Increased density of multiple-head dendritic spines on medium-sized spiny neurons of the striatum in rats reared in a complex environment.

It has generally been assumed that new synapses added to various brain regions in response to experience are equivalent to those already in existence. Theorists have recently posited that synaptic configurations involving multiple associated contacts may facilitate plastic change. The number of multiple-headed dendritic spines on medium-sized spiny neurons in the rat dorsolateral corpus striatum was determined following rearing in environments differing in complexity. Postweaning rats were either housed as a group in a toy- and object-filled environment or housed individually in standard laboratory cages for 30 days. Dendritic segments of Golgi-Cox impregnated Type I spiny neurons of the complex environment housed rats had approximately 60% more multiple-head spines than those of the individually caged animals. Multiple-head spines may reflect parallel synaptic contacts that modify relative strengths of existing connections or connections with a novel presynaptic terminal that alter the neuron's pattern of connections.