Agonistic actions of DPI (2-(3,4-dihydroxyphenylimino)-imidazolidine) on alpha-adrenoceptors and dopamine receptors

Effects of DPI (2-(3,4-dihydroxyphenylimino)-imidazolidine) on prejunctional alpha-adrenoceptors and prejunctional dopamine receptors in the rabbit ear artery

The effects of DPI on transmitter noradrenaline release were examined in rabbit isolated ear arteries previously incubated in [ 3H ]noradrenaline. DPI (0.1 mumol/l) reduced transmitter release with periarterial electrical stimulation at 1 Hz (1 ms pulses, supramaximal voltage for 30 s), which is consistent with an agonistic effect of DPI on prejunctional alpha-adrenoceptors. At stimulation frequencies of 2 or 5 Hz DPI alone did not affect release, however in the presence of phentolamine (3 mumol/l, which increased release per se) DPI again significantly reduced release; this effect was blocked by metoclopramide (1 mumol/l) indicating an agonistic effect of DPI on prejunctional dopamine receptors.

Involvement of the septum in central dopamine-acetylcholine interactions in morphine-treated cats

The involvement of the septum in central dopamine-acetylcholine (DA-ACh) interactions was investigated by analysis of the behavioural effects of intracerebrally injected drugs in cats pretreated with morphine (5 mg/kg, IP). The intracerebrally evoked effects on the morphine-induced behaviour were analyzed both quantitatively (changes in the incidence of locomotor patterns) and qualitatively (changes in the stereotyped nature of the behaviour patterns). Activation of a particular subclass of dopamine receptors (DAi receptors) within the septum by means of the DAi agonist (3,4-dihydroxyphenylamino)-2-imidazoline (DPI) suppressed the effect of intraseptal injections of the cholinergic agonist carbachol. Simultaneous activation of the DAi receptors within the septum and those within the caudate nucleus produced an effect characteristics of activation of septal DAi receptors; the effect characteristic of activation of caudate DAi receptors was suppressed. Analogous results were obtained when the DAi receptors within the septum and those within the caudate nucleus were simultaneously inhibited by the DAI antagonist ergometrine. Finally, it was found that the effect of intraseptal injections of the cholinergic agonist carbachol was suppressed by inhibition of the DAi receptors within the caudate nucleus. In the latter case, the effect characteristic of inhibition of caudate DAi receptors was also suppressed. The present results indicate the existence of interactions between (1) septal ACh activity and septal DAi activity, (2) septal DAi activity and caudate DAi activity, and (3) septal ACh activity . A mechanism of action for the observed interactions is discussed.

Comparison of the vasodilator action of dopamine and dopamine agonists in the renal and coronary beds of the dog

1 The effects of dopamine and the dopamine receptor agonists, SK&F 38393 and bromocriptine, on renal and coronary blood flow in the anaesthetized dog were examined. Dopamine was found to dilate both vascular beds, whereas SK&F 38393 increased renal blood flow but did not have any dilator activity in the coronary vasculature. Bromocriptine did not cause vasodilatation in either vascular bed. 2 The vasodilator responses to dopamine and SK&F 38393 were significantly reduced by the dopamine receptor antagonists, ergometrine or metoclopramide. 3 It is proposed that the selective action of SK&F 38393 on the renal vasculature suggests that the dopamine receptors of the renal and coronary vascular beds may be of different types.