<unk><unk>  release The binding of ghrelin to<unk> 1a in pituitary cells stimulates the secretion of<unk><unk>  (GH) by the pituitary gland. Constitutive activity One important feature of<unk> 1a is that there is still some activity in the<unk>  even when it is not actively being stimulated. This is called constitutive activity, and it means that the<unk>  is always "on," unless acted on by an inverse agonist. This constitutive activity seems to provide a tonic signal required for the development of normal height, probably through an effect on the GH axis. In fact, some genetic variations, or single nucleotide polymorphisms (SNPs), in<unk> , have been found to be associated with hereditary obesity and others with hereditary short stature. It was also found that, when<unk> 1A constitutive activity, was diminished, there were decreased levels of hunger-inducing<unk>  neuropeptide Y (NPY) as well as in food intake and body weight. Intracellular signaling mechanisms When<unk>  is activated, a variety of different intracellular signaling cascades can result, depending on the cell type in which the<unk>  is expressed. These intracellular signaling cascades include mitogen-activated protein kinase (MAPK)), protein kinase A (PKA), protein kinase B (PKB), also known as AKT), and AMP Activated Protein Kinase (AMPK) cascades. Behavioral reinforcement of food intake It is well-characterized that activating the<unk>  with ghrelin induces an orexigenic state, or general feeling of hunger. However, ghrelin may also play a role in behavioral reinforcement. Studies in animal models, found that food intake increased when ghrelin was specifically administered to just the ventral tegmental area (VTA), a brain area that uses dopamine signaling to reinforce behavior. In fact, the more ghrelin administered, the more food the rodent consumed. This is called a dose-dependent effect. Building on this, it was found that there are<unk> s in the VTA and that ghrelin acts on the VTA through these<unk> s. Current studies, furthermore, suggest that the VTA may contain dimers of<unk> 1a and dopamine<unk>  type 2 (DRD2). If these two<unk> s do indeed form dimers, this would somehow link ghrelin signaling to dopaminergic signaling. Enhancement of learning and memory The<unk>  may also be linked to learning and memory. First of all, the<unk>  is found in the hippocampus, the brain region responsible for long-term memory. Second, it was found that specifically activating the<unk>  in just the hippocampus increased both long-term potentiation (LTP) and dendritic spine density, two cellular phenomena thought to be involved in learning. Third, short-term calorie restriction, defined as a 30% reduction in caloric intake for two weeks, which naturally increases ghrelin levels and thus activates the<unk> , was found to increase both performance on spatial learning tasks as well as neurogenesis in the adult hippocampus.
Growth hormone secretagogue receptor
Prolactin-releasing peptide receptor
Cystic fibrosis transmembrane conductance regulator
Vitamin D receptor
Hydroxycarboxylic acid receptor 2
Heart-type fatty acid binding protein
Xenobiotic-sensing receptor
G protein-coupled receptor kinase 3