The mushroom bodies in the protocerebrum are believed to be the structures of the insect brain most closely associated with higher-order sensory integration and learning. Drosophila melanogaster mutants with olfactory learning deficits have anatomically abnormal mushroom bodies or altered patterns of gene expression in mushroom body neurons. In addition, anatomical reorganization of the mushroom bodies occurs in adult flies, and possibly in adult honeybees; disturbance of electrical activity in this region disrupts memory formation in honeybees. Little is known, however, about the relationship of naturally occurring anatomical changes in the mushroom bodies to naturally occurring behavioural plasticity. We now report that age-based division of labour in adult worker honeybees (Apis mellifera) is associated with substantial changes in certain brain regions, notably the mushroom bodies. Moreover, these striking changes in brain structure are dependent, not on the age of the bee, but on its foraging experience, thus demonstrating a robust anatomical plasticity associated with complex behaviour in an adult insect.

There is an age-related division of labor in the honey bee colony that is regulated by juvenile hormone. After completing metamorphosis, young workers have low titers of juvenile hormone and spend the first several weeks of their adult lives performing tasks within the hive. Older workers, approximately 3 weeks of age, have high titers of juvenile hormone and forage outside the hive for nectar and pollen. We have previously reported that changes in the volume of the mushroom bodies of the honey bee brain are temporally associated with the performance of foraging. The neuropil of the mushroom bodies is increased in volume, whereas the volume occupied by the somata of the Kenyon cells is significantly decreased in foragers relative to younger workers. To study the effect of flight experience and juvenile hormone on these changes within the mushroom bodies, young worker bees were treated with the juvenile hormone analog methoprene but a subset was prevented from foraging (big back bees). Stereological volume estimates revealed that, regardless of foraging experience, bees treated with methoprene had a significantly larger volume of neuropil in the mushroom bodies and a significantly smaller Kenyon cell somal region volume than did 1-day-old bees. The bees treated with methoprene did not differ on these volume estimates from untreated foragers (presumed to have high endogenous levels of juvenile hormone) of the same age sampled from the same colony. Bees prevented from flying and foraging nonetheless received visual stimulation as they gathered at the hive entrance. These results, coupled with a subregional analysis of the neuropil, suggest a potentially important role of visual stimulation, possibly interacting with juvenile hormone, as an organizer of the mushroom bodies. In an independent study, the brains of worker bees in which the transition to foraging was delayed (overaged nurse bees) were also studied. The mushroom bodies of overaged nurse bees had a Kenyon cell somal region volume typical of normal aged nurse bees. However, they displayed a significantly expanded neuropil relative to normal aged nurse bees. Analysis of the big back bees demonstrates that certain aspects of adult brain plasticity associated with foraging can be displayed by worker bees treated with methoprene independent of foraging experience. Analysis of the overaged nurse bees suggests that the post-metamorphic expansion of the neuropil of the mushroom bodies of worker honey bees is not a result of foraging experience.

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We describe a method for producing high-resolution chemical patterns on surfaces to control the attachment and growth of cultured neurons. Microcontact printing has been extended to allow the printing of micron-scale protein lines aligned to an underlying pattern of planar microelectrodes. Poly-L-lysine (PL) lines have been printed on the electrode array for electrical studies on cultured neural networks. Rat hippocampal neurons showed a high degree of attachment selectivity to the PL and produced neurites that faithfully grew onto the electrode recording sites.