The laminar location of GABA-immunoreactive and non-immunoreactive PPD neurons. A plot showing the distribution of all of the PPD cells that were analysed for GABA-immunoreactivity in the three rats that were fixed with glutaraldehyde/formaldehyde. Each symbol represents a single PPD neuron. The GABA-immunoreactive cells are shown as filled circles and the GABA-negative cells as open circles. Note that while the majority of cells (143/208) in the superficial dorsal horn (laminae I-II) are GABA-immunoreactive, all of the PPD cells ventral to this region are GABA-negative.