Figure 8.

Optogenetic activation of TH⁺ interneurons inhibits SPN firing. (A) Photomicrograph of a parasagittal slice obtained from a TH-Cre mouse injected with AAV-2:EF1:DOI:CHR2-EGFP. Note the large area of infection (∼1 mm, in diameter). (B) Higher magnification of the area indicated by the red rectangle in A showing numerous AAV-infected TH⁺ interneurons as well as axonal and dendritic processes expressing ChR2-YFP (green). (C) Photomicrograph of a single ChR2 expressing striatal TH interneuron recorded in current clamp. C1: A 2-ms pulse of blue light elicits action potentials in the TH⁺ interneuron. C2: Responses to injected current identify the trasfected neuron as a Type II TH⁺ interneuron. C3: I–V plot from data in C2 is typical of Type II interneurons. (D) SPN recorded in another ChR2-expressing striatal slice. Inset shows typical I–V characteristics. (E) Action potential firing elicited in the SPN in D with current injection can be blocked by brief (2 ms) optogenetic stimulation of TH⁺ interneurons and axons. Inset shows the IPSP elicited in the SPN at higher magnification. (F) Current traces obtained without (top traces) and with optical stimulation (bottom traces). V_h = −80 mV. Note the IPSC elicited in the SPN (bottom).