Loss of cocaine locomotor response in Pitx3-deficient mice lacking a nigrostriatal pathway

Abstract

Both the dorsal and ventral striatum have been demonstrated to have a critical role in reinforcement learning and addiction. Dissecting the specific function of these striatal compartments and their associated nigrostriatal and mesoaccumbens dopamine pathways, however, has proved difficult. Previous studies using lesions to isolate the contribution of nigrostriatal and mesoaccumbens dopamine in mediating the locomotor and reinforcing effects of psychostimulant drugs have yielded inconsistent and inconclusive results. Using a naturally occurring mutant mouse line, aphakia, that lacks a nigrostriatal dopamine pathway but retains an intact mesoaccumbens pathway, we show that the locomotor activating effects of cocaine, including locomotor sensitization, are dependent on an intact nigrostriatal dopamine projection. In contrast, cocaine reinforcement, as measured by conditioned place preference and cocaine sensitization of sucrose preference, is intact in these mice. In light of the well-established role of the nucleus accumbens in mediating the effects of psychostimulants, these data suggest that the nigrostriatal pathway can act as a critical effector mechanism for the nucleus accumbens highlighting the importance of intrastriatal connectivity and providing insight into the functional architecture of the striatum.

Figure 1

Tyrosine hydroxylase immunoreactivity in serial coronal sections of the striatum in mice heterozygote (top panel) and homozygote (middle panel) for the mutation in the promoter region of the Pitx3 gene. Bottom panel shows midbrain sections from one heterozygote (left) and four homozygote mice from two age groups (right).

Figure 2

Quantification of dopamine denervation in the striatum. Composite images averaging the intensity of tyrosine hydroxylase (TH) reactivity from overlaid, matched, and aligned sections for (a) control (N = 2, heterozygote, N = 3 wild-type) and (b) Pitx3-deficient mice (N = 7). Lines used to quantify the gradient in the dorsal striatum (DSt) from ventral to dorsal (3) and from medial to lateral (2) are shown. The rectangular regions used to quantify TH reactivity in the nucleus accumbens (Nac) are also shown. (c) The mean of TH reactivity measured along the reference lines (filled symbols plot ventral to dorsal measurements; open symbols plot medial to lateral; genotypes represented by color). (d) Radiating reference lines used to compute angular gradient of dopamine denervation are shown on the Pitx3-deficient composite image. (e) Pseudo-colored difference map resulting from subtracting the Pitx3-deficient composite from the control composite (hot colors (red) show greatest difference, cool colors (blue) least difference) with DSt and the NAc shell and core labeled according to standard mouse atlas (Paxinos, 2nd edn.). (f) Average intensity of TH-reactivity for each spoke of the radiating reference lines plotted in the direction of the arrow shown in (d). (g) TH reactivity in the NAc plotted from medial to lateral based on rectangular region of interest shown in (a) and (b). Each point represents the average of a vertical column within the rectangle. (h) Group averages of TH immunoreactivity in the DSt and NAc from sections quantified individually (rather than as composite) using a polygon regions of interest drawn around the DSt and NAc. The Pitx3-deficient mice were divided into young (< 100 days, N = 3) and old (> 100 days, N = 4). ***p<0.001.

Figure 3

Baseline activity of Pitx3 and littermate heterozygote controls. (a) Distance traveled (mean ± SEM) during 5-min bins averaged across baseline sessions. Inset: average distance (mean ± SEM) traveled during baseline sessions. N = 66 (Pitx3 +/−), 67 (Pitx3 −/−). (b) Scatter plot showing distance traveled plotted against age. Regression lines in bold for each group with 95% confidence for slope shown as dashed lines.

Figure 4

Cocaine dose–response curve. Average distance traveled (mean ± SEM) in response to cocaine administration (N = 4 per genotype/dose).

Figure 5

Conditioned place preference and sucrose preference. (a) Percent of time (mean ± SEM) spent in the cocaine- and saline-paired chambers during 30-min test session (N = 14 per genotype). (b) Pitx3-deficient and Pitx3 heterozygote controls were administered repeated cocaine (six exposures, 10 mg/kg, open symbols) or a single cocaine exposure (10 mg/kg, filled symbols) and placed in home cages with a choice of sucrose or water. Sucrose consumption as a percentage of total consumption (mean ± SEM) reflects preference and is shown at five increasing sucrose concentrations, averaged over 6 days for each concentration (N = 4 per genotype/preexposure condition).

Figure 6

Locomotor sensitization to repeated exposures of cocaine. (a) Ambulatory activity (mean ± SEM) during 30-min conditioning sessions alternating saline and cocaine pairing in the conditioned place preference test (N = 14 per genotype). (b) Mice preexposed to either 5 days of cocaine (10 mg/kg) or saline (72 h between exposures) and then challenged with cocaine (5 mg/kg, solid bars) and saline (hatched bars) in 30 min open-field sessions on consecutive days (mean distance ± SEM, N = 6 per genotype/preexposure condition). (c) Response to cocaine challenge normalized as percent increase over saline challenge (mean ± SEM).