Neuropeptide Y changes the excitability of fine afferent units in the rat knee joint.

1. The aim of the present study was to examine the effects of the sympathetic co-transmitter Neuropeptide Y on primary afferent nerve fibres of the rat knee joint. The responses to passive joint rotations at defined torque were recorded from 41 slowly conducting afferent nerve fibres (0.9 - 18.8 m s(-1)) innervating the knee joint capsule. 2. About 70% of the joint afferents were significantly affected in their mechanosensitivity by topical application of Neuropeptide Y. Significant effects occurred at a concentration of 10 nM. 3. Decreased mechanosensitivity was observed in about 40% of nerve fibres, whereas 30% of the units increased the mechanosensitivity. In addition, in about 35% of the fibres resting activity was induced or increased. Neither the conduction velocity nor the mechanical threshold of the units correlated with the described effects of Neuropeptide Y. 4. NPY(13--36), a specific Y2-receptor agonist, only modulated the mechanosensitivity, with no effect on the resting activity. The effects on the mechanosensitivity were similar to Neuropeptide Y, i.e. increase and decrease of the response. 5. Studies with the Y1-agonist (Leu(31), Pro(34))-NPY showed that activation of the Y1-receptor predominantly resulted in an enhanced mechanosensitivity and an induction or increase of a resting activity. The opposite effect was observed by application of BIBP 3226 BS, a Y1-receptor antagonist. 6. In conclusion, these data indicate that Neuropeptide Y affects the excitability of sensory nerve fibre endings.