Dopamine has no direct causal role in the formation of treatment expectations and placebo analgesia in humans.

Dopamine-based reward and learning mechanisms have been suggested to contribute to placebo effects. However, the exact role of dopaminergic neurotransmission in their generation and maintenance is still unclear. This study aimed to shed light on the causal role of dopamine in establishing positive treatment expectations, as well as on the magnitude and duration of their effect on pain. To this end, we used an established placebo analgesia paradigm in combination with 2 opposing pharmacological modulations of dopaminergic tone, i.e., the dopamine antagonist sulpiride and the dopamine precursor L-dopa which were both applied in an experimental, double-blind, randomized, placebo-controlled trial with a between-subject design in N = 168 healthy volunteers. The study medication successfully altered dopaminergic tone during the conditioning procedure. Contrary to our hypotheses, the medication did not modulate the formation of positive treatment expectation and placebo analgesia tested 1 day later. Placebo analgesia was no longer detectable on day 8 after conditioning. Using a combined frequentist and Bayesian approach, our data provide strong evidence against a direct dopaminergic influence on the generation and maintenance of placebo effects. Further exploration of the neurochemical mechanisms underlying placebo analgesia remains paramount in the quest to exploit these effects for optimal treatment outcomes. Trial registration: ClinicalTrials.gov German Clinical Trials Register, ID: DRKS00029366, https://drks.de/search/en/trial/DRKS00029366.

Significance of Duodenal Prolactin Receptor Modulation by Calcium and Vitamin D in Sulpiride-Induced Hyperprolactinemia.

Background and Objectives: Hyperprolactinemia, as a potential side-effect of some antipsychotic medications, is associated with decreased bone density and an increased risk of fractures. This study investigates whether calcium and vitamin D supplementation affects prolactin receptor (Prlr) gene expression in the duodenum, vertebrae, and kidneys of female rats with sulpiride-induced hyperprolactinemia. Materials and Methods: Twenty-one-week-old female Wistar rats were assigned to three groups: Group S consisted of ten rats who received sulpiride injections (10 mg/kg) twice daily for 6 weeks; Group D (10 rats) received daily supplementation of 50 mg calcium and 500 IU vitamin D along with sulpiride for the last 3 weeks; and Group C consisting of seven age-matched nulliparous rats serving as a control group. Real-time PCR was used to assess Prlr gene expression in the duodenum, vertebrae, and kidneys. Results: In Group S, Prlr gene expression was notably decreased in the duodenum (p < 0.01) but elevated in the vertebrae and kidneys compared to Group C. Conversely, Group D exhibited significantly increased Prlr expression in the duodenum (p < 0.01) alongside elevated expression in the vertebrae and kidneys. Conclusions: In sulpiride-induced hyperprolactinemia, decreased Prlr gene expression in the duodenum may lead to reduced intestinal calcium absorption. Consequently, prolactin may draw calcium from the skeletal system to maintain calcium balance, facilitated by increased Prlr gene expression in the vertebrae. However, vitamin D supplementation in sulpiride-induced hyperprolactinemia notably enhances Prlr gene expression in the duodenum, potentially ameliorating intestinal calcium absorption and mitigating adverse effects on bone health.

D2-like dopamine receptors blockade within the dentate gyrus shows a greater effect on stress-induced analgesia in the tail-flick test compared to D1-like dopamine receptors.

INTRODUCTION: Acute stress, as a protective mechanism to respond to an aversive stimulus, can often be accompanied by suppressing pain perception via promoting consistent burst firing of dopamine neurons. Besides, sensitive and advanced research techniques led to the recognition of the mesohippocampal dopaminergic terminals, particularly in the hippocampal dentate gyrus (DG). Moreover, previous studies have shown that dopamine receptors within the hippocampal DG play a critical role in induced antinociceptive responses by forced swim stress (FSS) in the presence of inflammatory pain. Since different pain states can trigger various mechanisms and transmitter systems, the present experiments aimed to investigate whether dopaminergic receptors within the DG have the same role in the presence of acute thermal pain. METHODS: Ninety-seven adult male albino Wistar rats underwent stereotaxic surgery, and a stainless steel guide cannula was unilaterally implanted 1 mm above the DG. Different doses of SCH23390 or sulpiride as D1- and D2-like dopamine receptor antagonists were microinjected into the DG 5-10 min before exposure to FSS, and 5 min after FSS exposure, the tail-flick test evaluated the effect of stress on the nociceptive response at the time-set intervals. RESULTS: The results demonstrated that exposure to FSS could significantly increase the acute pain perception threshold, while intra-DG administration of SCH23390 and sulpiride reduced the antinociceptive effect of FSS in the tail-flick test. DISCUSSION: Additionally, it seems the D2-like dopamine receptor within the DG plays a more prominent role in FSS-induced analgesia in the acute pain model.

Selenoprotein T, a potential treatment of attention-deficit/hyperactivity disorder and comorbid pain in neonatal 6-OHDA lesioned mice.

pathological pain and Attention-deficit/hyperactivity disorder (ADHD) are two complex multifactorial syndromes. The comorbidity of ADHD and altered pain perception is well documented in children, adolescents, and adults. According to pathophysiological investigations, the dopaminergic system's dysfunction provides a common basis for ADHD and comorbid pain. Growing evidence suggests that oxidative stress may be crucial in both pathologies. Recent studies revealed that a small peptide encompassing the redox-active site of selenoprotein T (PSELT), protects dopaminergic neurons and fibers as well as lesioned nerves in animal models. The current study aims to examine the effects of PSELT treatment on ADHD-like symptoms and pain sensitivity, as well as the role of catecholaminergic systems in these effects. Our results demonstrated that intranasal administration of PSELT reduced the hyperactivity in the open field, decreased the impulsivity displayed by 6-OHDA-lesioned male mice in the 5-choice serial reaction time task test and improved attentional performance. In addition, PSELT treatment significantly increased the nociception threshold in both normal and inflammatory conditions. Furthermore, anti-hyperalgesic activity was antagonized with sulpiride pre-treatment, but not by phentolamine, or propranolol pre-treatments. The present study suggests that PSELT reduces the severity of ADHD symptoms in mice and possesses potent antinociceptive effects which could be related to the involvement of D2/D3 dopaminergic receptors.

The antipsychotic agent sulpiride microinjected into the ventral pallidum restores positive symptom-like habituation disturbance in MAM-E17 schizophrenia model rats.

Dysfunction of subcortical D2-like dopamine receptors (D2Rs) can lead to positive symptoms of schizophrenia, and their analog, the increased locomotor activity in schizophrenia model MAM-E17 rats. The ventral pallidum (VP) is a limbic structure containing D2Rs. The D2R antagonist sulpiride is a widespread antipsychotic drug, which can alleviate positive symptoms in human patients. However, it is still not known how sulpiride can influence positive symptoms via VP D2Rs. We hypothesize that the microinjection of sulpiride into the VP can normalize hyperactivity in MAM-E17 rats. In addition, recently, we showed that the microinjection of sulpirid into the VP induces place preference in neurotypical rats. Thus, we aimed to test whether intra-VP sulpiride can also have a rewarding effect in MAM-E17 rats. Therefore, open field-based conditioned place preference (CPP) test was applied in neurotypical (SAL-E17) and MAM-E17 schizophrenia model rats to test locomotor activity and the potential locomotor-reducing and rewarding effects of sulpiride. Sulpiride was microinjected bilaterally in three different doses into the VP, and the controls received only vehicle. The results of the present study demonstrated that the increased locomotor activity of the MAM-E17 rats was caused by habituation disturbance. Accordingly, larger doses of sulpiride in the VP reduce the positive symptom-analog habituation disturbance of the MAM-E17 animals. Furthermore, we showed that the largest dose of sulpiride administered into the VP induced CPP in the SAL-E17 animals but not in the MAM-E17 animals. These findings revealed that VP D2Rs play an important role in the formation of positive symptom-like habituation disturbances in MAM-E17 rats.

Dopamine D2 receptor antagonist counteracts hyperglycemia and insulin resistance in diet-induced obese male mice.

Obesity leads to insulin resistance (IR) and type 2 diabetes. In humans, low levels of the hormone prolactin (PRL) correlate with IR, adipose tissue (AT) dysfunction, and increased prevalence of T2D. In obese rats, PRL treatment promotes insulin sensitivity and reduces visceral AT adipocyte hypertrophy. Here, we tested whether elevating PRL levels with the prokinetic and antipsychotic drug sulpiride, an antagonist of dopamine D2 receptors, improves metabolism in high fat diet (HFD)-induced obese male mice. Sulpiride treatment (30 days) reduced hyperglycemia, IR, and the serum and pancreatic levels of triglycerides in obese mice, reduced visceral and subcutaneous AT adipocyte hypertrophy, normalized markers of visceral AT function (PRL receptor, Glut4, insulin receptor and Hif-1α), and increased glycogen stores in skeletal muscle. However, the effects of sulpiride reducing hyperglycemia were also observed in obese prolactin receptor null mice. We conclude that sulpiride reduces obesity-induced hyperglycemia by mechanisms that are independent of prolactin/prolactin receptor activity. These findings support the therapeutic potential of sulpiride against metabolic dysfunction in obesity.

Conjugation of sulpiride with a cell penetrating peptide to augment the antidepressant efficacy and reduce serum prolactin levels.

Depression ranks as the fourth most prevalent global disease, with suicide incidents occurring at a younger age. Sulpiride (SUL), an atypical antidepressant drug acting as a dopamine D2 receptor antagonist and possessing anti-inflammatory properties, exhibits limited ability to penetrate the blood brain barrier (BBB). This weak penetration hampers its inhibitory effect on prolactin release in the pituitary gland, consequently leading to hyperprolactinemia. In order to enhance the central nervous system efficacy of sulpiride and reduce serum prolactin levels, we covalently linked sulpiride to VPALR derived from the nuclear DNA repair protein ku70. In vivo study on depressive mice using intraperitoneal injection of VPALR-SUL demonstrated a significant increase in struggle time and total distance compared to those treated with only sulpiride while also reducing serum prolactin concentration. The pharmacokinetic study results showed that VPALR-SUL prolonged half-life and increased bioavailability. In conclusion, VPALR-SUL exhibited potential for enhancing sulpiride transport across the BBB, augmenting its antidepressant effects, and reducing serum prolactin levels. This study laid a foundation for improving sulpiride delivery and developing novel antidepressants.

Sex similarities and dopaminergic differences in interval timing.

Rodent behavioral studies have largely focused on male animals, which has limited the generalizability and conclusions of neuroscience research. Working with humans and rodents, we studied sex effects during interval timing that requires participants to estimate an interval of several seconds by making motor responses. Interval timing requires attention to the passage of time and working memory for temporal rules. We found no differences between human females and males in interval timing response times (timing accuracy) or the coefficient of variance of response times (timing precision). Consistent with prior work, we also found no differences between female and male rodents in timing accuracy or precision. In female rodents, there was no difference in interval timing between estrus and diestrus cycle stages. Because dopamine powerfully affects interval timing, we also examined sex differences with drugs targeting dopaminergic receptors. In both female and male rodents, interval timing was delayed after administration of sulpiride (D2-receptor antagonist), quinpirole (D2-receptor agonist), and SCH-23390 (D1-receptor antagonist). By contrast, after administration of SKF-81297 (D1-receptor agonist), interval timing shifted earlier only in male rodents. These data illuminate sex similarities and differences in interval timing. Our results have relevance for rodent models of both cognitive function and brain disease by increasing representation in behavioral neuroscience. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Dopamine D2 Receptor Modulates Exercise Related Effect on Cortical Excitation/Inhibition and Motor Skill Acquisition.

Exercise is known to benefit motor skill learning in health and neurological disease. Evidence from brain stimulation, genotyping, and Parkinson's disease studies converge to suggest that the dopamine D2 receptor, and shifts in the cortical excitation and inhibition (E:I) balance, are prime candidates for the drivers of exercise-enhanced motor learning. However, causal evidence using experimental pharmacological challenge is lacking. We hypothesized that the modulatory effect of the dopamine D2 receptor on exercise-induced changes in the E:I balance would determine the magnitude of motor skill acquisition. To test this, we measured exercise-induced changes in excitation and inhibition using paired-pulse transcranial magnetic stimulation (TMS) in 22 healthy female and male humans, and then had participants learn a novel motor skill-the sequential visual isometric pinch task (SVIPT). We examined the effect of D2 receptor blockade (800 mg sulpiride) on these measures within a randomized, double-blind, placebo-controlled design. Our key result was that motor skill acquisition was driven by an interaction between the D2 receptor and E:I balance. Specifically, poorer skill learning was related to an attenuated shift in the E:I balance in the sulpiride condition, whereas this interaction was not evident in placebo. Our results demonstrate that exercise-primed motor skill acquisition is causally influenced by D2 receptor activity on motor cortical circuits.

Blocking the dopaminergic receptors within the hippocampal dentate gyrus reduced analgesic responses induced by restraint stress in the formalin test.

Previous studies have shown that various receptors, including dopamine receptors, are expressed in the hippocampal dentate gyrus (DG). Besides, indicatively, dopamine receptors play an essential role in the modulation of pain perception. On the other hand, stressful experiences can produce analgesia, termed stress-induced analgesia (SIA). The current study examined the probable role of dopamine receptors within the DG in antinociception induced by restraint stress (RS). Ninety-seven male albino Wistar rats were unilaterally implanted with a cannula in the DG. Animals received intra-DG microinjections of SCH23390 or Sulpiride (0.25, 1, and 4 µg/rat) as D1-and D2-like dopamine receptor antagonists, respectively, five minutes before RS. Ten minutes after the end of the induction of RS for three hours, 50 µl 2.5% formalin was injected subcutaneously into the plantar surface of the hind paw to induce persistent inflammatory pain. Pain scores were evaluated at 5-minute intervals for 60 minutes. These findings showed that; exposure to RS for three hours produced SIA in both phases of the formalin test, while this RS-induced analgesia was attenuated in the early and late phases of the formalin test by intra-DG microinjection of SCH23390 and Sulpiride. The results of the present study suggested that both D1- and D2-like dopamine receptors in the DG have a considerable role in the induced analgesia by RS.

The monoaminergic pathways are involved in the antidepressant-like effect of quercetin.

Quercetin, a plant-derived flavonoid, is an antioxidant and has demonstrated antidepressant and anti-inflammatory activities in several animal models. However, there is scanty information on the underlying mechanisms of its antidepressant property. This present study aimed at assessing the involvement of monoaminergic systems in the antidepressant-like activity of quercetin in experimental animals. Mice received varying doses of quercetin (25, 50 &100 mg/kg daily) and were then subjected to open field test (OPF), despair tests, the reserpine test, and the yohimbine lethality test (YLT). In addition, monoaminergic involvement was investigated by combining quercetin (100 mg/kg) with dopaminergic antagonists (haloperidol and sulpiride), adrenergic blockers (prazosin, propranolol and yohimbine), and serotonergic blockers/inhibitors (metergoline). The results showed that quercetin produced significant anti-immobility effects in the forced swim test (FST) and tail suspension test (TST), suggesting antidepressant activity. In addition, the potentiation of yohimbine lethality by quercetin further indicates its antidepressant-like property. This antidepressant action demonstrated was, however, blocked when quercetin was co-administered with dopaminergic, adrenergic and serotonergic antagonists, suggesting involvement of the monoaminergic system in the antidepressant action of quercetin. Nevertheless, quercetin did not significantly alter the locomotor activity of mice, which implies lack of stimulant effect. Taken together, these outcomes suggest that monoaminergic systems are likely involved in the anti-depressant effect of quercetin in mice.

Regulation of plasma glucose levels by central dopamine D2 receptors is impaired in type 1 but not type 2 diabetic mouse models.

Glucose metabolism is reported to be regulated by the central nervous system, but it is unclear whether this regulation is altered in diabetes. We investigated whether regulation of glucose metabolism by central dopamine D2 receptors is altered in type 1 and type 2 diabetic models. Intracerebroventricular injections of both the dopamine D2 receptor agonist quinpirole and the antagonist l-sulpiride induced hyperglycemia in control mice, but not in streptozotocin (STZ)-induced diabetic mice, a type 1 diabetic model. Hyperglycemia induced by quinpirole or l-sulpiride was diminished following fasting and these drugs did not affect hyperglycemia in the pyruvate tolerance test. In addition, both quinpirole and l-sulpiride increased hepatic glucose-6-phosphatase (G6Pase) mRNA. In STZ-induced diabetic mice, dopamine and dopamine D2 receptor mRNA in the hypothalamus, which regulates glucose homeostasis, were decreased. Hepatic glycogen and G6Pase mRNA were also decreased in STZ-induced diabetic mice. Neither quinpirole nor l-sulpiride increased hepatic G6Pase mRNA in STZ-induced diabetic mice. In diet-induced obesity mice, a type 2 diabetic model, both quinpirole and l-sulpiride induced hyperglycemia, and hypothalamic dopamine and dopamine D2 receptor mRNA were not altered. These results indicate that (i) stimulation or blockade of dopamine D2 receptors causes hyperglycemia by increasing hepatic glycogenolysis, and (ii) stimulation or blockade of dopamine D2 receptors does not affect glucose levels in type 1 but does so in type 2 diabetic models. Moreover, hypothalamic dopaminergic function and hepatic glycogenolysis are decreased in the type 1 diabetic model, which reduces hyperglycemia induced by stimulation or blockade of dopamine D2 receptors.

Persistent extrasynaptic hyperdopaminergia in the mouse hippocampus induces plasticity and recognition memory deficits reversed by the atypical antipsychotic sulpiride.

Evidence suggests that subcortical hyperdopaminergia alters cognitive function in schizophrenia and antipsychotic drugs (APD) fail at rescuing cognitive deficits in patients. In a previous study, we showed that blocking D2 dopamine receptors (D2R), a core action of APD, led to profound reshaping of mesohippocampal fibers, deficits in synaptic transmission and impairments in learning and memory in the mouse hippocampus (HP). However, it is currently unknown how excessive dopamine affects HP-related cognitive functions, and how APD would impact HP functions in such a state. After verifying the presence of DAT-positive neuronal projections in the ventral (temporal), but not in the dorsal (septal), part of the HP, GBR12935, a blocker of dopamine transporter (DAT), was infused in the CA1 of adult C57Bl/6 mice to produce local hyperdopaminergia. Chronic GBR12935 infusion in temporal CA1 induced a mild learning impairment in the Morris Water Maze and abolished long-term recognition memory in novel-object (NORT) and object-place recognition tasks (OPRT). Deficits were accompanied by a significant decrease in DAT+ mesohippocampal fibers. Intrahippocampal or systemic treatment with sulpiride during GBR infusions improved the NORT deficit but not that of OPRT. In vitro application of GBR on hippocampal slices abolished long-term depression (LTD) of fEPSP in temporal CA1. LTD was rescued by co-application with sulpiride. In conclusion, chronic DAT blockade in temporal CA1 profoundly altered mesohippocampal modulation of hippocampal functions. Contrary to previous observations in normodopaminergic mice, antagonising D2Rs was beneficial for cognitive functions in the context of hippocampal hyperdopaminergia.

The dopaminergic D1 receptor modulates the hyperactivity of Bapa mutant mice.

The mutant bate-palmas ("claps"; symbol - bapa) mice induced by the mutagenic chemical ENU present motor incoordination and postural alterations. A previous study showed that bapa mice present increased motor/exploratory behaviors during the prepubertal period due to increased striatal tyrosine hydroxylase expression, suggesting striatal dopaminergic system hyperactivity. This study aimed to evaluate the involvement of striatal dopaminergic receptors in the hyperactivity of bapa mice. Male bapa mice and their wild strain (WT) were used. Spontaneous motor behavior was observed in the open-field test, and stereotypy was evaluated after apomorphine administration. The effects of DR1 and DR2 dopaminergic antagonists (SCH-23,390; sulpiride) and the striatal DR1 and D2 receptor gene expression were evaluated. Relative to WT, bapa mice showed: 1) increased general activity for four days; 2) increased rearing and sniffing behavior and decreased immobility after apomorphine; 3) blockage of rearing behavior after the DR2 antagonist but no effect after DR1 antagonist; 4) blockage of sniffing behavior after the DR1 antagonist in bapa and WT mice but no effect after the DR2 antagonist; 5) increased immobility after the DR1 antagonist but no effect after the DR2 antagonist; 6) increased expression of striatal DR1 receptor gene and reduced the DR2 expression gene after apomorphine administration. Bapa mice showed increased activity in open field behavior. The increased rearing behavior induced by apomorphine of bapa mice resulted from the increased gene expression of the DR1 receptor.

Methylphenidate undermines or enhances divergent creativity depending on baseline dopamine synthesis capacity.

Catecholamine-enhancing psychostimulants, such as methylphenidate have long been argued to undermine creative thinking. However, prior evidence for this is weak or contradictory, stemming from studies with small sample sizes that do not consider the well-established large variability in psychostimulant effects across different individuals and task demands. We aimed to definitively establish the link between psychostimulants and creative thinking by measuring effects of methylphenidate in 90 healthy participants on distinct creative tasks that measure convergent and divergent thinking, as a function of individuals' baseline dopamine synthesis capacity, indexed with 18F-FDOPA PET imaging. In a double-blind, within-subject design, participants were administered methylphenidate, placebo or selective D2 receptor antagonist sulpiride. The results showed that striatal dopamine synthesis capacity and/or methylphenidate administration did not affect divergent and convergent thinking. However, exploratory analysis demonstrated a baseline dopamine-dependent effect of methylphenidate on a measure of response divergence, a creativity measure that measures response variability. Response divergence was reduced by methylphenidate in participants with low dopamine synthesis capacity but enhanced in those with high dopamine synthesis capacity. No evidence of any effect of sulpiride was found. These results show that methylphenidate can undermine certain forms of divergent creativity but only in individuals with low baseline dopamine levels.

Dopamine receptor-mediated roles on retinal ganglion cell hyperexcitability and injury in experimental glaucoma.

Extraordinary excitability (hyperexcitability) is closely related to retinal ganglion cell (RGC) injury in glaucoma. Dopamine (DA) and its receptors are involved in modulating RGC excitability. We investigated how DA system affects RGC injury in chronic ocular hypertension (COH) experimental glaucoma model. Western blotting and immunohistochemistry results revealed that expression of DA D2-like receptor (D2R) in RGCs was increased in COH retinas. Patch-clamp recordings showed that outward K+ currents were downregulated, while Na+ currents and NaV1.6 expression were upregulated in RGCs of COH retinas, which could be reversed by intravitreal pre-injection of the D2R antagonist sulpiride, but not by the D1-like receptor (D1R) antagonist SCH23390. However, pre-injection of the D1R agonist SKF81297 could partially reverse the increased expression of NaV1.6 proteins. Consistently, the numbers of evoked action potentials induced by current injections were increased in RGCs of COH retinas, indicating that RGCs may be in a condition of hyperexcitability. The increased frequency of evoked action potentials could be partially block by pre-injection of sulpiride, SKF81297 or DA, respectively. Furthermore, the increased number of TUNEL-positive RGCs in COH retinas could be partially reduced by intravitreal pre-injection of sulpiride, but not by pre-injection of SCH23390. Moreover, pre-injection of SKF81297 or DA could reduce the number of TUNEL-positive RGCs in COH retinas. All these results indicate that in COH retina, activation of D2R enhances RGC hyperexcitability and injury, while activation of D1R results in the opposite effects. Selective inhibition of D2R or activation of D1R may be an effective strategy for treatment of glaucoma.

D2 receptor blockade eliminates exercise-induced changes in cortical inhibition and excitation.

BACKGROUND: Although cardiorespiratory exercise is known to affect cortical excitatory and inhibitory activity, the neurochemical mechanisms driving this effect are poorly understood. Animal models of Parkinson's disease identify dopamine D2 receptor expression as a candidate mechanism, but the link between the D2 receptor and exercise-induced changes in cortical activity in humans is unknown. OBJECTIVE: Here, we examined the effect of a selective dopamine D2 receptor antagonist, sulpiride, on exercise-induced changes in cortical activity. METHODS: We acquired measures of excitatory and inhibitory activity of the primary motor cortex using transcranial magnetic stimulation (TMS) from 23 healthy adults, both before and after a 20-min bout of high-intensity interval cycling exercise. We examined the effect of D2 receptor blockade (800 mg sulpiride) on these measures within a randomised, double-blind, placebo-controlled crossover design. RESULTS: Sulpiride abolished exercise-induced modulation of the cortical excitation:inhibition balance relative to placebo (P < 0.001, Cohen's d = 1.76). Sulpiride blocked both the increase in glutamatergic excitation and reduction in gamma-aminobutyric acid (GABA) inhibition that was observed following exercise in the placebo condition. CONCLUSION: Our results provide causal evidence that D2 receptor blockade eliminates exercise-induced changes in excitatory and inhibitory cortical networks, and have implications for how exercise should be prescribed in diseases of dopaminergic dysfunction.

Effect of risperidone on morphine-induced conditioned place preference and dopamine receptor D2 gene expression in male rat hippocampus.

BACKGROUND: Previous studies suggest the possible effect of risperidone on brain reward system and D1 and D2 dopamine receptors' involvement in morphine-induced conditioned place preference (CPP). AIMS: The present study was designed to investigate the effect of risperidone as an atypical antipsychotic drug on morphine-induced CPP and D2-like dopamine receptor gene expression in rat. MATERIALS AND METHODS: An unbiased CPP paradigm was used to study the effect of risperidone. Intraperitoneal (i.p.) injection of risperidone (1, 2, and 4 mg/kg) was performed 30 min before the morphine (10 mg/kg, i.p.) injection and just after the rat was placed in the CPP box. The open field test was used to assay the locomotor activity of animal. The gene expression of D2 dopamine receptor in hippocampus was measured by real-time PCR technique. The hippocampi of rats were also used for histology evaluation. RESULTS: Morphine-produced (10 mg/kg) CPP and morphine-induced CPP were reversed only by the administration of a low dose of risperidone (1 mg/kg). Low dose of risperidone (1 mg/kg) showed no effect on locomotor activity but a higher dose of risperidone (2 and 4 mg/kg) decreased locomotor activity. Real-time PCR data analysis revealed that the gene expression of D2 dopamine receptor had significant difference between morphine and a 1 mg/kg dose of risperidone. Moreover, in histological evaluation, apoptosis was observed in the morphine group, whereas there was no evidence of apoptosis in the risperidone-treated groups. CONCLUSION: Our results suggest that risperidone (1 mg/kg) reverses the morphine-induced CPP and may reduce the rewarding properties of morphine. It is also demonstrated that risperidone decreases the expression of D2 receptor in rat hippocampus. Therefore, risperidone can be considered potential adjunct therapy in morphine dependence.

Modulatory role of intra-accumbal dopamine receptors in the restraint stress-induced antinociceptive responses.

Stress contributes to pain sensation by affecting several neural pathways, including mesolimbic-cortical dopamine neurons. Nucleus accumbens, an essential element of the mesolimbic dopaminergic pathway, plays a fundamental role in modulating pain and is differentially influenced by stressful events. Since we previously demonstrated the marked association of intra-NAc dopamine receptors with forced swim stress-evoked analgesia in acute pain state, this research was conducted to consider the contribution of intra-accumbal D1- and D2-like dopamine receptors to modulating effects of exposure to restraint stress in pain-related behaviors during the tail-flick test. Stereotaxic surgery was executed to implant a guide cannula within the NAc in male Wistar rats. On the test day, different concentrations of SCH23390 and Sulpiride as D1- and D2-like dopamine receptor antagonists, respectively, were unilaterally microinjected within the NAc. The vehicle animals received saline or 12 % DMSO (0.5 µl) instead of SCH23390 or Sulpiride into the NAc, respectively. Five minutes following receiving drug or vehicle, animals were restrained for 3 h and then their acute nociceptive threshold was measured for a 60-min period by the tail-flick test. Our data revealed that RS considerably enhanced antinociceptive reaction in acute pain states. The analgesia evoked by RS dramatically declined following blocking either D1- or D2-like dopamine receptors in the NAc, an effect was more noticeable by D1-like dopamine receptor antagonist. These findings indicated that intra-NAc dopamine receptors are considerably mediated in the RS-produced analgesia in acute pain states, suggesting their possible role in psychological stress and disease.

Involvement of dopaminergic system in the dentate gyrus of the hippocampus in modulating the orofacial pain-related behaviors in the rats.

Chemical stimulation of the lateral hypothalamus (LH) induces analgesia by forming neural circuitries with multiple brain regions. The involvement of hippocampal dopaminergic receptors in the LH stimulation-induced antinociception in specific pain models in animals has been documented. However, because the neural circuitries involved in the mediation of orofacial pain are not the same as those that mediate the other types of pain, the present study aims to detect the role of dopamine receptors within the dentate gyrus (DG) in the antinociceptive responses induced by LH stimulation in an animal model of orofacial pain. Male Wistar rats (220-250 g) were implanted with two separate cannulae into the LH and DG on the same side. D1- or D2-like dopamine receptor antagonist, SCH23390, or sulpiride (0.25, 1, and 4 µg) were microinjected into the DG, five minutes before intra-LH injection of carbachol (250 nM). The animals were then injected with formalin 1% (50 µL; sc) into the upper lip lateral to the nose and subjected to the orofacial formalin test. Intra-DG administration of SCH23390 or sulpiride attenuated the antinociceptive responses induced by intra-LH microinjection of carbachol during the orofacial formalin test. The findings of the current study suggest that chemical stimulation of the LH modulates orofacial pain, possibly through activation of the DG dopaminergic neurons. Due to the high incidence and prevalence of orofacial pain in the general population, understanding how such neuronal circuitry modulates nociceptive processing will advance the search for novel therapeutics.