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Distribution of muscimol, QNB, and 5HT binding in the vertebrate diencephalon: A comparative study of eight mammals and three non‐mammals

The distribution of muscimol, quinuclidinyl benzilate (QNB), and serotonin (5HT)‐bound receptors in the diencephalon was examined by conventional receptor‐binding methods in 11 species of amniotes including 2 reptiles, 1 bird, and 8 mammals, selected mostly on the basis of their differing last common ancestor with Anthropoids. We found that receptor binding can help define major subdivisions of the forebrain. The results show that in each of these species, the distribution of muscimol and QNB binding across the four major subdivisions of the diencephalon was consistent; densest in the dorsal thalamus, with hypothalamus and then either ventral thalamus or epithalamus with successively lesser amounts. However, the binding of serotonin (5HT) was most prevalent in the hypothalamus with equivalent amounts in the other diencephalic subdivisions. Myelin‐ and cell‐stained materials showed that the pattern of high‐density binding probably is not the secondary result of non‐neurochemical factors such as differences in cell or neuropil density or in total available membrane. Perhaps more importantly, the receptor distributions suggest functional roles for major subdivisions across taxa. Results show that GABA‐A and muscaranic Ach receptors are common in the dorsal diencephalon across vertebrate species and, therefore, are probably responsible for the gating of information to the cortex. Results show that serotonin is predominant in the hypothalamus. The lack of it in the dorsal thalamus indicates that it is probably not responsible for gating of information to the cortex. Results also show that in nonmammals the amount of GABA‐A and muscaranic Ach differs from that found in mammals. For muscaranic Ach, the labeling in marsupials differs from that in placentals. Primates differ from other species (nonmammals and mammals combined) in the amount of 5HT found in the ventral diencephalon and the hypothalamus. Microsc. Res. Tech. 62:247–261, 2003. © 2003 Wiley‐Liss, Inc.