Chronic Cocaine Dampens Dopamine Signaling during Cocaine Intoxication and Unbalances D₁ over D₂ Receptor Signaling

a, b, [Ca²⁺]_i changes induced by acute cocaine injection (8 mg/kg, i.p.) in striatal D₁R-expressing (a) and D₂R-expressing (b) neurons in mice chronically exposed to cocaine (30 mg/kg/d, i.p., for 2 weeks), which are referred to as chronic mice. 1, 2, Δ[Ca²⁺]_i images at t = −5 min and t = 25 min before and after cocaine injection, respectively. 3, EGFP image to identify striatal D₁R-/D₂R-expressing neurons. 4, [Ca²⁺]_i overlapped with EGFP image to identify Δ[Ca²⁺]_i from D₁R/D₂R neurons. 5, Time course of Δ[Ca²⁺]_i increases within D₁R neurons (black curves, n = 6) and their mean (bold red curve) or Δ[Ca²⁺]_i decreases within D₂R neurons (black curves, n = 6) and their mean (bold blue curve) in response to acute cocaine exposure (also included are dashed curves of previously reported control mice with no history of cocaine exposures before the acute administration (Luo et al., 2011) for comparison. 6, Comparison of mean [Ca²⁺]_i changes at t = 25 min versus baseline (t ≤ 0 min) among the groups, showing less Δ[Ca²⁺]_i increase for D₁R in chronic mice (3.79 ± 1.32%) than in control mice (10.6 ± 3.2%) and less Δ[Ca²⁺]_i decrease for D₂R in chronic mice (−3.85 ± 1.55%) than in control mice (−10.4 ± 5.8%). In vivo simultaneous EGFP and Rhod₂-[Ca²⁺]_i imaging of striatal neurons was enabled by a microneedle approach (Fig. 1) that relayed the subcellular images in the mouse's striatum at the distal end of a microendoscope (φ, 1 × 25 mm; 0.65 NA) back to the focal plane of a modified epifluorescence microscope (E800, Nikon) to be acquired by a time-sharing EM CCD (i885, Andor).

a, b, Dynamic changes in Δ[Ca²⁺]_i induced by acute cocaine in striatal D₁R-expressing neurons (red curve), D₂R-expressing neurons (blue curve), and their ratios [Δ[Ca²⁺]_i(D₁R/D₂R); bold black curves] in control mice (a) and chronic mice (b). Least-squares fitting (dashed lines) is applied to analyze the slope differences [i.e., k = d(Δ[Ca²⁺]_i)/dt]. In control mice (a), Δ[Ca²⁺]_i increases in striatal D₁R neurons are biphasic (pink lines): an initial steep rise with k^a₁(D₁R) in phase 1 (t ≤ 8.3 min) followed by a long-lasting, slow increase with k^a₂(D₁R) in phase 2 (t > 8.3 min) and the Δ[Ca²⁺]_i in striatal D₂R neurons are uniphasic: a gradual decrease with k^a(D₂R) over t > 30 min. In chronic mice (b), Δ[Ca²⁺]_i increases in D₁R neurons have changed to uniphasic, with a gradual but long-lasting rise with k^c(D₁R), and those of D₂R neurons remain uniphasic with a gradual decrease. As a result, Δ[Ca²⁺]_i(D₁R/D₂R) in control mice rapidly increases to a climax at t ≈ 5–7 min, but it then returns to baseline; whereas in chronic mice Δ[Ca²⁺]_i(D₁R/D₂R) progressively increases and does not return to baseline.

Comparison of baseline [Ca²⁺]_i-Rhod2 fluorescence levels (∝[Ca²⁺]_i) between control mice and chronic mice. Red/blue lines are for D₁R/D₂R neurons, and white/gray bars are for saline- versus cocaine-treated groups. [Ca²⁺]_i in D₁R neurons did not differ significantly (p = 0.3) between control mice (1.41 ± 0.08) and chronic mice (1.29 ± 0.06), whereas [Ca²⁺]_i in D₂R neurons in chronic mice (1.35 ± 0.24) was significantly lower (p = 0.04) than in control mice (1.78 ± 0.15).