Striatal Dopamine Release in Response to Morphine: A [11C]Raclopride Positron Emission Tomography Study in Healthy Men

BACKGROUND

Preclinical and human positron emission tomography studies have produced inconsistent results regarding the effects of opioids on mesolimbic dopamine (DA). Here, we quantify striatal DA release (measured by [11C]raclopride displacement) in response to an intravenous infusion of morphine, and its relationship with morphine-induced subjective effects, in healthy, nondependent opioid-experienced participants.

METHODS

Fifteen healthy male participants were initially included. Sessions were on separate days. On session 1, participants received intravenous morphine (10 mg/70 kg) in the clinic to ensure tolerability. Participants without adverse reactions (n = 10) then received intravenous morphine and placebo (saline) sessions, in counterbalanced order, while undergoing [11C]raclopride positron emission tomography scans. Subjective and physiological responses were assessed. Region-of-interest and voxelwise image analyses were used to assess changes in [11C]raclopride nondisplaceable binding potential.

RESULTS

Morphine produced marked subjective and physiological effects and induced a significant decrease in [11C]raclopride nondisplaceable binding potential, particularly in the nucleus accumbens and globus pallidus, where the change in [11C]raclopride nondisplaceable binding potential was approximately 9%. However, the subjective effects of morphine did not show a simple pattern of correlation with DA release.

CONCLUSIONS

This is, to our knowledge, the first study providing in vivo human evidence that DA transmission in the ventral striatum is affected by morphine. Further studies are required to fully delineate the DA contribution to the reinforcing effects of opioids.

A data driven method for estimation of B(avail) and appK(D) using a single injection protocol with [¹¹C]raclopride in the mouse

PURPOSE

The partial saturation approach (PSA) is a simple, single injection experimental protocol that will estimate both B(avail) and appK(D) without the use of blood sampling. This makes it ideal for use in longitudinal studies of neurodegenerative diseases in the rodent. The aim of this study was to increase the range and applicability of the PSA by developing a data driven strategy for determining reliable regional estimates of receptor density (B(avail)) and in vivo affinity (1/appK(D)), and validate the strategy using a simulation model.

METHODS

The data driven method uses a time window guided by the dynamic equilibrium state of the system as opposed to using a static time window. To test the method, simulations of partial saturation experiments were generated and validated against experimental data. The experimental conditions simulated included a range of receptor occupancy levels and three different B(avail) and appK(D) values to mimic diseases states. Also the effect of using a reference region and typical PET noise on the stability and accuracy of the estimates was investigated.

RESULTS

The investigations showed that the parameter estimates in a simulated healthy mouse, using the data driven method were within 10±30% of the simulated input for the range of occupancy levels simulated. Throughout all experimental conditions simulated, the accuracy and robustness of the estimates using the data driven method were much improved upon the typical method of using a static time window, especially at low receptor occupancy levels. Introducing a reference region caused a bias of approximately 10% over the range of occupancy levels.

CONCLUSIONS

Based on extensive simulated experimental conditions, it was shown the data driven method provides accurate and precise estimates of B(avail) and appK(D) for a broader range of conditions compared to the original method.

[Behavioral effects during the local activation and blockade of serotonin and dopamine receptors in the frontal cortex in cats in the model with a choice of reinforcements of different value]

In the model with a choice of reinforcements of different value animals were able to "impulsive" and "self-controlled" behavior with an equal probability. Five adult cats were tested. A local application of agonists of 5-HT(1A) and 5-HT(2A/C) receptors (8-OH-DPAT, DOI) in the frontal cortex have resulted in a significant decrease of the "impulsive" reactions and an increase of omissions. The administration of antagonists of 5-HT(2A/C) receptors (ketanserin) and D1/D2 receptors (SCH 23390, raclopride) have impaired the "impulsive" behavior. However the combined administration of agonists/antagonists of 5-HT(2A/C) receptors and antagonists of D1/D2 receptors have not shown the significant changes in behavior as compared with control experiments. The data showed the realization of the optimal behavior with the "impulsive" and "self-controled" reactions in ambivalent animals requires the involvement of both dopaminergic and serotoninergic systems for the regulation of the activity of neurons in frontal cortex areas.

[Dopamine-NO interaction in the nucleus accumbens during stress-induced inhibition of expliratory behavior]

In Sprague-Dawley rats by means of in vivo microdialysis it was shown that stress, evoked by conditioned fear response acquisition, resulted in inhibition of exploratory behavior and produced a reduction of exploration-induced rise in extracellular levels of citrulline (an NO co-product) in the nucleus accumbens which were observed 24 hours after the stress. Intra-accumbal infusion of dopamine D2 receptor antagonist raclopride (10 μM), made after the conditioned fear response acquisition, in 24 hours after the infusion, restore the exploratory behavior and exploration-induced rise of extracellular levels of citrulline in this brain area. The data obtained indicate for the first time that stress-induced inhibition of the exploratory behavior might be mediated by inhibition of the nucleus accumbens nitrergic system occurring under the control of D2 receptors of this brain area.

[Influence of D2-receptor blockade on the nitrergic system activity of the nucleus accumbens]

In Sprague-Dawley rats, by means of in vivo microdialysis combined with HPLC analysis it was shown that the infusion into the n. accumbens of the D2-receptor antagonist raclopride (20, 100 microM) did not affect extracellular level ofcitrulline (an NO co-product) in this brain area. The intraaccumbal infusion of NMDA, an NMDA receptor agonist (100 microM) caused a rise of the extracellular citrulline level in this brain area. This rise was prevented by intraaccumbal infusion of 0.5 mM 7-nitroindazole, a neuronal NO-synthase inhibitor, and it was significantly reduced by the infusion of raclopride (20, 100 microM) into this brain area. The data obtained suggest that the D2-receptors of the n. accumbens are implicated in the regulation of neuronal NO-synthase activity induced by local NMDA receptor stimulation.

[Effects of blockade of dopaminergic D1/D2 receptors on behavior of impulsive and self-controlled rats]

Selective antagonists of D1 and D2 receptors (SCH 23 390 and raclopride respectively) were injected to rats divided on the basis of preliminary experiments into groups of"self-controlled" (preference of valuable but delayed reinforcement) and "impulsive" (choice of less valuable but immediate reinforcement) animals. Number of omissions of reactions and their latencies were recorded. In "self-controlled" rats, both drugs increased the number of reactions for less valuable immediate reinforcement, i.e., increased impulsiveness, whereas practically did not change the behavior of "impulsive" animals. Introduction of SCH 23 390 to animals ofboth groups increased the number of reaction omissions the effect being stronger in "self-controlled" animals. Raclopride also increased the number of reaction omissions in "self-controlled" rats but only on the next day after the injection. In "impulsive" animals, both drugs increased the latencies of reactions.

The role of dopamine receptors in ventrolateral orbital cortex-evoked antinociception in a rat formalin test model

The present study examined the roles of dopamine and D(1)- and D(2)-like dopamine receptors in ventrolateral orbital cortex (VLO)-evoked antinociception in rats with persistent inflammatory pain. Following formalin injection into the rat unilateral hindpaw pad, the effects of dopamine receptor agonist and antagonist microinjections into the VLO on nociceptive behavior were observed. Results demonstrated that VLO microinjection of the non-selective dopamine receptor agonist apomorphine (R(-)-apomorphine hydrochloride, 1.0, 2.5 and 5.0μg) depressed later-phase nociceptive behavior induced by formalin injection; this effect was attenuated by the D(2)-like dopamine receptor antagonist S(-)-raclopride(+)-tartrate salt (raclopride, 3.0μg), but not by the D(1)-like dopamine receptor antagonist R(+)-SCH-23390 hydrochloride (SCH-23390, 1.0μg). Apomorphine-induced antinociception was mimicked by microinjection of the D(2)-like dopamine receptor agonist (-)-quinpirole hydrochloride (2.0 and 5.0μg) into the same VLO site, and this effect was antagonized by raclopride (3.0μg). In addition, microinjection of the D(1)-like dopamine receptor agonist R(+)-SKF-38393 hydrochloride (5.0μg) had no effect on formalin-induced nociceptive behavior during the later phase. However, the D(1)-like dopamine receptor antagonist SCH-23390 (2.5, 5.0 and 10μg) depressed nociceptive behavior in a dose-dependent manner. These results suggested that dopamine mediated VLO-induced antinociception via different mechanisms in the persistent inflammatory pain model; D(2)-like receptors mediated dopamine-induced antinociception, while D(1)-like dopamine receptors exhibited tonic facilitatory action on nociceptive behavior, thereby blocking D(1)-like dopamine receptors could induce antinociception.

Dopamine D1 and D2 antagonist effects on response likelihood and duration

Experimentally induced and parkinsonian disruptions in dopamine (DA) transmission are associated with motor abnormalities that include a reduced likelihood of behavioral response initiation and an increased duration of executed responses. Here we investigated the dopamine receptor subtypes involved in regulating these two aspects of behavior. We examined the effects of D1 family (D1/D5) antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390; 0, 0.04, 0.08, or 0.16 mg/kg) and D2/D3 antagonist 3,5-dichloro-N-(1-ethylpyrrolidin-2-ylmethyl)-2-hydroxy-6-methoxybenzamide (+)-tartrate salt (raclopride; 0, 0.2, or 0.4 mg/kg) on the likelihood and duration of a cued Pavlovian approach and a cued operant lever-press response. While the high doses of the D1 and D2 antagonists produced similar levels of overall locomotor suppression, only the D2 antagonist increased the duration of time that animals' heads remained in the food compartment during both Pavlovian and operant task performance. In contrast, D1 antagonist SCH23390 decreased the proportion of trials in which animals executed both the Pavlovian approach and operant lever-press, while raclopride did not. The results suggest that D2 receptor blockade preferentially increases response duration, and, under the simple discrete-trial procedures employed here, D1 receptor blockade preferential reduces Pavlovian and operant response likelihood.

Multiinjection approach for D2 receptor binding quantification in living rats using [11C]raclopride and the beta-microprobe: crossvalidation with in vitro binding data

The purpose of this study was to quantify D2 receptors density and affinity in living rats using [11C]raclopride and to validate the multiinjection modelling approach. To this aim, we used an intracerebral beta+-sensitive probe as a highly sensitive system to quantify the radioligand activity using a single three-injection experimental paradigm. The study was divided into three main parts: (i) [11C]raclopride catabolism evaluation without and with cimetidine pretreatment (cytochrome P450 inhibitor); (ii) quantification of kinetics parameters in the striatum, enthorinal cortex, and cerebellum of living rats using a three-compartment model with an arterial input function; (iii) correlation study of in vivo and in vitro binding density and affinity values in the same striatal tissues. (i) raclopride catabolism was very reproducible between individuals; cimetidine pre-treatment resulted in a 30% reduction of raclopride metabolites. (ii) D2 striatal B'max and KdVr estimates obtained by compartmental modelling were 19.87+/-6.45 and 6.2+/-3.3 nmol/L, respectively. Cerebellum is the best candidate as a reference region with no specific binding detectable in vivo. (iii) When comparing density (Bmax/B'max) and affinity (Kd/KdVr) values in vivo and in vitro for each striatum, a high strict correlation was found (r2=0.90 and 0.72, for density and affinity, respectively). These results validate the multi-injection modelling approach coupled to beta-microprobe acquisitions as a mean to provide accurate and separate estimates of dopamine D2-receptor density and affinity, in the living rodent striatum.

Automatic extraction of brain surface and mid-sagittal plane from PET images applying deformable models

In this study, we propose and evaluate new methods for automatic extraction of the brain surface and the mid-sagittal plane from functional positron emission tomography (PET) images. Designing methods for these segmentation tasks is challenging because the spatial distribution of intensity values in a PET image depends on the applied radiopharmaceutical and the contrast to noise ratio in a PET image is typically low. We extracted the brain surface with a deformable model which is based on a global optimization algorithm. The global optimization allows reliable automation of the extraction task. Based on the extracted brain surface, the mid-sagittal plane was determined. The method was tested with the image of the Hoffman brain phantom (FDG) and the images from the brain studies with the FDG (17 images) and the C11-Raclopride tracers (4 images). In addition to the brain surfaces, we applied the deformable model for extraction of the coarse cortical structure based on the tracer uptake from FDG-PET brain images. The proposed segmentation methods provide a promising direction for automatic processing and analysis of PET brain images.

The effects of systemic administration of selective antagonists of dopamine D1 and D2/D3 receptors on food-related and defensive (escape responses) conditioned paw-placing responses in cats

Experiments were performed on cats to study the effects of systemic administration of antagonists of dopaminergic transmission on food-related and defensive (an escape response) operant conditioned reflexes acquired on the basis of the innate response of placing the forepaw on a support. Selective blockade of D1 receptors with SCH23390 (0.005-0.1 mg/kg) completely and selective blockade of D2/D3 receptors with raclopride (0.1-0.25 mg/kg) partially suppressed both reflexes. At these doses, both blockers had stronger actions on the defensive conditioned escape reflex than the food-related reflex: SCH23390 had significantly stronger inhibitory effects on both reflexes than raclopride.

[The influence of systemically administered selective dopamine antagonists of D1 and D2/D3 types on alimentary and escape conditioned placing reactions in cats]

Systemic administration of the specific antagonists of D1 (SCH23390, 0.005-0.1 mg/kg) and D2/D3 (raclopride, 0.1-0.25 mg/kg) dopamine receptors leeds to dose-dependent increase of the reaction time and decrease of conditined reflex probability up to full blocking (in the case of SCH23390) of alimentary and escape conditioned placing reaction in cats. The action of both antagonists was far more suppressive regarding conditioned escape reflex. The action of SCH23390 was far more effective than that of raclopride concerning both types of conditioned reflexes.

Smoking-induced ventral striatum dopamine release

OBJECTIVE

Substantial evidence from animal models demonstrates that dopamine release in the ventral striatum underlies the reinforcing properties of nicotine. The authors used [(11)C]raclopride bolus-plus-continuous-infusion positron emission tomography (PET) to determine smoking-induced ventral striatum dopamine release in humans.

METHOD

Twenty nicotine-dependent smokers (who smoked > or =15 cigarettes/day) underwent a [(11)C]raclopride bolus-plus-continuous-infusion PET session. During the session, subjects had a 10-minute break outside the PET apparatus during which 10 subjects smoked a cigarette and 10 did not smoke (as a control condition).

RESULTS

The group that smoked had greater reductions in [(11)C]raclopride binding potential in ventral striatum regions of interest than the group that did not smoke, particularly in the left ventral caudate/nucleus accumbens and left ventral putamen (range for smoking group=-25.9% to -36.6% reduction). Significant correlations were found between change from before to after the smoking break in craving ratings and change from before to after the break in binding potential for these two regions.

CONCLUSIONS

Nicotine-dependent subjects who smoked during a break in PET scanning had greater reductions in [(11)C]raclopride binding potential (an indirect measure of dopamine release) than nicotine-dependent subjects who did not smoke. The magnitude of binding potential changes was comparable to that found in studies that used similar methods to examine the effects of other addictive drugs.

Effects of intrastriatal administration of selective dopaminergic ligands on spontaneous stereotypy in mice

Abnormal repetitive behaviors are often associated with specific developmental, genetic, and neuropsychiatric disorders. Repetitive motor behaviors, often referred to as stereotypies, have been studied extensively as they can be readily induced by certain drugs (e.g., amphetamine). Recent work has shown, however, that such drug-induced models of stereotypy may not accurately reflect the neurobiological perturbations responsible for the spontaneous manifestation of these behaviors. The present study employed the deer mouse model of spontaneous and persistent stereotypy to evaluate the capacity of several selective dopaminergic agonists (apomorphine, SKF81297, and quinpirole) to exacerbate levels of spontaneously emitted stereotypic topographies when administered intrastriatally. Additionally, the effects of intrastriatal administration of the D(2)R antagonist raclopride on the expression of spontaneous stereotypic jumping were evaluated. No induction or exacerbation of stereotypy was observed following administration of the selective D(1)- or D(2)- receptor agonists, and the mixed agonist apomorphine induced hyperlocomotion and excessive grooming but failed to exacerbate spontaneous stereotypy. Thus, a dissociation was observed between spontaneously emitted and drug-induced stereotypy, suggesting significant limitations to the use of dopamine agonist-induced stereotypy as a model of clinical stereotyped movement disorder. Furthermore, an unexpected and statistically significant (P<.05) potentiation of locomotor activity was observed following intrastriatal raclopride administration, suggesting major alterations to the modulatory characteristics of the striatal dopaminergic system in these spontaneously stereotypic animals.

Operant behavior in conditions of activation and blockade of neostriatal muscarinic receptors

Chronic experiments were performed on four dogs using a model of an operant defensive reflex associated with maintaining a flexion posture to study the effects of bilateral intraneostriatal microinjection of the non-selective muscarinic receptor agonist carbachol, the selective D2 dopamine receptor blocker raclopride, and the selective M1 muscarinic receptor blocker pirenzipine on the performance of the operant defensive reflex and differentiation of signals. The results show that microinjection of carbachol induced increases in the tonic component and inhibition of the phasic component of the reflex, an ordering rearrangement of the posture, and increases in the amplitudes of its components. Raclopride microinjection gave similar but less marked results. The greatest effects with both substances were seen using differential stimuli. There were sharp increases in the process of differentiation of sound signals. Pirenzipine microinjections gave the opposite result. These data are assessed on the basis of concepts of the existence of two efferent outputs from the neostriatum with opposite effects on their targets and the roles of muscarinic and dopamine receptors in triggering and blocking these effects.

Dopamine Antagonists Do Not Block Conditioned Place Preference Induced by Paced Mating Behavior in Female Rats

The authors assessed the behavioral effects of dopamine (DA) receptor antagonists, Cis (Z) flupentixol and S(+)-raclopride L-tartrate, on conditioned place preference (CPP) induced by paced mating behavior. Ovariectomized female rats of the Wistar strain were used. The administration of amphetamine (1 mg/kg) induced a clear CPP that was completely blocked by the DA antagonists flupentixol (0.25 mg/kg) or raclopride (0.125 mg/kg). These doses had no effect on motor coordination. Female rats that mated in a pacing chamber developed a clear CPP. Neither flupentixol nor raclopride blocked the reward state induced by paced mating behavior. These results indicate that DA is not involved in the reward state induced by paced mating behavior in female rats.

Dopaminergic involvement in medial prefrontal cortex and core of the nucleus accumbens in the regulation of ethanol self-administration: a dual-site microinjection study in the rat

The complex mesolimbic-mesocortical system involved with behavioral selection has been implicated in the control of ethanol self-administration. However, the nature of the interactions within this multiple-structured system in ethanol intake regulation remains unclear. Although the role of dopamine (DA) in the prefrontal cortex and the nucleus accumbens has been examined individually, the interaction of DA activity in both structures at the same time remains to be examined. Male, Long-Evans rats were initiated to self-administer ethanol in an operant situation using the sucrose-substitution procedure. Following initiation, bilateral cannula guides were located to allow microinjection in the medial prefrontal cortex (mPFC) and the core of the nucleus accumbens. The DA D2/D3 agonist quinpirole (10.0-microg dose in the prefrontal cortex; 4.0-microg dose in n. accumbens) and the D2 antagonist raclopride (0.05-microg dose in prefrontal cortex; 1.0-microg dose in the nucleus accumbens) were then tested in each site alone and in combination in both sites in each rat. Changes in total responding, ethanol intake, and the pattern of responding were analyzed. Single-site injections replicated most of our previous findings for these doses. Changes in single-site effects were found when dual-site injections were performed, with altered input from the prefrontal areas impacting the effects of accumbens injections. Based on these interactions, our hypothesis that the prefrontal area is involved with the onset and offset of drinking, while the nucleus accumbens is involved with maintaining the ongoing behavior, remains viable.

Occupancy of dopamine D2 receptors in the mouse brain measured using ultra-high-resolution single-photon emission tomography and [123]IBF

Functional imaging of small animals, such as mice and rats, using ultra-high-resolution positron emission tomography (PET) and single-photon emission tomography (SPET) should be a valuable tool in studies of drug occupancy of cerebral binding sites. In this study we aimed to demonstrate the feasibility of using ultra-high-resolution SPET to measure the occupancy of dopamine D2 receptors by a competing drug, using the dopamine D2 receptor-specific radioligand iodine-123 5-iodo-7-N-[(1-ethyl-2-pyrrolidinyl) methyl] carboxamido-2,3-dihydrobenzofuran ([123I]IBF). Fourteen normal male mice (CD-1) were jugular vein-cannulated and a bolus infusion protocol was used to deliver 360 MBq [123I]IBF into the mouse (bolus-to-infusion ratio 1.8:1). The mice were scanned using an ultra-high-resolution triple-headed SPET system equipped with pinhole collimators. After sustained equilibrium had been achieved, varying doses of raclopride, a potent dopamine D2 receptor antagonist, were injected through the tail vein and the tracer was allowed to regain equilibrium. A simple equilibrium ratio of striatum to cerebellum provided a measure of D2 receptor binding both before and after injection of raclopride. Following raclopride administration, the system returned to equilibrium with lower specific binding in the striatum, while the counts in the cerebellum were unaffected. Receptor occupancy was 5.2% +/- 2.9% (control), 52.1% +/- 11.1% (0.3 mg/kg), 79.3% +/- 4.8% (1.0 mg/kg), and 94.7% +/- 2.2% (3.0 mg/kg), which gave an ED50=0.26 +/- 0.03 mg/kg using a single receptor site saturation model. This study has demonstrated clearly that ultra-high-resolution SPET of small animals is capable of measuring displacement and occupancy of dopamine D2 receptors by competing ligands.

Effects of SCH23390 and raclopride on a run-climb-run behavioral task in rats

The present study was designed to compare the putative differential behavioral consequences of treatment with SCH23390 (a selective dopamine D1 receptor blocker) and raclopride (a selective dopamine D2 receptor blocker) by employing a run-climb-run (RCR) behavioral task of different lengths. Rats were trained to traverse an uncovered floor alleyway (150 cm), climb a vertical rope (70 or 130 cm), and run across an upper board (100 cm) to access water for the reinforcement. At doses of 0.05, 0.10 and 0.15 mg/kg administered intraperitoneally 60 min before the behavioral session, both SCH23390 and raclopride significantly increased the total time to complete the tasks in a dose-related fashion. Microstructural analysis on the RCR behavioral performance revealed that the most apparent impairment induced by either drug was observed as the subject shifted motion from the end of the floor alleyway to the rope when hopping or to initiate climbing. However, the motion shift from climbing to running on the upper board was significantly impaired by raclopride, but not by SCH23390. Surprisingly, neither SCH23390 nor raclopride affected the climbing response itself. Running responses on the floor alleyway board were significantly disrupted by raclopride, whereas those on the upper board were significantly disrupted by SCH23390. Deficits induced by both drugs were more profound for the longer compared to the shorter rope, and were most notably shown at the transition area from running to climbing. These data indicate that both dopamine D1 and D2 receptors are involved in the RCR behavior performance. The results also suggest that the cost of motoric demand for behavioral performance is important for evaluating of the effects of drugs blocking dopamine receptors.

Drug-induced receptor occupancy: substantial differences in measurements made in vivo vs ex vivo

RATIONALE

The number of receptors occupied by a given drug is a central construct in understanding drug action in the brain. Two techniques have been commonly used to measure drug receptor occupancy. In one method, the drug and the radioligand used to measure occupancy compete in vivo while in the other method, the drug is injected into the living animal, the animal killed and the radioligand competes for available receptors ex vivo. While these methods are often used interchangeably, there has been no systematic comparison of their sensitivities and consistency.

OBJECTIVES

In this study, we performed a systematic within-animal comparison of drug-induced receptor occupancy as measured by the in vivo vs the ex vivo methods.

METHODS

We examined the occupancy of dopamine Do receptors by different doses of the drug raclopride using the in vivo and ex vivo autoradiographic methods in the same rat with 11C-raclopride and 3H-raclopride as radioligands, respectively.

RESULTS

The in vivo method showed a significantly greater sensitivity and internal consistency while the ex vivo method was less sensitive, and increasingly so as a function of longer incubation times. The lack of sensitivity was accounted for by the unidirectional dissociation of the drug from the receptors in the incubation medium.

CONCLUSIONS

Our data suggest that these two methods are not interchangeable; the ex vivo method is much less sensitive, lacks internal consistency and hence is best avoided.

Dopamine-dependent inhibition of glycine release in the nucleus accumbens of the rat brain during food consumption

Studies of Sprague-Dawley rats using in vivo intracerebral dialysis and high-performance liquid chromatography with electrochemical detection were used to investigate glycine release into the intercellular space of the nucleus accumbens during food consumption. The results showed that food consumption led to decreases in glycine levels in the intercellular space of the nucleus accumbens. Administration of the sodium channel blocker tetrodotoxin (1 microM), but not the glutamate reuptake blocker D,L-threo-hydroxyaspartate (1 mM), prevented the food-related behavior-induced decrease in glycine levels in the nucleus accumbens. Eating of food after administration of the dopamine D2 receptor blocker raclopride (10 microM) into the nucleus accumbens was accompanied by an increase in the glycine level in the intercellular space of this structure. These data provide evidence for the neural regulation of glycine release in the nucleus accumbens during food-related behavior, mediated via dopamine D2 receptors.

Inhibition of amygdaloid dopamine D2 receptors impairs emotional learning measured with fear-potentiated startle

Considerable advances have been made in understanding the neurocircuitry underlying the acquisition and expression of Pavlovian conditioned fear responses. Within the complex cellular and molecular processes mediating fearfulness, amygdaloid dopamine (DA), originating from cells in the ventral tegmental area (VTA) of the midbrain, is thought to contribute to fear-motivated responding. Considering that blockade of DA D(2) receptors is a common mechanism of action for antipsychotic agents, we hypothesized that inhibition of D(2) receptors in the amygdala may be involved in the antiparanoid effects of these drugs. To assess the role of amygdaloid DA D(2) receptors in aversive emotionality, the D(2) receptor antagonist raclopride was infused into the amygdala prior to Pavlovian fear conditioning. Potentiated startle was used as a behavioral indicator of fear and anxiety. Classical fear conditioning and acoustic startle testing were conducted in a single session allowing for the concomitant assessment of shock reactivity with startle enhancement. Depending on dose, the results found conditioned fear acquisition and retention to be impaired following administration of raclopride into the amygdala. Additionally, the learning deficit was dissociated from shock detection and from fear expression assessed with the shock sensitization of acoustic startle. These findings further refine the known neural mechanisms of amygdala-based emotional learning and memory and were interpreted to suggest that, along with D(1) receptors, D(2) receptors in the amygdala may mediate the formation and the retention of newly-acquired fear associations.

Prepulse inhibition deficits and perseverative motor patterns in dopamine transporter knock-out mice: differential effects of D1 and D2 receptor antagonists

Dopamine is known to regulate several behavioral phenomena, including sensorimotor gating and aspects of motor activity. The roles of dopamine D1 and D2 receptors in these behaviors have been documented in the rat literature, but few reports exist on their role in mice. We used dopamine transporter (DAT) (-/-) mice to examine the behavioral consequences of a chronically hyperdopaminergic state, challenging them with the preferential dopamine D2 receptor antagonist raclopride and D1 receptor antagonist SCH23390. At baseline, DAT (-/-) mice exhibited deficient sensorimotor gating as measured by prepulse inhibition (PPI) of the startle response, exhibited nonfocal preservative patterns of locomotion, and were hyperactive in a novel environment. Pretreatment with raclopride significantly increased PPI in the DAT (-/-) mice, whereas SCH23390 had no significant effect. Blockade of D2 receptors did not affect the predominantly straight patterns of motor behavior produced by the DAT (-/-) mice, but antagonism of D1 receptors significantly attenuated the preservative patterns, producing more of a meandering behavior seen in the DAT (+/+) control mice. Both D1 and D2 receptor antagonists decreased the hyperactivity seen in the DAT (-/-) mice. These findings support the role of the D2, but not the D1, receptor in the modulation of PPI in mice. Furthermore, D1 receptor activation appears to be the critical substrate for the expression of preservative patterns of motor behavior, whereas both D1 and D2 receptors appear to regulate the amount of motor activity.

Effects of acute or chronic administration of substituted benzamides in experimental models of depression in rats

The effects of substituted benzamides, sulpiride and raclopride on experimental models of depression were studied in male rats after acute or chronic administration in comparison to those of the classical antidepressant, clomipramine. In contrast to clomipramine (50 mg/kg), acute doses of sulpiride or raclopride (1 or 5 mg/kg) failed to change the behavioral response of animals tested in the despair (constrained swim) test or in the model of reserpine-induced changes in the open field behavior. These doses also did not modify the grooming response of rats exposed to a novel environment. Sulpiride or raclopride 10 mg/kg increased the immobility time in the despair test and reduced novelty-induced grooming. The repeated injection for 21 days of sulpiride or raclopride (1 or 5 mg/kg, but not 10 mg/kg) induced a reduction of the immobility period during the constrained swim test similar to that following the chronic treatment with clomipramine 50 mg/kg. This appeared to be a clear-cut reversed dose-response relationship for both substituted benzamides, being the dose potency 1 mg/kg>5 mg/kg>10 mg/kg. Raclopride was more potent than sulpiride in this respect. Furthermore, like clomipramine, sulpiride (1 or 5 mg/kg) and raclopride (1 mg/kg) antagonized reserpine-induced changes in the open field behavior and enhanced novelty-induced grooming. These results indicate that, in contrast to acute injection, repeated administration of small doses of the substituted benzamides, sulpiride or raclopride induce an effect similar to that of the classical antidepressant, clomipramine. The reverse dose-response relationship suggests that these drugs in small doses act on presynaptic dopamine D(2) receptors. This may be consistent with a postsynaptic action of greater doses that exert sedative effects and increase immobility time in the despair test.

Evidence for striatal dopaminergic overactivity in paroxysmal dystonia indicated by microinjections in a genetic rodent model

Mutant dystonic hamsters (dt(sz)), a model of primary paroxysmal dystonia, display attacks of generalized dystonia in response to mild stress in an age-dependent manner. Recent studies in dystonic hamsters have revealed decreased densities of dopamine D(1) and D(2) in the dorsal striatum. This finding has been interpreted as a down-regulation in response to enhanced dopamine release because systemic treatments with neuroleptics reduced the severity of dystonia while levodopa exerted prodystonic effects. Therefore, in the present study we investigated the effects of amphetamine as well as of selective D(1) or D(2) receptor agonists and antagonists on the severity of dystonia after systemic administrations and after microinjections into the dorsal striatum. Amphetamine and the dopamine D(2) agonist quinpirole increased the severity of dystonia after systemic and striatal injections, while the dopamine D(1) agonist SKF 38393 exerted only moderate prodystonic effects after systemic administration of a high dose but not after striatal injections. These results suggest that a predominant overstimulation of D(2) receptors is pathogenetically involved in the dystonic syndrome. Combined systemic or striatal administrations of the D(1) and D(2) receptor agonists did not reveal synergistic prodystonic effects at the examined doses. The selective D(1) antagonist SCH 23390 as well as the D(2) antagonist raclopride tended to decrease the severity of dystonia after systemic administration but failed to exert significant effects after striatal injection. The coadministration of ineffective doses of the antagonists SCH 23390 and raclopride, however, exerted an enormous antidystonic efficacy after both systemic and striatal injections. Since striatal injections of compounds which enhance dopaminergic activity aggravated dystonia, while coinjections of dopamine D1 and D2 receptor antagonists reduced the severity of dystonia, the present findings clearly support the hypothesis that striatal dopaminergic overactivity plays a crucial role for the manifestation of dystonic attacks in the hamster model of paroxysmal dystonia.

Microinjections of dopaminergic agents in the nucleus accumbens affect ethanol consumption but not palatability

It was determined whether ethanol palatability in rats could be changed by manipulating the reinforcement experienced during limited access consumption. During the first 3 days of the experiment, initial taste reactivity (TR) testing to distilled water (1 day) and 10% alcohol (2 days) was performed. Following the establishment of baseline TR, separate groups of animals received bilateral microinjections (0.5 microl/side) into the nucleus accumbens of either the nonspecific dopamine agonist d-amphetamine sulfate (20 microg, n = 10), the D(2) antagonist raclopride (1.0 microg, n = 8), or physiological saline (n = 5). The injections occurred at the same time each day for 5 consecutive days. Five minutes after the microinjection, the fluid-deprived rats were given 30-min access to 10% ethanol. Over the 3 days following drug administration, TR to distilled water and 10% alcohol was repeated. After this, the rats were once again given 30 min of access to 10% ethanol for 5 consecutive days, but without drug microinjection prior to alcohol access. A final TR exposure (the same as the others) was performed over the final 3 days of the study. Both raclopride and d-amphetamine administration produced reductions in ethanol consumption (in comparison to saline treatment). However, treatment with d-amphetamine and raclopride during ethanol consumption did not cause significant, conditioned changes in palatability as measured by the taste reactivity procedure. These results suggest that dopamine plays a role in the motivation to consume ethanol but this neurotransmitter is not involved in evaluating its incentive value.

Effects of acute metabolic stress on striatal dopamine release in healthy volunteers

Several lines of evidence indicate that a variety of metabolic stressors, including acute glucose deprivation are associated with dopamine release. Pharmacologic doses of the glucose analogue, 2-deoxyglucose (2DG) cause acute glucoprivation and are associated with enhanced dopamine turnover in preclinical studies. In this study, we utilized [11C]raclopride PET to examine 2DG-induced striatal dopamine release in healthy volunteers. Six healthy volunteers underwent PET scans involving assessment of 2DG-induced (40 mg/kg) decrements in striatal binding of the D(2)/D(3) receptor radioligand [11C]raclopride. Decreases in [11C]raclopride specific binding reflect 2DG-induced changes in synaptic dopamine. Specific binding significantly decreased following 2DG administration, reflecting enhanced synaptic dopamine concentrations (p =.02). The administration of 2DG is associated with significant striatal dopamine release in healthy volunteers. Implications of these data for investigations of the role of stress in psychiatric disorders are discussed.

Measurement of dopamine release with continuous infusion of [11C]raclopride: optimization and signal-to-noise considerations

PET studies with [11C]raclopride provide an indirect measure of changes in synaptic dopamine. Previously, we used the bolus-plus-infusion (B/I) method to assess dopamine response from the percentage change in binding potential (deltaBP) before and after administration of amphetamine. The goal of this work is to optimize the measurement of changes in neurotransmitter with the B/I method by choosing the optimal timing for pre- and poststimulus scanning.

METHODS

Two sources of variability in deltaBP were considered: within-subject and between-subject noise. A noise model based on a phantom study and human data was used to evaluate the within-subject noise. For between-subject noise, simulated time--activity curves were generated from measured [11C]raclopride input functions. Optimal timing to measure deltaBP was determined and applied to human data.

RESULTS

According to the simulation study, the optimal scan times for pre-and poststimulus scans were 39-50 and 58-100 min, respectively. The optimal timing resulted in a 28% noise reduction compared with the original timing. By applying the optimal timing to human studies, the statistical significance of the difference in deltaBP between patients with schizophrenia and healthy volunteers increased from P = 0.038 to 0.012.

CONCLUSION

Careful assessment of the sources of noise in receptor imaging studies can increase the sensitivity of the B/I method for the detection of biologic signals.