A Single Neonatal Injury Induces Life-Long Deficits in Response to Stress

Abstract

Approximately 500,000 infants are born prematurely each year in the United States. These infants typically require an extensive stay in the neonatal intensive care unit (NICU), where they experience on average 14 painful and invasive procedures each day. These procedures, including repeated heel lance, insertion of intravenous lines, and respiratory and gastric suctioning, typically result in an inflammatory response, inducing pain and stress in the newborn. Remarkably, the majority of these procedures are performed in the complete absence of pre- or post-emptive analgesics. Recent clinical studies report that former NICU patients have increased thresholds for pain and stress later in life as compared with term-born infants. However, to date, the mechanisms whereby early-life inflammation alters later-life response to stress and pain are not known. The present studies were conducted to determine if neonatal injury impairs adult responses to anxiety- and stress-provoking stimuli. As we have previously reported that early-life pain results in a significant increase in opioid peptide expression within the midbrain periaqueductal gray, the role of endogenous opioids in our behavioral studies was also examined. Male and female rats received an intraplantar injection of the inflammatory agent carrageenan (1%) on the day of birth. In adulthood, animals were assessed for changes in response to anxiety- and stress-provoking stimuli using the open field and forced swim tests, respectively. Injury-induced changes in sucrose preference and stress-induced analgesia were also assessed. As adults, neonatally injured animals displayed a blunted response to both anxiety- and stress-provoking stimuli, as indicated by significantly more time spent in the inner area of the open field and a 2-fold increase in latency to immobility in the forced swim test as compared to controls. No change in sucrose preference was observed. Using in situ hybridization and immunohistochemistry, we observed a 2-fold increase in enkephalin mRNA and protein expression, respectively, in stress-related brain regions including the central amygdala and lateral septum. Administration of the opioid receptor antagonist naloxone reversed the attenuated responses to forced swim stress and stress-induced analgesia, suggesting the changes in stress-related behavior were opioid-dependent. Together, these data contribute to mounting evidence that neonatal injury in the absence of analgesics has adverse effects that are both long-term and polysystemic.

Analgesia for Early-Life Pain Prevents Deficits in Adult Anxiety and Stress in Rats

“…A two-way ANOVA revealed a significant main effect of treatment (F 3, 42 = 24.68; p < 0.0001) on time spent in the inner area; no significant effect of sex (F 1, 42 = 1.28; p = 0.26) and no significant interaction (F 3, 42 < 1; p = 0.78) were observed. Consistent with our previous report [20] , neonatally injured adults spent significantly more time in the inner area than noninjured controls (Injury + Saline vs. Handled + Saline, Fisher's PLSD, p < 0.0001; fig.…”

“…We have previously reported that early-life pain increases the latency to immobility in the FST and reduces FST-induced corticosterone release in adulthood [20,21] . To determine if morphine treatment reverses this effect, neonatally injured or handled adult male and female rats (+/-morphine) were exposed to the FST for 5 min.…”

“…On the day of birth (P0), male and female rat pups received an injection of CGN 1% (5 μl, dissolved in saline; Sigma, USA) into the intraplantar surface of the right hindpaw or were handled in an identical manner [19,20] . This time point is developmentally comparable to 24 weeks of gestation in humans [23] .…”

“…Morphine treatment at the time of injury prevents both effects in adulthood [19] . Most recently, we reported that neonatally injured adults show behavioral hyposensitivity to acute anxiety-and stress-provoking stimuli [20] and blunted corticosterone release [21] , indicating dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis [22] . Moreover, these behavioral and hormonal alterations are accompanied by site-specific changes in enkephalin, glucocorticoid receptor and corticotropin-releasing factor receptors 1 and 2 [20,21] .…”

Ritual circumcision and risk of autism spectrum disorder in 0- to 9-year-old boys: national cohort study in Denmark

“…These findings obviously do not prove the suggested associations. However, in combination with recent animal studies showing lifelong deficits in stress responses following exposure to just one single neonatal insult, 25 the observed strong correlation between circumcision and ASD prevalence, 31 and clinical observations of long-term changes in pain perception in circumcised infants, 4 our population-based findings should prompt other researchers to examine the possibility that circumcision trauma in infancy or early childhood might carry an increased risk of serious, yet hitherto unappreciated negative neurodevelopmental and psychological consequences.…”