Quantitative and electron microscopic studies of sensory ganglion cells of the Sprawling mouse

Neuropeptide Y-expressing dorsal horn inhibitory interneurons gate spinal pain and itch signalling

Somatosensory information is processed by a complex network of interneurons in the spinal dorsal horn. It has been reported that inhibitory interneurons that express neuropeptide Y (NPY), either permanently or during development, suppress mechanical itch, with no effect on pain. Here, we investigate the role of interneurons that continue to express NPY (NPY-INs) in the adult mouse spinal cord. We find that chemogenetic activation of NPY-INs reduces behaviours associated with acute pain and pruritogen-evoked itch, whereas silencing them causes exaggerated itch responses that depend on cells expressing the gastrin-releasing peptide receptor. As predicted by our previous studies, silencing of another population of inhibitory interneurons (those expressing dynorphin) also increases itch, but to a lesser extent. Importantly, NPY-IN activation also reduces behavioural signs of inflammatory and neuropathic pain. These results demonstrate that NPY-INs gate pain and itch transmission at the spinal level, and therefore represent a potential treatment target for pathological pain and itch.

Proprioceptive sensory neuropathy in mice with a mutation in the cytoplasmic Dynein heavy chain 1 gene

Mice heterozygous for the radiation-induced Sprawling (Swl) mutation display an early-onset sensory neuropathy with muscle spindle deficiency. The lack of an H reflex despite normal motor nerve function in the hindlimbs of these mutants strongly suggests defective proprioception. Immunohistochemical analyses reveal that proprioceptive sensory neurons are severely compromised in the lumbar dorsal root ganglia of newborn Swl/+ mice, whereas motor neuron numbers remain unaltered even in aged animals. We have used positional cloning to identify a nine base-pair deletion in the cytoplasmic dynein heavy chain 1 gene (Dync1h1) in this mutant. Furthermore, we demonstrate that Loa/+ mice, which have previously been shown to carry a missense point mutation in Dync1h1 that results in late-onset motor neuron loss, also present with a severe, early-onset proprioceptive sensory neuropathy. Interestingly, in contrast to the Loa mutation, the Swl mutation does not delay disease progression in a motor neuron disease mouse model overexpressing a human mutant superoxide dismutase (SOD1(G93A)) transgene. Together, we provide in vivo evidence that distinct mutations in cytoplasmic dynein can either result in a pure sensory neuropathy or in a sensory neuropathy with motor neuron involvement.

Age-related mitochondrial damage in the B-type cells of the rat trigeminal ganglia

Aging is associated with signs of sensory impairment and neurological symptoms. Advancing age is characterized by increased thresholds of thermal, tactile and vibratory sensations. One important cause of the sensory disturbances has been stated to be the loss of neurons. Decreases have been observed in the number of peripheral nerve fibers and in the number of neurons in the spinal ganglia of rats. In the present study, the cytoplasmic organelles of the neurons of the trigeminal ganglia were examined in young and senescent rats in order to reveal the cause of cell loss during aging. Mitochondrial alterations, swelling and loss of internal cristae were observed from 23 week of age in the B-type neurons of the trigeminal ganglia. Other cytoplasmic elements were intact. Mitochondrial damage was never seen in A-type neurons and satellite glial cells. It was concluded that the ultrastructural changes in the mitochondria of the B-type cells may contribute to the nervous disturbances that occur in senescent individuals. The diminution of mitochondrial damage and the protection of B-type neurons through the use of nerve growth factors may prevent the sensory impairment late in life.

Developmental regulation of sensory neuron number and limb innervation in the mouse

Although used widely in studies of naturally occurring cell death, systematic descriptions of the time course of changes in sensory neuron number and of limb innervation in the mouse are not available. The development of sensory innervation to the mouse forelimb was traced using the lipophilic carbocyanine dye, DiI, and correlated with neuron number in dorsal root ganglia contributing to the cervical enlargement. Axon invasion of the forelimb began at E10.5. Sensory axons reached the distal margin of the forelimb by E13.5. The difficulty of identifying immature neurons precluded estimating neuron numbers during the period of limb innervation. Neuron numbers in dorsal root ganglia (DRGs) C5-C8 increased from E14 to E16 and from E18 to P4. No evidence of a decline in neuron numbers was found during the developmental periods studied. Neuron number was compared in neonates and adults to determine if sensory neurons are added as body size increases as found in the frog [J. Comp. Neurol. 314 (1991) 106] and the rat [J. Comp. Neurol. 386 (1997) 8]. In contrast to previous findings, no difference was found in sensory neuron number between neonate and adult mice in either cervical or lumbar DRGs.

Hindlimb sensory neuron number increases with body size

As an animal grows, its sensory systems face the task of maintaining sensitivity and discrimination in peripheral fields that are continually enlarging. Without the addition of neurons, existing cells would have to innervate a wider skin area, leading to a decrease in the precision with which stimuli are localized. Neurons were counted in the three dorsal root ganglia (DRGs) that innervate the hindlimb of the bullfrog (Rana catesbeiana). Profiles of neuronal nuclei containing the single nucleolus found in these cells were counted in every third section of serially cut ganglia. This means of assessing neuron number was validated by comparing these profile counts with three-dimensional reconstructions of sensory neurons. Large frogs (10-17 cm) had more than twice as many DRG neurons as small frogs (3.3-5 cm). The rate of increase was greatest between 3 and 8 cm, when over 1,300 hindlimb sensory neurons were added for each 1 cm increase in body length. The possibility that selective survival of frogs with many neurons biases estimates of mean neuron number was ruled out by the finding that frogs drawn from the same closed population, half of which were sacrificed immediately and half of which were sacrificed after 1 year's survival, showed expected differences in neuron number. Horseradish peroxidase applied to particular hindlimb nerves retrogradely labeled more neurons in large frogs than small frogs, supporting the hypothesis that added neurons extend their axons to the periphery.(ABSTRACT TRUNCATED AT 250 WORDS)

Size-related increase in motoneuron number: evidence for late differentiation

The number of motoneurons in the lumbar lateral motor column (LMC) was compared in bullfrogs (Rana catesbeiana) ranging in body length from 2.5 to 19 cm. Large frogs had 36% more motoneurons than small frogs; however, within the caudal third of the LMC, large frogs had over 70% more motoneurons than small frogs. Injection of small frogs with [3H]thymidine every third day for 20-22 weeks gave no evidence of motoneuron birth. Instead, a pool of small, incompletely differentiated (type L) motoneurons appears to be converted into mature (type M) motoneurons as the animal grows. This hypothesis is supported by several lines of evidence: (1) the number of type-M motoneurons varies directly with body size while the number of type-L cells varies inversely; (2) the increase in type-M motoneurons and the decrease in type-L cells are restricted to the same regions of the LMC; and (3) type-L cells exhibited both immunoreactivity to neurofilament antibodies and histochemical evidence of acetylcholinesterase activity, a marker for spinal motoneurons.

Neuron addition in the postmetamorphic frog

Neuron number among somatic motoneurons, sensory neurons, and sympathetic postganglionic neurons that innervate the hindlimb was correlated with body length in the bullfrog, Rana catesbeiana. Two to three times more dorsal root and sympathetic ganglion neurons are found in the largest than the smallest specimens. Hindlimb motoneurons show a 20% increase in number, but this increase is restricted to the caudal third of the motor pool. Within this region, 60% more motoneurons are found among the largest frogs. Cell division does not appear to be the mechanism of neuron addition. Instead, we propose that a pool of undifferentiated neurons mature to maintain functional capabilities as the animal increases in size.

Inherited neuromuscular diseases in the mouse. A review of the literature

There are several neuromuscular disorders affecting the human being. Most of these are poorly understood and lack and effective treatment. Due to the limitation of experimental manipulation in "anima nobili", inherited neuromuscular diseases in laboratory animals constitute a valuable source of scientific information. Amongst several animal species affected by neuromuscular disorders the house mouse is of particular interest because of its small size, short pregnancy and low costs of maintanence. In the present review 20 murine mutants with diseases affecting peripheral nerves, skeletal muscles and motor end-plates are tabulated. Genetic, clinical and pathological aspects are discussed aiming to provide information about these mutants which might be of great interest as animal models for human neuromuscular diseases.

The ultrastructure of spiral ganglion cells in the mouse

In the adult mouse, three types of spiral ganglion cells can be distinguished on an ultrastructural basis. The first type, TI cells, corresponds to the most commonly encountered cells in the ganglion, which represents 92 to 94% of the whole population. This type of cell is characterized by numerous cytoplasmic organelles that give a dark and a granular appearance, while the nucleus is more lightly coloured. Moreover, the perikaryon is surrounded by a myelin sheath composed of several myelin lamellae that can be either loose or compact. The periodicity of compact myelin around the perikaryon is larger than that found in nerve fibres. The T II ganglion cells, corresponding to the second type, are less numerous and represent only 6 to 8% of the cell population. They present a clear perikaryon with few cytoplasmic organelles and large areas composed of filamentous structures. The perikaryon is not surrounded by a myelin sheath, although several Schwann cell processes can be observed. The third type is composed of few ganglion cells that are ultrastructurally very close to T I cells, but without a myelin sheath. Desmosome-like junctions have been observed between myelin lamellae and between myelin lamellae and the plasmalemma, but no synaptic contacts have been observed between auditory nerve fibres and ganglion cells.

The measurement of motor cell volumes in rats

The volumes of cell bodies and nuclei of rat motoneurons were calculated from reconstruction in araldite embedded spinal cord. The results were compared with those obtained by applying formulae to two-dimensional measurements. The results show that the volumes of reconstructed cells have comparable values when derived from transverse and longitudinal sections. These values differ quite remarkably from those obtained by applying the formulae of the sphere and of a non-rotational ellipsoid. The difference is thought to reflect shape and orientation of the cells in the spinal cord. Nuclei were found to have a spheroidal shape, with the major diameter parallel to the axis of the spinal cord. There is also a linear relationship between cells and nuclear volume (with values varying between 5 and 15). The results of this study suggest that once reconstruction studies have shown the basic parameters of the cells, a graphical method for cell volumes from equatorial area measurements gives more satisfactory results than other proposed methods.

Hereditary deafness in the cat. An electron microscopic study of the spiral ganglion

The spiral ganglion from white cats with hereditary deafness has been studied with the transmission electron microscope, and comparisons made with hearing animals at different ages. Ganglion cell loss occurs secondary to destruction of the organ of Corti, but only after the lapse of several months. Prior to neuronal loss, the type I ganglion cells lose their myelin sheaths and concurrently develop an increased content of neurofilaments. Type I neurons transform into type II through an intermediate type III stage. This process of neurofilamentous degeneration occurs slowly, and phagocytosis is therefore an inconspicuous feature.

The chick embryo nodose ganglion: effects of nerve growth factor in culture

Nodose ganglia from 8-day-old chick embryos were cultured in collagen gels for 2 days, with or without added nerve growth factor (NGF), in order to discover whether the nodose neurons, derived from an epidermal placode, are susceptible to this trophic factor. Neuronal survival and neurite outgrowth were stimulated only after addition of NGF, and the enhancing effects could be blocked by introducing antibodies to NGF. Stereological analysis of ganglia sectioned for light microscopy showed that the NGF-treated neurons increased their volume by about 50%, as did the nodose neurons in ovo from the eighth to the tenth day of incubation. The volume density, however, was significantly lower in vitro indicating a limited cell death during culture despite the presence of exogenously supplied NGF. The number of neurofilaments and microtubules increased in the cell centre of neurons treated with NGF; this region also showed numerous dense bodies and an extensive Golgi complex by electron microscopy. Ultrastructural similarities between neurons responding to NGF and neurons undergoing the axon reaction which follows axotomy are indicated. A role for a trophic factor resembling NGF in the normal development of placode-derived neurons of the sensory cranial ganglia is suggested.

Electron microscopic and quantitative studies of cell necrosis in developing sensory ganglia in normal and Sprawling mutant mice

The percentage of neurons undergoing necrosis during foetal and early post-natal development in normal and Sprawling (Swl) mutant mice was determined. Two major peaks in the occurrence of necrosis were found at the 13th and the 19th foetal days in both groups of mice but at all stages the incidence was greater in Swl. Electron microscopically the first degenerative changes seemed to involve the perikaryon while many of the cells undergoing necrosis in Swl already showed the characteristic abnormalities previously described in that mutant.

The development of sensory ganglion cells in normal and Sprawling mutant mice

The foetal and early post-natal development of dorsal root ganglion cells in normal and Sprawling (Swl) mutant mice was studied by light and electron microscopy. As early as the 11th foetal day the nucleus of some neurons showed an indentation of its membrane and by the 13th day this abnormality was marked and present in many ganglion cells of foetuses which were, therefore, identified as presumptive Swl'. A peripherally situated nucleus and the aggregation of filaments and organelles in the central perikaryon were also early abnormalities, similar to those of the sensory neurons of the adult Swl mouse. In the normal ganglion cells, nuclei were rounded and central by the 16th foetal day and nuclear indentations were never seen, thus making a clear distinction possible between normal and Swl foetuses.

The morphology of the hippocampus and dentate gyrus in normal and reeler mice

The morphology of the hippocampus and dentate gyrus in normal and reeler mice has been studied in Nissl, myelin, Golgi, Timm's sulfide silver and gold chloride-sublimate preparations. It is evident from both cell-and fiber-stained sections that despite the obvious defect in the positioning of the hippocampal pyramidal and dentate granule cells in the reeler mouse within the radial dimension, the hippocampal formation as a whole shows a normal and consistent progression of cytoarchitectonic fields along its transverse axis, and a normal and consistent progression of changes in the structure of the hippocampus and dentate gyrus along their longitudinal axes. Thus, at least in these structures, the reeler gene seems to exert its effect only in the radial dimension. Cell counts in the area dentata indicate that the number of dentate granule cells in the reeler mouse is reduced compared to that found in normal or heterozygous animals. Although it has been known for some time that the number of granule cells in the reeler cerebellar cortex is markedly reduced, this appears to be the first evidence for a reduction in cell number in a forebrain structure. All the major cell types normally found in the hippocampus and the dentate gyrus are recognizable in Golgi-stained preparations from the brains of reeler mutants. However, in both regions there are a number of abnormalities in the appearance of the cells which seem to be related to the cellular ectopia. Thus, whereas most of the pyramidal and granule cells which attain a normal position in the mutant usually have normal, or near-normal dendritic arbors, the dendrites of nearly all ectopic cells are severely distorted, both in their orientation and general configuration. In preparations stained by the Timm's sulfide silver technique it is evident that the general lamination pattern seen in normal mice is retained in the reeler hippocampus and dentate gyrus despite the gross malpositioning of many of the relevant neurons. However, although the overall laminar arrangement is preserved, there are some fairly consistent abnormalities; for example, the normal trilaminar staining pattern seen in the stratum moleculare of the dentate gyrus is replaced in the reeler by a bilaminar pattern. In gold chloride-sublimate impregnated preparations there is no obvious alignment of the astrocytes in the stratum moleculare of the dentate gyrus in either normal or reeler mice. Moreover, the distribution of the astrocytes within this zone is fairly normal in the reeler mouse, although, in general, these cells appear to be more consistently stellate in form than in normal animals.

The postnatal development of large light and small dark neurons in mouse dorsal root ganglia: a statistical analysis of cell numbers and size

A method is described for the analysis of cell types in mouse dorsal root ganglia using the distribution of cell cross-sectional areas measured at the level of the nucleolus in 1.5 micron Epon sections. Using a computer program it was possible to demonstrate the existence of two normally distributed sub-populations of neurons in all the 3rd lumbar segment ganglia (17 in number) measured at various ages from birth to 70 days. The two populations appeared to correspond with large light cells and small dark cells. The large light cell bodies increased in size until about 20 days postnatal, subsequently their size decreased whereas the mean size of the small dark cells reached a plateau by about day 10. The relationship of both nuclear volume and surface area to the surface area of the perikaryon differed between light and dark cells. The number of neurons in L3 remained virtually constant at about 6000 throughout the period examined. Since the proportion of neurons in each population was not shown to change with age there was no evidence that cells could change from one type into the other.