AJP - Regulatory, Integrative and Comparative Physiology, Vol 260, Issue 6 1194-R1199, Copyright © 1991 by American Physiological Society
Dopamine modulation of prolactin and vasopressin but not behavior on satiation of sheep
F. R. Bell, S. L. Lightman and A. Simmonds
Department of Biochemistry and Physiology, University of Reading, Whiteknights, United Kingdom.
We have investigated the rapid changes in plasma prolactin and arginine
vasopressin (AVP) associated with water satiation in ruminants. Sheep
deprived of water for 72 h were allowed voluntary access to ad libitum
water. Each sheep drank approximately 5 liters in a single draft. This was
performed in a head-down position, which was maintained for approximately
2-3 min and ceased rapidly with an abrupt movement to the upright position.
During dehydration, plasma sodium, osmolality, and AVP all increased
significantly, but prolactin was unaffected. On rehydration, AVP fell to
basal levels within 5 min, but prolactin increased as a short pulse, after
which it rapidly fell again. Plasma sodium and osmolality returned to
predehydration levels within 6 h of satiation. A possible role of
dopaminergic mechanisms in these responses was investigated by the
administration of the dopamine agonist bromocriptine or the antagonist
metoclopramide. Neither of these agents had any observable effects on the
drinking behavior of the sheep during water satiation. During
metoclopramide treatment, dehydration was associated with a marked fall in
prolactin, and on rehydration there was a prompt and very marked increase
in prolactin level. There was also an exaggerated increase of AVP during
dehydration and a decrease on rehydration. Bromocriptine had relatively
little effect on prolactin responses but prevented the drinking-associated
inhibition of AVP, demonstrating an unexpected dissociation between AVP
secretion and the stereotyped drinking response.
