alpha2-Adrenoceptor-mediated potassium currents in acutely dissociated rat locus coeruleus neurones

Noradrenergic current responses of neurons in rat oculomotor neural integrators

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are involved in the control of horizontal and vertical gaze, respectively. A previous study showed that PHN neurons exhibit depolarized or hyperpolarized responses to noradrenaline (NA). However, the adrenoceptor types that participate in NA-induced responses and the effects of NA on INC neurons have not yet been investigated. Furthermore, the relationship between NA-induced responses and neuron types defined by neurotransmitter phenotypes has not been determined. In this study, we investigated NA-induced current responses in PHN and INC neurons and the relationships between these responses and neuron types using whole cell recordings in wild-type and transgenic rat brainstem slices. Local application of NA to the cell soma induced slow inward (SI) and slow outward (SO) currents that were mainly mediated by α1 and α2 adrenoceptors, respectively. These current responses were observed in both PHN and INC neurons, although the proportion of INC neurons that responded to NA was low. Analyses of the distributions of the current responses revealed that in the PHN, all fluorescently identified inhibitory neurons exhibited SI currents, whereas glutamatergic and cholinergic neurons exhibited both SI and SO currents. In the INC, glutamatergic and inhibitory neurons preferentially exhibited SI and SO currents, respectively. When the PHN and INC neurons were characterized by their firing pattern, we found that the proportions of the currents depended on their firing pattern. These results suggest that various modes of noradrenergic modulation in horizontal and vertical neural integrators are dependent on neuron type.NEW & NOTEWORTHY Noradrenergic modulation of oculomotor neural integrators involved in gaze control has not been elucidated. Here, we report that noradrenaline (NA)-induced slow inward (SI) and outward (SO) currents are mediated mainly by α1 and α2 adrenoceptors in neurons that participate in horizontal and vertical gaze control. The NA-induced current responses differed depending on the neurotransmitter phenotype and firing pattern. These results suggest various modes of noradrenergic modulation in horizontal and vertical integrator neurons.

The Na+ leak channel NALCN controls spontaneous activity and mediates synaptic modulation by α2-adrenergic receptors in auditory neurons

Cartwheel interneurons of the dorsal cochlear nucleus (DCN) potently suppress multisensory signals that converge with primary auditory afferent input, and thus regulate auditory processing. Noradrenergic fibers from locus coeruleus project to the DCN, and α2-adrenergic receptors inhibit spontaneous spike activity but simultaneously enhance synaptic strength in cartwheel cells, a dual effect leading to enhanced signal-to-noise for inhibition. However, the ionic mechanism of this striking modulation is unknown. We generated a glycinergic neuron-specific knockout of the Na+ leak channel NALCN in mice and found that its presence was required for spontaneous firing in cartwheel cells. Activation of α2-adrenergic receptors inhibited both NALCN and spike generation, and this modulation was absent in the NALCN knockout. Moreover, α2-dependent enhancement of synaptic strength was also absent in the knockout. GABAB receptors mediated inhibition through NALCN as well, acting on the same population of channels as α2 receptors, suggesting close apposition of both receptor subtypes with NALCN. Thus, multiple neuromodulatory systems determine the impact of synaptic inhibition by suppressing the excitatory leak channel, NALCN.

The Na+ leak channel NALCN controls spontaneous activity and mediates synaptic modulation by α2-adrenergic receptors in auditory neurons

Cartwheel interneurons of the dorsal cochlear nucleus (DCN) potently suppress multisensory signals that converge with primary auditory afferent input, and thus regulate auditory processing. Noradrenergic fibers from locus coeruleus project to the DCN, and α2-adrenergic receptors inhibit spontaneous spike activity but simultaneously enhance synaptic strength in cartwheel cells, a dual effect leading to enhanced signal-to-noise for inhibition. However, the ionic mechanism of this striking modulation is unknown. We generated a glycinergic neuron-specific knockout of the Na+ leak channel NALCN, and found that its presence was required for spontaneous firing in cartwheel cells. Activation of α2-adrenergic receptors inhibited both NALCN and spike generation, and this modulation was absent in the NALCN knockout. Moreover, α2-dependent enhancement of synaptic strength was also absent in the knockout. GABAB receptors mediated inhibition through NALCN as well, acting on the same population of channels as α2 receptors, suggesting close apposition of both receptor subtypes with NALCN. Thus, multiple neuromodulatory systems determine the impact of synaptic inhibition by suppressing the excitatory leak channel, NALCN.

Expression and localization of α2A-adrenergic receptor in the rat post-natal developing cochlea

Lots of adrenergic receptors (ARs) are widely present across the auditory pathways and are positioned to affect auditory and vestibular functions. However, noradrenergic regulation in the cochlea has not been well characterized. In this study, a rat model of noise-induced hearing loss was developed to investigate the expression of α2A-adrenergic receptor (AR) after acoustic trauma, then, we investigated the expression of α2A-AR in the developing rat cochlea using immunofluorescence, qRT-PCR, and Western blotting. We found that the expression of α2A-AR significantly increased in rats exposed to noise compared with controls. Immunofluorescence analysis demonstrated that α2A-AR is localized on hair cells (HCs), spiral ganglion neurons (SGNs), and the stria vascularis (SV) in the postnatal developing cochlea from post-natal day (P) 0 to P28. Furthermore, we observed α2A-AR mRNA reached a maximum level at P14 and P28 when compared with P0, while no significant differences in α2A-AR protein levels at the various stages when compared with P0. This study provides direct evidence for the expression of α2A-AR in HCs, SGNs, and the SV of the cochlea, indicating that norepinephrine might play a vital role in hearing function within the cochlea through α2A-AR.

Prenatal exposure to morphine enhances excitability in locus coeruleus neurons

Opioid abuse during pregnancy may have noteworthy effects on the child's behavioral, emotional and cognitive progression. In this study, we assessed the effect of prenatal exposure to morphine on electrophysiological features of locus coeruleus (LC) noradrenergic neurons which is involved in modulating cognitive performance. Pregnant dams were randomly divided into two groups, that is a prenatal saline treated and prenatal morphine-treated group. To this end, on gestational days 11-18, either morphine or saline (twice daily, s.c.) was administered to pregnant dams. Whole-cell patch-clamp recordings were conducted on LC neurons of male offspring. The evoked firing rate, instantaneous frequency and action potentials half-width, and also input resistance of LC neurons significantly increased in the prenatal morphine group compared to the saline group. Moreover, action potentials decay slope, after hyperpolarization amplitude, rheobase current, and first spike latency were diminished in LC neurons following prenatal exposure to morphine. In addition, resting membrane potential, rise slope, and amplitude of action potentials were not changed by prenatal morphine exposure. Together, the current findings show a significant enhancement in excitability of the LC neurons following prenatal morphine exposure, which may affect the release of norepinephrine to other brain regions and/or cognitive performances of the offspring.

The Nature of Noradrenergic Volume Transmission From Locus Coeruleus to Brainstem Mesencephalic Trigeminal Sensory Neurons

Noradrenergic neurons in the locus coeruleus (LC) release noradrenaline (NA) that acts via volume transmission to activate extrasynaptic G-protein coupled receptors (GPCRs) in target cells throughout the brain. As the closest projection, the dorsal LC laterally adjoins the mesencephalic trigeminal nucleus (MTN), in which proprioceptive primary sensory neurons innervating muscle spindles of jaw-closing muscles are exceptionally located. MTN neurons express α₂-adrenergic receptors (α₂-ARs) and display hyperpolarization-activated cyclic nucleotide-gated (HCN) currents (Ihs), which is downregulated by α₂-AR activation. To quantify the activity-dependent outcome of volume transmission of NA from LC to MTN, we investigated how direct LC activation inhibits Ih in MTN neurons by performing dual whole-cell recordings from LC and MTN neurons. Repetition of 20 Hz spike-train evoked with 1-s current-pulse in LC neurons every 30 s resulted in a gradual decrease in Ih evoked every 30 s, revealing a Hill-type relationship between the number of spike-trains in LC neurons and the degree of Ih inhibition in MTN neurons. On the other hand, when microstimulation was applied in LC every 30 s, an LC neuron repeatedly displayed a transient higher-frequency firing followed by a tonic firing at 5–10 Hz for 30 s. This subsequently caused a similar Hill-type inhibition of Ih in the simultaneously recorded MTN neuron, but with a smaller Hill coefficient, suggesting a lower signal transduction efficacy. In contrast, 20 Hz activity induced by a 1-s pulse applied every 5–10 s caused only a transient facilitation of Ih inhibition followed by a forced termination of Ih inhibition. Thus, the three modes of LC activities modulated the volume transmission to activate α₂-adrenergic GPCR to differentially inhibit Ih in MTN neurons.

Preliminary evaluation of the efficacy and safety of brimonidine for general anesthesia

Background

To determine the hypnotic and analgesic effects of brimonidine, and evaluate its efficacy and safety for general anesthesia. Potentiation of pentobarbital sleeping time following brimonidine administration was observed in mice, as was the analgesic activity of brimonidine.

Methods

The median effective dose (ED₅₀) and lethal dose (LD₅₀) of intraperitoneally injected brimonidine were determined in hypnotized mice. In addition, the LD₅₀ of intravenously injected brimonidine, and ED₅₀ of intravenously, intramuscularly, and intrarectally injected brimonidine in hypnotized rabbits were determined. Finally, the synergistic anesthetic effect of brimonidine and chloral hydrate was evaluated in rabbits.

Results

Intraperitoneal injection of 10 mg/kg brimonidine enhanced the hypnotic effect of a threshold dose of pentobarbital. Intraperitoneally injected brimonidine produced dose-related analgesic effects in mice. The ED₅₀ of intraperitoneally administered brimonidine in hypnotized mice was 75.7 mg/kg and the LD₅₀ was 379 mg/kg. ED₅₀ values of intravenous, intramuscular, and intrarectal brimonidine for hypnosis in rabbits were 5.2 mg/kg, 8.8 mg/kg, and 8.7 mg/kg, respectively; the LD₅₀ of intravenous brimonidine was 146 mg/kg. Combined intravenous administration of 0.6 mg/kg brimonidine and 0.03 g/kg chloral hydrate had a synergistic anesthetic effect.

Conclusions

Brimonidine elicited hypnotic and analgesic effects after systemic administration and exhibited safety. Moreover, brimonidine enhanced the effects of other types of narcotics when combined.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12871-021-01516-1.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Modulation of Noradrenergic and Serotonergic Systems by Cannabinoids: Electrophysiological, Neurochemical and Behavioral Evidence

The main noradrenergic and serotonergic nuclei in the central nervous system (CNS) are the locus coeruleus (LC) and the dorsal raphe nucleus (DRN). These brain areas, located in the brainstem, play a pivotal role in the control of various functions and behaviors that are altered by cannabinoids (i.e., pain, arousal, mood, anxiety, or sleep-wake cycle). Anatomical, neurochemical, and functional data suggest that cannabinoids regulate both central noradrenergic and serotonergic neurotransmission. Thus, strong evidence has shown that the firing activity of LC and DRN monoamine neurons or the synthesis/release of noradrenaline (NA) and serotonin (5-HT) in the projection areas are all affected by cannabinoid administration. Herein, we propose that interaction between the endocannabinoid system and the noradrenergic-serotonergic systems could account for some of the anxiolytic, antidepressant, and antinociceptive effects of cannabinoids or the disruption of attention/sleep induced by these drugs.

Axonal projection-specific differences in somatodendritic α2 autoreceptor function in locus coeruleus neurons

The locus coeruleus (LC) contains the majority of central noradrenergic neurons sending wide projections throughout the entire CNS. The LC is considered to be essential for multiple key brain functions including arousal, attention and adaptive stress responses as well as higher cognitive functions and memory. Electrophysiological studies of LC neurons have identified several characteristic functional features such as low-frequency pacemaker activity with broad action potentials, transient high-frequency burst discharges in response to salient stimuli and an apparently homogeneous inhibition of firing by activation of somatodendritic α2 autoreceptors (α2AR). While stress-mediated plasticity of the α2AR response has been described, it is currently unclear whether different LC neurons projecting to distinct axonal targets display differences in α2AR function. Using fluorescent beads-mediated retrograde tracing in adult C57Bl6/N mice, we compared the anatomical distributions and functional in vitro properties of identified LC neurons projecting either to medial prefrontal cortex, hippocampus or cerebellum. The functional in vitro analysis of LC neurons confirmed their mostly uniform functional properties regarding action potential generation and pacemaker firing. However, we identified significant differences in tonic and evoked α2AR-mediated responses. While hippocampal-projecting LC neurons were partially inhibited by endogenous levels of norepinephrine and almost completely silenced by application of saturating concentrations of the α2 agonist clonidine, prefrontal-projecting LC neurons were not affected by endogenous levels of norepinephrine and only partially inhibited by saturating concentrations of clonidine. Thus, we identified a limited α2AR control of electrical activity for prefrontal-projecting LC neurons indicative of functional heterogeneity in the LC-noradrenergic system.

Chronic fluoxetine reverses the effects of chronic corticosterone treatment on α2-adrenoceptors in the rat frontal cortex but not locus coeruleus

Disruption of the hypothalamic-pituitary-adrenal axis is an established finding in patients with anxiety and/or depression. Chronic corticosterone administration in animals has been proposed as a model for the study of these stress-related disorders and the antidepressant action. Alterations of the central noradrenergic system and specifically of inhibitory α₂-adrenoceptors seem to be part of the pathophysiology of depression and contribute to the antidepressant activity. The present study evaluates in male rats the effect of chronic corticosterone treatment during 35 days (16-20 mg kg^-1 day^-1) on the sensitivity of α₂-adrenoceptors expressed in the somatodendritic and terminal noradrenergic areas locus coeruleus (LC) and prefrontal cortex (PFC), respectively. Further, the effect of chronic fluoxetine treatment (5 mg kg^-1, i.p., since the 15th day) on the sensitivity of α₂-adrenoceptors was examined under control conditions and in corticosterone-treated rats. The α₂-adrenoceptor functionality was analysed in vitro by agonist-mediated [³⁵S]GTPγS binding stimulation and in vivo through the modulation of noradrenaline (NA) release evaluated by dual-probe microdialysis. The concentration-effect curves of the [³⁵S]GTPγS binding stimulation by the agonist UK14304 (5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) demonstrated a desensitization of cortical α₂-adrenoceptors induced by corticosterone (-logEC₅₀ = 6.7 ± 0.2 vs 8.2 ± 0.3 in controls) that was reverted by fluoxetine treatment (-logEC₅₀ = 7.5 ± 0.3). Local administration of the α₂-adrenoceptor antagonist RS79948 ((8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulfonyl)-6H-isoquino[2,1-g][1,6]naphthyridine) (0.1-100 μmol L^-1) into the LC induced a concentration-dependent NA increase in the PFC of the control group (E_max = 191 ± 30%) but non-significant effect was observed in corticosterone-treated rats (E_max = 133 ± 46%), reflecting a desensitization of α₂-adrenoceptors that control the firing of noradrenergic neurons. Fluoxetine treatment did not alter the corticosterone-induced desensitization in this area (E_max = 136 ± 19%). No effect of fluoxetine on α₂-adrenoceptor functionality was observed in control animals (E_max = 223 ± 30%). In PFC, the local administration of RS79948 increased NA in controls (E_max = 226 ± 27%) without effect in the corticosterone group (E_max = 115 ± 26%), suggesting a corticosterone-induced desensitization of terminal α₂-adrenoceptors. Fluoxetine administration prevented the desensitization induced by corticosterone in the PFC (E_max = 233 ± 33%) whereas desensitized α₂-adrenoceptors in control animals (E_max = -24 ± 10%). These data indicate that chronic corticosterone increases noradrenergic activity by acting at different α₂-adrenoceptor subpopulations. Treatment with the antidepressant fluoxetine seems to counteract these changes by acting mainly on presynaptic α₂-adrenoceptors expressed in terminal areas.

Bidirectional and context-dependent changes in theta and gamma oscillatory brain activity in noradrenergic cell-specific Hypocretin/Orexin receptor 1-KO mice

Noradrenaline (NA) and hypocretins/orexins (HCRT), and their receptors, dynamically modulate the circuits that configure behavioral states, and their associated oscillatory activities. Salient stimuli activate spiking of locus coeruleus noradrenergic (NA^LC) cells, inducing NA release and brain-wide noradrenergic signalling, thus resetting network activity, and mediating an orienting response. Hypothalamic HCRT neurons provide one of the densest input to NA^LC cells. To functionally address the HCRT-to-NA connection, we selectively disrupted the Hcrtr1 gene in NA neurons, and analyzed resulting (Hcrtr1^Dbh-CKO) mice’, and their control littermates’ electrocortical response in several contexts of enhanced arousal. Under enforced wakefulness (EW), or after cage change (CC), Hcrtr1^Dbh-CKO mice exhibited a weakened ability to lower infra-θ frequencies (1–7 Hz), and mount a robust, narrow-bandwidth, high-frequency θ rhythm (~8.5 Hz). A fast-γ (55–80 Hz) response, whose dynamics closely parallelled θ, also diminished, while β/slow-γ activity (15–45 Hz) increased. Furthermore, EW-associated locomotion was lower. Surprisingly, nestbuilding-associated wakefulness, inversely, featured enhanced θ and fast-γ activities. Thus HCRT-to-NA signalling may fine-tune arousal, up in alarming conditions, and down during self-motivated, goal-driven behaviors. Lastly, slow-wave-sleep following EW and CC, but not nestbuilding, was severely deficient in slow-δ waves (0.75–2.25 Hz), suggesting that HCRT-to-NA signalling regulates the slow-δ rebound characterizing sleep after stress-associated arousal.

Chemogenetic and Optogenetic Activation of Gαs Signaling in the Basolateral Amygdala Induces Acute and Social Anxiety-Like States

Anxiety disorders are debilitating psychiatric illnesses with detrimental effects on human health. These heightened states of arousal are often in the absence of obvious threatening cues and are difficult to treat owing to a lack of understanding of the neural circuitry and cellular machinery mediating these conditions. Activation of noradrenergic circuitry in the basolateral amygdala is thought to have a role in stress, fear, and anxiety, and the specific cell and receptor types responsible is an active area of investigation. Here we take advantage of two novel cellular approaches to dissect the contributions of G-protein signaling in acute and social anxiety-like states. We used a chemogenetic approach utilizing the Gαs DREADD (rM3Ds) receptor and show that selective activation of generic Gαs signaling is sufficient to induce acute and social anxiety-like behavioral states in mice. Second, we use a recently characterized chimeric receptor composed of rhodopsin and the β2-adrenergic receptor (Opto-β2AR) with in vivo optogenetic techniques to selectively activate Gαs β-adrenergic signaling exclusively within excitatory neurons of the basolateral amygdala. We found that optogenetic induction of β-adrenergic signaling in the basolateral amygdala is sufficient to induce acute and social anxiety-like behavior. These findings support the conclusion that activation of Gαs signaling in the basolateral amygdala has a role in anxiety. These data also suggest that acute and social anxiety-like states may be mediated through signaling pathways identical to β-adrenergic receptors, thus providing support that inhibition of this system may be an effective anxiolytic therapy.

Persistent α1-adrenergic receptor function in the nucleus locus coeruleus causes hyperexcitability in AD/HD model rats

Spontaneously hypertensive rats (SHR) are widely used as a model of attention deficit hyperactivity disorder (ADHD) as their ADHD-like behaviors are restored by methylphenidate. However, a postnatal neural development in SHR is unknown. We performed whole cell patch clamp recordings from locus coeruleus (LC) neurons in neonatal [postnatal day (P) 3–5], juvenile (P21–28), and adult (P 49–56) SHR and age-matched Wistar rats to evaluate α1- and α2-adrenergic receptor (ARs) activities at each developmental period. LC neurons in neonatal Wistar rats and SHR showed no difference in resting membrane potential and spontaneous firing rate, while juvenile and adult SHR LC neurons showed depolarized resting membrane potential and faster spontaneous firing rate than in Wistar rats. Blockade of α1-AR activity by prazosin hyperpolarized the membrane and abolished spontaneous firings in all developmental periods in SHR LC neurons, but not in juvenile and adult Wistar rats. α1-AR stimulation by phenylephrine evoked an inward current in juvenile LC neurons in SHR, but not in juvenile Wistar rats. This phenylephrine-induced inward current was abolished by nonselective cation channel blockers. By contrast, α2-AR stimulation-induced outward currents in the presence of an α1-AR antagonist were equivalent in SHR and Wistar LC neurons. These data suggest that Wistar LC neurons lose α1-AR function during development, whereas α1-ARs remain functional in SHR LC neurons. Thus persistent intrinsic activity of α1-ARs may be a neural mechanism contributing to developmental disorders in juvenile SHRs.

The α2B-adrenergic receptor is mutant in cortical myoclonus and epilepsy

OBJECTIVE

Autosomal dominant cortical myoclonus and epilepsy (ADCME) is characterized by distal, fairly rhythmic myoclonus and epilepsy with variable severity. We have previously mapped the disease locus on chromosome 2p11.1-q12.2 by genome-wide linkage analysis. Additional pedigrees affected by similar forms of epilepsy have been associated with chromosomes 8q, 5p, and 3q, but none of the causing genes has been identified. We aim to identify the mutant gene responsible for this form of epilepsy.

METHODS

Genes included in the ADCME critical region were directly sequenced. Coimmunoprecipitation, immunofluorescent, and electrophysiologic approaches to transfected human cells have been utilized for testing the functional significance of the identified mutation.

RESULTS

Here we show that mutation in the α2 -adrenergic receptor subtype B (α2B -AR) is associated with ADCME by identifying a novel in-frame insertion/deletion in 2 Italian families. The mutation alters several conserved residues of the third intracellular loop, hampering neither the α2B -AR plasma membrane localization nor the arrestin-mediated internalization capacity, but altering the binding with the scaffolding protein spinophilin upon neurotransmitter activation. Spinophilin, in turn, regulates interaction of G protein coupled receptors with regulator of G protein signaling proteins. Accordingly, the mutant α2B -AR increases the epinephrine-stimulated calcium signaling.

INTERPRETATION

The identified mutation is responsible for ADCME, as the loss of α2B -AR/spinophilin interaction causes a gain of function effect. This work implicates for the first time the α-adrenergic system in human epilepsy and opens new ways of understanding the molecular pathway of epileptogenesis, widening the spectrum of possible therapeutic targets.

Tipepidine activates VTA dopamine neuron via inhibiting dopamine D₂ receptor-mediated inward rectifying K⁺ current

We previously reported that the novel antidepressant-like effect of tipepidine may be produced at least partly through the activation of mesolimbic dopamine (DA) neurons via inhibiting G protein-coupled inwardly rectifying potassium (GIRK) channels. In this study, we investigated the action of tipepidine on DA D2 receptor-mediated GIRK currents (IDA(GIRK)) and membrane excitability in DA neurons using the voltage clamp and current clamp modes of the patch-clamp techniques, respectively. DA neurons were acutely dissociated from the ventral tegmental area (VTA) in rats and identified by the presence of the hyperpolarization-activated currents. Tipepidine reversibly inhibited IDA(GIRK) with IC50 7.0 μM and also abolished IDA(GIRK) irreversibly activated in the presence of intracellular GTPγS. Then tipepidine depolarized membrane potential and generated action potentials in the neurons current-clamped. Furthermore, the drug at 40 mg/kg, i.p. increased the number of cells immunopositive both for c-Fos and tyrosine hydroxylase (TH) in the VTA. These results suggest that tipepidine may activate DA neurons in VTA through the inhibition of GIRK channel-activated currents.

Activation and inhibition of neurons in the hippocampal ventral subiculum by norepinephrine and locus coeruleus stimulation

The ventral subiculum (vSub) has been implicated in a wide range of neurocognitive functions, including responses to fear, stress, and anxiety. The vSub receives dense noradrenergic (NE) inputs from the locus coeruleus (LC), and the LC-NE system is heavily implicated in attention and is known to be activated by stressors. However, the way in which the neurons in the vSub respond to activation of the LC-NE is not well understood. In this study, the direct LC innervation of the vSub was investigated. The effect of norepinephrine (NE) on single vSub neurons was examined using microiontophoresis combined with electrophysiological recordings in anesthetized rats, and this response compared with the effect of electrical stimulation of the LC. Iontophoretic NE inhibited all vSub neurons tested, whereas LC stimulation inhibited 16% and activated 38% of neurons. Inhibition was mediated primarily by alpha-2 receptors, whereas activation was mediated by beta-adrenergic receptors. Furthermore, this effect was not mediated via the LC-basolateral amygdala (BLA) pathway, because BLA inactivation did not block LC stimulation-evoked activation of the vSub. These results indicate that the LC-NE system is a potent modulator of vSub activity. Based on these findings, stress-induced activation of the LC-NE system is expected to evoke inhibition and activation in the vSub, both of which may contribute to stress adaptation, whereas an imbalance of this system may lead to pathological stress responses in mental disorders.

Altered locus coeruleus-norepinephrine function following single prolonged stress

Data from preclinical and clinical studies have implicated the norepinephrine system in the development and maintenance of post-traumatic stress disorder. The primary source of norepinephrine in the forebrain is the locus coeruleus (LC); however, LC activity cannot be directly measured in humans, and previous research has often relied upon peripheral measures of norepinephrine to infer changes in central LC-norepinephrine function. To directly assess LC-norepinephrine function, we measured single-unit activity of LC neurons in a validated rat model of post-traumatic stress disorder - single prolonged stress (SPS). We also examined tyrosine hydroxylase mRNA levels in the LC of SPS and control rats as an index of norepinephrine utilisation. For electrophysiological recordings, 92 LC neurons were identified from 19 rats (SPS, 12; control, 7), and spontaneous and evoked responses to a noxious event (paw compression) were recorded. Baseline and restraint stress-evoked tyrosine hydroxylase mRNA expression levels were measured in SPS and control rats (n = 16 per group) in a separate experiment. SPS rats showed lower spontaneous activity but higher evoked responses, leading to an enhanced signal-to-noise ratio of LC neurons, accompanied by impaired recovery from post-stimulus inhibition. In concert, tyrosine hydroxylase mRNA expression in the LC of SPS rats tended to be lower at baseline, but was exaggerated following restraint stress. These data demonstrate persistent changes in LC function following stress/trauma in a rat model of post-traumatic stress, as measured by differences in both the electrophysiological properties of LC neurons and tyrosine hydroxylase mRNA transcription.

Properties of slow oscillation during slow-wave sleep and anesthesia in cats

Deep anesthesia is commonly used as a model of slow-wave sleep (SWS). Ketamine-xylazine anesthesia reproduces the main features of sleep slow oscillation: slow, large-amplitude waves in field potential, which are generated by the alternation of hyperpolarized and depolarized states of cortical neurons. However, direct quantitative comparison of field potential and membrane potential fluctuations during natural sleep and anesthesia is lacking, so it remains unclear how well the properties of sleep slow oscillation are reproduced by the ketamine-xylazine anesthesia model. Here, we used field potential and intracellular recordings in different cortical areas in the cat to directly compare properties of slow oscillation during natural sleep and ketamine-xylazine anesthesia. During SWS cortical activity showed higher power in the slow/delta (0.1-4 Hz) and spindle (8-14 Hz) frequency range, whereas under anesthesia the power in the gamma band (30-100 Hz) was higher. During anesthesia, slow waves were more rhythmic and more synchronous across the cortex. Intracellular recordings revealed that silent states were longer and the amplitude of membrane potential around transition between active and silent states was bigger under anesthesia. Slow waves were mostly uniform across cortical areas under anesthesia, but in SWS, they were most pronounced in associative and visual areas but smaller and less regular in somatosensory and motor cortices. We conclude that, although the main features of the slow oscillation in sleep and anesthesia appear similar, multiple cellular and network features are differently expressed during natural SWS compared with ketamine-xylazine anesthesia.

Spontaneous spiking and synaptic depression underlie noradrenergic control of feed-forward inhibition

Inhibitory interneurons across diverse brain regions commonly exhibit spontaneous spiking activity, even in the absence of external stimuli. It is not well understood how stimulus-evoked inhibition can be distinguished from background inhibition arising from spontaneous firing. We found that noradrenaline simultaneously reduced spontaneous inhibitory inputs and enhanced evoked inhibitory currents recorded from principal neurons of the mouse dorsal cochlear nucleus (DCN). Together, these effects produced a large increase in signal-to-noise ratio for stimulus-evoked inhibition. Surprisingly, the opposing effects on background and evoked currents could both be attributed to noradrenergic silencing of spontaneous spiking in glycinergic interneurons. During spontaneous firing, glycine release was decreased due to strong short-term depression. Elimination of background spiking relieved inhibitory synapses from depression and thereby enhanced stimulus-evoked inhibition. Our findings illustrate a simple yet powerful neuromodulatory mechanism to shift the balance between background and stimulus-evoked signals.

Cardiovascular responses to microinjection of noradrenaline into the medial amygdaloid nucleus of conscious rats result from α₂-receptor activation and vasopressin release

The medial amygdaloid nucleus (MeA) is involved in the modulation of physiological and behavioral processes, as well as regulation of the autonomic nervous system. Moreover, MeA electrical stimulation evokes cardiovascular responses. Thus, as noradrenergic receptors are present in this structure, the present study tested the effects of local noradrenaline (NA) microinjection into the MeA on cardiovascular responses in conscious rats. Moreover, we describe the types of adrenoceptor involved and the peripheral mechanisms involved in the cardiovascular responses. Increasing doses of NA (3, 9, 27 or 45 nmol/100 nL) microinjected into the MeA of conscious rats caused dose-related pressor and bradycardic responses. The NA cardiovascular effects were abolished by local pretreatment of the MeA with 10 nmol/100 nL of the specific α₂-receptor antagonist RX821002, but were not affected by local pretreatment with 10 nmol/100 nL of the specific α₁-receptor antagonist WB4101. The magnitude of pressor response evoked by NA microinjected into the MeA was potentiated by intravenous pretreatment with the ganglion blocker pentolinium (5 mg/kg), and blocked by intravenous pretreatment with the selective V₁-vasopressin antagonist dTyr(CH₂)₅ (Me)AVP (50 μg/kg). In conclusion, our results show that microinjection of NA into the MeA of conscious rats activates local α₂-adrenoceptors, evoking pressor and bradycardic responses, which are mediated by vasopressin release.

Dopamine depresses melanin concentrating hormone neuronal activity through multiple effects on α2-noradrenergic, D1 and D2-like dopaminergic receptors

Two neuronal populations of the lateral hypothalamus that, respectively, produce melanin-concentrating hormone (MCH) and orexin peptides are crucially involved in control of metabolism, feeding and related goal-oriented behaviors. In contrast to orexin neurons, mainly involved in short-term regulation of feeding, MCH neurons participate in long-term control of energy storage and body weight. Beyond its effect on feeding, MCH has also been shown to be involved in regulation of seeking behavior and addiction through modulation of dopamine (DA) metabolism. This regulation is essential for reinforcement-associated behaviors. Moreover, drugs of abuse, which increase extracellular DA levels, are known to decrease food intake. Consistent with this observation, DA has been shown to modulate orexin neurons of the lateral hypothalamus. However, no study is available concerning the effects of DA on MCH neurons. Whole-cell patch-clamp recordings were done in hypothalamic mouse brain slices. MCH neurons were identified by Tau-Cyan-GFP labeling using a transgenic mouse model (MCH-GFP). First, we show that DA (10-200 μM) induces an outward current in MCH neurons. However, this current is not due to activation of DA receptors, but mediated through activation of α2-noradrenergic receptors and subsequent opening of G-protein activated inward rectifier K+ (GIRK) channels. Current-clamp experiments revealed that this GIRK-activation leads to hyperpolarization, thus decreasing excitability of MCH neurons. Furthermore, we confirm that MCH neurons receive mainly GABAergic inputs rather than glutamatergic ones. We show that DA modulates these inputs in a complex manner: at low concentrations, DA activates D1-like receptors, promoting presynaptic activity, whereas, at higher concentrations (100 μM), D2-like receptor activation inhibits presynaptic activity. Overall, DA should lead to a decrease in MCH neuron excitability, likely resulting in down-regulation of MCH release and feeding behavior.

The potent inhibitory effect of tipepidine on marble-burying behavior in mice

Our previous study revealed that centrally acting non-narcotic antitussives inhibited G-protein-coupled inwardly rectifying K(+) (GIRK) channel currents in brain neurons, and that the tipepidine antitussives had a novel antidepressive-like effect on rats. Furthermore, the antitussives revealed multiplexed ameliorating actions on intractable brain disease models. This study evaluated the therapeutic potential of tipepidine in obsessive-compulsive disorder (OCD) subjects using marble-burying behavior (MBB) tests in mice. In fact, OCD is classified as an anxiety disorder characterized by obsession or compulsion. Although selective 5-HT reuptake inhibitors (SSRIs) are considered first choice agents for the pharmacological treatment of OCD, 50% of patients with OCD failed to respond to SSRIs. The burying of harmless objects such as marbles by mice might reflect the formation of compulsive behavior. The results show that tipepidine reduced MBB in a dose-dependent manner. The effect of tipepidine was significant even at a dosage as small as 5 mg/kg. The tipepidine at 10 mg/kg s.c. nearly abolished MBB without reducing the locomotor activity in mice. It is particularly interesting that the dopamine D₂ antagonist or 5-HT(1A) antagonist partly inhibited the effect of tipepidine on MBB. The results suggest that tipepidine has more of a potent inhibitory effect on MBB, compared with known drugs used for the treatment of OCD, and that the tipepidine action mechanism might differ from that of known drugs.

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.

Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

Firing activity of locus coeruleus noradrenergic neurons increases in a rodent model of Parkinsonism

OBJECTIVE

To investigate the changes in the firing activity of noradrenergic neurons in the locus coeruleus (LC) in a rat model of Parkinson disease (PD).

METHODS

2 and 4 weeks after unilateral lesion of the nigrostriatal pathway in the rat by local injection of 6-hydroxydopamine (6-OHDA) into the right substantia nigra pars compacta (SNc), the firing activity of noradrenergic neurons in LC was recorded by extracellular single unit recording.

RESULTS

The firing rate of LC noradrenergic neurons increased significantly 2 and 4 weeks after 6-OHDA lesions compared to normal rats, respectively (P < 0.05). The percentage of irregularly firing neurons was obviously higher than that of normal rats during the fourth week after SNc lesion (P < 0.05).

CONCLUSION

LC noradrenergic neurons are overactive and more irregular in 6-OHDA-lesioned rats. These changes suggest an implication of the LC in the pathophysiological mechanism of PD.

Noradrenergic modulation of basolateral amygdala neuronal activity: opposing influences of alpha-2 and beta receptor activation

Substantial data exists demonstrating the importance of the amygdala and the locus ceruleus (LC) in responding to stress, aversive memory formation, and the development of stress-related disorders; however, little is known about the effects of norepinephrine (NE) on amygdala neuronal activity in vivo. The basolateral nucleus of the amygdala (BLA) receives dense NE projections from the LC, NE increases in the BLA in response to stress, and the BLA can also modulate the LC via reciprocal projections. These experiments examined the effects of noradrenergic agents on spontaneous and evoked responses of BLA neurons. NE iontophoresis inhibited spontaneous firing and decreased the responsiveness of BLA neurons to electrical stimulation of entorhinal cortex and sensory association cortex (Te3). Confirmed BLA projection neurons exhibited exclusively inhibitory responses to NE. Systemic administration of propranolol, a beta-receptor antagonist, decreased the spontaneous firing rate and potentiated the NE-evoked inhibition of BLA neurons. In addition, iontophoresis of the alpha-2 agonist clonidine, footshock administration, and LC stimulation mimicked the effects of NE iontophoresis on spontaneous activity. Furthermore, the effects of LC stimulation were partially blocked by systemic administration of alpha 2 and beta receptor antagonists. This is the first study to demonstrate the actions of directly applied and stimulus-evoked NE in the BLA in vivo, and provides a mechanism by which beta receptors can mediate the important behavioral consequences of NE within the BLA. The interaction between these two structures is particularly relevant with regard to their known involvement in stress responses and stress-related disorders.

Effects of noradrenaline on the GABA response in rat isolated spiral ganglion neurons in culture

In the present study, the modulatory effects of noradrenaline (NA) on the GABA response were investigated in the isolated cultured spiral ganglion neurons of rat by using nystatin perforated patch recording configuration under voltage-clamp conditions. NA reversibly depressed GABA response in a concentration-dependent manner and neither changed the reversal potential of the GABA response nor affected the apparent affinity of GABA to its receptor. alpha2-adrenoceptor agonist and antagonist, clonidine and yohimbine mimicked and blocked the NA action on the GABA response, respectively. N-[2(methylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H-89), a protein kinase A inhibitor, mimicked the effect of NA on the GABA response. NA failed to affect the GABA response in the presence of both cAMP and protein kinase A modulator. However, NA still depressed the GABA response even in the presence of both phorbol-12-myristate-13-acetate, a protein kinase C activator and chelerythrine, a protein kinase C inhibitor. These results suggest that the NA suppression of the GABA response is mediated by alpha2-adrenoceptor which reduces intracellular cAMP formation through the inhibition of adenylyl cyclase. Therefore, NA input to the spiral ganglion neurons may modulate the auditory transmission by affecting the GABA response.

Long latency of evoked quantal transmitter release from somata of locus coeruleus neurons in rat pontine slices

The locus coeruleus (LC) harbors a compact group of noradrenergic cell bodies projecting to virtually all parts of the central nervous system. By using combined measurements of amperometry and patch-clamp, quantal vesicle release of noradrenaline (NA) was detected as amperometric spikes, after depolarization of the LC neurons. After a pulse depolarization, the average latency of amperometric spikes was 1,870 ms, whereas the latency of glutamate-mediated excitatory postsynaptic currents was 1.6 ms. A substantial fraction of the depolarization-induced amperometric spikes originated from the somata. In contrast to glutamate-mediated excitatory postsynaptic currents, NA secretion was strongly modulated by the action potential frequency (0.5-50 Hz). Somatodendritic NA release from LC upon enhanced cell activity produced autoinhibition of firing and of NA release. We conclude that, in contrast to classic synaptic transmission, quantal NA release from LC somata is characterized by a number of distinct properties, including long latency and high sensitivity to action potential frequency.

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC(50) (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [(3)H]rauwolscine binding assay. When treated with acute N(2)-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

The alpha2-adrenoceptor antagonist dexefaroxan enhances hippocampal neurogenesis by increasing the survival and differentiation of new granule cells

The generation of new neurons in the hippocampus is a dynamic process regulated by environmental, endocrine, and pharmacological factors. Since enhancement of hippocampal neurogenesis has been associated with learning and memory, and the locus coeruleus-noradrenergic system has been shown to modulate these cognitive functions, we hypothesized that activation of noradrenergic neurotransmission might enhance neurogenesis in the adult hippocampus. To test this hypothesis in vivo, we induced the release of noradrenaline in the hippocampus by blocking presynaptic inhibitory autoreceptors with the selective alpha2-adrenoceptor antagonist dexefaroxan. Confocal microscopy showed that noradrenergic afferents make contact with proliferating and differentiating cells, suggesting a direct noradrenergic influence on neurogenesis. Chronic systemic treatment of rats with dexefaroxan did not affect cell proliferation per se in the dentate gyrus (as monitored by bromodeoxyuridine-labeling), but promoted the long-term survival of newborn neurons by reducing apoptosis. Dexefaroxan treatment also enhanced the number and complexity of the dendritic arborizations of polysialated neural cell adhesion molecule-positive neurons. The trophic effects of dexefaroxan on newborn cells might involve an increase in brain-derived neurotrophic factor, which was upregulated in afferent noradrenergic fiber projection areas and in neurons in the granule cell layer. By promoting the survival of new endogenously formed neurons, dexefaroxan treatment represents a potential therapeutic strategy for maintaining adult neurogenesis in neurodegenerative conditions, such as Alzheimer's disease, that affect the hippocampus.

Orexin neurons are directly and indirectly regulated by catecholamines in a complex manner

We reported elsewhere that orexin neurons are directly hyperpolarized by noradrenaline (NA) and dopamine. In the present study, we show that NA, dopamine, and adrenaline all directly hyperpolarized orexin neurons. This response was inhibited by the alpha2 adrenergic receptor (alpha2-AR) antagonist, idazoxan or BRL44408, and was mimicked by the alpha2-AR-selective agonist, UK14304. A low concentration of Ba2+ inhibited NA-induced hyperpolarization, which suggests that activation of G protein coupled inward rectifier potassium channels is involved in the response. In the presence of a high concentration of idazoxan, NA induced depolarization or inward current. This response was inhibited by alpha1-AR antagonist, prazosin, which suggests the existence of alpha1-ARs on the orexin neurons along with alpha2-AR. We also examined the effects of NA on glutamatergic and GABAergic synaptic transmission. NA application dramatically increased the frequency and amplitude of spontaneous inhibitory synaptic currents (sIPSCs) and inhibited excitatory synaptic currents (sEPSCs) in orexin neurons; however, NA decreased the frequency of miniature EPSCs (mEPSCs) and IPSCs and the amplitude of evoked EPSCs and IPSCs through the alpha2-AR, because the NA response on mPSCs was inhibited by idazoxan. These results suggest that the NA-induced increase in sIPSC frequency and amplitude is mediated via alpha1-ARs on the somata of GABAergic neurons that innervate the orexin neurons. Calcium imaging using orexin/YC2.1 transgenic mouse brain revealed that NA-induced inhibition of orexin neurons is not altered by sleep deprivation or circadian time in mice. The evidence presented here revealed that orexin neurons are regulated by catecholamines in a complex manner.

Activation of presynaptic GABAA receptors increases spontaneous glutamate release onto noradrenergic neurons of the rat locus coeruleus

In order to further explore how GABA can modulate the excitability of noradrenergic neurons of the locus coeruleus (LC), we investigated the presence of GABA(A) receptors on glutamatergic nerve terminals and the functional consequences of their activation. We used mechanically dissociated immature rat LC neurons with adherent nerve terminals and patch-clamp recordings of spontaneous excitatory postsynaptic currents. Activation of presynaptic GABA(A) receptors by muscimol facilitated spontaneous glutamate release by activating tetrodotoxin-sensitive Na(+) channels and high-threshold Ca(2+) channels. Bumetanide (10 microM), a potent blocker of Na(+)-K(+)-Cl(-) cotransporter, diminished the muscimol-induced facilitatory action of glutamate release. Our results indicate that the Na(+)-K(+)-Cl(-) cotransporter accumulates Cl(-) inside the nerve terminals so that activation of presynaptic GABA(A) receptors causes depolarization. This GABA(A)-receptor-mediated modulation of spontaneous glutamatergic transmission is another mechanism by which GABA and its analogues can regulate the excitability and activity of noradrenergic neurons in the LC.

Facilitation of spontaneous glutamate release by antidepressant drugs in rat locus coeruleus

The effects of antidepressant drugs on spontaneous excitatory postsynaptic currents (EPSCs) were investigated in the mechanically dissociated rat locus coeruleus (LC) neurons which had their presynaptic nerve terminals attached. The membrane currents were recorded by the whole-cell patch-clamp technique. Desipramine, a tricyclic antidepressant, reversibly and concentration-dependently increased the frequency of spontaneous EPSCs, but did not alter their amplitude distribution. The inhibitors of high-voltage-activated Ca2+ channels failed to block the facilitatory action of desipramine, while they inhibited the high K+-induced facilitation of spontaneous EPSC frequency. The desipramine action was also observed in the absence of extracellular Ca2+. Pretreatment of thapsigargin in Ca2+-free solution fully inhibited the desipramine action, thus suggesting the involvement of Ca2+ release from intracellular Ca2+ stores at glutamatergic presynaptic nerve terminals. Imipramine and nortriptyline, other tricyclic antidepressants, and amoxapine, mianserin and fluoxetine, non-tricyclic antidepressants, also increased the EPSC frequency, while tranylcypromine, an inhibitor of monoamine oxidase, did not increase the glutamate release. The present results indicate that modulation of spontaneous glutamatergic transmission by tricyclic- and non-tricyclic-antidepressant drugs may regulate the excitability of LC neurons.

Serotonergic regulation of the orexin/hypocretin neurons through the 5-HT1A receptor

Both orexin and serotonin (5-HT) have important roles in the regulation of sleep-wakefulness, as well as in feeding behavior. We examined the effects of 5-HT on orexin/hypocretin neurons, using hypothalamic slices prepared from orexin/enhanced green fluorescent protein (EGFP) transgenic mice in which EGFP is expressed exclusively in orexin neurons. Patch-clamp recording from EGFP-expressing cells showed that 5-HT hyperpolarized all orexin neurons in a concentration-dependent manner. The response was inhibited by the 5-HT1A receptor antagonist WAY100635. A 5-HT1A receptor agonist, 8-hydroxy-2-(dl-N-propyl-amino)tetralin, also evoked hyperpolarization on orexin neurons with potency comparable with 5-HT. A low concentration of Ba2+ (30 microM) inhibited 5-HT-induced hyperpolarization. Single-channel recording revealed that the conductance of 5-HT-induced channel activity was 33.8 pS, which is in good agreement with that of the G-protein-coupled inward rectifier potassium channel (GIRK). Moreover, 5-HT1A receptor-like immunoreactivity was observed on orexin neurons, and 5-HT transporter immunoreactive nerve endings are in close apposition to orexin neurons. Intracerebroventricular injection of the 5-HT1A receptor-selective antagonist WAY100635 (100 ng) increased locomotor activity during the latter half of dark phase in wild-type mice but not in orexin/ataxin-3 mice in which orexin neurons are specifically ablated, suggesting that activation of orexin neurons is necessary for the WAY100635-induced increase in locomotor activity. These results indicate that 5-HT hyperpolarizes orexin neurons through the 5-HT1A receptor and subsequent activation of the GIRK and that this inhibitory serotonergic input to the orexin neurons is likely to be important for the physiological regulation of this neuropeptide system.

Inwardly rectifying potassium channels in rat retinal ganglion cells

Inwardly rectifying potassium channels (Kir channels) are important for neuronal signalling and membrane excitability. In the present work we characterized, for the first time, Kir channels in rat retinal ganglion cells (RGCs), the output neurons in the retina, using immunocytochemical and patch-clamp techniques. Various subunits of Kir channels (Kir1.1, 2.1, 2.3, 3.1, 3.2 and 3.3) were expressed in RGCs, but with distinct subcellular localization. Kir1.1 was mainly expressed in axons of RGCs. Kir2.1 and Kir2.3 were both present in somata of RGCs. Whereas staining for Kir3.1 was profoundly present in an endoplasmic reticulum-like structure and Kir3.2 was strongly expressed in the cytoplasm and the cytomembrane of somata, dendrites and axons of RGCs, faint, sparse labelling for Kir3.3 was seen in the cytomembrane. Immunoreactivity for Kir4.1 and Kir4.2 was not detectable in RGCs. Whole-cell currents mediated by Kir channels were recorded in isolated RGCs and they differed from hyperpolarization-activated currents (I(h)) by showing full activation in < 10 ms, no inactivation, and being significantly suppressed by 300 microM Ba2+. Unlike in retinal horizontal cells and bipolar cells, these currents were mainly mediated by G-protein-coupled Kir3 (GIRK) channels, as demonstrated by the fact that GDP(beta)S and GTP(gamma)S included in the pipette solution markedly decreased and increased the currents, respectively. Furthermore, the GIRK channels were probably coupled to GABA(B) receptors, because baclofen considerably increased the Kir currents and the increased currents were suppressed by Ba2+. These characteristics of the Kir currents provide more versatility for signalling of RGCs.

delta-Opioid receptor antagonists inhibit GIRK channel currents in acutely dissociated brainstem neurons of rat

In this study, we investigated the effects of delta-opioid receptor antagonists on the G protein-coupled inwardly rectifying potassium (GIRK) channel currents induced by serotonin (5-HT) and noradrenaline (NAd) in the dorsal raphe and the locus coeruleus neurons, respectively. Perforated patch and conventional whole-cell patch clamp recording techniques were used for the study. Neurons were acutely dissociated from neonatal rats. Both naltrindole (NTI) and naltriben (NTB), which are selective delta-antagonists possessing antitussive activity in in vivo animal studies, reversibly inhibited the 5-HT-induced GIRK channel currents (I(5-HT)) in dorsal raphe neurons. This inhibition was concentration-dependent and voltage-independent. The half-maximum inhibitory concentration (IC(50)) on I(5-HT) was 9.84x10(-5) M for NTI and 1.28x10(-5) M for NTB. The inhibition was not reversed by 10(-5) M DPDPE, a selective delta-opioid receptor agonist. NTI did not affect 50% effective concentration (EC(50)) on the concentration-response relationship for 5-HT but inhibited the maximum response. In neurons internally perfused with GTPgammaS, both NTI and NTB also inhibited the GIRK channel currents irreversibly activated by 5-HT. Furthermore, these antagonists concentration dependently inhibited 10(-6) M NAd-induced currents (I(NAd)) in locus coeruleus neurons. The IC(50) of NTI on I(NAd) was 8.44x10(-5) M, which was close to that on I(5-HT). The results suggest that NTI and NTB, which are delta-opioid receptor antagonists possessing antitussive activity, may inhibit GIRK channel currents through a non-opioid action, and give further support to our idea previously proposed that centrally acting non-narcotic antitussives have a common characteristic of the inhibitory action on GIRK channels.

Actions of noradrenaline on substantia gelatinosa neurones in the rat spinal cord revealed by in vivo patch recording

To elucidate the mechanisms of antinociception mediated by the descending noradrenergic pathway in the spinal cord, the effects of noradrenaline (NA) on noxious synaptic responses of substantia gelatinosa (SG) neurones, and postsynaptic actions of NA were investigated in rats using an in vivo whole-cell patch-clamp technique. Under urethane anaesthesia, the rat was fixed in a stereotaxic apparatus after the lumbar spinal cord was exposed. In the current-clamp mode, pinch stimuli applied to the ipsilateral hindlimb elicited a barrage of EPSPs, some of which initiated an action potential. Perfusion with NA onto the surface of the spinal cord hyperpolarized the membrane (5.0-9.5 mV) and suppressed the action potentials. In the voltage-clamp mode (V(H), -70 mV), the application of NA produced an outward current that was blocked by Cs(+) and GDP-beta-S added to the pipette solution and reduced the amplitude of EPSCs evoked by noxious stimuli. Under the blockade of postsynaptic actions of NA, a reduction of the evoked and spontaneous EPSCs of SG neurones was still observed, thus suggesting both pre- and postsynaptic actions of NA. The NA-induced outward currents showed a clear dose dependency (EC(50), 20 microM), and the reversal potential was -88 mV. The outward current was mimicked by an alpha(2)-adrenoceptor agonist, clonidine, and suppressed by an alpha(2)-adrenoceptor antagonist, yohimbine, but not by alpha(1)- and beta-antagonists. These findings suggest that NA acts on presynaptic sites to reduce noxious stimuli-induced EPSCs, and on postsynaptic SG neurones to induce an outward current by G-protein-mediated activation of K(+) channels through alpha(2)-adrenoceptors, thereby producing an antinociceptive effect.

Roles of alpha1- and alpha2-adrenoceptors in the nucleus raphe magnus in opioid analgesia and opioid abstinence-induced hyperalgesia

Noradrenaline and alpha-adrenoceptors have been implicated in the modulation of pain in various behavioral conditions. Noradrenergic neurons and synaptic inputs are present in neuronal circuits critical for pain modulation, but their actions on neurons in those circuits and consequently the mechanisms underlying noradrenergic modulation of pain remain unclear. In this study, both recordings in vitro and behavioral analyses in vivo were used to examine cellular and behavioral actions mediated by alpha1- and alpha2-adrenoceptors on neurons in the nucleus raphe magnus. We found that alpha1- and alpha2-receptors were colocalized in the majority of a class of neurons (primary cells) that inhibit spinal pain transmission and are excited during opioid analgesia. Activation of the alpha1-receptor depolarized whereas alpha2-receptor activation hyperpolarized these neurons through a decrease and an increase, respectively, in potassium conductance. Blockade of the excitatory alpha1-receptor or activation of the inhibitory alpha2-receptor significantly attenuated the analgesia induced by local opioid application, suggesting that alpha1-receptor-mediated synaptic inputs in these primary cells contribute to their excitation during opioid analgesia. In the other cell class (secondary cells) that is thought to facilitate spinal nociception and is inhibited by analgesic opioids, only alpha1-receptors were present. Blocking the alpha1-receptor in these cells significantly reduced the hyperalgesia (increased pain) induced by opioid abstinence. Thus, state-dependent activation of alpha1-mediated synaptic inputs onto functionally distinct populations of medullary pain-modulating neurons contributes to opioid-induced analgesia and opioid withdrawal-induced hyperalgesia.

Roles of alpha1- and alpha2-adrenoceptors in the nucleus raphe magnus in opioid analgesia and opioid abstinence-induced hyperalgesia

Noradrenaline and alpha-adrenoceptors have been implicated in the modulation of pain in various behavioral conditions. Noradrenergic neurons and synaptic inputs are present in neuronal circuits critical for pain modulation, but their actions on neurons in those circuits and consequently the mechanisms underlying noradrenergic modulation of pain remain unclear. In this study, both recordings in vitro and behavioral analyses in vivo were used to examine cellular and behavioral actions mediated by alpha1- and alpha2-adrenoceptors on neurons in the nucleus raphe magnus. We found that alpha1- and alpha2-receptors were colocalized in the majority of a class of neurons (primary cells) that inhibit spinal pain transmission and are excited during opioid analgesia. Activation of the alpha1-receptor depolarized whereas alpha2-receptor activation hyperpolarized these neurons through a decrease and an increase, respectively, in potassium conductance. Blockade of the excitatory alpha1-receptor or activation of the inhibitory alpha2-receptor significantly attenuated the analgesia induced by local opioid application, suggesting that alpha1-receptor-mediated synaptic inputs in these primary cells contribute to their excitation during opioid analgesia. In the other cell class (secondary cells) that is thought to facilitate spinal nociception and is inhibited by analgesic opioids, only alpha1-receptors were present. Blocking the alpha1-receptor in these cells significantly reduced the hyperalgesia (increased pain) induced by opioid abstinence. Thus, state-dependent activation of alpha1-mediated synaptic inputs onto functionally distinct populations of medullary pain-modulating neurons contributes to opioid-induced analgesia and opioid withdrawal-induced hyperalgesia.

Evidence for the involvement of G-proteins in the generation of the slow poststimulus afterdepolarisation (sADP) induced by muscarinic receptor activation in rat olfactory cortical neurones in vitro

The involvement of G-proteins in generating the slow poststimulus afterdepolarising potential (sADP) induced by muscarinic receptor activation in immature (P10-20) rat olfactory cortical brain slice neurones was investigated under whole-cell patch clamp, using GTP-gamma-S (G-protein activator) or GDP-beta-S (G-protein blocker)-filled electrodes. In control experiments using K methylsulphate electrodes, cell resting potential (V(m)) and spike firing properties were unaffected over 10-15 min recording, although input resistance (R(N)) was slightly increased ( approximately 14%). Oxotremorine-M (OXO-M; 10 microM) produced a reversible slow depolarisation, an increase in R(N) ( approximately 90%) and induction of a slow poststimulus inward tail current (I(ADP)) (measured under voltage clamp at -60 mV) that was sustained during drug exposure (up to 15 min); the amplitude of slow inward rectifier (I(h)) currents activated from -50 mV were also apparently increased. By contrast, in GTP-gamma-S-loaded cells, R(N) was consistently decreased ( approximately 22%) and spike firing threshold (V(th)) was raised ( approximately 5 mV) after 10 min recording. In approximately 60% of loaded cells, a persistent muscarinic slow inward current and I(ADP) were induced by OXO-M; I(h) relaxation amplitude was also significantly decreased. The effects of GTP-gamma-S on R(N), V(th) and I(h) were partly counteracted by adding Ba(2+) (100 microM) to the bathing medium or mimicked by adding baclofen (GABA(B) receptor agonist; 100 microM) to normally-recorded cells. Intracellular GDP-beta-S (up to 30 min) had no effect on cell membrane properties or I(h), but irreversibly blocked the muscarinic slow inward current and I(ADP) induced by OXO-M. We conclude that both muscarinic responses require G-protein-linked transduction mechanisms for their generation.

Different alpha(2) adrenoceptor subtypes control noradrenaline release and cell firing in the locus coeruleus of wildtype and monoamine oxidase-A knockout mice

In this study, we investigated which subtype(s) of alpha(2)-adrenoceptor control stimulated noradrenaline (NA) release and noradrenergic cell firing in the locus coeruleus (LC) of monoamine oxidase-A knockout (MAO-A KO) and C3H/HeJ wildtype mice. On short stimulus trains (10 pulses, 200 Hz), the alpha(2) agonist dexmedetomidine (10 nm) reduced NA efflux by 78 +/- 8% and 51 +/- 8% in wildtype and MAO-A KO mice, respectively. In both strains, BRL 44408 (100 nm) and ARC 239 (100 nm) each partially blocked the effect of dexmedetomidine. In MAO-A KO mice, BRL 44408 (100 nm) increased evoked NA efflux on short trains while ARC 239 (100 nm) had no effect. The two antagonists in combination increased NA efflux (by 81 +/- 34%, P < 0.001), significantly more than by BRL 44408 alone. Conversely, in wildtype mice, the alpha2-adrenoceptor antagonists did not significantly increase LC NA efflux. On long stimuli (30 pulses, 10 Hz), NA efflux was increased by BRL 44408 (P < 0.001) but not by ARC 239. The effect of BRL 44408 was significantly greater in MAO-A KO than wildtype mice (208 +/- 43% vs. 113 +/- 31% increase, P < 0.001). When we examined noradrenergic cell firing, we found that dexmedetomidine inhibited LC cell firing in both strains with comparable EC(50) values (2-5 nm), although E(max) was significantly lower in MAO-A KO mice (P < 0.001). The agonist effect was antagonized by BRL 44408 (P < 0.001) in wildtype but not in MAO-A KO mice, with a pK(B) of 7.75. ARC 239 had no effect on the agonist response in either strain. A combination of the antagonists was no more effective than BRL 44408 alone (in wildtypes) and had no effect in MAO-A KO mice. Neither BRL 44408 nor ARC 239 affected basal LC cell firing in wildtype or MAO-A KO mice. Collectively, these results suggest that, analogous to other monoamine cell groups, there are differences in the autoreceptor populations controlling NA efflux and LC cell firing and that important differences exist between MAO-A KO and wildtype mice.

Alpha 1-adrenoceptor-activated cation currents in neurones acutely isolated from rat cardiac parasympathetic ganglia

The noradrenaline (NA)-induced cation current was investigated in neurones freshly isolated from rat cardiac parasympathetic ganglia using the nystatin-perforated patch recording configuration. Under current-clamp conditions, NA depolarized the membrane, eliciting repetitive action potentials. NA evoked an inward cation current under voltage-clamp conditions at a holding potential of -60 mV. The NA-induced current was inhibited by extracellular Ca2+ or Mg2+, with a half-maximal concentration of 13 microM for Ca2+ and 1.2 mM for Mg2+. Cirazoline mimicked the NA response, and prazosin and WB-4101 inhibited the NA-induced current, suggesting the contribution of an alpha1-adrenoceptor. The NA-induced current was inhibited by U73122, a phospholipase C (PLC) inhibitor. The membrane-permeable IP3 receptor blocker xestospongin-C also blocked the NA-induced current. Furthermore, pretreatment with thapsigargin and BAPTA-AM could inhibit the NA response while KN-62, phorbol 12-myristate 13-acetate (PMA) and staurosporine had no effect. These results suggest that NA activates the extracellular Ca2+- and Mg2+-sensitive cation channels via alpha 1-adrenoceptors in neurones freshly isolated from rat cardiac parasympathetic ganglia. This activation mechanism also involves phosphoinositide breakdown, release of Ca2+ from intracellular Ca2+ stores and calmodulin. The cation channels activated by NA may play an important role in neuronal membrane depolarization in rat cardiac ganglia.

Chronic exposure to cold stress alters electrophysiological properties of locus coeruleus neurons recorded in vitro

Chronic stress exposure can alter central noradrenergic function. Previously, we reported that in chronically cold-exposed rats the release of norepinephrine and electrophysiological activation of locus coeruleus (LC) neurons is enhanced in response to multiple excitatory stimuli without alterations in basal activity. In the present studies, we used in vitro intracellular recording techniques to explore the effect of chronic cold exposure on the basal and evoked electrophysiological properties of LC neurons in horizontal slices of the rat brainstem. Consistent with our findings from in vivo experiments, chronic cold exposure did not affect basal firing rate. Furthermore, gross morphology of LC neurons and spike waveform characteristics were similar in slices from control and previously cold-exposed rats. However, excitability in response to intracellular current injection and input resistance were larger in slices from previously cold-exposed rats. In addition, the accommodation of spike firing in response to sustained current injection was smaller and the period of postactivation inhibition appeared to be less in LC neurons from cold-exposed rats. These data demonstrate that the stress-evoked sensitization of LC neurons observed in vivo is at least in part maintained in the slice preparation and suggest that alterations in electrophysiological properties of LC neurons contribute to the chronic stress-induced sensitization of central noradrenergic function observed in vivo. Furthermore, the present data suggest that an alteration in autoinhibitory control of LC activity is involved in the chronic stress-induced alterations. The enhanced functional capacity of LC neurons following cold exposure of rats may represent a unique model to study the mechanisms underlying the alterations in central noradrenergic function observed in humans afflicted with mood and anxiety disorders.