Dopamine D-2 autoreceptors regulating the release of dopamine from cultured rat fetal dopaminergic neurons do not desensitize upon sustained activation: implications for the combined pharmaco- and grafting therapy in parkinsonian patients

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.

Release of endogenous dopamine in cultured mesencephalic neurons: influence of dopaminergic agonists and glucocorticoid antagonists

Several electrochemical techniques allow the measurement of dopamine release in freely moving animals and brain slices. In this report, we applied one of these techniques, coulometry, coupled to high-performance liquid chromatography (HPLC), to the study of dopamine release in primary cultures of embryonic mesencephalic dopaminergic neurons. Between day 9 and 33 of culture, concentrations of dopamine, above the detection threshold, were found in the incubation buffer (Krebs ringer buffer, KRB). Concentrations of dopamine in the incubation buffer reflected neuronal release as they were: (i) positively correlated with the number of tyrosine hydroxylase-positive dopamine neurons in the culture; (ii) tetrodotoxin (TTX) sensitive and Ca2+ dependent; (iii) increased by a depolarizing stimulus, e.g. K+ (20 mM), or by the indirect dopamine agonists amphetamine and cocaine; (iv) decreased by a hyperpolarizing stimulus, e.g. the dopamine D2-like receptor agonist quinpirole. Dopamine release in this model was also sensitive to the manipulation of glucocorticoids, potent modulators of dopamine release in vivo. Long-term treatment of the cell cultures with RU 39305, a selective antagonist of glucocorticoid receptors (GR), but not with spironolactone, a selective antagonist of mineralocorticoid receptors (MR), dose-dependently decreased K+-stimulated dopamine release. In conclusion, these results demonstrate an in vitro model that allows the studying of the release of endogenous dopamine in cell cultures and the effects of glucocorticoid hormones on the release dynamics.

Evidence for genetically mediated dysfunction of the central dopaminergic system in the stargazer rat

The stargazer rat is an autosomal recessive mutant (homozygous stg/stg) that displays abnormal behavior, characterized by stereotypic head-movement, circling, and a high level of ambulatory activity. Heterozygous (stg/+) littermates display normal spontaneous behaviors. In this study, stargazers and their unaffected littermates were compared in their behavioral responses to both stimulation and inhibition of dopamine D2/D3 receptors, using quinpirole and haloperidol. Stargazers were observed to yawn a significantly fewer number of times than littermates in response to (--)-quinpirole (50 mu g/kg, IP). Haloperidol (HAL 0.1 mg/kg and 0.3 mg/kg, SC) caused a decrease in stereotypic head-movement in the mutants that was both time- and dose-dependent. In normal littermates, HAL inhibited locomotor activity and produced catalepsy in a time- and dose-dependent manner. In stargazers, both doses of HAL inhibited locomotor activity to a similar degree as in the littermates. However, no catalepsy was detectable in the mutants using 0.1 mg/kg of HAL. A dose of 0.3 mg/kg HAL was only weakly cataleptogenic. Overall, the spectrum of abnormal behaviors expressed by the stargazers and the present evidence of D2/D3 receptor subsensitivity suggest that stargazers possess a genetically mediated dysfunction of the central dopaminergic system.

Effects of chronic activation of dopamine D-2 receptors in cultures of rat fetal dopaminergic neurons: indications for alterations in functional activity

In Parkinsonian patients, previously subjected to neuronal grafting therapy, the survival and functional status of dopaminergic grafts might be impaired by the concurrent pharmacotherapy with L-DOPA and/or dopamine (DA) D-2 receptor agonists. To test this hypothesis in vitro, we studied the effects of chronic DA D-2 receptor activation on the functional capacity of cultured fetal rat mesencephalic DA neurons, using the activity of tyrosine hydroxylase (TH) and the intracellular dopamine content as neurochemical parameters. In cellular extracts prepared from our cultures, TH activity (as determined by the release of 3H2O from 3H-[3,5] tyrosine) appeared to be tetrahydrobiopterin-, Fe2+, and temperature sensitive, while in intact cells, the catalytic activity of TH could be induced by K(+)-evoked depolarization in a Ca(2+)-dependent way. In contrast, no acute DA D-2 receptor mediated inhibitory effects could be demonstrated in intact cells, either when tested under basal or depolarizing conditions. Nevertheless, after chronic exposure to DA D-2 receptor agonists for 14 days clear differences were observed in the functional status of cultured fetal dopaminergic neurons. Thus, whereas the overall survival and basal TH activity of cultured fetal dopaminergic neurons remained virtually unaltered, the depolarization induced activation of TH was enhanced in agonist-treated cultures. Moreover, after long-term treatment for 14 or 21 consecutive days, the intracellular DA content of agonist treated cultures appeared to be higher, as compared to untreated controls. It is concluded that chronic activation of DA D-2 receptors may induce adaptive alterations in the functional activity of cultured fetal dopaminergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)