Capillary and mitochondrial support of neural plasticity in adult rat visual cortex.

Young adult rats (60 days old) were placed in complex environments (EC) or kept in individual cages (IC) for 10, 30, or 60 days. Previously reported findings in these same animals of synaptogenesis, decreased neuronal density, and increased cortical thickness in the EC animals demonstrated that cortical volume substantially expanded after 30 days. Such expansion would have spread apart existing capillaries and mitochondria, thereby diluting metabolic support. However, capillary spacing and mitochondrial volume fraction were maintained in these EC animals after 30 days, suggesting that new capillaries and mitochondria had infiltrated the tissue. Furthermore, many small vessels appeared after 10 days of complex experience, followed by expansion in vessel size until vessels from rats in EC for 60 days were larger than those from rats in IC for 60 days. The findings of constant vessel spacing in the face of expanding tissue volume, along with a set of small vessels that subsequently increased in size, suggest that small-sized new vessels were introduced in EC cortex by 10 days but had not matured in size until after 30 days. The results indicate that young adult rats can generate new capillaries and mitochondria in response to increased metabolic demands, but in a less vigorous fashion than in previously described weanling animals.