Connections of the auditory cortex of the brush-tailed possum, Trichosurus vulpecula

The afferent connections of the auditory cortex of the acallosal marsupial Trichosurus vulpecula have been studied using the retrograde transport of horseradish peroxidase. The enzyme was applied by electrophoresis from a micropipette previously used to define physiologically the organization of the auditory cortex. Three major (lateral, dorsomedial and ventromedial) sectors of the medial geniculate contribute axons to the ipsilateral auditory cortex; the connections from at least the lateral sector are topographically arranged. Axons reach the auditory field from the contralateral cortex by way of the anterior commissure and fasciculus aberrans.     

The organization of the facial nucleus of the brush-tailed possum (Trichosurus vulpecula).

The facial nucleus of the brush-tailed possum has been studied using Nissl staining and the horseradish peroxidase (HRP) retrograde tracing technique. In Nissl stained sections the nucleus is seen to comprise five distinct subnuclei. Injections of HRP into individual facial muscle groups have shown that these subnuclei reflect the peripheral innervation pattern of efferents from this nucleus. Although in most cases, injection of HRP into a single facial muscle group resulted in the labelling of neurons in more than one facial subnucleus, the following subnuclei were most completely labelled subsequent to intramuscular injection of HRP: the dorsal intermediate subnucleus was labelled with HRP reaction product following injection of m. auricularis anterior; the middle intermediate subnucleus was labelled following injection of the muscle underlying the malar vibrissae; the ventral intermediate subnucleus was labelled following injection of the m. mentalis; the medial subnucleus was labelled following injection of the m. auricularis posterior; the lateral subnucleus was labelled following injection of the m. nasolabialis with HRP. In general there is a mediolateral representation in the facial nucleus of neurons innervating facial muscle groups which are found in anteroposterior succession along the head of the animal. Muscle groups found in dorsoventral succession on the animal are represented dorsoventrally in the facial nucleus.     

The retinal origins of the optic nerve conduction latency groups in the brush-tailed possum, Trichosurus vulpecula

The antidromic compound action potential recorded near the centre of the possum optic disc, following stimulation of the optic chiasm, has four negative peaks corresponding to the optic nerve conduction latency groups t₁, t₂, t₃ and t₄. The t₂ peak is reduced in the compound action potential recorded from the nasal margin of the optic disc but is prominent on the opposite side at the superior temporal margin. The t₄ peak may be absent in this vicinity. The frequency distributions of the diameters of intraretinal ganglion cell axons at the nasal margin and at the superior temporal margin of the optic disc are bimodal. Neither of these size distributions can be directly correlated with the multipeaked antidromic compound action potentials recorded at these same sites. However, by considering the frequency distribution difference between these sites, it is possible to infer (i) that the optic nerve t₂ group may be represented intraretinally on the temporal side by a population of medium sized axons with a modal diameter of 0.8 μm and (ii) that the t₄ group may be represented intraretinally on the nasal side by a population of small axons with modal diameter 0.3 μm. In addition, a correlation of axon sizes with previously reported regional variations in the sizes of ganglion cell somas suggests that the optic nerve t₄ conduction latency group may arise from small ganglion cells in the nasal streak and nasal peripheral retina and that the t₂ group may arise from medium sized cells in the temporal area centralis.     

The optic nerve of the brush-tailed possum, Trichosurus vulpecula: fibre diameter spectrum and conduction latency groups

The principal findings of this report on the morphology and electrophysiology of the possum optic nerve are: (i) There are about 230,000 fibres in the optic nerve. This fibre count, based on electron microscopy, is slightly less than a previously reported estimate of the total number of ganglion cells in the possum retina. (ii) The majority (greater than 98%) of the fibres of the optic nerve are myelinated axons of retinal ganglion cells. The diameters of these fibres range from 0.4--4.6 micrometer (axon diameter range: 0.3--3.8 micrometer) and the frequency distribution of the fibre diameters (and axon diameters) is positively skewed and unimodal. (iii) The antidromic compound action potential of the possum optic nerve shows four negative peaks following stimulation of the optic chiasm. These peaks are associated with four conduction latency groups of fibres which have been designated t1, t2, t3 and t4 in order of increasing conduction latency. (iv) The mean peak conduction velocities of the fibres in the conduction latency groups are 13.1 ms-1 (t1), 8.1 ms-1 (t2), 5.7 ms-1 (t3) and 3.1 ms-1 (t4). (v) There is no direct correlation between the frequency distribution of fibre (or axon) diameters as measured by electron microscopy of transverse sections of fixed optic nerve and the conduction latency groups. (vi) The reconstruction of the possum optic nerve compund action potential on the basis of either axon or fibre diameter frequency distribution does not provide an acceptable, indirect correlation between the morphology and the electrophysiology of this optic nerve.     

Neurogenesis in a marsupial: the brush-tailed possum (Trichosurus vulpecula). II. Sensorimotor pathways

The times of origin of neurons in sensorimotor pathways of the marsupial brush-tailed possum were determined with 3H-thymidine autoradiography. A series of 20 possums were injected with 3H-thymidine from postnatal days 5-95 and were normally allowed to survive until the brain cytoarchitecture was mature. Brain stem and spinal sensorimotor regions were not labelled in our study and presumably form before birth in order to enable the newborn young to make the journey from the birth canal to the pouch on its own. Neurogenesis in thalamic sensorimotor nuclei probably begins about the time of birth and continues into the 2nd week of postnatal life. Formation of neurons in the sensorimotor cortex and the basal ganglia occurs during the first 2 months of postnatal life and in the cerebellum during the first 3 months. This protracted postnatal development of telencephalic and cerebellar sensorimotor regions offers great advantages for developmental studies.     

Biphasic retinal neurogenesis in the brush-tailed possum, Trichosurus vulpecula: further evidence for the mechanisms involved in formation of ganglion cell density gradients.

We investigated cell generation in the retina of the brush-tailed possum (Trichosurus vulpecula) by using tritiated (3H)-thymidine labelling of newly generated cells. Animals aged between postnatal day (P) 5 and 85 each received a single injection of 3H-thymidine. Following autoradiographic processing, maps of labelled cells were constructed from retinal sections. Retinal cell generation takes place in two phases, the first is concluding in the retinal periphery at P53 as the second is seen to commence in midtemporal retina. In the first phase, cells in central retina are generated earlier than those in peripheral regions. In the second phase, cells complete their final division in midtemporal retina first and in the periphery last. Cells generated in the first phase comprise virtually all cells in the ganglion cell layer, amacrine cells, horizontal cells, and cones. Ganglion cells are produced at a slightly earlier stage than displaced amacrine cells, horizontal cells, or cones. Amacrine cells in the inner nuclear layer are the final cells produced in the first phase. When ganglion cells and amacrine cells are pooled, their combined rate of production matches that of the other cell types. These data indicate that the ratio of displaced amacrine cells: horizontal cells: cones: combined ganglion cells and amacrine cells does not change throughout development. However, the ratio of ganglion cells:macrines changes steadily as development proceeds to favour amacrine cells. In the second phase, sparse numbers of nonganglion cells in the ganglion cell layer and large numbers of bipolar and Müller cells are produced along with all rods. The two phases in the possum are similar to those seen in the wallaby, the quokka. However, fewer cells are added in central retina in the possum than in the quokka and cell addition continues for a more extended period in the periphery in the possum. We suggest that this difference in cell addition could account for the development of a more pronounced visual streak of retinal ganglion cells in the possum than in the quokka. A comparison of possum retinal cell generation with that of other marsupials adds support for the "homochrony theory."     

The motor cortex of the brush-tailed possum (Trichosurus vulpecula): motor representation, motor function and the pyramidal tract

Brief anodal shocks applied to the cerebral cortex of the brush-tailed possum (Trichosurus vulpecula) evoked discrete movements of the contralateral fore- and hindlimbs. In 17 possums, unilateral lesions were made in the motor cortex and the resultant corticospinal degeneration was studied with the Nauta-Gygax²⁴ technique. The corticospinal tract was traced contralaterally to T₁₀ in the dorsal funiculus and to T₇in the lateral funiculus. Preterminal degeneration was observed in Rexed's laminae IV–VII and occasionally in laminae III and VIII. In no instances were fibres traced to motoneurones. After cortical ablation, the possums exhibited no defect of gait or climbing ability in the cage and no consistent disturbance of posture when sitting. However, specific tests involving grasping, holding and placing revealed temporary motor deficits in the fore- and hindlimbs when large areas of motor cortex were ablated. The results have been compared with similar studies on other mammals, in particular those with prehensile limbs.     

SmI cortical barrels in an australian marsupial,Trichosurus vulpecula (brush-tailed possum): Structural organization,patterned distribution,and somatotopic relationships

This study reports on the cerebral cortex of an Australian marsupial, Trichosurus vulpecula (brush-tailed possum). It consists of an analysis of layer IV of somatosensory cortex in tangential sections of flattened specimens and in oblique radial sections stained to show Nissl substance or myelin, or tested for succinic dehydrogenase. It includes results of electrophysiological mapping experiments that ascertained the somatotopic significance of the cytoarchitecture of this cortical region. Layer IV has two interlocking cytoarchitectural fields: one granular (the barrelfield, comprising cell-dense barrels 150 to 500 μm in diameter) and one dysgranular. Only neurons within the granular field responded to light cutaneous stimulation. In the barrelfield cell-sparse septa (about 100 μm wide), low in succinic dehydrogenase activity and containing many radial myelinated axons, separate adjoining barrels. Possum barrels are “solid,” lacking the prominent hollows characteristic of most rodent barrels. In some specimens three to five small neuronal “lobules” may constitute each large barrel. In tangential sections the size, shape, and arrangement of barrels combine to form a histological caricature of the possum's body, especially of the face and forepaw. Six rows of “mystacial barrels” are homeomorphic to the six rows of large mystacial vibrissae, and “forepaw barrels” are homeomorphic to the glabrous palmar and apical digital pads. Correlating cortical recording sites and receptive fields confirmed that individual barrels represent specific cutaneous regions. These results show that the cortical barrels of brush-tailed possums are remarkably similar to those of rodents, in structure, arrangement, and functional significance. © 1993 Wiley-Liss, Inc.     

Neurogenesis in a marsupial: the brush-tailed possum (Trichosurus vulpecula). I. Visual and auditory pathways

The times of origin of neurons in the visual and auditory systems were studied in a marsupial, the brush-tailed possum, using tritiated thymidine autoradiography. Within the subcortical visual pathways, most neurons are generated between postnatal days 5 and 21, and the neurons of the primary visual cortex up to postnatal day 68. In the subcortical auditory pathways, most neurons are generated between postnatal days 5 and 28, and all auditory cortex neurons have appeared by postnatal day 46. Neurons in a single layer of cerebral cortex are generated during a period of about 2 weeks. Thus cortical neurogenesis in marsupials extends over a period similar to that seen in primates.     

The number and distribution of ganglion cells in the cat's retina

The number of ganglion cells in the cat's retina, and the pattern of their distribution over the retina, have been reinvestigated. Criteria are presented for the identification of ganglion cells in Nissl-stained whole mounts, most particularly for the distinction between small ganglion cells and neuroglial cells, by reference to retinas with no ganglion cells (obtained by nerve section) and to areas to retina containing a population of only small ganglion cells (obtained by optic tract section). Using these criteria, the number of ganglion cells was counted in four retinas (mean total 116,250). The number of large or "giant" cells (presumably the somas of Y-cells and of alpha-cells) varied from 4,200 to 7,100. Overall these cells comprised 4.0-6.3% of the total ganglion cell population. Their distribution over the retina showed a concentration around the area centralis, with a localized minimum density at the area centralis, and a concentration in the visual streak. These concentrations of large cells were quantitatively less than the concentrations of smaller cells in the area centralis and visual streak, so that the relative frequency of large cells was minimal (mean 1.6%) at the area centralis and increased steadily up to 5.5-6.9% in peripheral retina. Their relative frequency was distinctly lower along the visual streak than in peripheral retina above or below the streak.     

Photoreceptor topography and spectral sensitivity in the common brushtail possum (Trichosurus vulpecula)

Marsupials are believed to be the only non‐primate mammals with both trichromatic and dichromatic color vision. The diversity of color vision systems present in marsupials remains mostly unexplored. Marsupials occupy a diverse range of habitats, which may have led to considerable variation in the presence, density, distribution, and spectral sensitivity of retinal photoreceptors. In this study we analyzed the distribution of photoreceptors in the common brushtail possum (Trichosurus vulpecula). Immunohistochemistry in wholemounts revealed three cone subpopulations recognized within two spectrally distinct cone classes. Long‐wavelength sensitive (LWS) single cones were the largest cone subgroup (67–86%), and formed a weak horizontal visual streak (peak density 2,106 ± 435/mm²) across the central retina. LWS double cones were strongly concentrated ventrally (569 ± 66/mm²), and created a “negative” visual streak (134 ± 45/mm²) in the central retina. The strong regionalization between LWS cone topographies suggests differing visual functions. Short‐wavelength sensitive (SWS) cones were present in much lower densities (3–10%), mostly located ventrally (179 ± 101/mm²). A minority population of cones (0–2.4%) remained unlabeled by both SWS‐ and LWS‐specific antibodies, and may represent another cone population. Microspectrophotometry of LWS cone and rod visual pigments shows peak spectral sensitivities at 544 nm and 500 nm, respectively. Cone to ganglion cell convergences remain low and constant across the retina, thereby maintaining good visual acuity, but poor contrast sensitivity during photopic vision. Given that brushtail possums are so strongly nocturnal, we hypothesize that their acuity is set by the scotopic visual system, and have minimized the number of cones necessary to serve the ganglion cells for photopic vision. J. Comp. Neurol. 522:3423–3436, 2014. © 2014 Wiley Periodicals, Inc.     

Mesotocin in the brain and plasma of an Australian marsupial, the brushtail possum (Trichosurus vulpecula)

Oxytocic peptides extracted from the brain and plasma of the brushtail possum, Trichosurus vulpecula, were separated by reverse-phase high pressure lipid chromatography (HPLC) and quantified by specific radioimmunoassays for oxytocin (OT) and mesotocin (MT). The pituitary, hypothalamus and cerebral cortex were found to contain MT only in quantities of 3.9 +/- 0.2 (SE) ug, 17.6 +/- 0.6 ng and 21.0 +/- 2.6 ng respectively. The plasma concentration of MT varied according to the degree of stress of the possum. In anaesthetized animals values of 39.7 +/- 9.7 pg/ml (11 males) and 31.5 +/- 12.9 pg/ml (6 females) were obtained; in four conscious catheterized animals, 9.4 +/- 6.3 pg/ml. Samples taken from three anaesthetized animals during exsanguination contained 271 +/- 102 (SD) pg MT/ml. It was concluded that hypothalamic and extra-hypothalamic MT is present in the marsupial brain and that as in placental mammals, stress stimulates the secretion of mesotocin.     

Neocortical projections of the suprageniculate and posterior thalamic nuclei in the marsupial brush-tailed possum,Trichosurus vulpecula(phalangeridae), with a comparative commentary on the organization of the posterior thalamus in marsupial and placental mammals

Axonal transport methods were used to determine the extent and organisation of neocortical projections from the Suprageniculate (SG) and posterior (PO) thalamic nuclei in the brush-tailed possum. Our findings show that SG projects extensively to the auditory cortex, overlapping the cortical projection field of the modial geniculatc nucleus, and to the immediately neighbouring association cortex. Though the input relationships of SG appear similar to those reported for other mammals, placental and marsupial, a strong SG projection to auditory cortex has not been reported previously. Neocortical relationships of PO are characterised by an orderly point-to-point projection to all but the most rostral parts of the motor-somaesthetic cortex. There is also a substantial projection to the entire posterior parietal association cortex. The PO-neocortex projection is reciprocally organised. The PO-neocortical projection in the possum is similar to that reported in the Virginia opossum, rat, and several other mammals. There is a major difference in organisation in comparison with certain monkeys where the PO projection is much more restricted and does not involve the motor and som-aesthetic cortex. We conclude that PO is similarly organised in many, though not all, mammals, including the marsupials, rodents, insectivores, and prosimian primates. The possum SG, on the other hand, is clearly distinct from other mammals in its extensive projection to auditory cortex, though we cannot say at present whether this a general property of marsupial mammals or a peculiarity restricted to this species and possibly its close relatives.     

Characterization of the intrinsic and extrinsic innervation of the gall bladder epithelium in the Australian Brush-tailed possum (Trichosurus vulpecula)

Intrinsic neurones of the gall bladder modulate its function. Nitric oxide synthase (NOS) and vasoactive intestinal polypeptide (VIP) are present in gall bladder neurones and nitric oxide and VIP modulate its epithelial functions. As an extensive extrinsic innervation of the gall bladder is also present, the source of the epithelial innervation is unclear. In this study the source of the gall bladder epithelial innervation is defined. Immunoreactivity for VIP, NOS, substance P (SP), calcitonin gene related peptide (CGRP) and tyrosine hydroxylase (TH) in organotypic cultured and freshly fixed gall bladder were compared. Retrograde tracing in vitro from the epithelium was used to identify putative intrinsic secretomotor neurones, which were then characterized by immunohistochemistry. Abundant spinal afferent and sympathetic innervation of the gall bladder epithelium was demonstrated by CGRP/SP and TH immunohistochemistry, respectively. The intrinsic secretomotor innervation of the epithelium is derived exclusively from neurones of the subepithelial plexus. A majority of these neurones were immunoreactive for NOS. Some of the NOS-immunoreactive neurones of the subepithelial plexus also contained VIP and/or SP. Gall bladder subepithelial plexus neurones, containing NOS and/or VIP/SP, innervate the epithelium, as do extrinsic neurones.     

The distribution of the alpha type of ganglion cells in the cat's retina.

There is a distinct class of large ganglion cell bodies observable in whole-mount preparations of the cat's retina stained with cresyl violet. Measurements of perikaryal size and its variation with eccentricity from the central area support the identification of the large cells with the class of alpha cells previously defined in Golgi-stained preparations. A complete enumeration of alpha cells in one retina yielded a total of 6212. They were encountered in all parts of the retina and had a peak density within the central area ( 200/mm2). The retinal distribution was displayed as a contour map of alpha-cell density. Away from the central area, the lines of iso-density had the form of a 4-pointed star with rather blunt points corresponding to horizontal and vertical ridges of augmented density. Along horizontal, vertical and oblique strips through the central area, alpha-cell density was an approximately constant fraction of total ganglion cell-density. The average value of the fraction was 3.3% from which it has been calculated that the cat's retina may contain as many as 190,000 ganglion cells.