title: Characteristics and distribution of high- and low-affinity alpha bungarotoxin binding sites in the rat hypothalamus.
abstract: When binding of 125I-alpha bungarotoxin (125I-alpha BTX) to hypothalamic membranes is observed over a wide range of concentrations, 3 binding sites can be identified, with estimated equilibrium dissociation constants (Kds) of 4.1 X 10(-11) M, 6.2 X 10(-10) M, and 9.1 X 10(-7) M for high-, low-, and very-low-affinity interactions, respectively. The densities of the high- and low-affinity sites were similar at 14-21 fmol/mg protein, whereas the very-low-affinity site had approximately 1000 X greater capacity. Association and dissociation kinetics predicted a biphasic binding reaction, with association rate constants of 1.38 X 10(8) M-1 min-1 and 7.53 X 10(7) M-1 min-1 and dissociation rate constants of 5.23 X 10(-3) min-1 and 1.80 X 10(-3) min-1. The presence of Na+ inhibited the binding of 125I-alpha BTX with a half-maximally effective concentration of 22 mM. This decrease in binding was associated with the observation of a single binding site with a Kd of 4.3 X 10(-10) M and a density of 12.1 fmol/mg protein. In competition binding experiments, alpha BTX, curare, nicotine, and quinacrine were the most potent competitors. Acetylcholine competed with 125I-alpha BTX binding at 2 sites with estimated affinities of 3.6 X 10(-8) and 7.4 X 10(-5) M. In the rostral hypothalamus, high-affinity binding of 125I-alpha BTX was localized to the region of the supraoptic nucleus, paraventricular nucleus, suprachiasmatic nucleus, and the nucleus circularis complex. Within magnocellular regions, binding was closely associated with neurophysin-immunoreactive neurons and processes, while in the region of the suprachiasmatic nucleus, the binding was in a perinuclear region surrounding parvocellular neurophysin-immunoreactive neurons.
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism; D002038:Bungarotoxins / Q000378:metabolism; D005404:Fixatives; D007031:Hypothalamus / Q000378:metabolism*; D007457:Iodine Radioisotopes; D007700:Kinetics; D008297:Male; D008566:Membranes / Q000378:metabolism; D009994:Osmolar Concentration; D051381:Rats; D011919:Rats, Inbred Strains; D011950:Receptors, Cholinergic / Q000378:metabolism*; D011978:Receptors, Nicotinic; D012964:Sodium / Q000494:pharmacology; D014018:Tissue Distribution; D064569:alpha7 Nicotinic Acetylcholine Receptor
chemical_list: D002038:Bungarotoxins; C579165:Chrna7 protein, rat; D005404:Fixatives; D007457:Iodine Radioisotopes; D011950:Receptors, Cholinergic; D011978:Receptors, Nicotinic; D064569:alpha7 Nicotinic Acetylcholine Receptor; D012964:Sodium