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The absence of discernible abnormal symptoms such as pain, often leading to delayed diagnosis in cancer patients, may be indicative of a dysregulation in sensory transmission between the tumour and the central nervous system. We explored expression of Fos protein in spinal cord neurons in rats, during the development of the MAT-LyLu prostatic adenocarcinoma grafted on the hind limb. The tumour triggered the densest Fos labelling in the L3–L5 lumbar segments, ipsilateral to the grafted limb. The labelling, detected at day 5, increased until day 10 and dropped off thereafter. The ventral horn (except lamina IX) was the most densely labelled region. Histological examination of the grafted limbs demonstrated that no inflammatory reaction accompanied the tumour growth. Rats exhibited no behavioural alterations either spontaneous or induced by handling. These results demonstrate that signals are sent to the central nervous system by the peripheral tumour. Considering both the behavioural and histological observations, it is unlikely that spinal activity reflects a painful state. The nature of these signals, inefficient to trigger the appropriate reaction of the organism against the tumour, remain to be determined with regard to the pharmacologically active compounds synthesized and released by the tumour cells. © 1999 Cancer Research Campaign

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