A rat model of early immune stimulation

It took me a few a reads of the paper by Anna Kubesova and colleagues [1] (open-access) to understand just how potentially important their findings might be to various conditions including schizophrenia and autism.

Reporting results of a study where "early immune stimulation induced by postnatal systemic administration of LPS [lipopolysaccharide]" was performed in rats, researchers examined various potential effects on "the levels of monoamines (dopamine, serotonin) and their metabolites, and the levels of the main excitatory and inhibitory neurotransmitters glutamate and γ-aminobutyric acid (GABA) in the brain" and "activation of the kynurenine pathway of tryptophan metabolism." Brain morphology, and in particular, hippocampal volume was also the topic of some investigation.

Before coming to the results, a few additional points are worthwhile mentioning. Rats were the participants of choice in this study. I say again, rats were the participants of choice. The use of the word 'postnatal' to denote how LPS - one of the artificial immune activation weapons of choice - was administered after birth was also of initial interest to me, given that prenatal immune activation (that is immune stimulation administered to pregnant mother mice/rats/monkeys and then analysis of the effects on offspring after birth conducted) has been where the majority of the research has been done in this area (see here). The authors do comment on this however: "The early postnatal period in rats corresponds approximately with the third trimester of human brain development" although the difference between injecting LPS into offspring rats vs. pregnant rats is still present. LPS was also administered via intraperitoneal injection, that is the into the peritoneum, the tissue that lines the abdominal wall.

OK, so what did they find? Well, quite a bit, for example:

• Analysis of neurotransmitters and metabolites in brain and plasma of LPS treated and control rats were carried out by "liquid chromatography combined with electrospray ionization tandem mass spectrometry (UHPLC–ESI-MS/MS)" based on methods reported in a previous study [2]. Curiously however, authors chose to present a HPLC trace (Figure 1) showing the separation of the analytes over and above the more superior mass spec results. I would have preferred to have seen those any day.
• The authors reported: "significantly increased levels of DOPAC [3,4-dihydroxyphenylacetic acid], HVA [homovanilic acid] and 5-HIAA [5-hydroxyindolacetic acid] and decreased levels of 5-HT [serotonin] and 3-MT [3-methoxytyramine] in each measured brain area in LPS treated animals compared with the control group."
• Further: "There were significantly increased levels of GLU [glutamate] in each measured brain area, decreased levels of GABA in the hippocampus and a decreasing trend of GABA in the prefrontal cortex in LPS treated animals compared with the control group."
• And also: "We detected significantly increased levels of TRP [tryptophan], KYN [kynurenine], 3-OH-KYN [3-hydroxykynurenine] and QUIN [quinolinic acid] in each measured brain area and plasma in LPS treated animals compared with the control group."
• Differences in brain morphology are also reported as per the finding: "early postnatal LPS administration led to a volume reduction of the hippocampus."

And with all that, again bearing in mind that this was a study of rats, the authors conclude that: "Our results suggest a pathogenetic link between early immune stimulation and neuropsychiatric disorders such and schizophrenia, mood disorders, anxiety disorders, autism, Parkinson’s disease and Alzheimer’s disease."

These are interesting results that require both replication and follow-up. Of all the findings presented as potentially being connected to early immune stimulation, I was particularly interested in those related to kynurenine and some previous work talking about a kynurenic acid hypothesis of schizophrenia (see here). Kubesova and colleagues do talk about their results not necessarily fitting in with the idea that elevated kynurenic acid (KYNA) might be linked to at least some cases of schizophrenia [3] as per the fact that they: "did not find any change of KYNA levels in the brain or plasma compared to the controls." Activation of this pathway and a link with those dastardly pro-inflammatory cytokines (see here) (which were surprisingly not measured in the current study) might however offer some important clues on how immune activation links into some of the metabolites of little 'ole tryptophan and where future work might go from there.

The findings of elevated levels of tryptophan in both plasma and brain of LPS treated animals and reduced levels of tyrosine hydroxylase positive cells in certain parts of the brain was also something which got me thinking. Tyrosine hydroxylase as one of the triad of aromatic amino acid hydroxylases (metabolising tryptophan, tyrosine and phenylalanine) is a valued user of something called tetrahydrobiopterin (BH4), a substrate which has been mentioned a few times on the blog with autism (see here), schizophrenia (see here) and more classically, phenylketonuria (PKU) (see here) in mind. Aside from the potential effect of immune activation on levels of tyrosine hydroxylase bearing in mind how this family of enzymes might not necessarily be too exclusive in what they metabolise [4], I'm wondering whether BH4 might be something else to look at in future work given it's already known-about link to immune activation [5] alongside say, levels of tryptophan hydroxylase too? Indeed, the fact also that higher levels of tryptophan were noted in LPS treated animals but lower levels of 5-HT (serotonin) in "each measured brain area" could be construed as offering further support for closer inspection of tryptophan enzyme chemistry in this immune related model.

I know animal studies such as this one have to be cautiously interpreted in terms of how well they extend to complex labels such as autism or schizophrenia, with their very wide degree of heterogeneity and important mix of comorbidities. This still doesn't stop me however from being really quite interested in where the Kubesova findings might eventually lead us. I might also link you to the paper from Luan and colleagues [6] and another area crying out for further study with maternal immune activation in mind...

Music: Nick Cave & The Bad Seeds with Red Right Hand.

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[1] Kubesolva A. et al. Biochemical, Histopathological and Morphological Profiling of a Rat Model of Early Immune Stimulation: Relation to Psychopathology. PLoS ONE. 2015; 10(1): e0115439.

[2] Najmanová V. et al. LC-ESI-MS-MS Method for Monitoring Dopamine, Serotonin and Their Metabolites in Brain Tissue. Chromatographia. 2011; 73: 143-149.

[3] Erhardt S. et al. The kynurenic acid hypothesis of schizophrenia. Physiol Behav. 2007 Sep 10;92(1-2):203-9.

[4] Roberts KM. & Fitzpatrick PF. Mechanisms of Tryptophan and Tyrosine Hydroxylase. IUBMB Life. 2013.; 65: 350–357.

[5] Neurauter G. et al. Chronic immune stimulation correlates with reduced phenylalanine turnover. Curr Drug Metab. 2008 Sep;9(7):622-7.

[6] Luan R. et al. Maternal Lipopolysaccharide Exposure Promotes Immunological Functional Changes in Adult Offspring CD4+ T Cells. Am J Reprod Immunol. 2015 Jan 30.

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Kubesova A, Tejkalova H, Syslova K, Kacer P, Vondrousova J, Tyls F, Fujakova M, Palenicek T, & Horacek J (2015). Biochemical, Histopathological and Morphological Profiling of a Rat Model of Early Immune Stimulation: Relation to Psychopathology. PloS one, 10 (1) PMID: 25602957