Effects of Some γ-Aminobutyric Acid (GABA)-Ergic Drugs on the Dopaminergic Control of Human Growth Hormone Secretion

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Abstract
Effects of 3-day premedication with baclofen, diazepam, and sodium valproate, which are believed to act via γ-aminobutyric acid (GABA)-ergic mechanisms, on apomorphine- and L-dopa-stimulated human GH secretion were tested in 45 healthy male volunteers. Sodium valproate failed to modify apomorphine- and L-dopastimulated GH secretion. Baclofen premedication significantly reduced the GH response to L-dopa (the mean of individual peak values decreased from 16.7 ± 2.3 to 9.3 ± 1.7 ng/ml; P < 0.025), but it did not affect apomorphine-stimulated GH secretion. On the contrary, diazepam abolished both apomorphine- and Ldopa-stimulated GH secretion (in the L-dopa test, the mean of individual peak values decreased from 12.0 ± 2.1 to 3.6 ± 0.3 ng/ml; P < 0.01; in the apomorphine test, the mean of individual peak values decreased from 14.8 ± 3.3 to 6.4 ± 2.3 ng/ml; P <0.05). Together with earlier observations that these GABA-ergic drugs in an acute experiment stimulate basal GH secretion in man and that these effects result, at least partly, from a stimulation of dopaminergic transmission, the findings demonstrate that some GABA-ergic drugs exert both stimulatory and inhibitory influence on the dopaminergic neurons which are involved in the neuroendocrine regulation of human GH secretion. However, inhibitory influences of GABA-ergic mechanisms on GH secretion may be more widespread than simply affecting the dopaminergic neurons.

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