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Major difference in the expression of δ‐ and μ‐opioid receptors between turtle and rat brain
Author(s) -
Xia Ying,
Haddad Gabriel G.
Publication year - 2001
Publication title -
journal of comparative neurology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.855
H-Index - 209
eISSN - 1096-9861
pISSN - 0021-9967
DOI - 10.1002/cne.1061
Subject(s) - biology , receptor , brainstem , opioid , turtle (robot) , opioid receptor , medicine , neuroscience , hippocampus , endocrinology , thalamus , spinal cord , opioid peptide , central nervous system , cerebral cortex , biochemistry , fishery
The reptilian turtle brain has a remarkably higher endurance for anoxia than mammalian brains. Since the response to O 2 deprivation is dependent in a major way on the expression and regulation of membrane proteins, differences in such proteins may play a role in the species‐related differences in hypoxic responses. Because opioid system is involved in the regulation of hypoxic responses, we asked whether there are differences between rat and turtle brains in terms of opioid receptor expression. In this work, we compared the expression and distribution of δ‐and μ‐opioid receptors in the turtle and rat brains. Our results show that (1) the dissociation constant (K d ) for δ‐receptor binding was approximately four times lower and B max was more than double in the turtle brain homogenates than in rat ones; (2) the δ‐receptor binding density was heterogeneously distributed in the turtle brain, with a higher density in the rostral regions than in the brainstem and spinal cord, and was generally much higher than in rat brains from the cortex to spinal cord; (3) the δ‐opioid receptors in the rat brains were mostly located in the cortex, caudate putamen, and amygdala with an extremely low density in most subcortical (e.g., hippocampus and thalamus) and almost all brainstem regions; and (4) in sharp contrast to δ‐opioid receptors, μ‐opioid receptor density was much lower in all turtle brain regions compared with the rat ones. Our results demonstrate that the turtle brain is actually an organ of δ‐opioid receptors, whereas the rat brain has predominantly μ‐opioid receptors. Because we have recently found that δ‐opioid receptors protect neurons against glutamate and hypoxic stress, we speculate that the unique pattern of δ‐receptor receptor expression and distribution plays a critical role in the tolerance of turtle brain to stressful situations characterized by glutamate excitotoxicity. J. Comp. Neurol. 436:202–210, 2001. © 2001 Wiley‐Liss, Inc.