Browsing by Author "Pulkkinen, Nita"

Amphetamine and its derivatives are widely used as medicines but also abused as psychostimulant drugs. The most important action of amphetamine in the central nervous system is to release dopamine to the extracellular space which leads to enhanced dopaminergic neurotransmission. Amphetamine also releases serotonin and norepinephrine by similar mechanisms and it affects indirectly other neurotransmitter systems too. It still remains partly unsolved how amphetamine exactly releases monoamines but it is known to have multiple sites of action. Amphetamine is a substrate for dopamine transporter (DAT) and it acts as a competitive inhibitor of the transporter reducing uptake of dopamine. Amphetamine enters the cell mainly through DAT and partly by diffusing through the cell membrane. The drug induces changes in DAT leading to reverse transport of dopamine from the cytoplasm into the synaptic cleft through DAT. Amphetamine is also substrate for vesicular monoamine transporter 2 (VMAT2) preventing the uptake of dopamine into storage vesicles and promoting its release from the vesicles to cytoplasm. Additionally, amphetamine inhibits monoamine oxidase (MAO), enzyme which degrades monoamines. It also enhances dopamine synthesis and according to recent studies amphetamine augments exocytotic dopamine release. Drug addiction is a chronic disorder related to structural and functional adaptive changes of neurons, called neuronal plasticity. GDNF (glial cell line-derived neurotrophic factor) is one of the many molecules regulating plasticity. It is especially important to the dopaminergic system and some investigations have suggested that it has potential as a protective agent against addiction. The aim of this study was to investigate how the overexpression of endogenic GDNF affects dopaminergic system and how it changes drug responses. A hypermorphic mouse strain (GDNFh), which is overexpressing physiological GDNF, was used. Their wild-type littermates were used as controls. Using brain microdialysis it was measured how the extracellular dopamine concentration changes in striatum and nucleus accumbens (NAcc) after amphetamine stimulation. Amphetamine was administered straight to the brain through the microdialysis probe. Microdialysis was performed on days 1 and 4, and on days 2 and 3 the mice were given amphetamine intraperitoneally. This was done to find out if the response to amphetamine changed after repeated dosing. In addition to these experiments, the biological activity of three small-molecule GDNF mimetics in intact brains was tested by microdialysis. On the first day amphetamine increased striatal dopamine output more in the heterozygous GDNFh mouse than in the wild-type mice. This stronger reaction to amphetamine may be explained by the enhanced activity of DAT in the GDNFh-het mice leading to higher intracellular amphetamine concentration. Also the striatal dopamine levels are increased in the GDNFh-het. On the fourth day no differences were detected between the genotypes. In the NAcc no significant difference was found between the genotypes. Instead in NAcc amphetamine caused a smaller increase in the dopamine output on day 4 than on day 1 in both genotypes suggesting that tolerance was developed. These results confirm that endogenic GDNF has a remarkable role in the regulation of the dopamine system and hence addiction but further investigations are needed to clarify its versatile actions. The small-molecule GDNF mimetics increased striatal dopamine output thus showing biological activity and encouraging to further investigations.