Dopamine D1 Receptor Activation Research Articles

Amelioration of cognitive impairments induced by GABA hypofunction in the male rat prefrontal cortex by direct and indirect dopamine D1 agonists SKF-81297 and d-Govadine

Deficits in prefrontal cortex (PFC) GABAergic neurotransmission are linked to cognitive impairments seen in schizophrenia and other disorders, and pharmacological reduction of PFC GABAA transmission disrupts processes including working and spatial memory. This provides an opportunity to examine whether compounds capable of neutralizing GABAergic dysfunction may ameliorate these cognitive deficits. PFC dopamine (DA) D1 receptor activation enhances GABA transmission, raising the possibly that direct or indirect agonists of DA D1 receptors would be effective in reversing working memory and other forms of cognitive deficits. To test this, male rats were pre-treated with two drugs that augment PFC D1 signalling before PFC infusion of the GABAA antagonist, bicuculline (50 ng) and assessment of spatial working and reference memory function. A moderate dose of the full D1 agonist SKF-81297 (0.1 mg/kg) completely reversed PFC GABA hypofunction-induced working memory deficits assessed in an delayed-response task, whereas lower and higher doses (0.05 and 0.3 mg/kg respectively) were associated with mild improvements or deleterious effects. Treatment with the tetrahydroprotoberberine d-govadine (0.5 or 1.0 mg/kg), a synthetic compound known to enhance DA release selectively in the PFC, also significantly improved delayed-response working memory function induced by PFC GABAA antagonism. Furthermore, administration of the optimal dose of both drugs led to a partial rescue of PFC GABA hypofunction-induced reference and short-term spatial memory impairments assessed on a radial maze task. These findings suggest that modulation of PFC DA signalling via actions on the DA D1 receptor represents a promising therapeutic strategy for working memory and other cognitive impairments observed in psychiatric disorders, including those with causes that extend beyond DA dysfunction.

The Lacto-Tetrapeptide Gly-Thr-Trp-Tyr, β-Lactolin, Improves Spatial Memory Functions via Dopamine Release and D1 Receptor Activation in the Hippocampus.

Scope: Peptides containing tryptophan–tyrosine sequences, including the lacto-tetrapeptide glycine–threonine–tryptophan–tyrosine (GTWY) and β-lactolin, from β-lactoglobulin in whey enzymatic digestion, enhance hippocampus-dependent memory functions, which are blocked by the systemic administration of dopamine D1-like antagonist. In this study, we investigated the role of the hippocampal dopaminergic system in the memory-enhancing effect of β-lactolin. Methods and Results: The results of in vivo microdialysis revealed that oral administration of β-lactolin increased the extracellular concentration of dopamine in the hippocampus and enhanced both spatial working memory, as measured in the Y-maze test, and spatial reference memory, as measured in the novel object location test. These memory-enhancing effects of β-lactolin, but not the baseline memory functions, were impaired by the knockdown of the dopamine D1 receptor subtype in the hippocampus. β-Lactolin also enhanced object memory, as measured by the novel object recognition test. However, D1 knockdown in the hippocampus spared this memory function either with or without the administration of β-lactolin. Conclusions: The present results indicate that oral administration of β-lactolin increases dopamine release and D1 receptor signaling in the hippocampus, thereby enhancing spatial memory, but it may improve object memory via a separate mechanism.

Blockade of Intranigral and Systemic D3 Receptors Stimulates Motor Activity in the Rat Promoting a Reciprocal Interaction among Glutamate, Dopamine, and GABA.

In vivo activation of dopamine D3 receptors (D3Rs) depresses motor activity. D3Rs are widely expressed in subthalamic, striatal, and dendritic dopaminergic inputs into the substantia nigra pars reticulata (SNr). In vitro studies showed that nigral D3Rs modulate their neurotransmitter release; thus, it could be that these changes in neurotransmitter levels modify the discharge of nigro-thalamic neurons and, therefore, motor behavior. To determine how the in vitro responses correspond to the in vivo responses, we examined the effect of intra-nigral and systemic blockade of D3Rs in the interstitial content of glutamate, dopamine, and GABA within the SNr using microdialysis coupled to motor activity determinations in freely moving rats. Intranigral unilateral blockade of D3R with GR 103,691 increased glutamate, dopamine, and GABA. Increments correlated with increased ambulatory distance, non-ambulatory activity, and induced contralateral turning. Concomitant blockade of D3R with D1R by perfusion of SCH 23390 reduced the increase of glutamate; prevented the increment of GABA, but not of dopamine; and abolished behavioral effects. Glutamate stimulates dopamine release by NMDA receptors, while blockade with kynurenic acid prevented the increase in dopamine and, in turn, of GABA and glutamate. Finally, systemic administration of D3R selective antagonist U 99194A increased glutamate, dopamine, and GABA in SNr and stimulated motor activity. Blockade of intra-nigral D1R with SCH 23390 prior to systemic U 99194A diminished increases in neurotransmitter levels and locomotor activity. These data highlight the pivotal role of presynaptic nigral D3 and D1R in the control of motor activity and help to explain part of the effects of the in vivo administration of D3R agents.

Dopaminergic D1 receptor signalling is necessary, but not sufficient for cued fear memory destabilisation

RationalePharmacological targeting of memory reconsolidation is a promising therapeutic strategy for the treatment of fear memory-related disorders. However, the success of reconsolidation-based approaches depends upon the effective destabilisation of the fear memory by memory reactivation.ObjectivesHere, we aimed to determine the functional involvement of dopamine D1 receptors in cued fear memory destabilisation, using systemic drug administration.ResultsWe observed that direct D1 receptor agonism was not sufficient to stimulate tone fear memory destabilisation to facilitate reconsolidation disruption by the glucocorticoid receptor antagonist mifepristone. Instead, administration of the nootropic nefiracetam did facilitate mifepristone-induced amnesia, in a manner that was dependent upon dopamine D1 receptor activation. Finally, while the combined treatment with nefiracetam and mifepristone did not confer fear-reducing effects under conditions of extinction learning, there was some evidence that mifepristone reduces fear expression irrespective of memory reactivation parameters.ConclusionsThe use of combination pharmacological treatment to stimulate memory destabilisation and impair reconsolidation has potential therapeutic benefits, without risking a maladaptive increase of fear.

Allopregnanolone is required for prepulse inhibition deficits induced by D1 dopamine receptor activation

IntroductionThe extraction of salient information from the environment is modulated by the activation of dopamine receptors. Using rodent models, we previously reported that gating deficits caused by dopamine receptor activation - as measured by the prepulse inhibition (PPI) of startle - are effectively opposed by inhibitors of the steroidogenic enzyme 5α-reductase (5αR). The specific 5αR isoenzyme and steroids implicated in these effects, however, remain unknown. MethodsThe effects of the selective D1 dopamine receptor agonist SKF-82958 (SKF, 0.3 mg/kg, IP) and D2 receptor agonist quinpirole (QUIN, 0.5 mg/kg, IP) were tested in the startle reflex and PPI of knockout (KO) mice for either 5αR type 1 (5αR1) or type 2 (5αR2). Furthermore, we established whether these effects may be modified by the 5α-reduced steroids dihydroprogesterone (DHP), allopregnanolone (AP), dihydrotestosterone (DHT), 5α-androstane-3α,17β-diol (3α-diol), or androsterone. To test the mechanisms whereby 5αR products may alter the PPI-disrupting properties of D1 agonists, we studied the involvement of GABA-A and PXR, two receptors targeted by neuroactive steroids. Specifically, we tested the effects of SKF in combination with the GABA-A antagonist bicuculline, as well as in KO mice for the GABA-A δ subunit and PXR. Results5αR1, but not 5αR2, knockout (KO) mice were insensitive to the PPI-disrupting effects of SKF. This sensitivity was reinstated by AP (3 mg/kg, IP), but not other 5α-reduced steroids. The PPI deficits induced by SKF were not modified by bicuculline, δ-subunit KO mice and PXR KO mice. ConclusionsThese results collectively suggest that 5αR1 enables the negative effects of D1 dopamine receptor activation on information processing via production of AP. The contribution of AP to the PPI-disrupting mechanisms of D1 receptor agonists, however, do not appear to be mediated by either GABA-A or PXR receptors.

Dopamine D2 receptors regulate the action potential threshold by modulating T-type calcium channels in stellate cells of the medial entorhinal cortex.

Activation of axonal dopamine D2 receptors (D2Rs) increases action potential (AP) threshold, and thus decreases neuronal excitability in layer II stellate cells of medial entorhinal cortex. Endogenous dopamine release increases the AP threshold of stellate cells by activating D2Rs. Activation of D2Rs shifts the activation curve of T-type Ca2+ channels in a positive direction in a protein kinase A-dependent manner. Immunofluorescence staining reveals the presence of T-type Ca2+ channels and D2Rs in the axon initial segments (AISs). This research makes the pioneering discovery of D2R-induced AP threshold plasticity in AISs of stellate cells. The findings are likely to have significant implications for understanding the cellular processes by which dopamine influences neuronal intrinsic excitability. Stellate cells in the medial entorhinal cortex (MEC) are considered to constitute the largest population of grid cells, which provide spatial representation to support animal estimation of location. Although dopaminergic fibres from the ventral tegmental area and substantia nigra pars compacta innervate the majority of the cortex, including the MEC, little is known about how dopamine modulates the function of MEC stellate cells. Because dopamine D2 receptors (D2Rs) are involved in spatial cognition and MEC contains high levels of D2Rs, we investigated how D2R activation modulates the neuronal intrinsic excitability of stellate cells. Electrophysiological recordings, optogenetics and molecular biology experiments were performed to investigate the mechanism in mice. Activation of axonal D2Rs, not dendritic or somatic D2Rs, elevated the action potential (AP) threshold and decreased the intrinsic excitability of stellate cells, which was caused by shifting rightward the activation properties of T-type Ca2+ channels in a D2R-protein kinase A-dependent manner without affecting their steady-state inactivation curve. In support, immunofluorescence assays revealed colocalization of D2Rs and Cav 3.2 calcium channels within the axon initial segment. These findings are likely to have significant implications for understanding the cellular processes by which dopamine influences neuronal excitability and they may also be applicable to other hippocampal and cortical regions as dopaminergic fibres innervate wide brain regions. Taken together, these findings provide a novel cellular mechanism by which D2Rs modulate AP threshold of stellate cells through T-type Ca2+ channels in MEC, indicating that D2Rs of MEC play a vital role in modulating the information processing of stellate cells.

Histamine H3 receptor activation reduces the impairment in prepulse inhibition (PPI) of the acoustic startle response and Akt phosphorylation induced by MK-801 (dizocilpine), antagonist at N-Methyl-d-Aspartate (NMDA) receptors

We have investigated the effect of the local activation of histamine H3 receptors (H3Rs) in the rat prefrontal cortex (PFCx) on the impairment of pre-pulse inhibition (PPI) of the startle response induced by the systemic administration of MK-801, antagonist at glutamate N-Methyl-d-Aspartate (NMDA) receptors, and the possible functional interaction between H3Rs and MK-801 on PFCx dopaminergic transmission. Infusion of the H3R agonist RAMH (19.8 ng/1 μl) into the PFCx reduced or prevented the inhibition by MK-801 (0.15 mg/kg, ip) of PPI evoked by different auditory stimulus intensities (5, 10 and 15 dB), and the RAMH effect was blocked by the H3R antagonist/inverse agonist ciproxifan (30.6 ng/1 μl). MK-801 inhibited [3H]-dopamine uptake (−45.4 ± 2.1%) and release (−32.8 ± 2.6%) in PFCx synaptosomes or slices, respectively, and molecular modeling indicated that MK-801 binds to and blocks the rat and human dopamine transporters. However, H3R activation had no effect on the inhibitory action of MK-801 on dopamine uptake and release. In PFCx slices, MK-801 and the activation of H3Rs or dopamine D1 receptors (D1Rs) stimulated ERK-1/2 and Akt phosphorylation. The co-activation of D1Rs and H3Rs prevented ERK-1/2 and Akt phosphorylation, and H3R activation or D1R blockade prevented the effect of MK-801. In ex vivo experiments, the intracortical infusion of the D1R agonist SKF-81297 (37 ng/1 μl) or the H3R agonist RAMH increased Akt phosphorylation, prevented by D1R/H3R co-activation. These results indicate that MK-801 enhances dopaminergic transmission in the PFCx, and that H3R activation counteracts the post-synaptic actions of dopamine.

Acute Cocaine Enhances Dopamine D2R Recognition and Signaling and Counteracts D2R Internalization in Sigma1R-D2R Heteroreceptor Complexes

The current study was performed to establish the actions of nanomolar concentrations of cocaine, not blocking the dopamine transporter, on dopamine D2 receptor (D2R)-sigma 1 receptor (δ1R) heteroreceptor complexes and the D2R protomer recognition, signaling and internalization in cellular models. We report the existence of D2R-δ1R heteroreceptor complexes in subcortical limbic areas as well as the dorsal striatum, with different distribution patterns using the in situ proximity ligation assay. Also, through BRET, these heteromers were demonstrated in HEK293 cells. Furthermore, saturation binding assay demonstrated that in membrane preparations of HEK293 cells coexpressing D2R and δ1R, cocaine (1 nM) significantly increased the D2R Bmax values over cells singly expressing D2R. CREB reporter luc-gene assay indicated that coexpressed δ1R significantly reduced the potency of the D2R-like agonist quinpirole to inhibit via D2R activation the forskolin induced increase of the CREB signal. In contrast, the addition of 100 nM cocaine was found to markedly increase the quinpirole potency to inhibit the forskolin-induced increase of the CREB signal in the D2R-δ1R cells. These events were associated with a marked reduction of cocaine-induced internalization of D2R protomers in D2R-δ1R heteromer-containing cells vs D2R singly expressing cells as studied by means of confocal analysis of D2R-δ1R trafficking and internalization. Overall, the formation of D2R-δ1R heteromers enhanced the ability of cocaine to increase the D2R protomer function associated with a marked reduction of its internalization. The existence of D2R-δ1R heteromers opens up a new understanding of the acute actions of cocaine.

Molecular and behavioral mechanisms mediating paclitaxel‐induced changes in affect‐like behavior in mice

The antineoplastic paclitaxel is associated with negative affective outcomes, such as depression, anxiety, and decreased quality of life during treatment and convalescence. With the Baby Boomer population approaching peak cancer age, it is dire that the mechanisms behind paclitaxel‐induced changes in mood are uncovered. Cancer‐free male and female C57BL/6J mice were treated with one set of four injections of vehicle or paclitaxel (32mg/kg cumulative), or two sets of four injections of vehicle or paclitaxel (64mg/kg cumulative), and periodically assessed for depression‐like behaviors. Paclitaxel caused significant, time‐dependent deficits in sucrose preference and operant responding for palatable food. Because there is growing evidence to support the role of kappa opioid receptors (KORs) in stress‐mediated depression and reward dysfunction, we investigated KOR signaling as a putative mechanism of paclitaxel‐induced depression‐like behaviors. The selective KOR antagonist norbinaltorphimine (norBNI) reversed paclitaxel‐induced attenuation of sucrose preference. At the molecular level, paclitaxel time‐dependently induced an increase in the expression of Prodynorphin mRNA, the precursor for endogenous KOR agonists, in the nucleus accumbens (NAc). Using the [35S]GTPγS assay, we discovered that a history of paclitaxel time‐dependently attenuated activation of dopamine D2 receptors (D2R) and KORs in the NAc but not caudate putamen. These data suggest that paclitaxel‐induced changes in affect‐like behavior may be due to time‐ and region‐dependent dysregulation of KOR and D2R signaling. These observations help to establish the roles of KOR and D2R systems in paclitaxel‐induced disruption of behavioral reward, thus revealing potential neurochemical targets for therapeutic intervention in cancer survivors with treatment‐resistant depression.Support or Funding InformationSupported by: Ruth L. Kirschstein National Research Service Award Individual Predoctoral Fellowship to Promote Diversity in Health‐Related Research 1F31CA224930‐01A1; NIH grant 1R01‐CA206028‐01; NIHNCI Cancer Center Support Grant P30‐CA016059.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Relative Contribution of Dopamine, Norepinephrine, and Serotonin to the Discriminative Stimulus Effects of MDPV

The abuse of synthetic cathinones (“bath salts”) has increased worldwide over the past decade. Bath salt preparations often contain multiple synthetic cathinones in addition to other stimulants such as cocaine or caffeine. Synthetic cathinones are known to interact with dopamine, norepinephrine, and serotonin transporters (DAT, NET, and SERT, respectively). Although recent studies suggest that interactions with DAT are crucial for the reinforcing effects of synthetic cathinones, actions at SERT were found to negatively modulate reinforcing effectiveness. The goal of the current study was to characterize the relative contributions of dopamine, norepinephrine, and serotonin to the discriminative stimulus effects of MDPV by evaluating the substitution profiles of 1) a series of structurally related cathinones (MDPBP, MDPPP, α‐PPP, and α‐PVP) and cocaine, and 2) direct‐acting dopamine (quinpirole [D2/3], SKF 82958 [D1/5], and apomorphine [D1‐5]), and norepinephrine (phenylephrine [α1], and clonidine [α2]) receptor agonists in male Sprague‐Dawley rats (n=8) trained to discriminate 1.0 mg/kg MDPV from saline under a fixed ratio 10 schedule of sucrose pellet delivery. The mu‐opioid receptor agonist fentanyl served as a negative control. As expected, each of the cathinones and cocaine produced dose‐dependent increases in MDPV‐appropriate responding, whereas fentanyl failed to increase MDPV‐appropriate responding at doses smaller than those that suppressed responding. The rank order potency of the synthetic cathinones to increase MDPV‐appropriate responding is positively correlated with their potency to inhibit DAT and NET, but not SERT, and is consistent with the rank order to maintain self‐administration. The D2/3 receptor‐preferring agonist quinpirole dose‐dependently increased MDPV‐appropriate responding whilst SKF 82958 [D1/5], apomorphine [D1‐5], phenylephrine [α1], and clonidine [α2] failed to increase MDPV‐appropriate responding at doses smaller than those that suppressed responding. Together, these data provide evidence that the discriminative stimulus effects of MDPV are primarily mediated by its capacity to inhibit DAT, and the subsequent activation of dopamine D2 and/or D3 receptors. Moreover, these studies suggest do not support a role for serotonergic or adrenergic systems in mediating/modulating the discriminative stimulus effects of MDPV. Future studies will utilize antagonists of dopamine, adrenergic, and serotoninergic receptors to better characterize their relative contributions to the discriminative stimulus effects of MDPV.Support or Funding InformationThis work was supported by NIH/NIDA grant (R01 DA039146; GTC) and NIDA‐ and NIAAA‐IRPs (KCR)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Alcohol withdrawal drives depressive behaviors by activating neurons in the rostromedial tegmental nucleus.

Rostromedial tegmental nucleus (RMTg) GABA neurons exert a primary inhibitory drive onto midbrain dopamine neurons and are excited by a variety of aversive stimuli. There is, however, little evidence that the RMTg-ventral tegmental area (VTA)-nucleus accumbens shell (Acb) circuit plays a role in the aversive consequences of alcohol withdrawal. This study was performed in adult male Long-Evans rats at 48-h withdrawal from chronic alcohol drinking in the intermittent schedule. These rats displayed clear anhedonia and depression-like behaviors, as measured with the sucrose preference, and forced swimming tests. These aberrant behaviors were accompanied by a substantial increase in cFos expression in the VTA-projecting RMTg neurons, identified by a combination of immunohistochemistry and retrograde-tracing techniques. Pharmacological or chemogenetic inhibition of RMTg neurons mitigated the anhedonia and depression-like behaviors. Ex vivo electrophysiological data showed that chemogenetic inactivation of RMTg neurons reduced GABA release and accelerated spontaneous firings of VTA dopamine neurons. Finally, using a functional hemispheric disconnection procedure, we demonstrated that inhibition of unilateral RMTg, when combined with activation of D1 and D2 dopamine receptors in the contralateral (but not ipsilateral) Acb, mitigated the anhedonia and depression-like behaviors in alcohol-withdrawal rats. These data show that the integrity in the RMTg-VTA-Acb pathway in a single hemisphere is sufficient to elicit depression-like behavior during ethanol-withdrawal. Overall, the present results reveal that the RMTg-VTA-Acb pathway plays a crucial role in the depression-like behavior in animals undergoing alcohol withdrawal, further advocating the RMTg as a potential therapeutic target for alcoholism.

GABAergic imbalance is normalized by dopamine D1 receptor activation in the striatum contralateral to the cortical injury in motor deficit-recovered rats.

The sensorimotor cortex and the striatum are interconnected by the corticostriatal pathway, suggesting that cortical injury alters the striatal function, which may be modulated by dopamine. We studied whether the activation of dopamine D1 receptors (D1Rs) modulates the γ-aminobutyric acid (GABA) and glutamate levels in the striatum of recovered rats at 192h after cortical injury. The D1R agonist SKF-38393 (0, 2, 3, or 4mg/kg) was administered at 24, 48, 96, and 192h post-injury, and then rats were decapitated to determine GABA and glutamate levels and the levels of D1R mRNA on both sides of the striatum. GABAergic imbalance in the striatum contralateral to the injury site was normalized by the administration of the D1R agonist, but this treatment did not produce a significant effect on glutamate levels, suggesting that glutamate was metabolized into GABA. The administration of SKF-38393 (2mg/kg) decreased the levels of D1R mRNA in the striatum contralateral to the injury, and this effect was blocked by the coadministration of the D1R antagonist SCH-23390 (2mg/kg). In the striatum ipsilateral to the injury, the D1R agonist increased the D1R mRNA levels, an effect that was blocked by SCH-23390. The reversal of the GABAergic imbalance in the striatum contralateral to the cortical injury can be modulated by extrastriatal D1R activation, and the D1R agonist-induced increases in the D1R mRNA levels in the striatum ipsilateral to the injury suggest that the striatum may be necessary to achieve functional recovery.

D2 dopamine receptor activation induces female preference for male song in the monogamous zebra finch.

The evolutionary conservation of neural mechanisms for forming and maintaining pair bonds is unclear. Oxytocin, vasopressin and dopamine (DA) transmitter systems have been shown to be important in pair-bond formation and maintenance in several vertebrate species. We examined the role of dopamine in formation of song preference in zebra finches, a monogamous bird. Male courtship song is an honest signal of sexual fitness; thus, we measured female song preference to evaluate the role of DA in mate selection and pair-bond formation, using an operant conditioning paradigm. We found that DA acting through the D2 receptor, but not the D1 receptor, can induce a song preference in unpaired female finches and that blocking the D2 receptor abolished song preference in paired females. These results suggest that similar neural mechanisms for pair-bond formation are evolutionarily conserved in rodents and birds.

Functional characterization of AC5 gain-of-function variants: Impact on the molecular basis of ADCY5-related dyskinesia

Adenylyl cyclases are key points for the integration of stimulatory and inhibitory G protein-coupled receptor (GPCR) signals. Adenylyl cyclase type 5 (AC5) is highly expressed in striatal medium spiny neurons (MSNs), and is known to play an important role in mediating striatal dopaminergic signaling. Dopaminergic signaling from the D1 expressing MSNs of the direct pathway, as well as the D2 expressing MSNs of the indirect pathway both function through the regulation of AC5 activity, controlling the production of the 2nd messenger cAMP, and subsequently the downstream effectors. Here, we used a newly developed cell line that used Crispr-Cas9 to eliminate the predominant adenylyl cyclase isoforms to more accurately characterize a series of AC5 gain-of-function mutations which have been identified in ADCY5-related dyskinesias. Our results demonstrate that these AC5 mutants exhibit enhanced activity to Gαs-mediated stimulation in both cell and membrane-based assays. We further show that the increased cAMP response at the membrane effectively translates into increased downstream gene transcription in a neuronal model. Subsequent analysis of inhibitory pathways show that the AC5 mutants exhibit significantly reduced inhibition following D2 dopamine receptor activation. Finally, we demonstrate that an adenylyl cyclase “P-site” inhibitor, SQ22536 may represent an effective future therapeutic mechanism by preferentially inhibiting the overactive AC5 gain-of-function mutants.

Dopamine D1 receptor activation reduces local inner retinal inhibition to light-adapted levels.

During adaptation from dim to bright environments, changes in retinal signaling are mediated, in part, by dopamine. Dopamine is released with light and can modulate retinal receptive fields, neuronal coupling, inhibitory receptors, and rod pathway inhibition. However, it is unclear how dopamine affects inner retinal inhibition to cone bipolar cells, which relay visual information from photoreceptors to ganglion cells and are important signal processing sites. We tested the hypothesis that dopamine (D)1 receptor activation is sufficient to elicit light-adapted inhibitory changes. Local light-evoked inhibition and spontaneous activity were measured from OFF cone bipolar cells in dark-adapted mouse retinas while stimulating D1 receptors, which are located on bipolar, horizontal, and inhibitory amacrine cells. The D1 agonist SKF38393 reduced local inhibitory light-evoked response magnitude and increased response transience, which mimicked changes measured with light adaptation. D1-mediated reductions in local inhibition were more pronounced for glycinergic than GABAergic inputs, comparable with light adaptation. The effects of D1 receptors on light-evoked input were similar to the effects on spontaneous input. D1 receptor activation primarily decreased glycinergic spontaneous current frequency, similar to light adaptation, suggesting mainly a presynaptic amacrine cell site of action. These results expand the role of dopamine to include signal modulation of cone bipolar cell local inhibition. In this role, D1 receptor activation, acting primarily through glycinergic amacrine cells, may be an important mechanism for the light-adapted reduction in OFF bipolar cell inhibition since the actions are similar and dopamine is released during light adaptation. NEW & NOTEWORTHY Retinal adaptation to different luminance conditions requires the adjustment of local circuits for accurate signaling of visual scenes. Understanding mechanisms behind luminance adaptation at different retinal levels is important for understanding how the retina functions in a dynamic environment. In the mouse, we show that dopamine pathways reduce inner retinal inhibition similar to increased background luminance, suggesting the two are linked and highlighting a possible mechanism for light adaptation at an early retinal processing center.

Pharmacological activation of dopamine D4 receptor modulates morphine-induced changes in the expression of GAD65/67 and GABAB receptors in the basal ganglia

Dopamine D4 receptor (D4R) stimulation, in a putative D4R/μ opioid heteroreceptor (MOR) complex, counteracts the molecular, cellular and behavioural actions of morphine which are associated with morphine addiction, without any effect on its analgesic properties. In the present work, we have evaluated the role of D4R in modulating the effects of a continuous treatment with morphine on the GABAergic system in the basal ganglia. It has been demonstrated that the co-administration of a D4R agonist together with morphine leads to a restoration of GABA signaling by preventing drug-induced changes in GAD65/67 expression in the caudate putamen, globus palidus and substantia nigra. Results from GABABR1 and GABABR2 expression suggest a role of D4R in modulation of the GABAB heteroreceptor complexes along the basal ganglia, especially in the functional divisions of the caudate putamen. These results provide a new proof of the functional interaction between D4R and MOR and we postulate this putative heteroreceptor complex as a key target for the development of a new strategy to prevent the addictive effects of morphine in the treatment of pain.This article is part of the Special Issue entitled ‘Receptor heteromers and their allosteric receptor-receptor interactions’.

Dopamine D1 receptor activation improves adult hippocampal neurogenesis and exerts anxiolytic and antidepressant-like effect via activation of Wnt/β-catenin pathways in rat model of Parkinson's disease

Parkinson's disease (PD) is primarily characterized by midbrain dopamine depletion. Dopamine acts through dopamine receptors (D1 to D5) to regulate locomotion, motivation, pleasure, attention, cognitive functions and formation of newborn neurons, all of which are likely to be impaired in PD. Reduced hippocampal neurogenesis associated with dopamine depletion has been demonstrated in patients with PD. However, the precise mechanism to regulate multiple steps of adult hippocampal neurogenesis by dopamine receptor(s) is still unknown. In this study, we tested whether pharmacological agonism and antagonism of dopamine D1 and D2 receptor regulate nonmotor symptoms, neural stem cell (NSC) proliferation and fate specification and explored the cellular mechanism(s) underlying dopamine receptor (D1 and D2) mediated adult hippocampal neurogenesis in rat model of PD-like phenotypes. We found that single unilateral intra-medial forebrain bundle administration of 6-hydroxydopamine (6-OHDA) reduced D1 receptor level in the hippocampus. Pharmacological agonism of D1 receptor exerts anxiolytic and antidepressant-like effects as well as enhanced NSC proliferation, long-term survival and neuronal differentiation by positively regulating Wnt/β-catenin signaling pathway in hippocampus in PD rats. shRNA lentivirus mediated knockdown of Axin-2, a negative regulator of Wnt/β-catenin signaling potentially attenuated D1 receptor antagonist induced anxiety and depression-like phenotypes and impairment in adult hippocampal neurogenesis in PD rats. Our results suggest that improved nonmotor symptoms and hippocampal neurogenesis in PD rats controlled by D1-like receptors that involve the activation of Wnt/β-catenin signaling.

Co-localization of mu-opioid and dopamine D1 receptors in the medial preoptic area and bed nucleus of the stria terminalis across seasonal states in male European starlings

In seasonally breeding animals, changes in photoperiod and sex-steroid hormones may modify sexual behavior in part by altering the activity of neuromodulators, including opioids and dopamine. In rats and birds, activation of mu-opioid receptors (MOR) and dopamine D1 receptors in the medial preoptic area (mPOA) often have opposing effects on sexual behavior, yet mechanisms by which the mPOA integrates these opposing effects to modulate behavior remain unknown. Here, we used male European starlings (Sturnus vulgaris) to provide insight into the hypothesis that MOR and D1 receptors modify sexual behavior seasonally by altering activity in the same neurons in the mPOA. To do this, using fluorescent immunohistochemistry, we examined the extent to which MOR and D1 receptors co-localize in mPOA neurons and the degree to which photoperiod and the sex-steroid hormone testosterone alter co-localization. We found that MOR and D1 receptors co-localize throughout the mPOA and the bed nucleus of the stria terminalis, a region also implicated in the control of sexual behavior. Numbers of single and co-labeled MOR and D1 receptor labeled cells were higher in the rostral mPOA in photosensitive males (a condition observed just prior to the breeding season) compared to photosensitive males treated with testosterone (breeding season condition). In the caudal mPOA co-localization of MOR and D1 receptors was highest in photosensitive males compared to photorefractory males (a post-breeding season condition). Seasonal shifts in the degree to which neurons in the mPOA integrate signaling from opioids and dopamine may underlie seasonal changes in the production of sexual behavior.

Microglial glutamate release evoked by α‐synuclein aggregates is prevented by dopamine

When activated, microglial cells have the potential not only to secrete typical proinflammatory mediators but also to release the neurotransmitter glutamate in amounts that may promote excitotoxicity. Here, we wished to determine the potential of the Parkinson's disease (PD) protein α-Synuclein (αS) to stimulate glutamate release using cultures of purified microglial cells. We established that glutamate release was robustly increased when microglial cultures were treated with fibrillary aggregates of αS but not with the native monomeric protein. Promotion of microglial glutamate release by αS aggregates (αSa) required concomitant engagement of TLR2 and P2X7 receptors. Downstream to cell surface receptors, the release process was mediated by activation of a signaling cascade sequentially involving phosphoinositide 3-kinase (PI3K) and NADPH oxidase, a superoxide-producing enzyme. Inhibition of the Xc- antiporter, a plasma membrane exchange system that imports extracellular l-cystine and exports intracellular glutamate, prevented the release of glutamate induced by αSa, indicating that system Xc- was the final effector element in the release process downstream to NADPH oxidase activation. Of interest, the stimulation of glutamate release by αSa was abrogated by dopamine through an antioxidant effect requiring D1 dopamine receptor activation and PI3K inhibition. Altogether, present data suggest that the activation of microglial cells by αSa may possibly result in a toxic build-up of extracellular glutamate contributing to excitotoxic stress in PD. The deficit in dopamine that characterizes this disorder may further aggravate this process in a vicious circle mechanism.

Leucine-rich repeat kinase 2 controls protein kinase A activation state through phosphodiesterase 4

BackgroundEvidence indicates a cross-regulation between two kinases, leucine-rich repeat kinase 2 (LRRK2) and protein kinase A (PKA). In neurons, LRRK2 negatively regulates PKA activity in spiny projecting neurons during synaptogenesis and in response to dopamine D1 receptor activation acting as an A-anchoring kinase protein (AKAP). In microglia cells, we showed that LRRK2 kinase activity negatively regulates PKA, impacting NF-κB p50 signaling and the inflammatory response. Here, we explore the molecular mechanism underlying the functional interaction between LRRK2 and PKA in microglia.MethodsTo understand which step of PKA signaling is modulated by LRRK2, we used a combination of in vitro and ex vivo systems with hyperactive or inactive LRRK2 as well as different readouts of PKA signaling.ResultsWe confirmed that LRRK2 kinase activity acts as a negative regulator of PKA activation state in microglia. Specifically, we found that LRRK2 controls PKA by affecting phosphodiesterase 4 (PDE4) activity, modulating cAMP degradation, content, and its dependent signaling. Moreover, we showed that LRRK2 carrying the G2019S pathological mutation downregulates PKA activation causing a reduction of PKA-mediated NF-κB inhibitory signaling, which results, in turn, in increased inflammation in LRRK2 G2019S primary microglia upon α-synuclein pre-formed fibrils priming.ConclusionsOverall, our findings indicate that LRRK2 kinase activity is a key regulator of PKA signaling and suggest PDE4 as a putative LRRK2 effector in microglia. In addition, our observations suggest that LRRK2 G2019S may favor the transition of microglia toward an overactive state, which could widely contribute to the progression of the pathology in LRRK2-related PD.