Effects of stimulation of putative dopamine autoreceptors on electroencephalographic power spectrum in comparison with effects produced by blockade of postsynaptic dopamine receptors in rats

Alterations in cortical EEG activity in male rats produced by putative agonists at dopamine (DA) autoreceptors and by antagonists at postsynaptic DA receptors were compared in order to study, whether an impairment in dopaminergic neurotransmission via two different mechanisms might result in similar or different effects. Simultaneously to the EEG recordings, gross behaviour was observed. Putative agonists at DA autoreceptors (apomorphine 0.05 mg/kg, quinpirole 0.05 mg/kg, or talipexole 0.02 mg/kg s.c.) produced increases in the power iin all of the frequency bands, except beta-2, with the most pronounced increase in the delta band. These EEG alterations were accompanied by hypokinesia, ptosis and yawning. In contrast, antagonists at DA receptors (haloperidol 0.1 mg/kg i.p., D₂ blocker) or SCH 23390 (0.2 mg/kg i.p., D₁ blocker) led to little increases in the delta band, but more pronounced increases in the alpha-2 band. Behavioural signs were hypokinesia, but little ptosis and yawning. The combination of both blockers produced, in addition, strong increases in the delta band and behavioural signs of ptosis and yawning. These results suggest that activation of putative dopamine autoreceptors produced EEG patterns and behavioural patterns different from those produced by blockade of either D₁ or D₂ postsynaptic dopamine receptors. In contrast, the effects following a stimulation of putative DA autoreceptors, which are expected to decrease the release of the agonist and its action at postsynaptic D₁ and D₂ receptors, were very similar to those found after a combined blockade of both types of postsynaptic dopamine receptors.     

Chronic phencyclidine treatment decreases phencyclidine and dopamine receptors in rat brain

Chronic phencyclidine treatment (10 mg/kg/day, SC for 14 days) significantly decreased the number of [³H]phencyclidine and [³H]spiperone binding sites in rat brain. [³H]Dihydromorphine binding was not affected by the same treatment. An acute treatment with phencyclidine (10 mg/kg, SC) did not modify any of the binding sites under study. These results suggest that a chronic phencyclidine treatment induces a down-regulation of phencyclidine and dopamine receptors without affecting opiate receptors. These reductions in the number of phencyclidine and dopamine binding sites might be related to the development of tolerance and/or dependence to phencyclidine.     

On the involvement of a tonic dopamine D2-autoinhibition in the regulation of pulse-to-pulse-evoked dopamine release in the rat striatum in vivo

The dopamine overflow evoked by trains of electrical stimulation pulses applied to the ascending dopaminergic pathway was measured with continuous amperometry in the striatum of anesthetised rats. As previously observed in in vitro studies, a pulse by pulse analysis showed a fall in dopamine overflow evoked by pulses 2 to 6, compared to the response evoked by pulse 1. However, in contrast with in vitro findings, the present in vivo data showed that the dopamine receptor antagonist haloperidol i) completely reverses the fall in dopamine overflow between pulse 1 and subsequent pulses, ii) enhances the dopamine overflow elicited by pulse 1. These results suggest that in vivo, both basal and pulse-evoked dopamine overflow results in stimulation of dopamine D2-type autoreceptors and therefore in regulation of dopamine release. Received: 2 December 1996 / Accepted: 28 February 1997     

Multiple postsynaptic dopamine receptors and behavioral manifestation

Previously we demonstrated the existence of non- and positive-cooperative dopamine receptors in rat striatum in [3H] apomorphine binding experiments. Non-cooperative sites were sensitive to the inhibition of sulpiride while cooperative sites were not. In the present study results dealing with the involvement of those two types of postsynaptic dopamine receptors in different behavior manifestations is shown employing lisuride hydrogen maleate (LHM). LHM elicited contralateral turnings in 6-hydroxydopamine lesioned rats unilaterally in the striatum whereas it caused ipsilateral turnings in kainic acid lesioned rats as was observed following the administration of apomorphine. Furthermore, the effect of LHM on rotating behavior was abolished by the pretreatment with sulpiride. On the other hand, LHM inhibited apomorphine induced stereotyped behavior whereas the sulpiride failed to block it. These results suggested the dual action of LHM on multiple postsynaptic dopamine receptors. The results also indicated that non-cooperative postsynaptic dopamine receptors are involved in rotating behavior while cooperative receptors participate in the elicitation of stereotypy.     

Effects of intermittent haloperidol treatment on dopamine receptor sensitivity in guinea pigs

Intermittent haloperidol treatment in guinea pigs caused an increase in the behavioral response to apomorphine and an increase in ³H-spiroperidol binding in the striatum to the same degree as continual haloperidol treatment. These results do not support the claim that drug holidays can reduce the incidence of tardive dyskinesia.     

Regulation of CART mRNA in the rat nucleus accumbens via D3 dopamine receptors

A variety of studies indicate that CART in the nucleus accumbens (NAcc) is involved in the action of psychostimulants. In order to understand in more detail if and how dopamine is involved in the regulation of CART mRNA in the NAcc, the present studies of individual receptors were performed. The D1 agonist, dihydrexidine, and the D1 antagonist, SCH23,390, were administered separately and in combination to adult male rats; however, no changes were found in CART mRNA as measured by in situ hybridization. The D2/3 agonist, quinpirole, was administered either separately or in combination with the D2 selective antagonist, L741,626, or the D3 selective antagonist, GR103,691. Quinpirole produced a decrease in CART mRNA of up to 43%. This effect was blocked by pretreatment with the D3 antagonist GR103, 691, but not by the D2 antagonist, L741,626. CART peptide levels showed a similar decrement after acute quinpirole. CART mRNA levels in the NAcc of D3 mutant mice were found to be higher than that in wild-type animals, but treating the mutants with quinpirole failed to produce a decrease in CART expression like that observed in wild-type rodents. These findings demonstrate that CART is regulated by dopamine in the NAcc, at least partly by D3 dopamine receptors.     

Chronic haloperidol administration increases the density of D2 dopamine receptors in the medial prefrontal cortex of the rat

Rats received haloperidol (1.3–1.5 mg/kg/day) via their drinking water for 21 weeks. At the end of this period the density of D2 dopamine receptors and their affinity for [³H]-spiperone were measured in the striatum and medial prefrontal cortex. The chronic haloperidol treatment increased the density of D2 receptors in the striatum by 70% and in the medial prefrontal cortex by 50%. The chronic haloperidol did not significantly alter the apparent affinity of D2 receptors for [³H]-spiperone in either structure. These results indicate that the density of D2 receptors in the medial prefrontal cortex is influenced by chronic exposure to haloperidol in a manner that is very similar to the well-documented increase that occurs in the striatum.     

Apomorphine-induced behavioural sensitization in rats: individual differences, role of dopamine and NMDA receptors

Apomorphine-induced behavioural sensitization was studied in male Wistar rats. The acute administration of apomorphine (0.5 mg/kg s.c.), a dopamine agonist, did not affect the locomotor activity of rats, but it caused stereotyped behaviour characterized by repeated gnawing, licking and sniffing. A significant increase in the locomotor activity became evident after repeated treatments with apomorphine (0.5 mg/kg twice daily for 14 days). However, there were marked individual differences in the sensitization of rats to apomorphine. One third of animals did not react with increased locomotor activity even after the 2-week administration of apomorphine, whereas the other one third needed only a few injections to display increased behavioural response to apomorphine. The behavioural response of the remaining one third of rats was between weak and strong responders. Simultaneously, the stereotyped behaviour occurred earlier and its intensity tended to be lower after repeated treatment with apomorphine. Nevertheless, the established changes of stereotyped behaviour did not correlate with the increase of locomotor activity. The administration of amphetamine (2.5 mg/kg, s.c.), an indirect dopamine agonist, but not a non-competitive NMDA antagonist dizocilpine (0.25 mg/kg i.p.), tended to cause a similar response profile with apomorphine in sensitized rats. The ED₅₀ values of the dopamine antagonists blocking apomorphine-induced increase in the locomotor activity were the following: 0.09 mg/kg for raclopride (dopamine D₂ antagonist), 0.023 mg/kg for SCH 23390 (dopamine D₁ antagonist), 6.42 mg/kg for clozapine (dopamine D₄ antagonist). This supports the involvement of D₁ and D₂ receptors in the expression of apomorphine-induced behavioural sensitization. The concomitant administration of dizocilpine (0.5 mg/kg), SCH 23390 (0.05 mg/kg), raclopride (0.1 mg/kg) and clozapine (20 mg/kg) with apomorphine (0.5 mg/kg twice daily for 2 weeks) antagonized the development of behavioural sensitization to apomorphine. Accordingly, at least three different molecular targets, namely dopamine D₁ and D₂, and NMDA receptors, are involved in the development of apomorphine-induced behavioural sensitization.     

Kappa opioid mediated locomotor activity in the preweanling rat: role of pre- and postsynaptic dopamine receptors

 Treatment with a non-selective DA receptor agonist (i.e., NPA) has previously been shown to attenuate the kappa opioid mediated locomotor activity of preweanling rats. The purpose of the present study was to determine whether stimulation of D₁-like or D₂-like receptors is responsible for this behavioral effect and whether the critical DA receptors are located pre- or postsynaptically. To assess these questions, 17-day-old rats were injected with saline, the D₂/D₃ agonist quinpirole (0.1, 0.3, or 1.0 mg/kg, IP), or the D₁ agonist SKF 38393 (7.5, 15, or 30 mg/kg, IP), 20 min after receiving the kappa opioid agonist U-50,488 (5 mg/kg, SC) or saline. Results showed that the locomotor activating effects of U-50,488 were blocked by the D₂/D₃, but not the D₁, receptor agonist. To dissociate the effects of DA autoreceptors and postsynaptic receptors, 17-day-old rats were given α-methyl-dl-p-tyrosine (AMPT reduces endogenous DA stores) prior to U-50,488 or amphetamine (1.5 mg/kg, SC) treatment. Interestingly, AMPT (which reduced DA levels by more than 80%) fully attenuated amphetamine-induced locomotor activity, while having little effect on U-50,488-induced locomotion. In addition, quinpirole blocked the locomotor activating effects of U-50,488 in rats acutely depleted of DA. When considered together, these results indicate that kappa opioid stimulation enhances locomotor activity regardless of presynaptic DA levels. Similarly, quinpirole appears to attenuate U-50,488-induced locomotor activity by stimulating postsynaptic D₂-like receptors, since the D₂/D₃ agonist inhibited kappa opioid mediated behavior independent of endogenous DA levels. Received: 31 December 1996 / Final version: 27 March 1997     

Zuclopenthixol, a combined dopamine D1/D2 antagonist, versus haloperidol, a dopamine D2 antagonist, in tardive dyskinesia.

Animal data suggest that a D₁ antagonistic component in neuroleptic drugs counteracts development of dopamine supersensitivity and of tolerance to cataleptic effect. This has led to the hypothesis that neuroleptics with D₁ antagonistic activity should cause a better suppression of tardive dyskinesia (TD) and less rebound aggravation after withdrawal than pure D₂ antagonists. In this study the effect of zuclopenthixol (mixed D₁/D₂ antagonist) and haloperidol (D₂ antagonist) was evaluated in chronic psychotic patients with TD. Fifteen patients completed a randomized crossover study with blind evaluation of TD and parkinsonism. The test medications, haloperidol and zuclopenthixol, caused a significant suppression of TD and a significant increase of parkinsonism. No significant differences between haloperidol and zuclopenthixol were observed. No TD aggravation was seen. The lack of differences between the mixed D₁/D₂ antagonist and a D₂ antagonist suggest that tolerance and DA supersensitivity play no or a minor role for development of TD.     

Preferential involvement of D3 versus D2 dopamine receptors in the effects of dopamine receptor ligands on oral ethanol self-administration in rats

There is evidence that dopamine transmission is involved in reinforcement processes and the present study investigated the relative involvement of D₃ versus D₂ dopamine receptors in the effects of dopamine ligands on the reinforcing action of ethanol. Rats were trained to self-administer ethanol (10% v/v) orally in a free-choice two-lever operant task using a saccharin-fading procedure. When preference in responding for ethanol over water had developed the rats were tested with several dopamine agonists and antagonists. Pretreatment with the non-selective dopamine agonist, apomorphine (0.01–0.1 mg/kg), the preferential D₂ agonist, bromocriptine (1–10 mg/kg) and the selective D₃ agonists, 7-OH-DPAT (0.003–0.1 mg/kg), PD 128907 (0.1–3 mg/kg), (+)3PPP (0.3–3 mg/kg), quinelorane (0.0001–0.003 mg/kg) and quinpirole (0.003–0.03 mg/kg), resulted in dose-dependent decreases in responding for ethanol. The relative potencies of the dopamine agonists to decrease ethanol self-administration were highly correlated with their published potencies to produce in vitro functional D₃ but not D₂ responses. Active doses could be considered as those selectively stimulating receptors involved in the control of dopamine release, suggesting that reduction of dopamine transmission was associated with a decrease in ethanol-reinforced responding. This conclusion was further supported by the finding that pretreatment with the D₂/D₃ dopamine antagonists, haloperidol (0.1–0.4 mg/kg) and tiapride (10–60 mg/ kg), decreased responding for ethanol at doses which have been shown previously to block dopamine transmission. Received: 25 January 1998/Final version: 24 April 1998     

Pharmacological and functional characterisation of dopamine D4 receptors in the rat retina

In the retina, activation of dopamine receptors, particularly the D2-like family (D2, D3, D4 receptor subtypes), with quinpirole suppresses the light sensitive cAMP pool and inhibits melatonin synthesis in photoreceptor cells. We have characterised rat retinal D4 receptors using the D4 selective radioligand [¹²⁵I] L-750667 which bound specifically and saturably to rat retinal membranes with high affinity (K_d 0.06±0.02 nM) and exhibited a D4 receptor pharmacology. Comparison of the binding kinetics of [¹²⁵I] L-750667 and [³H] spiperone revealed B_max values of 134±27 fmol/mg and 219±47 fmol/mg respectively, indicating that the dopamine D4 receptor is a major component of D2-like dopamine receptors in the rat retina. Modulation of retinal cAMP levels by quinpirole was used to evaluate the functional relevance of rat retinal dopamine D4 receptors. Quinpirole (0.03–3 μM) produced a dose-related decrease of the light sensitive cAMP pool which was reversed by haloperidol, clozapine and the D4 selective antagonist, L-745870 with a rank order of potency suggesting that the quinpirole effect is due to activation of the dopamine D4 receptors. The D2 selective ligand L-741626 had no effect on the quinpirole response confirming that the D4 receptor is the major receptor subtype mediating dopamine induced suppression of adenylate cyclase in the retina.     

Iontophoretic evidence for subsensitivity of postsynaptic dopamine receptors following long-term amphetamine administration

Neuronal activity was recorded from the anteromedial neostriatum of rats pretreated twice daily for 6 consecutive days with saline or 5.0 mg/kg d-amphetamine. Glutamate was applied in both groups of animals to increase spontaneous firing rates. Iontophoretic application of increasing currents of DA (20-120 nA) produced a progressive inhibition of unit activity in control animals that was significantly reduced in rats pretreated with amphetamine. These results support the view that long-term amphetamine treatment reduces the sensitivity of postsynaptic DA receptors in the neostriatum.     

Effects of chronic alcohol exposure on dopamine uptake in rat nucleus accumbens and caudate putamen

Rationale

Existing data strongly suggest that alcohol affects dopamine (DA) neurotransmission in the brain. However, many questions remain about the effects of alcohol on the delicate equilibrium between such neurochemical processes as DA release and uptake. Dysregulation of these processes in the mesolimbic and nigrostriatal systems after chronic alcohol ingestion could be a neuroadaptation contributing to dependence.

Objectives

In the present study, we have employed an alcohol vapor inhalation model to characterize the effects of chronic alcohol exposure on DA dynamics in rat nucleus accumbens (NAc) and caudate putamen (CP) using fast-scan cyclic voltammetry (FSCV) in brain slices. This method provides a unique view of real-time, spatially resolved changes in DA concentration.

Results

We found that chronic alcohol exposure enhanced DA uptake rates in rat NAc and CP. These changes would have the effect of down-regulating extracellular DA levels, presumably a compensatory effect related to increased DA release by repeated alcohol exposure. The sensitivity of terminal release-regulating DA autoreceptors was not different in alcohol-exposed rats compared with alcohol-naïve animals.

Conclusions

The DA uptake changes after chronic alcohol exposure documented here using FSCV may be associated with a compensatory response of the DA system aimed at decreasing DA signaling. Alterations in autoreceptor function may require relatively long lasting alcohol exposure.

     

Catecholamine synthesis in rat brain after axotomy: Interaction between apomorphine and haloperidol

Summary Axotomy of the ascending monoaminergic fibers by means of a complete cerebral hemitransection stimulated the formation of dopa during 30 min after inhibition of the aromatic amino acid decarboxylase with 3-hydroxybenzylhydrazine HCl, 100 mg/kg i.p., in c. striatum and the dopamine-rich part of the limbic system. Apomorphine, 0.5 mg/kg i.p., antagonized the accumulation of dopa not only on the intact but also on the lesioned side. Haloperidol, 2 mg/kg i.p., stimulated dopa accumulation on the intact side but could not further stimulate the increase in dopa caused by transection. When both drugs were given together, the inhibitory effect of apomorphine was fully counteracted by haloperidol on both sides. In the predominantly noradrenaline-innervated occipito-temporal cortex dopa formation was slightly higher on the lesioned than on the intact side and was not markedly influenced by apomorphine.In the rest of the hemispheres the apomorphine-induced decrease in dopa formation was more pronounced on the intact than on the lesioned side and was fully antagonized by haloperidol.The dopamine concentration was slightly higher in the lesioned c. striatum as compared to the intact side irrespective of the drugs administered. In c. striatum and the limbic system haloperidol caused a decrease in dopamine on the intact side which was not antagonized by additional treatment with apomorphine.Hemitransection caused a decrease in noradrenaline especially in the hemisphere portion. Neither apomorphine nor haloperidol or both drugs in combination changed the latter effect.In general, the tyrosine concentration tended to be higher on the lesioned than on the intact side in all brain structures investigated.The data support the view that a local receptor-mediated feedback mechanism exists which is controlling dopamine synthesis even in the absence of impulse flow.     

Chronic treatment with D1 and D2 dopamine receptor agonists: combined treatments interact to differentially affect brain levels of monoamines

Summary Recent evidence of functional interactions between D1 and D2 dopamine receptor subtypes has led to the concept that many of the behavioural effects of dopamine agonists occur only with activation of both receptor subtypes. Thus, combined treatment with dopamine agonists selective for each of the D1 and D2 receptors may be an effective therapy for Parkinson's disease, chiefly characterized by loss of central dopamine-containing neurons. In addition, recent hypotheses of the possible pathogenesis of this disorder have suggested that metabolism of dopamine by monoamine oxidase in the presynaptic terminal may contribute to the loss of dopaminergic cells, through the production of reactive by-products. Therefore, the effects of chronic (15 day) treatment of rats with different doses of (+)-4-propyl-9-hydroxynaphthoxazine (PHNO, a D2 receptor agonist), SKF 38393 (a D1 receptor partial agonist) or combinations of both drugs on levels of brain monoamines and some of their acidic metabolites were investigated. Little or no effects of the drugs were observed on measures of dopamine or noradrenaline when given separately, while each selective agonist dose-dependently reduced serotonin levels. Combined treatment with the two agonists produced profound effects on the catecholamines, but with no effect on 3,4-dihydroxyphenylacetic acid, the metabolite of dopamine produced by monoamine oxidase. In addition, the effects of combined treatment on serotonin levels were opposite of those of the drugs given independently. Concomitant treatment of animals with both D1 and D2 receptor agonists can therefore increase tissue levels of dopamine without increasing the potentially harmful metabolism of dopamine by monoamine oxidase.     

Prenatal phenobarbital treatment and temperature-controlling dopamine receptors

Pregnant mice were fed a phenobarbital-containing diet on days nine through 18 of pregnancy. Following parturition, the offspring of such animals were allowed to reach adulthood and then were tested for their response to an acute injection of apomorphine. Male offspring were less sensitive, while female offspring were more sensitive than matched controls to apomorphine-induced hypothermia. The witnessed differences in apomorphine-induced hypothermia could not be attributed to differences in brain apomorphine levels, alterations in the thermoregulation following non-drug challenges to the mouse's thermoregulatory ability, or changes in alpha-adrenergic receptor function. Our results suggest that prenatal phenobarbital administration produces changes in the function of dopamine receptors which regulate body temperature, and that the prenatally-induced changes last well into adulthood.     

Estrogen treatment enhances dopamine receptor sensitivity in the rat striatum

Estrogen (125 μg/rat) was administered, subcutaneously, to male rats six days before measurements were made. Estrogen treatment increased the number of dopamine receptor sites labeled by [³H]spiroperidol in vitro and the intensity of stereotypy induced by dopamine agonists in vivo. These two measures suggest that estrogen administration significantly influences the number of striatal DA receptors and a response to in vivo striatal DA receptors stimulation.     

Does chronic morphine treatment induce a supersensitivity of dopamine receptors in rat brain?

These experiments were carried out to investigate whether chronic morphine treatment increases the sensitivity of dopamine receptors in rat brain. This was studied by considering apomorphine effectiveness in a) inducing stereotyped behaviour and b) decreasing the dopamine turnover in the striata.Acute morphine treatment slightly reduced stereotyped behaviour induced by l-Dopa and by apomorphine. In morphine-withdrawn rats, apomorphine was no more effective than saline in inducing stereotypies (sniffing, licking, gnawing). However, the ability of apomorphine to lower striatal dopamine turnover was increased in morphine-withdrawn rats. Despite these latter observations, chronic morphine treatment probably did not induce a supersensitivity of dopamine receptors in rat brain.Part of the results were presented at the VIth International Symposium on Clinical Pharmacology, Regensburg, Germany, Oct. 3–6, 1974.     

3- and 4-O-sulfoconjugated and methylated dopamine: highly reduced binding affinity to dopamine D2 receptors in rat striatal membranes

Summary The binding properties of 3- and 4-O-sulfoconjugated dopamine (DA-3-0-S, DA-4-0-S) as well as 3-O-methylated dopamine (MT) to rat striatal dopamine D₂ receptors were investigated. ³H-spiperone was used as a radioligand in the binding studies. In saturation binding experiments (+)butaclamol, which has been reported to bind to dopaminergic D₂ and serotoninergic 5HT₂ receptors, was used in conjunction with ketanserin and sulpiride, which preferentially label 5HT₂ and D₂ receptors, respectively, in order to discriminate between ³H-spiperone binding to D₂ and to 5HT₂ receptors. Under our particular membrane preparation and assay conditions, ³H-spiperone binds to D₂ and 5HT₂ receptors with a maximal binding capacity (B _max) of 340 fmol/mg protein in proportions of about 75%:25% with similar dissociation constants K _D (35 pmol/l; 43 pmol/l). This result was verified by the biphasic competition curve of ketanserin, which revealed about 20% high (K _D = 24 nmol/l) and 80% low (K _D = 420 nmol/l) affinity binding sites corresponding to 5HT₂ and D₂ receptors, respectively. Therefore, all further competition experiments at a tracer concentration of 50 pmol/l were performed in the presence of 0.1 mol/l ketanserin to mask the 5HT₂ receptors. DA competition curves were best fitted assuming two binding sites, with high (K _H = 0.12 mol/l) and low (K_L = 18 mol/l) affinity, present in a ratio of 3:1. The high affinity binding sites were interconvertible by 100 mol/l guanyl-5-yl imidodiphosphate [Gpp(NH)p], resulting in a homogenous affinity state of DA receptors (K _D = 2.8 mol/l). Competition experiments with various compounds confirmed the binding of ³H-spiperone to D₂ receptors. DA-3-O-S, DA-4-O-S, and MT were more than 5,000-, more than 10,000-, and 530-fold less potent in competing for ³H-spiperone binding when compared with DA at the high affinity binding site which mediates biological effects. Therefore, it is concluded that these DA metabolites are biologically ineffective at central D₂ receptors. Send offprint requests to E. Werle at the above address