[3H]spiperone binds selectively to rat striatal D2 dopamine receptors in vivo: a kinetic and pharmacological analysis

Dissociable effects of dopamine D1 and D2 receptors on compulsive ingestion and pivoting movements elicited by disinhibiting the ventral pallidum

Previous studies have shown that infusion of a GABAA receptor antagonist, such as bicuculline (bic), into the ventral (pallidum VP) of rats elicits vigorous ingestion in sated subjects and abnormal pivoting movements. Here, we assessed if the ingestive effects generalize to the lateral preoptic area (LPO) and tested both effects for modulation by dopamine receptor signaling. Groups of rats received injections of the dopamine D2 receptor antagonist, haloperidol (hal), the D1 antagonist, SCH-23390 (SCH), or vehicle (veh) followed by infusions of bic or veh into the VP or LPO. Ingestion effects were not observed following LPO bic infusions. Compulsive ingestion associated with VP activation was attenuated by hal, but not SCH. VP bic-elicited pivoting was attenuated by neither hal, nor SCH.

The lateral preoptic area and ventral pallidum embolden behavior

While recently completing a study of the effects of stimulating the lateral preoptic area (LPO) and ventral pallidum (VP) on locomotion and other movements, we also noticed LPO and VP effects on motivational drive and threat tolerance. Here, we have investigated these latter effects by testing conditioned place preference (CPP), behavior on the elevated plus maze (EPM) and the willingness of sated rats to occupy a harshly lit open field center to acquire sweet pellets, a measure of threat tolerance, following infusions of vehicle or bicuculline (bic) into the LPO and VP. LPO-bic infusions robustly increased total locomotion, and, in direct proportion, occupancy of both the harshly lit field center and open arms of the EPM. LPO bic also generated CPP, but did not increase sweet pellet ingestion. These effects were attenuated by dopamine D1 and D2 receptor antagonists, whether given individually or as a cocktail and systemically or infused bilaterally into the nucleus accumbens. VP-bic infusions did not increase total locomotion, but preferentially increased field center occupancy. VP-bic-infused rats compulsively ingested sweet pellets and did so even under the spotlight, whereas harsh illumination suppressed pellet ingestion in the control groups. VP bic produced CPP and increased open arm occupancy on the EPM. These effects were attenuated by pretreatment with dopamine receptor antagonists given systemically or as bilateral infusions into the VP, except for % distance in the field center (by D1 or D2 antagonists) and pellet ingestion (by D1 antagonist). Thus, boldness generated in association with LPO activation is tightly tied to locomotor activation and, as is locomotion itself, strongly DA dependent, whereas that accompanying stimulation of the VP is independent of locomotor activation and, at least in part, DA signaling. Furthermore, respective emboldened behaviors elicited from neither LPO nor VP could clearly be attributed to goal pursuit. Rather, emboldening of behavior seems more to be a fixed action response not fundamentally different than previously for reported locomotion, pivoting, backing, gnawing, and eating elicited by basal forebrain stimulation.

Lateral preoptic and ventral pallidal roles in locomotion and other movements

The lateral preoptic area (LPO) and ventral pallidum (VP) are structurally and functionally distinct territories in the subcommissural basal forebrain. It was recently shown that unilateral infusion of the GABAA receptor antagonist, bicuculline, into the LPO strongly invigorates exploratory locomotion, whereas bicuculline infused unilaterally into the VP has a negligible locomotor effect, but when infused bilaterally, produces vigorous, abnormal pivoting and gnawing movements and compulsive ingestion. This study was done to further characterize these responses. We observed that bilateral LPO infusions of bicuculline activate exploratory locomotion only slightly more potently than unilateral infusions and that unilateral and bilateral LPO injections of the GABAA receptor agonist muscimol potently suppress basal locomotion, but only modestly inhibit locomotion invigorated by amphetamine. In contrast, unilateral infusions of muscimol into the VP affect basal and amphetamine-elicited locomotion negligibly, but bilateral VP muscimol infusions profoundly suppress both. Locomotor activation elicited from the LPO by bicuculline was inhibited modestly and profoundly by blockade of dopamine D2 and D1 receptors, respectively, but was not entirely abolished even under combined blockade of dopamine D1 and D2 receptors. That is, infusing the LPO with bic caused instances of near normal, even if sporadic, invigoration of locomotion in the presence of saturating dopamine receptor blockade, indicating that LPO can stimulate locomotion in the absence of dopamine signaling. Pivoting following bilateral VP bicuculline infusions was unaffected by dopamine D2 receptor blockade, but was completely suppressed by D1 receptor blockade. The present results are discussed in a context of neuroanatomical and functional organization underlying exploratory locomotion and adaptive movements.

Mechanistic models enable the rational use of in vitro drug-target binding kinetics for better drug effects in patients

INTRODUCTION

Drug-target binding kinetics are major determinants of the time course of drug action for several drugs, as clearly described for the irreversible binders omeprazole and aspirin. This supports the increasing interest to incorporate newly developed high-throughput assays for drug-target binding kinetics in drug discovery. A meaningful application of in vitro drug-target binding kinetics in drug discovery requires insight into the relation between in vivo drug effect and in vitro measured drug-target binding kinetics.

AREAS COVERED

In this review, the authors discuss both the relation between in vitro and in vivo measured binding kinetics and the relation between in vivo binding kinetics, target occupancy and effect profiles.

EXPERT OPINION

More scientific evidence is required for the rational selection and development of drug-candidates on the basis of in vitro estimates of drug-target binding kinetics. To elucidate the value of in vitro binding kinetics measurements, it is necessary to obtain information on system-specific properties which influence the kinetics of target occupancy and drug effect. Mathematical integration of this information enables the identification of drug-specific properties which lead to optimal target occupancy and drug effect in patients.

Effects of binge pattern cocaine administration on dopamine D1 and D2 receptors in the rat brain: an in vivo study using positron emission tomography

The aim of the present study was to determine the effect of "binge" pattern cocaine administration on dopamine D1 and D2 receptors in the rat brain. Male Sprague Dawley rats were injected three times at 1 hr intervals with saline or cocaine (15 mg/kg) each day for 2, 7, or 14 d. The in vivo binding of [11C]SCH23390 (dopamine D1 receptor antagonist) and [11C]N-methylspiperone (NMSP; dopamine D2 receptor antagonist) in the striatal region was measured by a high-resolution positron emission tomography at 1 and 3.5 hr, respectively, after the last cocaine or saline injection. Acute (2 d) binge cocaine administration did not change the in vivo binding potential of [11C]SCH23390 or the binding of [11C]NMSP in the striatum. After 7 d of binge cocaine administration, a significant decrease in the binding potential of [11C]SCH23390 was observed, whereas no change in the binding of [11C]NMSP was found. After 14 d of binge cocaine administration, the in vivo binding was significantly reduced for both [11C]SCH23390 and [11C]NMSP. Separate saturation experiments indicated that the observed alterations of in vivo binding were attributable mainly to apparent alterations in the affinity and not the number of binding sites. These results suggest that both dopamine D1 and D2 receptors may have altered physiologically available binding sites after binge pattern cocaine administration.

Effect of a corn diet during development on [3H]-spiperone binding in the brain of rats at the perinatal stage

A model of undernutrition based on feeding with a corn-diet (tryptophan deficient) was used to study maturation and physiology of the rat brain 5-HTergic system. Previously, using that model, we observed a decrease in 5-HT uptake in rat brain slices, assuming that some compensatory mechanisms could be implicated at the synaptic level on those conditions. Body, brain weight, and total [3H]-spiperone binding were recorded at two fetal ages and newborn pups from mothers were fed with a corn-diet and with a corn-lysine diet. Significant decreases in body weight and in [3H]-spiperone binding were observed at all ages studied in the corn-based diet groups with respect to controls at the first postnatal day of age, whereas at the 18th gestation day, the [3H]-spiperone binding was higher in the protein restricted and corn-lysine supplemented group than in the control group. Results suggested that expression of 5-HT receptors and possibly their synthesis may be limited at the synaptic level on undernutrition conditions.

Systemic and cerebral kinetics of 16 alpha [18F]fluoro-17 beta-estradiol: a ligand for the in vivo assessment of estrogen receptor binding parameters

Estrogen receptors are expressed in several brain areas of various animal species, and steroid hormones exert physiologic and biochemical effects on the central nervous system. The aim of the present study was to evaluate in female adult rats, the suitability of 16 alpha [18F]fluoro-17 beta-estradiol ([18F]FES), a selective estrogen receptor ligand, for the in vivo assessment of brain estrogen receptors. This was considered to be a preliminary step in evaluating the potential usefulness of [18F]FES for studies of cerebral estrogen receptors with positron emission tomography (PET) in nonhuman primates and human subjects. We evaluated (a) the time course of the metabolic degradation of [18F]FES in blood; (b) the time course of distribution of the tracer in discrete cerebral areas; (c) the inhibitory effect of increasing doses of cold estradiol on cerebral [18F]FES uptake; and (d) the possibility of in vivo quantification of estrogen receptor binding parameters using both equilibrium and dynamic kinetic analyses. We quantified [18F]FES binding to estrogen receptors using both equilibrium and dynamic kinetic analyses. The results of this study indicate that [18F]FES is a suitable tracer for the measurement of estrogen receptors in the pituitary and hypothalamus, using either the equilibrium or the kinetic analysis. However, [18F]FES is inadequate for the in vivo investigation of estrogen binding sites in brain areas with low receptor density, such as the hippocampus.

High affinity dopamine D2 receptor radioligands. 3. [123I] and [125I]epidepride: in vivo studies in rhesus monkey brain and comparison with in vitro pharmacokinetics in rat brain

Studies of [123I]epidepride uptake in rhesus monkey brain were performed using single photon tomography. Striatal uptake peaked at 0.85% of administered dose/g at 107 min post-injection, then declined slowly to 0.70% of administered dose/g at 6 h. Striatal:posterior brain ratios rose from 2 at 25 min to 6.8 at 105 min, to 15 at 4 h and to 58 at 6.4 h. [123I]Epidepride was displaced by haloperidol (0.1 and 1 mg/kg) with a half-life of washout of 55 min. Little displacement of [123I]epidepride was observed following administration of 1 or 2 mg/kg d-amphetamine, respectively, indicating [123I]epidepride is not easily displaced by endogenous dopamine. In vitro equilibrium binding studies using rat striatum revealed a KD of 46 pM and Bmax of 33 pmol/g tissue at 37 degrees C, while at 25 degrees C the KD was 25 pM and the Bmax 32 pmol/g tissue. In vitro kinetic analysis of association and dissociation curves revealed a half-life for receptor dissociation at 37 degrees C of 15 min and 79-90 min at 25 degrees C. Allowing for the temperature difference, there is good correspondence between in vivo and in vitro dissociation kinetics at 25 degrees C. Increasing in vitro incubation temperature from 25 to 37 degrees C caused a 6-fold increase in the dissociation rate, suggesting that there is a change in binding kinetics at the dopamine D2 receptor at 37 degrees C compared to in vivo binding. The results of this study indicate that [123I]epidepride is an excellent radioligand for SPECT studies of the dopamine D2 receptor in man.

Inhibition of cutaneous contact hypersensitivity in the mouse with systemic or topical spiperone: topical application of spiperone produces local immunosuppression without inducing systemic neuroleptic effects

We tested the ability of the neuroleptic agent spiperone (8-[3-(p-fluorobenzoyl)propyl]-1-phenyl-1,3,8-triazaspiro-[4.5] decan-4- one) to influence the tissue swelling and leukocyte infiltration associated with T-cell--dependent immune responses, i.e., contact hypersensitivity reactions, in mice. Contact hypersensitivity reactions were elicited by applying the haptens oxazolone or dinitrofluorobenzene topically to one or both ears 5-8 d after epicutaneous sensitization. When spiperone was given subcutaneously at a dose of 30 or 150 mg/kg, 1 h after challenge with oxazolone, cutaneous contact hypersensitivity to this hapten was significantly diminished. When applied topically in concentrations as low as 0.08% (w/w), preparations of spiperone significantly suppressed both the tissue swelling and the leukocyte infiltration associated with the elicitation phase of contact hypersensitivity. Topical treatment with spiperone also suppressed the sensitization phase of contact sensitivity. However, mice treated topically with spiperone, unlike those treated systemically, exhibited no drowsiness or other evidence of central nervous system effects. Spiperone expresses both serotonin and dopamine receptor antagonist activity. However, unlike spiperone, the chemically unrelated serotonin antagonists, trazadone and mianserin, and the dopamine receptor antagonist, haloperidol, were not effective in suppressing contact hypersensitivity. Our results indicate that spiperone can have immunosuppressive effects on contact hypersensitivity reactions in the mouse, even when applied topically in doses that lack neuroleptic effects, and that the mechanism of action of spiperone on the immune response may be independent of its serotonin or dopamine receptor blocking properties.

Subsets of neurotensin-immunoreactive neurons revealed following antagonism of the dopamine-mediated suppression of neurotensin immunoreactivity in the rat striatum

The distribution of neurotensin-immunoreactive structures in the rat striatum was evaluated after blockade of dopamine neurotransmission by drugs that act presynaptically (6-hydroxydopamine, reserpine) and postsynaptically, preferentially at the D2 (eticlopride, haloperidol) and D1 [(R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepi n-7-ol, SCH-23390] receptor sites. Calbindin-D (mol. wt 28,000) immunoreactivity was used to delineate patch (striosome) and matrix in the caudate-putamen and core and shell in the nucleus accumbens. Antagonism at the D2 dopamine receptor and 6-hydroxydopamine lesions caused dense axonal immunoreactivity and moderate numbers of neurotensin-immunoreactive neurons to be distributed preferentially in the matrix of the caudate-putamen. D1 receptor antagonism was significantly less effective at eliciting neurotensin-immunoreactive neurons in the caudate-putamen. Reserpine or co-administration of the D1 and D2 receptor antagonists produced many neurotensin-immunoreactive neurons in both striatal compartments throughout the caudate-putamen and dense axonal neurotensin immunoreactivity in the medial patch compartment. To varying degrees, with SCH-23390 being least effective and reserpine most effective, all of the drug treatments elicited neurotensin immunoreactivity in neurons in the olfactory tubercle, rostral nucleus accumbens, accumbal shell and ventrolateral caudate-putamen, i.e. most of the ventral striatum.

Swim stress alters in vivo binding of [3H]N-methylspiperone

The effect of swim stress on the in vivo binding of [3H]N-methylspiperone in the striatum of the mouse was investigated. Mice were forced to swim for 5 min at 18 degrees C and the time course of radioactivity in the striatum and cerebellum, following intravenous injection of [3H]N-methylspiperone was measured. The ratio of radioactivity in the striatum to that in the cerebellum was plotted as a function of time for the estimation of in vivo binding to dopamine D2 receptors. Immediately after the swim stress, a significant decrease in binding to D2 receptors in vivo was observed. Neither the KD nor Bmax determined by in vitro binding were altered by swim stress. The time course of the changes in binding, within a 24 hr period, following the swim stress was also studied and a rapid reversal of binding, within 1 hr after the swim stress was observed. In vivo binding of [3H]N-methylspiperone in the cerebral cortex, which appeared to involve serotonin receptors, as well as D2 receptors, was not significantly altered by the swim stress. A saturation study of in vivo binding indicated that the decreases in binding to D2 receptors, due to swim stress, were primarily caused by changes in the apparent affinity rather than in the number of binding sites available in vivo. These results support the hypothesis that micro-environmental factors, including the diffusion barrier to the synapse, might be altered by swim stress.

Repeated stressful experiences differently affect brain dopamine receptor subtypes

The binding of tritiated spiperone (D2 antagonist) and tritiated SCH 23390 (D1 antagonist), in vivo, was investigated in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of mice submitted to ten daily restraint stress sessions. Mice sacrificed 24 hr after the last stressful experience presented a 64% decrease of D2 receptor density (Bmax) but no changes in D1 receptor density in the NAS. In the CP a much smaller (11%) reduction of D2 receptor density was accompanied by a 10% increase of D1 receptors. These results show that the two types of dopamine (DA) receptors adapt in different or even opposite ways to environmental pressure, leading to imbalance between them.

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

Equilibrium models are derived and applied to in vivo binding of spiperone in the rat brain. The models express the concentration of the ligand in the striatum and frontal cortex as a function of the accumulation in the cerebellum. The models differ with respect to the description of specific binding. Nonlinear regression analysis shows that the in vivo specific binding of 3H-labeled spiperone in the frontal cortex (mainly serotonergic) can be described by a noninteracting sites model, whereas the specific binding in the striatum (mainly dopaminergic) can best be described by models that lead to sigmoid saturation curves. These results were tested and partly confirmed by determining the region-of-interest/cerebellar radioactivity ratio of 11C-labeled N-methylspiperone, with and without pretreatment with haloperidol. The estimated Bmax was 32 fmol/mg wet tissue in the frontal cortex and approximately 90 fmol/mg wet tissue in the striatum. The free plus nonspecific binding of spiperone was similar in the frontal cortex but lower in the striatum than in the cerebellum. The occurrence of sigmoidicity can be best explained by the existence of high-affinity/low-capacity sites in the cerebellum rather than mutual interactions of striatal sites. The consequence of the present analysis for positron emission tomography is that the striatal/cerebellar activity ratio is not an accurate parameter of specific binding features at tracer doses of spiperone or N-methylspiperone.

Atypical neuroleptics: role of multiple receptors, endogenous dopamine, and receptor linkage

A variety of biological factors may account for the atypical lack of parkinsonism that is a characteristic of the administration of the many 'atypical' neuroleptics. Although dopamine D2 receptor blockade continues to be a dominant feature of successful neuroleptics, the concomitant blockade of muscarinic or serotonergic S2 receptors helps to prevent neuroleptic-induced parkinsonism for some atypical neuroleptics (clozapine, thioridazine, risperidone). The D2-selective benzamides, however, do not block other known receptors (with the possible exception of sigma sites). Therefore, the atypical nature of the benzamides may be based on their sensitivity to the level of endogenous dopamine released in the different regions of the brain. Finally, atypical neuroleptic action may possibly stem from direct linkage between different receptors coupled through components of the G protein system.

Differences in the time course of dopaminergic supersensitivity following chronic administration of haloperidol, molindone, or sulpiride

The onset and persistence of changes in 3H-spiroperidol binding to dopamine (DA) D2 receptors were examined in rat mesolimbic and striatal brain regions following daily administration of haloperidol, molindone, or sulpiride for 3, 7, 14, or 28 days. Neuroleptic dose equivalencies were determined by inhibition of 3H-spiroperidol in vivo binding in several rat brain regions. Changes in locomotor and stereotyped responses to the specific DA D2 agonist quinpirole were examined 3 days after the last treatment dose. Haloperidol or molindone administration increased mean stereotypy scores and striatal DA D2 receptor densities throughout the 28-day treatment period. In contrast, mesolimbic DA D2 receptor densities were transiently increased and returned to control values, after 28 days of haloperidol or molindone treatment. Sulpiride treatment increased mean stereotypy scores and striatal Bmax values, but had no effect on locomotion or mesolimbic dopamine receptor density. Additionally, the magnitude of change in the various measures of brain DA function varied among the three neuroleptic treatment groups. Results from this study suggest that mesolimbic and striatal brain regions differ in their response to long-term neuroleptic administration and that drug choice may influence the magnitude of neuroleptic-induced dopaminergic supersensitivity.

Intracerebral SCH 23390 and catalepsy in the rat

Stereotaxic injection of SCH 23390 (D-1 antagonist), but not water or ritanserin (5-HT antagonist), into all parts of the caudate-putamen, elicited an immediate and often long-lasting catalepsy in 79% of rats. Similar results were obtained with SCH 23390 delivered into the nucleus accumbens and globus pallidus, but not into a variety of other brain sites. The results support the idea that SCH 23390-induced catalepsy is related to the occlusion of dopamine D-1 receptors in the forebrain, particularly the striatum, although the topography of the catalepsy evoked by intrastriatal SCH 23390 did not match the distribution of D-1 sites labelled in autoradiographic studies.

Blockade of hippocampal dopamine (DA) receptors: a tool for antipsychotics with low extrapyramidal side effects

1. 32 neuroleptics (NL) and a Ciba-Geigy antipsychotic (savoxepine, CGP 19 486 A) were tested on adult male rats and mice. 2. Interaction with D2 DA receptors was assessed using in vivo (3H)spiperone (SPI) binding in rat hippocampus and striatum. 3. Antipsychotic efficacy of NL was checked by their ability to antagonize apomorphine-induced climbing behavior in mice. 4. We found a good correlation between blockade of DA receptors in hippocampus and antagonism of climbing with both classical and atypical NL. 5. No correlation was found between blockade of DA receptors in striatum and climbing with the atypical NL. 6. It is suggested that NL exerting preferential blockade of hippocampal as compared to striatal DA receptors may show a better dissociation between antipsychotic efficacy and induction of extrapyramidal side effects (EPS) in the clinic. The antipsychotic agent savoxepine, displayed such a favourable profile of action.

Striatal D1- and D2-dopamine receptor sites are separately detectable in vivo

We have characterized in particulate fractions of normal rat striatum the in vivo binding kinetics, binding affinity, and pharmacological profiles of [3H]SCH 23390, a ligand selective for the D1-subtype of dopamine (DA) receptor, and compared these to [3H]spiperone, a ligand classically associated with the D2 DA receptor subtype. The pharmacological specificity of each ligand's in vivo binding is very similar to binding to striatal homogenates in vitro. While similar maximum numbers (Bmax) of striatal binding sites exist in vivo compared to in vitro for both ligands, binding affinities in vivo for both ligands are reduced 125- to 200-fold compared to in vitro. In vivo binding of [3H]SCH 23390 to striatum is not increased by dopamine denervation produced by 6-hydroxydopamine lesions of the nigrostriatal pathway. In vivo binding of [3H]SCH 23390 and [3H]spiperone to striatum is not significantly reduced by increased synaptic concentration of dopamine following D-amphetamine administration. 125I-SCH 23982, the iodinated analogue of SCH 23390, localizes very highly to dopaminergic forebrain areas following i.v. administration. External imaging of mammalian and human brain D1-receptors is potentially feasible with this ligand.