Up-regulation of Μ-opioid receptors in the spinal cord of morphine-tolerant rats

Though morphine remains the most powerful drug for treating pain, its effectiveness is limited by the development of tolerance and dependence. The mechanism underlying development of tolerance to morphine is still poorly understood. One of the factors could be an alteration in the number of Μ-receptors within specific parts of the nervous system. However, reports on changes in the Μ-opioid receptor density in the spinal cord after chronic morphine administration are conflicting. Most of the studies have used subcutaneously implanted morphine pellets to produce tolerance. However, it does not simulate clinical conditions, where it is more common to administer morphine at intervals, either by injections or orally. In the present study, rats were made tolerant to morphine by injecting increasing doses of morphine (10-50 mg/kg, subcutaneously) for five days.In vitro tissue autoradiography for localization of Μ-receptor in the spinal cord was done using [³H]-DAMGO. As compared to the spinal cord of control rats, the spinal cord of tolerant rats showed an 18.8% increase or up-regulation in the density of Μ-receptors in the superficial layers of the dorsal horn. This up-regulation of Μ-receptors after morphine tolerance suggests that a fraction of the receptors have been rendered desensitized, which in turn could lead to tolerance     

Abnormal myelination in the spinal cord of PTPα-knockout mice

PTPα interacts with F3/contactin to form a membrane-spanning co-receptor complex to transduce extracellular signals to Fyn tyrosine kinase. As both F3 and Fyn regulate myelination, we investigated a role for PTPα in this process. Here, we report that both oligodendrocytes and neurons express PTPα that evenly distributes along myelinated axons of the spinal cord. The ablation of PTPα in vivo leads to early formation of transverse bands that are mainly constituted by F3 and Caspr along the axoglial interface. Notably, PTPα deficiency facilitates abnormal myelination and pronouncedly increases the number of non-landed oligodendrocyte loops at shortened paranodes in the spinal cord. Small axons, which are normally less myelinated, have thick myelin sheaths in the spinal cord of PTPα-null animals. Thus, PTPα may be involved in the formation of axoglial junctions and ensheathment in small axons during myelination of the spinal cord.     

“Axonal spheroids” in the spinal cord of normal rabbits

Summary Within the gray matter and the white matter of the spinal cord of apparently healthy rabbits, myelinated and unmyelinated axonal swellings, so called axonal spheroids, occur. Most of the spheroids contain mitochondria, dense bodies, vesicles and fragments of the tubular or smooth endoplasmic reticulum. In myelinated spheroids the process of swelling is effected by slippage of the myelin leaflets. At the periphery of the unmyelinated parts of the spheroids, synapses are regularly found. The presynaptic terminal bouton is formed by the spheroid. A few myelinated and unmyelinated spheroids are packed with fine granular material while mitochondria are lacking. The axonal spheroids may represent a physiological, perhaps age dependent phenomenon.Dedicated to Prof. Dr. Berta Scharrer on the occasion of her 70^th birthdayThe author wishes to thank Mrs. Helga Zuther-Witzsch, Mrs. Elisabeth Schöngarth and Miss Hildegard Schöning for excellent technical assistance. Supported by the Deutsche Forschungsgemeinschaft, Projekt Le 69/7-13     

Is the vertebral canal prepared to host the aged spinal cord? A morphometric assessment

Although the interaction between the growing spinal cord and the vertebrae has been widely demonstrated for mammal’s prenatal and early postnatal life, there is no extensive knowledge about this interaction during late postnatal stages. It has been shown that spinal cord injuries are causally related to significant degenerative changes in bone properties. Nevertheless, information about a possible influence of the spinal cord on bone remodelling in adult healthy animals is missing. The aim of this research work was to assess possible morphological changes of the cervical vertebral canal of juvenile and aged rats during the ontogenetic period of adulthood that would justify the suggested influence. Since the spinal cord of rats increases its size with ageing, we analysed whether morphometric changes are occurring in the vertebral canal that would indicate bone remodelling in response to said growth. To this end, we used three complementary morphometric methods to describe the canal of the cervical and the first thoracic vertebrae. Geometric morphometric analyses evidence scarce variation in size and shape between juvenile and aged rats suggesting that, in general terms, the canal morphology of cervical vertebrae is already prepared in early adulthood to host the growing spinal cord. C3 was the only vertebra that showed consistent variation for the variables of canal thickness, perimeter, height and area. This regional variation may be linked to the patterns described for the changing spinal cord.     

Histochemical study of the pre—and postnatal development of acetylcholinesterase in the rat spinal cord

The distribution of acetylcholinesterase(AChE)-positive structures in the developing rat spinal cord was studied with AChE-histochemistry.AChE-positive perikarya were first seen on embryonic day 14(E14) in the ventrolateral portion of the spinal cord.From that time onward.AChE=containing cells appeared gradually in the intermediate gray,dorsal horn and lateral spinal nucleus of the spinal cord in a ventral-to-dorsal,and lateral-to-medial order.No obvious rostral-to-caudal sequence was found.At birth,the distribution pattern of AChE-positive perikarya was basically similar to that in adults.After birth a dramatic increase in the AChE staining intensity extended from postnatal day 5(P5) to postnatal day 21(P21),In addition,two phases of transient AChE staining were observed in the external surface of the dorsal horn from embryonic day 15(E15) to embryonic day 21(E21) and in the marginal layer from embryonic day 21(E21) to postnatal day 14(P14),respectively.     

Cystathionine β-synthase in structural elements of the human brain and spinal cord

By means of the immunohistochemical method, the presence and distribution of cystathionine β-synthase (CBS) was studied in nerve cells of the spinal cord and brainstem nuclei in eight men aged 18–44 years who had died as a result of causes not connected with damage to the central nervous system. CBS-positive neurons are revealed in all studied brain parts, in which their content varied in different nuclei from 0.9 to 17%. Large cells of motor nuclei more often had high and very high density of the reaction product deposition. In sensory nuclei the high portion was of small neurons with low intensity of the enzymatic reaction.     

Primary afferent depolarization distribution of the γ-aminobutyric acid system in frog spinal cord

In the frog spinal cord primary afferent depolarization (PAD) constitutes a powerful inhibitory control mechanism. It has been suggested that -aminobutyric acid (GABA) is the transmitter substance involved in the genesis of PAD. In these studies we show that maximal glutamic acid decarboxylase activity is localized roughly 400–600 m from the dorsal surface, and that correlates well with the intraspinal distribution of field potentials associated with PAD. Measurement of GABA in serial spinal cord sections cut in a dorsal-ventral direction shows that high levels of GABA are seen at 400–600 m, with a peak at 800 m from the dorsal surface. Stimulation at frequencies shown to produce PAD augments the release of endogenous GABA from a superfused frog hemicord preparation.     

Propagation of activity along the “stepping strip” of the cat spinal cord

Three points located approximately 8 mm apart were identified in a dorsolateral funiculus of the lower thoracic spinal cord in mesencephalic cats, each producing stepping movements on the ipsilateral hindlimb when stimulated. An area 5–17 mm caudal to the caudal stepping point (SP) was scanned for neurons responding synaptically to stimulating the rostral or caudal SP prior and subsequent to electrolytic coagulation of the medial SP. Relative incidence of neurons excited by stimulating the caudal SP did not change following this type of lesioning, although stimulation of the rostral SP at the rate of 4 Hz induced response 5 times less frequently than before. Even stimulation of the rostral SP at the rate of 40–60 Hz, which had considerably increased firing index prior to coagulation, could only produce excitation in tiny numbers of neurons. This indicates that synaptic excitation of neurons becomes considerably more difficult once the stepping strip between stimulation and recording sites has been damaged.Institute for Research into Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 20, No. 6, pp. 763–769, November–December, 1988.     

Δ-9-Tetrahydrocannabinol increases prodynorphin and proenkephalin gene expression in the spinal cord of the rat

Hypoalgesia induced by cannabinoid drugs has been found to implicate the opioid system. The effect of five days treatment with Δ-9-tetrahydrocannabinol (THC) was examined on prodynorphin (PDYN) and proenkephalin (PENK) gene expression in the spinal cord of male rats. PDYN and PENK gene expression was estimated measuring by northern blot analysis mRNA levels in the whole spinal cord, containing perikarya of these neurons. The subchronic treatment with THC (5 mg/kg/day; 5 days; i.p.) produced an increase in PDYN (39%) and PENK (34%) gene expression when compared with the vehicle treated group. These results suggest that the effects of THC in the spinal cord involve an increase in opioid activity, and therefore sustain the hypothesis of an interaction between the cannabinoid and opioid systems in this region.     

Mammalian DSCAMs: roles in the development of the spinal cord,cortex, and cerebellum?

Central nervous system (CNS) development involves neural patterning, neuronal and axonal migrations, and synapse formation. DSCAM, a chromosome 21 axon guidance molecule, is expressed by CNS neurons during development and throughout adult life. We now report that DSCAM and its chromosome 11 paralog DSCAML1 exhibit inverse ventral-dorsal expression patterns in the developing spinal cord and distinct, partly inverse, expression patterns in the developing cortex, beginning in the Cajal-Retzius cells. In the adult cortex, DSCAM predominates in layer 3/5 pyramidal cells and DSCAML1 predominates in layer 2 granule cells. In the cerebellum, DSCAM is stronger in the Purkinje cells and DSCAML1 in the granule cells. Finally, we find that the predicted DSCAML1 protein contains 60 additional N-terminal amino acids which may contribute to its distinct expression pattern and putative function. We propose that the DSCAMs comprise novel elements of the pathways mediating dorsal-ventral patterning and cell-fate specification in the developing CNS.     

Colocalization of prostaglandin F2�� receptor FP and prostaglandin F synthase-I in the spinal cord

Prostaglandin F_2α is synthesized by prostaglandin F synthase, which exists in two types, prostaglandin F synthase I (PGFS I) and prostaglandin F synthase II (PGFS II). Prostaglandin F_2α binds to its specific receptor, FP. Our previous immunohistochemical study showed the distinct localization of prostaglandin F synthases in rat spinal cord. PGFS I exists in neuronal somata and dendrites in the gray substance, and PGFS II exists in ependymal cells and tanycytes surrounding the central canal. Both enzymes are also present in endothelial cells of blood vessels in the white and gray substances of the spinal cord. In this study, we found that FP localizes in neuronal somata and dendrites but not in ependymal cells, tanycytes, or endothelial cells. Immunohistochemical analysis of serial sections showed the colocalization of FP and PGFS I. FP immunoreactivity was intense in spinal laminae I and II of the dorsal horn, a connection site of pain transmission, and was similar to that of PGFS I in neuronal elements. These findings suggest that prostaglandin F_2α synthesized in the neuronal somata and dendrites exert an autocrine action there.—Suzuki-Yamamoto, T., K. Toida, Y. Sugimoto, and K. Ishimura. Colocalization of prostaglandin F_2α receptor FP and prostaglandin F synthase-I in the spinal cord.     

Poro-elastic modeling of Syringomyelia – a systematic study of the effects of pia mater,central canal,median fissure,white and gray matter on pressure wave propagation and fluid movement within the cervical spinal cord

Syringomyelia, fluid-filled cavities within the spinal cord, occurs frequently in association with a Chiari I malformation and produces some of its most severe neurological symptoms. The exact mechanism causing syringomyelia remains unknown. Since syringomyelia occurs frequently in association with obstructed cerebrospinal fluid (CSF) flow, it has been hypothesized that syrinx formation is mechanically driven. In this study we model the spinal cord tissue either as a poro-elastic medium or as a solid linear elastic medium, and simulate the propagation of pressure waves through an anatomically plausible 3D geometry, with boundary conditions based on in vivo CSF pressure measurements. Then various anatomic and tissue properties are modified, resulting in a total of 11 variations of the model that are compared. The results show that an open segment of the central canal and a stiff pia (relative to the cord) both increase the radial pressure gradients and enhance interstitial fluid flow in the central canal. The anterior median fissure, anisotropic permeability of the white matter, and Poisson ratio play minor roles.     

Effect of aging on the substance P receptor,NK–1, in the spinal cord of rats with peripheral nerve injury

Substance P (SP) levels in the spinal cords of very old rats are less than the levels in younger rats (Bergman et al., 1996). After injury to a peripheral nerve in young rats, immunoreactivity (ir) to the SP receptor, NK–1 (neurokinin-1), increases in the spinal cord ipsilateral to the injury and the increases are correlated with the development of thermal hyperalgesia (Goff et al., 1998). Thus we postulated that aged rats might display an increased sensitivity to thermal stimulation before peripheral nerve injury and that they might respond differently to injury than do younger rats. To test this hypothesis, we used the Bennett and Xie model (1988) of chronic constriction injury (CCI) to the sciatic nerve to induce a neuropathic pain condition. We investigated the effect of age on changes in NK-1 ir in superficial layers of the dorsal horn and on numbers of NK ir cells in deeper laminae at the L4-L5 levels of the spinal cord after CCI. NK-1 receptors were tagged immunohistochemically and their distribution quantified by use of computer-assisted image analysis. NK-1 ir changes were related to alterations in thermal and tactile sensitivity that developed after CCI in young, mature and aged (4-6, 14-16, and 24-26 months) Fischer F344 BNF1 hybrid rats. No differences in thermal or tactile sensitivity of young and aged rats were seen in the absence of nerve injury. After injury, aged rats developed thermal hyperalgesia and tactile allodynia more slowly than did the younger rats. NK-1 receptor ir and numbers of NK-1 ir cells in the dorsal horn increased with time post-injury in all three groups. NK-1 ir increases were correlated with the development of thermal hyperalgesia in those rats that displayed hyperalgesia. However, some rats developed an increased threshold to thermal stimuli (analgesia) and that also was correlated with increases in NK-1 ir. Thus NK-1 ir extent, while correlated with thermal sensitivity in the absence of injury, is not a specific marker for disturbances in one particular sensory modality; rather it increases with peripheral nerve injury per se.     

Actions of endomorphins on synaptic transmission of aδ-fibers in spinal cord dorsal horn neurons

The effects of endogenous mu-opioid ligands, endomorphins, on Adelta-afferent-evoked excitatory postsynaptic currents (EPSCs) were studied in substantia gelatinosa neurons in spinal cord slices. Under voltage-clamp conditions, endomorphins blocked the evoked EPSCs in a dose-dependent manner. To determine if the block resulted from changes in transmitter release from glutamatergic synaptic terminals, the opioid actions on miniature excitatory postsynaptic currents (mEPSCs) were examined. Endomorphins (1 microM) reduced the frequency but not the amplitude of mEPSCs, suggesting that endomorphins directly act on presynaptic terminals. The effects of endomorphins on the unitary (quantal) properties of the evoked EPSCs were also studied. Endomorphins reduced unitary content without significantly changing unitary amplitude. These results suggest that in addition to presynaptic actions on interneurons, endomorphins also inhibit evoked EPSCs by reducing transmitter release from Adelta-afferent terminals.