Mesocortical Dopamine Phenotypes in Mice Lacking the Sonic Hedgehog Receptor Cdon

The number of proliferating cells in the ventral midbrain of Cdon^−/− embryos is increased at E12.5. A, Representative merged images of immunofluorescence for Ki67 (green), TH (red), and DAPI (blue) in coronal slices of the ventral midbrain of embryos at E12.5. B, Schematic illustrating the coronal plane of analysis. C, The total number of Ki67 immunoreactive cells was significantly increased in Cdon^−/− embryos relative to WT controls (Student’s t test, p = 0.0069; Table 1, a), and, D, this effect was seen across the anterior to mid-posterior extent of the ventral midbrain (ANOVA_{Genotype×Level}, main effect of genotype, p = 0.0003; Table 1, b). E, The total number of TH immunoreactive cells was similar between Cdon^−/− embryos relative to WT controls (Student’s t test, p = 0.498; Table 1, c), and no genotype- or level-based effect was observed at, F, anterior, mid, or posterior levels of the ventral midbrain (ANOVA_{Genotype×Level}; Table 1, d). n = 6-8 embryos/group.

A greater number of TH-positive neurons in the VTA of Cdon^−/− mice at birth and in adulthood. A, B, Total number of TH-positive neurons in the VTA (left, in red) and SN (right, in blue) in P0 (A) and adult mice (B) as measured by stereology. A greater number of TH-positive neurons were observed in the VTA of Cdon^−/− mice compared to WT controls at birth (Student’s t test, p < 0.05; Table 1, e) and in adulthood (Student’s t test, p < 0.01; Table 1, f). C, Mouse brain atlas illustrations showing the VTA and SN sections that were included in this analysis, and representative TH immunoreactivity in coronal sections of adult mice. n = 4-5 mice/group. *p < 0.05, **p < 0.01.

Greater dopamine and DOPAC concentrations in the mPFC, but not the NAcc or DS, of adult Cdon^−/− mice. A, Brain samples were taken from each target region illustrated. B, HPLC revealed a selective increase in the dopamine and DOPAC concentrations of the mPFC of Cdon^−/− mice, an effect that was not seen in the NAcc or DS (Table 1, g). n = 7-10 animals/group. *p < 0.05, **p < 0.01.

Increased number of dopamine varicosities in the mPFC of Cdon^−/− mice. A, Stereological quantifications of the number of dopamine varicosities in the Cg, the PL, and the IL pregenual mPFC. B, The total number of dopamine varicosities was greater in the Cdon^−/− mice compared with WT controls (ANOVA_Genotype, p = 0.0079; Table 1, h). C, There were no differences in the volume that dopamine varicosities occupied in the mPFC between Cdon^−/− and WT mice (Table 1, i). D, Likewise, an increase in the density of dopamine varicosities was observed in all three subregions (ANOVA_Genotype, p = 0.0073; Table 1, j). E, Representative photomicrographs at high magnification illustrating differences in the total number/density of dopamine varicosities in the PL mPFC comparing Cdon^−/− and WT mice. n = 3 mice/group.

Locomotor activity testing of Cdon^−/− mice reveals attenuation of behavioral plasticity in adulthood. A–C, First exposure/habituation to the locomotor testing environment (A), habituation to handling and saline injection (injection denoted by “S” vertical line; B), and the first injection of amphetamine (injection denoted by “A” vertical line, 2.5 mg/kg, i.p.; C) all produce indistinguishable levels of locomotor activity between Cdon^−/− and WT controls (Table 1, k, l, and m, respectively). D, E, In contrast, a sensitizing schedule of amphetamine injections (D) induced robust locomotor sensitization in WT controls (E), while locomotor sensitization in Cdon^−/− mice was greatly attenuated (ANOVA_{Genotype×Time}, p = 0.043; Table 1, n). F, Stereotypy counts were increased in WT controls, but did not change significantly in Cdon^−/− mice. n = 6-10 animals/group.