Lack of pineal-induced gonadal regression in dark-exposed or blind hamsters after surgical isolation of the medial-basal hypothalamus

Blinding or exposing hamsters to increased dark periods (L:D::1:23) caused involution of the reproductive organs. Involution of reproductive organs was absent in hamsters that were pinealectomized or subjected to complete surgical isolation of the medial-basal hypothalamus. This observation suggests that isolation of the medial-basal hypothalamus may have destroyed pathways which serve to transmit photic information to the pineal (e.g., medial forebrain bundle) or the site of action of the substance secreted by the pineal in blind or dark-exposed hamsters is outside of the cut, located in a neural structure responsible for inhibition of gonadotropin release.