Further evidence for the stimulation of rat brain dopamine receptors by a cyclic analogue of dopamine

Biogenic amine modulation of honey bee sociability and nestmate affiliation

Biogenic amines modulate a range of social behaviours, including sociability and mechanisms of group cohesion, in both vertebrates and invertebrates. Here, we tested if the biogenic amines modulate honey bee (Apis mellifera) sociability and nestmate affiliation. We examined the consequences of treatments with biogenic amines, agonists and antagonists on a bee’s approach to, and subsequent social interactions with, conspecifics in both naturally hive-reared bees and isolated bees. We used two different treatment methods. Bees were first treated topically with compounds dissolved in the solvent dimethylformamide (dMF) applied to the dorsal thorax, but dMF had a significant effect on the locomotion and behaviour of the bees during the behavioural test that interfered with their social responses. Our second method used microinjection to deliver biogenic amines to the head capsule via the ocellar tract. Microinjection of dopamine and a dopamine antagonist had strong effects on bee sociability, likelihood of interaction with bees, and nestmate affiliation. Octopamine treatment reduced social interaction with other bees, and serotonin increased the likelihood of social interactions. HPLC measurements showed that isolation reduced brain levels of biogenic amines compared to hive-reared bees. Our findings suggest that dopamine is an important neurochemical component of social motivation in bees. This finding advances a comparative understanding of the processes of social evolution.

Designing prodrugs for the treatment of Parkinson's disease

INTRODUCTION

Current Parkinson's disease (PD) therapy is essentially symptomatic, and l-Dopa (LD), is the treatment of choice in more advanced stages of the disease. However, motor complications often develop after long-term treatment, and at this point physicians usually prescribe adjuvant therapy with other classes of antiparkinsonian drugs, including dopamine (DA) agonists, catechol-O-methyl transferase (COMT) or monoamine oxidase (MAO)-B inhibitors. In order to improve bioavailability, the prodrug approach appeared to be the most promising, and some antiparkinsonian prodrugs have been prepared in an effort to solve these problems.

AREAS COVERED

This review discusses the evidence of progress in PD therapy, mainly focused on prodrug approach for treatment of this neurological disorder. Several derivatives were studied with the aim of enhancing its chemical stability, water or lipid solubility, as well as diminishing the susceptibility to enzymatic degradation. Chemical structures mainly related to LD, DA and dopaminergic agonists are also reviewed in this paper.

EXPERT OPINION

In order to strengthen the pharmacological activity of antiparkinsonian drugs, enhancing their penetration of the blood-brain barrier (BBB), different approaches are possible. Among these, the prodrug approach appeared to be the most promising, and many prodrugs have been prepared in an effort to optimize physicochemical characteristics. In addition, novel therapeutic strategies based on formulations linking dopaminergic drugs with neuroprotective agents, increasing LD striatal levels and offering sustained release of the drug without any fluctuation of brain concentration, offer promising avenues for development of other effective new treatments for PD.

Antiparkinson prodrugs

Parkinson’s disease (PD) is a progressive, neurodegenerative disorder which involves the loss of dopaminergic neurons of the substantia nigra pars compacta. Current therapy is essentially symptomatic, and L-Dopa (LD), the direct precursor of dopamine (DA), is the treatment of choice in more advanced stages of the disease. Substitution therapy with LD is, however, associated with a number of acute problems. The peripheral conversion of LD by amino acid decarboxylase (AADC) to DA is responsible for the typical gastrointestinal (nausea, emesis) and cardiovascular (arrhythmia, hypotension) side effects. To minimize the conversion to DA outside the central nervous system (CNS) LD is usually given in combination with peripheral inhibitors of AADC (carbidopa and benserazide). In spite of that, other central nervous side effects such as dyskinesia, on-off phenomenon and end-of-dose deterioration still remain. The main factors responsible for the poor bioavailability and the wide range of inter- and intra-patient variations of plasma levels are the drug’s physical-chemical properties: low water and lipid solubility, resulting in unfavourable partition, and the high susceptibility to chemical and enzymatic degradation. In order to improve the bioavailability, the prodrug approach appeared to be the most promising and some LD prodrugs have been prepared in an effort to solve these problems. We report here a review of progress in antiparkinson prodrugs, focusing on chemical structures mainly related to LD, DA and dopaminergic agonists.

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.

A prodrug of ADTN: selectivity of dopaminergic action and brain levels of ADTN

The effects of administration of the prodrug dibenzoyl ADTN (DBADTN) on ADTN concentrations in rat brain and on behaviour in rats having a unilateral 6-hydroxydopamine lesion in the corpus striatum have been studied. Using a combination of HPLC and electrochemical detection as assay method it was found that there was a more selective accumulation of ADTN in the corpus striatum than in the cerebellum. In addition the accumulation of ADTN in the corpus striatum was slow in onset yet long in duration. The peak concentration of ADTN was relatively low and although it was sufficient to cause a strong stimulation of presynaptic DA receptors it did not cause significant rotation in the unilaterally lesioned rat. Prodrug methodology may thus prove useful in designing new selectively acting DA agonists.

Dopamine receptors in rat striatum and nucleus accumbens; conformational studies using rigid analogues of dopamine

A study was made of the actions of dopamine and of some 2-amino-1,2,3,4-tetrahydronaphthalenes on dopamine-sensitive adenylate cyclase in homogenates of rat striatum and nucleus accumbens. The compounds were also tested for their ability to stimulate motor activity following bilateral injection into the nucleus accumbens of conscious rats. The most active compounds on adenylate cyclase from both striatum and nucleus accumbens were dopamine and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene(6,7-diOHATN). The 5,6-dihydroxy analogue (5,6-diOHATN) was 50 times less active than 6,7-diOHATN in striatal homogenates and 350 times less active in homogenates of nucleus accumbens. All dihydroxy compounds tested were active in causing stimulation of motor activity, the most active compounds being 6,7-and5,6diOHATN. Both dimethoxy derivatives tested were inactive on the adenylate cyclase and as locomotor stimulants.

Conformational analysis of dopamine by the INDO molecular orbital method

The results of INDO calculations on dopamine are reported. A conformational energy map and an isodistance map for the key distances N-OH1, N-OH2 in dopamine as functions of the two main torsion angles tau1 and tau2 were constructed. In addition to the three known minima of dopamine corresponding to the trans and gauche forms, two new minima were found. The key distances of the rigid analogues of dopamine, apomorphine, isoapomorphine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene and isoquinoline were plotted on the isodistance map of dopamine. By taking the corresponding tau values as coordinates on the energy map, conformations of dopamine, resembling the rigid analogues, could be found. When a conformation is close to a local minimum it is assumed that this conformation is energetically favourable. The possible relation between the energy minima and the biological action of dopamine is discussed. An explanation is suggested for the lack of dopaminergic activity of isoapomorphine.

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.

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.

Comparison of antimicrobial activity of nuclear-substituted aromatic esters of 5-dimethylamino-1-phenyl-3-pentanol and 3-dimethylamino-1-phenyl-1-propanol with related cyclic analogs

A series of six aromatic esters of both 5-dimethyl-amino-1-phenyl-3-pentanol and 3-dimethylamino-1-(2-phenylcyclohexyl)-1-propanol was prepared. Antimicrobial evaluation showed that the cyclic analogs had approximately twice the activity of the open chain series; in particular, the o-chlorophenyl ester showed pronounced activity against three pathogenic fungi at approximately 10 ppm. Aromatic esters of 3-dimethylamino-1-phenyl-1-propanol were prepared and demonstrated lower activity than two esters of 2-dimethylamino-1-phenylcyclohexanol. The screening results showed that the best activity was found when a dimethylene chain was present between the phenyl ring and the carbon atom bearing the acyloxy function and that the cyclic derivatives were more active than their more flexible counterparts.

Aporphines. 15. Action of aporphine alkaloids on dopaminergic mechanisms in rat brain

Of eleven aporphine analogues tested on striatal adenylate cyclase only (-)-apomorphine and (+/-)-N-n-propyl-norapomorphine (+/-(NPA)) were effective in stimulating the cyclase from rat brain. Inactive compounds included (+/-)-isoapomorphine, (-)-1,2-dihydroxyaporphine and (+/-)-10-hydroxy-N-n-propylnoraporphine. (+)-Bulbocapnine was an effective antagonist of the stimulating effects of dopamine or (-)-apomorphine on striatal adenylate cyclase. Injection of (-)-apomorphine into the lateral ventricle of rats with unilateral 6-hydroxydopamine-induced lesions of the nigro-striatal pathway caused the animals to rotate away from the side of the lesion. Intraventricular injection of 25 mug (+/-)-10-hydroxy-N-n-propylnorapomorphine was ineffective in producing rotation. The results are discussed in relation to the structural requirements for CNS dopamine receptor agonists.

Dopamine receptor blockade and the neuroleptics, a crystallographic study

The X-ray structures of 12 drugs of the tricyclic class having varying pharmacological profiles have been examined in detail in an attempt to rationalize the known structure-activity relations of neuroleptic drugs with respect to their ability to block dopamine receptors in the brain. Further evidence is presented in support of the theory that the neuroleptics are able to block dopamine receptors because of a conformational complementarity between certain portions of these drugs and dopamine.

Effects of desipramine on neuronal responses to dopamine, noradrenaline, 5-hydroxytryptamine and acetylcholine in the caudate nucleus of the rat

1 The sensitivity of single neurones to microelectrophoretically applied dopamine, noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholing (ACh) was investigated in the caudate nucleus of the rat, anaesthetized with halothane. Both excitatory and depressant responses could be observed to each of the agonists. There was a high correlation between the direction of responses to dopamine and noradrenaline, whereas there was no significant correlation between the direction of responses to dopamine and ACh. 2 The effect of desipramine was studied on both excitatory and depressant responses to dopamine, NA and 5-HT, and on excitatory responses to ACh. Both potentiation and antagonism of neuronal responses to monoamines and ACh could be observed after a brief application of desipramine. 3 Excitatory responses to glutamate were not affected by desipramine. 4 The observation that responses to dopamine and NA can be potentiated by desipramine in the caudate nucleus suggests that uptake blockade is not a prerequisite for potentiation. 5 It is suggested that the potentiation of neuronal responses to dopamine by desipramine may be responsible for the therapeutic efficacy of desipramine in Parkinson's disease.

Studies on the behavioural pharmacology of a cyclic analogue of dopamine following its injection into the brains of conscious rats

1. The cyclic analogue of dopamine, 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADTN) was injected into the lateral ventricle or bilaterally into the nucleus accumbens or caudate nucleus of conscious rats and its effect on locomotor activity was investigated. 2. When given intraventricularly, ADTN produced some stereotyped responses which were followed by a strong and long lasting stimulation of locomotor activity. When administered bilaterally into the nucleus accumbens a similar stimulation of locomotor activity was observed. ADTN had no effect on locomotor activity when injected bilaterally into the caudate nucleus. 3. The ADTN-induced locomotor stimulation following its intraventricular injection was completely abolished by a low dose of pimozide (0.01 mg/kg, i.p.) or haloperidol (0.5 mg/kg, i.p.). Pimozide (1 mg/kg, i.p.) given 30 min before ADTN injected bilaterally into the nucleus accumbens completely blocked locomotor stimulation. 4. Unilateral injections of ADTN (5 mug) into the nucleus accumbens caused locomotor stimulation but no turning. 5. Bilateral injections into the nucleus accumbens of 2-amino-6,7-dimethoxy-1,2,3,4-tetrahydronaphthalene or 0.9 percent w/v NaCl solution had no effect on locomotor activity. 6. It is concluded that the central stimulant action of ADTN is due to an effect on the dopamine receptors in the nucleus accumbens.