The novel psychoactive substance 25E-NBOMe induces reward-related behaviors via dopamine D1 receptor signaling in male rodents

Novel psychoactive substances (NPSs) are new psychotropic drugs designed to evade substance regulatory policies. 25E-NBOMe (2-(4-ethyl-2,5-dimethoxyphenyl)-N-(2-methoxybenzyl)ethanamine) has recently been identified as an NPS, and its recreational misuse has been reported to be rapidly increasing. However, the psychopharmacological effects and mechanisms of 25E-NBOMe have not been studied. We examined the abuse potential of 25E-NBOMe using the conditioned place preference in male mice and self-administration paradigms in male rats. Additionally, immunoblot assay, enzyme-linked immunosorbent assay, and microdialysis were used to determine the molecular effects of 25E-NBOMe in the nucleus accumbens (NAc). Our data demonstrated that 25E-NBOMe induces conditioned place preference, and the dopaminergic signaling in the NAc mediates these. Following 25E-NBOMe administration, expression of dopamine transporter and dopamine D1 receptor (D1DR) were enhanced in the NAc of male mice, and NAc dopamine levels were reduced in both male mice and rats. Induction of intracellular dopaminergic pathways, DARPP32, and phosphorylation of CREB in the NAc of male mice was also observed. Significantly, pharmacological blockade of D1DR or chemogenetic inhibition of D1DR-expressing medium spiny neurons in the NAc attenuated 25E-NBOMe-induced conditioned place preference in male mice. We also examined the hallucinogenic properties of 25E-NBOMe using the head twitch response test in male mice and found that this behavior was mediated by serotonin 2A receptor activity. Our findings demonstrate that D1DR signaling may govern the addictive potential of 25E-NBOMe. Moreover, our study provides new insights into the potential mechanisms of substance use disorder and the improvement of controlled substance management.

The dopamine D1 receptor positive allosteric modulator, DETQ, improves cognition and social interaction in aged mice and enhances cortical and hippocampal acetylcholine efflux

Dopamine (DA) D1 and D2 receptors (Rs) are critical for cognitive functioning. D1 positive allosteric modulators (D1PAMs) activate D1Rs without desensitization or an inverted U-shaped dose response curve. DETQ, [2-(2,6-dichlorophenyl)-1-((1S,3R)-3-(hydroxymethyl)-5-(2-hydroxypropan-2-yl)-1-methyl-3,4-dihydroisoquinolin-2(1H)-yl)ethan-1-one] is highly selective for the human D1Rs as shown in humanized D1R knock-in (hD1Ki) mice. Here, we have ascertained the efficacy of DETQ in aged [13-23-month-old (mo)] hD1Ki mice and their corresponding age-matched wild-type (WT; C57BL/6NTac) controls. We found that in aged mice, DETQ, given acutely, subchronically, and chronically, rescued both novel object recognition memory and social behaviors, using novel object recognition (NOR) and social interaction (SI) tasks, respectively without any adverse effect on body weight or mortality. We have also shown, using in vivo microdialysis, a significant decrease in basal DA and norepinephrine, increase in glutamate (Glu) and gamma-amino butyric acid (GABA) efflux with no significant changes in acetylcholine (ACh) levels in aged vs young mice. In young and aged hD1Ki mice, DETQ, acutely and subchronically increased ACh in the medial prefrontal cortex and hippocampal regions in aged hD1Ki mice without affecting Glu. These results suggest that the D1PAM mechanism is of interest as potential treatment for cognitive and social behavioral deficits in neuropsychiatric disorders including but not restricted to neurodegenerative disorders, such as Parkinson's disease.

Optogenetic activation of the lateral habenulaD1R-ventral tegmental area circuit induces depression-like behavior in mice

A number of different receptors are distributed in glutamatergic neurons of the lateral habenula (LHb). These glutamatergic neurons are involved in different neural pathways, which may identify how the LHb regulates various physiological functions. However, the role of dopamine D1 receptor (D1R)-expressing habenular neurons projecting to the ventral tegmental area (VTA) (LHbD1R-VTA) remains not well understood. In the current study, to determine the activity of D1R-expressing neurons in LHb, D1R-Cre mice were used to establish the chronic restraint stress (CRS) depression model. Adeno-associated virus was injected into bilateral LHb in D1R-Cre mice to examine whether optogenetic activation of the LHb D1R-expressing neurons and their projections could induce depression-like behavior. Optical fibers were implanted in the LHb and VTA, respectively. To investigate whether optogenetic inhibition of the LHbD1R-VTA circuit could produce antidepressant-like effects, the adeno-associated virus was injected into the bilateral LHb in the D1R-Cre CRS model, and optical fibers were implanted in the bilateral VTA. The D1R-expressing neuronal activity in the LHb was increased in the CRS depression model. Optogenetic activation of the D1R-expressing neurons in LHb induced behavioral despair and anhedonia, which could also be induced by activation of the LHbD1R-VTA axons. Conversely, optogenetic inhibition of the LHbD1R-VTA circuit improved behavioral despair and anhedonia in the CRS depression model. D1R-expressing glutamatergic neurons in the LHb and their projections to the VTA are involved in the occurrence and regulation of depressive-like behavior.

Optogenetic activation of dopamine D1 receptors in island cells of medial entorhinal cortex inhibits temporal association learning

A critical feature of episodic memory formation is to associate temporally segregated events as an episode, called temporal association learning. The medial entorhinal cortical-hippocampal (EC-HPC) networks is essential for temporal association learning. We have previously demonstrated that pyramidal cells in the medial EC (MEC) layer III project to the hippocampal CA1 pyramidal cells and are necessary for trace fear conditioning (TFC), which is an associative learning between tone and aversive shock with the temporal gap. On the other hand, Island cells in MECII, project to GABAergic neurons in hippocampal CA1, suppress the MECIII input into the CA1 pyramidal cells through the feed-forward inhibition, and inhibit TFC. However, it remains unknown about how Island cells activity is regulated during TFC. In this study, we report that dopamine D1 receptor is preferentially expressed in Island cells in the MEC. Optogenetic activation of dopamine D1 receptors in Island cells facilitate the Island cell activity and inhibited hippocampal CA1 pyramidal cell activity during TFC. The optogenetic activation caused the impairment of TFC memory recall without affecting contextual fear memory recall. These results suggest that dopamine D1 receptor in Island cells have a crucial role for the regulation of temporal association learning.

Dopamine D1 receptor in orbitofrontal cortex to dorsal striatum pathway modulates methamphetamine addiction

The orbitofrontal cortex (OFC)-dorsal striatum (DS) is an important neural circuit that contributes to addictive behavior, including compulsive reinforcement, yet the specific types of neurons that play a major role still need to be further elucidated. Here, we used a place conditioning paradigm to measure the conditioned responses to methamphetamine (MA). The results demonstrated that MA increases the expression of c-Fos, synaptic plasticity in OFC and DS. Patch-clamp recording showed that MA activated projection neurons from the OFC to the DS, and chemogenetic manipulation of neuronal activity in OFC-DS projection neurons affects conditioned place preference (CPP) scores. And the combined patch-electrochemical technique was used to detect the DA release in OFC, the data indicated that the DA release was increased in MA group. Additionally, SCH23390, a D1R antagonist, was used to verify the function of D1R projection neurons, showing that SCH23390 reversed MA addiction-like behavior. Collectively, these findings provide evidence for the D1R neuron is sufficient to regulate MA addiction in the OFC-DS pathway, and the study provides new insight into the underlying mechanism of pathological changes in MA addiction.

Computational insights into ligand-induced G protein and β-arrestin signaling of the dopamine D1 receptor

The dopamine D1 receptor (D1R), is a class A G protein coupled-receptor (GPCR) which has been a promising drug target for psychiatric and neurological disorders such as Parkinson's disease (PD). Previous studies have suggested that therapeutic effects can be realized by targeting the β-arrestin signaling pathway of dopamine receptors, while overactivation of the G protein-dependent pathways leads to side effects, such as dyskinesias. Therefore, it is highly desirable to develop a D1R ligand that selectively regulates the β-arrestin pathway. Currently, most D1R agonists are signaling-balanced and stimulate both G protein and β-arrestin pathways, with a few reports of G protein biased ligands. However, identification and characterization of β-arrestin biased D1R agonists has been a challenge thus far. In this study, we implemented Gaussian accelerated molecular dynamics (GaMD) simulations to provide valuable computational insights into the possible underlying molecular mechanism of the different signaling properties of two catechol and two non-catechol D1R agonists that are either G protein biased or signaling-balanced. Dynamic network analysis further identified critical residues in the allosteric signaling network of D1R for each ligand at different conformational or binding states. Some of these residues are crucial for G protein or arrestin signals of GPCRs based on previous studies. Finally, we provided a molecular design strategy which can be utilized by medicinal chemists to develop potential β-arrestin biased D1R ligands. The proposed hypotheses are experimentally testable and can guide the development of safer and more effective medications for a variety of CNS disorders.

Activation of D1R signaling in the medial prefrontal cortex rescues maternal separation-induced behavioral deficits through restoration of excitatory neurotransmission

Lack of maternal care and attention during infancy and childhood increases the likelihood of developing a range of neuropsychiatric disorders, such as social deficits, working memory impairment, and anxiety-like behaviors, in adulthood. However, the neuroregulatory signaling through which early-life stress causes behavioral and cognitive abnormalities in the offspring is largely unexplored. Here, we show that in mice, unpredictable maternal separation (MS) during the early postnatal period impairs neuronal development in the medial prefrontal cortex (mPFC) and results in long-lasting behavioral changes. Additionally, MS disrupts excitatory neurotransmission and inhibits the neuronal activity of pyramidal neurons in the mPFC. Differentially expressed gene (DEG) analysis of RNA sequencing (RNA-seq) data of mPFC showed that dopamine D1 receptor (D1R) was significantly downregulated in MS animals. Finally, we show that pharmacological activation of D1R signaling specifically in the mPFC improves neuronal excitability and rescues behavioral and cognitive dysfunction of MS mice, whereas pharmacologically inhibiting of D1R in the mPFC mimics MS-induced behavioral abnormalities in control mice. Together, our results identify D1R signaling in the mPFC, at least in part, as a potential therapeutic target for the behavioral and cognitive abnormalities caused by deprivation of maternal care in early life.

Cellular bases for reward-related dopamine actions

Dopamine neurons exhibit transient increases and decreases in their firing rate upon reward and punishment for learning. This bidirectional modulation of dopamine dynamics occurs on the order of hundreds of milliseconds, and it is sensitively detected to reinforce the preceding sensorimotor events. These observations indicate that the mechanisms of dopamine detection at the projection sites are of remarkable precision, both in time and concentration. A major target of dopamine projection is the striatum, including the ventral region of the nucleus accumbens, which mainly comprises dopamine D1 and D2 receptor (D1R and D2R)-expressing spiny projection neurons. Although the involvement of D1R and D2R in dopamine-dependent learning has been suggested, the exact cellular bases for detecting transient dopamine signaling remain unclear. This review discusses recent cellular studies on the novel synaptic mechanisms for detecting dopamine transient signals associated with learning. Analyses of behavior based on these mechanisms have further revealed new behavioral aspects that are closely associated with these synaptic mechanisms. Thus, it is gradually possible to mechanistically explain behavioral learning via synaptic and cellular bases in rodents.

Dopamine D1 receptors mediate methamphetamine-induced dopaminergic damage: involvement of autophagy regulation via the AMPK/FOXO3A pathway

RATIONALE

Clinical studies have revealed that methamphetamine abuse increases risk for developing Parkinson's diseases. It is thus important to elucidate the mechanisms by which methamphetamine damages dopaminergic neurons.

OBJECTIVES

The present study was designed to elucidate the role of the dopamine D1 receptor in methamphetamine-mediated dopaminergic neuronal damage and its underlying mechanisms.

METHODS

Mice were treated for 4 days with vehicle, methamphetamine, or the D1 agonist SKF38393 and then assessed for locomotion and performance in the pole and rotarod tests. Cellular indices of autophagy, LC3, P62, and Beclin-1, tyrosine hydroxylase, and the AMPK/FOXO3A pathway were analyzed in striatal tissue from treated mice, in PC12 cells, and in D1 receptor mutant mice.

RESULTS

Repeated treatment with a relatively high dose of methamphetamine for 4 days induced both loss of dopaminergic neurons and activation of autophagy in the striatum as evidenced by increased expression of LC3 and P62. However, such treatment did not induce either loss of dopaminergic neurons or activation of autophagy in D1 receptor knockout mice. D1 receptor-mediated activation of autophagy was also confirmed in vitro using dopaminergic neuronal PC12 cells. Further studies demonstrated that the AMPK/FOXO3A signaling pathway is responsible for D1 receptor-mediated activation of autophagy.

CONCLUSIONS

The present data indicate a novel mechanism for methamphetamine-induced dopaminergic neuronal damage and reveal an important role for D1 receptors in the neurotoxicity of this drug.

CRF-receptor1 modulation of the dopamine projection to prelimbic cortex facilitates cognitive flexibility after acute and chronic stress

Stress reduces cognitive flexibility and dopamine D1 receptor-related activity in the prelimbic cortex (PL), effects hypothesized to depend on reduced corticotropic releasing factor receptor type 1 (CRFr1) regulation of dopamine neurons in the ventral tegmental area (VTA). We assessed this hypothesis in rats by examining the effect of chronic unpredictable restraint stress (CUS), mild acute stress, or their combination on cognitive flexibility, CRFr1 expression in the VTA and D1-related activity in PL. In Experiment 1, rats received either CUS or equivalent handling for 14 days before being trained to press two levers to earn distinct food outcomes. Initial learning was assessed using an outcome devaluation test after which cognitive flexibility was assessed by reversing the outcomes earned by the actions. Prior to each reversal training session, half the CUS and controls receiving acute stress with action-outcome updating assessed using a second devaluation test and CRFr1 expression in the VTA assessed using in-situ hybridisation. Although CUS did not itself affect action-outcome learning, its combination with acute stress blocked reversal learning and decreased VTA CRFr1 expression after acute shock. The relationship between these latter two effects was assessed in Experiment 2 by pharmacologically disconnecting the VTA and PL, unilaterally blocking neurons expressing CRFr1 in the VTA and D1 receptors in the contralateral PL during reversal learning after acute stress. Acute stress again blocked reversal learning but only in the group with VTA-PL disconnection, demonstrating that VTA CRFr1-induced facilitation of dopaminergic activity in the PL is necessary for maintaining cognitive flexibility after acute stress. [250].

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Acute stress increased CRF receptor1 expression in the VTA.

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Chronic stress attenuated the effect of acute stress on CRFr1 expression.

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Chronic stress plus acute stress produced a loss of cognitive flexibility.

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Blocking VTA CFRr1 and dopamine D1r in PL reduced cognitive flexibility following stress.

Neuroprotection in the Striatum of Hypoxic-Ischemic Piglets by Simultaneous Inhibition of Dopamine D1 and Adenosine A2A Receptors

INTRODUCTION

Striatal neurons of term newborns are highly vulnerable to hypoxia-ischemia (H-I). In a piglet model of H-I, a dopamine D1 receptor antagonist and an adenosine A2A receptor antagonist alone preferentially protect striatonigral and striatopallidal neurons, respectively. Here, we tested the hypothesis whether the combined treatment with SCH23390, a D1 receptor antagonist, and SCH58261, an A2A receptor antagonist, is more efficacious than individual D1 and A2A receptor antagonist treatment.

METHODS

Anesthetized newborn piglets were subjected to sham operation (n = 6) or 40 min of hypoxia and 7 min of airway occlusion. At 5 min of reoxygenation, piglets received the vehicle, SCH23390, SCH58261, or the combined treatment (n = 9 in each group). At 4 days of recovery, the number of viable neurons in the entire putamen was estimated by unbiased stereology.

RESULTS

Stereological results showed that sham-operated piglets had an estimated 2.9 × 106 neurons in the putamen, and the number of viable neurons in hypoxic-ischemic piglets was significantly reduced by 80% to 0.6 × 106/putamen. Treatment with SCH23390, SCH58261, and the combination increased the numbers of viable neurons to 1.4 × 106/putamen, 1.4 × 106/putamen, and 2.1 × 106/putamen, respectively. Notably, the combined treatment improved neuroprotection compared to individual therapy.

CONCLUSION

We conclude that simultaneous inhibition of dopamine D1 receptors and adenosine A2A receptors saves more neurons than individual treatment in the highly vulnerable putamen of a large-animal neonatal H-I model.

Dopamine D1 Receptor-Mediated Regulation of Per1, Per2, CLOCK, and BMAL1 Expression in the Suprachiasmatic Nucleus in Adult Male Rats

Photic and non-photic inputs are reported to affect clock gene expressions and behavioral activities in the SCN. However, it is not known whether dopaminergic input mediates these regulatory effects on clock genes. The present study examined the molecular effects of dopamine D1 agonist on Per1, Per2, CLOCK, and Bmal1 expressions in the SCN and its effect on behavioral activities to determine the role of dopamine D1 receptor in regulation of these gene expressions and behavioral activities in adult male Wistar rats. To examine the molecular effects of dopamine D1 agonist day and night, we injected 20 mg/kg SKF38393 to the first group of rats at 6 a.m. and the second group at 6 p.m. We also injected saline to the third and fourth groups of rats at 6 a.m. and 6 p.m. as control groups. All rats were sacrificed 2 h following the injections. The real-time PCR technique was used to evaluate the clock gene expression. In addition, to examine the effects of dopamine D1 agonists on behavioral activities, we injected 20 mg/kg SKF38393 to SKF receiving group and saline to control group. The behavioral activities of the rats were monitored on the running wheel for 21 days, 1 week following the injections. SKF injections increased the Per2 and CLOCK expressions in the daytime and significantly decreased the Per1 and Bmal1 expressions. However, at night, SKF injections increased only Per2 expressions significantly and decreased the Per1, CLOCK, and Bmal1 genes expressions. Both saline receiving groups showed that all gene expressions were significantly higher except Per2 during nighttime. SKF injection increased the running wheel activity during nighttime significantly. Based on the obtained result, clock gene expression and behavioral activities in adult male Wistar rats may be altered or monitored by administration of exogenous dopamine.

Discovery of eight alkaloids with D1 and D2 antagonist activity in leaves of Nelumbo nucifera Gaertn. Using FLIPR assays

ETHNOPHARMACOLOGICAL RELEVANCE

Dopamine receptors are long-standing primary targets in the treatment of mental diseases and there is growing evidence that suggests relationships between obesity and the dopamine system, especially dopamine D1 and D2 receptors. Leaves of Nelumbo nucifera Gaertn. (lotus leaves) have been medically used for helping long-term maintenance of weight loss. Whether and how components of lotus leaves function through the dopamine receptors remains unclear.

AIM OF THE STUDY

This work aimed to discover dopamine receptor-active alkaloids isolated from the lotus leaves, to evaluate their potencies and to analyze their structure activity relationship.

MATERIALS AND METHODS

Dried lotus leaves were prepared and total extract was divided into alkaloids and flavones. Eight alkaloids were separated and characterized by a combination of high-performance liquid chromatography, quadrupole time-of-flight mass spectrometry and nuclear magnetic resonance, and assayed by a fluorometric imaging plate reader platform. Human embryonic kidney 239 cell lines expressing dopamine D1, D2 and serotonin 2A (5-HT2A) receptors, respectively, were cultured and used in the assay.

RESULTS

Alkaloids in the lotus leaves were the bioactive phytochemicals and inhibited dopamine from accessing the D1 and D2 receptors. All eight compounds functioned as D1-receptor antagonists and except N-nornuciferine, seven alkaloids functioned as D2-receptor antagonists. (S)-coclaurine and (R)-coclaurine are optical isomers and antagonized both D1 and D2 with equivalent potencies, suggesting that the optical rotation of the methylene linker in the monobenzyl isoquinoline backbone did not influence their activity. Among the eight alkaloids, O-nornuciferine was the potent antagonist to both receptors (the lowest IC₅₀ values, D1: 2.09 ± 0.65 μM and D2: 1.14 ± 0.10 μM) while N-nornuciferine was found to be the least potent as it moderately antagonized D1 and was inactive on D2. O-nornuciferine was also a 5-HT2A antagonist (IC₅₀~20 μM) while N-nornuciferine had no activity. These hinted the importance of a methyl group attached to the nitrogen atom in the aporphine backbone. Armepavine showed a nearly 10-fold selectivity to D2.

CONCLUSIONS

In this work, eight alkaloids were isolated from the leaves of Nelumbo nucifera Gaertn. and assayed on the D1 and D2 receptors. They were D1/D2 antagonists with IC₅₀ values in the mid- to low-micromolar range and O-nornuciferine was the most potent alkaloid among the eight. This family of alkaloids was biochemically evaluated on the dopamine receptors by the same platform for the first time.

DRD1 agonist A-68930 improves mitochondrial dysfunction and cognitive deficits in a streptozotocin-induced mouse model

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder characterized by irreversible cognitive deficits and memory dysfunction. Dopamine is the most abundant catecholaminergic neurotransmitter in the brain which regulates motivation, reward, movement, and cognition. Recently, increasing evidences have shown that dopaminergic system is disturbed in AD conditions, and pharmacological interventions targeting dopamine D1 receptor (DRD1) exhibit certain therapeutic benefits in AD models. However, the underlying link between DRD1 and AD remains elusive. This study sought to test whether the selective DRD1 agonist A-68930 could improve streptozotocin (STZ)-induced cognitive impairment in mice. Here we found that A-68930 treatment through intraperitoneal injection efficiently alleviated STZ-induced cognitive deficits in mice. Moreover, our mechanism researches revealed that the DRD1 signaling induced by A-68930 significantly rescued STZ-induced mitochondrial biogenesis deficit, mitochondrial dysfunction, Aβ overexpression, and tau phosphorylation in mice hippocampus and cortex and SH-SY5Y cells, which may be mediated through stimulating AMPK/PGC-1α pathway. This study indicates that DRD1 agonist A-68930 can improve STZ-induced cognitive deficits and mitochondrial dysfunction in vivo and in vitro, and DRD1 may represent an appropriate target candidate for AD drug development.

Dopamine D1 Receptor in the Nucleus Accumbens Modulates the Emergence from Propofol Anesthesia in Rat

It has been reported that systemic activation of D1 receptors promotes emergence from isoflurane-induced unconsciousness, suggesting that the central dopaminergic system is involved in the process of recovering from general anesthesia. The nucleus accumbens (NAc) contains abundant GABAergic medium spiny neurons (MSNs) expressing the D1 receptor (D1R), which plays a key role in sleep-wake behavior. However, the role of NAc D1 receptors in the process of emergence from general anesthesia has not been identified. Here, using real-time in vivo fiber photometry, we found that neuronal activity in the NAc was markedly disinhibited during recovery from propofol anesthesia. Subsequently, microinjection of a D1R selective agonist (chloro-APB hydrobromide) into the NAc notably reduced the time to emerge from propofol anesthesia with a decrease in δ-band power and an increase in β-band power evident in the cortical electroencephalogram. These effects were prevented by pretreatment with a D1R antagonist (SCH-23390). Whole-cell patch clamp recordings were performed to further explore the cellular mechanism underlying the modulation of D1 receptors on MSNs under propofol anesthesia. Our data primarily demonstrated that propofol increased the frequency and prolonged the decay time of spontaneous inhibitory postsynaptic currents (sIPSCs) and miniature IPSCs (mIPSCs) of MSNs expressing D1 receptors. A D1R agonist attenuated the effect of propofol on the frequency of sIPSCs and mIPSCs, and the effects of the agonist were eliminated by preapplication of SCH-23390. Collectively, these results indicate that modulation of the D1 receptor on the activity of NAc MSNs is vital for emergence from propofol-induced unconsciousness.

Activation of Dopamine Signals in the Olfactory Tubercle Facilitates Emergence from Isoflurane Anesthesia in Mice

Activation of dopamine (DA) neurons is essential for the transition from sleep to wakefulness and maintenance of awakening, and sufficient to accelerate the emergence from general anesthesia in animals. Dopamine receptors (DR) are involve in arousal mediation. In the present study, we showed that the olfactory tubercle (OT) was active during emergence from isoflurane anesthesia, local injection of dopamine D1 receptor (D1R) agonist chloro-APB (1 mg/mL) and D2 receptor (D2R) agonist quinpirole (1 mg/mL) into OT enhanced behavioural and cortical arousal from isoflurane anesthesia, while D1R antagonist SCH-23390 (1 mg/mL) and D2R antagonist raclopride (2.5 mg/mL) prolonged recovery time. Optogenetic activation of DAergic terminals in OT also promoted behavioural and cortical arousal from isoflurane anesthesia. However, neither D1R/D2R agonists nor D1R/D2R antagonists microinjection had influences on the induction of isoflurane anesthesia. Optogenetic stimulation on DAergic terminals in OT also had no impact on the anesthesia induction. Our results indicated that DA signals in OT accelerated emergence from isoflurane anesthesia. Furthermore, the induction of general anesthesia, different from the emergence process, was not mediated by the OT DAergic pathways.

Dopamine D1 receptor alleviates doxorubicin-induced cardiac injury by inhibiting NLRP3 inflammasome

Doxorubicin (DOX) is a broad-spectrum antineoplastic drug; however, its serious cardiotoxic side effects in inflammatory responses limit its use in clinical applications. Dopamine D1 receptor (DRD1), a G protein-coupled receptor, is crucial for the development and function of the nervous system; additionally, it also play a role in immune regulation. However, the specific role of DRD1 in DOX-induced cardiac inflammation has not yet been clarified. Here, we discovered that DRD1 expression was induced by DOX treatment in H9C2 cardiomyocytes. DRD1 activation by A-68930, a DRD1-specific agonist, decreased DOX-induced nucleotide-binding domain-like receptor protein 3 (NLRP3) expression, caspase-1 activation, and IL-1β maturation in H9C2 cells. Expression of the cytokines IL-1β and IL-18 in the supernatants was also inhibited by A-68930 treatment. DRD1 knockdown, using siRNA, abolished the effects of A-68930 on the DOX-induced NLRP3 inflammasome. Furthermore, we found that DRD1 signaling downregulated the NLRP3 inflammasome in H9C2 cells through cyclic adenosine monophosphate (cAMP). Moreover, application of A-68930 to activate DRD1 reduced cardiac injury and fibrosis in a DOX-treated mouse model by suppressing the NLRP3 inflammasome in the heart. These findings indicate that DRD1 signaling may protect against DOX-induced cardiac injury by inhibiting the NLRP3 inflammasome-mediated inflammation.

Optogenetic induction of orbitostriatal long-term potentiation in the dorsomedial striatum elicits a persistent reduction of alcohol-seeking behavior in rats

Uncontrolled drug-seeking and -taking behaviors are generally driven by maladaptive corticostriatal synaptic plasticity. The orbital frontal cortex (OFC) and its projections to the dorsomedial striatum (DMS) have been extensively implicated in drug-seeking and relapse behaviors. The influence of the synaptic plasticity of OFC projections to the DMS (OFC→DMS) on drug-seeking and -taking behaviors has not been fully characterized. To investigate this, we trained rats to self-administer 20% alcohol and then delivered an in vivo optogenetic protocol designed to induce long-term potentiation (LTP) selectively at OFC→DMS synapses. We selected LTP induction because we found that voluntary alcohol self-administration suppressed OFC→DMS transmission and LTP may normalize this transmission, consequently reducing alcohol-seeking behavior. Importantly, ex vivo slice electrophysiology studies confirmed that this in vivo optical stimulation protocol resulted in a significant increase in excitatory OFC→DMS transmission strength on day two after stimulation, suggesting that LTP was induced in vivo. Rat alcohol-seeking and -taking behaviors were significantly reduced on days 1-3, but not on days 7-11, after LTP induction. Striatal synaptic plasticity is modulated by several critical neurotransmitter receptors, including dopamine D1 receptors (D1Rs) and adenosine A2A receptors (A2ARs). We found that delivery of in vivo optical stimulation in the presence of a D1R antagonist abolished the LTP-associated decrease in alcohol-seeking behavior, whereas delivery in the presence of an A2AR antagonist may facilitate this LTP-induced behavioral change. These results demonstrate that alcohol-seeking behavior was negatively regulated by the potentiation of excitatory OFC→DMS neurotransmission. Our findings provide direct evidence that the OFC exerts "top-down" control of alcohol-seeking behavior via the DMS.

Prenatal disruption of D1R-SynGAP complex causes cognitive deficits in adulthood

γ-aminobutyric acid (GABA)-ergic interneurons are essential for the physiological function of the mammalian central nervous system. Dysregulated GABAergic interneuron function has been implicated in the pathophysiology of a number of neurodevelopmental disorders including schizophrenia and autism spectrum disorder. Tangential migration is an important process to ensure the proper localization of GABAergic interneurons. Previously we found that disrupting the interaction between dopamine D1 receptor (D1R) and synaptic Ras GTPase- activating protein (SynGAP) using an interfering peptide (TAT-D₁R_pep) during embryonic development impaired tangential migration. Here, we assessed the effects of prenatal disruption of D1R-SynGAP complex with the TAT-D₁R_pep on the expression of several behaviours during adulthood. Mice with prenatal D1R-SynGAP disruption exhibited transiently reduced locomotor activity, abnormal sensorimotor gating, impaired sociability and deficits in visual discrimination associative learning compared to their control counterparts. Our findings reinforce the importance of GABAergic interneuron migration in the manifestation of normal motor, sensory, and cognitive behaviours of animals during adulthood.

Impaired Acquisition of Nicotine-Induced Conditioned Place Preference in Fatty Acid-Binding Protein 3 Null Mice

Nicotine causes psychological dependence through its interactions with nicotinic acetylcholine receptors in the brain. We previously demonstrated that fatty acid-binding protein 3 (FABP3) colocalizes with dopamine D2 receptors (D2Rs) in the dorsal striatum, and FABP3 deficiency leads to impaired D2R function. Moreover, D2R null mice do not exhibit increased nicotine-induced conditioned place preference (CPP) following chronic nicotine administration. To investigate the role of FABP3 in nicotine-induced CPP, FABP3 knockout (FABP3^-/-) mice were evaluated using a CPP apparatus following consecutive nicotine administration (0.5 mg/kg) for 14 days. Importantly, nicotine-induced CPP was suppressed in the conditioning, withdrawal, and relapse phases in FABP3^-/- mice. To resolve the mechanisms underlying impaired nicotine-induced CPP in these mice, we assessed c-Fos expression and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase (ERK) signaling in both dopamine D1 receptor (D1R)- and D2R-positive neurons in the nucleus accumbens (NAc). Notably, 64% of dopamine receptor-positive neurons in the mouse NAc expressed both D1R and D2R. Impaired nicotine-induced CPP was correlated with lack of responsiveness of both CaMKII and ERK phosphorylation. The number of D2R-positive neurons was increased in FABP3^-/- mice, while the number of D1R-positive neurons and the responsiveness of c-Fos expression to nicotine were decreased. The aberrant c-Fos expression was closely correlated with CaMKII but not ERK phosphorylation levels in the NAc of FABP3^-/- mice. Taken together, these results indicate that impaired D2R signaling due to lack of FABP3 may affect D1R and c-Fos signaling and underlie nicotine-induced CPP behaviors.

Dopamine D1 receptor agonist A-68930 ameliorates Aβ1-42-induced cognitive impairment and neuroinflammation in mice

Alzheimer's disease (AD) is an irreversible neurodegenerative disease characterized by progressive cognitive dysfunction and memory impairment. Dopamine is an important catecholaminergic neurotransmitter that controls movement, reward, motivation, and cognition. Recently, dopamine receptors were reported to regulate immune system in both periphery and central nervous system. However, whether dopamine D1 receptor (DRD1) activation could improve neuroinflammation in AD conditions remains unknown. The present study aimed to investigate the therapeutic effects and underlying mechanisms of a potent and selective DRD1 agonist A-68930 on Aβ_1-42-induced mice. Here we showed that intraperitoneal injection of A-68930 significantly ameliorated Aβ_1-42-induced cognitive dysfunction in mice. Moreover, both in vivo and in vitro data showed that A-68930-induced DRD1 activation significantly inhibited NLRP3 inflammasome-dependent neuroinflammation induced by Aβ_1-42, and this effect may be mediated by the activation of AMPK/autophagy signaling pathway, which enhanced NLRP3 inflammasome degradation and thus decreased the secretion of IL-1β and IL-18. The present study suggests that A-68930-induced DRD1 signaling efficiently alleviates Aβ_1-42-induced cognitive impairment and neuroinflammation in mice and BV2 cells, and DRD1 may become a promising therapeutic target for AD.

Blockade of spinal dopamine D1/D2 receptor suppresses activation of NMDA receptor through Gαq and Src kinase to attenuate chronic bone cancer pain

Introduction

Spinal N-methyl-D-aspartate receptor (NMDAR) is vital in chronic pain, while NMDAR antagonists have severe side effects. NMDAR has been reported to be controlled by G protein coupled receptors (GPCRs), which might present new therapeutic targets to attenuate chronic pain. Dopamine receptors which belong to GPCRs have been reported could modulate the NMDA-mediated currents, while their exact effects on NMDAR in chronic bone cancer pain have not been elucidated.

Objectives

This study was aim to explore the effects and mechanisms of dopamine D1 receptor (D1DR) and D2 receptor (D2DR) on NMDAR in chronic bone cancer pain.

Methods

A model for bone cancer pain was established using intra-tibia bone cavity tumor cell implantation (TCI) of Walker 256 in rats. The nociception was assessed by Von Frey assay. A range of techniques including the fluorescent imaging plate reader, western blotting, and immunofluorescence were used to detect cell signaling pathways. Primary cultures of spinal neurons were used for in vitro evaluation.

Results

Both D1DR and D2DR antagonists decreased NMDA-induced upregulation of Ca²⁺ oscillations in primary culture spinal neurons. Additionally, D1DR/D2DR antagonists inhibited spinal Calcitonin Gene-Related Peptide (CGRP) and c-Fos expression and alleviated bone cancer pain induced by TCI which could both be reversed by NMDA. And D1DR/D2DR antagonists decreased p-NR1, p-NR2B, and Gαq protein, p-Src expression. Both Gαq protein and Src inhibitors attenuated TCI-induced bone cancer pain, which also be reversed by NMDA. The Gαq protein inhibitor decreased p-Src expression. In addition, D1DR/D2DR antagonists, Src, and Gαq inhibitors inhibited spinal mitogen-activated protein kinase (MAPK) expression in TCI rats, which could be reversed by NMDA.

Conclusions

Spinal D1DR/D2DR inhibition eliminated NMDAR-mediated spinal neuron activation through Src kinase in a Gαq-protein-dependent manner to attenuate TCI-induced bone cancer pain, which might present a new therapeutic strategy for bone cancer pain.

Neurotransmission through dopamine D1 receptors is required for aversive memory formation and Arc activation in the cerebral cortex

Dopaminergic neurotransmission is considered to play an important role not only in reward-based learning, but also in aversive learning. Here, we investigated the role of dopaminergic neurotransmission via dopamine D1 receptors (D1Rs) in aversive memory formation in a passive avoidance test using D1R knockdown (KD) mice, in which the expression of D1Rs can conditionally and reversibly be controlled by doxycycline (Dox) treatment. We also performed whole-brain imaging after aversive footshock stimulation in activity-regulated cytoskeleton protein (Arc)-dVenus D1RKD mice, which were crossbred from Arc-dVenus transgenic mice and D1RKD mice, to examine the distribution of Arc-controlled dVenus expression in the hippocampus and cerebral cortex during aversive memory formation. Knockdown of D1R expression following Dox treatment resulted in impaired performance in the passive avoidance test and was associated with a decrease in dVenus expression in the cerebral cortex (visual, somatosensory, and motor cortices), but not the hippocampus, compared with control mice without Dox treatment. These findings indicate that D1R-mediated dopaminergic transmission is critical for aversive memory formation, specifically by influencing Arc expression in the cerebral cortex.

Lack of dopamine D1 receptors in the antidepressant actions of (R)-ketamine in a chronic social defeat stress model

It is reported that dopamine D₁ receptors in the medial prefrontal cortex play a role in the antidepressant actions of (R,S)-ketamine. However, its role in the antidepressant actions of (R)-ketamine, which is more potent than (S)-ketamine, is unknown. In the locomotion test, tail suspension test, forced swimming test and 1% sucrose preference test, pretreatment with dopamine D₁ receptor antagonist SCH-23390 did not block the antidepressant effects of (R)-ketamine in the susceptible mice after chronic social defeat stress. These findings suggest that dopamine D₁ receptors may not play a major role in the antidepressant actions of (R)-ketamine.

Biased G Protein-Independent Signaling of Dopamine D1-D3 Receptor Heteromers in the Nucleus Accumbens

Several studies found in vitro evidence for heteromerization of dopamine D₁ receptors (D1R) and D₃ receptors (D3R), and it has been postulated that functional D1R-D3R heteromers that are normally present in the ventral striatum mediate synergistic locomotor-activating effects of D1R and D3R agonists in rodents. Based also on results obtained in vitro, with mammalian transfected cells, it has been hypothesized that those behavioral effects depend on a D1R-D3R heteromer-mediated G protein-independent signaling. Here, we demonstrate the presence on D1R-D3R heteromers in the mouse ventral striatum by using a synthetic peptide that selectively destabilizes D1R-D3R heteromers. Parallel locomotor activity and ex vivo experiments in reserpinized mice and in vitro experiments in D1R-D3R mammalian transfected cells were performed to dissect the signaling mechanisms of D1R-D3R heteromers. Co-administration of D1R and D3R agonists in reserpinized mice produced synergistic locomotor activation and a selective synergistic AKT phosphorylation in the most ventromedial region of the striatum in the shell of the nucleus accumbens. Application of the destabilizing peptide in transfected cells and in the shell of the nucleus accumbens allowed demonstrating that both in vitro and in vivo co-activation of D3R induces a switch from G protein-dependent to G protein-independent D1R-mediated signaling determined by D1R-D3R heteromerization. The results therefore demonstrate that a biased G protein-independent signaling of D1R-D3R heteromers localized in the shell of the nucleus accumbens mediate the locomotor synergistic effects of D1R and D3R agonists in reserpinized mice.

Dopamine D1 Receptor (D1R) Expression Is Controlled by a Transcriptional Repressor Complex Containing DISC1

Disrupted-in-Schizophrenia 1 (DISC1) is a scaffold protein implicated in various psychiatric diseases. Dysregulation of the dopamine system has been associated with DISC1 deficiency, while the molecular mechanism is unclear. In this study, we propose a novel molecular mechanism underlying the transcriptional regulation of the dopamine D1 receptor (D1R) in the striatum via DISC1. We verified the increase in D1R at the transcriptional level in the striatum of DISC1-deficient mouse models and altered histone acetylation status at the D1r locus. We identified a functional interaction between DISC1 and Krüppel-like factor 16 (KLF16). KLF16 translocates DISC1 into the nucleus and forms a regulatory complex by recruiting SIN3A corepressor complexes to the D1r locus. Moreover, DISC1-deficient mice have altered D1R-mediated signaling in the striatum and exhibit hyperlocomotion in response to cocaine; the blockade of D1R suppresses these effects. Taken together, our results suggest that nuclear DISC1 plays a critical role in the transcriptional regulation of D1R in the striatal neuron, providing a mechanistic link between DISC1 and dopamine-related psychiatric symptoms.

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.

Dopamine D2-like receptor stimulation blocks negative feedback in visual and spatial reversal learning in the rat: behavioural and computational evidence

RATIONALE

Dopamine D2-like receptors (D2R) are important drug targets in schizophrenia and Parkinson's disease, but D2R ligands also cause cognitive inflexibility such as poor reversal learning. The specific role of D2R in reversal learning remains unclear.

OBJECTIVES

We tested the hypotheses that D2R agonism impairs reversal learning by blocking negative feedback and that antagonism of D1-like receptors (D1R) impairs learning from positive feedback.

METHODS

Male Lister Hooded rats were trained on a novel visual reversal learning task. Performance on "probe trials", during which the correct or incorrect stimulus was presented with a third, probabilistically rewarded (50% of trials) and therefore intermediate stimulus, revealed individual learning curves for the processes of positive and negative feedback. The effects of D2R and D1R agonists and antagonists were evaluated. A separate cohort was tested on a spatial probabilistic reversal learning (PRL) task after D2R agonism. Computational reinforcement learning modelling was applied to choice data from the PRL task to evaluate the contribution of latent factors.

RESULTS

D2R agonism with quinpirole dose-dependently impaired both visual reversal and PRL. Analysis of the probe trials on the visual task revealed a complete blockade of learning from negative feedback at the 0.25 mg/kg dose, while learning from positive feedback was intact. Estimated parameters from the model that best described the PRL choice data revealed a steep and selective decrease in learning rate from losses. D1R antagonism had a transient effect on the positive probe trials.

CONCLUSIONS

D2R stimulation impairs reversal learning by blocking the impact of negative feedback.

Phase 1 Parkinson's Disease Studies Show the Dopamine D1/D5 Agonist PF-06649751 is Safe and Well Tolerated

INTRODUCTION

There is a need for new therapies in Parkinson's disease that may help to address known limitations of current options. PF-06649751 is a novel, highly selective dopamine D1/D5 agonist targeted for Parkinson's disease treatment.

METHODS

The safety, pharmacokinetics, and pharmacodynamics of PF-06649751 were assessed in single ascending dose and multiple ascending dose clinical trials in patients with Parkinson's disease. The single ascending dose study (N = 18) was a double-blind, placebo-controlled study with a three-way crossover design consisting of three treatment periods separated by 7-day study drug washout periods. PF-06649751 doses were 0.75 mg, 1.5 mg, 3 mg, 6 mg, and 9 mg. In the open-label multiple ascending dose study, eligible subjects received once-daily doses of PF-06649751 (N = 45) over 21 days, with up-titration to 5 mg, 15 mg, and 25 mg once daily. Pharmacodynamics were assessed by measuring change from baseline in the Movement Disorder Society-sponsored revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Part III at different time points post dose.

RESULTS

PF-06649751 was safe and well tolerated across studies and in all cohorts. Peak plasma concentrations were attained 1-4 h post dose across both studies, and exposure increased with increasing dose. PF-06649751 demonstrated sustained pharmacodynamic effects compared with placebo, with mean reductions from baseline in the MDS-UPDRS Part III up to 12 h post dose at 9 mg single dose. MDS-UPDRS Part III changes in the open-label multiple dose study on day 22 also demonstrated sustained pharmacodynamic activity.

CONCLUSIONS

PF-06649751 represents a novel therapeutic candidate for Parkinson's disease with an initial safety, tolerability, and pharmacokinetic profile and potential for efficacy that merits further study in larger clinical trials.

TRIAL REGISTRATION

These studies are registered at www.clinicaltrials.gov as NCT02373072, NCT02224664.

FUNDING

Pfizer.

Modulation of appetitive motivation by prefrontal cortical mu-opioid receptors is dependent upon local dopamine D1 receptor signaling

Opioid neurotransmission has been implicated in psychiatric disorders featuring impaired control over appetitive motivation, such as addiction and binge-eating disorder. We have previously shown that infusions of the μ-opioid receptor (μOR) agonist DAMGO into the ventromedial prefrontal cortex (vmPFC) induced hyperphagia, increased motor activity, and augmented sucrose-reinforced responding in the task progressive ratio (PR) task, which assesses the motivational value of an incentive. These effects were not reproduced by intra-PFC infusion of a variety of dopamine (DA) agonists and antagonists, suggesting that manipulation of intra-PFC DA systems alone is not sufficient to reproduce μOR-like effects. Nevertheless, this does not rule out interactions between PFC DA and μ-opioid systems. Here we used intra-vmPFC drug cocktails containing DAMGO and SCH 23390 (a DA D1 receptor antagonist) to determine whether increases in appetitive motivation and motor activity elicited by intra-vmPFC μOR stimulation require intact signaling through vmPFC D1 receptors. Blockade of D1 receptors with SCH 23390 attenuated the enhancement of PR breakpoint, and increases in exploratory-like behavior and feeding initiation elicited by intra-vmPFC μOR stimulation. These results establish that intra-vmPFC D1 signaling is required for the expression of behavioral effects evoked by μOR stimulation within the PFC, and further suggest that D1 tone plays an enabling or permissive role in the expression of μOR -elicited effects. Simultaneous targeting of both μ-opioid and D1 systems may represent a more efficacious treatment strategy (compared to μOR blockade alone) for psychiatric disorders characterized by dysregulated appetitive motivation.

Adenosine A1-Dopamine D1 Receptor Heteromers Control the Excitability of the Spinal Motoneuron

While the role of the ascending dopaminergic system in brain function and dysfunction has been a subject of extensive research, the role of the descending dopaminergic system in spinal cord function and dysfunction is just beginning to be understood. Adenosine plays a key role in the inhibitory control of the ascending dopaminergic system, largely dependent on functional complexes of specific subtypes of adenosine and dopamine receptors. Combining a selective destabilizing peptide strategy with a proximity ligation assay and patch-clamp electrophysiology in slices from male mouse lumbar spinal cord, the present study demonstrates the existence of adenosine A₁-dopamine D₁ receptor heteromers in the spinal motoneuron by which adenosine tonically inhibits D₁ receptor-mediated signaling. A₁-D₁ receptor heteromers play a significant control of the motoneuron excitability, represent main targets for the excitatory effects of caffeine in the spinal cord and can constitute new targets for the pharmacological therapy after spinal cord injury, motor aging-associated disorders and restless legs syndrome.

Muscarinic Acetylcholine Receptors Inhibit Fyn Activity in the Rat Striatum In Vivo

The Src family kinase (SFK) is a subfamily of non-receptor tyrosine kinases. SFK members, Src and especially Fyn, are expressed in the striatum. These SFK members are involved in the regulation of neuronal and synaptic activities and are linked to the pathogenesis of a variety of neuropsychiatric and neurodegenerative disorders. Given the fact that muscarinic acetylcholine (mACh) receptors are highly expressed in striatal neurons and are critical for the regulation of striatal function, we investigated the role of mACh receptors in the regulation of SFKs in the adult rat striatum in vivo. We found that pharmacological blockade of mACh receptors by systemic administration of the mACh antagonist scopolamine induced a marked increase in phosphorylation of SFKs in the striatum of male and female rats. This scopolamine-induced increase in SFK phosphorylation occurred in the two subdivisions of the striatum (caudate putamen and nucleus accumbens) and was time-dependent and reversible. Another mACh antagonist atropine was also effective in stimulating SFK phosphorylation. Coadministration of subthreshold doses of scopolamine and a dopamine D1 receptor agonist SKF81297 enhanced striatal SFK phosphorylation. Between Fyn and Src proteins immunoprecipitated from striatal tissue, scopolamine selectively increased phosphorylation of Fyn. The increase in Fyn phosphorylation was accompanied by an increase in Fyn kinase activity in response to scopolamine. These results reveal a significant role of mACh receptors in the regulation of SFKs (mainly Fyn) in striatal neurons. Under normal conditions, endogenous mACh receptors appear to exert an inhibitory effect on Fyn activity.

In Utero Exposure to Fine Particulate Matter Causes Hypertension Due to Impaired Renal Dopamine D1 Receptor in Offspring

BACKGROUND/AIMS

Adverse environment in utero can modulate adult phenotypes including blood pressure. Fine particulate matter (PM2.5) exposure in utero causes hypertension in the offspring, but the exact mechanisms are not clear. Renal dopamine D1 receptor (D1R), regulated by G protein-coupled receptor kinase type 4 (GRK4), plays an important role in the regulation of renal sodium transport and blood pressure. In this present study, we determined if renal D1R dysfunction is involved in PM2.5-induced hypertension in the offspring.

METHODS

Pregnant Sprague-Dawley rats were given an oropharyngeal drip of PM2.5 (1.0 mg/kg) at gestation day 8, 10, and 12. The blood pressure, 24-hour sodium excretion, and urine volume were measured in the offspring. The expression levels of GRK4 and D1R were determined by immunoblotting. The phosphorylation of D1R was investigated using immunoprecipitation. Plasma malondialdehyde and superoxide dismutase levels were also measured in the offspring.

RESULTS

As compared with saline-treated dams, offspring of PM2.5-treated dams had increased blood pressure, impaired sodium excretion, and reduced D1R-mediated natriuresis and diuresis, accompanied by decreased renal D1R expression and GRK4 expression. The impaired renal D1R function and increased GRK4 expression could be caused by increased reactive oxidative stress (ROS) induced by PM2.5 exposure. Administration of tempol, a redox-cycling nitroxide, for 4 weeks in the offspring of PM2.5-treated dam normalized the decreased renal D1R expression and increased renal D1R phosphorylation and GRK4 expression. Furthermore, tempol normalized the increased renal expression of c-Myc, a transcription factor that regulates GRK4 expression.

CONCLUSIONS

In utero exposure to PM2.5 increases ROS and GRK4 expression, impairs D1R-mediated sodium excretion, and increases blood pressure in the offspring. These studies suggest that normalization of D1R function may be a target for the prevention and treatment of the hypertension in offspring of mothers exposed to PM2.5 during pregnancy.

The effects of local microinjection of selective dopamine D1 and D2 receptor agonists and antagonists into the dorsal raphe nucleus on sleep and wakefulness in the rat

The effects of the dopamine (DA) D1 and D2 receptor agonists SKF38393, bromocriptine and quinpirole, respectively, on spontaneous sleep were analyzed in adult rats prepared for chronic sleep recordings. Local administration of the DAergic agonists into the dorsal raphe nucleus (DRN) during the light phase of the light-dark cycle induced a significant reduction of rapid-eye movement sleep (REMS) and the number of REM periods. Additionally, bromocriptine and quinpirole significantly increased wakefulness (W). Opposite, the microinjection into the DRN of the DA D1 and D2 receptor antagonists SCH23390 and sulpiride, respectively, significantly augmented REMS and the number of REM periods. Pretreatment with SCH23390 and sulpiride prevented the effects of SKF38393 and bromocriptine, respectively, on sleep variables. Our results tend to indicate that DAergic neurons located in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) contribute to the regulation of predominantly W and REMS by DRN serotonergic neurons.

Extracellular dopamine, acetylcholine, and activation of dopamine D1 and D2 receptors after selective breeding for cocaine self-administration in rats

RATIONALE

The low self-administration (LS)/Kgras (LS) and high self-administration (HS)/Kgras (HS) rat lines were generated by selective breeding for low- and high-intravenous cocaine self-administration, respectively, from a common outbred Wistar stock (Crl:WI). This trait has remained stable after 13 generations of breeding.

OBJECTIVE

The objective of the present study is to compare cocaine preference, neurotransmitter release, and dopamine receptor activation in LS and HS rats.

METHODS

Levels of dopamine, acetylcholine, and cocaine were measured in the nucleus accumbens (NA) shell of HS and LS rats by tandem mass spectrometry of microdialysates. Cocaine-induced locomotor activity and conditioned-place preference were compared between LS and HS rats.

RESULTS

HS rats displayed greater conditioned-place preference scores compared to LS and reduced basal extracellular concentrations of dopamine and acetylcholine. However, patterns of neurotransmitter release did not differ between strains. Low-dose cocaine increased locomotor activity in LS rats, but not in HS animals, while high-dose cocaine augmented activity only in HS rats. Either dose of cocaine increased immunoreactivity for c-Fos in the NA shell of both strains, with greater elevations observed in HS rats. Activation identified by cells expressing both c-Fos and dopamine receptors was generally greater in the HS strain, with a similar pattern for both D1 and D2 dopamine receptors.

CONCLUSIONS

Diminished levels of dopamine and acetylcholine in the NA shell, with enhanced cocaine-induced expression of D1 and D2 receptors, are associated with greater rewarding effects of cocaine in HS rats and an altered dose-effect relationship for cocaine-induced locomotor activity.

The novel antipsychotic drug brexpiprazole, alone and in combination with escitalopram, facilitates prefrontal glutamatergic transmission via a dopamine D1 receptor-dependent mechanism

Brexpiprazole (Rexulti^®), a novel D2/3 receptor (R) partial agonist, was recently approved as monotherapy for schizophrenia, demonstrating effectiveness against both positive and negative symptoms, and also approved as add-on treatment to antidepressant drugs, inducing a potent antidepressant effect with a faster onset compared to an antidepressant given alone. Moreover, brexpiprazole has demonstrated pro-cognitive effects in preclinical studies. To explore whether the observed effects may be mediated via modulation of prefrontal glutamatergic transmission, we investigated the effect of brexpiprazole, alone and in combination with the SSRI escitalopram, on prefrontal glutamatergic transmission using in vitro electrophysiological intracellular recordings of deep layer pyramidal cells of the rat medial prefrontal cortex (mPFC). Nanomolar concentrations of brexpiprazole potentiated NMDAR-induced currents and electrically evoked EPSPs via activation of dopamine D1Rs, in similarity with the effect of the atypical antipsychotic drug clozapine. The effect of an ineffective concentration of brexpiprazole was significantly potentiated by the addition of escitalopram. When combined with escitalopram, brexpiprazole also potentiated AMPAR-mediated transmission, in similarity with the clinically rapid acting antidepressant drug ketamine. The effect on the AMPAR-mediated currents was also D1R dependent. In conclusion, our data propose that brexpiprazole exerts a clozapine-like potentiation of NMDAR-mediated currents in the mPFC, which can explain its efficacy on negative symptoms of schizophrenia and the pro-cognitive effects observed preclinically. Moreover, add-on brexpiprazole to escitalopram also potentiated AMPAR-mediated transmission, which may provide a neurobiological explanation to the faster antidepressant effect of add-on brexpiprazole in major depression.

Functional roles of T3.37 and S5.46 in the activation mechanism of the dopamine D1 receptor

The activation mechanism of dopamine receptors is unknown. The amino acids S5.42, S5.43, and S5.46 located in helix 5 appear to be crucial, but their specific roles in receptor activation have not been studied. We modeled the D1 dopamine receptor using the crystal structures of the D3 dopamine and β2 adrenergic receptors. Molecular dynamics simulations show that the interaction of dopamine with the D1 receptor leads to the formation of a hydrogen-bond network with its catechol group and helices 3, 5, and 6, including water molecules. The para hydroxyl group of dopamine binds directly to S5.42 and N6.55, the latter also interacting with S5.43. Unexpectedly, S5.46 does not interact directly with the catechol; instead, it interacts through a water molecule with S5.42 and directly with T3.37. The formation of this hydrogen-bond network, part of which was previously observed in docking studies with dopamine agonists, triggers the opening of the E6.30-R3.60 ionic lock associated with the activation of GPCRs. These changes do not occur in the unbonded (apo) receptor or when it is in a complex with the antagonist 3-methoxy-5,6,7,8,9,14-hexahydrodibenz[d,g]azecine. Our results provide valuable insight into the T3.37-S5.46-water-S5.43-ligand interaction, which may be crucial to the activation of the D1 dopamine receptor and should be considered during the design of novel agonists. Graphical Abstract General representation of the relationship between the formation of the HBN and the opening of the R3.50-E6.30 ionic lock.

GSK-3β Interacts with Dopamine D1 Receptor to Regulate Receptor Function: Implication for Prefrontal Cortical D1 Receptor Dysfunction in Schizophrenia

INTRODUCTION

Impaired dopamine D1 receptor (D1R) function in prefrontal cortex (PFC) is believed to contribute to the PFC hypofunction that has been hypothesized to be associated with negative symptoms and cognitive deficits in schizophrenia. It is therefore critical to understand the mechanisms for modulation of D1R function.

AIMS

To investigate the physical interaction and functional modulation between D1R and GSK-3β.

RESULTS

D1R and GSK-3β physically interact in cultured cells and native brain tissues. This direct interaction was found to occur at the S(417)PALS(421) motif in the C-terminus of D1R. Inhibition of GSK-3β impaired D1R activation along with a decrease in D1R-GSK-3β interaction. GSK-3β inhibition reduced agonist-stimulated D1R desensitization and endocytosis, the latter associated with the reduction of membrane translocation of β-arrestin-2. Similarly, inhibition of GSK-3β in rat PFC also resulted in impaired D1R activation and association with GSK-3β. Moreover, in a NMDA antagonist animal model of schizophrenia, we detected a decrease in prefrontal GSK-3β activity and D1R-GSK-3β association and decreased D1R activation in the PFC.

CONCLUSIONS

The present work identified GSK-3β as a new interacting protein for D1R functional regulation and revealed a novel mechanism for GSK-3β-regulated D1R function which may underlie D1R dysfunction in schizophrenia.

Genetic variants of dopamine D2 receptor impact heterodimerization with dopamine D1 receptor

BACKGROUND

The human dopamine D2 receptor gene has three polymorphic variants that alter its amino acid sequence: alanine substitution by valine in position 96 (V96A), proline substitution by serine in position 310 (P310S) and serine substitution by cysteine in position 311 (S311C). Their functional role has never been the object of extensive studies, even though there is some evidence that their occurrence correlates with schizophrenia.

METHODS

The HEK293 cell line was transfected with dopamine D1 and D2 receptors (or genetic variants of the D2 receptor), coupled to fluorescent proteins which allowed us to measure the extent of dimerization of these receptors, using a highly advanced biophysical approach (FLIM-FRET). Additionally, Fluoro-4 AM was used to examine changes in the level of calcium release after ligand stimulation of cells expressing different combinations of dopamine receptors.

RESULTS

Using FLIM-FRET experiments we have shown that in HEK 293 expressing dopamine receptors, polymorphic mutations in the D2 receptor play a role in dimmer formation with the dopamine D1 receptor. The association level of dopamine receptors is affected by ligand administration, with variable effects depending on polymorphic variant of the D2 dopamine receptor. We have found that the level of heteromer formation is reflected by calcium ion release after ligand stimulation and have observed variations of this effect dependent on the polymorphic variant and the ligand.

CONCLUSION

The data presented in this paper support the hypothesis on the role of calcium signaling regulated by the D1-D2 heteromer which may be of relevance for schizophrenia etiology.

Anatomical and molecular characterization of dopamine D1 receptor-expressing neurons of the mouse CA1 dorsal hippocampus

In the hippocampus, a functional role of dopamine D1 receptors (D1R) in synaptic plasticity and memory processes has been suggested by electrophysiological and pharmacological studies. However, comprehension of their function remains elusive due to the lack of knowledge on the precise localization of D1R expression among the diversity of interneuron populations. Using BAC transgenic mice expressing enhanced green fluorescent protein under the control of D1R promoter, we examined the molecular identity of D1R-containing neurons within the CA1 subfield of the dorsal hippocampus. In agreement with previous findings, our analysis revealed that these neurons are essentially GABAergic interneurons, which express several neurochemical markers, including calcium-binding proteins, neuropeptides, and receptors among others. Finally, by using different tools comprising cell type-specific isolation of mRNAs bound to tagged-ribosomes, we provide solid data indicating that D1R is present in a large proportion of interneurons expressing dopamine D2 receptors. Altogether, our study indicates that D1Rs are expressed by different classes of interneurons in all layers examined and not by pyramidal cells, suggesting that CA1 D1R mostly acts via modulation of GABAergic interneurons.

The role of the dopamine D1 receptor in social cognition: studies using a novel genetic rat model

Social cognition is an endophenotype that is impaired in schizophrenia and several other (comorbid) psychiatric disorders. One of the modulators of social cognition is dopamine, but its role is not clear. The effects of dopamine are mediated through dopamine receptors, including the dopamine D1 receptor (Drd1). Because current Drd1 receptor agonists are not Drd1 selective, pharmacological tools are not sufficient to delineate the role of the Drd1. Here, we describe a novel rat model with a genetic mutation in Drd1 in which we measured basic behavioural phenotypes and social cognition. The I116S mutation was predicted to render the receptor less stable. In line with this computational prediction, this Drd1 mutation led to a decreased transmembrane insertion of Drd1, whereas Drd1 expression, as measured by Drd1 mRNA levels, remained unaffected. Owing to decreased transmembrane Drd1 insertion, the mutant rats displayed normal basic motoric and neurological parameters, as well as locomotor activity and anxiety-like behaviour. However, measures of social cognition like social interaction, scent marking, pup ultrasonic vocalizations and sociability, were strongly reduced in the mutant rats. This profile of the Drd1 mutant rat offers the field of neuroscience a novel genetic rat model to study a series of psychiatric disorders including schizophrenia, autism, depression, bipolar disorder and drug addiction.

Frequency-Dependent Modulation of Dopamine Release by Nicotine and Dopamine D1 Receptor Ligands: An In Vitro Fast Cyclic Voltammetry Study in Rat Striatum

Nicotine is a highly addictive drug and exerts this effect partially through the modulation of dopamine release and increasing extracellular dopamine in regions such as the brain reward systems. Nicotine acts in these regions on nicotinic acetylcholine receptors. The effect of nicotine on the frequency dependent modulation of dopamine release is well established and the purpose of this study was to investigate whether dopamine D1 receptor (D1R) ligands have an influence on this. Using fast cyclic voltammetry and rat corticostriatal slices, we show that D1R ligands are able to modulate the effect of nicotine on dopamine release. Nicotine (500 nM) induced a decrease in dopamine efflux at low frequency (single pulse or five pulses at 10 Hz) and an increase at high frequency (100 Hz) electrical field stimulation. The D1R agonist SKF-38393, whilst having no effect on dopamine release on its own or on the effect of nicotine upon multiple pulse evoked dopamine release, did significantly prevent and reverse the effect of nicotine on single pulse dopamine release. Interestingly similar results were obtained with the D1R antagonist SCH-23390. In this study we have demonstrated that the modulation of dopamine release by nicotine can be altered by D1R ligands, but only when evoked by single pulse stimulation, and are likely working via cholinergic interneuron driven dopamine release.

Suppression of outward K(+) currents by activating dopamine D1 receptors in rat retinal ganglion cells through PKA and CaMKII signaling pathways

Dopamine plays an important role in regulating neuronal functions in the central nervous system by activating the specific G-protein coupled receptors. Both D1 and D2 dopamine receptors are extensively distributed in the retinal neurons. In the present study, we investigated the effects of D1 receptor signaling on outward K(+) currents in acutely isolated rat retinal ganglion cells (RGCs) by patch-clamp techniques. Extracellular application of SKF81297 (10 μM), a specific D1 receptor agonist, significantly and reversibly suppressed outward K(+) currents of the cells, which was reversed by SCH23390 (10 μM), a selective D1 receptor antagonist. We further showed that SKF81297 mainly suppressed the glybenclamide (Gb)- and 4-aminopyridine (4-AP)-sensitive K(+) current components, but did not show effect on the tetraethylammonium (TEA)-sensitive one. Both protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways were likely involved in the SKF81297-induced suppression of the K(+) currents since either Rp-cAMP (10 μM), a cAMP/PKA signaling inhibitor, or KN-93 (10 μM), a specific CaMKII inhibitor, eliminated the SKF81297 effect. In contrast, neither protein kinase C (PKC) nor mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) signaling pathway seemed likely to be involved because both the PKC inhibitor bisindolylmaleimide IV (Bis IV) (10 μM) and the MAPK/ERK1/2 inhibitor U0126 (10 μM) did not block the SKF81297-induced suppression of the K(+) currents. These results suggest that activation of D1 receptors suppresses the Gb- and 4-AP-sensitive K(+) current components in rat RGCs through the intracellular PKA and CaMKII signaling pathways, thus modulating the RGC excitability.

Manganese-Disrupted Interaction of Dopamine D1 and NMDAR in the Striatum to Injury Learning and Memory Ability of Mice

Manganese (Mn) is widely regarded as a neurotoxic heavy metal that causes learning and memory deficits. Recently, it has been proved that the striatum is related to memory and learning ability. However, no previous study focused on the effect of Mn-induced learning and memory deficits on the striatum. This study aims to investigate the probable interaction of dopamine D1 receptor (DR1) and N-methyl-D-aspartate receptor (NMDAR), two cognition-related receptors in the striatum during Mn exposure. Mice are randomly divided into four groups, including control group, 12.5 mg/kg MnCl₂ group, 25 mg/kg MnCl₂ group, and 50 mg/kg MnCl₂ group. The mice receive intraperitoneal injections of 0, 12.5, 25, and 50 mg/kg MnCl₂ once daily for 2 weeks. Then, learning and memory ability, pathological changes, expression, and interaction of DR1 and NMDAR are determined. It has been found that Mn disrupted spatial learning and memory ability of mice by Morris water maze test and the passive avoidance test. Pathological and ultrastructure were injured. Mn decreased the immunohistochemical activities, protein levels, and messenger RNA (mRNA) expression of DR1, NR1, and NR2A. Mn exposure inhibited interaction between DR1 and NMDAR in striatum by double immunofluorescent staining and co-immunoprecipitation. In conclusion, our study illustrated that Mn caused learning and memory dysfunction via injury of striatum and inhibition of interaction between DR1 and NMDAR in striatum.

Possible role of the dopamine D1 receptor in the sensorimotor gating deficits induced by high-fat diet

RATIONALE

High-fat diet (HFD) has been recently reported to induce sensorimotor gating deficits, but the underlying mechanisms are not well understood.

OBJECTIVE

The purpose of this study is to determine whether HFD induces long-lasting deficits in sensorimotor gating and to examine the involvement of altered dopamine (DA) function.

METHODS

C57BL/6J mice were fed HFD for 10 weeks and then normal diet (ND) for 4 weeks. DA D2 receptor (D2R) knockout (KO) mice were also fed HFD for 10 weeks. The mice were evaluated for prepulse inhibition (PPI) of acoustic startle after HFD and the subsequent 4-week ND. We evaluated the effect of SCH23390, a D1 receptor (D1R) antagonist, on PPI and measured protein expression levels of D1R and D2R in the prefrontal cortex (PFC) in HFD mice. The concentrations of monoamines and their metabolites in the cortices of 10-week HFD or ND mice were measured using high performance liquid chromatography.

RESULTS

Long-term HFD-induced PPI disruption in WT and D2R KO mice. Even after 4 weeks of subsequent ND, PPI remained to be disrupted. SCH23390 mitigated the PPI disruption. In HFD animals, D1R protein expression in the PFC was significantly decreased, while DA, homovanillic acid, and 3,4-dihydroxyphenylacetic acid levels in the cortex were increased.

CONCLUSION

This is the first evidence that HFD can induce long-lasting deficits in sensorimotor gating through alteration of cortical levels of DA and its metabolites. Our data suggest that HFD-induced PPI deficits are related to altered D1R signaling and that D1R antagonists may have therapeutic effects on the deficits.

Structure and function of the amygdaloid NPY system: NPY Y2 receptors regulate excitatory and inhibitory synaptic transmission in the centromedial amygdala

The amygdala is essential for generating emotional-affective behaviors. It consists of several nuclei with highly selective, elaborate functions. In particular, the central extended amygdala, consisting of the central amygdala (CEA) and the bed nucleus of the stria terminalis (BNST) is an essential component actively controlling efferent connections to downstream effectors like hypothalamus and brain stem. Both, CEA and BNST contain high amounts of different neuropeptides that significantly contribute to synaptic transmission. Among these, neuropeptide Y (NPY) has emerged as an important anxiolytic and fear-reducing neuromodulator. Here, we characterized the expression, connectivity and electrophysiological function of NPY and Y2 receptors within the CEA. We identified several NPY-expressing neuronal populations, including somatostatin- and calretinin-expressing neurons. Furthermore, in the main intercalated nucleus, NPY is expressed primarily in dopamine D1 receptor-expressing neurons but also in interspersed somatostatin-expressing neurons. Interestingly, NPY neurons did not co-localize with the Y2 receptor. Retrograde tract tracing experiments revealed that NPY neurons reciprocally connect the CEA and BNST. Functionally, the Y2 receptor agonist PYY3-36, reduced both, inhibitory as well as excitatory synaptic transmission in the centromedial amygdala (CEm). However, we also provide evidence that lack of NPY or Y2 receptors results in increased GABA release specifically at inhibitory synapses in the CEm. Taken together, our findings suggest that NPY expressed by distinct populations of neurons can modulate afferent and efferent projections of the CEA via presynaptic Y2 receptors located at inhibitory and excitatory synapses.

(18)F-Fluorodeoxyglucamines: Reductive amination of hydrophilic (18)F-fluoro-2-deoxyglucose with lipophilic amines for the development of potential PET imaging agents

Maillard reaction of (18)F-FDG with biological amines results in the formation of (18)F-fluorodeoxyglycosylamines ((18)F-FDGly) as pseudo-Amadori products. To increase in vivo stability, we report the reductive amination of FDGly to provide reduced fluorodeoxyglucamines (FDGlu). (18)F-Fluorodeoxyglucamines ((18)F-FDGlu), resulting from linking (18)F-FDG (hydrophilic) to lipophilic molecules containing amine group may be useful as positron emission tomography (PET) imaging agents. Two amine derivatives, 7-chloro-8-hydroxy-3-methyl-l-(3'-aminophenyl)-2,3,4,5-tetrahydro-lH-3-benzazepine (SCH 38548 for dopamine D1 receptors) and BTA-0 (for Aβ amyloid) were reacted with FDG under reductive amination conditions to yield stable products, FDGluSCH and FDGluBTA. FDGluSCH had high binding affinity to rat brain dopamine D1 receptors with a Ki of 19.5 nM while FDGluBTA had micromolar affinity for human frontal cortex Aβ plaques. (18)F-FDGluSCH was prepared in low to modest radiochemical yields and preliminary results showed binding to the rat striatum in brain slices. In vivo stability of(18)F-FDGluSCH needs to be determined. Our results suggest that (18)F-FDG is a useful 'radioactive synthon' for PET radiotracer development. Its usefulness will have to be determined on the basis of the structure-activity relationship of the target molecule.

The reinforcing effects of ethanol within the posterior ventral tegmental area depend on dopamine neurotransmission to forebrain cortico-limbic systems

Ethanol can be self-infused directly into the posterior ventral tegmental area (pVTA) and these effects involve activation of local dopamine neurons. However, the neuro-circuitry beyond the pVTA involved in these reinforcing effects has not been explored. Intra-pVTA microinjection of ethanol increases dopamine release in the nucleus accumbens (NAC), medial prefrontal cortex (mPFC) and ventral pallidum (VP). The present study tested the hypothesis that the reinforcing effects of ethanol within the pVTA involve the activation of dopamine projections from the pVTA to the NAC, VP and mPFC. Following the acquisition of self-infusions of 200 mg% ethanol into the pVTA, either the dopamine D2 receptor antagonist sulpiride (0, 10 or 100 μM) or the D1 receptor antagonist SCH-23390 (0, 10 or 100 μM) was microinjected into the ipsilateral NAC shell (NACsh), NAC core (NACcr), VP or mPFC immediately prior to the self-infusion sessions to assess the involvement of the different dopamine projections in the reinforcing effects of ethanol. Microinjection of each compound at higher concentration into the NACsh, VP or mPFC, but not the NACcr, significantly reduced the responses on the active lever (from 40-50 to approximately 20 responses). These results indicate that activation of dopamine receptors in the NACsh, VP or mPFC, but not the NACcr, is involved in mediating the reinforcing effects of ethanol in the pVTA, suggesting that the 'alcohol reward' neuro-circuitry consist of, at least in part, activation of the dopamine projections from the pVTA to the NACsh, VP and mPFC.

Cognitive impairment and dentate gyrus synaptic dysfunction in experimental parkinsonism

BACKGROUND

Parkinson's disease (PD) is characterized by the progressive degeneration of the nigrostriatal dopaminergic pathway and the emergence of rigidity, tremor, and bradykinesia. Accumulating evidence indicates that PD is also accompanied by nonmotor symptoms including cognitive deficits, often manifested as impaired visuospatial memory.

METHODS

We studied cognitive performance and synaptic plasticity in a mouse model of PD, characterized by partial lesion of the dopaminergic and noradrenergic inputs to striatum and hippocampus. Sham- and 6-hydroxydopamine-lesioned mice were subjected to the novel object recognition test, and long-term potentiation was examined in the dentate gyrus and CA1 regions of the hippocampus.

RESULTS

Bilateral 6-hydroxydopamine lesion reduced long-term but not short-term novel object recognition and decreased long-term potentiation specifically in the dentate gyrus. These abnormalities did not depend on the loss of noradrenaline but were abolished by the antiparkinsonian drug, L-DOPA, or by SKF81297, a dopamine D1-type receptor agonist. In contrast, activation of dopamine D2-type receptors did not modify the effects produced by the lesion. Blockade of the extracellular signal-regulated kinases prevented the ability of SKF81297 to rescue novel object recognition and long-term potentiation.

CONCLUSIONS

These findings show that partial dopamine depletion leads to impairment of long-term recognition memory accompanied by abnormal synaptic plasticity in the dentate gyrus. They also demonstrate that activation of dopamine D1 receptors corrects these deficits, through a mechanism that requires intact extracellular signal-regulated kinases signaling.