Central Administration of Serotonin Decreases Tyrosine Hydroxylase Catalytic Activity and Messenger Ribonucleic Acid Signal Levels in the Hypothalamus of Female Rats

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