Decreased dopamine transporter availability in male smokers -- a dual isotope SPECT study

Introduction

Although the mesolimbic dopaminergic system has been shown to play a role in reinforcing tobacco smoking, results of imaging studies that examine the relationship between tobacco smoking and the central dopamine system remain discrepant. To delineate the role of tobacco addiction in central pre- and post-synaptic dopaminergic activities, we analyzed the central D₂-family receptors, the dopamine transporters (DAT), and degrees of dependence in male smokers.

Methods

Eleven male smokers and 11 healthy non-smokers were recruited. The striatal dopamine D₂/D₃ receptor availability was approximated using SPECT and [¹²³I] IBZM while the DAT availability was approximated using SPECT and [^99mTc] TRODAT-1. All of the smokers completed the Fagerström Test for Nicotine Dependence (FTND) and other related questionnaires.

Results

A decrease in DAT availability in the striatum of male smokers is noted (p < 05). However, the striatal D₂/D₃ receptor availability in male smokers does not differ from that of non-smokers.

Conclusions

These findings suggest that cigarette smoking may alter central dopamine functions in males, particularly at the pre-synaptic sites.     

Blockade of in vivo binding of I-labeled 3-iodobenzamide (IBZM) to dopamine receptors by D2 antagonist and agonist

The in vivo binding of [¹²⁵I]3-iodobenzamide (IBZM), a substituted benzamide, to DA receptor binding sites in the caudate nucleus, nucleus accumbens, and olfactory tubercle was investigated by using ex vivo autoradiography. The in vivo binding of IBZM seems to be selective to D₂ dopamine receptors, since the binding was blocked by pretreatment of animals with D₂ agonist LY-171555 or antagonist YM-09151-2. Furthermore, in vitro binding assays in striatal membranes confirmed that IBZM binding was highly selective to D₂ sites. Thus, IBZM, when labeled with ¹²³I (T1/2: 13 h; 159 kev), could be a potential ligand for imaging D₂ dopamine receptors by single photon emission computerized tomography procedures.     

Autoradiographic localization and quantification of dopamine D2 receptors in normal human brain with [H]N-n-propylnorapomorphine

The precise distribution of dopamine receptors has been studied autoradiographically in the normal human brain using [³H]N-n-propylnorapomorphine ([³H]NPA) as a ligand. Preliminary experiments aimed at optimizing the binding assay conditions revealed that preincubation washing of caudate nucleus sections was a prerequisite to obtain a good ratio of specific to non-specific binding. The binding of [³H]NPA to caudate-putamen sections was saturable, stereospecific, reversible, of high affinity (K_d = 0.27–0.35 nM) and occurred at a single population of sites. Competition experiments with various drugs indicated that in the caudate-putamen the specific [³H]NPA binding sites possess the pharmacological features of the dopamine D₂ receptor. The highest levels of [³H]NPA binding sites were found in the caudate nucleus, putamen, globus pallidus and nucleus accumbens. There were also intermediate to low concentrations of the ³H-ligand in the hippocampus, the insular and cingular cortices and in the occipito-temporal gyrus, while almost undetectable levels of binding were found in the anteior frontal cortex. Thorough examination of the subregional distribution of [³H]NPA binding sites in the caudate-putamen-pallidum complex revealed heterogenous patterns of radioactivity. In these brain regions, the distribution of autoradiographic grains was punctate and islands of high and low densities were observed. Moreover, in the caudate nucleus, there was a subtle high lateral to low medial gradient in the topography of the [³H]NPA binding sites and a more pronounced gradient along the rostrocaudal axis; the highest levels of binding being located at the midbody of the nucleus. No gradients of [³H]NPA binding were observed in the putamen. The present data indicate that [³H]NPA is a suitable ligand for accurate autoradiographic labeling of dopamine D₂ receptors in human postmortem brain tissue and that dopamine receptors are heterogeneously distributed and topographically organized in patches and gradients in the basal ganglia regions.     

Evidence for the behavioral supersensitivity of dopamine D2 receptors without receptor up-regulation in morphine-abstinent rats

The effect of morphine tolerance-dependence and abstinence on the characteristics of dopamine D₂ receptors in brain regions and spinal cord was determined in the rat. Male Sprague-Dawley rats were implanted s.c. under light ether anesthesia with 6 morphine pellets for a 7-day period, each containing 75 mg of morphine free base. Rats implanted with placebo served as controls. This procedure resulted in the development of tolerance to morphine as evidenced by decreased analgesic response to a challenge dose of morphine. Similarly, the development of physical dependence was evidenced by a decreased in body weight and colonic temperature after morphine pellet removal (withdrawal). The binding characteristics (B_max andK_d values) of [³H]spiroperidol to dopamine D₂ receptors were determined in the tissues of morphine-tolerant and morphine-abstinent rats. In the tolerant rats, the pellets were left intact at the time of sacrificing, whereas, in the abstinent rats the pellets were removed 18 h prior to sacrificing. The binding of [³H]spiroperidol was determined in membranes prepared from brain regions (hypothalamus, hippocampus, cortex, pons and medulla, midbrain, corpus striatum and amygdala) and spinal cord of rats from various treatment groups. [³H]Spiroperidol bound to brain regions and spinal cord at a single high affinity site. TheB_max or theK_d values in brain regions and spinal cord of morphine-tolerant and -abstinent rats did not differ from their respective placebo controls. The behavioral responses to a selective dopamine D₂ receptor agonist, 2-bromo-α-ergocryptine were also determined in the morphine-abstinent rats. In morphine-abstinent rats, increased behavioral activity, such as total distance travelled, number of movements, and the number of stereotypic movements was seen as compared to placebo controls. The dose of 2-bromo-α-ergocryptine which by itself had no effect on any type of behavioral activity in placebo-treated rats, increased the total distance travelled, horizontal activity, number of movements, and movement time in morphine-abstinent rats. Although in morphine-tolerant or morphine-abstinent rats, the characteristics of [³H]spiroperidol binding to dopamine D₂ receptors in brain regions and spinal cord were unchanged, the supersensitivity was observed to behavioral responses of 2-bromo-α-ergocryptine, a selective dopamine D₂ receptor agonist. These results provide an evidence for behavioral responses of 2-bromo-α-ergocryptine, a selective up-regulation in morphine abstinent rats. Previously, we have show that dopamine D₁ receptors are unaffected in morphine tolerant rats but are modified in morphine-abstinent rats. Thus, in the morphine abstinent process a significant difference was noted in the biochemical characteristics of dopamine D₁ and D₂ receptors.     

C957T polymorphism of the human dopamine D2 receptor gene predicts extrastriatal dopamine receptor availability in vivo

The C957T (rs6277) single nucleotide polymorphism (SNP) of the human dopamine D2 receptor (DRD2) gene (DRD2) affects DRD2 mRNA stability and has been shown to predict striatal DRD2 availability (B_max/K_D) in vivo in man. Specifically, the C/C genotype is associated with low striatal DRD2 availability (C/C<C/T<T/T). It is not known, however, whether this pattern of genetic regulation of DRD2 expression also applies to low density DRD2 populations in extrastriatal regions. We analyzed extrastriatal DRD2 availability (indexed by binding potential, BP_ND) measured in 38 healthy male volunteers with 3D-PET and the high-affinity DRD2 radioligand [¹¹C]FLB457. The subjects were genotyped for the C957T as well as for two other widely studied DRD2 SNPs, the TaqIA (rs1800497) and the −141C Ins/Del (rs1799732). Statistical analyses showed that the C957T C/C genotype was associated with high extrastriatal DRD2 BP_ND throughout the cortex and the thalamus (C/C>C/T>T/T). Also the TaqIA A1 allele carriers (p = 0.101) tended to have higher extrastriatal DRD2 BP_ND compared to non-carriers whereas the −141C Ins/Del genotype did not influence extrastriatal DRD2 BP_ND. Our findings indicate that the DRD2 SNPs regulate DRD2 availability in the human cortex and in the thalamus in vivo. However, the regulation pattern is different from that observed previously for striatal DRD2 availability in vivo, which may reflect distinct functional roles of dopamine and DRD2 in the cortex versus the striatum. The results provide useful information for the interpretation of genetic studies exploring the role of the DRD2 in normal physiology as well as in psychiatric and neurological diseases.     

Properties of D2 dopamine receptor autoradiography: High percentage of high-affinity agonist sites and increased nucleotide sensitivity in tissue sections

[3H]Spiroperidol (spiperone) binding in the presence of mianserin, a serotonin (5-HT2) receptor antagonist, was characterized in rat brain using quantitative autoradiography. All binding parameters were directly determined from film densities. Competition and kinetic studies revealed that [3H]spiroperidol binds to a site having characteristics of the dopamine, D2, receptor in striatum. The general binding parameters were similar to values obtained in homogenate and swabbed section studies except as related to agonist binding and guanine nucleotide sensitivity. Competition studies with dopamine revealed biphasic competition curves with a Kh of 8.23 nM and a Kl of 12.3 microM. The percentage of high-affinity sites was 90%. Guanine nucleotides (1 microM guanylyl-imidodiphosphate) completely converted the high-affinity site to a low-affinity site. Quantitative regional distribution studies revealed high binding in striatum, olfactory tubercle and nucleus accumbens, with lower binding in other dopamine innervated regions including frontal and cingulate cortex. [3H]Spiroperidol was also found to bind to a spirodecanone site with an anatomical localization distinct from the dopamine and serotonin systems and in a region (entorhinal cortex) not previously reported. This report provides a detailed pharmacologic and regional characterization of [3H]spiroperidol binding to D2 receptor in rat brain using quantitative autoradiography to determine all binding parameters. This report also demonstrates an increased percentage of sites in the high-affinity state of the D2 receptor in tissue sections and increased affinity of the guanine regulatory protein for guanine nucleotides.     

Identification of extrastriatal dopamine D2 receptors in post mortem human brain with [I]epidepride

The regional distribution of striatal and extrastriatal dopamine D₂ receptors in human brain was studied in vitro with(S)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-[¹²⁵I]iodo-2,3-dimethoxybenzamide, [¹²⁵I]epidepride, using post mortem brain specimens from six subjects. Scatchard analysis of the saturation equilibrium binding in twenty-three regions of post mortem brain revealed highest levels of binding in the caudate (16.5 pmol/g tissue) and putamen (16.6 pmol/g tissue) with lower levels seen in the globus pallidus (7.0 pmol/g tissue), nucleus accumbens (7.2 pmol/g tissue), hypothalamus (1.8 pmol/g tissue), pituitary (1.3 pmol/g tissue), substantia innominata (1.0 pmol/g tissue), and amygdala (0.87 pmol/g tissue). Of note was the presence of dopamine D₂ receptors in the four thalamic nuclei studied, i.e. anterior nucleus (1.0 pmol/g tissue), dorsomedial nucleus (0.96 pmol/g tissue), ventral nuclei (0.72 pmol/g tissue), and pulvinar (0.86 pmol/g tissue), at levels comparable to the amygdala (0.87 pmol/g tissue) and considerably higher than levels seen in anterior cingulate (0.26 pmol/g tissue) or anterior hippocampus (0.36 pmol/g tissue). The frontal cortex had very low levels of dopamine D₂ receptors (0.17–0.20 pmol/g tissue) while the inferior and medial temporal cortex had relatively higher levels (0.31–0.46 pmol/g tissue). Inhibition of [¹²⁵I]epidepride binding by a variety of neurotransmitter ligands to striatal, ventral thalamic and inferior temporal cortical homogenates demonstrated that [¹²⁵I]epidepride binding was potently inhibited only by dopamine D₂ ligands. The present study demonstrates that dopamine D₂ receptors are present in basal ganglia, many limbic regions, cortex and in the thalamus. The density of thalamic D₂ receptors is comparable to many limbic regions and is considerably higher than in cortex. Very few frontal lobe D₂ receptors are present in man.     

Effects of D2 dopamine receptor agonist and antagonist on brain activity in the rat assessed by functional magnetic resonance imaging

The effects of D₂ dopamine receptor agonist, bromocriptine (BROMO), and antagonist, haloperidol (HPD), on brain activity were investigated in rats by functional magnetic resonance imaging. T2^*-weighted signal intensity was increased in the hypothalamus at 120 min after acute administration of BROMO, and in the ventral posterior and dorsomedial nuclei of the thalamus from 30 to 120 min. In contrast, the signal intensity was decreased in the caudate–putamen at 30 min after acute administration of HPD, in the hypothalamus from 30 to 60 min, and in the perirhinal cortex at 30 min. After chronic (2 weeks) HPD treatment, acute administration of HPD decreased signal intensity in the caudate–putamen at 60 min, in the hypothalamus at 30 min, the perirhinal cortex from 2 to 120 min, the dorsomedial and ventral posterior nuclei of the thalamus from 2 to 120 min, and the medial nucleus of the amygdala from 60 to 120 min. These results suggest that (1) the D₂ receptor agonist increased the activity of the thalamic nuclei and the hypothalamus, while the D₂ receptor antagonist suppressed brain activity in the regions where D₂ receptors were present, (2) the suppression of brain activity in the thalamic nuclei and the perirhinal cortex by acute HPD administration was enhanced by chronic HPD treatment, and (3) the effects of antipsychotic drugs on the thalamus, amygdala, and perirhinal cortex may be related to their therapeutic efficacy, since clinical improvement in schizophrenic patients appears several days after the start of HPD treatment.     

Exploring the relationship between social attachment and dopamine D2/3 receptor availability in the brains of healthy humans using [11C]-(+)-PHNO

Differences in striatal dopamine (DA) function may be related to differences in the degree of social attachment to others. Using positron emission tomography (PET), socially detached persons demonstrate reduced DA D_2/3 receptor (D_2/3R) availability in the striatum. However, previous PET studies have only used antagonist radiotracers for D_2/3R and have not specifically examined regions of interest (ROIs) such as the ventral striatum (VS). In 32 healthy persons, we investigated the relationship between self-reported attachment and DA D_2/3R availability in striatal and extrastriatal ROIs as measured using the agonist radiotracer [¹¹C]-(+)-PHNO. Surprisingly, more social attachment—as measured by the attachment subscale of the temperament and character inventory—was related to less [¹¹C]-(+)-PHNO binding in the VS (r(30) = ?.43, p = .01). This relationship held in a subsample who also completed the detachment subscale of the Karolinska Scales of Personality (r(10) = .62, p = .03). However, no relationships were observed with BP_ND in the dorsal striatum or D₃R-specific ROIs. One potential explanation for these findings is that persons who are more socially detached have less endogenous DA occupying D_2/3R in the VS. This interpretation warrants investigation by future research. These findings may help us better understand the neurochemical basis of attachment.     

Sex-specific variation of MRI-based cortical morphometry in adult healthy volunteers: the effect on cognitive functioning

Previous investigations have revealed sex-specific differences in brain morphometry. The effect of sex on cortical thickness may be influencing cognitive differences between sexes. With this exploratory study, we aimed to investigate the effect of sex in MRI-based cerebral cortex morphometry in healthy young volunteers and how the variability in cortical measures might affect cognitive functioning in men and women. 76 young healthy volunteers (45 men and 31 women) underwent a 1.5 T MR scan and 53 of them completed a comprehensive cognitive battery. Overall no gross significant differences between sexes were found in cortical thickness, surface area and curvature indexes. However, there was a significant group by hemisphere interaction in the total cortical thickness (F(1,72) = 5.02; p = 0.03). A greater leftward asymmetry was observed in cortical thickness in males. Only females show significant associations between cortical thickness and cognitive functioning (IQ and executive functioning). In conclusion, our findings do not support the notion of sexual dimorphism in cortical mantle morphology. The results also suggest that variability in cortical thickness may affect cognitive functioning in females but not in males.     

Lithium effects on brain glutamatergic and GABAergic systems of healthy volunteers as measured by proton magnetic resonance spectroscopy

Lithium is a first-line medicinal treatment for acute bipolar disorder and is also used prophylactically in manic depressive illnesses; however, its mechanism of action is still largely unknown. Animal and human studies have suggested that lithium modulates glutamatergic and GABAergic neurotransmissions. The aim of this study is to investigate the effects of lithium on brain glutamate (Glu), glutamine (Gln), and gamma-aminobutyric acid (GABA) levels in healthy individuals using proton magnetic resonance spectroscopy (1H-MRS). In vivo 3 Tesla 1H-MRS was performed on the anterior cingulate cortex and bilateral basal ganglia initially and after two weeks of lithium administration on 8 healthy male subjects who had a mean age of 34.9 years. After two weeks of lithium administration, Gln significantly decreased in the left basal ganglia and showed a decreasing trend in the right basal ganglia. Additionally, Glu+Gln (Glx) significantly decreased in the right basal ganglia and showed a decreasing trend in the left basal ganglia. Glu did not significantly change in any of the three tested areas, and GABA exhibited no significant change after the lithium administration when measured in the anterior cingulate cortex and left basal ganglia. This study is the first to demonstrate that subchronic lithium treatment decreases Gln and Glx levels in the bilateral basal ganglia of healthy individuals. Our finding might suggest that the decrease of Glx levels is associated with the pharmacological actions of subchronic lithium treatment.     

Changes in midbrain serotonin transporter availability in atypically depressed subjects after one year of psychotherapy

BACKGROUND: Psychotherapy is an effective treatment method for depression, but no differences in the psychotherapy response have been found between the subtypes of depression. The effect of psychotherapy on neurotransmitter transporter functions has never been recorded in depressed subjects. METHODS: Depressive outpatients (N=19) received psychodynamic psychotherapy for 12 months. All subjects fulfilled the DSM-IV criteria for depression, and 8 were classified as having atypical depression. The severity of depression was assessed with the 29-item Hamilton Depression Rating Scale (HAM-D-29). Midbrain serotonin transporter (SERT) and striatum dopamine transporter (DAT) densities were recorded using single photon emission computed tomography (SPECT) brain imaging with the [123I]nor-beta-CIT radioligand before and after psychotherapy. RESULTS: Midbrain SERT density significantly increased during psychotherapy in atypicals but not in nonatypicals. There were no changes in the levels of DAT. CONCLUSIONS: The psychotherapy-related SERT elevation of atypically depressed subjects may be due to some unknown adaptive mechanisms inducing an increase in either the levels of SERT or serotonergic nerve terminals and therefore enhancing serotonergic activity and improving mood.