Biogenic amines and the activity of the hypothalamo-pituitary-ovarian axis in ewes.

An important feature of communication between the central nervous system and the pituitary-ovarian axis is the pattern of pulsatile discharge of hypothalamic luteinizing hormone releasing hormone (LHRH). The discharge of LHRH is under control of noradrenergic, dopaminergic, and serotoninergic systems of the brain. These systems intervene between external and internal signals (e.g. photoperiod and gonadal steroids) and LHRH output. In the ewe, noradrenaline, dopamine and serotonin of the brain are implicated in the control of LHRH output, and hence luteinizing hormone (LH) release, during both the anoestrous and breeding seasons. These amines are involved in steroid-dependent and steroid-independent regulation of LHRH/LH discharge. An interplay of inhibitory versus excitatory influences of these amines on LHRH/LH release appears to govern the pattern of LHRH/LH output during the annual reproductive cycle and the ovulatory cycle. A concise overview of this topic will be provided.