Aim: 

The aim of the present study was to assess the effects of chronic treatment with growth hormone (GH) on monoamine synthesis in the brain. The accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used as a mesure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. The modulation of tyrosine and tryptophan hydroxylation was assessed also in young rats for comparisons.

Methods: 

Male young (3 months, n=6) and aged (20 months, n=12) Wistar rats were used. Animals were maintained under controlled standard conditions (22 °C, 70% humidity) and a 12/12 light/dark cycle. A group of aged rats received GH (1 mg/kg, s.c., twice daily for 4 weeks). Control rats received equal volumes of saline vehicle. After the treatments rats were sacrificed by decapitation and samples of hippocampus and striatum analyzed by HPLC with electrochemical detection.

Results: 

Tyrosine and tryptophan hydroxylation was decreased significantly in the hippocampus and striatum of aged rats (30%-70%) indicating an impairment in noradrenaline, serotonine and dopamine synthesis with age. However, when aged rats were repeatedly treated with GH along four weeks, an important increase in tyrosine hydroxylation in striatum (55%) and also iin tryptophan hydroxylation in hippocampus (117%) and striatum (163%) was observed. Tyrosine hydroxylation in hippocampus remained unchanged after GH treatment.

Conclusion: 

The results indicate that repeated treatment with growth hormone might aid to improve the descent in dopamine and serotonine neurotransmission that normally occurs as a consequence of aging.

(Supported by SAF 2007-66878-C02-02 from MEC)