*1.2.3.3 Congenital malformations*

Fetal malformations, which usually occur between 7 and 10 weeks, account for 30 to 40 percent of prenatal deaths. Studies show that the rate of severe malformations in children of healthy mothers is 7.8%, but 15% in children of diabetic mothers. These malformations usually affect the central nervous system, heart, kidneys, and urinary system [27, 33].

## *1.2.3.4 Macrosomia*

It is more common in infants of mothers with diabetes and increases the risk of death at birth. Diabetes during pregnancy can double the incidence of macrosomia and other neonatal anthropometric indexes in diabetic mothers compared to babies born to non-diabetic mothers. Previous studies have clearly established that maternal diabetes can induce macrosomia in most fetal organs except the brain. These events are closely related to fetal hyperinsulinemia and maternal hyperglycemia [24, 61].

## *1.2.3.5 Neuropsychological effects of gestational diabetes on infants*

Previous studies have shown that infants of diabetic mothers have a significant decrease in brain weight and size compared to healthy mothers [37]. Moreover,

### *The Impact of Diabetes on Hippocampus DOI: http://dx.doi.org/10.5772/intechopen.99895*

multiple lines of evidence indicated that offspring of diabetic mothers exhibit disturbances in behavioral and intellectual functioning. In this way, other studies have shown a link between brain size and intelligence. These children also show poorer cognitive function than children of healthy mothers, which is related to the effects of maternal metabolic changes on the development of the fetal central nervous system (CNS) [62–64]. Long-term studies of offspring born to diabetic mothers have shown that diabetes during pregnancy reduces IQ in these children [65].

Researchers believe that gestational diabetes is a teratogen for the development of the central nervous system [66]. Since the brain is one of the major organs using up glucose in the body, any defect in the process of supplying glucose to the brain, even for a short time, can cause brain disorders [67]. In humans, the differentiation and maturation of the cerebral cortex occur at the end of the second trimester of pregnancy, and therefore at this time, any change in blood glucose levels can have irreversible effects [65, 67, 68]. It is believed that hypoglycemia due to hyperinsulinemia in infants born to mothers causes neuronal damage in the internal temporal region and also memory-related areas [69].

Smoak and Sadler examined the role of glucose in brain development in mouse embryos. The researchers showed that a 50 percent reduction in blood glucose levels for 2 hours in mice at the stage of neural tube formation causes developmental disorders in the nervous system [70]. Habituation is a sign of proper functioning of the central nervous system. Studies have shown that fetuses of diabetic mothers have poorer habituation than fetuses of healthy mothers. This reduction is indicative of the effect of gestational diabetes on suitable central nervous system function [62, 71].

## *1.2.3.6 Attention deficit disorder and hyperactivity*

Neural damage in infants of mothers with diabetes during pregnancy is not only limited to a negative effect on their intelligence but also can reduce their concentration [65]. Studies have indicated a higher incidence of developmental delay and behavioral problems including short attention span, over-activity, and attentionseeking in children born to mothers with diabetes [58]. Moreover, growth motor skills and speech and language delay were the main development areas of concern that could link between maternal diabetes and development in children aged 1–6 years [72]. Other studies demonstrated that school-age children younger than 9 years, born to diabetic mothers, had a higher rate of attention deficit, lower cognitive scores, and lower gross and fine motor achievements than matched control children did. as well as the period might affect the later cognitive and behavioral function of progeny by influencing developing brain cells in utero [73, 74].

In general, it can be said that diabetes during pregnancy disrupts the development, function, and maturity of the CNS in the children of diabetic mothers, which manifests itself in the form of intelligence, educational and behavioral problems. It should be noted that both increase and decrease in blood sugar during brain development can lead to a decrease in various cognitive functions [64, 65, 75].

### *1.2.3.7 Intelligence and memory disorders*

Children born to diabetic mothers suffer from neuropsychiatric disorders, including decreased intelligence and memory disorders. In 1997, Rizzo and colleagues followed 139 women with gestational diabetes and reported that their children's intelligence was directly related to maternal glucose metabolism in the second and third trimesters of pregnancy [76]. In animal studies, the hippocampus, which is particularly involved in memory, has been identified as the most susceptible area

to the negative effects of hypoglycemia, which can occur in the fetus or immediately after birth. Overall, the results of various studies indicate the negative impact of maternal diabetes on cognitive functions, which can cause memory and intelligence deficits [75, 77, 78].

### **1.3 Effects diabetes during pregnancy on hippocampus**

As mentioned before, the hippocampus is an important brain structure crucial for spatial learning and memory. In diseases that cause memory loss and other intellectual functions, such as Alzheimer's disease, hippocampus cells are among the first cells to undergo degenerative changes. it is well documented that the hippocampus provides a stimulus that converts short-term memory to long-term memory, and whatever its mechanism, it would not have happened without the hippocampus [3].

Studies have shown hippocampus is very sensitive to changes in glucose concentration during development. in this regard, there is a bulk of studies that show children born to diabetic mothers are more likely to have neurodevelopmental abnormalities including impairments in memory, learning ability, activity level, attention span, and motor functioning. These infants also show lower IQ scores compared to infants born to healthy mothers [37, 58].

When pregestational and gestational diabetes-exposed children were grouped together in the study of DeBoer et al., it was demonstrated a negative link between maternal diabetes and development of memory, circuitry, and behavioral mnemonic performance in children at 1-year of age. Moreover, they showed that the metabolic abnormalities due to diabetes during pregnancy alters prenatal development, which can influence memory performance on a delay recall task. It is well documented that metabolic abnormalities which occur in diabetes during pregnancy can impair fetal CNS development, which leads to structural and functional defects, especially in the hippocampus [79].

Experimental models of diabetes during pregnancy in animals have shown a decrease in the numerical density of neurons in some parts of the fetal CNS, particularly in the hippocampus, which is reflected in decreased memory and learning and impaired memory storage and recall of information [37].

In the study by Sadeghi et al., stereological change in the hippocampus of rat offspring due to diabetes in pregnancy was evaluated. In that study, the authors found a significant reduction in total volumes of the hippocampus in offspring born to diabetic mothers when compared to the control group. In addition, their results have been shown the hippocampal subfields volumes, especially the CA1, DG, and subiculum, were significantly decreased. Moreover, they reported a significant decrease in the number of hippocampal cells in infants born to diabetic mothers [80].

A study by Tehranipour and Khakzad examines the effect of maternal diabetes on neural density in the hippocampus of newborn rats immediately after birth. Their results showed that diabetes during pregnancy can decrease the number of neurons in the hippocampus, especially in the CA3 area [81].

Synaptogenesis is one of the key events which happen throughout the development of the central nervous system. The chemical synapses in the central nervous system contain the presynaptic apparatus, the synaptic cleft, and the postsynaptic region. During synaptic transmission, In the presynaptic part, neurotransmitters stored in the synaptic vesicle through the exocytosis process, release into the synaptic cleft and then fuse with their receptors in the postsynaptic membrane. This process is highly regulated in CNS. The bulk of studies were performed to recognize and specify the components of the synaptic vesicle membrane. These

### *The Impact of Diabetes on Hippocampus DOI: http://dx.doi.org/10.5772/intechopen.99895*

studies have been found several proteins, including synaptophysin, synaptobrevin, and synaptogamin that functions as the regulators of exocytosis. In recent studies, synaptophysin has been utilized as a valid marker for synaptic density and synaptogenesis [66, 82, 83].

In a study by Vafaei-Nezhad et al., researchers revealed that the SYP expression levels were significantly reduced in hippocampus sub-regions of pups born to diabetic animals, especially at P7 and P14, compared with the control group [66]. SYP as a major protein of the synaptic vesicle membrane may play an important role in transmitter release. Thus, the early decrease in SYP expression may reflect a down-regulation of synaptic functions and may be related to the release of the neurotransmitters. Since SYP and other synaptic vesicle proteins have been implicated in the mechanisms of cellular plasticity underlying learning, a decrease in the expression of this protein might disrupt memory formation [84–86]. SYP is also a reliable indicator of synaptic plasticity, and has previously been demonstrated to correlate well with the loss of cognitive function in animal models with neurodegeneration and in humans with Alzheimer's disease [87]. Earlier study also demonstrated a correlation between aging-related deficits in cognitive functions and disturbances in SYP expression in the hippocampus. There are also documents showing that upregulation of SYP expression may contribute to the mechanisms that underlie learning and memory [85, 86].

In another investigation by Sadeghi et al., the effects of maternal diabetes on neurogenesis in the developing hippocampus were examined. in the study, researchers probe NeuN and DCX markers changes. They found a significant higher mean number of DCX-positive cells and an up-regulation in mRNA expression of DCX in neonates born to diabetic mothers. Moreover, they demonstrated a significant reduction in the mean number of NeuN-positive cells and down-regulation in NeuN expression in the newborns to diabetic animals [88].

This author in another study also revealed that maternal diabetes could result in developmentally induced increase in the hippocampal GFAP expression and numerical density of GFAP positive cells in the DG hippocampal subfield of the offspring born to diabetic mothers compared to healthy mothers [89].

In experimental animals, it is reported that diabetes during pregnancy can increase apoptosis in the central nervous system in neonates. In this context, research by Lotfi et al. revealed that maternal diabetes in newborns of diabetic mothers leads to a marked increase in the number of apoptotic cells in the CA3 subregion of the hippocampus. They also suggested that hyperglycemia during pregnancy could cause a developmental change in the density of neuronal cells in the offspring's hippocampus [90]. In consist with this study, in another research has been reported that diabetes during pregnancy leads to up-regulate in Bax and down-regulated in Bcl-2 gene expression (apoptosis-regulatory genes) in the hippocampus of rat neonates born to mothers with diabetes [91].

Nowadays, neurotrophic factor such as brain-derived neurotrophic factor (BDNF) is considered for its role in regulating the development of the fetal nervous system. In the experiment by Sardar et al., they assessed the effects of maternal diabetes on gene expression and distribution pattern of BDNF in the hippocampus of neonatal rats. The results of the present study showed that diabetes during pregnancy causes marked BDNF downregulation and numerical density of BDNF<sup>+</sup> cells reduction in both sides' hippocampi of the male/female diabetic group [92].

In another study by Hami et al., the effects of diabetes in pregnancy on gene expression and protein concentration of IGF1R and IR in the developing rat hippocampus at postnatal days 0, 7, and 14 were evaluated. In that study, the authors found a markedly upregulation of both IR and IGF1R expression in the hippocampus of diabetic group newborns at first postnatal day. At the same time point, they showed only slight changes in their hippocampal protein transcripts. In 7-day, old rats, there was a significant decreased in IGF-1R gene expression and protein levels in the newborns born to diabetic dams. Moreover, they found a down regulation in hippocampal IGF1R transcripts in 14-day old diabetic group offspring. Two weeks after birth, the IR gene expression was significantly declined in the hippocampus of diabetic newborns [18].

### **1.4 Effects diabetes on the hippocampus in adults**

A bulk of studies have been demonstrated that all types of diabetes have adverse effects on the central nervous system such as disruption in hypothalamic and hippocampal neuropeptides gene expression, a change in hippocampal function and decreased hippocampal synaptic plasticity, glutamate neurotransmission abnormalities, and neurotoxicity [93, 94]. Studies have shown that people with diabetes have a higher risk of developing memory and learning disorders and also Alzheimer's disease, depression, stroke, and dementia compared to healthy people [95]. A previous study showed that diabetes induction in animal models can elevate corticosterone levels and defects hippocampal synaptic plasticity, learning, and long-term potentiation in the CA1-field [96, 97]. Recent studies showed that STZinduced diabetes notably decreased the number of proliferating cells in the dentate gyrus of rats by changing the hippocampal synaptic plasticity [98].

There is some evidence that diabetes can decrease hippocampal cell proliferation and survival, while some researchers suggest that diabetes also has adverse effects on neuronal differentiation. Several preclinical studies revealed the bulk of evidence that diabetes has adverse effects morphological integrity of the hippocampus and that it reduced hippocampal neurogenesis. These hippocampal dysfunctions may cause cognitive and mood disorders in diabetic's people [99].

In the study by Revsin et al., Neuronal and astroglial alterations in the hippocampus of a mouse model for type 1 diabetes have been evaluated. Their results showed that diabetic condition in mice can cause a significant increase in the number of astrocytes positive for apolipoprotein-E (Apo-E), a marker of ongoing neuronal dysfunction, abnormal expression of Jun + neurons in CA1 and CA3 layers and dentate gyrus, and Fos-expressing neurons in CA3 layer, and augmented activity of NADPH-diaphorase which is linked to oxidative stress, in CA3 region. They state that these changes could be one of the reasons for the negative effects of diabetes on the hippocampus [94]. Similar to the results of this study Stereological studies have shown that there is no significant change in the total neuron number values and the volume of the hippocampus in diabetic animals compared to healthy animals [94, 100].

It is well known that type 2 diabetes mellitus can lead to cognitive deficits in patients. In this regard, a study by Liu et al., has been revealed that diabetes could alter effective connectivity (EC) between hippocampus and default mode network (DMN), which is interpreted to be related to cognitive disorders in patients with T2DM especially affecting learning and memory [101].

In experimental animals by Kamal et al., they assessed Synaptic transmission changes in diabetic rat s' hippocampus. Their data illustrated that intracellular recording from the pyramidal hippocampal cells of the AMPA summation responses in the diabetic animals was markedly lower than control animals. Hence, they suggest that diabetes can change pre and postsynaptic cells functions which may play a key role in in the synaptic plasticity disorder observed in diabetic animals [93].

Defects in glucose metabolism in the nervous system are not only important for neural cells function but also normal astrocyte activity is related to it. Astrocytes

### *The Impact of Diabetes on Hippocampus DOI: http://dx.doi.org/10.5772/intechopen.99895*

have many important roles in central nervous system such as glucose uptake. Astrocyte dysfunction in the hippocampus have been seen in pathological conditions such as aging, stress, autoimmune diseases, neurodegenerative diseases, and diabetes. In the hippocampus of the animal diabetic model has been reported a significant increase in GFAP immunoreactivity, glial fibrillary acidic protein (GFAP), an astrocyte intermediate filament cytoskeletal protein which is considered the main indicator of astroglial activation caused by CNS injury [102]. In line with this study, a study by Lebed et al. showed that diabetes can alter GFAP and S100B levels in the hippocampus. These findings propose that the reaction of astroglial cells can be the first reaction to impaired glucose metabolism, which is likely to play a crucial role in the mechanisms underlying diabetes-related disorders of CNS function [103].

In a study by Pamidi et al., the Effect of streptozotocin-induced diabetes on rat hippocampus was assessed. They used Cresyl violet staining for evaluating the number of surviving neurons in the subfield of the hippocampus. Their findings revealed that the number of survived neurons in the subfield of the hippocampus (CA1, CA2, CA3, dentate hilus, dentate gyrus) in diabetic animals was significantly reduced compared to controls. Hence, researchers suggested that uncontrolled and long-term diabetes could cause intense hippocampal neurodegeneration [104].

In another study, researchers examined the effects of STZ-induced diabetes on NCAM protein expression in various parts of the nervous system, including the hippocampus. The results of this study have been demonstrated that diabetic animals developed remarkable defects in learning and memory behaviors which were evaluated by passive avoidance and water maze tests. Their results also indicate that streptozotocin-induced diabetes disrupts cognitive functions and causes an imbalance in NCAM expression in some parts of the brain, particularly the hippocampus, which is involved in memory and learning. Based on the results of their study, the researchers concluded that changes in the expression of NCAM in the hippocampus of diabetic people could be one of the main causes of memory and learning disorders [105].

Axonal transport has a critical role in normal CNS function and disruption of this process has been linked to various neurodegenerative diseases and could also play a role in diabetes-related diseases that affect the nervous system. Given the importance of this process, Baptista et al. assessed the impact of diabetes on axonal transport in the hippocampus. in this regard, they evaluated KIF1A, KIF5B, and dynein in the hippocampus. Their results showed a significant increase in KIF1A and KIF5B mRNA and protein levels, in CA1, CA3, and DG hippocampal subregion, in the diabetic animals. Nevertheless, no changes in dynein protein were observed [106].

The animal experiment has been revealed that diabetes can cause a reduction of nuclear and perikaryon diameters as well as neuronal density in the sub-region of the hippocampus (CA1, CA2, CA3, and dentate gyrus) [107]. In addition, it has been reported that levels of Caspase-3, Bax, and Bcl-2 in the hippocampus increase after diabetes [108].

### **1.5 Effects of diabetes during pregnancy on the other parts of CNS**

The source of the mammalian neural tube is the ectoderm layer in the embryo. During the first few months of pregnancy which is called the embryonic period, part of the ectoderm becomes a neural plate. Then, this plate is folded, raised, and connected in the midline, which becomes the neural tube [109, 110]. The mentioned procedure is mediated by several signaling molecules and transcription elements [111, 112]. There is strong evidence that diabetes during pregnancy causes abnormalities in the development of the nervous system, including the neural tube, which itself can cause neural tube defects (NTD) [113, 114]. Neural tube formation during the embryogenesis period is tightly regulated by several cellular mechanisms such as proliferation, migration, differentiation, and apoptosis of neural progenitor cells. Previous studies have shown that pregnancy-related hyperglycemia can disrupt cell migration and proliferation, as well as induce programmed cell death during embryogenesis. Studies have revealed that any changes in the mentioned processes can cause NTDs [95, 115].

Spina bifida, which is one of the common NTD, results from failure of neural tube fusion in the spinal area. This abnormality is a common congenital defect that occurs in infants of diabetic mothers [116]. The exact mechanism underlying the teratogenic effects of diabetes during pregnancy is not entirely understood, but it is suggested that the incidence and intensity of congenital malformations in infants born to diabetic mothers are correlated with the degree of maternal metabolic control [37].

Diabetes during pregnancy causes biological changes in the mother that can affect the development of the fetal nervous system. These biological changes can affect nervous system development and neurological abnormalities in the fetus by affecting neurotransmitters, synaptic membranes, and the expression of growth factors that are involved in nervous system development. Studies have shown that increased oxidative stress plays an important role in fetal development and indirectly can cause some of the nervous system developmental disorders that occur in fetuses from diabetic mothers [22]. Free radicals can also alter the biological activity of proteins and peptides, increase neuronal death, and cause DNA damage [37, 117, 118]. Experimental research showed that maternal hyperglycemia can lead to a significant decrease in the volume of gray and white matter, and also can reduce the number of neurons in the gray matter of the CNS in this offspring [119].

Evaluations of cognitive functioning and the behavior of the children born to diabetic mothers provide the chance to functionally assess the CNS development. Hence, behavioral and cognitive assessments in neonates of diabetic mothers can shed light on the effects of maternal diabetes on CNS development [64, 65, 72, 73].

Animal models of diabetes during pregnancy indicate a decrease in the numerical density of neurons in some parts of the fetal central nervous system, particularly in the brain, hippocampus, and cerebellum, due to diabetes during pregnancy, which may be followed by decreased memory and learning ability, and memory defects and information retrieval [37, 58, 80, 120]. Studies by Khaksar on gestational diabetes have shown that gestational diabetes can affect the infant CNS and reduce the number of cells and the thickness of the white and gray matter of the infant cerebrum, cerebellum and spinal cord [119]. Earlier research on the neurologic development in infants of diabetic mothers illustrated fundamental CNS deficits even when there were no structural abnormalities. These alterations were markedly less serious when maternal diabetes was controlled and treated, but some alterations in cognitive function may continue all over childhood [37].

The study by Hami et al. showed that diabetes during pregnancy in infants born to diabetic mothers can lead to a significant decrease in cerebellar volume, the thickness of cerebellar cortical layers, and also decreases in the numerical densities of cerebellar Purkinje and granular cells. They suggested that this event may delay the normal development of the cerebellum and may be a cause of the motor, behavioral, structural, and cognitive disorders seen in the offspring of diabetic mothers [120].

It is well documented that diabetes during pregnancy has structural and functional effects the on central nervous system in both human and animal studies. Despite much research in the field, the exact mechanism by which diabetes affects

### *The Impact of Diabetes on Hippocampus DOI: http://dx.doi.org/10.5772/intechopen.99895*

the development of the nervous system during pregnancy is not yet clearly understood. Some studies have suggested that diabetes itself may have teratogenic effects that can increase the risk of abnormal fetal development in the fetal organs [37].

Some studies suggested that hyperglycemia conditions during pregnancy can induce programmed cell death and impair cell proliferation in mouse embryo neural tubes. Recent experiments suggest that tumor necrosis factor (TNF) plays an important role in neurodevelopmental disorders in babies born to diabetic mothers. TNF could cross the placenta and enter the embryo's bloodstream, which has neurotoxic effects on fetal brain development and can also lead to white matter damage and cerebral palsy [35, 121].

Other investigations implicated that infants born to diabetic mothers have iron metabolism abnormalities. This deficiency can lead to neurodevelopmental and neurobehavioral disorders. Experimental studies in animal models have shown that iron impairment during pregnancy and after childbirth has negative effects on myelination, the metabolism of neurotransmitters in the brain, and the regulation of brain energy. The perinatal iron disorder can elevate the vulnerability of the neonatal brain, especially the hippocampus, to the hypoxic–ischemic insult which leads to abnormal cognitive processing in the newborn period [122, 123].

Kinney *et al*. found that the only female offspring born to diabetic dams showed deficits in long-term memory and learning. These results have suggested that the *in utero* diabetic condition has gender-specific effects on CNS development [78]. In a study by Plagemann *et al*., alterations in catecholamines levels in the hypothalamic nuclei of newborns born to diabetic animals were evaluated. They reported an increased hypothalamic dopamine (DA) and norepinephrine (NE) concentrations in the offspring born to diabetic rats at birth. Twenty-one day- old pups born to diabetic mothers, NE levels were strikingly increased in the ventromedial hypothalamic nucleus and the lateral hypothalamic area (LHA), while DA levels were significantly elevated in the paraventricular hypothalamic nucleus and the LHA. The authors concluded that there are strikingly differences in hypothalamic catecholaminergic systems during early development in the rat newborns born to diabetic animals [124].

Studies have shown that brain weight in infants born to diabetic mothers is significantly lower than in infants born to healthy mothers [66]. Interestingly, Xiang et al. explained a close correlation between diabetes during pregnancy with an elevated risk of autism spectrum disorder (ASD) in infants [125]. Animal models of diabetes during pregnancy indicate a decrease in the numerical density of neurons in some parts of the fetal central nervous system, particularly in the brain, hippocampus, and cerebellum, due to diabetes during pregnancy, which may be followed by decreased memory and learning ability, and memory defects and information retrieval [37, 58].

Much research has shown that fetal hyperglycemia during pregnancy can alter gene expression that is involved in the proliferation and differentiation of nerve cells. These changes can be the basis of the neurocognitive and neurodevelopmental disorders seen in babies of diabetic mothers [37, 58].

Previous studies have shown that insulin and insulin receptor (InsR), and insulin-like growth factor-1 (IGF-I) and IGF-1 receptor (IGF-1R) play key roles in regulating the growth and development of the CNS. It is well documented that insulin and IGF-1 stimulate the proliferation of neuronal progenitor cells, increases the survival of neurons and oligodendrocytes, elevate synaptogenesis and neurotic outgrowth, inhibit neural cell apoptosis, and induces differentiation of neurons. With this in mind, the researchers examined the effects of maternal diabetes on the expression of this factor in the nervous system. Their results have been revealed that maternal diabetes can strongly influence the regulation of these factors at the level of mRNA and protein in neonates' central nervous system [18, 37, 58, 126, 127].

Synaptic transmission and information transfer are highly regulated processes in the nervous system. Information transfer occurs when neurotransmitters stored in synaptic vesicles release into the synaptic cleft and attach to their receptors in the postsynaptic cell membrane. Synaptic vesicles are responsible for collecting neurotransmitters and releasing them into the synaptic cleft through exocytosis. The exocytosis process during the release of neurotransmitters has been extensively studied [82, 83]. Several families of proteins that are present in the membranes of synaptic vesicles which involved in the regulation of this process have been identified [82]. One of these proteins is synaptophysin. This protein is widely found in the membranes of vesicles containing neurotransmitters in neurons.

Synaptophysin is a protein involved in the construction of synaptic vesicles that researchers use as a suitable marker of synaptic density, synaptogenesis [128]. Studies have also shown that synaptophysin may be involved in the exocytosis of synaptic vesicles [129]. It is well documented that synaptophysin levels also change in pathological brain conditions such as Alzheimer's, Parkinson's, schizophrenia, and bipolar disorder [87, 130–132]. Previous studies illustrated that diabetes during pregnancy can reduce significantly downregulate synaptophysin gene expression in different layers of the neonate cerebellum which is born to diabetic mothers [74].
