In vivo binding of spiperone and N-methylspiperone to dopaminergic and serotonergic sites in the rat brain: multiple modeling and implications for PET scanning

The dopamine hypothesis of bipolar affective disorder: the state of the art and implications for treatment

Bipolar affective disorder is a common neuropsychiatric disorder. Although its neurobiological underpinnings are incompletely understood, the dopamine hypothesis has been a key theory of the pathophysiology of both manic and depressive phases of the illness for over four decades. The increased use of antidopaminergics in the treatment of this disorder and new in vivo neuroimaging and post-mortem studies makes it timely to review this theory. To do this, we conducted a systematic search for post-mortem, pharmacological, functional magnetic resonance and molecular imaging studies of dopamine function in bipolar disorder. Converging findings from pharmacological and imaging studies support the hypothesis that a state of hyperdopaminergia, specifically elevations in D2/3 receptor availability and a hyperactive reward processing network, underlies mania. In bipolar depression imaging studies show increased dopamine transporter levels, but changes in other aspects of dopaminergic function are inconsistent. Puzzlingly, pharmacological evidence shows that both dopamine agonists and antidopaminergics can improve bipolar depressive symptoms and perhaps actions at other receptors may reconcile these findings. Tentatively, this evidence suggests a model where an elevation in striatal D2/3 receptor availability would lead to increased dopaminergic neurotransmission and mania, whilst increased striatal dopamine transporter (DAT) levels would lead to reduced dopaminergic function and depression. Thus, it can be speculated that a failure of dopamine receptor and transporter homoeostasis might underlie the pathophysiology of this disorder. The limitations of this model include its reliance on pharmacological evidence, as these studies could potentially affect other monoamines, and the scarcity of imaging evidence on dopaminergic function. This model, if confirmed, has implications for developing new treatment strategies such as reducing the dopamine synthesis and/or release in mania and DAT blockade in bipolar depression.

Comparison of [(18)F]altanserin and [(18)F]deuteroaltanserin for PET imaging of serotonin(2A) receptors in baboon brain: pharmacological studies

The regional distribution in brain, distribution volumes, and pharmacological specificity of the PET 5-HT(2A) receptor radiotracer [(18)F]deuteroaltanserin were evaluated and compared to those of its non-deuterated derivative [(18)F]altanserin. Both radiotracers were administered to baboons by bolus plus constant infusion and PET images were acquired up to 8 h. The time-activity curves for both tracers stabilized between 4 and 6 h. The ratio of total and free parent to metabolites was not significantly different between radiotracers; nevertheless, total cortical R(T) (equilibrium ratio of specific to nondisplaceable brain uptake) was significantly higher (34-78%) for [(18)F]deuteroaltanserin than for [(18)F]altanserin. In contrast, the binding potential (Bmax/K(D)) was similar between radiotracers. [(18)F]Deuteroaltanserin cortical activity was displaced by the 5-HT(2A) receptor antagonist SR 46349B but was not altered by changes in endogenous 5-HT induced by fenfluramine. These findings suggest that [(18)F]deuteroaltanserin is essentially equivalent to [(18)F]altanserin for 5-HT(2A) receptor imaging in the baboon.

5-HT2 and D2 dopamine receptor occupancy in the living human brain. A PET study with risperidone

It has been suggested that a combined blockade of 5-HT2 and D2 dopamine receptors may be superior to D2 dopamine antagonists alone in the treatment of schizophrenia. Risperidone, which has a high affinity for 5-HT2 and D2 dopamine receptors in vitro, is a new antipsychotic drug that has been developed according to this hypothesis. The aim of this study was to examine if risperidone indeed induces 5-HT2 and D2 dopamine receptor occupancy in vivo in humans. Central receptor occupancy was examined by positron emission tomography (PET) in three healthy men after oral administration of 1 mg risperidone. [11C]N-methylspiperone ([11C]NMSP) was used as a radioligand for determination of 5-HT2 receptor occupancy in the neocortex. Both an equilibrium ratio analysis and a kinetic three-compartmental analysis indicated a 5-HT2 receptor occupancy about 60%. [11C]raclopride was used as a radioligand for determination of D2 dopamine receptor occupancy in the striatum and the calculated occupancy was about 50%. This is the first quantitative determination of 5-HT2 receptor occupancy induced by an antipsychotic drug in the living human brain. The results indicate that 5-HT2 receptor occupancy should be very high at the dose level of 4-10 mg risperidone daily, as suggested for clinical use. Risperidone is thus an appropriate compound for clinical evaluation of the benefit of combined 5-HT2 and D2 dopamine receptor blockade in the treatment of schizophrenia.

Synthesis and evaluation of high affinity, aryl-substituted [18F]fluoropropylbenzamides for dopamine D-2 receptor studies

The potent dopamine D-2 ligands (S)-2,3-dimethoxy-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5(3- [18F]fluoropropyl)benzamide (18F-1) and (S)-2,3-dimethoxy-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(3- [18F]fluoropropyl)-6-hydroxybenzamide (18F-2) were prepared in high specific activity and 5-25% overall radiochemical yields. Benzamide 1 possessed a lower in vitro binding affinity for the D-2 receptor than salicylamide 2, but the in vivo striatal-to-cerebellar radioactivity concentration ratios (St/Cb) in rats and dogs were nearly identical for the two compounds. Compound 18F-2 was more lipophilic than 18F-1, and its increased penetration into and retention by striatal tissue was matched by an increase in non-specific binding in the cerebellum. Cerebral cortex radioactivity concentration levels in dogs were similar to cerebellum levels. The binding of 18F-labelled 1 and 2 displayed regional brain distribution patterns consistent with known dopamine D-2 receptor densities and was selectively blocked in the striatum of rats by dopamine D-2 antagonists. The binding of 18F-1 was found to be stereoselective, as the 18F-labelled (R)-enantiomer displayed no selective retention in the striatum of dogs. High levels of radioactivity were found in the bones of rats following the injection of 18F-1 and 18F-2, indicating that in vivo defluorination had occurred; however, no bone radioactivity was observed in dogs following the injection of these radioligands. Compound 18F-1 was displaced from the striatum of dogs by both d-amphetamine-stimulated dopamine release and haloperidol at doses of 1 and 0.5 mg/kg, respectively, while compound 18F-2 was displaced from the dog striatum only by haloperidol at these doses. The radioligand 18F-2 holds promise for positron emission tomography studies of the dopamine D-2 receptor system based upon its selective, potent binding and resistance to displacement by endogenous dopamine.

Regional brain measurement of Bmax and KD with the opiate antagonist cyclofoxy: equilibrium studies in the conscious rat

Brain distribution of the opiate receptor antagonist, cyclofoxy (CF), was evaluated at equilibrium in rats. A combination of i.v. injection and constant i.v. infusion was used to administer CF over a wide dose range (2.4-450 nmol/rat). Kinetic simulations and experimental results showed that this administration schedule accomplishes "true" tissue-blood equilibrium of CF within 60 min. To estimate the receptor-ligand binding parameters, we assumed that the CF concentration at the receptor site is identical to that in plasma water at equilibrium, and can be calculated from measured blood data after corrections for radiolabeled metabolites and plasma protein binding. This assumption was supported by CSF and plasma water measurements at equilibrium. Regional KD, Bmax, and a nonspecific tissue binding equilibrium constant (Keq) were estimated by fitting the tissue and plasma water concentrations to a single receptor model; the estimated values were 1.4-2.9 nM, 15-74 pmol/g of tissue, and 5.2-8.0, respectively. They are in good agreement with previous in vitro measurements (Rothman and McLean, 1988) as well as in vivo estimates from i.v. injection experiments (Sawada et al., 1990c). The conventional method to estimate the receptor-ligand binding parameters using data from cerebellum to approximate nonspecific tissue binding was found to be unacceptable. Although cerebellum is a brain region with no opiate receptors in rats, small differences in nonspecific tissue binding in different brain regions resulted in significant overestimations of KD and Bmax with this method. Receptor-active and -inactive enantiomers [[18F](-)-CF and [3H](+)-CF)] were simultaneously administered to the same animal and the receptor-bound CF concentration could be accurately measured; this method was used to estimate Bmax from a single study in a single animal and has potential for direct application in human studies using positron emission tomography.

Graphical method to determine sigmoid ligand binding in vivo at low doses

In this study, a graphical method is presented to detect sigmoidal binding of in vivo data at low nonsaturating doses. The method can also be applied when the overall binding includes nonspecific binding. Irrespective of the mathematical expression describing the saturation curve, it is shown that a sigmoid curve, in contrast to a nonsigmoid, can be converted into a peak-shaped curve by normalizing the ordinate values with the corresponding value of the abscissa. Computer simulations of in vivo ligand binding demonstrate this principle for in vivo data plots. Experimental results are described to illustrate this approach based on in vivo binding of the neuroleptic drug N-methylspiperone in the rat brain. It is concluded that the method is in particular useful for in vivo applications, e.g., positron emission tomography (PET) studies in human, because low doses are required, and specifically bound, nonspecifically bound, and unbound ligand do not need to be determined separately.