Sistema Político (SP)

The development of rapid eye movement (REM) and non-REM sleep, and associated sleep-wake cycles, has not been investigated for the tammar wallaby or any other species of diprotodont metatherians. However, one study reports on the development of sleep-wake cycles in the polyprotodont Virginia opossum (Didelphis virginiana) (Walker & Berger, 1978). Joeys of this species occupy the pouch on average for 70-90 days and then ride on the mothers’ backs until weaning at around 100-110 days (McManus, 1974; Krause & Saunders, 1994; Darlington et al., 1999). Prior to 60 days the EEG of opossum joeys was apparently uniform, regardless of behaviourally and physiologically scored states of arousal, and consisted mainly of isoelectric activity. Continuous low-voltage EEG activity appeared from about 60 days, first EEG signs of non-REM sleep occurred around 65 days and at about 75 days a distinction between REM and non-REM sleep states could be made. Fully established sleep-wake cycles would be expected to be present soon after this time as judged by behavioural development of the joeys. Although Walker and Berger (1978) report periods of so- called wakefulness from 48 days onwards (recordings started at 48 days), clear differentiation into sleep-wake states is not expected until after REM-non-REM sleep differentiation which was shown to begin from about 65 days (Walker & Berger, 1978). The pattern of EEG development described above for the Virginia opossum is also

likely to be seen in tammar wallaby joeys, as it appears to be common to all species observed to date including placental mammals and birds (Mellor & Diesch, 2007). There are several developmental factors that may be relevant to the onset of REM-non- REM EEG differentiation. In appears that, at least in some animal species born moderately immature, the onset of REM-non-REM differentiation is closely associated with eye-opening. In rats, eye-opening occurs at around 14 days after birth (Bolles & Woods, 1964) and REM-non-REM differentiation begins around 10 to 12 days after birth (Jouvet-Mounier et al., 1970), and this is similar in mice where eye-opening and REM-non-REM differentiation occurs around 12 days after birth (Daszuta & Gambarelli, 1985; Findlater et al., 1993). In humans however, eye opening occurs well before EEG differentiation (Prof Alistair Gunn, personal communication) and this may also be the case for other species born neurologically mature. Even if eye opening is not directly (causally) related to the development of REM-non-REM differentiation, it may nevertheless be useful for estimating when altricial animals may be in the process of sleep state differentiation, as it would suggest that neurological maturation has progressed to a point where sleep state differentiation has already occurred or is about to occur. However, this would have to be further explored.

Second, there may be a functional relationship between the development of endothermic thermoregulation and the development of sleep states. The link between thermoregulation and sleep regulation in addition to circadian and homeostatic processes has been reviewed previously (McGinty & Szymusiak, 2001; Parmeggiani, 2003; Gilbert et al., 2004). The preoptic area (POA) of the hypothalamus has been shown to be involved in promoting sleep by a variety of studies applying various experimental techniques (McGinty & Szymusiak, 2001). For example, lesions of the POA induce insomnia or induce partial sleep loss (Nauta, 1946; Sallanon et al., 1989; John & Kumar, 1998; Schmidt et al., 2000) showing that this area is critical in sleep production. Involvement in thermoregulatory control by the POA has also been observed (McGinty & Szymusiak, 2001). In addition, a link between thermoregulation and sleep regulation via the POA has been demonstrated in a variety of species where local warming of the POA triggered non-REM sleep or slow wave EEG activity (Roberts & Robinson, 1969; Benedek et al., 1982) and local cooling suppressed sleep

(McGinty et al., 1996). Importantly, even in the absence of input from the suprachiasmatic nuclei, which are essential for circadian rhythms of body temperature and sleep-wake activity, sleep-temperature coupling persists (Baker et al., 2005).

Generally, a functional link between development of endothermic thermoregulation and the development of sleep states has not been proven and hence caution has to be exercised when using the development of thermoregulation as a means to estimate the timing for REM-non-REM EEG differentiation. However, even if there were no causal links between the two, using the development of one to predict the development of the other may nevertheless be possible. This is because both will depend on functional development of the POA and necessary interconnections with a variety of brain regions. Hence the development of the two systems would be anticipated to overlap to some extent, allowing cautious estimates to be made regarding the timing of development of one, when the timing of the other is known.

The onset of thermoregulatory capabilities in tammar wallaby joeys occurs between approximately 140 and 149 days (Hulbert, 1988) in conjunction with development of pelage, which also appears around 140 days (Tyndale-Biscoe & Janssens, 1988). In addition, thyroid gland size increases between 140 and 180 days with an associated increase in the amount of labelled iodine (I-131) sequestered by the thyroid gland (Setchell, 1974). During this period - 140-180 days after birth - the ability to elevate oxygen consumption in response to low ambient temperatures also begins to develop and at about 180 days it follows a similar pattern to that found in a fully endothermic vertebrate (Setchell, 1974). The animal possesses a full pelage and is apparently homeothermic by about 180 days (Setchell, 1974; Tyndale-Biscoe & Janssens, 1988). If we accept that there is a link between development of endothermic thermoregulation and the differentiation of sleep states, the observations on development of thermoregulation made above are in agreement with the suggestion that REM-non-REM differentiation may begin around the time of or shortly after eye-opening (i.e. around 140 days after birth).

Present study: Anaesthetised tammar wallaby joeys

background just described for tammar wallaby joeys.