Opioid binding sites in the fish brain: An autoradiographic study
Bird, D. J., Jackson, M., Baker, B. I. and Buckingham, J. C. (1988) Opioid binding sites in the fish brain: An autoradiographic study. General and Comparative Endocrinology, 70 (1). p. 49. ISSN 0016-6480
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Publisher's URL: http://dx.doi.org/10.1016/0016-6480(88)90093-7
Cryostat sections of trout brains were incubated with tritiated opioid ligands (5 nM [3H]etorphine or 4 nM -[3H]Ala2, Met5 enkephalinamide) with the initial aim of locating opioid binding sites associated with the hypothalamo-pituitary axis. Naloxone-displaceable binding was observed in all regions of the brain, with a density ranking order of cerebellum > telencephalon > optic tectum > hypothalamus > brain stem > pituitary gland. Within the hypothalamus and pituitary gland binding was unexpectedly low, apart from a slightly enhanced binding anteriorly in the preoptic region and posteriorly associated with presumptive sensory fibres. A similar distribution of opioid binding sites was seen in the eel and lamprey brain. The high level of opioid binding in the cerebellum permitted a tentative identification of opioid subtypes using [3H]etorphine binding to membrane preparations from trout cerebellum. Scatchard analysis indicated the presence of a single class of high-affinity binding sites with a density of 0.38 pmol/mg protein and affinity constant (KD) of 2.6 nM. Displacement by unlabelled ligands suggested the existance of μ and/or κ receptors.