David M. Jackson

Dopamine receptors: Molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DAs interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Prepulse inhibition of acoustic startle, a measure of sensorimotor gating : effects of antipsychotics and other agents in rats

Schizophrenic patients are deficient in various neurologic measures reflecting information processing. One such measure in prepulse inhibition (PPI) of acoustic startle, in which schizophrenics display less inhibition than normal subjects. PPI is also diminished in rats treated with psychotomimetic drugs such as amphetamine and phencyclidine. PPI has been suggested as a model relevant for studying the pathophysiology of schizophrenia. We studied the effect of a variety of antipsychotics and putative antipsychotics and some key reference compounds on the acoustic startle response (ASR) and PPI. Some, but not all, antipsychotics tested (mainly selective dopamine D2 antagonists) enhanced PPI. Remoxipride and clozapine, both of which are antipsychotics, and the very potent and highly selective D2 antagonist, NCQ-298, did not. It is concluded that enhanced PPI in otherwise untreated rats does not reflect antipsychotic efficacy. We further noted that the effect on PPI was independent of the effect on ASR.

Dopamine receptor antagonists block amphetamine and phencyclidine-induced motor stimulation in rats.

d-Amphetamine (DEX) and phencyclidine (PCP) increased motor activity in rats as measured in automated activity cages. Analysis of the stimulation indicated that both drugs increased horizontal activity (total activity), locomotion, and peripheral activity. However, DEX increased while PCP decreased the incidence of rearing. The ability of different drugs to antagonise DEX- and PCP-induced increases in total activity (called stimulation) was measured. Dopamine (DA) D1 receptor antagonists (SCH23390, NNC-01-0112) were 7-8 times more potent in blocking DEX than PCP. DA D2 receptor antagonists (raclopride, remoxipride, haloperidol) were only 1-2 times more potent against DEX-induced stimulation. Nonselective DA receptor antagonists were also tested. Chlorpromazine was more potent against DEX than against PCP. Buspirone and sertindole were slightly more potent in blocking PCP than DEX. Ritanserin (5-HT2 receptor antagonist) was inactive against both stimulants. 8-OH-DPAT (5-HT1A receptor agonist) potentiated the stimulant effects of DEX and PCP. Prazosin (alpha 1-adrenergic receptor antagonist) partially blocked both DEX and PCP. Most drugs tested depressed spontaneous motor activity. Remoxipride and sertindole, however, caused very little depression even at doses several times higher than those needed to block DEX or PCP. The data show clear pharmacological differences between DEX- and PCP-induced stimulation.

The atypical antipsychotic, remoxipride, blocks phencyclidine-induced disruption of prepulse inhibition in the rat

The effect of various typical (haloperidol) and atypical (clozapine, raclopride, remoxipride) antipsychotics on phencyclidine (PCP)-induced disruption of sensorimotor gating was tested in rats using an acoustic startle paradigm. Clozapine (4–40 µmol/kg), haloperidol (1–5 µmol/kg) and raclopride (1–12 µmol/kg) failed to reverse PCP-induced disruption of prepulse inhibition (PPI) of the acoustic startle response. In contrast, remoxipride (12–60 µmol/kg) caused a dose-dependent block of this effect. PCP-induced disruption of PPI is a widely accepted animal model of a corresponding behavioural deficit observed in schizophrenia although little evidence has been presented that it is in fact sensitive to antipsychotic agents. The present results indicate that remoxipride behaves in a unique way in this model compared to clozapine, haloperidol and raclopride.

Nitric oxide synthase inhibition blocks phencyclidine-induced behavioural effects on prepulse inhibition and locomotor activity in the rat

Abstract The ability of the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), to block the behavioural effects of the potent psychotomimetic, phencyclidine, was tested in rats using two different behavioural models. L-NAME was found to block both phencyclidine-induced disruption of prepulse inhibition of acoustic startle and phencyclidine-induced stimulation of locomotor activity. A selective action of L-NAME on the effects of phencyclidine was indicated, since L-NAME did not alter the effects of amphetamine, another potent psychotomimetic, in these behavioural models. These observations suggest that a nitric oxide-dependent mechanism may be involved in the effects of phencyclidine in the central nervous system.

Kinetic properties of the accumulation of 3H-raclopride in the mouse brain in vivo

SummaryThe kinetic properties of the accumulation of 3H-raclopride, a selective dopamine (DA) D-2 receptor antagonist, in mouse striatum in vivo was examined under various experimental conditions. The accumulation in striatum was saturable in contrast to that in cerebellum, which linearily increased with the dose. The specific binding of 3-Hraclopride in the striatum, defined as the difference in the accumulation in striatum and cerebellum 30 min after the injection was completely inhibited by the D-2 receptor antagonists spiperone and (-I-)-butaclamol [but not (−)butaclamol] and the DA receptor agonist N-n-propylnorapomorphine. The mean Bmax value of the specific binding was 40.7 ± 2.8 pmol/g tissue and the mean apparent KD value, based on the dose injected, was 87.8 ± 11.5 nmol/kg i. v. (18 different experiments). Pretreatment of the mice with a single injection of reserpine 4 h or 3 days beforehand reduced the apparent KD value which in part seemed to be due to the decreased concentration of synaptic DA. Similarly, γ-butyrolactone injected immediately before raclopride reduced the apparent KD value, whereas amfonelic acid and (−)-amphetamine increased the observed KD values. These findings indicate competition between endogenous DA and raclopride for the D-2 receptors. Both reserpine and γ-butyrolactone increased the apparent Bmax value by about 50% which indicates a receptor pool of DA for which raclopride does not compete.

Binding characteristics of remoxipride and its metabolites to dopamine D2 and D3 receptors

The substituted benzamide, remoxipride, is a new atypical antipsychotic agent with good clinical efficacy and low extrapyramidal side-effect potential. In the present study, the in vitro receptor binding properties of remoxipride and several of its metabolites to rat striatal dopamine D2 and cloned human dopamine D2A and D3 receptors were investigated. Remoxipride bound to [3H]raclopride-labelled dopamine D2 receptors in rat striatum with an affinity (Ki) of 113 nM. The significantly lower affinities of remoxipride reported when [3H]spiperone was used as a radioligand are suggested to be due to methodological problems associated with the use of very high-affinity radioligands. Some of the phenolic metabolites of remoxipride found mainly in rat exhibited considerably higher affinities to dopamine D2 and D3 receptors than remoxipride itself. The pyrrolidone metabolites found mainly in the human had very low dopamine D2 and D3 affinities. The present in vitro results suggest that the behavioural effects of remoxipride in rats may reflect the effect of remoxipride and some of its high-affinity metabolites.

Kinetic properties of the in vivo accumulation of 3H-(−)-N-n-propylnorapomorphine in mouse brain

Summary(1) The influence of various dopamine (DA) receptor agonists and antagonists on the kinetic properties of the specific binding of 3H(−)-N-n-propylnorapomorphine (NPA) in the mouse striatum in vivo was studied. The specific binding of 3H-NPA, defined as the difference between the radioactivity in the striatum and cerebellum, was completely antagonized by the selective D-2 receptor antagonist raclopride but not by the selective D-1 antagonist SCH 23390, showing that the binding occurs exclusively to the D2 receptors. (2) The selective D-2 receptor agonists pergolide and quinpirole inhibited the 3H-NPA binding biphasically at low doses, indicating that these DA receptor agonists have high affinities for a subfraction (10 to 30%) of the NPA binding sites. (3) Increasing the synaptic DA concentration by DA release [(+)-amphetamine] or uptake blockade (amfonelic acid and methylphenidate) inhibited the 3HNPA binding in a competitive manner (unchanged Bmax, increased KD). Depletion of the DA in the synapses by γ-butyrolactone or reserpine decreased the apparent KD value. (4) The possibility of estimating changes in the synaptic DA concentration from changes in the apparent KD is discussed. According to the results obtained, the normal concentration of DA in the synaptic cleft in mouse striatum in vivo is about 40 nmol/l and this concentration is increased 2 to 3 times by (+)-amphetamine and amfonelic acid in doses which evoke hyperactivity and stereotypic behaviour.

Serotonergic modulation of 3,4-methylenedioxymethamphetamine (MDMA)-elicited reduction of response rate but not rewarding threshold in accumbal self-stimulation

In a fixed interval 5-s rate-frequency function paradigm with rats, 3,4-methylenedioxymethamphetamine (MDMA; 0.5, 2 and 4 mg/kg) dose-dependently decreased response rate for nucleus accumbens self-stimulation while both D-amphetamine (0.3 and 1 mg/kg) and cocaine (5 and 15 mg /kg) increased response rates. The highest doses of MDMA caused a cessation of responding in many of the rats tested, but in those rats that continued to respond a significant reduction in frequency threshold for self-stimulation was seen. Cocaine and amphetamine dose-dependently reduced frequency threshold in all rats tested. The non-specific serotonin antagonist, methysergide (5 mg/kg), reversed the inhibitory effects of MDMA on response rates and caused all rats to respond following MDMA (4 mg/kg). Methysergide did not affect MDMAs threshold-lowering properties and when administered alone methysergide had not effect on self-stimulation. These results suggest serotonergic involvement in the performance but not reinforcement-modulating effect of MDMA in the self-stimulation paradigm.

Preclinical findings with new antipsychotic agents: what makes them atypical?

Many early antipsychotics such as haloperidol, while effective in treating the symptoms of schizophrenia, cause detrimental side effects and moreover induce nonspecific sedation in many patients. Newer drugs such as remoxipride are as effective as the classical antipsychotics but induce fewer debilitating side effects. These clinical properties are reflected to some extent in their preclinical pharmacology, with drugs such as remoxipride having effects on various preclinical behavioural and biochemical models that are quite different to those exerted by drugs such as haloperidol. This article reports some new behavioural data and discusses the various mechanisms that can underlie the effect of new atypical antipsychotics.