Cadherin 13: human cis-regulation and selectively-altered addiction phenotypes and cerebral cortical dopamine in knockout mice

Abstract

The cadherin 13 (CDH13) gene encodes a cell adhesion molecule likely to influence development and connections of brain circuits that modulate addiction, locomotion and cognition, including those that involve midbrain dopamine neurons. Human CDH13 mRNA expression differs by more than 80% in postmortem cerebral cortical samples from individuals with different CDH13 genotypes, supporting examination of mice with altered Cdh13 expression as models for common human variation at this locus. Constitutive cdh13 knockout mice display evidence for changed cocaine reward: shifted dose response relationship in tests of cocaine-conditioned place preference using doses that do not alter cocaine conditioned taste aversion. Reduced adult Cdh13 expression in conditional knockouts also alters cocaine reward in ways that correlate with individual differences in cortical Cdh13 mRNA levels. In control and comparison behavioral assessments, knockout mice display modestly-quicker acquisition of rotarod and water maze tasks, with a trend toward faster acquisition of 5 choice serial reaction time tasks that otherwise displayed no genotype-related differences. They display significant differences in locomotion in some settings, with larger effects in males. In assessments of brain changes that might contribute to these behavioral differences, there are selective alterations of dopamine levels, dopamine/metabolite ratios, dopaminergic fiber densities and mRNA encoding the activity dependent transcription factor npas4 in cerebral cortex of knockout mice. These novel data and previously reported human associations of CDH13 variants with addiction, individual differences in responses to stimulant administration and attention deficit hyperactivity disorder (ADHD) phenotypes suggest that levels of CDH13 expression, through mechanisms likely to include effects on mesocortical dopamine, influence stimulant reward and may contribute modestly to cognitive and locomotor phenotypes relevant to ADHD.

Keywords: addiction; attention deficit hyperactivity disorder; cell adhesion molecule; cocaine; conditioned place preference.

Figure 1.

Constitutive (A, C) and adult (B) CDH13 knockout alters conditioned place preference (A, B) but not conditioned taste aversion (C) provided by modest to moderate cocaine doses. (A, B) Mean difference ± SEM in time spent on the cocaine-paired side before and after conditioning (n = 1 –12/group). There was no significant difference between genotypes in time spent on the drug-paired side during the pre-tests for constitutive (ANOVA, p = 0.740) or conditional (p = 0.542) knockouts. (C) Preference for cocaine-paired saccharin solution in two bottle testing after taste aversion conditioning. Bp < 0.05, BBp < 0.01.

Figure 2.

Acquisition and performance of the rotorod (A) Morris water maze (B) and 5-choice serial reaction time (C) tasks in constitutive CDH13 knockouts and wild-type littermates. (A) Mean ± SEM of the latencies to fall from the accelerating rotorod in wild-type, heterozygous and homozygous constitutive CDH13 knockouts. Knockouts showed faster acquisition of the task (repeated measures ANCOVA, p = 0.040). N = 24/genotype. Bp < 0.05. (B) Mean ± SEM of the latencies to reach Morris water maze platform for wild-type, heterozygous and homozygous CDH13 knockouts. Knockouts displayed no significant deficit in task acquisition (days 1–5, repeated measures ANOVA genotypeBday effect, p = 0.96), and displayed better performance on d 2 of testing when compared with mice of both other genotypes Bp < 0.05. (B) Mean ± SEM of the probe trial results. (C) Mean ± SEM of % correct responses in 5-choice serial reaction time testing in the first 2 trials of this task in male mice. Knockouts showed a trend toward faster acquisition of the task that did not achieve significance (p = 0.1). Females failed to display any trends (data not shown).

Figure 3.

Altered dopamine levels (A–D) and dopamine fiber densities (E–G) in cortices of constitutive CDH13 knockout mice. Regional concentrations of monoamines and metabolites in dopamine-associated brain regions. (A–C) Mean ± SEM of the concentrations of monoamines and metabolites in cerebral cortex (A), striatum (B) and ventral midbrain (C) samples dissected from brains of wild-type (black bars) and CDH13 knockout (open bars) mice. (D) Ratios between dopamine metabolites and dopamine in: CX, cerebral cortex; HC, hippocampus; ST, striatum; MB, ventral midbrain. N = 10/ genotype. Bp < 0.05. (E) Densities of dopamine transporter immumoreactive dopamine fibers in 50 μm sections through prefrontal cortex from constitutive CDH13 knockouts and wild-type littermates. Bp < 0.05. (F–G) Images of dopamine transporter immunoreactive elements > 2 SD above mean pixel density for each section from infralimbic/prefrontal cortical sections from wild-type (F) and CDH13 knockout (G) mice. Midline is at the left in both images.