Background
In the USA, one out of eight infants is born premature [
1]. Preterm infants are at increased risk for developmental disorders, abnormal behaviors, and cognitive dysfunction syndromes that can be associated with autism spectrum disorder (ASD) [
2,
3]. ASD includes a heterogeneous group of early onset childhood neurodevelopmental disorders [
4]. The prevalence of ASD is 1 in 150 individuals and occurs more frequently in males than in females [
5]. The incidence estimation may be even higher, affecting 1 in 68 children when combining three basic categories of neurodevelopmental disorders: autistic disorder (autism), Asperger syndrome, and pervasive developmental disorder (PDD-NOS) [
6]. Although the clinical symptoms are heterogeneous, ASD patients show common characteristics including social interaction deficits, communication difficulties, stereotyped repetitive behaviors, limited repertoire of interests, and, in some cases, cognitive problems [
7]. Early symptoms of ASD may include locomotion impairment [
7,
8]. ASD patients may suffer from comorbid conditions such as anxiety, epilepsy, intellectual disability, and depression [
8]. According to the recent guideline of American Psychiatric Association (APA), clinical diagnostic criteria include the following: (a) persistent deficits in social communication and social interaction across multiple contexts; (b) Restricted, repetitive patterns of behavior, interests, or activities; (c) symptoms must be present in the early developmental period; (d) symptoms cause clinically significant impairment in social, occupational, or other important areas of current functioning; and (e) these disturbances are not better explained by intellectual disability (intellectual developmental disorder) or global developmental delay [
9]. In this report, we used the term autism spectrum disorder or ASD to refer to autism-like behaviors in the animal model tested in this investigation.
The contribution of genetic and environmental factors to the development of ASD has drawn increasing attention in basic and clinical research. ASD most likely emerges from a complex interaction between pre-existing genetic vulnerabilities and environment factors. Although a number of genes such as neurexin 1 (NRXN1), fragile X mental retardation 1 (FMR1), and oxytocin/oxytocin receptors have been identified to be ASD related, the contribution of environmental factors to the development of ASD is not well understood. It was noticed that children born prematurely more often display poorer executive functionality and cognition and are more likely to have behavioral problems [
10]. There are reports that premature infants under stress or surgery may show increased inflammatory factors such as TNF-α and IL-6 [
11‐
13]. It was suggested that premature birth and susceptibility genes may make infants more vulnerable to allergic, environmental, infectious, or stress-related triggers that could stimulate mast cell release of pro-inflammatory and neurotoxic molecules, thus contributing to brain inflammation and ASD pathogenesis [
12]. In clinical practices, premature infants in the neonatal intensive care unit (NICU) are routinely exposed to an average of ten therapies or procedures per day without analgesics [
14]. Many of the procedures are painful and may cause inflammatory responses and local tissue edema/damage. It is now believed that infants are more sensitive to pain due to the incomplete development of the brain and the descending inhibitory tracts in their spinal cord [
15]. Further, the immature sensory processing system within the newborn spinal cord results in lower thresholds for excitation and sensitization [
16]. Although a few reports noticed different pain experiences in ASD children and discussed special care for these young patients [
17,
18], the possibility that early life inflammatory pain experience influences the progression of ASD has not been explored.
A subcutaneous formalin injection-induced acute inflammatory pain model has been widely used for many years in pain research [
19,
20]. Formalin-produced local response patterns lasting for approximately 1 h are composed of phase I reactions for about 5 min followed by a longer phase II reaction of about 40 min, characterized by shaking and/or linking of the paw(s). This model is suitable for the examination of acute inflammatory pain and the chronic consequences following the acute pain insult. In the present investigation, the inflammatory pain insult was applied to postnatal day 3 to 5 (P3–P5) pups that are equivalent in brain developmental stage to human preterm infants [
21]. Using this model, we aimed to elucidate whether repeated inflammatory pain experienced by preterm/premature babies could lead to acute and delayed brain damage that might be associated with social and behavioral abnormalities at the juvenile age.
Methods
Animals and ethics, consent, and permissions
Wistar rats (female adult mothers, neonatal pups, and juvenile rats of male and female sex) were kept in the Emory University animal facility under environmental control of standardized room temperature (22–23 °C), low humidity, and 12-h lighting circle. Animals were allowed free access to water and food. Postnatal rat pups stayed with their mothers during experimental periods. All studies were approved by the Institutional Animal Care and Use Committee (IACUC) at Emory University.
Inflammatory pain model of neonatal rats
A subcutaneous injection (sc) of formalin induces progressive and selective activation of the somatosensory pathway and limbic system structures in the brain and brain stem [
20]. Formalin injection causes a biphasic response. The early phase (0–5 min) results mainly from C-fiber activation due to the peripheral stimulus, while the late phase (more than 20 min) results from the combination of an inflammatory reaction in the peripheral tissue and functional changes in the dorsal horn of the spinal cord [
19,
20].
The inflammatory pain model followed our previous procedures with some modifications [
22]. Male and female rat pups at postnatal day 3 (P3) received 5-μl subcutaneous injections of 5 % formalin or saline solution, to each hind paw. The second paw injection was performed 1 h after the first one to give the pups some rest. Ten-μl Luer lock syringes (Hamilton Co., Reno, NV) fitted with an intradermal needle were used for injections. For response to the inflammatory pain, animals were sacrificed 24 h after these formalin injections. For sub-acute and chronic consequences of the pain insult, two more injections were performed at P4 and P5 and sacrificed at different specified times later. After each injection, the pups were immediately returned to their mothers in the home cage.
Drug administration
Indomethacin was purchased from Sigma. Indomethacin (10 mg/kg) was administered intraperitoneally within 10 min after injections of 5 % formalin or saline solution.
Paw volume measurements
The paw volume of rats was measured 1 day before the first formalin injection and 1,3, 5, 7, 9, 11, 13, and 18 days after the first formalin injection using a plethysmometer (UGO Basile, Varese, Italy). For each day, the edema was expressed as the increase in paw volume, and the percentage of induction of edema was expressed as the increase in volume with respect to the control group.
Immunohistochemistry
After sacrifice and dissection, brains were immediately frozen at −80 °C in the optimal cutting temperature (OCT) compound (Tissue-Tek®, Sakura Finetek USA, Inc., Torrance, CA). Sections were cut at 10-μm thickness using a cryostat (Leica Biosystems, Buffalo Grove, IL). Brains from animals that were perfused with 0.9 % saline (pH 7.4) followed by 10 % buffered formalin for detection of myelin basic protein (MBP) and bromodeoxyuridine (BrdU) immunoreactivities were removed and placed in formalin for 24 h, then placed in a 30 % sucrose solution at −20 °C in OCT compound, and were cut into 14-μm thick sections on a cryostat (Leica Biosystems).
The brain sections were then fixed for 10 min in 10 % buffered formalin, washed in phosphate buffered saline (PBS) three times, then incubated in −20 °C ethanol acetic acid (2:1) for 5 min or methanol for 7 min. Sections were washed in PBS three times and then incubated in 0.2 % TritonX-100 for 5 min. After three more washes in PBS, sections were incubated in 1 % fish gelatin (Sigma, St. Louis, MO) for 60 min. Sections were again washed in PBS or automation buffer, and an appropriate primary antibody was applied for overnight incubation at 4 °C: anti-neuronal nuclei (NeuN; Millipore, Billerica, MA), anti-neurofilament (NF; Millipore), anti-BrdU (AbD Serotec; Raleigh, NC or Santa Cruz Biotechnology, Dallas, TX), anti-MBP (Millipore), anti-neurokinin 1 receptor (NK-1R; Millipore), and anti-Iba-1 (Biocare Medical, Concord, CA). Slides were then washed and incubated with the appropriate conjugate secondary antibody for 60 min at 37 °C: donkey anti-mouse Cy5, donkey anti-rat and donkey anti-rabbit Cy3 (Jackson ImmunoResearch, West Grove, PA), and donkey anti-goat 488 (Invitrogen, Grand Island, NY). In some slides, nuclei were counterstained with Hoechst 33342 (1:20,000; Molecular Probes, Eugene, OR) for 5 min. Slides were washed three times in PBS and cover-slipped prior to imaging under a fluorescent microscope (Olympus BX61; Olympus America, Inc., Melville, NY). The image data were collected using the SlideBook 4.2 software (Olympus America, Inc.). All measurements were performed by an individual who was blinded to the experimental groups.
TUNEL staining
In brain section containing the cortex, hippocampus, and other regions, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was performed using a commercial kit (DeadEnd™ Fluorometric TUNEL system; Promega, Madison, WI) to label DNA fragmentation in dead or dying cells in brain regions. In brief, brain sections were placed in equilibration buffer and incubated with nucleotide mix and rTdT enzyme at 37 °C for 1 h and 15 min. Reactions were terminated by ×2 SSC solution for 15 min. Nuclei were counterstained with Hoechst 33342 (1:20,000; Molecular Probes) for 5 min. TUNEL-positive cells were visualized using the fluorescein isothiocyanate (FITC) channel on the Olympus fluorescence microscope (Olympus America, Inc.).
Cell counting
Cell count was performed following the principles of design-based stereology. Systematic random sampling was employed to ensure accurate and non-redundant cell counting. Every section under analysis was at least 100 μm away from the next. Six 10- to 14-μm thick sections, frozen or perfusion fixed, spanning the entire region of interest, were selected for cell counting. Counting was performed on six non-overlapping randomly selected ×20 fields per section. Images were taken in an anterior to posterior direction from the same region of the cortex defined according to a standard atlas of the rat brain. Cell counting was performed by an individual who was blinded to the experimental groups.
The data analysis of reactive microglia in the brain regions was based on the morphological assessment of Iba-1-positive cells according to published method [
23]. Briefly, based on the length of branches, thickness of branches, and cell body volume, the Iba-1-positive cells were categorized to three classes: (a) ramified microglia (surveillant/resting microglia), characterized by small round or oval cell bodies containing a small volume of cytoplasm; (b) hypertrophied microglia, which had larger cell bodies and thicker processes than ramified microglia; and (c) bushy microglia, which had numerous but short processes forming thick bundles around their swollen cell bodies. Hypertrophied and bushy Iba-1-positive cells were identified as activated microglia.
Axon measurements
To study axon diameter and distribution, a minimum of 100 axons labeled by neurofilament (NF) were randomly selected per brain section. Three sections were randomly selected for each animal, and 100 axons per section were analyzed in cortical areas adjacent to layer VI, aligned radially, and perpendicular to the cutting field. Axonal diameter was estimated by measuring the diameter perpendicular to the center of the maximum diameter of the axon profile, as previously described by Zikopoulos and Barbas [
24]. Measured axons were then categorized as small (<0.35 μm), medium (0.35–0.69 μm), large (0.7–1.4 μm), and extra-large (>1.4 μm).
Myelinated axons were quantified using the principles of design-based stereology stated above. Axons labeled with MBP were quantified using ImageJ software (NIH, Bethesda, MD, USA) area fraction measurements to determine the density of myelinated axons in the region of interest. Counting was performed on six non-overlapping randomly selected ×20 fields per section. Each section was 14-μm thick and at least 100 μm from the next section.
Isolation of total RNA and quantitative RT-PCR
Total RNA was extracted from the whole blood and the specific brain regions of rats in formalin and control groups using RiboPure™-Blood Kit (Invitrogen) and TRIzol reagent (Invitrogen), respectively. RNA integrity was confirmed by the detection of 28s and 18s rRNA bands in 1 % agarose gel with ethidium bromide. Also, RNA was confirmed to be free of genomic DNA contamination by PCR in the absence of reverse transcriptase. The RNA samples were reverse transcribed in 20 μl of a reaction mixture containing ×2 RT buffer and ×20 RT enzyme mix according to the manufacturer’s instructions (Life Technologies, Grand Island, NY, USA) at 37 °C for 60 min. The samples were then incubated at 95 °C for 5 min and transferred to 4 °C. For measuring gene expressions, quantitative real-time polymerase chain reaction (qRT-PCR) was done with an ABI 7500 Fast Real-Time system (Applied Biosystems, Foster City, CA, USA) with the FastStart DNA Master SYBR Green kit (Roche Diagnostics, Mannheim, Germany), and results were analyzed with the 7500 software supplied with the machine. GAPDH was used as an internal control. PCR primers used were listed as follows: for TNF-α, 5′-ATGGCCTCCCTCTCAGTTC-3′ (forward) and 5′-TTGGTGGTTTGCTACGACGTG-3′ (reverse); for IL-1β, 5′-CATCTTTGAAGAAGAGCCCG-3′ (forward) and 5′-AGCTTTCAGCTCACATGGGT-3′ (reverse); for IL-6, 5′-GCCCTTCAGGAACAGCTATG-3′ (forward) and 5′-CGGACTTGTGAAGTAGGGA-3′ (reverse); for substance P (SP), 5′-ATGAAAATCCTCGTGGCGGT-3′ (forward) and 5′-CAGCATCCCGTTTGCCCATT-3′ (reverse); and for 18s, 5′-ACCACAGTCCATGCCATCAC-3′ (forward) and 5′-CACCACCCTGTTGCTGTAGCC-3′ (reverse).
Western blot analysis
Western blotting was used to detect the expression of inflammatory- and ASD-related genes. After sacrifice, animals were subjected to transcardial perfusion using PBS. Brain cortical and hippocampal tissues were lysed in a buffer containing 0.02 M Na4P2O7, 10 mM Tris-HCl (pH 7.4), 100 mM NaCl, 1 mM EDTA (pH 8.0), 1 % Triton, 1 mM EGTA, 2 mM Na3VO4, and a protease inhibitor cocktail (Sigma). The supernatant was collected after centrifugation at 15000g for 10 min at 4 °C. Protein concentration was determined with a bicinchoninic acid assay (Pierce Biotechnology, Rockford, IL, USA). Equivalent amounts of total protein were separated by molecular weight on an SDS-polyacrylamide gradient gel and then transferred to a PVDF membrane. The blot was incubated in 10 % nonfat dry milk for 1 h and then reacted with primary antibodies at 4 °C for overnight.
The primary antibodies and their dilutions are as follows: rabbit anti-TNF-α antibody (Cell Signaling) 1:2000, rabbit IL-1β antibody (Cell Signaling, Danvers, MA, USA) 1:1000, rabbit IL-6 antibody (Cell Signaling) 1:1000, mouse anti-actin (Sigma) 1:5000, rabbit anti-NK-1R (Millipore) 1:2500, mouse anti-neurexin 1 (NRXN1; Cell Signaling) 1:1000, rabbit anti-fragile X mental retardation 1 (FMR1; Cell Signaling) 1:1000, mouse anti-neuroligin3 (NLGN3; Millipore) 1:1000, rabbit anti-autism susceptibility gene 2 (AUTS2; Abcam) 1:2000, goat anti-oxytocin (Abcam) 1:1000, and rabbit anti-oxytocin receptor (Santa Cruz) 1:500. After washing with Tris-buffered saline with Tween-20 (TBST), membranes were incubated with AP-conjugated or HRP-conjugated secondary antibodies (GE Healthcare, Piscataway, NJ, USA) for 2 h at room temperature. After final washing with TBST, the signals were detected with bromochloroidolylphosphate/nitroblue tetrazolium (BCIP/NBP) solution (Sigma) or film. Signal intensity was measured by ImageJ (NIH) and normalized to the actin signal intensity.
Behavioral tests
Locomotor activity using TopView system
Behavioral changes of experimental rats were monitored and analyzed using the TopScan System (Clever Sys Inc., Reston, VA, USA). P10, P15, and P20 rats were allowed to freely move in an open field container (50 cm × 50 cm × 50 cm high) during the dark cycle. Travelled distance and velocity for locomotor activity were recorded for 1 h. After finishing the recording, the videos were analyzed by the TopScan Realtime Option Version 3.0 (Clever Sys Inc.).
Hot-plate test
Pain sensitivity was measured using a hot-plate set to 55 +/− 1 °C. Response latency was measured as the time taken for the rat to jump after placing on the hot plate. The maximum allowed time was 30 s. The reported latency was the average value calculated from three measurements per animal. Repeated tests were separated by at least 15 min.
Three chamber sociability test
The three-chamber test was utilized to test general sociability and response to social novelty. The test was performed in a three-chambered box that has openings between the chambers. Glass slides were used to cover the openings during phase changes. First, the test subject was placed into the empty box and allowed to explore all chambers freely for 10 min. After the habituation period, a stranger (non-littermate) rat contained in a wire cage was placed into the left chamber. The rat was then allowed to explore all three chambers. Both the time spent with the stranger rat (stranger #1) and the time in the empty chamber were recorded over a 10-min session. The test rat was then returned to the center chamber, and the openings were blocked. In the social novelty test, a second stranger rat (stranger #2) was placed in the empty chamber. The central chamber door was opened, and the test rat was free again to explore the strangers #1 and #2. Since the test rat had already had contact with stranger #1 but not #2, the time it spent with strangers #1 and #2 tested its novel social interaction.
Social interaction test
This test was conducted with untreated, unfamiliar, weight-matched partner same sex rats. Subject and stranger rats were put together in a clean empty cage and recorded by the TopScan System (Clever Sys Inc.). We scored time spent in social interaction (social sniffing, social grooming, and social following) of the animals for 5 min between 1:00 and 5:00 pm. The cage was washed with a 70 % alcohol solution and water before we performed the next test in order to prevent possible contamination by previous tests.
Home cage observation
The HomeCage Monitoring System (Clever Sys Inc.) was used to detect the behavior patterns of animals in their home cage environment without human intervention. The system had four cameras that monitor four rats simultaneously in four separate cages (191 mm x 292 mm x 127 mm). Empty cages were first recorded and saved as the background image for video analysis. Animals were placed one per cage and allowed to habituate to the new environment for at least 30 min. The behavior patterns were recorded from 10 pm to 4 am during the night time when rodents are most active. The video recordings were analyzed using the HomeCage Software 3.0 (Clever Sys Inc.). The software discriminates various body movements and behavior patterns. We analyzed both the number of bouts of each behavior and the time spent performing each behavior during the 6-h period.
Morris water maze test
The Morris water maze test was performed and analyzed to measure memory function [
25]. This test was videotaped using TopScan (Clever Sys, Inc.), and performance was analyzed using TopScan Realtime Option Version 3.0 software (Clever Sys, Inc.). The water maze apparatus is a round, water-filled tub (120.1-cm diameter filled with blue tempera paint) placed in a room rich with extra-maze cues. Rats were placed in the maze starting from four different positions (NW, NE, SW, and SE). An invisible escape platform was located in the same spatial location 1 cm below the water surface independent of the starting position on a particular trial. In this manner, subjects were able to utilize extra-maze cues to determine the platform’s location. Each subject was given four trials per day (NW, NE, SW, and SE) for 6 days with a 15-min inter-trial interval. The maximum trial length was 60 s, and subjects were manually guided to the platform if they did not reach it in the allocated time. Upon reaching the invisible escape platform, subjects were kept on it for an additional 15 s to allow them to survey the spatial cues in the environment to guide future navigation to the platform. After 6 days of task acquisition, a probe trial was carried out during which the platform was removed. The time spent and distance travelled in the quadrant that previously contained the escape platform during task acquisition was measured over 60 s.
Social transmission of food preference transmission test
The social transmission of food preference test was used in rodents to assess olfactory memory processes. For the test, two demonstrator rats were removed from each test cage and individually housed overnight with water but without food (18 h). The demonstrator rats were then placed into clean cages containing almond-flavored food in small glass jars (3.9-cm diameter, 3.4-cm high). The glass jars were set in shallow Petri dishes so that food scattered by the digging of the rats was retained. Demonstrator rats were left to eat the cued food (almond) for 1 h. Dishes were weighed before and after to measure how much food was eaten. The demonstrator rat was then placed in a clean experimental cage. One at a time, “observer” rats from the same home cage of the demonstrator rat were placed in the cage containing the demonstrator rat and left there for 5 min. The observer rat was then removed. After an interval of 15 min, the sequence of interaction was repeated, with each observer rat being placed with the second demonstrator rat from the home cage. All observer rats were then returned to the home cage, and demonstrator rats were individually housed. Six hours after the social interaction sessions, the observer rat was food-deprived for 18 h (overnight). The following morning, each rat was placed individually in a clean cage (45 cm × 28 cm × 12 cm) containing two “dishes” in either corner at the back of the cage: one with almond-flavored food (cued) and the other containing normal diet (non-cued). Rats were allowed to eat for 1 h. Dishes were weighed before and after to determine the amount of food eaten. Food preference was calculated as the amount of cued food eaten/total food eaten ×100 (% total).
Direct interaction test
To measure the social memory function, we performed the direct interaction test as described previously [
26]. In the first trial, subject rats were placed in a clean cage, and a novel rat was introduced. Social interaction activity was quantified to examine the time spent in social sniffing, social following, and social grooming. After an inter-trial interval of 1 h, either the previously encountered rat or novel rat was introduced and then the social interaction activity was measured for 5 min.
Five-trial social memory test
The five-trial social memory test was performed to measure more obvious social memory ability as described previously [
26]. Briefly, subject rats were presented with four successive 1-min trials. On the last trial, we introduced a novel rat. In each trial, we measured the social interaction activity (nose-to-nose sniffing, following, and grooming).
Statistical analysis
All analyses were performed using GraphPad Prism 6.0 statistical software (GraphPad Software, Inc., La Jolla, CA). Multiple comparisons were performed by one- or two-way analysis of variance (ANOVA) followed by Bonferroni’s post hoc analysis. Single comparisons were performed using Student’s t test. Changes were considered significant if the P value was less than 0.05. Mean values were reported with the standard error of the mean (SEM) unless otherwise indicated.
Discussion
The present investigation provides novel and comprehensive cellular, molecular, and behavioral evidence that acute but relatively severe neonatal inflammatory pain can trigger lasting systemic inflammatory responses and pathological alterations that may generate a vulnerable environment for the development of ASD-like syndrome. Formalin-induced inflammatory pain increases major inflammatory factors TNFα and IL-1β not only in the blood circulation but also in the brain. This upregulation of inflammatory factors persisted even after the disappearance of local tissue damage. Significant neuronal cell death in the cortex and hippocampus CA1/CA2 regions was observed after the peripheral inflammatory stimuli, accompanied by impaired neuronal axons and reduced neurogenesis. Moreover, the inflammatory pain led to long-term regulation of ASD-associated genes NRXN1, FMR1, and oxytocin/oxytocin receptor in the brain. These genetic alterations, increased repetitive behaviors, and deficient social memory/interactions all are analogous to patients with ASD and/or fragile X syndrome. Consistent with clinical cases, these pathological phenotypes are more prominent in males than in females. As a mechanistic verification and of clinical importance, these abnormal phenotypes can be largely prevented with an anti-inflammatory pain intervention using the clinical drug indomethacin.
Previous work reported that exposure to repetitive inflammatory pain during the development of high brain plasticity is associated with neuronal cell death, neuroinflammation, modulations in pain sensation, and adverse changes in brain structure and function [
22,
32,
33]. Due to the plasticity of sensory connections in the neonatal period, early damage in infancy can cause prolonged structural and functional alterations in pain pathways that can last into later life stages [
32,
33]. For example, inflammatory pain experienced during the postnatal period may cause abnormal adult behaviors such as increased anxiety, altered pain sensitivity, hyperactivity, self-destructive behavior, or reduced social behaviors [
32,
33]. In clinical studies, premature neonates exposed to painful experiences are more likely to develop chronic abnormalities compared to full-term infants. For example, circumcised infants showed a stronger pain response to subsequent routine vaccination than uncircumcised infants [
34]. On the other hand, there has been no research focusing on the relationship between neonatal inflammatory pain and the prevalence of ASD.
In the study of postmortem brain from ASD patients, elevated cytokines and chemokines and activated microglia were observed [
35,
36]. A recent review pointed out that premature babies are more vulnerable to infections and inflammation that can lead to neurodevelopmental problems and higher risk for ASD [
12]. In an animal study, maternal infections caused by multiple intraperitoneal injections of lipopolysaccharide damaged the layer formation of the fetal brain, possibly linked to neuropsychiatric disorders, such as schizophrenia and autism [
37]. A clinical study with 1.2 million pregnancies showed that the risk of autism in the children of women with the highest levels of C-reactive protein, a well-known marker of inflammation, was 43 % higher than women with the lowest levels [
38]. Another study provided new evidence that mothers who have autoimmune diseases associated with excess and/or chronic inflammation could be at increased risk of having children with ASD [
39]. ASD patients frequently showed widespread inflammation as indicated by elevated inflammatory cytokines in both the brain and blood similar to those in autoimmune disease, signifying the importance of the inflammatory response on the development of ASD [
40,
41]. Specifically, ASD patient’s peripheral blood cells secrete higher levels of TNF-α, IL-1β, and IL-6 [
42,
43]. Our current findings are in line with these observations, showing systemic and lasting elevations of TNF-α, IL-1β, and activation of microglia in the brain. The link between inflammation and ASD pathology was strongly supported by the success of indomethacin in protecting against inflammatory pain-induced changes.
IL-1β-induced inflammation inhibited hippocampal neurogenesis [
44]. Similarly, we observed reduced neurogenesis in the dentate gyrus of the formalin group, which may relate to the IL-1β increase in the brain. Conversely, one previous study in neonatal rats found increased hippocampal neurogenesis at P22 after a Freund’s adjuvant injection [
45]. The contrasting result may be due to apparent differences in the inflammatory insults and in the timing and severity of the insult and measurements in a particular model. Inflammatory pain may result from the increased excitability of peripheral nociceptive sensory fibers activated by inflammatory mediators [
46]. Alterations in pain pathways such as pain-related receptor expression can last into the adolescent or adult stage [
16]. NK-1R, an activator of nociception-induced spinal central sensitization, is reduced in rat models of chronic pain and stress [
28]. Consistent with these findings, rats exposed to early pain in our study had reduced NK-1R expression, which is likely an event related to enhanced pain sensitivity in adolescents/juvenile rats.
We show here in juvenile rats that early neonatal inflammatory pain causes important morphological alterations in the developing brain, including axonal damage and reduced MBP. These alterations resemble some changes caused by activated cytokine responses seen in prenatal stress [
47]. A previous study on inflammatory lesions in multiple sclerosis patients revealed that axonal density changes may be caused by the release of inflammatory mediators [
48]. It is likely that the persistent elevation of inflammatory factors in the brain is largely responsible for the axonal impairment. The significant axonal changes imply that altered neuronal transduction along these nerve fibers must take place after the inflammatory pain and chronic cytokine upregulation.
Repetitive and uncontrolled behaviors such as repetitive grooming, jumping, and muscle twitching are prominent features in human and animals with autism-like disorders [
49,
50]. In our study, the time spent in twitching and doing repetitive jumping and grooming behaviors in juvenile rats were significantly increased in the formalin group. In general, rodents prefer social environments over solitary ones. They prefer to engage a novel partner rather than a familiar one. Strikingly, juvenile rats subjected to early inflammatory pain exhibit noticeable dysfunction in social interaction tests. In addition to social deficits, children with ASD appear to experience sleep problems more frequently than healthy children [
7]. This is consistent with our observation that juvenile rats in the formalin group showed disrupted sleep behavior. Collectively, the functional and behavioral disorders in the animal model resemble the syndromes of ASD children.
Although memory loss has not been a diagnostic criterion for ASD, it is a common difficulty experienced by ASD patients [
51]. In social memory tests, formalin-treated animals, especially male rats, exhibited an impaired social memory. Coincidently, we observed that formalin stimuli increased more TUNEL-positive cells in the hippocampal CA2 region. Previous research showed the social responsiveness is reduced in rodents with hippocampal lesions [
52]. It is likely that the increased cortical and hippocampal neuronal cell death contribute, at least partly, to the development of abnormal social behaviors. Recent data revealed that the CA2 region is essential for social memory [
26]. Our immunostaining data showed reduced oxytocin receptor in CA1 and CA2 regions of male rats in the formalin group. These findings raise the possibility that CA2 damages and abnormal gene expression induced by early inflammatory pain play a critical role in social memory impairment.
Many studies have been investigating the connection between genetic variation and ASD. Genome-wide association studies (GWAS) for ASD have identified few potential loci associated with ASDs [
53]. NRXN1 was implicated as an autism susceptibility gene, though changes in this gene alone are not always detrimental [
54]. For example, mice with a deletion of NRXN1 spend more time grooming but also show improved motor learning [
54]. NRXN1 knockout mice display increased responsiveness and accelerated habituation to novel environments [
55]. These data suggest that mutation or deletion of NRXN1 alone is not sufficient to cause ASD. In the inflammatory pain model, we detected significant decreases of NRXN1 and FMR1 expression in the cortex. Being a sub-category of ASD, fragile X syndrome is identified as a single gene inherited disorder due to mutations or deficiency of FMR1 [
56]. FMR1 mutation or deletion has shown autism-like behaviors such as impaired social activity, anxiety, and reduced behavioral flexibility [
56]. A wealth of studies has implicated oxytocin and the oxytocin receptor in the mediation of social behaviors and social memory, suggesting that failures in this system may be associated with ASD [
57]. Decreased oxytocin and its receptor signal can result in low social activity and autism-like behaviors, and this change in oxytocin system is commonly detected in ASD patients [
57]. Our study shows a significantly reduced oxytocin level in the cortex of formalin-treated rats. We did not detect significant changes in the expressions of NLGN3 or AUTS2 in formalin-treated rats. Alteration of NLGN3 contributes to the induction of autism-related behaviors [
54,
58]. However, recent clinical investigations showed that NLGN3 may not be a major disease gene in ASD [
54]. It is possible that although ASD is associated with multiple genes, the development of ASD does not require participation of all related genes.
Some important issues remain to be better addressed. Males are approximately four times more likely than females to be diagnosed with ASD [
59,
60]. It has been hypothesized that prenatal sex steroids may affect fetal brain structure and function and consequently influences postnatal behavior [
61]. Whether the high incidence of ASD in male can be explained by the levels of sex hormones in postnatal babies is obscure. Different levels of sex hormones are detected in fetal surroundings and human neonates [
62]. As estrogen and progesterone have neuroprotective and anti-inflammatory effects, it might be possible that these sex hormones contribute to the low incidence of ASD in female [
63,
64]. There are also several limitations in this investigation. Most children with ASD are at a normal gestational age at birth and are not treated by painful procedures. Whether inflammation pain shows similar pathological and etiological impacts on full term infants and adults remains to be examined. It is also to note that inflammation and pain are distinct insults although may sometimes reciprocal. The pathogenic effects of pain and inflammation may play distinctive roles in ASD, while this is unclear based on available data.
The inflammation insult tested in this investigation (two hindpaws formalin injections for three consecutive days) is relatively severe. In our preliminary tests, we found inflammatory reaction and pathological consequences to formalin-induced inflammatory pain depended on the severity of the insult. A single paw one-time formalin injection elicited mild changes sometimes without statistical significance. A clear demonstration of the “dose-dependent” pathogenesis for ASD development requires a future investigation. Considering that formalin can act as a pro-oxidative neurotoxicant, it may be necessary to verify the observations in this investigation using other inflammatory pain agents such as carrageenan, zymosan, or complete Freund’s adjuvant that are known to trigger longer inflammatory pain responses than formalin [
65]. Based on our data, however, an irritating insult that triggers comparable pain and neuroinflammatory reactions should bear similar pathogenic consequence as shown with formalin. It will also be interesting to see whether the beneficial effects of anti-inflammation treatment may last longer beyond the juvenile age or continual treatments are needed for long-term effects.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
JHL conceived the concept, performed the experimental design and most experiments and data analysis and wrote manuscript; ARE performed the immunohistochemical and behavioral experiments and data analysis and proofread the manuscript; DC performed the immunohistochemical experiments and data analysis; K-EC performed some Western blot analysis, immunohistochemical experiments, and behavioral tests; AYC performed the immunohistochemical staining, cell counting, and behavioral tests; SW performed the qRT-PCR measurement of inflammatory factors in the blood and brain tissues and participated in the data analysis; VP performed the cell counting and behavioral tests; G-YX performed the data analysis and manuscript revision; LW performed the experimental design, data interpretation, and research funding; SPY conceived the concept, performed the experimental design and data analysis, wrote manuscript, oversee the project, and provided fund supports. All authors read and approved the final manuscript.