The pharmacology of sulpiride--a dopamine receptor antagonist

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.

Further Quinolizidine Derivatives as Antiarrhythmic Agents- 3

Fourteen quinolizidine derivatives, structurally related to the alkaloids lupinine and cytisine and previously studied for other pharmacological purposes, were presently tested for antiarrhythmic, and other cardiovascular effects on isolated guinea pig heart tissues in comparison to well-established reference drugs. According to their structures, the tested compounds are assembled into three subsets: (a) N-(quinolizidinyl-alkyl)-benzamides; (b) 2-(benzotriazol-2-yl)methyl-1-(quinolizidinyl)alkyl-benzimidazoles; (c) N-substituted cytisines. All compounds but two displayed antiarrhythmic activity that was potent for compounds 4, 1, 6, and 5 (in ascending order). The last compound (N-(3,4,5-trimethoxybenzoyl)aminohomolupinane) was outstanding, exhibiting a nanomolar potency (EC50 = 0.017 µM) for the increase in the threshold of ac-arrhythmia. The tested compounds shared strong negative inotropic activity; however, this does not compromise the value of their antiarrhythmic action. On the other hand, only moderate or modest negative chronotropic and vasorelaxant activities were commonly observed. Compound 5, which has high antiarrhythmic potency, a favorable cardiovascular profile, and is devoid of antihypertensive activity in spontaneously hypertensive rats, represents a lead worthy of further investigation.

Levosulpiride for Premature Ejaculation: A Systematic Review and Meta-Analysis

Premature ejaculation (PE) is one of the major causes of sexual dysfunction. Levosulpiride is an off-label medicine used to treat PE, but no review on its efficacy exists. A systematic review and meta-analysis was performed to determine the efficacy of levosulpiride in treating PE. Databases PubMed, Science Direct, and Google Scholar were searched. Randomized control trials (RCTs) comparing levosulpiride with placebo or other medicine were selected. Odds ratio (OR) of improved intravaginal ejaculation latency time (IELT) was calculated. A total of 97 articles were retrieved from database search, of which only four RCTs containing 203 men met the selection criteria. All four RCTs were included in systematic review while only two were included in meta-analysis. A high selection and detection bias was found in both of these studies. Meta-analysis also showed the odds of improving IELT in PE patients using levosulpiride to be significantly higher (p < .05) compared with those who used placebo, OR: 100.81, 95% confidence interval (CI) [13.12-774.90], I² = 0%. Odds of improving IELT for > 5 min (500% improvement) were also significantly higher (p < .05) compared with the placebo groups (OR: 38.88, 95% CI [5.12-295.29], I² = 0%). The odds of improving IELT for > 1 min, but < 5 min were also significantly higher (p < .05) than placebo groups (OR: 32.84, 95% CI [4.15-259.75], I² = 0%). Levosulpiride improved IELT, but even so, limited studies are available on this topic. Additional research is thus required to support the present review's findings.

Activation of dopamine D2 receptor promotes pepsinogen secretion by suppressing somatostatin release from the mouse gastric mucosa

In vivo administration of dopamine (DA) receptor (DR)-related drugs modulate gastric pepsinogen secretion. However, DRs on gastric pepsinogen-secreting chief cells and DA D2 receptor (D2R) on somatostatin-secreting D cells were subsequently acquired. In this study, we aimed to further investigate the local effect of DA on gastric pepsinogen secretion through DRs expressed on chief cells or potential D2Rs expressed on D cells. To elucidate the modulation of DRs in gastric pepsinogen secretion, immunofluorescence staining, ex vivo incubation of gastric mucosa isolated from normal and D2R-/- mice were conducted, accompanied by measurements of pepsinogen or somatostatin levels using biochemical assays or enzyme-linked immunosorbent assays. D1R, D2R, and D5R-immunoreactivity (IR) were observed on chief cells in mouse gastric mucosa. D2R-IR was widely distributed on D cells from the corpus to the antrum. Ex vivo incubation results showed that DA and the D1-like receptor agonist SKF38393 increased pepsinogen secretion, which was blocked by the D1-like receptor antagonist SCH23390. However, D2-like receptor agonist quinpirole also significantly increased pepsinogen secretion, and D2-like receptor antagonist sulpiride blocked the promotion of DA. Besides, D2-like receptors exerted an inhibitory effect on somatostatin secretion, in contrast to their effect on pepsinogen secretion. Furthermore, D2R-/- mice showed much lower basal pepsinogen secretion but significantly increased somatostatin release and an increased number of D cells in gastric mucosa. Only SKF38393, not quinpirole, increased pepsinogen secretion in D2R-/- mice. DA promotes gastric pepsinogen secretion directly through D1-like receptors on chief cells and indirectly through D2R-mediated suppression of somatostatin release.

Preclinical PK/PD model for the combinatorial use of dexamethasone and sulpiride in the treatment of breast cancer

Previous studies show that dopamine D₂-like receptor (D2DR) antagonist sulpiride (SUL) enhances the antitumor efficacy of dexamethasone (DEX) in drug-resistant breast cancer involving cancer stem-like cells (CSCs). In this study, we investigated the pharmacokinetic (PK) properties of SUL in nude mice and developed a semi-mechanism PK/PD model to quantitatively characterize the synergistic effect of DEX and SUL in preclinical breast cancer xenografts. After nude mice received oral administration of a single dose of SUL (50 mg/kg, ig), plasma concentrations were assessed using LC-MS/MS. A two-compartment model with double first-order absorption rate was developed to describe the PK profiles of SUL. The pharmacodynamic (PD) study was conducted in nude mice bearing human breast cancer MCF-7/Adr xenografts, which received oral administration of DEX (1, 8 mg·kg⁻¹·d⁻¹) or SUL (25, 50 mg·kg⁻¹·d⁻¹) alone or in various combination. Tumor volumes were measured every other day. The PK model of SUL as well as that of DEX with a time-dependent clearance were integrated into the final PK/PD model both using Hill’s function, where DEX exerted its antitumor efficacy by inhibiting the proliferation of tumor cells, and SUL enhanced DEX responses by decreasing the sensitivity parameter EC₅₀. The PK/PD model was evaluated and subjected external validation. Finally, simulations were performed to predict the antitumor efficacy of DEX combined with SUL under various dose regimens, where changing dosing frequency of SUL had little effect, while the antitumor efficacy was predicted to be improved when DEX was given more frequently. The established PK/PD model in this study quantitatively characterizes the antitumor efficacy of the DEX combined with SUL as well as their synergism, and the simulations could provide reference for dose optimization of the combination in future studies.

Female Reproductive Behavior

Reproductive behavior is the behavior related to the production of offspring and includes all aspects from the establishment of mating systems, courtship, sexual behavior, and parturition to the care of young. In this chapter, I outline the hormonal regulation of the estrous cycle, followed by a description of the neural regulation of female sexual behavior. Ovarian hormones play an important role in the induction of ovulation and behavioral estrus, in which they interact closely with several neurotransmitters and neuropeptides to induce sexual behavior. This chapter discusses the latest research on the role of estrogen, progesterone, serotonin, dopamine, noradrenaline, oxytocin, and GABA in female mating behavior. In addition, the most relevant brain areas, such as the preoptic area and the ventromedial nucleus of the hypothalamus, in which these regulations take place, are discussed.

Event-related potentials reflect impaired temporal interval learning following haloperidol administration

BACKGROUND

Signals carried by the mesencephalic dopamine system and conveyed to anterior cingulate cortex are critically implicated in probabilistic reward learning and performance monitoring. A common evaluative mechanism purportedly subserves both functions, giving rise to homologous medial frontal negativities in feedback- and response-locked event-related brain potentials (the feedback-related negativity (FRN) and the error-related negativity (ERN), respectively), reflecting dopamine-dependent prediction error signals to unexpectedly negative events. Consistent with this model, the dopamine receptor antagonist, haloperidol, attenuates the ERN, but effects on FRN have not yet been evaluated.

METHODS

ERN and FRN were recorded during a temporal interval learning task (TILT) following randomized, double-blind administration of haloperidol (3 mg; n = 18), diphenhydramine (an active control for haloperidol; 25 mg; n = 20), or placebo (n = 21) to healthy controls. Centroparietal positivities, the Pe and feedback-locked P300, were also measured and correlations between ERP measures and behavioral indices of learning, overall accuracy, and post-error compensatory behavior were evaluated. We hypothesized that haloperidol would reduce ERN and FRN, but that ERN would uniquely track automatic, error-related performance adjustments, while FRN would be associated with learning and overall accuracy.

RESULTS

As predicted, ERN was reduced by haloperidol and in those exhibiting less adaptive post-error performance; however, these effects were limited to ERNs following fast timing errors. In contrast, the FRN was not affected by drug condition, although increased FRN amplitude was associated with improved accuracy. Significant drug effects on centroparietal positivities were also absent.

CONCLUSIONS

Our results support a functional and neurobiological dissociation between the ERN and FRN.

Risperidone-induced priapism in an autistic child: a case report

Background

Priapism is a prolonged stimulation with painful, persistent penile erection unaccompanied by sexual desire. It is a rare but serious urological emergency. Risperidone is an atypical antipsychotic widely prescribed for the treatment of behavior problems in children with autism spectrum disorder. It seems associated with priapism in children.

Case presentation

We present a case of a 12-year-old Moroccan boy diagnosed with autism spectrum disorder who developed priapism while on an existing regimen of risperidone, and we report the treatment decisions that followed.

Conclusions

Clinicians who prescribe risperidone should be aware of the possibility of this rare complication in their patients. Information about this possible side effect and instructions regarding appropriate response should be made available to caregivers of those in the at-risk group of young patients.

The Role of Dopamine in Temporal Uncertainty

The temporal preparation of motor responses to external events (temporal preparation) relies on internal representations of the accumulated elapsed time (temporal representations) before an event occurs and on estimates about its most likely time of occurrence (temporal expectations). The precision (inverse of uncertainty) of temporal preparation, however, is limited by two sources of uncertainty. One is intrinsic to the nervous system and scales with the length of elapsed time such that temporal representations are least precise for longest time durations. The other is external and arises from temporal variability of events in the outside world. The precision of temporal expectations thus decreases if events become more variable in time. It has long been recognized that the processing of time durations within the range of hundreds of milliseconds (interval timing) strongly depends on dopaminergic (DA) transmission. The role of DA for the precision of temporal preparation in humans, however, remains unclear. This study therefore directly assesses the role of DA in the precision of temporal preparation of motor responses in healthy humans. In a placebo-controlled double-blind design using a selective D2-receptor antagonist (sulpiride) and D1/D2 receptor antagonist (haloperidol), participants performed a variable foreperiod reaching task, under different conditions of internal and external temporal uncertainty. DA blockade produced a striking impairment in the ability of extracting temporal expectations across trials and on the precision of temporal representations within a trial. Large Weber fractions for interval timing, estimated by fitting subjective hazard functions, confirmed that this effect was driven by an increased uncertainty in the way participants were experiencing time. This provides novel evidence that DA regulates the precision with which we process time when preparing for an action.

Pharmacokinetics of levosulpiride after single and multiple intramuscular administrations in healthy Chinese volunteers

The main purpose of this study was to evaluate the pharmacokinetics of levosulpiride in humans after single and multiple intramuscular injections. Six males and six females received single dose of either 25 mg or 50 mg levosulpiride, or multiple doses of 25 mg every 12 h for 5 consecutive days. In the single 25 mg study, the mean peak plasma concentration (C_max) was 441 ng/mL, the mean area under the concentration–time curve from 0 to 36 h (AUC_0–36) was 1724 ng h/mL, and the mean elimination half-life (t_1/2) was 7.0 h. In the single 50 mg study, the mean C_max was 823 ng/mL, the mean AUC_0–36 was 3748 ng·h/mL, and the mean t_1/2 was 6.8 h. After multiple doses of 25 mg levosulpiride, the average plasma concentration (C_av) was 136 ng/mL, the fluctuation index (DF) was 3.60, and the accumulation ratio (R) was 1.2. Levosulpiride injections appeared to be well tolerated by the subjects, and can be used for successive administration.

The Role of Dopamine in Motor Flexibility

Humans carry out many daily tasks in a seemingly automatic fashion. However, when unexpected changes in the environment occur, we have the capacity to inhibit prepotent behavior and replace it with an alternative one. Such behavioral flexibility is a hallmark of executive functions. The neurotransmitter dopamine is known to be crucial for fast, efficient, and accurate cognitive flexibility. Despite the perceived similarities between cognitive and motor flexibility, less is known regarding the role of dopamine within the motor domain. Therefore, the aim of this study was to determine the role of dopamine in motor flexibility. In a double-blind, five-session, within-subject pharmacological experiment, human participants performed an RT task within a probabilistic context that was either predictable or unpredictable. The probabilistic nature of the predictable context resulted in prediction errors. This required participants to replace the prepotent or prepared action with an unprepared action (motor flexibility). The task was overlearned, and changes in context were explicitly instructed, thus controlling for contributions from other dopamine-related processes such as probabilistic or reversal learning and interactions with other types of uncertainty. We found that dopamine receptor blockade by high-dose haloperidol (D1/D2 dopamine receptors) impaired participants' ability to react to unexpected events occurring in a predictable context, which elicit large prediction errors and necessitate motor flexibility. This effect was not observed with selective D2 receptor blockade (sulpiride), with a general increase in tonic dopamine levels (levodopa), or during an unpredictable context, which evoked minimal prediction error. We propose that dopamine is vital in responding to low-level prediction errors about stimulus outcome that requires motor flexibility.

Dopamine and pain sensitivity: neither sulpiride nor acute phenylalanine and tyrosine depletion have effects on thermal pain sensations in healthy volunteers

Based on animal studies and some indirect clinical evidence, dopamine has been suggested to have anti-nociceptive effects. Here, we investigated directly the effects of increased and decreased availability of extracellular dopamine on pain perception in healthy volunteers. In Study 1, participants ingested, in separate sessions, a placebo and a low dose of the centrally acting D2-receptor antagonist sulpiride, intended to increase synaptic dopamine via predominant pre-synaptic blockade. No effects were seen on thermal pain thresholds, tolerance, or temporal summation. Study 2 used the acute phenylalanine and tyrosine depletion (APTD) method to transiently decrease dopamine availability. In one session participants ingested a mixture that depletes the dopamine amino acid precursors, phenylalanine and tyrosine. In the other session they ingested a nutritionally balanced control mixture. APTD led to a small mood-lowering response following aversive thermal stimulation, but had no effects on the perception of cold, warm, or pain stimuli. In both studies the experimental manipulation of dopaminergic neurotransmission was successful as indicated by manipulation checks. The results contradict proposals that dopamine has direct anti-nociceptive effects in acute experimental pain. Based on dopamine's well-known role in reward processing, we hypothesize that also in the context of pain, dopamine acts on stimulus salience and might play a role in the initiation of avoidance behavior rather than having direct antinociceptive effects in acute experimental pain.

Effects of amisulpride on human resting cerebral perfusion

RATIONALE

Quantitative neuroimaging studies show that different neuroleptics have similar effects on resting metabolism/perfusion in the basal ganglia, but vary in their effect on the cortex, especially in the prefrontal and temporal lobes. These differences may represent signatures of the action of medication on distinctive receptor combinations.

OBJECTIVES

This study seeks to determine the effect on cerebral perfusion at rest of low-dose amisulpride, a neuroleptic with a receptor profile relatively selective to dopaminergic D2-receptors and both antidepressant and antipsychotic efficacy.

METHODS

Continuous arterial spin labelling in a placebo-controlled, double blind, crossover study at steady state of N = 20 healthy male adults.

RESULTS

Relative to placebo, amisulpride was associated with extensive and significant cortical decrements in resting perfusion levels, particularly in the prefrontal lobes (p = 0.01, corrected). Decrements spared the basal ganglia, where perfusion was slightly increased.

CONCLUSIONS

In contrast to earlier reports on other neuroleptics, amisulpride was associated with intense cortical perfusion decrements at rest. These results are consistent with an existing model in which dopaminergic blockade is associated not only with metabolism/perfusion increases in the basal ganglia, but also with decreases in the cerebral cortex that in most neuroleptics are compensated by action on other receptor systems. The selective receptor profile of amisulpride may explain the extensive cortical decrements.

Role of dopamine D(2) receptors for antipsychotic activity

This review summarizes the current state of knowledge regarding the proposed mechanisms by which antipsychotic agents reduce the symptoms of schizophrenia while giving rise to adverse side effects. The first part summarizes the contribution of neuroimaging studies to our understanding of the neurochemical substrates of schizophrenia, putting emphasis on direct evidence suggestive of a presynaptic rather than a postsynaptic dysregulation of dopaminergic neurotransmission in this disorder. The second part addresses the role of D(2) and non-D(2) receptor blockade in the treatment of schizophrenia and highlights a preponderant role of D(2) receptors in the mechanism of antipsychotic action. Neuroimaging studies have defined a narrow, but optimal, therapeutic window of 65-78 % D(2) receptor blockade within which most antipsychotics achieve optimal clinical efficacy with minimal side effects. Some antipsychotics though do not conform to that therapeutic window, notably clozapine. The reasons for its unexcelled clinical efficacy despite subthreshold levels of D(2) blockade are unclear and current theories on clozapine's mechanisms of action are discussed, including transiency of its D(2) receptor blocking effects or preferential blockade of limbic D(2) receptors. Evidence is also highlighted to consider the use of extended antipsychotic dosing to achieve transiency of D(2) blockade as a way to optimize functional outcomes in patients. We also present some critical clinical considerations regarding the mechanisms linking dopamine disturbance to the expression of psychosis and its blockade to the progressive resolution of psychosis, keeping in perspective the speed and onset of antipsychotic action. Finally, we discuss potential novel therapeutic strategies for schizophrenia.

Induction of reproductive function in anestrous mares using a dopamine antagonist

We investigated the role of dopamine in the regulation of seasonal reproductive activity in mares. Nine seasonal anestrous mares, maintained under a natural photoperiod, were treated daily with a dopamine D2 antagonist, [-]-sulpiride (200 mg/mare, im), beginning February 5 (day of year = 36) until the first ovulation of the year or for a maximum of 58. Nine untreated anestrous mares were maintained under the same conditions. The ovaries were examined by ultrasonography twice a week, and blood was collected three times a week for progesterone, LH, FSH and prolactin determinations. Mean day of first ovulation was significantly advanced for [-]-sulpiride-treated mares than control mares (mean day of year +/- SEM = 77.3 +/- 7.9 and 110.0 +/- 6.8, respectively; P < 0.01). Eight mares ovulated during [-]-sulpiride treatment while one mare failed to ovulate. Ovulation occurred 91 d after the start of treatment or on Day 127. All mares continued to have normal estrous cycles after the first ovulation. First cycle length and luteal progesterone concentrations did not differ between [-]-sulpiride-treated and control mares. Plasma prolactin concentrations were significantly increased at 2 and 9 h after [-]-sulpiride administration (P < 0.05), and had returned to basal levels by 24 h. At the time of the LH surge associated with the first ovulation, mean LH and FSH secretion was significantly higher in [-]-sulpiride-treated mares than in control mares (P < 0.05). These results suggest that dopamine plays a role in the control of reproductive seasonality in mares and exerts a tonic inhibition on reproductive activity during the anovulatory season.

Dopamine receptors in equine ovarian tissues

Dopamine (DA) agonist and antagonist treatments can affect ovarian reproductive events in the mare. To support our theory that DA produces these effects by acting directly on the ovary, we analyzed equine ovarian tissues for the presence of dopamine receptor-1 (D1r) and dopamine receptor-2 (D2r) mRNA by reverse transcription polymerase chain reaction (RT-PCR) and D1r and D2r proteins by Western blot and immunohistochemistry (IHC). RT-PCR was performed on RNA isolated from ovarian cortex, medulla, granulosa/theca or corpus luteum (CL) tissues and from pituitary (D2r control) and renal artery (D1r control). D1r and D2r specific primers were designed from partial DNA sequences known for the horse (D2r) or conserved sequences from other species (D1r). Western blot analyses were conducted on CL, cortex and granulosa/theca samples and IHC was performed on CL tissues using D1r or D2r specific antibodies. The incidence of positive D2r mRNA was high in CL and ovarian cortex, low in granulosa/theca, and not detectable in ovarian medulla. Dopamine D1r mRNA incidence was high (50%) only in CL tissues. D1r and D2r antibody staining was positive for each tissue type analyzed by Western blot procedures. All CL tissues prepared by IHC showed positive staining for D1r and D2r proteins. Both DA receptor proteins appeared uniformly distributed throughout the CL tissue. These results indicate that equine ovarian tissues do possess D1r and D2r, and suggests that DA can act directly on ovarian tissues through its interaction with DA receptors.

Progestins' actions in the VTA to facilitate lordosis involve dopamine-like type 1 and 2 receptors

In the ventral tegmental area (VTA), 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP) facilitates lordosis. Whether this involves dopamine type 1 (D1) or dopamine type 2 (D2) receptors is of interest. Ovariectomized (ovx) rats with guide cannulae to the VTA were estradiol (E2) primed and pretested for lordosis. Rats were then infused with the D1 (Experiment 1) or D2 (Experiment 2) antagonists or agonists (0, 100, or 200 ng) to the VTA and were retested. After a second infusion of 3alpha,5alpha-THP (0, 100, or 200 ng) or vehicle, rats were tested 10, 60, and 120 min later. In Experiment 3, rats were administered a progestin receptor antagonist, RU38486, systemically or to the VTA 1 h prior to vehicle, SKF38393 and/or 3alpha,5alpha-THP infusions. 3alpha,5alpha-THP infusions increased lordosis over that seen with E2 priming. The D1 antagonist, SCH23390, attenuated 3alpha,5alpha-THP, but not E2-facilitated lordosis. The D1 agonist, SKF38393, augmented 3alpha,5alpha-THP, but not E2-facilitated lordosis. The D2 antagonist, sulpiride, had no significant effects on lordosis. The D2 agonist, quinpirole, prevented 3alpha,5alpha-THP-facilitated lordosis. RU38486 (subcutaneous) inhibited lordosis, whereas infusions to the VTA decreased lordosis produced by SKF38393 and 3alpha,5alpha-THP, but not 3alpha,5alpha-THP alone. Thus, 3alpha,5alpha-THP's actions in the VTA for lordosis may involve D1 and/or D2 receptors.

Stereocontrolled dopamine receptor binding and subtype selectivity of clebopride analogues synthesized from aspartic acid

Employing the achiral 4-aminopiperidine derivative clebopride as a lead compound, chiral analogues were developed displaying dopamine receptor binding profiles that proved to be strongly dependent on the stereochemistry. Compared to the D1 receptor, the test compounds showed high selectivity for the D2-like subtypes including D2(long), D2(short), D3 and D4. The highest D4 and D3 affinities were observed for the cis-3-amino-4-methylpyrrolidines 3e and the enantiomer ent3e resulting in K(i) values of 0.23 and 1.8 nM, respectively. The benzamides of type 3 and 5 were synthesized in enantiopure form starting from (S)-aspartic acid and its unnatural optical antipode.

SPPS of protected peptidyl aminoalkyl amides

Monophthaloyl diamines derived from naturally occurring amino acids were attached through their free amino functions to resins of the trityl type. The phthaloyl groups were removed by hydrazinolysis, and peptide chains were assembled using Fmoc/tBu-amino acids on the liberated amino functions. The peptidyl aminoalkyl amides obtained were cleaved from the resins by mild acidolysis, with the tBu-side chain protection remaining intact.

The differential effects of atypical antipsychotics on prolactin elevation are explained by their differential blood-brain disposition: a pharmacological analysis in rats

All atypical antipsychotics avoid extrapyramidal side-effects yet differ in their propensity to cause other side-effects, like prolactin elevation. We proposed that the atypical antipsychotics with a propensity for prolactin elevation would show a higher pituitary versus striatal D2 receptor occupancy. To investigate this hypothesis, we tested four atypical antipsychotics, two that are commonly associated with prolactin elevation (amisulpride and risperidone) and two that are less frequently associated (quetiapine and olanzapine). In particular, we calculated their ED(50) values to increase plasma prolactin and block peripheral pituitary D2 receptors to their ED(50) values to antagonize apomorphine-induced stereotypy and occupy central striatal D2 receptors. All antipsychotics dose dependently increased prolactin levels and antagonized apomorphine-induced stereotypy. However, the central to peripheral potency (ED(50) for apomorphine antagonism to ED(50) for prolactin elevation) differed remarkably across these drugs: amisulpride (21764), risperidone (14), quetiapine (10), and olanzapine (1.7). Compounds displaying a higher peripheral potency brought about higher prolactin levels for a given level of functional central antagonism. This dissociation between central and peripheral effects was explained by the differential occupancy of D2 receptors in the striatum versus in the pituitary [ratio of striatal/pituitary ED(50) values (milligram per kilogram) for D2 occupancy): amisulpride (17/0.026 = 654), risperidone (0.89/0.081 = 14), quetiapine (24/4.1 = 6), olanzapine (0.30/0.43 = 0.7). These results indicate that dissociation between central and peripheral D2 receptor occupancy is a major determinant of the degree of prolactin elevation observed at therapeutic doses.

Role of Dopaminergic D2 Receptors in the Regulation of Glutamic Acid Decarboxylase Messenger RNA in the Striatum of the Rat

Levels of messenger RNA (mRNA) encoding glutamic acid decarboxylase (GAD) and preproenkephalin (PPE) were measured by Northern blot and in situ hybridization analyses in the striatum of the rat, after chronic injections of two neuroleptics, sulpiride and haloperidol. The Northern blot analysis showed that the chronic injection of sulpiride at high doses (80 mg/kg, twice a day, 14 days) increased striatal GAD and PPE mRNA levels by 120% and 78% respectively, when compared to vehicle-injected rats. Haloperidol injections at relatively low doses (1 mg/kg, once a day, 14 days) produced parallel increases in GAD (40%) and PPE (52%) mRNA levels. After in situ hybridization densitometric measurements were performed on autoradiograms from rats treated with sulpiride, haloperidol or vehicle. The distribution of GAD and PPE mRNA signals in control rats was homogeneous along the rostrocaudal extension of the striatum. A similar increase was found along this axis after sulpiride (20%) and haloperidol (30%) treatments. The cellular observation of hybridization signals showed that grain density for GAD mRNA was increased in a majority of striatal cells after both treatments. By contrast, the PPE mRNA hybridization signal only increased in a subpopulation of neurons. The effects of such treatments were also analysed by measuring GAD activity in the striatum and in its output structures, the globus pallidus and the substantia nigra. After the administration of sulpiride, GAD activity was not modified in the striatum but increased in the globus pallidus (by 17%). After haloperidol treatment, GAD activity was increased in the globus pallidus (20%) and the substantia nigra (17%). It is concluded that the interruption of dopaminergic transmission, more precisely the D2 receptor blockade, promotes in striatopallidal neurons an increase in GAD mRNA accompanied by an increase in GAD activity and PPE mRNA. A possible regulation of GAD mRNA and GAD activity in striatonigral neurons is also discussed.

Dopamine D2-like receptors and amino acid-induced glomerular hyperfiltration in humans

AIMS

In rodents, blockade of dopamine D2-like receptors abolishes both the physiological increase in glomerular filtration rate (GFR) induced by amino acids and the pathological hyperfiltration in experimental diabetes mellitus. This study addressed the contribution of dopamine D2-like receptors to changes in renal haemodynamics after amino acid infusion in humans.

METHODS

Twelve healthy volunteers participated in this double-blind, randomized, cross-over study. GFR and renal blood flow (RPF) were assessed by renal clearance of inulin and p-aminohippuric acid (PAH), respectively. Following infusion of 0.45% saline at baseline, an electrolyte-balanced solution of mixed amino acids (10%) was infused. Prior to the experiments, the subjects received orally either placebo, or sulpiride (10 mg kg-1), a centrally and peripherally acting D2-like receptor antagonist, or domperidone (1 mg kg-1) which affects only peripheral D2-like receptors.

RESULTS

In the placebo series, amino acid infusion significantly increased GFR and RPF by up to 15.8 +/- 5.3% and 14.4 +/- 6.1%, respectively, while mean blood pressure and heart rate remained unchanged. Pretreatment with domperidone only marginally altered the renal response to amino acids (maximal increase by 13.2 +/- 5.6 and 11.9 +/- 4.0% in GFR and RPF, respectively), while sulpiride completely abolished the renal haemodynamic changes induced by amino acids. Total and fractional urinary sodium excretion as well as urinary osmolality were similar at baseline and increased in response to amino acids, to the same extent, in all series. No changes in renal dopamine excretion occurred.

CONCLUSION

The results indicate that in man dopamine D2-like receptors are involved in the renal haemodynamic response to amino acid infusion. Whether dopamine D2-like receptor blockade diminishes glomerular hyperfiltration in pathological states requires clinical investigations.

Phenyloxazoles and phenylthiazoles as benzamide bioisosteres: synthesis and dopamine receptor binding profiles

Conformationally restricted benzamide bioisosteres were investigated when the aminomethylpyrrolidine derivative 4o proved D3 as well as D4 binding properties which were comparable to those of the atypical neuroleptics sulpiride and clozapine, respectively.

N-[[1-(2-phenylethyl)pyrrolidin-2-yl]methyl]cyclohexanecarboxamides as selective 5-HT1A receptor agonists

A series of benzamides was synthesized as selective agonists for the 5-HT1A receptor. It was found that (S)-N-[[1-(2-phenylethyl)pyrrolidin-2-yl]methyl]cyclohexanecarb oxamide(7-(S)) has potent and selective agonistic activity for the 5-HT1A receptor (5-HT1A; Ki 0.49 nmol/L, D2; IC50 = >1000 nmol/L, 5-HT2; Ki = 240 nmol/L).

Presynaptic dopamine receptors involved in the inhibition of noradrenaline and dopamine release in the human gastric and uterine arteries

Electrical stimulation-induced depolarization releases both dopamine (DA) and noradrenaline (NA) from sympathetic neurones of the human gastric and uterine arteries. The overflow of catecholamines elicited by electrical stimulation was measured by using high performance liquid chromatography with electrochemical detection. The addition of yohimbine (0.01-10 microM), an alpha2-adrenoceptor antagonist, to the perfusion fluid increased, in a concentration-dependent manner, the electrically-evoked DA and NA overflow from gastric and uterine arteries. In the presence of sulpiride (0.01-10 microM), a dopamine D2-type receptor antagonist, the overflow of both amines was found to be increased in the uterine artery, but not in the gastric artery. Apomorphine (0.1-10 microM), a dopamine receptor agonist, produced a dose-dependent inhibition in the amount of DA and NA released from gastric and uterine arteries. SCH 23390 (0.1-10 microM), a dopamine D1 receptor antagonist, had no effect on the release of both amines in both preparations. The inhibitory effect of apomorphine was blocked by sulpiride in the gastric and uterine arteries but not by SCH 23390. The results presented suggest the existence of dopamine D2-type receptors in the human gastric and uterine arteries. They seem to have, in each artery, a different physiological importance.

Enantio- and diastereocontrolled dopamine D1, D2, D3 and D4 receptor binding of N-(3-pyrrolidinylmethyl)benzamides synthesized from aspartic acid

Subreceptor selectivity tuning of N-(3-pyrrolidinyl)benzamides leading to the selective dopamine D3 ligand ent1h and the derivatives 1g and 1e/ent1e which preferably recognize human D2 or D4 receptors, respectively, is described. Binding profiles were controlled by both, absolute and relative configuration. The enantiopure target compounds were synthesized from aspartic acid.

(-)-Pentazocine analgesia in mice: interactions with a sigma receptor system

(-)-Pentazocine is active in the tailflick assay in CD-1 mice, although it shows a biphasic dose-response curve with a peak effect of only 30%. Co-administration of haloperidol shifts the dose-response curve to the left and elevates the maximal response to 70% through a blockade of sigma 1 receptors, but the curve remains biphasic. (+)-Pentazocine is inactive in all antinociceptive assays, either alone or with haloperidol. The analgesic actions of (-)-pentazocine are readily reversed by nor-binaltorphimine, but not by the mu-selective opioid receptor antagonist beta-funaltrexamine, implying a kappa 1-opioid receptor mechanism of action. This conclusion is supported by the ability of antisense oligodeoxynucleotides directed against the KOR-1 clone, which encodes the kappa 1-opioid receptor, to block (-)-pentazocine analgesia.

Sulpiride antagonizes the biphasic locomotor effects of quinpirole in weanling rats

Low doses of dopamine (DA) agonists such as the D2 receptor subfamily agonist quinpirole are thought to stimulate DA autoreceptors selectively, thereby inhibiting locomotor activity. High doses of quinpirole initially suppress and later activate locomotion during a single test-session; the activation is presumably due to stimulation of postsynaptic receptors. The aim of this study was to investigate whether pretreatment with a selective DA D2 receptor antagonist, sulpiride, could block the putative autoreceptor-mediated inhibition at a lower dose than was required to block the postsynaptically mediated activation. Male and female 30-day-old rats were injected SC with one of eight doses of sulpiride (0.313-40 mg/kg) or the vehicle. Sixty minutes later, rats were injected SC with 0.2 mg/kg quinpirole or the vehicle. Five minutes after the second injection, rats were placed in automated activity monitors which recorded locomotor behavior for 60 min at 5-min intervals. Quinpirole at this dose first suppressed and later increased locomotor activity. Sulpiride pretreatment dose-dependently reversed both the early inhibition and later activation of quinpirole-induced locomotion. However, sulpiride did not block the quinpirole-induced early suppression at a lower dose than was required to block the later activation. Thus, there was no evidence that the locomotor suppression elicited by quinpirole is mediated by a more sensitive subset of DA receptors.

Pharmacotoxicological aspects of levosulpiride

Levosulpiride is the levorotatory enantiomer of sulpiride, a substituted benzamide indicated as an antipsychotic, antidepressant, antiemetic and antidyspeptic drug, as well as for the treatment of somatoform disorders. In vivo sulpiride displays a number of neuroleptic properties which it shares with all typical neuroleptic drugs; however, it has also a number of divergent characteristics that set it apart as the principal compound of the so-called 'atypical neuroleptic agents'. The main mechanism of action of levosulpiride consists of blocking the D2 dopaminergic receptors, preferentially located on the presynaptic membranes in the dopaminergic pathways of the brain; this means that sulpiride is a selective autoreceptor blocker. The results of series of experimental trials conducted to evaluate the toxicologic characteristics of levosulpiride are presented. Both the acute, subacute, chronic and local toxicity trials, and the studies on reproduction toxicity, mutagenic potential and oncogenic/carcinogenic potential, demonstrate that levosulpiride is well tolerated by the animals tested (rats, mice, rabbits and dogs) at doses higher than those effective in human therapy. Moreover, the findings from the experimental studies on levosulpiride lead to exclude the toxicity from accumulation, tolerance, dependence or withdrawal syndrome. In conclusion, according to the evaluated preclinical studies, levosulpiride shows pharmacotoxicologic properties which make it suitable for the management of diseases for which the drug is indicated.

Role of dopaminergic mechanisms in the regulation of stress responses in experimental animals

The effects of some dopaminergic agents were evaluated on stress responses in normal and immunized experimental animals. Restraint stress (RS) consistently induced gastric mucosal lesions and elevated plasma corticosterone in rats. Pretreatment with alpha-methyltyrosine (alpha-MT), haloperidol, or sulpiride aggravated both responses, whereas bromocriptine attenuated them. In rats immunized with sheep red blood cells (SRBCs), RS prevented the booster-induced rise in anti-SRBC antibody titre. This response was further suppressed by alpha-MT, haloperidol, or sulpiride pretreatment, whereas bromocriptine potentiated the humoral immune response. In mice immunized with SRBCs, antigen challenge-induced increase in footpad thickness was inhibited by RS. Similar inhibitions in this response were also seen after alpha-MT or haloperidol treatment. The results are discussed in light of complex dopaminergic mechanisms in the regulation of visceral, endocrinological, and immune responses during stress.

Differential influence of haloperidol and sulpiride on dopamine receptors and peptide mRNA levels in the rat striatum and pituitary

We examined the effect of chronic administration (14 days) of haloperidol (2 mg/kg/day) or sulpiride (100 mg/kg/day), on the mRNA levels of various genes in the rat striatum and pituitary by quantitative in situ and Northern blot hybridizations. In the pituitary, haloperidol and sulpiride induced similar increases of mRNAs of pro-opiomelanocortin (POMC) (+65% and +73%), prolactin (PRL) (+821% and +840%) and growth hormone (GH) (+32% and +47%), but sulpiride induced a greater increase of D2R mRNA (+125%) than haloperidol (+92%). In the striatum, sulpiride and haloperidol had different effects: sulpiride induced a higher increase than haloperidol of both preproenkephalin A (PPA) mRNA (+67% versus +47%) and D2 dopamine receptor (D2R) mRNAs (+72% versus +40%). Moreover, haloperidol and sulpiride had opposite effects on substance P (SP) mRNA. Haloperidol decreased the amount of SP mRNA by 20% while sulpiride increased it by 20%. The D1 dopamine receptor (D1R) mRNA level was not significantly modified after either treatment. Our results demonstrate that the effect of a chronic haloperidol treatment on striatal dopamine receptors and neuropeptide mRNA levels is different to that of sulpiride, whereas it is similar on pituitary hormones mRNA levels.

A differential dopamine receptor involvement during stress ulcer formation in rats

The involvement of dopaminergic (DA) receptors and their possible interactions were evaluated during stress ulcer formation in rats. The DA1 antagonist SCH 23390 (0.025, 0.05, or 0.1 mg/kg) produced only marginal aggravations in gastric stress pathology when compared to vehicle controls. The DA2 antagonist sulpiride (10 or 50 mg/kg) had dose-related effects. The lower dose aggravated whereas the higher dose attenuated stress ulcerogenesis. The DA2 agonist bromocriptine (2.5 or 5.0 mg/kg), however, attenuated gastric stress ulcers. Pretreatment of rats with the DA depletor alpha-methyl-para-tyrosine or the DA1-antagonist SCH23390 clearly neutralized the stress ulcer-attenuating effects of bromocriptine. These results reaffirm a gastric cytoprotective role for DA and further suggest that DA1-DA2 receptor interactions are crucial during DAergic regulation of gastric mucosal integrity during stress.

Comparison of the effect of substituted benzamides on midbrain dopamine neurones after treatment of rats for 21 days

We have tested five different substituted benzamides (nemonapride, D,L-sulpiride, remoxipride, raclopride and zacopride) for their potential to decrease the number of spontaneously active dopamine neurones in the rat midbrain after treatment for 21 days. Nemonapride, D,L-sulpiride, and remoxipride significantly reduced the number of spontaneously active dopamine neurones in the ventral tegmental area, indicating an antipsychotic potential, while raclopride and zacopride induced but minor effects. The number of active dopamine neurones in the substantia nigra pars compacta was reduced by nemonapride at higher doses which should indicate the propensity for developing extrapyramidal side-effects. In conclusion, several of the substituted benzamides showed an antipsychotic profile in this test model. In addition, some of the benzamides also showed a propensity for extrapyramidal side-effects and these results are in accordance with the profile reported from clinical trials. However, the results obtained with this model indicate that raclopride neither has antipsychotic potential nor induces extrapyramidal side-effects. The reason for this discrepancy is at present not known.

Subcortical excitatory amino acid levels after acute and subchronic administration of typical and atypical neuroleptics

The effects of three atypical neuroleptic compounds, clozapine, sulpiride, and (-)-3-(3-hydroxyphenyl)-N-n-propyl-piperidine ((-)-3-PPP) were compared to the effects of haloperidol and saline on excitatory amino acid levels in the rodent nucleus accumbens and corpus striatum after acute (1 day) and subchronic (28 days) treatment. Equivalent doses of each drug were determined by assessing their in vivo displacement of [3H]spiperone binding in the nucleus accumbens and corpus striatum. After acute treatment, all three atypical neuroleptics, but not haloperidol, produced a significant decrease in nucleus accumbens glutamate concentrations. Acute haloperidol treatment significantly elevated glutamate concentrations in the corpus striatum when compared to all three atypical drugs. After subchronic treatment, (-)-3-PPP significantly increased glutamate concentrations in the nucleus accumbens when compared to the effects of haloperidol and clozapine. There were no major between-group differences in glutamate levels after subchronic treatment in the corpus striatum. The effects of acute and subchronic neuroleptic administration on aspartate levels in the nucleus accumbens and corpus striatum were highly variable. These findings indicate that atypical and typical neuroleptics may alter subcortical excitatory amino acid levels in a site-specific manner.

Brain dopamine D-2 receptor mechanisms in spontaneously hypertensive rats

Brain dopaminergic function was studied in spontaneously hypertensive rats (SHR) with the selective dopamine D-2 antagonist sulpiride. Sulpiride dose-dependently inhibited locomotor activity of normotensive Wistar-Kyoto rats (WKY). SHR showed an increase in locomotor activity in response to low doses of sulpiride, whereas no effect was observed of higher doses. In a two-bottle salt-preference test, WKY showed increased preference for an 0.9% saline solution after treatment with sulpiride, whereas total fluid intake remained the same. In SHR, sulpiride influenced neither salt-preference nor total fluid intake. SHR and WKY with a unilateral lesion of the median forebrain bundle showed similar turning behaviour in response to treatment with amphetamine. Pretreatment with 100 mg/kg sulpiride virtually abolished amphetamine-induced turning in WKY, but had little effect in SHR. Sulpiride dose-dependently increased serum prolactin concentrations in WKY and SHR. However, the increase was significantly greater in SHR. Dopamine D-2 receptor binding was measured with in vitro autoradiography, using [125I]-sulpiride as the ligand. Binding density was similar in the caudate nucleus and substantia nigra of SHR and WKY brain. Concentrations of the dopamine metabolites DOPAC and HVA, but not of dopamine itself, were significantly increased in frontal cortex, striatum and hypothalamus after treatment with 100 mg/kg sulpiride. There were no significant differences between SHR and WKY in the increase in the DOPAC/DA and HVA/DA ratio. These data show that SHR show differential changes in their response to central dopamine D-2 blockade when compared to WKY. Thus, in some tests (locomotor activity after high doses, salt preference, turning behaviour), SHR respond less to sulpiride.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of D1 and D2 dopamine receptors produces additive anorectic effects

In food-deprived mice the D1 dopamine agonist SKF 38393 induced dose dependent anorexia (ED50 = 2.6 mg/kg). This effect was reversed by the D1 antagonist SCH 23390. In similar conditions, the D2 dopamine agonist RU 24926 also induced dose dependent anorexia (ED50 = 0.19 mg/kg). This effect was reversed by the D2 antagonist (+/-) sulpiride. The mixed D1/D2 agonist apomorphine also induced an anorectic effect (150 micrograms/kg sc) which was completely reversed by (+/-) sulpiride (25 mg/kg, ip) but unaffected by SCH 23390 (5-30 micrograms/kg). The dose response curve obtained by associating SKF 38393 (2.5 mg/kg) with increasing doses of RU 24926 was roughly parallel to that obtained with RU 24926 alone. This indicates that effects of two drugs were additive. Although both D1 and D2 receptors regulate food consumption, the anorectic effect of apomorphine appears to involve only D2 receptors.

Dopamine D1 and D2 receptor agonists induce opposite changes in the firing rate of ventral pallidal neurons

Selective dopamine D1 and D2 agonists were used to determine the contributions of each receptor subtype in the modulation of firing rate of ventral pallidum/substantia innominata (VP/SI) neurons. Administration of cumulative doses of the D2 agonist, quinpirole, decreased activity in 59% of the VP/SI cells tested. The decrease in firing rate was dose-dependent between 0.002-0.2 mg/kg i.v. and was blocked by the D2 antagonist, sulpiride (12.5 mg/kg i.v.). In addition, the magnitude and the distribution of responses of VP/SI neurons was not changed following administration of quinpirole as a single versus a divided cumulative dose of 0.1 mg/kg. In contrast, administration of the D1 agonist, SKF38393, excited 69% of the neurons sampled. Similar maximal responses were observed following administration of either a single or a divided cumulative dose of 3.2 mg/kg of SKF38393. The D1 receptor antagonist, SCH23390 (0.1-0.4 mg/kg i.v.) often attenuated the SKF38393-induced increases. The results illustrate that, (1) VP/SI neurons are sensitive to systemically administered dopamine agonists, (2) D1 or D2 receptor activation is sufficient to change the activity of these neurons and (3) these selective agonists mediate opposite effects on VP/SI neuronal activity. These differential responses contrast with effects observed for other dopaminoceptive brain regions, and distinguish VP/SI neurons from morphologically related neurons of the dorsal globus pallidus.

Decrease of behavioral and biochemical denervation supersensitivity of rat striatum by nigral transplants

The effect of fetal mesencephalic transplants on dopamine receptor supersensitivity has been studied behaviorally and biochemically in rats with a unilateral lesion of the nigrostriatal pathway. Female rats were lesioned with 6-hydroxydopamine in the left substantia nigra. At least one month later they were tested with apomorphine (0.25 mg/kg, s.c.), amphetamine (5 mg/kg, s.c.), LY 171555 (D2 agonist) (0.5 mg/kg, i.p.) and CY 208243 (D1 agonist) (0.5 mg/kg, s.c.). A suspension containing approximately 1.5 x 10(6) cells from the ventral mesencephalon of rat embryos was distributed in three sites in a triangular fashion in the center of the denervated striatum. Six months later, grafted dopamine neurons reinnervated the medial part of the dorsal striatum, increased the dopamine level and reversed the rotational asymmetry evoked by amphetamine. Apomorphine given four months post-transplant still elicited contraversive circling but the number of turns was reduced. Circling evoked six months post-transplant by CY 208243 or LY 171555 was significantly less in grafted rats than in lesioned non-grafted rats. The density of dopaminergic receptors in the striatum of grafted and lesioned rats was examined by autoradiography by means of in vitro binding with [3H]SCH 23390 for D1 receptors and [3H] spiperone for D2 receptors. The results show that intrastriatal nigral transplants decrease the supersensitivity of the D2 receptors and to a lesser extent of the D1 receptors. Normalization of D2 receptors may explain the decrease of behavioral supersensitivity following administration of apomorphine and D2 agonist in grafted rats. D1 receptors were less affected by the lesion and also less normalized than D2 receptors by the transplants.(ABSTRACT TRUNCATED AT 250 WORDS)

Unilateral inactivation of dopamine receptors after intrastriatal injection of N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ): a novel rotational model to investigate dopamine receptor interactions

The interaction between D1 and D2 dopamine (DA) receptors was investigated in a novel rotational model. Rats were unilaterally injected into the striatum with the irreversible DA receptor blocker N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). This treatment induced a marked decrease in the density of D1 (-48%) and D2 (-45%) DA receptors available for binding to 3H-SCH 23390 and 3H-spiperone, respectively. Under these experimental conditions, the effect of DA receptor agonists were predominant on the intact side and resulted in rotations ipsilateral to the injected side. The effects of different agonists and antagonists for D1 and D2 DA receptors were evaluated 24 hr after EEDQ administration. The D2 agonist LY 171555 induced ipsilateral rotations in a dose-dependent manner (0.1-10.0 mg/kg, IP) in rats treated intrastriatally with EEDQ. In contrast, the D1 agonist SKF 38393 (1-20 mg/kg, IP) was unable to elicit circling behavior per se. However, SKF 38393 increased the number of rotations caused by LY 171555. The circling behavior induced by LY 171555 was blocked by the D2 antagonists (-)sulpiride and raclopride and by the D1 antagonist SCH 23390. Moreover, the inhibition of circling behavior induced by SCH 23390 was reversed by SKF 38393 in a dose-dependent manner. LY 171555 (1 mg/kg, IP) was unable to induce rotations in EEDQ-treated rats following DA depletion by alpha-methyl-p-tyrosine, whilst the combined administration of LY 171555 (1 mg/kg, IP) and SKF 38393 (10 mg/kg, IP) elicited intense circling behavior in DA depleted rats.(ABSTRACT TRUNCATED AT 250 WORDS)

The actions of (-)N-n-propylnorapomorphine and selective dopamine D1 and D2 receptor agonists to modify the release of [3H]dopamine from the rat nucleus accumbens

The ability of (-)N-n-propylnorapomorphine and selective D1 and D2 dopamine receptor agonists and antagonists to modify the release of [3H]dopamine, induced by potassium from the nucleus accumbens, was studied using an in vitro superfusion technique. (-)N-n-Propylnorapomorphine, in picomolar concentrations, inhibited the release of [3H]dopamine, the inhibition being antagonised by fluphenazine and the selective D2 receptor antagonist sulpiride; the selective D1 receptor antagonist SCH 23390 was ineffective. The selective D1 receptor agonist SKF 38393 and the selective D2 agonist quinpirole, both inhibited the potassium-induced release of [3H]dopamine; no synergistic effect was observed to a combined treatment with SKF 38393 and quinpirole. The effects of SKF 38393 and quinpirole were selectively antagonised by SCH 23390 and sulpiride, respectively, although both antagonists failed to modify the release of [3H]dopamine when administered alone. Receptor antagonists for other transmitter sites, e.g. noradrenaline, 5-hydroxytryptamine and acetylcholine, failed to modify potassium-induced release of [3H]dopamine, when administered alone or to prevent the inhibition of the release caused by (-)N-n-propylnorapomorphine. It is concluded that the action of dopamine agonists on both dopamine D1 and D2 receptors in the nucleus accumbens can reduce the release of [3H]dopamine in the in vitro system. Comparable actions in vivo may contribute to the ability of dopamine agonists to moderate locomotor responding.

The effect of dopamine receptor blockade on the development of sensitization to the locomotor activating effects of amphetamine and morphine

The effect of dopamine (DA) receptor blockade on the development of sensitization to the locomotor activating effects of systemic amphetamine and intra-ventral tegmental area (intra-VTA) morphine was investigated. Rats were pretreated with the D-1 DA receptor antagonist, SCH-23390 (0.04 or 0.2 mg/kg, i.p.) or one of two D-2 DA receptor antagonists, pimozide (0.5 mg/kg, i.p.) and Ro 22-2586 (0.2 mg/kg, i.p.), prior to each of 5 exposures to the sensitizing drug. SCH-23390 blocked the development of sensitization to amphetamine but not to intra-VTA morphine. Pimozide had the opposite effect and Ro 22-2586 had no effect on the development of sensitization to either amphetamine or intra-VTA morphine. All 3 antagonists, at the doses tested, completely blocked the acute locomotor activating effects of these two drugs. Pretreatment in separate animals with low autoreceptor doses of sulpiride (25 mg/kg, i.p. with amphetamine and 10 mg/kg, i.p. with intra-VTA morphine) slightly potentiated the acute locomotor effect and produced a slight enhancement of the sensitized response to amphetamine and intra-VTA morphine. Pretreatment with a higher dose of sulpiride (50 mg/kg, i.p.) blocked the acute locomotor effect of intra-VTA morphine but had no effect on the development of sensitization to this drug. These results suggest that the mechanisms underlying the development of sensitization to the locomotor activating effects of amphetamine and intra-VTA morphine are different even though these may ultimately result in similar changes in the activity of mesencephalic DA neurons. Implications of these findings for the differential involvement of D-1 and D-2 DA receptors and for neural hypotheses of behavioral sensitization are discussed.

Light-induced dopamine release from teleost retinas acts as a light-adaptive signal to the retinal pigment epithelium

In the retinal pigment epithelium (RPE) of lower vertebrates, melanin pigment granules migrate in and out of the cells' long apical projections in response to changes in light condition. When the RPE is in its normal association with the retina, light onset induces pigment granules to disperse into the apical projections; dark onset induces pigment granules to aggregate into the cell bodies. However, when the RPE is separated from the retina, pigment granule movement in the isolated RPE is insensitive to light onset. It thus seems likely that a signal from the retina communicates light onset to the RPE to initiate pigment dispersion. We have examined the nature of this retina-to-RPE signal in green sunfish, Lepomis cyanellus. In isolated retinas with adherent RPE, light-induced pigment dispersion in the RPE is blocked by treatments known to block Ca2+-dependent transmitter release in the retina. In addition, the medium obtained from incubating previously dark-adapted retinas in the light induces light-adaptive pigment dispersion when added to isolated RPE. In contrast, the medium obtained from incubating dark-adapted retinas in constant darkness does not affect pigment distribution when added to isolated RPE. These results are consistent with the idea that RPE pigment dispersion is triggered by a substance that diffuses from the retina at light onset. The capacity of the conditioned medium from light-incubated retinas to induce pigment dispersion in isolated RPE is inhibited by a D2 dopamine antagonist, but not by D1 or alpha-adrenergic antagonists. Light-induced pigment dispersion in whole RPE-retinas is also blocked by a D2 dopamine antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative study of sulpiride and haloperidol on dopamine turnover in the rat brain

The effects of the neuroleptics, sulpiride and haloperidol, on dopamine (DA) turnover were compared following the acute and chronic administration of these drugs alone or in combination with levodopa or apomorphine. In the acute treatment, the increase in DA metabolites in the striatum and nucleus accumbens was more marked in the haloperidol-treated rats than in the sulpiride-treated rats. Following the additional administration of levodopa, however, the potency of the neuroleptics in elevating DA metabolites was reversed. A low dose of apomorphine induced a marked reduction in the striatal DA metabolite levels by approximately 50%. When rats were pretreated with the neuroleptics, haloperidol was more effective in preventing an apomorphine-induced reduction in DA metabolites. On repeated administration of the neuroleptics, a tolerance occurred in the striatum and nucleus accumbens, but not in the prefrontal cortex. This differential development of tolerance was observed in the different brain regions and with the different drugs administered. These results suggest that the pharmacological mechanism of sulpiride on DA turnover differs from that of haloperidol.

Differences in the time course of dopaminergic supersensitivity following chronic administration of haloperidol, molindone, or sulpiride

The onset and persistence of changes in 3H-spiroperidol binding to dopamine (DA) D2 receptors were examined in rat mesolimbic and striatal brain regions following daily administration of haloperidol, molindone, or sulpiride for 3, 7, 14, or 28 days. Neuroleptic dose equivalencies were determined by inhibition of 3H-spiroperidol in vivo binding in several rat brain regions. Changes in locomotor and stereotyped responses to the specific DA D2 agonist quinpirole were examined 3 days after the last treatment dose. Haloperidol or molindone administration increased mean stereotypy scores and striatal DA D2 receptor densities throughout the 28-day treatment period. In contrast, mesolimbic DA D2 receptor densities were transiently increased and returned to control values, after 28 days of haloperidol or molindone treatment. Sulpiride treatment increased mean stereotypy scores and striatal Bmax values, but had no effect on locomotion or mesolimbic dopamine receptor density. Additionally, the magnitude of change in the various measures of brain DA function varied among the three neuroleptic treatment groups. Results from this study suggest that mesolimbic and striatal brain regions differ in their response to long-term neuroleptic administration and that drug choice may influence the magnitude of neuroleptic-induced dopaminergic supersensitivity.

Dopamine D2 and muscarinic receptor binding characteristics in rat brain after withdrawal of subchronic fluphenazine and sulpiride treatment

1. The effects of subchronic (10 days) treatment with two antischizophrenic drugs, fluphenazine and sulpiride, on rat brain dopamine and muscarinic receptor sites were studied at various time points after withdrawal from the drug treatments. Striatal D-2 dopamine receptors and cortical muscarinic receptors were characterized with the radioligands 3H-spiperone and 3H-quinuclidinyl benzilate (3H-QNB), respectively. 2. Neither drug treatment affected significantly the number or affinity of cortical muscarinic receptors during the withdrawal time of 18 days. 3. The number of D-2 receptors in striatum was not altered after 4 or 8 fluphenazine-free days, but after 18 days fluphenazine caused an increase of 55% in the density of D-2 receptors. Sulpiride caused a slight increase in Bmax of striatal D-2 receptors after 4 and 8 withdrawal days but the maximum increase of 29% was observed after 18 withdrawal days. 4. Fluphenazine treatment caused an increase in the apparent dissociation constant 4 days after the termination of the treatment suggesting that residual neuroleptic levels may have been retained after this dose regimen, since the Kd returned near control levels as the withdrawal time passed. Sulpiride, however, caused a moderate increase in Kd throughout the withdrawal time. 5. In conclusion, these results suggest that even a rather short treatment with fluphenazine or sulpiride, a classical and an atypical neuroleptic, respectively, can induce pronounced changes in striatal dopamine receptor characteristics after a drug washout period of almost three weeks. Thus, possible delayed actions of classical as well as novel neuroleptic drugs on neurotransmitter function should be considered in studies examining the neuroleptic withdrawal effects.