Inhibition of oestradiol-induced prolactin release in a dual-cannulated ovariectomized rat model by carmoxirole, a peripherally restricted dopamine agonist.

Centrally acting dopamine agonists (e.g. bromocriptine) and dopamine transport inhibitors (e.g. GBR12909) are known to inhibit oestradiol-induced prolactin release. The capacity of peripherally restricted compounds to do likewise, however, is unknown. Here, the effects of the peripherally restricted dopamine receptor agonist carmoxirole on oestradiol-induced prolactin release were investigated. Dual-cannulated ovariectomized rats were used, so that a robust, reproducible response to exogenous oestrogen could be induced and sequential blood samples were taken with minimal stress. Carmoxirole (15 mg/kg) inhibited oestradiol-induced prolactin release, similar to bromocriptine and GBR12909. However, carmoxirole also induced a rapid, transient, oestradiol-independent release of prolactin. These data show that peripherally restricted dopamine receptor agonists are sufficient to inhibit oestradiol-induced prolactin release. Like centrally acting compounds, they may therefore be expected to affect the incidence of prolactin-dependent tumours in rat carcinogenesis studies without inducing central-mediated side effects.