Chronic nicotine treatment changes the axonal distribution of 68 kDa neurofilaments in the rat ventral tegmental area

Region-specific decreases of neurofilament proteins (NF) were described in the ventral tegmental area (VTA) of rats treated chronically with morphine, cocaine or alcohol. In a previous study, we demonstrated that NF levels were also changed in the VTA after chronic treatment with nicotine. The aim of this study was to clarify the submicroscopic basis of decreased immunoreactivity for NF-68, NF-160 and NF-200, as determined by using NR4, BF10 and RT97 antibodies, respectively. Microdensitometric analysis of brain sections showed that immunoreactivity for all NF was reduced in the VTA of animals exposed chronically to nicotine (0.4 mg/kg per day, 6 days of treatment), when compared to rats exposed to saline. Reduction in immunoreactivity was significant for NF-68 (P < 0.05), NF-160 (P < 0.01) and NF-200 (P < 0.05), showing a relative reduction of 34%, 42% and 38%, respectively, when compared to saline-treated rats. No difference was observed for any of the NF under study when immunoreactivity measurements in the substantia nigra were compared. Ultrastructural analysis was applied to evaluate changes in NF-68, NF-160 and NF-200 immunoreactivity in regions of the VTA that contain dopaminergic neurons following chronic nicotine treatment. At the electron microscopic level, no degenerative changes were found in neurons or glial cells of the VTA. With ultrastructural immunohistochemistry, evaluation of the homogeneity parameter of NF distribution showed a loss of homogeneity for NF-68 linked to the nicotine treatment. In areas in which NF organization appeared well preserved, analysis of the numerical density of NF revealed no significant difference for NF-68 (897/ micro m2 vs. 990/ micro m2), NF-160 (970/ micro m2 vs. 820/ micro m2) and NF-200 (1107/ micro m2 vs. 905/ micro m2) in nicotine-treated rats when compared to saline-treated rats. These results confirm that nicotine shares the same properties with cocaine and morphine in reducing NF in the VTA, a key brain structure of the rewards system, and that chronic nicotine treatment changes the axonal distribution of 68 kDa neurofilaments in the rat VTA.