Anatomical distribution and characteristics of [(3)H]muscimol high affinity binding sites in the dorsolateral pontine tegmentum of the rat brain.
The pharmacological characterization and fine anatomical distribution of [(3)H]muscimol high affinity binding sites in the dorsolateral pontine tegmentum (region of locus coeruleus) of the rat was established using biochemical and autoradiographic techniques. On crude synaptosomal membranes obtained from microsamples of locus coeruleus, as well as on frozen sections of the dorsolateral pontine tegmentum, [(3)H]muscimol binding was saturable, displaced by 80-90% with 10(?5) M GABA or 5.10(?4) M (+) bicuculline, but unchanged in the presence of 10(?4) M nipecotic acid. Analysis of the saturation curve suggested the existence of one population of high affinity binding sites with K(d) = 10.5 nM, B(max) = 2.15 pmol/mg of crude synaptosomal membrane proteins, and K(d) = 14.3 nM, B(max) = 0.36 pmol/mg proteins on frozen tissue sections. Autoradiographic data on (3)H ultrofilms evidenced low densities of sites in this region compared to cerebellum or cortex as stated by previous authors. The highest densities of sites were observed in nuclei locus coeruleus and cuneiform, and in central grey. Dorsal and ventral parabrachial, ventral tegmental, motor trigeminal and superior vestibular nuclei exhibited slightly lower densities of sites, and brachium conjunctivum and medial longitudinal fasciculus presented no binding sites. In addition, light microscopy autoradiographic observations revealed [(3)H]muscimol binding sites between the structures previously quoted. In the locus coeruleus, [(3)H]muscimol binding sites were not strictly superimposed with cell bodies. These data are discussed in relation with the distribution of GABAergic terminals in the various areas of the dorso-lateral pontine tegmentum.
Miachon
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INSERM U171-Groupe de Neurochimie Fonctionnelle, Hôpital Ste Eugénie, Pav. 4 H, l'avenue G. Clémenceau, 69230 St Genis Laval, France.
Staunton
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