AUTORADIOGRAPHIC DEMONSTRATION AND LOCALIZATION OF GLUCAGON RECEPTORSIN 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 existen ce of glucagon receptors in duck brain by radioligand binding on fresh tissue sections and radioautography. Specific high-affinity [I-125]gl ucagon binding sites similar to those in the liver were demonstrated i n the avian brain. Mapping of these putative glucagon receptors reveal ed a discrete distributional pattern. Most of the [I-125]glucagon bind ing capacity in duck brain is concentrated within the telencephalon, m ainly in components of motor and limbic systems. Specific labeling den sities were associated with avian equivalents of the mammalian pyramid al system (hyperstriatum accessorium; archistriatum intermedium and tr actus occipitomesencephalicus) and extrapyramidal system (paleostriatu m augmentatum, paleostriatum primitivum and lobus parolfactorius), as well as several limbic structures (hippocampal formation, nucleus taen iae and the caudal part of the archistriatum). Few glucagon-reactive f oci were detected in the diencephalon (the nucleus dorsomedialis of hy pothalamus, the two circumventricular organs, organum vasculosum of th e lamina terminalis and median eminence and the nucleus habenularis me dialis). These findings suggest that glucagon might be involved in the central control of somatic motricity and basic behaviors and point th erefore to glucagon as a new neuroactive messenger in avian brain. The extensive difference between the distribution of glucagon binding sit es observed in duck brain and that previously reported in rat brain su ggests that glucagon does not subserve the same physiological role(s) in avian and mammalian brains.