Stefania Bianchi, Federico Piazza
INTRODUCTION
Some time ago an article published on Journal of Neuroscience Transient early-life forebrain CRH elevation causes long lasting anxiogenic and despair-like changes in mice. 2010 called our attention to the relationship between stress and trauma in very early life and their consequence on behavioural and cognitive patterns. The reading of the article and our knowledge about the role of glucocorticoids in chronic stress led us to study the question in depth.
Surfing the Internet we found interesting articles about prenatal overexposure to maternal glucocorticoids and the risk of negative cognitive and psychiatric outcomes in children. In particular, we learnt that maternal prenatal anxiety is involved in prenatal overexposure to glucocorticoids, through the downregulation of the activity of a specific enzyme normally expressed in the placenta and responsible of low fetal exposition to maternal glucocorticoids. Furthermore, a particular molecule normally contained in licorice, the glycyrrhiza, is involved in worsening these outcomes by inhibiting the same metabolic pathway.
THE ROLE OF GLUCOCORTICOIDS ON BRAIN DEVELOPMENT
Glucocorticoids are essential for brain development but elevated levels of these hormons are detrimental, affecting neuronal division, maturation, migration, interactions, and apoptosis Glucocorticoid programming. 2004. An article published in 1997 The acute effects of corticosteroids on cognition: integration of animal and human model studies. 2007 showed that a specific region of the central nervous system (CNS), the limbic system (especially the hippocampus) is particularly sensitive to glucocorticoids manipulations in early life. Thanks to modern imaging technologies we know that hippocampus is a crucial region of the CNS for cognition and behavior; maintenance of low glucocorticoid exposure during critical periods of brain development is thus crucial.
MATERNAL ANXIETY AND OVEREXPOSURE TO GLUCOCORTICOIDS
Normally, fetal cortisol levels are 2-13 times lower than maternal ones. The gap between fetal and maternal exposition is due to the placental enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11b-HSD2), the barrier enzyme which catalyzes the rapid inactivation of cortisol to its inert 11-keto form cortisone, thus acting as a physiologic placental barrier to maternal cortisol Placental 11 beta-hydroxysteroid dehydrogenase: a key regulator of fetal glucocorticoid exposure. 1997.
Within the placenta, 11b-HSD2 is primarily expressed in the syncytiotrophoblast Differential expression of 11 (O’Donnell et al.) -hydroxysteroid dehydrogenase types 1 and 2 in human placenta and fetal membranes. 1997 where it acts to prevent the majority of maternal cortisol from crossing the placenta, resulting in circulating cortisol concentrations in the fetus being approximately 13-fold lower than those in the mother Fetal exposure to maternal cortisol. 1998.
The 11b-HSD2 is also present in the brain of fetus until the 19th gestional week; from that moment the expression of the enzyme in this anatomical location is silenced.
Chemical reactions mediated by 11b-HSD. From Adipose tissue expression of 11b-Hydroxysteroid dehydrogenase type 1 in cushing's syndrome and in obesity. 2007
Maternal anxiety during pregnancy is associated with increased risk of abnormal neurodevelopment in the children. There is evidence for an association between prenatal maternal mood and downregulation of placental 11b-HSD2. Animals studies demonstrates that maternal stress alters some placenta’s function; in particular high level of glucocorticoids in anxious women causes the downregulation of the enzyme 11b-HSD2, so that fetus is “programmed” by corticosteroids due to maternal stress and this can permanently affect the phenotype of the baby resulting in an altered neuropsychological development.
Some Authors tried to give to the mothers the Spielberger Trait and State Anxiety and the Edinburgh Depression Self-rating Scales the day before delivery by elective caesarean Maternal prenatal anxiety and downregulation of placental 11b-HSD2. 2011 and, thanks to biochemical studies made on placentae, was demonstrated that prenatal trait anxiety is negatively correlated with placental 11b-HSD2 mRNA expression.
Little is understood about the mechanisms which may underlie fetal programming by prenatal stress in humans. One possible mediating factor is increased exposure of the fetus to cortisol. The potential for cortisol to alter neurodevelopment is demonstrated by a microanalysis of fetal brain explants. Cortisol treatment affected the expression of over a thousand genes, most notably those involved in cell growth and metabolism Microarray analysis of cultured human brain aggregates following cortisol exposure: Implications for cellular functions relevant to mood disorders. 2006. While in animals there is an evident correlation between stress (simulated by external somministration of ACTH) and elevation of cortisol levels, which cross the placenta and have bad influence on fetus development, the human hypothalamic-pituitary-adrenal (HPA) axis functions differently in pregnancy from most animal models. Association between maternal mood and her cortisol level is minimal because corticotrophin releasing hormone (CRH) producted by placenta causes an increase in maternal cortisol Evidence for local stimulation of ACTH secretion by corticotropin-releasing factor in human placenta. 1987 and the maternal HPA axis becomes gradually less responsive to stress as pregnancy progresses Pregnant women become insensitive to cold stress. 2002, especially later in pregnancy Stress and salivary cortisol during pregnancy. 2005. Then, a more important role in fetal cortisol exposure increase seems to be played by the inactivation of 11b-hydroxysteroid dehydrogenase type 2 (11b- HSD2). If there is less of this barrier enzyme, then the fetus will be exposed to more maternal cortisol.
THE ROLE OF LICORICE
It is a common knowledge that during pregnancy women could feel craving for some kind of food. Anyway, physicians should make sure that future mothers totally avoid substances containing glycyrrhiza such as licorice because this molecule can inhibit the placentar 11b-HSD2, increasing the glucocorticoids levels the fetus is exposed to.
As in many other case, we came to a study on a cohort of women from the results of experimentation on animals. For example, we know that some steroids like betamethasone or dexamethasone are poor 11b-HSD2 substrates: using this substrates Programming effects of antenatal dexamethasone in the developing mesolimbic pathways. 2007 it was possible to demonstrate that the deficiency of the placentar 11b-HSD2 is associated with defective brain function in later life.
Betamethasone
Dexamethasone
Cortisol
Furthermore, it is possible to obtain similar results in mice treated with licorice-derived enzyme inhibitors Inhibition of 11betahydroxysteroid dehydrogenase, the foeto-placental barrier to maternal glucocorticoids, permanently programs amygdala GR mRNA expression and anxiety-like behaviour in the offspring. 2000. In rat models, administration of the 11b-HSD2 inhibitor carbenoxolone, a synthetic derivative of glycyrrhizinic acid, resulted in an increase in anxiety [14], mirroring the phenotype of offspring exposed prenatal stress.
Carbenoxolone
To study the effect of licorice consumption during pregnancy in humans, a study was designed in Finland Maternal Licorice Consumption and Detrimental Cognitive and Psychiatric Outcomes in Children. 2009.
The children were included in a random, population-based study initially comprising 1,049 infants born in 1998. In 2006, children and their parents where invited to participate in a follow-up focusing on individual differences in physical and psychological development. Only a subset of 892 mothers entered the follow up and was divided in three groups according to the level of consumption of licorice during pregnancy. We have to underline that consumption of licorice by young Finnish women is common. The substance studied in this group of women was glycyrrhiza or 3b-D-diglucuronyl-18b-glycyrrhetinic acid.
Glycyrrhetinic acid
To study the maternal consumption of this molecule, mothers were given a list including all brands of confectionery that contained licorice and were available in Finland in 1998. The mothers then reported the brands and frequency of weekly consumption. In this way it was possible to divide mothers basing on the glycyrrhiza intake per week in groups of
-Zero or low (0-249 mg/week)
-Moderate (250-499 mg/week)
-High consumption (more than 500 mg/week)
The largest part of the cohort (75%) was placed in the first group.
To evaluate the psychiatric and behavioral outcomes, a battery of neuropsychological test, taken for example from the common Wechsler Intelligence Scale for Children, was performed. These tests allowed the evaluation of linguistic abilities, abstract reasoning, problem solving and memory. Mothers completed the Child Behavior Checklist, a scale that screens for emotional, social and behavioral problems.
With this study has been possible to recognize that there are no differences in prenatal, perinatal or maternal characteristics and no differences in birth weight, birth anthropometry or gestation length between the three groups. On the contrary, important differences appeared in cognitive tests: children of mothers who consumed high amounts of glycyrrhiza performed significantly more poorly in cognitive tests than offspring of mother consuming zero or low amounts.
In conclusion, we should not forget about other effects of glycyrrhiza; higher doses of these molecule can affect gap junction formation, but such concentrations are unlikely to be achieved in vivo Comparison of glycyrrhetinic acid isoforms and carbenoxolone as inhibitors of EDHF-type relaxations mediated via gap junctions. 2000 and it has been known for a long time that glycyrrhiza can alter maternal physiology inducing hypertensive effects in sensitive individuals. It is absolutely probable that also these mechanisms can affect the physiological development of brain.
ALTERED EXPRESSION OF GAP JUNCTIONS IN CNS: A POSSIBLE MECHANISM OF DAMAGE?
High doses of molecules as glycyrrhizza can alter the normal formation of the gap junctions in brain an it is pretty interesting to recognize that some reviews about the relationship between gap junction and neurological and psychiatric disorders are available in literature.
Gap junctions are intracellular channels wich directly connect the cytoplasm of neighboring cells. In the central nervous system gap junctions are expressed by both neurons and glial cells. In neurons, these channels are involved in electrical coupling while in astrocytes they are involved in the development of most neuronal disorders, from brain ischemia to epilepsy Gap junctions and neurological disorders. 2003.
Neuronal gap junctions. See original
The higher brain functions, such as sensory processing and control of behaviour, are understood as neurocomputation in the synaptic networks between axons and dendritic terminations. This process, when the person is awake, is characterized by a particular EEG pattern, with 30 to 90 Hz gamma synchrony. This electric activity largely derives from neuronal groups that are connected by gap junctions, thus forming a transient syncytia called dendritic web. It has been demonstrated that as the gap junctions open and close, the dendritic web, that is synchronized on a specific frequency, can rapidly change topology and move through the brain as a cognitive cloud. The mobile dendritic web has been called conscious pilot The “conscious pilot”—dendritic synchrony moves through the brain to mediate consciousness 2010. The conscious pilot is able to assume control on otherwise non-conscious auto-pilot neurocomputation. This relief gives us the support to say that gap junctions are essential in higher brain functions as intelligence too.
Berhard Mitterhauer has developed a medical hypothesis about the connection between loss of functions in gap junctions and severe cognitive impairments in the most important psychiatric disease, schizophrenia Loss of function of glial gap junction may cause severe cognitive impairments in schizophrenia. 2009. In particular, he focused on the glial network called glial syncytium. As the neuronal syncytium cited before, the glial one is composed by the main glial cells, astrocytes and oligodendrocytes with their processes interconnected via gap junctions. It is known that this functional and anatomical syncytium exert a very important role in information processing; thus, dysfunctions in these gap junctions may cause severe cognitive deficits. Evidences of a role of the glial gap junctions in cognitive and behavioral impairments derive from murine models. In one of these models, connexin 30 knock-out mice, compared with wild type, show altered reactivity to novel environment Connexin30-deficient mice show increased emotionality and decreased rearing activity in the open-field along with neurochemical changes. 2003.
The hypothesis of the Author is simple but very interesting: the brain may be able to distinguish between different cognitive domains and qualities thanks to the various proteins (connexins) that constitute the gap junctions, wich could be able to differentiate between the operative qualities of the nearest synapses, as defined by the neurotransmitter used in those cellular contacts. If the function of glial gap junction proteins is lost, the brain could became incapable of distinguishing between same and different qualities of synaptic information processing. Dependent on the brain regions affected, this disorder may be responsible for severe cognitive impairment in schizophrenia.
The brain is who we are: impairments of its development have inevitable consequences on the development of who we are or could be.
