Spreading depression in the cortex differently modulates dopamine release in rat mesolimbic and nigrostriatal terminal fields

The effects of cortical spreading depression (SD) on evoked dopamine release in mesolimbic (nucleus accumbens) and nigrostriatal (nucleus caudatus) terminal fields were studied by in vivo voltammetry in anesthetized rats. Dopamine release was evoked by electrical stimulation of medial forebrain bundle (20 Hz, 100 pulses). Local application of 3 M KCl on the dura initiated SD in the cortex. It was found that SD modulated evoked dopamine release in subcortical structures at the same time when the wave of depression of cortical activity reached reciprocally connected subcortical areas. This cortical depression increased stimulated dopamine release in the nucleus accumbens and decreased dopamine release in the nucleus caudatus. In agreement with these results, electrical stimulation of the prefrontal cortex at 20 Hz, synchronized with medial forebrain bundle stimulation, decreased evoked dopamine release in the nucleus accumbens. Areas of the cortex which modulated dopamine release in these two terminal fields were spatially separated by at least 5 mm from each other. It is proposed that depression and activation of evoked dopamine release in the nucleus caudatus and nucleus accumbens following SD are indicative of tonic activation of the nigrostriatal and tonic inhibition of the mesolimbic dopaminergic terminals by cortex in normal conditions. SD in the cortex, modulating neurotransmitter release in subcortical structures, may have a general impact on redistribution of oxygen supply in these subcortical areas and on behavior associated with brain trauma, migraine, insult or seizures, i.e. the kind of neuropathology which may cause SD type phenomena also in human brain.     

Regional variations in the physiology of the rat caudate-putamen. 1. In vivo subregional effects of amphetamine on extracellular dopamine concentration

Summary. The caudate putamen is a neurochemically and functionally heterogeneous nucleus. Understanding the correlation between these regional variations in neurochemistry and function could greatly aid in the treatment of neurological disorders associated with this area of the brain. Since dopaminergic dysfunction has been implicated in some of these disorders, regional variations in the neurochemistry of this transmitter system are of particular interest. The dopaminergic response to 2.5 mg/Kg D-amphetamine was examined by in vivo voltammetry in 7 dorsal and 7 ventral regions of the caudate-putamen in the urethane anaesthetized rat. Extracellular dopamine concentration increased in all the areas examined. However, the effect was regionally heterogeneous-areas separated by as little as 1 mm showing significantly different responses in terms of both the absolute change and the rate of change in dopamine concentration. A significant general trend was also evident. Amphetamine produced an increasing effect in extracellular dopamine concentrations in the dorsal and lateral areas of the nucleus. It was concluded that the regionally heterogeneous effects of amphetamine on extracellular dopamine could be attributed to regional variations in the density of dopamine transport sites within the caudate-putamen. Since this transport site is the site of entry of a number of neurotoxins this finding may contribute to our understanding of the functional loss associated with disorders such as Parkinson's disease. Received April 17, 2002; accepted December 2, 2002 Published online March 5, 2003 Acknowledgements We thank Dr. B. K. Yamamoto for assistance reviewing an early draft of the paper. All experiments were performed strictly adhering to the policy on the use of animals in neuroscience research as drafted by the Society for Neuroscience. Authors' address: Dr. G. Glynn, School of Pharmacy and Allied Health Professions, Creighton University, 2500 California Plaza, Omaha, NE 68178, U.S.A., e-mail: GGlynn@creighton.edu     

In vivo voltammetric measurement of extracellular DOPAC levels in the anteromedial prefrontal cortex of the rat

Differential pulse voltammetric recordings with carbon fiber electrodes performed in vivo in the anteromedial prefrontal cortex of the rat yielded 3 oxidation peaks at -100, +100 and +300 mV, respectively. Pharmacological manipulations revealed that 3,4-dihydroxyphenylacetic acid (DOPAC) is the main contributor to the oxidation current recorded at +100 mV (peak 2). Thus, systemic administration of FLA 63 did not alter whereas pargyline caused a disappearance of cortical peak 2. Moreover, haloperidol and sulpiride increased peak 2 amplitude both in normal and in N-(2-chloroethyl)N-ethyl-2-bromobenzylamine (DSP4)-lesioned rats. Peak 2 was detected only in those prefrontal cortex regions known to receive a dopamine input. It is concluded that in vivo voltammetry with carbon fiber electrodes is a useful means of monitoring dopaminergic activity in the prefrontal cortex of the rat.     

Dynamics of 5-hydroxytryptamine released from dopamine neurons in the caudate putamen of the rat

Fast-scan cyclic voltammetry has been used for the detection of 5-hydroxytryptamine (5-HT) in vivo. The applied potential waveform was previously optimized to maximize response times to concentration changes. This technique has been used to reinvestigate 5-HT release from striatal dopaminergic terminals after pharmacological pretreatment as reported by Stamford et al. [13]. Our results concur with those set forth by Stamford and further show that the dopamine transporter is responsible for 5-HT uptake in this experiment.     

Influence of anaesthetics on rat striatal dopamine metabolism in vivo

The effects of 4 anaesthetics, halothane, α-chloralose, chloral hydrate and pentobarbitone, on rat striatal extracellular dihydroxyphenylacetic acid (DOPAC) levels were determined using in vivo voltammetry. Stable baseline levels of DOPAC were maintained under halothane/N ₂O and α-chloralose while the extracellular levels of DOPAC gradually declined under chloral hydrate or pentobarbitone anaesthesia. Administration of haloperidol (0.3 mg/kg i.p.) significantly increased DOPAC using halothane/N ₂O, α-chloralose or chloral hydrate but not under pentobarbitone anaesthesia. The greatest increase in DOPAC was seen in rats anaesthetized with α-chloralose>halothane/N₂O>chloral hydrate > pentobarbitone. Apomorphine (0.5 mg/kg s.c.) given 2 h after haloperidol partially reversed the increase in DOPAC produced by haloperidol. The results suggest care needs to be exercised in the choice of anaesthetic used for voltammetric studies with pentobarbitone being the least recommended.     

Regional variations in the physiology of the rat caudate-putamen. 2. Effects of amphetamine and amphetamine induced dopamine release on basal and cortical stimulation evoked multiple unit activity

Summary. Dopaminergic terminals within the caudate-putamen are located in an ideal position to modulate the corticostriatal system. Since this is the major afferent system of the striatum, dopamine has very powerful effects on striatal electrophysiological activity. The striatum is a regionally specialized multifunctional nucleus. It is therefore important to determine if dopamine has the same modulatory effects within different areas of the nucleus. The effects of 2.5 mg/Kg D-amphetamine (IP) on cortical stimulation evoked and basal multiple unit activity (MUA) was measured in 7 dorsal and 7 ventral striatal areas of the urethane anaesthetized rat. In general, amphetamine caused an increase in the basal activity and a decrease in the cortical stimulation evoked activity. However, there were both qualitative and quantitative regionally dependent differences in these responses. The effect on basal MUA was more pronounced in the dorsal and caudal areas whereas the effect on cortical stimulation evoked MUA was more pronounced in the ventral areas. The electrophysiological effects of amphetamine within the striatum were correlated with its regionally dependent effects on extracellular dopamine. This produced a measure of the effects of striatal dopamine on regional electrophysiological activity. This information was also used to determine the mathematical relationship between dopamine concentration change and the change in MUA. These data indicate that the excitatory effects of amphetamine-induced dopamine release on the non-stimulated MUA progressively increase along the rostro-caudal axis of the nucleus. In addition, the effects were more pronounced in the ventromedial as compared to the ventrolateral areas. These effects correlated best with the rate of change in dopamine concentration. In the dorsal striatum amphetamine-induced increases in dopamine had a regionally homogeneous inhibitory effect on the stimulated MUA. In the ventral striatum however, it had a progressively stronger effect along the rostro-caudal axis. These effects correlated best with the absolute change in dopamine concentration. Received April 17, 2002; accepted December 2, 2002 Published online March 5, 2003 Authors' address: Dr. G. Glynn, School of Pharmacy and Allied Health Professions, Creighton University, 2500 California Plaza, Omaha, NE 68178, U.S.A., e-mail: GGlynn@creighton.edu     

Effect of exogenousL-DOPA on behavior in the rat: an in vivo voltammetric study

L-DOPA was administered intraperitoneally (i.p.) or intraventricularly (i.v.t.) to freely moving rats to investigate the effects of exogenousL-DOPA itself on behavior. Striatal dopamine (DA) in the extracellular fluid was examined with microcomputer-controlled in vivo voltammetry, and behavioural change was observed. WhenL-DOPA was administered (i.p.) after pretreatment with benserazide, a peripheral DOPA decar☐ylase inhibitor, behavioral change was elicited before the elevation in DA and suppressed before its reduction. After pretreatment with NSD-1015, a central DOPA decar☐ylase inhibitor, behavioral change was also elicited, although DA was still not increased. WhenL-DOPA was injected (i.v.t.), the behavioral effects was manifested at once; DA was still unchanged at this time, but it increased after behavioral activity reached the maximum level.L-DOPA was also injected (i.v.t.) into rats with striatal lesions induced by 6-hydroxydopamine (i.v.t.). Behaviral change was manifested promptly after the injection. When the dose-response curves to different dosages ofL-DOPA were examined in normal rats without striatal lesions, it was found to exhibit a steeper rise than that of DA. Finally, when rats were injected (i.p. or i.v.t.) with 3-O-methyl-DOPA (3-methoxytyrosine), a major metabolite ofL-DOPA, no behavioral change was elicited, and no increase in DA was recognized. These experimental results indicated thatL-DOPA is related to the manifestation of behavioral change.     

Voltammetry of extracellular dopamine in rat striatum during ICSS-like electrical stimulation of the medial forebrain bundle

Fast-scan voltammetry cyclic in the striatum of anesthetized rats has been used to monitor extracellular dopamine during forced electrical stimulation of the media forebrain bundle using parameters that mimic intracranial self-stimulation. The temporal resolution provided by microelectrodes positioned very near sites of dopamine release allows resolution of the response to individual 500-ms stimulation trains separated by 500-ms intervals. Uptake inhibition by Nomifensine alters the resolution obtained at short times after initiation of stimulation.     

Dopamine release is heterogeneous within microenvironments of the rat nucleus accumbens

Many individual neurons within the intact brain fire in stochastic patterns that arise from interactions with the neuronal circuits that they comprise. However, the chemical communication that is evoked by these firing patterns has not been characterized because sensors suitable to monitor subsecond chemical events in micron dimensions have only recently become available. Here we employ a voltammetric sensor technology coupled with principal component regression to examine the dynamics of dopamine concentrations in the nucleus accumbens (NAc) of awake and unrestrained rats. The sensor has submillimeter dimensions and provides high temporal (0.1 s) resolution. At select locations spontaneous dopamine transient concentration changes were detected, achieving instantaneous concentrations of approximately 50 nm. At other locations, transients were absent even though dopamine was available for release as shown by extracellular dopamine increases following electrical activation of dopaminergic neurons. At sites where dopamine concentration transients occur, uptake inhibition by cocaine enhances the frequency and magnitude of the rapid transients while also causing a more gradual increase in extracellular dopamine. These effects were largely absent from sites that did not support ongoing transient activity. These findings reveal an unanticipated spatial and temporal heterogeneity of dopamine transmission within the NAc that may depend upon the firing of specific subpopulations of dopamine neurons.     

Striatal dopaminergic responses observed in latent inhibition are dependent on the hippocampal ventral subicular region

We showed recently that behavioural and striatal dopaminergic (DA) responses obtained in latent inhibition are crucially dependent on the parahippocampal region, the entorhinal cortex. In the present study, we investigated the influence exerted by the hippocampal ventral subicular region (SUB) on the DA responses in the anterior part of the dorsal striatum using in vivo voltammetry in freely moving rats and the same latent inhibition paradigm. To that end, the left SUB was temporarily blocked with tetrodotoxin (TTX) during pre-exposure to a new olfactory stimulus (banana odour). During the second session the animals were aversively conditioned to banana odour. With respect to the results obtained during the test session (third presentation of banana odour), similar changes in behaviour and DA levels were obtained in control and conditioned rats microinjected with the solvent, phosphate-buffered saline (PBS), in the SUB, consistently with a latent inhibition phenomenon. In contrast, after reversible inactivation of the SUB during the pre-exposure session, TTX-pre-exposed conditioned animals displayed aversive behaviour in the test session, and anterior striatal DA variations in these animals differed significantly from those obtained in pre-exposed rats injected locally with PBS. Striatal DA variations obtained in conditioned animals microinjected with TTX were also significantly different from those observed in conditioned non-pre-exposed animals. The present data suggest that, in parallel to the entorhinal cortex, the SUB regulates the latent inhibition-related behavioural and DA responses in the anterior part of the dorsal striatum. These data may provide new insight into the pathophysiology of schizophrenic psychoses.     

Dopamine release from pharmacologically distinct storage pools in rat striatum following stimulation at frequency of neuronal bursting

The increase of dopamine overflow in the rat caudate and nucleus accumbens following repeated stimulation of the median forebrain bundle (MFB) at short intervals and the role of -methyl-p-tyrosine (AMPT) and reserpine sensitive storage pools in evoked dopamine overflow were investigated by in vivo voltammetry. In contrast to stimulation at 30-s intervals and longer, stimulation of the MFB at 5-s intervals led to increased dopamine release following each consecutive stimulation. This effect was much larger in the caudate than in the nucleus accumbens. We consider that the increase in dopamine release is due to translocation of dopamine from a reserpine-sensitive storage pool and this accounts for the increase of the rate of refilling of the readily releasable pool. The shorter the intervals between stimulation and the higher the frequency (with a plateau at 30 Hz), then the greater the fraction of dopamine which originates from the reserpine-sensitive storage pool. Exocytotic release of dopamine from the AMPT-sensitive storage pool does not seem to depend on the intervals between stimulation. We propose that the vesicles in presynaptic dopaminergic terminals near the outer membrane are more sensitive to AMPT. Distantly located vesicles have a relatively higher sensitivity to reserpine. Experiments using repeated stimulation at as low a frequency as 10–20 Hz, revealed that this phenomenon may take place under physiological conditions following bursting of dopamine neurones.     

Catecholamine Metabolism in Locus Coeruleus Neurons: A Study of its Activation by Sciatic Nerve Stimulation in the Rat

Sciatic nerve stimulation, which strongly activates noradrenergic locus coeruleus (NA-LC) neurons, was used in anaesthetized rats as a model to study the transneuronal control of catechol metabolism in this nucleus. We show, using in vivo electrochemistry and biochemical post-mortem assays, that a prolonged (20 min) unilateral sciatic nerve electrical stimulation led to a reversible enhancement (80 - 130%) of both endogenous and in vivo extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) within the contralateral LC region. An elevation in DOPAC levels was also observed in the ipsilateral nucleus but was always significantly lower. The response was abolished by a pretreatment with kynurenic acid, a non-selective excitatory amino acid (EAA) antagonist known to block footshock-induced excitations of NA-LC neurons: in antagonist-treated rats, the stimulation induced a non-significant effect (+ 30%) on endogenous DOPAC levels, which contrasted with the highly significant effect (+ 113%) observed in vehicle-treated animals. As the major source of EAA afferents to the LC originates in the nucleus paragigantocellularis, we made an attempt to suppress activation by a section of these fibres. An incision performed obliquely (45 degrees ) between LC and PGi greatly and significantly attenuated, but did not totally suppress, the increase in DOPAC endogenous content due to the stimulation. These experiments indicate that a peripheral stimulus provokes an activation of catecholamine metabolism within the soma - dendritic region of the NA-LC cells. They suggest that this effect may be mediated, at least in part, by afferent pathways originating from the medulla which utilize an EAA as transmitter.     

Effect of 6-fluoro-m-tyrosine on dopamine release and metabolism in rat striatum using in vivo microdialysis

6-[¹⁸F]Fluoro-m-tyrosine (FMT) is a positron emission tomography (PET) imaging agent for the aromatic -amino acid decarboxylase enzyme. Its parent compound, -m-tyrosine (LMT) induces behavioral effects in rodents via dopamine release. To assess the potential pharmacologic effect of FMT, its role in dopamine release and metabolism in rat striatum was compared with LMT and -DOPA using in vivo microdialysis. Results indicate that FMT will not have the same dopamine-induced behavioral effects as LMT.     

Electrophysiological and pharmacological characterization of identified nigrostriatal and mesoaccumbens dopamine neurons in the rat

Extracellular single-unit recording techniques were used to compare the basal activity and pharmacological responsiveness of identified nigrostriatal and mesoaccumbens dopamine (DA)-containing neurons. The projection area of each DA cell was determined by antidromic activation techniques. The forebrain stimulation used for the cell identification procedure did not alter the pharmacological responsiveness of DA neurons; the inhibitory effect of apomorphine (and d-amphetamine) was identical when stimulation was applied either prior to or following drug administration. Analysis of the spike discharge pattern revealed that a higher proportion of mesoaccumbens DA cells exhibited burst-firing activity. Although the firing pattern of the two populations of burst-firing DA cells was similar in many regards, mesoaccumbens DA cells exhibited a longer postburst inhibition than did nigrostriatal DA cells. Each of the DA agonists, apomorphine, pergolide, B-HT 920, and d-amphetamine, inhibited nigrostriatal and mesoaccumbens DA neuronal activity in a similar fashion. However, there was a marked population difference in the recovery of cell firing in the 10 minutes following apomorphine-induced inhibition; the recovery of mesoaccumbens spike discharges was considerably slower. Although this population difference was apparent to some extent following administration of pergolide or B-HT 920 (but not d-amphetamine), it was considerably less marked. The present findings are discussed with respect to the known regulatory control of midbrain DA neurons.     

Effects of anaesthetics and lazaroid U-83836E on survival of transplanted rat dopaminergic neurones

We investigated whether different methods of anaesthesia, used on the pregnant rat when collecting embryonic donor tissue, and a lipid peroxidation inhibitor (lazaroid U-83836E) affect the survival of grafted embryonic dopaminergic neurones in hemiparkinsonian rats. There was no difference in either functional recovery or survival of dopaminergic neurones between the different euthanasia groups: (a) isoflurane sedation followed by cervical dislocation, (b) equithesin- or (c) euthatal-anaesthesia. However, lazaroid-treatment enhanced both behavioural recovery and transplant survival.     

Extracellular dopamine in the rat striatum during ischemia and reperfusion as measured by in vivo electrochemistry and in vivo microdialysis

The effects of transient global forebrain ischemia and reperfusion on striatal extracellular dopamine levels were analyzed using both in vivo electrochemistry and in vivo microdialysis in urethane-anesthetized rats. Electrochemical records showed that extracellular dopamine levels increased once during the period of ischemia, and a second time during reperfusion. This biphasic pattern was not detected by microdialysis, probably because of the relatively low time resolution of this technique. Microdialysis provided evidence that the voltammetric signal was a measure of dopamine, and also allowed measurement of the metabolites dihydroxyphenylacetic acid and homovanillic acid, both of which decreased during ischemia. The biphasic dopamine pattern seen in rats is similar to that reported previously in gerbils, suggesting that it is a phenomenon common to transient ischemia and reperfusion across different species and models of transient global ischemia. This phenomenon may have important implications for therapeutic intervention in cerebral ischemia.     

Bilateral effects of unilateral GDNF administration on dopamine- and GABA-regulating proteins in the rat nigrostriatal system

Dopamine (DA) affects GABA neuronal function in the striatum and together these neurotransmitters play a large role in locomotor function. We recently reported that unilateral striatal administration of GDNF, a growth factor that has neurotrophic effects on DA neurons and enhances DA release, bilaterally increased striatal neuron activity related to locomotion in aged rats. We hypothesized that the GDNF enhancement of DA function and resulting bilateral enhancement of striatal neuronal activity was due to prolonged bilateral changes in DA- and GABA-regulating proteins. Therefore in these studies we assessed dopamine- and GABA-regulating proteins in the striatum and substantia nigra (SN) of 24 month old Fischer 344 rats, 30 days after a single unilateral striatal delivery of GDNF. The nigrostriatal proteins investigated were the DA transporter (DAT), tyrosine hydroxylase (TH), and TH phosphorylation and were examined by blot-immunolabeling. The striatal GABA neuron-related proteins were examined by assay of the DA D₁ receptor, DARPP-32, DARPP-32 Thr³⁴ phosphorylation, and glutamic acid decarboxylase (GAD). Bilateral effects of GDNF on TH and DAT occurred only in the SN, as 30 μg GDNF increased ser19 phosphorylation, and 100 μg GDNF decreased DAT and TH protein levels. GDNF also produced bilateral changes in GAD protein in the striatum. A decrease in DARPP-32 occurred in the ipsilateral striatum, while increased D₁ receptor and DARPP-32 phosphorylation occurred in the contralateral striatum. The 30 μg GDNF infusion into the lateral striatum was confined to the ipsilateral striatum and substantia nigra. Thus, long-lasting bilateral effects of GDNF on proteins regulating DA and GABA neuronal function likely alter physiological properties in neurons, some with bilateral projections, associated with locomotion. Enhanced nigrostriatal excitability and DA release by GDNF may trigger these bilateral effects.     

Acute effects of sigma ligands on the electrophysiological activity of rat nigrostriatal and mesoaccumbal dopaminergic neurons.

The effects of acute i.v. administration of several sigma ligands on the single-unit activity of nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. DTG (1,3-di(o-tolyl)guanidine) did not alter DA neuronal activity at nontoxic doses and JO 1784 [(+)-N-cyclopropylmethyl-N-methyl-1,4-diphenyl-1-ethylbut-3-en-1-+ ++ylamine] was inactive. (+)-Pentazocine was more effective in increasing mesoaccumbal vs. nigrostriatal DA cell firing rates. BMY 14802(alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-but anol) dose-dependently increased DA cell firing rate in both populations. The inhibition of nigrostriatal DA cell firing rate by (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine [(+)-3-PPP] was reversed by (-)-eticlopride and (+)-but not (-)-butaclamol, which supports previous evidence that (+)-3-PPP-induced inhibition is due to the DA agonist properties of the drug. From what is known of the pharmacological properties of these compounds, it is concluded that acute sigma receptor occupation does not markedly alter the firing rate of DA neurons. The dose-response curve for inhibition of nigrostriatal DA neuronal activity by the D2 DA agonist, quinpirole, was shifted to the right tenfold by BMY 14802 pretreatment (8 mg/kg, i.v.) and twofold by (+)-pentazocine (8 mg/kg, i.v.), but was not changed by DTG (2 mg/kg, i.v.). It is concluded that the marked effects of certain sigma ligands on DA cell electrophysiology are likely due to their non-sigma properties.     

In vivo labelling of pituitary dopamine D-2 receptors in the male rat using [3H]-raclopride

Summary The substituted benzamide drug [³H]-raclopride (Köhler et al., 1985) was used to label dopamine D-2 receptors within the individual lobes of the pituitary gland as well as in the brain of male rats in vivo. The in vivo [³H]-raclopride binding was found to be saturable, reversible and of high specificity. Between 5–30% of the binding was non-specific at saturating concentrations dependent upon the lobe of the pituitary gland as well as of the brain region (e.g., caudate nucleus and olfactory tubercle) studied. Saturation analyses revealed B_max-values of 12.9±1.6 and 2.2±0.9pmol·g^–1 wet weight in the intermediate and anterior lobes, respectively with respective K_D values of 6.5±4.6 and 7.3±2.4 nmol·kg^–1. Quantitative autoradiographic studies using a single concentration of [³H]-raclopride showed a similar relationship with regard to binding densities in the different lobes, and showed, in addition, that the posterior lobe contained the lowest number of specific [³H]-raclopride binding sites. The binding capacities and affinities of binding were 12.9±1.7 and 9.2±2.8 respectively in the caudate nucleus and 6.1±0.7 and 9.3±2.7 respectively in the olfactory tubercle.The pharmacological analysis revealed that (S)sulpiride, remoxipride and raclopride were 10 to 125 times more potent than their corresponding isomers [(R)sulpridie, FLA 731(–), and FLB 472, respectively] in blocking the in vivo [³H]raclopride binding in the pituitary gland as well as in brain. The in vivo potency of different D-2 antagonists in preventing the [³H]-raclopride binding in the anterior and intermediate lobes was: spiperone > domperidone > raclopride > (S)sulpiride > remoxipride. The D-1 selective antagonist SCH 23390 did not block the in vivo binding of [³H]-raclopride neither in the pituitary lobes nor in the brain. In agreement with these findings the D-2 agonists N,N-propylnorapomorphine and quinpirole (LY 171555) but not the D-1 agonist SKF 38393-A blocked the specific in vivo [³H]-raclopride binding in the pituitary gland as well as in the brain. Comparisons between the relative potencies of different drugs in blocking pituitary and brain D-2 receptors in vivo showed that some drugs, including sulpiride and domperidone, were more potent in the pituitary gland than in the brain, while remoxipride and raclopride were equipotent in the two areas. The D-2 agonists tested appeared to be slightly more potent in the brain than in the pituitary gland.