Induction of galanin receptor-1 (GalR1) expression in external granule cell layer of post-natal mouse cerebellum.

Galanin is a modulator of fast transmission in adult brain and recent evidence suggests that it also acts as a trophic factor during neurogenesis and neural injury and repair. Previous studies in our laboratory have identified galanin mRNA in Purkinje cells of adult and developing rat (but not adult mouse) cerebellum; and galanin-binding sites in adult mouse (but not rat) cerebellum. The post-natal development of the cerebellum provides a unique and convenient model for the investigation of developmental processes and to learn more about putative cerebellar galanin systems, the current study examined the presence and distribution of galanin-like-immunoreactivity (- LI), [(125)I]-galanin binding sites and galanin receptor-1 (GalR1) mRNA in post-natal mouse cerebellum. Using autoradiography and in situ hybridization, [(125)I]-galanin binding sites and GalR1 mRNA were first detected on post-natal day 10 (P10) in the external germinal layer of all lobes and high levels were maintained until P14. Quantitative real-time PCR assays detected GalR1 mRNA in whole cerebellum across the post-natal period, with a strong induction and peak of expression at P10. Assessment of galanin levels in whole cerebellum by radioimmunoassay revealed relatively similar concentrations from P5 to P20 and in adult mice (80-170 pg/mg protein), with a significantly higher concentration (250 pg/mg, p < 0.01) detected at P3. These concentrations were some four- to six-fold lower than those in adult forebrain samples. Using immunohistochemistry, galanin-like-immuno-reactivity was observed in prominent fibrous elements within the white matter tracts of the cerebellum at P3-5 and in more punctate elements in the internal granule cell layer and associated with the Purkinje cell layer at P12 and P20. Increased levels of GalR1 mRNA and galanin binding (attributed to GalR1) in the external granule cell layer at P10-12/(14) coincide with granule cell migration from the external to the inner granule cell layer and together with demonstrated effects of other neuropeptide-receptor systems suggest a role for GalR1 signalling in regulating this or related developmental processes.

[125I]-Galanin binding in brain of wildtype, and galanin- and GalR1-knockout mice: strain and species differences in GalR1 density and distribution.

Widespread production of knockout and transgenic mice has led to an increased use of mice as animal models for studies of normal- and patho-physiology. Hence, the precise mapping of central transmitter/peptide systems in the mouse has become essential for the interpretation of functional studies and for the correct correlation with findings obtained in the rat, primates and/or human. In this regard, the current study reports the autoradiographic localization of [(125)I]-galanin (GAL) binding sites in brain of the common C57BL/6J and 129OlaHsd mouse strains, as well as in GAL and galanin receptor-1 (GalR1) knockout (KO) mice. In C57BL/6J and 129OlaHsd mice, [(125)I]-GAL binding sites were detected throughout the brain, including moderate-high relative densities in the basal ganglia (caudate putamen, nucleus [n.] accumbens, olfactory tubercle, substantia nigra), limbic regions (septum, bed n. stria terminalis, ventral hippocampus, amygdala), cingulate, retrosplenial, entorhinal cortex, centro-lateral/medial thalamic n., preoptic/lateral hypothalamus, midbrain (superior colliculus, periaqueductal gray), pons/medulla oblongata (parabrachial, pontine reticular and solitary tract n.) and cerebellar cortex. [(125)I]-GAL binding levels were low or absent in main olfactory bulb, neocortex, ventrolateral/geniculate thalamic n., dorsal hippocampus, inferior colliculus and cranial motor n. In simultaneous determinations, relative [(125)I]-GAL binding site densities in brain were generally lower in C57BL/6J than in 129OlaHsd mice, while the density and distribution of central binding in the GAL-KO mouse was essentially identical to that in its background-129OlaHsd strain. In contrast, no specific [(125)I]-GAL binding was detected in any region of GalR1-KO mouse brain, revealing that under the experimental conditions used, the peptide ligand binding is predominantly (exclusively) to the GalR1 subtype. This evaluation of GAL receptor site distribution in mouse brain has revealed similarities and some differences with the equivalent system in rat and provides a valuable reference for future comparative studies of central GAL transmission.