The central consequences of the application of capsaicin to one peripheral nerve in adult rat.
This paper reviews the central consequences of local application of capsaicin to one nerve in adult animals. 1) Marked chemical changes occur in the central terminals of C fibres. These include depletion of the enzyme FRAP and the peptides SP, CCK, somatostatin, CGRP and an increase of VIP. Maximal depletions occur if the nerve is soaked with capsaicin solutions with a concentration higher than 3 mM. The depletion begins by 7 days and is complete by 11. Recovery begins at about 110 days and is largely complete by 200. Our studies have concentrated on the effects of 40 mM capsaicin examined 14 days after the application. 2) Capsaicin treatment of a peripheral nerve decreased the ability of C fibres in that nerve to excite or to inhibit spinal cord cells. It produces a marked expansion of receptive fields of some cells in the dorsal horn which respond to A fibre stimulation. It is proposed that this change is not due to anatomical changes but to disinhibition. A further example of receptive field expansion is seen after treatment of the mouse infraorbital nerve which defocuses the normally precise projection of individual whiskers onto single cells in the barrel field of the somatosensory cortex. 3) Behavioural consequences follow the treatment of one adult nerve with capsaicin. In the area subserved by the treated nerve, there is a raised threshold to response to chemical and thermal stimuli, no change in the response to mechanical stimuli and an increase of autotomy following nerve section. 4) The aim of the experiments was to determine the role of C fibres in producing the changes seen in spinal cord following peripheral nerve section. Capsaicin treatment of nerve imitates the central effect of complete nerve section in certain important ways. Both result in a marked expansion of the receptive field of some cells. The effect is produced by a change of chemical transport. The results show that C fibres influence the connection of A fibres onto spinal cord cells.
Wall
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Department of Anatomy and Embryology, University College London, England.
D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D002211:Capsaicin / Q000008:administration & dosage / Q000494:pharmacology*; D004594:Electrophysiology; D009412:Nerve Fibers / Q000187:drug effects / Q000502:physiology; D009479:Neuropeptides / Q000378:metabolism; D010525:Peripheral Nerves / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D013116:Spinal Cord / Q000187:drug effects / Q000502:physiology
D009479:Neuropeptides; D002211:Capsaicin