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Abstract
This study was designed to evaluate the functional significance of angiotensin II
(Ang II) receptors identified by previous receptor autoradiography studies to be located
presynaptically on terminals of dopaminergic neurones projecting to the striatum.
Microdialysis was performed in the striatum of conscious freely moving rats and dopamine
and serotonin metabolites measured by HPLC with electrochemical detection. During
perfusion with artificial CSF, the major extracellular dopamine metabolite identified
was DOPAC with smaller concentrations of HVA. When Ang II (1 microM) was introduced
into the dialysis perfusion medium, DOPAC output increased markedly, peaking at 219%,
and returned to control with vehicle perfusion during the recovery period. This increase
in DOPAC output with Ang II was completely blocked by co-administration of the AT1
selective antagonist, Losartan (1 microM). Administration of Losartan alone led to
a significant (16%) depression of DOPAC output relative to vehicle, suggesting that
dopamine release is under a tonic facilitatory influence of Ang II via the AT1 receptor
subtype. Parallel, but smaller changes were seen with HVA outputs. During Ang II perfusion
the output of HVA was elevated 34-79% of that in vehicle-treated rats and this effect
was completely abolished by concomitant administration of Losartan. As was observed
with DOPAC output, administration of Losartan alone led to a 13-24% depression of
HVA output compared to vehicle perfusion. When nomifensine (10 microM) was included
in the infusion fluid, dopamine was clearly measurable. Ang II perfusion increased
the levels of dopamine to 225%. Values returned towards baseline during the recovery
period. Ang II administration also increased (by 15% and 55%) the levels of the major
serotonin metabolite, 5HIAA.(ABSTRACT TRUNCATED AT 250 WORDS)