In vivo receptor binding, neurochemical and functional studies with the dopamine D-1 receptor antagonist SCH23390

Reduced threshold for induction of LTP by activation of dopamine D1/D5 receptors at hippocampal CA1-subiculum synapses

The phasic release of dopamine in the hippocampal formation has been shown to facilitate the encoding of novel information. There is evidence that the subiculum operates as a detector and distributor of sensory information, which incorporates the novelty and relevance of signals received from CA1. The subiculum acts as the final hippocampal relay station for outgoing information. Subicular pyramidal cells have been classified as regular- and burst-spiking neurons. The goal of the present study was to study the effect of dopamine D1/D5 receptor activation on synaptic transmission and plasticity in the subicular regular-spiking neurons of 4–6 week old Wistar rats. We demonstrate that prior activation of D1/D5 receptors reduces the threshold for the induction of long-term potentiation (LTP) in subicular regular-spiking neurons. Our results indicate that D1/D5 receptor activation facilitates a postsynaptic form of LTP in subicular regular-spiking cells that is NMDA receptor-dependent, relies on postsynaptic Ca²⁺ signaling, and requires the activation of protein kinase A. The enhanced propensity of subicular regular-spiking cells to express postsynaptic LTP after activation of D1/D5 receptors provides an intriguing mechanism for the encoding of hippocampal output information.

The dopamine D2 receptor regulates Akt and GSK-3 via Dvl-3

The dopamine D2 receptor (D2DR) regulates Akt and may also target the Wnt pathway, two signalling cascades that inhibit glycogen synthase kinase-3 (GSK-3). This study examined whether the Wnt pathway is regulated by D2DR and the role of Akt and dishevelled-3 (Dvl-3) in regulating GSK-3 and the transcription factor β-catenin in the rat brain. Western blotting showed that subchronic treatment of raclopride (D2DR antagonist) increase phosphorylated Akt, Dvl-3, GSK-3, phosphorylated GSK-3 and β-catenin, whereas subchronic treatment of quinpirole (D2DR agonist) induced the opposite response. Co-immunopreciptations revealed an association between GSK-3 and the D2DR complex that was altered following raclopride and quinpirole, albeit in opposite directions. SCH23390 (D1DR antagonist) and nafadotride (D3DR antagonist) were also used to determine if the response was specific to the D2DR. Neither subchronic treatment affected Dvl-3, GSK-3, Akt nor β-catenin protein levels, although nafadotride altered the phosphorylation state of Akt and GSK-3. In addition, in-vitro experiments were conducted to manipulate Akt and Dvl-3 activity in SH-SY5Y cells to elucidate how the pattern of change observed following manipulation of D2DR developed. Results indicate that Akt affects the phosphorylation state of GSK-3 but has no effect on β-catenin levels. However, altering Dvl-3 levels resulted in changes in Akt and the Wnt pathway similar to what was observed following raclopride or quinpirole treatment. Collectively, the data suggests that the D2DR very specifically regulates Wnt and Akt signalling via Dvl-3.

Dopamine D1/D5 receptors contribute to de novo hippocampal LTD mediated by novel spatial exploration or locus coeruleus activity

The neurons of the locus coeruleus (LC) fire in response to novelty, and LC activation coupled with hippocampal afferent stimulation leads to long-term depression (LTD). The encoding of novel spatial information also involves activation of dopamine D1/D5 receptors. It is unclear if, or how, the noradrenergic and dopaminergic systems interact mechanistically in processing novelty. Novel spatial exploration when coupled with Schaffer collateral (SC) test-pulse stimulation results in short-term depression at SC-CA1 synapses, which is not observed in the absence of afferent stimulation. However, activation of D1/D5 receptors under these conditions without concomitant afferent stimulation enables slow-onset depression. LTD (>24 h) is facilitated when novel exploration occurs concurrently with low-frequency stimulation of CA1. Effects are not improved by a D1/D5 agonist. Facilitation of LTD (>4 h) by coupling LC stimulation with CA1 test-pulse stimulation was blocked by a D1/D5 antagonist, however, as was habituation to the holeboard environment. Novel spatial learning during LC stimulation did not enhance LTD further, whereas D1/D5 agonist treatment enabled LTD to persist for over 24 h. These data suggest that the regulation of hippocampal LTD by the LC is supported by D1/D5 receptors and that their contribution to information storage becomes important when the thresholds for persistent LTD have not been reached.

Activation of dopamine D1/D5 receptors facilitates the induction of presynaptic long-term potentiation at hippocampal output synapses

Encoding of novel information has been proposed to rely on the time-locked release of dopamine in the hippocampal formation during novelty detection. However, the site of novelty detection in the hippocampus remains a matter of debate. According to current models, the CA1 and the subiculum act as detectors and distributors of novel sensory information. Although most CA1 pyramidal neurons exhibit regular-spiking behavior, the majority of subicular pyramidal neurons fire high-frequency bursts of action potentials. The present study investigates the efficacy of dopamine D1/D5 receptor activation to facilitate the induction of activity-dependent long-term potentiation (LTP) in rat CA1 regular-spiking and subicular burst-spiking pyramidal cells. Using a weak stimulation protocol, set at a level subthreshold for the induction of LTP, we show that activation of D1/D5 receptors for 5-10 min facilitates LTP in subicular burst-spiking neurons but not in CA1 neurons. The results demonstrate that D1/D5 receptor-facilitated LTP is NMDA receptor-dependent, and requires the activation of protein kinase A. In addition, the D1/D5 receptor-facilitated LTP is shown to be presynaptically expressed and relies on presynaptic Ca(2+) signaling. The phenomenon of dopamine-induced facilitation of presynaptic NMDA receptor-dependent LTP in subicular burst-spiking pyramidal cells is in accordance with observations of the time-locked release of dopamine during novelty detection in this brain region, and reveals an intriguing mechanism for the encoding of hippocampal output information.

Genetic variance contributes to dopamine receptor antagonist-induced inhibition of sucrose intake in inbred and outbred mouse strains

Preference and intake of sucrose varies across inbred and outbred strains of mice. Pharmacological analyses revealed that the greatest sensitivity to naltrexone-induced inhibition of sucrose (10%) intake was observed in C57BL10/J and C57BL/6J strains, whereas 129P3/J, SWR/J and SJL/J strains displayed far less sensitivity to naltrexone-induced inhibition of sucrose intake. Given that dopamine D1 (SCH23390) and D2 (raclopride) receptor antagonism potently reduce sucrose intake in outbred rat and mouse strains, the present study examined the possibility of genetic variance in the dose-dependent (50-1600 nmol/kg) and time-dependent (5-120 min) effects of these antagonists upon sucrose (10%) intake in the eight inbred (BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J, SWR/J and 129P3/J) and one outbred (CD-1) mouse strains previously tested with naltrexone. SCH23390 significantly reduced sucrose intake across all five doses in 129P3/J and SJL/J mice, across four doses in C57BL/6J and BALB/cJ mice, across three doses in DBA/2J, SWR/J, C3H/HeJ and C57BL/10J mice, but only at the two highest doses in CD-1 mice. SCH23390 was 2-3-fold more potent in inhibiting sucrose intake in 129P3/J and SJL/J mice relative to CD-1 mice. In contrast, only the highest equimolar 1600 nmol/kg dose of raclopride significantly reduced sucrose intake in the BALB/cJ, C3H/HeJ, C57BL/6J, C57BL/10J, DBA/2J, SJL/J and 129P3/J, but not the SWR/J and CD-1 strains. The present and previous data demonstrate specific and differential patterns of genetic variability in inhibition of sucrose intake by dopamine and opioid antagonists, suggesting that distinct neurochemical mechanisms control sucrose intake across different mouse strains.

Dopamine D1/D5 receptors gate the acquisition of novel information through hippocampal long-term potentiation and long-term depression

Hebbian learning models require that neurons are able to both strengthen and weaken their synaptic connections. Hippocampal synaptic plasticity, in the form of long-term potentiation (LTP) and long-term depression (LTD), has been implicated in both spatial memory formation as well as novelty acquisition. In addition, the ventral tegmental area-hippocampal loop has been proposed to control the entry of information into long-term memory, whereas the dopaminergic system is believed to play an important role in information acquisition and synaptic plasticity. D1/D5 dopamine receptors are positively coupled to adenylyl cyclase and have been to modulate certain forms of synaptic plasticity, particularly in vitro. We investigated how D1/D5 dopamine receptors modify long-lasting synaptic plasticity at CA1 synapses of adult freely moving rats and found that receptor activation lowered the threshold for the induction of both LTP and LTD. Specific types of learning are associated with specific types of hippocampal synaptic plasticity. We found that object-configuration learning, facilitation of late-phase LTD by object exploration, and late-phase LTP by exploration of empty space were all prevented by D1/D5 receptor antagonism. Furthermore, receptor antagonism prevented electrically induced late-LTP, whereas receptor activation facilitated induction of both LTP and LTD by patterned electrical stimulation. These findings suggest that the dopaminergic system, acting via D1/D5 receptors, gates long-term changes in synaptic strength and that these changes are a critical factor in the acquisition of novel information.

Dopamine-dependent and dopamine-independent actions of cocaine as revealed by brain thermorecording in freely moving rats

Brain temperature fluctuates biphasically in response to repeated, intravenous (i.v.) cocaine injections, perhaps reflecting cocaine's inhibiting effect on both dopamine (DA) transporters and Na+ channels. By using a DA receptor blockade, one could separate these actions and determine the role of DA-dependent and DA-independent mechanisms in mediating this temperature fluctuation. Rats were chronically implanted with thermocouple probes in the brain, a non-locomotor head muscle and subcutaneously. Temperature fluctuations associated with ten repeated i.v. cocaine injections (1 mg/kg with 8-min inter-injection intervals) were examined after a combined, systemic administration of selective D1-like and D2-like receptor blockers (SCH-23390 and eticlopride) at doses that effectively inhibit DA transmission. In contrast to the initial temperature increases and subsequent biphasic fluctuations (decreases followed by increases) seen with repeated cocaine injections in saline-treated control, brain and muscle temperatures during DA receptor blockade decreased with each repeated cocaine injection. DA receptor blockade had no effects on skin temperature, which tonically decreased and biphasically fluctuated (decreases followed by increases) during repeated cocaine injections in both conditions. DA receptor blockade by itself slightly increased brain and muscle temperatures, with no evident effect on skin temperature. DA antagonists also strongly decreased spontaneous movement activity and completely blocked the locomotor activation normally induced by repeated cocaine injections. Although our data confirm that cocaine's inhibitory action on presynaptic DA uptake is essential for its ability to induce metabolic and behavioral activation, they also suggest that the physiological effects of this drug cannot be explained through this system alone. The continued hypothermic effect of cocaine points to its action on other central systems (particularly blockade of Na+ channels) that may be important for the development of cocaine abuse and adverse effects of this drug.

Acute effects of 3,4-methylenedioxymethamphetamine on striatal single-unit activity and behavior in freely moving rats: differential involvement of dopamine D1 and D2 receptors

3,4-Methylenedioxymethamphetamine (MDMA) is a widely abused amphetamine derivative that increases dopamine (DA) and serotonin release via a reverse transport mechanism. Changes in the activity of striatal neurons in response to increased DA transmission may shape the behavioral patterns associated with amphetamine-like stimulants. To determine how the striatum participates in MDMA-induced locomotor activation, we recorded the activity of >100 single units in the striatum of freely moving rats in response to a dose that increased motor activation (5.0 mg/kg). MDMA had a predominantly excitatory effect on neuronal activity that was positively correlated with the magnitude of locomotor activation. Categorizing neurons according to baseline locomotor responsiveness revealed that MDMA excited significantly more neurons showing movement-related increases in activity compared to units that were non-movement-related or associated with movement-related decreases in activity. Further analysis revealed that the drug-induced striatal activation was not simply secondary to the behavioral change, indicating a primary action of MDMA on striatal motor circuits. Prior administration of SCH-23390 (0.2 mg/kg), a D(1) antagonist, resulted in a late onset of MDMA-induced locomotion, which correlated positively with delayed neuronal excitations. Conversely, prior administration of eticlopride (0.2 mg/kg), a D(2) antagonist, completely abolished MDMA-induced locomotion, which paralleled its blockade of MDMA-induced excitatory neuronal responses. Our results highlight the importance of striatal neuronal activity in shaping the behavioral response to MDMA, and suggest that DA D(1) and D(2) receptors have distinct functional roles in the expression of MDMA-induced striatal and locomotor activation.

Decreased striatal dopamine efflux after intrastriatal application of benzazepine-class D1 agonists is not mediated via dopamine receptors

Previous pharmacological studies have reported that striatal dopamine efflux is negatively modulated not only by presynaptic D2 dopamine autoreceptors but also by striatal D1 dopamine receptors. The present experiments employed in vivo microdialysis to further examine the ability of widely used benzazepine-class D1 agonists to modulate striatal dopamine efflux. In the present study, both the partial D1 agonist (+/-)-SKF 38393 (10 microM) and the full D1 agonist (+/-)-SKF 82958 (10 and 100 microM) significantly reduced striatal dopamine efflux during intrastriatal application. Intrastriatal application of the less active enantiomer, S(-)-SKF 38393 (10 microM) did not decrease striatal dopamine suggesting a selective receptor-mediated mode of action of (+/-)-SKF 38393. Additional experiments were conducted with the full D1 agonist (+/-)-SKF 82958 in order to characterize the receptor(s) mediating the observed decrease in dopamine efflux. Neither local application of the D1 antagonist R(+)-SCH 23390 (100 microM) nor local application of the selective D2 antagonist raclopride (5 microM) blocked the ability of (+/-)-SKF 82958 (10 microM) to decrease striatal dopamine efflux. However, intrastriatal application of the less selective D2 antagonist haloperidol (1 microM) did prevent the decrease in striatal dopamine efflux observed during intrastriatal (+/-)-SKF 82958 application. The present data suggest that the ability of intrastriatally applied benzazepine-class D1 agonists to decrease striatal dopamine efflux is receptor-mediated, but this action apparently is not mediated at D1 or D2 receptors. There is therefore no indication for an intrastriatal population of D1 receptors capable of modulating dopamine efflux.

Stoichiometry of tyrosine hydroxylase phosphorylation in the nigrostriatal and mesolimbic systems in vivo: effects of acute haloperidol and related compounds

Electrical stimulation of the medial forebrain bundle increases (32)P incorporation into striatal tyrosine hydroxylase (TH) at Ser (19), Ser(31), and Ser(40). In the present studies, the effects of acute haloperidol and related drugs on sitespecific TH phosphorylation stoichiometry (PS) in the nigrostriatal and mesolimbic systems were determined by quantitative blot immunolabeling using phosphorylation statespecific antibodies. The striatum (Str), substantia nigra (SN), nucleus accumbens (NAc), and ventral tegmental area (VTA) from Sprague-Dawley rats were harvested 30-40 min after a single injection of either vehicle, haloperidol (2 mg/kg), raclopride (2 mg/kg), clozapine (30 mg/kg), or SCH23390 (0.5 mg/kg). In vehicle-injected control rats, Ser(19) PS was 1.5- to 2. 5-fold lower in Str and NAc than in SN and VTA, Ser(31) PS was two-to fourfold higher in Str and NAc than in SN and VTA, and Ser(40) PS was similar between the terminal field and cell body regions. After haloperidol, Ser(40) PS increased twofold in Str and NAc, whereas a smaller increase in SN and VTA was observed. The effects of haloperidol on Ser(19) PS were similar to those on Ser(40) in each region; however, haloperidol treatment increased Ser(31) PS at least 1.6-fold in all regions. The effects of raclopride on TH PS were comparable to those of haloperidol, whereas clozapine treatment increased TH PS at all sites in all regions. By contrast, the effects of SCH23390 on TH PS were relatively small and restricted to the NAc. The stoichiometries of site-specific TH phosphorylation in vivo are presented for the first time. The nigrostriatal and mesolimbic systems have common features of TH PS, distinguished by differences in TH PS between the terminal field and cell body regions and by dissimilar increases in TH PS in the terminal field and cell body regions after acute haloperidol.

Striatal neuronal activity and responsiveness to dopamine and glutamate after selective blockade of D1 and D2 dopamine receptors in freely moving rats

Although striatal neurons receive continuous dopamine (DA) input, little information is available on the role of such input in regulating normal striatal functions. To clarify this issue, we assessed how systemic administration of selective D1 and D2 receptor blockers or their combination alters striatal neuronal processing in freely moving rats. Single-unit recording was combined with iontophoresis to monitor basal impulse activity of dorsal and ventral striatal neurons and their responses to glutamate (GLU), a major source of excitatory striatal drive, and DA. SCH-23390 (0.2 mg/kg), a D1 antagonist, strongly elevated basal activity and attenuated neuronal responses to DA compared with control conditions, but GLU-induced excitations were enhanced relative to control as indicated by a reduction in response threshold, an increase in response magnitude, and a more frequent appearance of apparent depolarization inactivation. In contrast, the D2 antagonist eticlopride (0.2 mg/kg) had a weak depressing effect on basal activity and was completely ineffective in blocking the neuronal response to DA. Although eticlopride reduced the magnitude of the GLU response, the response threshold was lower, and depolarization inactivation occurred more often relative to control. The combined administration of these drugs resembled the effects of SCH-23390, but whereas the change in basal activity and the GLU response was weaker, the DA blocking effect was stronger than SCH-23390 alone. Our data support evidence for DA as a modulator of striatal function and suggest that under behaviorally relevant conditions tonically released DA acts mainly via D1 receptors to provide a continuous inhibiting or restraining effect on both basal activity and responsiveness of striatal neurons to GLU-mediated excitatory input.

In vivo striatal binding of the D1 antagonist SCH 23390 is not modified by changes in dopaminergic transmission

The in vivo striatal binding of [3H]SCH 23390, an antagonist of the D1 dopamine receptors, was investigated in mice submitted to pretreatment to either decrease (gammabutyrolactone 750 mg/kg, i.p.) or, increase (3,4-dihydroxyphenylalanine (L-DOPA) 200 mg/kg i.p. plus dexamphetamine 4 mg/kg, s.c.) dopaminergic transmission. Such conditions failed to modify [3H]SCH 23390 binding. However, we observed that dopamine (at concentrations > or = 1 microM), reduced the in vitro binding of [3H]SCH 23390 in membrane fractions. These results suggest that modifications in dopamine neurotransmission do not alter the in vivo quantification of D1 receptors with [3H]SCH 23390, for example, in studies that use positron emission tomography.

Differential effects of dopamine D2 and D3 receptor antagonists in regard to dopamine release, in vivo receptor displacement and behaviour

To establish possible functional differences between the dopamine D2 and D3 receptor we investigated the relation between the ability, for a set of nine mixed dopamine D2 and D3 receptor antagonists, to displace N, N-dipropyl-2-amino-5,6-dihydroxy tetralin (DP-5,6-ADTN) from striatal binding sites and the subsequent behavioural consequences in vivo. Dopamine D2 receptor preferring antagonists are powerful displacers of DP-5,6-ADTN from the striatum. Maximal displacement is followed by strong hypomotility. Displacement of the agonist by the D3 preferring antagonist U99194A is only partial and results in synergistic increases in locomotor activity. Superimposing haloperidol upon GBR12909 leads to a synergistic increase in striatal dialysate dopamine concentrations. This effect is absent when combining GBR12909 with the putative D3 antagonist U99194A. These data give support for the hypothesis that the dopamine D3 receptor is functionally relevant at the postsynaptic level. Here, in contrast to the D2 receptor, it is proposed to exert an inhibitory influence on psychomotor functions.

The effects of SCH 23390 and raclopride on cocaine-induced analepsis and EEG arousal in rabbits

The effects of the dopamine D1 and D2 receptor antagonists on cocaine-induced, cholinergically-mediated analeptic and hippocampal theta activity in anesthetized rabbits were investigated. Cocaine (2 mg/kg, i.v.) reduced by 35% the duration of loss of righting reflex produced by a 25 mg/kg dose of pentobarbital. This shortening of narcosis time was blocked by pretreating the animals with the D1 antagonist, SCH 23390 (0.1 mg/kg) but not with the D2 antagonist raclopride (1-2 mg/kg). Cocaine (5 mg/kg, i.v.) also produced a short burst of increased hippocampal theta activity in urethane-anesthetized rabbits, which was also blocked by SCH 23390 but not by raclopride. An unexpected finding was that raclopride itself, at 2 mg/kg (i.v.), produced a marked activation of theta activity that lasted for 15-20 min. When cocaine was administered after this time it produced a potentiated theta response, both in duration and in magnitude. These results suggest that in the rabbit, cocaine exerts a cholinergically-mediated behavioral and EEG arousal through a D1 dopamine mechanism. The role of the D2 system is less clear but appears to be antagonistic to the D1-mediated response.

Dopaminergic neurotransmission in somatodendritic and terminal areas of the rat brain: susceptibility to modulation by D1 and D2 receptors and to axotomy

We have investigated the influence of D1 and D2 dopamine receptor active drugs on dopamine (DA) release in substantia nigra (SN), striatum and limbic forebrain in intact and in hemisected rats in vivo. DA release was indirectly assessed as 3-methoxytyramine (3-MT) accumulation following monoamine oxidase inhibition by pargyline. Hemisection per se had no effect on the 3-MT accumulation in the SN. Neither, had SCH 23390, SK & F 28393, or cis-flupentixol any effect in the SN in intact animals or in the lesioned side in hemisected animals. SCH 23390 slightly increased the 3-MT accumulation both in the striatum and limbic forebrain, indicating a stimulatory action on DA release, but SK & F 38393 had no effect in these brain regions. A difference between the striatum and the limbic forebrain was that the effects of SCH 23390, and cis-FPX were almost abolished following hemisection in the limbic forebrain, but only partially reduced in the striatum. In summary, our data give further support for the concept that neither D1 nor D2 dopamine receptors have any pronounced influence on the DA release in the SN. The data also indicate operational differences in the feedback regulation of limbic versus striatal dopaminergic transmission.

Potency mismatch for behavioral and biochemical effects by dopamine receptor antagonists: implications for the mechanism of action of clozapine

Clozapine (3.8-60.0 mumol kg-1) did not produce any alterations in DOPA accumulation (following inhibition of cerebral aromatic L-amino acid decarboxylase) in the prefrontal cortex or in three regions of the neostriatum, i.e. the ventral, the dorso-lateral and the posterior regions, in the rat. In contrast, clozapine produced a reduction in the 5-HTP accumulation in all these brain areas, except for the prefrontal cortex. Raclopride (0.08-20.0 mumol kg-1) produced a marked increase in DOPA accumulation in all four brain regions and an increase in 5-HTP accumulation in the dorso-lateral neostriatum (2.5-20.0 mumol kg-1), but not in the other forebrain regions. Treatment with SCH-23390 (0.4-1.6 mumol kg-1) resulted in increased DOPA accumulation in the ventral and posterior parts of the neostriatum. No other changes in the DOPA or 5-HTP accumulation were seen with SCH-23390. Considering the doses of these three compounds needed for suppression of conditioned avoidance behavior and for the induction of cataleptic rigidity, it is concluded that raclopride produces an increased DA synthesis at much lower doses than those needed for behavioral effects. In contrast, the behavioral effects of SCH-23390 or clozapine precedes effects on brain DA synthesis on the dose-effect curve. In fact, the only biochemical effect of clozapine, which was observed in low, yet behaviorally active doses, was a decrease in forebrain 5-HTP accumulation. In conclusion, the present results demonstrate a mismatch, in different directions for raclopride and SCH-23390, as regards to the doses needed to produce effects on brain dopamine synthesis and on behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Functionally selective neurochemical afferents and efferents of the mesocorticolimbic and nigrostriatal dopamine system

In summary, evidence is presented that the mesocorticolimbic and nigrostriatal dopamine systems form functionally selective afferents to different parts of the basal ganglia and these inputs are paralleled by functionally selective outputs. The ventral striatal region of the nucleus accumbens and olfactory tubercle has a dopamine input that is critical for locomotor activation produced by psychomotor stimulant drugs and some non-drug states. These regions also appear critical for the reinforcing actions of psychomotor stimulants such as cocaine and amphetamine, and these regions may also be involved in the activation associated with non-drug rewards. Both psychomotor stimulant-induced locomotor activation and reinforcement may selectively involve dopamine D1 receptors. The functional efferents of this system appear to involve the region of the ventral pallidum and more specifically GABAergic mechanisms of the posterior medial (sublenticular) ventral pallidum. The relationship of this circuitry with the revised concept of the "extended amygdala" is an area of current work. The nigrostriatal dopamine system forms a functionally selective afferent system to the dorsal striatum and appears to be critical for the focused stereotyped behavior associated with high doses of psychomotor stimulants. This dopamine input also appears to be involved in non-drug-induced conditioned reaction time performance and may selectively involve dopamine D2 receptors. The functional efferents of this system appear to involve both direct and indirect GABAergic connections to the substantia nigra reticulata and dorsal pallidum, respectively. Activation of the GABAergic connection to the dorsal pallidum (indirect connection) appears to mimic the action of dopamine in the dorsal striatum, whereas activation of the GABAergic connection to the substantia nigra reticulata (direct connection) appears to modulate striatal dopamine function. These results show an important functional role for the globus pallidus in the output of the dorsal striatum and emphasize the parallel functional processing of both dorsal and ventral striatum.

Effects of acute administration of SCH 23390 on dopamine and serotonin turnover in major dopaminergic areas and mesencephalic raphe nuclei--comparison with ritanserin

1. The effects of acute administration of SCH 23390 (0.05 and 0.25 mg/kg s.c.), a dopamine D-1 receptor antagonist having also a moderate serotonin-S2 (5-HT-2) receptor blocking activity, and ritanserin (0.5 mg/kg), a specific 5-HT-2 antagonist, on dopamine (DA) and serotonin (5-HT) turnover were investigated in dopaminergic (nucleus caudatus, nucleus accumbens, substantia nigra, A10 area) and serotonergic (nucleus raphe dorsalis and nucleus raphe medialis) rat brain nuclei. 2. Acute SCH 23390 (both doses) increased the metabolism of DA and tended to augment the rate of DA synthesis (accumulation of DOPA after inhibition of aromatic acid decarboxylase) in the nucleus accumbens, but not in the nucleus caudatus. In addition, SCH 23390 had a moderate effect on DA metabolism in substantia nigra. SCH 23390 did not alter the turnover of 5-HT in any of the nuclei studied. 3. Acute administration of ritanserin did not modify 5-HT or DA turnover in any of the nuclei studied. 4. In conclusion, these results suggest that acute SCH 23390 administration preferentially activates the mesolimbic DA system. The lack of effect of ritanserin on DA or 5-HT turnover in nigrostriatal and mesolimbic DAergic areas suggests that under basal conditions the blockade of 5-HT2 receptors do not change monoamine metabolism in these areas. The role of 5-HT-2 blockade in the actions of SCH 23390 on DA turnover appears thus to be of a minor importance.

Postnatal development of striatal dopamine function. I. An examination of D1 and D2 receptors, adenylate cyclase regulation and presynaptic dopamine markers

We have characterized the postnatal development from 1 to 7 weeks after birth in rat striatal homogenates of D1 and D2 dopamine (DA) receptor sites, adenylate cyclase (AC) enzyme activity coupled to DA receptor function, guanine nucleotide binding sites and presynaptic markers of DA terminal function. D1 receptor density, expressed per unit of membrane protein, does not increase over this developmental interval, while maximum DA-stimulated AC activity per mg membrane protein increases 50-100%. D1 agonist affinity for D1 receptor sites doubles by 7 weeks of age but is consistently reduced by guanine nucleotide during development. Guanine nucleotide stimulation of AC develops a biphasic dose-response curve after 3 weeks of age. Between 2 and 4 weeks postnatal age there is a rapid increase in AC catalytic component activity as manifested by the capacity of forskolin or manganese ion to stimulate AC in presence of guanine nucleotide and DA. Reversible [3H]GppNHp (guanyldiphosphonateimidophosphate) binding to striatal homogenates is dependent on Mg2+, inhibited by Ca2+ and GppNHp analogues, and occurs in about a 300-fold excess over D1 sites. Presynaptic markers of dopaminergic function indicate a 7-fold increase in tissue DA levels, a 2-fold reduction in DA turnover and no apparent change in density of DA uptake sites, assayed by [3H]mazindol binding. Subcomponents of D1 and D2 DA receptors have distinct postnatal developmental profiles. Striatal D1 sites do not change significantly during development, but D2 receptors and GTP inhibition of AC increase and appear, respectively, at 3-4 weeks of age, at the same time as the massive matrix innervation of striatum by DA terminals.

The effects of dopamine D-1 and D-2 receptor agonists on body temperature in male mice

The effect of dopamine D-1 and D-2 receptor stimulation on body temperature has been investigated in male mice. The selective D-2 receptor agonists, quinpirole and LY 163502, and the mixed D-1/D-2 agonist, apomorphine, induced a dose-dependent hypothermia, whereas the selective D-1 receptor agonists, SK&F 81297, SK&F 38393 and SK&F 75670, induced hyperthermia. The hyperthermic responses of these agents were of a similar magnitude although the relative efficacies determined in vitro with the adenylate cyclase assay were different. The peripherally acting D-1 agonist, fenoldopam, did not influence body temperature, indicating that the hyperthermia is mediated, centrally. Studies with combinations of quinpirole and SK&F 38393 showed that the effect of one of the substances could be counteracted by the other. Furthermore, antagonist studies showed that the hypothermia induced by quinpirole could be inhibited by the D-2-selective antagonist, YM 09151-2, and by the mixed D-1/D-2 antagonist, cis(Z)-flupentixol, but not by the D-1-selective antagonist, SCH 23390. Similar results were found for apomorphine-induced hypothermia. SK&F 38393-induced hyperthermia could be antagonized by all three antagonists. These results suggest that the two receptor subtypes act differentially on body temperature, and that they influence a common out-put system, but in opposite directions. These findings are opposite to those of behavioural studies, where a synergistic function of D-1 and D-2 receptors has been demonstrated in the regulation of motor function.

Microstructural analysis of the anorectic effect of N-0437, a highly selective dopamine D2 agonist

Drug actions, mediated by dopamine D2 receptors, have been shown to reduce food consumption in rodents. The present study used a microstructural approach to feeding responses to determine the behavioural changes which underlie the anorectic effect of a selective D2 agonist, N-0437. Non-deprived male rats were trained to consume a palatable, sweetened mash in a 30 min test under familiar test conditions. N-0437 (1.0 and 3.0 mg/kg) significantly reduced food intake, but had no effect on the duration of feeding, the duration and frequency of feeding bouts, or on the time course of feeding. Its anorectic effect depended upon a selective reduction in the rate of eating. Microstructural analysis of other behavioural changes which followed treatments with N-0437 indicated that, at 0.3 mg/kg, the drug may have selective dopamine autoreceptor activity, but at 1.0 and 3.0 mg/kg it acts postsynaptically at D2 receptors. The results show that the anorectic effects of N-0437 can be clearly distinguished from the effects of psychomotor stimulants like D-amphetamine or cocaine, but they overlap in part with the effect of the mixed D1/D2 agonist, apomorphine. The results are discussed in relation to a proposed D2 receptor-mediation of one component of the behavioural changes that underlie feeding satiation.

Interaction between accumbens D1 and D2 receptors regulating rat locomotor activity

The effect of intra-accumbens injections of various dopaminergic agonists and antagonists on the rat locomotor activity has been evaluated in automated open fields. Locomotor stimulation has been observed after local administration of d-amphetamine (10 micrograms), apomorphine (10 micrograms), as well as of solution containing the D1 agonist SKF 38 393 and D2 receptor agonist LY 171 555 (quinpirole) in doses (10 and 4 micrograms, respectively) which were inactive when both drugs were administered separately. On the other hand separate injections of metoclopramide (0.1 microgram) and SCH 23 390 (0.5 microgram) (D2 and D1 receptor antagonists) very potently inhibited animals' locomotor activity. The data indicate that concomitant stimulation of both accumbens D1- and D2-receptor related mechanisms is a necessary condition to increase rat motility. Moreover, it seems that accumbens D1 receptors may be differently involved in the control of facilitatory versus inhibitory motor processes.

Pharmacological activity profiles of dopamine D-1 and D-2 reception agonists and antagonists on striatal neuronal activity and the response to dexamphetamine in freely moving rats

1. The effects of dopamine D-1 and D-2 receptor agonists and antagonists were investigated by recording extracellular striatal action potentials in freely moving rats. Dopamine receptor antagonist effects were also evaluated on dexamphetamine-induced excitation of striatal neurons. 2. Striatal neurons responded to SKF 38393, a D-1 agonist, with dose-dependent reductions in activity. At a 2.0 mg/kg dose neuronal activity decreased to 50% of control values. 3. The D-1 antagonist, SCH 23390, at a dose of 4.0 mg/kg decreased striatal neuronal activity by more than 50% and also effectively blocked the effects of 2.5 mg/kg dexamphetamine. 4. LY 171555, a D-2 agonist, at 1.0 or 2.5 mg/kg, did not significantly increase striatal neuronal activity. Although behavioral activation was noted, the neuronal response at the high dose was biphasic with inhibition predominant. 5. The D-2 antagonists haloperidol and sulpiride decreased striatal neuronal activity in a dose-dependent manner and also effectively antagonized the effects of dexamphetamine. The D-2 antagonist, RO 22-1319, at a dose of 2.0 mg/kg completely antagonized increases in striatal neuronal activity after dexamphetamine. 6. These findings suggest that dexamphetamine-induced increases in striatal neuronal activity are due to either stimulation of both D-1 and D-2 receptors, or alternatively, a third dopamine receptor subtype sensitive to both D-1 and D-2 antagonists but not agonists. Furthermore, the concept of specific D-1 and D-2 receptor agonists may require revision as neither SKF 38393 or LY 171555 increased striatal neuronal activity.

Influence of the D-2 dopamine receptor agonist quinpirole on the odor detection performance of rats before and after spiperone administration

The influence of five doses of the D-2 receptor agonist quinpirole (0.025, 0.05, 0.075, 0.10, and 0.20 mg/kg IP) on the odor detection performance of 21 adult male Long Evans rats was assessed using high precision olfactometry and a go/no-go operant task. Additionally, ten rats were pre-treated with the D-2 receptor antagonist spiperone (0.62 mg/kg IP) and their performance monitored following quinpirole administration. Treatments were administered every third day in a counterbalanced order, with the quinpirole injections occurring 15 min before, and the spiperone injections 35 min before, the 260-trial test sessions. Quinpirole injection resulted in a dose-dependent decrease in odor detection performance, as measured by the percentage of correct trials and by the non-parametric signal detection sensitivity index SI. Prior treatment with spiperone eliminated these effects. Dose-related influences of quinpirole on (a) the average latency to initiate a detection response (i.e., the S + response latency), (b) the total session duration, and (c) the number of aborted trials were also eliminated or greatly attenuated by prior spiperone injection. These results suggest that D-2 receptors may be involved in the modulation of odor detection performance and related behaviors.

Chronic treatment with SCH 23390, a selective dopamine D-1 receptor antagonist, decreases dopamine metabolism in rat caudate nucleus

Chronic treatment (18 days; 0.1 and 0.5 mg/kg per day) with the selective dopamine (DA) D-1 receptor antagonist, SCH 23390, significantly reduced the concentration of homovanillic acid (HVA) and the ratios of HVA/DA and DOPAC/DA in the nucleus caudatus of the rat but did not change the metabolism of DA in the prefrontal cortex, substantia nigra or the A10 area. Furthermore, the concentrations of noradrenaline were dose dependently decreased in the A10 area during SCH 23390 treatment. It is concluded that chronic DA D-1 receptor blockade causes changes in the metabolism of DA similar to those caused by classical neuroleptics.

In vivo dopamine (DA) receptor binding and behavioural effects of the putative DA autoreceptor antagonists (+)-AJ 76 and (+)-UH 232 in rats with a unilateral nigral 6-OH-DA lesion

The in vivo dopamine (DA) receptor binding and behavioural properties of the recently characterised putative preferential DA autoreceptor antagonists (+)-AJ 76 and (+)-UH 232 were studied in rats with a unilateral 6-OH-DA lesion of the substantia nigra. The main findings were a) that (+)-UH 232 and (+)-AJ 76 per se failed to produce significant turning behaviour, b) that both agents antagonised contralateral rotation caused by the DA agonist apomorphine, including a change of the characteristic two-peak apomorphine rotation pattern into a single peak, indicating that the DA antagonist properties of (+)-UH 232 and (+)-AJ 76 are retained also at denervation-sensitised postsynaptic DA receptors and--in support of this notion--c) that (+)-UH 232 and (+)-AJ 76 were able to displace the specific in vivo binding of the DA receptor agonist DP-5,6-ADTN in the denervated as well as in the intact striata of the 6-OH-DA-lesioned animals. Interestingly, in this regard (+)-UH 232 was significantly less efficient on the lesioned as compared to the intact side. The DP-5,6-ADTN-displacing effect of (+)-AJ 76 did not, however, differ between the intact and the denervated striatum. The implications of the present findings are discussed with particular reference to DA receptor sensitivity and adaptational phenomena.