Tropisetron increases the inhibitory effect of mild restraint on lordosis behavior of hormonally primed, ovariectomized rats

Ovariectomized rats, hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone are resistant to the lordosis-inhibiting effects of a 5 min restraint experience. However, modulation of the serotonergic (5-HT) system alters this resistance to stress. In the following experiment, ovariectomized Fischer inbred rats were hormonally primed with 10 μg estradiol benzoate and 500 μg progesterone. The effect of 5 min restraint on sexual behavior was examined after bilateral hypothalamic infusion or intraperitoneal (ip) treatment with the 5-HT3 receptor antagonist, 3-tropanylindole-3-carboxylate hydrochloride (tropisetron). Infusion with 50 or 100 ng tropisetron inhibited lordosis behavior. When rats were infused with 10 or 25 ng tropisetron, rats showed normal lordosis behavior. However, when infusion with 10 or 25 ng tropisetron was combined with 5 min restraint, lordosis behavior was inhibited. These findings are consistent with prior work that has implicated hypothalamic serotonin in control of lordosis behavior and in the effect of mild restraint on the behavior. In contrast to the effects of the intracranial infusion, intraperitoneal injection with 1.0 or 2.0 mg/kg tropisetron did not amplify the effects of restraint.

Research highlights▶ Bilateral infusion of 100 ng tropisetron to the medial basal hypothalamus inhibited lordosis behavior of ovariectomized, hormonally primed rats while infusion of 10 or 25 ng had no effect on lordosis behavior. ▶ Five minutes restraint had no effect on lordosis behavior when ovariectomized, hormonally primed rats were infused with deionized water; however, when restraint occurred 15 min after infusion with 10 or 25 ng tropisetron, lordosis behavior was reduced. ▶ In contrast to intracranial effects of tropisetron, intraperitoneal treatment with the drug did not affect the response to stress.