Small effect of dopamine release and no effect of dopamine depletion on [18F]fallypride binding in healthy humans

Molecular imaging has been used to estimate both drug-induced and tonic dopamine release in the striatum and most recently extrastriatal areas of healthy humans. However, to date, studies of drug-induced and tonic dopamine release have not been performed in the same subjects. This study performed positron emission tomography (PET) with [18F]fallypride in healthy subjects to assess (1) the reproducibility of [18F]fallypride and (2) both D-amphetamine-induced and alpha-methyl-p-tyrosine (AMPT)-induced changes in dopamin release on [(18)F]fallypride binding in striatal and extrastriatal areas. Subjects underwent [18F]fallypride PET studies at baseline and following oral D-amphetamine administration (0.5 mg/kg) and oral AMPT administration (3 g/70 kg/day over 44 h). Binding potential (BP) (BP(ND)) of [18F]fallypride was calculated in striatal and extrastriatal areas using a reference region method. Percent change in regional BP(ND) was computed and correlated with change in cognition and mood. Test-retest variability of [18F]fallypride was low in both striatal and extrastriatal regions. D-Amphetamine significantly decreased BP(ND) by 8-14% in striatal subdivisions, caudate, putamen, substantia nigra, medial orbitofrontal cortex, and medial temporal cortex. Correlation between change in BP(ND) and verbal fluency was seen in the thalamus and substantia nigra. In contrast, depletion of endogenous dopamine with AMPT did not effect [18F]fallypride BP(ND) in both striatum and extrastriatal regions. These findings indicate that [18F]fallypride is useful for measuring amphetamine-induced dopamine release, but may be unreliable for estimating tonic dopamine levels, in striatum and extrastriatal regions of healthy humans.     

Placebo, placebo effect and clinical trials

Recent studies have begun to unveil some of the biochemical bases of the placebo effect. Thus, while placebo analgesia is related to the release of endogenous opioids, placebo-induced dopamine release leads to motor improvement in Parkinson's disease. A theory proposes that the placebo effect is mediated by the activation of the reward circuitry. These biochemical findings indicate that the placebo effect is real, and suggest that many ethical arguments and controversies regarding the use of placebos should perhaps be reconsidered. While it may be advisable to minimize the placebo effect in clinical trials in order to estimate the pure effect of the active treatment, acting in the patient's best interest may require maximizing the placebo effect in the usual clinical setting.     

Placebo effect in Parkinson's disease]

"Placebo" is Latin for "I shall please". The placebo effect has been widely documented by randomized placebo-controlled drug studies. One of the best examples of placebo effectiveness is that have been shown in clinical trials of anti-parkinsonian drugs. The placebo effect is observable not only in drug trials but also with deep brain stimulation. Recent advances in research on the placebo effect in Parkinson's disease (PD) have suggested that motor symptoms of PD can be essentially improved by placebo. A recent study using positron emission tomography (PET) with raclopride demonstrated that release of endogeneous dopamine in the dorsal striatum occurs in placebo-responsive patients with PD. This suggests that placebo-induced expectation of clinical improvement may activate endogenous dopamine in the striatum, and that placebo effectiveness is thus achieved by endogenous dopamine supplementation. Indeed, decreased neuronal activities in the subthalamic nucleus (STN), that were recorded during surgery to implant deep brain stimulation electrodes, correlated well with placebo-induced clinical improvement in patients with PD. Although the detailed pathophysiological mechanism underlying the placebo effects remains uncertain, theoretically, the placebo effect has generally been explained by two different mechanisms: one is conditioning theory (pavlovian conditioning), and the other is cognitive theory (expectation of clinical improvement). Although both mechanisms may contribute to placebo effects, the placebo effect in PD may be attributed more to cognitive mechanisms such as expectation of improvement, because the placebo effect can be obtained in de novo PD patients. There have been accumulating findings that suggest a functional relationship between dopamine and the expectation of clinical improvement (reward). Further basic studies are required to clarify the complex link between dopamine and the reward system, but such findings will contribute to a better understanding of the pathophysiological mechanism underlying the placebo effect in PD.     

Placebo effect and clinical trials in migraine therapy

Migraine is a cyclical condition in which the cycles may occur or remit in an unpredictable fashion. The goal of prophylactic antimigraine therapy is to induce remission of an active cycle of migraine or to inhibit occurrence of individual headaches until the cycle remits for other reasons. The placebo effect probably takes advantage of these cycles. The placebo effect is significant and quite potent. The placebo effect may be divided into (1) the initial effect and (2) the continuing effect which is seen following a stabilization period. The initial placebo effect is dramatic with 62% of 188 subjects improving by 75% after 4 weeks of placebo. The continuing effect is demonstrated by occurrence of 75% further improvement in 28% of 282 subjects in 7 studies in which comparison of results after 4-12 weeks was made with a placebo stabilization period. The placebo effect is a very potent one and must be taken into account in designing and carrying out studies of migraine therapy.     

Isoflurane anesthesia induces biphasic effect on dopamine release in the rat striatum

The effect of isoflurane anesthesia on changes in the extracellular concentrations of dopamine (DA) and its metabolites (3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)) modulated by pargyline, monoamine oxidase inhibitor, was studied using in vivo microdialysis techniques. A microdialysis probe was implanted into the right striatum of male SD rats. Each rat (n=5-6) was given saline or the same volume of 30 or 75 mg kg(-1) pargyline intraperitoneally with or without 1 h isoflurane anesthesia (1 or 3%). Isoflurane anesthesia increased the extracellular concentration of DA in high dose (3%) and increased the metabolite concentrations in a dose-dependent manner. Pargyline administration increased the extracellular concentration of DA and 3-MT, and decreased that of other metabolites. After 30 mg kg(-1) pargyline treatment, 1% isoflurane-induced DA release and increasing of 3-MT were preserved, whereas high dose isoflurane (3%) decreased the concentration of metabolites (DOPAC and HVA), despite of the increase by low dose isoflurane (DOPAC). When 75 mg kg(-1) pargyline was administered, isoflurane anesthesia decreased the concentration of DA and DOPAC. The isoflurane-induced 3-MT increase was preserved in all experiments. Our results suggest that isoflurane anesthesia induced biphasic effect on DA regulation probably by the potentiation of DA release and the inhibition of DA synthesis. Isoflurane might modulate DA homeostasis presynaptically.     

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.     

Direct effect of 17 beta-estradiol on striatum: sex differences in dopamine release

The nigrostriatal dopamine (DA) system is sexually dimorphic. In female but not male rats, striatal DA activity is modulated by gonadal steroid hormones. Ovariectomy (OVX) decreases striatal DA release and turnover. Estrogen replacement restores the response to that of the intact female in estrus. In contrast, castration (CAST) of male rats has no effect on the stimulated release of DA from striatal tissue. This report addresses the question: Dose estrogen act directly on the striatum to induce changes in DA release? Physiological concentrations of 17b?-estradiol and other steroids or a nonsteroidal estrogen analog were applied directly to striatal tissue maintained in an in vitro superfusion system. The effect of hormonal treatments on the responsiveness of striatal DA terminals to stimulation was examined in tissue from OVX females and intact and CAST male rats. The results are summarized as follows: (1) Infusion of 17b?-estradiol (p < 0.01) and diethylstilbesterol (p < 0.05) increased amphetamine (AMPH)-stimulated striatal DA release from striatal tissue of OVX female rats compared with the effect of cholesterol. 17alpha-Estradiol also tended to potentiate the striatal DA response to AMPH, but this result was not statistically significant (p < 0.062). 17b?-Estradiol had no effect on AMPH-stimulated DA release from striatal tissue of intact male rats. (2) The KCl-stimulated release of DA from striatal tissue of OVX rats exposed in vitro to 100 pg/ml 17b?-estradiol (a physiological dose) was significantly greater (p < 0.05) than the response after exposure to vehicle. In contrast, 1,000 pg/ml 17b?-estradiol produced a decrease in KCl-induced striatal DA release (p < 0.05), whereas 17alpha-estradiol (100 pg/ml or 1,000 pg/ml) did not significantly influences the response to KCl. (3) The pulsatile administration of 17b?-estradiol stimulated DA release from strital tissue of OVX females (p < 0.05; compared with tissue from an OVX group that received vehicle or CAST male rats exposed to either 17b?-estradiol or vehicle). It is concluded that with tissue from OVX rats, physiological concentrations of estrogen can act directly on striatal tissue in vitro to stimulate DA release and to increase striatal DA responsiveness to stimulation, whereas prolonged exposure or high concentrations of 17b?-estradiol decreases striatal DA responsiveness. The striatum and/or the striatal DA system are sexually dimorphic in this regard.     

Mazindol, nomifensine and desmethylimipramine inhibit potassium-induced release of dopamine: effect of stimulus strength

Summary. Catecholamine uptake blockers exhibit a second action: They suppress the potassium (K⁺)-stimulated release of [³H]-dopamine (DA) from rat striatal tissue in vitro. In the present study, a K⁺ dose-response curve was obtained for the release of both [³H]-DA and endogenous DA from striatal tissue, in the absence and presence of catecholamine uptake blockers. Three drugs were used, namely, mazindol, nomifensine, and desmethylimipramine. The data showed that diminished release in the presence of mazindol or nomifensine was dependent upon the concentration of K⁺. Marked inhibition of stimulus-induced release of either ³H-DA or endogenous DA was observed at low concentrations of K⁺ (15 and 20 mM), but not at higher concentrations of K⁺ (40 and 60 mM). In contrast, the diminished release of DA in the presence of desmethylimipramine persisted over the K⁺ range of 20–60 mM. These data show that the drug action observed previously was not restricted to a specific neuronal pool of DA that was labelled with [³H]-DA. Further, the data emphasize the importance of the concentration of the depolarizing stimulus when evaluating the effects of drugs on release of catecholamines. Accepted January 29, 1998; received December 8, 1997     

The effect of verapamil on GABA and dopamine release does not involve voltage-sensitive calcium channels

The effect of verapamil, which belongs to the group of drugs collectively referred to as 'organic Ca2+ channel blockers', was investigated on the basal and stimulated release of the neurotransmitters dopamine and GABA in rat striatum synaptosomes. Verapamil inhibits the Na(+)-dependent release of GABA in response to depolarization with an IC50 of 25 microM, whereas it is unable to modify the Na2(+)-independent, Ca2(+)-dependent fraction of GABA release induced by high K+ depolarization. Verapamil does not modify the basal release of GABA but stimulates the basal release of dopamine in a dose-dependent manner (ED50 5 microM). This verapamil-induced outflow of dopamine is independent of Ca2+ and occurs in the presence of tetrodotoxin, indicating that it is not mediated by voltage-sensitive Ca2+ or Na+ channels of the presynaptic membrane. Dopamine release induced by verapamil is cumulative with that induced by depolarizing agents (high K+ or veratridine). As verapamil, pimozide, a neuroleptic of the diphenylbutylpiperidine type, increases the basal and stimulated release of dopamine. We conclude that the opposite effects of verapamil of GABA and dopamine release are due to differences in the releasable fractions of these 2 types of neurotransmitters. Besides, none of these effects are directly linked with the blockade of voltage-operated Ca2+ channels of the presynaptic membrane.     

Chromogranin A inhibits retinal dopamine release

Chromogranin A (CGA), a peripherally active prohormone, is a soluble component in the secretory granules of many endocrine tissues and is cosecreted with their peptide or amine hormones. Using an antibody prepared against purified rat adrenal CGA, immunostaining was localized to the inner and outer plexiform layers of the rat retina and to selected ganglion cells. Exogenous CGA (purified from human adrenals) when applied to perfused rat retina potently inhibited the potassium-induced release of endogenous dopamine (DA). This action was dose-dependent, with an IC₅₀ of 3 nM; at 100 nM CGA retinal DA release was completely abolished.     

Serotonin-mediated striatal dopamine release involves the dopamine uptake site and the serotonin receptor

Modulation of striatal dopamine (DA) release by serotonin (5HT) and its antagonists was studied utilizing in vitro perfusion techniques. In isolated striatal tissue, 5HT (10 microM) increased the fractional basal release of labeled DA. The 5HT(2/1c) antagonist ketanserin (5 microM) also stimulated the basal release. These two effects were mediated by different mechanisms as cocaine (10 microM) greatly inhibited the 5HT-mediated response, but slightly increased the ketanserin-mediated response. 6-Nitroquipazine maleate (10 microM, 5HT uptake inhibitor) partially inhibited both responses. Inhibition by GBR 12909 (DA uptake inhibitor) at 1 microM of the 5HT-mediated DA release was similar to that of cocaine, but at 10 microM it increased release before addition of 5HT, and maintained elevated DA release while present in the incubation medium. At 1 microM GBR 12909, ketanserin-mediated DA release was stimulated and a much greater release was seen at 10 microM, but the prolonged release was not observed as after 5HT-mediated release. Among other antagonists methiothepin (5HT(1,2,6) antagonist) also enhanced DA release, whereas oxymetazoline (5HT(1A,1B,1D) agonist) had no effect. RS2359-190 (5HT(4) antagonist) had a small effect (slight stimulation) on 5HT-mediated DA release, and no effect on ketanserin-mediated DA release. CGS 12066A (5HT(1B) agonist) inhibited 5HT-mediated DA release. The glutamate antagonist MK-801 and the GABA(A) antagonist bicuculline had no affect on either response. These results indicate that 5HT-mediated DA release occurs via reversal of the DA transporter and that inhibitory presynaptic 5HT heteroreceptors and both inhibitory and stimulatory somato-dendritic 5HT receptors regulate release. In addition to the reversal of the transporter, an inhibitory 5HT(2) component was identified.     

Peptide-induced grooming behavior and caudate nucleus dopamine release

We simultaneously measured the display of grooming behavior and, by monitoring the extracellular dopamine concentration via transversal microdialysis, the release of dopamine in the caudate nucleus in freely moving rats after i.c.v. administration of 1 μg adrenocorticotropic hormone-(1–24) (ACTH-(1–24)). During a period of 1 h after administration of the peptide, the incidence of excessive grooming behavior was increased. Concomitantly, the concentration of dopamine in the caudate nucleus dialysates was significantly increased (maximal effect 151% of basal release) whereas that of its metabolite DOPAC was unchanged. The potent α-melanocyte stimulating hormone (α-MSH) receptor agonist, [Nle⁴,D-Phe⁷]α-MSH, induced grooming behavior and stimulated caudate nucleus dopamine release (maximal effect 148% of basal release) whereas ACTH-(7–16)-NH₂ did neither induce grooming behavior nor cause an increase in caudate nucleus dopamine release. Single-dose tolerance was observed for ACTH-induced grooming but not for ACTH-induced dopamine release. These data are in support of the proposed involvement of brain dopamine systems in grooming behavior of the rat but at the same time suggest that the effect of ACTH/MSH-like peptides on dopaminergic transmission in the caudate nucleus is proximal to the final neural pathway involved in ACTH-induced grooming behavior.     

The effect of a 5-HT1A receptor agonist on striatal dopamine release

5-HT1A receptor agonists consistently reduce neuroleptic induced catalepsy in rats. A serotonin-dopamine interaction has been proposed to underlie this effect. Specifically, 5-HT1A receptor agonists may reduce the activity of serotonergic projections that inhibit dopaminergic nigrostriatal neurones, therefore increasing dorsal striatal dopamine levels and partially overcoming the neuroleptic blockade of D2 receptors. We tested the hypothesis that 5-HT1A receptor agonists increase striatal dopamine release in man using PET scanning with the selective D2 receptor radioligand [11C]raclopride, which is sensitive to endogenous dopamine levels. Six healthy volunteers received two PET scans, one after placebo, the other after 1 mg flesinoxan, a selective 5-HT1A receptor agonist. Binding potential values for striatal subdivisions were determined using a simplified reference tissue model. We did not find any difference in striatal [11C]raclopride binding between conditions, even though flesinoxan lead to typical 5-HT1A receptor agonist side effects and produced elevation of growth hormone in five of the six subjects. Our results suggest that the anticataleptic effect of 5-HT1A receptor agonists is not mediated by striatal dopamine release, and indicates a need for further research with other suitable 5-HT1A receptor agonists.     

Placebo and modafinil effect on sleepiness in obstructive sleep apnea

INTRODUCTION: Previous studies have evaluated the effect of modafinil on residual excessive daytime sleepiness (EDS) in patients with obstructive sleep apnea syndrome (OSAS) under effective CPAP treatment. Even though those trials also used placebo groups, we suppose that the placebo effect might influence the patients' response to modafinil. METHODS: Twenty sleepy patients with OSAS under CPAP treatment were selected. All of them had Epworth Sleepiness Scale (ESS) >10. Following baseline evaluation (T1), all subjects were instructed to take placebo for 7 days. After this single-blind placebo phase and second evaluation (T2), patients were randomly allocated to placebo or modafinil treatment for 21 days in a double-blind protocol. Patients underwent a final evaluation (T3) on the last day of drug intake. The evaluations at T1, T2 and T3 consisted of: medical and laboratory examinations, nocturnal polysomnography, ESS, maintenance of wakefulness test (MWT) and complex reaction time (CRT-NY). In addition, in T2 and T3 the change of illness severity scale (CGI-C) and the evaluation of quality of life (SF-36) were applied. RESULTS: The comparison between the two groups during the three periods studied, showed the following results: in the modafinil group, ESS score did not change during the initial placebo period, but there was a significant reduction during the modafinil treatment period (p=0.0006); in the placebo group a significant reduction occurred during the initial placebo period (p=0.05), and no further change was observed in the treatment (placebo) period. A significant difference was found between the two groups after the placebo period (T2) (p=0.02). Three patients (33%) of the modafinil group and 9 patients (81%) of the placebo group were classified as placebo-responsive (X2: p=0.039). In the treatment period, reaction time was significantly reduced in the modafinil group compared to the placebo group (p<0.02). There was a trend toward improvement in overall clinical condition and also in some domains of SF-36 in the modafinil group. CONCLUSION: In summary, our study confirms that modafinil used adjunctively with CPAP therapy improves subjective daytime sleepiness in patients with OSAS who were regular users of CPAP therapy but still experienced sleepiness. Moreover, it could help in the improvement of objective measures of behavioral alertness and reduce functional impairments. The usefulness of a blinded placebo period for systematic investigation of placebo role in studies based on subjective response is a point that should be considered in this type of drug trial.