Repeated renal and splenic sympathetic nerve stimulation in anaesthetized pigs: maintained overflow of neuropeptide Y in controls but not after reserpine.

The overflow and the arterial vascular effects of neuropeptide Y (NPY) in response to repeated sympathetic nerve stimulation of kidney and spleen were investigated in anaesthetized pigs. The responses under control conditions were compared to those evoked in pigs with tissue stores of noradrenaline (NA) selectively depleted by reserpine pretreatment combined with sympathetic nerve transection. The renal and splenic sympathetic nerves were repeatedly stimulated at 1 h intervals with one 5 Hz stimulation for 48 s and transmitter overflow determined. Between these stimulations, 5 min stimulations with bursts of 20 Hz (for 1 s every 10 s) were given in order to induce a depletion of nerve transmitter. In the control group, overflow of NPY and NA and vasoconstrictor responses were almost identical for the 5 consecutive stimulations in the kidney, whereas in the spleen the parameters showed a slight tendency to be reduced. In the reserpine-treated group, the initial evoked overflow of NPY was increased 8-fold and 3-fold in the kidney and spleen, respectively, compared to the control group. Upon each subsequent stimulation the overflow decreased gradually, in parallel with the evoked vasoconstrictor response. After a 2 h recovery period no change in evoked overflow of NPY compared to the amount released by the previous stimulation was observed. The present study illustrates, the high capacity of maintenance of not only NA but also NPY overflow and vascular responses in control conditions, whereas the enhanced release of NPY in the absence of NA cannot be maintained. It is therefore possible that the NA-mediated prejunctional feedback mechanism is important for the maintenance of a constant NPY release in situations of high sympathetic activation.