Effects of Neonatal Stress and Morphine on Murine Hippocampal Gene Expression

Abstract

Critically ill preterm infants experience multiple stressors while hospitalized. Morphine is commonly prescribed to ameliorate their pain and stress. We hypothesized that neonatal stress will have a dose-dependent effect on hippocampal gene expression, and these effects will be altered by morphine treatment. Male C57BL/6 mice were exposed to 5 treatment conditions between postnatal day 5 and 9: 1) Control, 2) mild stress + saline, 3) mild stress + morphine, 4) severe stress + saline and 5) severe stress + morphine. Hippocampal RNA was extracted and analyzed using Affymetrix Mouse Gene 1.0 ST Arrays. Single gene analysis and gene set analysis were used to compare groups with validation by qPCR. Stress resulted in enrichment of genes sets related to fear response, oxygen carrying capacity and NMDA receptor synthesis. Morphine downregulated gene sets related to immune function. Stress plus morphine resulted in enrichment of mitochondrial electron transport gene sets, and down-regulation of gene sets related to brain development and growth. We conclude that neonatal stress alone influences hippocampal gene expression, morphine alters a subset of stress-related changes in gene expression and influences other gene sets. Stress plus morphine show interaction effects not present with either stimulus alone. These changes may alter neurodevelopment.

Effects of Repetitive Exposure to Pain and Morphine Treatment on the Neonatal Rat Brain

“…Severe pain also altered the expression of the neurodevelopmentally important proteins PKCvarepsilon and DCX. Comparable to our results, Juul et al [17] showed an effect between the severity of stress and the amount of gene expression in the neonatal murine hippocampus. This indicates a dose-dependant relationship between distress and the neuronal damage.…”

“…Morphine might protect the brain up to a certain limit. Previous animal studies also showed interactive effects between morphine and pain [14,15,16,17]. Morphine seems to downregulate specific stress-related changes in gene expression that protect the cell against apoptosis [13].…”

“…Preemptive morphine attenuates increased hot plate thresholds, reduced place preference conditioning and reduced ethanol preferences in long-term behavioral rodent studies after neonatal pain exposure [14,16]. Interacting effects of hippocampal gene expression are found after the combination of stress and morphine exposure in neonatal mice [17]. …”

Neonatal Abstinence Syndrome

“…86 It also may affect gene expression within various body systems (Fig 2). 87 The incidence and severity of withdrawal is less extensive in preterm neonates. 88 Various factors explain the decreased incidence in preterm neonates, including decreased cumulative exposure, 88 decreased transmission across the placenta during early gestation, 71 decreased morphine clearance, 89 decreased excretion because of immaturity of the kidneys and liver, decreased fatty tissues in preterm infants (methadone is accumulated in fatty tissue), decreased receptor development, and decreased receptor sensitivity.…”

Gut microbiota, the immune system, and diet influence the neonatal gut–brain axis

“…The limbic system receives input from other brain regions responsible for a range of behaviors. Maternal deprivation and pain studies in neonatal mice (7,8) indicate that the limbic system plays an important role in development of the hippocampus. In preterm infants without intracranial hemorrhage or ischemia, injury to the hippocampus is likely responsible for neurobehavioral disorders seen in preterm infants during childhood (9).…”