GROWTH-HORMONE RELEASE EVOKED BY ELECTRICAL-STIMULATION OF THE ARCUATE NUCLEUS IN ANESTHETIZED MALE-RATS

Plasma growth hormone (GH) concentrations were measured following elec trical stimulation of either the arcuate nucleus or the median eminenc e in urethane-anesthetized male rats. While electrical stimulation of the arcuate nucleus elicited a large pulse of GH secretion, stimulatio n of the median eminence was relatively ineffective. For stimulation o f the arcuate nucleus, the frequency dependence of stimulus-secretion coupling for GH release was investigated by delivering 3 differing pat terns 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); an d 50 Hz (2 s on/8 s off). To examine the effect of increasing the dura tion 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 frequ encies of stimulation, it was greatly potentiated by increasing the du ration of 50 Hz stimulus trains. These findings suggest that GH releas e is not linearly related to the activation of GRF neurons but is stro ngly facilitated with increases in burst duration.