Photoperiodism in the ewe: Endogenous circannual rhythms and olfactory mechanisms influencing reproductive activity
Jansen, Heiko Thomas
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https://hdl.handle.net/2142/19655
Description
Title
Photoperiodism in the ewe: Endogenous circannual rhythms and olfactory mechanisms influencing reproductive activity
Author(s)
Jansen, Heiko Thomas
Issue Date
1993
Doctoral Committee Chair(s)
Jackson, Gary L.
Department of Study
Veterinary Biosciences
Discipline
Veterinary Biosciences
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Animal Physiology
Biology, Veterinary Science
Language
eng
Abstract
Sheep exhibit periods of breeding and non-breeding activity that are correlated with annual changes in daylength. The seasonal nature and timing of reproductive activity in this species is thought to result from the complex interplay of endogenously generated rhythms with exogenous factors including photoperiod and social cues. These studies evaluated whether the expression of circannual rhythms is photoperiod-dependent and whether or not the olfactory system exerts a tonic influence on the hypothalmo-pituitary axis to modulate photoperiod responses. The majority of gonadal-intact ewes maintained for more than three years under two different constant photoperiod and restricted temperature conditions failed to express a circannual rhythm of breeding and non-breeding activity but expressed a clear circannual rhythm of prolactin secretion. Moreover, most ewes expressed prolonged periods of breeding activity demonstrating that the basal reproductive state of ewes housed under favorable environmental conditions is one of activation. The results also suggest that the expression of an underlying circannual oscillator controlling reproductive activity was either masked or damped. Unlike in the hamster, olfactory bulb removal in the ewe did not influence the gonadotropin or prolactin responses to changing photoperiod nor did it affect basal gonadotropin or prolactin concentrations. Together the results indicate that the olfactory bulbs do not exert a tonic influence on the hypothalamus in this species. To determine whether this apparent species difference reflected a fundamental anatomical difference, the connections between the olfactory bulb formations with the rest of the brain were determined using the anterograde and retrograde neuronal tracer wheat germ agglutinin-conjugated horseradish peroxidase. The connections of the sheep olfactory bulb bore similarities to both rodents and primates. Furthermore, a direct projection from the main olfactory bulb to the medial amygdala provides a potential mechanism whereby this structure may influence luteinizing hormone secretion.
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