Abstract
The present study was conducted to determine whether the ovarian steroid oestradiol alters the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM). Adult female rats were ovariectomized and implanted s.c. with an oestradiol-filled or placebo capsule. Sixteen to 37 days later, rats were anaesthetised for single unit recording from RVM neurons. On-cells were characterised by a burst of activity, and off-cells by a pause in activity immediately preceding reflexive withdrawal of the tail from 51 and 54 °C water. Although on- and off-cells were evident in both oestradiol- and placebo-treated rats, the reflex-related on-cell burst and off-cell pause were dampened in oestradiol-treated rats. On-cells from oestradiol-treated rats had a mean activity burst of 9.1±2.2 Hz in the 2 s preceding the tail withdrawal reflex to 51 °C water, compared with 17.9±4.3 Hz for on-cells in placebo controls. Off-cell activity during the 2 s preceding tail withdrawal was 4.8±2.2 vs. 0.1±0.1 Hz in oestradiol vs. placebo-treated females, respectively. Similar changes in on- and off-cell activity occurred when the tail was placed in 54 °C water. The present data demonstrate that oestradiol constrains the magnitude of the shift in RVM on- and off-cell activity associated with nociceptive reflexes.
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