Animals placed in complex environments develop greater numbers of visual cortex synapses per neuron than animals housed in standard cages. Increased numbers of synapses could theoretically arise from (i) active formation of new synapses, or (ii) selective stabilization of constitutively produced synapses. The postsynaptic location of polyribosomal aggregates appears to be an indicator of newly forming synapses. In developmental synaptogenesis and adult reactive (to injury) synaptogenesis, polyribosomes are more frequently found at spine synapses and are more likely to appear in the spine head and stem. In the visual cortex of rats from complex environments, there was a greater frequency of spine synapses associated with polyribosomes, relative to rats from individual or group cages. Furthermore, a greater percentage of these spines had polyribosomes in the head and stem region. This suggests that synapses in this region may be actively induced by neural activity arising from the complex environment experience.The complex, or "enriched," laboratory environment, while quite different from the feral norm, has provided considerable evidence regarding possible forms of neuronal plasticity. Rats placed at weaning or in adulthood in these environments develop a thicker heavier occipital cortex with larger neuronal nuclei, more glial cells, and greater RNA content and enzyme activity per neuron, relative to rats housed in simple cages (1). Occipital cortex neurons have larger dendritic fields (2-6), greater density of postsynaptic spines on dendrites (7), and larger synaptic contacts in some layers (8), and synapses show additional morphological differences (9, 10) in the rats from a complex environment. Larger dendritic fields indicate more synapses, and this has been confirmed by electron microscopic analysis of the number of synapses per neuron (11)(12)(13) (26,27), connections may be overproduced, such that only a subset, activated by or appropriate to experience, is preserved. In adults, overproduction might take the form of constitutive (i.e., continuous, without regulation) generation of potentially transient synapses, selective retention of which depends on their appropriateness to ongoing neural activity (26-29). Second, synapses might be actively induced by experience-related neural activity. In this case, synaptogenesis would be local and temporally discrete, rather than widespread and continuous, although again only a subset of synapses produced in response to an experience might survive to encode it (30).These two hypothetical mechanisms predict different immediate consequences of a to-be-encoded event on synaptogenesis. The overproduction or constitutive generation model implies that exposure to such an event should not affect the rate of synapse production, but only the rate of synapse survival, whereas the induction model predicts that synaptogenesis will increase over baseline levels in response to novel experiences. Recently, a possible morphological indicant of newly forming synapses, the pres...