Pain threshold was determined in female rats using the tail hick test.
Latency to respond depended on the locus of the tail heated, with the
most distal sites resulting in the shortest response times (Experimen
t 1). Tail flick latency also varied according to the time of day, wit
h shorter response times recorded around the middle of the dark phase
than near its beginning or after its end. This was the case for intact
, for ovariectomized, and for ovariectomized, estradiol-treated rats (
Experiment 2). However, response times also varied across the estrous
cycle, with significantly shorter latencies recorded during estrus and
metestrus. Ovariectomy abolished these fluctuations, and whereas admi
nistration of estradiol increased response times, progesterone had lit
tle effect (Experiment 3). These results suggest that site of tail hea
ting, time of day, and presence of ovarian hormones can influence tail
hick latency independently, thus demonstrating the complexity of the
mechanisms that may contribute to pain threshold even within the same
paradigm.