In vivo long-lasting alterations of central serotonin transporter activity and associated dopamine synthesis after acute repeated administration of methamphetamine

Abstract

Background: Methamphetamine (METH)-associated alterations in the striatal dopamine (DA) system or dopamine transport (DAT) have been identified in clinical and preclinical studies with positron emission tomography (PET) imaging but have not been well correlated with in vivo serotonin transporter (SERT) availability due to the lack of appropriate imaging agents to assess SERTs. N,N-dimethyl-2-(2-amino-4-[¹⁸F]-fluorophenylthio) benzylamine (4-[¹⁸F]-ADAM) has been developed by our group and validated for its high affinity and selectivity for SERTs, allowing the in vivo examination of SERT density, location, and binding function. The aims of this study were to investigate the potential of SERT imaging using 4-[¹⁸F]-ADAM PET to estimate the long-lasting effects of METH-induced serotonergic neurotoxicity, and further determine whether a correlative relationship exists between SERT availability/activity and tyrosine hydroxylase (TH) activity in various brain regions due to the long-lasting consequences of METH treatment.

Results: Male rats received four administrations of METH (5 or 10 mg/kg, s.c.) or saline (1 ml/kg, s.c.) at 1-h intervals. At 30 days post-administration, in vivo SERT availability and activity were measured by 4-[¹⁸F]ADAM PET imaging. In contrast to the controls, the uptake of 4-[¹⁸F]ADAM in METH-treated mice was significantly reduced in a dose-dependent manner in the midbrain, followed by the hypothalamus, thalamus, striatum, hippocampus, and frontal cortex. The regional effects of METH on TH activity were assessed by quantitative immunohistochemistry and presented as integrated optical density (IOD). A significant decrease in TH immunostaining and IOD ratios was seen in the caudate, putamen, nucleus accumbens, substantia nigra pars compacta, and substantia nigra pars reticulata in the METH-treated rats compared to controls.

Conclusion: The present results suggested that the long-lasting response to METH decreased the uptake of 4-[¹⁸F]-ADAM and varied regionally along with TH immunoreactivity. In addition, 4-[¹⁸F]ADAM PET could be used to detect serotonergic neuron loss and to evaluate the severity of serotonergic neurotoxicity of METH.

Keywords: 4-[18F]-ADAM PET; Long-lasting serotonergic neurotoxicity; Methamphetamine.

Fig. 1

An example of 2D images showing 4-[¹⁸F]ADAM distribution obtained 60–90 min after administration of 4-[¹⁸F]ADAM in control animals (upper panel) and those treated with 5 mg/kg of METH (middle panel) and 10 mg/kg of METH (lower panel). Higher 4-[¹⁸F]ADAM accumulations were found in the amygdala, caudate/putamen, midbrain, and hippocampus. Lower accumulation of radioactivity was found in the cerebellum. The accumulation of 4-[¹⁸F]ADAM illustrates decreased SERT availability in the METH-induced groups. The accumulated radioactivity was expressed as a percentage of the injection dose/tissue gram (%ID/g). fctx, frontal cortex; str, striatum (caudate/putamen); tha, thalamus; mid, midbrain; hip, hippocampus; cr, cerebellum

Fig. 2

The accumulation of 4-[¹⁸F]ADAM, showing SERT availability, tended to decrease in most brain regions with increasing METH dose in rats. The higher dose of METH (10 mg/kg) caused a dramatic reduction of SERT availability. The accumulated radioactivity is expressed as a percentage of the injection dose/tissue gram (%ID/g) as the mean ± SD of 6–8 rats per group. *p < 0.05, **p < 0.01, and ***p < 0.001 compared to the control group

Fig. 3

3D PET parametric images showing the specific uptake ratio (SUR) of 4-[¹⁸F]-ADAM in the brains of normal and methamphetamine-treated rats. The 10 mg/kg treatment group showed a globally lower 4-[¹⁸F]ADAM uptake in the brain compared to the 5 mg/kg group

Fig. 4

The specific uptake ratios (SURs) of 4-[¹⁸F]ADAM PET in various rat brain regions after repeated 5 or 10 mg/kg METH treatments. Data are expressed as the mean ± SD. *p < 0.05 and **p < 0.01 compared to the control group; ^#p < 0.05 and ^##p < 0.01 compared to the 5 mg/kg group; %, percentage of decrease compared to the control

Fig. 5

Immunohistochemical photomicrographs of tyrosine hydroxylase (TH) immunoreactivity at the para-sagittal levels of the control and METH-induced brains (upper panel) and their magnifications (middle and lower panels). CPu, striatum; NAc, nucleus accumbens; SN, substantia nigra

Fig. 6

The integrated optical density (IOD) ratio of tyrosine hydroxylase (TH) immunoreactivity in the caudate-putamen (CP), nucleus accumbens (NA), substantia nigra pars compacta (SNPC), and pars reticulata (SNPR) of the control and METH-induced groups. *p < 0.05 and **p < 0.01 compared to the control group; ^#p < 0.05 and ^##p < 0.01 compared to the 5 mg/kg group