Within the "Xenopus" tadpole model, several signaling systems have been studied. For example, in optical tectal neurons, dendrite arbor growth occurs approximately at the onset of retinal input. Many on the caudal tectate have “silent” synapses which are modulated only by N-methyl-D-aspartate (NMDA) receptors. As neurons mature, alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptors are added, increasing synaptic transmission. Neuron and dendrite development are NMDA dependent. Rapidly growing dendrite arbors are more dynamic than slowly growing ones and dendrites themselves play an active role in their own development. It has been shown in studies that transport of HCN (hyperpolarization activated cyclic nucleotide) gated channel isoforms to dendritic fields of CA1 pyramidal neurons in the hippocampus occurs in an age-specific manner in the developing hippocampus.