HORMONAL AND NONHORMONAL AGENTS AT IMPLANTATION AS TARGETS FOR CONTRACEPTION

The processes leading to implantation and the establishment of pregnan cy involve hormonal and non-hormonal agents that offer opportunities a s targets for contraception. Hormonal agents include progesterone, lut eolytic factors (prostaglandin F-2 alpha) and embryonic signals (chori onic gonadotrophin, oestradiol-17 beta, interferon-tau) responsible fo r maintaining the corpus luteum. Non-hormonal agents include surface a ntigens (attachment and adhesion molecules), vasoactive agents, tissue -remodelling enzymes (matrix metalloproteinases) and inhibitors (TIMPs ), growth factors (epidermal growth factor and insulin-like growth fac tor families) and cytokines (such as leukaemia inhibitory factor, colo ny-stimulating factor-1, interleukin-1 (IL-1) and IL-6) associated wit h the pre-attachment period and the apposition, adhesion and invasion of the blastocyst. This review describes some of the hormonal and non- hormonal agents present at the time of implantation that may be exploi ted as targets for contraception in feral species. Particular attentio n is paid to the mouse as an experimental model and potential target s pecies. The considerable species differences which exist in the modes of implantation and placentation and the way in which the female 'reco gnizes' the presence of a viable conceptus offer a means of conferring species specificity on potential contraceptive targets for feral spec ies.