Alan S. Horn

Biochemical evidence for the 5-HT agonist properties of PAT (8-hydroxy-2-(di-n-propylamino)tetralin) in the rat brain

In vitro investigations revealed that PAT (8-hydroxy-2-(n-dipropylamino)tetralin) interacted with postsynaptic 5-HT receptors in the rat brain: the drug stimulated 5-HT-sensitive adenylate cyclase in homogenates of colliculi from new-born rats (KAapp 8.6 microM) and inhibited the specific binding of [3H]5-HT to 5-HT1 sites. The PAT-induced inhibition of [3H]5-HT binding showed marked regional differences compatible with a preferential interaction of PAT (IC50 2 nM) with the 5-HT1A subclass. As previously seen with 5-HT agonists, the efficacy of PAT for displacing [3H]5-HT bound to hippocampal membranes was markedly increased by Mn2+ (1 mM) and reduced by GTP (0.1 mM). PAT also affected presynaptic 5-HT metabolism since it inhibited competitively (Ki 1.4 microM) [3H]5-HT uptake into cortical synaptosomes and reduced (in the presence of the 5-HT uptake inhibitor fluoxetine) the K+-evoked release of [3H]5-HT previously taken up or newly synthesized from [3H]tryptophan in cortical or striatal slices. This latter effect was prevented by 5-HT antagonists (methiothepin, metergoline) suggesting that it was mediated by the stimulation of presynaptic 5-HT autoreceptors by PAT. Like 5-HT, PAT counteracted the stimulatory effect of K+-induced depolarization on the synthesis of [3H]5-HT from [3H]tryptophan in cortical slices. It is concluded that PAT is a potent 5-HT agonist acting on both post- and presynaptic 5-HT receptors in the rat brain.

A noradrenaline sensitive adenylate cyclase in the rat limbic forebrain: Preparation, properties and the effects of agonists, adrenolytics and neuroleptic drugs

A method for the preparation of a noradrenaline sensitive adenylate cyclase from homogenates of the rat limbic forebrain is described using Krebs--Ringer as the homogenising medium. Some of its properties resemble those reported previously by other workers, using slices. Its response to agonists show that it has the characteristics of a beta1-receptor i.e. the potency of 1-isoprenaline exceeds that of 1-noradrenaline which exceeds that of 1-adrenaline. Structure--activity analysis of the response of the adenylate cyclase to a range of adrenergic agonists shows a strict requirement for a catechol moiety and a beta-hydroxyl group. The activation of the enzyme by 1-noradrenaline is sensitive to stereoselective inhibition by 1-propranolol. The effect of a number of neuroleptic drugs was examined. Promazine was the most effective agent tested in antagonising the stimulation produced by 50 muM 1-noradrenaline, whilst the potent dopamine receptor antagonist, alpha-flupenthixol was only weakly active. Furthermore, there was no stereoselectivity in the antagonism produced by alpha- and beta-siomers of flupenthixol. Pimozide was not found to be a potent antagonist. Thus the spectrum of antagonism produced by neuroleptic drugs was quite different from that seen in the dopamine sensitive adenylate cyclase of the rat corpus striatum.

Structural and conformational relationships between the enkephalins and the opiates

Two pentapeptides, the enkephalins (Fig. 1a), have been isolated from the mammalian brain and have been shown to be potent opiate agonists1,2. Further work has demonstrated that the opiate analgesics themselves interact competitively with the receptor sites for these endogenous pentapeptides. This explains in a simple and elegant manner the occurrence of opiate receptors in nervous tissue3,4; clearly it was unlikely that they were there to interact solely with the opiates. As the enkephalins and opiates are competing for the same receptors and give a similar pharmacological response it is likely that they have structural and conformational similarities. Hughes et al. have shown that both of these small peptides contain a tyrosine residue at the amino terminal position2. We consider here the significance of that finding for the conformational relationship between the enkephalins and opiates.

Dopamine receptor blockade and the neuroleptics, a crystallographic study.

The X‐ray structures of 12 drugs of the tricyclic class having varying pharmacological profiles have been examined in detail in an attempt to rationalize the known structure‐activity relations of neuroleptic drugs with respect to their ability to block dopamine receptors in the brain. Further evidence is presented in support of the theory that the neuroleptics are able to block dopamine receptors because of a conformational complementarity between certain portions of these drugs and dopamine.

Effect of butaclamol on dopamine-sensitive adenylate cyclase in the rat striatum

Neuroleptic drugs exhibit a wide spectrumofpharmacologicaleffects(Matthysse, 1973; Keller, Bartholini & Pletscher, 1973; Miller & Hiley, 1974). Of these actions dopamine receptor blockade is widely thought to be responsible for the parkinsonian side effects seen with these drugs and in addition correlates well with the antipsychotic effects of neuroleptics. Evaluation of the topography of the receptor at which these drugs act has been facilitated recently by the use of an in vitro system-the dopaminestimulated adenylate cyclase present in dopamine-rich regions of the mammalian cns (Kebabian, Petzold & Greengard, 1972; Horn, Cuello & Miller, 1974). Previous results have illustrated the close correlation between blockade of the stimulating effects of dopamine on this adenylate cyclase and neuroleptic potency (Miller, Horn & Iversen, 1974). We now describe the action of butaclamol, (I) a neuroleptic of novel structure, (Humber, Bruderlein & Voigt, 1974; Bruderlein, Humber & Voigt, 1975) in this system. Among other features butaclamol (a racemate) exhibits optical isomerism.

The tricyclic antidepressants: imipramine hydrochloride. The crystal and molecular structure of 5‐(3‐dimethylaminopropyl)‐10,1l‐dihydro‐5H‐dibenz[b,f]azepine hydrochloride

The structure of imipramine hydrochloride, C19H24N2.HCI, monoclinic with space group P21/e and a= 11.303 (3), b=29-227 (8), c= 14.282 (3) .,?k, ,8= 130.91 (1) °, Z=8, was determined by multi-solution direct methods and refined by full-matrix least-squares and conjugate gradient methods to an R of 0-057 for 2880 counter reflexions. The two molecules in the asymmetric unit have slightly different conformations of the se~,en-membered hetero-ring and one molecule appears to exhibit a small movement in the dimethylene bridge. Angles between benzene ring planes are 130.3 and 123-0 °. The dimethylaminopropyl side chains exhibit different conformation and the amine nitrogen atoms and chloride ions form a layered arrangement in the lattice.

The conformation of dopamine at its uptake site; further studies with rigid analogues

JOHANSSON, B. (1973). MCEWEN, L. M. (1956). J. Physiol. (Lond.), 131, 678-689. MEKATA, H. & NIU, H. (1969). Jap. J. Physiol., 19, 599-608. MUJIC, M. & VAN ROSSUM, J. M. (1965). Archs int. Pharmacodyn. Thdr., 155,432-449. NORTON, J. M., GELLAI, M. & DETAR, R. (1972). Pfliigers Arch. ges. Physiol., 335, 279-286. SMITH, D. J. (1950). TRINKER, F. R. (1973). Archs int. Pharmacodyn. Thdr., 205, 218-225. WALTER, P.& BASSENGE, . (1968). Pfliigers Arch. ges. Physiol., 299, 52-65. Am. J. Physiol., 223,878-881.

THE SIGNIFICANCE OF COMT ACTIVITY IN CONTROLLING DOPAMINE AGONIST LEVELS IN BRAIN AND SERUM - STUDIES WITH A PRODRUG AND A METABOLITE OF 6,7-ADTN

After i.p. administration of the dibenzoylester of 2-amino-6,7-dihydroxytetralin (DB-6,7-ADTN) a metabolite was found in rat brain and serum, which was identified as 2-amino-6-hydroxy-7-methoxytetralin (7-O-MeADTN). By means of HPLC coupled with amperometric detection, time-concentration curves of 7-O-MeADTN in rat brain and serum were determined after 100 mumol/kg DB-6,7-ADTN. These showed a rapid formation and homogeneous distribution of high peak levels (4 nmol/g) of 7-O-MeATN. Brain and serum concentrations of 6,7-ADTN after 100 mumol/kg DB-6,7-ADTN, determined during catechol-O-methyltransferase (COMT) inhibition by tropolone, were 5--7 times higher than those during normal COMT activity, thus equalling 5,6-ADTN concentrations after 100 mumol/kg DB-5,6-ADTN. The greater susceptibility of 6,7-ADTN to metabolic degradation by COMT was confirmed by preliminary results of in vitro studies, which showed that two methoxy derivatives are formed from 6,7-ADTN and one, in very small amounts, from 5,6-ADTN. 7-O-MeADTN had no dopaminergic activity of its own, as after i.p. injection of 100 mumol/kg it was devoid of behavioural and biochemical effects typical for DA agonists. A homogeneous distribution and high peak concentrations (20 nmol/g after 15 min) were found in rat brain after this dose of the metabolite. The results indicate that a substantial amount of 6,7-ADTN, in contrast to 5,6-ADTN, is metabolized by COMT and that differences between brain concentrations of both isomers are almost exclusively due to differences in susceptibility for COMT. This has implications for the design of new DA agonists.

The potential antipsychotic activity of the partial dopamine receptor agonist (+) N-0437

The (+) enantiomer of the very potent and selective dopamine D-2 agonist, 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437), displays partial agonistic activity at dopamine D-2 receptors. In this study (+)N-0437 was investigated for its antagonistic activity at postsynaptic DA receptors in four behavioural tests which are commonly used to evaluate potential neuroleptic activity, i.e. d-amphetamine-induced stereotypy, passive avoidance responding, intracranial self-stimulation behaviour, and catalepsy. (+)N-0437 (25-50 mumol/kg) was active in the first three models, but did not cause catalepsy. Haloperidol, which was used as a reference compound for classical DA antagonists, showed clear activity in all four models at low doses (0.5-1.0 mumol/kg). (-)N-0437, a full D-2 agonist, displayed no activity in these behavioural models. These results suggest that (+)N-0437 could be used to examine the hypothesis that the use of partial agonists could provide a new treatment for schizophrenia.

Antipsychotic phenothiazine drugs and the significance of the X‐ray structure of promazine HCl

The antipsychotic actions of the phenothiazine neuroleptics are thought to be predominantly mediated through a blockade of dopamine receptors in the brain (Carlsson & Lindqvist, 1963; Horn & Snyder, 1971; Bunney, Walters & others, 1973; Horn, Cuello & Miller, 1974). It has also been suggested (Horn & Snyder, 1971) that chlorpromazine is able to block dopamine receptors due to a possible complementarity between certain portions of the X-ray structures of chlorpromazine and dopamine. Support for this idea was obtained from a detailed conformational analysis of 15 drugs of the tricyclic class (Horn, Post & Kennard, 1975). Results from a variety of animal tests (Zirkle & Kaiser, 1970) and clinical data (Klein & Davis, 1969) have shown that potent neuroleptics of this group usually have a chain of three carbon atoms separating the terminal amino function from the nucleus, together with a substituent at the 2-position of the phenothiazine ring system. Although the need for a 2-substituent is common to at least 4 classes of neuroleptics (the phenothiazines, thioxanthenes, dibenzo-diazepines and dibenzo-oxazepines (Zirkle & Kaiser, 1970) its function is unclear. The strict positional specificity of this phenomenon (Horn & others, 1975; Zirkle & Kaiser, 1970) is unlikely to be explicable solely in terms of a lipid solubility effect (Green, 1967). In order to obtain information about any possible conformational or structural effects these substituents might have on drugs of this class we have determined the crystal and molecular structure of the unsubstituted drug promazine