Estrogen regulation of brain vasotocin secretion in the catfish Heteropneustes fossilis: An interaction with catecholaminergic system

Vasotocin (VT) is a basic neurohypophysial nonapeptide in non-mammalian vertebrates and is involved in diverse functions like osmoregulation, reproduction, metabolism and behavior. In this study, we report that estradiol-17β (E2) regulates brain and plasma VT secretion through the involvement of the catecholaminergic (CA) system. To demonstrate this, E2 level was altered through ovariectomy (OVX, 3 weeks) and replacement study with low and high E2 doses (0.1 and 0.5 μg/g body weight). CA activity was inhibited by treatment with α-methylparatyrosine (α-MPT; 250 μg/g body weight), a competitive inhibitor of tyrosine hydroxylase. VT was assayed by an enzyme immunoassay method. In the sham group, the low E2 dose produced 82% and 104% increase, respectively, in brain and plasma VT levels. The high E2 dose decreased the VT levels significantly. The low E2 dose decreased brain E2 but elevated plasma E2. In the high E2 group, the E2 level increased further in both brain and plasma. OVX resulted in a significant inhibition (69% and 25%, respectively) of both brain and plasma VT, which was correlated with low E2 levels. The low E2 dose not only reversed the inhibition, but increased the VT level in both brain and plasma in comparison to the sham groups. The high E2 replacement inhibited VT levels further low in both brain and plasma. The α-MPT treatment inhibited VT levels significantly in both sham and OVX groups. The drug treatment abolished partially the restorative effect of the low E2 dose in the ovariectomized fish. In the high E2 dose group, α-MPT decreased brain and plasma VT levels further low compared to the sham + 0.5 μg E2 group or OVX + 0.5 μg E2 group except the brain VT level, which increased in the OVX + 0.5 μg E2 group. It is inferred that E2 may exert biphasic effects on VT through the mediation of the CA system.

► Ovariectomy inhibits vasotocin secretion.
► E2 modulated VT secretion biphasically.
► α-MPT inhibits VT secretion.
► Low E2 dose reversed, and high E2 amplified the α-MPT effect.
► The interaction of E2, VT and catecholaminergic system is discussed.