Development of rostrocaudal dendritic bundles in rat thoracic spinal cord: analysis of cholinergic sympathetic preganglionic neurons

Interrelationships between spinal sympathetic preganglionic neurons, autonomic systems and electrical synapses formed by connexin36-containing gap junctions

Spinal sympathetic preganglionic neurons (SPNs) are among the many neuronal populations in the mammalian central nervous system (CNS) where there is evidence for electrical coupling between cell pairs linked by gap junctions composed of connexin36 (Cx36). Understanding the organization of this coupling in relation to autonomic functions of spinal sympathetic systems requires knowledge of how these junctions are deployed among SPNs. Here, we document the distribution of immunofluorescence detection of Cx36 among SPNs identified by immunolabelling of their various markers, including choline acetyltransferase, nitric oxide and peripherin in adult and developing mouse and rat. In adult animals, labelling of Cx36 was exclusively punctate and dense concentrations of Cx36-puncta were distributed along the entire length of the spinal thoracic intermediolateral cell column (IML). These puncta were also seen in association with SPN dendritic processes in the lateral funiculus, the intercalated and central autonomic areas and those within and extending medially from the IML. All labelling for Cx36 was absent in spinal cords of Cx36 knockout mice. High densities of Cx36-puncta were already evident among clusters of SPNs in the IML of mouse and rat at postnatal days 10-12. In Cx36^BAC::eGFP mice, eGFP reporter was absent in SPNs, thus representing false negative detection, but was localized to some glutamatergic and GABAergic synaptic terminals. Some eGFP⁺ terminals were found contacting SPN dendrites. These results indicate widespread Cx36 expression in SPNs, further supporting evidence of electrical coupling between these cells, and suggest that SPNs are innervated by neurons that themselves may be electrically coupled.

Heterogeneity of membrane properties in sympathetic preganglionic neurons of neonatal mice: evidence of four subpopulations in the intermediolateral nucleus

Spinal cord sympathetic preganglionic neurons (SPNs) integrate activity from descending and sensory systems to determine the final central output of the sympathetic nervous system. The intermediolateral column (IML) has the highest number and density of SPNs and, within this region, SPN somas are found in distinct clusters within thoracic and upper lumbar spinal segments. Whereas SPNs exhibit a rostrocaudal gradient of end-target projections, individual clusters contain SPNs with diverse functional roles. Here we explored diversity in the electrophysiological properties observed in Hb9-eGFP-identified SPNs in the IML of neonatal mice. Overall, mouse SPN intrinsic membrane properties were comparable with those seen in other species. A wide range of values was obtained for all measured properties (up to a 10-fold difference), suggesting that IML neurons are highly differentiated. Using linear regression we found strong correlations between many cellular properties, including input resistance, rheobase, time constant, action potential shape, and degree of spike accommodation. The best predictor of cell function was rheobase, which correlated well with firing frequency-injected current (f-I) slopes as well as other passive and active membrane properties. The range in rheobase suggests that IML neurons have a recruitment order with stronger synaptic drives required for maximal recruitment. Using cluster analysis, we identified at least four subpopulations of SPNs, including one with a long time constant, low rheobase, and high f-I gain. We thus propose that the IML contains populations of neurons that are differentiable by their membrane properties and hypothesize they represent diverse functional classes.

Sympathetic-correlated c-Fos expression in the neonatal rat spinal cord in vitro

An isolated thoracic spinal cord of the neonatal rat in vitro spontaneously generates sympathetic nerve discharge (SND) at ~25 degrees C, but it fails in SND genesis at < or = 10 degrees C. Basal levels of the c-Fos expression in the spinal cords incubated at < or = 10 degrees C and ~25 degrees C were compared to determine the anatomical substrates that might participate in SND genesis. Cells that exhibited c-Fos immunoreactivity were virtually absent in the spinal cords incubated at < or = 10 degrees C. However, in the spinal cords incubated at ~25 degrees C, c-Fos-positive cells were found in the dorsal laminae, the white matter, lamina X, and the intermediolateral cell column (IML). Cell identities were verified by double labeling of c-Fos with neuron-specific nuclear protein (NeuN), glial fibrillary acidic protein (GFAP), or choline acetyltransferase (ChAT). The c-Fos-positive cells distributed in the white matter and lamina X were NeuN-negative or GFAP-positive and were glial cells. Endogenously active neurons showing c-Fos and NeuN double labeling were scattered in the dorsal laminae and concentrated in the IML. Double labeling of c-Fos and ChAT confirmed the presence of active sympathetic preganglionic neurons (SPNs) in the IML. Suppression of SND genesis by tetrodotoxin (TTX) or mecamylamine (MECA, nicotinic receptor blocker) almost abolished c-Fos expression in dorsal laminae, but only mildly affected c-Fos expression in the SPNs. Therefore, c-Fos expression in some SPNs does not require synaptic activation. Our results suggest that spinal SND genesis is initiated from some spontaneously active SPNs, which are capable of TTX- or MECA-resistant c-Fos expression.

Developmental and injury-induced expression of alpha1beta1 and alpha6beta1 integrins in the rat spinal cord

Loss and damage to blood vessels are thought to contribute to secondary tissue loss after spinal cord injury. Integrins might be therapeutic targets to protect the vasculature and/or promote angiogenesis, as their activation can promote tubule formation and survival of endothelial cells in vitro. Here, we show that immunostaining with an antibody against the alpha1beta1 integrin heterodimer is present only in blood vessels from postnatal day 1 (P1) through adulthood in Sprague-Dawley rats. After a spinal cord contusion at T9 in adults, the area of alpha1beta1 integrin positive blood vessels increases within 11 mm from the injury site at 3 days post-injury and remains prominent within the injured core only at 7 days. Staining for the alpha6beta1 integrin heterodimer increases in blood vessels between P10 and adulthood and is present in preganglionic neurons of the intermediolateral cell column (IML) at all ages. The alpha6beta1 integrin is also expressed by motor neurons postnatally, and oligodendrocyte precursors (OPCs), as previously reported. After the contusion, the area of alpha6beta1-stained blood vessels is increased at 3 days and most prominently, 1 mm from the injury site, followed by a significant reduction at 7 days, when alpha6beta1 integrin staining is most prominent around the injured core. Staining is also present in a subset of microglia and/or macrophages. These results raise the possibility that alpha1beta1 and alpha6beta1 integrins in blood vessels might be targeted to reduce blood vessel loss and promote angiogenesis, which may promote tissue sparing after spinal cord injury.

Neural activity between ovaries and the prevertebral celiac-superior mesenteric ganglia varies during the estrous cycle of the rat

The ovaries' innervation arrives via the superior ovarian nerve, which originates from the celiac ganglion. Using True Blue as an antidromic marker, the present study analyzed the changes in the anatomical relation between each ovary and the prevertebral celiac-superior mesenteric ganglia during the estrous cycle. The number of labeled neurons increased from the day of diestrus 1 to the day of proestrus. The largest number of labeled cells was observed when tracer was injected into the left ovary on proestrus. The number of labeled cells was significantly higher when the tracer was injected into the left ovary on proestrus than when it was done in the right one. When tracer was injected into the left ovary, the average labeled area of cells increased significantly from diestrus 1 to proestrus, and declined at estrus. In contrast, when True Blue was injected into the right ovary, the average labeled area was similar in diestrus 1 and diestrus 2, and the values increased in proestrus and estrus. The results indicate an apparent asymmetry in the activity of neural connections between ovaries and the prevertebral celiac-superior mesenteric ganglia, and that the number of active neurons of these connections varies during the estrous cycle.

Evidence for a cell-specific action of Reelin in the spinal cord

Reelin, the extracellular matrix protein missing in reeler mice, plays an important role in neuronal migration in the central nervous system. We examined the migratory pathways of phenotypically identified spinal cord neurons to determine whether their positions were altered in reeler mutants. Interneurons and projection neurons containing choline acetyltransferase and/or NADPH diaphorase were studied in E12.5-E17.5 reeler and wild-type embryos, and their final locations were assessed postnatally. While three groups of dorsal horn interneurons migrated and differentiated normally in reeler mice, the migrations of both sympathetic (SPNs) and parasympathetic preganglionic neurons (PPNs) were aberrant in the mutants. Initially reeler and wild-type SPNs were detected laterally near somatic motor neurons, but by E13.5, many reeler SPNs had mismigrated medially. Postnatally, 79% of wild-type SPNs were found laterally, whereas in reeler, 92% of these neurons were positioned medially. At E13.5, both reeler and wild-type PPNs were found laterally, but by E14.5, reeler PPNs were scattered across the intermediate spinal cord while wild-type neurons correctly maintained their lateral location. By postnatal day 16, 97% of PPNs were positioned laterally in wild-type mice; in contrast, only 62% of PPNs were found laterally in mutant mice. In E12.5-E14.5 wild-type mice, Reelin-secreting cells were localized along the dorsal and medial borders of both groups of preganglionic neurons, but did not form a solid barrier. In contrast, Dab1, the intracellular adaptor protein thought to function in Reelin signaling, was expressed in cells having positions consistent with their identification as SPNs and PPNs. In combination, these findings suggest that, in the absence of Reelin, both groups of autonomic motor neurons migrate medially past their normal locations, while somatic motor neurons and cholinergic interneurons in thoracic and sacral segments are positioned normally. These results suggest that Reelin acts in a cell-specific manner on the migration of cholinergic spinal cord neurons.

Manipulation of intracellular calcium has no effect on rate of migration of rat autonomic motor neurons in organotypic slice cultures

Migration of neurons is a key step in the formation of the central nervous system, and an increase in internal Ca(2+) concentration has been shown to increase the rate of migration of granule cells along radial glial processes in slices of postnatal cerebellum. In embryonic spinal cord, the non-radial migration of autonomic motor neurons from the ventral horn dorsally into the region of the intermediolateral nucleus differs from that of granule cells, so it is possible that the role of Ca(2+) may also differ in the migration of these two types of neurons. To investigate this possibility, we made organotypic slice cultures of thoracic spinal cord from rat embryos. In control slices after about one day in vitro, diaphorase-positive autonomic motor neurons had migrated 100 microm at a rate of 3.6 microm/h. In experimental slice cultures, we added pharmacological reagents that are known to either increase or decrease internal Ca(2+) levels, including some reagents used successfully in the aforementioned granule cell studies. None of the nine reagents had a significant effect on migration speed of autonomic motor neurons in slice cultures. Our results suggest that autonomic motor neuron migration is not regulated by internal Ca(2+) levels, and hence this mechanism may not be used universally by all types of neurons.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Differential vulnerability of autonomic and somatic motor neurons to N-methyl-D-aspartate-induced excitotoxicity

Two closely-related subsets of spinal motor neurons are differentially vulnerable in the degenerative neurological disease amyotrophic lateral sclerosis. Autonomic motor neurons (i.e. preganglionic sympathetic neurons) survive in this disorder, whereas most spinal somatic motor neurons do not. The present study was undertaken in order to begin to understand the phenotypic differences between the two motor neuronal subsets which might contribute to this differential survival. Organotypic slice cultures of postnatal rat thoracic spinal cord were maintained in defined medium for one to 12 days in the presence or absence of N-methyl-D-aspartate or its antagonist, D-amino-phosphonopentanoic acid. Autonomic motor neurons that were stained for either nicotinamide adenine dinucleotide phosphate reduced diaphorase or choline acetyltransferase only were both able to tolerate 50 microM N-methyl-D-aspartate treatment for over seven days in culture with no apparent adverse effects. In contrast, cultures maintained for only one day in medium containing 50 microM N-methyl-D-aspartate showed a dramatic and highly significant decrease in the numbers of neurofilament-positive somatic motor neurons, as well as nicotinamide adenine dinucleotide phosphate reduced diaphorase-positive interneurons. These N-methyl-D-aspartate-induced effects were dose-dependent and blockable. The results of this investigation indicated that autonomic motor neurons and somatic motor neurons were differentially susceptible to N-methyl-D-aspartate-induced excitotoxicity, and that the resistance of autonomic motor neurons to this insult appeared to be independent of the nicotinamide adenine dinucleotide phosphate reduced diaphorase phenotype.

Voltage-dependent potassium currents of sympathetic preganglionic neurons in neonatal rat spinal cord thin slices

Voltage-dependent potassium currents were analyzed in the visually identified sympathetic preganglionic neurons (SPNs) of neonatal rat spinal cord thin slices by the whole-cell patch-clamp technique. Some of the SPNs were identified by the presence of retrogradely transported fluorescent dye, DiI, injected into the superior cervical ganglion several days prior to experimentation. In a tetrodotoxin (TTX)-containing solution, a step depolarization from the holding potential of -72 mV generated a slow outward current that was suppressed by tetraethylammonium (TEA) and by Ca(2+)-free/2.5 ImM Co2+ solution. Ca(2+)-dependent current consisted of a transient and a sustained components. In a Ca(2+)-free (substituted with Mg2+) solution with TTX and TEA, a step depolarization from a hyperpolarized potential evoked a transient outward current that was blocked by 4-aminopyridine (4-AP). A step hyperpolarization evoked a voltage-dependent inward current, the conductance of which was dependent not only on the membrane potential, but also on the extracellular K+ concentration. Tail current analyses revealed that all of these currents were carried by K+ ions. These results indicate that SPN possesses at least five types of voltage-dependent K+ current, including the delayed rectifier current (IK), Ca(2+)-dependent transient current (IC), Ca(2+)-dependent sustained current (IAHP), A-current (IA) and inward rectifying current (Iu), which may be targets of putative transmitters released from various descending and segmental inputs impinging upon the SPN.

Development and segmental organization of rostrocaudal dendrites of rat sympathetic preganglionic neurons

The longitudinal organization of preganglionic sympathetic neurons in the adult mammalian spinal cord takes the general form of a ladder. In the rat, the preganglionic neurons of the intermediolateral cell column (IML) have extensive dendritic arborizations in both rostrocaudal and mediolateral directions. We have studied the development of the rostrocaudal dendritic projection by retrogradely labeling single sympathetic preganglionic spinal segments with the membrane label DiI. The rostrocaudal dendrites of sympathetic preganglionic neurons in the IML begin to develop prenatally on embryonic day 19-20 (E19/20), several days after these neurons develop mediolaterally oriented dendrites. Between E19 and postnatal day 1 (P1), the rostrocaudal dendrites attain a length of approx. 200 microns. As the rostrocaudal dendrites elongate, the preganglionic neurons form distinct clusters between which dendritic bundles are seen. Following a growth spurt from E19 to P1, the average rostrocaudal dendritic length approximates twice the distance between clusters from P1 to P9.

Transient and continuous expression of NADPH diaphorase in different neuronal populations of developing rat spinal cord

Nitric oxide is a novel intercellular messenger whose role in neuronal development is not yet known. As a first step toward elucidating its developmental function, we examined the pattern of NADPH diaphorase histochemical staining, an indicator of the presence of nitric oxide synthase, in the rat spinal cord at pre and postnatal ages. Some types of neurons expressed diaphorase activity transiently during development. For example, a subset of somatic motor neurons, located in the ventrolateral corner of a few caudal segments of the cervical spinal cord, were diaphorase-positive beginning on E15, but gradually became diaphorase-negative by birth. In contrast, other spinal neurons expressed diaphorase activity continuously from development into adulthood. Preganglionic autonomic motor neurons became diaphorase-positive early in their development, as they were migrating toward their adult positions. Other spinal neurons, such as those in superficial dorsal horn, first expressed diaphorase relatively late in their development, after reaching their final location. The transient expression in some cell types, as well as the early expression in others, suggest that nitric oxide may have an important role(s) during development, which may differ from its functions in the adult nervous system.

A comparison between the adult rat and neonate rat of the architecture of sympathetic preganglionic neurones projecting to the superior cervical ganglion, stellate ganglion and adrenal medulla

Sympathetic preganglionic neurones (SPN) projecting to the superior cervical ganglion (SCG) and adrenal medulla (AM) in the neonate (< 14 days) and SCG, stellate ganglion (SG) and AM in the adult rat (> 3 months) were retrogradely labelled with cholera B horseradish peroxidase (CBHRP). Labelled neurones were found in 4 four distinct nuclei: the nucleus intermediolateralis thoracolumbalis pars principalis (ILp), a nucleus equivalent to the intemediolateral cell column (IML); the nucleus intermediolateralis thoracolumbalis pars funicularis (ILf); the nucleus intercalatus spinalis (IC) and the nucleus intercalatus pars paraependymatis (ICpe) or central autonomic area (CA). These were represented to a similar extent in both neonate and adult. Neonate and adult SCG, SG and AM-SPN had a similar segmental distribution cervical 8 (C8) to thoracic 5 (T5) for SCG-SPN and thoracic 3 (T3) to thoracic (T13) for AM-SPN whereas adult SG-SPN were distributed over segments C8 to T9. Most labelled neurones (70%) were located in the ILp with one segment containing the highest proportion of SPN. Three morphologically distinct neurones were evident. Fusiform and roundbodied were the most common. Fusiform somata of the ILp were orientated both mediolaterally and rostrocaudally in the neonate but only rostrocaudally in the adult. Dendrites of the SPN in the adult and neonate extended in a dense rostrocaudal band along the ILp, more diffusely into the white matter of the Ilf and in bundles medially towards the central canal (CC). The neonate showed some significant differences. In the ILp, the cell bodies were less tightly packed into a narrow band and into clusters and the dendrites were more diffuse. It was concluded that at 12 days postnatally the organisation of the sympathetic nuclei had still nor reached the adult form. However, there is no extensive realignment of dendrites in the adult so the ILp remains an 'open' nucleus like the neonate.