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The present study is designed to characterize the role of dopamine in
neuroimmunomodulation of mice. The neurotoxin-induced
neurodegeneration was used to investigate the mechanism in lymphoid
tissue atrophy. In addition, effects of the dopamine and its agonists on
lymphocyte proliferation were assessed. The presence of tyrosine
hydroxylase in immune cells was identified. Correlation of tyrosine
hydroxylase expression with cell growth was also elucidated.
6-Hydroxydopamine (6-OHDA) was reported to induce the
degeneration of noradrenergic nerves and alter the immune responses.
In this study, intraperitoneal administration of 6-OHDA induces thymus
atrophy in mice. The lowest levels of thymus weight and cell number
were reached between day 3 to 5 in mice receiving 6-OHDA treatment;
they gradually recovered thereafter. On flow cytometry analysis, the
most substantial reductions were recorded for CD4+CD8+ thymocytes,
although the numbers of other subpopulations such as CD4+CD8-, CD4-
CD8+, CD4-CD8- cells were also reduced. DNA fragmentation, a
characteristic of apoptosis, was detected in the thymocytes following 6-
OHDA injection. Pretreatment with desipramine, a catecholamine
uptake inhibitor, greatly blocked the effects of 6-OHDA on reduction in
thymus size and thymocyte number, the changes in thymocyte
subpopulations, the percentage of apoptotic cells and the appearance of
DNA fragmented bands. These results indicate that 6-OHDA induces
mouse thymocytes to undergo apoptosis in vivo.
The mechanism of 6-OHDA-induced thymocyte apoptosis were
further investigated in vitro. Results showed that 6-OHDA-induced
thymocyte apoptosis could be detected in vitro and blocked by
desipramine treatment. Flow cytometry analysis indicated that
CD4+CD8+ cells were most susceptible to 6-OHDA, and Bcl-2 expression
was reduced after 6-OHDA treatment. Both VAD-FMK, a broad
spectrum interleukin-1b-converting enzyme (ICE)-family protease
inhibitor, and DEVD-CHO, a potent inhibitor of CPP32 (caspase-3),
blocked 6-OHDA-induced thymocyte apoptosis. However, the ICE
(caspase-1) inhibitor YVAD-CMK failed to cause such effect. This cell
death process was prevented by the treatment of protein synthesis
inhibitor, cycloheximide, and by the antioxidants such as N-
acetylcysteine or butylated hydroxyanisole. These results suggest that
down-regulation of Bcl-2, activation of ICE-like proteases such as CPP32
but not ICE, generation of reactive oxygen species, and synthesis of new
proteins are involved in the apoptotic mechanism of 6-OHDA in
thymocytes.
Dopamine is known as the precursor of catecholamine and one of the
neurotransmitters in brain and peripheral tissues. Recent studies
implicated the important role of dopamine in immune responses.
Intraperitoneal administration of 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine (MPTP) at dose sufficient to lower endogenous
dopamine suppressed splenocyte proliferation in response to mitogens
such as lipopolysaccharide (LPS) or concanavalin A (Con A). Moreover,
intravenous injection of the specific agonists for dopamine DA-1 receptor
(SKF38393) or DA-2 receptor (LY171555) into mice enhanced the
splenocyte proliferation stimulated by LPS or Con A. In the in vitro
cultures, dopamine, SKF38393 and LY171555 all promoted cell
proliferation in the presence of LPS or Con A. These results indicate
that dopamine has an ability to regulate B- and T-cell proliferation both in
vivo and in vitro.
The role of endogenous dopamine in immune cell growth was further
investigated. Haloperidol, a general antagonist of dopamine receptor,
reduced cell growth rate of T cell hybridoma (10I) and rat nervous
pheochromocytoma cells (PC12). Tyrosine hydroxylase (TH) is the
initial rate-limiting enzyme of catecholamine biosynthesis in nervous
system. Flow cytometric analysis indicated TH also expresses in
various immune cells including T-cell lineage (murine thymocytes, spleen
T cells, T cell hybridoma 10I, human Jurkat-T and MOLT-4 cells), B-cell
lineage (A20 cells), and monocyte/macrophage lineage (P388D1 and
U937 cells). The presence of TH in PC12 cells was used as control.
TH expression was down-regulated in low serum condition in which cell
growth rate was reduced. Temporal studies indicated that the expression
of TH increased during 10I cell growth. Both a-methyl-p-tyrosine and
MPTP reduced TH expression and cell growth in a dose-dependent
manner. These results suggest that immune T cells expresses TH which
is correlated to cell growth, and that dopamine released from immune
cells may bind to the receptors to act in an autocrine or paracrine way.
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