Distribution of bulbospinal γ-aminobutyric acid-synthesizing neurons of the ventral respiratory group of the rat

Spinal respiratory motoneuron activity is controlled primarily by excitatory and inhibitory neurons in the medulla oblongata. To identify bulbospinal inhibitory neurons, immunohistochemistry for glutamic acid decarboxylase (GAD) was combined with retrograde labeling of projections to the C₄ ventral horn with Fluoro-Gold. GAD-immunoreactive bulbospinal neurons were located in the ventrolateral portion of the intermediate reticular nucleus, the ventral portion of the medial reticular nuclei, and the raphe and spinal vestibular nuclei. Small numbers of bulbospinal ventral respiratory group neurons were GAD immunoreactive. These neurons were distributed throughout the rostral ventral respiratory group and the Botzinger complex. Surprisingly, low numbers of Botzinger neurons, a population thought to be exclusively inhibitory, were GAD immunoreactive. These results suggest that the rostral ventral respiratory group and the Botzinger complex both contain heterogeneous bulbospinal neuron populations, only some of which have γ-aminobutyric acid (GABA)-mediated inhibitory control over phrenic motoneurons. Furthermore, the ventral respiratory group contained many GABAergic neurons that lacked bulbospinal projections.