Adult development of the hippocampal-serotonin system of C57BL/6N mice; analysis of high-affinity uptake of ³H-5HT in slices and synaptosomes

The saturable and specific high-affinity uptake of [³H]serotonin ([³H]5HT) (5 × 10^-8 M) was studied in slices from the hippocampus, parietal cortex, septum-preoptic area, and hypothalamus of male 2, 6, 12 and 24-32 month old C57BL/6N mice. Hippocampal [³H]5-HT uptake showed a significant biphasic relationship to age, with lower values in the 2 and 24-32 month old mice compared to 6 month old mice. No significant age effects were seen in the other regions, or in [³H]norepinephrine high-affinity uptake in the hippocampus. Studies of the high-affinity uptake mechanism in synaptosomal preparations were made in a subgroup of 12 and 24 month old mice. A micro-assay using a tissue-harvester measured high-affinity uptake on 8-30 μl of the P₂ suspension (crude-synaptosomal preparation). The high-affinity uptake was linear for 4 min at 37°C and inhibited in both the adult and aged tissue by 10^-5 M cold 5-HT (83 and 78% respectively), 10^-5 M fluoxetine (85 and 82% respectively) and 10^-3 M NaCN (57 and 57% respectively). Kinetic analysis of the [³H]5HT high-affinity uptake in the hippocampus (3 min, 37°C) revealed the same apparent K_m for serotonin at both ages (6.7 x 10^-8 M), but a 44% decrease in V_max in the aged hippocampal synaptosomal high-affinity uptake compared to adults (120 vs 215 pmol of 5-HT/g-tissue/3 min). These results are discussed in relationship to the reported age effects on the intrinsic neurons of the hippocampus.