Developmental changes in the electroencephalogram and responses to a noxious stimulus in anaesthetized tammar wallaby joeys (Macropus eugenii eugenii)

T J Diesch ^1 2, D J Mellor ^1 2 3, C B Johnson ⁴ and R G Lentle ²

¹ Riddet Centre; ² Institute of Food, Nutrition and Human Health; ³ Animal Welfare Science and Bioethics Centre; ⁴ Institute of Animal Veterinary and Biomedical Sciences, College of Sciences, Massey University, New Zealand

Corresponding author: Professor D J Mellor, Institute of Food, Nutrition and Human Health, Massey University, Palmerston North 4442, New Zealand. Email: [email protected]

The tammar wallaby joey is born extremely immature and mostof its neurological development occurs in the maternal pouch.It is not known at what in-pouch age functions such as conscioussensory perception commence. We determined the electroencephalographic(EEG) responses to noxious stimulation in lightly anaesthetizedtammar wallaby joeys. Baseline median (F50) and spectral edge(F95) frequencies, total power (Ptot) and frequency spectrabetween 1 and 30 Hz of the EEG power spectrum were determined.Joeys aged less than 127 days showed little or no EEG activity.Prolonged periods of spontaneous EEG activity were present by142 days. This activity increased, as did the power in all frequencies,while the duration of any intervening isoelectric periods decreasedwith increasing in-pouch age. EEG responses to a noxious stimulus(toe clamping) changed with increasing in-pouch age as therewas no response from joeys aged 94–127 days (no EEG),a minimal decrease in the F50 in those aged between 142 and181 days (P = 0.052) and a greater decrease in the F50 in thoseaged between 187 and 261 days (P < 0.001). The pattern ofthese changes, which presumably reflects anatomical and functionalmaturation of the cerebral cortex, is similar to, but developsmore slowly than, that reported in the rat. The opening of theeyes and development of the pelage are discussed as markersof when brain development may be sufficient for joeys to consciouslyperceive noxious sensations including pain.

Key Words: EEG • noxious stimulus • neurological development • marsupial