D2 receptor antagonist paliperidone. The enhancement of NE firing was restored by the 5-HT(2A) receptor antagonist MDL-100,907 after the 14-day regimen. All the effects of agomelatine were antagonized by a single administration of the melatonergic antagonist S22153 (except for the increase in the percentage of spikes occurring in burst for DA neurons). The present results suggest that (1) agomelatine exerts direct (2 days) and indirect (14 days) modulations of monoaminergic neuronal activity and (2) the melatonergic agonistic activity of agomelatine contributes to the enhancement of DA and 5-HT neurotransmission.

Atomoxetine modulates spontaneous and sensory-evoked discharge of locus coeruleus noradrenergic neurons

Atomoxetine (ATM) is a potent norepinephrine (NE) uptake inhibitor and increases both NE and dopamine synaptic levels in prefrontal cortex, where it is thought to exert its beneficial effects on attention and impulsivity. At the behavioral level, ATM has been shown to cause improvements on the measures of executive functions, such as response inhibition, working memory and attentional set shifting across different species. However, the exact mechanism of action for ATM's effects on cognition is still not clear. One possible target for the cognitive enhancing effects of ATM is the noradrenergic locus coeruleus (LC), the only source of NE to key forebrain areas such as cerebral cortex and hippocampus. Although it is known that ATM increases NE availability overall by blocking reuptake of NE, the effects of this agent on impulse activity of LC neurons have not been reported. Here, the effect of ATM (0.1–1 mg/kg, ip) on NE-LC neurons was investigated by recording extracellular activity of LC neurons in isoflurane-anesthetized rats. ATM caused a significant decrease of the tonic activity of LC single-units, although leaving intact the sensory-evoked excitatory component of LC phasic response. Moreover, the magnitude of the inhibitory component of LC response to paw stimulation was increased after 1 mg/kg of ATM and its duration was prolonged at 0.3 mg/kg. Together, these effects of ATM produced an increase in the phasic-to-tonic ratio of LC phasic response to sensory stimulation. ATM also modulated the average sensory-evoked local field potential (LFP) and spike-field coherence in LC depending on the dose tested. The lower dose (0.1 mg/kg) significantly decreased early positive and negative components of the sensory-evoked LFP response. Higher doses (0.3–1 mg/kg) initially increased and then decreased the amplitude of components of the evoked fields, whereas the spike-field coherence was enhanced by 1 mg/kg ATM across frequency bands. Finally, coherence between LC fields and EEG signals was generally increased by 1 mg/kg ATM, whereas 0.1 and 0.3 mg/kg respectively decreased and increased coherence values in specific frequency bands. Taken together these results suggest that ATM effects on LC neuronal activity are dose-dependent, with different doses affecting different aspects of LC firing. This modulation of activity of LC-NE neurons may play a role in the cognitive effects of ATM.This article is part of a Special Issue entitled ‘Cognitive Enhancers’.

Early and late consolidation and reconsolidation of memory in the prelimbic cortex

Motivation and attention can have a profound influence on perception, learning and memory. Neuromodulatory systems, especially the noradrenergic (NE) system, co-vary with psychological states to modulate cortical arousal, influence sensory processing and promote synaptic plasticity. There is even some suggestion that the NE system might facilitate functional recovery after brain damage. Post-synaptic effects of NE in its ubiquitous projection regions have been well studied, but factors controlling activation of NE neurons are less known. The pontine nucleus locus coeruleus (LC) contains the entire population of NE neurons projecting to the forebrain and all cortical and thalamic regions receive NE input. Using single unit recording in freely moving rats, we have been studying neuroanatomical circuits, along with sensory stimuli within their cognitive contexts that control LC firing. Rats are implanted with movable microelectrodes to record activity during a variety of behavioral situations: exploration of novelty in a hole-board, response to tones or odors that predict reward or absence of reward, extinction or reversal of stimulus-reward association. We find that LC-NE neurons respond to novel or salient stimuli, habituate rapidly to respond anew when the stimulus is associated with reward (CS+), particularly in the early stage of learning. Recent experiments suggest that these responses are more related to reward expectancy than to the CS+. Importantly, there is robust LC response to any change in the predictive value of the stimulus, i.e. when new learning must occur. NE released by LC activation will promote the underlying synaptic plasticity.

Brain circuits involved in corticotropin-releasing factor-norepinephrine interactions during stress.

Corticotropin-releasing factor (CRF)- and norepinephrine (NE)-containing neurons in the brain are activated during stress, and both have been implicated in the behavioral responses. NE neurons in the brain stem can stimulate CRF neurons in the hypothalamic paraventricular nucleus (PVN) to activate the hypothalamic-pituitary-adrenocortical axis and may affect other CRF neurons. CRF-containing neurons in the PVN, the amygdala, and other brain areas project to the area of the locus coeruleus (LC), and CRF injected into the LC alters the electrophysiologic activity of LC-NE neurons. Neurochemical studies have indicated that CRF applied intracerebroventricularly or locally activates the LC-NE system, and microdialysis and chronoamperometric measurements indicate increased NE release in LC-NE terminal fields. However, chronoamperometric studies indicated a significant delay in the increase in NE release, suggesting that the CRF input to LC-NE neurons is indirect. The reciprocal interactions between cerebral NE and CRF systems have been proposed to create a "feed-forward" loop. It has been postulated that a sensitization of such a feed-forward loop may underlie clinical depression. However, in the majority of studies, repeated or chronic stress has been shown to decrease the behavioral and the neurochemical responsivity to acute stressors. Repeated stress also seems to decrease the responsivity of LC neurons to CRF. These results do not provide support for a feed-forward hypothesis. However, a few studies using certain tasks have indicated sensitization, and some other studies have suggested that the effect of CRF may be dose dependent. Further investigations are necessary to establish the validity or otherwise of the feed-forward hypothesis.

Glial cell line-derived neurotrophic factor is essential for neuronal survival in the locus coeruleus–hippocampal noradrenergic pathway

It has been shown that the noradrenergic (NE) locus coeruleus (LC)–hippocampal pathway plays an important role in learning and memory processing, and that the development of this transmitter pathway is influenced by neurotrophic factors. Although some of these factors have been discovered, the regulatory mechanisms for this developmental event have not been fully elucidated. Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor influencing LC-NE neurons. We have utilized a GDNF knockout animal model to explore its function on the LC-NE transmitter system during development, particularly with respect to target innervation. By transplanting various combinations of brainstem (including LC) and hippocampal tissues from wildtype or GDNF knockout fetuses into the brains of adult wildtype mice, we demonstrate that normal postnatal development of brainstem LC-NE neurons is disrupted as a result of the GDNF null mutation. Tyrosine hydroxylase immunohistochemistry revealed that brainstem grafts had markedly reduced number and size of LC neurons in transplants from knockout fetuses. NE fiber innervation into the hippocampal co-transplant from an adjacent brainstem graft was also influenced by the presence of GDNF, with a significantly more robust innervation observed in transplants from wildtype fetuses. The most successful LC/hippocampal co-grafts were generated from fetuses expressing the wildtype GDNF background, whereas the most severely affected transplants were derived from double transplants from null-mutated fetuses. Our data suggest that development of the NE LC–hippocampal pathway is dependent on the presence of GDNF, most likely through a target-derived neurotrophic function.

The Noradrenergic System of Aged GDNF Heterozygous Mice.

Glial cell line-derived neurotrophic factor (GDNF) is a trophic factor for noradrenergic (NE) neurons of the pontine nucleus locus coeruleus (LC). Decreased function of the LC-NE neurons has been found during normal aging and in neurodegenerative disorders. We have previously shown that GDNF participates in the differentiation of LC-NE neurons during development. However, the continued role of GDNF for LC-NE neurons during maturation and aging has not been addressed. We examined alterations in aged mice that were heterozygous for the GDNF gene (Gdnf+/-). Wild-type (Gdnf+/+) and Gdnf+/- mice (18 months old) were tested for locomotor activity and brain tissues were collected for measuring norepinephrine levels and uptake, as well as for morphological analysis. Spontaneous locomotion was reduced in Gdnf+/- mice in comparison with Gdnf+/+ mice. The reduced locomotor activity of Gdnf +/- mice was accompanied by reductions in NE transporter activity in the cerebellum and brain stem as well as decreased norepinephrine tissue levels in the LC. Tyrosine hydroxylase (TH) immunostaining demonstrated morphological alterations of LC-NE cell bodies and abnormal TH-positive fibers in the hippocampus, cerebellum, and frontal cortex of Gdnf+/- mice. These findings suggest that the LC-NE system of Gdnf+/- mice is impaired and suggest that GDNF plays an important role in continued maintenance of this neuronal system throughout life.

Hippocampus and locus coeruleus activity on rats chronically treated with diazepam

The neural mechanisms underlying benzodiazepine (BZD) dependence remain equivocal. The present studies tested the hypothesis that similar neural circuitry might be involved in the effects of chronic 7-chloro-1-methyl-5-phenyl-3 H-1,4-benzodiazepine-2(1 H)-one, diazepam (DZ, Roche), administration and withdrawal. The results of our study showed an increased hippocampal synaptic plasticity in slices from rats chronically treated with DZ (5 mg/kg/18 days), assessed as a decrease of the threshold in the stimulation rate for long-term potentiation (LTP) elicitation. Rats with the same schedule of DZ administration but without signs of withdrawal behaved similarly to vehicle-treated ones (VEH), in the threshold to induce LTP. Furthermore, the activity of locus coeruleus (LC) norepinephrine (NE) neurons in rats tested 24 h after the last DZ injection showed a significant increase. On the other hand, rats that after chronic DZ administration did not develop signs of withdrawal and exhibited a similar pattern of discharge on LC-NE nucleus compared with their controls. We conclude that chronic DZ administration enhances both hippocampal synaptic plasticity and activity of LC-NE neurons. This neural system could be the biological substrate underlying the behavioral alterations accompanying chronic DZ administration and withdrawal.

Discrete local application of corticotropin-releasing factor increases locus coeruleus discharge and extracellular norepinephrine in rat hippocampus

The most prominent afferents impinging upon the noradrenergic neurons of the locus coeruleus (LC) utilize GABA and glutamate. However, peptide neurotransmitters such as galanin, neuropeptide Y, and corticotropin-releasing factor (CRF) have also been localized to LC afferents. The evidence for CRF modulation of LC activity was examined in the present studies. Specifically, the impact of local CRF administration on both LC-NE discharge characteristics and release of norepinephrine (NE) in hippocampus was determined. First, the ability of CRF microinfused into the LC area to increase NE efflux in the dorsal hippocampus was determined using in vivo microdialysis techniques in awake rats. CRF into the LC dose-dependently increased extracellular NE in the ipsilateral hippocampus. Second, a more detailed analysis was performed in halothane-anesthetized rats by characterizing the electrophysiological activity of LC-NE neurons in response to local application of CRF. Changes in the firing rate and pattern of single LC-NE neurons were measured while simultaneously monitoring the extracellular level of NE in hippocampus. A dose of 30 ng CRF applied directly into LC via pressure ejection elicited an 88% increase in the discharge rate of LC-NE neurons and increased the incidence of burst firing from 14% to 33%. This manipulation simultaneously increased extracellular NE in hippocampus by 63%. The CRF-induced increases in discharge rate of LC-NE neurons and extracellular NE efflux in hippocampus were prevented by prior i.c.v. administration of the CRF antagonist, d-PheCRF(12-41 )(3 microg / 3 microl). The present findings demonstrate that CRF applied directly into the LC increases both the activity of LC-NE neurons and the release of NE in an LC terminal region. The shift in activity of LC-NE neurons to more burst-like firing in response to CRF may provide a means for enhanced release of NE in LC projection fields. This is the first report to demonstrate a dose-dependent increase in extracellular NE levels evoked by intra-LC infusion of CRF in unanesthetized animals.

Cortical Catecholamine Secretion Following Intravenous Nitroprusside Infusion: A Voltammetric Study

Intravenous administration of sodium nitroprusside (NP) decreases blood pressure and activates noradrenergic neurons in the locus coeruleus (LC). Microdialysis studies have shown that NP infusion is accompanied by increased extracellular concentrations of norepinephrine (NE) in the medial prefrontal cortex. The present study used in vivo voltammetry to obtain a finer temporal analysis of the NP-induced changes in the extracellular concentrations of catecholamine-like compounds in the LC terminal fields in the rat medial prefrontal cortex. Intravenous infusion of rats with NP caused a rapid decrease in blood pressure that lasted for the duration of the infusion but rapidly reversed when the infusion was terminated. After a delay of between about 2 and 8 min (mean 5 min), there was an increase in extracellular concentrations of a NE-like substance. Presumed cortical release of NE lasted for several minutes but had almost returned to baseline by the time the NP infusion was terminated at 15 min. In many cases, the first peak was followed by a second one, usually of smaller amplitude but more prolonged than the first one. There was no clear response to the cessation of infusion of NP. The time course of the initial response is comparable to the previously reported electrophysiological response of LC-NE neurons to NP. In rats treated with DSP-4 to deplete cortical NE, blood pressure was reduced as in untreated rats, but no voltammetric response to NP infusion was observed. These results suggest that activation of the NE-LC neurons by NP results in a delayed synaptic release of NE in the cerebral cortex which attenuates within several minutes.

Single unit activity of noradrenergic neurons in locus coeruleus and serotonergic neurons in the nucleus raphe dorsalis of freely moving cats in relation to the cardiac cycle

Single unit activity of serotonergic (5-HT) neurons in the nucleus raphe dorsalis (NRD) and of noradrenergic (NE) neurons in the locus coeruleus (LC) was recorded in relation to the cardiac cycle in awake, freely moving cats. The discharge of NRD-5-HT neurons showed no relationship to the cardiac cycle, while LC-NE neurons displayed a cardiac periodicity such that the units were most likely to fire from 80 to 180 ms after the peak of the cardiac r-wave (diastole), and least likely to fire during the period from 40 ms before to 60 ms after the r-wave (systole). The strength of this periodicity was inversely related to the discharge rate of individual cells. Exposure to a noxious environmental stimulus (15 min of 100 dB white noise) greatly attenuated the cardiac relationship of LC-NE neurons. A blood volume expansion of approximately 15% (9.0 ml/kg b. wt.) decreased unit rate by about 25%, but did not alter either the timing or the magnitude of the LC-NE cardiac relationship. These data are discussed in terms of the participation of NRD-5-HT and LC-NE neurons in cardiovascular function, and the possible role of LC-NE neurons in short-and long-term volume homeostasis.