Hypertrophy of the ageing rat medial preoptic nucleus.

The medial preoptic nucleus (MPN) plays an essential role in the coordination of behaviours and physiological responses necessary for reproduction. Since ageing is associated with a progressive deterioration of reproductive functions we have explored the possibility that changes in the structural organization of the MPN might be implicated in this process. Thus, we have estimated the volume of the MPN, and the total number and size of its neurons, using stereological methods, and quantitatively evaluated the dendritic trees of MPN neurons in Golgi-impregnated material. Male and female rats, aged 6, 24 and 30 months, were independently analysed. No cell loss was observed in aged rats of both sexes. However, the volume of the MPN and the somatic size of its neurons were remarkably enlarged in aged rats. No significant age-related changes in the size or shape of the dendritic trees or in dendritic spine density were found. To evaluate whether the changes observed in aged rats could be ascribed to an altered interaction between gonadal steroids and steroid-sensitive neurons, we have additionally estimated the total number of MPN neurons immunoreactive for the estrogen receptor-alpha. No significant age-related variations were detected. The age effects upon the MPN were more marked in females than in males and, consequently, the sexual dimorphisms in neuronal size and in the number of estrogen receptor-immunoreactive neurons were blunted in aged rats.