Importance of Retinal Photoreceptors to the Photoperiodic Control of Seasonal Breeding in the Ewe 2

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Abstract
Two experiments were performed to determine whether the eyes are necessary for photoperiodic control of reproduction in ewes. Intact and estradiol-treated ovariectomized (OVX + E) ewes were housed in each of 2 photoperiod-controlled rooms with a vasectomized ram and subjected to 90 day alternations between long and short days. Prior to blinding, long days initiated anestrus in intact ewes and a suppression of serum luteinizing hormone (LH) levels in OVX + E ewes; short days caused onset of estrous cycles and an increase in LH levels in the intact and OVX + E ewes, respectively. After 1.5 yr of such photoperiodic control, all ewes were blinded by bilateral orbital enucleation. Photoperiodic control was lost following blinding, but circannual alternations between cyclicity and anestrus or high and low LH levels, were maintained in most ewes for the remaining 2.5 yr of the study. In 1 group of OVX + E ewes, serum LH levels remained synchronized to the 90 day shifts in photoperiod for .apprx. 1 yr after blinding. Once the sighted ram was removed from the room, however, the 90 day rhythm in LH disappeared and a circannual pattern of LH became evident, suggesting that blind ewes may received photoperiodic information from a sighted ram. This possibility was supported by the results of the 2nd experiment in which 12 additional OVX + E ewes were blinded and exposed to 90 long days and 90 short days in the absence of a sighted ram. In these ewes, serum LH levels were not controlled by the changes in photoperiod. The eyes are necessary for perception of photoperiod in the ewe. Ewes have an endogenous circannual rhythm of reproduction and/or they can be controlled by other environmental signals in the absence of photoperiodic input. It is hypothesized that blind ewes can receive photoperiodic information indirectly from a sighted ram.