We investigated the effect of central
serotonin (5-hydroxytryptamine, 5-HT) administration on hypothalamic tuberoinfundibular dopamine neurons and related changes in neuronal activity to circulating
prolactin (PRL) levels. Ovariectomized rats were treated with either vehicle or
5-HT through a lateral ventricular
cannula in one of two dose paradigms: 1) a bolus of 20 μg, with tissues taken at 30 min, or 2) the same bolus immediately followed by 20 μg/30 min via a syringe pump for 120 min, and tissues taken at 120 min. Blood samples were taken throughout experiments and plasma PRL determined by radioimmunoassay. Under both paradigms,
NSD 1015, a
dihydroxyphenylalanine (
DOPA) decarboxylase inhibitor (25 mg/kg intraarterially) was injected 10 min before
decapitation and brain excision followed by stalk-median eminence dissection. The rate of
DOPA accumulation, determined by measuring
DOPA levels in the stalk-median eminence by high-performance liquid chromatography with electrochemical detection was used as a measure of
tyrosine hydroxylase (TH) catalytic activity. Stalk-median eminence
DOPA accumulation in control rats was 29.9 ± 4.2 and 28.8 ± 4,4 ng/mg
protein (30 and 120 min experiments, respectively).
DOPA accumulation in 5-HT-treated rats was significantly reduced (P<0.05) after 30 min to 17.8 ± 1.2 ng/mg
protein, but it was similar (21.7 ± 3.9) to controls after 120 min of 5-HT infusion. 5-HT levels in the stalk-median eminence of rats treated with 5-HT were 13- to 17-fold greater than controls (16.9 to 18.5 ng/mg
protein). Plasma PRL levels in both groups increased 10-fold after 5-HT treatment with a peak at 5 min, returning to baseline by 120 min. TH
mRNA levels were determined by in situ hybridization in a second group of rats which were treated with the 20μg bolus and subsequent 120 min infusion of 5-HT. TH
mRNA signal levels in the arcuate nucleus of control rats averaged 144 ± 21 grains/cell.
After treatment with 5-HT, TH
mRNA levels in the arcuate nucleus were significantly lower (P<0.0001) with 69±14 grains/cell. In a third group of rats, the effects of the 30 min 5-HT treatment on TH catalytic activity and circulating PRL levels was challenged with two 5-HT(2) receptor antagonists, LY53857 (5 mg/kg intraperitoneally) or
ketanserin (10 mg/kg intraperitoneally). Neither the 5-HT-induced decrease in TH catalytic activity nor the increase in PRL was altered by pretreatment (120 min) with 5-HT(2) antagonists. These data suggest that central 5-HT is capable of decreasing TH activity and TH
mRNA levels in the tuberoinfundibular dopamine neurons and that the decrease in dopaminergic neuronal activity may contribute to the 5-HT-induced PRL rise. The changes in TH catalytic activity and PRL after intracerebroventricular administration of 5-HT do not appear to be mediated by
5-HT(2) receptors.