Effects of Serotonin and Slow-Release 5-Hydroxytryptophan on Gastrointestinal Motility in a Mouse Model of Depression.

BACKGROUND & AIMS: Mood disorders and constipation are often comorbid, yet their shared etiologies have rarely been explored. The neurotransmitter serotonin (5-HT) regulates central nervous system and enteric nervous system (ENS) development and long-term functions, including gastrointestinal (GI) motility and mood. Therefore, defects in neuron production of 5-HT might result in brain and intestinal dysfunction. Tryptophan hydroxylase 2 (TPH2) is the rate-limiting enzyme in 5-HT biosynthesis. A variant of TPH2 that encodes the R441H substitution (TPH2-R441H) was identified in individuals with severe depression. We studied mice with an analogous mutation (TPH2-R439H), which results in a 60%-80% decrease in levels of 5-HT in the central nervous system and behaviors associated with depression in humans. Feeding chow that contains 5-HTP slow release (5-HTP SR) to TPH2-R439H mice restores levels of 5-HT in the central nervous system and reduces depressive-like behaviors. METHODS: We compared the effects of feeding chow, with or without 5-HTP SR, to mice with the TPH2-R439H mutation and without this mutation (control mice). Myenteric and submucosal plexuses were isolated from all 4 groups of mice, and immunocytochemistry was used to quantify total enteric neurons, serotonergic neurons, and 5-HT-dependent subsets of neurons. We performed calcium imaging experiments to evaluate responses of enteric neurons to tryptamine-evoked release of endogenous 5-HT. In live mice, we measured total GI transit, gastric emptying, small intestinal transit, and propulsive colorectal motility. To measure colonic migrating motor complexes (CMMCs), we isolated colons and constructed spatiotemporal maps along the proximodistal length to quantify the frequency, velocity, and length of CMMCs. We measured villus height, crypt perimeter, and relative densities of enterochromaffin and enteroendocrine cells in small intestinal tissue. RESULTS: Levels of 5-HT were significantly lower in enteric neurons from TPH2-R439H mice than from control mice. TPH2-R439H mice had abnormalities in ENS development and ENS-mediated GI functions, including reduced motility and intestinal epithelial growth. Total GI transit and propulsive colorectal motility were slower in TPH2-R439H mice than controls, and CMMCs were slower and less frequent. Villus height and crypt perimeter were significantly decreased in colon tissues from TPH2-R439H mice compared with controls. Administration of 5-HTP SR to adult TPH2-R439H mice restored 5-HT to enteric neurons and reversed these abnormalities. Adult TPH2-R439H mice given oral 5-HTP SR had normalized numbers of enteric neurons, total GI transit, and colonic motility. Intestinal tissue from these mice had normal measures of CMMCs and enteric epithelial growth CONCLUSIONS: In studies of TPH2-R439H mice, we found evidence for reduced release of 5-HT from enteric neurons that results in defects in ENS development and GI motility. Our findings indicate that neuron production of 5-HT links constipation with mood dysfunction. Administration of 5-HTP SR to mice restored 5-HT to the ENS and normalized GI motility and growth of the enteric epithelium. 5-HTP SR might be used to treat patients with intestinal dysfunction associated with low levels of 5-HT.

Deletion of GSK3ß in D2R-expressing neurons reveals distinct roles for ß-arrestin signaling in antipsychotic and lithium action.

Several studies in rodent models have shown that glycogen synthase kinase 3 ß (GSK3ß) plays an important role in the actions of antispychotics and mood stabilizers. Recently it was demonstrated that GSK3ß through a ß-arrestin2/protein kinase B (PKB or Akt)/protein phosphatase 2A (PP2A) signaling complex regulates dopamine (DA)- and lithium-sensitive behaviors and is required to mediate endophenotypes of mania and depression in rodents. We have previously shown that atypical antipsychotics antagonize DA D2 receptor (D2R)/ß-arrestin2 interactions more efficaciously than G-protein-dependent signaling, whereas typical antipsychotics inhibit both pathways with similar efficacy. To elucidate the site of action of GSK3ß in regulating DA- or lithium-sensitive behaviors, we generated conditional knockouts of GSK3ß, where GSK3ß was deleted in either DA D1- or D2-receptor-expressing neurons. We analyzed these mice for behaviors commonly used to test antipsychotic efficacy or behaviors that are sensitive to lithium treatment. Mice with deletion of GSK3ß in D2 (D2GSK3ß(-/-)) but not D1 (D1GSK3ß(-/-)) neurons mimic antipsychotic action. However, haloperidol (HAL)-induced catalepsy was unchanged in either D2GSK3ß(-/-) or D1GSK3ß(-/-) mice compared with control mice. Interestingly, genetic stabilization of ß-catenin, a downstream target of GSK3ß, in D2 neurons did not affect any of the behaviors tested. Moreover, D2GSK3ß(-/-) or D1GSK3ß(-/-) mice showed similar responses to controls in the tail suspension test (TST) and dark-light emergence test, behaviors which were previously shown to be ß-arrestin2- and GSK3ß-dependent and sensitive to lithium treatment. Taken together these studies suggest that selective deletion of GSK3ß but not stabilization of ß-catenin in D2 neurons mimics antipsychotic action without affecting signaling pathways involved in catalepsy or certain mood-related behaviors.

The effects of brain serotonin deficiency on behavioural disinhibition and anxiety-like behaviour following mild early life stress.

Aberrant serotonin (5-HT) signalling and exposure to early life stress have both been suggested to play a role in anxiety- and impulsivity-related behaviours. However, whether congenital 5-HT deficiency × early life stress interactions influence the development of anxiety- or impulsivity-like behaviour has not been established. Here, we examined the effects of early life maternal separation (MS) stress on anxiety-like behaviour and behavioural disinhibition, a type of impulsivity-like behaviour, in wild-type (WT) and tryptophan hydroxylase 2 (Tph2) knock-in (Tph2KI) mice, which exhibit ~60-80% reductions in the levels of brain 5-HT due to a R439H mutation in Tph2. We also investigated the effects of 5-HT deficiency and early life stress on adult hippocampal neurogenesis, plasma corticosterone levels and several signal transduction pathways in the amygdala. We demonstrate that MS slightly increases anxiety-like behaviour in WT mice and induces behavioural disinhibition in Tph2KI animals. We also demonstrate that MS leads to a slight decrease in cell proliferation within the hippocampus and potentiates corticosterone responses to acute stress, but these effects are not affected by brain 5-HT deficiency. However, we show that 5-HT deficiency leads to significant alterations in SGK-1 and GSK3ß signalling and NMDA receptor expression in the amygdala in response to MS. Together, these findings support a potential role for 5-HT-dependent signalling in the amygdala in regulating the long-term effects of early life stress on anxiety-like behaviour and behavioural disinhibition.

Slow-release delivery enhances the pharmacological properties of oral 5-hydroxytryptophan: mouse proof-of-concept.

5-hydroxytryptophan (5-HTP) has shown therapeutic promise in a range of human CNS disorders. But native 5-HTP immediate release (IR) is poorly druggable, as rapid absorption causes rapid onset of adverse events, and rapid elimination causes fluctuating exposure. Recently, we reported that 5-HTP delivered as slow-release (SR) in mice augmented the brain pro-serotonergic effect of selective serotonin reuptake inhibitors (SSRIs), without the usual adverse events associated with 5-HTP IR. However, our previous study entailed translational limitations, in terms of route, dose, and duration. Here we modeled oral 5-HTP SR in mice by administering 5-HTP via the food. We modeled oral SSRI treatment via fluoxetine in the water, in a regimen recapitulating clinical pharmacokinetics and pharmacodynamics. 5-HTP SR produced plasma 5-HTP levels well within the range enhancing brain 5-HT function in humans. 5-HTP SR robustly increased brain 5-HT synthesis and levels. When administered with an SSRI, 5-HTP SR enhanced 5-HT-sensitive behaviors and neurotrophic mRNA expression. 5-HTP SR's pro-serotonergic effects were stronger in mice with endogenous brain 5-HT deficiency. In a comprehensive screen, 5-HTP SR was devoid of overt toxicological effects. The present preclinical data, appreciated in the context of published 5-HTP clinical data, suggest that 5-HTP SR could represent a new therapeutic approach to the plethora of CNS disorders potentially treatable with a pro-serotonergic drug. 5-HTP SR might in particular be therapeutically relevant when brain 5-HT deficiency is pathogenic and as an adjunctive augmentation therapy to SSRI therapy.

Insensitivity of NMRI mice to selective serotonin reuptake inhibitors in the tail suspension test can be reversed by co-treatment with 5-hydroxytryptophan.

RATIONALE: Exploring differences between mouse strains in drug effects in models of antidepressant-like activity may provide clues to the neurobiology of antidepressant responses. OBJECTIVES: The objective of this study was to explore whether insensitivity to selective serotonin reuptake inhibitors (SSRIs) in NMRI mice in the tail suspension test can be related to 5-hydroxytryptamine (5-HT) function. MATERIALS AND METHODS: We compared NMRI and C57Bl/6 mice, a SSRI-sensitive strain, in the tail suspension test following citalopram, paroxetine, or fluoxetine and determined 5-HT transporter (5-HTT) densities, 5-HT tissue and extracellular levels, 5-HT synthesis, tryptophan hydroxylase 2 (TPH2) genotypes and hypothermia induced by the 5-HT(1A) agonist 8-OH-DPAT. In NMRI mice, we tested if co-treatment with 5-HTP would increase 5-HT levels and confer SSRI sensitivity in the tail suspension test. RESULTS: C57Bl/6, but not NMRI, mice responded to SSRIs in the tail suspension test. 5-HTT densities in the frontal cortex and hippocampus were similar between the strains. NMRI mice had lower tissue 5-HT levels in these regions and decreased extracellular 5-HT in the frontal cortex at baseline and following citalopram. C57Bl/6 mice were more sensitive to 8-OH-DPAT-induced hypothermia. Both strains had the 1473C TPH2 genotype and similar 5-HT synthesis. In NMRI mice, 5-HTP co-treatment restored the tail suspension and extracellular 5-HT responses to SSRIs to levels equivalent to those seen in C57Bl/6 mice. CONCLUSION: Low 5-HT function in NMRI mice may account for their insensitivity to SSRIs in the tail suspension test. As the tail suspension test is a predictor of clinical efficacy, the current data suggest that 5-HTP adjunct treatment may benefit SSRI treatment refractory patients.

Chronic corticosterone decreases brain-derived neurotrophic factor (BDNF) mRNA and protein in the hippocampus, but not in the frontal cortex, of the rat.

This study examined the effects of chronic corticosterone (32 mg/kg/day, s.c., 21 days) on brain-derived neurotrophic factor (BDNF) mRNA and protein in the frontal cortex and hippocampus of the rat. Because evidence suggests that BDNF is an important determinant of the function of the 5-hydroxytryptamine (5-HT) system, we also quantified tissue levels of 5-HT and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), to investigate if changes in BDNF mRNA and protein paralleled changes in the 5-HT system. Corticosterone modestly decreased BDNF protein (-16.6%) in whole hippocampus and BDNF mRNA (-19%) in the CA3 area. In contrast, BDNF mRNA and protein in the frontal cortex were unchanged. In both the frontal cortex and hippocampus, tissue levels of 5-HT and 5-HIAA were increased and decreased, respectively. Combined, these data suggests that the effects of corticosterone on the BDNF system are not linked to the effects on the 5-HT systems. However, our findings do suggest that chronic corticosterone impairs hippocampal BDNF function, a finding with potential relevance for the hippocampal atrophy reported in major depression. Additionally, as inferred from the alterations in tissue levels of 5-HT and 5-HIAA, chronic corticosterone may influence the function of the 5-HT system.

Adjunctive 5-Hydroxytryptophan Slow-Release for Treatment-Resistant Depression: Clinical and Preclinical Rationale.

Serotonin transporter (SERT) inhibitors treat depression by elevating brain extracellular 5-hydroxytryptamine (5-HTExt). However, only one-third of patients respond adequately. Treatment-resistant depression (TRD) is a major unmet need. Interestingly, elevating 5-HTExt beyond what is achieved by a SERT inhibitor appears to treat TRD. Adjunctive administration of 5-hydroxytryptophan (5-HTP) safely elevates 5-HTExt beyond the SERT inhibitor effect in humans; however, 5-HTP cannot be a clinically viable drug because of its poor pharmacokinetics. A slow-release (SR) delivery mode would be predicted to overcome the pharmacokinetic limitations of 5-HTP, substantially enhancing the pharmacological action and transforming 5-HTP into a clinically viable drug. Animal studies bear out this prediction. Thus, adjunct 5-HTP SR could be an important new treatment for TRD. Here, we review the clinical and preclinical evidence for this treatment.

The interaction of escitalopram and R-citalopram at the human serotonin transporter investigated in the mouse.

RATIONALE: Escitalopram appears to be a superior antidepressant to racemic citalopram. It has been hypothesized that binding of R-citalopram to the serotonin transporter (SERT) antagonizes escitalopram binding to and inhibition of the SERT, there by curtailing the elevation of extracellular 5-hydroxytryptamine (5-HTExt), and hence anti-depressant efficacy. Further, it has been suggested that a putative allosteric binding site is important for binding of escitalopram to the primary, orthosteric, site, and for R-citalopram's inhibition here of. OBJECTIVES: Primary: Investigate at the human (h)SERT, at clinical relevant doses, whether R-citalopram antagonizes escitalopram-induced 5-HTExt elevation. Secondary: Investigate whether abolishing the putative allosteric site affects escitalopram-induced 5-HTExt elevation and/or modulates the effect of R-citalopram. METHODS: Recombinant generation of hSERT transgenic mice; in vivo microdialysis; SERT binding; pharmacokinetics; 5-HT sensitive behaviors (tail suspension, marble burying). RESULTS: We generated mice expressing either the wild-type human SERT (hSERT(WT)) or hSERT carrying amino acid substitutions (A505V, L506F, I507L, S574T and I575T) collectively abolishing the putative allosteric site (hSERT(ALI/VFL+SI/TT)). One mg/kg escitalopram yielded clinical relevant plasma levels and brain levels consistent with therapeutic SERT occupancy. The hSERT mice showed normal basal 5-HTExt levels. Escitalopram-induced 5-HTExt elevation was not decreased by R-citalopram co-treatment and was unaffected by loss of the allosteric site. The behavioral effects of the clinically relevant escitalopram dose were small and tended to be enhanced by R-citalopram co-administration. CONCLUSIONS: We find no evidence that R-citalopram directly antagonizes escitalopram or that the putative allosteric site is important for hSERT inhibition by escitalopram.

The 5-HT deficiency theory of depression: perspectives from a naturalistic 5-HT deficiency model, the tryptophan hydroxylase 2Arg439His knockin mouse.

A decreased level of brain 5-hydroxytryptamine (5-HT) has been theorized to be a core pathogenic factor in depression for half a century. The theory arose from clinical observations that drugs enhancing extracellular levels of 5-HT (5-HT(Ext)) have antidepressant effects in many patients. However, whether such drugs indeed correct a primary deficit remains unresolved. Still, a number of anomalies in putative biomarkers of central 5-HT function have been repeatedly reported in depression patients over the past 40 years, collectively indicating that 5-HT deficiency could be present in depression, particularly in severely ill and/or suicidal patients. This body of literature on putative 5-HT biomarker anomalies and depression has recently been corroborated by data demonstrating that such anomalies indeed occur consequent to severely reduced 5-HT(Ext) levels in a mouse model of naturalistic 5-HT deficiency, the tryptophan hydroxylase 2 His(439) knockin (Tph2KI) mouse. In this review, we will critically assess the evidence for 5-HT deficiency in depression and the possible role of polymorphisms in the Tph2 gene as a causal factor in 5-HT deficiency, the latter investigated from a clinical as well as preclinical angle.

Tryptophan hydroxylase 2 genotype determines brain serotonin synthesis but not tissue content in C57Bl/6 and BALB/c congenic mice.

Tryptophan hydroxylase 2 (TPH2) catalyzes the rate-limiting step in the synthesis of brain serotonin (5-HT). In a previous report, a single nucleotide polymorphism in mTph2 (C1473G) reduced 5-HT synthesis by 55%. Mouse strains expressing the 1473C allele, such as C57Bl/6, have higher 5-HT synthesis rates than strains expressing the 1473G allele, such as BALB/c. Many studies have attributed strain differences to Tph2 genotype without ruling out the potential role of alterations in other genes. To test the role of the C1473G polymorphism in strain differences, we generated C57Bl/6 and BALB/c mice congenic for the Tph2 locus. We found that the 1473G allele reduced 5-HT synthesis in C57Bl/6 mice but had no effect on 5-HT tissue content except for a slight reduction (15%) in the frontal cortex. In BALB/c mice, the 1473C allele increased 5-HT synthesis but again did not affect 5-HT tissue content. At the same time, 5-hydroxyindoleacetic acid (5-HIAA) was significantly elevated in BALB/c congenic mice. In C57Bl/6 mice, there was no effect of genotype on 5-HIAA levels. BALB/c mice had lower expression of monoamine oxidase A and B than C57Bl/6 mice, but there was no effect of Tph2 genotype. On the tail suspension test, escitalopram treatment reduced immobility regardless of genotype. These data demonstrate that the C1473G polymorphism determines differences in 5-HT synthesis rates among strains but only minimally affects 5-HT tissue levels.