ReviewEarly maternal separation and responsiveness to thermal nociception in rodent offspring: A meta-analytic review

- Rodent offspring exposed to early maternal separation had lower sensitivity to nociceptive thermal stimulation compared to non-separated controls.
- Tail flick, but not hot plate responses, were associated with changes in pain sensitivity.
- Receipt of injections during separation was associated with changes in pain sensitivity.
- Males, but not females, showed changes in pain sensitivity.
- The effect size was stronger for animals tested at younger ages, and larger effect size differences were found in older than more recent papers.

Although numerous animal studies have assessed the impact of maternal separation upon pain sensitivity, overall conclusions are difficult to draw about the literature in light of mixed patterns of findings. In this research, a meta-analysis was performed to assess effects of early maternal separation on sensitivity to nociceptive thermal stimulation in rodent offspring and to identify moderators that might explain variable results between studies. Fifteen studies comprising 19 rodent offspring samples (N = 1642) fulfilled all selection criteria. Analyses indicated that rodent offspring exposed to early maternal separation had longer response latencies (RLs) reflecting lower sensitivity to nociceptive thermal stimulation compared to non-separated controls. Heightened effect size heterogeneity was also evident. Moderator analyses indicated variable findings between studies were partially or fully explained by operationalizations of maternal separation (early handling vs. maternal separation), type of noxious stimuli, age of testing, receipt of injections during separation, sex composition of samples, and publication year. Biobehavioral underpinnings of overall group differences and moderator effects are posited in the discussion.