Cephalic sensory influence on forelimb movement in newborn opossums, Monodelphis domestica.

Like other marsupials, the opossum Monodelphis domestica is born very immature and crawls, unaided by the mother, from the urogenital opening to a nipple where it attaches and pursues its development. If the alternate, rhythmic movements of the forelimbs which allow this locomotion are generated by the developing spinal motor networks, sensory information is nonetheless needed to guide the newborn to a nipple. Behavioral, anatomical and physiological studies suggest that the auditory and the visual systems are insufficiently developed in newborn opossums to influence spinal motor centers, while the vestibular, trigeminal, and olfactory systems are likelier candidates. The trigeminal, vestibular and olfactory regions of the brain were electrically stimulated to test their relative effectiveness at eliciting forelimb movement in newborn opossums, using in vitro preparations of brain-spinal cord with the limbs attached. The minimal stimulation of the cervical spinal cord needed to induce forelimb movement was considered as threshold (T). Stimulations of the trigeminal ganglion (5G) at ∼2T and of the vestibular complex at ∼20T could induce the same movement, and were not statistically different, in contrast to the ∼600T necessary for the olfactory bulb (OB). Neurofilament-200 immunohistochemistry and retrograde tracing with Texas-Red conjugated Dextran Amines were used to study trigeminal innervation of the facial skin and pathways by which trigeminal inputs may be relayed to the spinal cord. Numerous nerve fibers were observed in the snout dermis, especially in the maxillary region, but also elsewhere in the head skin. Some 5G cells project to the upper spinal cord, but more project to the caudal medulla where they could contact secondary trigeminal neurons or reticular cells projecting to the spinal cord. These results support a significant influence of the trigeminal and the vestibular systems, but not of olfaction, on forelimb movement of neonatal opossums.

Comparing the retinal structures and functions in two species of gulls (Larus delawarensis and Larus modestus) with significant nocturnal behaviours.

Ring-billed gulls (Larus delawarensis) and gray gulls (Larus modestus) are two species active both by day and night. We have investigated the retinal adaptations that allow the diurnal and nocturnal behaviours of these two species. Electroretinograms and histological analyses show that both species have a duplex retina in which cones outnumber rods, but the number of rods appears sufficient to provide vision at night. Their retinas respond over the same scotopic dynamic range of 3.4logcdm(-2), which encompasses all of the light levels occurring at night in their photic environment. The amplitudes of the scotopic saturated a- and b-wave responses as well as the photopic saturated b-wave response and the photopic sensitivity parameter S are however higher in ring-billed gulls than in gray gulls. Moreover, the process of dark adaptation is about 30min faster in gray gulls than in ring-billed gulls. Our results suggest that both species have acquired in the course of their evolution functional adaptations that can be related to their specific photic environment.

The development of vesicular acetylcholine transporter immunoreactivity in the hindlimbs of the opossum Monodelphis domestica.

Vesicular acetylcholine transporter (VAChT) was revealed immunohistochemically in light microscopy on hindlimb sections of developing opossums, Monodelphis domestica. In the immobile hindlimbs of the newborn, which comprise cartilaginous bones and loose, unstriated myofibers, scant immunolabeled nerve segments and small spherical terminals, presumably growth cones or immature neuromuscular junctions, are found in the muscle tissue of the thigh, leg and proximal foot, decreasing in number and size proximodistally. When the hindlimbs start moving at 1 week, terminals are more numerous and larger, still decreasing proximodistally, and occur in the newly formed interosseous foot muscles. At 4 weeks, when the hindlimbs start supporting weight and quadrupedal locomotion appears, terminals are more numerous, flattened and in comparable size and density in thigh, leg and foot muscles. By 7 weeks, large and completely flat terminals are observed in groups of 3 to 4 at regular intervals along muscle fibers. VAChT expression develops largely postnatally in the opossum hindlimbs, along a proximodistal gradient that parallels somatic and reflex development.

The development of mammalian motor systems: the opossum Monodelphis domestica as a model.

The opossum Monodelphis domestica is a marsupial born considerably immature 14-15 days after conception. It is possible to study postnatally, in this species, almost the entire development of its motor behaviors as well as of the nerve centers involved in their control. The lumbosacral spinal cord of the newborn comprises a thick ventricular zone containing mitotic figures, an intermediate zone of small and undifferentiated cells, and a thin marginal zone. The hindlimbs are little more than embryonic buds. The presumptive bones consist of cartilageneous or mesenchymal condensations and the presumptive muscles of immature myofibers mixed and surrounded with mesenchyme. Cholinergic fibers from lumbosacral motoneurons are already seen among the myofibers, but most of hindlimb motor innervation develops postnatally. The long descending and ascending projection systems connecting the lumbosacral enlargement to the cervical cord and the encephalon also form largely postnatally, but lateral vestibular and medullary reticular axons are present in the lumbosacral cord at birth. Synaptogenesis in the lumbosacral enlargement occurs largely postnatally, according to a general outside-in gradient, and the earliest evidence for it is on lateral motoneurons. Myelinogenesis therein is even later. These observations on neural development are correlated with observations on the development of simple reflex behaviors and locomotion.

Synaptogenesis in the brachial and lumbosacral enlargements of the spinal cord in the postnatal opossum, Monodelphis domestica.

Synaptic proteins were localized in light microscopy on sections of the brachial and lumbosacral enlargements of the spinal cord of postnatal opossums, Monodelphis domestica, to determine whether their expression correlates with the development of major motor pathways and simple motor behaviors. The tissues were fixed, cryoprotected, frozen, cut in 15-micrometer sections, and processed immunohistochemically using antibodies against synaptophysin, synaptotagmin-I, or SNAP-25. Immunolabeling was observed in the presumptive white matter before the presumptive gray matter, suggesting that the proteins are evidenced in growing axons before the onset of synaptogenesis, and it was observed in presumed propriospinal axons before most presumed descending axons of supraspinal origin. In the newborn opossum, the immunolabeling was scant in the gray matter and was limited to the periphery of the ventral horn, and indeed few synapses were seen in electron microscopy in nonexperimental material. Labeling increased in intensity and spread throughout the gray matter until 5-7 weeks, when it was no longer found in the white matter and resembled the adult pattern of labeling. Considering the location and relative intensity of the immunolabeling for the three proteins over time in the two enlargements, synaptogenesis occurs according to three general gradients: rostrocaudal, ventrodorsal, and lateromedial. These gradients match those of spinal cord and limb development, and of the growth of descending axons into the cord. Synaptogenesis is most intense when the spinal sensorimotor reflexes begin to be expressed.

Behavioral, morphological and physiological correlates of diurnal and nocturnal vision in selected wading bird species.

We examined in selected wading bird species if diurnal or nocturnal foraging and the use of visual or tactile feeding strategies could be correlated with retinal structure and function. The selected species were the Yellow-crowned Night Heron (Nycticorax violaceus), a crepuscular and nocturnal forager, the Great Blue Heron (Ardea herodias), a mainly crepuscular, but also diurnal and nocturnal feeder, the Roseate Spoonbill (Ajaia ajaja), a mainly crepuscular feeder which forages more at night than during the day, the Cattle (Bubulcus ibis) and Tricolored (Egretta tricolor) egrets and the American White Ibis (Eudocimus ruber) which forage only during daytime. Herons and egrets are visual foragers; ibises and spoonbills are tactile feeders. Electroretinograms were obtained from anesthetized birds in photopic and scotopic conditions to a wide range of light intensities, following which the retinae were processed for histological analysis. Based on rod densities and rods:cones ratios, nocturnal vision capability is greater in the Yellow-crowned Night Heron, followed by the Great Blue Heron and the spoonbill, then by the egrets and the ibis. Visual feeders that forage near dawn or dusk or at night have a higher rods:cones ratio, and consequently a greater night vision capability, than visual feeding species which forage only during daytime. Visual nocturnal feeders have a night vision capability greater than tactile diurnal as well as tactile nocturnal feeders. However, based on maximum scotopic b-wave amplitudes, all species studied have roughly comparable night vision capability. The factor that best discriminates between wading bird species appears to be the daytime visual capabilities. Indeed, the diurnal ibis and egrets have similar cone densities, cones:rods ratios, and photopic a-wave amplitudes, values which are greater than those measured in the two nocturnally active heron species.

Diurnal and nocturnal visual capabilities in shorebirds as a function of their feeding strategies.

Some shorebird species forage with the same feeding strategy at night and during daytime, e.g. visual pecking in the Wilson's Plover (Charadrius wilsonia) or tactile probing in the Short-billed Dowitcher (Limnodromus griseus). The American Woodcock (Scolopax minor) uses tactile probing, by day and by night, but sometimes pecks for insects during daytime. The Black-winged Stilt (Himantopus himantopus) is a visual pecker, both by day and by night, and sometimes forages tactilely on windy (agitated water surface) moonless nights. Territorial Willets (Catoptrophorus semipalmatus) are visual peckers during daylight and on moonlight conditions but switch to tactile feeding under lower light conditions. It could be postulated that some shorebird species would switch from visual feeding during daytime to tactile foraging at night because they have poor night vision compared to species that are always sight foragers irrespective of the time of the day. This issue was examined by comparing retinal structure and function in the above species. Electroretinograms (ERGs) were obtained at different light intensities from anesthetized birds, and the retinae were processed for histological observations. Based on ERGs, retinal sensitivity, and rod:cone ratios, both plovers and stilts are well adapted for nocturnal vision. Although they have low rod density compared to that of stilts and plovers, Willets and woodcocks have a scotopic retinal sensitivity similar to that of stilts and plovers but rank midway between plovers and dowitchers for the b-wave amplitude. Dowitchers have the lowest scotopic b-wave amplitude and retinal sensitivity and appear the least well adapted for night vision. Based on photopic ERGs and cone densities, although stilts, Willets and dowitchers appear as well adapted for daytime vision, plovers occupy the last rank of all species examined. Compared to the nighttime tactile feeders and those that switch from daytime visual pecking to tactile feeding at night, nighttime sight feeders have a superior rod function and, consequently, potentially superior nocturnal visual capabilities.

The development of synaptophysin-like immunoreactivity in the lumbosacral enlargement of the spinal cord of the opossum Monodelphis domestica.

The presence of synaptophysin in the lumbosacral enlargement of developing opossums, Monodelphis domestica, was studied immunohistochemically at the light microscopic level. In newborn, synaptophysin-labeling was observed in the presumptive white matter, presumably in growing axons, and was scant in the ventrolateral gray matter. Over the next 3 weeks the labeling filled the gray matter following a general ventrodorsal gradient. Labeling was found in the white matter until the fifth week. Synaptogenesis in the lumbosacral enlargement of the opossum thus occurs mostly postnatally, when many descending axons have already reached that level. It is particularly intense in the ventral horn when the hindlimbs begin to move, and in the dorsal horn when sensorimotor reflexes can be elicited.

Organization of the projections from the pericruciate cortex to the pontomedullary reticular formation of the cat: a quantitative retrograde tracing study.

Dextran-amines were used as retrograde tracers to investigate the organization of cortical projections to different cytoarchitectonic regions of the pontomedullary reticular formation of the cat. Injections into the nucleus reticularis pontis oralis resulted in labelling of neurones in the proreus cortex and area 6a beta of the premotor cortex, with little labelling in the motor cortex (area 4). This labelling was predominantly ipsilateral to the injection site. In contrast, injections into the nucleus reticularis pontis caudalis (NRPc), nucleus reticularis gigantocellularis (NRGc), and nucleus reticularis magnocellularis (NRMc) resulted in bilateral labelling--primarily in areas 6a beta, 6a gamma, and in the rostromedial region of area 4--with little labelling in the proreus cortex. In general, the cortical projections to the caudal NRGc and the NRMc were larger than those to the NRPc. More than 25% of the total projections to each of the latter three reticular regions arose from the medial part of area 4. Labelling in the hindlimb regions of area 4 was largest following the NRMc injections and smallest after injections in the NRPc. The projections to the NRPc originated from more medial parts of areas 4 and 6 than did the projections to the caudal region of the NRGc. These results suggest that areas 4 and 6 may be able to differentially activate different regions of the pontomedullary reticular formation depending on the movement that is made and perhaps also on the context of that movement.

Myelination of the ventral and dorsal roots of the C8 and L4 segments of the spinal cord at different stages of development in the gray opossum, Monodelphis domestica.

We have quantified the number and size of myelinated fibers of the ventral and dorsal roots of selected segments that innervate the forelimbs (C8) and hindlimbs (L4) in the developing opossum, Monodelphis domestica. The gray opossum was chosen because it is born very immature and its somatomotor development occurs almost entirely postnatally. After aldehyde fixation, osmium postfixation, and resin embedding, the roots were cut transversely (1.5 pm), stained with toluidine blue, and observed and photographed by using light microscopy. The counts and measurements were made with a digitizing table. Myelination of the C8 and L4 roots begins during the second week of life and occurs according to two gradients: rostrocaudal and ventrodorsal. The number of myelinated fibers in these roots increased over approximately 7 weeks after which an excess, compared with their adult value, was recorded during the following weeks in three of the four roots. The supernumerary myelinated fibers are presumed to be collaterals. The fibers increased in diameter until at least 98 days. The classification according to size for the ventral roots (alpha and gamma) became evident in the fourth week, but that in types I, II, and III for the dorsal roots was never clear. There was no significant difference in the number and size distribution of myelinated fibers between sexes until late in development. The fibers innervating the limbs thus become myelinated postnatally in the opossum, a process that occurs over a protracted period and that continues after sensorimotor reflexes and locomotion appear adult-like.

Development of spontaneous locomotor behaviors in the opossum, Monodelphis domestica.

The development of spontaneous locomotor behaviors was studied in the opossum Monodelphis domestica. The newborn opossum performs alternate, rhythmic movements with its forelimbs to crawl on the mother's belly where it attaches to a nipple, and its hindlimbs are little more than embryonic buds. The forelimbs retain the above movements for about 3 weeks, while the hindlimbs begin to move late in the second week. When detached from the nipple at 2-3 weeks, the pup can support its weight on the forelimbs and pivot around its hindquarter. Around the fourth week, the young can detach from the mother, its hindlimbs can support weight and linear locomotion appears, but the four limbs are not well coordinated. However, it can swim with coordinated movements of all limbs. Coordination when walking appears around the sixth week. During development, the duration of the step cycle decreases significantly. The durations of the stance and swing phases of the step cycle decrease in absolute terms, but swing increases as a percentage of the step cycle. The results are discussed in relation to the development of nervous and skeletomuscular components as well as sensorimotor reflexes.

Responses of medullary reticulospinal neurones to stimulation of cutaneous limb nerves during locomotion in intact cats.

The present study was designed to determine whether the transmission of cutaneous afferent information from the limbs to the medullary reticular formation is phasically modulated during locomotion. Experiments were carried out in three chronically prepared, intact cats in which nerve cuff electrodes were placed, bilaterally, on the superficial radial and the superficial peroneal nerves. Thirty-seven reticulospinal neurones (RSNs) were identified by stimulation of their axons in the lumbar spinal cord (L2); 29 of 37 of these were recorded with the cat at rest, 28 of 37 during locomotion and 20 of 37 both at rest and during locomotion. Low-threshold stimulation of the cutaneous nerves evoked excitatory responses in the majority of RSNs both at rest and during locomotion. In the 28 of 37 RSNs recorded during locomotion, it was possible to record the evoked response to stimulation of all four limb nerves, giving a total of 184 tested cases [RSNs tested x number of nerves stimulated x phase of stimulation (swing or stance)]. The responses of most RSNs to cutaneous stimulation were modulated in a phase-dependent manner during locomotion. The maximal responses in most, but not all, cases were obtained during the swing phase of the limb that was stimulated and were largely independent of the discharge pattern of the cell. We interpret this result as indicating that the efficacy of transmission of the afferent information is determined more by the excitability of the spinal relay neurones than by the level of excitability of the RSNs in the brainstem. It is suggested that the base discharge pattern of RSNs might be largely determined by their central afferent input, while peripheral afferent inputs would primarily serve to modify the RSN discharge pattern in response to perturbations.

The vestibular primary afferents and the vestibulospinal projections in the developing and adult opossum, Monodelphis domestica.

The gray short-tailed opossum Monodelphis domestica is born in a very immature state (eyes and ears closed, budlike hindlimbs, etc.) 14 days post-coitum, but it can locomote with its forelimbs from the mother's genital aperture to a nipple to which it attaches. The forelimb movements allowing this behavior may be the expression of central pattern generators in the spinal cord, but sensory clues must guide it. One such system may be the vestibular system, which senses linear and angular acceleration and has a strong influence on posture and balance at rest and during locomotion in adult animals. Using the neuronal tracer DiI, we have looked at the vestibular primary afferents and the vestibular nuclear projections to the cervical spinal cord in newborn and postnatal opossums, as well as in the adult animal. The projections in the adult opossum conform to those described for other mammals. Fibers of the vestibular portion of the eighth nerve distribute to all four vestibular nuclei and toward the cerebellar primordium on the day of birth. In addition, some of the fibers project to the contralateral vestibular ganglion, a projection that is not found in the adult opossum. Projections from the lateral, medial and inferior vestibular nuclei to the cervical cord are also present in the newborn. Although we cannot exclude the possibility that chemical and/or tactile guidance is used for directing the movements of the newborn opossum, our results support the hypothesis that the vestibular system may be directly involved in the control of these movements. However, not all components of the system are equally developed at birth, and the circuit from the utricle to the lateral vestibular nucleus and from the latter to the cervical cord may be better formed than that from the semicircular canals to the medial and inferior vestibular nuclei to the cervical cord.

Populations of myelinated nerve fibers in the C8 and L4 ventral and dorsal roots in the opossum, Monodelphis domestica.

A quantitative light-microscopic analysis of the ventral and dorsal roots at the C8 and L4 segments of the spinal cord was made in the opossum, Monodelphis domestica, to evaluate the number of myelinated fibers and their class distribution, and will serve as a baseline to a study of myelinogenesis in that species. In male opossums, the C8 ventral root comprises an average of 595 myelinated fibers (70.3% alpha: 29.7% gamma) and the dorsal root 1,124 fibers (29.4% type I: 41.2% type II: 29.4% type III). The L4 ventral root has an average of 831 fibers (57.9% alpha: 42.1% gamma), and the dorsal root 2,079 fibers (17.4% type I:44.7 type II: 37.9% type III). The females have less fibers but their size and class proportions are comparable to those of the males. These data are discussed in relation to peripheral innervation and are compared to those reported for the rat and the cat.

The development of sensorimotor reflexes in the Brazilian opossum Monodelphis domestica.

The development of a number of sensorimotor reflexes was studied in the Brazilian opossum Monodelphis domestica. At birth, an opossum's forelimbs execute rhythmic, alternate movements which resemble swimming, whereas the hindlimbs are little more than embryonic buds that do not move independent of the trunk. It is possible, therefore, to witness the entire development of hindlimb motility, the advent of coordination between forelimbs and hindlimbs, and the development of ambulation. The following sequence in the appearance and maturation of the reflexes was observed: rooting, an innate reflex in mammals which disappears over time; withdrawal of the forelimbs followed by withdrawal of the hindlimbs; crossed extension of the forelimbs and then of the hindlimbs; grasp, the forelimbs preceding the hindlimbs; body righting on a surface; forward hopping of the forelimbs followed by the hindlimbs; lateral and medial hopping of the forelimbs, then the hindlimbs; chin tactile placing; body righting in the air; and, finally, visual placing. Limb tactile placing is not significantly expressed, even in the adult. This behavioral sequence generally matches the sequence of somatic (trunk, limbs, head) and neural (spinal cord and brain) structures involved in the control of these behaviors.

The development of the long descending propriospinal projections in the opossum, Monodelphis domestica.

The origin of the long descending propriospinal (LDP) projections have been studied in adult and developing opossums, Monodelphis domestica. This species has been chosen because of the considerable immaturity of the hindlimbs at birth, the postnatal appearance of their motility and the late development of coordination between them and the forelimbs. Neuroanatomical tracing has indicated that some LDP projections form postnatally. The ones present at birth arise from the regions of the cord where they are the most numerous in the adult opossum, presumptive laminae VII and VIII of the brachial enlargement. Subsequently, LDP projections arise from neurons located in adjacent laminae (IV to VI and IX and X) and at more rostral cervical levels. The origin of LDP projections in the adult opossums generally matches that reported for other mammals. These long propriospinal projections are in place well before the behavioral appearance of coordination between the hindlimbs and the forelimbs, but the timing of their synaptogenesis is not yet known.

The ontogenic development of sensorimotor reflexes and spontaneous locomotion in the Mongolian gerbil (Meriones unguiculatus).

Most behavioral studies on the ontogeny of sensorimotor reflexes and locomotion were done in quadrupedal species with equally developed forelimbs (FL) and hindlimbs (HL). In contrast, the Mongolian gerbil has long and strong HL but relatively small FL, indicating their differential use for locomotion. We have used the gerbil to study the ontogeny of a number of reflexes and locomotor acts to see if their sequence of appearance, their maturation, as well as their intensity of expression, differed from some other mammals. The following sequence was observed: forward FL hopping, FL grasp, forward HL hopping, surface body righting, chin tactile placing, lateral FL hopping, lateral HL hopping, medial FL hopping, medial HL hopping, visual placing, air body righting, and FL and HL tactile placing. When comparing FL and HL, a given reflex does not necessarily appear earlier in the limb that expresses it more strongly. The results are discussed in relation to the development of the central nervous system, the limb, and the locomotor behavior.

Immunochemical identification of ubiquitin and heat-shock proteins in corpora amylacea from normal aged and Alzheimer's disease brains.

Corpora amylacea (CA) accumulation in the central nervous system (CNS) is associated with both normal aging and neurodegenerative conditions such as Alzheimer's disease (AD). CA is reported to be primarily composed of glucose polymers, but approximately 4% of the total weight of CA is consistently composed of protein. CA protein resolved on sodium dodecylsulfate-polyacrylamide gel electrophoresis showed a broad range of polypeptides ranging from 24 to 133 kDa, with four abundant bands. Immunoblots of the profile of polypeptides solubilized from purified CA, showed positive ubiquitin (Ub) immunoreactivity for all the bands. Antisera to heat-shock proteins (hsp) 28 and 70 reacted selectively with bands of 30 and 67 kDa. These results show that Ub is associated with the primary protein components of CA and that the polypeptides are likely to be Ub conjugates. Immunostaining experiments were performed to specifically characterize the protein components of CA in brain tissue sections as well as those of CA purified from both AD and normal aged brains. In all cases CA showed positive reactions with antibodies to Ub, with antibodies raised against either paired helical filaments or hsp 28 or 70, the most prominent staining being with antibodies to Ub, hsp 28 or hsp 70. The presence of Ub and hsp 28 and 70, which are actively induced after stress, suggests that accumulation of altered proteins, possibly attributed to an increased frequency of unusual post-translational modifications or to a sustained physiological stress (related to both normal aging and neurodegenerative process), may be involved in the pathogenesis of CA.

The ontogenesis of sensorimotor reflexes in the Mongolian gerbil Meriones unguiculatus.

The ontogeny of a number of sensorimotor reflexes has been studied in the Mongolian gerbil. In contrast to a number of other mammals, the gerbil has relatively long and strong hindlimbs but small forelimbs, indicating their different importance for a number of locomotor acts, and during the developmental period studied, the hindlimbs grow at a much faster rate than the forelimbs. The following sequence of appearance and maturation of the reflexes was observed: rooting, forelimb hopping, surface body righting, forelimb grasp, hindlimb hopping, chin tactile placing, visual placing, air body righting and, at the same time, forelimb and hindlimb tactile placing. This sequence concords with the general gradient of development and maturation of the spinal and brain centers subserving these reflexes, as evaluated from Nissl preparations. The results indicate that there is no clearcut rostro-caudal gradient of postnatal maturation of the spinal cord and the spinally mediated reflexes, but that there is a general caudo-rostral gradient of brain maturation and of the brain-mediated reflexes. Comparisons with other mammals are made.

A four-parameter generalization of the Gompertz curve suitable for somatic growth.

A four-parameter generalization of the Gompertz curve is proposed which passes through the origin with respect to total age and is often more suitable for somatic growth than the three-parameter Gompertz curve. Applied to published cross-sectional data on 44 male white rats, the new curve differs from another four-parameter curve proposed earlier by Jolicoeur and Pirlot (1988) in that it does not involve initial growth delays and it avoids an arbitrary assumption concerning the initial curvature, but it nevertheless yields approximately similar descriptions of chronological growth and complex allometry. The new four-parameter version of the Gompertz curve may be useful in cases where other growth curves do not provide a satisfactory fit.