The effect of acute and chronic treatment with SCH 23390 on the spontaneous activity of midbrain dopamine neurons

The dopamine D1 receptor antagonist SCH-23390 blocks the acquisition, but not expression of mitragynine-induced conditioned place preference in rats

Mitragynine (MG) is the primary active constituent of Mitragyna speciosa Korth (kratom), a psychoactive Southeast Asian plant with potential therapeutic use. Numerous studies support roles of dopaminergic system in drug reward. However, the involvement of the dopaminergic system in mediating MG reward and drug-seeking is poorly understood. Using conditioned place preference (CPP) paradigm, the present study aims to evaluate the roles of the dopamine (DA) D1 receptor in the acquisition and expression of MG-induced CPP in rats. The effects of SCH-23390, a selective DA D1 receptor antagonist, on the acquisition of MG-induced CPP were first investigated. Rats were pre-treated systemically with SCH-23390 (0, 0.1 and 0.3 mg/kg, i.p.) prior to MG (10 mg/kg) conditioning sessions. Next, we tested the effects of the DA D1 receptor antagonist on the expression of MG-induced CPP. Furthermore, the effects of a MG-priming dose (5 mg/kg) on the reinstatement of extinguished CPP were tested. The results showed that SCH-23390 dose-dependently suppressed the acquisition of a MG-induced CPP. In contrast, SCH-23390 had no effect on the expression of a MG-induced CPP. The findings of this study suggested a crucial role of the DA D1 receptor in the acquisition, but not the expression of the rewarding effects of MG in a CPP test. Furthermore, blockade of the D1-like receptor during conditioning did not prevent MG priming effects on CPP reinstatement test, suggesting no role for the DA D1 receptor in reinstatement sensitivity.

Pacemaker rate and depolarization block in nigral dopamine neurons: a somatic sodium channel balancing act

Midbrain dopamine (DA) neurons are slow intrinsic pacemakers that undergo depolarization (DP) block upon moderate stimulation. Understanding DP block is important because it has been correlated with the clinical efficacy of chronic antipsychotic drug treatment. Here we describe how voltage-gated sodium (Na(V)) channels regulate DP block and pacemaker activity in DA neurons of the substantia nigra using rat brain slices. The distribution, density, and gating of Na(V) currents were manipulated by blocking native channels with tetrodotoxin and by creating virtual channels and anti-channels with dynamic clamp. Although action potentials initiate in the axon initial segment and Na(V) channels are distributed in multiple dendrites, selective reduction of Na(V) channel activity in the soma was sufficient to decrease pacemaker frequency and increase susceptibility to DP block. Conversely, increasing somatic Na(V) current density raised pacemaker frequency and lowered susceptibility to DP block. Finally, when Na(V) currents were restricted to the soma, pacemaker activity occurred at abnormally high rates due to excessive local subthreshold Na(V) current. Together with computational simulations, these data show that both the slow pacemaker rate and the sensitivity to DP block that characterizes DA neurons result from the low density of somatic Na(V) channels. More generally, we conclude that the somatodendritic distribution of Na(V) channels is a major determinant of repetitive spiking frequency.

Dopamine-glutamate interactions in methamphetamine-induced neurotoxicity

Repeated administration of methamphetamine (m-AMPH) to rats induces dopamine (DA) terminal damage, and coadministration of antagonists of the N-methyl-D-aspartate (NMDA) or dopamine D1 or D2 receptors are protective. Striatal microdialysis of rats given a neurotoxic regimen of 4 x m-AMPH (4 mg/kg, s.c.) treatments revealed a dramatic and prolonged elevation of extracellular DA after the final m-AMPH administration. Neuroprotective regimens of MK-801, SCH 23390, or eticlopride greatly attenuated the overflow of DA resulting from the fourth m-AMPH treatment. By itself, MK-801 had no significant influence on striatal DA overflow, whereas either DA antagonist given alone elevated dialysate DA concentrations. A significant correlation was found between the magnitude of the m-AMPH-induced DA overflow of individual microdialyzed rats and their striatal DA content at sacrifice one week later. We conclude that the ability of non-competitive NMDA antagonists and of the D1 or D2 antagonists to protect against m-AMPH-induced striatal DA terminal injury can be accounted for by their attenuation of m-AMPH-evoked DA overflow. These findings underscore the important role played by elevated extracellular DA concentrations to the injurious effects of this stimulant drug.

Methamphetamine-induced dopamine overflow and injury to striatal dopamine terminals: attenuation by dopamine D1 or D2 antagonists

Pharmacological blockade of either D1 or D2 dopamine (DA) receptors prevents damage of striatal DA terminals by repeated doses of methamphetamine (m-AMPH). Because the substantial DA overflow produced by multiple m-AMPH treatments appears to contribute to the subsequent injury, we have investigated the effects of blockade of D1 or D2 receptors on m-AMPH-induced DA efflux using in vivo microdialysis. Four treatments with m-AMPH (4 mg/kg, s.c., 2-h intervals) produced large increases in striatal DA overflow, with particularly marked overflow (10 times the basal values) following the fourth injection. Administered by themselves, four injections of the D1 antagonist SCH 23390 or the D2 antagonist eticlopride (0.5 mg/kg, i.p., 2-h intervals) significantly increased striatal DA overflow. However, treatment with either SCH 23390 or eticlopride 15 min before each of four m-AMPH injections attenuated the marked DA peak otherwise seen after the fourth m-AMPH injection. These effects on DA overflow were related to subsequent DA depletions. Although our m-AMPH regimen produced a 54% reduction in striatal DA tissue content 1 week later, pretreatments with either the D1 or the D2 antagonist completely prevented subsequent DA content depletions. Furthermore, the DA content of striatal tissue remaining 1 week after m-AMPH treatment was significantly correlated with the magnitude of the cumulative DA overflow during the m-AMPH treatment (r = -0.69). Thus, the extensive DA overflow seen during neurotoxic regimens of m-AMPH appears critical to the subsequent neurotoxicity, and the neuroprotective action of DA receptor antagonists seems to result from their attenuation of stimulant-induced DA overflow.

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.

Dopamine receptor subtypes that induce hyperactive urinary bladder response in anesthetized rats

In anesthetized rats, SKF 38393 (10 mg/kg, i.v.) did not facilitate urinary bladder motility, but bromocriptine (BR, 5 mg/kg, i.v.) alone and the combination of BR (1 mg/kg, i.v.) and SKF 38393 (1 mg/kg, i.v.) induced a hyperactive bladder response (HBR). Both HBR induced by BR alone or BR and SKF 38393 combined was suppressed by SCH 23390, sulpiride or haloperidol. These results indicate that HBR is mediated by the activation of D-2 receptors, and the effects of D-2 agonists on bladder motility are potentiated by the simultaneous stimulation of D-1 receptors.

Dopamine D1 receptor antagonism in schizophrenia: is there reduced risk of extrapyramidal side-effects?

The first selective D1 dopamine receptor antagonist, SCH23390, has been reported to be active in preclinical tests that predict antipsychotic activity in schizophrenic patients. This is particularly exciting because it has been claimed that this compound is 'atypical', in that it has a reduced propensity to induce extrapyramidal side-effects. However, in considering the evidence from preclinical screening tests for antipsychotic activity and extrapyramidal side-effects of potential neuroleptic drugs, Jarmo Hietala and colleagues conclude that the majority of available data is not compatible with the postulated atypical profile of SCH23390.

Differential attenuation of water intake and water-rewarded operant responding by repeated administration of haloperidol and SCH 23390 in the rat

It has previously been described that water intake in thirsty rats require higher doses of dopamine (DA) D-1 and D-2 antagonists to be attenuated than operant lever-pressing with water as reward. In the present study, effects of repeated administration of the DA D-1 antagonist SCH 23390 and the DA D-2 antagonist haloperidol were investigated in the same experimental paradigm. In agreement with previous reports, attenuation of operant responding increased progressively by haloperidol (0.05 mg/kg) given for four consecutive days. However, this attenuation was not accompanied by decreased water intake, tested for in parallel experiments. After haloperidol (0.2 mg/kg), in contrast, a progressively decreasing attenuation of water intake was found. After SCH 23390, both the initial attenuation of lever-pressing (0.02 mg/kg) and consummatory water intake (0.1 mg/kg) became less pronounced over time. The results thus show that: 1) the previously reported progressively increasing attenuation of operant responding caused by repeated administration of D-2 antagonists is not mimicked by the D-1 antagonist SCH 23390, and 2) attenuation of water intake caused by higher doses of neuroleptics is, in direct opposition, less pronounced after repeated administrations. The results also show that attenuation of operant responding by neuroleptics cannot solely be dependent upon a blunting of the impact of the reward.