Dopamine receptors involved in prolactin secretion pharmacologically characterized by means of 3-PPP enantiomers

Effects of the dopamine stabilizers (S)-(-)-OSU6162 and ACR16 on prolactin secretion in drug-naive and monoamine-depleted rats

Dopaminergic stabilizers may be conceptualized as drugs with normalizing effects on dopamine-mediated behaviours and neurochemical events. (S)-(-)-OSU6162 (OSU6162) and ACR16 are two structurally related compounds ascribed such properties, principally because of their stabilizing effects on motor activity in rodents. Reports in the literature indicate possible partial D2 receptor agonist effects using various in vitro systems. This study aimed to measure D2 receptor antagonist and agonist effects of OSU6162 and ACR16 in vivo. To address this, we have studied the effects of both compounds on prolactin secretion in drug-naive and dopamine-depleted rats; dopamine depletion was induced by pretreatment with reserpine plus α-methyl-DL: -p-tyrosine. We find that OSU6162 and ACR16 both stimulate prolactin secretion in drug-naive rats with OSU6162 being considerably more potent and efficacious. Both compounds show a non-significant trend towards reversal of the increased secretion caused by dopamine depletion, whereas the D2 receptor antagonist haloperidol further increased prolactin secretion. Thus, this study suggests that OSU6162 and ACR16 act as D2 receptor antagonists under normal conditions in vivo, possibly with minor agonist effects in a state of dopamine depletion.

The neuroendocrinological profile of roxindole, a dopamine autoreceptor agonist, in schizophrenic patients

Roxindole is a potent autoreceptor-selective dopamine agonist with additional properties as a serotonin reuptake inhibitor and 5-HT1A agonist. In order to get more insight into its mode of action in various psychiatric populations, we evaluated the effects of subchronic roxindole treatment on pituitary and adrenal hormone secretion, i.e. release of prolactin, thyroid stimulating hormone (TSH), growth hormone (GH), luteinizing hormone (LH), and cortisol. Fifteen schizophrenic patients with positive and negative symptomatology, respectively, were treated with roxindole for 28 days. Both basal and thyrotropin releasing hormone (TRH) -induced prolactin secretion diminished significantly to 26.4% and 22.8% of baseline levels, respectively, under roxindole. Basal GH secretion was insignificantly elevated by 89%, whereas GH levels increased nearly 3-fold after stimulation by TRH. TSH levels decreased insignificantly to 57.5% of baseline levels, while TRH-induced TSH release was not affected by subchronic roxindole. Roxindole treatment influenced neither LH secretion nor cortisol release. Our results indicate that roxindole's dopaminergic actions might prevail over its serotonergic effects, at least as far as the regulation of anterior pituitary hormone secretion is concerned.

Partial dopamine receptor agonists reverse behavioral, biochemical and neuroendocrine effects of neuroleptics in the rat: potential treatment of extrapyramidal side effects

The partial DA receptor agonist preclamol, (-)-3-PPP (50-200 mumol/kg, s.c.) partially reversed the catalepsy induced by the dopamine (DA) receptor antagonists haloperidol (5.3 mumol/kg, i.p.) and raclopride (20.1 mumol/kg, i.p.) in rats. Terguride (transdihydrolisuride), a partial DA receptor agonist with an efficacy lower than that of preclamol, blocked haloperidol (10.6 mumol/kg, i.p.) induced catalepsy at 5 mumol/kg, s.c., but not at 20 mumol/kg, s.c. The effects of terguride in this assay are possibly related to the compound's mixed partial DA agonist/5-HT1A receptor agonist properties. The high efficacy agonist, pramipexole (SND 919) also blocked haloperidol induced catalepsy at 50 mumol/kg, s.c. Haloperidol (0.33-1.3 mumol/kg, i.p.) reduced the locomotor activity down to 5% of saline controls and elevated limbic and striatal DOPA accumulation. When combined with haloperidol, preclamol (100-200 mumol/kg, s.c.) antagonized both the strong hypomotility and increase in DOPA accumulation. Finally, the elevation of serum prolactin in rats induced by haloperidol (0.25 mumol/kg, i.p.) was significantly antagonized by co-administration of preclamol (39 mumol/kg, s.c.). These results show that partial DA agonists can reverse both behavioral, biochemical and neuroendocrine effects of neuroleptics. It also suggests the utility of partial DA receptor agonists in combination with classical neuroleptics in order to minimize the appearance of extrapyramidal side-effects and hyperprolactinemia.

Partial dopamine D2 receptor agonists antagonize prolactin-regulating D2 receptors in a transfected clonal cell line (GH4ZR7)

(-)-(3-Hydroxyphenyl)-N-n-propylpiperidine ((-)-3-PPP) and transdihydrolisuride (terguride, TDHL) are partial dopamine D2 receptor agonists displaying low intrinsic activity at normosensitive postsynaptic dopamine D2 receptors in brain while activating prolactin-regulating dopamine D2 receptors in male rats with an efficacy close to that of full dopamine D2 receptor agonists. In the present study we examined the effects of these partial dopamine D2 receptor agonists on prolactin release in vitro. For this purpose prolactin-producing tumour cells which have been transfected with the dopamine D2 receptor cDNA and hence which express a dopamine D2 receptor (short isoform) that is functionally active with respect to inhibition of adenylate cyclase and prolactin release (GH4ZR7; Albert et al., J. Biol. Chem. (1990) 265, 2098) were used. While the full dopamine D2 receptor agonists, quinpirole, (+)-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3-PPP) and dopamine induced a dose-dependent suppression of vasoactive intestinal peptide (VIP)-induced prolactin release from GH4ZR7, both (-)-3-PPP and terguride were inactive. Furthermore, the prolactin-suppressive effects of dopamine and quinpirole were significantly antagonized by pretreatment with (-)-3-PPP and terguride. It is concluded that partial dopamine D2 receptor agonists, which activate male lactotroph dopamine D2 receptors in vivo, may antagonize dopamine D2 receptors on GH4ZR7 cells. There were similar results from an experiment using monolayers of anterior pituitary cells from male rats. Thus, in this in vitro preparation (+)-3-PPP suppressed spontaneous prolactin release while (-)-3-PPP again was inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of the two partial D2 agonists (+)- and (-)-3-PPP on prolactin release in EEDQ treated male rats

The effects of pretreatment with the irreversible inactivator of brain D2 receptors N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on the suppression of prolactin (PRL) release induced by the two partial D2 agonists (+)- and (-)-3-(3-hydroxyphenyl)-N-n-propyl-piperidine [(+)-3-PPP, (-)-3-PPP] were investigated in gamma-butyrolactone (GBL) treated male rats. Pretreatment with a high dose of EEDQ (20 mg/kg, 24 h) did not influence the PRL response to (+)-3-PPP. In contrast, the effect of (-)-3-PPP was dose-dependently antagonized by EEDQ administration; thus, while pretreatment with EEDQ 2 mg/kg (24 h) failed to influence the efficacy of (-)-3-PPP, a complete antagonism of the PRL suppressive effect of the (-) enantiomer was obtained by administration of EEDQ 16 or 20 mg/kg (24 h). Moreover, in EEDQ (20 mg/kg, 24 h) treated rats (-)-3-PPP effectively antagonized the PRL inhibiting effect of the (+) enantiomer. Also, in EEDQ (20 mg/kg, 24 h) treated animals not receiving GBL (-)-3-PPP induced a dose-dependent increase in plasma levels of PRL. The data indicate that higher doses of EEDQ are required in order to reduce the responsiveness of lactotroph D2 receptors than that of various populations of brain D2 receptors. Also, the data lend support for the assumption that an altered receptor responsiveness may dramatically modify the response to a partial agonist with low intrinsic efficacy without affecting the response to a partial agonist with higher intrinsic efficacy.

Phasic versus tonic dopamine release and the modulation of dopamine system responsivity: a hypothesis for the etiology of schizophrenia

A novel mechanism for regulating dopamine activity in subcortical sites and its possible relevance to schizophrenia is proposed. This hypothesis is based on the regulation of dopamine release into subcortical regions occurring via two independent mechanisms: (1) transient or phasic dopamine release caused by dopamine neuron firing, and (2) sustained, "background" tonic dopamine release regulated by prefrontal cortical afferents. Behaviorally relevant stimuli are proposed to cause short-term activation of dopamine cell firing to trigger the phasic component of dopamine release. In contrast, tonic dopamine release is proposed to regulate the intensity of the phasic dopamine response through its effect on extracellular dopamine levels. In this way, tonic dopamine release would set the background level of dopamine receptor stimulation (both autoreceptor and postsynaptic) and, through homeostatic mechanisms, the responsivity of the system to dopamine in these sites. In schizophrenics, a prolonged decrease in prefrontal cortical activity is proposed to reduce tonic dopamine release. Over time, this would elicit homeostatic compensations that would increase overall dopamine responsivity and thereby cause subsequent phasic dopamine release to elicit abnormally large responses.

Dopamine D-2 agonists with high and low efficacies: differentiation by behavioural techniques

The effects of the dopamine (DA) D-2 antagonist YM 09151-2 and the DA D-2 agonists terguride, preclamol, EMD 23448, B-HT 920, quinpirole and (-)-NPA were studied in a battery of behavioural tests in order to evaluate their relative efficacies. Furthermore, their affinities for DA D-2 receptors labelled by 3H-N-0437 were measured in vitro. All agonists reduced spontaneous locomotor activity and induced marked contralateral circling behaviour in 6-hydroxy-DA-lesioned rats. Quinpirole and (-)-NPA increased motor activity after high doses. YM 09151-2 did not induce circling. In hemitransected rats quinpirole and (-)-NPA had weak effects when given alone, whereas the other agonists were ineffective. After combination with DA D-1 agonist SK&F 38393, B-HT 920 became effective, and the effects of quinpirole and (-)-NPA were facilitated. EMD 23448, preclamol and terguride were not active. In contrast, the two latter compounds fully inhibited the response to apomorphine. In stereotypy experiments a similar activity pattern was observed. Finally, drug discrimination studies showed that quinpirole, (-)-NPA and B-HT 920 substituted for the stimulus effects induced by d-amphetamine or (-)-NPA in different groups of rats. EMD 23448 induced intermediate effects, whereas preclamol and terguride had weak effects. None of the partial agonists inhibited the response of d-amphetamine. YM 09151-2 potently inhibited the effect of d-amphetamine. The results suggest that DA D-2 agonists can be ranked according to gradually increasing agonist efficacies rather than classified into autoreceptor-selective versus nonselective D-2 agonists. Implications of this hypothesis are discussed.

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.

The partial dopamine receptor agonists (--)-3-PPP and transdihydrolisuride enhance prolactin secretion in female but not in male rats

The effects exerted by the partial dopamine (DA) receptor agonists (--)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(--)-3-PPP] and transdihydrolisuride (TDHL) on prolactin secretion were compared in male and female rats. Both agents were found to increase prolactin release more than tenfold in female rats. In male rats, in contrast, (--)-3-PPP and TDHL were either without effect or decreased prolactin secretion. It is suggested that this sexually differentiated prolactin response to partial DA agonists reflects a lower responsiveness of lactotroph DA receptors in female than in male rats.

The intrinsic activity of (-)-3-PPP (preclamol) on pituitary DA receptors in female rats is enhanced following chronic DA depletion

The present investigation was aimed at comparing the influence of the partial dopamine (DA) receptor agonist (-)-3-PPP (preclamol) on prolactin release in acutely hyperprolactinemic but otherwise intact female rats and female rats subjected to chronic DA depletion. One group of animals received daily vehicle injections (controls) and another group daily reserpine (1 mg/kg) injections for a period of seven days. On the eighth day all animals were administered 10 mg/kg of reserpine in order to eliminate endogenous DA and elevate serum prolactin levels. In the control group (-)-3-PPP lowered serum prolactin levels only moderately. In contrast, in the chronically reserpinized female rats, a pronounced reduction of prolactin secretion was observed. It is suggested that this increase in intrinsic activity of (-)-3-PPP following chronic DA depletion reflects an enhanced responsiveness of hypophyseal DA receptors, possibly due to conformational changes in the receptor molecules. Our observations lend further support to the hypothesis that an inverse relationship exists between the intrinsic activity of a mixed agonist/antagonist and the degree of previous receptor occupancy.

The intrinsic activities of the partial dopamine receptor agonists (-)-3-PPP and TDHL on pituitary dopamine receptors are lower in female than in male rats

The abilities of the mixed agonists/antagonists on dopamine (DA) receptors, (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [-)-3-PPP) and transdihydrolisuride (TDHL), to suppress serum prolactin levels in acutely hyperprolactinemic male and female rats were investigated. gamma-Butyrolactone was used to deplete endogenous DA and raise serum prolactin concentrations. Both (-)-3-PPP and TDHL were found to cause sexually differentiated responses: (-)-3-PPP reduced serum prolactin levels dose dependently and effectively in males but caused only a modest decrease of prolactin release in females. Moreover, (-)-3-PPP antagonized the prolactin-suppressing effects induced by the DA receptor agonist (+)-3-PPP in females. Likewise TDHL decreased prolactin secretion markedly in males while it had only slight effects in females. It can be concluded from these results that the intrinsic activities of the partial DA agonists (-)-3-PPP and TDHL are lower in female than in male rats, suggesting a reduced responsiveness of hypophyseal DA receptors in females. Since DA levels in the pituitary portal circulation are higher in female than in male rats, this study gives further support to the hypothesis claiming an inverse relationship between the intrinsic activity of a mixed agonist/antagonist and the degree of previous stimulation of its receptor.

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.

Sexually differentiated actions of 3-PPP enantiomers on prolactin secretion

The ability of the enantiomers of the atypical dopamine receptor agonist 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) to counteract gamma-butyrolactone-induced hyperprolactinemia was compared in male and female rats. Following gamma-butyrolactone (GBL) pretreatment serum prolactin concentrations were higher in female than in male rats. In males (-)-3-PPP tended to be somewhat less effective than (+)-3-PPP in decreasing serum prolactin concentrations (levels after (+)-3-PPP and (-)-3-PPP: 21% and 33%, respectively, of levels in GBL-pretreated control(s). In females the (-)-form induced a much weaker response than did the (+)-form (levels after (+)-3-PPP and (-)-3-PPP: 8% and 74%, respectively, of levels in GBL pretreated controls). Parallel experiments replacing GBL by reserpine yielded similar results. Data are discussed in terms of sex differences in responsiveness of pituitary dopamine receptors.

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.

A central serotonin receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin, has different effects on prolactin secretion in male and female rats

Male and female rats were compared with respect to alterations in prolactin secretion induced by the serotonin receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The dose-response curves after 8-OH-DPAT were irregular and had different shapes in the two sexes. In males, 0.1 and 0.3 mg/kg enhanced serum prolactin concentrations to about 200% of control values, whereas higher doses (1 and 3 mg/kg) had no effect on prolactin release. In females, in contrast, 0.1 mg/kg of 8-OH-DPAT tended to decrease serum levels of prolactin, while 0.3, 1, and 3 mg/kg elevated them in a dose dependent manner to maximally 700% of control values. The serotonergic agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) (5 mg/kg), too, caused increased prolactin release in both sexes, and, again, females responded more forcefully. In males, but not in females, pretreatment with 8-OH-DPAT (1 mg/kg) reduced the 5-MeODMT-induced elevation of serum prolactin levels. The mechanism underlying the sexually differentiated effects of 8-OH-DPAT on prolactin secretion is discussed.

Effects of dopaminomimetics on the secretion of VIP-like immunoreactivity in conscious dogs

The effects of some dopaminomimetics on VIP levels in peripheral venous blood of conscious dogs were analysed with a radioimmunoassay. The dopamine D2 agonist pergolide, like apomorphine and bromocriptine, increased VIP levels. The putative DA autoreceptor agonist 3PPP, as well as the D1 agonist SK&F 38393 were devoid of action. The D1 antagonist SCH 23390 did not abolish the effect of apomorphine. It is suggested that monitoring of VIP levels could be an interesting screening test for activity at D2 receptors. Amphetamine did not modify VIP levels suggesting that DA neurons are not involved in the mechanism leading to a release of VIP. The VIP response to apomorphine was not suppressed by an infusion of somatostatin. Decreasing blood pressure with nitroglycerin or with the adrenergic antagonist prazosin did not release VIP. The mechanism by which administration of dopaminomimetics lead to a release of VIP is further discussed.

Dopamine and the action of opiates: a reevaluation of the dopamine hypothesis of schizophrenia. With special consideration of the role of endogenous opioids in the pathogenesis of schizophrenia

It is suggested that the antipsychotic efficacy of opioids in patients suffering from schizophrenia may result from an interaction of opioids with the dopaminergic system. The modulatory effect of opioids on dopaminergic functions has already been demonstrated in basic experiments: Anatomical and biochemical data reveal an interaction between opioid receptors and dopamine (DA) actions on dopaminergic nerve terminals, cell bodies, and afferent nerve endings. Endogenous enkephalin levels correlate well with the endogenous dopamine content in various brain areas. Systemic or iontophoretic administration of morphine alters the spontaneous activity of ventral tegmental dopaminergic neurons. Morphine and enkephalin effectively enhance pituitary prolactin release, whereas dopamine inhibits it. Opioid agonists effectively alter DA release, DA reuptake, and DA metabolism in the striatum and substantia nigra. In reverse, chronic neuroleptic treatment enhances the synthesis and release of pituitary beta-endorphin. Opioids affect contralateral rotation elicited by dopamine agonists in animals with unilateral lesions of the nigrostriatal pathway. Phencyclidine, a psychotropic drug that shares certain pharmacological characteristics with the putative sigma-opioid receptor ligand SKF 10,047, indirectly mimics the effects of dopamine agonists on prolactin release, release of acetylcholine, etc. It is suggested that an imbalance of opiate-DA interaction might be involved in the pathogenesis of schizophrenia. Consequently, clinical studies on the effects of opioids on psychotic symptoms should also examine opioid influence on dopaminergic functions in these patients.

Effects of the enantiomers of 3-PPP on DA1 and DA2 dopamine receptors in the dog

The (+)- and (-)-enantiomers of 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) were studied for their effects on DA1 and DA2 dopamine receptors in pentobarbital anesthetized dogs. 3-PPP enantiomers were administered into the renal artery after phenoxybenzamine pretreatment to determine possible DA1 activity; dopamine was also injected for comparison. DA2 activity was determined by injection of the enantiomers into the femoral vascular bed with intact nerve supply and without phenoxybenzamine; dipropyl dopamine (DPDA) or apomorphine were used as standard DA2 agonists. Antagonist activity of the enantiomers on DA1 or DA2 receptors was determined by simultaneous administration of the enantiomer with DA in the renal vascular bed and with DPDA or apomorphine in the femoral vascular bed. Neither enantiomer was active as a DA1 agonist, but both exhibited antagonist activity. Both enantiomers were found to be agonists of the DA2 receptor; in addition, both showed DA2 antagonist activity. In all actions the (-)-enantiomer was approximately 4 times more potent than the (+)-enantiomer.

Lack of functional evidence for the involvement of sigma opiate receptors in the actions of the 3-PPP enantiomers on central dopaminergic systems: discrepancies between in vitro and in vivo observations

In vitro radioligand binding and autoradiographic distribution studies have suggested the possible involvement of central sigma-opiate sites in the effects of several purportedly dopaminergic agents. Specifically, Largent et al. (Proc. Nat. Acad. Sci. 81, 4983, 1984) proposed that "actions of 3-PPP at sigma receptors may account for the effect of the drug on behavior and dopaminergic nerve function". Using the sigma-opiate- and dopamine (DA)-preferring (-)- and (+)-enantiomer, respectively, of butaclamol, and the two enantiomers of 3-PPP, the present study was undertaken to address the in vivo functional significance of this proposal. To this end we investigated various biological responses considered to reflect drug interactions with DA cell body and terminal autoreceptors and with presumed non-synaptic and postsynaptic DA receptors in the rat CNS. (+)- but not (-)-butaclamol antagonized the 3-PPP (either enantiomer)-induced DA synthesis and prolactin decreases in GBL-treated rats, the (+)-3-PPP-induced inhibition of substantia nigra DA cell firing and the (+)-3-PPP-induced reversal of reserpine akinesia. Taken together with previous findings available data suggest that DA rather than sigma-opiate receptors mediate the neurochemical, electrophysiological, behavioral and other physiological (prolactin, body temperature) effects of 3-PPP and its enantiomers. The in vivo pharmacological relevance of the claimed non-dopaminergic, proposedly sigma-opiatergic, radioligand binding demonstrated in vitro (with e.g. (+)-3-PPP) thus remains to be established.

An electrophysiological analysis of the actions of the 3-PPP enantiomers on the nigrostriatal dopamine system

Extracellular single unit recording and microiontophoretic studies were carried out in chloral hydrate-anesthetized gallamine-paralyzed rats to investigate the actions of the enantiomers of the dopamine (DA) analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, on the nigrostriatal DA system. Intravenously administered (+)- or (-)-3-PPP consistently inhibited nigral DA neuronal activity; these actions were readily antagonized by haloperidol but were not affected by a pretreatment of reserpine plus alpha-methyltyrosine. In contrast to (+)-3-PPP, the (-)-enantiomer produced only partial inhibition of the majority of cells studied and was also capable of partially reversing the inhibitory action of apomorphine. A prior hemitransection of the brain did not alter the inhibitory action of either enantiomer. Whereas iontophoretically ejected (+)-3-PPP consistently reduced DA cell firing rate, similarly applied (-)-3-PPP reduced the activity of only some DA cells, while the majority were not influenced. In addition, iontophoresis of (-)-3-PPP could reduce the inhibitory effect of similarly applied DA or (+)-3-PPP. The (+)-enantiomer reduced caudate neuronal activity both after intravenous administration and iontophoresis. Intravenously administered (-)-3-PPP failed to influence or increased the activity of these neurons and reversed the inhibitory action of apomorphine. However, iontophoretically ejected drug reduced caudate cell activity and did not influence the inhibitory action of DA. The activity of non-DA zona reticulata neurons was inconsistently influenced by the 3-PPP enantiomers. It is concluded that (+)-3-PPP is a directly acting DA agonist, stimulating both DA autoreceptors and postsynaptic DA receptors. In contrast, (-)-3-PPP appears to be a partial agonist at nigral DA autoreceptors, whereas the action of the drug at putative postsynaptic DA receptors in the caudate remains to clarified.

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

Recent behavioural and biochemical investigations have suggested that the alpha-2 receptor agonist B-HT 920 is also a centrally acting dopamine (DA) agonist with a selectivity for autoreceptors. It is presently demonstrated that B-HT 920, in contrast to the structurally related alpha-2 agonist B-HT 933, effectively reduces the firing rate of nigral DA neurons both after intravenous and microiontophoretic administration. Furthermore, B-HT 920, but not B-HT 933, decreases plasma levels of prolactin in reserpine pretreated rats. The electrophysiological as well as the neuroendocrine effects of the drug were antagonised by DA antagonists but not by alpha-2 receptor antagonists. The data support the contention that B-HT 920 acts as an agonist at central DA autoreceptors. Furthermore, they reinforce the hypothesis that lactotroph DA receptors are more similar to DA autoreceptors than to postsynaptic DA receptors in the brain.

Dopamine D2 receptors in brain and anterior pituitary recognize agonist and antagonist actions of (-)-3-PPP

(-)-3-PPP, is a unique dopamine analogue, reported to have selective agonist actions at dopamine autoreceptors and antagonist actions at postsynaptic receptors. The interactions of D2 dopamine receptors with (-)-3-PPP in vitro were examined, using [3H]spiperone to label D2 receptors in brain regions containing both pre- and postsynaptic D2 receptors (caudate nucleus, corpus striatum) and a region containing nonsynaptic D2 receptors (anterior pituitary). In the absence of sodium ions, (-)-3-PPP detected D2 receptors in high- and low-affinity states in all regions examined, as is typical of dopamine agonists. That these two subpopulations of (-)-3-PPP-detected sites were dopaminergic in nature was assured by precluding [3H]spiperone binding to serotonergic receptors. In the presence of sodium ions, there was a significant increase in the affinity of some D2 receptors detected by (-)-3-PPP, and (-)-3-PPP in the presence of sodium was unable to discriminate between the two D2 affinity states in pituitary and striatum. The addition of guanine nucleotide led to (-)-3-PPP recognition of a single D2 binding site; the enhanced affinity of D2 receptors for (-)-3-PPP in the presence of sodium was retained in the presence of guanine nucleotide. These in vitro characteristics of (-)-3-PPP recognition of dopamine D2 receptor binding sites, when compared with dopamine and spiperone are seen to have clear features of both typical agonist and antagonist interactions with D2 receptors in both brain and pituitary.

Dopamine-receptor agonists: mechanisms underlying autoreceptor selectivity. I. Review of the evidence

The behavioural, biochemical, neuroendocrinological and electrophysiological actions of the enantiomers of the dopamine (DA) analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, are extensively reviewed. (+)-3-PPP acts in a fashion similar to classical direct-acting DA agonists, stimulating both DA autoreceptors and postsynaptic DA receptors, although in some situations the drug appears to exhibit partial agonist activity. (-)-3-PPP exerts a variety of actions in different pharmacological models. Either agonistic, antagonistic or both agonistic and antagonistic activity are observed depending on the anatomical location of the relevant DA receptors and the experimental conditions. The actions of transdihydrolisuride (TDHL) and the trans-fused 7-OH-1,2,3,4,4a,5,6,10b-octahydrobenzo(f)quinoline (HW 165) are also discussed. These agents possess a similar spectrum of action to (-)-3-PPP suggesting a new generation of DA agonists which exhibit variable intrinsic activity at different DA receptors. Finally, evidence is presented indicating that the 3-PPP enantiomers display selectivity for DA receptors.

Dopamine receptor-mediated hypothermia induced in rats by (+)-, but not by (-)-3-PPP

The novel dopaminergic agents (+)- and (-)-3-PPP were evaluated for their effects upon thermoregulation in rats maintained at room temperature (approximately 22 degrees C). Although approximately 30 times less potent than apomorphine, (+)-3-PPP induced a clearcut, dose-dependent and haloperidol/pimozide-reversible hypothermia. In contrast, the (-)-enantiomer per se lacked a significant effect upon rat body temperature. However, (-)-3-PPP clearly attenuated apomorphine-induced hypothermia. Simultaneous biochemical investigations confirmed the presence of central dopamine (DA) agonist and antagonist properties for (+)- and (-)-3-PPP, respectively, at the doses employed. The results are compared to the agonist and antagonist effects of the 3-PPP enantiomers in various other central DA receptors systems. Particular reference is made to the recent hypothesis by Carlsson (J. Neural Transm. 57 (1983) 309, relating agonist intrinsic activity to the DA receptor responsiveness state, in turn determined by the endogenous tone. Based on the findings with (+)- and (-)-3-PPP it is suggested that DA receptors mediating hypothermia in the rat may be more akin to 'normosensitive' postsynaptic than to highly 'agonist-responsive' autoreceptors.

Pharmacological properties of presynaptic dopamine receptor agonists

Selective presynaptic dopamine receptor agonists appear to offer promise as putative antipsychotic agents with a low risk of extrapyramidal side-effects, including tardive dyskinesia. However, no such agent with a reasonable degree of selectivity has yet reached the stage of clinical trial. In the present paper the particular pharmacological profile of presynaptic dopamine receptor (autoreceptor) agonists is described, and underlying mechanisms are discussed. Special attention is paid to the compound 3-(3-hydroxyphenyl-N-n-propylpiperidine(3-PPP), especially its levotatory enantiomer. This agent shows affinity for both pre- and postsynaptic dopamine receptors. Its intrinsic activity in different locations varies between virtually zero and 100%, leading to a mixture of agonist and antagonist properties. It is suggested that this variability depends on the adaptive properties of the dopamine receptor.

(+)- and (-)-3-PPP exhibit different intrinsic activity at striatal dopamine autoreceptors controlling dopamine synthesis

Both enantiomers of the dopamine analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP; 0.5-32 mg/kg s.c.) dose dependently reduced the increase in striatal dopamine (DA) synthesis rate produced by gamma-butyrolactone (GBL). Whereas (+)-3-PPP completely prevented the action of GBL, (-)-3-PPP was only partially effective. In addition, (-)-3-PPP partially antagonised the inhibitory action of apomorphine on the GBL-induced increase in DA synthesis rate. These findings suggest that (+)- and (-)-3-PPP act as full and partial agonists respectively, at striatal DA autoreceptors controlling DA synthesis.

Postsynaptic dopamine agonistic effects of 3-PPP enantiomers revealed by bilateral 6-hydroxy-dopamine lesions and by chronic reserpine treatment in rats

The motor effects of some DA autoreceptor agonists and apomorphine in rats with bilateral 6-hydroxydopamine lesions of the median forebrain bundle were studied. Whereas (-)-3-PPP, (+)-3-phenethyl-PP and EMD 23448 decreased motility in sham-operated controls, a pronounced hypermotility was induced in 6-OHDA-lesioned rats. 3-PPP enantiomers and apomorphine had similar potency as that found in test models for DA autoreceptor activity in normal rats, e.g. motility inhibition. The DA receptor involvement in the effect of (-)-3-PPP was confirmed by neuroleptic antagonism. (-)-3-PPP and EMD 23448 had similar intrinsic activity as apomorphine, whereas (+)-3-phenethyl-PP and (+)-3-PPP had lower maximal effect. However, the DA autoreceptor agonists differed from apomorphine: The development of postsynaptic supersensitivity to these drugs appeared 4-7 days after the lesion compared to 1-2 days for apomorphine and (+)-3-PPP. Furthermore, no active oral stereotypy was induced by the autoreceptor selective compounds in contrast to the effect observed after apomorphine and (+)-3-PPP. In a separate experiment using circling behaviour in unilaterally 6-OHDA-lesioned rats the different time-course of appearance of supersensitivity to (-)-3-PPP, (+)-3-PPP and apomorphine was confirmed. After chronic reserpine treatment a similar postsynaptic supersensitivity to (-)-3-PPP was observed with a development time between 4 and 7 days and with a similar intensity as that observed in 6-OHDA-lesioned rats. In contrast, after chronic neuroleptic treatment for 12 days, (-)-3-PPP was unable to induce hyperactivity 3-7 days after withdrawal. The results indicate that DA autoreceptor agonists are able to stimulate postsynaptic DA receptors in conditions without endogenous transmitter supply for at least 4-7 days, but not after chronic receptor blockade in a similar period. This should lead to consideration of DA autoreceptor agonists as potential antiparkinsonian drugs without stimulant effects on normosensitive postsynaptic DA receptors.

Effects of (-)3-PPP on acquisition and retention of a conditioned avoidance response in the rat

The administration of (-)3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) was found to partially, but significantly, suppress the acquisition (4-8 mg/kg IP) and performance (8-16 mg/kg IP) of a conditioned avoidance response (CAR) in male Sprague-Dawley rats. All statistically significant effects were observed within 2 h of injection. Furthermore, using a situation in which the CAR was dependent on a visual successive discrimination, it was shown that discriminative performance was unaffected, and that (-)3-PPP (12.5-25 mg/kg IP) but not (+)3-PPP, suppressed the CAR. When (-)3-PPP (6.25 mg/kg IP) was combined with haloperidol (0.1-0.4 mg/kg IP), additive effects on the CAR performance were observed. Considering these effects, and the doses of (-)3-PPP required to suppress the CAR performance, it is concluded that the effects obtained in the present experiments are primarily due to a blockade of postsynaptic DA receptors.

Dopamine receptor agonists: intrinsic activity vs. state of receptor

Based on observations with the novel dopamine-receptor agonist 3-(3-hydroxyphenyl-)-N-n-propylpiperidine, 3-PPP, especially its levorotatory enantiomer, it is proposed that the intrinsic activity of a receptor agonist depends in part on the responsiveness of the receptor; this in turn is determined by the degree of previous agonist occupancy on the receptor. A change in occupancy will induce a slow conformational change, influencing the responsiveness. This may constitute an important aspect of receptor adaptation and may help to explain otherwise puzzling phenomena, e.g. that compounds such as (-)-3-PPP or transdihydrolisuride can act as strong dopamine-receptor agonists in some locations and as antagonists in others. The observations discussed in the present paper may be interpreted to indicate that the dopamine receptors in different locations are, in fact, derived from a homogenous receptor population, though in a varying state of adaptation. Thus it may prove worth-while to reconsider the various subclassifications of DA receptors proposed so far.