Long-term effects of two antidepressants on copulatory and exploratory behaviour in the female rat.

The immediate and long-term effects of antidepressant drugs on two different behaviours--copulatory and exploratory--were compared in hormone-treated, ovariectomized rats. The drugs used were desmethylimipramine (DMI 2 and 5 mg/kg) and zimelidine (ZIM 5 and 25 mg/kg). The acute effects were studied 1-4 hours after drug injection, and the long-term effects after four weeks of daily injections. The acute treatment with DMI and ZIM significantly suppressed the lordosis behaviour compared with that in saline-treated controls. After long-term administration of DMI, the 5 mg/kg dose of this drug was still able to inhibit the lordosis response, while the low dose of DMI and ZIM did not produce a significant effect after chronic administration. We suggest that subsensitivity to certain effects of the antidepressant drugs develops during the long-term treatment.