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Basic and AppliedAspects o/Vestibular Functio" J.C. Hwang. N.G. Daunton and V.I Wilson (Eds.)© Hong Kong University Press. Hong Kong. 1988 IMMUNOCYTOCHEMICAL LOCALIZATION OF GLUTAMIC ACID DECARBOXYLASE (GAD) AND SUBSTANCE P IN NEURAL AREAS MEDIATING MOTION-INDUCED EMESIS. EFFECTS OF VAGAL STIMULATION ON GAD IMMUNOREACTIVITY F. D'Amelio*, M.A..Gibbs**, W.R. Mehler**, N.G. Daunton** and R.A. Fox* *San Jose State University, San Jose, CA 95192, U.SA.; and **Neurosciences Branch, National Aeronautics and Space Administration (NASA)Ames Research Center, Moffett Field, CA 94035, U.SA. Abstract Immunocytochemical methods were employed to localize the neurotransmitter amino acid 'Y-aminobutyric acid (GABA) by means of its biosynthetic enzyme glutamic acid decarboxylase (GAD) and the neuropeptide substance P in the area postrema (AP), area subpostrema (ASP), nucleus of the tractus solitarius (NTS) and gelatinousnucleus (GEL). Inaddition, electrical stimulation was applied to the right vagus nerve at the cervical level to assess the effects on GAD-immunoreactivity (GAD-IR). GABA: GAD-IR terminals and fibers were observed in the AP, ASP, NTS and GEL. They showed pronounced density at the level of the ASP and gradual decrease towards the solitary complex. Nerve cells were not labelled in our preparations. Ultrastructural studies showed symmetric or asymmetric synaptic contacts between labelled terminals and non-immunoreactive dendrites, axons or neurons. Some of the labelled terminals contained both clear- and dense-core vesicles. Our preliminary findings, after electrical stimulation of the vagus nerve, revealed a bilateral decrease ofGAD-IR that was particularly evident at the level of the ASP. Substance P: SP-immunoreactive (SP-IR) terminals and fibers showed varying densities in the AP, ASP, NTS and GEL. In our preparations, the lateral subdivision of the NTS showed the greatest accumulation. The ASP showed medium density of immunoreactive varicosities and terminals and the AP and GEL Keywords: GABA, substance P, vagus nerve, area postrema, nucleus tractus solitarius, immunocytochemistry , electrical stimulation 114 D' Amelio et al. displayed scattered varicose axon terminals. The electron microscopy revealed that all immunoreactive terminals contained clear-core and dense-core vesicles which made symmetric or asymmetric synaptic contact with unlabelleddendrites. It is suggested that the GABAergic terminals might correspond to vagal afferent projections and that GAD/GABA and substance P might be co-localized in the same terminal allowing the possibility of a regulat~d release of the transmitters in relation to demands. Introduction The present report is part of a study designed to investigate the interaction between neuropeptides and conventional neurotransmitters under conditions producing motion sickness and in the process of sensory-motor adaptation. A vast amount of literature has dealt with the cytoarchitectural organization and ultrastructural analysis of the area postrema (AP), area subpostrema (ASP), nucleus of the tractus solitarius (NTS) and gelatinous nucleus (GEL), all structures localized in the dorsal part of the medulla oblongata (e.g., Olszewski and Baxter, 1954; Taber, 1961; Gwyn and Wolstencroft, 1968; Klara and Brizzee, 1975, 1977; Chemicky et ai., 1980; D'Amelio et al., 1986). Anatomical studies haveprovideddetails oftheir somatotopicorganization in relation to visceral afferents and physiological findings have demonstrated their involvement in a variety of autonomic functions (e.g., von Euler et al., 1973; Gwyn et ai., 1979; Gwyn and Leslie, 1979; Katz and Karten, 1979; Gale et al., 1980; Hamilton and Gillis, 1980; Helke et al., 1980; Kalia and Mesulam, 1980a, b; Panneton and Loewy, 1980; Ciriello et al., 1981; Kalia, 1981; Kalia and Sullivan, 1982; Helke, 1982). Neurotransmitters such as GABA, catecholamines, neuropeptides and serotonin have been identified by immunocytochemical procedures (e.g., Armstrong et al., 1981, 1982a,b; Maley andElde, 1982; Maley etal., 1983; Kalia et al., 1984; Maley, 1985; Maley and Newton, 1985; Newton et al., 1985; D'Amelio et al. 1987; Maley et al., 1987; Newton and Maley, 1987; Nomura et al., 1987) and in some cases, synaptic interactions between neurotransmitters have been established (pickel et ai.,1979, 1984; Kubota et ai., 1985). By combining autoradiography and immunocytochemistry , Sumal etal. (1983) reported synaptic interactions betweenvagal afferentsandcatecholaminergic neurons in the NTS of the rat. Glial fibrillary acidic protein and glutamine synthetase were identified in the glioependymal cells and astrocytesofthe cat AP (D'Amelio etal., 1985, 1987).Therelevanceofthe AP as theemeticchemoreceptortriggerzone hasbeen corroborated (Borison and Brizzee, 1951; Carpenter et al., 1983; Borison et al., 1984) and evidence ofits participation in the emetic response to motion has also been reported (Wang and Chinn, 1952, 1954; Brizzee et al., 1980; Crampton and Daunton, 1984). In this report we will describe the light microscopic distribution and ultrastructural...

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