Impact of neonatal pain and opiate administration in animal models: A meta-analysis concerning pain threshold

BACKGROUND AND AIM

Neonatal intensive care treatment, including frequently performed painful procedures and administration of analgesic drugs, can have different effects on the neurodevelopment. This systematic review and meta-analysis aimed to investigate the influence of pain, opiate administration, and pre-emptive opiate administration on pain threshold in animal studies in rodents, which had a brain development corresponding to preterm and term infants.

METHODS

A systematic literature search of electronic data bases including CENTRAL (OVID), CINAHL (EBSCO), Embase.com, Medline (OVID), Web of Science, and PsycInfo (OVID) was conducted. A total of 42 studies examining the effect of pain (n = 38), opiate administration (n = 9), and opiate administration prior to a painful event (n = 5) in rodents were included in this analysis.

RESULTS

The results revealed that pain (g = 0.42, 95%CI 0.16-0.67, p = 0.001) increased pain threshold leading to hypoalgesia. Pre-emptive opiate administration had the opposite effect, lowering pain threshold, when compared to pain without prior treatment (g = -1.79, 95%CI -2.71-0.86, p = 0.0001). Differences were found in the meta regression for type of stimulus (thermal: g = 0.66, 95%CI 0.26-1.07, p = 0.001; vs. mechanical: g = 0.13, 95%CI -0.98-1.25, p = 0.81) and gestational age (b = -1.85, SE = 0.82, p = 0.027). In addition, meta regression indicated an association between higher pain thresholds and the amount of cumulative pain events (b = 0.06, SE = 0.03, p = 0.05) as well as severity of pain events (b = 0.94, SE = 0.28, p = 0.001).

CONCLUSION

Neonatal exposure to pain results in higher pain thresholds. However, caution is warranted in extrapolating these findings directly to premature infants. Further research is warranted to validate similar effects in clinical contexts and inform evidence-based practices in neonatal care.

Validation of the Unesp-Botucatu pig composite acute pain scale (UPAPS) in piglets undergoing castration

To accurately assess pain and support broadly-based analgesic protocols to mitigate swine pain, it is imperative to develop and validate a species-specific pain scale. The objective of this study was to investigate the clinical validity and reliability of an acute pain scale (UPAPS) adapted for newborn piglets undergoing castration. Thirty-nine male piglets (five days of age, 1.62 ± 0.23 kg BW) served as their own control, were enrolled in the study and underwent castration in conjunction with an injectable analgesic administered one-hour post-castration (flunixin meglumine 2.2 mg/kg IM). An additional 10, non-painful female piglets were included to account for the effect of natural behavioral variation by day on pain scale results. Behavior of each piglet was video recorded continuously at four recording periods (24 h pre-castration, 15 min post-castration, 3 and 24 h post-castration). Pre- and post-operative pain was assessed by using a 4-point scale (score 0-3) including the following six behavioral items: posture, interaction and interest in surroundings, activity, attention to the affected area, nursing, and miscellaneous behavior. Behavior was assessed by two trained blinded observers and statistical analysis was performed using R software. Inter-observer agreement was very good (ICC = 0.81). The scale was unidimensional based on the principal component analysis, all items except for nursing were representative (rs ≥ 0.74) and had excellent internal consistency (Cronbach's alpha ≥ 0.85). The sum of scores were higher in castrated piglets post-procedure compared to pre-procedure, and higher than in non-painful female piglets confirming responsiveness and construct validity, respectively. Scale sensitivity was good when piglets were awake (92.9%) and specificity was moderate (78.6%). The scale had excellent discriminatory ability (area under the curve > 0.92) and the optimal cut-off sum for analgesia was 4 out of 15. The UPAPS scale is a valid and reliable clinical tool to assess acute pain in castrated pre-weaned piglets.

Wound Pain and Wound Healing Biomarkers From Wound Exudate: A Scoping Review

PURPOSE

Effective management of wound pain is essential for optimal wound healing. Nevertheless, the outcomes of wound pain interventions are based on subjective measures, which can prove problematic in patients with cognitive impairment. Identification of biomarkers associated with wound pain and wound healing can be used to more objectively estimate wound pain and contribute to the development of precise management options to reduce wound pain and promote wound healing. This scoping review aimed to identify wound pain and wound healing biomarkers from wound exudates and to describe different wound collection methods to identify these biomarkers.

METHODS

We searched the literature (PROSPERO database registration number: CRD42018103843) via a scoping review.

SEARCH STRATEGY

The PubMed database was searched for articles that explored relationships between cutaneous wound pain, wound healing, and biomolecules. Inclusion criteria were articles that reported original data, used adult human samples, and were published in English.

FINDINGS

Twenty-one articles were retrieved: 17 investigated molecules from wound exudate associated with wound healing status, and 4 reported molecules associated with wound pain. The most frequently observed wound pain biomarkers were proinflammatory cytokines; the most frequently observed wound healing biomarkers were proteases including those in the matrix metalloproteinase family. Six wound exudate collection methods were identified to extract potential wound pain and wound healing biomarkers from wound exudate.

IMPLICATIONS

The results can guide future wound exudate research to validate these wound pain and wound healing biomarkers and to develop therapies targeting these biomarkers to reduce wound pain and promote wound healing.

Inflammatory neonatal pain disrupts maternal behavior and subsequent fear conditioning in a rodent model

Infants spending extended time in the neonatal intensive care unit are at greater risk of developing a variety of mental health problems later in life, possibly due to exposure to painful/stressful events. We used a rodent model of inflammatory neonatal pain to explore effects on fear conditioning, somatosensory function and maternal behavior. Hindpaw injections of 2% λ-carrageenan on postnatal days 1 and 4 produced an attenuation in conditioned freezing during the postweaning period, similar to our previous work with acute pain, but did not cause lasting impacts on contextual freezing nor somatosensory function. Additionally, we assessed maternal behavior to observe dam-pup interactions during the neonatal period. Results showed dams of litters which experienced pain spent similar amounts of time with pups as undisturbed controls. However, the specific behaviors differed per condition. Dams of pain litters exhibited less time licking/grooming, but more time nursing than controls. These results suggest changes in maternal care following pain could be a contributing factor underlying the long-term effects of neonatal trauma. Furthermore, our laboratory has previously shown acute, but not inflammatory pain, disrupted conditioned freezing; the current experiment observed the long-term effects of neonatal inflammatory pain on conditioned fear using a weak conditioning protocol.

Neonatal pain and stress disrupts later-life pavlovian fear conditioning and sensory function in rats: Evidence for a two-hit model

Early life trauma has been linked to increased risks for anxiety, depression, and chronic pain. We used rodent models of acute and inflammatory neonatal pain to explore effects on fear conditioning and somatosensory function. Hindpaw needle pricks or handling on postnatal days (PNDs) 1-7 caused lasting impacts on affective and somatosensory function when assessed at later ages, PNDs 24 (postweaning), 45 (adolescence), or 66 (adulthood). First, auditory, but not contextual, freezing was mildly disrupted regardless of age. Second, a profound postfear conditioning tactile hypersensitivity was observed in neonatally stressed, postweaning rats. In the absence of fear conditioning, the mechanical hypersensitivity was not observed, consistent with a two-hit model of psychopathology. Injections of 2% α-carrageenan did not have the same lasting impact but was slightly protective against observed effects of neonatal vehicle injections. Basal and elicited corticosterone levels postweaning were not altered by neonatal pain or handling. These data demonstrate that neonatal adversity can have lasting impacts on affective and somatosensory function that differs regardless of age.

Microglia in health and pain: impact of noxious early life events

NEW FINDINGS

What is the topic of this review? This review discusses the origins and development of microglia, and how stress, pain or inflammation in early life disturbs microglial function during critical developmental periods, leading to altered pain sensitivity and/or increased risk of chronic pain in later life. What advances does it highlight? We highlight recent advances in understanding how disrupted microglial function impacts the developing nervous system and the consequences for pain processing and susceptibility for development of chronic pain in later life. The discovery of microglia is accredited to Pío del Río-Hortega, who recognized this 'third element' of CNS cells as being morphologically distinct from neurons and astrocytes. For decades after this finding, microglia were altogether ignored or relegated as simply being support cells. Emerging from virtual obscurity, microglia have now gained notoriety as immune cells that assume a leading role in the development, maintenance and protection of a healthy CNS. Pioneering studies have recently shed light on the origins of microglia, their role in the developing nervous system and the complex roles they play beyond the immune response. These studies reveal that altered microglial function can have a profoundly negative impact on the developing brain and may be a determinant in a range of neurodevelopmental disorders and neurodegenerative diseases. The realization that aberrant microglial function also critically underlies chronic pain, a debilitating disorder that afflicts over 1.5 billion people worldwide, was a major conceptual leap forward in the pain field. Adding to this advance is emerging evidence that early life noxious experiences can have a long-lasting impact on central pain processing and adult pain sensitivity. With microglia now coming of age, in this review we examine the association between adverse early life events, such as stress, injury or inflammation, and the influence of sex differences, on the role of microglia in pain physiology in adulthood.

Neonatal Injury Results in Sex-Dependent Nociceptive Hypersensitivity and Social Behavioral Deficits During Adolescence, Without Altering Morphine Response

Neonatal injury is associated with persistent changes in sensory function and altered nociceptive thresholds that give rise to aberrant pain sensitivity in later life. Although these changes are well documented in adult rodents, little is known about the consequences of neonatal injury during adolescence. Because adolescence is a critical developmental period during which persistent pain conditions can arise, we examined the effect of neonatal injury on nociception, social behavior, and response to morphine in adolescent Sprague Dawley rats. Male and female rats exposed to plantar incision injury at postnatal day 3 displayed mechanical hypersensitivity that resolved by 24 hours after incision. When these animals reached adolescence (postnatal day 28-40), neonatally-injured male rats showed ipsilaterally restricted mechanical, heat, and cold hypersensitivity, as well as social behavioral deficits. In contrast, these effects were not seen in female rats. Neonatal injury did not alter acute morphine antinociception or the development of analgesic tolerance in either sex. Morphine-induced conditioned place preference, behavioral sensitization, and physical withdrawal were also not affected by neonatal incision. Thus, early-life injury results in sex-dependent pain-related hypersensitivity and social behavior deficits during adolescence, without altering the response to opioids.

PERSPECTIVE

Neonatal surgery has greater effects on adolescent male than female rats, resulting in pain-related hypersensitivity and social behavioral deficits. Neonatal surgery does not alter the antinociceptive effects of morphine or abuse liability.

A "Wear and Tear" Hypothesis to Explain Sudden Infant Death Syndrome

Sudden infant death syndrome (SIDS) is the leading cause of death among USA infants under 1 year of age accounting for ~2,700 deaths per year. Although formally SIDS dates back at least 2,000 years and was even mentioned in the Hebrew Bible (Kings 3:19), its etiology remains unexplained prompting the CDC to initiate a sudden unexpected infant death case registry in 2010. Due to their total dependence, the ability of the infant to allostatically regulate stressors and stress responses shaped by genetic and environmental factors is severely constrained. We propose that SIDS is the result of cumulative painful, stressful, or traumatic exposures that begin in utero and tax neonatal regulatory systems incompatible with allostasis. We also identify several putative biochemical mechanisms involved in SIDS. We argue that the important characteristics of SIDS, namely male predominance (60:40), the significantly different SIDS rate among USA Hispanics (80% lower) compared to whites, 50% of cases occurring between 7.6 and 17.6 weeks after birth with only 10% after 24.7 weeks, and seasonal variation with most cases occurring during winter, are all associated with common environmental stressors, such as neonatal circumcision and seasonal illnesses. We predict that neonatal circumcision is associated with hypersensitivity to pain and decreased heart rate variability, which increase the risk for SIDS. We also predict that neonatal male circumcision will account for the SIDS gender bias and that groups that practice high male circumcision rates, such as USA whites, will have higher SIDS rates compared to groups with lower circumcision rates. SIDS rates will also be higher in USA states where Medicaid covers circumcision and lower among people that do not practice neonatal circumcision and/or cannot afford to pay for circumcision. We last predict that winter-born premature infants who are circumcised will be at higher risk of SIDS compared to infants who experienced fewer nociceptive exposures. All these predictions are testable experimentally using animal models or cohort studies in humans. Our hypothesis provides new insights into novel risk factors for SIDS that can reduce its risk by modifying current infant care practices to reduce nociceptive exposures.