Fos expression reveals what dorsal horn neurons are activated during allodynia.

<p>(A) Images of Fos positive cell nuclei in the MDH; Fos was expressed following light brushing of the ipsilateral lip after intracisternal strychnine (top right). Contralateral side of the MDH in the same animal is shown on top left. Only sparse Fos positive cell nuclei were found in the MDH following light brushing of the lip after intracisternal aCSF (bottom right). Bar: 200 µm. (B) Bar histograms summarizing mean Fos expression in the different laminae of the ipsilateral MDH following light brushing of the lip after intracisternal strychnine or aCSF (n = 5/group). I-IIo: laminae I and outer II; IIi-IIIo: laminae inner II and outer III; IIIi-IV: laminae inner III and IV; V: lamina V. *, P<0.05; **, P<0.01, strychnine <i>vs.</i> aCSF. (C) Camera lucida diagrams representing the coronal level at which the retrograde tracer fluorogold deposition was maximal in the parabrachial area of 3 rats (left). Image of lamina I retrogradely labeled neurons in the MDH (top right; bar: 100 µm) and image of a retrogradely labeled neuron expressing Fos (bottom right, arrow; bar: 40 µm).