These molecules are synthesized in the postsynaptic cell as a result of presynaptic events. Retrograde messengers diffuse back into the presynaptic cell, where they stimulate neurotransmitter release. Although this receptor is ionotropic, LTP is a neuromodulatory process, in which serotonin commonly serves as the neuromodulator, and cAMP is the second messenger. Studies conducted on kittens in which one eye was occluded during the developmental critical period suggest that processes associated with binocular vision depend upon a kind of LTP, mediated by NMDA receptors. Extrinsic stimuli that cause activation of the developing pathways must accompany intrinsic developmental mechanisms.
Developmental plasticity is observed commonly in maturing organisms, but is also evident in adults.
Predictably, the rats raised in the complex environment demonstrated enhanced neural development relative to those raised in the simple environment. Surprisingly, rats raised in a common environment until adulthood showed developmental differences upon separation into simple and complex environments. Enhancements appeared within a week of the rats' placement in the complex environment. Although not all developmental effects seen in young rats were observed in adults, these experiments show that structural changes occur in the nervous systems of adult organisms when they experience new stimuli. Learning, a change in an animal's behavior as a consequence of its experience, may be considered at the neuronal level to be an extension of previously described neuromodulatory processes.