Role of Histamine in Implantation: Inhibition of H istid me Decarboxylase Induces Delayed Implantation in the Rabbit

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Abstract
Implantation in the rabbit occurs between 156-168 h postcoitum (p.c.), and by 175 h p.c. implanting blastocysts cannot normally be flushed intact from the uterine lumen. The effect of an intraluminal injection of DL-α-methylhistidine (DL-α-MH), a specific inhibitor of histidine decarboxylase, on implantation in the rabbit was studied. An intraluminal injection of 2.5 mg of DL-α-MH at 102 h p.c. interfered with implantation in the rabbit sacrificed at 175 h p.c. Implantation rates were reduced by more than 60% and this was reflected in the increased number of morphologically expanded normal blastocysts which were flushed from the uterine lumen intact. On the other hand, the same treatment did not significantly reduce the number of embryos implanted nor the viability of fetuses when examined on Day 12. However, 4 mg of DL-α-MH drastically reduced the implantation rates as well as the viability of the implanted embryos on Day 12. The intraluminal injections of various doses of L-histidine did not significantly affect the implantation rates or the viability of the fetuses. These results as well as our previous findings suggest that implantation may have been delayed for some time due to inhibition of embryonic histamine synthesis by DL-α-MH.

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