RELEASE OF FALSE TRANSMITTER SEROTONIN FROM THE DOPAMINERGIC NERVE-TERMINALS OF THE RAT PITUITARY INTERMEDIATE LOBE

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.