Responsiveness to painful stimuli in anaesthetised newborn and young animals of varying neurological maturity (wallaby joeys, rat pups and lambs)

The patterns of neurological development, as indicated by electroencephalographic (EEG) activity, are similar in different species. EEG activity is absent (isoelectric) initially. It then shows intermittent spikes followed by longer more sustained epochs that are separated by isoelectric periods. Thereafter, a continuous undifferentiated EEG appears and this finally differentiates into alternating rapid-eye-movement (REM) and non-REM sleep-like patterns, with a later addition of characteristics indicating conscious awareness. The timing of these developmental changes evidently depends on the neurological maturity of the young at birth. In wallaby joeys (extremely immature), the EEG evidently remains isoelectric until about 100-120 days of in-pouch age and becomes continuous by about 150-160 days, with behavioural signs of conscious awareness apparent by about 160-180 days. In rat pups (immature), the EEG is isoelectric, intermittent or continuous but undifferentiated at birth with REM/non-REM differentiation occurring between postnatal days 12-18, and conscious awareness appearing no earlier than this. In lambs (mature), REM/non-REM differentiation occurs before birth (at 0.8 pregnancy) with conscious awareness appearing only minutes after birth. Pain-specific EEG responses of anaesthetised young of these three species apparently depend on neurological maturity and, in lambs, on proximity to birth. These observations have some novel implications for pain management during studies of newborn animals.