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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Chem Neuroanat. Author manuscript; available in PMC 2012 July 1.
Published in final edited form as:
PMCID: PMC3350375
NIHMSID: NIHMS304464

Development by environment interactions controlling tryptophan hydroxylase expression

Abstract

Tryptophan hydroxylase is the rate-limiting enzyme in the biosynthesis of serotonin (5-hydroxytryptamine; 5-HT). Two isoforms of tryptophan hydroxylase, derived from different genes, tph1 and tph2, have been identified. The tph1 isoform is expressed in peripheral tissues, whereas tph2 is brain and neuron-specific. Recent studies suggest that tph2 expression and brain serotonin turnover are upregulated in depressed suicide patients, and drug-free depressed patients, respectively. Increased tph2 expression could result from genetic influences, early life developmental influences, adverse experience during adulthood, or interactions among these factors. Studies in rodents support the hypothesis that interactions between early life developmental influences and adverse experience during adulthood play an important role in determining tph2 expression. In this review, we highlight the evidence for the effects of adverse early life experience and stressful experience during adulthood on both tph1 and tph2 expression.

Keywords: tryptophan hydroxylase, tph1, tph2, serotonin, dorsal raphe nucleus

Introduction

Tryptophan hydroxylase is the rate-limiting enzyme in the biosynthesis of serotonin (5-hydroxytryptamine; 5-HT). Two isoforms have been identified, which are derived from different genes, tryptophan hydroxylase 1 (tph1) and tryptophan hydroxylase 2 (tph2). Tph1 is expressed predominantly in peripheral tissues, while tph2 is expressed in serotonergic neurons within the central nervous system. Recent studies suggest that tph2 expression within the dorsal raphe nucleus (DR), an important source of serotonergic innervation of limbic forebrain structures, is elevated in depressed suicide patients (Bach-Mizrachi et al., 2006;Bach-Mizrachi et al., 2008;Boldrini et al., 2005;Bonkale et al., 2006;Underwood et al., 2004;Underwood et al., 2010;Underwood et al., 1999) as well as in DR projection regions (Perroud et al., 2010). Expression of tph2 mRNA in projection regions of the DR is thought to reflect transport of tph2 transcripts from the brainstem raphe nuclei to the nerve terminal, allowing local serotonin synthesis at the synapse (Perroud et al., 2010). These findings are consistent with the observation that brain serotonin turnover is elevated in depressed patients and returns to baseline following successful antidepressant treatment (Barton et al., 2008). Finally, genetic linkage studies support an association between tph2 genetic polymorphisms and depression (Zhang et al., 2005). Together, these findings suggest that elevated tph2 expression and activity may be a biomarker or endophenotype of depression.

Elevated tph2 expression in depressed suicide victims could result from genetic influences, adverse early life experience, chronic stress or major life events during adulthood, or medication. However, a picture is emerging of interactions among these factors being important determinants of tph2 regulation. Genetic regulation of tph2 expression has been reviewed in a number of excellent review articles (see especially, Matthes et al., 2010;Popova and Kulikov, 2010). Briefly, in a number of studies genetic polymorphisms in tph2 have been associated with major depressive disorder (MDD; Haghighi et al., 2008;Tsai et al., 2009;Zhang et al., 2005;Zill et al., 2004a), bipolar disorder (Cichon et al., 2008;Harvey et al., 2004;Roche and McKeon, 2009;Van Den Bogaert et al., 2006), and suicidal behavior (Ke et al., 2006;Lopez de Lara et al., 2007;Zill et al., 2004b). In this review, we focus on development (D) × environment (E) interactions determining tph2 expression and the potential consequences of altered basal tph2 expression during adulthood and altered vulnerability to stress-induced changes in tph2 expression during adulthood.

Developmental influences on tph1 and tph2 expression

Whereas the predominant isoform of tryptophan hydroxylase expressed in the brain during adulthood is tph2, some evidence suggests that both tph1 and tph2 are expressed in the brainstem raphe complex during development (Nakamura et al., 2006). Studies using reverse transcriptase polymerase chain reaction (RT-PCR) in mice suggest that tph1 expression peaks at weaning (postnatal day (P) 21) and then is nearly absent by adulthood (Nakamura et al., 2006). Meanwhile, tph2 expression is relatively constant from P7 through adulthood. Consequently, developmental influences on serotonergic signaling may alter adult phenotypes through actions on either tph1 or tph2 expression during the critical neonatal period. Nakamura and colleagues (2006) argue that at weaning tph1 is a major determinant of serotonergic signaling due to a higher affinity for tryptophan and a higher enzymatic activity. However, more recent studies by Gutknecht and colleagues (2009), using quantitative RT-PCR, in situ hybridization and immunohistochemistry, found no detectable tph1 mRNA or protein expression during the postnatal period in mouse or human brain, including P21 in mice. This raises the possibility that the tph1 expression in the studies by Nakamura and colleagues (2006) was present at very low levels, that are only readily detectable by RT-PCR, or that environmental influences during the neonatal period (housing conditions, diet, etc.) resulted in increased tph1 expression in these mice. A further possibility is that studies using brain microdissection and RT-PCR techniques, detect tph1 expression from non-neuronal sources. For example, tph1 expression has been characterized in peripheral vasculature (Linder et al., 2008), including arterial smooth muscle cells (Ni et al., 2008). The presence or absence of tph1 in the vasculature of the brainstem raphe complex has not been specifically addressed. In addition, tph1 is expressed by T cells (O'Connell et al., 2006), and upon activation, T cells can synthesize and release serotonin. Importantly, tph1 expression in T cells is dynamically regulated and is upregulated approximately 30-fold following T cell activation (Leon-Ponte et al., 2007). Thus, a portion of tph1 expression, measured by brain microdissection and RT-PCR techniques, could represent tph1 expression in non-neuronal sources, including T cells. In contrast, T cells do not express tph2 (Leon-Ponte et al., 2007;O'Connell et al., 2006). As stress-related stimuli can activate T cells (Merlot et al., 2004;Satoh et al., 2006;Schmidt et al., 2010), it is possible that stress-induced increases in tph1 expression in T cells can account for some previously reported stress-induced changes in tph1 expression in studies using brain microdissection and RT-PCR (see below for further discussion). Clearly, the cellular localization and regulation of tph1 expression during development requires further studies, taking into account the potential contribution of non-neuronal tissues to tph1 expression when using brain microdissection and RT-PCR techniques. Regardless of the source of tph1 expression that has been described in the brainstem raphe complex, genetic studies suggest that polymorphisms in the tph1 gene are associated with depression (Gizatullin et al., 2006;Nash et al., 2005); thus, further studies of D x E interactions in the control of tph1 expression are warranted.

Few studies have investigated factors controlling either tph1 or tph2 expression during the neonatal period. However, a study by Sidor and colleagues (2010) found that treatment of neonatal mice with the bacterial cell wall component, lipopolysaccharide (LPS), increased tph2 mRNA expression within the dorsolateral part of the dorsal raphe nucleus/ventrolateral periaqueductal gray region (DRVL/VLPAG) at P14, but not at P17, 21, or 28. This effect was specific for the DRVL/VLPAG region and was not seen in other subregions of the DR. Interestingly, however, the LPS-induced increase in tph2 expression was observed in the same subset of serotonergic neurons that have been shown to be activated by LPS in adult mice (Hollis et al., 2005). To our knowledge, no studies have yet examined the control of tph1 expression by environmental factors during the neonatal period, and this remains an important objective for future studies as altered tph1 expression during this critical period of development may alter the adult phenotype. This is of particular importance because, as will be described below, tph1 expression appears to be more stress-sensitive than tph2, at least during adulthood.

Studies by Lowry and colleagues (Gardner et al., 2009) suggest that early life experience during the neonatal period can influence basal tph2 expression during adulthood. In these studies, neonatal rats were exposed to one of three conditions, i.e. neonatal handling, maternal separation, or animal facility rearing control conditions, during P2-P14. Neonatal handling involves separation of pups from the dam for 15 min each day, whereas maternal separation involves separation of pups from the dam for 180 min each day. Neonatal handling has been associated with decreased hypothalamic-pituitary-adrenal (HPA) axis responses to stress and decreased anxiety in adulthood (Ladd et al., 2000;Meaney et al., 1996). In contrast, maternal separation has been associated with increased HPA axis responses to stress and increased anxiety-state, anhedonia, increased ethanol preference and impairment of sexual behavior in males during adulthood (Huot et al., 2001;Kalinichev et al., 2002;Ladd et al., 1996;Plotsky and Meaney, 1993;Rhees et al., 2001;Wigger and Neumann, 1999).

As adults, rats previously exposed to neonatal handling during development had decreased tph2 expression in subdivisions of the DR, measured using in situ hybridization histochemistry, relative to animal facility reared control rats (Gardner et al., 2009). Specifically, rats exposed to neonatal handling as pups had decreased tph2 mRNA expression, relative to animal facility reared controls, in the DRVL/VLPAG region, bilaterally (cf. Fig 1A and 1C).

Figure 1
Early life experience and adverse experience in adulthood interact to alter tph2 mRNA expression in subregions of the dorsal raphe nucleus of rats (Adapted with permission, from figure 5 of Gardner et al., 2009). Autoradiographic images illustrate tph2 ...

When tph2 expression was considered at specific rostrocaudal levels of specific subdivisions of the DR, additional differences between neonatal handled and control rats emerged. Rats exposed to neonatal handling had decreased tph2 mRNA expression, relative to animal facility reared controls, specifically in the mid-rostrocaudal portion of the dorsal raphe nucleus, dorsal part (DRD; approximately 8.25 mm bregma; cf. Fig 1A and 1C). Interestingly, this portion of the DR is an important component of a stress-and anxiety-related neuronal circuit (Commons et al., 2003;Hale and Lowry, 2010;Lowry et al., 2008;Lowry and Hale, 2010), and increased activity of DRD serotonergic neurons is thought to result in increases in anxiety state (Lowry et al., 2005;Lowry et al., 2008;Lowry and Hale, 2010;Maier and Watkins, 2005). The DRD is activated by a number of stress- and anxiety-related stimuli including; anxiogenic drugs (Abrams et al., 2005), the anxiety-related neuropeptide, urocortin 2 (Amat et al., 2004;Ucn 2; Hale et al., 2010;Staub et al., 2005;Staub et al., 2006), social defeat (Gardner et al., 2005) and uncontrollable stress (Amat et al., 2005;Grahn et al., 1999). In addition, alcohol-dependent depressed suicide patients show a 46% increase in TPH expression in the DRD compared with controls (Bonkale et al., 2006). Thus, rats exposed to neonatal handling, which have a decreased anxiety state as adults, have decreased tph2 expression in the mid-rostrocaudal DR, a region associated with responses to anxiogenic drugs and anxiogenic stimuli.

As adults, rats previously exposed to maternal separation during development had increased tph2 expression in subdivisions of the DR, measured using in situ hybridization histochemistry, relative to animal facility reared control rats (Gardner et al., 2009). Rats exposed to maternal separation had increased tph2 mRNA expression, relative to animal facility reared controls, in the caudal portion of the dorsal raphe nucleus (DRC, including its dorsal and ventral parts; approximately 8.42 and 8.50 mm bregma). Like the DRD, the DRC appears to be part of a stress- and anxiety-related neuronal circuit, and is activated by anxiogenic drugs (Abrams et al., 2005) and Ucn 2 (Hale et al., 2010;Staub et al., 2005;Staub et al., 2006). Meanwhile, blockade of corticotropin-releasing factor (CRF) receptors in the DRC, but not the rostral part of the DR, blocks the behavioral effects of inescapable stress (Hammack et al., 2002), including escape deficits and potentiation of fear conditioning in a model of learned helplessness. In addition, exposure to unpredictable acoustic stimulation in vivo, or application of CRF to rat brain slices in vitro, increases TPH activity selectively in the DRC (Evans et al., 2009). Depressed suicide patients have elevated tph2 expression that is also restricted to the DRC (Bach-Mizrachi et al., 2008). As both the number and density of tryptophan hydroxylase-positive neurons is higher in depressed suicide patients, it is possible that depressed patients have increased tph2 expression that is dependent on adverse early life experience, independent of stressful life events during adulthood. This might confer a vulnerability to stress-related psychiatric disorders. Indeed children exposed to adverse experience have increased risk of developing depressive or anxiety disorders as adults (Heim and Nemeroff, 2001;McEwen, 2003;Ressler et al., 2010).

Environmental influences on tph1 and tph2 expression

As illustrated in Fig. 2, TPH1 immunoreactivity is evident in the pineal gland of adult male rats, but not in DR serotonergic neurons; conversely, TPH2 immunoreactivity is evident in the DR, but not in the pineal gland. A number of studies have characterized the distribution of tph1 and tph2 in adult rodents and humans (Gutknecht et al., 2009;Liang et al., 2004;Malek et al., 2005;Nakamura et al., 2006;Patel et al., 2004;Perroud et al., 2010;Sakowski et al., 2006;Sugden, 2003;Sugden et al., 2009;Zill et al., 2005;Zill et al., 2009). Generally, tph2 is believed to be the neuron-specific isoform, and tph1 is believed to be expressed in peripheral tissues, including the pineal gland, enterochromaffin cells, etc. However, a number of studies, using RT-PCR and in situ hybridization histochemistry techniques, have reported low levels of tph1 expression in the brainstem raphe complex of rodents (Abumaria et al., 2008;Gundlah et al., 2005;Nakamura et al., 2006). However, as recently pointed out by Gutknecht and colleagues (2009), the probe that was used to detect tph1 using in situ hybridization histochemical techniques (Abumaria et al., 2008) is directed against a nucleotide sequence that includes an extensive stretch of the coding region of tph1, including highly conserved functional domains that are highly homologous (85% identity) to tph2 at the amino acid level, corresponding to 72% sequence identity in the two isoforms. Therefore, it is possible that the in situ hybridization histochemistry using this riboprobe results in cross-hybridization to tph2. This possibility is highlighted by studies by Malek and colleagues (2005), in which in situ hybridization using a riboprobe directed against a portion of the coding region of tph1 detected a low level of mRNA expression in the midbrain raphe complex, whereas in situ hybridization using a selective oligonucleotide probe did not. In some studies, low levels of tph1 mRNA have been detected in microdissected tissues including the midbrain raphe complex using RT-PCR (Abumaria et al., 2008;Nakamura et al., 2006), whereas in other studies, they have not (Gutknecht et al., 2009). As described briefly above, it remains possible that studies detecting low levels of tph1 in microdissections of the midbrain raphe complex are detecting tph1 that is expressed in blood components, including T cells, as these studies did not perfuse the brain tissue in order to remove blood components prior to collecting brain tissue. In one study comparing tph1 and tph2 expression in microdissected tissues from the midbrain raphe complex in perfused and non-perfused brain (Gutknecht et al., 2009), there was no detectable tph1 expression in either perfused or non-perfused tissue using RT-PCR techniques. Similar procedures should be used when evaluating stress-induced increases of tph1 in microdissected brain tissues to evaluate whether or not stress-induced increases in tph1 are dependent on expression in neurons or blood components such as T cells (Leon-Ponte et al., 2007). Conclusive demonstration of tph1 expression specifically in serotonergic neurons will require further studies, including use of nucleotide probes for in situ hybridization that are directed toward non-conserved regions of tph1 mRNA, such as the 3’ untranslated region, and ensuring that low levels of tph1 expression detected using RT-PCR in microdissected brain tissues are not due to the presence of blood components that are known to express tph1, such as T cells.

Figure 2
Tryptophan hydroxylase 2- (TPH2-), but not tryptophan hydroxylase 1- (TPH1-) like immunoreactivity (ir) is present in cell soma and fibers in the dorsal raphe nucleus (DR) and median raphe nucleus (MnR), while TPH1- but not TPH2-like immunoreactivity ...

Tryptophan hydroxylase protein is expressed in a circadian pattern in forebrain structures, including the suprachiasmatic nucleus, and intergeniculate leaflet, brain structures that are important for control of circadian function (Malek et al., 2004). In the median raphe nucleus and in the lateral wings of the DR, TPH protein concentrations peak approximately 6 h into the dark phase of a 12 h light: 12 h dark light cycle, a pattern that persists in total darkness. In contrast, TPH protein in terminal regions peaks before the onset of the dark phase and declines thereafter, with peak concentrations occurring approximately 18 h following the peak TPH concentration in the midbrain raphe nuclei. The peak in TPH in terminal regions precedes the increase in serotonin release that occurs at the onset of the dark phase (Rueter et al., 1997) and the associated increase in behavioral arousal. Tph2 mRNA expression throughout the DR and median raphe nuclei also varies in a diurnal pattern. Studies by Malek and colleagues (2005) have shown tph2 mRNA expression in the midbrain raphe nuclei peaks approximately 2 h prior to the onset of the dark phase, or 8 h prior to the peak of midbrain TPH protein concentrations. In further studies using adrenalectomy and corticosterone replacement (Malek et al., 2007), the authors demonstrated that the diurnal pattern of tph2 mRNA expression is dependent on glucocorticoids. Tph2 mRNA expression increases in the late afternoon/early evening, coinciding with the increase in plasma corticosterone concentrations.

As far as we are aware, no rodent or primate studies to date have compared tph2 expression in males and females. However, it is clear that female sex hormones play an important role in tph2 expression. Daily, systemic treatment of ovariectomized female rats with diarylpropionitrile (DPN), a selective estrogen receptor beta (ERβ) agonist, increases tph2 expression selectively in the mid-rostrocaudal DRD and the DRC (Donner et al., 2007). Local treatment with DPN, using stereotaxically implanted wax pellets containing the drug, increased tph2 expression in the same regions. Additional studies using local treatment with 17-β-estradiol using stereotaxically implanted wax pellets (Donner et al., 2007) or systemic treatment with estrogen (Hiroi et al., 2006), found increased tph2 expression in the mid-rostrocaudal DRD and DRC, respectively. These studies in rats are consistent with studies in primates showing that systemic treatment of ovariectomized female macaques with estrogen, progesterone, or estrogen plus progesterone for 1 month increases tph2 expression, as measured using in situ hybridization histochemistry (Sanchez et al., 2005), or RT-PCR using microdissected raphe nuclei or pools of serotonergic neurons obtained using laser capture microdissection techniques (Bethea and Reddy, 2008). Studies in primates (Bethea et al., 2000) and guinea pigs (Lu et al., 1999) demonstrate that similar treatments also increase TPH protein. Together, these studies provide strong support for the hypothesis that female sex hormones are strong determinants of tph2 expression, particularly in the mid-rostrocaudal and caudal parts of the DR, regions that give rise to projections to forebrain limbic structures controlling cognitive function and mood (Hale and Lowry, 2010;Lowry et al., 2008).

A number of studies suggest that tph2 mRNA expression is relatively insensitive to exposure to acute, and to some chronic, stress-related stimuli including social defeat (Gardner et al., 2009), chronic restraint stress (Abumaria et al., 2008), and chronic social stress (Abumaria et al., 2006). As detailed topographical analysis of the midbrain raphe complex was not conducted in several of these studies, it may be the case that there were increases in tph2 mRNA expression in specific subregions of the raphe nuclei, but these were undetected. Consistent with this hypothesis, chronic mild stress in adult male mice increases tph2 expression in the mid-rostrocaudal DRD and in the DRC, but not other regions of the DR (McEuen et al., 2008). Alternatively, genetic or developmental factors may alter the vulnerability to stress-induced elevations of tph2 expression (Gardner et al., 2009).

Multiple studies have found that tph1 mRNA expression, in marked contrast to tph2 mRNA expression, is upregulated following exposure to chronic immobilization stress (Chamas et al., 1999;Chamas et al., 2004), chronic restraint stress (Abumaria et al., 2008) and chronic social stress (Abumaria et al., 2006), However, as discussed in detail above, further studies are required to determine if the increased tph1 expression reflects stress-induced increases in tph1 expression in serotonergic neurons or non-neuronal cells.

D × E influences on tph1 and tph2 expression

Lowry and colleagues have investigated D × E influences on tph2 expression in rats (Gardner et al., 2009). Briefly, rats were exposed to neonatal handling, maternal separation, or animal facility rearing control conditions during P2-P14. As adults, rats were exposed to either social defeat or a control condition, which consisted of exposure to a novel cage environment. Rats exposed to neonatal handling as pups had decreased tph2 mRNA expression, relative to animal facility reared controls, in the DRD, dorsal raphe nucleus, ventral part (DRV), and DRVL/VLPAG region. When tph2 expression was considered at specific rostrocaudal levels of specific subdivisions of the DR, it emerged that differences between neonatal handled and animal facility reared control rats in the DRD and DRV were restricted to the mid-rostrocaudal and caudal parts of these subregions (DRD, 8.25, 8.34, 8.42 mm bregma; DRV, 8.25, 8.42 mm bregma; cf. Fig 1B and 1D). Thus, neonatal handling decreased stress-induced tph2 expression in regions that have been associated with regulation of anxiety states and anxiety-related behavior.

As adults, rats previously exposed to maternal separation during development had increased tph2 expression following social defeat in specific subdivisions of the DR, measured using in situ hybridization histochemistry, relative to animal facility reared control rats exposed to social defeat (Gardner et al., 2009). Defeated rats exposed to maternal separation as pups had increased tph2 mRNA expression, relative to defeated animal facility reared controls in the DRD, DRV and DRVL/VLPAG. In addition, social defeat-induced tph2 expression was increased in the DRD, DRV, DRVL/VLPAG and DRI of rats exposed to maternal separation as pups relative to neonatally handled rats (cf. Fig 1B and 1F), suggesting polarized responses of maternally separated and neonatally handled rats to a stressful experience during adulthood. When tph2 expression was considered at specific rostrocaudal levels of specific subdivisions of the DR, maternally separated rats exposed to social defeat had increased tph2 expression in the rostral DRV relative to animal facility reared controls exposed to social defeat. The rostral DRV receives input from cortical areas including the lateral orbital cortex and subregions of the amygdala including anterior, anterior cortical and basomedial nuclei (Peyron et al., 1998) and therefore may be involved in the regulation of emotional behavior.

Rats exposed to maternal separation and exposed to a novel cage control condition as adults had increased tph2 mRNA expression, relative to animal facility reared controls, specifically in the DRC (including its dorsal and ventral parts; approximately 8.42 and 8.50 mm bregma). As discussed above the DRC is part of a stress- and anxiety-related neuronal circuit, and is activated by anxiogenic drugs (Abrams et al., 2005) and Ucn 2 (Hale et al., 2010;Staub et al., 2005;Staub et al., 2006). Depressed suicide patients have elevated tph2 expression that is also restricted to the DRC (Bach-Mizrachi et al., 2008). It is possible that depressed patients have increased tph2 expression that is dependent on adverse early life experience, independent of stressful life events during adulthood. This might confer a vulnerability to stress-related psychiatric disorders.

Among rats exposed to any given early life experience condition, only rats exposed to maternal separation responded with social defeat-induced increases in tph2 expression, and this effect was restricted to the DRVL/VLPAG region (cf. Fig 1E and 1F). This suggests that maternally separated rats had a vulnerability to stress-induced increases in tph2 expression. As mentioned previously, tph2 seems somewhat resilient to stress-induced alterations in expression; however, these data suggest that adverse early life experience results in a unique vulnerability to stress-induced changes (Gardner et al., 2009).

It is currently unknown how tph2 expression relates to the activity state of serotonergic neurons. For example, if stress-induced TPH2 is preferentially trafficked to the dendrites, this could potentially result in a decrease in serotonergic activity; conversely, if stress-induced TPH2 is preferentially trafficked to the axon terminals, this could potentially result in an increase in serotonergic activity. It will be important, in future studies, to understand the consequences of stress-induced increases in tph2 expression and serotonergic activity.

Conclusions

Evidence suggests that adverse experience during adulthood increases tph1 mRNA expression in microdissected tissues from the midbrain raphe complex. Further studies are required to determine if the stress-induced increases in tph1 expression are due to altered expression in neuronal or non-neuronal sources, such as the cerebral vasculature or T cells. A stress-induced increase in tph1 mRNA expression in T cells would be of interest in its own right, given recent studies demonstrating a role for T cells in regulation of cognitive function (Derecki et al., 2010b;Derecki et al., 2010a). The expression of tph2 appears to be relatively insensitive to either acute or chronic stress in adulthood. However, some studies have identified stress-induced increases in tph2 expression in the mid-rostrocaudal and caudal DR, similar to what has been described in depressed suicide patients. Therefore, further studies investigating the effects of stress on tph2 expression in topographically organized subpopulations of serotonergic neurons are warranted. Finally, studies in rodents suggest that adverse early life experience increases the vulnerability to stress-induced increases in tph2, while neonatal handling decreases tph2 expression, and may increase the resilience to stress-induced increases in tph2 later in life.

Research Highlights

  • Tryptophan hydroxylase is the rate-limiting enzyme in the biosynthesis of serotonin
  • The tph1 isoform is believed to be expressed in peripheral tissues, whereas the tph2 isoform is believed to be brain and neuron-specific.
  • tph2 expression is upregulated in the dorsal raphe nucleus of depressed suicide victims.
  • Adverse early life experience increases vulnerability to stress-induced increases in tph2 expression.
  • Neonatal handling decreases tph2 expression during adulthood.

Acknowledgments

The authors gratefully acknowledge Professor Donald M. Kuhn for providing the monospecific polyclonal antibodies for TPH1 and TPH2 used in Fig 2. This work was supported by Award Number R01MH086539 to CAL and R01MH065702 to AS and CAL from the National Institute of Mental Health. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institute of Mental Health or the National Institutes of Health.

Footnotes

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Reference List

  • Abrams JK, Johnson PL, Hay-Schmidt A, Mikkelsen JD, Shekhar A, Lowry CA. Serotonergic systems associated with arousal and vigilance behaviors following administration of anxiogenic drugs. Neuroscience. 2005;133:983–997. [PubMed]
  • Abumaria N, Ribic A, Anacker C, Fuchs E, Flugge G. Stress Upregulates TPH1 but not TPH2 mRNA in the Rat Dorsal Raphe Nucleus: Identification of Two TPH2 mRNA Splice Variants. Cell Mol Neurobiol 2008 [PubMed]
  • Abumaria N, Rygula R, Havemann-Reinecke U, Ruther E, Bodemer W, Roos C, Flugge G. Identification of genes regulated by chronic social stress in the rat dorsal raphe nucleus. Cell Mol Neurobiol. 2006;26:145–162. [PubMed]
  • Amat J, Baratta MV, Paul E, Bland ST, Watkins LR, Maier SF. Medial prefrontal cortex determines how stressor controllability affects behavior and dorsal raphe nucleus. Nat Neurosci. 2005;8:365–371. [PubMed]
  • Amat J, Tamblyn JP, Paul ED, Bland ST, Amat P, Foster AC, Watkins LR, Maier SF. Microinjection of urocortin 2 into the dorsal raphe nucleus activates serotonergic neurons and increases extracellular serotonin in the basolateral amygdala. Neuroscience. 2004;129:509–519. [PubMed]
  • Bach-Mizrachi H, Underwood MD, Kassir SA, Bakalian MJ, Sibille E, Tamir H, Mann JJ, Arango V. Neuronal tryptophan hydroxylase mRNA expression in the human dorsal and median raphe nuclei: major depression and suicide. Neuropsychopharmacology. 2006;31:814–824. [PubMed]
  • Bach-Mizrachi H, Underwood MD, Tin A, Ellis SP, Mann JJ, Arango V. Elevated expression of tryptophan hydroxylase-2 mRNA at the neuronal level in the dorsal and median raphe nuclei of depressed suicides. Mol Psychiatry. 2008;13:507–513. [PMC free article] [PubMed]
  • Barton DA, Esler MD, Dawood T, Lambert EA, Haikerwal D, Brenchley C, Socratous F, Hastings J, Guo L, Wiesner G, Kaye DM, Bayles R, Schlaich MP, Lambert GW. Elevated brain serotonin turnover in patients with depression: effect of genotype and therapy. Arch Gen Psychiatry. 2008;65:38–46. [PubMed]
  • Bethea CL, Mirkes SJ, Shively CA, Adams MR. Steroid regulation of tryptophan hydroxylase protein in the dorsal raphe of macaques. Biol Psychiatry. 2000;47:562–576. [PubMed]
  • Bethea CL, Reddy AP. Effect of ovarian hormones on survival genes in laser captured serotonin neurons from macaques. J Neurochem. 2008;105:1129–1143. [PubMed]
  • Boldrini M, Underwood MD, Mann JJ, Arango V. More tryptophan hydroxylase in the brainstem dorsal raphe nucleus in depressed suicides. Brain Res. 2005;1041:19–28. [PubMed]
  • Bonkale WL, Turecki G, Austin MC. Increased tryptophan hydroxylase immunoreactivity in the dorsal raphe nucleus of alcohol-dependent, depressed suicide subjects is restricted to the dorsal subnucleus. Synapse. 2006;60:81–85. [PMC free article] [PubMed]
  • Chamas F, Serova L, Sabban EL. Tryptophan hydroxylase mRNA levels are elevated by repeated immobilization stress in rat raphe nuclei but not in pineal gland. Neurosci Lett. 1999;267:157–160. [PubMed]
  • Chamas FM, Underwood MD, Arango V, Serova L, Kassir SA, Mann JJ, Sabban EL. Immobilization stress elevates tryptophan hydroxylase mRNA and protein in the rat raphe nuclei. Biol Psychiatry. 2004;55:278–283. [PubMed]
  • Cichon S, Winge I, Mattheisen M, Georgi A, Karpushova A, Freudenberg J, Freudenberg-Hua Y, Babadjanova G, Van Den BA, Abramova LI, Kapiletti S, Knappskog PM, McKinney J, Maier W, Jamra RA, Schulze TG, Schumacher J, Propping P, Rietschel M, Haavik J, Nothen MM. Brain-specific tryptophan hydroxylase 2 (TPH2): a functional Pro206Ser substitution and variation in the 5'-region are associated with bipolar affective disorder. Hum Mol Genet. 2008;17:87–97. [PubMed]
  • Commons KG, Connolley KR, Valentino RJ. A neurochemically distinct dorsal raphe-limbic circuit with a potential role in affective disorders. Neuropsychopharmacology. 2003;28:206–215. [PubMed]
  • Derecki NC, Cardani AN, Yang CH, Quinnies KM, Crihfield A, Lynch KR, Kipnis J. Regulation of learning and memory by meningeal immunity: a key role for IL-4. J Exp Med. 2010a;207:1067–1080. [PMC free article] [PubMed]
  • Derecki NC, Quinnies KM, Kipnis J. Alternatively activated myeloid (M2) cells enhance cognitive function in immune compromised mice. Brain Behav Immun 2010b [PMC free article] [PubMed]
  • Donner NC, Weiser MJ, Kudwa AE, Handa RJ. Program No. 730.21 ed. 2007. Estrogen receptor beta regulates c-Fos-immunoreactivity within tryptophan-hydroxylase-2 neurons of the dorsal raphe nucleus.
  • Evans AK, Heerkens JL, Lowry CA. Acoustic stimulation in vivo and corticotropin-releasing factor in vitro increase tryptophan hydroxylase activity in the rat caudal dorsal raphe nucleus. Neurosci Lett. 2009;455:36–41. [PubMed]
  • Gardner KL, Hale MW, Oldfield S, Lightman SL, Plotsky PM, Lowry CA. Adverse experience during early life and adulthood interact to elevate tph2 mRNA expression in serotonergic neurons within the dorsal raphe nucleus. Neuroscience. 2009;163:991–1001. [PMC free article] [PubMed]
  • Gardner KL, Thrivikraman KV, Lightman SL, Plotsky PM, Lowry CA. Early life experience alters behavior during social defeat: focus on serotonergic systems. Neuroscience. 2005;136:181–191. [PubMed]
  • Gizatullin R, Zaboli G, Jonsson EG, Asberg M, Leopardi R. Haplotype analysis reveals tryptophan hydroxylase (TPH) 1 gene variants associated with major depression. Biol Psychiatry. 2006;59:295–300. [PubMed]
  • Grahn RE, Will MJ, Hammack SE, Maswood S, McQueen MB, Watkins LR, Maier SF. Activation of serotonin-immunoreactive cells in the dorsal raphe nucleus in rats exposed to an uncontrollable stressor. Brain Res. 1999;826:35–43. [PubMed]
  • Gundlah C, Alves SE, Clark JA, Pai LY, Schaeffer JM, Rohrer SP. Estrogen receptor-beta regulates tryptophan hydroxylase-1 expression in the murine midbrain raphe. Biol Psychiatry. 2005;57:938–942. [PubMed]
  • Gutknecht L, Kriegebaum C, Waider J, Schmitt A, Lesch KP. Spatio-temporal expression of tryptophan hydroxylase isoforms in murine and human brain: convergent data from Tph2 knockout mice. Eur Neuropsychopharmacol. 2009;19:266–282. [PubMed]
  • Haghighi F, Bach-Mizrachi H, Huang YY, Arango V, Shi S, Dwork AJ, Rosoklija G, Sheng HT, Morozova I, Ju J, Russo JJ, Mann JJ. Genetic architecture of the human tryptophan hydroxylase 2 Gene: existence of neural isoforms and relevance for major depression. Mol Psychiatry 2008 [PubMed]
  • Hale MW, Dady KF, Evans AK, Lowry CA. Evidence for in vivo thermosensitivity of serotonergic neurons in the rat dorsal raphe nucleus and raphe pallidus nucleus implicated in thermoregulatory cooling. Exp Neurol. 2011;277:271–281. [PubMed]
  • Hale MW, Lowry CA. Functional topography of midbrain and pontine serotonergic systems: implications for synaptic regulation of serotonergic circuits. Psychopharmacology (Berl) 2010;213:243–264. [PubMed]
  • Hale MW, Stamper CE, Staub DR, Lowry CA. Urocortin 2 increases c-Fos expression in serotonergic neurons projecting to the ventricular/periventricular system. Exp Neurol. 2010;224:271–281. [PMC free article] [PubMed]
  • Hammack SE, Richey KJ, Schmid MJ, LoPresti ML, Watkins LR, Maier SF. The role of corticotropin-releasing hormone in the dorsal raphe nucleus in mediating the behavioral consequences of uncontrollable stress. J Neurosci. 2002;22:1020–1026. [PubMed]
  • Harvey M, Shink E, Tremblay M, Gagne B, Raymond C, Labbe M, Walther DJ, Bader M, Barden N. Support for the involvement of TPH2 gene in affective disorders. Mol Psychiatry 2004 [PubMed]
  • Heim C, Nemeroff CB. The role of childhood trauma in the neurobiology of mood and anxiety disorders: preclinical and clinical studies. Biol Psychiatry. 2001;49:1023–1039. [PubMed]
  • Hiroi R, McDevitt RA, Neumaier JF. Estrogen selectively increases tryptophan hydroxylase-2 mRNA expression in distinct subregions of rat midbrain raphe nucleus: association between gene expression and anxiety behavior in the open field. Biol Psychiatry. 2006;60:288–295. [PubMed]
  • Hollis JH, Lightman SL, Lowry CA. Lipopolysaccharide has selective actions on sub-populations of catecholaminergic neurons involved in activation of the hypothalamic-pituitary-adrenal axis and inhibition of prolactin secretion. J Endocrinol. 2005;184:393–406. [PubMed]
  • Huot RL, Thrivikraman KV, Meaney MJ, Plotsky PM. Development of adult ethanol preference and anxiety as a consequence of neonatal maternal separation in Long Evans rats and reversal with antidepressant treatment. Psychopharmacology (Berl) 2001;158:366–373. [PubMed]
  • Kalinichev M, Easterling KW, Holtzman SG. Early neonatal experience of Long-Evans rats results in long-lasting changes in reactivity to a novel environment and morphine-induced sensitization and tolerance. Neuropsychopharmacology. 2002;27:518–533. [PubMed]
  • Ke L, Qi ZY, Ping Y, Ren CY. Effect of SNP at position 40237 in exon 7 of the TPH2 gene on susceptibility to suicide. Brain Res. 2006;1122:24–26. [PubMed]
  • Ladd CO, Huot RL, Thrivikraman KV, Nemeroff CB, Meaney MJ, Plotsky PM. Long-term behavioral and neuroendocrine adaptations to adverse early experience. Prog Brain Res. 2000;122:81–103. [PubMed]
  • Ladd CO, Owens MJ, Nemeroff CB. Persistent changes in corticotropin-releasing factor neuronal systems induced by maternal deprivation. Endocrinology. 1996;137:1212–1218. [PubMed]
  • Leon-Ponte M, Ahern GP, O'Connell PJ. Serotonin provides an accessory signal to enhance T-cell activation by signaling through the 5-HT7 receptor. Blood. 2007;109:3139–3146. [PubMed]
  • Liang J, Wessel JH, III, Iuvone PM, Tosini G, Fukuhara C. Diurnal rhythms of tryptophan hydroxylase 1 and 2 mRNA expression in the rat retina. Neuroreport. 2004;15:1497–1500. [PubMed]
  • Linder AE, Ni W, Szasz T, Burnett R, Diaz J, Geddes TJ, Kuhn DM, Watts SW. A serotonergic system in veins: serotonin transporter-independent uptake. J Pharmacol Exp Ther. 2008;325:714–722. [PubMed]
  • Lopez de Lara C, Brezo J, Rouleau G, Lesage A, Dumont M, Alda M, Benkelfat C, Turecki G. Effect of tryptophan hydroxylase-2 gene variants on suicide risk in major depression. Biol Psychiatry. 2007;62:72–80. [PubMed]
  • Lowry CA, Hale MW. Serotonin and the neurobiology of anxious states. In: Mûller CP, Jacobs BL, editors. Handbook of the Behavioral Neurobiology of Serotonin. Elsevier; Amsterdam: 2010. pp. 379–398.
  • Lowry CA, Hale MW, Evans AK, Heerkens J, Staub DR, Gasser PJ, Shekhar A. Serotonergic systems, anxiety, and affective disorder: focus on the dorsomedial part of the dorsal raphe nucleus. Ann N Y Acad Sci. 2008 (in press) [PubMed]
  • Lowry CA, Johnson PL, Hay-Schmidt A, Mikkelsen J, Shekhar A. Modulation of anxiety circuits by serotonergic systems. Stress. 2005;8:233–246. [PubMed]
  • Lu NZ, Shlaes TA, Gundlah C, Dziennis SE, Lyle RE, Bethea CL. Ovarian steroid action on tryptophan hydroxylase protein and serotonin compared to localization of ovarian steroid receptors in midbrain of guinea pigs. Endocrine. 1999;11:257–267. [PubMed]
  • Maier SF, Watkins LR. Stressor controllability and learned helplessness: the roles of the dorsal raphe nucleus, serotonin, and corticotropin-releasing factor. Neurosci Biobehav Rev. 2005;29:829–841. [PubMed]
  • Malek ZS, Dardente H, Pevet P, Raison S. Tissue-specific expression of tryptophan hydroxylase mRNAs in the rat midbrain: anatomical evidence and daily profiles. Eur J Neurosci. 2005;22:895–901. [PubMed]
  • Malek ZS, Pevet P, Raison S. Circadian change in tryptophan hydroxylase protein levels within the rat intergeniculate leaflets and raphe nuclei. Neuroscience. 2004;125:749–758. [PubMed]
  • Malek ZS, Sage D, Pevet P, Raison S. Daily rhythm of tryptophan hydroxylase-2 messenger ribonucleic acid within raphe neurons is induced by corticoid daily surge and modulated by enhanced locomotor activity. Endocrinology. 2007;148:5165–5172. [PubMed]
  • Matthes S, Mosienko V, Bashammakh S, Alenina N, Bader M. Tryptophan hydroxylase as novel target for the treatment of depressive disorders. Pharmacology. 2010;85:95–109. [PubMed]
  • McEuen JG, Beck SG, Bale TL. Failure to mount adaptive responses to stress results in dysregulation and cell death in the midbrain raphe. J Neurosci. 2008;28:8169–8177. [PMC free article] [PubMed]
  • McEwen BS. Early life influences on life-long patterns of behavior and health. Ment Retard Dev Disabil Res Rev. 2003;9:149–154. [PubMed]
  • Meaney MJ, Diorio J, Francis D, Widdowson J, LaPlante P, Caldji C, Sharma S, Seckl JR, Plotsky PM. Early environmental regulation of forebrain glucocorticoid receptor gene expression: implications for adrenocortical responses to stress. Dev Neurosci. 1996;18:49–72. [PubMed]
  • Merlot E, Moze E, Dantzer R, Neveu PJ. Cytokine production by spleen cells after social defeat in mice: activation of T cells and reduced inhibition by glucocorticoids. Stress. 2004;7:55–61. [PubMed]
  • Nakamura K, Sugawara Y, Sawabe K, Ohashi A, Tsurui H, Xiu Y, Ohtsuji M, Lin QS, Nishimura H, Hasegawa H, Hirose S. Late developmental stage-specific role of tryptophan hydroxylase 1 in brain serotonin levels. J Neurosci. 2006;26:530–534. [PubMed]
  • Nash MW, Sugden K, Huezo-Diaz P, Williamson R, Sterne A, Purcell S, Sham PC, Craig IW. Association analysis of monoamine genes with measures of depression and anxiety in a selected community sample of siblings. Am J Med Genet B Neuropsychiatr Genet. 2005;135B:33–37. [PubMed]
  • Ni W, Geddes TJ, Priestley JR, Szasz T, Kuhn DM, Watts SW. The existence of a local 5-hydroxytryptaminergic system in peripheral arteries. Br J Pharmacol. 2008;154:663–674. [PMC free article] [PubMed]
  • O'Connell PJ, Wang X, Leon-Ponte M, Griffiths C, Pingle SC, Ahern GP. A novel form of immune signaling revealed by transmission of the inflammatory mediator serotonin between dendritic cells and T cells. Blood. 2006;107:1010–1017. [PubMed]
  • Patel PD, Pontrello C, Burke S. Robust and tissue-specific expression of TPH2 versus TPH1 in rat raphe and pineal gland. Biol Psychiatry. 2004;55:428–433. [PubMed]
  • Perroud N, Neidhart E, Petit B, Vessaz M, Laforge T, Relecom C, La HR, Malafosse A, Guipponi M. Simultaneous analysis of serotonin transporter, tryptophan hydroxylase 1 and 2 gene expression in the ventral prefrontal cortex of suicide victims. Am J Med Genet B Neuropsychiatr Genet. 2010;53B:909–918. [PubMed]
  • Peyron C, Petit JM, Rampon C, Jouvet M, Luppi PH. Forebrain afferents to the rat dorsal raphe nucleus demonstrated by retrograde and anterograde tracing methods. Neuroscience. 1998;82:443–468. [PubMed]
  • Plotsky PM, Meaney MJ. Early, postnatal experience alters hypothalamic corticotropin-releasing factor (CRF) mRNA, median eminence CRF content and stress-induced release in adult rats. Mol Brain Res. 1993;18:195–200. [PubMed]
  • Popova NK, Kulikov AV. Targeting tryptophan hydroxylase 2 in affective disorder. Expert Opin Ther Targets. 2010;14:1259–1271. [PubMed]
  • Ressler KJ, Bradley B, Mercer KB, Deveau TC, Smith AK, Gillespie CF, Nemeroff CB, Cubells JF, Binder EB. Polymorphisms in CRHR1 and the serotonin transporter loci: gene x gene x environment interactions on depressive symptoms. Am J Med Genet B Neuropsychiatr Genet. 2010;153B:812–824. [PMC free article] [PubMed]
  • Rhees RW, Lephart ED, Eliason D. Effects of maternal separation during early postnatal development on male sexual behavior and female reproductive function. Behav Brain Res. 2001;123:1–10. [PubMed]
  • Roche S, McKeon P. Support for tryptophan hydroxylase-2 as a susceptibility gene for bipolar affective disorder. Psychiatr Genet. 2009;19:142–146. [PubMed]
  • Rueter LE, Fornal CA, Jacobs BL. A critical review of 5-HT brain microdialysis and behavior. Rev Neurosci. 1997;8:117–137. [PubMed]
  • Sakowski SA, Geddes TJ, Thomas DM, Levi E, Hatfield JS, Kuhn DM. Differential tissue distribution of tryptophan hydroxylase isoforms 1 and 2 as revealed with monospecific antibodies. Brain Res. 2006;1085:11–18. [PubMed]
  • Sanchez RL, Reddy AP, Centeno ML, Henderson JA, Bethea CL. A second tryptophan hydroxylase isoform, TPH-2 mRNA, is increased by ovarian steroids in the raphe region of macaques. Mol Brain Res. 2005;135:194–203. [PubMed]
  • Satoh E, Edamatsu H, Omata Y. Acute restraint stress enhances calcium mobilization and proliferative response in splenic lymphocytes from mice. Stress. 2006;9:223–230. [PubMed]
  • Schmidt D, Reber SO, Botteron C, Barth T, Peterlik D, Uschold N, Mannel DN, Lechner A. Chronic psychosocial stress promotes systemic immune activation and the development of inflammatory Th cell responses. Brain Behav Immun. 2010;24:1097–1104. [PubMed]
  • Sidor MM, Amath A, MacQueen G, Foster JA. A developmental characterization of mesolimbocortical serotonergic gene expression changes following early immune challenge. Neuroscience. 2010;171:734–746. [PubMed]
  • Staub DR, Evans AK, Lowry CA. Evidence supporting a role for corticotropin-releasing factor type 2 (CRF(2)) receptors in the regulation of subpopulations of serotonergic neurons. Brain Res. 2006;1070:77–89. [PubMed]
  • Staub DR, Spiga F, Lowry CA. Urocortin 2 increases c-Fos expression in topographically organized subpopulations of serotonergic neurons in the rat dorsal raphe nucleus. Brain Res. 2005;1044:176–189. [PubMed]
  • Sugden D. Comparison of circadian expression of tryptophan hydroxylase isoform mRNAs in the rat pineal gland using real-time PCR. J Neurochem. 2003;86:1308–1311. [PubMed]
  • Sugden K, Tichopad A, Khan N, Craig IW, D'Souza UM. Genes within the serotonergic system are differentially expressed in human brain. BMC Neurosci. 2009;10:50. [PMC free article] [PubMed]
  • Tsai SJ, Hong CJ, Liou YJ, Yu YW, Chen TJ, Hou SJ, Yen FC. Tryptophan hydroxylase 2 gene is associated with major depression and antidepressant treatment response. Prog Neuropsychopharmacol Biol Psychiatry 2009 [PubMed]
  • Underwood MD, Johnson VL, Bakalian MJ, Wiste AK, Kassir S, Mann JJ, Arango V. 2010 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience; 2010. Serotonergic and noradrenergic imbalance in bipolar disorder. 2010.Online. Program No. 881.11.
  • Underwood MD, Khaibulina AA, Ellis SP, Moran A, Rice PM, Mann JJ, Arango V. Morphometry of the dorsal raphe nucleus serotonergic neurons in suicide victims. Biol Psychiatry. 1999;46:473–483. [PubMed]
  • Underwood MD, Mann JJ, Arango V. Serotonergic and noradrenergic neurobiology of alcoholic suicide. Alcohol Clin Exp Res. 2004;28:57S–69S. [PubMed]
  • Van Den Bogaert A, Sleegers K, De ZS, Heyrman L, Norrback KF, Adolfsson R, Van BC, Del-Favero J. Association of brain-specific tryptophan hydroxylase, TPH2, with unipolar and bipolar disorder in a Northern Swedish, isolated population. Arch Gen Psychiatry. 2006;63:1103–1110. [PubMed]
  • Wigger A, Neumann ID. Periodic maternal deprivation induces gender-dependent alterations in behavioral and neuroendocrine responses to emotional stress in adult rats. Physiol Behav. 1999;66:293–302. [PubMed]
  • Zhang X, Gainetdinov RR, Beaulieu JM, Sotnikova TD, Burch LH, Williams RB, Schwartz DA, Krishnan KR, Caron MG. Loss-of-function mutation in tryptophan hydroxylase-2 identified in unipolar major depression. Neuron. 2005;45:11–16. [PubMed]
  • Zill P, Baghai TC, Zwanzger P, Schule C, Eser D, Rupprecht R, Moller HJ, Bondy B, Ackenheil M. SNP and haplotype analysis of a novel tryptophan hydroxylase isoform (TPH2) gene provide evidence for association with major depression. Mol Psychiatry. 2004a;9:1030–1036. [PubMed]
  • Zill P, Buttner A, Eisenmenger W, Moller HJ, Ackenheil M, Bondy B. Analysis of tryptophan hydroxylase I and II mRNA expression in the human brain: A post-mortem study. J Psychiatr Res 2005 [PubMed]
  • Zill P, Buttner A, Eisenmenger W, Moller HJ, Bondy B, Ackenheil M. Single nucleotide polymorphism and haplotype analysis of a novel tryptophan hydroxylase isoform (TPH2) gene in suicide victims. Biol Psychiatry. 2004b;56:581–586. [PubMed]
  • Zill P, Buttner A, Eisenmenger W, Muller J, Moller HJ, Bondy B. Predominant expression of tryptophan hydroxylase 1 mRNA in the pituitary: a postmortem study in human brain. Neuroscience. 2009;159:1274–1282. [PubMed]