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.

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.

Effects of specific dopamine lesions and dopamine receptor sensitivity on angiotensin II- and carbachol-induced thirst in rats

A study was made of the effects of manipulating brain dopaminergic activity upon drinking induced by intracerebroventricular administration of angiotensin II or carbachol. Non-specific lesions induced by injecting 6-hydroxydopamine (6-OHDA) into the cerebroventricles caused a significant reduction in angiotensin-induced thirst without affecting carbachol drinking. specific 6-OHDA-induced lesions of the dopaminergic nigro-striatal pathway also attenuated the angiotensin-induced response, while unilateral lesions reduced and bilateral lesions almost completely abolished the effect. Again, the response to carbachol was unaffected. Chronic haloperidol treatment increased behavioural responses to the dopamine agonist apomorphine and significantly stimulated angiotensin-induced drinking without affecting response to carbachol. These studies provide support for the hypothesis that a dopaminergic event is involved in the angiotensin-induced thirst response and point to the need for a functioning dopaminergic nigro-striatal pathway for the full expression of this response.

The effect of neuroleptic drugs on drinking induced by central administration of angiotensin or carbachol

The effect of a series of neuroleptic drugs on the drinking response elicited by intracerebroventricular injection of either angiotensin or carbachol into conscious rats was studied. The i.p. injection of haloperidol, cis-flupenthixol, or fluphenazine antagonized both angiotensin-induced and carbachol-induced drinking. When injected into the lateral ventricles, the neuroleptics haloperidol, fluphenazine, cis-fluphenthixol and sulpiride were potent inhibitors of angiotensin-induced drinking, but had little effect on the dipsogenic action of carbachol. Clozapine, administered centrally, antagonized drinking caused by both angiotensin and carbachol. Pimozide and chlorpromazine were also potent inhibitors of angiotensin-induced drinking, while trans-flupenthixol was inactive. Our results support the concept of an involvement of dopamine in angiotensin-induced drinking.