Ian Creese

3H-Spiroperidol labels dopamine receptors in pituitary and brain.

3H-Spiroperidol of high specific radioactivity labels dopamine receptors in membranes of bovine caudate nucleus and anterior pituitary. The saturation and kinetic properties of 3H-spiroperidol binding are similar in the two tissues. In both caudate and pituitary 3H-spiroperidol displays very high affinity with a dissociation constant of 0.2-0.3 nM. The relative potencies of numerous dopamine agonists and antagonists in competing for 3H-spiroperidol binding are closely similar in anterior pituitary and caudate.

Dopamine receptor binding: Differentiation of agonist and antagonist states with 3H-dopamine and 3H-haloperidol

Abstract 3H-Dopamine and 3H-haloperidol bind with high affinity and selectivity to synaptic dopamine receptors in membrane preparations of the calf caudate. Binding of both ligands shows marked regional variations with greatest density in caudate, putamen, globus pallidus, nucleus accumbens and olfactory tubercle, areas rich in dopamine nerve terminals. The rank-order of phenothiazines and related agents as well as catecholamines in displacing both dopamine and haloperidol binding closely parallels their pharmacological potencies and affinities for the dopamine-sensitive adenylate cyclase. Dopamines affinity for specific 3H-dopamine binding sites is 100 times its apparent affinity for the dopamine sensitive adenylate cyclase. Agonists have about 50 times more affinity for dopamine than haloperidol sites, whereas antagonists display about 100 times greater affinity for haloperidol than dopamine sites.

Nigrostriatal lesions enhance striatal 3H-apomorphine and 3H-spiroperidol binding.

Following unilateral 6-hydroxydopamine nigrostriatal lesions in rats, the binding of both 3H-apomorphine and 3H-spiroperidol in the striatum is increased. In rats with incomplete lesions or at early time points after lesion, binding is not significantly different from control levels.

Opiate receptor binding affected differentially by opiates and opioid peptides

The potencies of various opiates in displacing several 3H-opiate ligands binding to rat membranes vary depending on the nature of the ligand. Whereas opiate antagonists, as well as the opioid peptides and some agonists (etorphine, levorphanol and phenazocine) display similar affinities in displacing either 3H-opiate or 3H-methionine enkephalin binding, other agonists (such as morphine and oxymorphone) are considerably (20-50 times) weaker in displacing 3H-enkephalin than 3H-dihydromorphine binding. These agonists also compete for 3H-enkephalin binding with shallow displacement curves, and are greatly weakened in displacing 3H-naloxone binding in the presence of sodium. These agonists differ from the other opiate classes by possessing a relatively hydrophilic component in their C-ring moieties which may provide a basis for the differential interactions of drugs with the opiate receptor.

The dopamine receptor: differential binding of d-LSD and related agents to agonist and antagonist states.

Dopamine receptor binding is calf striatal membranes of 3H-dopamine and 3H-haloperidol appears to differentiate agonist and antagonist states of the receptor. Agonists and antagonists have selective affinities for dopamine and haloperidol sites respectively. In evaluating relative affinities for dopamine and haloperidol binding sites, we have observed that d-LSD interacts with considerable affinity at the dopamine receptor. Its similar competition petition for binding of the two tritiated ligands suggests that it is a mixed agonist-antagonist, which is consistent with its interactions with the dopamine-sensitive adenylate cyclase. The effects of LSD on dopamine receptor binding are stereospecific, with d-LSD being 1,000 times more potent than d-LSD. 2-Bromo-LSD has more of an antagonist profile than d-LSD for the dopamine receptor. In binding experiments methiothepin behaves like a potent and relatively pure antagonist at dopamine receptors.

Dopamine receptor binding: Specificity, localization and regulation by ions and guanyl nucleotides

3H-Spiroperidol labels dopamine receptors in rats striatum but in frontal cortex and hippocampus 3H-spiroperidol labels serotonin receptors. The agonists 3H-ADTN and 3H-apomorphine label rat striatal dopamine receptors. Comparison with calf striatal binding indicates a species difference in 3H-apomorphine binding. Drug displacement and lesion studies suggest that in the rat 3H-apomorphine labels two distinct dopamine receptors, one associated with the dopamine-sensitive adenylate cyclase and the other with presynaptic dopamine receptors also labeled by 3H-spiroperidol. Whereas divalent cations increase specific dopamine receptor binding of 3H-agonists and 3H-antagonists, 3H-agonist binding is selectively decreased by some guanyl nucleotides.

Species variations in dopamine receptor binding

Binding of 3H-spiroperidol, 3H-apomorphine and 3H-ADTN (2-amino-6,7-dihydroxytetrahydronaphthalene) associated with dopamine receptors has been evaluated in corpus striatal membranes of calf, rat and human brains. Substantial species differences are apparent for numberous agonists and antagonists in competing for receptor binding. In general, dopamine receptor antagonists are more potent in rat and agonists more potent in calf. In competing for 3H-spiroperidol binding sulpiride, molindone and metaclopramide show the most pronounced species differences, being 3--10 times more potent in rat and human than in calf. In all three species agonists compete for 3H-spiroperidol binding with Hill coefficients less than one while antagonists inhibit 3H-spiroperidol binding with Hill coefficients of about 1.0. Conversely, 3H-apomorphine and 3H-ADTN binding in all three species is inhibited by antagonists with Hill coefficients less than 1.0 while agonists display Hill coefficients of about 1.0. In general agonists are more potent in competing for binding of 3H-apomorphine and 3H-ADTN than 3H-spiroperidol. However, a small component of dopamine, apomorphine and ADTN inhibition of 3H-spiroperidol binding displays very high affinity (IC50 about 1 nM). In human amygdala 3H-spiroperidol appears to label serotonin receptors predominantly.

Regulation by Cations of [3H]Spiroperidol Binding Associated with Dopamine Receptors of Rat Brain

Abstract: The effects of monovalent and divalent cations on binding of [3H]spiroperidol to dopamine receptors in rat corpus striatum were studied. Both monovalent and divalent cations as well as several chelating agents increase the number of [3H] spiroperidol binding sites. Manganese is most potent, enhancing binding at 1 μm concentration, while magnesium and calcium are at least two orders of magnitude less potent and the monovalent cations sodium, potassium and lithium are still weaker. Divalent cations enhance the potency of dopaminergic agonists in competing for [3H]spiroperidol binding, an effect which appears to be independent of the ionic augmentation of [3H]spiroperidol binding. Divalent cations decrease both the association and dissociation rates of [3H]spiroperidol binding to dopamine receptor sites.

Biochemical Actions of Neuroleptic Drugs: Focus on the Dopamine Receptor

Who cares how antipsychotic drugs act? What difference does it make whether the clinical effects of neuroleptics derive from one or another biochemical influence? Such questions are rarely asked by pharmacologists because they have taken as their vocation the study of how all drugs act. If it is a drug, the most sacred obligation of the pharmacologist is to find out how it does whatever it does. However, for drugs such as the neuroleptics, which have been the subject of massive amounts of biochemical investigation, one must seriously ask how one can justify such an expenditure of intellectual and financial resources. The obvious answer is that these are the drugs which alleviate the symptoms of schizophrenia, so that if we know how they act, we would know something about the aberrations which account for schizophrenic symptoms. Such an assertion involves certain assumptions. The most crucial assumption is that the drugs are in fact antischizophrenic. One can readily conceive of a drug that would make life more livable for schizophrenics and for the staff of mental hospitals without doing anything fundamental to the schizophrenic process. For many years sedatives, straitjackets, and wet sheets have “eased” the predicaments of schizophrenics and their keepers, but few people would argue that these treatments did anything fundamental to schizophrenic mechanisms.

Discrimination by temperature of opiate agonist and antagonist receptor binding

Abstract Variations in incubation temperature can markedly differentiate opiate receptor binding of agonists and antagonists. In the presence of sodium increasing incubation temperatures from 0° to 30° reduces receptor binding of 3 H-naloxone by 50% while tripling the binding of the agonist 3 H-dihydromorphine. Lowering incubation temperature from 25° to 0° reduces the potency of morphine in inhibiting 3 H-naloxone binding by 9-fold while not affecting the potency of the antagonist nalorphine. At temperatures of 25° and higher the number of binding sites for opiate antagonists is increased by sodium and the number of sites for agonists is decreased by sodium with no changes in affinity. By contrast, in the presence of sodium lowering of incubation temperature to 0° increases opiate receptor binding of the antagonist naloxone by enhancing its affinity for binding sites even though the total number of binding sites are not changed.