Distribution of neurons that express Calca mRNA in the PB region. Calca-expressing neurons concentrate in the LC and external lateral PB (PBeL), with fewer in the ventral cochlear nucleus (VCo), V, medial PB (PBm), and surrounding parts of the reticular formation. (a–f) Plots of every neuron labeled by in situ hybridization for Calca mRNA (red) across six rostro-caudal levels through the PB region (approximate distance caudal to bregma is shown at the bottom right of each panel). Other abbreviations: DTN, dorsal tegmental nucleus; PSV, principal sensory trigeminal nucleus.