Distribution of GABA and glycine in the lamb nucleus of the solitary tract.

The inhibitory amino acids gamma-aminobutyric acid (GABA) and glycine are involved in several nucleus of the solitary tract (NST)-mediated functions. The distribution of these amino acids in the NST of the lamb, a species frequently used in investigations of NST-mediated behaviors, has not been described. Therefore, this study was designed to investigate the distribution of GABA and glycine in the lamb NST using immunohistochemistry. Both GABA and glycine immunoreactive cells and puncta were unevenly distributed in the lamb NST. The highest density of GABA immunoreactive cells was found in the intermediate zone of the NST, medial to the solitary tract (ST). High to moderate levels of puncta labeling were observed throughout the NST, particularly around the ST in intermediate and caudal zones. Moderate to low levels of glycine immunoreactivity were observed, with most glycine immunoreactive cells and puncta found in the caudal two-thirds of the NST in the medial, ventrolateral and dorsomedial NST. Only a few glycine immunoreactive cells and puncta were found in the rostral zone of the NST. The widespread distribution of GABA and glycine immunoreactivity in intermediate and caudal zones of the NST suggests that these inhibitory amino acids play an important role in modulating NST-mediated functions like swallowing, respiration and cardiovascular regulation in the lamb. The much higher density of GABA immunoreactivity compared to glycine immunoreactivity in the rostral zone of the NST suggests that GABA, but not glycine, is an important neurotransmitter in the processing of taste information by the lamb NST.