Methods
Replication deficient viral vectors, based on adenovirus (ADV) or adeno-associated virus (AAV) and encoding green fluorescent protein (GFP), were injected into exposed hindlimb muscles of adult mice under chloral hydrate/pentobarbital anaesthesia (soleus or extensor digitorum longus). Tissues were fixed by cardiac perfusion after a few days to several months and examined in a fluorescence microscope.

Results
The ADV and AAV vectors both transduced skeletal muscle fibers. The ADV vector also transduced motoneurons innervating the injected muscle but not other spinal cord neurons (as shown by co-injection of the fluorescent retrograde tracer tetramethylrhodamine-dextran). In addition, GFP was detected in sensory neurons in the corresponding dorsal root ganglia (L4-L5). The extent of GFP expression obtained with the ADV vector varied considerably between experiments and faded after 3 - 4 weeks. By contrast, the AAV vector caused transduction of skeletal muscle fibers for several months, but did not appear to transduce nerve cells by retrograde axonal transport.

Conclusions
These results indicate that the ADV vector can target identified sets of motor and sensory neurons by endocytosis and retrograde axonal transport after injection into a small leg muscle. However, transgene expression in neurons and muscle fibers is of limited duration. The AAV vector causes long-term transduction of skeletal muscle fibers, but does not appear to transduce neurons by retrograde axonal transport. Within these contraints, ADV and AAV vectors can serve as efficient tools for carrying gene constructs into the adult neuromuscular system in vivo.