The role of the paraventricular nucleus (PVN) of the
hypothalamus in the cardiovascular response induced
by blood volume expansion was examined in
anaesthetised rabbits, in which simultaneous recordings
were made from a renal sympathetic nerve and one of other
sympathetic nerves, the inferior cardiac nerve, a splanchnic nerve and
an adrenal nerve. Activation of PVN neurones, by discrete injections
(25-100 nl) of d,l-homocysteic acid (DLH, 0.2 M) produced one main
pattern of sympathetic nerve activity accompanying a pressor response
(57-86 % of PVN sites). This was a decrease in renal sympathetic
activity (27 +/- 12 %) and an increase in splanchnic (60 +/- 12 %), adrenal
(31 +/- 9 %) and cardiac (42 +/- 8 %) sympathetic activity. Sites in the PVN
from which these combinations of nerve activity were obtained were not
confined to a specific subnucleus. An increase in renal sympathetic
activity which was reversed to a decrease by reducing the volume of
DLH injected was obtained at 10 sites in the PVN. These sites were
mainly located in the dorsal parvocellular subnucleus. Varying
combinations of sympathetic activation were obtained at a minority of
sites. It is concluded that the PVN can evoke a differential pattern of
sympathetic discharge which may be functionally significant in the
control of blood volume regulation, as it mimics that seen on acute
volume expansion.