Masculinizing effect of dihydrotestosterone on growth hormone secretion isinhibited in ovariectomized rats with anterolateral deafferentation of themedial basal hypothalamus or in intact female rats

There is a striking sex difference in the pattern of growth hormone (GH) se cretion in rats. Our previous studies showed that short-term administration of pharmacological doses of testosterone or dihydrotestosterone (DHT) masc ulinized the GH secretory pattern in ovariectomized (OVX) rats. The locus w here testosterone or DHT interacts with the somatotropic axis is believed t o be the hypothalamus. To obtain insights into this phenomenon, we administ ered a single dose of DHT s.c. to adult OVX rats at 0.01, 0.1 or 1 mg/rat. Blood GH concentrations were measured in unanaesthetized rats. Six to 12 h after the s.c. administration of all three doses of DHT, the GH secretory p attern revealed a male-like secretory pattern as shown by episodic bursts o ccurring at 2-3-h intervals with low or undetectable trough levels. When an terolateral deafferentation of the medial basal hypothalamus (ALC) was perf ormed, the blood concentrations revealed irregularly occurring small fluctu ations, instead of the usual high bursts, but the basal GH concentration wa s significantly higher than that of OVX-sham-operated rats. DHT treatment d id not elicit pulsatile GH secretion or alter GH concentrations in OVX rats with ALC. When intact adult female rats received DHT at a dose of 1 mg/rat , the male-like GH secretory pattern was not induced. These results suggest that neural inputs from the anterolateral direction to the medial basal hy pothalamus are necessary for the masculinizing effect of DHT on the GH secr etory pattern in OVX rats, and that oestrogen in intact female rats prevent s the masculinizing effect of DHT.