Spatial memory in middle-aged female rats: assessment of estrogen replacement after ovariectomy

Previous studies have shown that estrogen influences diverse aspects of neuronal function and morphology and modulates acquisition of various memory tasks in young adult female rodents. It is not clear whether estrogen is critical for optimal memory function in middle-aged female animals, i.e. when cyclicity gradually declines. We trained young adult (5 months) and older (10 months) female Long-Evans rats on a win-shift (delay) 12-arm radial maze (7 arms blocked pre-delay). Rats were preoperatively trained to criterion (< or =2 errors/trial for 3 days) with no delay then with a 60 s delay. All rats were ovariectomized when the age groups were 9 (Y) and 14 months (MA), respectively. Following recovery and retraining to criterion, each rat underwent consecutive treatment cycles with vehicle (Oil) or 17-beta-estradiol benzoate (E). Each 6-day acute treatment cycle, modeled after protocols previously shown to induce morphological and electrophysiological plasticity in the hippocampus at 24-48 h after estrogen injection, consisted of two consecutive daily injections of 10 microg E or Oil (0.1 ml subcutaneously) on Days 1-2 (Oil-Oil or E-E), testing on Days 3-4 at 60 s or 6 h delays, with Days 5-6 comprising of washout days. Each rat received a total of 4 treatment cycles, alternating between Oil and E cycles, in counterbalanced order. Estrogen treatment had no effect in either age group on pre-delay or post-delay errors at either 60 s or 6 h delays. The data indicate that the cyclic estrogen replacement regimen does not influence spatial memory function in young or middle-aged animals in the hippocampal-dependent appetitive radial maze task. Discussion of these unexpected results includes consideration of important experimental design factors that differ between our study and some previous reports, such as the extensive training and task experience our subject received prior to testing for estrogen effects.