Microstructural changes to the brain of mice after methamphetamine exposure as identified with diffusion tensor imaging

Abstract

Methamphetamine (METH) is an addictive psychostimulant inducing neurotoxicity. Human magnetic resonance imaging and diffusion tensor imaging (DTI) of METH-dependent participants find various structural abnormities. Animal studies demonstrate immunohistochemical changes in multiple cellular pathways after METH exposure. Here, we characterized the long-term effects of METH on brain microstructure in mice exposed to an escalating METH binge regimen using in vivo DTI, a methodology directly translatable across species. Results revealed four patterns of differential fractional anisotropy (FA) and mean diffusivity (MD) response when comparing METH-exposed (n=14) to saline-treated mice (n=13). Compared to the saline group, METH-exposed mice demonstrated: 1) decreased FA with no change in MD [corpus callosum (posterior forceps), internal capsule (left), thalamus (medial aspects), midbrain], 2) increased MD with no change in FA [posterior isocortical regions, caudate-putamen, hypothalamus, cerebral peduncle, internal capsule (right)], 3) increased FA with decreased MD [frontal isocortex, corpus callosum (genu)], and 4) increased FA with no change or increased MD [hippocampi, amygdala, lateral thalamus]. MD was negatively associated with calbindin-1 in hippocampi and positively with dopamine transporter in caudate-putamen. These findings highlight distributed and differential METH effects within the brain suggesting several distinct mechanisms. Such mechanisms likely change brain tissue differentially dependent upon neural location.

Keywords: Diffusion tensor imaging; Fractional anisotropy; Magnetic resonance imaging; Mean diffusivity; Methamphetamine; Mouse.

Figure 1

Experimental Design. METH: Methamphetamine; MRI: Magnetic Resonance Imaging.

Figure 2

Comparison of methamphetamine (METH) exposed mice to saline control mice. All images are coronal slices in neurological view (left side of brain is the left hemisphere). A) Fractional anisotropy (FA) parametric maps and B) Mean diffusivity (MD) parametric maps demonstrating Cohen’s d effect sizes. Maps were resampled to 100μm isotropic and smoothed full width half mass by 150μm for presentation. Positive values reflect higher FA/MD in the METH sample compared to saline sample and negative values represent lower FA/MD in the METH sample. C) Clusters where differences were significant at a corrected p-value of 0.05.

Figure 3

Examples of group averaged diffusivity and fractional anisotropy values demonstrating the geometric shape of ellipsoids. Average axial diffusivity values were used for the λ1 axis and radial diffusivity values were used for λ2 and λ3 axes as radial diffusivity is the average of λ2 and λ3 represented by the circle next to ellipsoid. A) Two example regions demonstrating increased fractional anisotropy in methamphetamine-exposed mice compared to saline control mice and one example region demonstrating decreased fractional anisotropy B) One example region demonstrating increased mean diffusivity in methamphetamine-exposed mice. METH: methamphetamine; FA: fractional anisotropy; MD: mean diffusivity; AD: axial diffusivity; RD: radial diffusivity.

Figure 4

Representative immunoreactivity patterns for A1) calbindin-1 in the dorsal hippocampal formation and B1) dopamine transporter in the caudate-putamen from the same mouse brain. Arrows indicate the lateral borders of hippocampal CA1 (A1) and caudate-putamen (B1) in coronal plane. Scale bars = 300μm. Associations between A2) mean diffusivity and calbindin-1 in dorsal hippocampus; and B2) mean diffusivity and dopamine transporter in caudate-putamen. R² values have the sign of the correlation preserved to illustrate directionality and were thresholded at R² ± 0.25. The same cluster threshold used in the main analyses of 0.081μL was then applied. Images are coronal slices in neurological view (left side of brain is the left hemisphere). Maps were resampled to 100μm isotropic and smoothed full width half mass by 150μm for presentation. Positive values represent a positive relationship between mean diffusivity and immunoreactivity intensities, and negative values a negative relationship.