Characterization and regional distribution of glutamatergic and cholinergic ligand binding sites in goldfish brain

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Abstract
The binding of several ligands that selectively interact with glutamate receptor subtypes was characterized in extensively washed synaptosomal membrane preparations from goldfish brain. The binding affinity (Kd), an estimate of the number of sites (Bmax), the rank-order potency of glutamatergic ligands at inhibiting binding, and the regional localization of binding sites were determined. In whole brain preparations, 3H-kainate had a Kd of 136 nM and a Bmax of 63 pmol/mg protein, 3H-AMPA had a Kd of 26 nM and a Bmax of 0.4 pmol/mg protein, and 3H-L-glutamate bound with an apparent affinity of 323 nM and a Bmax of 5 pmol/mg protein. Most of the binding sites for each of the glutamate analogs were present in the cortex and the fewest were in the cerebellum, except for 3H-kainate binding sites, which were most prevalent in the cerebellum and least abundant in the cortex. The proposed neuronal nicotinic acetylcholine receptor (nAChR) ligands 3H-(- )nicotine and 125I-alpha-Bgt were also investigated. 125I-alpha-Bgt had a Kd of 0.08 nM and a Bmax of 132 fmol/mg protein. 3H-(-)nicotine did not bind to the extensively washed membrane preparations, so a less stringently washed P2 tissue fraction was used. In this tissue preparation, 3H-(-)nicotine had a Kd of 9 nM and a Bmax of 84 fmol/mg protein. Eye removal resulted in a time-dependent decrease in the number of 3H-(-)nicotine and 125I-alpha-Bgt binding sites in the contralateral optic tectum, but no reduction in the number of binding sites for any of the glutamatergic ligands. The results suggest that both 3H-(-)nicotine and 125I-alpha-Bgt binding sites are similarly regulated in the optic tectum, which supports previously reported data indicating that the binding sites are located on closely related proteins or may, at least partially, be colocalized on the same protein (Henley and Oswald, 1987).(ABSTRACT TRUNCATED AT 250 WORDS)