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Evidence for the Existence of Imidazoline‐Specific Binding Sites in Synaptosomal Plasma Membranes of the Bovine Brainstem
Author(s) -
Heemskerk F. M. J.,
Dontenwill M.,
Greney H.,
Vonthron C.,
Bousquet P.
Publication year - 1998
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1998.71052193.x
Subject(s) - imidazoline receptor , idazoxan , synaptosome , membrane , monoamine oxidase , chemistry , postsynaptic potential , brainstem , biochemistry , receptor , biophysics , endocrinology , medicine , biology , enzyme , neuroscience , antagonist , prazosin
Nonadrenergic imidazoline‐specific binding sites were characterized pharmacologically in crude cerebral membrane preparations, but little is known about their subcellular localization in neurons. As in the brain‐stem these sites are involved in cardiovascular regulation and peripherally imidazolines modulate neurotransmitter release, we tried to determine a possible (pre)synaptic localization in brainstem. We found a specific enrichment in (entire) synaptosome, purified synaptosomal plasma membrane (37 fmol/mg), and mitochondrial (83 fmol/mg) fractions as compared with other membrane fractions (3–8 fmol/mg). Synaptosomes appeared to be free of postsynaptic structures, and purified synaptosomal plasma membranes were devoid of mitochondrial material, as determined by electron microscopy and by comparison with the distribution of marker enzymes such as monoamine oxidase. These results show for the first time that these extramitochondrial imidazoline‐specific sites are neuronal and are located on presynaptic terminals. We found high affinities for unlabeled p ‐iodoclonidine (subnanomolar), clonidine (0.2 n M ), and efaroxan (11 n M ), but idazoxan did not compete significantly for the p ‐[ 125 I]iodoclonidine binding in these membranes. Therefore, these sites can be classified as I 1 imidazoline receptors. In summary, we describe for the first time that high‐affinity I 1 receptors of the bovine brainstem are located on (pre)synaptic membranes.