S. Vanhatalo et S. Soinila, RELEASE OF FALSE TRANSMITTER SEROTONIN FROM THE DOPAMINERGIC NERVE-TERMINALS OF THE RAT PITUITARY INTERMEDIATE LOBE, Neuroscience research, 22(4), 1995, pp. 367-374
Rat pituitary intermediate lobe contains two types of serotonin-immuno
reactive nerve terminals. Most of them are dopaminergic, in which sero
tonin acts as a false transmitter, while the rest are true serotoniner
gic nerves. In the present study, release of the false transmitter ser
otonin from the dopaminergic nerve terminals was studied by loading th
e neurons in vivo with serotonin precursor L-tryptophan and MAO inhibi
tor pargyline, which results in accumulation of false transmitter sero
tonin. Subsequently pituitary neurointermediate lobe complexes were in
cubated in the presence of various agents. Potassium induced dramatic
release of serotonin. This release was Ca2+-dependent, as demonstrated
by an inhibition by Mg2+, and transporter-independent, since it was u
naffected by GBR 12909 (a dopamine transport inhibitor). Tyramine and
sodium nitroprusside, a nitric oxide donor, caused slight to remarkabl
e release of serotonin. This release was inhibited by GBR 12909, sugge
sting that it was transporter-dependent. Presynaptic stimulation with
apomorphine or haloperidol, dopamine receptor agonist or antagonist, r
espectively, or isoproterenol, agonist of the beta-adrenergic receptor
, did not significantly release serotonin. Thus, it seems that presyna
ptic receptors per se cannot induce release of significant amounts of
serotonin from the IL dopaminergic fibers. Our results suggest that fa
lse transmitter serotonin in the IL dopaminergic nerve terminals is re
leased primarily by the classical exocytotic release mechanism, but ma
y also be partly released by the transporter-dependent, non-exocytotic
release.