D-2 dopamine autoreceptor selective drugs: do they really exist?

The catecholamine dopamine plays an important role as a neurotransmitter or neurohormone in the brain and pituitary gland. Dopamine exerts its effects through activation of two types of receptors called D-1 and D-2. These receptors are distinguished by their different pharmacological characteristics and signal transduction mechanism(s). Release of dopamine inhibits the activity of dopaminergic neurons through activation of so-called dopamine autoreceptors which are of the D-2 type. In general, these receptors occur both in the soma-dendritic region of the dopaminergic neuron, where they are involved in the inhibition of the firing rate and on the dopaminergic terminals where they mediate the inhibition of dopamine synthesis and release. D-2 receptors occur also on the target cells of dopaminergic neurons both in the brain (postsynaptic D-2 receptors) and pituitary gland. On the basis of data gathered from in vivo (behavioral- as well as electrophysiological) studies it has been concluded that D-2 agonists are much more potent at dopamine autoreceptors as compared to postsynaptic D-2 receptors, indicating the possibility of a pharmacological distinction between these differentially located D-2 receptors. This concept led to the introduction of a whole group of drugs allegedly displaying a selective agonist profile at the dopamine autoreceptor. In contrast, biochemical (in vitro) studies with brain tissue as well as the pituitary gland, did not reveal any significant difference between the pharmacological profiles of autoreceptors and postsynaptic D-2 receptors. In the present minireview a balanced discussion is presented of these in vivo and in vitro findings and it is concluded that both autoreceptors as well as postsynaptic D-2 receptors are similar if not identical entities.     

Signaling Cascade of Diacylglycerol Kinase �� in the Pituitary Intermediate Lobe: Dopamine D2 Receptor/Phospholipase C��4/Diacylglycerol Kinase ��/Protein Kinase C��

The pituitary gland dynamically changes its hormone output under various pathophysiological conditions. One of the pathways implicated in the regulatory mechanism of this gland is a dopaminergic system that operates the phosphoinositide (PI) cycle to transmit downstream signal through second messengers. We have previously shown that diacylglycerol kinase β (DGKβ) is coexpressed with dopamine D1 and D2 receptors in medium spiny neurons of the striatum, suggesting a plausible implication of DGKβ in dopaminergic transmission. However, it remains elusive whether DGKβ is involved in the dopaminergic system in the pituitary gland. The aim of this study is to investigate the expression and localization of DGK in the pituitary gland, together with the molecular components involved in the PI signaling cascade, including dopamine receptors, phospholipase C (PLC), and a major downstream molecule, protein kinase C (PKC). Here we show that DGKβ and the dopamine D2 receptor are coexpressed in the intermediate lobe and localize to the plasma membrane side by side. In addition, we reveal that PLCβ4 and PKCα are the subtypes expressed in the intermediate lobe among those families. These findings will substantiate and further extend our understanding of the molecular-anatomical pathway of PI signaling and the functional roles of DGK in the pituitary intermediate lobe. (J Histochem Cytochem 58:119–129, 2010)     

Differential effects of morphine on the rates of dopamine turnover in the neural and intermediate lobes of the rat pituitary gland

The acute administration of morphine to male rats decreased the rate of dopamine turnover in the median eminence and in the neural lobe of the pituitary, but was without effect in the intermediate lobe of the pituitary. Pretreatment with the opiate antagonist, naltrexone, reduced the effects of morphine. These results indicate that morphine, by acting on opiate receptors, inhibits the activity of tuberoinfundibular dopaminergic neurons that terminate in the median eminence and those tuberohypophysial dopaminergic neurons that terminate in the neural lobe of the pituitary.     

Effects of ergots on adenylate cyclase activity in the corpus striatum and pituitary

Adenylate cyclase activity was determined in homogenates of the corpus striatum and pituitary gland. Dopamine and several ergots stimulated cyclic AMP synthesis in the striatum, but no stimulation was seen in the pituitary gland. None of the ergots tested were as active as dopamine itself, and all were able to partially inhibit the dopamine-induced activation of adenylate cyclase. Lergotrile, a simple ergoline derivative which displays dopamine agonist activities in the pituitary gland and striatum, did not stimulate adenylate cyclase in either tissue. These findings show that the $\text{in vivo}$ dopaminergic activity of ergots is not reflected in the dopamine-dependent adenylate cyclase assay using either the corpus striatum or the pituitary gland. It is suggested that those dopamine receptors in the pituitary gland which mediate prolactin release are not associated with adenylate cyclase.     

Dopamine D1 and D2 Receptor Immunoreactivities in the Arcuate-Median Eminence Complex and their Link to the Tubero-Infundibular Dopamine Neurons

Dopamine D1 and D2 receptor immunohistochemistry and Golgi techniques were used to study the structure of the adult rat arcuate-median eminence complex, and determine the distribution of the dopamine D1 and D2 receptor immunoreactivities therein, particularly in relation to the tubero-infundibular dopamine neurons. Punctate dopamine D1 and D2 receptor immunoreactivities, likely located on nerve terminals, were enriched in the lateral palisade zone built up of nerve terminals, while the densities were low to modest in the medial palisade zone. A codistribution of dopamine D1 receptor or dopamine D2 receptor immunoreactive puncta with tyrosine hydroxylase immunoreactive nerve terminals was demonstrated in the external layer. Dopamine D1 receptor but not dopamine D2 receptor immnunoreactivites nerve cell bodies were found in the ventromedial part of the arcuate nucleus and in the lateral part of the internal layer of the median eminence forming a continuous cell mass presumably representing neuropeptide Y immunoreactive nerve cell bodies. The major arcuate dopamine/ tyrosine hydroxylase nerve cell group was found in the dorsomedial part. A large number of tyrosine hydroxylase immunoreactive nerve cell bodies in this region demonstrated punctate dopamine D1 receptor immunoreactivity but only a few presented dopamine D2 receptor immunoreactivity which were mainly found in a substantial number of tyrosine hydroxylase cell bodies of the ventral periventricular hypothalamic nucleus, also belonging to the tuberoinfundibular dopamine neurons. Structural evidence for projections of the arcuate nerve cells into the median eminence was also obtained. Distal axons formed horizontal axons in the internal layer issuing a variable number of collaterals classified into single or multiple strands located in the external layer increasing our understanding of the dopamine nerve terminal networks in this region. Dopamine D1 and D2 receptors may therefore directly and differentially modulate the activity and/or Dopamine synthesis of substantial numbers of tubero-infundibular dopamine neurons at the somatic and terminal level. The immunohistochemical work also gives support to the view that dopamine D1 receptors and/or dopamine D2 receptors in the lateral palisade zone by mediating dopamine volume transmission may contribute to the inhibition of luteinizing hormone releasing hormone release from nerve terminals in this region.Key words: Dopamine D1 and D2 receptors, tubero-infundibular dopamine neurons, dopamine receptor colocalization, arcuate-median eminence complex, volume transmission, luteinizing hormone releasing hormone     

Desensitization of the D-2 dopamine receptor and the β2-adrenoceptor in the intermediate lobe of the rat pituitary gland

A D-2 dopamine receptor and a β₂-adrenoceptor occur in the intermediate lobe of the rat pituitary gland (IL). Exposure of intact IL tissue to a D-2 agonist diminished the ability of dopaminergic agonists [but not 5′-guanylyl imidodiphosphate (Gpp(NH)p)] to inhibit adenylate cyclase activity. Conversely, exposure of intact IL tissue to a β-adrenergic agonist diminished the ability of a β-adrenergic agonist (but not forskolin) to stimulate adenylate cyclase activity. Treatment of ovariectomized rats with 17β-estradiol desensitizes the β₂-adrenoceptor but not the D-2 receptor. Desensitization of the IL catecholamine receptors is discussed within the framework of a previously published “working model” of these receptors.     

Binding site of salsolinol: its properties in different regions of the brain and the pituitary gland of the rat

It has been recently shown that salsolinol (SAL) is present in the hypothalamic neuroendocrine dopaminergic (NEDA) system and appears to be a selective and potent stimulator of prolactin (PRL) secretion in the rat. Furthermore, the lack of interference of SAL with 3H-spiperone binding in the striatum and the anterior lobe (AL) of the pituitary gland has been also demonstrated. These data clearly indicate that SAL does not act at the dopamine (DA) D(2) receptors, and suggest that SAL supposedly has a binding site through which the secretion of PRL may be affected. Therefore, binding of 3H-SAL to different regions of the central nervous system (CNS) has been investigated. Specific and saturable binding has been detected in the striatum, cortex, median eminence and in the hypothalamus as well as in the AL and the neuro-intermediate lobe (NIL) of the pituitary gland. K(D) values of the bindings were in the nanomolar range in all tissue tested. 3H-SAL displacing activity of several agonists and antagonists of known DA receptors have also been tested. It has been found that DA and in a lesser extent, apomorphine could displace 3H-SAL, but other DA receptor specific ligands have not been able to affect it. Furthermore, several pharmacologically active compounds, selected on the basis of their influence on DA synthesis, transport mechanisms and signal transduction, have also been tested. Neither mazindol (a selective DA transporter inhibitor) nor clonidine (an alpha(2)-adrenoreceptor agonist) could alter SAL binding. At the same time, L-dopa, carbidopa, benserazide and alpha-methyldopa were able to displace 3H-SAL. The possible changes in SAL binding due to physiological and pharmacological stimuli, like suckling stimulus and reserpine pretreatment (that blocks vesicular monoamine transport in DA terminals), respectively, have also been investigated. In the NIL of the pituitary gland and in the median eminence of the hypothalamus the binding decreased following 10 min of suckling stimulus compared to the binding detected in the same tissues obtained from mothers separated from their pups for 4h and not allowed to be suckled. At the same time, there were no changes in the binding at the AL and striatum. Following reserpine pretreatment that has completely prevented PRL releasing effect of SAL, the binding was significantly augmented. These results support our assumption that SAL should have specific binding sites through which it can affect PRL secretion. Furthermore, it clearly suggests that it may regulate DAergic neurotransmission of NEDA neurons by an altered intracellular or intraterminal synthesis and/or distribution of hypophysiotropic DA.     

Photoaffinity labeling of D-2 dopamine binding subunits from rat striatum, anterior pituitary and olfactory bulb with a new probe, [3H]azidosulpride

A novel photoaffinity probe [3H] azidosulpride has been developed for biochemical studies of D-2 dopamine receptors. This ligand binds to the receptors with high affinity (Kd = 3.1 +/- 0.2 nM) and, upon photoactivation, about 20% of the radioactivity bound to membranes becomes covalently incorporated. More than 90% of this irreversible binding is protectable by dopaminergic agents including D-2 selective compounds, whereas D-1 selective and non-dopaminergic compounds are ineffective. Analysis by sodium dodecylsulfate polyacrylamide gel electrophoresis reveals a single band at Mr = 85 kDa for labeled receptors in striatum, anterior pituitary or olfactory bulb, where pharmacologically distinct binding sites have been previously detected.     

An Evaluation of the Interactions of Antiestrogens with Pituitary and Striatal Dopamine Receptors

Abstract

We have examined the ability of various antiestrogens (AE's) to compete with ³H-spiroperidol for binding to membrane preparations from striatal tissue and anterior pituitary glands of immature female rats in order to determine the affinity of binding of AE's to D-2 dopamine receptors. Scatchard analyses revealed the presence of a single class of high affinity receptor sites in both the striatum and pituitary with a dissociation constant (K_d) of 0.33 nM and 0.40 nM, respectively, for the dopamine antagonist spiroperidol. The AE's tamoxifen, 4-hydroxy-tamoxifen (TAM-OH), CI-628, LY 117018, and a structurally related compound t-butyl-phenoxyethyl diethylamine (BPEA) were all able to compete with spiroperidol for binding to D-2 receptors and demonstrated relative binding affinities of 0.4-0.06%, with spiroperidol set at 100%. Dopamine displayed a lower affinity, 0.01%. Estradiol failed to compete with spiroperidol for D-2 receptor binding while the non-steroidal estrogen diethylstilbestrol (DES) showed very week competition. For the lipophilic AE's, alteration of the level of their non-specific binding greatly affected their relative affinities in these competitive binding assays. The amine side chain on an aromatic ring appears to be a critical structural requirement in allowing the AE's to bind to the dopamine receptor. The relatively low affinity of AE's for the dopamine receptor and the high degree of interaction of AE's with other proteins suggest that only limited occupancy of D-2 receptors by AE's is likely in vivo.     

Lack of Dopaminergic Inputs Elongates the Primary Cilia of Striatal Neurons

In the rodent brain, certain G protein-coupled receptors and adenylyl cyclase type 3 are known to localize to the neuronal primary cilium, a primitive sensory organelle protruding singly from almost all neurons. A recent chemical screening study demonstrated that many compounds targeting dopamine receptors regulate the assembly of Chlamydomonas reinhardtii flagella, structures which are analogous to vertebrate cilia. Here we investigated the effects of dopaminergic inputs loss on the architecture of neuronal primary cilia in the rodent striatum, a brain region that receives major dopaminergic projections from the midbrain. We first analyzed the lengths of neuronal cilia in the dorsolateral striatum of hemi-parkinsonian rats with unilateral lesions of the nigrostriatal dopamine pathway. In these rats, the striatal neuronal cilia were significantly longer on the lesioned side than on the non-lesioned side. In mice, the repeated injection of reserpine, a dopamine-depleting agent, elongated neuronal cilia in the striatum. The combined administration of agonists for dopamine receptor type 2 (D2) with reserpine attenuated the elongation of striatal neuronal cilia. Repeated treatment with an antagonist of D2, but not of dopamine receptor type 1 (D1), elongated the striatal neuronal cilia. In addition, D2-null mice displayed longer neuronal cilia in the striatum compared to wild-type controls. Reserpine treatment elongated the striatal neuronal cilia in D1-null mice but not in D2-null mice. Repeated treatment with a D2 agonist suppressed the elongation of striatal neuronal cilia on the lesioned side of hemi-parkinsonian rats. These results suggest that the elongation of striatal neuronal cilia following the lack of dopaminergic inputs is attributable to the absence of dopaminergic transmission via D2 receptors. Our results provide the first evidence that the length of neuronal cilia can be modified by the lack of a neurotransmitter''s input.     

Multiple classes of dopamine receptors in mammalian central nervous system: the involvement of dopamine-sensitive adenylyl cyclase.

Two classes of dopamine receptor mechanism are defined according to their association with, or independence from, a dopamine-sensitive adenylyl cyclase. Dopamine receptors unrelated to adenylyl cyclase are designated type alpha. Dopamine receptors linked to adenylyl cyclase are designated type beta. Drugs discriminate between the two receptor mechanisms. The dopaminergic ergots (lisuride, lergotrile and CB-154) and their antagonists (such as metoclopramide) are relatively specific for the alpha-dopaminergic receptor in the anterior pituitary. Other agonists (e.g. apomorphine and dopamine) and antagonists (e.g. antipsychotic phenothiazines and butyrophenones) affect both classes of receptor.     

Dopamine D-1 Receptor and Cyclic AMP-Dependent Phosphorylation in Parkinson''s Disease

D-1 and D-2 receptor densities, evaluated respectively by [3H]SCH 23390 and [3H]spiperone binding, and DARPP-32 (dopamine and adenosine 3':5'-monophosphate-regulated phosphoprotein-32K) concentrations, were studied in the brains of control and parkinsonian subjects postmortem. D-2 receptor density was unchanged in the putamen of parkinsonian patients. D-1 receptor density was unchanged in the putamen and substantia nigra pars reticulata (SNR) of parkinsonian patients, but decreased by 28% in the substantia nigra pars compacta (SNC). DARPP-32, which is localized in the same structures as D-1 receptors of which it is thought to represent the intracellular messenger, decreased by 45% in the putamen, 66% in the SNR, and 79% in the SNC. The decrease in D-1 receptors in the SNC may be due to degeneration of pallidonigral GABAergic neurons, but some of the D-1 receptors may be on the nigrostriatal dopaminergic neurons themselves. The dissociation between the alteration of D-1 receptor densities and DARPP-32 concentrations in both the striatum and substantia nigra, which are of the same order in the two structures, may be an index of functional hypoactivity of D-1 neurotransmission.     

Pharmacological and molecular basis for dopamine D-2 receptor diversity

This review will focus on the main lines of evidence that suggest the existence of multiple types of dopamine D-2 receptors. Dopamine D-2 receptors share structural elements suggesting that they belong to a gene superfamily classified as G-protein-coupled receptors and show an archetypical topology predicted to consist of seven putative transmembrane domains. Activation of D-2 receptors results in a variety of responses, including inhibition of cyclic AMP formation, inhibition of phosphoinositol turnover, increase of K-channel activity, and identified, nor has the possible hierarchy of these regulatory proteins in transforming the incoming signal into a change of second-messenger levels. A lot of experimental data support the hypothesis that there are multiple signal-processing pathways activated by dopamine through D-2-receptor stimulation. Recently, the identification of dopaminergic drugs that discriminate among the different transduction pathways and the isolation of distinct cDNAs encoding proteins that share binding profile indicative of D-2 receptors clearly indicate multiple forms of D-2 receptors. Pharmacologically, at least two distinct categories of dopamine D-2 receptors exist in rat pituitary. The first (D-2a) is insensitive to BHT 920 and coupled to inhibition of adenylyl cyclase activity; the second (D-2b) is activated by BHT 920 and linked to voltage-dependent K channels. The two types of dopamine D-2 receptors differ in their structure, G-protein-coupled and effector. Each of the three basic receptor units shows a certain degree of heterogeneity, which may affect the quality and the kinetic of the response. This variety may represent the molecular basis for the diversity in pharmacological and functional profiles of different dopamine D-2 receptors located in various brain areas and peripheral tissues.     

Effects of Cannabinoids on Dopamine Release in the Corpus Striatum and the Nucleus Accumbens In Vitro

Cannabinoid receptors are widely distributed in the nuclei of the extrapyramidal motor and mesolimbic reward systems; their exact functions are, however, not known. The aim of the present study was to characterize the effects of cannabinoids on the electrically evoked release of endogenous dopamine in the corpus striatum and the nucleus accumbens. In rat brain slices dopamine release elicited by single electrical pulses was determined by fast cyclic voltammetry. Dopamine release was markedly inhibited by the OP2 opioid receptor agonist U-50488 and the D2/D3 dopamine receptor agonist quinpirole, indicating that our method is suitable for studying presynaptic modulation of dopamine release. In contrast, the CB1/CB2 cannabinoid receptor agonists WIN55212-2 (10(-6) M) and CP55940 (10(-6)-10(-5) M) and the CB1 cannabinoid receptor antagonist SR141716A (10(-6) M) had no effect on the electrically evoked dopamine release in the corpus striatum and the nucleus accumbens. The lack of a presynaptic effect on terminals of nigrostriatal and mesolimbic dopaminergic neurons is in accord with the anatomical distribution of cannabinoid receptors: The perikarya of these neurons in the substantia nigra and the ventral tegmental area do not synthesize mRNA, and hence protein, for CB1 and CB2 cannabinoid receptors. It is therefore unlikely that presynaptic modulation of dopamine release in the corpus striatum and the nucleus accumbens plays a role in the extrapyramidal motor and rewarding effects of cannabinoids.     

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.     

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.     

Sexual differences in the activity of periventricular-hypophysial dopaminergic neurons in rats.

The activities of periventricular-hypophysial dopaminergic (DA) neurons were compared in male and female rats by measuring dopamine synthesis (accumulation of 3,4-dihydroxyphenylalanine [DOPA] after inhibition of L-aromatic amino acid decarboxylase) and metabolism (concentrations of 3,4-dihydroxyphenylacetic acid [DOPAC]) in terminals of these neurons in the intermediate lobe of the pituitary. For comparison, the synthesis and metabolism of dopamine in the neural lobe of the pituitary and median eminence were also determined. The concentrations of DOPAC and accumulation of DOPA were higher in females than in males in both the intermediate lobe and median eminence, revealing a sexual difference in the basal activity of periventricular-hypophysial and tuberoinfundibular DA neurons. In contrast, there were no differences between male and female rats in activity of DA neurons terminating in the neural lobe. One week following gonadectomy, DOPA accumulation in the median eminence was decreased in females and increased in males, but remained unchanged in the intermediate lobe. These results indicate that sexual differences in the activity of periventricular-hypophysial DA neurons terminating in the intermediate lobe are not dependent upon the presence of circulating gonadal steroids, and in this respect, these neurons differ from tuberoinfundibular DA neurons.     

Regulation of pro-opiomelanocortin synthesis by dopamine and cAMP in the amphibian pituitary intermediate lobe

The modulation of pro-opiomelanocortin (POMC) synthesis in Xenopus laevis pituitary intermediate lobe (IL) during background adaptation and the role of dopamine and cAMP in mediating this effect were examined. Neurointermediate lobes (NILs) were pulselabeled in vitro with [3H]arginine and analyzed for POMC synthesis by acid-urea gel electrophoresis. After black background adaptation of the animal (7 days), POMC synthesis increased 5-6-fold, while after white background adaptation (7 days), POMC synthesis decreased by 76%. Dopamine (50 microM) suppressed POMC synthesis in NILs in culture. In the absence of dopamine, POMC synthesis was stimulated. Several experiments were conducted to determine the category of dopamine receptor in the X. laevis IL. A D-2 dopamine receptor agonist inhibited immunoreactive alpha-MSH release from the NIL in a D-2 antagonist-reversible manner. A D-1 receptor agonist or antagonist did not alter the release of immunoreactive alpha-MSH from the NIL. Dopamine (10 microM) inhibited forskolin-stimulated cAMP accumulation. In addition, dopamine inhibition of POMC synthesis in cultured ILs was reversed by 8-Br-cAMP. These studies suggest that white background adaptation results in stimulation of the X. laevis D-2 receptor, which reduces cAMP production and POMC synthesis. Conversely, during black background adaptation the IL D-2 receptor is not stimulated, leading to increased cAMP production and POMC synthesis.     

A comparison of domperidone and haloperidol effects on different dopaminergic neurons in the rat brain

Domperidone, a dopamine (DA) receptor antagonist with reportedly preferential actions outside of the blood-brain barrier, and haloperidol, a centrally active DA antagonist, were compared with respect to their abilities to increase the activity of dopaminergic neurons in the rat brain. The activity of nigrostriatal, mesolimbic, tuberohypophyseal and tuberoinfundibular dopamine nerves was estimated by measuring the $\text{in vivo}$ rate of DA synthesis (dihydroxyphenylalanine accumulation following administration of an inhibitor of aromatic L-amino acid decarboxylase) in the striatum, olfactory tubercle, posterior pituitary and median eminence, respectively. In an initial study, the rates of DA synthesis in striatum, olfactory tubercle, and posterior pituitary were determined at 2, 8, and 16 h after subcutaneous administration of 0.25, 2.5, or 25 mg/kg domperidone. At the lowest dose of domperidone, DA synthesis was increased only in the posterior pituitary at 8 and 16 h; at the intermediate dose, DA synthesis increased in the posterior pituitary at 8 and 16 h and in the olfactory tubercle at 8 h. Only at 8 h after the highest dose of domperidone was DA synthesis increased in the striatum. When 2.5 mg/kg of doperidone or haloperidol were administered, DA synthesis in posterior pituitary and median eminence was increased in a similar fashion (in the latter region only at 16 h). In contrast, domperidone promoted only modest and delayed increases in DA synthesis in the olfactory tubercle and had no effect in the striatum. These results indicate that systemically administered domperidone preferentially increases DA synthesis in neurons terminating outside the blood-brain barrier, but after a pronounced delay, high doses of the drug can also activate DA neurons which project to the forebrain.