Schizophrenia Research Forum - A Catalyst for Creative Thinking

A New Link between Serotonin and Depression

12 January 2006. There is a new link between serotonin and depression—it is a tiny protein of only 97 amino acids called p11. In the January 6 Science, Paul Greengard and colleagues at The Rockefeller University, New York, reported that p11 plays a key role in localizing 5-hydroxytryptmine 1B (5-HT1B) neuroreceptors to the cell surface, where they facilitate serotonergic transmission. Perhaps more importantly, the authors also report that levels of p11 are lower in brain samples taken from depressed patients, and that in animal models, tricyclic antidepressants or electroconvulsive therapy boosts levels of the protein in the brain. The findings could lead to a better understanding of the causes of depression, as well as point to more effective treatments, and may prove beneficial to the substantial number of schizophrenia patients who suffer from depression.

Most antidepressants work by increasing levels of serotonin, which is crucial for controlling mood. The specifics of serotonin’s role in depressed states have remained elusive, partly because there are at least 14 different types of serotonin receptor, each interacting with a plethora of other proteins that may have modulating effects. The 5-HT1B subtype is particularly interesting, however, because it is both an autoreceptor, appearing on neurons that make serotonin, and a heteroreceptor, found on projection neurons that are stimulated by serotonin-producing neurons. These are thought to modulate and facilitate, respectively, communication between neurons. 5-HT1B receptors have also been implicated in the pathology of a variety of psychiatric conditions, such as obsessive-compulsive disorder, anxiety, and aggression (see review by Moret and Briley, 2000).

In order to learn more about this receptor, first author Per Svenningsson and colleagues used the classic yeast two-hybrid screen to search for proteins that might bind to, and modulate, its activity. Using one of the intracellular loops of the receptor as “bait,” the authors fished out 29 clones, of which 26 turned out to harbor p11 cDNA. The interaction between the two proteins appears to be quite specific, because baits made from 5-HT1A, 5-HT2A, 5-HT5A, and 5-HT6 receptors, or baits using the dopamine D1 or D2 receptors, failed to attract the protein.

What affect might this small protein have on serotonin receptors? Clues to the role of p11 come from studies showing that it regulates translocation of transmembrane proteins to the cell surface. This may also be how it modulates 5-HT1B receptors, because when Svenningsson and colleagues transfected 5-HT1B-producing cells with p11 DNA, more of the serotonin receptors ended up at the cell membrane, and they appeared to be functional: 5-HT1B inhibits adenyl cyclase activity and in cells expressing p11, cyclic AMP production in response to forskolin was attenuated.

These data led the authors to examine the potential role of p11 in depression. They found modest though significant reductions in levels of p11 messenger RNA (15 percent lower) and protein (about 20 percent) in postmortem samples of anterior cingulate cortex from patients who had suffered from unipolar major depression. They also found that levels of the protein are markedly reduced in H/Rouen mice, a genetic model of depression. Furthermore, they found that p11 knockout mice had significantly fewer 5-HT1B receptors and exhibited a depressionlike phenotype, including decreased appetite, while animals overexpressing p11 behaved as if they were on antidepressants. In fact, when the authors used electroconvulsive therapy or administered the tricyclic antidepressant imipramine to mice, levels of p11 mRNA in the forebrain increased by 30 percent.

“Overall, this finding represents compelling evidence that p11 has a pivotal role in both the cause of depression and perhaps its successful treatment,” writes Trevor Sharp from the University of Oxford, England, in an accompanying Science perspective. In this regard, it might be worthwhile pursuing the development of drugs that can boost levels of p11. Of course, this would not necessarily represent a panacea for depression because other components of the serotonin signaling pathway, such as the serotonin transporter and other 5-HT receptors have also been implicated in the pathology of this condition (for a review, see Stockmeier, 2003).—Tom Fagan.

Svenningsson P, Chergui K, Rachleff I, Flajolet M, Zhang X, El Yacoubi M, Vaugeois J-M, Nomikos GG, Greengard P. Alterations in 5-HT1B receptor function by p11 in depression-like states. Science. January 6, 2006;311:77-80. Abstract

Comments on News and Primary Papers
Comment by:  Todd Gould
Submitted 13 January 2006
Posted 14 January 2006
  I recommend the Primary Papers

Proof that model organisms can “suffer” from psychiatric illness is at very best modest. However, the use of animal models gets around this either through symptom modeling or studying endophenotypes (see recent SRF endophenotype discussion and Gould and Gottesman, 2005). Thus, only facets, whether they be face valid “re-creations” of symptoms or models of inherent and quantifiable measures of brain functions, are utilized.

The recent paper by Svennignsson, Greengard, and colleagues takes advantage of these approaches to describe a novel function of p11, namely, the modulation of depression-like states. This includes increased tail suspension test (TST) immobility in mice where p11 has been removed (knockout; KO mice), and decreased TST immobility in mice that overexpress p11. Further, p11 KO mice spent more time along the “safer” sides of an open field, while mice overexpressing p11 tended to move away from the sides. These data are consistent with evidence that p11 is involved in modulating cellular pathways involved in depression-like and anxiety-like behavior. There are a number of additional behavioral tasks related to depression-like (e.g., forced swim test, learned helplessness) and anxiety-like (e.g., the elevated plus maze and black/white box) behavior, which could be studied in these mice. Additionally, the researchers used modern molecular and cellular biology, in addition to electrophysiological techniques, to strongly make the case that the behavioral changes likely involve interactions with the 5-HT1B receptor.

The authors “link” their basic science and rodent behavior data to humans by showing that both mRNA and protein levels of p11 are decreased in the postmortem brain of depressed patients compared to control subjects. Their finding that both ECT and imipramine increase p11 levels in the mouse brain suggests that similar effects could occur in humans. However, caution is warranted: The present field of psychiatric genetics was initiated with the understanding that human diseases are complex in nature—needing multiple genes working in disharmony with nongenetic contributors for the human syndrome. Thus, while a single gene disruption (e.g., p11) in the mouse may result in “human-related” psychiatric phenotypes, in humans, any involvement of p11 in the pathophysiology or treatment of mood disorders undoubtedly requires complex interactions with other susceptibility genes.

Gould TD, Gottesman II (in press): Psychiatric endophenotypes and the development of valid animal models. Genes, Brain, and Behavior. (full text courtesy of Genes, Brain, and Behavior)

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Primary Papers: Alterations in 5-HT1B receptor function by p11 in depression-like states.

Comment by:  Guang ChenHusseini K. Manji
Submitted 15 January 2006
Posted 15 January 2006

The recent manuscript from Per Svenningsson’s laboratory at the Karolinska Institute in Stockholm, and Paul Greengard’s laboratory at the Rockefeller University in New York has identified a molecule—p11—as a putative mediator of depressive states, and as a target of antidepressant drug action.

The major strength of the study is the diverse array of methodologies/paradigms utilized to provide convergent data. Thus, they used a yeast two-hybrid system, transfected cells, selectively bred animals, and transgenic and knockout animals, and even human postmortem brain samples. They report that the p11 protein mediates 5-HT1B receptor surfacing, 5-HT1B receptor-induced inhibition of the cAMP pathway and the ERK pathway, fEPSP, and 5-HT turnover. Importantly, they show that mRNA and/or protein levels of p11 are different in groups of sham versus antidepressant treated animals, non-helpless versus helpless selectively bred animals, and control versus depression patients’ postmortem brain tissue. Furthermore, transgenic mice potentially overexpressing p11 proteins exhibit increased locomotion and reduced thigmotaxis in the open field test and reduced immobility in the tail suspension test. Finally, p11 knockout mice exhibit increased thigmotaxis in the open field test, increased immobility in the tail suspension test, and reduced consumption of sweetened water in the sucrose preference test.

The 11 kDa p11 protein, which is also known as S100A10, is a member of the S100 family of Ca2+-binding proteins expressed in the CNS. The function of p11 in the CNS has hitherto been largely unknown. Traditional antidepressants have long been known to exert their initial effects on increasing the synaptic levels of serotonin and/or norepinephrine. However, a major problem has been the delay in therapeutic effects (often several weeks) observed in patients. This has led to the suggestion that the delayed therapeutic effects may be due to delayed, adaptive changes in serotonergic receptors and post-receptor signaling cascades. Along with the 5-HT1A and 5-HT2A receptors, the 5-HT1B receptor has been postulated to represent a delayed, therapeutically relevant target for the actions of antidepressants. In this study, the authors focused upon the 5-HT1B receptor. In an open-ended search for the molecules directly interacting with 5-HT1B receptor using yeast two-hybrid screen, these investigators found that p11 selectively controls the number of 5-HT1B receptor at the cell surface; indeed, trafficking of important receptors to the cell surface appears to represent a major form of regulating various forms of neural plasticity. Concomitant with the effects on surface 5-HT1B receptors, p11 alters 5-HT1B receptor-mediated signaling processes, including inhibition of cAMP production and inactivation of the extracellular signal-regulated kinase (ERK) pathways. At a more “systems level,” they found that p11 attenuates corticoaccumbal glutamatergic synaptic transmission.

They then investigated the effects of manipulating the p11 protein in the context of depressive-like behaviors, and antidepressant models. While animal models of depression/antidepressant drug action do have some limitations, they can often provide useful information in the gene-to-human behavior causal chain. One such rodent model is the learned helplessness model, in which the rodents develop some of the behavioral/biochemical changes of depression, many of which are reversed by antidepressant treatment. Notably, the researchers found lower levels of p11 mRNA in forebrain of rodents that exhibited learned helplessness. Interestingly, the investigators found that chronic antidepressants (either chemical or electroconvulsive seizures) increased forebrain p11 mRNA and protein levels.

Svenningsson and colleagues then investigated the effects of genetically decreasing/increasing p11 levels in brain by generating knockout/transgenic mice. Mice lacking p11 exhibited increased thigmotaxis (the tendency to stay close to the walls; a putative measure of anxiety-like behavior) in the open field test, increased immobility in the tail suspension test (another test of depression-like behavior), and attenuated consumption of sweetened water (a putative test of hedonic drive). By contrast, transgenic mice overexpressing p11 in cortex, hippocampus, and striatum showed increased locomotion and reduced attenuated immobility in the tail suspension test.

A study of postmortem human brain tissue from depressed subjects also revealed a small, but statistically significant, decrease in p11 mRNA levels.

In toto, these novel, intriguing convergent data from multiple sources raise the distinct possibility that p11 may play a role in depressive behavior, and that directly targeting p11 may represent a novel strategy for the development of improved therapeutics. A number of issues still need to be addressed:

1. Interestingly, even the p11 knockout mice respond to imipramine and anpirtoline (a 5-HT1B receptor agonist) in the tail suspension test (albeit more modestly), suggesting the existence of the functional homologs of p11 for mediating behavioral effects of antidepressants.

2. A major issue in behavioral animal models of antidepressant action is a temporal one. Thus, while chronic (weeks) of antidepressant administration are required to produce changes in p11 levels, the drugs exert rapid effects in the animal models used here. This suggests that in the tail suspension test, p11 is unlikely to be mediating the effects of acute antidepressants. However, this is a problem faced by the field with respect to target validation, and hardly unique to this study.

3. The issue of how p11 is up-regulated by antidepressant treatments and down-regulated in depression at transcription level remains completely unknown.

4. Does p11 modulate other G protein-coupled receptors on the cell surface, especially those which have been potentially more implicated in the pathophysiology of depression (e.g., 5-HT1A or 5-HT2A)?

5. The 5-HT1B receptor has been shown to play a role in regulating hippocampal neurogenesis. Does p11 play a role in the hippocampal neurogenesis produced by antidepressants?

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Primary Papers: Alterations in 5-HT1B receptor function by p11 in depression-like states.

Comment by:  Etienne Sibille
Submitted 17 January 2006
Posted 17 January 2006

As the 5-HT system is involved in the pathology and pharmacological treatment of depression, any new evidence for genes or molecules regulating its function has putative implication for mechanisms and/or treatment of depression. Here, Svenningsson et al. provide compelling evidence about the identification and role of p11 in mediating some of the downstream effects of 5-HT1B receptor signaling. In particular, the authors, using several complementary approaches, demonstrate that p11 levels correlate with 5-HT1B receptor level and function at the membrane. Thus, p11 levels may be considered an “index” of 5-HT1B receptor function. Furthermore, manipulations that increase 5-HT function (i.e., chronic antidepressant treatment and ECT) up-regulate p11 levels, possibly responding to increased demand on 5-HT1B function to regulate presynaptic release. The fact that these up-and-down manipulations of p11 correlate with behaviors in the mouse that have been associated either with changes that occur after antidepressant treatment or in “depression-like” or increased fearfulness states strengthens the link between serotonin function and depressive states. In turn, the small changes observed in the human depressed subjects could correspond to a balance between decreased levels due to depression, which are partially reversed by antidepressant exposure in some subjects. These results will await confirmation and clarifications.

An ongoing debate in the depression field relates to the cellular and regional sites of action and/or role of serotonin receptors in the etiology of depression, but also on the nature of the networks supporting therapeutic effects after chronic antidepressant treatment. And here an interesting dissociation is worth noting. Indeed, the work of Svenningsson et al. provides evidence for “postsynaptic”, or “non-serotonergic” neurons mediating some of the depression-related behavior. Chronic antidepressant treatment or ECT increase p11 mRNA in frontal cortex but not in raphe, indicating that changes occur in non-serotonergic cortical neurons (mRNAs for presynaptic 5-HT1B receptor in serotonergic neurons are located in the raphe nucleus). Furthermore, due to the expression pattern of CAMK2, increased p11 levels in non-serotonergic neurons of Tg-p11 mice correlate with antidepressant-like state, suggesting that the neural mechanisms that are responsible for the expression of the depression-like and antidepressant-like phenotypes in this study are probably modulated, but not mediated, by 5-HT (i.e., 5-HT1B inhibition/modulation of other neurotransmitter system). In other words, the nature of the neurons/network that are regulated by 5-HT1B presynaptic inhibition and that may mediate the behavioral phenotypes are non-serotonergic.

Evidence is limited on the role of the 5-HT1B receptor in depression, especially when compared to the 5-HT1A receptor or the 5-HT transporter, for instance. So it is not known at this point whether p11 represents a novel candidate for dysfunction in depression. However, this study identified an important new element in the complexity of serotonergic neurotransmission, and may guide future studies towards specific cortical networks that either express presynaptic 5-HT1B receptors or that are targeted by these neurons. As the authors judiciously noted in their conclusion, their work sheds light on “molecular adaptations occurring in neuronal networks that are dysfunctional in depression-like states.”

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Primary Papers: Alterations in 5-HT1B receptor function by p11 in depression-like states.

Comment by:  Patricia Estani
Submitted 17 January 2006
Posted 17 January 2006
  I recommend this paperComment by:  Mary Reid
Submitted 21 January 2006
Posted 23 January 2006

It's most interesting that Paul Greengard and colleagues report lower levels of p11 in brain samples from depressed patients. Renegunta et al. report that knockdown of p11 with siRNA enhanced trafficking of TASK-1 to the surface membrane. Hopwood et al. find that present data suggest that the excitatory effects of 5-HT on DVN are mediated in part by inhibition of a TASK-like, pH-sensitive K+ conductance, and the Perrier group reports that 5-HT1A receptors inhibit TASK-1-like K+ current in the adult turtle. Might we suspect that a specific inhibitor of TASK-1 conductance would be beneficial in depression, and might this in part explain the benefit reported by SSRIs and agents with 5-HT1A receptor agonist activity in the treatment of depression?

Renigunta V, Yuan H, Zuzarte M, Rinne S, Koch A, Wischmeyer E, Schlichthorl G, Gao Y, Karschin A, Jacob R, Schwappach B, Daut J, Preisig-Muller R. The Retention Factor p11 Confers an Endoplasmic Reticulum-Localization Signal to the Potassium Channel TASK-1. Traffic. 2006 Feb;7(2):168-81. Abstract

Hopwood SE, Trapp S. TASK-like K+ channels mediate effects of 5-HT and extracellular pH in rat dorsal vagal neurones in vitro. J Physiol. 2005 Oct 1;568(Pt 1):145-54. Epub 2005 Jul 14. Abstract

Perrier JF, Alaburda A, Hounsgaard J. 5-HT1A receptors increase excitability of spinal motoneurons by inhibiting a TASK-1-like K+ current in the adult turtle. J Physiol. 2003 Apr 15;548(Pt 2):485-92. Epub 2003 Mar 7. Abstract

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Related News: SfN Atlanta: Paul Greengard on DARPP-32 and p11

Comment by:  Karl-Ludvig Reichelt (Disclosure)
Submitted 7 November 2006
Posted 7 November 2006

Serotonin Transmission in Mental Disorders
As always, Greengard makes outstanding contributions. Very, very interesting.

We, as well as several other groups, have demonstrated peptide increases in schizophrenia (Hole et al., 1979; Drysdale et al., 1982; Idei et al., 1982; Cade et al., 2000) and also in several other disorders (e.g., Cade et al., 2000; Reichelt and Knivsberg, 2003). This confirms older data from Sweden (Lindstrom et al., 1986), where opioids were found, but measured as receptor binding total level. Unfortunately they named these endorphins, too, while we find that these are probably exorphins.

Opioids affect uptake and release of monoamines, and long ago we could demonstrate uptake inhibition of dopamine and serotonin (Hole et al., 1979), and later a serotonin uptake stimulating tripeptide, which in oocytes from frog stimulate the transport protein in a bell-shaped dose response (hormetic) manner (Pedersen et al., 1999; Keller, 1997). There has been some dispute about the structure of the tripeptide, and we are re-running mass spectrometry as soon as possible to see if we made any mistake. The structure we arrived at was pyroglu-trp- glyNH2 and in depression (in press) pyroglu-trp-gly.

Peptides are a bit tricky because of their considerable tendency to aggregate (Reichelt, in press), which might explain some of the problems. We use tri-fluoroacetic acid (TFA) on HPLC, therefore, and offline mass spectrometry in methanol formic acid (10mM). Formic acid 10mM is not electrometrically as strong and dissociating as TFA. (The mass spectrometry does not tolerate TFA well). In our hands, formic acid 10 mM does not deaggregate all peptide complexes.

Be that as it may, peptides regulating uptake and release of transmitters have been neglected too long. Also, the immune data on peptides in brain should by and large have been confirmed by independent methods such as HPLC and also, preferably, mass spectrometry. Immuno-like does not really ensure identity. For an overview of schizophrenia in this regard, see Reichelt et al, 1996.

We do not know what percentage of the schizophrenics show peptide increases, but a fairly large untreated cohort does. (We have great problems in getting untreated patient urine, 10 ml of the first morning urine (frozen) of carefully diagnosed cases). Our data seem able to explain the onset and suggest reasonable treatment, as shown for autism (Knivsberg et al., 1995; Knivsberg et al., 2002). It does not apply to all, of course, but a large percentage. The curse of medicine is that diagnosis is usually based on symptoms and not aetiology, almost like Morbus febris once was a diagnosis, but with a thousand different causes. We have suffered considerable opposition as would be expected, but now seem to get support from many experiments carried out properly.


Cade RJ, Privette M, Fregly M, Rowland N, Sun Z, Zele V, Wagemaker H and Edelstein C (2000) Autism and schizophrenia: intestinal disorders. 3: 57-72.

Keller J. (1997) Impact of autism-related peptides and 5-HT system manipulations on cortical development and plasticity -Ist Ann report for EU proj. BMH4-CT96-0730 pp 1-10.

Knivsberg A-M, Reichelt KL, Nödland M and Höien T. (1995) Autistic syndromes and diet :a follow up study. Scand J Educat. Res 39: 225-236.

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Related News: Cortical Neurons Underlying Antidepressant Effects Identified

Comment by:  Lei ZhangDavid M. BenedekCarol S. FullertonRobert J. Ursano
Submitted 29 May 2012
Posted 29 May 2012

P11 (S100A10) in cortical neurons' response to antidepressants: A new story for p11
The potential role of p11 (S100A10) in the pathogenesis of psychiatric illness is gaining increasing interest in the neuroscience community. Recent studies suggest that p11 may be important in a number of disorders including posttraumatic disorder (PTSD) and depression (Su et al., 2009; Zhang et al., 2011; Zhang et al., 2008; Svenningsson and Greengard, 2007; Verma et al., 2007; Warner-Schmidt et al., 2009). P11 has been considered as a potential biomarker or therapeutic target for the treatment of depression and PTSD (Su et al., 2009; Zhang et al., 2011; Svenningsson and Greengard, 2007). However, the neuronal cell target or targets for p11-involved treatment in the central nervous system (CNS) remain unknown. In the current study, the authors find that corticostriatal projection neurons are the critical cells for the response to antidepressants in the CNS, and suggest that the regulation of serotonergic tone in this single cell type plays a pivotal role in antidepressant therapy. They also demonstrate that in corticostriatal pyramidal cells, p11 and serotonin receptor 4 are abundantly expressed, suggesting their strong and specific responses to chronic antidepressant treatment. As antidepressants are also a mainstay of PTSD treatment, their findings suggest that p11 may also mark treatment response in this disorder. Therefore, their findings of p11 action at the neuronal cellular level will facilitate the understanding of the pathology and drug development of stress-related mental disorders such as depression and PTSD.


1. Su, T.P., et al. Levels of the potential biomarker p11 in peripheral blood cells distinguish patients with PTSD from those with other major psychiatric disorders. Journal of psychiatric research 43, 1078-1085 (2009). Abstract

2. Zhang, L., et al. P11 (S100A10) as a potential biomarker of psychiatric patients at risk of suicide. Journal of psychiatric research 45, 435-441 (2011). Abstract

3. Zhang, L., et al. p11 is up-regulated in the forebrain of stressed rats by glucocorticoid acting via two specific glucocorticoid response elements in the p11 promoter. Neuroscience 153, 1126-1134 (2008). Abstract

4. Svenningsson, P. and Greengard, P. p11 (S100A10)--an inducible adaptor protein that modulates neuronal functions. Current opinion in pharmacology 7, 27-32 (2007). Abstract

5. Verma, R., et al. Investigating the role of p11 (S100A10) sequence variation in susceptibility to major depression. American journal of medical genetics. Part B, Neuropsychiatric genetics : the official publication of the International Society of Psychiatric Genetics 144B, 1079-1082 (2007). Abstract

6. Warner-Schmidt, J.L., et al. Role of p11 in cellular and behavioral effects of 5-HT4 receptor stimulation. Journal of neuroscience 29, 1937-1946 (2009). Abstract

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