Mechanisms of Growth Hormone Enhancement of Excitatory Synaptic Transmission in Hippocampus

Growth hormone (GH) deficiency is associated with impaired learning and memory. One possible target for GH effects on memory is the hippocampus, a brain region containing GH receptors (GHRs). To determine if GH acutely alters hippocampal function, recombinant human GH (rhGH) was applied to in vitro rat hippocampal brain slices. Extracellular recordings were used to assess effects of GH on the field EPSP (fEPSP) and long-term potentiation (LTP) of the fEPSP. GHR expression was measured in GH-treated and control rat hippocampal slices using RT-PCR. The GH signaling pathway was investigated by studying the effect of GH on the fEPSP after block of Janus kinase (JAK) by tyrphostin AG 490, phosphoinositide-3-kinase (PI3-kinase) by wortmanin, and mitogen- activated/extracellular response protein kinase kinase (MEK) by U0126. To determine if protein synthesis is required, hippocampal slices were perfused with cycloheximide, a protein synthesis inhibitor. I examined the effects of GH on pharmacologically isolated N-methyl-D-aspartate receptor (NMDAR)-and α- amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR)-mediated fEPSPs. Using western blotting, total and phosphorylated STAT5A/B, and NMDAR subunits NR1, NR2A, and NR2B were measured in GH-treated and control slices. GH caused a gradual (2 hr) increase in fEPSP amplitude during application that was maintained for more than 4 hours. Prior GH treatment (3hr) prevented additional potentiation by tetanus (100Hz, 1s), indicating that similar mechanisms contribute to both. GH caused equivalent enhancement of isolated NMDAR-fEPSPs and dual component fEPSP, indicating that GH effects are mediated in part by NMDARs. GH enhancement of fEPSPs was blocked by inhibitors of protein synthesis, JAK, PI3-kinase, and MEK, implicating all of these signaling mechanisms in GH enhancement of synaptic transmission. In vitro GH treatment of hippocampal brain slices for 15-30 min failed to alter either total or phosphorylated STAT 5a/b. In vitro GH treatment of hippocampal brain slices (3 hr) increased GHR mRNA, decreased NR2B protein, and increased NR2A/NR2B ratio. My results clearly demonstrate a previously unknown role for GH as a short-term modulator of hippocampal synaptic function