Involvement of serotoninergic pathways in the control of LH secretion in red deer hinds

Abstract

Serotoninergic pathways are implicated in the neuroendocrine regulation of LH secretion in domestic animals. Two experiments were carried out to investigate this in red deer hinds.

In Experiment 1, ovariectomized (n = 5) and ovariectomized-thyroidectomized (n = 5) hinds received a vehicle solution followed 4 h later by either serotonin (66 µg/kg, i.v.) every 10 minutes for a further 4 h or the serotonin antagonist, cyproheptadine (3 mg/kg, i.v.) as a single injection. This procedure was performed in the breeding season and again in the non-breeding season, and plasma LH and prolactin concentrations were measured. During the breeding season, serotonin reduced LH pulse amplitude in ovariectomized hinds (P < 0.05) and cyproheptadine reduced LH pulse frequency in both ovariectomized and ovariectomized-thyroidectomized hinds (P < 0.05 and P < 0.01, respectively). In the non-breeding season cyproheptadine reduced LH pulse frequency and amplitude in ovariectomized-thyroidectomized hinds (P < 0.01). Serotonin did not affect plasma prolactin concentration, whereas cyproheptadine caused an increase (P<0.01) in plasma prolactin concentration in both the breeding and non-breeding seasons.

To determine whether these compounds acted at the level of the pituitary gland, in Experiment 2 was examined their effects on the responsiveness of the pituitary gland to an exogenous GnRH challenge in ovariectomized hinds (n = 5). During the breeding season and after induction of halothane anaesthesia, hinds received either a serotonin infusion (6.6. µg/kg i.v. per min for 15 minutes), cyproheptadine (3 mg/kg i.v. as a single dose) or vehicle. Five µg of GnRH was injected i.v. 10 minutes later. In the non-breeding season, either serotonin (66 µg/kg, i.v. 2 injections 10 min apart), cyproheptadine (3.0 mg/kg i.v., as a single dose) or vehicle was administered to hinds followed by a GnRH injection (5 . µg, i.v.) 30 minutes later. During the breeding season halothane anaesthesia eliminated endogenous pulses of LH. The response of plasma LH to exogenous GnRH was not altered by serotonin or cyproheptadine in either season.

These results support a stimulatory role for serotoninergic neurons on the hypothalamic GnRH pulse generator mechanism during the breeding season and indicated no effects of serotonin on LH release at the pituitary gland level in red deer hinds.