INFLUENCE OF GROWTH-HORMONE (GH) AND GH-RELEASING FACTOR ON LOCOMOTOR-ACTIVITY IN RATS

Acute recombinant human growth hormone (r-hGH) administration (1-250 mug/kg; IP) decreased locomotor activity (LA) in a dose-dependent manner, F(5, 43) = 3.55, p < 0.009, with maximum effect at a dose of 10 mug/kg [control = 1655 +/- 659 inputs/outputs (I/O); r-hGH = 909 +/- 436 I/O; p < 0.05]. The daily treatment with r-hGH (10 mug/kg/day; 5 days; IP) reduced LA counts during the first trial, with no apparent changes in motor behavior after habituation. In contrast, growth hormone-releasing factor (GRF; 10 mug/rat/day; 5 days; IP) enhanced LA (control = 1045 +/- 566 I/O; GRF = 2284 +/- 894 I/O; p < 0.01) during the 5-day treatment period, inhibiting habituation. Moreover, the individual differences in LA persisted during the treatment period in response to GRF or r-hGH, but not in control rats. These results seem to indicate that the effects of GRF on LA are not mediated by the release of peripheral GH, and suggest that GRF might influence psychomotor behavior by a centrad mechanism.