Testosterone is essential for alpha(2)-adrenoceptor-induced antinociception in the trigeminal region of the male rat

Abstract

Activation of the alpha(2)-adrenoceptor has been shown to produce antinociception. We have previously shown that the antinociceptive effect of clonidine, an alpha(2)-adrenoceptor agonist, is sex-specific and is abolished by exogenous estrogen in ovariectomized rats or high level of endogenous estrogen in proestrous females. Here, we investigated whether testosterone mediates the antinociceptive effect of clonidine in the trigeminal region of the male rat. Clonidine (7 microg/5 microl) was injected intracisternally through a PE-10 cannula implanted dorsal to the trigeminal region in orchidectomized (GDX) male Sprague-Dawley rats. In separate groups, testosterone propionate (250 microg/100 microl; GDX+T) or beta-estradiol benzoate (100 microg/100 microl; GDX+E) were injected subcutaneously 24 and 48 h respectively prior to the N-methyl-D-aspartic acid (NMDA)--or heat-evoked nociceptive test. NMDA-induced number of scratches or duration of scratching behavior did not change significantly in control groups with or without hormonal replacement. Clonidine significantly reduced both measures only in the GDX+T group but not in GDX or GDX+E group. Clonidine also significantly increased head withdrawal latency (HWL) in the GDX+T group, but not in GDX or GDX+E group. The antinociceptive effect of clonidine was reversed by yohimbine, an alpha(2)-adrenoceptor antagonist, in GDX+T group. We conclude that testosterone is required for the expression of antinociception produced by selective activation of the alpha(2)-adrenoceptor in the trigeminal region of the male rat. These findings further our understanding of sex-related differences in the modulation of nociception and may provide insight into development and administration of analgesic agents in young vs. aging men.

Figure 1. Testosterone is required for the expression of α₂-adrenoceptor antinociception on the NMDA-evoked nociceptive test in the male rat

Clonidine antinociception was observed only in testosterone treated orchidectomized rats (GDX+T) on the NMDA-induced nociceptive test. Significant main effect of group was obtained on the number of NMDA-induced scratches and the duration of scratching (F_5,17=5.58 and 5.41 respectively; p<0.01). Post hoc comparisons revealed that while the number of scratches (Fig. 1A) or the duration (Fig. 1B) did not differ between control groups (NMDA injection only) with or without hormonal replacement, clonidine, given 15 min prior to NMDA, significantly reduced both measures only in the GDX+T group but not in GDX or GDX+E groups. Latency to scratching was not significantly affected between groups (F<1; data not shown). n = 3/group.

Figure 2. Testosterone is required for the expression of α₂-adrenoceptor antinociception on the heat-evoked head withdrawal test in the male rat

Fig 2A: Clonidine antinociception was observed only in testosterone treated orchidectomized rats (GDX+T) on the heat-evoked head withdrawal test. ANOVA yielded significant main effects of Group (F_3,10=58.67; p<0.001), Time (F_11,110=7.04; p<0.001) and an interaction between Group × Time (F_33,110=2.83; p<0.001). Post hoc comparisons revealed that the baseline head withdrawal latency did not significantly differ between different groups with or without hormonal replacement. Clonidine significantly increased head withdrawal latencies (i.e. produced antinociception) only in the GDX+T (n=4) group but not in GDX (n=3) or GDX+E (n=3) groups. The effect of clonidine was seen within 5 min of injection and persisted for the duration of the experiment (90 min) in the GDX+T group. This effect was completely reversed by yohimbine, a non imidazoline α₂-adrenoceptor antagonist (n=4). Fig 2B: The data was also subjected to area under the curve analysis to highlight the cumulative effect of clonidine. Clonidine significantly increased the AUC only in the GDX+T group (F_3,13=64.86; p<0.001).