Ketamine increases the striatal N-[C]methylspiperone binding in vivo: positron emission tomography study using conscious rhesus monkey

A system for positron emission tomography study of conscious monkeys was newly developed. By use of this system in combination with a microdialysis technique, the effect of ketamine on the binding and release of dopamine was investigated. The administration of ketamine (5 mg/kg) caused sedation accompanied by psychotic symptoms such as nystagmus and stereotyped movements of extremities. During this psychotomimetic period produced by ketamine, a significant increase in the accumulation of the dopamine D₂ receptor ligand N-[¹¹C]methylspiperone was observed in the striatum compared with the level in the conscious state, while no significant change was observed in the frontal cortex and cerebellum. In contrast to the use of ketamine as the anesthetic, pentobarbital (25 mg/kg), which produced deeper anesthesia but no psychotic symptoms, caused a decrease in the accumulation of N-[¹¹C]methylspiperone in the striatum. Kinetic analysis, conducted by a graphical method, revealed that the value of the association constant (K3) for N-[¹¹C]methylspiperone binding in the striatum was increased to approximately 130% by ketamine and decreased to approximately 70% by pentobarbital compared with the control values. Furthermore, the release of dopamine from the striatum measured by microdialysis was not affected by ketamine anesthesia. These results indicate that ketamine facilitates striatal dopaminergic neurotransmission through increasing the binding activity of dopamine D₂ receptors in the striatum, and suggest that these changes may be related to the psychotomimetic behavioral symptoms of this drug.     

Age-dependent changes in the expression of dopamine receptor subtypes in human peripheral blood lymphocytes

The pharmacological profile and the density of dopamine D₃ and D₅ receptor subtypes expressed by human peripheral blood lymphocytes of subjects of different ages (ranging from 20 to 75 years) were assessed using radioligand binding techniques. Dopamine D₃ receptor was assayed with [³H]7-hydroxy-N,N-di-n-propyl-2-aminotetraline ([³H]7-OH-DPAT) as a ligand. Dopamine D₅ receptor was assayed using [³H][R]-(+)-(-chloro-2,3,4,5,tetrahydro-5-phenyl-1H-3-benzazepin-al-hemimaleate) ([³H]SCH 23390) as a ligand. The affinity and the pharmacological profile of [³H]7-OH-DPAT and [³H]SCH 23390 at dopamine D₃ and D₅ receptor, respectively, were similar in subjects of different ages. The density of dopamine D₃ receptor binding sites was slightly decreased in subjects of 30–39 years in comparison with younger individuals. A remarkable loss of dopamine D₃ receptor was then found between 40 and 49 years of age in comparison with younger subjects. A further slight decrease was noticeable between 50 and 59 years of age. The number of [³H]7-OH-DPAT binding sites was then stabilized after 60 years of age. The density of dopamine D₅ receptor binding sites did not show age-dependent changes. The above findings indicate the occurrence of a decline in the density of lymphocyte dopamine D₃ but not D₅ receptor between adult and mature subjects. The possibility that dopamine D₃ receptor assay in peripheral blood lymphocytes may represent a tool for investigating dopamine receptor function in aging and age-related neurological disorders is discussed.     

Long-term treatment with haloperidol or clozapine does not affect dopamine D4 receptors in rat frontal cortex

Summary We examined the effects of long-term treatment with haloperidol and clozapine on dopamine D₄ receptors in rat frontal cortex. Dopamine D₄ receptor binding sites were indirectly determined from the displacement experiments of [³H]clozapine binding using nemonapride. Three-weeks administration of haloperidol (0.5mg/kg) or clozapine (10mg/kg) did not significantly affect the D₄ receptors in the frontal cortex. The density of D₂ receptors, determined by [³H]spiperone binding to striatum, was increased by long-term treatment with haloperidol, but it was not significantly changed by that with clozapine.     

A single (−)-nicotine injection causes change with a time delay in the affinity of striatal D2 receptors for antagonist, but not for agonist, nor in the D2 receptor mRNA levels in the rat substantia nigra

The in vitro and in vivo effects of (−)-nicotine on dopamine D₂ receptors in the rat neostriatum have been studied using biochemical binding, in situ hybridization and immunocytochemistry. A single i.p. injection (1 mg/kg) of (−)-nicotine resulted in a reduction of theK_D value of the D₂ antagonist [³H]raclopride binding sites in rat neostriatal membrane preparations at 12 h without any significant change in theB_max value. This action of (−)-nicotine was counteracted by pretreatment 15 min earlier with the nicotine antagonist mecamylamine (1 mg/kg, i.p.). However, theK_D and theB_max values of the D₂ agonist [³H]NPA binding sites in the rat neostriatal membrane preparations were not significantly affected 0.5–48 h after a single i.p. injection with 1 mg/kg of (−)-nicotine. No significant change in neostriatal D₂ receptor mRNA levels was observed at any time interval after the (−)-nicotine injection. No significant change was observed in tyrosine hydroxylase (TH) immunoreactivity in either the substantia nigra or the neostriatum, nor in nigral TH mRNA levels during the time interval studied (4–24 h posttreatment). Furthermore, addition of low (10 nM) or high (1 μM) concentrations of (−)-nicotine in vitro to rat neostriatal membranes did not alter the characteristics of [³H]raclopride or [³H]NPA binding. These results indicate that a single (−)-nicotine injection can produce a selective and delayed increase in the affinity of D₂ receptors for the antagonist, but not for the agonist without modifying the levels of D₂ receptor mRNA, probably via the activation of central nicotinic receptors.     

Blunted brain metabolic response to ketamine in mice lacking D1A dopamine receptors

The interaction of glutamatergic and dopamine neurotransmission is thought to have relevance to both the pathophysiology and pharmacotherapy of schizophrenia. For example, subanesthetic doses of the N-methyl- -aspartate receptor (NMDA-R) antagonist ketamine induce schizophrenia-like behavioral effects in humans and both behavioral and brain metabolic activation in rodents. Blockade of NMDA-R results in dopamine release, and antipsychotic drugs that block dopamine neurotransmission decrease NMDA-R antagonist-induced behavioral activation. The involvement of dopamine receptors in brain metabolic activation induced by ketamine is, however, unknown. The present study used D_1A knockout mice to determine the role of dopamine D_1A receptors in the effects of subanesthetic doses of ketamine on both behavioral responses and on alterations in regional [¹⁴C]2-deoxyglucose (2-DG) uptake. There was less ketamine-induced behavioral activation in D_1A knockout mice than in wild-type mice. In wild-type mice, ketamine (30 mg/kg) induced dramatic increases in 2-DG uptake in limbic cortical regions, hippocampal formation, nucleus accumbens, basolateral amygdala, and caudal parts of the substantia nigra pars reticulata. D_1A knockout mice exhibited blunted metabolic activation in response to ketamine in a neuroanatomically specific manner. The selective D₁ antagonist, SCH23390 (0.3 mg/kg), inhibited both ketamine-induced brain metabolic activation and behavioral responses in the wild-type mice, with a similar neuroanatomical specificity observed in the D_1A knockout mice. Thus, the neuroanatomically selective role that D_1A receptors play in ketamine-induced behavior and regional brain metabolic activation in mice provides a useful model for further studies of how the D_1A receptor function may be altered in schizophrenia.     

Effects of monoamine uptake inhibitors given early postnatally on monoamines in the brain stem,caudate/putamen and cortex,and on dopamine D1 and D2 receptors in the caudate/putamen

Summary Rats were treated with desipramine 5mg/kg, nomifensine 10mg/kg, zimelidine 25 mg/kg or with 0.9% sodium chloride once a day during the second and third weeks after birth, and brain stem, caudate/putamen and cortical monoamines, and caudate/putamen dopamine D₁ (³[H]SCH 23390) and D₂ (³[H]spiroperidol) receptor binding were measured when rats were at two months of age. In the brain stem, the concentration of 3-methoxy-4-hydroxy-phenyl glycol was increased in nomifensine rats and the ratio of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine was increased in zimelidine rats. In the caudate/putamen, the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid and the ratio of homovanillic acid to dopamine were increased in desipramine rats; neither³[H]SCH 23390 nor³[H]spiroperidol binding were affected by any of the three monoamine uptake inhibiting antidepressants studied. In the cortex, the ratio of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine was increased in desipramine and zimelidine rats. The findings suggest that desipramine but not nomifensine increases the metabolism of dopamine in the caudate/ putamen and nomifensine but not desipramine increases the metabolism of norepinephrine in the brain stem, and furthermore that the metabolism of serotonin is affected by desipramine as well as by zimelidine. It is possible that also treatment of women with these drugs during late pregnancy causes long-lasting changes in the brain of human fetus.     

Locomotor effects of amantadine in the mouse are not those of a typical glutamate antagonist

Summary Amantadine has been shown to displace [³H]MK 801 from its binding site on the NMDA receptor. We have therefore studied the motor effects of amantadine in normal and 24h reserpine-treated mice to determine whether the behavioural profile of this drug resembles that of other NMDA receptor antagonists (e.g. MK 801). In common with the latter, amantadine (5–40 mg/kg IP) produced a modest dose-dependent sedation in dopamineintact mice, with a reduction in locomotion and other species-typical behaviours (e.g. rearing and grooming), but with no signs of the hyperactivity, stereotypy, ataxia or loss of muscle tone commonly seen with MK 801. Amantadine (5–80 mg/kg IP) effected a small incrase in motility in akinetic reserpine-treated mice by itself, but this response was highly variable and not statistically significant. As with MK 801, amantadine significantly inhibited the locomotion induced by the selective D₂ agonist RU 24213 (5 mg/kg SC) and the mixed D₁/D₂ agonist apomorphine (0.5 mg/kg SC) in monominedepleted mice, without altering the animals' responsiveness to threshold doses of these drugs. However, amantadine did not facilitate the locomotion induced by threshold (3 mg/kg IP) or fully active doses (30 mg/kg IP) of the selective D₁ agonist SKF 38393, which distinguishes amantadine from other NMDA receptor blockers. Since the potentiation of dopamine D₁-dependent locomotion may be a major factor in the antiparkinson activity of MK 801 and other glutamate receptor antagonists the inability of amantadine to potentiate SKF 38393 in this study suggests the mechanism of its anti-akinetic activity differs, from that of conventional glutamate blocking drugs.     

Characterization of [3H]clozapine binding sites in rat brain

Summary We examined the characteristics of [³H]clozapine binding sites in four rat brain regions (frontal cortex, limbic area, hippocampus and striatum) in order to elucidate the pharmacological profile of this unique atypical antipsychotic drug. The specific [³H]clozapine binding was found to be saturable and reversible in all these brain regions. Scatchard analysis of the saturation data indicated that the specific binding consisted of high- and low-affinity components. Displacement experiments showed that the muscarinic cholinergic receptor represented about 50% of [³H]clozapine binding in each brain area. Serotonin 5-HT₂ and dopamine D₄ receptor binding sites could also be detected by displacement experiments using ketanserin and nemonapride, respectively, in frontal cortex and limbic area, but not in hippocampus or striatum. Alpha-1, alpha-2, histamine H₁, dopamine D₁, D₂, or D₃ receptor components could not be determined within the high-affinity [³H]clozapine binding sites in any brain region. It is possible that the atypical property of clozapine may depend on the modulatory effect on dopaminergic function via 5-HT₂ receptor blockade and/or may be mediated via D₄ receptor blockade in the mesocortical and mesolimbic area.     

Imaging cortical dopamine D1 receptors using [11C]NNC112 and ketanserin blockade of the 5-HT2A receptors

[¹¹C]NNC112 (8-chloro-7-hydroxy-3-methyl-5-(7-benzofuranyl)-2,3,4,5-tetrahydro-IH-3-benzazepine), a selective positron-emission tomography (PET) ligand for the D₁ receptor (R) over the 5-HT_2A R in vitro, has shown lower selectivity in vivo, hampering measurement of D₁ R in the cortex. [¹¹C]NNC112 PET and intravenous (i.v) ketanserin challenge were used to (1) confirm the previous findings of [¹¹C]NNC112 in vivo D₁ R selectivity, and (2) develop a feasible methodology for imaging cortical D₁ R without contamination by 5-HT_2A R. Seven healthy volunteers underwent [¹¹C]NNC112 PET scans at baseline and after a 5-HT_2A R-blocking dose of ketanserin (0.15 mg/kg, i.v.). Percent BP_ND change between the post-ketanserin and baseline scans was calculated. Irrespective of the quantification method used, ketanserin pretreatment led to significant decrease of BP_ND in the cortical (∼30%) and limbic regions (∼20%) but not in the striatum, which contains a much lower amount of 5-HT_2A R. Therefore, ketanserin allows D₁ R signal to be detected by [¹¹C]NNC112 PET without significant 5-HT_2A R contamination. These data confirm the presence of a significant 5-HT_2A R contribution to cortical [¹¹C]NNC112 signal, and call for caution in the interpretation of published [¹¹C]NNC112 PET findings on cortical D₁ R in humans. In the absence of more selective ligands, [¹¹C]NNC112 PET with ketanserin can be used for cortical D₁ R imaging in vivo.     

Differential labeling of dopamine and sigma sites by [H]nemonapride and [H]raclopride in postmortem human brains

The difference between the binding of [³H]nemonapride and [³H]raclopride has been used to quantify dopamine D₄ receptors in postmortem schizophrenic brain studies. Recent work, however, has suggested that at least part of the differential between [³H]nemonapride and [³H]raclopride binding may represent σ rather than D₄ receptor sites. We applied the nemonapride-raclopride subtraction method to postmortem, non-schizophrenic human striatum to examine the variation in dopaminergic receptor binding labeled by these ligands. Variation in σ receptor binding labeled by [³H]nemonapride was studied in frontal cortex, striatum and cerebellum. Specific binding was defined by sulpiride (dopamine receptor ligand), PPAP (σ receptor ligand) and haloperidol (mixed dopaminergic/σ agent), respectively. Haloperidol defined a combination of sites, which were approximately the sum of the dopaminergic and σ components defined by sulpiride and PPAP, respectively. Significant inter-individual variation in the amount of specific binding for dopaminergic and σ receptor sites was observed. However, no significant nor consistent observation of striatal dopamine D₄ receptors or D₄-like binding sites was observed in the striatum even though two independent sets of tissues, with different dissections were used. The inconsistencies in some previous postmortem studies appear to be at least partially explained by the inclusion of both σ and dopaminergic components in [³H]nemonapride binding and the inherent high inter-individual variability of the different components.     

Measurement error analysis for the determination of dopamine D2 receptor occupancy using the agonist radioligand [11C]MNPA

The purpose of this study is to investigate errors in quantitative analysis for estimating dopamine D₂ receptor occupancy of antipsychotics with agonist radioligand [¹¹C]MNPA by numerical simulation, with particular attention to the validity of a quantitative approach based on the use of a reference region. Synthetic data were validated using clinical data combined with a bootstrap approach. Time–activity curves (TACs) of [¹¹C]MNPA were simulated, and the reliability of binding potential (BP_ND) and occupancy estimated by nonlinear least square (NLS) fitting and a simplified reference tissue model (SRTM) were investigated for various noise levels and scan durations. In the human positron emission tomography (PET) study with and without antipsychotic, risperidone, the uncertainty of BP_ND and occupancy estimated by SRTM was investigated using resampled TACs based on bootstrap approach with weighted residual errors of fitting. For both NLS and SRTM, it was possible to have <3% of bias in occupancy estimates of [¹¹C]MNPA by 60 mins. However, shortened scan duration degrades the quantification of very small binding potentials, especially in case of SRTM. Observations were replicated on the clinical data. Results showed that dopamine D₂ receptor occupancy by antipsychotics can be estimated precisely in region of interest analysis by SRTM with a longer than 60-min [¹¹C]MNPA PET scan duration.     

Characterisation of dopamine receptors in insect (Apis mellifera) brain

In vitro binding experiments using the vertebrate D₁ dopamine receptor ligand [³H]SCH23390 and the vertebrate D₂ dopamine receptor ligand [³H]spiperone were conducted on membrane preparations of honey bee (Apis mellifera) brain. Specific binding of [³H]SCH23390 was saturable and reversible. Analysis of saturation data gave an apparent K_d of 6.3 ± 1.0 nM and B_max of 1.9 ± 0.2 pmol/mg protein for a single class of binding sites. The specificity of high affinity [³H]SCH23390 binding was confirmed in displacement experiments using a range of dopaminergic antagonists and agonists. The rank order of potency for antagonists was: R(+)-SCH23390 > cis-(Z)-flupentixol ≥ chlorpromazine > fluphenazine> S(+)-butaclamol > spiperone. R(±)-SKF38393 and dopamine were the most effective agonists tested. [³H]SCH23390 labels a site in bee brain that is similar, but not identical to the vertebrate D₁ dopamine receptor subtype. [³H]Spiperone also bound with high affinity to bee brain homogenates. Scatchard analysis of [³H]spiperone saturation data revealed a curvilinear plot suggesting binding site heterogeneity. The high affinity site had a apparent K_d of 0.11 ± 0.02 nM and B_max of 9.2 ± 0.5 fmol/mg protein. The calculated values for the low affinity site were a K_d of 19.9 nM and B_max of 862 fmol/mg protein. Kinetic analyses also indicated that [³H]spiperone recognises a heterogeneous population of sites in bee brain. Furthermore, agonist competition studies revealed a phenolaminergic as well as a dopaminergic component to [³H]spiperone binding in bee brain. The rank order of potency of dopaminergic antagonists in competing for [³H]spiperone binding was: spiperone > fluphenazine> S(+)-butaclamol > domperidone> R(+)-SCH23390 > S(−)-sulpiride.     

SDZ GLC 756, a novel octahydrobenzo[g]quinoline derivative exerts opposing effects on dopamine D1 and D2 receptors

Summary SDZ GLC-756, a novel octahydrobenzo[g]quinoline derivative, is equipotent in displacing [³H]SCH23390 from dopamine D₁ receptors and [³H]205–501 from dopamine D₂ receptor binding sites. It blocks dopamine sensitive adenylate cyclase with the same potency as SCH23390, indicating antagonist properties at dopamine D₁ receptors. On the other hand, SDZ GLC 756 inhibits electrically evoked acetylcholine release from rat striatal slices with the same potency as the selective dopamine D₂ receptor agonist bromocriptine. This effect is blocked by spiperone suggesting that it is mediated by dopamine D₂ receptor activation. The opposing action of SDZ GLC 756 on dopamine D₁ and D₂ receptors is also evident in vivo. SDZ GLC 756, like SCH23390, blocks apomorphine-induced rearing in mice. On the other hand, it inhibits prolactin secretion and produces circling in unilateral 6-OHDA-lesioned rats, which is compatible with stimulant properties at dopamine D₂ receptors. This drug might be a new tool to study linkage between dopamine D₁ and D₂ receptors.     

Involvement of dopamine D1 and D2 receptors in the ethanol-associated place preference in rats exposed to conditioned fear stress

The present study was designed to investigate: (1) the involvement of dopamine D₁ and D₂ receptors, and (2) the roles of these receptors and endogenous opioid systems (endorphinergic and enkephalinergic systems) in the ethanol-induced place preference in rats exposed to conditioned fear stress using the conditioned place preference paradigm. The administration of ethanol (300 mg/kg, i.p.) induced a significant place preference. The selective D₁ receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H3-benzazepine)hydrochloride (SCH23390; 0.01 and 0.03 mg/kg, s.c.) and the selective D₂ receptor antagonist S(−)-5-(aminosulfonyl)-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride; 20 and 40 mg/kg, s.c.) significantly attenuated the ethanol-induced place preference. The administration of ethanol (75 mg/kg, i.p.) tended to produce a place preference, but this effect was not significant. SCH23390 (0.03 mg/kg, s.c.) and sulpiride (40 mg/kg, s.c.) significantly attenuated the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by the μ-opioid receptor agonist morphine (0.1 mg/kg, s.c.). In addition, SCH23390 (0.03 mg/kg, s.c.) also significantly attenuated the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by the selective δ-opioid receptor agonist 2-methyl-4aα-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12aα-octahydroquinolino[2,3,3,-g]isoquinoline (TAN-67; 20 mg/kg, s.c.). On the other hand, sulpiride (40 mg/kg) had no significant effect on the enhancement of the ethanol (75 mg/kg, i.p.)-induced place preference produced by TAN-67. These results suggest that D₁ and D₂ receptors may be involved in the rewarding mechanism of ethanol under psychological stress. In addition, D₁ receptors may participate in the rewarding effect of ethanol modulated by the activation of μ- and δ-opioid receptors, whereas D₂ receptors may participate in the rewarding effect of ethanol modulated by the activation of μ-opioid receptors, but not in that modulated by the activation of δ-opioid receptors.     

Autoradiographic analysis of D1 and D2 dopaminergic receptors in rat brain after paradoxical sleep deprivation

Previous work had shown that paradoxical sleep deprivation (PSD) results in potentiation of several apomorphine-induced behaviors, leading to the suggestion that PSD induces an upregulation of brain dopamine receptors. In this study, quantitative receptor autoradiography was used to verify whether PSD does, in fact, induce alterations in D₁ or D₂ receptor binding, and to investigate the regional brain specificity of such effects. After 96 h of PSD, [³H]SCH-23390 binding to D₁ receptors was examined in 30 different brain areas of 10 experimental and 10 cage control rats. [³H]Spiperone was used to label D₂ sites in adjacent tissue sections. Results revealed a 39% increase in [³H]SCH 23390 binding in the entorhinal cortex of PSD rats (p < 0.05), but no other changes in any of the remaining 29 brain areas examined. In contrast, [³H]spiperone binding was significantly elevated in the n. accumbens (+45%) and in all subrogions of the caudate-putamen (range: +13% to +23%). These results, thus, provide evidence that PSD increases D₂ but not D₁ receptor binding in brain. The present results also suggest that upregulated D₂ receptors can account for the previously reported changes in apomorphine-induced behaviors after PSD.     

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.     

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.     

Selective unilateral inactivation of striatal D1 and D2 dopamine receptor subtypes by EEDQ: turning behavior elicited by D2 dopamine receptor agonists

The unilateral intrastriatal injection of the irreversible dopamine (DA) receptor blockerN-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) induces a marked decrease in the density of D₁ (-48%) and D₂ (-51%) DA receptors available for binding to [³H]SCH 23390 and [³H]raclopride, respectively. A challenge dose of the D₂ agonist LY 171555 (1 mg/kg, i.p., 24 h after EEDQ) causes intensive ipsiversive circling behavior, whereas the selective D₁ agonist SKF 38393 (20 mg/kg, i.p., 24 h after EEDQ) is unable to induce rotations. The density of D₁ and D₂ DA receptors returns to basal levels by 7 days after the intrastriatal infusion of EEDQ. This biochemical recovery is associated with a progressive decrease in the number of rotations elicited by a challenge dose of LY 171555, suggesting the EEDQ does not cause any relevant neuronal damage. A selective inactivation of striatal D₁ or D₂ DA receptors can be obtained by injecting EEDQ 30 min after the administration of the D₂ antagonist raclopride (20 mg/kg, i.p.) or of the D₁ antagonist SCH 23390 (2 mg/kg, s.c.), respectively. The intensity of the circling behavior induced by LY 171555 24 h after EEDQ in animals with a selective inactivation of D₂ DA receptors is similar to that found in rats in which both D₁ and D₂ DA receptors have been inactivated. In contrast, LY 171555 does not cause rotations when the density of D₁ DA receptors is selectively decreased by EEDQ in rats pretreated with raclopride. These results indicate that the imbalance in striatal D₂ receptors, but not in D₁ receptors, is a critical factor for the expression of the motor effects elicited by LY 171555 in EEDQ-treated rats.     

The role of dopamine in the maintenance and breakdown of D1/D2 synergism

The neurochemical factors involved in the maintenance and breakdown of dopamine D₁/D₂ receptor synergism were investigated by giving rats various pharmacological treatments that diminish the ability of dopamine to interact with its D₁ and/or D₂ receptors. Following these treatments, rats were observed for the expression of stereotyped motor behavior in response to independent stimulation of D₁ or D₂ receptors. Independent D₂-mediated responses were observed: (a) 2 h after the last of three daily reserpine (1 mg/kg) injections, (b) 48 h after bilateral 6-hydroxydopamine (6-OHDA) lesions of the mesostriatal pathways, (c) 24 h after a concentrated 48-h regimen (one injection/6 h) of eticlopride (0.5 mg/kg) or eticlopride + SCH 23390 (0.5 mg each), and (d) 2 h after a concentrated 48-h regimen (one injection/6 h) of α-methyl-p-tyrosine (αMPT; 100 mg/kg), but not after control treatments or a concentrated regimen of SCH 23390 alone. By contrast, independent D₁-mediated responses were observed only after three daily reserpine injections or 48 h after bilateral 6-OHDA lesions. Independent D₁-mediated stereotypy was not observed under control conditions or following a concentrated 48-h regimen of (a) SCH 23390 or eticlopride (0.5 mg/kg each) alone or in combination, (b) a high dose of SCH 23390 (1.0 mg/kg), (c) αMPT (100 mg/kg), or (d) αMPT (100 mg/kg)+SCH 23390 (1.0 mg/kg). Reserpine, bilateral 6-OHDA, and αMPT treatments produced striatal dopamine depletions of 96%, 92%, and 71%, respectively. These data indicate that the breakdown in D₁/D₂ synergism consists of two components: (a) D₁ independence from the controlling influence of D₂ receptors, and (b) D₂ independence from the controlling influence of D₁ receptors. The interaction of synaptic DA with its D₂ receptors plays a major role in determining whether these receptors can function independently of D₁ receptors, whereas reduced DA-D₁ activity alone appears insufficient to elicit D₁ independence.     

5-HT2 receptor regulation of acetylcholine release induced by dopaminergic stimulation in rat striatal slices

The role of 5-hydroxytryptamine (5-HT) receptor subtypes in acetylcholine (ACh) release induced by dopamine or neurokinin receptor stimulation was studied in rat striatal slices. The dopamine D₁ receptor agonist SKF 38393 potentiated in a tetrodotoxin-sensitive manner the K⁺-evoked [³H]ACh release while SCH 23390, a dopamine D₁ receptor antagonist, had no effect. [³H]ACh release was decreased by the dopamine D₂ receptor agonist LY 171555 (quinpirole) and slightly potentiated by the dopamine D₂ receptor antagonist haloperidol. The selective neurokinin NK₁ receptor agonist [Sar⁹, met(O₂)¹¹]SP also potentiated K⁺-evoked release of [³H]ACh. GR 82334, a NK₁ receptor antagonist, blocked not only the effect of [Sar⁹, met(O₂)¹¹]SP but also the release of ACh induced by the D₁ receptor agonist SKF 38393. Among the 5-HT agents studied, only the 5-HT_2A receptor antagonists ketanserin and ritanserin were able to reduce the ACh release induced by dopamine D₁ receptor stimulation. Mesulergine, a more selective 5-HT_2C antagonist, showed an intrinsic releasing effect but did not affect K⁺-evoked ACh release induced by SKF 38393. Methysergide and methiothepin, mixed 5-HT_1/2 antagonists, as well as ondansetron, a 5-HT₃ receptor antagonist, showed an intrinsic effect on ACh release, their effects being additive to that of SKF 38393. 5-HT₂ receptor agonists were ineffective. However, the 5-HT₂ agonist DOI was able to prevent the antagonism by ketanserin of the increased [³H]ACh efflux elicited by SKF 38393, suggesting a permissive role of 5-HT_2A receptors. None of the above indicated 5-HT agents was able to reduce the ACh release induced by the selective NK₁ agonist. The results suggest that 5-HT₂ receptors, probably of the 5-HT_2A subtype, modulate the release of ACh observed in slices from the rat striatum after stimulation of dopamine D₁ receptors. It seems that this serotonergic control is exerted on the interposed collaterals of substance P-containing neurons which promote ACh efflux through activation of NK₁ receptors located on cholinergic interneurons.