Neural targets of the enteric dopaminergic system in regulating motility of rat proximal colon.

In isolated segments of the rat proximal colon, the dopamine reuptake inhibitor GBR 12909 (GBR) causes a dilatation, while the D1-like receptor antagonist SCH 23390 (SCH) induces a tonic constriction, suggesting that neurally released dopamine tonically stimulates enteric inhibitory efferent neurons. Here, the targets of the enteric dopaminergic neurons were investigated. Cannulated segments of rat proximal colon were bathed in physiological salt solution and luminally perfused with 0.9% saline, while all drugs were applied to the bath. Spatio-temporal maps of colonic motility were constructed from video recordings of peristaltic contractions, and the maximum diameter was measured as an index of colonic contractility. GBR (1 µM)-induced dilatations of colonic segments were prevented by SCH (5 µM), L-nitro arginine (L-NA; 100 µM), a nitric oxide synthase inhibitor, or tetrodotoxin (0.6 µM). In contrast, constrictions induced by a higher concentration of SCH (20 µM) were unaffected by either L-NA or tetrodotoxin. The vasoactive intestinal peptide (VIP) receptor antagonist VIP10-28 (3 µM) or P2Y1 receptor antagonist MRS 2500 (1 µM) had no effect on either the GBR-induced dilatation or the SCH-induced constriction. In colonic segments that had been pretreated with 6-hydroxydopamine (100 µM, 3 h) to deplete enteric dopamine, GBR failed to increase the colonic diameter, while SCH was still capable of constricting colonic segments. Enteric dopaminergic neurons appear to project to nitrergic neurons to dilate the proximal colon by activating neuronal D1-like receptors. In addition, constitutively activated D1-like receptors expressed in cells yet to be determined may provide a tonic inhibition on colonic constrictions.

Parasitoid wasp venom manipulates host innate behavior via subtype-specific dopamine receptor activation.

The subjugation strategy employed by the jewel wasp is unique in that it manipulates the behavior of its host, the American cockroach, rather than inducing outright paralysis. Upon envenomation directly into the central complex (CX), a command center in the brain for motor behavior, the stung cockroach initially engages in intense grooming behavior, then falls into a lethargic sleep-like state referred to as hypokinesia. Behavioral changes evoked by the sting are due at least in part to the presence of the neurotransmitter dopamine in the venom. In insects, dopamine receptors are classified as two families, the D1-like and the D2-like receptors. However, specific roles played by dopamine receptor subtypes in venom-induced behavioral manipulation by the jewel wasp remain largely unknown. In the present study, we used a pharmacological approach to investigate roles of D1-like and D2-like receptors in behaviors exhibited by stung cockroaches, focusing on grooming. Specifically, we assessed behavioral outcomes of focal CX injections of dopamine receptor agonists and antagonists. Both specific and non-specific compounds were used. Our results strongly implicate D1-like dopamine receptors in venom-induced grooming. Regarding induction of hypokinesia, our findings demonstrate that dopamine signaling is necessary for induction of long-lasting hypokinesia caused by brain envenomation.

Striatal spreading depolarization: Possible implication in levodopa-induced dyskinetic-like behavior.

OBJECTIVE: Spreading depolarization (SD) is a transient self-propagating wave of neuronal and glial depolarization coupled with large membrane ionic changes and a subsequent depression of neuronal activity. Spreading depolarization in the cortex is implicated in migraine, stroke, and epilepsy. Conversely, spreading depolarization in the striatum, a brain structure deeply involved in motor control and in Parkinson's disease (PD) pathophysiology, has been poorly investigated. METHODS: We characterized the participation of glutamatergic and dopaminergic transmission in the induction of striatal spreading depolarization by using a novel approach combining optical imaging, measurements of endogenous DA levels, and pharmacological and molecular analyses. RESULTS: We found that striatal spreading depolarization requires the concomitant activation of D1-like DA and N-methyl-d-aspartate receptors, and it is reduced in experimental PD. Chronic l-dopa treatment, inducing dyskinesia in the parkinsonian condition, increases the occurrence and speed of propagation of striatal spreading depolarization, which has a direct impact on one of the signaling pathways downstream from the activation of D1 receptors. CONCLUSION: Striatal spreading depolarization might contribute to abnormal basal ganglia activity in the dyskinetic condition and represents a possible therapeutic target. © 2019 International Parkinson and Movement Disorder Society.

Dopaminergic urethral closure mechanisms in a rat model of Parkinson's disease.

AIMS: Urinary incontinence is prevalent among patients with Parkinson's disease (PD). In the present study, we investigated urethral functions in a rat model of PD induced by 6-hydroxydopamine injection at their substantia nigra pars compacta as well as the roles of selective agonists/antagonist of dopamine D1- and D2-like receptors in active urethral closure mechanisms. METHODS: We measured changes in the urethral pressure amplitude during electrical stimulation, urethral baseline pressure, and leak point pressure after intravenous administration of selective agonists or antagonists of the dopamine D1- and D2-like receptors in a rat model of PD. RESULTS: The mean leak point pressure and the mean active urethral response values were significantly smaller for the untreated PD rat group compared with the control group. In PD model, the active urethral response increased significantly after treatment with the dopamine D1-like receptor agonist, whereas that induced by the dopamine D2-like receptor agonist decreased significantly. The response to the D2-like receptor agonist was suppressed in the PD rat by the dopamine D2-like receptor antagonist. CONCLUSION: Our results suggest that the active urethral closure mechanisms are significantly impaired when dopamine is depleted. In the PD rat, dopamine D1-like receptor activity on the central nervous system appear to partially compensate for urethral functions negatively impacted by the lack of dopamine, whereas dopamine D2-like receptor activity might exacerbate urinary leakage owing to the negative effect of this activated receptor on urethral pressure under increased intra-abdominal pressure.

Renal autocrine neuropeptide FF (NPFF) signaling regulates blood pressure.

The kidney and brain play critical roles in the regulation of blood pressure. Neuropeptide FF (NPFF), originally isolated from the bovine brain, has been suggested to contribute to the pathogenesis of hypertension. However, the roles of NPFF and its receptors, NPFF-R1 and NPFF-R2, in the regulation of blood pressure, via the kidney, are not known. In this study, we found that the transcripts and proteins of NPFF and its receptors, NPFF-R1 and NPFF-R2, were expressed in mouse and human renal proximal tubules (RPTs). In mouse RPT cells (RPTCs), NPFF, but not RF-amide-related peptide-2 (RFRP-2), decreased the forskolin-stimulated cAMP production in a concentration- and time-dependent manner. Furthermore, dopamine D1-like receptors colocalized and co-immunoprecipitated with NPFF-R1 and NPFF-R2 in human RPTCs. The increase in cAMP production in human RPTCs caused by fenoldopam, a D1-like receptor agonist, was attenuated by NPFF, indicating an antagonistic interaction between NPFF and D1-like receptors. The renal subcapsular infusion of NPFF in C57BL/6 mice decreased renal sodium excretion and increased blood pressure. The NPFF-mediated increase in blood pressure was prevented by RF-9, an antagonist of NPFF receptors. Taken together, our findings suggest that autocrine NPFF and its receptors in the kidney regulate blood pressure, but the mechanisms remain to be determined.

Salience to remember: VTA-IC dopaminergic pathway activity is necessary for object recognition memory formation.

Previous studies have shown that dopaminergic activity modulates the salience of novel stimuli enabling the formation of recognition memories. In this work, we hypothesize that dopamine released into the insular cortex (IC) from the ventral tegmental area (VTA) inputs enables the acquisition to consolidate object recognition memory. It has been reported that short training produces weak recognition memories; on the contrary, longer training produces lasting and robust recognition memories. Using a Cre-recombinase under the tyrosine hydroxylase (TH+) promoter mouse model, we photostimulated the VTA-IC dopaminergic pathway during short training or photoinhibited the same pathway during long training while mice explored objects. Our results showed that the photostimulation of the VTA-IC pathway during a short training enables the acquisition of recognition memory. Conversely, photoinhibition of the same pathway during a long training prevents the acquisition of recognition memory. Interestingly, the exploration time of the objects under photoinhibition or photostimulation of the dopaminergic VTA-IC pathway was not altered. Significantly, this enhancement of acquisition of the object recognition memory through the photostimulation of the VTA dopaminergic neurons could be impaired by the blockage of the D1-like receptors into the IC, either before or after the photostimulation. Altogether, our results suggest that dopamine released by the VTA is required during the acquisition to consolidate the object recognition memory through D1-like receptors into the IC without affecting the activity or the motivation to explore objects.

Endocytosis of dopamine receptor: Signaling in brain.

This chapter describes the physiological significance of dopamine receptor endocytosis and the consequence of the receptor signaling. Endocytosis of dopamine receptors is regulated by many components such as clathrin, ß-arrestin, caveolin, and Rab family proteins. The dopamine receptors escape from lysosomal digestion, and their recycling occurs rapidly, reinforcing the dopaminergic signal transduction. In addition, the pathological impact of the receptors interacting with specific proteins has been the focus of much attention. Based on this background, this chapter provides an in-depth understanding of the mechanisms of molecules interacting with dopamine receptors and discusses the potential pharmacotherapeutic targets for α-synucleinopathies and neuropsychiatric disorders.

Photostimulation of Ventral Tegmental Area-Insular Cortex Dopaminergic Inputs Enhances the Salience to Consolidate Aversive Taste Recognition Memory via D1-Like Receptors.

Taste memory involves storing information through plasticity changes in the neural network of taste, including the insular cortex (IC) and ventral tegmental area (VTA), a critical provider of dopamine. Although a VTA-IC dopaminergic pathway has been demonstrated, its role to consolidate taste recognition memory remains poorly understood. We found that photostimulation of dopaminergic neurons in the VTA or VTA-IC dopaminergic terminals of TH-Cre mice improves the salience to consolidate a subthreshold novel taste stimulus regardless of its hedonic value, without altering their taste palatability. Importantly, the inhibition of the D1-like receptor into the IC impairs the salience to facilitate consolidation of an aversive taste recognition memory. Finally, our results showed that VTA photostimulation improves the salience to consolidate a conditioned taste aversion memory through the D1-like receptor into the IC. It is concluded that the dopamine activity from the VTA into IC is required to increase the salience enabling the consolidation of a taste recognition memory. Notably, the D1-like receptor activity into the IC is required to consolidate both innate and learned aversive taste memories but not appetitive taste memory.

Neuromodulatory Selection of Motor Neuron Recruitment Patterns in a Visuomotor Behavior Increases Speed.

Animals generate locomotion at different speeds to suit their behavioral needs. Spinal circuits generate locomotion at these varying speeds by sequential activation of different spinal interneurons and motor neurons. Larval zebrafish can generate slow swims for prey capture and exploration by activation of secondary motor neurons and much faster and vigorous swims during escape and struggle via additional activation of primary motor neurons. Neuromodulators are known to alter the motor output of spinal circuits, but their precise role in speed regulation is not well understood. Here, in the context of optomotor response (OMR), an innate evoked locomotor behavior, we show that dopamine (DA) provides an additional layer to regulation of swim speed in larval zebrafish. Activation of D1-like receptors increases swim speed during OMR in free-swimming larvae. By analyzing tail bend kinematics in head-restrained larvae, we show that the increase in speed is actuated by larger tail bends. Whole-cell patch-clamp recordings from motor neurons reveal that, during OMR, typically only secondary motor neurons are active, whereas primary motor neurons are quiescent. Activation of D1-like receptors increases intrinsic excitability and excitatory synaptic drive in primary and secondary motor neurons. These actions result in greater recruitment of motor neurons during OMR. Our findings provide an example of neuromodulatory reconfiguration of spinal motor neuron speed modules where members are selectively recruited and motor drive is increased to effect changes in locomotor speed. VIDEO ABSTRACT.

Dopamine in the hippocampal dentate gyrus modulates spatial learning via D1-like receptors.

Increasing evidence supports that dopamine (DA) plays an important role in the hippocampal function via activation of D1-like receptors (D1Rs). As the entry structure of the hippocampal formation, the hippocampal dentate gyrus (DG) is critically involved in spatial learning and memory. Despite a number of studies investigated how DA influences CA1 plasticity and learning, there are fewer studies examining the influence of DA signaling to the DG. To investigate the roles of DA and D1Rs of the DG in modulation of spatial learning and memory, the spatial learning and memory abilities of rats were measured by Morris water maze (MWM), and then the concentration of DA in the DG region was determined by in vivo brain microdialysis and HPLC. Next, the effects of local microinjection of SCH23390 (an antagonist of D1Rs) on extracellular levels of excitatory amino acids (EAAs), including glutamate (Glu) and aspartate (Asp), were measured in the DG region during MWM test in freely-moving conscious rats. During the place navigation trial of MWM test, the escape latency was decreased with the increase in training days, and DA concentration in the DG was significantly increased. In SCH23390 group rats, the escape latency was increased in place navigation trial and the percentage of time spent in target quadrant and the number of platform crossings were decreased in spatial probe trial during MWM test, compared with vehicle group. Furthermore, in vehicle group rats, the extracellular levels of Glu and Asp in the DG were significantly increased during place navigation trial of MWM test, and these responses were partly inhibited by microinjection of SCH23390. Our results suggest that DA activation of D1Rs in the hippocampal DG promotes spatial learning and memory, in part by modulating the responses of EAAs during spatial learning.

The ameliorative effects and underlying mechanisms of dopamine D1-like receptor agonist SKF38393 on Aß1-42-induced cognitive impairment.

Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by extracellular amyloid plaques and intracellular neurofibrillary tangles. It is the most common form of human cognitive decline and dementia. In this study, we aim to systematically investigate the ameliorative effects of dopamine D1-like receptor agonist SKF38393 on cognitive dysfunction and explore its underlying mechanisms. The Aß1-42 was injected intracerebroventricularly to establish cognitive disorder model. Then, a series of behavior tests were used. In order to further study the mechanisms, some relevant protein was assessed by ELISA method and Western blot. The results in behavior tests revealed that SKF38393 significantly ameliorated all the test indexes compared with the model mice. Then SKF38393 increased phosphorylation of cAMP response element binding protein (CREB) and expression of Bcl-2 in Western blot analyses. Furthermore, in ELISA assay, SKF38393 significantly increased the brain-derived neurotrophic factor (BDNF) levels and reduced the ß-site APP cleaving enzyme1 (BACE1) and Aß1-42 levels in hippocampus and cortex of mice. However, compared with SKF38393-H, all these results were significantly reversed by the dopamine D1 receptor antagonist SCH23390. These results indicated that SKF38393 could ameliorate Aß1-42-induced cognitive dysfunction in mice, which may be related to D1 receptor activation. It leads to the phosphorylation of CREB, which promote the expression of BDNF, Bcl-2 and decrease the expression of Aß1-42 of mice. Our findings suggest that dopamine D1-like receptor may be a potential target for the treatment of AD and its agonists may become a novel drug in the future.

Dopamine-sensitive signaling mediators modulate psychostimulant-induced ultrasonic vocalization behavior in rats.

The mesolimbic dopamine system plays a major role in psychostimulant-induced ultrasonic vocalization (USV) behavior in rodents. Within this system, psychostimulants elevate synaptic concentrations of dopamine thereby leading to exaggerated activation of postsynaptic dopamine receptors within the D1-like and D2-like subfamilies. Dopamine receptor stimulation activate several transmembrane signaling systems and cognate intracellular mediators; downstream activation of transcription factors then conveys the information from receptor activation to appropriate modulation of cellular and physiologic functions. We previously showed that cocaine-induced USV behavior was associated with enhanced expression of the neurotrophin BDNF. Like cocaine, amphetamine also increases synaptic dopamine levels, albeit primarily through facilitating dopamine release. Therefore, in the present study we investigated whether amphetamine and cocaine similarly activate dopamine-linked signaling cascades to regulate intracellular mediators leading to induction of USV behavior. The results show that amphetamine increased the emission of 50 kHz USVs and this effect was blocked by SCH23390, a D1 receptor antagonist. Similar to cocaine, amphetamine increased BDNF protein expression in discrete brain regions, while pretreatment with K252a, a trkB neurotrophin receptor inhibitor, significantly reduced amphetamine-induced USV behavior. Inhibition of cyclic-AMP/PKA signaling with H89 or inhibition of PLC signaling with U73122 significantly blocked both the acute and subchronic amphetamine-induced USV behavior. In contrast, pharmacologic inhibition of either pathway enhanced cocaine-induced USV behavior. Although cocaine and amphetamine similarly modulate neurotrophin expression and USV, the molecular mechanisms by which these psychostimulants differentially activate dopamine receptor subtypes or other monoaminergic systems may be responsible for the distinct aspects of behavioral responses.

The Blockade of D1/D2-Like Dopamine Receptors within the Dentate Gyrus of Hippocampus Decreased the Reinstatement of Morphine-Extinguished Conditioned Place Preference in Rats.

INTRODUCTION: The hippocampus (HIP), the primary brain structure related to learning and memory, receives sparse but comprehensive dopamine innervations and contains dopamine D1/D2-like receptors. It is demonstrated that dopamine receptors in dentate gyrus (DG) region of HIP have a remarkable function in spatial reward processing. Much less is known about the involvement of HIP and its D1/D2 dopamine receptors in drug-seeking behaviors, more particularly, in the morphine extinguished conditioned place preference (CPP). METHODS: To find out the role of D1/D2-like receptors within the DG in morphine-seeking behaviors, forty adult male albino Wistar rats weighing 220-280g were unilaterally implanted by a cannula into the DG. The CPP paradigm was done; conditioning score and locomotors activity were recorded by Ethovision software. All drugs/vehicles were microinjected one day after extinction (just before the CPP test) into the DG as reinstatement day. RESULTS: The results showed that intra-DG administration of different dose of SCH23390 (0.25, 1 and 4µg/0.5µl saline), as a selective D1-like receptor antagonist and sulpiride (0.25, 1 and 4µg/0.5µl DMSO), as a selective D2-like receptor antagonist dose-dependently attenuated the morphine-extinguished CPP reinstated by priming injection of morphine (1 mg/kg;sc). DISCUSSION: It can be concluded that D1/D2-like receptors within this region have an important role in morphine-seeking behaviors in extinguished rats.

Role of dopamine D1-like receptor within the nucleus accumbens in acute food deprivation- and drug priming-induced reinstatement of morphine seeking in rats.

Dopamine is a predominant neurotransmitter in the nervous system, which plays an important role in both drug priming- and cue-induced reinstatement of cocaine and heroin seeking. Therefore, in the present study, the conditioned place preference (CPP) paradigm was used to evaluate the effects of intra-accumbal administration of SCH23390 as a dopamine D1-like receptor antagonist on food deprivation (FD) and drug priming-induced reinstatement. Sixty-eight adult male albino Wistar rats weighing 200-280 g were bilaterally implanted by cannulae into the nucleus accumbens (NAc). For induction of the CPP, subcutaneous (sc) administration of morphine (5mg/kg) was used daily during a three-day conditioning phase. The conditioning score and locomotor activity were recorded by using the Ethovision software. Under extinction conditions, rats were given an 'off' period and were tested for FD-induced reinstatement following the 24-h or 48-h FD condition, and for drug priming-induced reinstatement under the sated condition following an injection of 0.5 and 1mg/kg (sc) morphine. In the next experiments, animals received different doses of intra-accumbal SCH23390 (0.25, 1 and 4 µg/0.5 µl saline) bilaterally and were subsequently tested for FD- and morphine priming-induced reinstatement. Our findings indicated that only a dose of 1mg/kg and not 0.5mg/kg of morphine induced the reinstatement of morphine. 24-h FD similar to 48-h FD induced the reinstatement of seeking behaviors facilitated by an ineffective dose of morphine (0.5mg/kg). Furthermore, the D1-like receptor antagonist attenuated FD- and drug priming-induced reinstatement dose-dependently. It is concluded that FD- and drug priming-induced reinstatement may be mediated, at least in some way, by activation of dopamine D1-like receptors in the NAc.

D1-like antagonist blocks conditioned place preference induced by ejaculation in male rats.

Mating behavior, particularly ejaculation, induces a state of sexual reward, which is evaluated by the conditioned place preference test. Several studies have shown that opioid receptors are involved in inducing the state of sexual reward, mainly because this state is blocked with naloxone, a mu opioid receptor antagonist. Dopamine has been implicated in sexual motivation, coital behavior and sexual reward, however, some experiments show that D2-like or non-specific dopaminergic antagonists are not capable of blocking the conditioned place preference induced by ejaculation; therefore, the role of dopamine on sexual reward has not been demonstrated, or has been frequently discarded. We show that a dose of SCH 23390 (a specific dopamine D1-like receptor antagonist), which does not modify locomotion, blocks the conditioned place preference induced by ejaculation and the conditioned place preference induced by SKF 38393 (D1-like agonist). Our results indicate that dopamine, across the D1-like receptors, is involved in the sexual reward induced by ejaculation.

The effect of dopamine on adult hippocampal neurogenesis.

Cumulative studies indicated that adult hippocampal neurogenesis might be involved in the action mechanism of antidepressant drugs and/or the pathophysiology of depression. Dopamine (DA) is involved in the regulation of motivation, volition, interest/pleasure, and attention/concentration, all of which are likely to be impaired in depressed patients. Several previous reports suggest that depression may often be accompanied by a relative hypo-dopaminergic state, and some DA receptor agonists are beneficial effects in the treatment for refractory and bipolar depression. In the present study, to clarify the direct effect of DA on neural progenitor cells, we examined the effect of DA on the proliferation of adult rat dentate gyrus-derived neural precursor cells (ADPs). In addition, we examined the effect of DA receptor agonists on adult rat hippocampal neurogenesis in vivo. Results showed that DA promoted the increase of ADPs via D1-like receptor and D1-like receptor agonist promoted the survival of newborn cells in the adult hippocampus. On the contrary, D2-like receptor agonist did not affect both proliferation and survival. These results suggested that DA might play, at least in part, a role in adult hippocampal neurogenesis via D1-like receptor and the activation of D1-like receptor has a therapeutic potential for depression.

Blockade of dopamine D1-like receptor signalling protects mice against OVA-induced acute asthma by inhibiting B-cell activating transcription factor signalling and Th17 function.

Previous studies have consistently demonstrated that dopamine D1-like receptor (D1-like-R) signalling is implicated in the pathogenesis of experimental autoimmune encephalomyelitis and type I diabetes. Given that allergic asthma shares certain disease aetiology similarities with autoimmune diseases, we conducted studies in OVA-induced mice aiming to address the impact of D1-like-R signalling on the pathogenesis of allergic asthma. It was noted that blockade of D1-like-R signalling provided protection for mice against OVA-induced acute asthma. Particularly, treatment of OVA-induced mice with SCH23390, a D1-like-R antagonist, significantly attenuated inflammatory infiltration in the airways along with repressed goblet cell hyperplasia and mucus production, as well as airway resistance. By contrast, administration of SKF83959, a D1-like-R agonist, displayed the opposite effect. Blockade of D1-like-R signalling impaired Th17 function, as manifested by a significant reduction of Th17 cells in the spleen and bronchoalveolar lavage fluid. Mechanistic studies revealed that D1-like-R signalling enhances B-cell activating transcription factor activity, which then transcribes the expression of RORγt, a Th17 transcription factor; accordingly, D1-like-R signalling regulates Th17 differentiation to promote the development of allergic asthma. Taken together, the data obtained in the present suggest that blockade of D1-like-R signalling could be an effective therapeutic strategy for the prevention and treatment of allergic asthma in clinical practice.