R.A. Lahti

Dopamine D4 versus D2 receptor selectivity of dopamine receptor antagonists: possible therapeutic implications

The dopamine D4 receptor, which is considered a close variant of the dopamine D2 receptor, has recently been cloned. Receptor binding studies demonstrated that clozapine, which is an effective antipsychotic agent but atypical in that it lacks the usual side effects of other antipsychotic agents, has high selectivity for the dopamine D4 receptor versus the dopamine D2 receptor. Comparative binding affinity studies have been carried out for a number of interesting dopaminergic agents using membranes prepared from cloned dopamine D2 and D4 receptor containing cells. It was found that clozapine is selective for the dopamine D4 vs. the D2 receptor by a factor of 2.8. Other compounds with dopamine D4 receptor selectivity were (+)-apomorphine (8.7), (+)-N-propyl-norapomorphine (NPA) (2.4) and melperone (1.3). Compounds with considerable selectivity for the dopamine D2 receptor were haloperidol (0.31), chlorpromazine (0.084), trifluoperazine (0.034) and raclopride (0.001). Overall, the results with the antipsychotic agents tested, support the concept that dopamine D4 receptor selectivity may confer clozapine-like antipsychotic efficacy and furthermore that dopamine D2 receptor selectivity may confer side effect liability (extrapyramidal side effects and tardive dyskinesia).

D2-Family receptor distribution in human postmortem tissue: An autoradiographic study

THE distribution throughout the normal human brain of the dopamine D2-family of receptors were investigated autoradiographically. Three ligands were used, [3H]-YM- 09151–2 to define the D2, D3, D4 receptors; [3H]raclopride the D2 D3 receptors; and [3H](+)-7-OH-DPAT, in the presence of GTP, demonstrates D3 distribution. [3H]YM- 09151–2 and [3H]raclopride binding were highest in caudate (121 vs 130 fmol mg-1), putamen (96 vs 136 fmol mg-1), and nucleus accumbens (113 vs 120 fmol nig-1). [3H]YM-09151–2 also displayed significant binding in several cortical areas (56–39 fmol mg-1) and hippocampus (27 fmol mg-1). [3H](+)-7-OH-DPAT was highest in the nucleus accumbens. Based upon the ligands properties it is inferred that D2 distribution is highest in putamen, caudate and nucleus accumbens; D3in the nucleus accumbens; D4 receptor in cortical areas and hippocampus.

Antipsychotic properties of the partial dopamine agonist (-)-3-(3- hydroxyphenyl)-N-n-propylpiperidine (preclamol) in schizophrenia

BACKGROUND In an ongoing effort to characterize the clinical pharmacologic profile of the partial dopamine agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)-3PPP], we administered it to drug-free schizophrenic patients in two consecutive studies. METHODS In a preliminary dose-finding study, 9 patients were treated using a 6-week placebo-controlled crossover design. Then, to properly demonstrate the antipsychotic effect, we carried out an early efficacy study; here 10 patients received (-)-3PPP, 300 mg B.I.D., in a 1-week placebo-controlled crossover study. RESULTS Dose-Finding Study: (-)-3PPP showed apparent antipsychotic effect in repeated dosing, with 300 mg B.I.D. being the most effective dose for antipsychotic action; however, the apparent antipsychotic action was not sustained for longer than 1 week, presumably because of desensitization of the receptor by the agonist. Early Efficacy Study: Positive symptoms as measured by the Psychosis Change Scale decreased in 1 week by 30% with (-)-3PPP compared to placebo, and negative symptoms measured with the Brief Psychiatric Rating Scale Withdrawal subscale decreased by 28% with the drug. In both studies, (-)-3PPP lacked any evidence of motor side effects. CONCLUSIONS These data show that psychotic symptoms decrease with (-)-3PPP and suggest that the treatment of schizophrenia with partial dopamine agonist is a promising strategy. Future attention will be directed toward testing techniques to diminish the tachyphylaxis to allow an ongoing therapeutic effect.

Direct determination of dopamine D4 receptors in normal and schizophrenic postmortem brain tissue: A [3H]NGD-94-1 study

Using an indirect subtraction binding technique and human postmortem tissue, several laboratories1–3 reported finding increases in dopamine D4 receptors in caudate nuclei of schizophrenic patients, although others4–6 have not replicated these findings. NGD-94-1 is a selective D4 antagonist with low affinity for the D2 and D3 receptors.7,8 [3H]NGD-94-1 has been used in this study to directly determine the density of D4 receptors in normals (n = 13) and schizophrenic subjects (n = 7) off antipsychotic drugs for at least 3 months prior to death, or on antipsychotic (n = 7) drugs at the time of death. Human postmortem coronal brain sections were incubated with [3H]NGD-94-1 and autoradiograms developed; and binding in pertinent regions was quantified. In normals, the highest density of [3H]NGD-94-1 binding was in the hippocampus (68 fmol mg−1, temporal (33), insular (30), and entorhinal cortices (24.9). Significant increases in [3H]NGD-94-1 density in schizophrenics (n = 14) vs normals (n = 13) were observed in the entorhinal cortex (46%) at both low and high magnifications. The increases observed in the schizophrenics were found in both schizophrenics off antipsychotic drugs for at least 3 months prior to death and those on antipsychotic drugs at the time of death. Thus, the changes may be disease-related and not a consequence of pharmacological treatment. No significant differences were found between the two schizophrenic groups in any brain area studied.

Regional expression of RGS4 mRNA in human brain

Regulators of G‐protein signalling (RGS) proteins are a recently discovered class of proteins that modulate G‐protein activity. More than 20 RGS proteins have been identified and are expressed throughout the body and brain. In particular, RGS4 appears to regulate dopamine receptor function and has been implicated in several dopamine related diseases, including schizophrenia. This study presents an extensive examination of the regional distribution of RGS4 mRNA in postmortem human brain. Using in situ hybridization, the expression levels of RGS4 mRNA were determined in human hemicoronal (Talairach sections +8 and −20) brain sections. In the rostral slice (Talairach +8) highest levels were found in the inferior frontal cortex, the superior frontal, and the cingulate cortex. Slightly lower levels were found in the insular cortex and inferior temporal cortex. The caudate, putamen and nucleus accumbens had lower levels. In the caudal slice (−20), the cortical layers showed the highest levels, with moderate levels observed in the parahippocampal gyrus, low levels in the CA‐pyramidal region, and almost undetectable levels in the thalamus. In the frontal cortex a dense band was apparent near one of the inner layers of the cortex. In conclusion, RGS4 mRNA distribution in human postmortem tissue from normal persons was very dense in most cortical layers examined, with lower density in the basal ganglia and thalamus.

D2-type dopamine receptors in postmortem human brain sections from normal and schizophrenic subjects

The density of the presumed dopamine D4 receptor ([3H]YM-09151-2 minus [3H]raclopride), as well as the densities of the two ligands themselves were compared in various areas of cerebral tissue from normal versus schizophrenic subjects off or on antipsychotic drugs at the time of death. Using autoradiographic techniques, and analyzing various brain areas, no differences were found in the density of the D4 receptor, nor were differences found between the groups, in any brain region, in the amount of bound [3H]YM-09151-2 or [3H]raclopride. There were, therefore, no differences in the density of the D3-type receptors, including the D4 receptor, in normal and schizophrenic subjects off or on antipsychotic drugs at the time of death.

[3H]Neurotensin receptor densities in human postmortem brain tissue obtained from normal and schizophrenic persons. An autoradiographic study

Summary.[3H]Neurotensin binding and autoradiographic techniques were used to determine the distribution and density of neurotensin receptors in normal and schizophrenic postmortem brain tissue. Coronal hemi-brain blocks of tissue were cut at the level of the caudate and hippocampus from frozen brain tissue obtained from normal individuals with no known psychiatric or neurologic illnesses and from schizophrenic subjects off- or on-antipsychotic drugs at the time of death. Each hemi-block was further divided, sectioned, thaw mounted on to slides, incubated with [3H]neurotensin and apposed to film. Digitized images were analyzed for binding densities. Areas of intense binding include the substantia nigra, the entorhinal cortex, superficial layers of the cingulate, middle frontal, and insular cortices; and with moderate binding in nucleus accumbens, and caudate. Schizophrenic patients off- (3 months or more) or on-antipsychotic drugs at the time of death were tested; all patients showed a reduced level of neurotensin receptors in the caudate (68% of normals), cingulate (34%) and prefrontal cortices (25%).

Affinities and intrinsic activities of dopamine receptor agonists for the hD21 and hD4.4 receptors

The affinity and intrinsic activity of dopamine receptor agonists were determined at the human dopamine hD21 and hD4.4 receptors. (-)-3-Hydroxy-N-n-propylpiperidine ((-)3-PPP) had an intrinsic activity of 46% and 83%, whereas (+)-N-propylnorapomorphine ((+)-NPA) had intrinsic activities of 61% and 58% at the dopamine hD21 and hD4.4 receptors, respectively. Affinities also varied. A single, or multiple, dopamine D2-type receptor(s) may be involved in schizophrenia and agonists are being tested as therapy. Understanding their properties at the individual dopamine D2-family receptors is important.

Dopamine D2 receptor binding properties of [3H]U-86170, a dopamine receptor agonist

U-86170F, an imidazoguinolinone, is a potent dopamine D2 agonist, binding with high affinity to the dopamine D2 receptor. A Kd of 0.99 nM was determined in membranes from Chinese hamster ovarian (CHO) cells transfected with the D2 receptor and a Kd of 1.72 nM was obtained in rat striatal homogenates. GTP sensitivity was demonstrated when its addition (300 microM) reduced [3H]U-86170 binding by 60%. This agonist ligand is especially effective in identifying agonists and partial agonists, as well as antagonists, and affords a more precise evaluation of their affinity for the dopamine D2 receptor, without the use of multiple site analysis, than does an antagonist [3H]-ligand.

Effect of amphetamine, α-methyl-p-tyrosine (α-MPT) and antipsychotic agents on dopamine D2-type receptor occupancy in rats

1. EEDQ inactivates unoccupied receptors in vivo in brain tissue and is useful in determining which receptors are occupied by a drug treatment. 2. alpha-MPT, inhibits the synthesis of dopamine, reducing D2-type receptor occupancy by dopamine and enhances the amount of receptor inactivation by EEDQ. 3. Amphetamine releases dopamine resulting in increased occupancy of dopamine D2-type receptors and we have shown that it protects those receptors from EEDQ. 4. Clozapine and remoxipride, two antipsychotic agents, occupied the dopamine receptors in both the caudate and cortex. 5. These findings are important because they substantiate other results obtained with amphetamine and SPECT, which demonstrated an exaggerated dopamine neurotransmission in schizophrenic patients versus normal controls.