Neural Mechanisms of Motivation and Reward

National Institute Of Mental Health

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Abstract

Dopamine cells have been identified as important for predicting the value of ongoing behavior. Engineered viral vectors make it possible to manipulate neuronal activity reversibly. We would like to be able to modulate dopamine cell activity to learn how the prediction of value is related to the activity. One important strategy is to inject viral vectors that are taken up at axon terminals and are then transported to the cell body, so-called retrograde viral vectors. However, a conspicuous problem has been that the retrograde viruses generally used to induce retrograde transport do not seem to be taken up by dopamine cells. We tested four viral vectors in rhesus monkeys to determine whether they enter and express their product in the cell bodies of dopamine neurons. The vectors were AAV2-retro and three lentiviral vectors each pseudotyped with a rabies fusion glycoprotein labeled B, C, and E (FuG-B2, FuG-C, and FuG-E). The inclusion of rabies glycoprotein in the lentiviral vectors makes them have retrograde transfection properties. AAV2-retro intrinsically has the ability to be internalized by axons, enhancing retrograde transfection). FuG-B2 (HiRet) transduces all brain cell types (glia and neurons), while FuG-C and FuG-E ('NeuRet') are neuron specific. To measure retrograde transduction of dopaminergic cells, each virus was injected into the striatum in a stereotaxic surgery. The striatum was targeted because it is extremely rich in dopamine axonal terminals. We injected between 70 and 150 L (10 L/site at a rate of 0.5 L/min to 1.0 L/min) unilaterally across three anterior-posterior levels of each monkey. Post-mortem immunohistochemistry was examined using fluorescence and brightfield microscopy, and the transduction efficiency of the viral vectors in dopaminergic cells was assessed. Qualitative examination and quantitative analysis of the data revealed that the lentivirus-based vector FuG-B2 produced robust retrograde transfection of DA cells, as evidenced by the neuronal co-expression of fluorescent reporter protein and tyrosine-hydroxylase (TH) antibody in substantia nigra. The other three viral vectors do not transfect dopaminergic cells as efficiently. FuG-C expression was approximately one-half that of FuG-B2. FuG-E and AAV2-retro were each expressed in only an occasional neuron, that is, they showed virtually no expression. It is now possible to use a lentivirus FuG-B2 to get gene pseudotyped with a rabies fusion glycoprotein labeled B or HiRet to induce retrograde transport in dopamine neurons. This will open up new, important experiments into how dopamine neurons are related to prediction of value during reinforcement learning.