Reinstatement of vaginal cycles in aged female rats

Old constant estrous and young cycling control rats were injected with L-dopa, 200 mg/kg, for a period of 14 days. L-Dopa reinstated vaginal cycling in the old rats and it did not affect the vaginal cycling in young rats. Likewise, 200 mg/kg of L-tyrosine reinstated vaginal cycling. The effect of L-dopa on old rats was blocked by pimozide, by high doses of MK-486 and Ro 4-4602, and by fusaric acid. High doses of phenoxybenzamine or L-propranolol did not alter the L-dopa effect on old rats. A low dose of MK-486 or Ro 4-4602 increased the efficacy of L-dopa in reinstating vaginal cycling. These results suggest that L-dopa reinstates vaginal cycling in old rats by stimulating dopamine receptors. The possibility that a simultaneous stimulation of norepinephrine alpha and beta receptors and/or a decrease of central nervous system serotonin is necessary for this effect is discussed.