The usefulness and limitations of single neuron recordings in evaluating the neural control of temperature regulation

This paper deals with the possible significance that single neurons, which respond to local or remote temperature stimuli, may have in thermoregulatory control. Recordings of single neurons that appear to be involved in temperature regulation are easy to interpret as long as a functional association can be demonstrated. The processing of afferent thermal signals at different levels of the neuraxis exhibits differences in degree, depending on the location of the receptive field. Descending control of afferent temperature signals is already apparent at the segmental level of the spinal cord. It seems promising to search for central primary thermodetectors in the spinal cord rather than in those regions of the central nervous system where the principles of structural organization are largely unknown. In the hypothalamus, it is difficult to correlate neuronal responses to temperature with regulatory output, even in conscious animals. Characterization of thermoresponsive neurons by their sensitivity to biogenic amines might be used to establish a functional association. In vitro recordings from temperature-responsive neurons in the hypothalamus of rats and ducks indicate that a differentiation of intrinsic and synaptically induced thermosensitivity per se is not relevant to functional characterization.     

Cadherins in the Developing Central Nervous System: An Adhesive Code for Segmental and Functional Subdivisions

In this review, we describe general features of the expression of cadherins in the developing central nervous system (CNS) of vertebrates. In the early neuroepithelium, the expression of several cadherins is restricted to specific regions corresponding to segmental domains. Segmental boundaries often coincide with changes in cadherin expression, subdividing the primordial CNS into different adhesive domains. In the different neuromeric domains, early neurons are generated which differentially express cadherins. In the mantle layer, these early neurons seem to sort out according to which cadherin they express, and they aggregate into various gray matter regions (brain nuclei and cortical lamina and regions). The gray matter structures expressing a given cadherin become connected to one another to form parts of particular functional systems or neuronal circuits. Together, these findings show that cadherins provide a molecular system reflecting both early embryonic and mature nervous system architecture. The possible roles of cadherins in the formation and maintenance of segmental and functional nervous system structures are discussed.     

Microtubule-associated protein 2 within axons of spinal motor neurons: associations with microtubules and neurofilaments in normal and beta,beta'-iminodipropionitrile-treated axons

We have examined the distribution of microtubule-associated protein 2 (MAP2) in the lumbar segment of spinal cord, ventral and dorsal roots, and dorsal root ganglia of control and beta,beta'-iminodipropionitrile- treated rats. The peroxidase-antiperoxidase technique was used for light and electron microscopic immunohistochemical studies with two monoclonal antibodies directed against different epitopes of Chinese hamster brain MAP2, designated AP9 and AP13. MAP2 immunoreactivity was present in axons of spinal motor neurons, but was not detected in axons of white matter tracts of spinal cord and in the majority of axons of the dorsal root. A gradient of staining intensity among dendrites, cell bodies, and axons of spinal motor neurons was present, with dendrites staining most intensely and axons the least. While dendrites and cell bodies of all neurons in the spinal cord were intensely positive, neurons of the dorsal root ganglia were variably stained. The axons of labeled dorsal root ganglion cells were intensely labeled up to their bifurcation; beyond this point, while only occasional central processes in dorsal roots were weakly stained, the majority of peripheral processes in spinal nerves were positive. beta,beta'- Iminodipropionitrile produced segregation of microtubules and membranous organelles from neurofilaments in the peripheral nervous system portion and accumulation of neurofilaments in the central nervous system portion of spinal motor axons. While both anti-MAP2 hybridoma antibodies co-localized with microtubules in the central nervous system portion, only one co-localized with microtubules in the peripheral nervous system portion of spinal motor axons, while the other antibody co-localized with neurofilaments and did not stain the central region of the axon which contained microtubules. These findings suggest that (a) MAP2 is present in axons of spinal motor neurons, albeit in a lower concentration or in a different form than is present in dendrites, and (b) the MAP2 in axons interacts with both microtubules and neurofilaments.     

Glutamate-like immunoreactivity in the leech central nervous system

Summary Using a monoclonal antibody for glutamate the distribution was determined of glutamate-like immunoreactive neurons in the leech central nervous system (CNS). Glutamate-like immunoreactive neurons (GINs) were found to be localized to the anterior portion of the leech CNS: in the first segmental ganglion and in the subesophageal ganglion. Exactly five pairs of GINs consistently reacted with the glutamate antibody. Two medial pairs of GINs were located in the subesophageal ganglion and shared several morphological characteristics with two medial pairs of GINs in the first segmental ganglion. An additional lateral pair of GINs was also located in segmental ganglion 1. A pair of glutamate-like immunoreactive neurons, which are potential homologs of the lateral pair of GINs in segmental ganglion 1, were occasionally observed in more posterior segmental ganglia along with a selective group of neuronal processes. Thus only a small, localized population of neurons in the leech CNS appears to use glutamate as their neurotransmitter.     

Glutamate-like immunoreactivity in the leech central nervous system.

Using a monoclonal antibody for glutamate the distribution was determined of glutamate-like immunoreactive neurons in the leech central nervous system (CNS). Glutamate-like immunoreactive neurons (GINs) were found to be localized to the anterior portion of the leech CNS: in the first segmental ganglion and in the subesophageal ganglion. Exactly five pairs of GINs consistently reacted with the glutamate antibody. Two medial pairs of GINs were located in the subesophageal ganglion and shared several morphological characteristics with two medial pairs of GINs in the first segmental ganglion. An additional lateral pair of GINs was also located in segmental ganglion 1. A pair of glutamate-like immunoreactive neurons, which are potential homologs of the lateral pair of GINs in segmental ganglion 1, were occasionally observed in more posterior segmental ganglia along with a selective group of neuronal processes. Thus only a small, localized population of neurons in the leech CNS appears to use glutamate as their neurotransmitter.     

FGF22 signaling regulates synapse formation during post‐injury remodeling of the spinal cord

The remodeling of axonal circuits after injury requires the formation of new synaptic contacts to enable functional recovery. Which molecular signals initiate such axonal and synaptic reorganisation in the adult central nervous system is currently unknown. Here, we identify FGF22 as a key regulator of circuit remodeling in the injured spinal cord. We show that FGF22 is produced by spinal relay neurons, while its main receptors FGFR1 and FGFR2 are expressed by cortical projection neurons. FGF22 deficiency or the targeted deletion of FGFR1 and FGFR2 in the hindlimb motor cortex limits the formation of new synapses between corticospinal collaterals and relay neurons, delays their molecular maturation, and impedes functional recovery in a mouse model of spinal cord injury. These results establish FGF22 as a synaptogenic mediator in the adult nervous system and a crucial regulator of synapse formation and maturation during post‐injury remodeling in the spinal cord.     

Primary afferent depolarization of C fibres in the spinal cord of the cat

The excitability of primary afferent terminals of cutaneous C fibres was tested in the spinal cord of decerebrated cats. C fibre terminal excitability was decreased in the spinal state, and increased by conditioning volleys that activated only A fibres of another cutaneous nerve and by stimulating hair mechanically. It is suggested that C fibre input and therefore nociceptive information to the central nervous system is susceptible to presynaptic control by segmental and suprasegmental mechanisms.     

The effect of GABA on lumbar terminations of rubrospinal neurons in the cat spinal cord

Although GABA and piperidine-4-sulphonic acid depolarize I a afferent terminations in the cat spinal cord by activation of bicuculline-sensitive GABA receptors, no evidence was obtained for a bicuculline-sensitive alteration by either gabamimetic of the electrical threshold of rubrospinal terminations in the spinal intermediate nucleus. The terminal axonal arborizations in the spinal cord of neurons in the red nucleus thus do not have GABA receptors similar to those on the cell bodies. The results are discussed in relation to the depolarizing action of GABA on some central neurons, and on neurons with peripheral cell bodies, and to probable differences in the intracellular chloride content of neurons having peripheral or central cell bodies, and thus of different embryological origin. A presynaptic depolarizing inhibitory process mediated by GABA appears to be confined to the terminals of primary afferent fibres in the mammalian central nervous system.     

Cerebrospinal fluid contacting neurons in the reduced brain ventricular system of the atlantic hagfish,Myxine glutinosa

Cerebrospinal fluid (CSF)-contacting neurons are sensory-type cells sending ciliated dendritic process into the CSF. Some of the prosencephalic CSF-contacting neurons of higher vertebrates were postulated to be chemoreceptors detecting the chemical composition of the CSF, other cells may percieve light as "deep encephalic photoreceptors". In our earlier works, CSF-contacting neurons of the mechanoreceptor-type were described around the central canal of the hagfish spinal cord. It was supposed that perceiving the flow of the CSF they are involved in vasoregulatory mechanisms of the nervous tissue. In the present work, we examined the brain ventricular system of the Atlantic hagfish with special reference to the presence and fine structure of CSF-contacting neurons. Myxinoids have an ontogenetically reduced brain ventricular system. In the adult hagfish (Myxine glutinosa) the lumen of the lateral ventricle is closed, the third ventricle has a preoptic-, infundibular and subhabenular part that are not connected to each other. The choroid plexus is absent. The infundibular part of the third ventricle has a medial hypophyseal recess and, more caudally, a paired lateral recess. We found CSF-contacting neurons in the lower part of the third ventricle, in the preoptic and infundibular recess as well as in the lateral infundibular recesses. No CSF-contacting neurons were found in the cerebral aqueduct connecting the subhabenular recess to the fourth ventricle. There is a pineal recess and a well-developed subcommissural organ at the rostral end of the aqueduct. Extending from the caudal part of the fourth ventricle in the medulla to the caudal end of the spinal cord, the central canal has a dorsal and ventral part. Dendrites of CSF-contacting neurons are protruding into the ventral lumen. Corroborating the supposed choroid plexus-like function of the wall of the dorsal central canal, segmental vessels reach a thin area on both sides of the ependymal lining. The perikarya of the CSF-contacting neurons found in the brain ventricles are mainly bipolar and contain granular vesicles of various size. The bulb-like terminal of their ventricular dendrites bears several stereocilia and contains basal bodies as well as mitochondria. Basal bodies emit cilia of the 9+0-type. Cilia may arise from the basal body and accessory basal body as well. The axons run ependymofugally and enter--partially cross--the periventricular synaptic zones. No neurohemal terminals similar to those formed by spinal CSF-contacting neurons of higher vertebrates have been found in the hagfish. We suppose that CSF-contacting neurons transform CSF-mediated non-synaptic information taken up by their ventricular dendrites to synaptic one. A light-sensitive role for some (preoptic?) groups of CSF-contacting neurons cannot be excluded.     

Histochemical localization of galactose-containing glycoconjugates in sensory neurons and their processes in the central and peripheral nervous system of the rat

We studied the distribution of sugar residues in the oligosaccharide chains of complex carbohydrates in tissue sections of rat spinal cord, brainstem, and sensory ganglia using twelve lectin-horseradish peroxidase conjugates. Glycoconjugates containing terminal galactose residues were localized apparently in the Golgi apparatus in a population of predominantly small B-type neurons in spinal and trigeminal ganglia. Large A-type neurons rarely showed reactivity with galactose-binding lectins. A cells stained for glycoconjugates with N-glycosidically linked oligosaccharides and glycogen. The central and peripheral processes of the small neurons, mostly unmyelinated C fibers in sensory roots and spinal nerves, contained an abundance of glycoconjugates with terminal alpha-galactose residues. The central projections and terminals of small to medium-sized primary sensory neurons in the spinal and trigeminal ganglia were visualized in Lissauer's tract and the substantia gelatinosa in the spinal cord, and in the spinal trigeminal tract and the nucleus trigeminus in the lower medulla with lectins specific for terminal alpha-galactose residues. In addition, fibers of the solitary system and the area postrema were reactive with these lectins. The peripheral and central nervous system elements with affinity for galactopyranosyl-specific lectins correspond in distribution with neuroanatomical regions thought to be involved in the transmission and relay of somatic and visceral afferent inputs such as pain and temperature. Such specific localization of a glycosubstance to a distinct subpopulation of neurons and their peripheral and central processes suggests that the particular glycoconjugate may be of physiological significance.     

L-threonine administration increases glycine concentrations in the rat central nervous system

Intraperitoneal administration of L-threonine increased the glycine and threonine concentrations in rat spinal cord. Glycine contents also increased in synaptosomes prepared from spinal cords from threonine-pretreated animals. These findings suggest that plasma threonine concentrations normally might affect production of glycine by central nervous system neurons, and also that exogenous threonine might be useful in modifying glycinergic transmission.     

Dpysl2 (CRMP2) and Dpysl3 (CRMP4) phosphorylation by Cdk5 and DYRK2 is required for proper positioning of Rohon-Beard neurons and neural crest cells during neurulation in zebrafish

Dpysl2 (CRMP2) and Dpysl3 (CRMP4) are involved in neuronal polarity and axon elongation in cultured neurons. These proteins are expressed in various regions of the developing nervous system, but their roles in vivo are largely unknown. In dpysl2 and dpysl3 double morphants, Rohon-Beard (RB) primary sensory neurons that were originally located bilaterally along the midline shifted their position to a more medial location in the dorsal-most part of spinal cord. A similar phenotype was observed in the cdk5 and dyrk2 double morphants. Dpysl2 and Dpysl3 phosphorylation mimics recovered this phenotype. Cell transplantation analysis demonstrated that this ectopic RB cell positioning was non-cell autonomous and correlated with the abnormal position of neural crest cells (NCCs), which also occupied the dorsal-most part of the spinal cord during the neural rod formation stage. The cell position of other interneuron and motor neurons within the central nervous system was normal in these morphants. These results suggest that the phosphorylation of Dpysl2 and Dpysl3 by Cdk5 and DYRK2 is required for the proper positioning of RB neurons and NCCs during neurulation in zebrafish embryos.     

Neurotrophin treatment to promote regeneration after traumatic CNS injury

Neurotrophins are a family of growth factors that have been found to be central for the development and functional maintenance of the nervous system, participating in neurogenesis, neuronal survival, axonal growth, synaptogenesis and activity-dependent forms of synaptic plasticity. Trauma in the adult nervous system can disrupt the functional circuitry of neurons and result in severe functional deficits. The limitation of intrinsic growth capacity of adult nervous system and the presence of an inhospitable environment are the major hurdles for axonal regeneration of lesioned adult neurons. Neurotrophic factors have been shown to be excellent candidates in mediating neuronal repair and establishing functional circuitry via activating several growth signaling mechanisms including neuron-intrinsic regenerative programs. Here, we will review the effects of various neurotrophins in mediating recovery after injury to the adult spinal cord.     

Development of leg chordotonal sensory organs in normal and heat shocked embryos of the cricket Teleogryllus commodus (Walker)

This paper describes the embryonic development of some parts of the sensory peripheral nervous system in the leg anlagen of the cricket Teleogryllus commodus in normal and heat shocked embryos. The first peripheral neurons appear at the 30% stage of embryogenesis. These tibial pioneer neurons grow on a stereotyped path to the central nervous system and form a nerve which is joined by the growth cones of axons that arise later, including those from the femoral chordotonal organ, subgenual organ and tympanal organ. The development of these organs is described with respect to the increase in number of sensory receptor cells and the shape and position of the organs. At the 100% stage of embryogenesis all three organs have completed their development in terms of the number of sense cells and have achieved an adult shape. To study the function of the tibial pioneer neurons during embryogenesis a heat shock was used to prevent their development. Absence of these neurons has no effect on the development of other neurons and organs proximal to them. However, the development of distal neurons and organs guided by them is impaired. The tibial pioneer neurons grow across the segmental boundary between femur and tibia early in development, and the path they form seems to be essential for establishing the correct connections of the distal sense organs with the central nervous system.     

Genetic Study of Axon Regeneration with Cultured Adult Dorsal Root Ganglion Neurons

It is well known that mature neurons in the central nervous system (CNS) cannot regenerate their axons after injuries due to diminished intrinsic ability to support axon growth and a hostile environment in the mature CNS^1,2. In contrast, mature neurons in the peripheral nervous system (PNS) regenerate readily after injuries³. Adult dorsal root ganglion (DRG) neurons are well known to regenerate robustly after peripheral nerve injuries. Each DRG neuron grows one axon from the cell soma, which branches into two axonal branches: a peripheral branch innervating peripheral targets and a central branch extending into the spinal cord. Injury of the DRG peripheral axons results in substantial axon regeneration, whereas central axons in the spinal cord regenerate poorly after the injury. However, if the peripheral axonal injury occurs prior to the spinal cord injury (a process called the conditioning lesion), regeneration of central axons is greatly improved⁴. Moreover, the central axons of DRG neurons share the same hostile environment as descending corticospinal axons in the spinal cord. Together, it is hypothesized that the molecular mechanisms controlling axon regeneration of adult DRG neurons can be harnessed to enhance CNS axon regeneration. As a result, adult DRG neurons are now widely used as a model system to study regenerative axon growth^5-7.Here we describe a method of adult DRG neuron culture that can be used for genetic study of axon regeneration in vitro. In this model adult DRG neurons are genetically manipulated via electroporation-mediated gene transfection^6,8. By transfecting neurons with DNA plasmid or si/shRNA, this approach enables both gain- and loss-of-function experiments to investigate the role of any gene-of-interest in axon growth from adult DRG neurons. When neurons are transfected with si/shRNA, the targeted endogenous protein is usually depleted after 3-4 days in culture, during which time robust axon growth has already occurred, making the loss-of-function studies less effective. To solve this problem, the method described here includes a re-suspension and re-plating step after transfection, which allows axons to re-grow from neurons in the absence of the targeted protein. Finally, we provide an example of using this in vitro model to study the role of an axon regeneration-associated gene, c-Jun, in mediating axon growth from adult DRG neurons⁹.     

Neural Control of Interappendage Phase During Locomotion

Interappendage phasing of crayfish swimmeret movements dependsupon a central nervous system network of command, oscillator,and coordinating neurons. The command neurons serve to set thegeneral excitation level in each of the segmental oscillators.The oscillator neurons in each hemi-ganglion generate the rhythmicalternations of powerstroke and returnstroke motor neuron activity.The coordinating neurons transmit the precise timing informationabout the state of one oscillator to other oscillators. Thisinformation can serve to advance or to delay the motor burstsdriven by the other oscillators. Which effect is observed dependsupon the arrival time of the coordinating neuron discharge withinthe cycle period of the modulated oscillator. This type of modulationleads to the prediction that a stable interappendage phase canresult from situations where there is not a fixed excitabilitygradient among the segmental oscillators. This prediction hasbeen verified using a cut command neuron preparation.     

LPA receptor expression in the central nervous system in health and following injury

Lysophosphatidic acid (LPA) is released from platelets following injury and also plays a role in neural development but little is known about its effects in the adult central nervous system (CNS). We have examined the expression of LPA receptors 1-3 (LPA_1–3) in intact mouse spinal cord and cortical tissues and following injury. In intact and injured tissues, LPA₁ was expressed by ependymal cells in the central canal of the spinal cord and was upregulated in reactive astrocytes following spinal cord injury. LPA₂ showed low expression in intact CNS tissue, on grey matter astrocytes in spinal cord and in ependymal cells lining the lateral ventricle. Following injury, its expression was upregulated on astrocytes in both cortex and spinal cord. LPA₃ showed low expression in intact CNS tissue, viz. on cortical neurons and motor neurons in the spinal cord, and was upregulated on neurons in both regions after injury. Therefore, LPA_1–3 are differentially expressed in the CNS and their expression is upregulated in response to injury. LPA release following CNS injury may have different consequences for each cell type because of this differential expression in the adult nervous system.     

The distribution of cholinergic neurons in the human central nervous system

Choline acetyltransferase (ChAT), the enzyme responsible for the biosynthesis of acetylcholine, is presently the most specific marker for identifying cholinergic neurons in the central and peripheral nervous systems. The present article reviews immunohistochemical and in situ hybridization studies on the distribution of neurons expressing ChAT in the human central nervous system. Neurons with both immunoreactivity and in situ hybridization signals of ChAT are observed in the basal forebrain (diagonal band of Broca and nucleus basalis of Meynert), striatum (caudate nucleus, putamen and nucleus accumbens), cerebral cortex, mesopontine tegmental nuclei (pedunculopontine tegmental nucleus, laterodorsal tegmental nucleus and parabigeminal nucleus), cranial motor nuclei and spinal motor neurons. The cerebral cortex displays regional and laminal differences in the distribution of neurons with ChAT. The medial septal nucleus and medial habenular nucleus contain immunoreactive neurons for ChAT, which are devoid of ChAT mRNA signals. This is probably because there is a small number of cholinergic neurons with a low level of ChAT gene expression in these nuclei of human. Possible connections and speculated functions of these neurons are briefly summarized.     

Intersegmental coordinating system of the lamprey central pattern generator for locomotion

Summary In the lamprey,Ichthyomyzon unicuspis, the wave of activity required for normal swimming movements can be generated by a central pattern generator (CPG) residing in the spinal cord. A constant phase coupling between spinal segments can be organized by intersegmental coordinating neurons intrinsic to the cord. The rostral and caudal segmental oscillators of the CPG have different preferred frequencies when separated from each other. Therefore the system must maintain the segmental oscillators of the locomotor CPG at a single common frequency and with the proper relative timing. Using selective lesions and a split-bath, it is demonstrated that the coordinating system is comprised of at least 3 subsystems, short-axon systems in the lateral and medial tracts and a long axon system in the lateral tracts. Each alone can sustain relatively stable coordinated activity.Abbreviations CPG central pattern generator - NMDA N-methyl-D-aspartate - VR ventral root