Physostigmine-induced analgesia in young, middle-aged, and senescent rats.

To investigate the effect of aging on cholinergically medicated analgesia, rats from three age groups (3-month, 17-month, and 25-month) were injected with physostigmine (0.0156, 0.0625, or 0.25 mg/kg) or saline. Following the injection, tail-flick latencies were measured at 5-minute intervals for 30 minutes and at 45, 60, 75 and 90 minutes. The analysis of the tail-flick latencies revealed that physostigmine produced a dose-dependent analgesia in all age groups. However, the 17- and 25-month-old age groups were more sensitive to the highest dose of physostigmine. The age-related differences in the analgesia produced by physostigmine is in agreement with other research which has demonstrated that pharmacological stimulation of the cholinergic system produces an equivalent or increased responsivity in aged animals.