Neural pathways associated with the mesencephalic inhibitory influence on gonadotropin secretion

Summary The ascending pathways which convey mesencephalic stimuli capable of inhibiting gonadotropin secretion have been investigated. Stimulation by electrolytic deposition of iron (1 mA, 10 sec) from stainless steel unipolar electrodes was used. The inhibitory impulses originated by stimulating the midbrain are conveyed to the hypothalamus by the dorsal longitudinal fasciculus (DLF) and medial forebrain bundle (MFB). This was indicated by the facts that: (1) ovulation and LH secretion were blocked after stimulation of the DLF or the MFB at hypothalamic levels, and (2) transection of either MFB or DLF prevented the blockade of ovulation which follows stimulation of the ventral tegmental area (VTA). The blocking effect of VTA stimulation was also suppressed by transection of the medial corticohypothalamic tract (CHT), indicating that the hippocampus is also involved. On the other hand stimulation of the MFB resulted in blocked ovulation when the DLF or the CHT were interrupted independently, but not when both tracts were cut simultaneously, showing that the impulses can be conveyed to the hypothalamus by either one of these two pathways. Stimulation of the DLF exerts a blocking effect on ovulation independently of other hypothalamic afferents and depresses serum LH concentration in ovariectomized-estrogen primed rats in which the preoptic-hypothalamic connections have been interrupted, indicating that the inhibitory impulses have direct access to the hypophysiotropic area. It can be concluded that the mesencephalic inhibitory signals are transmitted to the hypothalamus directly via the DLF or through the MFB-hippocampus-CHT system.