Pattern of tyrosine hydroxylase expression during aging of mesolimbic pathway of the rat.

Senescence is a physiological and progressive event that leads to the impairment of normal functions of the organism. The nervous system is one of the most affected systems during aging, presenting both structural and functional alterations associated with a decline in normal brain functions. In the present study we aimed to evaluate the impact of senescence on the mesolimbic pathway (nucleus accumbens - NAc and ventral tegmental area - VTA) of the rat, through immunohistochemistry for tyrosine hydroxylase (TH) enzyme, in young (3 months old), middle-aged (10 months old) and aged animals (18 months old). There was a significant decrease in the TH-immunoreactivity across NAc in aged animals as compared to the young and middle-aged ones, as revealed by optical densitometry. Medium and caudal regions of the VTA in the young animals possessed a higher number of TH-immunoreactive neurons as compared to the more aged groups. Comparisons among VTA regions in young animals revealed a difference in the number of cell bodies when the medium region was compared to the rostral and caudal regions whilst in both the middle-aged and aged groups comparisons between rostral vs caudal and medium vs caudal regions were significant. Our results show that aging impacts the mesolimbic pathway across its rostrocaudal axis, with a decrease of TH-reactivity in NAc and loss of neurons in VTA. These events may be involved with behavioral alterations observed throughout aging.