Age- and sex-dependence of dopamine release and capacity for recovery identified in the dorsal striatum of C57/Bl6J mice

Abstract

The dorsal striatum is the main input structure of the basal ganglia and the major target area of dopaminergic projections originating in the substantia nigra pars compacta. Heavily involved in the regulation of voluntary movement and habit formation, this structure is of strong importance in Parkinson's disease, obsessive-compulsive disorder, Tourette's syndrome and addiction. The C57/Bl6J mouse strain, the most commonly used strain in preclinical research today, is frequently used as a model organism for analysis of dopaminergic parameters implicated in human pathophysiology. Several components of the dopamine system have been shown to vary with age and sex, however knowledge of the contribution of these factors for dopamine release kinetics in the C57/Bl6J mouse strain is lacking. In the present study, we used an intracranial KCl-stimulation challenge paradigm to provoke release from dopaminergic terminals in the dorsal striatum of anaesthetized C57/Bl6J mice. By high-speed in vivo chronoamperometric recordings, we analyzed DA release parameters in male and female mice of two different ages. Our experiments demonstrate elevated DA amplitudes in adult compared to young mice of both sexes and higher DA amplitudes in females compared to males at both ages. Adult mice exhibited higher recovery capabilities after repeated stimulation than did young mice and also showed a lower variability in the kinetic parameters trise and t80 between stimulations. These results identified age- and sex- dimorphisms in DA release parameters and point to the importance of taking these dimorphisms into account when utilizing the C57/Bl6J mouse strain as model for neurological and neuropsychiatric disorders.

Figure 1. High-speed in vivo chronoamperometry recording setup and parameters.

A: Experimental setup: 6 consecutive KCl-stimulations (1–6) in 2-minute intervals were followed by a 15-minute stimulation-free period upon which followed stimulation 7. Basal release capacity was defined by stimulation 1; recovery capacity was defined as a significant increase in DA amplitude upon stimulation 7 compared to stimulation 6; regain of basal release capability was defined as the absence of a significant difference in DA amplitude between stimulation 1 and stimulation 7. B: Left: Schematic illustration of a coronal brain section at Bregma 1.1 with recording electrode implicated by green dotted line. Right: Close-up of representative photomicrograph depicting the dorsal striatum (DStr) shown to the left as indicated by gray dotted lines; histological verification of the position of the DiO-coated carbon-fiber recording electrode (green) implanted in the DStr of a C57/BL6J mouse. C: Representative traces of DA release kinetics obtained by the high-speed chronoamperometry system following KCl-stimulation 1 for all four experimental groups: Green - adult females; Red - adult males; Black - young females; Blue - young males (shown in inset). Illustration of parameters analysed (green trace used as example): Amplitude, defined as the peak DA concentration (µM) from baseline; t _rise, the time (seconds) between injection and maximum peak concentration; and t₈₀, the time (seconds) from maximum peak concentration until 80% decrease of the maximum amplitude as a measure of DA clearance. CC; corpus callosum, LV; lateral ventricle.

Figure 2. DA release and reuptake parameters within adult and young groups of mice of both sexes.

Mean values obtained for each stimulation (1–7) within each group (Green - adult females (A–C); Red - adult males (D–F); Black - young females (G–I); Blue - young males (J–L) for parameters amplitude (A,D, G, J), t _rise (B, E, H, K) and t₈₀ (C, F, I, L); Recovery capacity (stimulation 7 vs. 6) and regain of basal release capability (stimulation 7 vs. 1) shown as bar graph inserts for all groups and parameters (adult females (A′–C′); adult males (D′–F′); young females (G′–I′); young males (J′–L′). p<0.05: #, p<0.01: ##, p<0.001: ### as detected by 1-way repeated measure ANOVA, p<0.05: *, p<0.01: **, p<0.001: *** as detected by post-hoc test with Bonferroni correction (A-L) and paired student's t-test (A′-L′)

Figure 3. Comparison of DA release and reuptake parameters between adult and young groups of mice of both sexes.

Scatter plots of DA amplitude vs. t_rise (A–C) and DA amplitude vs. t₈₀ (D–F) as logarithmic values (natural logarithm) for each individual mouse after stimulation 1 (A, D), 6 (B, E) and 7 (C, F), respectively. Same color-coding as in Fig. 1 and 2: Green - adult females; Red - adult males; Black - young females; Blue - young males. Bars on the side of each plot depict mean values of the groups upon each specific stimulation (1, 6 and 7) and stars indicate statistically significant differences between indicated groups. p<0.05: *, p<0.01: **, p<0.001: *** as detected by 3-way repeated measures ANOVA and post-hoc test with Bonferroni correction.