The discovery and structure-activity relationships of 1,2,3,6-tetrahydro-4-phenyl-1-[(arylcyclohexenyl)alkyl]pyridines. Dopamine autoreceptor agonists and potential antipsychotic agents.

A novel dopamine (DA) autoreceptor agonist, 1,2,3,6-tetrahydro-4-phenyl-1- [(3-phenyl-3-cyclohexen-1-yl)methyl]pyridine (14), was identified. The structure-activity relationships surrounding this compound were studied by synthesis of analogues and evaluation of their dopaminergic activity. The cyclohexene substitution pattern was varied along with the length of the chain connecting the 1,2,3,6-tetrahydro-4-phenylpyridine to the cyclohexene. Compound 14, having the 1,3-substitution pattern and a single methylene chain, was the most potent. The 1,2,3,6-tetrahydro-4-phenylpyridine could be replaced by other aryl-cyclic amines with a slight loss in activity. The phenyl group on the cyclohexene ring could be para substituted; electron-donating groups were better tolerated than electron-withdrawing groups. Finally, the enantiomers of 14 were resolved via the 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate salts. Although both isomers were partial DA agonists, the (+)-enantiomer had higher intrinsic activity than the (-)-enantiomer. Syntheses were developed that allowed rapid preparation of analogues. An X-ray crystal structure determination of an intermediate identified the (+)-isomer of 14 as having R configuration. This compound, designated CI-1007 (PD 143188), was found to have antipsychotic-like activity in behavioral tests; in particular, it was orally active in the conditioned avoidance test in squirrel monkeys with an ED50 of 0.6 mg/kg. The overall profile suggests that (R)-(+)-14 may be a clinically useful antipsychotic agent.

Pharmacological characterization of PD 118717, a putative piperazinyl benzopyranone dopamine autoreceptor agonist.

PD 118717 (7-[3-[4-(2-pyrimidinyl)-1-piperazinyl]-propoxy]-2H-1- benzopyran-2-one sulfate) proved to be a dopamine (DA) D-2 autoreceptor agonist in biochemical and electrophysiological studies in rats and to exhibit an antipsychotic-like profile in behavioral tests in rodents and monkeys. In vitro binding studies indicated that PD 118717 bound selectively to DA D-2 vs. D-1 receptors and exhibited agonist binding properties (biphasic inhibitory curves and GTP shift) similar to DA. It also had significant affinity for serotonin-(5-HT)1A but not 5-HT1B and 5-HT2 receptors. PD 118717 was active in antagonizing the tau-butyrolactone-induced accumulation of dopa in rat striatum and mesolimbic regions. PD 118717 also depressed the firing of DA neurons in substantia nigra pars compacta of rats. In both of the latter tests the effects of PD 118717 were reversed by haloperidol. PD 118717 decreased brain DA metabolism, decreased DA utilization, decreased accumulation of dopa after inhibition of L-aromatic amino acid decarboxylase, stimulated serum corticosterone and inhibited stimulated serum prolactin levels. PD 118717 did not alter striatal acetylcholine levels; nor did it induce locomotor stimulation or stereotypy in normal animals, suggesting a lack of postsynaptic DA stimulation of normosensitive DA receptors. In tests designed to reveal even weak postsynaptic DA agonist effects, PD 118717 stimulated locomotor activity in 6-hydroxydopamine-lesioned animals and relatively higher doses induced a low degree of stereotyped behavior when combined with the DA D-1 agonist SKF 38393. PD 118717 decreased the accumulation of 5-hydroxytryptophan in brain, an effect probably due to an agonist action at 5-HT1A receptors. PD 118717 decreased spontaneous locomotor activity in rodents, antagonized amphetamine-stimulated hyperactivity in mice and inhibited Sidman avoidance in monkeys, effects seen with antipsychotic agents. Unlike DA antagonist antipsychotics, PD 118717 did not induce extrapyramidal dysfunction in monkeys. PD 118717 displayed behavioral activity after p.o. dosing and its effects did not show tolerance on repeated dosing. In conclusion, PD 118717 has the profile of a DA autoreceptor agonist in neurochemical and neurophysiological tests and produces effects suggestive of antipsychotic efficacy without neurological side effect liability in preclinical behavioral tests.

Dopamine autoreceptor agonists as potential antipsychotics. 2. (Aminoalkoxy)-4H-1-benzopyran-4-ones.

The synthesis and pharmacological properties of a novel type of [(arylpiperazinyl)alkoxy]-4H-1-benzopyran-4-ones with dopaminergic activity are described. The nature of the arylpiperazine (AP) moiety determines the dopamine (DA) agonist/antagonist character of this series of compounds; when the aryl portion of the AP is unsubstituted the compounds appear to be DA autoreceptor agonists while substituted aryl groups seem to impart DA antagonist activity. A heterocyclic piperazine, 7-[3-[4-(2-pyridinyl)-1-piperazinyl]propoxy]-4H-1-benzopyran-4-one (31, PD 119819) has been identified as an extremely selective DA autoreceptor agonist in tests that include [3H]haloperidol binding, inhibition of spontaneous locomotor activity, inhibition of brain DA synthesis, inhibition of brain DA neuronal firing, stereotypy assessment, and reversal of 6-hydroxydopamine (6-OHDA) induced akinesia in rats. In addition, 31 possesses good oral activity in the Sidman avoidance test in squirrel monkeys, a predictor of clinical antipsychotic efficacy. In another primate model, 31 has been found to lack the liability for extrapyramidal side effects observed with currently available antipsychotic drugs.

Synthesis and dopamine agonist properties of (+-)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano [4,3-b]-1,4-oxazin-9-ol and its enantiomers.

The dopamine agonist profile of (+-)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano [4,3-b]-1,4-oxazin-9-ol (16a) and its enantiomers (16b-c) was examined. Racemic 16a exhibited moderate affinity for the dopamine (DA) D2 receptor labeled with the DA antagonist ligand [3H]haloperidol and moderate in vivo activity; it attenuated gamma-butyrolactone-stimulated DA synthesis, decreased DA neuronal firing of substantia nigra DA neurons, and inhibited exploratory locomotor activity in rats, a profile consistent with a DA autoreceptor agonist mechanism of action. The (+)-enantiomer 16b possessed greater DA receptor affinity with the agonist ligand [3H]-N-propylnorapomorphine than with the antagonist ligand. In rats it potently inhibited DA synthesis and neuronal firing and also inhibited exploratory locomotion. The (-)-enantiomer, on the other hand, did not have significant activity in any of these tests. This profile indicates that like many other rigid DA agonists, the dopaminergic activity resides in one enantiomer, in this case the (+)-enantiomer 16b. On the basis of single-crystal X-ray analysis of a key intermediate, the absolute configuration of 16b was found to be 4aR, 10bR.

Nicotine-induced tolerance and dependence in rats and mice: studies involving schedule-controlled behavior.

Tolerance to nicotine's disruptive effects on operant responding develops rapidly over a 14-36 day repeated dosing period in both rats and mice. This occurred regardless of whether nicotine was administered pre- or post- to each behavioral exposure. Thus, tolerance development appeared to depend on both behavioral as well as pharmacological mechanisms. It is suggested that the pharmacological mechanism(s) involved in the development of tolerance may be related to an up-regulation of brain area nicotinic receptors. As observed with receptor binding studies, mecamylamine did not appear to attenuate the development of pharmacological tolerance to nicotine (does not attenuate nicotinic receptor up-regulation) even though this cholinergic antagonist will antagonize nicotine's acute behavioral disruptive effects completely. However, the fact that mecamylamine may induce some cross-tolerance to nicotine does complicate our interpretation of these data. The development of nicotine tolerance, in part, appears to depend upon an interaction at some acetylcholine-sensitive nicotinic receptor as evidenced by the ability of physostigmine to induce cross-tolerance to nicotine in both the rat and mouse. These data support the view that nicotine may be inducing its effects via at least two separate nicotinic receptors, one of which may be acetylcholine sensitive. Furthermore, binding data suggest that physostigmine's effects were related to a reduction of available central nicotinic receptor sites. In contrast to what humans experience, the rat does not appear as sensitive to nicotine-induced physical dependence, at least when operant behavior is utilized as the dependent variable used to measure withdrawal signs. Other approaches such as drug discrimination and conditioned avoidance paradigms may provide a better alternative to the evaluation of nicotine-induced dependence. Research utilizing schedule-controlled behavior in the mouse, on the other hand, has provided us with an additional model of a nicotine-induced withdrawal syndrome which may be of value in evaluating mechanisms of nicotine dependence. However, as with all of these findings, much work is needed to confirm and further characterize each model in so far as they may provide us with a reliable and specific measure of nicotine dependence.

Nicotine and arecoline as discriminative stimuli: involvement of a non-cholinergic mechanism for nicotine.

The cholinergic innervation of central muscarinic (M-Ch) and nicotinic (N-Ch) receptors was evaluated by studying the interaction of physostigmine with the discriminative stimulus (DS) effects of arecoline and nicotine. Rats were trained to discriminate either arecoline (1.74 mg/kg) or nicotine (1.14 mg/kg) from saline using a two-lever, milk reinforced, operant task. Physostigmine (0.125 mg/kg) pretreatment potentiated, and when administered alone (0.25 mg/kg), generalized with the DS induced by arecoline. In contrast, physostigmine, at the same dose, neither potentiated nor generalized with the DS effects of nicotine. These findings provide evidence that central muscarinic receptors are cholinergically innervated (physiologic) while central nicotinic receptors are not cholinergically innervated but are cholinoceptive (pharmacological).

6- and 8-hydroxy-3,4-dihydro-3-(dipropylamino)-2H-1-benzopyrans. Dopamine agonists with autoreceptor selectivity.

The dopamine agonist profiles of 3,4-dihydro-3-(3-dipropylamino)-2H-1-benzopyran-6- and -8-ol (4 and 5, respectively) were examined. Both 4 and 5 exhibited greater relative affinity for receptors labeled with the dopamine agonist ligand [3H]propylnorapomorphine than for those labeled with the dopamine antagonist ligand [3H]haloperidol. Both compounds attenuated the stimulation of brain dopamine synthesis caused by gamma-butyrolactone (GBL) and decreased the firing rate of substantia nigra dopamine neurons in rats. This profile of activity, together with the ability of the dopamine antagonist haloperidol to reverse the inhibition of dopamine neuronal firing, indicate that both compounds are brain dopamine agonists.

Investigations on the CNS sites of action of the discriminative stimulus effects of arecoline and nicotine.

The role of the dorsal hippocampus (DH) and mesencephalic reticular formation (MRF) in mediating the discriminative stimulus (DS) effects of nicotine and arecoline was assessed. In rats trained to discriminate nicotine (1.14 mg/kg) from saline, peripherally administered nicotine generalized to injection of nicotine, but not arecoline, directly into the DH and MRF. The stimulus effect of centrally administered nicotine was antagonized by peripherally administered mecamylamine, but not atropine. Response rate decreases were also observed after nicotine injection into either site. In rats trained to discriminate arecoline (1.74 mg/kg) from saline, peripherally administered arecoline did not generalize to the direct injection of arecoline into the DH and MRF. However, a decrease in response rates was observed after arecoline injection into either site. Thus, The DH and MRF are important in mediating the DS effect of nicotine but not arecoline.

Central sites and mechanisms of action of nicotine.

Research conducted in this laboratory over the last ten years has been directed towards determining possible CNS sites and mechanisms by which nicotine is producing its psychopharmacological effects. To accomplish these goals, a drug discrimination paradigm was utilized in which rats were trained to detect nicotine using a two-lever operant procedure. In this situation nicotine acted as a discriminative stimulus (DS) to correct lever responding. In other words rats had to be able to differentiate nicotine's effects from saline in order to obtain a food reinforcement. The ability of nicotine to exert its DS effects appear to be dependent upon a stimulation of central nicotinic-cholinergic receptors which are stereospecific to (-)-nicotine. Interestingly, the nicotine DS could not be mimicked or potentiated by elevating brain acetylcholine levels centrally suggesting that the receptor action was non-cholinergic. Additional studies indicated that nicotine is acting at both reticular formation and hippocampus sites. The hippocampal site of nicotine action also appears to be dependent on a dopamine neuron connection as well.

Discriminative stimulus properties of the optical isomers of nicotine.

Rats were trained to discriminate 200 or 400 microgram/kg (-)nicotine from saline in a two-bar operant paradigm. Dose-response relationships for optically pure (-)- and (+)nicotine as well as antagonistic effects were examined in both groups of rats. The natural isomer (-)nicotine was approximately nine-times more potent than (+)nicotine. Mecamylamine produced equal blockade of the (-)- and (+)nicotine cues. Hexamethonium and atropine were without effect. These data demonstrate the possible stereospecificity of the central nicotinic receptor that mediates the stimulus effect produced by nicotine.