S. Vanhatalo et S. Soinila, PHARMACOLOGICAL CHARACTERIZATION OF SEROTONIN SYNTHESIS AND UPTAKE SUGGEST A FALSE TRANSMITTER ROLE FOR SEROTONIN IN THE PITUITARY INTERMEDIATE LOBE, Neuroscience research, 21(2), 1994, pp. 143-149
A subpopulation of nerve fibers in the rat pituitary intermediate lobe
(IL) have been shown to exhibit colocalization of serotonin (5-HT-IR)
and tyrosine hydroxylase immunoreactivities and they are sensitive to
neurotoxins specific to catecholamine neurons. This study was set out
to examine the uptake and synthesis mechanisms of serotonin in these
fibers. We developed an in vitro technique in which the neurointermedi
ate lobe explants were incubated (14 and 48 h) in the presence of vari
ous drugs and serotonin was subsequently visualized by immunohistochem
istry. Control incubation in the presence of serotonin (10(-6) M) resu
lted in a rich plexus of 5-HT-IR fibers in both posterior and intermed
iate lobes. Fluoxetine and citalopram (10(-6) M and 10(-5) M), inhibit
ors of 5-HT transporter, did not affect 5-HT-IR in the IL fibers, unle
ss they were used in concentrations high enough (10(-4) M and 10(-3) M
) to block unspecifically a number of monoamine transporters. The same
applied for desipramine (10(-5)-10(-7) M), an inhibitor of the noradr
enaline transporter. However, cocaine (10(-5)-10(-6) M) blocked seroto
nin uptake into these terminals, suggesting that serotonin uptake occu
rs through a dopamine transporter. Incubation of the IL in presence of
L-tryptophan (10(-4) M) did not result in 5-HT-IR in the IL fibers sh
owing colocalization of 5-HT-IR and tyrosine hydroxylase, which sugges
ts that these fibers do not synthesize serotonin. The present results
suggest that serotonin is taken up into the IL terminals by a dopamine
transporter and is not synthesized in them, at least in vitro. Thus,
serotonin may act as a false transmitter in some of the IL fibers, whi
ch may prove to be a valuable model tissue for studying serotonin's ro
le as an exogenously synthesized cotransmitter.