Immunocytochemistry has revealed that nerve fibers within the neural and 
intermediate lobes of the rat pituitary gland contain 5-hydroxytryptamine 
(5-HT). Recent anatomical evidence suggests that the content of this amine in 
the intermediate but not the neural lobe of the pituitary gland may represent 
5-HT that has been taken up from the blood rather than synthesized 
intraneuronally. The purpose of this study was to determine if 5-HT is 
synthesized in neurons of the neurointermediate lobe of the pituitary gland. 
5-HT synthesis was estimated by measuring the accumulation of the 5-HT 
precursor, 5-hydroxytryptophan (5-HTP), in the neurointermediate lobe of male 
Long-Evans rats following the administration of NSD 1015, an inhibitor of 
aromatic L-amino acid decarboxylase. Thirty min following the injection of NSD 
1015 (100 mg/kg, i.p.), 5-HTP accumulated in the neurointermediate lobe and the 
rate of this accumulation was increased by the administration of the 5-HTP 
precursor, tryptophan, and by electrical stimulation of the pituitary stalk. In 
addition, repeated injections of the 5-HT uptake inhibitor, fluoxetine (10 
mg/kg, i.p., every 12 h for a total of 7 injections), induced a marked depletion 
of platelet 5-HT but did not alter the concentration of 5-HT in either the 
neural or intermediate lobes of the pituitary gland. Taken together these 
results indicate that much of the 5-HT in the neurointermediate lobe of the 
pituitary gland does not represent 5-HT taken up from the blood, but rather the 
amine is synthesized in neurons projecting to this region.(ABSTRACT TRUNCATED AT 
250 WORDS)