Increased susceptibility to hyperoxic lung injury and alveolar simplification in newborn rats by prenatal administration of benzo[a]pyrene

Maternal smoking is one of the risk factors for preterm birth and for the development of bronchopulmonary dysplasia (BPD). In this study, we tested the hypothesis that prenatal exposure of rats to benzo[a]pyrene (BP), a component of cigarette smoke, will result in increased susceptibility of newborns to oxygen-mediated lung injury and alveolar simplification, and that cytochrome P450 (CYP)1A and 1B1 enzymes and oxidative stress mechanistically contribute to this phenomenon. Timed pregnant Fisher 344 rats were administered BP (25 mg/kg) or the vehicle corn oil (CO) on gestational days 18, 19 and 20, and newborn rats were either maintained in room air or exposed to hyperoxia (85% O2) for 7 or 14 days. Hyperoxic newborn rats prenatally exposed to the vehicle CO showed lung injury and alveolar simplification, and inflammation, and these effects were potentiated in rats that were prenatally exposed to BP. Prenatal exposure to BP, followed by hyperoxia, also resulted in significant modulation of hepatic and pulmonary cytochrome P450 (CYP)1A and 1B1 enzymes at PND 7-14. These rats displayed significant oxidative stress in lungs at postnatal day (PND) 14, as evidenced by increased levels of the F2-isoprostane 8-iso-PGF2α. Furthermore, these animals showed BP-derived DNA adducts and oxidative DNA adducts in the lung. In conclusion, our results show increased susceptibility of newborns to oxygen-mediated lung injury and alveolar simplification following maternal exposure to BP, and our results suggest that modulation of CYP1A/1B1 enzymes, increases in oxidative stress, and BP-DNA adducts contributed to this phenomenon.

Effects of p38 MAPK signaling pathway on cognitive function and recovery of neuronal function after hypoxic-ischemic brain injury in newborn rats

To explore the effects of p38 MAPK signaling pathway on cognitive function and recovery of neuronal function after hypoxic-ischemic brain injury (HIBI) in newborn rats. Seventy-two healthy SPF grade SD newborn rats were randomly and equally divided into Normal group (healthy rats) and Sham group (rats underwent sham operation), Model group (HIBI model rats), p38 MAPK Inhibitor group (HIBI model rats treated with p38 MAPK inhibitor) and p38 MAPK Activator group (HIBI model rats treated with p38 MAPK activator). On postnatal day 28, Morris water maze, tail suspension test and inclined plane test were conducted on rats in each group. Twenty-four hours after modeling, the expression of p-p38 MAPK protein and apoptosis related genes in rat hippocampal tissues was detected by TUNEL staining, qRT-PCR and Western blot. Compared with Normal group, escape latency and inclined plane test time were prolonged, the number of passing through the platform and tail suspension time were reduced (all P < 0.05); Bax and Caspase-3 mRNA and protein expression levels and p-p38 MAPK protein level were increased, Bcl-2 mRNA level was decreased, and neuronal apoptosis proportion was increased in Model group (all P < 0.05). Compared with Model group, the above indicators showed reversed and enhanced trends in p38 MAPK Inhibitor and p38 MAPK Activator groups, respectively (all P < 0.05). Inhibition of p38 MAPK signaling pathway can effectively improve the learning and memory ability and motor function of newborn rats with HIBI, and reduce neuronal apoptosis in the hippocampal tissues, thereby promoting neuronal recovery.

Corticosteroid responses following hypoxic preconditioning provide neuroprotection against subsequent hypoxic-ischemic brain injury in the newborn rats

Limited research has evaluated the corticosteroids (CS) response in hypoxic preconditioning (PC) induced neuroprotection against subsequent hypoxic-ischemic (HI) brain injury in newborns. To measure, CS response to hypoxic PC, at postnatal day 6 (P6), rat pups were randomly divided into sham, NoPC (exposure to 21% O2) and PC (exposure to 8% O2 for 3h) groups. In a separate experiment, at P6, rat pups were randomly divided into three groups (sham, NoPC+HI, PC+HI). Rat pups in NoPC+HI and PC+HI groups, respectively had normoxic or hypoxic exposure for 3h at P6 and then had the right carotid artery permanently ligated followed by 140 min of hypoxia at P7 (HI). Plasma CS levels were measured at 0.5, 1, 3, 6 and 12h after hypoxic PC and hypoxic PC followed by HI. To investigate whether CS response to hypoxic PC provides neuroprotection against HI, at P6, rat pups were randomly divided into five groups. Fifteen minutes prior to PC or normoxic exposure, rat pups in DMSO+PC+HI and DMSO+NoPC+HI groups received DMSO while in RU486+PC+HI and RU486+NoPC+HI groups received RU486 (glucocorticoid receptor blocker, 60 mg/kg) s.c., respectively. Afterwards, rat pups were exposed to normoxia (DMSO+NoPC+HI, RU486+NoPC+HI) or hypoxia (DMSO+PC+HI, RU486+PC+HI) for 3h and then HI 24h later (P7). Rat pups at the corresponding age without any exposure to PC or HI or RU486/DMSO were used as sham. We found that hypoxic PC caused CS surge as well as augmented CS surge and preserved the glucocorticoid feedback regulation after HI. Hypoxic PC reduced HI induced early and delayed brain damage. RU486 partially but significantly inhibited hypoxic PC induced neuroprotection.

Regional differences of microglial accumulation within 72 hours of hypoxia-ischemia and the effect of acetylcholine receptor agonist on brain damage and microglial activation in newborn rats

OBJECTIVE

We examined regional specificity of microglial activation in the developing rat brain for 72 hours after hypoxia-ischemia (HI) and the effect of acetylcholine receptor (AChR) agonist on microglial activation.

STUDY DESIGN

Seven-day-old Wistar rats were divided into two groups: one receiving a single dose of AChR agonist just before hypoxia (carbachol; 0.1mg/kg) to investigate the reducing effect on brain damage with decreasing activation of microglia and the other group receiving saline as a control. Rats were subjected to left carotid artery ligation followed by 8% hypoxia. Brains were analyzed immunohistochemically at 24, 48, and 72 hours after HI. TNFα production was measured at respective times after HI.

RESULTS

Activation of microglia on the hippocampus of the control group was strong for the first 48 hours and then weakened. In contrast, activation of microglia on white matter and the cortex was weak at 24 hours and then became stronger. A single dose of carbachol significantly reduced brain damage with a marked reduction of microglial activation on the hippocampus, whereas it was less effective regarding microglial activation on white matter and the cortex. TNFα production was low in both groups.

CONCLUSION

Regional specificity was observed for both microglial activation and susceptibility to carbachol for the first 72 hours after HI. Our data suggested that timely intervention along with region-specific microglial activation, apart from TNFα production, may be critical for the prevention of further brain damage after HI in the newborn.

Acute exposure to zinc oxide nanoparticles critically disrupts operation of the respiratory neural network in neonatal rat

Due to their extremely small size that gives them unique physicochemical properties, nanoparticles (NPs) are used in the production of everyday materials. However, NPs can accumulate in body organs and could cause various diseases. Moreover, NPs that cross biological membranes such as the blood-brain barrier can aggregate in the brain and potentially produce neuronal damage. Although studies have reported the effects of diverse NPs on the bioelectrical properties of individual neurons, their potential influences on the operation of whole neuronal networks have not been documented. Here, we aimed to evaluate the effects of an acute exposure to zinc oxide (ZnO) NPs on the central neural networks responsible for mammalian respiratory rhythm generation. Using an isolated ex vivo brainstem-spinal cord preparation from neonatal rat in which the circuitry for the central respiratory command remained intact, we show that ZnO NPs accelerate, then profoundly disrupt respiratory-related activity produced by the pre-Bötzinger complex (preBötC) responsible for inspiratory rhythm generation. Consequently, a sudden and definitive cessation of respiratory-related activity occurs in ZnO NPs-exposed preparations. Part of these effects is related to zinc ions released from NPs. Using brainstem slice preparations containing the preBötC network, whole-cell patch-clamp recordings revealed that ZnO NPs depolarize preBötC inspiratory neurons and affect their bioelectrical properties by reducing the amplitude of action potentials, thereby leading to a depression of intra-network activity and the ultimate termination of respiratory rhythmogenesis. These findings support the conclusion that ZnO NPs may have deleterious effects on the central respiratory centers of newborn mammals.

An in vitro experimental model for analysis of central control of sympathetic nerve activity

Newborn rat brainstem-spinal cord preparations are useful for in vitro analysis of various brainstem functions including respiratory activity. When studying the central control of sympathetic nerve activity (SNA), it is important to record peripheral outputs of the SNA. We developed an in vitro preparation in which neuronal connections between the cardiovascular center in the medulla and SNA peripheral outputs are preserved. Zero- to 1-day-old rats were deeply anesthetized with isoflurane, and the brainstem and spinal cord were isolated with a partial right thoracic cage to record sympathetic nerve discharge from the right thoracic sympathetic nerve trunk (T9-T11). SNA in this preparation was strongly modulated by inspiratory activity. Single-shot electrical stimulation of the ipsilateral rostral ventrolateral medulla (RVLM) induced a transient increase of SNA. Bath application of angiotensin II induced an increase of SNA, and local ipsilateral microinjection of angiotensin II to the RVLM induced a transient increase of SNA. This preparation allows analysis of the central control of the SNA in vitro.

Effects of acetylcholine on hypoglossal and C4 nerve activity in brainstem-spinal cord preparations from newborn rat

Effects of acetylcholine (ACh) on respiratory activity have been an intriguing theme especially in relation to central chemoreception and the control of hypoglossal nerve activity. We studied the effects of ACh on hypoglossal and phrenic (C4) nerve activities and inspiratory and pre-inspiratory neurons in the rostral ventrolateral medulla in brainstem-spinal cord preparations from newborn rats. ACh application increased respiratory rhythm, decreased inspiratory hypoglossal and C4 nerve burst amplitude, and enhanced pre-inspiratory hypoglossal activity. ACh induced membrane depolarization of pre-inspiratory neurons that might be involved in facilitation of respiratory rhythm by ACh. Effects of ACh on hypoglossal and C4 nerve activity were partially reversed by a nicotinic receptor blocker, mecamylamine. Further application of a muscarinic receptor antagonist, oxybutynin, resulted in slight increase of hypoglossal (but not C4) burst amplitude. Thus, ACh induced different effects on hypoglossal and C4 nerve activity in the brainstem-spinal cord preparation.

Effects of aconitine on the respiratory activity of brainstem-spinal cord preparations isolated from newborn rats

Aconitine is a sodium channel opener, but its effects on the respiratory center are not well understood. We investigated the dose-dependent effects of aconitine on central respiratory activity in brainstem-spinal cord preparations isolated from newborn rats. Bath application of 0.5-5 μM aconitine caused an increase in respiratory rhythm and decrease in the inspiratory burst amplitude of the fourth cervical ventral root (C4). Separate application of aconitine revealed that medullary neurons were responsible for the respiratory rhythm increase, and neurons in both the medulla and spinal cord were involved in the decrease of C4 amplitude by aconitine. A local anesthetic, lidocaine (100 μM), or a voltage-dependent sodium channel blocker, tetrodotoxin (0.1 μM), partially antagonized the C4 amplitude decrease by aconitine. Tetrodotoxin treatment tentatively decreased the respiratory rhythm, but lidocaine tended to further increase the rhythm. Treatment with 100 μM riluzole or 100 μM flufenamic acid, which are known to inhibit respiratory pacemaker activity, did not reduce the respiratory rhythm enhanced by aconitine + lidocaine. The application of 1 μM aconitine depolarized the preinspiratory, expiratory, and inspiratory motor neurons. The facilitated burst rhythm of inspiratory neurons after aconitine disappeared in a low Ca2+/high Mg2+ synaptic blockade solution. We showed the dose-dependent effects of aconitine on respiratory activity. The antagonists reversed the depressive effects of aconitine in different manners, possibly due to their actions on different sites of sodium channels. The burst-generating pacemaker properties of neurons may not be involved in the generation of the facilitated rhythm after aconitine treatment.

Effects of riluzole on spinal seizure-like activity in the brainstem-spinal cord preparation of newborn rat

Riluzole blocks persistent Na⁺ current, inhibits generation of neuronal bursts and decreases glutamate-induced excitotoxicity. In previous studies of respiratory activity, riluzole suppressed inspiratory-related burst generation activity in rat slice or en bloc preparations. We examined riluzole's effects on inspiratory burst generation and drug-induced seizure-like activity in newborn rat en bloc preparations. Medulla-spinal cord preparations from postnatal day 0-3 Wistar rats were isolated under deep isoflurane anesthesia and were superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4, at 25-26°C. Inspiratory activity was monitored from the fourth cervical ventral root. Seizure-like activity was induced by application of 20μM DL-threo-β-benzyloxyasparatate (TBOA, a glutamate uptake blocker preferentially acting on astrocytes) or coadministration of GABA_A antagonist bicuculline (10μM) and glycine antagonist strychnine (10μM). Pretreatment and co-application with 10μM riluzole abolished the seizure-like burst activity induced by TBOA or bicuculline/strychnine. N-methyl-d-aspartic acid receptor antagonist MK801 (10μM) also depressed this activity. Riluzole may attenuate excessive glutamate action involved in pathological hyperexcitability of motor neurons with no major effect on generation of respiratory activity. Riluzole at the optimal dose could be a potential treatment to protect drug-induced epileptic brain tissue from excitotoxic damage without inducing respiratory suppression.

Respiratory sinus arrhythmia in spontaneously breathing, unanesthetized newborn and adult Wistar rats

We examined respiratory sinus arrhythmia (RSA) and possible interaction with respiratory frequency (fR) and heart rate (HR) in spontaneously breathing, unanesthetized newborn Wistar rats (2- to 5-day-old; n = 54) and the adult rats (8-week-old; n = 34). Instantaneous heart rate (inst-HR) was calculated as the reciprocal of the inter-beat-interval. For each breath, RSA was determined as the difference between the maximum and minimum inst-HR value. The absolute RSA or RSA% (RSA per HR) were calculated as the average RSA of 10 consecutive breaths. RSA (or RSA%) in the newborn rats was significantly lower than that in the adult rats. Correlation coefficient between RSA (or RSA%) and 1/fR or HR/fR, but not HR, was significant in newborn rats, whereas only that between RSA (or RSA%) and HR was significant in adult rats. The power spectrum density of heartbeat fluctuation was detectable in both age groups. The present findings suggest that RSA exists and could be influenced by fR, rather than HR, in newborn rats.

Effect of cisplatin on respiratory activity in neonatal rats

One side effect of cisplatin, a cytotoxic platinum anticancer drug, is peripheral neuropathy; however, its central nervous system effects remain unclear. We monitored respiratory nerve activity from the C4 ventral root in brainstem and spinal cord preparations from neonatal rats (P0-3) to investigate its central effects. Bath application of 10-100 μM cisplatin for 15-20 min dose-dependently decreased the respiratory rate and increased the amplitude of C4 inspiratory activity. These effects were not reversed after washout. In separate perfusion experiments, cisplatin application to the medulla decreased the respiratory rate, and application to the spinal cord increased the C4 burst amplitude without changing the burst rate. Application of other platinum drugs, carboplatin or oxaliplatin, induced no change of respiratory activity. A membrane potential analysis of respiratory-related neurons in the rostral medulla showed that firing frequencies of action potentials in the burst phase tended to decrease during cisplatin application. In contrast, in inspiratory spinal motor neurons, cisplatin application increased the peak firing frequency of action potentials during the inspiratory burst phase. The increased burst amplitude and decreased respiratory frequency were partially antagonized by riluzole and picrotoxin, respectively. Taken together, cisplatin inhibited respiratory rhythm via medullary inhibitory system activation and enhanced inspiratory motor nerve activity by changing the firing property of motor neurons.

Involvement of cannabinoid receptors in depression of the putative nociceptive response in spinal cord preparations isolated from neonatal rats

A metabolite of acetaminophen, AM404, which is an anandamide transporter inhibitor, induces analgesia mainly via activation of transient receptor potential channel 1 in the spinal cord, although the role of cannabinoid receptors remains to be studied. The ventral root reflex response induced by stimulation of the dorsal root in in vitro preparations of rat spinal cord is useful to assess the effect of analgesics. We analyzed the effects of AM404 and cannabinoid receptor antagonist AM251 on reflex responses in lumbar spinal cord preparations from newborn rats and found that the amplitude of the slow ventral root potential after administration of 10 µM AM404 was not significantly changed, whereas 10 µM AM251 significantly increased the amplitude. Administration of the cannabinoid receptor 1 agonist WIN55,212-2 (10 µM) did not significantly affect the reflex response. We suggest that endogenous cannabinoids in the spinal cord are involved in the antinociceptive mechanism through suppressive effects.

Intrinsic responses to hypoxia and hypercapnia of neurons in the cardiorespiratory center of the ventral medulla of newborn rats

The rostral ventrolateral medulla (RVLM) includes a variety of neurons essential for cardiorespiratory control. Although some of these neurons are thought to be intrinsically sensitive to hypercapnia and/or hypoxia, relationships between types of neurons and responses to hypoxia and/or hypercapnia are not well understood. Tyrosine hydroxylase (TH) is one of the cell-type markers of the RVLM neurons. Here, we report effects of hypoxia and hypercapnia on TH-positive or -negative neurons in the RVLM of newborn rats. Brainstem-spinal cord preparations were isolated from 0-3-day-old Wistar rats and superfused with artificial cerebrospinal fluid equilibrated with 95% O2 and 5% CO2, pH 7.4 at 25-26 °C. Membrane potential responses to hypoxia (95% → 0% O2) and/or hypercapnia (2% → 8% CO2) were examined in the presence of tetrodotoxin (TTX) after identification of the firing pattern. We found that TH-positive C1 neurons in the RVLM were sensitive to hypoxia with membrane depolarization but less sensitive to hypercapnia. TH-negative neurons in the C1 area showed responses similar to those of C1 neurons. Moreover, C1 area neurons remained depolarized by hypoxia in the presence of TTX plus gliotransmitter blockers. In contrast, Phox2b-positive and TH-negative neurons in the parafacial respiratory group were intrinsically sensitive to CO2 but not sensitive to hypoxia. Respiratory-related neurons (Phox2b and TH negative) showed a variable response to hypoxia: unchanging, depolarizing, or hyperpolarizing. Our findings suggest that C1 area neurons in the RVLM are intrinsically sensitive to hypoxia and belong to one of the elements constituting central hypoxic sensors.