Download Summary - Publikationsserver UB Marburg

Summary
Dopaminergic midbrain Neurons are located mainly in two regions, the substantia
nigra (SN) and the ventral tegmental area (VTA). These neurons play an important
role in the pathophysiology of drug abuse. The dopaminergic projections from the
VTA to the Nucleus accumbens, amygdala and prefrontal cortex are implicated in
reward and reinforcing effects of drugs abuse, whereas the nigrostriatal projections
are important for habit formation. In animal models, in vivo injections of cocaine lead
to changes in glutamatergic and GABAergic signal transduction. It is still unclear,
whether other signal molecules are also involved in the regulation of electrical activity
of the dopaminergic neurons.
One aim of this study was, to determine the dopamine-mediated autoreceptorresponse of dopaminergic midbrain neurons in animal models using young mice
(Mus musculus L.). The crucial parts of the autoreceptor-response signal
transduction where molecular identified by using qualitative PCR. Therefore the
expression of dopamine receptors and GIRK-channel subunits in single dopaminergic
SN and VTA Neurons was investigated. Furthermore, the autoreceptor-response of
dopaminergic SN and VTA neurons, stimulated by local application of dopamine, was
electrophysiologically examined in in vitro brain slices.
A second aim of this study was to identify possible changes of the dopamine
autoreceptor-response triggered by a single in vivo injection of cocaine. Therefore the
expression of D2-like dopamine receptors and GIRK2-channel subunits was
analyzed by using quantitative real-time PCR. Additionally, electrophysiologically
perforated patch-clamp measurements were performed to study the dopamine
autoreceptor-response of dopaminergic midbrain neurons after a single in vivo
injection of saline (control) or cocaine.
The study confirmed that generally the D2-dopamine receptor and the GIRK2channel are mainly verifiable in dopaminergic SN and VTA neurons. In most neurons
the expression of both D2-receptor isoforms (DR-D2s and DR-D2l) were detected.
Therefore both of this isoforms are highly likely to be part of the presynaptic
dopamine autoreceptor-response. After a single in vivo injection of cocaine the
expression of DR-D2s-, DR-D2l- and GIRK2-mRNA was significantly reduced in
dopaminergic VTA neurons of young animals, whereas there were no changes
observed in dopaminergic SN neurons. However, the expression of mRNA of these
genes was mostly unchanged after a single in vivo injection of cocaine in adolescent
and adult animals.
Local application of dopamine on dopaminergic midbrain neurons is followed by a
reversible inhibition of their spontaneous firing. In analyzed dopaminergic VTA
neurons, a single in vivo injection of cocaine leads to a significant reduction in
dopamine-induced inhibition of spontaneous firing and thus to a significant decreased
autoreceptor-response, whereas the autoreceptor-response remained unchanged in
dopaminergic SN neurons.
In summary, this is the first study to show early cocaine-induced changes in
dopamine-mediated autoreceptor-response of dopaminergic midbrain neurons.
These dramatically changes, induced by a single in vivo injection of cocaine, may
reinforce the effect of cocaine by decreasing the dopamine-mediated autoreceptorinhibition.
A reduced DR-D2-binding in the brain of drug applied animals and drug abusing
humans is well described. The decrease in the amount of DR-D2-mRNA after a
single cocaine injection in the analysed dopaminergic VTA neurons could be
interpreted as a starting point in the DR-D2-reduction in the brain of drug abusers. A
significant cocaine-induced change in gene expression in the autoreceptor response
was observed in dopaminergic VTA neurons of young mice, but not in adolescent
and adult animals. This result suggests that the dopaminergic VTA neurons of young
mice seem to be more sensitive to cocaine. Additionally, the cocaine-induced
changes seem to depend on the age during its intake.