Growth hormone release evoked by electrical stimulation of the arcuate nucleus in anesthetized male rats.

Plasma growth hormone (GH) concentrations were measured following electrical stimulation of either the arcuate nucleus or the median eminence in urethane-anesthetized male rats. While electrical stimulation of the arcuate nucleus elicited a large pulse of GH secretion, stimulation of the median eminence was relatively ineffective. For stimulation of the arcuate nucleus, the frequency dependence of stimulus-secretion coupling for GH release was investigated by delivering 3 differing patterns of electrical stimulation, each of 2 min duration and containing 1,200 stimulus pulses: 10 Hz continuous; 20 Hz (10 s on/10 s off); and 50 Hz (2 s on/8 s off). To examine the effect of increasing the duration of the 50 Hz stimulus train on evoked GH release, a further three stimulation protocols were also tested: 50 Hz (2 s on/8 s off); 50 Hz (3 s on/7 s off) and 50 Hz (4 s on/6 s off). While evoked GH release (per stimulus pulse) was not significantly different for various frequencies of stimulation, it was greatly potentiated by increasing the duration of 50 Hz stimulus trains. These findings suggest that GH release is not linearly related to the activation of GRF neurons but is strongly facilitated with increases in burst duration.