C. J. E. Niemegeers

Minireview A systematic study of the pharmacological activities of dopamine antagonists

Abstract The so-called dopamine (DA)-antagonists possess a wide variety of pharmacological activities. The present paper analyses some comparative data that were obtained in the Janssen Research Laboratories, using a large series of DA-antagonists in a representative set of pharmacological tests. This study illustrates the relative importance of the different pharmacological properties of these compounds, provides a tool for further meaningful pharmacological and biochemical research in this area, and discusses the therapeutic potential of these drugs.

Delay of castor oil diarrhoea in rats: a new way to evaluate inhibitors of prostaglandin biosynthesis

Forty‐four non‐steroidal anti‐inflammatory compounds were tested for possible effects on castor oil‐induced diarrhoea in rats. A small but significant delay of intestinal evacuations was found with all compounds. Quantitatively, the oral doses required to delay diarrhoea beyond the first hour after castor oil challenge reflected the acute anti‐inflammatory potency of the tested compounds. Qualitatively, the evolution of the effective doses with increasing delay was linear for potent inhibitors of prostaglandin biosynthesis. The evolution for less potent compounds was markedly different and suggested the earlier occurrence of nonspecific drug effects. Suprofen, the most potent of the series of compounds, produced the 1 h delay at an oral dose of 1·11 mg kg−1; the ED50 increased linearly to 115 mg kg−1 for a 4 h delay. Compared with other compounds the activity pattern of suprofen was consistent with that of a very potent, short‐acting inhibitor of prostaglandin biosynthesis, which maintains its specific action over a wide dose range. It is concluded that delay of castor oil‐induced diarrhoea in rats allows a detailed characterization of aspirin‐like compounds, and that inhibition of prostaglandin biosynthesis is insufficient to suppress the intestinal effects of the oil.

Survey on the pharmacodynamics of the new antipsychotic risperidone

This review reports on the pharmacodynamics of the new antipsychotic risperidone. The primary action of risperidone is serotonin 5-HT2 receptor blockade as shown by displacement of radioligand binding (Ki: 0.16 nM), activity on isolated tissues (EC50:0.5 nM), and antagonism of peripherally (ED50: 0.0011 mg/kg) and centrally (ED50:0.014 mg/kg) acting 5-HT2 receptor agonists in rats. Risperidone is at least as potent as the specific 5-HT2 receptor antagonist ritanserin in these tests. Risperidone is also a potent dopamine D2 receptor antagonist as indicated by displacement of radioligand binding (Ki: 1.4 nM), activity in isolated striatal slices (IC50: 0.89 nM), and antagonism of peripherally (ED50: 0.0057 mg/kg in dogs) and centrally acting D2 receptor agonists (ED50: 0.056–0.15 mg/kg in rats). Risperidone shows all effects common to D2 antagonists, including enhancement of prolactin release. However, some central effects such as catalepsy and blockade of motor activity occur at high doses only. Risperidone is 4–10 times less potent than haloperidol as a central D2 antagonist in rats and it differs from haloperidol by the following characteristics: predominant 5-HT2 antagonism; LSD antagonism; effects on sleep; smooth dose-response curves for D2 antagonism; synergism of combined 5-HT2/D2 antagonism; pronounced effects on amphetamine-induced oxygen consumption; increased social interaction; and pronounced effects on dopamine (DA) turnover. Risperidone displays similar activity at pre- and postsynaptic D2 receptors and at D2 receptors from various rat brain regions. The binding affinity for D4 and D3 receptors is 5 and 9 times weaker, respectively, than for D2 receptors; interaction with D1 receptors occurs only at very high concentrations. The pharmacological profile of risperidone includes interaction with histamine H1 and α-adrenergic receptors but the compound is devoid of significant interaction with cholinergic and a variety of other types of receptors. Risperidone has excellent oral activity, a rapid onset, and a 24-h duration of action. Its major metabolite, 9-hydroxyrisperidone, closely mimics risperidone in pharmacodynamics. Risperidone can be characterized as a potent D2 antagonist with predominant 5HT2 antagonistic activity and optimal pharmacokinetic properties.

Behavioral and 5-HT antagonist effects of ritanserin: a pure and selective antagonist of LSD discrimination in rat

The newly synthesized compound and putative 5-HT2 antagonist ritanserin, but not the structurally related compound R 56413, resembles pirenperone in that it acts as a pure antagonist in an LSD-saline drug discrimination assay in the rat. Ritanserin exceeded pirenperone in terms of behavioral specificity; the lowest effective dose of ritanserin in antagonizing LSD was one order of magnitude higher than that of pirenperone, but the compound depressed rate of operant responding only at doses that were about 1000-fold higher than those at which pirenperone was effective. Ritanserin exerted effects in an open field test which were reminiscent of anxiolytic drug activity in the rat; its effects were greater than those of pirenperone, R 56413 and the benzodiazepines chlordiazepoxide and diazepam. The results of experiments on antagonism of 5-HT-induced hypothermia and of the 5-HTP-induced headtwitch response fail to support the possibility that the putative anxiolytic effects of ritanserin in the rat can be ascribed simply to a pharmacologically defined action at 5-HT receptors.

Theoretical and methodological considerations on drug discrimination learning

A method is described which allows the assessment of discriminative stimulus properties of drugs, and the ability of amphetamine (0.16 mg/kg, s.c), chlordiazepoxide (5 mg/kg, p.o.), desipramine (5 mg/kg, s.c), and haloperidol (0.02 mg/kg, s.c.) to produce a discriminative stimulus complex (DSC) is evidenced. The method is found to yield clear-cut data that are specifically related to drug discrimination learning without being possibly confounded by state dependent effects. In addition, the experimental procedure is designed so as to provide an appropriate measurement of operant response modulating drug effects.

Discriminative stimulus properties of cocaine: Neuropharmacological characteristics as derived from stimulus generalization experiments ☆ ☆☆

The experiments reported here were undertaken to examine the neuropharmacological characteristics of drugs inducing stimulus generalization with cocaine as a cue. Experiment 1 indicated that d-amphetamine (ED50: 0.17 mg/kg), 1-amphetamine (0.45 mg/kg), methylamphetamine (0.15 mg/kg), methylphenidate (0.55 mg/kg) and nomifensine (0.32 mg/kg) induce stimulus generalization with cocaine in rats trained to discriminate 10 mg/kg cocaine from saline; this generalization occurred in 100% of the animals, proceeded along steep slopes (s: 1.27 to 1.88 in log-probit plots), and was not associated with behaviorally toxic effects. Amantadine (57.8 mg/kg; s=1.85), apomorphine (0.33 mg/kg; s=1.77), piribedil (8.4 mg/kg; s=10.6) and bromocryptine (>40 mg/kg) also induced stimulus generalization to some extent, but this generalization was partial in some cases, proceeded along a shallow slope with piribedil, and was invariably associated with severe rate depressant effects. Ten mg/kg, but not 1.25 mg/kg hydroxyamphetamine induced generalization in 3 out of 8 rats. Experiment 2 revealed that tranylcrypromine (2.5 mg/kg; s=1.7), fentanyl (0.068 mg/kg; s=1.34), morphine (>10 mg/kg), phencyclidine (0.81 mg/kg; s=1.43), and benztropine (9.2 mg/kg) induce stimulus generalization with cocaine, whereas lidocaine, procaine, chlordiazepoxide, imipramine, desipramine, mescaline, LSD, isopropamide, and atropine do not. Experiment 3 shows that propranolol (1.25 to 40 mg/kg) and isoproterenol (0.63 to 2.5 mg/kg) induce a biphasic generalization with cocaine. Experiment 4 discloses that rats trained to discriminate 10 mg/kg propranolol from saline generalize their training drug along a linear gradient, but generalize cocaine along a biphasic gradient. It is suggested (a) that stimulus generalization with cocaine may be contingent upon increasing the functional availability of endogenous dopamine and, perhaps, of norepinephrine irrespective of the presynaptic mechanism from which such increase may result and (b) that differential stimulus generalization of drugs with cocaine (in terms of dose, subjects, slope, and rate effects) may parallel their differential primary reinforcing properties.

[3H]sufentanil, a superior ligand for μ-opiate receptors: Binding properties and regional distribution in rat brain and spinal cord

Stereospecific [3H]sufentanil binding, inhibited by dextromoramide, represents 90% of the total binding in membrane preparations of rat brain and spinal cord. Scatchard plots of the binding in the forebrain, at 37 degrees C in Tris-HCl buffer without and with 120 mM NaCl, were rectilinear; KD = 0.13 nM and 0.31 nM, Bmax = 13 fmol/mg tissue and 9.9 fmol/mg tissue in the absence and the presence of sodium ions respectively. The reduction in binding affinity in the presence of sodium ions was found to be due to a 9.7 fold enhancement of the initial dissociation rate from t1/2 = 2.1 min in the absence to 13 s in the presence of sodium ions. The [3H]sufentanil binding properties were superior to those of [3H]fentanyl, [3H]dihydromorphine and [3H]naloxone; [3H]sufentanil showed an unmatched favourable ratio of stereospecific versus non-specific binding; it had a 7.7, 20 and 40 fold binding affinity than the above ligands respectively. Due to its relatively slow dissociation rate, a more accurate estimation of the Bmax value was obtained with [3H]sufentanil than with the other, fast dissociating 3H-ligands (t1/2 less than 10 s). A total of 37 narcotic analgesic agonists and antagonists belonging to 5 different major structural classes all inhibited stereospecific [3H]sufentanil binding in a competitive way. There was no relationship between binding affinities and lipophilicity and degree of ionization of the compounds. Binding affinities correlated highly significantly with the analgesic potency measured in vivo, demonstrating that [3H]sufentanil labels mu-opiate receptor sites which mediate narcotic analgesia. Moreover, the binding affinity of sufentanil for delta-type binding sites labelled by [3H] [D-Ala2,D-Leu5]enkephalin was found to be 100 times lower than its binding affinity for the mu-receptor sites. [3H]Sufentanil was used for a detailed investigation of the regional distribution of mu-opiate receptor sites in the brain; Bmax and KD values were measured in the dorsal and ventral spinal cord.

Discriminative stimulus properties of cocaine and d-amphetamine, and antagonism by haloperidol: A comparative study

This paper presents a comparative study of the discriminative stimulus properties of cocaine and d-amphetamine in the rat. Using a discrete-trial, food-reward, two-lever drug discrimination procedure, one group of rats (n = 7) were trained to discriminate 10 mg/kg cocaine from saline, whereas 1.25 mg/kg d-amphetamine served as a cue to a second group (n = 6). Following training, stimulus generalization gradients were determined for cocaine and d-amphetamine in both groups, and so were the antagonistic effects of haloperidol vs the training drug conditions. The results indicate that the acquisition of discrimination proceeded at a comparable rate in the two groups. In both groups, d-amphetamine was about 5 times more potent than cocaine, and individual threshold doses for the two drugs showed a significant correlation. Haloperidol appeared equally effective in antagonizing 1.25 mg/kg d-amphetamine and 10 mg/kg cocaine. These findings converge to suggest that to a large extent the cueing properties of d-amphetamine and cocaine are similar.

The influence of various neuroleptic drugs on shock avoidance responding in rats

SummaryThe inhibitory effects of four neuroleptic drugs on amphetamineinduced stimulation in a discriminated Sidman avoidance procedure in rats were measured. Amphetamine 0.63 mg/kg s.c. increased R (responses) and decreased W (warning stimuli), WR (warning responses) and S (shocks). Relatively low doses of all four neuroleptics antagonized the amphetamineinduced changes. The order of potency was haloperidol > pimozide > chlorpromazine > pipamperone. The duration of action was pimozide > haloperidol > pipamperone > chlorpromazine.Haloperidol, pimozide and pipamperone restored the amphetamine-induced changes to the initial control levels in the order: S, W, WR and R. With chlorpromazine this order was reversed, except for R.The different pharmacological profiles of haloperidol, pimozide and pipamperone are discussed.

The influence of various neuroleptic drugs on shock avoidance responding in rats. I. Nondiscriminated Sidman avoidance procedure.

SummaryThe effect of twenty neuroleptic drugs on conditioned behaviour was studied in rats by means of a lever-press shock-avoidance procedure (shock-shock 20 sec, response-shock 20 sec). All twenty drugs inhibited lever-press response and reduced the shock-avoidance rate at very low dose levels. The order of potency was: benperidol=spiroperidol > trifluperidol > droperidol=spiramide > clofluperol=fluphenazine=haloperidol=spirilene > moperone > perphenazine > amiperone > fluanisone=trifluperazine > pimozide > thioperazine > chlorpromazine > pipamperone=thioridazine > promazine.