ALTERATIONS IN THE IMMUNOREACTIVITY FOR MUSCARINIC ACETYLCHOLINE-RECEPTORS AND COLOCALIZED PKC-GAMMA IN MOUSE HIPPOCAMPUS INDUCED BY SPATIAL DISCRIMINATION-LEARNING

This study describes changes in the immunoreactivity for muscarinic: a cetylcholine receptors (mAChRs) in the hippocampus of mice in relation to spatial discrimination behavior, employing the monoclonal antibody M35 raised against purified bovine mAChR protein. Performance in a ho le hoard in which the animals learned the pattern of 4 baited holes ou t of 16 holes served as the measure of spatial discrimination learning and memory. Twenty-six adult male house mice were used, divided into four groups. Three groups served as various controls: group N (naive; blank controls); group H (habituated; animals were introduced to the h ole board with all holes baited for 5 consecutive days), and group P ( pseudotrained; the animals were admitted to the hole board for 13 cons ecutive days with all holes baited). The T group (trained) was subject ed to the hole board for 5 consecutive habituation days with all holes baited (similar to the H and P groups), followed by 8 successive trai ning days with only four holes baited in a fixed pattern. During the 8 training days, the T group gradually acquired a pattern to visit the baited holes, whereas the P group continued visiting holes in a random fashion. The mice were killed 24 h after the last behavioral session. All principal cells in the cornu ammonis (CA) and dentate gyrus (DG) of the habituated animals revealed increased levels of mAChR immunorea ctivity (mAChR-ir) over the naive mice. A minor increase in mAChR-ir w as found in the apical dendrites of the CA1 pyramidal cells. Pseudotra ining resulted in a CA1-CA2 region with a low level of mAChR-ir, resem bling naive animals, whereas the trained mice showed a further increas e in mAChR-ir in the CA1-CA2 pyramidal cell bodies and apical dendrite s. Optical density measures of the mAChR-ir in the CA1 region revealed a significant (P < 0.05) increase in the pyramidal cell bodies of the hi and T group over the N and P group, and a significant (P < 0.05) i ncrease in the apical dendrites of the T group over all other groups. In contrast to the CA1-CA2 region, both pseudotrained and trained mice revealed high mAChR staining in the CA3-CA4 region and the DG. These results indicate that prolonged exposure to the hole board is sufficie nt for an enhanced mAChR-ir in the CA3-CA4 and DG, whereas the increas e in CA1-CA2 pyramidal cells is a training-specific feature related to spatial orientation, Nonpyramidal neurons within the CA1-CA2 region w ith enhanced mAChR-ir in the pyramidal cells, however, revealed a decr eased level of mGChR-ir. The opposing effect of pyramidal and nonpyram idal cells suggests a shift in the excitability of the hippocampal mic rocircuitry. Previously we demonstrated an increase and redistribution of hippocampal protein kinase C gamma-immunoreactivity (PKC gamma-ir) induced by hole board learning in mice (Van der Zee et al., 1992, J N eurosci 12:4808-4815). Immunofluorescence double-labeling experiments conducted in the present study in naive and trained animals revealed t hat the principal cells and DG interneurons co-express mAChRs and pKC gamma, and that the immunoreactivity for both markers increased in rel ation to spatial orientation within these neurons. The mAChR-positive nonpyramidal cells of the CA1-CA2 region were devoid of PKC gamma and revealed an opposite training-induced effect. These results suggest th at the postsynaptic changes in mAChR- and PKC gamma-ir reflect functio nal alterations of the hippocampal formation induced by spatial learni ng. (C) 1995 Wiley-Liss, Inc.