Access of dopamine to the median eminence and brain throughout local vascular pathways in sheep.

In female sheep, estradiol-dependent dopaminergic inhibition exerted by the A15 nucleus during long days (LD) results in a blockade of reproductive activity. This effect could involve the GnRH cell bodies or their terminals in the median eminence (ME). However, a vast majority of terminals of the A15 nucleus are located in neurohypophysis and only a few in the ME. Previously we demonstrated that tritiated dopamine (DA) was transferred from the venous blood of the cavernous sinus to the arterial blood supplying the brain. In the present paper, we tested the hypothesis that the transferred dopamine could reach further the brain and ME. Using isolated sheep heads harvested on short days vs. long days, we examined radioactivity in brain tissues following infusion of tritiated dopamine into the cavernous sinus. The experiment was performed in ovariectomized ewes treated with estradiol (E2) or vehicle. The mean level of radioactivity in brain was affected by season (p<0.001) and E2 (p<0.05) and was the highest during LD in E2-treated animals. In the next experiment on isolated sheep head we measured dopamine and its metabolites levels in blood and pituitary after infusion of non-radiolabeled dopamine. We observed an increase (p<0.01) in dopamine concentration in arterial blood but not in the brain. The pituitary was the only structure examined in which a tendency (p=0.06) towards increased dopamine concentration following dopamine infusion was observed. Thus, even if part of DA released from terminals within the posterior and intermediate lobes of the pituitary reaches the vessels of the ME through local vascular pathways, it is unlikely that it could affect the LHRH terminals located in ME. In addition, our results suggest that brain capillaries in the isolated head are able to maintain a functional blood brain barrier.