2.3. Unconditioned Effects of Opioids
Sex differences in opioid sensitivity often extend to unconditioned drug effects. For example, male rats are more sensitive than female rats to morphine-induced expression of the immediate-early gene c-Fos and to morphine-induced Straub tail (D’Souza et al., 2002). Similarly, male rats are more sensitive to the locomotor-suppressive effects of morphine than female rats (Craft et al., 2006; Holtman et al., 2004; Stewart & Rodaros, 1999), and male mice are more sensitive to the locomotor-stimulating effects of morphine than female mice (Kavaliers & Innes, 1986). Moreover, male rodents are more sensitive to the antidiuretic (Craft et al., 2000) and respiratory depressant effects (Craft et al., 1999) of morphine than female rodents, although this latter effect may vary across species (see Dahan et al., 2008; Sarton et al., 1999). In contrast, male rodents are less sensitive than female rodents to the immunological/inflammatory effects of morphine (Elliott et al., 2003; 2006) and to the thermoregulatory (both hypothermic and hyperthermic) effects of morphine (Kest et al., 2000; Quock et al., 1985; but see Kasson & George, 1984).