Interaction of the D1 receptor antagonist SCH 23390 with the central 5-HT system: radioligand binding studies, measurements of biochemical parameters and effects on L-5-HTP syndrome

A dynamic role for dopamine receptors in the control of mammalian spinal networks

Dopamine is well known to regulate movement through the differential control of direct and indirect pathways in the striatum that express D₁ and D₂ receptors respectively. The spinal cord also expresses all dopamine receptors; however, how the specific receptors regulate spinal network output in mammals is poorly understood. We explore the receptor-specific mechanisms that underlie dopaminergic control of spinal network output of neonatal mice during changes in spinal network excitability. During spontaneous activity, which is a characteristic of developing spinal networks operating in a low excitability state, we found that dopamine is primarily inhibitory. We uncover an excitatory D₁-mediated effect of dopamine on motoneurons and network output that also involves co-activation with D₂ receptors. Critically, these excitatory actions require higher concentrations of dopamine; however, analysis of dopamine concentrations of neonates indicates that endogenous levels of spinal dopamine are low. Because endogenous levels of spinal dopamine are low, this excitatory dopaminergic pathway is likely physiologically-silent at this stage in development. In contrast, the inhibitory effect of dopamine, at low physiological concentrations is mediated by parallel activation of D₂, D₃, D₄ and α₂ receptors which is reproduced when endogenous dopamine levels are increased by blocking dopamine reuptake and metabolism. We provide evidence in support of dedicated spinal network components that are controlled by excitatory D₁ and inhibitory D₂ receptors that is reminiscent of the classic dopaminergic indirect and direct pathway within the striatum. These results indicate that network state is an important factor that dictates receptor-specific and therefore dose-dependent control of neuromodulators on spinal network output and advances our understanding of how neuromodulators regulate neural networks under dynamically changing excitability.

D1 receptors in the anterior cingulate cortex modulate basal mechanical sensitivity threshold and glutamatergic synaptic transmission

The release of dopamine (DA) into target brain areas is considered an essential event for the modulation of many physiological effects. While the anterior cingulate cortex (ACC) has been implicated in pain related behavioral processes, DA modulation of synaptic transmission within the ACC and pain related phenotypes remains unclear. Here we characterized a Crispr/Cas9 mediated somatic knockout of the D1 receptor (D1R) in all neuronal subtypes of the ACC and find reduced mechanical thresholds, without affecting locomotion and anxiety. Further, the D1R high-efficacy agonist SKF 81297 and low efficacy agonist (±)-SKF-38393 inhibit α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) currents in the ACC. Paradoxically, the D1R antagonists SCH-23390 and SCH 33961 when co-applied with D1R agonists produced a robust short-term synergistic depression of AMPAR currents in the ACC, demonstrating an overall inhibitory role for D1R ligands. Overall, our data indicate that absence of D1Rs in the ACC enhanced peripheral sensitivity to mechanical stimuli and D1R activation decreased glutamatergic synaptic transmission in ACC neurons.

Advances in Dopamine D1 Receptor Ligands for Neurotherapeutics

The dopamine D1 receptor (D1R) is essential for neurotransmission in various brain pathways where it modulates key functions including voluntary movement, memory, attention and reward. Not surprisingly, the D1R has been validated as a promising drug target for over 40 years and selective activation of this receptor may provide novel neurotherapeutics for neurodegenerative and neuropsychiatric disorders. Several pharmacokinetic challenges with previously identified small molecule D1R agonists have been recently overcome with the discovery and advancement of new ligands, including drug-like non-catechol D1R agonists and positive allosteric modulators. From this, several novel molecules and mechanisms have recently entered clinical studies. Here we review the major classes of D1R selective ligands including antagonists, orthosteric agonists, non-catechol biased agonists and positive allosteric modulators, highlighting their structure-activity relationships and medicinal chemistry. Recent chemistry breakthroughs and innovative approaches to selectively target and activate the D1R also hold promise for creating pharmacotherapy for several neurological diseases.

Neuroleptic malignant syndrome and serotonin syndrome

The clinical manifestation of drug-induced abnormalities in thermoregulation occurs across a variety of drug mechanisms. The aim of this chapter is to review two of the most common drug-induced hyperthermic states, serotonin syndrome and neuroleptic malignant syndrome. Clinical features, pathophysiology, and treatment strategies will be discussed, in addition to differentiating between these two syndromes and differentiating them from other hyperthermic or febrile syndromes. Our goal is to both review the current literature and to provide a practical guide to identification and treatment of these potentially life-threatening illnesses. The diagnostic and treatment recommendations made by us, and by other authors, are likely to change with a better understanding of the pathophysiology of these syndromes.

Clozapine ameliorates epigenetic and behavioral abnormalities induced by phencyclidine through activation of dopamine D1 receptor

Accumulating evidence suggests that dysregulation of histone modification is involved in the pathogenesis and/or pathophysiology of psychiatric disorders. However, the abnormalities in histone modification in the animal model of schizophrenia and the efficacy of antipsychotics for such abnormalities remain unclear. Here, we investigated the involvement of histone modification in phencyclidine-induced behavioral abnormalities and the effects of antipsychotics on these abnormalities. After repeated phencyclidine (10 mg/kg) treatment for 14 consecutive days, mice were treated with antipsychotics (clozapine or haloperidol) or the histone deacetylase inhibitor sodium butyrate for 7 d. Repeated phencyclidine treatments induced memory impairment and social deficit in the mice. The acetylation of histone H3 at lysine 9 residues decreased in the prefrontal cortex with phencyclidine treatment, whereas the expression level of histone deacetylase 5 increased. In addition, the phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II in the nucleus decreased in the prefrontal cortex of phencyclidine-treated mice. These behavioral and epigenetic changes in phencyclidine-treated mice were attenuated by clozapine and sodium butyrate but not by haloperidol. The dopamine D1 receptor antagonist SCH-23390 blocked the ameliorating effects of clozapine but not of sodium butyrate. Furthermore, clozapine and sodium butyrate attenuated the decrease in expression level of GABAergic system-related genes in the prefrontal cortex of phencyclidine-treated mice. These findings suggest that the antipsychotic effect of clozapine develops, at least in part, through epigenetic modification by activation of the dopamine D1 receptor in the prefrontal cortex.

The role of prefrontal dopamine D1 receptors in the neural mechanisms of associative learning

Dopamine is thought to play a major role in learning. However, while dopamine D1 receptors (D1Rs) in the prefrontal cortex (PFC) have been shown to modulate working memory-related neural activity, their role in the cellular basis of learning is unknown. We recorded activity from multiple electrodes while injecting the D1R antagonist SCH23390 in the lateral PFC as monkeys learned visuomotor associations. Blocking D1Rs impaired learning of novel associations and decreased cognitive flexibility but spared performance of already familiar associations. This suggests a greater role for prefrontal D1Rs in learning new, rather than performing familiar, associations. There was a corresponding greater decrease in neural selectivity and increase in alpha and beta oscillations in local field potentials for novel than for familiar associations. Our results suggest that weak stimulation of D1Rs observed in aging and psychiatric disorders may impair learning and PFC function by reducing neural selectivity and exacerbating neural oscillations associated with inattention and cognitive deficits.

Involvement of dopamine receptors in binge methamphetamine-induced activation of endoplasmic reticulum and mitochondrial stress pathways

Single large doses of methamphetamine (METH) cause endoplasmic reticulum (ER) stress and mitochondrial dysfunctions in rodent striata. The dopamine D₁ receptor appears to be involved in these METH-mediated stresses. The purpose of this study was to investigate if dopamine D₁ and D₂ receptors are involved in ER and mitochondrial stresses caused by single-day METH binges in the rat striatum. Male Sprague-Dawley rats received 4 injections of 10 mg/kg of METH alone or in combination with a putative D₁ or D₂ receptor antagonist, SCH23390 or raclopride, respectively, given 30 min prior to each METH injection. Rats were euthanized at various timepoints afterwards. Striatal tissues were used in quantitative RT-PCR and western blot analyses. We found that binge METH injections caused increased expression of the pro-survival genes, BiP/GRP-78 and P58^IPK, in a SCH23390-sensitive manner. METH also caused up-regulation of ER-stress genes, Atf2, Atf3, Atf4, CHOP/Gadd153 and Gadd34. The expression of heat shock proteins (HSPs) was increased after METH injections. SCH23390 completely blocked induction in all analyzed ER stress-related proteins that included ATF3, ATF4, CHOP/Gadd153, HSPs and caspase-12. The dopamine D₂-like antagonist, raclopride, exerted small to moderate inhibitory influence on some METH-induced changes in ER stress proteins. Importantly, METH caused decreases in the mitochondrial anti-apoptotic protein, Bcl-2, but increases in the pro-apoptotic proteins, Bax, Bad and cytochrome c, in a SCH23390-sensitive fashion. In contrast, raclopride provided only small inhibition of METH-induced changes in mitochondrial proteins. These findings indicate that METH-induced activation of striatal ER and mitochondrial stress pathways might be more related to activation of SCH23390-sensitive receptors.

Striatal dopamine and glutamate receptors modulate methamphetamine-induced cortical Fos expression

Methamphetamine (mAMPH) is a psychostimulant drug that increases extracellular levels of monoamines throughout the brain. It has previously been observed that a single injection of mAMPH increases immediate early gene (IEG) expression in both the striatum and cerebral cortex. Moreover, this effect is modulated by dopamine and glutamate receptors since systemic administration of dopamine or glutamate antagonists has been found to alter mAMPH-induced striatal and cortical IEG expression. However, because dopamine and glutamate receptors are found in extra-striatal as well as striatal brain regions, studies employing systemic injection of dopamine or glutamate antagonists fail to localize the effects of mAMPH-induced activation. In the present experiments, the roles of striatal dopamine and glutamate receptors in mAMPH-induced gene expression in the striatum and cerebral cortex were examined. The nuclear expression of Fos, the protein product of the IEG c-fos, was quantified in both the striatum and the cortex of animals receiving intrastriatal dopamine or glutamate antagonist administration. Intrastriatal infusion of dopamine (D1 or D2) or glutamate [N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)] antagonists affected not only mAMPH-induced striatal, but also cortical, Fos expression. Overall, the effects of the antagonists occurred dose-dependently, in both the infused and non-infused hemispheres, with greater influences occurring in the infused hemisphere. Finally, unilateral intrastriatal infusion of dopamine or glutamate antagonists changed the behavior of the rats from characteristic mAMPH-induced stereotypy to rotation ipsilateral to the infusion. These results demonstrate that mAMPH's actions on striatal dopamine and glutamate receptors modulate the widespread cortical activation induced by mAMPH. It is hypothesized that dopamine release from nigrostriatal terminals modulates activity within striatal efferent pathways, thereby disinhibiting thalamo-cortical circuits. By extension, these results suggest processes through which repeated exposure to mAMPH might influence cortical function in mAMPH abusers.

Synergistic effect of combined treatment with risperidone and galantamine on phencyclidine-induced impairment of latent visuospatial learning and memory: Role of nAChR activation-dependent increase of dopamine D1 receptor-mediated neurotransmission

The clinically achievable efficacy of the atypical antipsychotics on cognitive symptoms of schizophrenia is practically limited by their dose-dependent side effects. Thus, there is the need for adjuvant treatments or strategies for the cognitive impairments. Further, human autopsy and genetic data in schizophrenia have indicated the existence of the abnormality of nicotinic acetylcholine receptors (nAChR). In the present study, we aimed to investigate the synergistic effect and mechanisms of a combined treatment with an atypical antipsychotic risperidone and galantamine, which is a nAChR-allosteric modulator and a modest cholinesterase inhibitor, on the impairment of latent visuospatial learning and memory in mice resembling the cognitive impairment of schizophrenia. Repeated treatment with phencyclidine (PCP, 10 mg/kg, 14 days)-induced cognitive impairment in mice in a one trial water-finding test was used as a model of the cognitive impairment of schizophrenia. In vivo microdialysis was used to investigate the extracellular concentration of dopamine in the medial prefrontal cortex (mPFC). Combined treatment with galantamine and risperidone, at low, ineffective doses (both at 0.05 mg/kg) showed a synergistic effect to reverse cognitive impairment and increase extracellular concentration of dopamine in the mPFC. The synergistic behavioral effect was abolished by a dopamine-D1 receptor antagonist, SCH 23390, and a nAChR antagonist, mecamylamine, but not a muscarinic AChR (mAChR) antagonist, scopolamine. Mecamylamine also blocked the synergistic effect on dopamine release in the mPFC of PCP-treated mice. The study indicates that galantamine and risperidone may have synergistic effect on the cognitive impairments in schizophrenia patients by synergistically promoting the nAChR activation-dependent increase of dopamine D1 receptor-mediated neurotransmission.

Dopamine D1 receptors in the anterior cingulate cortex regulate effort-based decision making

The anterior cingulate cortex (ACC) has been implicated in encoding whether or not an action is worth performing in view of the expected benefit and the cost of performing the action. Dopamine input to the ACC may be critical for this form of effort-based decision making; however, the role of distinct ACC dopamine receptors is yet unknown. Therefore, we examined in rats the effects of an intra-ACC D1 and D2 receptor blockade on effort-based decision making tested in a T-maze cost-benefit task. In this task, subjects could either choose to climb a barrier to obtain a high reward in one arm or a low reward in the other arm without a barrier. Unlike vehicle-treated rats, rats with intra-ACC infusion of the D1 receptor antagonist SCH23390 exhibited a reduced preference for the high-cost- high-reward response option when having the choice to obtain a low reward with little effort. In contrast, in rats with intra-ACC infusion of the D2 receptor antagonist eticlopride, the preference for the high-cost-high-reward response option was not altered relative to vehicle-treated rats. These data provide the first evidence that D1 receptors in the ACC regulate effort-based decision making.

Dibenzazecine compounds with a novel dopamine/5HT2A receptor profile and 3D-QSAR analysis

Background

Antipsychotics are divided into typical and atypical compounds based on clinical efficacy and side effects. The purpose of this study was to characterize in vitro a series of novel azecine-type compounds at human dopamine D₁-D₅ and 5HT_2A receptors and to assign them to different classes according to their dopamine/5HT_2A receptor profile.

Results

Regardless of using affinity data (pK_i values at D₁-D₅ and 5HT_2A) or selectivity data (15 log (K_i ratios)), principal component analysis with azecine-type compounds, haloperidol, and clozapine revealed three groups of dopamine/5HT_2A ligands: 1) haloperidol; 2) clozapine plus four azecine-type compounds; 3) two hydroxylated dibenzazecines. Reducing the number of K_i ratios used for principal component analysis from 15 to two (the D₁/D₂ and D₂/5HT_2A K_i ratios) obtained the same three groups of compounds. The most potent dibenzazecine clustering in the same group as clozapine was the non-hydroxylated LE410 which shows a slightly different D₂-like receptor profile (D_2L > D₃ > D_4.4) than clozapine (D_4.4 > D_2L > D₃). The monohydroxylated dibenzacezine LE404 clusters in a separate group from clozapine/LE410 and from haloperidol and shows increased D₁ selectivity.

Conclusion

In conclusion, two compounds with a novel dopamine/5HT_2A receptor profile, LE404 and LE410, with some differences in their respective D₁/D₂ receptor affinities including a validated pharmacophore-based 3D-QSAR model for D₁ antagonists are presented.

Mesocortical dopamine neurons operate in distinct temporal domains using multimodal signaling

In vivo extracellular recording studies have traditionally shown that dopamine (DA) transiently inhibits prefrontal cortex (PFC) neurons, yet recent biophysical measurements in vitro indicate that DA enhances the evoked excitability of PFC neurons for prolonged periods. Moreover, although DA neurons apparently encode stimulus salience by transient alterations in firing, the temporal properties of the PFC DA signal associated with various behaviors is often extraordinarily prolonged. The present study used in vivo electrophysiological and electrochemical measures to show that the mesocortical system produces a fast non-DA-mediated postsynaptic response in the PFC that appears to be initiated by glutamate. In contrast, short burst stimulation of mesocortical DA neurons that produced transient (<4 s) DA release in the PFC caused a simultaneous reduction in spontaneous firing (consistent with extracellular in vivo recordings) and a form of DA-induced potentiation in which evoked firing was increased for tens of minutes (consistent with in vitro measurements). We suggest that the mesocortical system might transmit fast signals about reward or salience via corelease of glutamate, whereas the simultaneous prolonged DA-mediated modulation of firing biases the long-term processing dynamics of PFC networks.

Prefrontal cortex D1 modulation of the reinforcing properties of cocaine

The involvement of the dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) in the reinforcing properties of many drugs of abuse is well established. Though the prefrontal cortex (PFC) exhibits significant influence over activity in this pathway, its role in drug abuse is less defined. The present experiment investigated the impact of PFC D1 activity on cocaine self-administration (0.25, 0.75 mg/kg/inj) under progressive (PR) and fixed ratio (FR) schedules of reinforcement by assessing immediate and delayed effects of bilateral intra-PFC infusions of a D1 agonist (SKF 38393; 0.23 microg/side) and antagonist (SCH 23390; 0.25 microg/side). Immediately following infusion of dopaminergic agents or vehicle, no significant changes in self-administration occurred under any tested condition. However, 24 h after intra-PFC antagonist treatment, significantly lower PR breakpoints were observed for low (0.25 mg/kg), but not moderate (0.75 mg/kg) unit doses of self-administered cocaine. Locomotor activity levels during these assessments were unaffected by intra-PFC treatments. On an FR-3 schedule of reinforcement, the 0.25 cocaine unit dose elicited higher total cocaine intake and hyperlocomotor activation during a shorter session, but intra-PFC treatment had no significant effects on the number of reinforced responses or behavioral activity. The observation of decreased cocaine breakpoints after intra-PFC DA antagonist treatment reflects decrements in cocaine reinforcement efficacy. This finding corresponds temporally with previous work showing increased NAcc DA levels after similar treatment. Current findings demonstrate that transient changes in PFC DA neurotransmission can specifically influence reinforced behaviors without affecting overall behavioral activation.

Sustained extracellular signal-regulated kinase 1/2 phosphorylation in neonate 6-hydroxydopamine-lesioned rats after repeated D1-dopamine receptor agonist administration: implications for NMDA receptor involvement

Extracellular signal-regulated kinase (ERK) 1/2, a well known regulator of gene expression, is likely to contribute to signaling events underlying enduring neural adaptations. Phosphorylated (phospho)-ERK was examined immunohistochemically after both single and repeated (i.e., sensitizing) doses of the partial D1-dopamine (DA) receptor agonist SKF-38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benazepine HCl) to adult rats lesioned as neonates (neonate lesioned) with 6-hydroxydopamine. Remarkably, prolonged phospho-ERK accumulated primarily in layers II-III of medial prefrontal cortex (MPC), where it declined gradually yet remained significantly elevated for at least 36 d after repeated doses of SKF-38393. Sustained (> or =7 d) phospho-ERK was observed for shorter periods in various other cortical regions but was not detectable in striatum or nucleus accumbens. At 36 d, an additional injection of SKF-38393 to sensitized rats restored phospho-ERK to maximal levels only in MPC when examined 7 d later. Phosphorylated cAMP response element-binding protein (CREB), examined 7 d after the sensitizing regimen, was observed exclusively in MPC, where it was abundant throughout all layers. Systemic injections of SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile), an inhibitor of the upstream ERK activator mitogen ERK kinase, attenuated both ERK and CREB phosphorylation in layers II-III of MPC. Pretreatment with the D1 antagonist SCH-23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-OL maleate) inhibited the prolonged increase in MPC phospho-ERK, whereas the 5-HT2 receptor antagonist ketanserin (3-[2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl]-2,4(1H,3H)-quinazolinedione tartrate) was ineffective. Competitive and noncompetitive NMDA receptor antagonists also blocked sustained ERK phosphorylation. Collectively, the present results demonstrate coupling of D1 and NMDA receptor function reflected in sustained activation of the ERK signaling pathway in MPC of SKF-38393-sensitized neonate-lesioned rats. Ultimately, long-lasting phosphorylation of ERK and CREB in MPC may play a pivotal role in any permanent adaptive change(s) in these animals.

Alterations in food intake by opioid and dopamine signaling pathways between the ventral tegmental area and the shell of the nucleus accumbens

Reward is an important factor motivating food intake in satiated animals. Two sites involved in the reward response are the ventral tegmental area (VTA) and the nucleus accumbens shell region (sNAcc), between which communication is partially regulated by opioids and dopamine (DA). Previous studies have shown that the mu-opioid agonist Tyr-D-Ala-Gly-MePhe-Gly(ol)-enkephalin (DAMGO) dose-dependently enhances food intake in satiated animals when injected into either the VTA or the sNAcc. The enhanced intake elicited by DAMGO injected into the sNAcc was dose-dependently blocked by injection of naltrexone (NTX) bilaterally into the VTA, indicating an opioid-dependent signaling pathway from the sNAcc to the VTA in mediation of food intake. In the present study, we cannulated animals bilaterally in both the VTA and the sNAcc to further study the nature of opioid- and DA-dependent communication between the sites. Food intake elicited by DAMGO (2 or 5 nmol) injected unilaterally into the VTA was dose-dependently diminished by bilateral injection of NTX (2.5, 5, and 25 g/side) or the D1 antagonist SCH 23390 (3, 1, 0.3, 0.15, 0.05, and 0.015 nmol/side) into the sNAcc. When DAMGO (5 nmol) was injected into the sNAcc, the resulting food intake was decreased by doses of SCH 23390 ranging from 0.05 to 100 nmol/side injected bilaterally into the VTA, but not by equimolar doses of Raclopride, a D2 antagonist. These results, combined with previous findings, suggest a signaling pathway between the VTA and the sNAcc in which opioids and DA facilitate feeding in an interdependent manner.

The effects of dopamine D(1) receptor blockade in the prelimbic-infralimbic areas on behavioral flexibility

This study examined the effects of a dopamine D(1) antagonist, SCH23390, infused into the prelimbic-infralimbic areas on the acquisition of a response and visual-cue discrimination task, as well as a shift from a response to a visual-cue discrimination and vice versa. Each test was carried out in a cross-maze. The response discrimination required learning to always turn in the same direction (right or left) for a cereal reinforcement. The visual-cue discrimination required learning to always enter the arm with the visual cue. In experiment 1, rats were tested on the response discrimination task, followed by the visual-cue discrimination task. In experiment 2, the testing order was reversed. Bilateral infusions of SCH23390 (0.1 or 1 microg/0.5 microL) into the prelimbic-infralimbic areas did not impair acquisition of the response or visual-cue discrimination tasks. SCH23390 injections at 1 microg, but not 0.1 microg impaired performance when shifting from a response to a visual-cue discrimination, and vice versa. Analysis of the errors revealed that the deficit was due to perseveration of the previously learned strategy. These results suggest that activation of dopamine D(1) receptors in the prelimbic-infralimbic areas may be critical for the suppression of a previously relevant strategy and/or generating new strategies.

Intra-prefrontal cortex injections of SCH 23390 influence nucleus accumbens dopamine levels 24 h post-infusion

The dopaminergic pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc) is well known to be involved in the reinforcing properties of many drugs of abuse. The medial prefrontal cortex (mPFC) has been shown to exhibit significant influence over activity in this pathway, and has also been implicated in drug abuse. The present experiment investigated the ability of D1 activity in the mPFC to influence accumbal dopamine levels. NAcc dopamine (DA) was monitored before, immediately after, and 24 h following mPFC infusion of a D1 agonist (SKF 38393), D1 antagonist (SCH 23390), or a vehicle solution. Immediately following infusion of dopaminergic agents or vehicle, no significant changes in accumbal DA were observed. However, 24 h following infusion of the antagonist but not the agonist, significant elevations of accumbal DA were observed. Since elevated NAcc DA was only observed 24 h after treatment, these results provide evidence that long-term neural adaptations can be induced by transient neuropharmacological treatment.

Regulation of rat cortex function by D1 dopamine receptors in the striatum

Interactions between the basal ganglia and the cerebral cortex are critical for normal goal-directed behavior. In the present study, we used immediate-early genes (c-fos, zif 268) as functional markers to investigated how basal ganglia output altered by stimulation/blockade of D1 dopamine receptors in the striatum affects cortical function. Systemic administration of the mixed D1/D2 receptor agonist apomorphine (3 mg/kg) increased immediate-early gene expression in the striatum and throughout most of the cortex. Unilateral intrastriatal infusion of the selective D1 receptor antagonist SCH-23390 (0.5-10 microg) blocked this response bilaterally in striatum and cortex in a dose-dependent manner. Even apparently regionally restricted blockade of striatal D1 receptors attenuated gene expression throughout striatum and cortex in both hemispheres. Intrastriatal administration of the D1 antagonist inhibited apomorphine-induced sniffing/whisking, whereas other motor behaviors were unaffected. To determine whether such changes in cortical gene expression could reflect altered cortical function, we examined the effects of blocking striatal D1 receptors on whisker stimulation-evoked immediate-early gene expression in the sensorimotor cortex. Apomorphine increased sensory stimulation-evoked gene expression in the barrel cortex, and intrastriatal infusion of SCH-23390 attenuated this effect. These results suggest that stimulation of D1 dopamine receptors in the striatum exerts a widespread facilitatory effect on cortical function.

Enhanced and impaired attentional performance after infusion of D1 dopaminergic receptor agents into rat prefrontal cortex

The role in spatial divided and sustained attention of D1 and D2-like dopamine (DA) receptors in the rat prelimbic medial prefrontal cortex (mPFC) was investigated in a five-choice serial reaction time task. Rats were trained to detect brief flashes of light (0.5-0.25 sec) presented randomly in a spatial array of five apertures. When performance stabilized, animals received bilateral microinfusions of either the D1 DA receptor antagonist SCH 23390, the D1 DA receptor agonist SKF 38393, or the D2 DA antagonist sulpiride into the mPFC. Rats were divided into two groups, with low (<75% correct) and high (>75%) baseline levels of accuracy. Infusions of the D2 receptor antagonist sulpiride had no significant effect on any task variable. SCH 23390 (0.3 microg) selectively impaired the accuracy of attentional performance in rats in the high baseline condition. By contrast, SKF 38393 (0.06 microg) enhanced the accuracy of attentional performance in the low baseline condition, a lower dose (0.03 microg) also increasing the speed of making correct responses. Finally, the beneficial effects of SKF-383893 on choice accuracy were antagonized by SCH 23390 (1.0 microg). The results provide apparently the first demonstration of enhanced cognitive function after local administration of a D1 receptor agonist to the mPFC and suggest dissociable roles of D1 and D2 DA receptors of the mPFC in modulating attentional function.

Substantia nigra D1 receptors and stimulation of striatal cholinergic interneurons by dopamine: a proposed circuit mechanism

Dopamine release can regulate striatal acetylcholine efflux in vivo through at least two receptor mechanisms: (1) direct inhibition by dopamine D2 receptors on the cholinergic neurons, and (2) excitation initiated by dopamine D1 receptors. The neuroanatomical locus of the latter population of D1 receptors and the pathway(s) involved in the expression of their influence are controversial issues. We have tested the hypothesis that D1 receptors in substantia nigra pars reticulata are involved in the excitatory component of dopaminergic actions on striatal acetylcholine output. In vivo microdialysis was used in awake rats. Infusion of the selective D1 receptor agonist R(+)-1-Phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF 38393) hydrochloride into pars reticulata of substantia nigra elicited a significant increase in striatal acetylcholine efflux. Likewise, D-amphetamine applied into pars reticulata of substantia nigra by reverse dialysis produced an elevation in acetylcholine output measured at a second microdialysis probe in the striatum. Application of D-amphetamine in the striatum by reverse dialysis elicited a decrease in striatal acetylcholine efflux that could be reversed subsequently by local application of D-amphetamine in substantia nigra pars reticulata. A 2 mg/kg intraperitoneal dose of D-amphetamine, which has no net effect on striatal acetylcholine output under control conditions, elicited a significant decrease in acetylcholine efflux when the D1 receptor antagonist R(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine (SCH 23390) hydrochloride was applied simultaneously via a second microdialysis probe in substantia nigra pars reticulata. Thus, an excitatory D1-mediated influence on striatal acetylcholine output is initiated in substantia nigra pars reticulata, and this influence contributes to the effects of indirect dopaminergic agonists such as D-amphetamine on striatal acetylcholine efflux. These results indicate an important role of somatodendritic dopamine release, in addition to nerve terminal dopamine release, in the regulation of activity in basal ganglia circuits.

8-OH-DPAT-induced release of hippocampal noradrenaline in vivo: evidence for a role of both 5-HT1A and dopamine D1 receptors

Here we investigate the effects of the novel selective 5-HT1A receptor antagonist, N-[2-[4-(2 methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl cyclo-hexanecarboxamide (WAY 100635), and the dopamine D1 receptor antagonist, R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin++ +-7-ol (SCH 23390), on the increase in extracellular noradrenaline in rat hippocampus induced by the 5-HT1A receptor agonist, 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT). 8-OH-DPAT (0.1 and 1 mg/kg s.c.) caused a dose-related increase in extracellular noradrenaline. WAY 100635 (0.3 and 1 mg/kg s.c.) did not block the release of noradrenaline induced by the higher dose of 8-OH-DPAT (1 mg/kg s.c.) but abolished the response to the lower dose (0.1 mg/kg s.c.). When administered alone, WAY 100635 (0.3 and 1 mg/kg s.c.) had no effect on extracellular noradrenaline. The postsynaptically mediated 5-HT behavioural syndrome induced by the higher dose of 8-OH-DPAT, in contrast to the increase in noradrenaline, was completely blocked by WAY 100635 (0.3 mg/kg s.c.). Finally, the noradrenaline response to 8-OH-DPAT (0.1 mg/kg s.c.) was blocked by SCH 23390 (0.5 mg/kg s.c.). Our data confirm that noradrenaline can be released by activation of 5-HT1A receptors but show that these receptors are not tonically activated, and may be more sensitive to stimulation than classical postsynaptic 5-HT1a receptors. A role for the dopamine D1 receptor in the noradrenaline response to 8-OH-DPAT is also suggested.

Pretreatment with a dopamine D1-receptor antagonist prevents metabolic activation by cocaine

The pharmacological mechanism underlying metabolic activation of the rat extrapyramidal system by acute cocaine was examined using the quantitative 2-deoxyglucose autoradiographic method. Pretreatment with a selective dopamine D1-like receptor antagonist, SCH 23390, prevented cocaine-induced metabolic activation in the entopeduncular nucleus and substantia nigra pars reticulata in a dose-dependent manner. These results suggest that activity in striatonigral circuits is induced by stimulation of D1-like receptors by dopamine in the presence of acute cocaine.

Dopamine receptors in the substantia nigra are involved in the regulation of muscle tone

The aim of the present study was to localize the dopamine receptors involved in the regulation of muscle tone. A strategy was used whereby the effects on muscle tone of injecting the irreversible dopamine receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in discrete brain regions were assessed. Increases in muscle tone were measured as changes in electromyographic activity of the gastrocnemius and tibialis muscles of conscious, unrestrained rats. No increases in muscle tone were found after injections of EEDQ into the anterior and posterior striatum, which produced marked reductions in dopamine receptor concentration. The effects of muscle tone of injecting EEDQ into the substantia nigra pars reticulata were also assessed. Large increases in muscle tone were observed associated with inactivation of either D1 or D2 dopamine receptors in the substantia nigra. The increased muscle tone was not reduced by subcutaneous administration of apomorphine, despite the presence of a normal population of striatal dopamine receptors. These findings provide evidence that dopamine receptors in the substantia nigra play an important role in the regulation of muscle tone. Further, they challenge the hypothesis that the muscle rigidity of Parkinson disease results primarily from loss of striatal dopamine receptor stimulation.

Time course of dopamine1,2 and serotonin2 receptor binding of antipsychotics in vivo

An in vivo receptor binding technique was applied to evaluate the affinities of clozapine (20 mg/kg), RMI-81582 (20 mg/kg), and haloperidol (1 mg/kg) for dopamine D1, D2 and serotonin 5-HT2 receptors in rat brain with [3H]-SCH23390, [3H]-YM-09151-2, and [3H]-ketanserin as selective ligands. The time course study of receptor occupancy at 25 to 250 min after intraperitoneal administration of the drugs showed higher 5-HT2 and lower D2 receptor occupancies of clozapine and RMI-81582 than those of haloperidol both in the striatum and frontal cortex. The 5-HT2/D2 ratios of receptor occupancy for clozapine and RMI-81582 were about 6 to 8 times higher than that for haloperidol. Stable occupancies of D1 receptors were observed only with RMI-81582 and clozapine, the former demonstrating the higher occupancy. These findings are in agreement with the previous findings obtained under in vitro conditions and may account for some part of the properties of atypical antipsychotic drugs.

Effects of atypical antipsychotic agents on social behavior in rodents

There are numerous preclinical screening procedures that are predictive of clinical efficacy for the positive symptoms of schizophrenia but no assays for the negative symptoms such as social withdrawal. In the social interaction test in rats, the atypical antipsychotic drug clozapine (10.0 mg/kg) and two putative atypical agents risperidone (0.0625 mg/kg) and HP 873 (0.5 and 1.0 mg/kg) significantly increased social interaction behaviors between pairs of unfamiliar but not familiar rats. The benzodiazepine diazepam (1.25-5.0 mg/kg) increased social behaviors in both paradigms. Haloperidol, chlorpromazine, raclopride, and SCH23390 decreased social behaviors in these assays. In vitro receptor binding studies revealed that only clozapine, risperidone, and HP 873 displayed dopamine to serotonin affinity ratios for both D2/5-hydroxytryptamine2(5-HT2)/ and D1/5-HT1A of greater than or equal to 12.9 and 1.0, respectively. The present study suggests that antipsychotic agents that may be effective in social withdrawal can be identified in this modified social interaction paradigm. Further, our data suggests that a compound's effectiveness for the treatment of social withdrawal is at least in part due to its relative affinity for binding to dopamine D1 and serotonin 5-HT1A receptors.

Mechanisms of amphetamine-induced rotation in rats with unilateral intrastriatal grafts of embryonic dopaminergic neurons: a pharmacological and biochemical analysis

Amphetamine induces a pronounced rotation directed ipsilateral to the lesion and lasting about 2 h in rats bearing a unilateral lesion of the nigrostriatal dopaminergic pathway. Implantation of embryonic dopaminergic neurons into the lesioned striatum leads to a compensation of this rotation. However, graft-bearing animals display a strong biphasic contralateral rotation, lasting up to 5 h. To try to ascertain the mechanisms of this anomalous rotation, two separate experiments were performed. First, we tested whether the contralateral rotation presented by the grafted animals could be correlated to the persistence of the lesion-induced decoupling of striatal D1 and D2 receptors. Lesioned and grafted animals were submitted to a series of four amphetamine (5 mg/kg, i.p.) rotation tests. Preceding each test animals received, in a randomized order, one of four of the following treatments: physiological saline, a D1 receptor blocker (SCH-23390, 0.1 mg/kg, s.c.), a D2 receptor blocker (raclopride, 2.5 mg/kg, i.p.) or the combination of the D1 and D2 antagonists. The ipsilateral rotation observed in the lesioned animals was abolished by the separate blockade of both classes of dopamine receptor as well as by their combined blockade. Grafted animals could be separated into two subgroups, based on the effect of the antagonists during the first 2 h of amphetamine-induced rotation. In one subgroup, antagonists had the same effect on the amphetamine-induced contralateral rotation as they did on the ipsilateral rotation displayed by lesioned animals. In this group, D1 and D2 receptors were therefore recoupled by the implant in the lesioned striatum. In the other subgroup, the contralateral rotation could be antagonized only by the combined D1 and D2 blockade, while the separate blockade of D1 or D2 receptors did not decrease or even increased the amphetamine-induced rotation. This indicates that in this group the lesion-induced decoupling of D1 and D2 receptors persisted. Nevertheless, the characteristics of the amphetamine-induced rotation (magnitude, duration) were the same in the two subgroups. Likewise, hypersensitivities of both D1 and D2 receptors were completely abolished by the graft in both subgroups. From this experiment it is concluded that the amphetamine-induced rotation observed in grafted animals is not correlated with the state of coupling of striatal D1 or D2 receptors. In a second experiment, dopamine release was monitored by microdialysis in the graft-bearing and the contralateral normal striatum of awake, behaving animals following the administration of amphetamine to test whether the observed rotation could be explained by a higher than normal dopamine release from the implanted dopaminergic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of the 5-HT1C receptor expressed in Xenopus oocytes by the benzazepines SCH 23390 and SKF 38393

1. A cloned 5-HT1C receptor expressed in Xenopus laevis oocytes was used to characterize the action of four dopamine D1-selective benzazepines at the 5-HT1C receptor. Additionally, the apparent binding of the D1-selective benzazepines to 5-HT1C receptors was measured in the choroid plexus of the pig. 2. In voltage-clamped oocytes expressing the cloned 5-HT1C receptor, 5-hydroxytryptamine (5-HT) elicited a characteristic inward current response with an EC50 of 13 nM. SCH 23390 acted as a stereoselective agonist (or partial agonist) with an EC50 of about 550 nM. SKF 38393 (1 microM-1 mM), SKF 77434 (100 microM), and SKF 82958 (100 microM) also acted as agonists (or partial agonists) at the cloned 5-HT1C receptor. SKF 38393 was not stereoselective at the 5-HT1C receptor. 3. The response to SCH 23390 activated slowly and, although the response contained many oscillations characteristic of the activation of the phosphatidylinositol signal transduction system, SCH 23390 rarely elicited the rapid spike-like response seen routinely in response to 5-HT. However, the responses to SKF 38393, SKF 77434, and SKF 82958 were identical in appearance to the response to 5-HT, except that the responses to the benzazepines were smaller. These comparisons were made by applying both a benzazepine and 5-HT to each individual oocyte expressing the cloned 5-HT1C receptor. 4. Consistent with the responses measured in oocytes, SCH 23390 bound stereoselectively to 5-HT1C receptors in the choroid plexus of the pig (Ki = 6.3 nM), and SKF 38393 bound non-stereoselectively with lower affinity (Ki = 2.0-2.2 microM).5. It is concluded that while these benzazepines demonstrate selectivity for the dopamine D1 receptor, they also can act as agonists or partial agonists at the 5-HT1c receptor in situ and as expressed in Xenopus oocytes. The oocyte expression system is useful for studies of the functional pharmacology of these 5-HTic receptors. Information about the pharmacological actions and variations in stereoselectivity among dopamine and 5-HT receptors should be of interest in modelling the interactions of ligands with these G-protein coupled receptors, and in the testing of such models through receptor mutagenesis.

Identification of serotonin 5HT2 receptors in bovine pineal gland

The concentration of serotonin in the pineal gland is extremely high, which prompted speculation that in addition to serving as a precursor of melatonin, serotonin may have an independent function of its own. By using [3H]-spiperone as a ligand, and ketanserine as a selective serotonin 5HT2 receptor antagonist, we have identified 5HT2 receptor in the bovine pineal gland, revealing a single population of binding sites with a dissociation equilibrium constant (Kd) value of 1.26 +/- 0.41 nM and a receptor density (Bmax) value of 193 +/- 38.85 fmol/mg protein. In displacement experiments, the concentrations of the drugs required to inhibit 50% of the specific binding of [3H]-spiperone in descending order of potency were methysergide greater than ritanserin greater than pirenperone greater than pipamperone greater than ketanserin greater than cyproheptadine greater than M-trifluoromethylphenyl-piperazine greater than prazosin greater than 5-methoxy-N-N-dimethyltryptamine hydrogen oxalate greater than 1-(3-chlorophenol) piperazine greater than serotonin. In the rat pineal gland, [3H]-spiperone revealed a low affinity serotonin binding site with a Kd value of 25.77 +/- 10.7 nM and a Bmax value of 1244 +/- 472 fmol/mg protein. The results of these studies are interpreted to indicate that the bovine pineal gland possess serotonin 5HT2 receptor. However, the rat pineal gland possess a serotoninergic binding site of unknown nature.

Evidence for involvement of both D1 and D2 receptors in maintaining cocaine self-administration

Rats trained to self-administer cocaine (0.75 mg/kg/infusion) on an FR-5 schedule were treated with selective D1 or D2 antagonists. A69045, a D1 antagonist with no appreciable affinity for 5-HT receptors increased cocaine self-administration to 147, 172 and 167% of baseline at doses of 2.5, 5.0 or 10.0 mumol/kg, SC respectively. SCH-23390 (0.007, 0.015 and 0.030 mumol/kg, SC) increased self-administration to 116, 147 and 165% of baseline, respectively. Both D1 antagonists decreased responding in some animals at the highest dose tested. The D2 antagonist YM-09151-2 showed a similar profile, increasing cocaine self-administration at 0.01 and 0.016 mumol/kg, SC and suppressing responding by most animals at the dose of 0.03 mumol/kg, SC. These data give further support to the hypothesis that both D1 and D2 receptors are involved in maintaining cocaine self-administration.

Endogenous dopamine (DA) modulates [3H]spiperone binding in vivo in rat brain

[3H]spiperone (SPI) binding in vivo, biochemical parameters and behavior were measured after modulating DA levels by various drug treatments. DA releasers and uptake inhibitors increased SPI binding in rat striatum. In other brain areas, the effects were variable, but only the pituitary remained unaffected. Surprisingly, nomifensine decreased SPI binding in frontal cortex. The effects of these drugs were monitored by measuring DA, serotonin (5-HT) and their metabolites in the same rats. The increased SPI binding in striatum was parallel to the locomotor stimulation with the following rank order: amfonelic acid greater than nomifensine greater than D-amphetamine greater than or equal to methylphenidate greater than amineptine greater than bupropion. Decreasing DA levels with reserpine or alpha-methyl-para-tyrosine reduced SPI binding by 45% in striatum only when both drugs were combined. In contrast, reserpine enhanced SPI binding in pituitary. Thus, the amount of releasable DA seems to modulate SPI binding characteristics. It is suggested that in vivo, DA receptors are submitted to dynamic regulation in response to changes in intrasynaptic concentrations of DA.

Serotonergic innervation of the rat caudate following a neonatal 6-hydroxydopamine lesion: an anatomical, biochemical and pharmacological study

6-Hydroxydopamine (6-OHDA) treatment of neonatal rats resulted in a dose-related loss of striatal dopamine (DA). These reductions corresponded closely with the loss of tyrosine hydroxylase-containing terminals at this brain site. Striatal serotonin (5-HT) concentration increased only after DA was maximally depleted by the highest dose of 6-OHDA. Quantitative immunohistochemistry revealed that the increased 5-HT content after neonatal 6-OHDA lesioning was due to a proliferation of 5-HT nerve terminals. The density of immunoreactive 5-HT-containing terminals appeared to increase more than did the 5-HT content. The present study examined whether 5-HT hyperinnervation was playing a role in behavioral responses induced by D1-DA agonists and antagonists in neonatally lesioned rats, because reports have suggested that these drugs may interact with 5-HT receptors. However, SCH-23390, the D1-DA antagonist (0.3 mg/kg), did not alter behavioral responses to 5-HTP and SKF-38393 (3 mg/kg), a D1-DA agonist did not produce any signs of activating 5-HT receptors in 5,7-DHT-lesioned rats. These data indicate that these compounds affecting D1-DA receptors do not have a significant effect on 5-HT function at doses which have maximal effects on D1-DA receptor function. Pretreatment with the 5-HT antagonist methysergide did not produce a change in apomorphine-induced locomotion and did not antagonize the self-mutilation or the other behaviors produced by L-DOPA or SKF-38393 in neonatally lesioned rats, suggesting that 5-HT hyperinnervation is not responsible for these drug-induced changes in neonatal 6-OHDA-lesioned rats.

Clinical studies on the mechanism of action of clozapine: the dopamine-serotonin hypothesis of schizophrenia

Clozapine administration to schizophrenic patients was found to produce dopamine2 (D-2) and serotonin2 (5-HT2) receptor blockade, as evidenced by the ability to block the increases in growth hormone and cortisol secretion produced by apomorphine and MK-212, respectively, direct acting dopamine (DA) and 5-HT2 agonists. Clozapine did not increase plasma prolactin (PRL) levels nor did it block the apomorphine-induced decrease in plasma PRL concentration, as would be expected from a D-2 receptor antagonist. These PRL results are consistent with the observation that clozapine may increase DA release. Clozapine also decreased plasma tryptophan, plasma homovanillac acid (HVA) and basal plasma cortisol levels. Rodent studies suggest clozapine also increases 5-HT release. We hypothesize that antagonism of D-2 and 5-HT2 receptors and enhancement of DA and 5-HT release are critical elements in the action of clozapine to minimize both positive and negative symptoms without producing significant extrapyramidal symptoms or plasma PRL increases. It is proposed that schizophrenia may also involve a dysregulation of 5-HT2- and D-2-mediated neurotransmission, and that a more normal balance in serotonergic and dopaminergic neurotransmission is at least partially restored by clozapine.

The D-1 dopamine receptor partial agonist, CY 208-243, exhibits antiparkinsonian activity in the MPTP-treated marmoset

Administration of L-DOPA plus carbidopa, or the D-2 agonist (+)-PHNO, to MPTP-treated common marmosets caused motor hyperactivity and a reversal of the parkinsonian syndrome. In contrast, administration of the putative D-1 agonist SKF 38393 was without effect on movement or motor disability. The subsequent administration of another putative selective D-1 partial agonist CY 208-243 produced a dose-related improvement in motor activity and reversal of parkinsonian motor deficits in MPTP-treated animals. The effect of CY 208-243 was inhibited by pretreatment with the D-1 antagonist SCH 23390 and, to a lesser extent, by the D-2 antagonist sulpiride. In another group of normal drug naive marmosets, the administration of CY 208-243 produced only a small increase in motor activity. Following treatment with MPTP and without other drug administration, administration of CY 208-243 produced a marked reversal of motor deficits and locomotor hyperactivity. Thus, CY 208-243, suggested to be a partial D-1 agonist exhibits antiparkinsonian activity in MPTP-treated marmosets which does not require prior or concurrent exposure to D-2 agonists.

Serotonergic component of SCH 23390: in vitro and in vivo binding analyses

A series of benzazepines related to SCH 23390 were tested for binding to the 5HT-2 receptor. The compounds tested inhibited the binding of 3H-ketanserin with KI values generally greater than those observed for the D-1 receptor, but less than those for the D-2 receptor. When this serotonergic activity was correlated to the D-1 activity, the resulting coefficient was 0.84, indicating a strong correlation between the two activities. Conversely, the 5HT-2 activity did not show a good correlation with the D-2 activity. To further test the significance of the 5HT-2 binding of the SCH 23390, in vivo binding studies were performed using 125I-SCH 38840 in the frontal cortex, an area containing both D-1 and 5HT-2 receptors. The in vivo binding of 125I-SCH 38840 to frontal cortex exhibited peak levels one hour following subcutaneous administration, similar to the time course previously observed in striatum. The binding was both D-1 and tissue specific. Competition studies with selected standards demonstrated that inhibition of the binding to frontal cortex, in contrast to the inhibition observed in the striatum, exhibited a Hill coefficient less than unity, implying interaction at more than one receptor subtype. When SCH 23390 and ketanserin were administered simultaneously, the inhibition of the in vivo binding of 125I-SCH 38840 to striatum was not different than that observed with SCH 23390, alone. However, the inhibition of binding to frontal cortex was significantly greater than that demonstrated with either SCH 23390 or ketanserin, alone, suggesting that 125I-SCH 38840 was binding to both D-1 and 5HT-2 receptors, in vivo.