Radioautographic demonstration and localization of glucagon receptors in duck brain

The discovery of glucagon biosynthesis and receptors within mammalian brain has led one to suspect a neurotransmitter role for glucagon. In order to address this hypothesis in birds, we investigated the existence of glucagon receptors in duck brain by radioligand binding on fresh tissue sections and radioautography. Specific high-affinity [125I]glucagon binding sites similar to those in the liver were demonstrated in the avian brain. Mapping of these putative glucagon receptors revealed a discrete distributional pattern. Most of the [125I]glucagon binding capacity in duck brain is concentrated within the telencephalon, mainly in components of motor and limbic systems. Specific labeling densities were associated with avian equivalents of the mammalian pyramidal system (hyperstriatum accessorium; archistriatum intermedium and tractus occipitomesencephalicus) and extrapyramidal system (paleostriatum augmentatum, paleostriatum primitivum and lobus parolfactorius), as well as several limbic structures (hippocampal formation, nucleus taeniae and the caudal part of the archistriatum). Few glucagon-reactive foci were detected in the diencephalon (the nucleus dorsomedialis of hypothalamus, the two circumventricular organs, organum vasculosum of the lamina terminalis and median eminence and the nucleus habenularis medialis). These findings suggest that glucagon might be involved in the central control of somatic motricity and basic behaviors and point therefore to glucagon as a new neuroactive messenger in avian brain. The extensive difference between the distribution of glucagon binding sites observed in duck brain and that previously reported in rat brain suggests that glucagon does not subserve the same physiological role(s) in avian and mammalian brains.