Localization of NADPH diaphorase/nitric oxide synthase and choline acetyltransferase in the spinal cord of the frog,Rana perezi

Pattern of nitrergic cells and fibers organization in the central nervous system of the Australian lungfish, Neoceratodus forsteri (Sarcopterygii: Dipnoi)

The Australian lungfish Neoceratodus forsteri is the only extant species of the order Ceratodontiformes, which retained most of the primitive features of ancient lobe finned-fishes. Lungfishes are the closest living relatives of land vertebrates and their study is important for deducing the neural traits that were conserved, modified, or lost with the transition from fishes to land vertebrates. We have investigated the nitrergic system with neural nitric oxide synthase (NOS) immunohistochemistry and NADPH-diaphorase (NADPH-d) histochemistry, which yielded almost identical results except for the primary olfactory projections and the terminal and preoptic nerve fibers labeled only for NADPH-d. Combined immunohistochemistry was used for simultaneous detection of NOS with catecholaminergic, cholinergic, and serotonergic structures, aiming to establish accurately the localization of the nitrergic elements and to assess possible interactions between these neurotransmitter systems. The results demonstrated abundant nitrergic cells in the basal ganglia, amygdaloid complex, preoptic area, basal hypothalamus, mesencephalic tectum and tegmentum, laterodorsal tegmental nucleus, reticular formation, spinal cord, and retina. In addition, low numbers of nitrergic cells were observed in the olfactory bulb, all pallial divisions, lateral septum, suprachiasmatic nucleus, prethalamic and thalamic areas, posterior tubercle, pretectum, torus semicircularis, cerebellar nucleus, interpeduncular nucleus, the medial octavolateral nucleus, nucleus of the solitary tract, and the dorsal column nucleus. Colocalization of NOS and tyrosine hydroxylase was observed in numerous cells of the ventral tegmental area/substantia nigra complex. Comparison with other vertebrates, using a neuromeric analysis, reveals that the nitrergic system of Neoceratodus shares many neuroanatomical features with tetrapods and particularly with amphibians.

Organization of the nitrergic neuronal system in the primitive bony fishes Polypterus senegalus and Erpetoichthys calabaricus (Actinopterygii: Cladistia)

Cladistians are a group of basal actinopterygian fishes that constitute a good model for studying primitive brain features, most likely present in the ancestral bony fishes. The analysis of the nitrergic neurons (with the enzyme nitric oxide synthase; NOS) has helped in understanding important aspects of brain organization in all vertebrates studied. We investigated the nitrergic system of two cladistian species by means of specific antibodies against NOS and NADPH-diaphorase (NADPH-d) histochemistry, which, with the exception of the primary olfactory and terminal nerve fibers, labeled only for NADPH-d, yielded identical results. Double immunohistochemistry was conducted for simultaneous detection of NOS with tyrosine hydroxylase, choline acetyltransferase, calbindin, calretinin, and serotonin, to establish accurately the localization of the nitrergic neurons and fibers and to assess possible interactions between these neuroactive substances. The pattern of distribution in both species showed only subtle differences in the density of labeled cells. Distinct groups of NOS-immunoreactive cells were observed in pallial and subpallial areas, paraventricular region, tuberal and retromammillary hypothalamic areas, posterior tubercle, prethalamic and thalamic areas, optic tectum, torus semicircularis, mesencephalic tegmentum, interpeduncular nucleus, superior and middle reticular nuclei, magnocellular vestibular nucleus, solitary tract nucleus, nucleus medianus magnocellularis, the spinal cord and amacrine cells in the retina. Large neurons in cranial nerve sensory ganglia were also labeled. The comparison of these results with those from other vertebrates, using a neuromeric analysis, reveals a conserved pattern of organization of the nitrergic system from this primitive fish group to amniotes, including mammals.

A comparative cluster analysis of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry in the brains of amphibians

Nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) is a key enzyme in the synthesis of the gaseous neurotransmitter nitric oxide. We compare the distribution of NADPH-d in the brain of four species of hylid frogs. NADPH-d-positive fibers are present throughout much of the brain, whereas stained cell groups are distributed in well-defined regions. Whereas most brain areas consistently show positive neurons in all species, in some areas species-specific differences occur. We analyzed our data and those available for other amphibian species to build a matrix on NADPH-d brain distribution for a multivariate analysis. Brain dissimilarities were quantified by using the Jaccard index in a hierarchical clustering procedure. The whole brain dendrogram was compared with that of its main subdivisions by applying the Fowlkes-Mallows index for dendrogram similarity, followed by bootstrap replications and a permutation test. Despite the differences in the distribution map of the NADPH-d system among species, cluster analysis of data from the whole brain and hindbrain faithfully reflected the evolutionary history (framework) of amphibians. Dendrograms from the secondary prosencephalon, diencephalon, mesencephalon, and isthmus showed some deviation from the main scheme. Thus, the present analysis supports the major evolutionary stability of the hindbrain. We provide evidence that the NADPH-d system in main brain subdivisions should be cautiously approached for comparative purposes because specific adaptations of a single species could occur and may affect the NADPH-d distribution pattern in a brain subdivision. The minor differences in staining pattern of particular subdivisions apparently do not affect the general patterns of staining across species.

Neuroanatomical organization of the cholinergic system in the central nervous system of a basal actinopterygian fish, the senegal bichir Polypterus senegalus

Polypterid bony fishes are believed to be basal to other living ray-finned fishes, and their brain organization is therefore critical in providing information as to primitive neural characters that existed in the earliest ray-finned fishes. The cholinergic system has been characterized in more advanced ray-finned fishes, but not in polypterids. In order to establish which cholinergic neural centers characterized the earliest ray-finned fishes, the distribution of choline acetyltransferase (ChAT) is described in Polypterus and compared with the distribution of this molecule in other ray-finned fishes. Cell groups immunoreactive for ChAT were observed in the hypothalamus, the habenula, the optic tectum, the isthmus, the cranial motor nuclei, and the spinal motor column. Cholinergic fibers were observed in both the telencephalic pallium and the subpallium, in the thalamus and pretectum, in the optic tectum and torus semicircularis, in the mesencephalic tegmentum, in the cerebellar crest, in the solitary nucleus, and in the dorsal column nuclei. Comparison of the data within a segmental neuromeric context indicates that the cholinergic system in polypterid fishes is generally similar to that in other ray-finned fishes, but cholinergic-positive neurons in the pallium and subpallium, and in the thalamus and cerebellum, of teleosts appear to have evolved following the separation of polypterids and other ray-finned fishes.

Origins of spinal cholinergic pathways in amphibians demonstrated by retrograde transport and choline acetyltransferase immunohistochemistry

The existence of propriospinal cholinergic pathways and the origin of supraspinal cholinergic descending projections have been investigated in anuran and urodele amphibians. Retrograde tract tracing techniques with dextran amines injected in the spinal cord at different levels were combined with immunohistochemistry for choline acetyltransferase (ChAT). The analysis of the brachial, thoracic and lumbar spinal cord demonstrated that doubly labeled cells were present only close to the injection site. Thus, the participation of the spinal cholinergic cells in distant intersegmental connections is not present, or is very limited, in amphibians. In anurans, tracer applications to the brachial cord revealed cholinergic cells of origin of spinal projections located in four distinct brain nuclei. The most rostrally located cells were found bilaterally in the preoptic area, among the magnocellular cells. In the ipsilateral isthmic region, the laterodorsal tegmental nucleus also showed doubly labeled cells. Throughout the brainstem, abundant codistribution was observed but actual coexistence of the tracer and ChAT was only found in the nucleus of the solitary tract and the inferior reticular nucleus. In the case of the urodele, abundant codistribution between retrogradely labeled cells and ChAT-positive neurons in zones like the suprachiasmatic nucleus, the isthmic region and the rhombencephalic reticular formation was observed, but the only doubly labeled cells were the Mauthner neurons. The present results in amphibians contrast with previous data in mammals in which is striking the presence of a widespread intrinsic cholinergic innervation of the spinal cord and the virtual absence of cholinergic projections descending from the brainstem.

Nitric Oxide Synthase and NADPH Diaphorase Distribution in the Bullfrog (Rana catesbeiana) CNS: Pathways and Functional Implications

The gas nitric oxide (NO) is emerging as an important regulator of normal physiology and pathophysiology in the central nervous system (CNS). The distribution of cells releasing NO is poorly understood in non-mammalian vertebrates. Nitric oxide synthase immunocytochemistry (NOS ICC) was thus used to identify neuronal cells that contain the enzyme required for NO production in the amphibian brain and spinal cord. NADPH-diaphorase (NADPHd) histochemistry was also used because the presence of NADPHd serves as a reliable indicator of nitrergic cells. Both techniques revealed stained cells in all major structures and pathways in the bullfrog brain. Staining was identified in the olfactory glomeruli, pallium and subpallium of the telencephalon; epithalamus, thalamus, preoptic area, and hypothalamus of the diencephalon; pretectal area, optic tectum, torus semicircularis, and tegmentum of the mesencephalon; all layers of the cerebellum; reticular formation; nucleus of the solitary tract, octaval nuclei, and dorsal column nuclei of the medulla; and dorsal and motor fields of the spinal cord. In general, NADPHd histochemistry provided better staining quality, especially in subpallial regions, although NOS ICC tended to detect more cells in the olfactory bulb, pallium, ventromedial thalamus, and cerebellar Purkinje cell layer. NOS ICC was also more sensitive for motor neurons and consistently labeled them in the vagus nucleus and along the length of the rostral spinal cord. Thus, nitrergic cells were ubiquitously distributed throughout the bullfrog brain and likely serve an essential regulatory function.

Distribution of NADPH-diaphorase and nitric oxide synthase reactivity in the central nervous system of the goldfish (Carassius auratus)

The nitrergic system has been inferred from cells positive to nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry and/or to the neuronal isoform of nitric oxide synthase (nNOS) immunohistochemistry in different species of vertebrates. The aim of the present work was to systematically study the distribution of cell producing nitric oxide in the goldfish (Carassius auratus) brain. To reach this goal, we firstly studied co-localization for NADPHd and nNOS techniques and demonstrated an extensive double labeling. Then, we studied the distribution through the brain by the two separate methods and found labeled cells widely distributed in brain and spinal cord. In the telencephalon, such cells were in both dorsal and ventral areas. In the diencephalon, the cells were found in some nuclei of the preoptic area and hypothalamus, habenula, pretectum, and dorsal and ventral thalamic regions. In the midbrain, cells were observed in the optic tectum, torus longitudinalis, and tegmental nuclei. In the rhombencephalon, cells were found in the cerebellum, the reticular formation, the locus coeruleus, the raphe nuclei, and the nuclei of the cranial nerves. Labeled cells were also observed in the gray area of the spinal cord. Cognizing that a direct comparison of the present results with those reported in other vertebrates is not clear-cut because of homologies; we conclude that the nitrergic system is roughly similar from fish to mammals.

The Sympathetic Nervous System of Anamniotes

The sympathetic nervous system develops as an evolutionary trait with gnathostomes (jawed vertebrates), but not with agnathan fishes (i.e., hagfishes and lampreys). Organization of the sympathetic preganglionic neuronal columns is different in teleosts and anurans. In the teleosts so far examined, the majority of sympathetic preganglionic neurons (SPNs) are located in the dorsal part of the spinal central gray matter. In Tetraodontiformes, the cell column occupies only two rostral spinal segments, which are distinct in their cytoarchitecture and projections. On the other hand, the SPNs of anurans form two cell columns segregated mediolaterally. The lateral and medial columns are also distinct in their cytoarchitecture and projections. The neuroactive substances expressed in the SPNs both in teleosts and anurans are coded to the projections. In anurans, the SPNs containing gonadotrophin-releasing hormone and those containing calcitonin gene-related peptide are involved in the regulation of blood vessels and cutaneous glands, respectively. In the filefish, the SPNs containing galanin project specifically to non-adrenergic non-cholinergic postganglionic neurons in the cranial sympathetic ganglia. Therefore, both anuran and teleost systems have different morphological and chemical-coded patterns for functional variation, although the anuran sympathetic nervous system has more organizational similarity with that of amniotes.

Developmental and regional expression of NADPH-diaphorase/nitric oxide synthase in spinal cord neurons correlates with the emergence of limb motor networks in metamorphosing Xenopus laevis

Metamorphosis in anuran amphibians requires a complete transformation in locomotor strategy from undulatory tadpole swimming to adult quadrupedal propulsion. The underlying reconfiguration of spinal networks may be influenced by various neuromodulators including nitric oxide, which is known to play an important role in CNS development and plasticity in diverse species, including metamorphosis of amphibians. Using NADPH-diaphorase (NADPH-d) staining and neuronal nitric oxide synthase (nNOS) immunofluorescence labelling, the expression and developmental distribution of NOS-containing neurons in the spinal cord and brainstem were analysed in all metamorphic stages of Xenopus laevis. Wholemount preparations of the spinal cord from early stages of metamorphosis (coincident with emergence of the fore- and hindlimb buds) revealed two clusters of NOS-positive neurons interspersed with areas devoid of stained somata. These cells were distributed in three topographic subgroups, the most ventral of which had axonal projections that crossed the ventral commissure. Motoneurons innervating the fore- and hindlimb buds were retrogradely labelled with horseradish peroxidase (HRP) to determine their position in relation to the two NOS-expressing cord regions. Limb motoneurons and NOS-positive cells did not overlap, indicating that during early stages of metamorphosis nitrergic neurons are excluded from regions where spinal limb circuits are forming. As metamorphosis progresses, NOS expression became distributed along the length of the spinal cord together with an increase in the number and intensity of labelled cells and fibers. NOS expression reached a peak as the forelimbs emerge then declined. These findings are consistent with a role for nitric oxide (NO) in the developmental transition from undulatory swimming to quadrupedal locomotion.

Immunohistochemical localization of calbindin-D28k and calretinin in the spinal cord of Xenopus laevis

Immunohistochemical techniques were used to investigate the distribution and morphology of neurons containing the calcium-binding proteins calbindin-D28k (CB) and calretinin (CR) in the spinal cord of Xenopus laevis and determine the extent to which this organization is comparable to that of mammals. Most CB- and CR-containing neurons were located in the superficial dorsal gray field, but with distinct topography. The lateral, ventrolateral, and ventromedial fields also possessed abundant neurons labeled for either CB or CR. Double immunohistofluorescence demonstrated that a subpopulation of dorsal root ganglion cells and neurons in the dorsal and ventrolateral fields contained CB and CR. By means of a similar technique, a cell population in the dorsal field was doubly labeled only for CB and nitric oxide synthase (NOS), whereas in the ventrolateral field colocalization of NOS with CB and CR was found. Choline acetyltransferase immunohistochemistry revealed that a subpopulation of ventral horn neurons, including motoneurons, colocalized CB and CR. The involvement of CB- and CR-containing neurons in ascending spinal projections was demonstrated combining the retrograde transport of dextran amines and immunohistochemistry. Cells colocalizing the tracer and CB or CR were quite numerous, primarily in the dorsal and ventrolateral fields. Similar experiments demonstrated supraspinal projections from CB- and CR-containing cells in the brainstem and diencephalon. The distribution, projections, and colocalization with neurotransmitters of the neuronal systems containing CB and CR in Xenopus suggest that CB and CR are important neuromodulator substances with functions conserved in the spinal cord from amphibians through mammals.

Partial cloning of neuronal nitric oxide synthase (nNOS) cDNA and regional distribution of nNOS mRNA in the central nervous system of the Nile tilapia Oreochromis niloticus

A constitutive NOS complementary DNA (cDNA) was partially cloned by RT-PCR from the brain of a teleost, the Nile tilapia (Oreochromis niloticus), using degenerate primers against conserved regions of NOS. The predicted 206-long amino acid sequence showed a high degree of identity with other vertebrate neuronal NOS (nNOS) protein sequences. In addition, phylogenetic analysis revealed that Nile tilapia NOS clustered with other known nNOS. Using the coupled reaction of semi-quantitative RT-PCR and Southern blotting, the basal tissue expression pattern of the cloned nNOS gene was investigated in discrete areas of the central nervous system (CNS) and in the heart and skeletal muscle tissue. As revealed, expression of nNOS transcripts was detected in all the CNS regions examined, whereas nNOS gene was not expressed in the heart and skeletal muscle. The distribution pattern of nNOS gene expression showed the highest expression levels in the forebrain followed by the optic tectum, the brainstem and the spinal cord, whereas scarce expression was detected in the cerebellum. Cellular expression of nNOS mRNA was analyzed in the CNS by means of in situ hybridization. According to the RT-PCR results, most nNOS mRNA expressing neurons are localized in the telencephalon and diencephalon, whereas in the mesencephalic optic tectum, the brainstem and the spinal cord, nNOS mRNA expressing neurons are relatively more scattered. A very low hybridization signal was detected in the cerebellar cortex. These results suggest that NO is involved in numerous brain functions in teleosts.

Presence and role of nitric oxide in the central nervous system of the freshwater snail Planorbarius corneus: possible implication in neuron–microglia communication

The aim of the present study was to investigate the involvement of nitric oxide (NO) as a messenger molecule in neuron-microglia communication in the central nervous system (CNS) of the freshwater snail Planorbarius corneus. The presence of both neuronal (nNOS) and inducible nitric oxide synthase (iNOS) was studied using NADPH-diaphorase (NADPH-d) histochemistry and NOS immunocytochemistry. The experiments were performed on whole ganglia and cultured microglial cells after different activation modalities, such as treatment with lipopolysaccharide and adenosine triphosphate and/or maintaining ganglia in culture medium till 7 days. In sections, nNOS immunoreactivity was found only in neurons and nNOS-positive elements were less numerous than NADPH-d-positive ones, with which they partially overlapped. The iNOS immunoreactivity was observed only after activation, in both nerve and microglial cells. We also found that the number of iNOS-immunoreactive neurons and microglia varied, depending on the activation modalities. In microglial cell cultures, iNOS was expressed in the first generation of cells only after activation, whereas a second generation, proliferated after ganglia activation, expressed iNOS even in the unstimulated condition.

Nitric oxide synthase expression and cell changes in dorsal root ganglia and spinal dorsal horn of developing and adultRana esculenta indicate a role of nitric oxide in limb metamorphosis

Metamorphosis of amphibians requires reconfiguration of sensory and locomotor neural networks. In view of such plastic changes and implications of nitric oxide (NO) in neural developmental shaping, we examined via histochemistry and immunohistochemistry its synthetic enzyme nitric oxide synthase (NOS) in dorsal root ganglia (DRGs) and dorsal horn of the developing and adult frog Rana esculenta. In limb DRGs, NOS positivity was first and selectively detected just before limb bud appearance, increased during metamorphosis, and was then down-regulated. In adulthood, NOS was expressed in some DRG neurons at all segmental levels. Similar features were detected in the dorsal horn neuropil. In limb DRGs, cell counts in Nissl-stained sections revealed a twofold increase of differentiated neurons during metamorphosis and an additional twofold increase in adulthood. Perikaryal sizes in limb DRGs did not vary during metamorphosis but increased and were more heterogeneous in the adult frog, probably reflecting adaptation to body size. NOS and cell changes during metamorphosis were much less marked in DRGs at other levels. Carbocyanine tracing documented selective labeling of NOS-expressing hindlimb DRG neurons from the spinal nerve at the time of initiation of hindlimb movements. The findings show that, in limb DRG neurons, NOS parallels cell differentiation and limb development during metamorphosis. The data also provide evidence of NOS expression in DRG cells innervating the hindlimbs when sensorimotor circuits become functionally mature. This study indicates a key role of NO production in the maturation of sensory functions that subserves in amphibians the transition from swimming to tetrapod locomotion.

Distribution of NADPH-diaphorase/nitric oxide synthase in the brain of the caecilian Dermophis mexicanus (amphibia: gymnophiona): comparative aspects in amphibians

The organization of nitrergic systems in the brains of anuran and urodele amphibians was recently studied and significant differences were noted between both amphibian orders. However, comparable data are not available for the third order of amphibians, the gymnophionans (caecilians). In the present study we have investigated the distribution of neuronal elements that express nitric oxide synthase (NOS) in the brain of the gymnophionan amphibian Dermophis mexicanus by means of immunohistochemistry with specific antibodies against NOS and enzyme histochemistry for NADPH-diaphorase. Both techniques yielded identical results and were equally suitable to demonstrate the nitrergic system. In addition, they were useful tools in the identification of cell groups and brain structures, otherwise indistinct in the brains of caecilians. The distribution of nitrergic structures observed in Dermophis conforms to the overall amphibian pattern but numerous distinct peculiarities were also noted. These included a dense innervation of the olfactory bulbs but a lack of reactivity in olfactory and vomeronasal fibers and glomeruli. A large population of nitrergic cells in the striatum and the presence of thalamic neurons, as well as the specific distribution of nitrergic cells in the isthmic region, are some of the differential features in the gymnophionan brain. Given the variability among species in the same class of vertebrates any discussion including amphibians should also include evidence for gymnophionans.

Choline acetyltransferase immunoreactivity in the developing brain of Xenopus laevis

The spatiotemporal sequence of the appearance of cholinergic structures in the brain of Xenopus laevis during development was studied by means of choline acetyltransferase (ChAT) immunohistochemistry. The first ChAT labeling in the central nervous system of Xenopus was obtained at late embryonic stages in the spinal motoneurons, the cranial nerve motor nuclei of the brainstem, and in amacrine cells of the retina. During premetamorphosis, these cholinergic structures maturated significantly and new ChAT-immunoreactive cells were observed in several other nuclei such as the solitary tract nucleus, isthmic nucleus, laterodorsal and pedunculopontine tegmental nuclei, epiphysis, dorsal habenular nucleus, medial amygdala, bed nucleus of the stria terminalis, and dorsal pallidum. Further maturation continued through prometamorphosis and the climax of the metamorphosis together with the appearance of new cell groups in the efferent octaval nucleus, ventral hypothalamic nucleus, anterior preoptic area, suprachiasmatic nucleus, and medial septum. Transient expression of ChAT was only seen in the large Mauthner cells that showed moderate ChAT labeling during pre- and prometamorphosis but became immunonegative at the end of the metamorphosis. The gradual appearance, in general from caudal to rostral brain levels, of ChAT immunoreactivity in Xenopus, was correlated with other developmental events to get insight into the possible roles of acetylcholine during ontogeny. Comparison with the developmental pattern of cholinergic systems in other vertebrates shows that Xenopus possesses abundant features in common with amniotes, suggesting a conservative developmental plan for tetrapods.

Localization of choline acetyltransferase (ChAT) immunoreactivity in the brain of a caecilian amphibian, Dermophis mexicanus (Amphibia: Gymnophiona)

The organization of the cholinergic system in the brain of anuran and urodele amphibians was recently studied, and significant differences were noted between both amphibian orders. However, comparable data are not available for the third order of amphibians, the limbless gymnophionans (caecilians). To further assess general and derived features of the cholinergic system in amphibians, we have investigated the distribution of choline acetyltransferase immunoreactive (ChAT-ir) cell bodies and fibers in the brain of the gymnophionan Dermophis mexicanus. This distribution showed particular features of gymnophionans such as the existence of a particularly large cholinergic population in the striatum, the presence of ChAT-ir cells in the mesencephalic tectum, and the organization of the cranial nerve motor nuclei. These peculiarities probably reflect major adaptations of gymnophionans to a fossorial habit. Comparison of our results with those in other vertebrates, including a segmental approach to correlate cell populations across species, shows that the general pattern of organization of cholinergic systems in vertebrates can be modified in certain species in response to adaptative processes that lead to morphological and behavioral modifications of members of a given class of vertebrates, as shown for gymnophionans.

The regional distribution of nitric oxide synthase activity in the spinal cord of the dog

The aim of this study was to examine the distribution of calcium-dependent nitric oxide synthase activity (cNOS) in the white and gray matter in cervical, thoracic, lumbar and sacral segments of the spinal cord and cauda equina of the dog. The enzyme's activity, measured by the conversion of [3H]arginine to [3H]citrulline revealed considerable region-dependent differences along the rostrocaudal axis of the spinal cord in general and in cervical (C1, C2, C4, C6 and C8) and lumbar (L1-L3, L4-L7) segments in particular. In the non-compartmentalized spinal cord, the cNOS activity was lowest in the thoracic and highest in the sacral segments. No significant differences were noted in the gray matter regions (dorsal horn, intermediate zone and ventral horn) and the white matter columns (dorsal, lateral and ventral) in the upper cervical segments (C1-C4), except for a significant increase in the ventral horn of C4 segment. In C6 segment, the enzyme's activity displayed significant differences in the intermediate zone, ventral and lateral columns. Surprisingly, extremely high cNOS activity was noted in the dorsal horn and dorsal column of the lowest cervical segment. Comparing the enzyme's activity in upper and lower lumbar segments of the spinal cord, cNOS activity prevailed in L4-L7 segments in the dorsal horn and in all the above mentioned white matter columns.

Ontogeny of NADPH diaphorase/nitric oxide synthase reactivity in the brain of Xenopus laevis

The development of nitric oxide synthase (NOS) expression in the brain of Xenopus laevis tadpoles was studied by means of immunohistochemistry using specific antibodies against NOS and enzyme histochemistry for nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase. Both techniques yielded identical results and were equally suitable for demonstrating the nitrergic system in the brain. The only mismatches were observed in the olfactory nerve and glomeruli and in the terminal nerve; they were intensely labeled with the NADPH-diaphorase technique but failed to stain with NOS immunohistochemistry. As early as stage 33, nitrergic cells were observed in the caudal rhombencephalon within the developing inferior reticular nucleus. At later embryonic stages, different sets of reticular and tegmental neurons were labeled in the middle reticular nucleus and, more conspicuously, in the laterodorsal and pedunculopontine tegmental nuclei. As development proceeded, new nitrergic cell groups gradually appeared in the mesencephalon, diencephalon, and telencephalon. A general caudorostral temporal sequence was observed, both in the whole brain and within each main brain subdivision. The premetamorphic period was mainly characterized by the maturation of the cell populations developed in the embryonic period. During prometamorphosis, the nitrergic system reached an enormous development, and many new cell groups were observed for the first time, in particular in the telencephalon. By the climax of metamorphosis, the pattern of organization of nitrergic cells and fibers observed in the brain was similar to that present in the adult brain. Transient expression of NOS was not detected in any brain region. Our data suggest that nitric oxide plays an important role during brain development of Xenopus. Comparison with the developmental pattern of nitrergic systems in other vertebrates shows that amphibians possess more common features with amniotes than with anamniotes.

Development of NADPH-diaphorase/nitric oxide synthase in the brain of the urodele amphibian Pleurodeles waltl

In the present study, the ontogenesis of nitrergic neurons has been studied in the urodele amphibian Pleurodeles waltl by means of NADPH-diaphorase (NADPHd) histochemistry and neuronal nitric oxide synthase (NOS) immunohistochemistry. Embryonic and larval stages were studied. Except for the olfactory fibers and glomeruli, both methods were equally suitable to reveal nitrergic structures in the brain. The earliest positive neurons were observed in the inferior reticular nucleus (Ri) in the caudal rhombencephalon at embryonic stage 30. At stage 33b, weakly reactive cells appeared in the tegmentum of the mesencephalon and isthmus, in the ventral hypothalamus (VH), and in the proximity of the solitary tract (sol). At initial larval stages (stages 34-38), two new groups appeared in the caudal telencephalon (future amygdaloid complex (Am)) and in the middle reticular nucleus (Rm) of the rhombencephalon. During the active larval life (stages 39-55c) the nitrergic system developed progressively both in number of cells and fiber tracts. At stages 39-42 reactive cells were found in the inner granular layer (igl) of the olfactory bulb, the telencephalic pallium, the pretectal region, the optic tectum (OT) and retina. New populations of nitrergic cells appear during the second half of the larval period (stages 52-55). Rostrally, reactive cells were found in the telencephalic diagonal band (DB) nucleus, medial septum and in the thalamic eminence (TE), whereas caudally cells appeared in the raphe (Ra) and the descending trigeminal nucleus (Vd). The last changes occurred during the juvenile period (metamorphic climax), when cells of the spinal cord (sc) and the preoptic area became positive. The sequence of appearance of nitrergic cells revealed a first involvement of this system in reticulospinal control, likely influencing locomotor behavior. As development proceeds, cells in different sensory systems expressed progressively nitric oxide synthase in a pattern that shows many similarities with amniotes.

Origin and development of descending catecholaminergic pathways to the spinal cord in amphibians

The origin and development of the supraspinal catecholaminergic (CA) innervation of the spinal cord was studied in representative species of the three amphibian orders (Anura: Xenopus laevis and Rana perezi; Urodela: Pleurodeles waltl; Gymnophiona: Dermophis mexicanus). Using retrograde dextran amine tracing in combination with tyrosine hydroxylase (TH)-immunohistochemistry, we showed that only four brain centers contribute to the CA innervation of the adult spinal cord: (1) the ventrolateral component of the posterior tubercle, (2) the periventricular nucleus of the zona incerta, (3) the locus coeruleus, and (4) the nucleus of the solitary tract (except for gymnophionans). The pattern observed is largely similar in all amphibian species studied. The development of the CA innervation of the spinal cord was studied with in vitro double labeling methods in Xenopus laevis tadpoles. At stage 40/41, the first CA neurons projecting to the spinal cord were found to originate in the posterior tubercle. At stage 43, spinal projections were found from the periventricular nucleus of the zona incerta and the locus coeruleus, whereas spinal projections from the nucleus of the solitary tract were not observed before stage 53. These results demonstrate a temporal sequence in the appearance of the CA cell groups projecting to the anuran spinal cord, organized along a rostrocaudal gradient.

Spatiotemporal pattern of nicotinamide adenine dinucleotide phosphate-diaphorase reactivity in the developing central nervous system of premetamorphic Xenopus laevis tadpoles

We have catalogued the progressive appearance of putative nitric oxide synthase (NOS)-containing neurons in the developing central nervous system (CNS) of Xenopus laevis. Xenopus embryos and larvae were processed in wholemount and in cross section using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry as a marker for NOS within the CNS. The temporal sequence of NADPH-d reactivity identified discrete groups and subgroups of neurons in the forebrain, midbrain, and hindbrain on the basis of their morphology, location, and order of appearance during development. A proportion of these groups of neurons appeared to be important in sensory processing and motor control. Staining also appeared at specific stages in the spinal cord, the retina, and the skin. After the appearance of labelling, NADPH-d reactivity continued in each of the cell groups throughout the stages examined. We found no evidence for staining that subsequently disappeared at later stages in any cell group, indicating a persistent rather than transient role for NO in the Xenopus tadpole CNS. These results are discussed in light of recent findings on possible roles for NADPH-d-positive cell groups within the developing motor circuitry.

Descending supraspinal pathways in amphibians. II. Distribution and origin of the catecholaminergic innervation of the spinal cord

Immunohistochemical studies with antibodies against tyrosine hydroxylase, dopamine, and noradrenaline have revealed that the spinal cord of anuran, urodele, and gymnophionan (apodan) amphibians is abundantly innervated by catecholaminergic (CA) fibers and terminals. Because intraspinal cells occur in all three orders of amphibians CA, it is unclear to what extent the CA innervation of the spinal cord is of supraspinal origin. In a previous study, we showed that many cell groups throughout the forebrain and brainstem project to the spinal cord of two anurans (the green frog, Rana perezi, and the clawed toad, Xenopus laevis), a urodele (the Iberian ribbed newt, Pleurodeles waltl), and a gymnophionan (the Mexican caecilian, Dermophis mexicanus). To determine the exact site of origin of the supraspinal CA innervation of the amphibian spinal cord, retrograde tracing techniques were combined with immunohistochemistry for tyrosine hydroxylase in the same sections. The double-labeling experiments demonstrated that four brain centers provide CA innervation to the amphibian spinal cord: 1.) the ventrolateral component of the posterior tubercle in the mammillary region, 2.) the periventricular nucleus of the zona incerta in the ventral thalamus, 3.) the locus coeruleus, and 4.) the nucleus of the solitary tract. This pattern holds for all three orders of amphibians, except for the CA projection from the nucleus of the solitary tract in gymnophionans. There are differences in the strength of the projections (based on the number of double-labeled cells), but in general, spinal functions in amphibians are controlled by CA innervation from brain centers that can easily be compared with their counterparts in amniotes. The organization of the CA input to the spinal cord of amphibians is largely similar to that described for mammals. Nevertheless, by using a segmental approach of the CNS, a remarkable difference was observed with respect to the diencephalic CA projections.