Effects of ergots on adenylate cyclase activity in the corpus striatum and pituitary

Ropinirole in the treatment of Parkinson's disease

In this review, the pharmacokinetics, safety and tolerability, and clinical efficacy of ropinirole (Requip), a non-ergoline dopamine agonist approved for use in the treatment of Parkinson's disease (PD) are examined. A summary of preclinical and clinical studies is presented. Ropinirole is safe and efficacious as monotherapy in the treatment of early PD and as an adjunct to levodopa in more advanced cases.

Bromocriptine enhances feeding behavior without changing dopamine metabolism

Bromocriptine is an ergot derivative and has been thought to act as a selective D2 receptor agonist, but its effects on dopamine release in vivo have not been confirmed. We administered bromocriptine into the striatum of rats and studied the effects on feeding behavior and dopamine release. Bromocriptine was perfused via a microdialysis probe into the ventrolateral striatum of rats fasted for 22 h, and the rats were then allowed to feed freely for 6 h. Bromocriptine perfusion increased food intake in a dose-dependent manner, whereas the extracellular concentrations of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) did not change. Perfusion of (-) sulpiride, a selective D2 receptor antagonist, decreased food intake, but increased dopamine release and the levels of DOPAC and HVA. Pretreatment with (-)sulpiride perfusion for 1 h prior to bromocriptine perfusion inhibited the increase of food intake induced by bromocriptine, and it increased dopamine release and the levels of DOPAC and HVA. These findings suggest that bromocriptine directly perfused into the ventrolateral striatum acts selectively on postsynaptic D2 receptors and enhances feeding behavior.

Further characterisation of the dopamine-inhibitory receptor in Helix and evidence for a noradrenaline-preferring receptor

1. The cells in this study responded with a hyperpolarization to the following agents in this order of potency; dopamine greater than noradrenaline phenylephrine = octopamine. 2. 6,7 ADTN had a relative potency of 0.1 compared to dopamine. 5,6 ADTN did not inhibit the cells in this study. 3. The D1 receptor agonists SKF38393 and dihydroxynomifensine mimicked the effect of dopamine on these cells but were over 100 times less active, whereas the D2 selective agonists quinpirole and RU24213 were without effect. 4. Both the D1 antagonist SCH23390 and the D2 antagonist sulpiride antagonised the dopamine response with pA2 values of 6.1 and 6.7, respectively. 5. Five cells that responded to dopamine with a hyperpolarization were depolarized by noradrenaline. The order of potency of compounds at eliciting this depolarization, noradrenaline greater than phenylephrine greater than octopamine indicated that this response may be mediated by a noradrenaline-preferring receptor.

Climbing and stereotyped behaviours in mice require the stimulation of D-1 dopamine receptors

Apomorphine-induced climbing and sniffing behaviours in mice were antagonize by low doses of SCH 23390 and metoclopramide. The selective D-2 dopamine receptor agonists, LY 171555 and RU 24926, and some other dopamine agonists (piribedil, lergotrile, bromocriptine) exerted only inhibitory effects on spontaneous behaviours. The selective D-1 dopamine receptor agonist, SK&F 38393, did not modify the climbing score but increased the sniffing and decreased the gnawing scores compared to the scores of control mice. Typical climbing and stereotyped behaviours were produced by the combination of SK&F 39383 with LY 171555, RU 24926 or with the other dopamine agonists tested. These data suggest that the stimulation of D-1 dopamine receptors is required for the induction of climbing and stereotyped behaviours in mice.

Effects of several ergosines on adenylate cyclase activity in synaptosomal membranes of the bovine caudate nucleus

The effects of several ergosines and different dopamine agonists and antagonists on the activity of dopamine-sensitive adenylate cyclase in synaptosomal membranes of the bovine caudate nucleus were comparatively studied. Among ergot alkaloid derivatives used, ergosinine was the most active in stimulating adenylate cyclase activity. Ergosine, bromoergosine, dihydroergosine, dihydroergocryptine and lisuride also stimulated this enzyme. Dihydroergosinine, bromodihydroergosine and bromoergocryptine did not affect adenylate cyclase activity. Saccharino derivatives of both ergosine and ergosinine were inactive. When used in higher concentrations, ergosine, ergosinine, dihydroergocryptine and lisuride inhibited dopamine-stimulated adenylate cyclase whereas other ergot alkaloid derivatives examined did not. If the extent of dopamine-sensitive adenylate cyclase stimulation is considered as a measure of dopaminergic activity, examination of the structure/dopaminergic activity relationship showed that modifications of ergot alkaloid molecules such as isomerization in position 8, hydrogenation of delta 9(10)-double bond, or introduction of bromine into position 2 of the molecule, lead to a significant decrease of stimulatory effects of adenylate cyclase. Introduction of a saccharino group into position 2 of the molecule caused a total loss of stimulatory activity of both ergosine and ergosinine, probably because of the size of the saccharino residue.

Tuberoinfundibular dopaminergic system and anterior pituitary dopamine receptor in Shy-Drager syndrome

Tuberoinfundibular dopaminergic system (TIDA) together with anterior pituitary dopamine receptor was evaluated in 9 patients with Shy-Drager syndrome (SDS) through the responses of serum growth hormone (GH) and prolactin (PRL) to dopaminergic stimulation or dopamine receptor inhibition. A variety of abnormal responses of serum GH and/or PRL following the administration of these drugs suggests the TIDA involvement as well as the nigrostriatal system and moreover the different degeneration processes in each dopaminergic system in SDS.

Differential effects of 5-hydroxytryptaminergic antagonists upon apomorphine- and lergotrile-induced hypothermia and stereotyped behaviour in rats

The dopaminergic stimulants apomorphine and lergotrile both evoked hypothermia and stereotyped behaviour in rats. These drug effects were sensitive to antagonism by haloperidol, a dopaminergic receptor blocker. In rats pretreated with 5-hydroxytryptaminergic receptor blockers, cinanserin reduced apomorphine-induced hypothermia but cyproheptadine did not. Both cinanserin and cyproheptadine significantly potentiated lergotrile-induced hypothermia. Similarly, the stereotypic effects of apomorphine were partly reduced by cyproheptadine, although higher doses of cyproheptadine did potentiate lergotrile-induced stereotyped behaviour. These findings of different influences of 5-HT antagonists upon the effects of apomorphine and lergotrile indicate that these two dopaminergic stimulants may not work in identical manner to produce outwardly similar drug effects.

Central antidopaminergic properties of 2-bromolisuride, an analogue of the ergot dopamine agonist lisuride

2-Bromolisuride (2-Br-LIS), a derivative of the ergot dopamine (DA) agonist lisuride, was investigated in rodents in comparison with the DA antagonist haloperidol with regard to its influence on DA related behaviour, cerebral DA metabolism and prolactin (PRL) secretion. 2-Br-LIS produced catalepsy in mice (ED50 3.3 mg/kg i.p.), antagonized apomorphine-induced stereotypies in mice (ED50 0.4 mg/kg i.p.), antagonized DA agonist-induced stereotypies in rats (0.1-1.56 mg/kg i.p.), inhibited locomotor activity in rats (0.025-6.25 mg/kg i.p.), antagonized the hyperactivity produced by various DA agonists in rats (0.025-6.25 mg/kg i.p.) and inhibited the apomorphine-induced hypothermia in mice (0.05-0.78 mg/kg i.p.). 2-Br-LIS (0.03-10 mg/kg i.p.) stimulated DA biosynthesis and DOPAC formation in the striatum and DA rich limbic system of rats, but had no effect on serotonin turnover. In striatum and limbic forebrain of gamma-butyrolactone-pretreated rats 2-Br-LIS reversed the apomorphine-induced inhibition of DOPA accumulation. 2-Br-LIS (0.03 - 3 mg/kg) enhanced PRL secretion in intact male rats. These findings indicate DA antagonistic properties of 2-Br-LIS presumably due to blockade of central pre- and postsynaptic DA receptors being of approximately the same order of potency as haloperidol. 2-Br-LIS is the first ergot compound with definite antidopaminergic properties suggesting its potential usefulness as a neuroleptic.

Nigral dopaminergic mechanisms in drug-induced circling

Unilateral injections of dopamine into the substantia nigra pars reticulata of pargyline-pretreated rats caused a prolonged, contralateral circling, similar in magnitude to that elicited by the injection of the same amount of dopamine intrastriatally. Contralateral circling was also elicited by the unilateral intranigral injection of amphetamine (after pargyline pretreatment), or by the dopamine agonists ergometrine and SKF 38393. In contrast, bilateral intranigral injection of the dopamine antagonist haloperidol greatly reduced the amphetamine-induced circling of rats with unilateral 6-hydroxydopamine-induced nigrostriatal lesions. These results support the hypothesis that dopaminergic mechanisms in the substantia nigra are involved in motor behavior.

Pharmacological evidence against the involvement of the D1 subtype of dopamine receptors in apomorphine-induced hypothermia

In chloral anaesthetized rats, apomorphine, bromocriptine, lergotrile, piribedil and its presumed active metabolite S 584, induced a hypothermia antagonized, except for S 584, by stereotactically controlled microinjections of haloperidol into the preopticus medialis nucleus. Hypothermia was also produced by these dopamine agonists when they were injected directly into the preopticus medialis nucleus. Furthermore, apomorphine-induced hypothermia was antagonized by an infusion of sulpiride into the preopticus medialis nucleus. These data suggest that the dopamine receptors located in the preopticus medialis nucleus which are involved in dopamine agonists-induced hypothermia do not possess the same pharmacological characteristics as the D1 subtype.

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.

Comparison of the binding of [3H]spiperone and [3H]domperidone in homogenates of mammalian retina and caudate nucleus

The specific binding of [3H]spiperone and [3H]domperidone, as defined by 1 microM-(+)butaclamol, was compared in homogenates of bovine retina and caudate nucleus. Scatchard analyses of saturation data for [3H]spiperone binding yielded dissociation constants (Kd) of 0.35 nM in the retina and 0.64 nM in the caudate nucleus. Comparison of the maximum number of binding sites (Bmax) present in each tissue indicated that the density of sites in bovine caudate nucleus (270 fmol/mg protein) was approximately three times higher than in bovine retina (92 fmol/mg protein). This difference was even more marked in guinea pig tissues, with a ratio of 7:1 between corpus striatum and retina. The pharmacological analysis of [3H]spiperone binding in both the bovine retina and caudate nucleus indicated an interaction with dopaminergic rather than serotonergic sites. However, inhibition curves obtained to dopaminergic agonists in the bovine retina were significantly steeper than those observed in the bovine caudate nucleus, as reflected in the greater Hill coefficients obtained for these agents in the retina. Furthermore, only a small amount of specific [3H]domperidone binding was observed in either the bovine caudate nucleus or the guinea pig striatum, whilst no specific [3H]domperidone binding was detectable in homogenates of either bovine or guinea pig retina. These data suggest that the retina possesses only a small population of dopaminergic D2 sites and that these binding sites may differ from those present in the caudate nucleus.

Dopaminergic modulation of adenylate cyclase stimulation by vasoactive intestinal peptide in anterior pituitary

The activation of adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] by vasoactive intestinal peptide (VIP) was used as a model to investigate the molecular mechanisms triggered by the occupancy of dopamine recognition sites in rat anterior pituitary. Dopamine failed to change the basal enzyme activity, but it inhibited the stimulation of adenylate cyclase elicited by VIP. Apomorphine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene, and 2-bromo-alpha-ergocryptine mimicked the effect of dopamine, whereas (-)-sulpiride and and classical neuroleptics antagonized it. Dopamine failed to modulate the activation of pituitary adenylate cyclase by prostaglandin E1, which does not increase prolactin secretion. From these results we infer that stimulation of D-2 (dopamine) receptors may affect pituitary secretion by inhibiting the activation of anterior pituitary adenylate cyclase by VIP or other secretagogues.

Effects of dopamine on cyclic AMP concentration in the anterior pituitary gland in vitro

Effects of different concentrations of dopamine on the cyclic AMP concentration in the anterior pituitary gland were investigated in vitro. Low concentrations of dopamine (10(-9)--10(-8) mol/l) were found to decrease, whereas the high concentration (10(-5) mol/l) increased the cyclic AMP concentration in pituitaries collected from ovariectomized and estradiol-treated females. In contrast, dopamine had no effect on the anterior pituitary cAMP concentration when pituitaries were collected from ovariectomized rats which had not received estrogen replacement. These data show that the action of dopamine on the anterior pituitary cAMP largely depends on the dopamine concentration and the hormonal state of animals.

Dopaminergic mechanisms in the teleost retina. I. Dopamine-sensitive adenylate cyclase in homogenates of carp retina; effects of agonists, antagonists, and ergots

A specific dopamine-sensitive adenylate cyclase has been identified in homogenates of the teleost (carp) retina. Maximal stimulation by 100 microM-dopamine resulted in a 5--10-fold increase in adenylate cyclase activity with half-maximal stimulation occurring at a concentration of 1 microM. l-Noradrenaline and l-adrenaline were some 10 times less potent than dopamine whilst the alpha- and beta-adrenoreceptor agonists, l-phenylephrine and dl-isoprenaline were inactive. Apomorphine elicited a partial stimulation of adenylate cyclase activity whilst various ergot alkaloids produced mixed agonist/antagonist responses. Dopamine-stimulated adenylate cyclase activity was potently antagonised by various neuroleptic drugs including fluphenazine, alpha-flupenthixol and alpha-piflutixol, and to a lesser extent by the butyrophenone derivatives haloperidol and spiperone. The benzamide derivatives, metoclopramide and sulpiride, together with the alpha- and beta-adrenoreceptor blocking agents, phentolamine and propranolol respectively were essentially inactive at blocking dopamine-stimulated adenylate cyclase activity. These data suggest the presence of a highly specific dopamine-sensitive adenylate cyclase in homogenates of teleost retina possessing similar pharmacological properties to the dopamine-sensitive adenylate cyclase observed in the mammalian central nervous system.

Stimulatory action of lisuride on dopamine-sensitive adenylate cyclase in the rat striatal homogenate

Effect of lisuride, an ergot derivative of isolysergic structure, on dopamine-sensitive adenylate cyclase was studied in the homogenate of rat corpus striatum. Stimulatory action of lisuride, similar to the actions of dopamine and apomorphine, on striatal adenylate cyclase was potentiated significantly by guanosine triphosphate (GTP) and by guanyl-5'-yl imidodiphosphate (GMP-PNP), although with lisuride alone, there was only a slight stimulation. The maximal stimulation attained in the presence of GTP corresponded to about 1.4 times the basal rate of cyclic AMP formation in the homogenate and was abolished by an addition of haloperidol. Lisuride at a concentration about 3 microM inhibited stimulation of cyclic AMP formation by dopamine. The effect of lisuride and the extent of potentiation by the guanyl nucleotides were almost comparable to the effects of apomorphine, under corresponding conditions. Thus, lisuride, like apomorphine, acts as a partial agonist-antagonist, and has the ability to stimulate the dopamine-sensitive adenylate cyclase in the rat corpus striatum.

The role of D-1 and D-2 receptors

Dopamine receptors in intracerebral motor and endocrine systems have been divided into two main types, D-1 and D-2, dependent on the presence or absence of adenylate cyclase linkage. Here we have investigated a number of dopamine agonist and antagonist drugs in man that have different actions on D-1 and D-2 receptors in animals. Motor and endocrine effects in parkinsonian subjects seem to depend on drug interaction with D-2, but not D-1, receptors. These results may have important implications for the design of anti-parkinsonian and antipsychotic agents.

Cross tolerance between two dopaminergic ergot derivatives--bromocriptine and lergotrile

1. When initially given to patients with idiopathic parkinsonism, lergotrile induced marked supine arterial hypotension. Tolerance to this hypotensive effect developed and larger doses of lergotrile were administered without this adverse effect. 2. Cross tolerance occurred between lergotrile and a structurally related ergot derivative, bromocriptine. A high dose of lergotrile given to patients who had been on long term bromocriptine therapy did not induce any significant hypotension when compared to placebo whereas hypotension still occurred when lergotrile was given after previous treatment with levodopa.

Dopamine neurons: effect of lergotrile on unit activity and transmitter synthesis

Intravenous administration of increasing doses of the ergoline, lergotrile mesylate, caused a rapid, dose-dependent and haloperidol-reversible inhibition of unit activity of dopamine cells in the pars compacta of the rat substantia nigra. 6 microgram/kg caused significant depression of dopamine cell firing rates; 50% inhibition was achieved with a cumulative dose of 100 microgram/kg. 60% of the cells were completely inhibited by increasing doses of the drug, the remainder became resistant to further inhibition after reaching rates 25--50% of baseline. Pretreatment with reserpine and alpha-methylparatyrosine did not significantly attenuate the lergotrile-induced inhibition. Lergotrile had no consistent effect on firing rates of cells in the pars reticulata of the substantia nigra. The ergoline reduced the apparent activation of striatal dopamine synthesis associated with complete cessation of impulse flow in nigral-striatal dopamine neurons induced by gamma-butyrolactone treatment. These effects are compared with those of apomorphine and amphetamine. The results are consistent with the idea that lergotrile is a direct acting dopamine agonist.

Interaction between the thyrotrophin-releasing hormone-induced growth hormone rise and dopaminergic drugs: studies in pathologic conditions of the animal and man

A rise in plasma growth hormone (GH) after thyrotrophin-releasing hormone (TRH) and a striking reduction after dopaminergic drugs is present in acromegalic ('responder') patients. We have investigated the GH response to dopaminergic stimuli in two conditions of animals and man, which, like acromegaly, are characterized by a TRH-induced GH rise, i.e. rats with electrolytic lesions of the median eminence (ME) and patients with hepatic cirrhosis. In addition, we have studied the TRH-induced GH rise in rats with ME lesions, in the cirrhotic patients and in a group of 'responder' acromegalics before and after administration of dopaminergic drugs. In rats with ME lesions an infusion of dopamine (DA) neither modified baseline GH levels nor the TRH-induced GH rise. In five out of six cirrhotic patients oral administration of L-Dopa was followed by the usual rise in plasma GH. infusion of DA increased plasma GH levels in three out of seven cirrhotic patients and in four out of five subjects an earlier GH rise after TRH was seen. However, in the 'responder' acromegalics, the infusion of DA, besides lowering baseline plasma GH, was capable of reducing the TRH-induced GH rise. Collectively these data indicate that the TRH-induced GH rise emphasizes defects in the neurohormonal links between the central nervous system and the anterior pituitary. Instead, the paradoxical fall of GH after dopaminergic drugs appears to be a prerequisite of acromegaly and may be attributable to receptors for DA located on the tumorous tissue.

Dopamine agonist-induced hyperglycemia in rats: effects of lergotrile mesylate

Lergotrile and apomorphine, two direct-acting dopamine agonists, caused marked hyperglycemia in fasted rats, while compounds which release endogenous dopamine (amphetamine, methylphenidate) or inhibit dopamine reuptake (LR5182), failed to elevate blood glucose. The effect of lergotrile was dose dependent, causing blood glucose to rise 3-fold above resting levels at 5 mg/kg (i.p.). Blood glucose increased prior to the onset of the behavioral signs of dopamine stimulation. The effect of lergotrile was attenuated by phentolamine, propranolol or butaclamol. Adrenalectomy also prevented lergotrile-induced hyperglycemia. These data indicate that hyperglycemia is not a property of all compounds that cause dopaminergic effects. The action of lergotrile might be indirect, perhaps mediated through release of catecholamines from the adrenal glands.

Effect of dihydroergocristine infusion on tolbutamide-induced insulin secretion in man

The insulinemic response to 1 g of tolbutamide i.v. is greatly enhanced (+ 145%) after a 60-min infusion of the alpha-lytic dihydrogenated ergot alkaloid, dihydroergocristine (83.3 micrograms/min, corresponding to a total dose of 5 mg) in 7 healthy subjects. No differences are observed in the glycemic responses.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

Differential enhancement of locomotor activity by dopamine agonists following chronic morphine treatment

Aniamls were administered increasing doses of morphine for 16 days. During withdrawal they were challenged with an apomorphine ester or lergotrile. These dopamine agonists produced quantitatively different effects on behaviour. The results suggest that chronic morphine administration leads to dopamine supersensitivity which may be anatomically distinct in origin.

Differential enhancement of locomotor activity by dopamine agonists following chronic neuroleptic treatment: an animal model of tardive dyskinesia

Animals were administered clozapine or haloperidol for 22 days. Following treatment they were challenged with an apomorphine ester or lergotrile. Only haloperidol-treated animals exhibited significantly enhanced responses to apomorphine ester whereas administration of lergotrile potentiated locomotor activity in both treated groups. The results suggest that the use of different dopaminergic agonists may help to dissociate receptor supersensitivity arising from the antipsychotic actions of neuroleptics from that leading to the development of undesirable side effects.