Effects of the fish spawning inducer ovaprim on vasotocin receptor gene expression in brain and ovary of the catfish Heteropneustes fossilis with a note on differential transcript expression in ovarian follicles

- Vasotocin receptor genes (v1a1, v12 and v2a) are expressed in the brain and ovary of the catfish.
- In the ovary, the genes show a compartmental distribution.
- v1a1 and v1a2 are expressed in the follicular layer and v2a in oocytes.
- Ovaprim induced periovulatory changes in the VT receptor gene expression, with high fold-increase from 8 h to 16 h.
- The peak expression pattern coincided with final oocyte maturation and ovulation.

Ovaprim (OVP), a commercial formulation of a salmon GnRH analogue and the dopamine receptor-2 blocker domperidone, is a successful spawning inducer for fish breeding. It induces a preovulatory surge in LH, which stimulates the synthesis of a maturation-inducing steroid (MIS, 17,20β-dihydroxy-4-pregnen-3-one) that initiates germinal vesicle breakdown (GVBD) and ovulation. Coincidently, the OVP treatment also stimulates vasotocin (VT) secretion in the brain and ovary of the catfish Heteropneustes fossilis that also stimulates the synthesis of the MIS. VT mediates its effect through V1- and V2-type receptors. In the present study in the catfish, we report that OVP stimulates the expression of VT receptor genes v1a1, v1a2 and v2a in the brain and ovary. A single intraperitoneal administration of OVP (0.5 μL/g body weight) or incubation of post-vitellogenic ovarian follicles with 5 μL/mL OVP, for 0, 4, 8, 12, 16, and 24 h stimulated ovulation and GVBD, respectively, in a time-dependent manner. The OVP treatment in vivo stimulated brain VT receptor transcript levels 4 h onwards. The peak expression was noticed at 12 h (v1a1), 8 and 12 h (v1a2), and 8, 12 and 16 h (v2a), coinciding with FOM and ovulation. The VT receptor genes are expressed in the ovarian follicles compartmentally; both v1a1 and v1a2 are expressed in the isolated follicular layer (theca and granulosa) but absent in denuded oocytes. V2a is expressed in the denuded oocytes and not in the follicular layer. The OVP injection stimulated the v1a1 and v1a2 expression from 4 h onwards in both intact follicle and isolated follicular layer, the peak expression was observed at 16 h. The v2a expression was up-regulated in both intact follicles and denuded oocytes at 4 h (denuded oocytes) or 8 h (intact follicle) onwards with the peak expression at 12 h and 16 h (denuded oocytes) or at 16 h (intact follicles). Under in vitro conditions, the OVP incubations elicited similar pattern of changes with the peak stimulation at 16 h for all the genes. In conclusion, the VT receptor genes are differentially expressed in the ovarian follicles and OVP induced periovulatory stimulation of the VT receptor genes, coinciding with FOM and ovulation.