Viral vector-mediated gene transfer has become increasingly valuable for primate 
brain research, in particular for application of genetic methods (e.g. 
optogenetics) to study neuronal circuit functions. Neuronal cell tropisms and 
infection patterns are viable options for obtaining viral vector-mediated 
transgene delivery that is selective for particular neuronal pathways. For 
example, several types of viral vectors can infect axon terminals (retrograde 
infections), which enables targeted transgene delivery to neurons that directly 
project to a particular viral injection region. Although recent studies in 
rodents have demonstrated that adeno-associated virus serotype 8 (AAV8) and 9 
(AAV9) efficiently transduce neurons, the tropisms and infection patterns remain 
poorly understood in primate brains. Here, we constructed recombinant AAV8 or 
AAV9, which expressed an enhanced green fluorescent protein (EGFP) gene driven 
by a ubiquitous promoter (AAV8-EGFP and AAV9-EGFP, respectively), and 
stereotaxically injected it into several brain regions in marmosets and macaque 
monkeys. Immunohistochemical analyses revealed almost exclusive colocalization 
of EGFP fluorescence via AAV9-mediated gene transfer with a neuron-specific 
marker, indicating endogenous neuronal tropism of AAV9, which was consistent 
with our previous results utilizing AAV8. Injections of either AAV8-EGFP or 
AAV9-EGFP into the marmoset striatum resulted in EGFP expression in local 
striatal neurons as a result of local infection, as well as expression in 
dopaminergic neurons of the substantia nigra via retrograde transport along 
nigrostriatal axonal projections. Retrograde infections were also observed in 
the frontal cortex and thalamus, which are known to have direct projections to 
the striatum. These local and retrograde gene transfers were further 
demonstrated in the geniculocortical pathway of the marmoset visual system. 
These findings indicate promising capabilities of AAV8 and AAV9 to deliver 
molecular tools into a range of primate neural systems in pathway-specific 
manners through their neuronal tropisms and infection patterns.