Effects of Neurotransmitters or Drugs on the in Vivo Release of Dopamine and Its Metabolites

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Abstract
The effects of neurotransmitters or drugs on the release of endogenous dopamine (DA) and extracellular levels of its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were examined in vivo by intracerebral dialysis. A dialysis tube was implanted stereotaxically through bilateral caudate nuclei of rats and perfused with the Ringer solution. Amounts of DA, DOPAC and HVA in the perfusates were measured by high performance liquid chromatography (HPLC) with electrochemical detection. The basal level of DA was 2.76 .+-. 0.64 pg/min, whereas the levels of DOPAC and HVA were 218.7 .+-. 20.7 and 142.4 .+-. 10.6 pg/min, respectively. Apomorphine (4 mg/kg, i.v.) reduced the efflux of DA and its metabolites. Haloperidol (0.4 mg/kg, i.v.) did not change DA release and produced only a minor increase of its metabolites. This increase of metabolites was inhibited by pargyline. Met-enkephalin (10-4 M), substance P (10-4 M) and acetylcholine chloride (10-4 M) added to the perfusing medium increased the release of DA. Met-enkephalin also increased the release of DOPAC. .gamma.-Amino-n-butyric acid (GABA, 10-4 M) reduced the release of DOPAC and HVA when added to the perfusing medium. Thyrotropin releasing hormone (TRH, 5 mg/kg, i.v.) increased the release of HVA. These findings indicated that different mechanisms mediated effects of neurotransmitters or drugs on the release and metabolism of DA in the rat striatum.
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