Pineal ablation in varying photoperiods and the incidence of 9,10-dimethyl-1,2-benzanthracene induced mammary cancer in rats.

Our earlier observation of increased incidence of 9,10-dimethyl-1,2-benzanthracene (DMBA) induced mammary carcinoma in young, virgin 'functionally' pinealectomized Holtzman rats poses the question whether or not a comparable incidence would occur in surgically pinealectomized rats reared in varying photoperiods (e.g. light/dark (LD) 24/0 or LD 10/14 schedules). Results show that functionally or surgically pinealectomized rats in LD 24/0 schedule have comparable mammary tumor incidence (95% and 83%, respectively) and latency period of tumor appearance (60 +/- 3.1 and 69.2 +/- 6.6 days, respectively). However, when surgically pinealectomized rats were kept in short photoperiods (LD 10/14), a significant difference was observed in both tumor incidence (60.9%) and latency period (91.8 +/- 11.0 days). Our data suggest that the susceptibility of the mammary gland to carcinogenic insult may be modulated by the concentration of the pineal hormone, melatonin, in the CNS.