Sustained activation does not desensitize the dopamine D2 receptor-mediated control of evoked in vitro release of radiolabeled acetylcholine from rat striatum

Evaluation of Drug Concentrations Delivered by Microiontophoresis

Microiontophoresis uses an electric current to eject a drug solution from a glass capillary and is often utilized for targeted delivery in neurochemical investigations. The amount of drug ejected, and its effective concentration at the tip, has historically been difficult to determine, which has precluded its use in quantitative studies. To address this, a method called controlled iontophoresis was developed which employs a carbon-fiber microelectrode incorporated into a multibarreled iontophoretic probe to detect the ejection of electroactive species. Here, we evaluate the accuracy of this method. To do this, we eject different concentrations of quinpirole, a D2 receptor agonist, into a brain slice containing the dorsal striatum, a brain region with a high density of dopamine terminals. Local electrical stimulation was used to evoke dopamine release, and inhibitory actions of quinpirole on this release were examined. The amount of drug ejected was estimated by detection of a coejected electrochemical marker. Dose response curves generated in this manner were compared to curves generated by conventional perfusion of quinpirole through the slice. We find several experimental conditions must be optimized for accurate results. First, selection of a marker with an identical charge was necessary to mimic the ejection of the cationic agonist. Next, evoked responses were more precise following longer periods between the end of the ejection and stimulation. Lastly, the accuracy of concentration evaluations was improved by longer ejections. Incorporation of these factors into existing protocols allows for greater certainty of concentrations delivered by controlled iontophoresis.

Loss of autoreceptor functions in mice lacking the dopamine transporter

Autoreceptors provide an important inhibitory feedback mechanism for dopamine neurons by altering neuronal functions in response to changes in extracellular levels of dopamine. Elevated dopamine may be a component of several neuropsychiatric disorders. However, evidence concerning the state of autoreceptors in such conditions has remained elusive. The function of dopamine autoreceptors was assessed in mice lacking the dopamine transporter (DAT). Genetic deletion of the DAT gene in mice results in a persistent elevation in levels of extracellular dopamine. Direct assessment of impulse-, synthesis- and release-regulating autoreceptors in these mice reveals a nearly complete loss of function. These findings may provide insight into the neurochemical consequences of hyperdopaminergia.

Regulation of dopamine D1 and D2 receptors on striatal acetylcholine release in rats

The effects of dopamine (DA) D1 and D2 receptors on striatal acetylcholine (ACh) releases were investigated by in vivo microdialysis. All drugs were applied via dialysis membrane directly to the striatum. The levels of ACh release were increased by 10(-4) M SKF38393, a D1 receptor agonist. Although 10(-4) M SCH23390, a D1 receptor antagonist, exhibited an increase in the levels of ACh release, the agonist (10(-4) M) induced-increase in the levels of ACh release was suppressed by coperfusion of the antagonist (10(-4) M). In contrast, the levels of ACh release were decreased by the D2 receptor agonist, N-434, in a dose-dependent manner (10(-4) M to 10(-7) M) and increased by the D2 receptor antagonist, sulpiride, in a dose-dependent manner (10(-5) M to 10(-7) M). The agonist (10(-5) M) induced-decrease in the levels of ACh release was suppressed by coperfusion of the antagonist (10(-4) M). Coperfusion of D1 (10(-4) M) and D2 (10(-5) M) agonists blocked both effects of respective drug alone. In order to clarify the effect of endogenous DA, two drugs with different mechanisms for enhancing DA concentration in the synaptic cleft, the DA release-inducer methamphetamine, and the DA uptake inhibitor nomifensine were perfused separately. Both (10(-4) M to 10(-5) M) produced a dose- and a time-dependent decrease in the levels of ACh release. Significant higher levels of ACh release were observed in the striatum of the 6-hydroxydopamine (8 micrograms/10 microliters)-treated rats with significant depletion of striatal DA content. These results suggest that in striatal DA-ACh interaction ACh release, as cholinergic interneuron's activity, is tonically inhibited via the D2 receptor, mainly by dopaminergic input, and the D1 receptor probably modifies the effect of the D2 receptor indirectly.

Intermittent morphine treatment causes long-term desensitization of functional dopamine D2 receptors in rat striatum

3 weeks following cessation of intermittent morphine administration (10 mg/kg, s.c., once daily for 14 days), [3H]dopamine and [14C]acetylcholine release induced by 10 microM N-methyl-D-aspartate (NMDA) from superfused rat striatal slices appeared to be significantly higher than the release from striatal slices from saline-treated rats. A similar adaptive increase of the NMDA-evoked release of these neurotransmitters was observed in slices of the nucleus accumbens, whereas that of [3H]noradrenaline from hippocampal slices remained unchanged. Blockade of dopamine D2 receptors by 10 microM (--)-sulpiride enhanced NMDA-induced [3H]dopamine and [14C]acetylcholine release from striatal slices from saline-treated animals, but was found to be ineffective in this respect following intermittent morphine treatment. Moreover, morphine administration appeared to cause a profound decrease in the apparent affinity of the full dopamine D2 receptor agonist LY171555 (quinpirole) for these release-inhibitory dopamine D2 receptors, indicating the occurrence of dopamine D2 receptor desensitization. It is suggested that such a desensitization of dopamine D2 receptors on dopaminergic nerve terminals as well as on cholinergic interneurons may play a pivotal role in the long-lasting nature of behavioural sensitization upon cessation of treatment with morphine and possibly other drugs of abuse.

Differential localization of mRNAs encoding dopamine D1 or D2 receptors in cholinergic neurons in the core and shell of the rat nucleus accumbens

By combining immunocytochemistry for ChAT and in situ hybridization for dopamine D1 or D2 receptor mRNA in the striatum, it was found that (1) the percentage of ChAT/D2 mRNA co-localization is higher in the caudate-putamen than in the shell and core of the nucleus accumbens, (2) in the shell the degree of ChAT/D2 mRNA co-localization is higher rostrally than caudally, and 3) no significant regional differences exist in the degree of co-localization of ChAT and D1 mRNA.