Response properties of neurons in the rostroventromedial medulla of neuropathic rats: attempted modulation of responses by [1DMe]NPYF, a neuropeptide FF analogue.

We determined whether chronic neuropathy changes response properties of neurons in the rostroventromedial medulla of rats, and whether (d-Tyr)L(Me-Phe)QPQRF-amide, a neuropeptide FF analogue, in the periaqueductal gray produces changes in responses of rostroventromedial medullary neurons that might underlie its antiallodynic effect described earlier. Single unit recordings of medullary neurons were performed in lightly anesthetized neuropathic and control animals. Spontaneous activity and the responses to noxious thermal and mechanical stimulation of the hind paw were determined with and without administration of (d-Tyr)L(Me-Phe)QPQRF-amide. The neurons were classified into three groups: ON-neurons increased, OFF-neurons decreased, and NEUTRAL-neurons did not change their discharge rate prior to a limb withdrawal induced by noxious stimulation of the skin. Spontaneous activity and heat-evoked responses of ON-neurons were not different between neuropathic and control animals, whereas their mechanically evoked responses were reduced in neuropathy. Response properties of OFF-neurons were not different between neuropathic and control animals. Spontaneous activity of NEUTRAL-neurons was not different between neuropathic and control animals. (d-Tyr)L(Me-Phe)QPQRF-amide in the periaqueductal gray had no significant effect on evoked responses or spontaneous activity of ON- or OFF-neurons, independent of the experimental group. However, (d-Tyr)L(Me-Phe)QPQRF-amide produced a significant attenuation of spontaneous activity of NEUTRAL-neurons in neuropathic animals. In a behavioral study performed in unanesthetized animals it was found that intrathecal administration of methysergide, a serotonin antagonist, selectively attenuated neuropathic symptoms. Also, light pentobarbitone anesthesia markedly attenuated, but did not abolish, behaviorally determined neuropathic symptoms. From these results we suggest that NEUTRAL-neurons of the rostroventromedial medulla may have a role in neuropathy and they may be involved in attenuation of mechanical hypersensitivity by (d-Tyr)L(Me-Phe)QPQRF-amide in the periaqueductal gray. It is proposed that in neuropathy the synaptic effects of descending impulses from medullary NEUTRAL-neurons on their axonal targets in the spinal cord are changed so that this contributes to mechanical hypersensitivity, due to mechanisms that are at least partly serotoninergic.