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).