Comparisons between brain dopaminergic neurons of juvenile and aged rats: sex-related differences

It is known that several aspects of dopaminergic neurotransmission deteriorate with advanced age. In the present report, we have studied the possible existence of sexual differences in these aging-induced changes. Thus, we measured several pre- and postsynaptic biochemical parameters, indicative of the activity of dopaminergic neurons, in striatum, limbic forebrain and hypothalamic-anterior pituitary area of aged (24-26 months) and young (2 months) rats of both sexes. Tyrosine hydroxylase (TH) activity, as well as the number of D2-dopaminergic receptors, decreased in the striatum of aged rats, especially in the males in which the decrease in the number of receptors was associated with an increase in their affinity. In addition, the ratio between dopamine (DA) and its intraneuronal metabolite, L-3,4-dihydroxyphenyl-acetic acid (DOPAC), which can be used as an index of neurotransmitter turnover, was increased in aged females in parallel with a decreased DA content. In the limbic forebrain, TH activity was also decreased during aging, but only in males, whereas the DOPAC/DA ratio was increased in females, although in parallel with an increased DOPAC production. Finally, in the hypothalamic-anterior pituitary area, aging only affected the females, in which increased plasma prolactin levels were observed. This effect might be the result of a low responsiveness of pituitary lactotrophs to DA released from hypothalamic neurons, in spite of high prolactin levels producing a constant, although ineffective, stimulation of the activity of these neurons, as reflected by the high DOPAC content and DOPAC/DA ratio observed in the medial basal hypothalamus. In summary, these data allow us to suggest that the activity of brain dopaminergic neurons is modified with aging and there are significant differences as a function of sex and brain area.