Release of E prostaglandins into the cerebrospinal fluid and its inhibition by melatonin after cervical stimulation in the rabbit.

The release of the E type of prostaglandin (PGE) into the cerebrospinal fluid of mature female rabbits was studied using an indwelling catheter in the cisterna magna. The cervix of the rabbit was stimulated 5 min after the central injection of either vehicle or melatonin solution. PGE was measured by RIA, and in unstimulated animals, the level of PGE remained steady at about 4 ng/ml. After stimulation of the cervix of control rabbits, PGE levels rose by 31 ng/ml 40--70 min after the initiation of stimulation in those animals which subsequently ovulated. PGE levels rose by only 15 ng/ml 70--115 min after stimulation in those rabbits which did not ovulate. In the melatonin-treated rabbits, PGE levels remained close to those in unstimulated controls, and ovulation did not occur. Thus, stimulation of the cervix of the rabbit produces a rise in PGE levels in cerebrospinal fluid. This endogenous PGE surge may, in turn, cause the release of LHRH and LH and induce ovulation if levels exceed a critical threshold. A weaker stimulus leads to only a smaller surge of PGE, insufficient to cause ovulation. Melatonin blocked the stimulation-induced rise in cerebrospinal fluid PGE levels and ovulation, and appears to exert its antigonadotropic activity in the rabbit by the inhibition of central PGE synthesis.