Further evidence for the stimulation of rat brain dopamine receptors by ergometrine

Dopamine Agonists: From the 1970s to Today

The discovery of dopamine inhibitory effects on prolactin secretion has led to an era of successful dopaminergic therapy for prolactinomas. Herein we provide an overview of the evolution of dopamine agonists and their use in patients with PRL-secreting pituitary tumors, starting from the 1970s up to today, highlighting that normalization of PRL levels, restoration of eugonadism, and reduction of tumor mass can be achieved in the majority of patients by treatment with dopamine agonists.

Effects of catecholamine and monophenolamine agonists on identifiable giant neurones, sensitive to these amines, of an African giant snail (Achatina fulica Férussac)

The effects of 23 substances proposed as catecholamine (CA) and monophenolamine (MA) agonists were tested on the five CA-sensitive neurones, periodically oscillating neurone (PON), tonically autoactive neurone (TAN), left-visceral multiple spike neurone (1-VMN), dorsal-right pedal autoactive neurone (d-RPeAN) and visceral intermittently firing neurone (VIN), and the three MA-sensitive neurones, frequently autoactive neurone (FAN), dorsal-left pedal large neurone (d-LPeLN) and dorsal-left cerebral distinct neurone (d-LCDN), of an African giant snail (Achatina fulica Férussac). Of these neurones, PON, VIN and d-LPeLN were excited by the most effective catecholamine or monophenolamine, i.e. dopamine, epinine or DL-octopamine, whereas TAN, 1-VMN, d-RPeAN, FAN and d-LCDN were inhibited by the same substances. Of the five CA-sensitive neurones, PON and VIN were markedly excited by ergometrine (effective potency quotients (EPQs) of these substances as compared with the effective potency of the most effective catecholamine or monophenolamine for these neurones: 3.0 for PON, 1.0 for VIN), methylergometrine (EPQs: 3.0 for PON, 0.3 for VIN) and mescaline (EPQs: 0.3 for PON, 1.0 for VIN). These two neurones were excited slightly by DL-metaraminol (EPQs: 0.03 for both neurones) and DL-neosynephrine (EPQs: 0.03 for PON, 0.1 for VIN); VIN was also slightly sensitive to 3-methoxytyramine (EPQ: 0.03). TAN and 1-VMN were inhibited by ergometrine (EPQs: 1.0 for TAN, 0.3 for 1-VMN) and methylergometrine (EPQs: 0.3 for TAN, 0.1 for 1-VMN), whereas d-RPeAN was inhibited slightly only by DL-neosynephrine (EPQ: 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Simple method for evaluation of stimulatory effect of drugs on presynaptic dopamine receptors in mice

In order to clarify the stimulating activity of presynaptic dopamine (DA) receptors, the ability of drugs to inhibit in vivo accumulation of l-dopa produced by gamma-butyrolactone was estimated in the striatum of mouse treated with an amino acid decarboxylase inhibitor. Apomorphine (0.05-0.5 mg/kg, s.c.) significantly suppressed this accumulation in a dose-related manner. Bromocriptine, lisuride, and ergometrine also reduced it. Methamphetamine (5 and 10 mg/kg, s.c.) diminished striatal l-dopa accumulation and the effects of apomorphine, bromocriptine, and methamphetamine were all blocked by 0.1 mg/kg, i.p., of haloperidol. Ephedrine (100 mg/kg, s.c.) caused a slight decrease in the dopa accumulation although the same dose of cocaine did not change it. These results indicate that the estimation of striatal l-dopa from a single mouse brain is feasible, and dopamine receptor agonists, apomorphine, and ergot alkaloids have stimulatory effects on the activity of presynaptic dopamine receptors of the nigrostriatal dopaminergic neurons of the mouse.

Effects of intranigral injections of dopamine agonists and antagonists, glycine, muscimol and N-methyl-D,L-aspartate on locomotor activity

Previously it has been shown that bilateral intranigral injections of dopamine into rats pretreated with a monoamine oxidase inhibitor induced prolonged stimulation of locomotor activity, while bilateral intranigral injections of haloperidol reduced the locomotor stimulation evoked by systemic amphetamine. In the present studies, the role of the substantia nigra in locomotor activity was further investigated using a variety of dopaminergic and other agonists and neuroleptics. Ergometrine, epinine, (+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronapthalene hydrobromide (ADTN), 1,2,3,4-tetrahydro-6,7,-dihydroxyisoquinoline hydrochloride (THIQ), muscimol and glycine elicited locomotor activity when injected into the substantia nigra pars reticulata bilaterally. Additionally the non-dopaminergic agonists also elicited a degree of stereotyped behavior. Locomotor activity induced by intranigral ergometrine was blocked by systemic haloperidol but was not affected by intranigral haloperidol. Locomotor activity elicited by systemic amphetamine was blocked by bilateral intranigral alpha-flupenthixol, but that elicited by bilateral intra-accumbens ergometrine was not affected by alpha-flupenthixol or haloperidol injected into the substantia nigra pars reticulata bilaterally. The results provide further evidence that alterations of neurotransmission in the substantia nigra exert effects on locomotor activity.

Multiple dopamine receptors and behavior

The therapeutic effects of dopamine (DA) agonists and DA antagonists used in the treatment of schizophrenia (antipsychotics, DA antagonists), Huntington's chorea (DA antagonists) and Parkinson's disease (antiparkinsonian agents, DA agonists) have been thought to result largely from actions on DA receptors located in the striatum (caudate nucleus and putamen). Many of the classical drugs used to treat these disorders are known to have a high incidence of extrapyramidal side effects (EPS). However, a number of drugs, the atypical antipsychotics and antiparkinsonian agents, have been developed which have a low incidence of EPS. It has been of enormous interest to researchers and clinicians alike to determine what characteristics of the atypical antipsychotics and antiparkinsonian agents are responsible for their unique behavioral profile. Because all of the antipsychotics and antiparkinsonian agents act on DA receptors, much attention has focused on potential differences in the interactions of the atypical agents with DA receptors. An hypothesis that has been raised, due to the knowledge that there are multiple subtypes of DA receptors located in the striatum, is that the atypical agents could have their therapeutic actions as a result of an interaction with one specific subtype of DA receptor. This review emphasizes two major points: (1) it is unlikely that the atypical antipsychotics and antiparkinsonian agents interact with only one subtype of DA receptor, or have their therapeutic actions only through that receptor; (2) other pharmacological characteristics of these agents are more critically involved in their unique behavioral effects. The applicability of animal models to assess the pharmacological and behavioral profiles of these agents is discussed, and the relevance to the clinical profiles of these agents is emphasized.

The role of dopaminergic receptors in the behavioral effects induced by lisuride in male rats

Lisuride increased the incidence of stretching and yawning (SY) as well as of penile erection (PE) and elicited stereotyped behavior (SB), aggressive behavior and mounting in male rats, depending on the dose used. SY was prevented by two dopaminergic antagonists, haloperidol and sulpiride, but not by methysergide, a serotoninergic antagonist, while PE was antagonized by all three drugs. With regard to SB, aggressive behavior and mounting, all three were suppressed by haloperidol; sulpiride, while partially antagonizing aggressiveness, failed to affect SB and mounting; methysergide did not significantly influence any of the three. This suggests that lisuride principally affects the dopaminergic system. Although further detailed studies are required to elucidate which type of the complex population of DA-receptors is involved in each kind of behavior, we suggest that SY at least is due to the activation by lisuride of presynaptic DA-receptors.

Turning behaviour following nigral injections of dopamine agonists and glycine

Glycine, ADTN and ergometrine, but not LSD, induced strong dose-related contraversive turning following unilateral microinjections into the substantia nigra (zona reticulata) of conscious rats. The responses to glycine and ADTN were blocked by low doses of systemically injected cis-Z-flupenthixol but were unaffected by pretreatment with a variety of dopamine antagonists injected intranigrally. Turning behaviour following glycine was antagonised by strychnine but that to ADTN was not. Possible ways by which the rotational responses may be induced in view of the postulated neuronal circuitry of the substantia nigra and its connections with other brain areas are discussed.

Central effect of ergometrine on gastric acid secretion in rats

1. The ergot alkaloid ergometrine administered by rapid intravenous injection (0.5--4 mg/kg) or by continuous intravenous infusion (0.5--2 mg/kg per hr) stimulated gastric acid secretion in rats and its effect was antagonized by cimetidine (2 mg/kg per hr). 2. The gastric stimulant action can be elicited also by intracerebro-ventricular administration (25--100 micrograms) suggesting a central mechanism of action of ergometrine.

Cardiovascular responses produced by the injection of dopamine into the cerebral ventricles of the unanaesthetized dog

1 The injection of dopamine (100 to 500 microgram) into the cerebral ventricles (i.c.v.) of 10 unanaesthetized dogs produced a dose-dependent increase in arterial blood pressure and heart rate. The dogs licked, swallowed, sometimes vomited and became sedated. 2 Autonomic ganglion blockade with hexamethonium (10 mg/kg i.v.) abolished cardiovascular responses to i.c.v. dopamine, indicating that dopamine was exerting its effect within the central nervous system. 3 The dopamine receptor antagonists, haloperidol (500 microgram), chlorpromazine (200 micrograms) and ergometrine (500 micrograms), each given i.c.v., sugsequently abolished the cardiovascular responses to dopamine. 4 Pretreatment with either the beta-adrenoceptor antagonist, propranolol (600 microgram) or the alpha-adrenoceptor antagonist, phentolamine (1 mg) given i.c.v. had no significant effect on the response to dopamine. 5 It is suggested that dopamine injected into the cerebral ventricles of the unanesthetized dog causes hypertension and tachycardia by activating central dopamine receptors.

Blockade of photically induced epilepsy by 'dopamine agonist' ergot alkaloids

The effect of the intravenous administration of ergot alkaloids on epileptic responses to intermittent photic stimulation )IPS) has been studied in adolescent baboons, Papio papio, from Senegal. Ergocornine, 1--2 mg/kg, produced marked autonomic and behavioural effects, slowed the EEG, and abolished myoclonic responses to IPS for 30--90 min. Ergometrine, 1 mg/kg, activated the EEG and blocked the induction of myoclonic responses for 1--3 h. Bromocriptine, 0.5--4 mg/kg, did not consistently prevent myoclonic responses to IPS. After pretreatment with a subconvulsant dose of allylglycine (180--200 mg/kg), lysergic acid diethylamide, 0.1 mg/kg, retained the capacity to block myoclonic responses to IPS, and ergocornine 1 mg/kg reduced such responses. The convulsant effect of allylglycine was enhanced, however, so that prolonged seizure sequences began 19--96 min after ergocornine administration. The protective action of ergot alkaloids against epileptic responses induced by sensory stimulation is interpreted in terms of effects at several sites, including dopaminergic and serotoninergic synapses.

Effect of lisuride and other ergot derivatives on monoaminergic mechanisms in rat brain

The effects of the ergot derivative lisuride hydrogen maleate on synthesis, turnover and receptor activity of monoamines were compared with those of LSD, methysergide, yohimbine and methiothepin. In the dopamine (DA) rich areas, c. striatum and mesolimbic forebrain, lisuride, (30-100 mug/kg) decreased the rate of dopa formation after inhibition of the aromatic amino acid decarboxylase with NSD 1015, 100 mg/kp i.p. After axotomy of the ascending monoaminergic fibers, lisuride (0.5 mg/kg) antagonized the accumulation of dopa in c. striatum and mesolimbic forebrain even on the lesioned side; haloperidol effectively counteracted this lisuride-induced decrease in dopa formation. In the predominantly noradrenaline (NA)-innervated neocortex, lisuride in doses of 0.3 and 1.0 mg/kg increased dopa accumulation. Methysergide, yohimbine and methiothepin also stimulated tyrosine hydroxylation. After inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine methylester HCl, lisuride decelerated DA disappearance and accelerated NA disappearance. 3-Methoxytryramine (3-MY) accumulating after inhibition of monoamine axidase with pargyline HCl (100 mg/kg) was used as an indicator of DA release. Lisuride and LSD, 50 mug/kg administered twice, reduced 3-MT formation while methysergide, 50 mg/kp i.p. had no effect on 3-MT accumulation. All compounds except methiothepin reduced 5-hydroxytryptophan (5-HTP) accumulation in whole brain after NSD 1015. In addition, lisuride caused an increase in 5-hydroxytryptamine (5-HT) and a decrease in 5-hydroxyindole acetic acid concentration. In spinal reserpinized rats, lisuride was indistinguishable from LSD in inducing extension and athetoid movements of the hind legs. The data support the view that lisuride stimulates pre- and postsynaptic DA and 5-HT receptors and suggest that lisuride blocks NA receptors in the central nervous system.

Studies on the pharmacology of neurones in the nucleus accumbens of the rat

A study was made of the effects of iontophoretically applied drugs on single neurones in the nucleus accumgens and caudate nucleus of rats anaesthetized with urethane. Neurones in the caudate nucleus were inhibited by dopamine, dibutyryl cyclic AMP, ADTN and ergometrine. Acetylcholine and homocysteic acid caused excitation of striatal neurones. In the nucleus accumbens neurones were inhibited by dopamine, ADTN, ergometrine, dibutyryl cyclic AMP, glycine and gamma-aminobutyric acid. The responses of glycine and gamma-aminobutyric acid were antagonised by strychnine and picrotoxin, respectively. Acetylcholine and homocysteic acid caused excitation of neurones in the nucleus acumbens; the effects of acetylcholine were blocked by atropine. The results are consistent with the suggestion that dopamine is an inhibitory transmitter in the nucleus accumbens and in the caudate nucleus and support the hypothesis that the effects of dopamine are mediated by cyclic AMP. The locomotor stimulants ADTN and ergometrine mimicked the inhibitory actions of dopamine in both the striatum and in the nucleus accumbens. These results support the suggestion that dopamine receptors in the nucleus accumbens are involved in the actions of locomotor stimulant drugs.

Ergot alkaloids as dopamine agonists: comparison in two rodent models

A series of ergot alkaloids, together with the DA agonists apomorphine and piribedil, were tested for protective effects against audiogenic seizures in an inbred strain of mice (DBA/2) and for induction of circling behaviour in mice with unilateral destruction of one nigrostriatal DA pathway. The order of potency against audiogenic seizures was apomorphine greater than ergocornine greater than bromocryptine greater than ergometrine greater than LSD greater than methysergide greater than piribedil while that observed in the rotating mouse model was apomorphine greater than ergometrine greater than ergocornine greater than bromocryptine greater than piribedil. LSD caused only weak circling behaviour even when administered in high doses (greater than 1 mg/kg). Methysergide was ineffective. Prior administration of the neuroleptic agent haloperidol blocked the effect of DA agonists and of ergot alkaloids in both animal models. The possible action of ergot alkaloids as DA agonists is discussed.

Effects of dopamine receptor stimulants on locomotor activity of rats with electrolytic or 6-hydroxydopamine-induced lesions of the nucleus accumbens

Ergometrine (8 mg/kg) injected intraperitoneally into normal rats had little effect on locomotor activity. In contrast, rats with selective 6-hydroxydopamine-induced lesions of dopamine terminals in the nucleus accumbens showed a strong stimulation of locomotor activity following injection of this dose or ergometrine. The dopamine analogue 2-amino-6-7-dihydroxy-1, 2, 3, 4-tetrahydronaphthalene (ADTN) (150 mug), caused strong and long lasting stimulation of locomotor activity when injected intracerebroventricularly into rats. The ADTN response was markedly reduced in rats with bilateral electrolytic lesions of the nucleus accumbens, but unchanged in rats with bilateral electrolytic lesions of the caudate nucleus. At a lower dose level (50 mug) ADTN, injected intracerebroventricularly, had little effect on the locomotor activity of normal or sham-operated rats. This dose of ADTN was, however, effective in causing locomotor stimulation of rats with bilateral 6-hydroxydopamine-induced lesions of the nucleus accumbens. These results support the view that the dopamine receptors in the nucleus accumbens are involved in the actions of locomotor stimulant drugs.

Direct dopaminergic action of lisuride hydrogen maleate, an ergot derivative, in mice

Lisuride hydrogen maleate induced stereotyped behaviour in normal as well as in reserpinized mice. It antagonized the motor depression and hypothermia induced by reserpine. On i.p. administration the compound was about as effective as apomorphine and D-amphetamine. As with apomorphine and in contrast to D-amphetamine the effects of lisuride hydrogen maleate in reserpinized mice were not impaired by additional treatment with alpha-methyl-p-tyrosine methylester. In untreated mice, the substance was very potent in lowering body temperature with significant hypothermia measured after dosages as low as 0.10 mg/kg i.p. Occurrence of stereotyped behaviour and hypothermia could be prevented by the dopaminergic antagonist haloperidol. From these data it is concluded that lisuride hydrogen maleate in addition to its interaction with serotoninergic systems is a potent dopaminergic agonist with a probably direct action on dopaminergic receptors. Further arguments in support of such an action of lisuride hydrogen maleate are, in addition to biochemical data, its serum prolactin lowering effect in rats, its strong emetic action in dogs and its effects on rat behaviour.

Lysergic acid diethylamide: evidence for stimulation of cerebral dopamine receptors

In the rat, lysergic acid diethylamide (LSD) decreased the striatal and retinal content of homovanillic acid. LSD did not change the level of dopamine (DA), but delayed the a-methyl-p-tyrosine-induced disappearance of this amine in the teldiencephalon. In the cat, LSD diminished the DA output into the perfusate of the caudate nucleus. Furthermore, LSD increased the activity of adenylate cyclase in striatal homogenates of rat. These and other findings indicate that in the central nervous system LSD stimulates DA receptors which may be involved in LSD-induced phychosis.