Evidence for co-transmitter role of neuropeptide Y in the pig spleen

1. The possible involvement of neuropeptide Y (NPY) in relation to noradrenaline (NA) and adenosine triphosphate (ATP) mechanisms in the sympathetic nervous control of the vascular tone and capsule contraction in the blood perfused pig spleen was investigated in vivo. 2. Local injections or infusions of NA, NPY and alpha-, beta-methylene ATP (mATP) caused vasoconstriction (perfusion pressure increase) and capsule contraction (increased venous blood flow). ATP only evoked vasodilatation. NPY was about 50 fold more potent than NA as a vasoconstrictor, and the NPY response was more long-lasting. Reserpine treatment did not change the effects of NPY. 3. Electrical stimulation of the splenic nerves in control animals caused a frequency-dependent, guanethidine-sensitive output of both NPY-like immunoreactivity (-LI) and NA, suggesting co-release. The output of NPY-LI relative to NA was enhanced at high frequency stimulation. Furthermore, alpha-adrenoceptor blockade by phentolamine enhanced both the output of NPY-LI and NA while inhibition of the neuronal uptake of NA with desipramine reduced the low frequency stimulation-evoked overflow of NPY-LI. Preganglionic denervation did not change the output of NPY-LI or NA. 4. Reserpine treatment reduced both the splenic content of NA and NPY-LI. Preganglionic denervation inhibited the reserpine-induced depletion of the NPY content but not of NA in terminal areas. The stimulation-evoked NPY overflow was markedly enhanced, especially at low-frequency stimulation after reserpine, and the plasma levels of NPY-LI in the venous effluent were then in the nmolar range (i.e. where exogenous NPY induced vasoconstriction). The perfusion-pressure increase upon stimulation in reserpine-treated, preganglionically-denervated animals was highly correlated (r = 0.91) to the NPY overflow. The functional 0.5 Hz responses were reduced after reserpine, while at higher frequencies the functional effects were of similar magnitude to controls but longer-lasting. 5. Tyramine induced a release of NA but not of NPY-LI. Furthermore, the increase in perfusion pressure induced by tyramine was absent after reserpine. 6. After tachyphylaxis to the vasoconstrictor effects of mATP, the nerve stimulation-evoked, functional response as well as the NA and NPY-LI overflow were unchanged. After reserpine treatment, both the perfusion-pressure increase and NPY-LI overflow to nerve stimulation were reduced after mATP tachyphylaxis. 7. In conclusion, release of NPY rather than ATP may explain the long-lasting, non-adrenergic, splenic functional responses in reserpinized animals upon sympathetic stimulation. However, NA is most likely the main splenic transmitter when low-frequency stimulation is used under control conditions.