Major depressive disorder (MDD) and suicide are associated with deficient serotonergic neurotransmission. Tryptophan hydroxylase (TPH) is the rate-limiting biosynthetic enzyme for serotonin. Previously, we reported elevated levels of TPH protein in the dorsal raphe nucleus (DRN) of depressed suicides and now examine expression of neuronal TPH2 mRNA in a cohort of matched controls and depressed suicides (n ¼ 11 pairs). DRN TPH2 mRNA was measured by densitometric analysis of autoradiograms from in situ hybridization histochemistry experiments. TPH2 mRNA is confirmed as the raphe-specific isoform of TPH in human brain, and is expressed in neurons throughout the anteroposterior extent of the DRN and median raphe nucleus (MRN). TPH2 mRNA expression correlates with TPH protein distribution in the DRN, and has a negative correlation with age. In drug-free suicides, TPH2 expression is 33% higher in the DRN and 17% higher in the MRN as compared to matched nonpsychiatric controls. Higher levels of TPH2 mRNA were found throughout the entire extent of the rostrocaudal axis of the DRN, and were not specific to any single subnucleus. Higher TPH2 mRNA expression may explain more TPH protein observed in depressed suicides and reflect a homeostatic response to deficient brain serotonergic transmission.

Deficient levels of serotonin are associated with suicide and depression. Paradoxically, in the dorsal raphe nucleus (DRN) there are more serotonin neurons and more neuronal tryptophan hydroxylase-2 (TPH2) expression postmortem in depressed suicides. In this study, we sought to determine whether greater TPH2 expression in depressed suicides was the result of more TPH2 expression per neuron. In situ hybridization and computer-assisted image analysis were performed on tissue sections throughout the extent of the raphe nuclei at the level of silver grains per neuron to systematically quantify TPH2 neuronal expression. Depressed suicides have 26.5% more TPH2 grain density per neuron in the DRN compared with matched controls (P = 0.04). The difference in grain density is greater at mid-and caudal anatomical levels across the rostrocaudal axis of the DRN. Densitometric analysis of TPH2 expression in the DRN subnuclei showed that higher expression levels were observed at posterior anatomical levels of depressed suicides (121% of control in the caudal subnucleus). Higher TPH2 expression in depressed suicides may explain more TPH2 protein and reflect a homeostatic response to deficient serotonin levels in the brains of depressed suicides. Localized changes in TPH2 expression in specific subnuclei of the DRN suggest that the serotonergic compensatory mechanism in depression and suicide is specifically regulated within the DRN and has implications for regions innervated by this subnucleus.

Impaired brain serotonin neurotransmission is a potential component of the diathesis of major depression. Tryptophan hydroxylase-2 (TPH2), is the rate limiting biosynthetic isoenzyme for serotonin that is preferentially expressed in the brain and a cause of impaired serotonin transmission. Here, we identify a novel TPH2 short isoform with truncated catalytic domain expressed in human brainstem, prefrontal cortex, hippocampus and amygdala. An exploratory study of 166 Caucasian subjects revealed association with major depression or suicide of a novel single nucleotide polymorphism (SNP) g.22879A > G located in exon 6 of this short isoform. This SNP and additional SNPs were discovered through a systematic characterization of the genetic architecture of the TPH2 gene for further genetic and functional investigations of its relationship to major depression and other psychopathology.

The upper panel is an in situ hybridization film autoradiogram of a human brainstem section expressing neuronal tryptophan hydroxylase (TPH2) mRNA, obtained using a TPH2-specific riboprobe (bar = 1 mm). TPH2 signal is specific to both dorsal and median raphe nuclei. The lower panel is a high-resolution photomicrograph depicting silver grains over two serotonin neurons in the dorsal raphe nucleus following emulsion-dipping. An unlabeled neuron can be seen with few silver grains present over the neuron, demonstrating specificity of the label (bar = 25 mm). Density of TPH2 expression in the brainstems of depressed suicides and matched controls was measured using computer-based densitometry with a fixed area sampling circle shown in red above. In depressed suicides, serotonergic neurons have elevated transcriptional capacity to express the biosynthetic enzyme TPH2 mRNA, determined by measurement of grain density within the fixed area circle. For more information on this topic, see the paper by Bach-Mizrachi et al. on pages 507-513.

Abnormalities of the serotonergic system have been implicated in suicide. Post-mortem brain studies of suicides have begun to elucidate the underlying molecular changes in the brain serotonergic system that may provide an understanding of the biology of suicide. There is evidence for alterations in the presynaptic serotonin transporter and serotonergic receptors in both the serotonin-synthesizing neurons in the brainstem and their targets in the prefrontal cortex. Some of these changes may represent primary pathophysiology, while other changes may reflect homeostatic regulatory responses to low serotonin or even maladaptive nonspecific stress responses. We review the post-mortem studies of suicides and discuss a model of homeostatic plasticity in the human brain in a serotonin-deficient environment.