Potency of catecholamines and other L-tyrosine derivatives at the cloned mouse adrenergic receptors

Dopaminergic and adrenergic receptors synergistically stimulate browning in 3T3-L1 white adipocytes

The browning of white adipose tissue (WAT) has attracted considerable attention in the scientific community as a popular strategy for enhancing energy expenditure to combat obesity. As a part of this strategy, β3-adrenergic receptor (β3-AR) is the most widely studied receptor that mediates thermogenesis. Parenthetically, further studies in search for additional receptors expressed in adipocytes that can mediate thermogenesis has been appearing, and this paper reports that dopaminergic receptor 1 (DRD1) and β3-AR synergistically stimulate browning in 3T3-L1 white adipocytes. qRT-PCR and immunoblot analysis methods were applied to evaluate the effects of DRD1 on the target proteins downstream of β3-AR and other markers involved in lipid metabolism, mitochondrial biogenesis, and browning events. These results show that DRD1 is expressed in epididymal WAT (eWAT), brown adipose tissue (BAT), and inguinal WAT (iWAT) of normal and high-fat-fed mice, and a deficiency of DRD1 downregulates the expression of brown adipocyte-specific proteins. Silencing of DRD1 affected lipid metabolic activity in 3T3-L1 adipocytes by reducing mitochondrial biogenesis as well as levels of lipolytic and fat oxidative marker proteins in a similar pattern to β3-AR. Moreover, mechanistic studies showed that the depletion of DRD1 downregulates β3-AR and its downstream molecules, suggesting both receptors might synergistically stimulate browning. Parallel to the UCP1-dependent thermogenesis, the depletion of DRD1 also downregulates the expression of core proteins responsible for UCP1-independent thermogenesis. Overall, DRD1 mediates β3-AR-dependent 3T3-L1 browning and UCP1-independent thermogenesis.

Metabolic profiles and fingerprints for the investigation of the influence of nitisinone on the metabolism of the yeast Saccharomyces cerevisiae

Nitisinone (2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione, NTBC) is considered a potentially effective drug for the treatment of various metabolic diseases associated with disorders of L-tyrosine metabolism however, side-effects impede its widespread use. This work aimed to broaden the knowledge of the influence of NTBC and its metabolites 2-amino-4-(trifluoromethyl)benzoic acid (ATFA), 2-nitro-4-(trifluoromethyl)benzoic acid (NTFA), and cyclohexane-1,3-dione (CHD) on the catabolism of L-tyrosine and other endogenous compounds in Saccharomyces cerevisiae. Based on a targeted analysis performed by LC-ESI-MS/MS, based on multiple reaction monitoring, it was found that the dissipation kinetics of the parent compound and its metabolites are compatible with a first-order reaction mechanism. Moreover, it has been proven that formed NTBC metabolites, such as CHD, cause a decrease in L-tyrosine, L-tryptophan, and L-phenylalanine concentrations by about 34%, 59% and 51%, respectively, compared to the untreated model organism. The overall changes in the metabolism of yeast exposed to NTBC or its derivatives were evaluated by non-targeted analysis via LC-ESI-MS/MS in the ion trap scanning mode. Based on principal components analysis, a statistically significant similarity between metabolic responses of yeast treated with ATFA or NTFA was observed. These findings facilitate further studies investigating the influence of NTBC on the human body and the mechanism of its action.

Chemogenetic Enhancement of cAMP Signaling Renders Hippocampal Synaptic Plasticity Resilient to the Impact of Acute Sleep Deprivation

Sleep facilitates memory storage and even brief periods of sleep loss lead to impairments in memory, particularly memories that are hippocampus dependent. In previous studies, we have shown that the deficit in memory seen after sleep loss is accompanied by deficits in synaptic plasticity. Our previous work has also found that sleep deprivation (SD) is associated with reduced levels of cyclic adenosine monophosphate (cAMP) in the hippocampus and that the reduction of cAMP mediates the diminished memory observed in sleep-deprived animals. Based on these findings, we hypothesized that cAMP acts as a mediator for not only the cognitive deficits caused by sleep deprivation, but also the observed deficits in synaptic plasticity. In this study, we expressed the heterologous Drosophila melanogaster Gαs-protein-coupled octopamine receptor (DmOctβ1R) in mouse hippocampal neurons. This receptor is selectively activated by the systemically injected ligand (octopamine), thus allowing us to increase cAMP levels in hippocampal neurons during a 5-h sleep deprivation period. Our results show that chemogenetic enhancement of cAMP during the period of sleep deprivation prevents deficits in a persistent form of long-term potentiation (LTP) that is induced at the Schaffer collateral synapses in the hippocampal CA1 region. We also found that elevating cAMP levels in either the first or second half of sleep deprivation successfully prevented LTP deficits. These findings reveal that cAMP-dependent signaling pathways are key mediators of sleep deprivation at the synaptic level. Targeting these pathways could be useful in designing strategies to prevent the impact of sleep loss.

Dopamine regulates astrocytic IL-6 expression and process formation via dopamine receptors and adrenoceptors

Dopamine levels in the central nervous system change under pathological conditions such as Parkinson's disease, Huntington's disease, and addiction. Under those pathological conditions, astrocytes become reactive astrocytes characterized by morphological changes and the release of inflammatory cytokines involved in pathogenesis. However, it remains unclear whether dopamine regulates astrocytic morphology and functions. Elucidating these issues will help us to understand the pathogenesis of neurodegenerative diseases caused by abnormal dopamine signaling. In this study, we investigated the effects of dopamine on IL-6 expression and process formation in rat primary cultured astrocytes and acute hippocampal slices. Dopamine increased IL-6 expression in a concentration-dependent manner, and this was accompanied by CREB phosphorylation. The effects of a low dopamine concentration (1 μM) were inhibited by a D1-like receptor antagonist, whereas the effects of a high dopamine concentration (100 μM) were inhibited by a β-antagonist and enhanced by a D2-like receptor antagonist. Furthermore, dopamine (100 μM) promoted process formation, which was inhibited by a β-antagonist and enhanced by both an α-antagonist and a D2-like receptor antagonist. In acute hippocampal slices, both a D1-like receptor agonist and β-agonist changed astrocytic morphology. Together, these results indicate that dopamine promotes IL-6 expression and process formation via D1-like receptors and β-adrenoceptors. Furthermore, bidirectional regulation exists; namely, the effects of D1-like receptors and β-adrenoceptors were negatively regulated by D2-like receptors and α₂-adrenoceptors.

Serotonin receptors contribute to dopamine depression of lateral inhibition in the nucleus accumbens

Dopamine modulation of nucleus accumbens (NAc) circuitry is central to theories of reward seeking and reinforcement learning. Despite decades of effort, the acute dopamine actions on the NAc microcircuitry remain puzzling. Here, we dissect out the direct actions of dopamine on lateral inhibition between medium spiny neurons (MSNs) in mouse brain slices and find that they are pathway specific. Dopamine potently depresses GABAergic transmission from presynaptic dopamine D2 receptor-expressing MSNs (D2-MSNs), whereas it potentiates transmission from presynaptic dopamine D1 receptor-expressing MSNs (D1-MSNs) onto other D1-MSNs. To our surprise, presynaptic D2 receptors mediate only half of the depression induced by endogenous and exogenous dopamine. Presynaptic serotonin 5-HT1B receptors are responsible for a significant component of dopamine-induced synaptic depression. This study clarifies the mechanistic understanding of dopamine actions in the NAc by showing pathway-specific modulation of lateral inhibition and involvement of D2 and 5-HT1B receptors in dopamine depression of D2-MSN synapses.

Opposing functions of α- and β-adrenoceptors in the formation of processes by cultured astrocytes

Astrocytes are glial cells with numerous fine processes which are important for the functions of the central nervous system. The activation of β-adrenoceptors induces process formation of astrocytes via cyclic AMP (cAMP) signaling. However, the role of α-adrenoceptors in the astrocyte morphology has not been elucidated. Here, we examined it by using cultured astrocytes from neonatal rat spinal cords and cortices. Exposure of these cells to noradrenaline and the β-adrenoceptor agonist isoproterenol increased intracellular cAMP levels and induced the formation of processes. Noradrenaline-induced process formation was enhanced with the α1-adrenoceptor antagonist prazosin and α2-adrenoceptor antagonist atipamezole. Atipamezole also enhanced noradrenaline-induced cAMP elevation. Isoproterenol-induced process formation was not inhibited by the α1-adrenoceptor agonist phenylephrine but was inhibited by the α2-adrenoceptor agonist dexmedetomidine. Dexmedetomidine also inhibited process formation induced by the adenylate cyclase activator forskolin and the membrane-permeable cAMP analog dibutyryl-cAMP. Moreover, dexmedetomidine inhibited cAMP-independent process formation induced by adenosine or the Rho-associated kinase inhibitor Y27632. In the presence of propranolol, noradrenaline inhibited Y27632-induced process formation, which was abolished by prazosin or atipamezole. These results demonstrate that α-adrenoceptors inhibit both cAMP-dependent and -independent astrocytic process formation.

α1-Adrenergic Receptors in Neurotransmission, Synaptic Plasticity, and Cognition

α₁-adrenergic receptors are G-Protein Coupled Receptors that are involved in neurotransmission and regulate the sympathetic nervous system through binding and activating the neurotransmitter, norepinephrine, and the neurohormone, epinephrine. There are three α₁-adrenergic receptor subtypes (α_1A, α_1B, α_1D) that are known to play various roles in neurotransmission and cognition. They are related to two other adrenergic receptor families that also bind norepinephrine and epinephrine, the β- and α₂-, each with three subtypes (β₁, β₂, β₃, α_2A, α_2B, α_2C). Previous studies assessing the roles of α₁-adrenergic receptors in neurotransmission and cognition have been inconsistent. This was due to the use of poorly-selective ligands and many of these studies were published before the characterization of the cloned receptor subtypes and the subsequent development of animal models. With the availability of more-selective ligands and the development of animal models, a clearer picture of their role in cognition and neurotransmission can be assessed. In this review, we highlight the significant role that the α₁-adrenergic receptor plays in regulating synaptic efficacy, both short and long-term synaptic plasticity, and its regulation of different types of memory. We will also present evidence that the α₁-adrenergic receptors, and particularly the α_1A-adrenergic receptor subtype, are a potentially good target to treat a wide variety of neurological conditions with diminished cognition.

Identification of potassium channel proteins Kv7.2/7.3 as common partners of the dopamine and glutamate transporters DAT and GLT-1

Dopamine and glutamate transporters (DAT and GLT-1, respectively) share some biophysical characteristics, as both are secondary active carriers coupled to electrochemical ion gradients. In order to identify common or specific components of their respective proteomes, we performed a proximity labelling assay (BioID) in the hippocampal cell line HT22. While most of the identified proteins were specific for each transporter (and will be analyzed elsewhere), we detected two membrane proteins in the shared interactome of GLT-1 and DAT: the transmembrane protein 263 (Tmem263) and the potassium channel protein Kv7.3. However, only Kv7.3 formed immunoprecipitable complexes with GLT-1 and DAT in lysates of transfected HEK293 cells. Moreover, either DAT or GLT-1 co-clustered with Kv7.2/7.3 along the axonal tracts in co-transfected primary neurons, indicating a close spatial proximity between these proteins. Kv7.3, forming heterotetramers with the closely related subunit Kv7.2, underlies the M-currents that control the resting membrane potential and spiking activity in neurons. To investigate whether the presence of the potassium channel affected DAT or GLT-1 function, we performed uptake determinations using radioactive substrate and electrophysiological measurements. Uptake through both transporters was mildly stimulated by the presence of the channel, an effect that was reversed by the potassium channel blocker XE-991. Electrophysiological recording (in transfected HT22 and differentiated SH-SY5Y cells) indicated that the depolarizing effect induced by the presence of the neurotransmitter was reverted by the activity of the potassium channel. Altogether, these data suggest a tight spatial and functional relationship between the DAT/GLT-1 transporters and the Kv7.2/7.3 potassium channel that immediately readjusts the membrane potential of the neuron, probably to limit the neurotransmitter-mediated neuronal depolarization. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Noradrenergic terminals are the primary source of α2-adrenoceptor mediated dopamine release in the medial prefrontal cortex

In various psychiatric disorders, deficits in dopaminergic activity in the prefrontal cortex (PFC) are implicated. Treatments involving selective augmentation of dopaminergic activity in the PFC primarily depend on the inhibition of α₂-adrenoreceptors singly or in combination with the inhibition of the norepinephrine transporter (NET). We aimed to clarify the relative contribution of dopamine (DA) release from noradrenergic and dopaminergic terminals to DA output induced by blockade of α₂-adrenoreceptors and NET. To this end, we assessed whether central noradrenergic denervation modified catecholamine output in the medial PFC (mPFC) of rats elicited by atipamezole (an α₂-adrenoreceptor antagonist), nisoxetine (an NET inhibitor), or their combination. Intraventricular administration of anti-dopamine-beta-hydroxylase-saporin (aDBH) caused a loss of DBH-positive fibers in the mPFC and almost total depletion of tissue and extracellular NE level; however, it did not reduce tissue DA level but increased extracellular DA level by 70% in the mPFC. Because noradrenergic denervation should have caused a loss of NET and reduced NE level at α₂-adrenoceptors, the actual effect of an aDBH-induced lesion on DA output elicited by blockade of α₂-adrenoceptors and NET was evaluated by comparing denervated and control rats following blockade of α₂-adrenoceptors and NET with atipamezole and nisoxetine, respectively. In the control rats, extracellular NE and DA levels increased by approximately 150% each with 3 mg/kg atipamezole; 450% and 230%, respectively, with 3 mg/kg nisoxetine; and 2100% and 600%, respectively, with combined atipamezole and nisoxetine. In the denervated rats, consistent with the loss of NET, nisoxetine failed to modify extracellular DA level, whereas atipamezole, despite the lack of NE-induced stimulation of α₂-adrenoceptors, increased extracellular DA level by approximately 30%. Overall, these results suggest that atipamezole-induced DA release mainly originated from noradrenergic terminals, possibly through the inhibition of α₂-autoreceptors. Furthermore, while systemic and local administration of the α₂-adrenoceptor agonist clonidine into the mPFC of the controls rats reduced extracellular NE level by 80% and 60%, respectively, and extracellular DA level by 50% and 60%, respectively, it failed to reduce DA output in the denervated rats, consistent with the loss of α₂-autoreceptors. To eliminate the possibility that denervation reduced DA release potential via the effects at dopaminergic terminals in the mPFC, the effect of systemic administration of the D₂-DA antagonist raclopride (0.5 mg/kg IP) on DA output was analyzed. In the control rats, raclopride was found to be ineffective when administered alone, but it increased extracellular DA level by 380% following NET inhibition with nisoxetine. In the denervated rats, as expected due to the loss of NET, raclopride-alone or with nisoxetine-increased DA release to approximately the same level as that observed in the control rats after NET inhibition. Overall, these results suggest that noradrenergic terminals in the mPFC are the primary source of DA released by blockade of α₂-adrenoreceptors and NET and that α₂-autoreceptors, and not α₂-heteroreceptors, mediate DA output induced by α₂-adrenoceptor blockade.

Daily Profiles of Neuropeptides, Catecholamines, and Neurotransmitter Receptors in the Chicken Pineal Gland

The avian pineal gland is one of three central biological clocks that contain all the components of a circadian system: a photoreceptive input, oscillator, and rhythmically secreted melatonin (MEL) as an effector. The biosynthesis of MEL is regulated by the neurotransmitters noradrenaline (NA), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating polypeptide (PACAP). The aim of the present study was to characterize the daily profile of neurotransmitters and their receptors in the pineal gland of male Hy-Line chickens housed under controlled light (12:12 light:dark) conditions. The pineal glands were isolated from 16-day-old birds every 2 h over a 24-h period, immediately after decapitation. The catecholamine content was measured using HPLC with electrochemical detection, whereas expression of VIP and PACAP was measured using quantitative real-time PCR (RT-qPCR) assays and Western blotting. Expression of the neurotransmitter receptors was also measured using RT-qPCR. We found daily changes in NA content, with elevated nocturnal levels, whereas the NA receptor was expressed in antiphase. Although we did not observe daily changes in VIP and PACAP protein levels, we found prominent diurnal changes in the expression of the Vip and Pacap genes. We also detected precursors of NA, 3,4-dihydroxy-L-phenylalanine (DOPA), and dopamine (DA) in the pineal glands, in addition to the DA metabolites. Our results provide the first evidence that the pineal gland itself may synthetize the neurotransmitters needed to regulate MEL biosynthesis.

Dopamine: Functions, Signaling, and Association with Neurological Diseases

The dopaminergic system plays important roles in neuromodulation, such as motor control, motivation, reward, cognitive function, maternal, and reproductive behaviors. Dopamine is a neurotransmitter, synthesized in both central nervous system and the periphery, that exerts its actions upon binding to G protein-coupled receptors. Dopamine receptors are widely expressed in the body and function in both the peripheral and the central nervous systems. Dopaminergic signaling pathways are crucial to the maintenance of physiological processes and an unbalanced activity may lead to dysfunctions that are related to neurodegenerative diseases. Unveiling the neurobiology and the molecular mechanisms that underlie these illnesses may contribute to the development of new therapies that could promote a better quality of life for patients worldwide. In this review, we summarize the aspects of dopamine as a catecholaminergic neurotransmitter and discuss dopamine signaling pathways elicited through dopamine receptor activation in normal brain function. Furthermore, we describe the potential involvement of these signaling pathways in evoking the onset and progression of some diseases in the nervous system, such as Parkinson's, Schizophrenia, Huntington's, Attention Deficit and Hyperactivity Disorder, and Addiction. A brief description of new dopaminergic drugs recently approved and under development treatments for these ailments is also provided.

Reversal of dopamine-mediated firing inhibition through activation of the dopamine transporter in substantia nigra pars compacta neurons

BACKGROUND AND PURPOSE

One of the hallmarks of ventral midbrain dopamine-releasing neurons is membrane hyperpolarization in response to stimulation of somato-dendritic D₂ receptors. At early postnatal age, under sustained dopamine, this inhibitory response is followed by a slow recovery, resulting in dopamine inhibition reversal (DIR). In the present investigation, we aimed to get a better insight into the cellular mechanisms underlying DIR.

EXPERIMENTAL APPROACH

We performed single-unit extracellular recordings with a multi-electrode array device and conventional patch-clamp recordings on midbrain mouse slices.

KEY RESULTS

While continuous dopamine (100 μM) perfusion gave rise to firing inhibition that recovered in 10 to 15 min, the same effect was not obtained with the D₂ receptor agonist quinpirole (100 nM). Moreover, firing inhibition caused by the GABA_B receptor agonist baclofen (300 nM) was reversed by dopamine (100 μM), albeit D₂ receptors had been blocked by sulpiride (10 μM). Conversely, the block of the dopamine transporter (DAT) with cocaine (30 μM) prevented firing recovery by dopamine under GABA_B receptor stimulation. Accordingly, in whole-cell recordings from single cells, the baclofen-induced outward current was counteracted by dopamine (100 μM) in the presence of sulpiride (10 μM), and this effect was prevented by the DAT antagonists cocaine (30 μM) and GBR12909 (2 μM).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the DAT plays a major role in DIR, mediating it under conditions of sustained dopamine exposure, and point to DAT as an important target for pharmacological therapies leading to prolonged enhancement of the dopaminergic signal.

Multiplexed profiling of GPCR activities by combining split TEV assays and EXT-based barcoded readouts

G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors and are implicated in the physiological regulation of many biological processes. The high diversity of GPCRs and their physiological functions make them primary targets for therapeutic drugs. For the generation of novel compounds, however, selectivity towards a given target is a critical issue in drug development as structural similarities between members of GPCR subfamilies exist. Therefore, the activities of multiple GPCRs that are both closely and distantly related to assess compound selectivity need to be tested simultaneously. Here, we present a cell-based multiplexed GPCR activity assay, termed GPCRprofiler, which uses a β-arrestin recruitment strategy and combines split TEV protein-protein interaction and EXT-based barcode technologies. This approach enables simultaneous measurements of receptor activities of multiple GPCR-ligand combinations by applying massively parallelized reporter assays. In proof-of-principle experiments covering 19 different GPCRs, both the specificity of endogenous agonists and the polypharmacological effects of two known antipsychotics on GPCR activities were demonstrated. Technically, normalization of barcode reporters across individual assays allows quantitative pharmacological assays in a parallelized manner. In summary, the GPCRprofiler technique constitutes a flexible and scalable approach, which enables simultaneous profiling of compound actions on multiple receptor activities in living cells.

Dopamine acting at D1-like, D2-like and α1-adrenergic receptors differentially modulates theta and gamma oscillatory activity in primary motor cortex

The loss of dopamine (DA) in Parkinson’s is accompanied by the emergence of exaggerated theta and beta frequency neuronal oscillatory activity in the primary motor cortex (M1) and basal ganglia. DA replacement therapy or deep brain stimulation reduces the power of these oscillations and this is coincident with an improvement in motor performance implying a causal relationship. Here we provide in vitro evidence for the differential modulation of theta and gamma activity in M1 by DA acting at receptors exhibiting conventional and non-conventional DA pharmacology. Recording local field potentials in deep layer V of rat M1, co-application of carbachol (CCh, 5 μM) and kainic acid (KA, 150 nM) elicited simultaneous oscillations at a frequency of 6.49 ± 0.18 Hz (theta, n = 84) and 34.97 ± 0.39 Hz (gamma, n = 84). Bath application of DA resulted in a decrease in gamma power with no change in theta power. However, application of either the D1-like receptor agonist SKF38393 or the D2-like agonist quinpirole increased the power of both theta and gamma suggesting that the DA-mediated inhibition of oscillatory power is by action at other sites other than classical DA receptors. Application of amphetamine, which promotes endogenous amine neurotransmitter release, or the adrenergic α1-selective agonist phenylephrine mimicked the action of DA and reduced gamma power, a result unaffected by prior co-application of D1 and D2 receptor antagonists SCH23390 and sulpiride. Finally, application of the α1-adrenergic receptor antagonist prazosin blocked the action of DA on gamma power suggestive of interaction between α1 and DA receptors. These results show that DA mediates complex actions acting at dopamine D1-like and D2-like receptors, α1 adrenergic receptors and possibly DA/α1 heteromultimeric receptors to differentially modulate theta and gamma activity in M1.

Temperament and arousal systems: A new synthesis of differential psychology and functional neurochemistry

This paper critically reviews the unidimensional construct of General Arousal as utilised by models of temperament in differential psychology for example, to underlie 'Extraversion'. Evidence suggests that specialization within monoamine neurotransmitter systems contrasts with the attribution of a "general arousal" of the Ascending Reticular Activating System. Experimental findings show specialized roles of noradrenaline, dopamine, and serotonin systems in hypothetically mediating three complementary forms of arousal that are similar to three functional blocks described in classical models of behaviour within kinesiology, clinical neuropsychology, psychophysiology and temperament research. In spite of functional diversity of monoamine receptors, we suggest that their functionality can be classified using three universal aspects of actions related to expansion, to selection-integration and to maintenance of chosen behavioural alternatives. Monoamine systems also differentially regulate analytic vs. routine aspects of activities at cortical and striatal neural levels. A convergence between main temperament models in terms of traits related to described functional aspects of behavioural arousal also supports the idea of differentiation between these aspects analysed here in a functional perspective.

Modafinil as a catecholaminergic agent: empirical evidence and unanswered questions

Modafinil, in its two clinical formulations (Provigil^® and Nuvigil^®), is a widely prescribed wake-promoting therapeutic agent. It binds competitively to the cell-membrane dopamine (DA) transporter and is dependent on catecholaminergic (dopaminergic and adrenergic) signaling for its wake-promoting effects. The clinical spectrum of effects for modafinil is distinct from the effects seen with other catecholaminergic agents. Relative to other commonly used agents that act through catecholaminergic mechanisms, modafinil has a relatively low abuse potential, produces wakefulness with an attenuated compensatory sleep recovery thereafter, and does not ameliorate cataplexy in narcolepsy. These clinically relevant phenomenological differences between modafinil and agents such as amphetamines and cocaine do not eliminate catecholaminergic effects as a possible mediator of its wake-promoting action; they merely reflect its unique pharmacological profile. Modafinil is an exceptionally weak, but apparently very selective, DA transporter inhibitor. The pharmacodynamic response to modafinil, as measured by DA levels in brain microdialyzate, is protracted relative to other agents that act via catecholaminergic mechanisms. The conformational constraints on the interaction of modafinil with the DA transporter – and probably, as a consequence, its effects on trace amine receptor signaling in the catecholaminergic cell – are unique among catecholaminergic agents. These unique pharmacological properties of modafinil should be considered both in seeking to thoroughly understand its putatively elusive mechanism of action and in the design of novel therapeutic agents.

Dopaminergic modulation of GABAergic transmission in the entorhinal cortex: concerted roles of α1 adrenoreceptors, inward rectifier K⁺, and T-type Ca²⁺ channels

Whereas the entorhinal cortex (EC) receives profuse dopaminergic innervations from the midbrain, the effects of dopamine (DA) on γ-Aminobutyric acid (GABA)ergic interneurons in this brain region have not been determined. We probed the actions of DA on GABAA receptor-mediated synaptic transmission in the EC. Application of DA increased the frequency, not the amplitude, of spontaneous IPSCs (sIPSCs) and miniature IPSCs (mIPSCs) recorded from entorhinal principal neurons, but slightly reduced the amplitude of the evoked IPSCs. The effects of DA were unexpectedly found to be mediated by α1 adrenoreceptors, but not by DA receptors. DA endogenously released by the application of amphetamine also increased the frequency of sIPSCs. Ca(2+) influx via T-type Ca(2+) channels was required for DA-induced facilitation of sIPSCs and mIPSCs. DA depolarized and enhanced the firing frequency of action potentials of interneurons. DA-induced depolarization was independent of extracellular Na(+) and Ca(2+) and did not require the functions of hyperpolarization-activated (Ih) channels and T-type Ca(2+) channels. DA-generated currents showed a reversal potential close to the K(+) reversal potential and inward rectification, suggesting that DA inhibits the inward rectifier K(+) channels (Kirs). Our results demonstrate that DA facilitates GABA release by activating α1 adrenoreceptors to inhibit Kirs, which further depolarize interneurons resulting in secondary Ca(2+) influx via T-type Ca(+) channels.

Hypnotic hypersensitivity to volatile anesthetics and dexmedetomidine in dopamine β-hydroxylase knockout mice

BACKGROUND

Multiple lines of evidence suggest that the adrenergic system can modulate sensitivity to anesthetic-induced immobility and anesthetic-induced hypnosis as well. However, several considerations prevent the conclusion that the endogenous adrenergic ligands norepinephrine and epinephrine alter anesthetic sensitivity.

METHODS

Using dopamine β-hydroxylase knockout (Dbh) mice genetically engineered to lack the adrenergic ligands and their siblings with normal adrenergic levels, we test the contribution of the adrenergic ligands upon volatile anesthetic induction and emergence. Moreover, we investigate the effects of intravenous dexmedetomidine in adrenergic-deficient mice and their siblings using both righting reflex and processed electroencephalographic measures of anesthetic hypnosis.

RESULTS

We demonstrate that the loss of norepinephrine and epinephrine and not other neuromodulators co-packaged in adrenergic neurons is sufficient to cause hypersensitivity to induction of volatile anesthesia. However, the most profound effect of adrenergic deficiency is retarding emergence from anesthesia, which takes two to three times as long in Dbh mice for sevoflurane, isoflurane, and halothane. Having shown that Dbh mice are hypersensitive to volatile anesthetics, we further demonstrate that their hypnotic hypersensitivity persists at multiple doses of dexmedetomidine. Dbh mice exhibit up to 67% shorter latencies to loss of righting reflex and up to 545% longer durations of dexmedetomidine-induced general anesthesia. Central rescue of adrenergic signaling restores control-like dexmedetomidine sensitivity. A novel continuous electroencephalographic analysis illustrates that the longer duration of dexmedetomidine-induced hypnosis is not due to a motor confound, but occurs because of impaired anesthetic emergence.

CONCLUSIONS

Adrenergic signaling is essential for normal emergence from general anesthesia. Dexmedetomidine-induced general anesthesia does not depend on inhibition of adrenergic neurotransmission.

The activation of α1-adrenoceptors is implicated in the antidepressant-like effect of creatine in the tail suspension test

The antidepressant-like activity of creatine in the tail suspension test (TST) was demonstrated previously by our group. In this study we investigated the involvement of the noradrenergic system in the antidepressant-like effect of creatine in the mouse TST. In the first set of experiments, creatine administered by i.c.v. route (1 μg/site) decreased the immobility time in the TST, suggesting the central effect of this compound. The anti-immobility effect of peripheral administration of creatine (1 mg/kg, p.o.) was prevented by the pretreatment of mice with α-methyl-p-tyrosine (100 mg/kg, i.p., inhibitor of tyrosine hydroxylase), prazosin (1 mg/kg, i.p., α1-adrenoceptor antagonist), but not by yohimbine (1 mg/kg, i.p., α2-adrenoceptor antagonist). Creatine (0.01 mg/kg, subeffective dose) in combination with subeffective doses of amitriptyline (1 mg/kg, p.o., tricyclic antidepressant), imipramine (0.1 mg/kg, p.o., tricyclic antidepressant), reboxetine (2 mg/kg, p.o., selective noradrenaline reuptake inhibitor) or phenylephrine (0.4 μg/site, i.c.v., α1-adrenoceptor agonist) reduced the immobility time in the TST as compared with either drug alone. These results indicate that the antidepressant-like effect of creatine is likely mediated by an activation of α1-adrenoceptor and that creatine produces synergistic effects in the TST with antidepressants that modulate noradrenaline transporter, suggesting that an improvement in the response to the antidepressant therapy may occur when creatine is combined with these antidepressants. Furthermore, the synergistic effect of creatine (0.01 mg/kg, p.o.) and reboxetine (2 mg/kg, p.o.) combination was abolished by the α1-adrenoceptor antagonist prazosin, indicating that the antidepressant-like effect of combined therapy is likely mediated by an activation of α1-adrenoceptor.

Norepinephrine modulates the motility of resting and activated microglia via different adrenergic receptors

Microglia, the resident immune cells of the central nervous system (CNS), monitor the brain for disturbances of tissue homeostasis by constantly moving their fine processes. Microglia respond to tissue damage through activation of ATP/ADP receptors followed by directional process extension to the damaged area. A common feature of several neurodegenerative diseases is the loss of norepinephrine, which might contribute to the associated neuroinflammation. We carried out a high resolution analysis of the effects of norepinephrine (NE) on microglial process dynamics in acute brain slices from mice that exhibit microglia-specific enhanced green fluorescent protein expression. Bath application of NE to the slices resulted in significant process retraction in microglia. Analysis of adrenergic receptor expression with quantitative PCR indicated that resting microglia primarily express β2 receptors but switch expression to α2A receptors under proinflammatory conditions modeled by LPS treatment. Despite the differential receptor expression, NE caused process retraction in both resting and LPS-activated microglia cultured in the gelatinous substrate Matrigel in vitro. The use of subtype-selective receptor agonists and antagonists confirmed the involvement of β2 receptors in mediating microglial process dynamics in resting cells and α2A receptors in activated cells. Co-application of NE with ATP to resting microglia blocked the ATP-induced process extension and migration in isolated microglia, and β2 receptor antagonists prolonged ATP effects in brain slice tissues, suggesting the presence of cross-talk between adrenergic and purinergic signaling in microglia. These data show that the neurotransmitter NE can modulate microglial motility, which could affect microglial functions in pathogenic situations of either elevated or reduced NE levels.

Characterisation of AmphiAmR4, an amphioxus (Branchiostoma floridae) α₂-adrenergic-like G-protein-coupled receptor

Little is known about the evolutionary relationship between vertebrate adrenergic receptors and invertebrate octopamine and tyramine receptors. The complexity of the adrenergic signalling system is believed to be an innovation of the vertebrate lineage but the presence of noradrenaline has been reported in some invertebrate species. The cephalochordate, amphioxus (Branchiostoma floridae), is an ideal model organism for studying the evolution of vertebrate GPCRs, given its unique position at the base of the chordate lineage. Here, we describe the pharmacological characterisation and second messenger coupling abilities of AmphiAmR4, which clusters with α₂-adrenergic receptors in a phylogenetic tree but also shares a high sequence similarity to invertebrate octopamine/tyramine receptors in both BLAST and Hidden Markov Model analyses. Thus, it was of particular interest to determine if AmphiAmR4 displayed similar functional properties to the vertebrate α₂-adrenergic receptors or to invertebrate octopamine or tyramine receptors. When stably expressed in Chinese hamster ovary (CHO) cells, noradrenaline couples the receptor to both the activation of adenylyl cyclase and to the activation of the MAPKinase pathway. Pharmacological studies with a wide range of agonists and antagonists suggest that AmphiAmR4 functions as an α₂-adrenergic-like receptor when expressed in CHO cells.

Imipramine induces brain-derived neurotrophic factor mRNA expression in cultured astrocytes

Depression is one of the most prevalent and livelihood-threatening forms of mental illnesses and the neural circuitry underlying depression remains incompletely understood. Recent studies suggest that the neuronal plasticity involved with brain-derived neurotrophic factor (BDNF) plays an important role in the recovery from depression. Some antidepressants are reported to induce BDNF expression in vivo; however, the mechanisms have been considered solely in neurons and not fully elucidated. In the present study, we evaluated the effects of imipramine, a classic tricyclic antidepressant drug, on BDNF expression in cultured rat brain astrocytes. Imipramine dose-dependently increased BDNF mRNA expression in astrocytes. The imipramine-induced BDNF increase was suppressed with inhibitors for protein kinase A (PKA) or MEK/ERK. Moreover, imipramine exposure activated transcription factor cAMP response element binding protein (CREB) in a dose-dependent manner. These results suggested that imipramine induced BDNF expression through CREB activation via PKA and/or ERK pathways. Imipramine treatment in depression might exert antidepressant action through BDNF production from astrocytes, and glial BDNF expression might be a target of developing novel antidepressants.

Disulfiram stimulates dopamine release from noradrenergic terminals and potentiates cocaine-induced dopamine release in the prefrontal cortex

RATIONALE

Disulfiram efficacy in treatment of cocaine addiction is attributed to the inhibition of dopamine-β-hydroxylase and reduction in brain noradrenaline (NA)/dopamine (DA) ratio.

OBJECTIVES

Using microdialysis, we investigated if disulfiram causes DA release from noradrenergic terminals and modifies cocaine-induced DA release.

RESULTS

Disulfiram reduced extracellular NA in the medial prefrontal (mPF) cortex, occipital cortex, accumbens and caudate nuclei, while it markedly increased DA not only in mPF but also in the occipital cortex, despite its scanty dopaminergic afferences, and modestly increased DA in the accumbens and caudate nuclei, despite their dense dopaminergic innervation. Disulfiram-induced DA accumulation was reversed in both cortices by tetrodotoxin infusion and by systemic administration of the α(2)-adrenoceptor agonist clonidine, but was not modified by the α(2)-adrenoceptor antagonist RS 79948 or the D(2)-like agonist quinpirole. Disulfiram prevented cocaine-induced NA release in the mPF cortex and nucleus accumbens, potentiated cocaine-induced DA release in the mPF cortex but failed to modify cocaine effect in the nucleus accumbens. DA release induced by disulfiram-cocaine combination in the mPF cortex was prevented by clonidine but not by quinpirole.

CONCLUSIONS

We suggested that disulfiram, by removing NA-mediated inhibitory control on noradrenergic terminals, causes an unrestrained cocaine-induced DA release from those terminals in the mPF cortex. In the accumbens and caudate nuclei, "allogenic" DA concentration might be clouded by DA originated from dopaminergic terminals. The possible role of "allogenic" DA in disulfiram ability to prevent stress-induced reinstatement of cocaine seeking is discussed.

Stress and glucocorticoids impair memory retrieval via β2-adrenergic, Gi/o-coupled suppression of cAMP signaling

Acute stress impairs the retrieval of hippocampus-dependent memory, and this effect is mimicked by exogenous administration of stress-responsive glucocorticoid hormones. It has been proposed that glucocorticoids affect memory by promoting the release and/or blocking the reuptake of norepinephrine (NE), a stress-responsive neurotransmitter. It has also been proposed that this enhanced NE signaling impairs memory retrieval by stimulating β(1)-adrenergic receptors and elevating levels of cAMP. In contrast, other evidence indicates that NE, β(1), and cAMP signaling is transiently required for the retrieval of hippocampus-dependent memory. To resolve this discrepancy, wild-type rats and mice with and without gene-targeted mutations were stressed or treated with glucocorticoids and/or adrenergic receptor drugs before testing memory for inhibitory avoidance or fear conditioning. Here we report that glucocorticoids do not require NE to impair retrieval. However, stress- and glucocorticoid-induced impairments of retrieval depend on the activation of β(2) (but not β(1))-adrenergic receptors. Offering an explanation for the opposing functions of these two receptors, the impairing effects of stress, glucocorticoids and β(2) agonists on retrieval are blocked by pertussis toxin, which inactivates signaling by G(i/o)-coupled receptors. In hippocampal slices, β(2) signaling decreases cAMP levels and greatly reduces the increase in cAMP mediated by β(1) signaling. Finally, augmenting cAMP signaling in the hippocampus prevents the impairment of retrieval by systemic β(2) agonists or glucocorticoids. These results demonstrate that the β(2) receptor can be a critical effector of acute stress, and that β(1) and β(2) receptors can have quite distinct roles in CNS signaling and cognition.

Xamoterol impairs hippocampus-dependent emotional memory retrieval via Gi/o-coupled β2-adrenergic signaling

Xamoterol, a partial β(1)-adrenergic receptor agonist, has been reported to impair the retrieval of hippocampus-dependent spatial reference memory in rats. In contrast, xamoterol restores memory retrieval in gene-targeted mice lacking norepinephrine (NE) and in a transgenic mouse model of Down syndrome in which NE levels are reduced. Restoration of retrieval by xamoterol in these two models complements the observation that NE and β(1) signaling are required for hippocampus-dependent retrieval of contextual and spatial reference memory in wild-type mice and rats. Additional evidence indicates that cAMP-mediated PKA and Epac signaling are required for the retrieval of hippocampus-dependent memory. As a result, we hypothesized that xamoterol has effects in addition to the stimulation of β(1) receptors that, at higher doses, act to counter the effects of β(1) signaling. Here we report that xamoterol-induced disruption of memory retrieval depends on β(2)-adrenergic receptor signaling. Interestingly, the impairment of memory retrieval by xamoterol is blocked by pretreatment with pertussis toxin, an uncoupling agent for G(i/o) signaling, suggesting that β(2) signaling opposes β(1) signaling during memory retrieval at the level of G protein and cAMP signaling. Finally, similar to the time-dependent roles for NE, β(1), and cAMP signaling in hippocampus-dependent memory retrieval, xamoterol only impairs retrieval for several days after training, indicating that its effects are also limited by the age of the memory. We conclude that the disruption of memory retrieval by xamoterol is mediated by G(i/o)-coupled β(2) signaling, which opposes the G(s)-coupled β(1) signaling that is transiently required for hippocampus-dependent emotional memory retrieval.

Neurocognitive function in dopamine-β-hydroxylase deficiency

Dopamine-β-hydroxylase (DβH) deficiency is a rare genetic syndrome characterized by the complete absence of norepinephrine in the peripheral and the central nervous system. DβH-deficient patients suffer from several physical symptoms, which can be treated successfully with L-threo-3,4-dihydroxyphenylserine, a synthetic precursor of norepinephrine. Informal clinical observations suggest that DβH-deficient patients do not have obvious cognitive impairments, even when they are not medicated, which is remarkable given the important role of norepinephrine in normal neurocognitive function. This study provided the first systematic investigation of neurocognitive function in human DβH deficiency. We tested 5 DβH-deficient patients and 10 matched healthy control participants on a comprehensive cognitive task battery, and examined their pupil dynamics, brain structure, and the P3 component of the electroencephalogram. All participants were tested twice; the patients were tested once ON and once OFF medication. Magnetic resonance imaging scans of the brain revealed that the patients had a smaller total brain volume than the control group, which is in line with the recent hypothesis that norepinephrine has a neurotrophic effect. In addition, the patients showed an abnormally small or absent task-evoked pupil dilation. However, we found no substantial differences in cognitive performance or P3 amplitude between the patients and the control participants, with the exception of a temporal-attention deficit in the patients OFF medication. The largely spared neurocognitive function in DβH-deficient patients suggests that other neuromodulators have taken over the function of norepinephrine in the brains of these patients.

Narcolepsy and depression and the neurobiology of gammahydroxybutyrate

A voluminous literature describes the relationship between disturbed sleep and depression. The breakdown of sleep is one of the cardinal features of depression and often also heralds its onset. Frequent arousals, periods of wakefulness and a short sleep onset REM latency are typical polysomnographic features of depression. The short latency to REM sleep has been attributed to the combination of a monoaminergic deficiency and cholinergic supersensitivity and these irregularities have been proposed to form the biological basis of the disorder. A similar imbalance between monoaminergic and cholinergic neurotransmission has been found in narcolepsy, a condition in which frequent awakenings, periods of wakefulness and short sleep onset REM latencies are also characteristic findings during sleep. In many cases of narcolepsy, this imbalance appears to result from a deficiency of hypocretin but once established, whether in depression or narcolepsy, this disequilibrium sets the stage for the dissociation or premature appearance of REM sleep and for the dissociation of the motor inhibitory component of REM sleep or cataplexy. In the presence of this monoaminergic/cholinergic imbalance, gammahydroxybutyrate (GHB) may acutely further reduce the latency of REM sleep and induce cataplexy, in both patients with narcolepsy or depression. On the other hand, the repeated nocturnal application of GHB in patients with narcolepsy improves the continuity of sleep, prolongs the latency to REM sleep and prevents cataplexy. Evidence to date suggests that GHB may restore the normal balance between monoaminergic and cholinergic neurotransmission. As such, the repeated use of GHB at night and the stabilization of sleep over time makes GHB an effective treatment for narcolepsy and a potentially effective treatment for depression.

Interplay among catecholamine systems: dopamine binds to alpha2-adrenergic receptors in birds and mammals

Dopaminergic and adrenergic receptors are G-protein-coupled receptors considered to be different based on their pharmacology and signaling pathways. Some receptor subtypes that are members of one family are actually closer in phylogenetic terms to some subtypes belonging to the other family, suggesting that the pharmacological specificity among these receptors from different families is not perfect. Indeed, evidence is accumulating that one amine can cross-talk with receptors belonging to the other system. However, most of these observations were collected in vitro using artificial cell models transfected with cloned receptors, so that the occurrence of this phenomenon in vivo as well as its distribution in the central nervous system is not known. In this study the pharmacological basis of possible in vivo interactions between dopamine and alpha(2)-adrenergic receptors was investigated in quail, zebra finches, and rats. Binding competitions showed that dopamine displaces the binding of the selective alpha(2)-adrenergic ligand, [(3)H]RX821002, in the brain of the three species with an affinity approximately 10-28-fold lower than that of norepinephrine. Dopamine also displaces with an affinity 3-fold lower than norepinephrine the binding of [(3)H]RX821002 to human alpha(h2A)-adrenergic receptors expressed in Sf9 cells. The anatomical distribution of this interaction was assessed in brain slices of quail and rat based on autoradiographic methods. Both norepinephrine and dopamine significantly displace [(3)H]RX821002 binding in all brain nuclei considered. Together, these data provide evidence for an interaction between the dopaminergic and noradrenergic systems in the vertebrate brain, albeit with species variations.

Electrophysiological correlates of sleep disturbance induced by acute and chronic administration of D-amphetamine

Sleep disturbance is the strongest predictor of manic relapse and is considered one of the most important objective measures of treatment response in bipolar disorder (BD). However, the neurobiological mechanisms underlying sleep disturbance in BD are poorly understood. The administration of psychostimulants to rodents can trigger a number of manic-like behaviors. Therefore, the present study aims to investigate the effects of single and repeated D-amphetamine (AMPH) administration on sleep patterns in rats. Sleep was continuously monitored during light periods after single and repeated (7 days) injections of AMPH (2 mg/kg, i.p.) or saline in adult Wistar rats using electrocorticogram and electromyographic recordings. Acute injections of AMPH suppressed sleep for the first 2 h, and were followed by a gradual increase in the amount of sleep. Both slow wave sleep (SWS) and paradoxical sleep (PS) were compromised. Repeated exposure to AMPH led to a drastic disruption of the sleep-wake cycle that was mainly characterized by a decrease of PS during all time-points recorded in comparison to the saline group. Furthermore, both acute and chronic AMPH administration induced longer latencies to both SWS and PS. These findings suggest that AMPH produces profound sleep disturbances and decreases PS sleep. Given that some of these abnormalities are observed in individuals with BD, this animal model can provide a means to investigate neurobiological aspects of sleep disturbance in BD, as well as their response to mood stabilizers.

Effects of cocaine, methamphetamine and modafinil challenge on sleep rebound after paradoxical sleep deprivation in rats

Sleep loss is both common and critically relevant to our society and might lead to the abuse of psychostimulants such as amphetamines, cocaine and modafinil. Since psychoactive substance abuse often occurs within a scenario of sleep deficit, the purpose of this investigation was to compare the sleep patterns of rats challenged with cocaine (7 mg/kg, ip), methamphetamine (7 mg/kg, ip), or modafinil (100 mg/kg, ip) subsequent to paradoxical sleep deprivation (PSD) for 96 h. Our results show that, immediately after 96 h of PSD, rats (10 per group) that were injected with a psychostimulant presented lower percentages of paradoxical sleep compared to those injected with saline (P < 0.01). Regarding slow wave sleep (SWS), rats injected with psychostimulants after PSD presented a late rebound (on the second night subsequent to the injection) in the percentage of this phase of sleep when compared to PSD rats injected with saline (P < 0.05). In addition, the current study has produced evidence of the characteristic effect of each drug on sleep architecture. Home cage control rats injected with modafinil and methamphetamine showed a reduction in SWS compared with the saline group. Methamphetamine affected sleep patterns most, since it significantly reduced paradoxical sleep, SWS and sleep efficiency before and after PSD compared to control (P < 0.05). Cocaine was the psychostimulant causing the least changes in sleep pattern in relation to those observed after saline injection. Therefore, our results suggest that abuse of these psychostimulants in a PSD paradigm aggravates their impact on sleep patterns.

Cross-talk between dopaminergic and noradrenergic systems in the rat ventral tegmental area, locus ceruleus, and dorsal hippocampus

A decreased central dopaminergic and/or noradrenergic transmission is believed to be involved in the pathophysiology of depression. It is known that dopamine (DA) neurons in the ventral tegmental area (VTA) and norepinephrine (NE) neurons in the locus ceruleus (LC) are autoregulated by somatodendritic D(2)-like and alpha(2)-adrenoceptors, respectively. Complementing these autoreceptor-mediated inhibitory feedbacks, anatomical and functional studies have established a role for noradrenergic inputs in regulating dopaminergic activity, and reciprocally. In the present study, a microiontophoretic approach was used to characterize the postsynaptic catecholamine heteroreceptors involved in such regulations. In the VTA, the application of DA and NE significantly reduced the firing activity of DA neurons. In addition to a role for D(2)-like receptors in the inhibitory effects of both catecholamines, it was demonstrated that the alpha(2)-adrenoceptor antagonist idazoxan dampened the DA- and NE-induced attenuations of DA neuronal activity, indicating that both of these receptors are involved in the responsiveness of VTA DA neurons to catecholamines. In the LC, the effectiveness of iontophoretically applied NE and DA to suppress NE neuronal firing was blocked by idazoxan but not by the D(2)-like receptor antagonist raclopride, which suggested that only alpha(2)-adrenoceptors were involved. In the dorsal hippocampus, a forebrain region having a sparse dopaminergic innervation but receiving a dense noradrenergic input, the suppressant effects of DA and NE on pyramidal neurons were attenuated by idazoxan but not by raclopride. The suppressant effect of DA was prolonged by administration of the selective NE reuptake inhibitor desipramine and, to lesser extent, of the selective DA reuptake inhibitor 1-(2-[bis(4-fluorophenyl)methoxy]ethyl)-4-(3-phenylpropyl)-piperazine (GBR12909), suggesting that both the NE and DA transporters were involved in DA uptake in the hippocampus. These findings might help in designing new antidepressant strategies aimed at enhancing DA and NE neurotransmission.

Possible dopaminergic stimulation of locus coeruleus alpha1-adrenoceptors involved in behavioral activation

alpha(1)-Adrenoceptors of the locus coeruleus (LC) have been implicated in behavioral activation in novel surroundings, but the endogenous agonist that activates these receptors has not been established. In addition to the canonical activation of alpha(1)-receptors by norepinephrine (NE), there is evidence that dopamine (DA) may also activate certain brain alpha(1)-receptors. This study examined the contribution of DA to exploratory activity in a novel cage by determining the effect of infusion of various dopaminergic and adrenergic drugs into the mouse LC. It was found that the D2/D3 agonist, quinpirole, which selectively blocks the release of CNS DA, produced a dose-dependent and virtually complete abolition of exploration and all movement in the novel cage test. The quinpirole-induced inactivity was significantly attenuated by coinfusion of DA but not by the D1 agonist, SKF38390. Furthermore, the DA attenuation of quinpirole inactivity was blocked by coinfusion of the alpha(1)-adrenergic receptor antagonist, terazosin, but not by the D1 receptor antagonist, SCH23390. LC infusions of either quinpirole or terazosin also produced profound inactivity in DA-beta-hydroxylase knockout (Dbh -/-) mice that lack NE, indicating that their behavioral effects were not due to an alteration of the release or action of LC NE. Measurement of endogenous DA, NE, and 5HT and their metabolites in the LC during exposure to the novel cage indicated an increase in the turnover of DA and NE but not 5HT. These results indicate that DA is a candidate as an endogenous agonist for behaviorally activating LC alpha(1)-receptors and may play a role in the activation of this nucleus by novel surroundings.

Behavioral responses of dopamine beta-hydroxylase knockout mice to modafinil suggest a dual noradrenergic-dopaminergic mechanism of action

Modafinil is approved for use in the treatment of excessive daytime sleepiness. The precise mechanism of modafinil action has not been elucidated, although both dopamine (DA) and norepinephrine (NE) systems have been implicated. To explore the roles of DA and NE in the mechanism of modafinil-induced arousal, dopamine beta-hydroxylase knockout (Dbh -/-) mice were examined in behavioral paradigms of arousal (photobeam breaks and behavioral scoring of sleep latency). Dbh -/- mice completely lack NE but have hypersensitive DA signaling. It was hypothesized that Dbh -/- mice would be unresponsive to modafinil if the compound acts primarily via NE, but would be hypersensitive to modafinil if it acts primarily via DA. Dbh -/- mice had increased sensitivity to the locomotor-activating and wake-promoting effects of modafinil. Paradoxically, the alpha1-adrenergic receptor antagonist, prazosin, attenuated the effects of modafinil in control mice, but not in Dbh -/- mice. Blockade of DA receptors with flupenthixol decreased modafinil-induced locomotion and wake in both control and Dbh -/- mice. These results suggest that both NE and DA are involved in the behavioral effects of modafinil in control mice, but the requirement for NE can be bypassed by hypersensitive DA signaling.

A novel conditional genetic system reveals that increasing neuronal cAMP enhances memory and retrieval

Consistent evidence from pharmacological and genetic studies shows that cAMP is a critical modulator of synaptic plasticity and memory formation. However, the potential of the cAMP signaling pathway as a target for memory enhancement remains unclear because of contradictory findings from pharmacological and genetic approaches. To address these issues, we have developed a novel conditional genetic system in mice based on the heterologous expression of an Aplysia octopamine receptor, a G-protein-coupled receptor whose activation by its natural ligand octopamine leads to rapid and transient increases in cAMP. We find that activation of this receptor transgenically expressed in mouse forebrain neurons induces a rapid elevation of hippocampal cAMP levels, facilitates hippocampus synaptic plasticity, and enhances the consolidation and retrieval of fear memory. Our findings clearly demonstrate that acute increases in cAMP levels selectively in neurons facilitate synaptic plasticity and memory, and illustrate the potential of this heterologous system to study cAMP-mediated processes in mammalian systems.

Modafinil: a review of neurochemical actions and effects on cognition

Modafinil (2-[(Diphenylmethyl) sulfinyl] acetamide, Provigil) is an FDA-approved medication with wake-promoting properties. Pre-clinical studies of modafinil suggest a complex profile of neurochemical and behavioral effects, distinct from those of amphetamine. In addition, modafinil shows initial promise for a variety of off-label indications in psychiatry, including treatment-resistant depression, attention-deficit/hyperactivity disorder, and schizophrenia. Cognitive dysfunction may be a particularly important emerging treatment target for modafinil, across these and other neuropsychiatric disorders. We aimed to comprehensively review the empirical literature on neurochemical actions of modafinil, and effects on cognition in animal models, healthy adult humans, and clinical populations. We searched PubMed with the search term 'modafinil' and reviewed all English-language articles for neurochemical, neurophysiological, cognitive, or information-processing experimental measures. We additionally summarized the pharmacokinetic profile of modafinil and clinical efficacy in psychiatric patients. Modafinil exhibits robust effects on catecholamines, serotonin, glutamate, gamma amino-butyric acid, orexin, and histamine systems in the brain. Many of these effects may be secondary to catecholamine effects, with some selectivity for cortical over subcortical sites of action. In addition, modafinil (at well-tolerated doses) improves function in several cognitive domains, including working memory and episodic memory, and other processes dependent on prefrontal cortex and cognitive control. These effects are observed in rodents, healthy adults, and across several psychiatric disorders. Furthermore, modafinil appears to be well-tolerated, with a low rate of adverse events and a low liability to abuse. Modafinil has a number of neurochemical actions in the brain, which may be related to primary effects on catecholaminergic systems. These effects are in general advantageous for cognitive processes. Overall, modafinil is an excellent candidate agent for remediation of cognitive dysfunction in neuropsychiatric disorders.

Dopamine binds to alpha(2)-adrenergic receptors in the song control system of zebra finches (Taeniopygia guttata)

A commonly held view is that dopamine exerts its effects via binding to D1- and D2-dopaminergic receptors. However, recent data have emerged supporting the existence of a direct interaction of dopamine with adrenergic but this interaction has been poorly investigated. In this study, the pharmacological basis of possible in vivo interactions between dopamine and alpha(2)-adrenergic receptors was investigated in zebra finches. A binding competition study showed that dopamine displaces the binding of the alpha(2)-adrenergic ligand, [(3)H]RX821002, in the brain. The affinity of dopamine for the adrenergic sites does not differ between the sexes and is 10- to 28-fold lower than that for norepinephrine. To assess the anatomical distribution of this interaction, binding competitions were performed on brain slices incubated in 5nM [(3)H]RX821002 in the absence of any competitor or in the presence of norepinephrine [0.1microM] or dopamine [1microM]. Both norepinephrine and dopamine displaced the binding of the radioligand though to a different extent in most of the regions studied (e.g., area X, the lateral part of the magnocellular nucleus of anterior nidopallium, HVC, arcopallium dorsale, ventral tegmental area and substantia grisea centralis) but not in the robust nucleus of the arcopallium. Together these data provide evidence for a direct interaction between dopamine and adrenergic receptors in songbird brains albeit with regional variation.

Peripheral circadian clock rhythmicity is retained in the absence of adrenergic signaling

OBJECTIVE

The incidence of heart attack and stroke undergo diurnal variation. Molecular clocks have been described in the heart and the vasculature; however it is largely unknown how the suprachiasmatic nucleus (SCN) entrains these peripheral oscillators.

METHODS AND RESULTS

Norepinephrine and epinephrine, added to aortic smooth muscle cells (ASMCs) in vitro, altered Per1, E4bp4, and dbp expression and altered the observed oscillations in clock gene expression. However, oscillations of Per1, E4bp4, dbp, and Per2 were preserved ex vivo in the aorta, heart, and liver harvested from dopamine beta-hydroxylase knockout mice (Dbh-/-) that cannot synthesize either norepinephrine or epinephrine. Furthermore, clock gene oscillations in heart, liver, and white adipose tissue phase shifted identically in Dbh-/- mice and in Dbh+/- controls in response to daytime restriction of feeding. Oscillation of clock genes was similarly preserved ex vivo in tissues from Dbh+/- and Dbh-/- chronically treated with both propranolol and terazosin, thus excluding compensation by dopamine in Dbh-/- mice.

CONCLUSIONS

Although adrenergic signaling can influence circadian timing in vitro, peripheral circadian rhythmicity is retained despite its ablation in vivo.

Effects of acepromazine on the cardiovascular actions of dopamine in anesthetized dogs

OBJECTIVE

To evaluate the effects of acepromazine maleate on the cardiovascular changes induced by dopamine in isoflurane-anesthetized dogs.

STUDY DESIGN

Prospective, randomized cross-over experimental design.

ANIMALS

Six healthy adult spayed female dogs weighing 16.4 +/- 3.5 kg (mean +/- SD).

METHODS

Each dog received two treatments, at least 1 week apart. Acepromazine (0.03 mg kg(-1), IV) was administered 15 minutes before anesthesia was induced with propofol (7 mg kg(-1), IV) and maintained with isoflurane (1.8% end-tidal). Acepromazine was not administered in the control treatment. Baseline cardiopulmonary parameters were measured 90 minutes after induction. Thereafter, dopamine was administered intravenously at 5, 10, and 15 microg kg(-1) minute(-1), with each infusion rate lasting 30 minutes. Cardiopulmonary data were obtained at the end of each infusion rate.

RESULTS

Dopamine induced dose-related increases in cardiac index (CI), stroke index, arterial blood pressure, mean pulmonary arterial pressure, oxygen delivery index (DO(2)I) and oxygen consumption index. In the control treatment, systemic vascular resistance index (SVRI) decreased during administration of 5 and 10 microg kg(-1) minute(-1) of dopamine and returned to baseline with the highest dose (15 microg kg (-1) minute(-1)). After acepromazine treatment, SVRI decreased from baseline during dopamine administration, regardless of the infusion rate, and this resulted in a smaller increase in blood pressure at 15 microg kg (-1) minute(-1). During dopamine infusion hemoglobin concentrations were lower following acepromazine and this contributed to significantly lower arterial O(2) content.

CONCLUSIONS

Acepromazine prevented the return in SVRI to baseline and reduced the magnitude of the increase in arterial pressure induced by higher doses of dopamine. However, reduced SRVI associated with lower doses of dopamine and the ability of dopamine to increase CI and DO(2)I were not modified by acepromazine premedication.

CLINICAL RELEVANCE

Previous acepromazine administration reduces the efficacy of dopamine as a vasopressor agent in isoflurane anesthetized dogs. Other beneficial effects of dopamine such as increased CO are not modified by acepromazine.

There and back again: a tale of norepinephrine and drug addiction

Fueled by anatomical, electrophysiological, and pharmacological analyses of endogenous brain reward systems, norepinephrine (NE) was identified as a key mediator of both natural and drug-induced reward in the late 1960s and early 1970s. However, reward experiments from the mid-1970s that could distinguish between the noradrenergic and dopaminergic systems resulted in the prevailing view that dopamine (DA) was the primary 'reward transmitter' (a belief holding some sway still today), thereby pushing NE into the background. Most damaging to the NE hypothesis of reward were studies demonstrating that NE receptor antagonists and NE reuptake inhibitors failed to impact drug self-administration. In recent years new tools, such as genetically engineered mice, and new experimental paradigms, such as reinstatement of drug seeking following withdrawal, have propelled NE back into the awareness of addiction researchers. Of particular interest is disulfiram, an inhibitor of the NE biosynthetic enzyme dopamine beta-hydroxylase, which has demonstrated promising efficacy in the treatment of cocaine dependence in preliminary clinical trials. The purpose of this review is to synthesize the new data linking NE to critical aspects of DA signaling and drug addiction, with a focus on psychostimulants (eg, cocaine), opiates (eg, morphine), and alcohol.

Antidepressant-like effect of lamotrigine in the mouse forced swimming test: evidence for the involvement of the noradrenergic system

Lamotrigine is an anticonvulsant drug that is also effective in the treatment of mood disorders, especially bipolar disorder. However, few studies have been conducted in animal models of depression to evaluate its mechanism of action. The present study investigated the effect of lamotrigine in the forced swimming test in mice and the involvement of the noradrenergic system in this effect. Lamotrigine (20-30 mg/kg, i.p.) decreased the immobility time in the forced swimming test and the number of crossings in the open-field test. In addition, the pretreatment of mice with the inhibitor of the enzyme tyrosine hydroxylase, alpha-methyl-p-tyrosine (100 or 250 mg/kg), prevented the antidepressant-like effect of lamotrigine (30 mg/kg, i.p.) in the forced swimming test. Besides that, the pretreatment of mice with prazosin (1 mg/kg, i.p., an alpha1-adrenoceptor antagonist) or yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist) also prevented the anti-immobility effect of lamotrigine (30 mg/kg, i.p.). Moreover, the administration of subeffective doses of phenylephrine (5 mg/kg, i.p., an alpha1-adrenoceptor agonist) or clonidine (0.06 mg/kg, i.p., an alpha2-adrenoceptor agonist) was able to potentiate the action of a subeffective dose of lamotrigine (10 mg/kg, i.p.) in the forced swimming test. Thus, the present study suggests that the antidepressant-like effect of lamotrigine in the forced swimming test is related to the noradrenergic system, likely due to an activation of alpha1- and alpha2-postsynaptic adrenoceptors.

Analysis of brain adrenergic receptors in dopamine-beta-hydroxylase knockout mice

The biosynthesis of norepinephrine occurs through a multi-enzymatic pathway that includes the enzyme dopamine-beta-hydroxylase (DBH). Mice with a homozygous deletion of DBH (Dbh-/-) have a selective and complete absence of norepinephrine. The purpose of this study was to assess the expression of alpha-1, alpha-2 and beta adrenergic receptors (alpha1-AR, alpha2-AR and beta-AR) in the postnatal absence of norepinephrine by comparing noradrenergic receptors in Dbh-/- mice with those in Dbh heterozygotes (Dbh+/-), which have normal levels of norepinephrine throughout life. The densities of alpha1-AR, alpha2-AR and beta-AR were assayed with [3H]prazosin, [3H]RX21002 and [125I]-iodo-pindolol autoradiography, respectively. The alpha2-AR agonist high affinity state was examined with [125I]-para-iodoclonidine autoradiography and alpha2-AR functionality by alpha2-AR agonist-stimulated [35S]GTPgammaS autoradiography. The density of alpha1-AR in Dbh-/- mice was similar to Dbh+/- mice in most brain regions, with an up-regulation in the hippocampus. Modest decreases in alpha2-AR were found in septum, hippocampus and amygdala, but these were not reflected in alpha2-AR functionality. The density of beta-AR was up-regulated to varying degrees in many brain regions of Dbh-/- mice compared to the heterozygotes. These findings indicate that regulation of noradrenergic receptors by endogenous norepinephrine depends on receptor type and neuroanatomical region.

Monoaminergic neuronal activity up-regulates BDNF synthesis in cultured neonatal rat astrocytes

Astrocytes as an active part of the tripartite synapse can respond to the synaptically released neurotransmitters. Because brain-derived neurotrophic factor (BDNF) is produced by astrocytes, in addition to neurons, we focused our present study on the regulatory effects of monoamines noradrenaline (NA), serotonin (5-HT), and dopamine (DA) on the synthesis of BDNF protein in rat neonatal astrocytes from specific brain regions (cortex, cerebellum). All tested neurotransmitters are able to potently and transiently increase BDNF cellular contents; their maximal effects are dose and time dependent and differ between the two brain regions. In cultured cortical astrocytes, NA (1 microM; 6 h) elevates BDNF levels by a 4-fold, 5-HT (1 microM; 4 h) by a 2.3-fold, and DA (150 microM; 4 h) by a 2.2-fold. In cerebellar astrocytes, NA (1 microM; 4 h) increases BDNF content by a 4.7-fold, 5-HT (1 microM; 4 h) by a 1.7-fold, and DA (150 microM; 4 h) by a 1.4-fold. The initial increase in the BDNF levels return to basal levels when incubation with monoamines is extended beyond 12 h (for 5-HT) or 24 h (for NA and DA). Our results confirm the involvement of monoaminergic systems in the regulation of BDNF production in astrocytes and suggest the existence of a positive reciprocal interaction between monoaminergic neuronal activity and astrocytic neurotrophic support in neuron-astrocyte crosstalk, which has a dynamic role in mediating neuronal plasticity and trophic functions in the brain.

Dopamine modulates von Willebrand factor secretion in endothelial cells via D2-D4 receptors

OBJECTIVE

von Willebrand factor (VWF) is acutely released from endothelial cells in response to numerous calcium-raising agents (e.g. thrombin, histamine) and cAMP-raising agents (e.g. epinephrine, adenosine, vasopressin). In contrast, very few inhibitors of endothelial VWF secretion have been described. The neurotransmitter dopamine is a modulator of exocytosis in several endocrine cells, and is possibly involved in the regulation of several endothelial cell functions. We therefore investigated the effect of dopamine on endothelial VWF secretion.

RESULTS

Dopamine, D2/D3- and D4-specific agonists inhibited histamine- but not thrombin-induced VWF secretion. Expression of dopamine D2, D3 and D4 receptors was demonstrated by reverse transcription polymerase chain reaction (RT-PCR) in both human aortic (HAEC) and umbilical vein (HUVEC) endothelial cells. D2-D4 agonists did not inhibit histamine-induced rise in [Ca(2+)](i): they inhibited histamine-induced secretion even in the absence of extracellular calcium. Thus, the dopamine effects are not mediated by [Ca(2+)](i)-dependent signalling. D2/D3- and D4-specific agonists inhibited neither the rise in cAMP nor VWF secretion in response to epinephrine and adenosine, arguing against an effect on cAMP-mediated signalling. D1 and D5 receptors were not detected in HAEC or HUVEC by RT-PCR, and the D1/D5-specific agonist SKF 38 393 failed to modulate VWF secretion, arguing against a role for these receptors in endothelial exocytosis.

CONCLUSIONS

Dopamine inhibits histamine-induced endothelial exocytosis by activating D2-D4 receptor, via a mechanism distinct from [Ca(2+)](i)-or cAMP-mediated signaling. In contrast, D1 and D5 receptors are not functionally expressed in cultured endothelial cells. Dopamine agonists may be useful as inhibitors of endothelial activation in inflammation and cardiovascular disease.

Alpha(1)-adrenergic and alpha(2)-adrenergic balance in the dorsal pons and gross behavioral activity of mice in a novel environment

RATIONALE

Central alpha(1)- and alpha(2)-adrenoceptors in a number of different brain regions are known to have opposing actions on gross behavioral activity, with the former stimulating and the latter inhibiting activity. Therefore, blockade of alpha(1)-receptors may induce inactivity by leading to unopposed alpha(2) activity.

OBJECTIVE

The aim of this study was to test if central blockade of alpha(2)-receptor function restores behavioral activity in alpha(1)-receptor-blocked mice.

METHODS

Dose-response studies were undertaken on the effects of alpha(1)- and alpha(2)-agonists and antagonists microinjected into the dorsal pons on gross behavioral activity in a novel cage test.

RESULTS

The behavioral inactivity resulting from blockade of alpha(1)-receptors in the pons with the antagonist, terazosin, was reversed by either a low dose of an alpha(2)-antagonist, atipamezole, or a low dose of an alpha(2)-agonist, dexmedetomidine, but was exacerbated by a high dose of the alpha(2)-agonist.

CONCLUSION

The results support the hypothesis that blockade of alpha(1)-receptors in the dorsal pons of mice produces inactivity by causing unopposed activity of alpha(2)-receptors. This condition may be relevant to inactive states seen after stress or during depressive illness.

Dopaminergic-adrenergic interactions in the wake promoting mechanism of modafinil

Adrenergic signaling regulates the timing of sleep states and sleep state-dependent changes in muscle tone. Recent studies indicate a possible role for noradrenergic transmission in the wake-promoting action of modafinil, a widely used agent for the treatment of excessive sleepiness. We now report that noradrenergic projections from the locus coeruleus to the forebrain are not necessary for the wake-promoting action of modafinil. The efficacy of modafinil was maintained after treatment of C57BL/6 mice with N-(2-chloroethyl)-N-ethyl 2-bromobenzylamine (DSP-4), which eliminates all noradrenaline transporter-bearing forebrain noradrenergic projections. However, the necessity for adrenergic receptors in the wake-promoting action of modafinil was demonstrated by the observation that the adrenergic antagonist terazosin suppressed the response to modafinil in DSP-4 treated mice. The wake-promoting efficacy of modafinil was also blunted by the dopamine autoreceptor agonist quinpirole. These findings implicate non-noradrenergic, dopamine-dependent adrenergic signaling in the wake-promoting mechanism of modafinil. The anatomical specificity of these dopaminergic-adrenergic interactions, which are present in forebrain areas that regulate sleep timing but not in brain stem areas that regulate sleep state-dependent changes in muscle tone, may explain why modafinil effectively treats excessive daytime sleepiness in narcolepsy but fails to prevent the loss of muscle tone that occurs in narcoleptic patients during cataplexy.

Postnatal development of the cerebellum and the CNS adrenergic system is independent of norepinephrine and epinephrine

A fundamental question in the formation of the nervous system is the extent to which a neurotransmitter contributes to the development of the neurons that synthesize and release it. A complementary question is whether neurotransmitter signaling contributes to the development of postsynaptic targets. Prior studies have suggested that adrenergic signaling may promote adrenergic neuronal proliferation or survival and may be critical for the postnatal development of the cerebellum. To test these possibilities genetically, we studied mice that are unable to synthesize norepinephrine and epinephrine (NE/E), the endogenous adrenergic receptor ligands, due to a disruption the gene for dopamine beta-hydroxylase. These mice develop postnatally in the absence of NE/E. Here we report that the adrenergic neurons of these mutant mice are present in normal numbers and locations and exhibit typical innervation patterns throughout the central nervous system (CNS), as assessed by immunostaining for tyrosine hydroxylase and the NE transporter. Furthermore, cerebellar cortical development (size, foliation, layering, cell number, and position), which proceeds to a large degree postnatally, is unaltered in the mutants. These results indicate that the fate and innervation pattern of the adrenergic neurons, as well as the development of the cerebellum, do not depend on postnatal signaling by NE/E. The results also suggest that when restoration of adrenergic signaling is performed in this mutant mouse model (by administering a synthetic precursor of NE), reversal of phenotypes is due to the synthesis and release of NE/E from adrenergic terminals that are distributed normally within the CNS.