GABA‐synthesizing neurons were identified in the medulla of the rat by peroxidase‐antiperoxidase (PAP) immunohistochemistry for glutamic acid decarboxylase (GAD). Using diaminobenzidine (DAB) either alone or intensified with silver, a relatively large number of GAD‐immunoreactive neurons were evident within the reticular formation, raphe nuclei and vestibular nuclei. In all these areas, profuse GAD‐immunoreactive varicosities appeared to contact the soma and dendrites of both non‐GABA and GABA neurons. These observations suggest that GABA neurons may act as interneurons or local projection neurons within the medulla and accordingly exert a potent inhibitory and/or disinhibitory control on bulbar projection neurons.

Within the ventral reticular formation (pars alpha and ventralis of the gigantocellular reticular field) and raphe magnus, large numbers of prominent GAD‐immunoreactive neurons resembled in size and morphology and overlapped in distribution the serotonin‐immunoreactive neurons of the same regions. However, by sequential double immunostaining utilizing DAB as a chromogen for serotonin (5‐HT) and benzidine dihydrochloride (BDHC) for GAD, it was found that GAD‐containing neurons were distinct from 5‐HT‐containing neurons.

Following injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA‐HRP) into the upper cervical spinal cord and combined processing for WGA‐HRP (using tetramethylbenzidine [TMB] with cobalt) and immunohistochemistry (with DAB), a contingent of spinally projecting neurons were found to contain GAD. The GAD‐immunoreactive reticulo‐and raphe‐spinal neurons were most frequent within the pars alpha and ventralis of the gigantocellular reticular fields and the raphe magnus, where they were approximately equal in number to the coexistent, but distinct 5‐HT spinally projecting neurons. GABA neurons of the medulla may thus contribute directly to the bulbar inhibitory influence upon spinal sensory and motor systems.