The effect of intrathecal neuropeptide Y on the flexor reflex in rats after carrageenan-induced inflammation

We examined the effects of intrathecal (i.t.) administration of neuropeptide Y (NPY) on the excitability of the flexor reflex in normal rats and 24 h after inflammation induced by subcutaneous carrageenan. In normal rats, i.t. NPY at low doses (10 and 100 ng) caused a brief facilitation of the flexor reflex with no subsequent depression. At higher doses (1 and 10 ng), the effect of NPY was mainly inhibitory, causing substantial and usually prolonged depression of the flexor reflex. At 24 h after the injection of carrageenan, when inflammation was at its peak, the magnitude of the reflex was increased and discharge duration became prolonged. I.t. NPY produced similar pattern of dose-dependent facilitatory and depressive effects on the flexor reflex. The facilitatory effect of i.t. NPY, particularly for the higher doses, was significantly enhanced in inflamed rats compared to normals. In contrast, the depressive effect of high doses of i.t. NPY was unchanged. These data suggest that the changes in levels of NPY and NPY receptors in the spinal cord known to occur after inflammation, are associated with an increased excitatory effect of this peptide.