Cholecystokinin stimulates and inhibits lordosis behavior in female rats

Recently, IP CCK-8 has been shown to inhibit lordosis in sexually experienced, estradiol benzoate (EB) and progesterone (P) primed rats. However, receptivity is influenced by prior sexual experience and/or exposure to sex steroids, as well as the steroid dosage administered before testing. Thus, we examined the effect of CCK-8 (3 micrograms/kg; IP) on lordosis in rats with different degrees of receptivity. Three weeks after ovariectomy, females were treated with EB followed 48 hr later with P, or with EB alone. CCK-8 significantly facilitated lordosis in rats given 5 micrograms EB. Following a 5 week nonexperimental period, animals were more receptive and CCK-8 significantly inhibited lordosis in the 5 or 10 micrograms EB groups. In a separate experiment, rats were ovariectomized, adrenalectomized, and treated with EB alone. As in the first experiment, CCK-8 facilitated and inhibited lordosis. CCK-8's effects were highly dependent on the female's receptivity, facilitating lordosis when receptivity was low and inhibiting lordosis when receptivity was high (but not maximal). In conclusion, IP CCK-8 modulates lordosis behavior independent of P, but its effects depend on the female's degree of receptivity.