Project 23

Sexually stimulated neurogenesis in sheep; fact or fantasy?

Dr Jenny Rodger, Dr Penny Hawken, Prof Graeme Martin (School of Animal Biology)

It has long been assumed that the mammalian brain completes cell division during early life, but we now know that it continues in several regions, especially the olfactory system. New neurons originate from proliferating cells in the subventricular zone then migrate rostrally along a pathway known as the rostral migratory stream towards the main olfactory bulb (Rochefort et al 2002). On arrival, they differentiate into local interneurons and functionally integrate into circuits (Carlen et al 2002). Integration is faster after stimulation by novel odours.

In addition to this process, environmental factors modulate the constitutive turnover of neurons in many brain regions, including the hypothalamus; as observed in female voles exposed to males (Fowler et al 2002) and in hamsters following a change in photoperiod (Huang et al 1998). Oestrogen also regulates the rate of neurogenesis in the hippocampus of rats and voles (Tanapat et al 1999; Ormerod & Galea 2001), although whether the rate increases or decreases may depend on whether the species is a reflex or spontaneous ovulator (Banasr et al 2001).

The introduction of rams to anoestrous (seasonally reproductively inactive) ewes stimulates an almost instantaneous increase in luteinizing hormone, which under certain physiological conditions and in certain breeds of sheep is sufficient to reinstate reproductive activity. The neurological mechanism controlling this phenomenon, termed the ‘ram effect’, is currently unknown. However as exposure to male pheromones stimulates neurogenesis in female prarie voles (Fowler et al., 2002) it is likely that neurogenesis is also involved in this phenomenon. Furthermore the occurrence and magnitude of neurogenesis may be a key determinant of whether ewes ovulate or not in response to the ram effect.

We hypothesise that

All tissue is available as part of a separate experiment. Methods will involve brain cryosectioning, immunocytochemistry, immunofluorescence and confocal microscopy, stereological methods of cell counting