Mustard oil excites but does not inhibit nociceptive dorsal horn neurones in the rat: a presumed effect on A-delta fibres.

1. Mustard oil was applied topically in concentrations of 10-20% to the excitatory and inhibitory nociceptive receptive fields in glabrous and hairy skin of the anesthetized rat while recording the activity of nociceptive dorsal horn neurones. The noxious stimulus was radiant heat which is known to activate C-fibres in glabrous and hairy skin. 2. Mustard oil had little effect when applied to glabrous skin and this was attributed to poor penetration of the skin. 3. Mustard oil excited cells in the dorsal horn which were excited by noxious heat in the receptive field on hairy skin. 4. Mustard oil excited cells in the dorsal horn which were inhibited by noxious heat in the receptive field on hairy skin. 5. Inhibitory effects of mustard oil were never seen, even when applied to receptive fields in which noxious heating caused inhibition. 6. The excitatory effects of mustard oil on cells inhibited by noxious heating of the skin are attributed to the reported activation of A-delta fibres which probably masked any C-fibre activation.