The putative dopamine autoreceptor agonist B-HT 920 decreases nigral dopamine cell firing rate and prolactin release in rat

BP 897, a selective dopamine D3 receptor ligand with therapeutic potential for the treatment of cocaine-addiction

BP 897 is a potent (K(i) = 0.92 nM) dopamine D(3) receptor compound developed for the treatment of cocaine abuse and craving. BP 897 has a high selectivity for the dopamine D(3) versus D(2) receptors (70-fold) and a moderate affinity for 5-HT(1A) receptors, (K(i) = 84 nM), adrenergic-alpha(1) (K(i) = 60 nM) and -alpha(2) adrenoceptors (K(i) = 83 nM). BP 897 displays significant intrinsic activity at the human dopamine D(3) receptor by decreasing forskolin-stimulated cAMP levels and by stimulating mitogenesis of dopamine D(3)-expressing NG108-15 cells. Although these findings suggest that BP 897 is a partial agonist, recent studies in Chinese Hamster Ovary (CHO) cells with expressed dopamine D(3) receptors demonstrated that BP 897 is devoid of any intrinsic activity but potently inhibits dopamine agonist effects (pIC(50) = 9.43 and 9.51) in agonist-induced acidification rate or increase of GTPgammaS binding, respectively. In addition, BP 897 inhibits in vivo (EC(50) = 1.1 mg/kg, i.v.) agonist-induced decrease of firing rate of dopaminergic neurons in the substantia nigra. It has been clearly shown that BP 897, 1 mg/kg, i.p., reduces cocaine-seeking behavior in rats, without producing reinforcement on its own. In rhesus monkeys, BP 897 is not self-administered (up to 30 microg/kg, i.v.) but reduces cocaine self-administration. The potential usefulness of BP 897 in the treatment of drug-seeking behavior is further supported by its effects in drug conditioning models. Although BP 897 reduces L-DOPA-induced dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys, it provokes a return of parkinsonian symptoms. At high doses BP 897 has been reported to produce catalepsy in rats. Pharmacokinetic and toxicological data have not yet been published. These interesting preclinical findings with BP 897 provide additional validation for dopamine D(3) receptor as a therapeutic target for the treatment of cocaine abuse and its associated central nervous system (CNS) disorders. BP 897 recently entered phase II clinical studies.

New generation dopaminergic agents. 6. Structure-activity relationship studies of a series of 4-(aminoethoxy)indole and 4-(aminoethoxy)indolone derivatives based on the newly discovered 3-hydroxyphenoxyethylamine D2 template

A series of 4-(aminoethoxy)indoles 7 and a related series of 4-(aminoethoxy)indolones 8 were synthesized and evaluated for their affinity for both the high- and low-affinity agonist states (D2High and D2Low, respectively) of the dopamine (DA) D2 receptor. The 4-aminoethoxy derivatives (i.e., 7 and 8) were designed as bioisosteric analogues based on the phenol prototype 4. The indolones 8 were observed to have high affinity for the D2High receptor. Comparison of their previously reported chroman analogues with the more flexible 4-(aminoethoxy)indoles revealed the chroman analogues to be more potent, whereas little loss in D2High affinity was observed when comparing the 4-(aminoethoxy)indolones with their respective chroman analogues. Several regions of the phenoxyethylamine framework were modified and recognized as potential sites to modulate the level of intrinsic activity. A conformational analysis was performed and a putative bioactive conformation was proposed which fulfilled the D2 agonist pharmacophore criteria based on the McDermed model. Structure-activity relationships gained from these studies have aided in the synthesis of D2 partial agonists of varying intrinsic activity levels. These agents should be of therapeutic value in treating disorders resulting from hypo- and hyperdopaminergic activity, without the side effects associated with complete D2 agonism or antagonism.

New generation dopaminergic agents. 1. Discovery of a novel scaffold which embraces the D2 agonist pharmacophore. Structure-activity relationships of a series of 2-(aminomethyl)chromans

A series of 2-(aminomethyl)chromans (2-AMCs) was synthesized and evaluated for their affinity and selectivity for both the high- and low-affinity agonist states (D2High and D2Low, respectively) of the dopamine (DA) D2 receptor. The 7-hydroxy-2-(aminomethyl)chroman moiety was observed to be the primary D2 agonist pharmacophore. The 2-methylchroman moiety was discovered to be an entirely novel scaffold which could be used to access the D2 agonist pharmacophore. Attaching various simple alkyl and arylalkyl side chains to the 7-hydroxy 2-AMC nucleus had significant effects on selectivity for the D2High receptor vs the 5HT1A and alpha 1 receptors. A novel DA partial agonist, (R)-(-)-2-(benzylamino)methyl)chroman-7-ol [R-(-)-35c], was identified as having the highest affinity and best selectivity for the D2High receptor vs the alpha 1 and 5HT1A receptors. Several regions of the 2-AMC nucleus were modified and recognized as potential sites to modulate the level of intrinsic activity. The global minimum conformer of the 7-hydroxy-2-AMC moiety was identified as fulfilling the McDermed model D2 agonist pharmacophoric criteria and was proposed as the D2 receptor-bound conformation. Structure-activity relationships gained from these studies have aided in the synthesis of D2 partial agonists of varying intrinsic activity levels. These agents should be of therapeutic value in treating disorders resulting from hypo- and hyperdopaminergic activity, without the side effects associated with complete D2 agonism or antagonism.

Dopamine agonists in schizophrenia: a review

Although it is unlikely that the different types of course and severity of schizophrenia are caused by one neurochemical abnormality alone, indirect pharmacological evidence still suggests a relative excess of dopaminergic activity being implicated in the pathogenesis of most of the schizophrenic symptoms, e.g. positive symptomatology. Synthesis and release of dopamine as well as firing rates of dopaminergic neurons are controlled by stimulation of autoreceptors via a negative feedback regulation. Investigations on therapeutic effects of autoreceptor-nonselective dopamine agonists in schizophrenia have yielded inconsistent results. Dopamine autoreceptor agonists like pramipexole, roxindole, talipexole and OPC-4392 as well as partial agonists like terguride and SDZ HDC 912 have been tested in positive schizophrenic symptomatology in order to reduce the postulated excess of central dopaminergic activity. However, administration of autoreceptor-'selective' agonists did not result in a significant improvement of positive symptoms. In predominantly negative schizophrenic symptomatology, a dopamine deficit rather than an excess has been hypothesized. Consequently, a nonselective dopamine agonistic action could be effective in some negative symptoms. Current evidence from several open and one placebo-controlled clinical trial suggests that some dopamine autoreceptor agonists like pramipexole, roxindole and talipexole may produce a mild improvement of symptoms like affective flattening, depressed mood, alogia and avolition. Nevertheless, these findings do not yet allow a reliable judgement and remain to be clarified by further double-blind placebo-controlled studies over a sufficient treatment duration.

Dopamine D2 autoreceptors in rats are behaviorally functional at 21 but not 10 days of age

Previous studies used either racemic 3-(3-hydroxyphenyl)-N-n-propylpiperidine [(+/-)-3-PPP] or lower doses of the mixed dopamine (DA) D1/D2 agonist apomorphine (APO) to conclude that brain DA D2 autoreceptors are not behaviorally functional until 28 days of age. The purpose of this study was to provide behavioral evidence for functional D2 autoreceptors before 28 days of age using DA agonists with greater selectivity for D2 autoreceptors. The locomotor activity of 10-, 21-, 35-day-old and adult rats was monitored after injection of a D2 autoreceptor agonist. There were significant decreases in the locomotor activity of 21-, 35-day-old, and adult rats injected with (-)-3-PPP, SND 919, or PD 128483. Lower doses of APO significantly decreased the activity of adult and 35-day-old rats but not younger rats. The only significant effect on the locomotor activity of 10-day-old rats was an increase in activity after injection of APO, 0.01 mg/kg or higher, or B-HT 920, 0.01 mg/kg. The results suggest that brain DA D2 autoreceptors are behaviorally functional at 21, but not 10, days of age.

Excitation by talipexole, a dopamine D2 agonist, of caudate nucleus neurons activated by nigral stimulation

An electrophysiological study using cats anesthetized with alpha-chloralose was performed to elucidate whether or not talipexole (B-HT 920 CL2: 6-allyl-2-amino -5, 6, 7, 8-tetrahydro-4H-thiazolo [4, 5 -d] -azepine-dihydrochroride), a dopamine D2 agonist, acts on postsynaptic dopamine receptors in the caudate nucleus (CN) neurons receiving excitatory input from the pars compacta of substantia nigra (SN). Extracellular neuron activities were recorded in the CN using a glass-insulated silver wire microelectrode attached along a seven-barreled micropipette, each of which was filled with talipexole, quinpirole (dopamine D2 agonist), domperidone (dopamine D2 antagonist), glutamate and 2M NaCl. These drugs were microiontophoretically applied to the immediate vicinity of the target neuron. In the same neurons in which the spikes elicited by the SN stimulation were blocked by microiontophoretically applied domperidone, microiontophoretic application of talipexole and quinpirole induced a dose-dependent increase in spontaneous firing. This increase in firing by talipexole and quinpirole was blocked during simultaneous application of domperidone, although glutamate-induced firing remained unaffected by domperidone. In the CN neurons, in which the SN stimulation-induced spikes were not blocked by domperidone, spontaneous firing was not affected by talipexole or quinpirole. These findings suggest that talipexole activates CN neurons receiving a dopaminergic input from SN via D2 receptors, as does quinpirole.

A high dose of EEDQ reduces pituitary dopamine D2 receptor density and the prolactin suppressive potency of agonists

In male rats, a high dose of the alkylating compound N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 30 mg/kg s.c., 24 h) caused a reduction of pituitary dopamine D2 receptor density by 27% as measured by means of in vivo radioligand binding (using a single dose of the ligand [3H]spiperone). The same dose of EEDQ reduced the potency, but not the maximal response, of the dopamine D2 receptor agonists R-(-)-N-n-propylnorapomorphine (NPA), (+)-3-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3-PPP), and 6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5]azepine (B-HT 920) with respect to suppression of prolactin release after pretreatment with gamma-butyrolactone. The measured reduction in dopamine D2 receptor density after EEDQ was of the same magnitude as the reduction in receptor number predicted from the EEDQ induced shift in the dose-response curve of the full dopamine D2 receptor agonist NPA. The findings are discussed in relation to our previous observation that a somewhat lower dose of EEDQ (20 mg/kg s.c., 24 h) effectively reduces the efficacy of partial dopamine D2 receptor agonists while affecting neither the prolactin response to full dopamine D2 receptor agonists nor the density of pituitary dopamine D2 receptors.

Dopamine autoreceptor agonists in the treatment of schizophrenic disorders

1. Synthesis and release of dopamine as well as firing rates of dopaminergic neurons are controlled by stimulation of autoreceptors via a negative feedback regulation. Investigations on therapeutic effects of autoreceptor-nonselective dopamine agonists in schizophrenia have yielded inconsistent results. 2. With respect to the dopamine hypothesis of schizophrenia, dopamine autoreceptor agonists have been tested in positive schizophrenic symptomatology in order to reduce the postulated excess of central dopaminergic activity. However, administration of selective dopamine autoreceptor agonists like talipexole or roxindole did not result in a significant improvement of psychopathological symptoms. 3. In negative schizophrenic symptomatology, a dopamine deficit rather than an excess has been hypothesized. Current evidence from open clinical trials suggests that dopamine autoreceptor agonists may produce a minor to moderate improvement of symptoms like affective flattening, depressed mood, alogia and avolition.

Hypotensive and bradycardic effects of talipexole (B-HT 920) in anaesthetized rabbits are antagonized by metoclopramide but not by yohimbine

The interactions of talipexole (B-HT 920) and clonidine with selective alpha-adrenoceptor antagonists, yohimbine (alpha 2) and prazosin (alpha 1), as well as with dopamine receptor antagonists, metoclopramide (D2), domperidone (D2) and SCH23,390 (D1) were investigated in anaesthetized rabbits after i.v. administration. Both talipexole (0.03-0.1 mg/kg) and clonidine (0.01-0.03 mg/kg) dose-dependently induced hypotension and bradycardia. Talipexole had a shorter duration of action. The hypotensive effect of the alpha 2-adrenoceptor and D2 agonist talipexole (0.03 mg/kg) was antagonized by pretreatment with metoclopramide (3 mg/kg) or domperidone (0.3-3 mg/kg), but not with yohimbine (3 mg/kg), prazosin (0.1 mg/kg) or SCH23,390 (1 mg/kg). Its bradycardic effect was antagonized only by metoclopramide (3 mg/kg). The hypotensive and bradycardic effects of clonidine (0.03 mg/kg) were most effectively antagonized by yohimbine (0.3-3 mg/kg). These findings indicate that in anaesthetized rabbits after i.v. administration, talipexole may lower blood pressure by peripheral, and heart rate by central, dopamine D2 agonism.

Memory enhancement by post-training peripheral administration of low doses of dopamine agonists: possible autoreceptor effect

These experiments examined the effect of post-training injections of low doses of dopamine (DA) agonists on the acquisition of two 8-arm radial maze tasks. On a winstay simultaneous discrimination task, a light cue signaled the location of food in four randomly selected arms on each trial, and animals were required to visit each of the lit arms twice within a trial. Animals received one food trial per day and were injected immediately after training on Day 5. The direct DA receptor agonist, apomorphine (0.05 mg/kg), and the direct D2-DA receptor agonists, LY 177555 (quinpirole: 0.05, 0.1 mg/kg) and B-HT 920 (0.05 mg/kg), all improved acquisition of winstay radial maze behavior relative to saline-injected controls. On a win-shift task, rats were allowed to obtain food from four randomly selected maze arms, followed by a delay period in which they were removed from the maze. Animals were returned to the maze for a retention test in which only those arms that had not been visited prior to the delay contained food. After training on shorter delays, a delay of 18 h was imposed between the first four and second four choices, and DA agonists were injected immediately after the first four choices. Apomorphine, LY 171555, and B-HT 920 (all at 0.05 mg/kg), all improved win-shift retention relative to saline-injected controls. On both tasks, delaying the injections for 2 h post-training eliminated the memory-improving effects of all drugs. The results indicate that post-training administration of DA agonists at doses that may preferentially stimulate autoreceptors improves memory.

Effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline on the prolactin suppression induced by a series of full and partial dopamine D2 receptor agonists in male rats

The effect of pretreatment with a high dose of the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (20 mg/kg, 24 h) on the intrinsic activity displayed by a series of full and partial dopamine D2 receptor agonists on prolactin regulating pituitary D2 receptors in male rats was studied. To increase baseline prolactin levels, gamma-butyrolactone in a dose inhibiting brain dopamine neurotransmission was given to all animals. In controls, i.e. rats not given EEDQ, supramaximal doses of all full and partial D2 receptor agonists tested decreased serum prolactin levels with greater than 80%. While the intrinsic activities of the dopamine precursor 1-DOPA and of the full agonists (+)-3-PPP, 5-OH-DPAT, B-HT 920 (talipexole), apomorphine, and NPA (R-(-)-N-n-propylnorapomorphine) were not affected by pretreatment with EEDQ, the effects of supramaximal doses of the partial agonists (-)-HW-165, TDHL (terguride), SDZ208-911, (-)-3-PP) (preclamol), and SDZ 208-912 were reduced to 66%, 74%, 59%, 100%, and 100%, respectively. The effect of EEDQ on the intrinsic activity displayed by the various agonists corresponds inversely to the intrinsic efficacy displayed by the drugs in other models of D2 receptor function with one exception only; thus, the prolactin suppressive effect of (-)-3-PPP was more effectively antagonized by EEDQ than would have been predicted from the intrinsic efficacy usually attributed to the drug. Since the dose of EEDQ used in the present study has previously been shown not to decrease D2 receptor density in the pituitary as measured using in vivo radioligand binding, it is suggested that alkylation of D2 receptors may change the conformation of the individual receptor complexes in a way that decreases the responsiveness to partial but not full agonists.

Inhibition by talipexole, a thiazolo-azepine derivative, of dopaminergic neurons in the ventral tegmental area

A microiontophoretic study using rats anesthetized with chloral hydrate and immobilized with gallamine triethiodide was carried out to compare the effect of talipexole (B-HT 920 CL2:2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine-dihydrochloride), a dopamine autoreceptor agonist, on dopaminergic neurons in the ventral tegmental area (VTA) to non-dopaminergic neurons in the VTA. VTA neurons were classified into two types according to the responses to antidromic stimulation of the nucleus accumbens (Acc): type I neurons with a long spike latency (8.69 +/- 0.24 msec) upon Acc stimulation and low spontaneous firing rate (6.80 +/- 1.34/sec), and type II neurons with a short latency (2.76 +/- 0.20 msec) and high spontaneous firing rate (26.77 +/- 7.05/sec), probably corresponding to dopaminergic and non-dopaminergic neurons, respectively. In type I neurons, microiontophoretic application of talipexole and dopamine inhibited antidromic spike generation elicited by Acc stimulation, and talipexole-induced inhibition was antagonized by domperidone (dopamine D-2 antagonist). In type II neurons, however, the antidromic spikes were not affected by either talipexole or dopamine. Furthermore, spontaneous firing was also inhibited by iontophoretically applied talipexole and dopamine in most type I neurons, but rarely affected by either drug. Inhibitory effects of talipexole were antagonized by domperidone. These results suggest that talipexole acts on dopamine D-2 receptors, thereby inhibiting the dopaminergic neurons in the VTA.

Effect of BHT 920 on monoaminergic neurons of the rat brain: an electrophysiological in vivo and in vitro study

BHT 920 was originally described as a dopamine autoreceptor agonist. In this study, the effect of this compound on the firing rate of noradrenergic locus coeruleus, serotonergic dorsal raphe and dopaminergic ventral tegmental area neurons was examined both in the anaesthetized rat and in rat brain slices. Extracellular recordings were performed in cells whose identity was determined by electrophysiological, pharmacological and histological criteria. In vivo, BHT 920 inhibited the firing of locus coeruleus neurons (ID50: 14.5 +/- 4.7 micrograms/kg, mean +/- SEM) and ventral tegmental area neurons (ID50: 7 +/- 3 micrograms/kg) at very low doses. As a comparison, the ID50 of clonidine on locus coeruleus cells was 5.5 +/- 0.6 microgram/kg and the ID50 of apomorphine on ventral tegmental area neurons was 13 +/- 3 micrograms/kg. BHT 920 also decreased the firing of dorsal raphe cells, but this effect was obtained at higher doses (ID50: 57 +/- 11 micrograms/kg). The in vitro study confirmed the results obtained in vivo. BHT 920 potently inhibited the firing of locus coeruleus cells (IC50: 71 +/- 28 nM) and was less potent than clonidine (IC50: 5.3 +/- 0.98 nM). The compound also inhibited the firing of ventral tegmental area neurons at very low concentrations (IC50: 21 +/- 3.3 nM), being more potent than apomorphine (IC50: 56 +/- 29 nM). BHT 920 only slightly decreased the firing rate of dorsal raphe neurons at 50 microM, showing that the drug has little direct effect on these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of a dopamine autoreceptor selective profile of B-HT 920 in functional in vitro model systems of D2 receptors in rat striatum

Based on the results of in vivo studies, the thiazoloazepine derivative B-HT 920 has been proposed to be a selective agonist of dopamine autoreceptors. In the present study, we investigated the effects of B-HT 920 in two functional in vitro model systems of D2 receptors and compared these effects with the effects of the classical D2 agonist LY 171555. B-HT 920 and LY 171555 concentration dependently inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine and the forskolin-induced stimulation of adenylate cyclase activity in rat striatal tissue slices with comparable efficacies. In striatal tissue slices prepared after 6-hydroxydopamine-induced destruction of dopaminergic terminals, both drugs were still able to inhibit forskolin-stimulated adenylate cyclase activity with a efficacy similar to that in tissue obtained from unlesioned rats. It is concluded that, in vitro, B-HT 920 is an agonist at both presynaptic and 'normosensitive' postsynaptic D2 receptors showing relatively high intrinsic activity.

Decrease of prolactin secretion via stimulation of pituitary dopamine D-2 receptors after application of talipexole and SND 919

The present experiments were performed to investigate the effects of talipexole (B-HT 920) and SND 919 on prolactin release from the anterior pituitary glands of rats both in vivo and in vitro. The basal serum prolactin levels were reduced dose dependently by s.c. administration of talipexole or SND 919 at doses of 5-100 micrograms/kg. Daily treatment with estradiol (35 micrograms/kg for 3 days) increased serum prolactin levels in male rats to levels 4-fold higher than those of non-primed rats. This increase was suppressed by administration of talipexole or SND 919. In vitro, the spontaneous prolactin release into perfusates from isolated anterior pituitary was inhibited by talipexole or SND 919 added at concentrations ranging from 10(-9) to 10(-6) M. This inhibitory effect of SND 919 was blocked by concurrent application of a dopamine D-2 receptor antagonist, YM-09151-2. The spontaneous prolactin release from the anterior pituitary isolated from estradiol-primed rats was 2-fold higher than that from non-primed rats. This increased release was also inhibited by application of either drug. The inhibitory effects of these drugs were greater in estradiol-primed rats than in non-primed rats when expressed as percent inhibition of control prolactin release. The results suggest that talipexole and SND 919 have a selective dopamine D-2 receptor agonistic property and are almost completely effective to counteract the enhancement of prolactin release induced by estrogens via stimulation of dopamine D-2 receptors in the anterior pituitary.

Effects of B-HT 920 on nigrostriatal and mesolimbic dopamine systems in normosensitive and supersensitive rats

1. B-HT 920, a D2 dopamine receptor agonist, was tested for its ability to exert presynaptic actions in normosensitive rats, and for possible postsynaptic actions in rats made 'supersensitive' to apomorphine. 2. In normosensitive rats, B-HT 920 (0.01-0.3 mg kg-1, i.p.) increased dopamine concentrations and lowered metabolite levels to a similar extent in all four terminal regions examined (medial prefrontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen). Analogous effects were seen for 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid. 3. Rats which received bilateral 6-hydroxydopamine (6-OHDA) infusions into the caudate-putamen showed signs of postsynaptic dopamine receptor activation (stereotyped behaviour) in response to B-HT 920 (0.1 and 1.0 mg kg-1, i.p.) and to apomorphine (0.2 mg kg-1, s.c.). Similarly, B-HT 920 (0.1 mg kg-1) induced contralateral circling in rats that had received unilateral 6-OHDA infusions into the medial forebrain bundle; the rate of circling increased gradually over several weeks. 4. In contrast, bilateral 6-OHDA infusions into the nucleus accumbens resulted in a supersensitive (locomotor stimulant) response to a low dose of apomorphine (0.1 mg kg-1, s.c.), but not to B-HT 920 (0.01 and 0.1 mg kg-1). 5. In intact rats, withdrawal of chronic haloperidol treatment induced behavioural supersensitivity to apomorphine but not to B-HT 920.

Potentiation of yawning responses to the dopamine receptor agonists B-HT 920 and SND 919 by pindolol in the rat

Subcutaneous injection of B-HT 920, a dopamine D2-receptor agonist, in doses ranging from 5 to 100 micrograms/kg, induced yawning behavior in rats. Yawning was also elicited by low doses (25-500 micrograms/kg sc) of SND 919, a newly synthesized dopamine receptor agonist. The yawning evoked by B-HT 920 or SND 919 was increased by the beta-adrenoceptor antagonist pindolol (20 mg/kg ip) which alone did not induce yawning. Stereotyped behavior did not appear after B-HT 920 or SND 919 given alone or in combination with pindolol. The results suggest that SND 919 as well as B-HT 920 has stimulatory activity at dopamine D2-receptors, and that pindolol may exert its enhancement of the yawning response to dopamine receptor agonists via blockade of beta-adrenoceptors.

Possible involvement of differing classes of dopamine D-2 receptors in yawning and stereotypy in rats

The present experiments were performed to investigate differences in the properties of the dopamine D-2 receptors related to yawning and stereotypy. Subcutaneous injection of talipexole (B-HT 920) (10-250 micrograms/kg) or SND 919 ((S)-2-amino-4,5,6,7-tetrahydro-6-propylamino-benzothiazole) (25-500 micrograms/kg) evoked yawning behavior with bell-shaped responses. However, SK&F 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (0.5-10 mg/kg SC) did not elicit yawning and decreased yawning responses to low doses of talipexole (25 micrograms/kg SC) or SND 919 (100 micrograms/kg SC). These low but effective doses for inducing yawning of talipexole or SND 919 in combination with SK&F 38393 (0.5-10 mg/kg SC) did not elicit stereotypy. In contrast, yawning behavior was not produced after very high doses of talipexole (500 micrograms/kg SC) or SND 919 (1000 micrograms/kg SC) given alone or in combination with SK&F 38393 (0.5-10 mg/kg SC). These extremely high doses of talipexole or SND 919 evoked slight stereotypy, which was enhanced by the combined treatment with SK&F 38393. The present results suggest that the dopamine D-2 receptors related to yawning are more sensitive to dopamine receptor agonists than those related to stereotypy, and that concurrent stimulation of postsynaptic dopamine D-1 receptors with D-2 receptors reduces the incidence of yawning but enhances that of stereotypy.

D-2 dopamine autoreceptor selective drugs: do they really exist?

The catecholamine dopamine plays an important role as a neurotransmitter or neurohormone in the brain and pituitary gland. Dopamine exerts its effects through activation of two types of receptors called D-1 and D-2. These receptors are distinguished by their different pharmacological characteristics and signal transduction mechanism(s). Release of dopamine inhibits the activity of dopaminergic neurons through activation of so-called dopamine autoreceptors which are of the D-2 type. In general, these receptors occur both in the soma-dendritic region of the dopaminergic neuron, where they are involved in the inhibition of the firing rate and on the dopaminergic terminals where they mediate the inhibition of dopamine synthesis and release. D-2 receptors occur also on the target cells of dopaminergic neurons both in the brain (postsynaptic D-2 receptors) and pituitary gland. On the basis of data gathered from in vivo (behavioral- as well as electrophysiological) studies it has been concluded that D-2 agonists are much more potent at dopamine autoreceptors as compared to postsynaptic D-2 receptors, indicating the possibility of a pharmacological distinction between these differentially located D-2 receptors. This concept led to the introduction of a whole group of drugs allegedly displaying a selective agonist profile at the dopamine autoreceptor. In contrast, biochemical (in vitro) studies with brain tissue as well as the pituitary gland, did not reveal any significant difference between the pharmacological profiles of autoreceptors and postsynaptic D-2 receptors. In the present minireview a balanced discussion is presented of these in vivo and in vitro findings and it is concluded that both autoreceptors as well as postsynaptic D-2 receptors are similar if not identical entities.

Dopamine-mediated behaviour following chronic treatment with B-HT 920

Following subchronic (5-day) dosing with B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo(4,5-d)azepine (1 mg kg-1 day-1 i.p.) in rats there was a significant increase in both apomorphine-induced motor activity and stereotypy. On continued B-HT 920 treatment, however, the enhancement of apomorphine motor activity faded into insignificance but the increase in stereotypy persisted beyond 15 days. The results are discussed in terms of dopamine autoreceptor tolerance, postsynaptic D2 supersensitivity and possible differential effects in different brain loci on the above two receptor sub-classes.

Dopamine-containing neurons in the mammalian central nervous system: electrophysiology and pharmacology

A decade of research culminated in the late 1950's with the demonstration that dopamine was a chemical neurotransmitter within the mammalian brain. Since this time, dopaminergic neuronal systems have been extensively studied using numerous techniques. This paper will review the last 14 years of electrophysiological investigation on neurochemically identified dopamine-containing neurons in the central nervous system. This will include an examination of both the electrophysiological and pharmacological characteristics in these cells, as well as the resulting insights into the regulation of dopamine cell electrical activity which is derived from this work.

B-HT 920 stimulates postsynaptic D2 dopamine receptors in the normal rat: electrophysiological and behavioral evidence

The putative autoreceptor-selective dopamine (DA) agonist B-HT 920 was tested using electrophysiological and behavioral models thought to reflect actions at postsynaptic D2 DA receptors. Direct iontophoretic application of B-HT 920 onto nucleus accumbens neurons caused a current-dependent inhibition of firing which could be attenuated by pretreatment with alpha-methyl-p-tyrosine (to deplete DA) and reinstated (enabled) by concurrent administration of the selective D1 DA receptor agonist SKF 38393. These findings suggest that, like other selective D2 DA receptor agonists, the postsynaptic effects of B-HT 920 require concurrent stimulation of D1 DA receptors. Behavioral indices of postsynaptic D2 DA receptor stimulation (stereotyped sniffing and rearing) were also evident following combined treatment with B-HT 920 and SKF 38393. Moreover, similar "low-level" stereotyped behaviors were also observed when B-HT 920 was administered alone following pretreatment with the alpha-2 adrenoceptor antagonists idazoxane and piperoxane, suggesting that alpha-2 agonist actions of B-HT 920, in some way, mask the expression of D2 receptor-mediated stereotyped responses. When B-HT 920 was combined with SKF 38393 following pretreatment with idazoxane, both the intensity and form (continual licking and gnawing) of stereotyped behavior was enhanced. Taken together, these electrophysiological and behavioral findings indicate that B-HT 920 possesses the properties of a selective D2 DA receptor agonist and cannot be considered as a DA autoreceptor-selective compound.

Electrophysiological and pharmacological characterization of identified nigrostriatal and mesoaccumbens dopamine neurons in the rat

Extracellular single-unit recording techniques were used to compare the basal activity and pharmacological responsiveness of identified nigrostriatal and mesoaccumbens dopamine (DA)-containing neurons. The projection area of each DA cell was determined by antidromic activation techniques. The forebrain stimulation used for the cell identification procedure did not alter the pharmacological responsiveness of DA neurons; the inhibitory effect of apomorphine (and d-amphetamine) was identical when stimulation was applied either prior to or following drug administration. Analysis of the spike discharge pattern revealed that a higher proportion of mesoaccumbens DA cells exhibited burst-firing activity. Although the firing pattern of the two populations of burst-firing DA cells was similar in many regards, mesoaccumbens DA cells exhibited a longer postburst inhibition than did nigrostriatal DA cells. Each of the DA agonists, apomorphine, pergolide, B-HT 920, and d-amphetamine, inhibited nigrostriatal and mesoaccumbens DA neuronal activity in a similar fashion. However, there was a marked population difference in the recovery of cell firing in the 10 minutes following apomorphine-induced inhibition; the recovery of mesoaccumbens spike discharges was considerably slower. Although this population difference was apparent to some extent following administration of pergolide or B-HT 920 (but not d-amphetamine), it was considerably less marked. The present findings are discussed with respect to the known regulatory control of midbrain DA neurons.

Further in vitro and in vivo studies with the putative presynaptic dopamine agonist N,N-dipropyl-7-hydroxy-2-aminotetralin

The in vitro binding of the putative dopamine autoreceptor agonist [3H]DP-7-ATN to rat striatal membrane homogenates was investigated. The maximum number of binding sites Bmax was 497.5 +/- 50.2 fmol/mg protein and the affinity constant KD was 8.3 +/- 1.5 nM using 10 microM (+) butaclamol to define non-specific binding. Lesion of the left medium forebrain bundle by 6-hydroxydopamine resulted in an almost complete loss of dopamine in the striatum but did not affect the binding of [3H]DP-7-ATN. The binding of [3H]DP-7-ATN to the homogenates of the dopaminergic cell bodies in the substantia nigra revealed a Bmax of 542.4 +/- 40.1 fmol/mg protein and a KD of 11.1 +/- 1.3 nM. The pharmacological profile of the binding was characterized as being to D-2 receptors. No direct in vitro evidence could be found for a selective binding to DA autoreceptors. The dopamine uptake inhibitor GBR 12909 interacted in a noncompetitive manner with the in vitro binding of [3H]DP-7-ATN and the latter compounds uptake into isolated synaptosomes was not through the specific dopamine uptake system but rather through diffusion. GBR 12909 failed to reveal any agonistic or antagonistic activity in the GBL model but was able to antagonize the hypomotility in rats induced by 0.25 mg/kg DP-7-ATN. The inhibitory effect of DP-7-ATN on DA release was also demonstrated using in vivo brain dialysis in conscious rats. Based on the above results, the possibility is discussed that the release regulating DA autoreceptors, which might be coupled to the reuptake complex, and the DA biosynthesis regulating autoreceptors, are different entities.

B-HT 920 and B-HT 958: presynaptic effects on electrically evoked 3H-dopamine release from slices of rat nucleus accumbens

The effects of two thiazoloazepine derivatives, B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine) and B-HT 958 (2-amino-6-(p-chloro-benzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]a zepine) on electrically evoked overflow of 3H-dopamine were studied. Slices from nucleus accumbens of the rat were preincubated with 3H-dopamine and superfused at 23 degrees C or 37 degrees C. Electrical field stimulation was applied using frequencies of 0.5 or 5 Hz. At 37 degrees C/5 Hz, B-HT 920 markedly and dose-dependently (0.01-0.1 mumol/l) reduced the stimulation evoked overflow of tritium. Its dose-response curve was shifted to the right at 23 degrees C/0.5 Hz and 23 degrees C/5 Hz, respectively. A similar result was obtained with the dopamine receptor agonist, apomorphine (1 mumol/l). B-HT 958 (0.1-10 mumol/l) also reduced electrically induced overflow of tritium at 37 degrees C/5 Hz, had no effect at 23 degrees C/0.5 Hz, and facilitated tritium overflow at 23 degrees C/5 Hz. Sulpiride (10 mumol/l) completely prevented the effects of B-HT 920 (1 mumol/l) or B-HT 958 (1 mumol/l) at 37 degrees C/5 Hz, whereas phentolamine (1 mumol/l) had no effect on the actions of the two drugs under these experimental conditions. From the patterns of effects obtained under the different experimental conditions it is concluded that B-HT 920 acts as full agonist at presynaptic dopamine autoreceptors whereas B-HT 958 acts as partial agonist.

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.

The dopamine autoreceptor agonist B-HT 920 stimulates denervated postsynaptic brain dopamine receptors in rodent and primate models of Parkinson's disease: a novel approach to treatment

B-HT 920 (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]azepine), an agonist at alpha 2-adrenoceptors and at dopamine autoreceptors, was tested with respect to stimulation of postsynaptic brain dopamine receptors in mice, rats and rhesus monkeys. In mice B-HT 920 (0.2-20 mg/kg s.c.) injected 4 h after reserpine did not stimulate locomotor activity; this was in contrast to apomorphine (0.1-10 mg/kg s.c.) which elicited locomotor activity in a dose-dependent manner. However, B-HT 920 was effective in inducing locomotor activity when injected 12, 24 and 48 h after reserpine. This effect was dose-dependent and increased with the duration of reserpine pretreatment. In naive rats, B-HT 920 (0.02-2.0 mg/kg s.c.) only decreased exploratory activity and did not elicit stereotyped activity in doses up to 4 mg/kg s.c. This was in contrast to the stereotypy-inducing effect of apomorphine (2.0 and 4.0 mg/kg s.c.). In rats with unilateral striatal ibotenic acid lesion, B-HT 920 (0.2-2.0 mg/kg s.c.) was ineffective in producing significant ipsilateral rotation, whereas apomorphine (0.5-10.0 mg/kg s.c.) was very potent in this model. In rats with unilateral 6-OH-dopamine lesions of the medial forebrain bundle B-HT 920 elicited strong contralateral rotation in a dose-dependent manner (0.02-1.0 mg/kg s.c.). In this model B-HT 920 was equi-effective but long acting when compared with apomorphine. The contralateral rotation produced by B-HT 920 was antagonized by the D2-antagonist sulpiride but not by the D1-antagonist SCH 23390. In rhesus monkeys with severe parkinson-like symptoms induced by MPTP, B-HT 920 in doses of 10 micrograms/kg i.m. and higher restored normal behavior, resulting in complete relief of parkinson symptoms in all animals with 100 micrograms/kg i.m. It is concluded that the property of B-HT 920 to stimulate the 'denervated' supersensitive (reserpine, 6-OH-dopamine, MPTP) but not the normosensitive postsynaptic dopamine receptor in the striatum may represent a novel principle for a specific approach to dopamine substitution treatment of Parkinson's disease.

The putatively selective dopamine autoreceptor antagonists (+)-AJ 76 and (+)-UH 232 stimulate prolactin release in rats

The 2-aminotetralin derivatives cis-(+)-(1S,2R)-5-methoxyl-1-methyl-2-(n-propylamino)tetralin, (+)-AJ 76, and cis-(+)-(1S,2R)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin, (+)-UH 232, are novel centrally acting stimulants with a putative action as selective dopamine (DA) autoreceptor antagonists. In the present study these compounds were evaluated with respect to their effects on prolactin release in male rats. Both (+)enantiomers caused a pronounced increase in plasma prolactin levels in previously untreated animals. The effects of (+)-AJ 76 and (+)-UH 232 were virtually similar, except for a higher initial increase after the latter compound. In agreement with earlier reports, the reserpine-induced elevation of plasma levels of prolactin was strongly suppressed by the DA autoreceptor agonist B-HT 920. This effect of B-HT 920 was completely blocked by (+)-AJ 76 and by (+)-UH 232, indicating that both (+)enantiomers antagonize lactotroph DA receptors. The present findings support the notion that lactotroph DA receptors resemble DA autoreceptors rather than postsynaptic DA receptors. A possible difference between the auto-/lactotroph vs. postsynaptic DA receptors with respect to both the responsiveness to agonists and to the affinity of pure antagonists is discussed.

The emetic effect of B-HT 920 and apomorphine in the dog: antagonism by haloperidol

Recent investigations have suggested that the alpha 2-adrenoreceptor agonist B-HT 920 is also a dopamine (DA) agonist with a selectivity for presynaptic receptors. In the present study, the emetic effect of B-HT 920 was investigated. Intravenous injections of B-HT 920 (0.32-10.0 micrograms/kg) and a DA2-agonist apomorphine (3.2-100.0 micrograms/kg) caused dose-dependent emesis. The ED50 of B-HT 920 and apomorphine were 3.2 and 12.3 micrograms/kg, respectively. When haloperidol (10.0-24.5 micrograms/kg i.v.), a DA2-antagonist, was given 5 minutes before B-HT 920 (10 micrograms/kg) or apomorphine (32 micrograms/kg), it caused a dose-dependent prevention of B-HT 920- and apomorphine-induced emesis. The ED50 of haloperidol in preventing the emetic effect of both drugs was identical (13.5 micrograms/kg). In contrast, haloperidol (32 micrograms/kg i.v.) did not prevent the emetic effect of ouabain (40 micrograms/kg i.v.). Neither did yohimbine (0.1 mg/kg i.v.), an alpha 2-adrenoreceptor antagonist, prevent the emetic effect of B-HT 920 (10 micrograms/kg). These results suggest that B-HT 920, acting like apomorphine, induces emesis by activating DA2-receptors probably in the chemoreceptor trigger zone of the area postrema.

Central dopaminergic properties of HW-165 and its enantiomers; trans-octahydrobenzo(f)quinoline congeners of 3-PPP

In the further development of CNS dopamine autoreceptor active compounds related to 3-PPP, the transfused 7-hydroxy-1,2,3,4,4a,5,6,10b-octahydrobenzo(f)quinoline HW-165 and its enantiomers were synthesised. This paper describes the basic pharmacological properties of these latter, novel "atypical" dopaminergic agents, based on an extensive series of biochemical and behavioural experiments in rats. By and large, the pharmacological activities of HW-165 - essentially, if not exclusively, residing in its (4aS,10bS)-(-)-enantiomer - were similar to those displayed by (S)-(-)-3-PPP, indicating the simultaneous presence of central dopamine (autoreceptor) agonist and weak (postsynaptic) antagonist properties in the molecule. Thus, in non-pretreated animals HW-165 and its active species monotonically suppressed the spontaneous locomotion without causing catalepsy or other appreciable motor disabilities, and at the same time selectively reduced the dopamine synthesis, release/turnover and utilisation. Some differences in these biochemical responses to HW-165 [racemate or (-)-enantiomer] were, however, noted in the limbic vs. striatal brain areas (e.g. decrease of dopamine synthesis particularly in the limbic parts). On the other hand, while failing to reverse reserpine-induced akinesia or to elicit stereotyped behaviour, the agents markedly inhibited the dopamine synthesis in either of the dopamine-dominated cerebral regions in the reserpinised as well as in gamma-butyrolactone (GBL)-treated rats. As shown for racemic HW-165 after reserpine pretreatment, the inhibition of dopamine synthesis was completely and stereoselectively blocked by (+)-butaclamol, thereby supporting direct dopamine receptor interaction. Racemic HW-165 readily antagonised the d-amphetamine-induced locomotor hyperactivity. Apomorphine-induced hyperactivity was, however, distinctly more resistant to antagonism by HW-165 [racemate or (-)-enantiomer]. Moreover, the latter agents fully prevented the apomorphine-induced inhibition of striatal dopamine synthesis in otherwise non-pretreated rats, while only partly counteracting this effect of apomorphine in the limbic regions of such animals, and in either brain area of rats treated with gamma-butyro-lactone. The findings are interpreted within the context of the mixed dopamine agonist/antagonist properties (referred above) of HW-165 and its active (-)-species in relation to the adaptive state of central dopamine receptors and possible regional variations in feedback strength and organisation.(ABSTRACT TRUNCATED AT 400 WORDS)

Stereoselective inhibition of prolactin secretion by (-)-HW-165, a novel 3-PPP congener; further support for similarities between central DA autoreceptors and pituitary lactotroph DA receptors

The novel atypical dopamine (DA) receptor agonist HW-165 (trans-7-hydroxy-1,2,3,4,4a,5,6,10b-octahydrobenzo(f)quinoline), a 'rigid' 3-PPP congener, and its enantiomers were investigated with regard to their actions on prolactin (PRL) release in rats. Racemic HW-165 dose dependently inhibited the elevation of PRL secretion induced by monoamine synthesis disruption (NSD 1015) combined either with abolished DA nerve impulse flow (GBL) or with monoamine depletion (reserpine). Racemic HW-165 was roughly equipotent to 3-PPP but 5-10-fold less potent than apomorphine. The PRL inhibitory action of racemic HW-165 was stereoselectively prevented by the DA antagonist (+)-butaclamol. Since the chiral aspect of lactotroph DA receptor activation by 'atypical' DA receptor agonists has not been covered in previous investigations the effects of the HW-165 enantiomers were now studied. Our findings suggest that the PRL inhibitory properties of racemic HW-165 reside in its (-) enantiomer, (+)-HW-165 being devoid of activity. Taken together, the results reinforce the concept that the pituitary lactotroph DA receptors mediating the inhibition of PRL release are similar, notably also from a stereochemical point of view, to central DA receptor sites with presumed high agonist sensitivity, such as DA autoreceptors.

Involvement of extrapyramidal motor mechanisms in the suppression of locomotor activity by antipsychotic drugs: a comparison between the effects produced by pre- and post-synaptic inhibition of dopaminergic neurotransmission

The effects of two proposed dopaminergic autoreceptor agonists, (-)3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) and the azepine derivative B-HT 920, on spontaneous locomotor activity, treadmill locomotion, and catalepsy in the rat have been compared with the effects produced by the postsynaptic dopamine (DA) receptor blocking agent haloperidol. It was found that the threshold dose for suppression of exploratory locomotor activity was 0.5, 0.005 and 0.2 mg/kg for (-)3-PPP, B-HT 920 and haloperidol, respectively. The corresponding doses for suppression of treadmill locomotion were 8.0, 5.12 and 0.2 mg/kg, respectively. Furthermore, (-)3-PPP and B-HT 920, in contrast to haloperidol, did not produce any catalepsy. Thus, using exploratory locomotor activity as an index of limbic forebrain DA functions and treadmill locomotion and catalepsy as indices of extrapyramidal DA functions, the DA autoreceptor agonists, in contrast to the postsynaptic antagonist, show a difference in the doses required to produce these effects. The designation of the behavioral functions as "limbic" or extrapyramidal is supported by the finding that scopolamine, 0.8 mg/kg, antagonized the haloperidol-induced suppression (0.2 mg/kg) of treadmill locomotion, but not the suppression of exploratory locomotor activity.

Evidence for a specific effect of BHT 920, an azepine derivative, on tyrosine hydroxylase in the dopaminergic system of the rat

The effect of BHT 920, a putative presynaptic dopamine receptor agonist, on tyrosine hydroxylase was investigated in rats. The activity of the high affinity (BH4) form of striatal tyrosine hydroxylase was investigated dose-dependent manner in rats treated with BHT 920. This effect was pronounced in the dopaminergic system and was not observed to the same extent in the adrenal medulla. In vitro, BHT 920 had no effect upon striatal tyrosine hydroxylase activity. BHT 920 also did not affect either striatal adenylate cyclase activity or the extent of its stimulation by dopamine. The results concerning tyrosine hydroxylase were complemented by measurements of dopamine and DOPA in the striatal and the limbic system. The reduction in DOPA accumulation and in the high affinity form of tyrosine hydroxylase activity elicited by BHT 920 could be blocked by haloperidol, suggesting that BHT 920 may interact with the D2 dopamine receptor although a functional antagonism could not be ruled out. The present results suggest that BHT 920 may exert a specific effect upon tyrosine hydroxylase in dopaminergic nervous tissue which is not mediated by alpha 2-adrenoceptors.

Dopamine receptor agonists: mechanisms underlying autoreceptor selectivity. II. Theoretical considerations

In a companion article, we extensively reviewed the pharmacological actions of the enantiomers of the dopamine analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP. The profiles of action exhibited by transdihydrolisuride (TDHL) and the trans-fused 7-OH-1,2,3,4,4a,5,6,10-octahydrobenzo(f)quinoline (HW 165) were also described. These latter agents, along with (-)-3-PPP, exert a variety of effects at different DA receptors depending on the anatomical location of these receptor sites and the experimental conditions. In the first part of the present article, it is suggested that the intrinsic activity of these agents in different pharmacological models is dependent on the responsiveness of the relevant DA-receptors which, in turn, is related to the degree of previous agonist occupancy of these sites. In situations where these agents exhibit partial agonist activity, their pharmacological effect is also dependent on the relative concentrations of drug and endogenous DA competing for common receptor sites. A number of theoretical implications will be discussed relevant to the suggestion that DA receptors exist in various adaptational states which can influence drug action. In the second part of this review, we will consider the behavioural profile exhibited by (-)-3-PPP in relation to that observed with classical DA antagonists. In addition, the potential clinical application of (-)-3-PPP and similar-acting agents will be discussed.