Validation of the transporter ligand cyanoimipramine as a marker of serotonin innervation density in brain.

Unlabelled: Radiolabeled ligands of monoamine transporters have already been used to visualize cerebral monoamine innervation by tissue autoradiography and by PET or SPECT in vivo.

Methods: A sampling technique was developed to allow for both the autoradiographic counting of serotonin (5-HT) axonal varicosities, labeled by uptake and storage of [3H]5-HT, and the measurement of the binding of [3H]cyanoimipramine ([3H]CYI), a specific 5-HT transporter ligand, in adjacent slices of adult rat neostriatum. The experiments were conducted in normal, decreased (after 5,7-dihydroxytryptamine lesions in adults) or increased (after 6-hydroxydopamine lesions in neonates) states of neostriatal 5-HT innervation.

Results: In normal tissue, the regional density of [3H]CYI binding faithfully reproduced rostrocaudal variations in the number of [3H]5-HT-labeled axonal varicosities. Pairs of values from all three experimental groups showed a highly significant linear correlation (r = 0.93) between the density of [3H]CYI binding and the number of 5-HT varicosities per cubic millimeter of tissue. The intercept of the regression line was close to zero; this confirmed the selectivity of the ligand.

Conclusion: Under drug-free conditions, specific [3H]CYI binding is a good quantitative index of 5-HT innervation density in brain tissue and is not significantly up- or downregulated on 5-HT denervation or hyperinnervation. When it is adequately labeled, such a ligand might therefore be appropriate to quantify regional 5-HT innervation in vivo by PET or SPECT. The present approach should also be useful to select ligands to quantify 5-HT and monoamine systems.