A semi-quantitative analysis of Fos expression by mustard oil

In the present study, a semi-quantitative analysis of Fos expression by mustard oil was performed. For this purpose, mustard oil was applied to the skin of the right hind foot of Wistar rats at various concentrations: 5%, 10%, 30%, 50%, 80% and 100% in liquid paraffin. The distribution and number of activated Fos-positive cells in the stimulated side (ipsilateral) and contralateral side of the spinal cord were investigated following the application. The ED50 of the response was also determined. The number of Fos-labelled cells gradually increased in a dose dependent manner in both sides of superficial layers (laminae I-II) of the spinal cord with increasing concentration of mustard oil. The increase between the doses was found significant in the ipsilateral superficial layers. The increase was significant in the contralateral superficial layers at concentrations above 50%. Very few Fos-labelled cells were observed around the central canal region in all concentrations. Higher doses of the mustard oil did not increase the number of activated cells in the deeper layers. However, the expression in the deeper layers (laminae III-X) does not show a consistent trend. Also, none of the concentrations used produced labelling in neurons of the deep ventral horn neurons or in motor neurons. Forty percent (40%) of mustard oil gave an approximately 1/2 maximum response i.e. an approximate ED 50. This may be important for studies using intrathecal application of antagonist following the mustard oil activation of skin nerve fibres.