The effects of iontophoretic clonidine on neurones in the rat superficial dorsal horn

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Abstract
Clonidine and glutamate were applied by iontophoresis to cells in the superficial 3 laminae of the spinal cord in the anaesthetised rat. Only cells that were excited by glutamate (up to 150 nA) were studied. Some spontaneously active cells could be excited by clonidine (up to 100 nA). However, when applied to non-spontaneous cells, clonidine had no effect at any dose level. When ejected in a cyclic pattern alternating with glutamate ejection, clonidine powerfully amplified the response of many cells to the glutamate stimulus. This effect was seen only on cells with small-amplitude spikes and low-threshold (LT) receptive fields. The amplification was often sustained and could outlast the clonidine ejection by several minutes. Clonidine had a long-lasting inhibitory effect on the responses to glutamate of cells with high-threshold (HT) or wide-dynamic-range (WDR) receptive fields. Clonidine appeared to selectively decrease the responsiveness of WDR cells to noxious stimulation. It is suggested that an amplification of the response of LT cells to other excitatory inputs could contribute to the analgesic action of clonidine.