A new dopaminergic prodrug

Modulating amacrine cell-derived dopamine signaling promotes optic nerve regeneration and preserves visual function

As part of the central nervous system, the optic nerve, composed of axons from retinal ganglion cells (RGCs), generally fails to regenerate on its own when injured in adult mammals. An innovative approach to promoting optic nerve regeneration involves manipulating the interactions between amacrine cells (ACs) and RGCs. Here, we identified a unique AC subtype, dopaminergic ACs (DACs), that responded early after optic nerve crush by down-regulating neuronal activity and reducing retinal dopamine (DA) release. Activating DACs or augmenting DA release with levodopa demonstrated neuroprotective effects and modestly enhanced axon regeneration. Within this context, we pinpointed the DA receptor D1 (DRD1) as a critical mediator of DAC-derived DA and showed that RGC-specific Drd1 overexpression effectively overcame subtype-specific barriers to regeneration. This strategy markedly boosted RGC survival and axon regeneration after crush and preserved vision in a glaucoma model. This study unveils the crucial role of DAC-derived DA signaling in optic nerve regeneration, holding promise for therapeutic insights into neural repair.

Evaluation of mono- and dibenzoyl esters of dopamine as potential pro-drugs for dopamine in the central nervous system

In this study, two ester pro-drugs of dopamine (DA) were synthesized and evaluated. These derivatives were the monobenzoyl (MBDA) and dibenzoyl (DBDA) esters of DA. MBDA was 300-fold and DBDA was 20,000-fold more lipophilic than DA itself. The half-lives of hydrolysis for MBDA and DBDA at physiologic pH and temperature were 15 and 420 min respectively. These compounds were radiolabelled and their uptake into brain measured. 14C-DBDA penetrated the brain rapidly; 0.28% of the dose injected was taken up per gram of brain tissue at 5 min. However DBDA did not produce measurable increases in DA levels in the brain. 14C-MBDA was found not to penetrate the brain. However, when MBDA was administered intracerebroventricularly (i.c.v.) to rats, it caused DOPAC levels to increase significantly both in the striatum and in the rest of the brain. The increase in the amount of DOPAC measured in the striatum was 3 to 10-fold greater than that seen in the rest of the brain. In rats that were pretreated with the MAO inhibitor, pargyline, MBDA given i.c.v. caused increases in DA levels in both the striatum and in the rest of the brain. The increased DA levels in striatum were considerably greater than those seen in the rest of the brain. From these results, it is inferred that MBDA is being hydrolyzed in vivo in the brain to form DA which is then taken up into dopaminergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Tissue levels of N-n-propylnorapomorphine after treatment with (-)10,11-methylenedioxy-N-n-propylnoraporphine, an orally long-acting prodrug active at central dopamine receptors

High performance liquid chromatography with electrochemical detection was used to assay N-n-propylnorapomorphine (NPA) and other aporphines. Pretreatment of rats with (-)10,11-methylenedioxy-N-n-propylnoraporphine (MDO-NPA) yielded dose-dependent increases in tissue levels of NPA after oral or parenteral administration. Cerebral levels of NPA significantly paralleled the stereo-typed behavioral effects produced by MDO-NPA at several doses and times. Pretreatment with the microsomal oxidase inhibitor SKF-525A (see Methods) prevented these behavioral effects of MDO-NPA and blocked the formation of NPA in vitro. These results support the suggestion that MDO-NPA is a uniquely orally effective and relatively long-acting aporphine which acts at cerebral dopamine receptors as a prodrug of NPA.

Behavioral effects of (-)10,11-methylenedioxy-N-n-propylnoraporphine, an orally effective long-acting agent active at central dopamine receptors, and analogous aporphines

Substituted and unsubstituted 10,11-methylenedioxy derivatives of apomorphine (APO) or its N-propyl congener (NPA) were synthesized and evaluated for their ability to alter motor activity or to induce stereotyped behavior in the rat. Of these, (-)10,11-methylenedioxy-N-n-propylnoraporphine hydrochloride (MDO-NPA) was the most active, and the only compound which was found to be active after oral administration. Also, MDO-NPA was more potent than NPA or APO in producing stereotypy, but large doses of these three aporphines were equipotent in stimulating motor activity. The duration of action of MDO-NPA exceeded that of NPA and APO, and increased with increasing doses. The effects of MDO-NPA on general activity were biphasic: larger doses stimulated activity: smaller doses markedly inhibited it and induced catalepsy. Catalepsy did not occur with NPA or APO and their motor-inhibitory effects were apparent only in aroused rats. The stereotypic effects of MDO-NPA were blocked by small doses of haloperidol, but not by large doses of reserpine. The effects due to large or small doses of MDO-NPA were also blocked by a microsomal enzyme inhibitor which did not interfere with the actions of NPA. These results suggest that MDO-NPA is a long-acting, orally effective prodrug of NPA with depot properties and dose-dependent agonistic and antagonistic interactions with central dopamine-mediated systems.

Neurochemical and behavioural profiles of five dopamine analogues

The dopaminergic actions of five hydroxylated dopamine analogues have been examined for: i) Ability to induce stereotypy, ii) Effects upon dopamine metabolism, iii) Ability to antagonise the rise in striatal dopamine caused by gammabutyrolactone. With the exception of the resorcinol derivative 2-(3,5-dihydroxyphenyl)-N,N-dipropylethylamine, all of the compounds tested exhibited dopamine-like actions, and similarities were found in the induction of stereotypy and in the reduction of dopamine metabolism. For example, 2-(3-hydroxyphenyl)-N,N-dipropylethylamine had a short duration of action as far as reducing dopamine metabolism and inducing stereotypy were concerned. On the other hand, 2-(3-hydroxyphenyl)-N-n-propyl-N-phenyl-ethyl-ethylamine (e) and also 5-hydroxy-2-(N-n-propyl-N-phenylethyl)-aminotetralin had a long duration of agonist-like effects upon both parameters, the aminotetralin derivative being the more potent of the two. Thus, in going from the simple dopamine-like structure to the aminotetralin compound there has been an increase in dopamine agonist-like activity. The differences in dopamine agonist potency of the drugs used are discussed in relation to the structure of these compounds, and are compared with the potencies or related compounds. Also, the potencies of the compounds under investigation upon presynaptic dopamine receptors (using the gammabutyrolactone model as a test system) were investigated, and the ester, 2-(3-benzoyloxyphenyl)-N-n-propyl-N-phenylethyl-ethylamine was the most potent. This ester, which is probably converted to (e) in the brain, also had a long duration of action in the stereotypy and dopamine metabolism tests. The results suggest that certain of the compounds might be useful leads for the design of dopamine agonists of possible clinical use.

Catecholaminergic involvement in the biphasic effect of ADATN on temperature in mice

A prodrug of 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene (ADHTN), 2-amino-6,7-diacetoxy-1,2,3,4-tetrahydronaphthalene (ADATN), was tested for activation of central dopaminergic receptors as measured by colonic temperature in mice. Intraperitoneal injections of ADATN (200 and 800 mumol/kg) produced hypothermia; and intermediate dose (400 mumol/kg) caused hyperthermia followed by hypothermia. A similar biphasic effect occurred with the one dose of ADHTN (800 mumol/kg) tested. Pretreatments with catecholaminergic antagonists were used to characterize the biphasic effect of the intermediate dose of ADATN. The hyperthermic phase depended on the release of endogenous catecholamines which activated alpha- and beta-adrenergic receptors. The hypothermic phase was mediated by a direct activation of dopaminergic receptors. The results are discussed in terms of the pharmacodynamics of a prodrug, the actions of ADHTN, and the known effects of indirect- and direct-acting catecholaminergic agonists on temperature. The involvement of different catecholaminergic systems in the effects of ADATN may limit its usefulness as a dopaminergic prodrug.

Involvement of both dopaminergic and alpha-adrenergic receptors in the hypomotility induced by dibenzoyl-6,7-ADTN

The dopaminergic prodrug dibenzoyl-6,7-ADTN (DB-6,7-ADTN) can enter the brain following intraperitoneal injection and be hydrolysed to produce low concentrations of the dopamine agonist 6,7-ADTN. Intraperitoneal injections of DB-6,7-ADTN produce a decrease in motor activity and in the present study this response has been characterised, and the underlying mechanisms examined. Doses of 10-100 mumol/kg DB-6,7-ADTN elicit a strong hypomotive response, which is dose dependent. Treated animals are significantly less active than controls. DB-6,7-ADTN hypomotility was significantly attenuated by the non-sedative dopamine receptor antagonist sulpiride (62 mumol/kg, i.p.), but haloperidol (0.3 mumol/kg, i.p.) and cis-flupenthixol (0.45 mumol/kg, i.p.) were without effect. The hypomotility due to DB-6,7-ADTN was also antagonised by yohimbine (13 mumol/kg, i.p.) and piperoxane (21 mumol/kg i.p.), drugs which act mainly by blocking presynaptic (alpha 2) adrenergic receptors. Prazosin (1.5 mumol/kg, i.p.), drugs which act mainly by blocking presynaptic (alpha 2) adrenergic receptors. Prazosin (1.5 mumol/kg, i.p.), a postsynaptic (alpha 1) adrenergic blocker, did not affect the hypomotility, and nor did a range of other neurotransmitter antagonists. DB-6,7-ADTN (50 mumol/kg, i.p.) was also found to antagonise the alpha-methyltyrosine (alpha-MT, 1.02 mmol/kg, i.p.) induced fall in noradrenaline and dopamine levels in brain and spinal cord. Inhibition of the effects of DB-6,7-ADTN on noradrenaline and dopamine turnover by yohimbine and sulpiride, respectively, suggests that 6,7-ADTN (derived from the prodrug) has alpha adrenergic as well as dopaminergic activity. The results are discussed in connection with the hypomotive effects of other dopamine agonists.

The significance of COMT activity in controlling dopamine agonist levels in brain and serum: studies with a prodrug and a metabolite of 6,7-ADTN

After i.p. administration of the dibenzoylester of 2-amino-6,7-dihydroxytetralin (DB-6,7-ADTN) a metabolite was found in rat brain and serum, which was identified as 2-amino-6-hydroxy-7-methoxytetralin (7-O-MeADTN). By means of HPLC coupled with amperometric detection, time-concentration curves of 7-O-MeADTN in rat brain and serum were determined after 100 mumol/kg DB-6,7-ADTN. These showed a rapid formation and homogeneous distribution of high peak levels (4 nmol/g) of 7-O-MeATN. Brain and serum concentrations of 6,7-ADTN after 100 mumol/kg DB-6,7-ADTN, determined during catechol-O-methyltransferase (COMT) inhibition by tropolone, were 5--7 times higher than those during normal COMT activity, thus equalling 5,6-ADTN concentrations after 100 mumol/kg DB-5,6-ADTN. The greater susceptibility of 6,7-ADTN to metabolic degradation by COMT was confirmed by preliminary results of in vitro studies, which showed that two methoxy derivatives are formed from 6,7-ADTN and one, in very small amounts, from 5,6-ADTN. 7-O-MeADTN had no dopaminergic activity of its own, as after i.p. injection of 100 mumol/kg it was devoid of behavioural and biochemical effects typical for DA agonists. A homogeneous distribution and high peak concentrations (20 nmol/g after 15 min) were found in rat brain after this dose of the metabolite. The results indicate that a substantial amount of 6,7-ADTN, in contrast to 5,6-ADTN, is metabolized by COMT and that differences between brain concentrations of both isomers are almost exclusively due to differences in susceptibility for COMT. This has implications for the design of new DA agonists.

Selective storage in vivo of 5,6-ADTN in dopamine-rich areas of the rat brain

This study investigates the mechanism of the selective storage of the dopamine agonist 2-amino-5,6-dihydroxytetrahydronaphthalene (5,6-ADTN, used as the esterified prodrug) in densely innervated dopaminergic brain structures. The accumulation of 5,6-ADTN in dopamine-rich areas appeared to be non-saturable at the doses studied. In rats with a unilateral 6-OHDA lesion there was a corresponding unilateral decrease in 5,6-ADTN concentrations which suggests that the agonist interacts with dopamine uptake or storage mechanisms. Apart from an initial decreased penetration in the brain, reserpine-induced depletion of dopamine stores did not influence the accumulation of 5,6-ADTN in dopamine-rich areas, which suggests that 5,6-ADTN is taken up in the so-called 'non-reserpine sensitive DA-pool'. Unexpectedly, reserpine pretreatment caused a long-lasting (at least 170 h) decrease of 5,6-ADTN distribution in non-dopaminergic structures. 5,6-ADTN (used as the prodrug) could have a role to play in the elucidation of the significance of the various dopamine pools.

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.

Do neuroleptics prevent the penetration of dopamine agonists into the brain?

A new, specific and highly sensitive method for the determination of apomorphine based on high performance liquid chromatography on a C18 reverse-phase column, coupled with electrochemical detection has been developed. The limit of detection of the assay is approximately 0.5 ng/sample (2 pmol). Haloperidol, cis-flupenthixol, metoclopramide and reserpine prevented the accumulation of apomorphine equally in "dopaminergic" as well as in "non-dopaminergic" brain areas. The non-neuroleptic trans-isomer of flupenthixol was without effect. Suppression of the accumulation of the dopamine agonist 6,7-ADTN (administered as the prodrug dibenzoyl-6,7-ADTN) was seen after combined treatment with haloperidol or reserpine, whereas cis- and trans-flupenthixol were without effect. The results imply that certain behavioural, biochemical and neuropharmacological studies, based on apomorphine in combination with other drugs, may need reinterpretation.