Principalis- or parabrachial-projecting spinal trigeminal neurons do not stain for GABA or GAD

Retrograde transport and immunohistochemical double-labeling methods (Weinberg et al., 1985) were used to assess the distribution and projection status of spinal trigeminal (SpV) neurons that stain positively for glutamic acid decarboxylase (GAD) or γaminobutyric acid (GABA). Large bilateral injections of diamidino yellow into the rostral and lateral pons, inclusive of V nucleus principalis and the parabrachial nucleus, retrogradely labeled large numbers of cells in each SpV subnucleus. Many cells in SpV subnuclei caudalis, interpolaris, and oralis also exhibited GABA immunoreactivity; the largest numbers were in caudalis and the smallest numbers were in oralis. However, none of the GABA- or GAD-immunoreactive SpV cells were double-labeled with diamidino yellow, though some reticular neurons displayed both GABA and the retrograde tracer. This negative result refutes a previously offered hypothesis that SpV local-circuit neurons with principalis collaterals are GABA-ergic (Jacquin et al., 1989b). These data also indicate that parabrachial-projecting SpV neurons are not GABA-ergic.