Glycine-immunoreactive synaptic terminals in the nucleus tractus solitarii of the cat: Ultrastructure and relationship to GABA-immunoreactive terminals

Abstract

Postembedding immunogold labeling methods applied to ultrathin and semithin sections of cat dorsomedial medulla showed that neuronal perikarya, dendrites, myelinated and nonmyelinated axons, and axon terminals in the nucleus tractus solitarii contain glycine immunoreactivity. Light microscopic observations on semithin sections revealed that these immunoreactive structures were unevenly distributed throughout the entire nucleus. At the electron microscopic level, synaptic terminals with high levels of glycine-immunoreactivity, assumed to represent those releasing glycine as a neurotransmitter, were discriminated from terminals containing low, probably metabolic levels of glycine-immunoreactivity, by a quantitative analysis method. This compared the immunolabeling of randomly sampled terminals with a reference level of labeling derived from sampling the perikarya of dorsal vagal neurones. The vast majority of these "glycinergic" terminals contained pleomorphic vesicles, formed symmetrical synaptic active zones, and targeted dendrites. They appeared to be more numerous in areas of the nucleus tractus solitarii adjoining the tractus solitarius, but rather scarce caudally, medially, ventrally, and in the dorsal motor vagal nucleus. In a random analysis of the entire nucleus tractus solitarii, 26.2% of sampled terminals were found to qualify as glycine-immunoreactive. In contrast, boutons immunoreactive for gamma-aminobutyric acid (GABA) were more evenly distributed throughout the dorsal vagal complex and accounted for 33.7% of the synaptic terminals sampled. A comparison of serial ultrathin sections suggested three subpopulations of synaptic terminals: one containing high levels of both GABA- and glycine-immunoreactivities (21% of all terminals sampled), one containing only GABA-immunoreactivity (12.7%), and relatively few terminals (5.2%) that were immunoreactive for glycine alone. These results were confirmed by dual labeling of sections using gold particles of different sizes. This study reports the first analysis of the ultrastructure of glycinergic nerve terminals in the cat dorsal vagal complex, and the pattern of coexistence of glycine and GABA observed provides an anatomical explanation for our previously reported inhibitory effects of glycine and GABA on neurones with cardiovascular and respiratory functions in the nucleus tractus solitarii.

Synaptic Responses of Neurons Controlling the Parotid and von Ebner Salivary Glands in Rats to Stimulation of the Solitary Nucleus and Tract

“…Although glycine has been shown in the NST using immunohistochemical techniques (Aprison et al 1969;Cassell et al 1992;Saha et al 1999;Sweazey 1996), IPSPs in this study did not have a glycinergic component, suggesting the source of the glycine for inhibitory input may possibly derive from descending connections (Kobayashi et al 1997;Matsuo 1999a) or be produced locally (Danysz and Parsons 1998). In addition to its role as an inhibitory neurotransmitter, glycine is also a coagonist with glutamate at NMDA receptors (Baptista and Varanda 2004;Turecek and Trussell 2001).…”

Co-localisation of markers for glycinergic and GABAergic neurones in rat nucleus of the solitary tract: Implications for co-transmission

“…Overlapping, yet characteristic distributions of GABA and glycine containing structures in the NTS have been reported in immunohistochemical studies on the lamb (Sweazey, 1996) and the cat (Saha et al, 1999). The distribution of glycinergic neurones in relation to GABAergic neurones in the rat dorsomedial medulla was also reported by Tanaka et al (2003), who used in situ hybridization to localize the mRNAs for glutamic acid decarboxylase (GAD) and glycine transporter 2 (GLYT2), a reliable marker for glycinergic neurones (Poyatos et al, 1997).…”

Increased GABAB Receptor Subtype Expression in the Nucleus of the Solitary Tract of the Spontaneously Hypertensive Rat

“…Numerous studies have demonstrated that GABA immunoreactive neurones and GABA-containing terminals are present in areas of the NTS known to receive terminations of the baroreceptor afferents (Maqbool et al 1991;Saha et al 1995;Saha et al 1999). The presence of GABA B receptors in these areas is well documented from pharmacological studies (Singh and Ticku 1985;Bowery et al 1987), with their involvement in cardiovascular regulation supported by studies using GABA B agonists and antagonists.…”