Long-term ovariectomy changes formalin-induced licking in female rats: the role of estrogens

Abstract

Gonadal hormones have been shown to exert modulatory effects on nociception and analgesia. To investigate the role of gonadal hormones in the response by female rats to both phasic and persistent nociceptive stimulation, we evaluated the effects of long-term ovariectomy (OVX, 6 months) on the thermal pain threshold and on formalin-induced responses. The thermal pain threshold was evaluated with the plantar test apparatus, while persistent pain was induced by a subcutaneous injection of dilute formalin (50 microliter, 10%) in the dorsal hind paw. The formalin test was carried out in an open field apparatus where the animal's spontaneous behavior and formalin-induced responses (licking duration, flinching frequency and flexing duration of the injected paw) were recorded for 60 min. Estradiol and corticosterone plasma levels were determined in blood collected from the anesthetized animals at the end of the test. In OVX females, the duration of formalin-induced licking was longer than in Intact females during both the first and the second phase; flinching and flexing did not differ from Intact. The thermal pain threshold was only slightly affected by OVX. Estradiol and corticosterone were lower in OVX females than Intact ones. These data indicate that long-term depletion of gonadal hormones in female rats modulates the pain-induced behavioral responses related to supraspinal neural circuits (licking of the injected paw) rather than more spinally mediated responses such as formalin-induced flinching and withdrawal latency in the plantar test.

Figure 1

Plantar test Latency of paw withdrawal after administration of a thermal pain stimulus to the plantar surface of the rat's paw with an infra-red source (set at an intensity of 190 mWatt/cm²). The measures were determined three times, with two minutes in between.

Figure 2

Formalin test A, Licking duration (sec), B, Flexing duration (sec) and C, Flinching frequency (num) were scored for a 1-hr period after formalin injection (50 μl, 10%) in Intact (INT) and Ovariectomized (OVX) female rats. The time after formalin injection (in 5-min periods) is indicated on the abscissa and the time spent licking or flexing or the frequency of flinching is indicated on the ordinate. Indications of significance for single time points were not included in order to maintain the clarity of the graph.

Figure 3

Hormone assays. Corticosterone and estradiol plasma levels were determined in all groups at the end of the experimental tests. Corticosterone and estradiol were lower in ovariectomized animals, independently of the formalin treatment.

Figure 4

Correlations. Graphic representation of the linear correlation between estrogen and corticosterone plasma levels (n = 21, r = 0.53, p < 0.05) and between estrogen plasma levels and the total amount of formalin-induced licking (n = 10, r = -0.63 p < 0.05).