Figure 1.
ChR2 expression in substantia nigra dopamine neurons.
(A) ChR2 was limited to the SNC with no expression in the neighboring VTA. Fluorescence intensity (with background subtracted) was calculated along a line through the VTA and SNC. Scale bar is 100 µm. (B) Average (± s.e.m.; 8 mice) normalized fluorescence values along the extent of the line from VTA to SNC. (C) Average intensity values from VTA and SNC (n = 8, p<0.001). (D) Schematic representation showing two implanted optic fibers targeting the SN connected to a rotating commutator. (E) The location of injection sites is shown for ChR2 mice (blue circles; n = 8) and YFP controls (yellow circles; n = 4). The numbers indicate the anterior-posterior location (mm from Bregma). (F) In vitro stimulation of DA neurons. Current-clamp recordings (left column) from a ChR2-expressing dopamine neuron in the substantia nigra: 10-ms pulses of 470-nm light (blue lines) elicit frequency-dependent spiking at 1-Hz, 10-Hz, and 50-Hz. In voltage-clamp recordings (right column), light pulses induce inward currents in ChR2-expressing dopamine neurons. (G) Current injection experiments showed a sag in response to hyperpolarizing current indicating a hyperpolarization activated current (Ih), characteristic of SNC DA neurons.
Figure 2.
Operant self-stimulation of nigral dopamine neurons.
The number of active (A) and inactive (B) lever presses during the first 10 testing sessions: ChR2-expressing mice rapidly acquire lever pressing for dopamine stimulation, whereas YFP control mice do not. (C) Following reversal of the active and inactive levers, mice (n = 3) changed their preference. (D) There was no effect of “satiety.” Self-stimulation behavior did not decrease when the animals received a second session immediately after the first. (E) Mice press the active lever less frequently for 1-Hz stimulation than they do for 50-Hz and 10-Hz stimulation (* p<0.05). (F) The number of inactive presses is not affected by stimulation frequency. Values are mean ± s.e.m.
Figure 3.
Self-stimulation of nigra DA neurons is sensitive to changes in contingency.
(A) Mice (n = 4) increase their rate of lever pressing when the ratio requirement (number of presses needed to earn one stimulation) is changed. (B) The number of stimulations received remains unchanged when the ratio requirement is changed. (C) On a progressive ratio 5 task (PR5), ChR2 mice (n = 6) reduce pressing for stimulation. (D) Lever pressing is rapidly extinguished when stimulation is no longer available (n = 6). (E) Rate of pressing during extinction is reduced compared to pre-extinction rewarded sessions. The rate of pressing returns to baseline level when the contingency between lever pressing and laser stimulation is restored. (F) Omission (n = 4) reduces the number of active lever presses (FR1 press rate is used as baseline).
Figure 4.
(A) The laser is turned on (50 Hz) as long as the mice (n = 4) hold down the lever. (B) Press duration decreases with training, demonstrating that mice are unable to learn to hold the lever down in order to receive extended stimulation. (C) Total number of presses is unaffected by training. For panels B and C, individual data (filled circles) are superimposed over mean+s.e.m. values.