Abstract
The beneficial effects of growth hormone (GH) on memory and learning have previously been confirmed in both humans and in animal models. An important role of GABA<sub>B</sub> receptors for multiple forms of learning and memory has also been reported. In this study, we examined the effect of GH on the density and functionality of the metabotropic GABA<sub>B</sub> receptors in the rat brain. Male Sprague-Dawley rats (n = 24) divided into 3 groups were injected twice daily with recombinant human GH (0.07 or 0.7 IU/kg) for 7 days. The effects of the hormone were determined by quantitative autoradiography and by GABA<sub>B</sub> stimulated [<sup>35</sup>S]-GTPγS binding using the selective GABA<sub>B</sub> receptor agonist baclofen. The results demonstrate moderate but significant alterations in both receptor density and functionality in a number of brain regions. For example, a dose-dependent upregulation of GABA<sub>B</sub> receptors was found in the cingulate cortex, primary motor cortex and caudate putamen, whereas attenuation in the receptor density was encountered in, for example, the medial geniculate nucleus. Although the GH-induced effects on the GABA<sub>B</sub> receptor in brain areas associated with cognition were fairly pronounced, they were significant and we propose that the physiological responses observed after GH administration at least partly can be mediated through a mechanism involving GABA<sub>B</sub> receptors.
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