Galanin and spatial learning in the rat. Evidence for a differential role for galanin in subregions of the hippocampal formation

Anatomical, neurochemical and behavioural evidence support a role for galan in in hippocampally mediated functions such as spatial learning and memory. To obtain more precise information on this role, galanin (3 nmol/rat) was infused via bilateral chronic cannulae into different areas of the hippocam pal formation which are characterized by different galanin receptor subtype s and also by different galanin innervation patterns. The effects of infuse d galanin on spatial learning were examined in the Morris swim maze. Infusi ons of galanin into both the dorsal and ventral dentate gyrus, which mainly contain GAL-R2 receptor mRNA and a high degree of galanin-noradrenaline co existence, significantly retarded spatial acquisition without affecting swi m speed or performance in the visible platform test. This spatial learning deficit was fully blocked by pretreatment with the non-selective galanin an tagonist M35. Analysis of retention performance suggested that the major ef fect of intrahippocampal galanin is mediated via a specific disruption of a cquisition mechanisms of importance for performance in the probe trial. Gal anin infused into the ventral CA1 (a mainly GAL-R1 receptor mRNA expressing region) or into anterior, ventral CA3 regions did not produce any deficits in spatial learning compared to control animals. These results suggest tha t galanin mediates its action on spatial learning mainly through the GAL-R2 receptor subtype in areas where most of the galanin iu present in noradren ergic terminals. A possible role for the GAL-R1 receptor subtype in cogniti on in the dorsal and ventral hippocampus remains to be defined. The results suggest a differential functional role for galanin and galanin receptor su btypes within subregions of the hippocampal formation. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.