The brain of the brown antechinus (Antechinus stuartii) Dasyuridae
The brown antechinus is a common small marsupial carnivore on the eastern seaboard that exhibits extreme male semelparity. Antechinus males live for only one year and engage in an intense period of territorial behaviour culminating in the mating period (Bradley et al., 1980). The brief once-only mating season is followed by complete male mortality before the young are born (male “die-off”). Males have a discrete hormonal profile, with circulating testosterone rising eight-fold, followed by a six-fold rise in plasma cortisol (Bradley, 2003). In males, the glucocorticoid feedback mechanism fails, and plasma cortisol continues to rise causing a catastrophic physiological collapse, culminating in immune system failure and death (Bradley, 2003).
Figure 1 shows the brains of adult male and female antechinus. Antechinus exhibit sexual dimorphism, with adult males being slightly larger (mean of 54 g body weight) than adult females (mean of 43 g body weight). Brain weights for two sexes are similar (0.88 g), about twice that for the laboratory mouse.
Pathways in the brain of the brown antechinus (Antechinus stuartii) Dasyuridae
All vertebrates have fibre pathways connecting functional regions within their forebrain. These are divided into: association fibres, that connect cortical areas on the same side of the brain; projection fibres, that connect the cerebral cortex with subcortical areas of the brain (e.g. striatum, thalamus, brainstem and spinal cord); and commissural fibres, that connect cortical and subcortical areas on the two sides of the brain.
Figure 2 shows the fundamental difference in the commissural connections between marsupials (metatheria) and placental mammals (eutheria). The major forebrain commissure for marsupials is the anterior commissure, through which interhemispheric connections pass for both dorsal and lateral iso- (6-layered) cortex (red, green axons). By contrast, placental mammals have a dorsally placed corpus callosum (cc) transmitting interhemispheric connections for the dorsal and lateral isocortex, as well as an anterior commissure for interhemispheric connections for the inferior isocortex and allocortex. In both eutherians and metatherians, commissural fibres for the olfactory allo- (3 to 5-layered) cortex (blue axons) pass through the most ventral part of the ac, called the basal telencephalic commissure (btc).
Immunohistochemistry for neurofilament proteins in frontal sections Antechinus brain
Fibre pathways can be identified by immunohistochemistry (e.g. by the SMI 312 antibody) for neurofilament proteins that are concentrated in the long axons of large neurons. Figures 3 and 4 show immunohistochemistry for neurofilament proteins in frontal sections through the level of the anterior commissure (Fig. 3) and the posterior commissure (Fig. 4). Apart from revealing major commissures, association and projection pathways, the pattern of staining also reveals chemically distinct functional areas with the cerebral cortex.
Figure 3 shows the large anterior commissure (ac) receiving fibres from the external capsule (ec) as well as the much smaller hippocampal commissure (hc) joining the hippocampal formation components on each side of the brain. The staining also reveals association fibres in the deep cortical white matter (dcw), as well as the optic nerve (2n) and major projection tracts to and from the cerebral cortex and other forebrain structures (f – fornix; ic – internal capsule; lo – lateral olfactory tract; st - stria terminalis).
Figure 4 shows the brain at the level of the posterior commissure, a small commissure which allows communication between the pretectal nuclei of each side for co-ordination of visual reflexes. Immunohistochemistry for neurofilament proteins also highlights the functional areas of the primary visual cortex (V1) and primary auditory cortex (Au1). The prominent fibre bundles of the cerebral peduncle (cp, containing corticobulbar, corticopontine and corticospinal pathways) and visual pathways (opt - optic tract and bsc – brachium of superior colliculus) can also be seen.
References
Bradley AJ (2003) Stress, hormones and mortality in small carnivorous marsupials. In Predators with Pouches: The Biology of Carnivorous Marsupials. Editors. M Jones, C Dickman. M Archer (eds). CSIRO publishers pp. 254-257
Bradley AJ, McDonald IR, Lee AK (1980) Stress and mortality in a small marsupial (Antechinus stuartii, Macleay). General and Comparative Endocrinology 40, 188-200.