Multiple receptors for brain dopamine in behavior regulation: concept of dopamine-E and dopamine-I receptors

From current knowledge, it is possible to substantiate the original concept of DAe and DAi receptors including the predicted correlation with the anatomical, histochemical, biochemical and functional features of the distinct neuronal structures, in which they occur; labelling them as neostriatal DAe receptors and mesolimbic DAi receptors appears to be justified. Available data warrant a revision of currently employed behavior and pharmacological tests. When revised in terms of the DAe-DAi concept, assessment of such tests reveals that agents such as (-)NPA, 6, 7ADTN and certain ergot alkaloids like lergotrile, lisuride and, to a less degree, CB-154 are weak DAe agonists and strong DAi antagonists (Table I). The discovery that mesolimbic α-like NE receptors which regulate the DA activity at the level of the mesolimbic DAi, but not neostriatal DAe, receptors show adaptational changes following priming or subacute treatments with apomorphine or haloperidol, opens new perspectives for understanding phenomena such as the development of hypersensitivity to apomorphine etc. Presynaptic DA receptors located within the DA synaps and DA receptors located at DA cell-bodies resemble closely the DAe receptors, although absence of linkage to the enzyme adenylyl cyclase hints at some distinction. It is possible that the distinct classes of DA receptors identified by behavior and pharmacological studies in mice correspond with the DAe and DAi receptors in snails, rats and cats. There is no evidence to suggest that DAe and DAi receptors are directly related to a) so-called DA1 and DA2 receptors which are coupled and uncoupled respectively to the enzyme adenylyl cyclase, or to b) any of the DA-specific binding sites identified with radiolabelled DA agonists or antagonists. Nonetheless, it cannot be excluded that DAe receptors may correspond with DA-specific binding sites identified with tritiated DA and/or haloperidol, and DAi receptors with a particular subclass of DA-specific binding sites identified within certain mesolimbic structures with radiolabelled spiperone. Thus, future work is still required to relate DAe and DAi receptors to particular, molecular entities within the brain.     

D-2 dopamine antagonist-like effects of SCH 23390 on A9 and A10 dopamine neurons

The effects of SCH 23390 on d-amphetamine-induced suppression of A9 and A10 DA neuronal firing were determined. SCH 23390 potently reversed d-amphetamine on both A9 and A10 DA neurons. Compared to haloperidol, SCH 23390 was 5 times more potent on A9 DA neurons and 20 times more potent on A10 DA neurons. However, the magnitude of the reversal effect was greater with haloperidol than SCH 23390. In addition, haloperidol produced a further increase in firing of both A9 and A10 DA neurons after SCH 23390 maximally increased firing. It was concluded that SCH 23390 has D-2 DA antagonist-like properties, possibly mediated via an interaction at D-1 DA receptors, which may be functionally linked with D-2 DA receptors. The marked potency of SCH 23390 in reversing d-amphetamine could be due to its combined antagonist effects at 5HT2 and D-1 DA receptor sites.     

Differential effects of chronic clorgyline and amfonelic acid on desensitization of striatal dopamine receptors

Chronic treatment of rats with the MAOI clorgyline significantly reduced the density (Bmax) of cortical beta-adrenergic receptors but did not alter either the Bmax or dissociation constant (Kd) of 3H-spiperone binding to striatal DA receptors. Clorgyline co-treatment also did not significantly affect either behavioral supersensitivity to apomorphine or the increase in 3H-spiperone binding induced by chronic haloperidol. In contrast, repeated treatment with the DA uptake inhibitor amfonelic acid elicited behavioral subsensitivity and reduced striatal 3H-spiperone binding. Furthermore, amfonelic acid co-treatment prevented haloperidol-induced behavioral and receptor binding changes. The possible relevance of these findings in relation to drug choice in clinical trials of receptor sensitivity modification are discussed.     

Contribution of D2 dopamine receptors of the rat dorsolateral caudate nucleus to immunomodulation

The analysis of the immune response changes in Wistar rats under activation or blockade of D2 DA receptors has shown that electrolytic lesion of the dorsolateral caudate nucleus characterized by a high density of D2 DA receptors resulted in a decrease of the immune response to SRBC. At the same time, in rats with similar lesion stimulation of the immune reactions caused by a selective D2 agonist guinpirol (1.0 mg/kg) did not develop completely. Administration of haloperidol (2.0 mg/kg), the immune-inhibitory effect of which is associated with increasing serotoninergic system activity, to rats with impaired dorso-lateral caudate nucleus did not produce more expressed immunosuppression. However, the level of the immune response in sham-operated rats receiving haloperidol was significantly lower than that of animal with the destructed nucleus caudatus. Considering that qunmpirol-induced immunostimulation is related to the selective activation of the DA-ergic brain system, it is concluded that D2 DA receptors of the nucleus caudatus are involved in the mechanisms of immunostimulation, although D2 DA receptors of other brain structures may also impact this process.     

Effects of dextromethorphan on dopamine dependent behaviours in rats

Dextromethorphan, a noncompetitive blocker of N-methyl-D- aspartate (NMDA) type of glutamate receptor, at 7.5-75 mg/kg, ip did not induce oral stereotypies or catalepsy and did not antagonize apomorphine stereotypy in rats. These results indicate that dextromethorphan at 7.5-75 mg/kg does not stimulate or block postsynaptic striatal D2 and D1 dopamine (DA) receptors. Pretreatment with 15 and 30 mg/kg dextromethorphan potentiated dexamphetamine stereotypy and antagonised haloperidol catalepsy. Pretreatment with 45, 60 and 75 mg/kg dextromethorphan, which release 5-hydroxytryptamine (5-HT), however, antagonised dexamphetamine stereotypy and potentiated haloperidol catalepsy. Apomorphine stereotypy was not potentiated or antagonised by pretreatment with 7.5-75 mg/kg dextromethorphan. This respectively indicates that at 7.5-75 mg/kg dextromethorphan does not exert facilitatory or inhibitory effect at or beyond the postsynaptic striatal D2 and D1 DA receptors. The results are explained on the basis of dextromethorphan (15-75 mg/kg)-induced blockade of NMDA receptors in striatum and substantia nigra pars compacta. Dextromethorphan at 15 and 30 mg/kg, by blocking NMDA receptors, activates nigrostriatal dopaminergic neurons and thereby potentiates dexampetamine stereotypy and antagonizes haloperidol catalepsy. Dextromethorphan at 45, 60 and 75 mg/kg, by blocking NMDA receptors, releases 5-HT and through the released 5-HT exerts an inhibitory influence on the nigrostriatal dopaminergic neurons with resultant antagonism of dexampetamine stereotypy and potentiation of haloperidol catalepsy.     

Neurotensin interacts with dopaminergic neurons in rat brain

Neurotensin (NT) injected intracerebroventricularly in rat increases dopamine (DA) turnover in the corpus striatum and nucleus accumbens. Significant increases in 3,4-dihydroxyphenylacetic acid (DOPAC) levels occurred within 15 minutes after injection with peak levels at 60 minutes. The effect on NT on DOPAC and homovanillic acid (HVA) accumulation was dose-dependent at 3–100 μg. NT, like haloperidol, stimulated 3,4-dihydroxyphenylalanine (DOPA) accumulation in striatal neurons, in the presence of DOPA decarboxylase inhibitor, after injection of gamma-butyrolactone (GBL). NT had a similar stimulatory effect on DOPA levels in the accumbens while haloperidol (0.25 mg·kg^?1) had no significant effect in this brain region. NT did not block the inhibitory effect of apomorphine on DOPA accumulation in both the striatum and accumbens, while haloperidol inhibited apomorphine effect in both regions. NT also failed to displace ³H-spiperone from DA receptors and the presence of NT in the binding assay did not alter the ability of DA to displace ³H-spiperone in either brain region. These experiments demonstrate that NT increases DA turnover in both the nigrostriatal and mesolimbic pathways.     

Dependence of the effects of tripeptide MIF and lithium chloride on the degree of supersensitivity of dopamine receptors of the brain

The desensitizing effects of MIF and lithium in respect to supersensitive striatal DA receptors in rats with unilateral lesion of the nigro-striatal pathway by 6-hydroxydopamine were revealed. Two groups of 6-hydroxydopamine-denervated rats were selected by their qualitative responsiveness to apomorphine-induced rotational behavior. It was found, that MIF and lithium (subchronic administration) did not modify behavioral supersensitivity in the highly sensitive group which showed two-peak rotational pattern in response to 0.05 mg/kg apomorphine, which was converted into a single-peak rotational pattern by haloperidol. On the contrary, administration of MIF and lithium for 21 days inhibited the apomorphine-induced rotations to 54 and 65% respectively in the less supersensitive group, which showed a single peak rotational pattern to apomorphine. Moreover, haloperidol showed the high antagonistic potency in these animals. These results suggest, that MIF and lithium might not exert desensitizing effects in the presence of high degree of supersensitivity of the striatal DA receptors, which is probably involved, for example, in the phenomenon of persistent tardive dyskinesia.     

Identification of a butyrophenone analog as a potential atypical antipsychotic agent: 4-[4-(4-chlorophenyl)-1,4-diazepan-1-yl]-1-(4-fluorophenyl)butan-1-one

The synthesis and exploration of novel butyrophenones have led to the identification of a diazepane analogue of haloperidol, 4-[4-(4-chlorophenyl)-1,4-diazepan-1-yl]-1-(4-fluorophenyl)butan-1-one (compound 13) with an interesting multireceptor binding profile. Compound 13 was evaluated for its binding affinities at DA subtype receptors, 5HT subtype receptors, H-1, M-1 receptors and at NET, DAT, and SERT transporters. At each of these receptors, compound 13 was equipotent or better than several of the standards currently in use. In in vivo mouse and rat models to evaluate its efficacy and propensity to elicit catalepsy and hence EPS in humans, compound 13 showed similar efficacy as clozapine and did not produce catalepsy at five times its ED(50) value.     

The neurochemical mechanisms of the formation and consolidation of haloperidol-induced catalepsy

In rat brain cortex, haloperidol initiates the long-term potentiation of K(+)-induced Ca(2+)-dependent noradrenaline (NA) and dopamine (DA) secretion in vitro and in vivo. In both cases, the long-term potentiation is caused by the long-term increase in catecholamine content in the NA and DA terminals, as it has been shown in cortical tangential slices. Acute intraperitoneal haloperidol injection (2.5 mg/kg) evokes catalepsy and increases the content of NA and DA in the brain structures with localization of catecholamine receptors on terminals. This increase appears to be caused, predominantly, by modification of the terminal DA receptors, since only a trend to catecholamine increase is observed in the brain structures with a mixed type of NA and DA receptor localization (on somata and terminals). It is suggested that the long-term and diffuse action of haloperidol after its acute administration consists in the anxiogenic reaction and consolidation of catalepsy without an additional procedure of training and in the absence of unconditioned stimulus.     

Dopamine receptor occupancy in vivo: measurement using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)

N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) inactivates a variety of monoamine neurotransmitter receptors. In this report, protection against EEDQ-induced inactivation of D-1 and D-2 DA receptors by DA antagonists and agonists was used to obtain a measure of occupancy of these receptors in vivo by such drugs. Rats were pretreated with drugs and then given EEDQ (10 mg/kg, i.p.). Twenty-four hours after the EEDQ injections, the animals were decapitated and the number of receptors remaining was measured using conventional receptor binding assays. The D-1 antagonist SCH 23390 potently protected D-1 sites from EEDQ-induced inactivation in a dose-dependent manner. Similarly, NO-756, another D-1 antagonist, selectively protected D-1 sites from inactivation. Conversely, haloperidol, a relatively selective D-2 antagonist, protected D-2 sites from inactivation. Likewise, a number of antipsychotic DA antagonists also protected D-2 sites from inactivation. Clozapine, fluperlapine, and (+) butaclamol were effective at protecting both D-1 sites and D-2 sites. In addition, the D-1 agonist SKF 38393 protected D-1 sites from EEDQ-induced inactivation, whereas the D-2 agonist quinpirole protected D-2 sites. (-) Apomorphine, a mixed D-1/D-2 agonist, protected both sites. Thus, this type of method provides a simple means of evaluating the occupation of DA receptors by DA antagonists and agonists in vivo.     

Actions of butyrophenones and other antipsychotic agents at NMDA receptors: relationship with clinical effects and structural considerations

Haloperidol inhibits NMDA receptors with higher affinity for NMDA receptors composed of NR1/2B compared with NR1/2A. To assess whether the clinical effects of haloperidol and other antipsychotic agents are mediated through this site on NMDA receptors and to examine structure activity relationships at this site, we examined the ability of a variety of drugs with neuroleptic actions to inhibit NMDA receptor function. Many antipsychotic agents inhibit 125I-MK 801 binding to the NMDA receptor with IC50 values in the micromolar range. The rank order of potency for inhibition of binding to adult rat forebrain was trifluperidol (TFP) > clozapine = fluphenazine = reduced haloperidol = spiperone = trifluoperazine = butaclamol > pimozide = risperidone = sulpiride. These findings match the molecular biological specificity of the agents, with trifluperidol having a marked preference for NR1/2B (epsilon2) receptors. Mutations at epsilon2E201, which alter the effects of haloperidol, also decrease the affinity of TFP but not other modulators, showing that the effect of TFP but not other modulators is mediated by this residue of the NMDA receptor. The present results demonstrate that while TFP acts on NMDA receptors in a manner similar to haloperidol, other antipsychotic agents do not share the specific pharmacological properties of this action, suggesting that their clinical mechanism is not mediated by this receptor.     

Renal effects of dopamine and ANP in high volume expanded rats

Both dopamine (DA) and atrial natriuretic peptide (ANP) have been postulated to exert similar effects on the kidney, participating in the regulation of body fluid and sodium homeostasis. In the present study, experiments were performed in anesthetized and isotonic sodium chloride volume expanded rats. After acute volume expansion at 15 % of body weight during 30 min, glomerular filtration rate, urine output, sodium excretion, fractional sodium excretion, proximal and distal sodium excretion and blood pressure were measured. In additional groups we administered ANP or haloperidol or the combination of both to volume expanded animals. Blockade of DA receptors with haloperidol, attenuated diuretic and natriuretic responses to volume load. Proximal sodium excretion was not modified by haloperidol in all experimental groups of rats. Reduction in distal tubular excretion was induced by haloperidol in saline infusion expanded rat but not in ANP treated expanded animals. In conclusion, when exaggerated volume expansion is provoked, both DA and ANP exert renal tubular events, but ANP have a major central role in the regulation of renal sodium handling.     

Role of striatal serotonin2A and serotonin2C receptor subtypes in the control of in vivo dopamine outflow in the rat striatum

This study investigated, using in vivo microdialysis in the striatum of freely moving rats, the role of striatal serotonin2A (5-HT2A) and 5-HT2C receptor subtypes in the modulation of dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOPAC) outflow, both in basal conditions and under activation induced by subcutaneous administration of 0.01 mg/kg haloperidol. The different 5-HT2 agents used were applied intrastriatally at a 1 microM concentration through the microdialysis probe. Basal DA efflux was enhanced (27%) by the 5-HT2A/2B/2C agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI) and reduced (-30%) by the 5-HT2B/2C antagonist SB 206553. It was unaffected by infusion of the 5-HT2A antagonist SR 46349B. The effect of DOI was abolished by SB 206553 but not modified by SR 46349B. Haloperidol-stimulated DA efflux (65-70%) was reduced by both SR 46349B (-32%) and the 5-HT2A/2B/2C antagonist ritanserin (-30%) but not affected by SB 206553. Conversely, the effect of haloperidol was potentiated (22%) when DOI was coperfused with SB 206553. Also, haloperidol-stimulated DOPAC outflow (40-45%) was reduced (-20%) by SR 46349B and potentiated (25%) by the combination of SB 206553 with DOI. These results indicate that striatal 5-HT2A receptors, probably through activation of DA synthesis, positively modulate DA outflow only under activated conditions. In contrast, striatal 5-HT2C receptors exert a facilitatory control on basal DA efflux, which appears to be both tonic and phasic.     

A behavioural and biochemical comparison of dopamine receptor blockade produced by haloperidol with that produced by substituted benzamide drugs

Haloperidol inhibited dopamine (DA) mediated behaviours and induced pronounced catalepsy in rodents. Metoclopramide, sulpiride, sultopride, tiapride and clebopride, in general, also inhibited these behaviours but only clebopride induced marked catalepsy. Haloperidol displaced ³H-haloperidol and ³H-spiperone from striatal binding sites and inhibited DA stimulated cyclase from striatal and mesolimbic regions. In general, substituted benzamide drugs displaced labelled ligands, but did not inhibit adenylate cyclase. Elevations of striatal HVA produced by haloperidol and sulpiride, but not other benzamide drugs, were partially reversed by atropine. Hypophysectomy did not prevent the elevation of forebrain HVA produced by sulpiride and metoclopramide. Substituted benzamide drugs appear to act on cerebral DA receptors that are independent of DA-sensitive adenylate cyclase and are not balance by a cholinergic input.     

Evidence for a coupling between dopaminergic receptors and phospholipid methylation in mouse B lymphocytes

Dopamine stimulated and dopaminergic antagonist·inhibited the enzymic synthesis of phosphatidyl N-monomethyl, N,N-dimethyl ethanolamine and phosphatidylcholine in mouse B lymphocytes in the presence of L-methionine. This effect was dose-dependent, stereospecific and the stimulation by dopamine was inhibited by very low doses of haloperidol from 10^?12 M to 10^?9 M. The stimulation of phospholipid methylation provoked by dopamine was increased by GTP. At higher doses DA inhibited and haloperidol stimulated phospholipase A₂. DA did not change the CDP choline pathway. The incubation of mouse B lymphocytes with L-methionine unmasks cryptic dopaminergic receptors. This effect is dose-dependent and inhibited by SIBA an inhibitor of phospholipid methylation. In a similar manner the efflux of Ca²⁺ which is sensitive to the change in membrane viscosity was increased by DA. These findings indicate that the dopaminergic receptors of the mouse B lymphocytes are coupled with phospholipid methylation.     

Modulation of In Vivo Dopamine Release by D2 but Not D1 Receptor Agonists and Antagonists

The capacity of D1 and D2 agonists and antagonists to regulate the in vivo release and metabolism of dopamine (DA) in mesolimbic and nigrostriatal DA neurons of the mouse was determined using gas chromatographic and mass fragmentographic (GC-MF) analysis. DA release was inferred from levels of 3-methoxytyramine (3-MT) and DA metabolism was inferred from levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). DA release was increased by the D2 antagonists haloperidol and metoclopramide but not by the D1 antagonists SCH 23390 and SKF 83566. DA metabolism was increased by each of the four antagonists but to a greater extent with the D2 antagonists. The D2 agonists CGS 15855A and LY 171555 decreased DA release whereas the D1 agonist SKF 38393, at relatively high doses, only slightly affected DA release. Each of the three agonists decreased DA metabolism but again metabolism was more affected by the D2-selective drugs. The in vivo release of DA from mesolimbic and neostriatal DA neurons appears to be modulated by D2 but not by D1 receptors, whereas both receptor types can modulate DA metabolism.     

Multi-receptor drug design: Haloperidol as a scaffold for the design and synthesis of atypical antipsychotic agents

Using haloperidol as a scaffold, new agents were designed to investigate the structural contributions of various groups to binding at CNS receptors associated with atypical antipsychotic pharmacology. It is clear that each pharmacophoric group, the butyrophenone, the piperidine and the 4-chlorophenyl moieties contributes to changes in binding to the receptors of interest. This strategy has resulted in the identification of several new agents, compounds 16, 18, 19, 23, 24 and 25, with binding profiles which satisfy our stated criteria for agents to act as potential atypical antipsychotics. This research demonstrates that haloperidol can serve as a useful lead in the identification and design of new agents that target multiple receptors associated with antipsychotic pharmacology.     

Inhibitory effect of ceruletide on haloperidol-induced catalepsy in rats

Ceruletide (CLT: 160 micrograms/kg, SC) produced a relatively long-lasting inhibition of haloperidol (HPD: 2 mg/kg, PO) catalepsy in rats. Neither bilateral vagotomies nor hypophysectomy abolished the anticataleptic effect of CLT. However, (-)-L-364,718 and proglumide blocked the effect of CLT. CLT (160 micrograms/kg) significantly inhibited HPD (2 mg/kg)-induced increase in dopamine (DA) release from the rat striatum. This effect of CLT was also antagonized by proglumide. These results suggest that CLT (160 micrograms/kg) primarily acts on cholecystokinin-A receptor in the brain, exerts some modulatory influence on HPD binding to striatal DA receptors via unknown neural pathways and, consequently, inhibits HPD catalepsy.     

Relationship Between Dopamine Receptor Occupation by Spiperone and Acetylcholine Levels in the Rat Striatum After Long-Term Haloperidol Treatment Depends on Dopamine Innervation

The effect of chronic neuroleptic treatment on the relationship between the blockade of dopamine (DA) receptors by the neuroleptic drug spiperone and the decline in acetylcholine (ACh) levels was determined in the rat striatum in vivo. In rats, a unilateral lesion of the nigrostriatal pathway was produced with 6-hydroxydopamine. The rats were treated for 6 weeks with haloperidol (twice a day at 1 mg kg-1). Partial and complete receptor occupation was determined with radioactive spiperone (a D2 antagonist), given in various doses of different specific activity 2 h before death. ACh, choline, and radioactivity contents were measured in the same striatum. Following long-term haloperidol treatment, an increase in the maximal number of binding sites for spiperone was found. Virtually identical negative (linear) correlations between striatal ACh content and the number of receptors occupied by spiperone were found in saline- or subchronic haloperidol-treated rats when DA innervation was intact. The slope of the line describing the decrease in ACh content per occupied receptor, however, was much lower in haloperidol-treated rats than in saline-treated animals. After lesioning of the dopaminergic pathway, there was no longer a correlation between the receptor occupation and ACh levels in the striatum. These results show that receptor occupation by a neuroleptic correlates highly with function only when dopaminergic innervation is intact. Also, it appears that there is no fixed number of striatal ACh molecules per DA receptor, and, finally, that in vivo receptor detection methods distinguish differences in receptor density (as do in vitro techniques).