Hippocampus-dependent spatial learning and memory are impaired in growth hormone-deficient spontaneous dwarf rats.

Growth hormone (GH)/insulin-like growth factor-I deficiencies are known to cause alterations in brain development resulting in impairment of cognitive function. In order to investigate the behavioral phenotype of GH-deficient spontaneous dwarf rats (SDRs), we examined the behavior of the SDRs in the Morris water maze and Y-maze tasks. The SDRs showed severe deficits in spatial learning and memory compared to normal rats. The possibility that the cognitive impairment is associated with alteration of neurotransmitter systems was examined histologically following completion of the behavioral tests, using choline acetyltransferase (ChAT), vesicular glutamate transporter 1 (VGlut1) and glutamic acid decarboxylase (GAD6) immunohistochemistry as markers. In the SDRs the number of ChAT-stained basal forebrain cholinergic neurons was decreased. ChAT staining was also decreased in the hippocampus, one of the target areas of basal forebrain cholinergic neurons. Next, we examined the number of glutamatergic and GABAergic boutons in the hippocampal molecular layer and found a significant reduction in the density of VGlut1+ boutons and an increase in GAD6+ profiles, leading to a significantly reduced ratio in glutamatergic/GABAergic synapses. Finally, the number of newly generated cells in the subgranular zone of the hippocampus was significantly lower than in normal rats. Taken together, our data suggest that GH is an important regulator of hippocampus-dependent spatial learning and memory. The behavioral deficits in the SDRs may be explained by altered basal forebrain cholinergic innervation, imbalance in hippocampal glutamatergic/GABAergic synapses, and decreased neurogenesis in the hippocampus.

Ghrelin protects spinal cord motoneurons against chronic glutamate excitotoxicity by inhibiting microglial activation.

Glutamate excitotoxicity is emerging as a contributor to degeneration of spinal cord motoneurons in amyotrophic lateral sclerosis (ALS). Recently, we have reported that ghrelin protects motoneurons against chronic glutamate excitotoxicity through the activation of extracellular signal-regulated kinase 1/2 and phosphatidylinositol-3-kinase/Akt/glycogen synthase kinase-3ß pathways. Previous studies suggest that activated microglia actively participate in the pathogenesis of ALS motoneuron degeneration. However, it is still unknown whether ghrelin exerts its protective effect on motoneurons via inhibition of microglial activation. In this study, we investigate organotypic spinal cord cultures (OSCCs) exposed to threohydroxyaspartate (THA), as a model of excitotoxic motoneuron degeneration, to determine if ghrelin prevents microglial activation. Exposure of OSCCs to THA for 3 weeks produced typical motoneuron death, and treatment of ghrelin significantly attenuated THA-induced motoneuron loss, as previously reported. Ghrelin prevented THA-induced microglial activation in the spinal cord and the expression of pro-inflammatory cytokines tumor necrosis factor-α and interleukin-1ß. Our data indicate that ghrelin may act as a survival factor for motoneurons by functioning as a microglia-deactivating factor and suggest that ghrelin may have therapeutic potential for the treatment of ALS and other neurodegenerative disorders where inflammatory responses play a critical role.

Ghrelin protects spinal cord motoneurons against chronic glutamate-induced excitotoxicity via ERK1/2 and phosphatidylinositol-3-kinase/Akt/glycogen synthase kinase-3ß pathways.

Excitotoxic degeneration of spinal cord motoneurons has been proposed as a pathogenic mechanism in amyotrophic lateral sclerosis (ALS). Recently, we have reported that ghrelin, an endogenous ligand for growth hormone secretagogue receptor (GHS-R) 1a, functions as a neuroprotective factor in various animal models of neurodegenerative diseases. In this study, the potential neuroprotective effects of ghrelin against chronic glutamate-induced cell death were studied by exposing organotypic spinal cord cultures (OSCC) to threohydroxyaspartate (THA), as a model of excitotoxic motoneuron degeneration. Ghrelin receptor was expressed on spinal cord motoneurons. Exposure of OSCC to THA for 3 weeks resulted in a significant loss of motoneurons. However, THA-induced loss of motoneurons was significantly reduced by treatment of ghrelin. Exposure of OSCC to the receptor-specific antagonist D-Lys-3-GHRP-6 abolished the protective effect of ghrelin against THA. Treatment of spinal cord cultures with ghrelin caused rapid phosphorylation of extracellular signal-regulated kinase 1/2, Akt, and glycogen synthase kinase-3ß (GSK-3ß). The effect of ghrelin on motoneuron survival was blocked by the MEK inhibitor PD98059 and the phosphatidylinositol-3-kinase (PI3K) inhibitor LY294002. Taken together, these findings indicate that ghrelin has neuroprotective effects against chronic glutamate toxicity by activating the MAPK and PI3K/Akt signaling pathways and suggest that administration of ghrelin may have the potential therapeutic value for the prevention of motoneuron degeneration in human ALS. Our data also suggest that PI3K/Akt-mediated inactivation of GSK-3ß in motoneurons contributes to the protective effect of ghrelin.