Imaging serotonergic neurotransmission in depression: hippocampal pathophysiology may mirror global brain alterations

The recent development of [carbonyl-11C]WAY-100635 for serotonin (5-HT)1A and [18F]setoperone and [18F]altanserin for 5-HT2A positron emission tomography receptor imaging has allowed studies of 5-HT neurotransmission in depressive disorders. The hippocampus is likely to be an important brain structure in the pathophysiology of depression because it may mediate both cognitive deficits and hypercortisolemia found in this disorder. Decreased 5-HT1A binding was reported in the medial temporal cortex, which receives dense 5-HT innervation, and also throughout neocortical regions. Because the 5-HT1A antagonist pindolol may hasten antidepressant effects of selective serotonin reuptake inhibitor medications, its receptor occupancy has been measured in both presynaptic and postsynaptic sites. The results are controversial but suggest that pindolol has preferential occupancy of somatodendritic autoreceptors in the raphe. The results of 5-HT2A receptors are mixed, with one showing a significant decrease in the right orbitoinsular cortex and three not detecting a significant change. The disparate findings in patients with depression almost certainly reflect the heterogeneity of the disorder, and we highlight the utility of the hippocampus as a useful target region not only to compare depressed subjects with healthy subjects but also to correlate findings with cognitive function and activity of the limbic–hypothalamic–pituitary axis system.