The action of microelectrophoretically applied (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI) on single cortical neurones

Dopaminergic modulation of ACTH-induced grooming

ACTH-(1-24)-induced grooming was studied after administration of the peptide into the substantia nigra or intracerebroventricularly (i.c.v.). The modulation of dopamine receptors in neostriatum (with haloperidol and apomorphine) and nucleus accumbens (with 3,4-dihydroxyphenylamino-2-imidazoline hydrochloride; DPI and ergometrine) was investigated. In the nucleus accumbens, the modulatory effects of ergometrine and DPI on ACTH-(1-24)-induced grooming were based on their affinity for dopamine receptors and not on their affinity for adrenoceptors. Intrastriatal application of dopaminergic agents inhibited i.c.v. ACTH-(1-24)-induced excessive grooming, whereas the grooming score was enhanced if ACTH-(1-24) was administered into the substantia nigra. The finding of differential effects of dopaminergic agents on ACTH-induced excessive grooming depending on the route of administration indicate that i.c.v. ACTH-induced excessive grooming is not mediated solely through the substantia nigra. The increase in grooming behavior seen after the intrastriatal administration of dopaminergic agents - when ACTH was injected into the substantia nigra - suggests the involvement of the striato-nigral GABAergic pathway. Local injections of ACTH-(1-24) into the periaqueductal gray also induced excessive grooming. Since a second injection of ACTH-(1-24) into the periaqueductal gray did not lead to a grooming response, irrespective of where the first injection of ACTH-(1-24) was given (i.c.v. into the nigra or via the periaqueductal gray) it is suggested that this structure seems to play a primary role in the induction of excessive grooming. Therefore the modulatory effects of the dopaminergic influence on ACTH-(1-24)-induced grooming may be exerted via the striato-nigro-collicular pathway.

The pharmacological characterization of 3,4-dihydroxyphenylimino-2-imidazolidine (DPI) as a potent mixed alpha 1/alpha 2-adrenoceptor agonist rather than as a dopamine receptor agonist

In the past decade many studies have been carried out on the pharmacological actions of the imidazoline derivative DPI (3,4-dihydroxyphenylimino-2-imidazolidine) because it has been proposed as a selective agonist at a postulated subtype of dopamine (DA) receptor. According to the first publication on this concept of multiple DA receptors, authored by Cools & van Rossum (1976), mammalian DA receptors can be divided into excitation-mediating (DAe) and inhibition-mediating (DAi) receptors. DAe receptors appeared to coincide with ‘classical’ DA receptors i.e. those that can be selectively stimulated with apomorphine and inhibited with haloperidol, whereas the newly postulated DAi receptors could be selectively stimulated with DPI and inhibited with ergometrine (Cools & van Rossum 1976, 1980). Though interesting from a conceptual point of view, the idea of the existence of DAi receptors in the mammalian brain was rather speculative, and Cools and coworkers consequently put much effort into attempting to provide it with an experimental basis. They found that in some test models DPI and ergometrine had indeed opposing effects, and they explained these results as being in strong support of the presence of DAi receptors and thus of the validity of the DAe/DAi concept (Cools et al 1976; Cools 1977). In spite of the latter conclusion, however, these authors have repeatedly revised their original concept, i.e. by the additional postulations of the ‘alpha-like norepinephrine receptor’ (Cools & van Rossum 1980) and of the ‘DAi/e receptor’ (Cools 1981).

Classification of drugs according to receptor binding profiles

The affinity for eight different neurotransmitter receptors of about forty drugs, used for the treatment of various central nervous system disorders, was determined following in vitro receptor binding assays. Our findings indicate that, in spite of widely varying chemical structures and often poorly understood mechanisms of action, the similarities in the "affinity profiles" permit a clinically meaningful classification of these drugs. Such an approach would thus be useful in the assessment of newly synthesized compounds at an early stage of drug development.

Differentiation of basal ganglia dopaminergic involvement in behavior after hippocampectomy

Large bilateral aspiration lesions of the hippocampus in rats lead to a variety of changes in spontaneous behavior measured in an open field/hole board, relative to sham and neocortically lesioned controls. These changes include increased locomotion, and decreased grooming frequency and rearing bout duration. When animals were injected with the dopamine (DA) agonist 3,4-dihydroxyphenylamino-2-imidazoline (DPI: 0.5, 1.0 and 5.0 microgram) into the nucleus accumbens one week after surgery, the behavior of hippocampally lesioned rats was restored to levels not different from control lesioned rats. Haloperidol injections (0.05, 0.1 and 0.5 microgram) into the caudate nucleus were not able to do this. Further, DPI injected into the caudate month after surgery was also able to attenuate some of the effects of hippocampal damage. On the other hand, haloperidol injections into the nucleus accumbens did not influence behavior. The results are interpreted in terms of hippocampal lesion-induced alteration of a balance in basal ganglia DA systems, indicated by modified response to pharmacological intervention and which mediate the behavioral effects of the lesion.

The purported dopamine agonist DPI inhibits [3H]noradrenaline release from rat cortical slices but not [3H]dopamine and [14C]acetylcholine release from rat striatal slices in-vitro

The effects of the purported dopamine (DA) receptor agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) upon the in-vitro K+-induced release of [3H]DA and [14C]acetylcholine from rat neostriatal slices, and of [3H]noradrenaline from rat neocortical slices have been investigated and compared with those of the DA receptor agonist TL-99 and the alpha-adrenoceptor agonist clonidine, respectively. The rapid decomposition of the catechol compounds DPI and TL-99 in the Krebs-Ringer bicarbonate superfusion medium was shown to be inhibited by both the chelating agent EDTA and the reducing agent ascorbic acid. The results suggest that in-vitro DPI is unable to stimulate striatal DA receptors, whereas it is effective in stimulating cortical alpha 2-adrenoceptors (EC50 = 61 nM). It is concluded that DPI should be considered as a mixed alpha 1/alpha 2-adrenoceptor agonist and that the designation of DPI as a DA receptor agonist should be abandoned.

Dopamine involvement in ACTH-induced grooming behavior

Low doses of dopamine agonists and antagonists were tested for their effects on the excessive grooming behavior induced by intracerebroventricular (ICV) injections of ACTH1-24. Grooming scores were significantly depressed at doses of haloperidol, metoclopramide, pimozide, and butaclamol that did not decrease locomotor activity. In fact at two doses of haloperidol (0.067 and 0.10 mg/kg), grooming scores were decreased while locomotor activity was increased significantly. Metoclopramide increased grooming scores at a dose reported to block presynaptic dopamine receptors. Apomorphine potentiated the grooming induced by low doses of ACTH. These data support the hypothesis that dopaminergic neurotransmission is necessary for the display of ACTH-induced grooming behavior.

Evidence that the purported dopaminergic agonist (3,4-dihydroxyphenylimino)-2-imidazolidine (DPI) may reduce rat striatal dopamine turnover by an alpha 2-adrenergic mechanism

The potent alpha-adrenergic agonist DPI, which has also been claimed to be a selective dopaminergic agonist, was shown to reduce rat striatal dopamine (DA) synthesis, DA utilization and DA metabolism following intraperitoneal administration (25 mumol/kg). An analytical procedure for the determination of DPI was developed and its application showed that DPI did not penetrate into the brain in substantial amounts. The possibility of a direct stimulatory action upon striatal presynaptic DA receptors was excluded by the demonstration that DPI lacked effectiveness both in the gamma-butyrolactone model and following intrastriatal administration. The selective alpha-adrenergic agonists phenylephrine (alpha 1) and tramazoline (alpha 2) decreased and increased DA metabolism, respectively, the yohimbine-induced increase being antagonized by DPI. The carbon-bridge analogue (3,4-dihydroxybenzyl)-2-imidazoline (DHBI) had about the same activity as DPI, whereas the potential DPI metabolite (4-hydroxy-3-methoxyphenylimino)-2-imidazolidine (HMPI) was without effect upon striatal DA metabolism. The results are discussed in relation to the remarkable resemblance with the literature data concerning clonidine. It is concluded that the DPI-elicited attenuation of striatal DA turnover is, in all likelihood, the result of a stimulation of alpha 2-adrenoceptors possibly located within the central nervous system. The results cast some doubt on the designation of DPI as a selective DA-inhibitory receptor agonist.

Changes in dopamine and DOPAC following systemic administration of apomorphine and 3,4-dihydroxyphenylamino-2-imidazoline (DPI) in rats

Rats were injected systemically with either saline, apomorphine, or one of four doses of DPI (3,4-dihydroxyphenylamino-2-imidazoline) and the levels of dopamine and DOPAC determined in the nucleus accumbens and the caudate regions. DPI reduced dopamine and DOPAC levels in the nucleus accumbens, while apomorphine did not. On the other hand, apomorphine reduced the levels of dopamine and DOPAC in the caudate but not the nucleus accumbens. DPI largely was without effect in the caudate. The results are discussed in terms of possible specificity of the two dopamine agonists in the two forebrain regions.

Presence of a particular subpopulation of dopamine receptors within the septal nuclei: a behavioural study on cats

The behavioural effects of intraseptal administration of dopaminergic drugs (apomorphine, haloperidol, (3,4-dihydroxy-phenylamino)-2-imidazoline (DPI), ergometrine and dopamine) and alpha-noradrenergic drugs (oxymetazoline, noradrenaline and phentolamine) were analysed in cats pretreated with morphine (5 mg/kg, i.p.). Changes in frequencies of stereotyped locomotor patterns were used for statistical evaluation of drug-induced effects. Taking advantage of the specificity of the drugs mentioned, a distinction could be made between effects mediated via excitation-mediating dopamine (DAe), inhibition-mediating dopamine (DAi) and alpha-noradrenaline (alpha-NA) receptors. Intraseptal injection of the DAi agonist DPI resulted in a decrease in the frequency of stereotyped locomotor patterns. This effect was dose-dependent and mimicked by that of intraseptally applied dopamine but not of any of the other drugs. Moreover, intraseptal injection of the DAi antagonist ergometrine inhibited the effect of DPI. The DAe agonist apomorphine as well as the DAe antagonist haloperidol remained ineffective when applied in low doses. The alpha-NA antagonist phentolamine and a rather high dose of haloperidol produced a slight but significant increase in the frequency of locomotor patterns; intraseptally applied oxymetazoline counteracted the phentolamine-induced effect. It is concluded that the septal nuclei of cats contain functionally active, alpha-NA receptors as well as functionally active, dopamine (DA) receptors having pharmacological properties identical to those of DA receptors present within the mesolimbic structures such as the nucleus accumbens: the so-called inhibition-mediating DAi receptors.