Shirley Ann Gilmore

Connections of the mesencephalic locomotor region (MLR) II. Afferents and efferents

Injections of a tritiated amino acid-fluorescent dye mixture were made unilaterally into the area of the mesencephalic locomotor region (MLR). After allowing for retrograde and anterograde transport, the same site was electrically stimulated to induce locomotion on a treadmill following a precollicular-postmamillary transection. The tritiated amino acid transported anterogradely primarily was found autoradiographically to descend in the area of Probsts tract and to ascend to the centremedian nucleus (CM) of the thalamus. Neurons labeled retrogradely by the fluorescent dye in the same injection-stimulation site were observed in the substantia nigra, entopeduncular nucleus, sub- and hypothalamus and amygdala. In subsequent experiments, injections of fluorescent tracers were made into the area of Probsts tract and CM. Neurons in the mesencephalic trigeminal root, cuneiform nucleus, nucleus tegmenti pedunculopontinus (NTPP), dorsal locus coeruleus and lateral central gray were labeled from Probsts tract injections. Neurons in medial and lateral central gray, as well as NTPP, were labeled from CM injections.

Interactions between intraspinal Schwann cells and the cellular constituents normally occuring in the spinal cord: An ultrastructural study in the irradiated rat

Relationships between intraspinal Schwann cells and neuroglia, particularly, astrocytes, were studied following X-irradiation of the spinal cord in 3-day-old rats. Initially, this exposure results in a depletion of the neuroglial population. By 10 days post-irradiation (P-I), gaps occur in the glia limitans, although the overlying basal lamina remains intact. Development of and myelination by intraspinal Schwann cells is well underway by 15 days P-I. These Schwann cell-occupied regions have a paucity of astrocyte processes, a finding which persists throughout the study (60 days P-I), and several types of Schwann cell-neuroglial interfaces are observed, including: (1) astrocyte separation of Schwann cells from oligodendrocyte-myelinated regions; (2) intermingling of Schwann cell-myelinated axons and oligodendrocyte-myelinated axons in the absence of astrocyte processes; and (3) ensheathment of unmyelinated axons by astrocyte processes which separate these axons from the Schwann cells. The gaps in the glia limitans widen as the P-I interval increases. At 45 and 60 days P-I, the basal lamina no longer forms a singular, continuous covering over the spinal cord surface, but follows instead a rather tortuous course over the disrupted glia limitans and the intraspinal Schwann cells. Although the mode of initial occurrence of Schwann cells within the spinal cord is not yet understood, the data indicate that the astrocyte population is involved in that process, as well as in limiting the further development of Schwann cells within the substance of the spinal cord.

Spontaneous generalized spike-wave discharges in the electrocorticograms of albino rats

Generalized epileptiform discharges occur spontaneously in the electrocorticograms of some albino laboratory rats. These discharges occur during periods of quiet wakefulness, are composed of 6-10/sec spike-wave complexes, are accompanied by locomotor arrest and often by mild clonic facial movements. The occurrence of these discharges is unrelated to surgical procedures used to create chronic recording preparations. Similar discharges and behavior have been induced in controls by injections of pentylenetetrazol.

Autoradiographic and ultrastructural studies of areas of spinal cord occupied by Schwann cells and Schwann cell myelin

Schwann cells, peripheral-type myelin and connective tissue elements develop within the dorsal portion of the X-irradiated spinal cord in immature rats. Factors controlling the distribution of these elements within the irradiated site are not fully understood. In the present study [3H]thymidine autoradiography was used to examine proliferative activities of cells in these areas occupied by peripheral nervous system components, and correlative ultrastructural evaluations were made. At 15 and 20 days post-irradiation (P-I), the Schwann cells occupied the dorsolateral portions of the dorsal funiculi, and heavily labeled cells occurred throughout these areas. By 25 days P-I the Schwann cells extended ventrally into the depths of the dorsal funiculi and into the dorsal gray matter, and labeled cells were concentrated in the deeper portions of these areas. Ultrastructurally, the Schwann cells and peripheral-type myelin were more mature in the superficial portions where proliferative activity was diminished. In contrast, much less mature, peripheral-type myelin occurred in the depths where the labeled cells were concentrated. At 30 and 45 days P-I, labeled cells were much less frequent but usually occurred in the depths when observed. Similarly, a dorsal-ventral gradient in maturity of peripheral-type myelin was evident ultrastructurally. By 60 and 90 days P-I, labeling was rare, and mature Schwann cell myelin was present throughout the areas. Astrocytes and their processes were less numerous in regions invaded by Schwann cells, as compared to controls, and studies are in progress to evaluate the relationships between these glial elements and intraspinal peripheral nervous system components.

Regrowth of dorsal root axons into a radiation-induced glial-deficient environment in the spinal cord.

Exposure of the lumbosacral spinal cord of early postnatal rats to X-rays reduces the glial populations within the irradiated region. The present study examines the ability of axons of a dorsal root subjected to a crush-freeze lesion to grow back into this glial-deficient spinal cord environment, in contrast to the non-irradiated rat. Ultrastructural examination of the dorsal root entry zone (DREZ) 60 days after root injury revealed a well-formed astrocytic scar in this zone and adjacent regions of spinal cord in non-irradiated rats. In contrast, scar formation did not occur in irradiated root-lesioned animals in which the astrocytic response was quite limited. Axons were present in the DREZ and underlying spinal cord in irradiated root-lesioned rats at this time but were absent from these regions in the non-irradiated lesioned controls. These ultrastructural findings are highly suggestive that axons are capable of regrowth into the irradiated spinal cord. Axonal regrowth was assessed further by tracing techniques after application of a combination of peroxidase-labeled wheat germ agglutinin and horseradish peroxidase to the cut end of the root distal to the previously injured site. Labeled axons were readily identified within the spinal gray matter in irradiated lesioned but not in the non-irradiated lesioned rats. These data, together with the ultrastructural observations, are supportive of regrowth of the dorsal root axons into the spinal cord. The radiation-induced changes in the glial populations are discussed with regard to conversion of a normally non-permissive environment into one conducive for axonal regrowth.

Development of NADPH diaphorase-positive pedunculopontine nucleus neurons

Nicotinamide adenine dinucleotide phosphate (NAD-PH) diaphorase histochemistry was used to localize cholinergic neurons in the pedunculopontine nucleus of neonatal and adult rats. Measurements of cell body areas revealed an average area around 200 microns2 at birth, followed by a significant increase to approximately 500 microns2 by 2 weeks of age. Thereafter, there was a decrease in cell area such that by 5 weeks of age the neurons had attained their adult size of around 300 microns2. The marked increase in cell size at the end of 2 weeks of age is discussed in relation to significant events in the development of locomotor and other rhythmic function control systems.

Dorsal—Ventral Differences in the Glia Limitans of the Spinal Cord: An Ultrastructural Study in Developing Normal and Irradiated Rats

The dorsal and ventral surfaces of the lumbosacral spinal cord were examined in normal and irradiated postnatal rats. In normal rats between three and 13 days postnatal (DP), the glia limitans (GL) of the ventral surface was a more complex structure than the dorsal GL. This greater degree of complexity was manifested in a greater number of subpial astrocytes, a greater number of radial glial processes and a more advanced state in differentiation of its constituents. In rats irradiated at three DP and examined at 13 DP, the ventral GL remained intact and relatively unaffected by the radiation. In contrast, the dorsal GL was disrupted, and Schwann cells were seen within the dorsal funiculus. The ventral GL of the rat lumbosacral spinal cord is a more substantial structure than the dorsal GL during normal development. This factor alone may account for the integrity of the barrier properties of the ventral GL following radiation. However, our observations suggest that subpial astrocytes of the dorsal GL are more susceptible to radiation damage at three DP than the subpial astrocytes and radial glia of the ventral GL.

Glial–glial and glial–neuronal interfaces in radiation-induced, glia-depleted spinal cord

This review summarises some of the major findings derived from studies using the model of a glia‐depleted environment developed and characterised in this laboratory. Glial depletion is achieved by exposure of the immature rodent spinal cord to x‐radiation which markedly reduces both astrocyte and oligodendrocyte populations and severely impairs myelination. This glia‐depleted, hypomyelinated state presents a unique opportunity to examine aspects of spinal cord maturation in the absence of a normal glial population. An associated sequela within 2–3 wk following irradiation is the appearance of Schwann cells in the dorsal portion of the spinal cord. Characteristics of these intraspinal Schwann cells, their patterns of myelination or ensheathment, and their interrelations with the few remaining central glia have been examined. A later sequela is the development of Schwann cells in the ventral aspect of the spinal cord where they occur predominantly in the grey matter. Characteristics of these ventrally situated intraspinal Schwann cells are compared with those of Schwann cells located dorsally. Recently, injury responses have been defined in the glia‐depleted spinal cord subsequent to the lesioning of dorsal spinal nerve roots. In otherwise normal animals, dorsal nerve root injury induces an astrocytic reaction within the spinal segments with which the root(s) is/are associated. Lesioning of the 4th lumbar dorsal root on the right side in irradiated or nonirradiated animals results in markedly different glial responses with little astrocytic scarring in the irradiated animals. Tracing studies reveal that these lesioned dorsal root axons regrow rather robustly into the spinal cord in irradiated but not in nonirradiated animals. To examine role(s) of glial cells in preventing this axonal regrowth, glial cells are now being added back to this glia‐depleted environment through transplantation of cultured glia into the irradiated area. Transplanted astrocytes establish barrier‐like arrangements within the irradiated cords and prevent axonal regrowth into the cord. Studies using other types of glial cultures (oligodendrocyte or mixed) are ongoing.

Astrocytes in the aged rat spinal cord fail to increase GFAP mRNA following sciatic nerve axotomy

Aging in the brain is associated with specific changes in the astrocyte population. The present study establishes that similar changes occur in the aging spinal cord. The levels of glial fibrillary acidic protein (GFAP) mRNA were significantly increased 0.4-fold in aged 8- to 17-month-old rats compared to young 2-month-old rats. The ability of astrocytes in the aging spinal cord to respond to a non-invasive CNS injury was compared to young rats 4 days following sciatic nerve axotomy. The level of GFAP mRNA was significantly increased 0.5-fold in the young rats in response to axotomy. In contrast, the level of GFAP mRNA in aged rats did not increase following injury above that present in non-axotomized rats of the same age.

Temporary adhesions between axons and myelin-forming processes

Following irradiation, the dorsal funiculus of the lumbosacral spinal cord in the rat undergoes the following sequence of events: (a) a marked reduction of the normal glial population, (b) an absence of oligodendrocyte myelin formation, (c) the invasion and proliferation of Schwann cells, and (d) the myelination of axons within the cord by Schwann cells. The present study demonstrates that, during the latter process, junctional complexes develop between these intraspinal Schwann cells and the axolemma. These complexes are present at sites of probable initial contact between the two membranes. As the Schwann cell process begins to wrap the axons, these junctional complexes are located between the inner spiraling process of the Schwann cell and the axon. With the advancement of myelin formation to the stage of 8 to 9 compact spirals, these contacts are rarely observed. Spinal cords from normal 8-day-old rats were examined in order to determine if such contacts occur during myelination by oligodendrocytes. Although they are more difficult to detect in the normal animal due to the abundance of glial processes, similar junctional complexes occur between oligodendrocyte processes and axons. These observations suggest that these complexes may serve to stabilize and to guide the myelin-forming process around the perimeter of the axon. Additionally, these junctions may play an active role in the advancement of the inner spiraling process by forming temporary adhesions between the axolemma and the adjacent myelin-forming process. Coated vesicles are commonly observed fused with the axolemma of axons which are in the early stages of myelination. These coated vesicles may be involved in the insertion or the deletion of junctional membrane.