Cells of origin of pathways descending to the spinal cord in some quadrupedal reptiles

Organization of enkephalinergic neuronal system in the central nervous system of the gecko Hemidactylus frenatus

Enkephalins are endogenous opioid pentapeptides that play a role in neurotransmission and pain modulation in vertebrates. However, the distribution pattern of enkephalinergic neurons in the brains of reptiles has been understudied. This study reports the organization of the methionine-enkephalin (M-ENK) and leucine-enkephalin (L-ENK) neuronal systems in the central nervous system of the gecko Hemidactylus frenatus using an immunofluorescence labeling method. Although M-ENK and L-ENK-immunoreactive (ir) fibers extended throughout the pallial and subpallial subdivisions, including the olfactory bulbs, M-ENK and L-ENK-ir cells were found only in the dorsal septal nucleus. Enkephalinergic perikarya and fibers were highly concentrated in the periventricular and lateral preoptic areas, as well as in the anterior and lateral subdivisions of the hypothalamus, while enkephalinergic innervation was observed in the hypothalamic periventricular nucleus, infundibular recess nucleus and median eminence. The dense accumulation of enkephalinergic content was noticed in the pars distalis of the hypophysis. In the thalamus, the nucleus rotundus and the dorsolateral, medial, and medial posterior thalamic nuclei contained M-ENK and L-ENK-ir fibers, whereas clusters of M-ENK and L-ENK-ir neurons were observed in the pretectum, mesencephalon, and rhombencephalon. The enkephalinergic fibers were also seen in the area X around the central canal, as well as the dorsal and ventral horns. The widespread distribution of enkephalin-containing neurons within the central nervous system implies that enkephalins regulate a variety of functions in the gecko, including sensory, behavioral, hypophysiotropic, and neuroendocrine functions.

Organization of the galaninergic neuronal system in the brain of the gecko Hemidactylus frenatus

Galanin (GAL) is a 29 amino acid peptide present in the central nervous system (CNS) as well as peripheral tissues in vertebrates. However, the brain distribution pattern of GAL is understudied in reptiles. The aim of this study was to determine the organization of galaninergic neuronal system in the brain of the gecko Hemidactylus frenatus, a tropical and sub-tropical lizard, using rabbit anti-galanin antibody. In the telencephalon, GAL-ir perikarya and fibres were found in the lateral septal nucleus, but only GAL-ir fibres were observed in the striatum, nucleus accumbens, anterior commissure, nucleus centralis amygdalae, dorsal and medial septal nuclei, nucleus of the diagonal band of Broca and in the optic chiasma. In the preoptic region, a cluster of GAL-ir cells and fibres was observed in the periventricular preoptic area and lateral preoptic area. GAL-ir perikarya and fibres were observed in hypothalamic areas such as the supraoptic nucleus, suprachiasmatic nucleus, paraventricular nucleus, periventricular nucleus of the hypothalamus, infundibular recess nucleus and in the median eminence, whereas GAL-ir fibres were present in the pars distalis of the pituitary gland. In the thalamus, GAL-ir fibres were observed in the dorsomedial, dorsolateral, and medial thalamic nuclei. GAL-ir fibres were also detected in mesencephalic areas such as the optic tectum, torus semicircularis, ventral tegmental area and substantia nigra, brain stem as well as the spinal cord. The organization of GAL-ir cells and fibres throughout the gecko brain suggests several neuroendocrine, neuromodulatory and behavioural functions for GAL in lizards. The study provides new insights into the evolutionarily conserved nature of GAL peptide in squamate reptiles and forms a valuable basis for future comparative studies.

Red nucleus structure and function: from anatomy to clinical neurosciences

The red nucleus (RN) is a large subcortical structure located in the ventral midbrain. Although it originated as a primitive relay between the cerebellum and the spinal cord, during its phylogenesis the RN shows a progressive segregation between a magnocellular part, involved in the rubrospinal system, and a parvocellular part, involved in the olivocerebellar system. Despite exhibiting distinct evolutionary trajectories, these two regions are strictly tied together and play a prominent role in motor and non-motor behavior in different animal species. However, little is known about their function in the human brain. This lack of knowledge may have been conditioned both by the notable differences between human and non-human RN and by inherent difficulties in studying this structure directly in the human brain, leading to a general decrease of interest in the last decades. In the present review, we identify the crucial issues in the current knowledge and summarize the results of several decades of research about the RN, ranging from animal models to human diseases. Connecting the dots between morphology, experimental physiology and neuroimaging, we try to draw a comprehensive overview on RN functional anatomy and bridge the gap between basic and translational research.

Intrathecal administration of clonidine or yohimbine decreases the nociceptive behavior caused by formalin injection in the marsh terrapin (Pelomedusa subrufa)

BACKGROUND

The role of noradrenergic system in the control of nociception is documented in some vertebrate animals. However, there are no data showing the role of this system on nociception in the marsh terrapins.

METHODOLOGY

In this study, the antinociceptive action of intrathecal administration of the α 2-adrenoreceptor agonist clonidine and α 2-adrenoreceptor antagonist yohimbine was evaluated in the African marsh terrapin using the formalin test. The interaction of clonidine and yohimbine was also evaluated.

RESULTS

Intrathecal administration of clonidine (37.5 or 65 μg/kg) caused a significant reduction in the mean time spent in pain-related behavior. Yohimbine, at a dose of 25 μg/kg, significantly blocked the effect of clonidine (65 μg/kg). However, administration of yohimbine (40 or 53 μg/kg) caused a significant reduction in the mean time spent in pain-related behavior. Intrathecal administration of yohimbine (53 μg/kg) followed immediately by intrathecal injection of the serotonergic methysergide maleate (20 μg/kg) resulted in a significant reversal of the antinociceptive effect of yohimbine.

CONCLUSION

The present study documented the intrathecal administration of drugs in the marsh terrapin, a technique that can be applied in future studies on these animals. The data also suggest the involvement of both α 2-adrenoreceptors and 5HT receptors in the modulation of nociception in testudines.

Concepts and methods for the study of axonal regeneration in the CNS

Progress in the field of axonal regeneration research has been like the process of axonal growth itself: there is steady progress toward reaching the target, but there are episodes of mistargeting, misguidance along false routes, and connections that must later be withdrawn. This primer will address issues in the study of axonal growth after central nervous system injury in an attempt to provide guidance toward the goal of progress in the field. We address definitions of axonal growth, sprouting and regeneration after injury, and the research tools to assess growth.

The Edinger-Westphal nucleus: a historical, structural, and functional perspective on a dichotomous terminology

The eponymous term nucleus of Edinger-Westphal (EW) has come to be used to describe two juxtaposed and somewhat intermingled cell groups of the midbrain that differ dramatically in their connectivity and neurochemistry. On one hand, the classically defined EW is the part of the oculomotor complex that is the source of the parasympathetic preganglionic motoneuron input to the ciliary ganglion (CG), through which it controls pupil constriction and lens accommodation. On the other hand, EW is applied to a population of centrally projecting neurons involved in sympathetic, consumptive, and stress-related functions. This terminology problem arose because the name EW has historically been applied to the most prominent cell collection above or between the somatic oculomotor nuclei (III), an assumption based on the known location of the preganglionic motoneurons in monkeys. However, in many mammals, the nucleus designated as EW is not made up of cholinergic, preganglionic motoneurons supplying the CG and instead contains neurons using peptides, such as urocortin 1, with diverse central projections. As a result, the literature has become increasingly confusing. To resolve this problem, we suggest that the term EW be supplemented with terminology based on connectivity. Specifically, we recommend that 1) the cholinergic, preganglionic neurons supplying the CG be termed the Edinger-Westphal preganglionic (EWpg) population and 2) the centrally projecting, peptidergic neurons be termed the Edinger-Westphal centrally projecting (EWcp) population. The history of this nomenclature problem and the rationale for our solutions are discussed in this review.

Autonomic control of the eye and the iris

The vertebrate eye receives innervation from ciliary and pterygopalatine parasympathetic and cervical sympathetic ganglia as well as sensory trigeminal axons. The sympathetic and parasympathetic pathways represent the classical "core" of neural regulation of ocular homeostasis. Sensory trigeminal neurons are also involved in autonomic regulation by both providing the afferent limb of various reflexes and exerting their peptide-mediated local effector function. This arrangement is remarkably conserved throughout vertebrate classes although significant modifications are observed in anamniotes, in particular their irises. In higher primates and birds, intrinsic choroidal neurons emerged as a significant additional innervation component. They most likely mediate local vascular regulation and other local homeostatic tasks in foveate eyes. This review across the vertebrate classes outfolds the complex neuronal regulatory underpinnings across vertebrates that ensure proper visual function.

Location of Spinal Cord Pathways That Control Hindlimb Movement Amplitude and Interlimb Coordination During Voluntary Swimming in Turtles

We performed mechanical lesions of the midbody (D2–D3; second to third postcervical spinal segments) spinal cord in otherwise intact turtles to locate spinal cord pathways that 1) activate and control the amplitude of voluntary hindlimb swimming movements and 2) coordinate hindlimb swimming with the movement of other limbs. Pre- and postlesion turtles were held by a band clamp around the carapace just beneath the water surface in a clear Plexiglas tank and videotaped from below so that kinematic measurements could be made of voluntary forward swimming with motion analysis software. Movements of the forelimbs (wrists) and hindlimbs (knees and ankles) were tracked relative to stationary reference points on the plastron to obtain bilateral measurements of hip and forelimb angles as functions of time along with foot trajectories. We measured changes in limb movement amplitude, cycle period, and interlimb phase before and after spinal lesions. Our results indicate that locomotor command signals that activate and regulate the amplitude of voluntary hindlimb swimming travel primarily in the dorsolateral funiculus (DLF) at the D2–D3 level and cross over to drive contralateral hindlimb movements. This suggests that electrical stimulation of the D3 DLF, which was previously shown to evoke swimming movements in the contralateral hindlimb of low-spinal turtles, activated the same locomotor command pathways that the animal uses during voluntary behavior. We also show that forelimb–hindlimb coordination is maintained by longitudinal spinal pathways that are largely confined to the ventrolateral funiculus (VLF) and mediate phase coupling of ipsilateral limbs, presumably by interenlargement propriospinal fibers.

Immunohistochemical and hodological characterization of calbindin-D28k-containing neurons in the spinal cord of the turtle,Pseudemys scripta elegans

Neurons and fibers containing the calcium-binding protein calbindin-D28k (CB) were studied by immunohistochemical techniques in the spinal cord of adult and juvenile turtles, Pseudemys scripta elegans. Abundant cell bodies and fibers immunoreactive for CB were widely and distinctly distributed throughout the spinal cord. Most neurons and fibers were labeled in the superficial dorsal horn, but numerous cells were also located in the intermediate gray and ventral horn. In the dorsal horn, most CB-containing cells were located in close relation to the synaptic fields formed by primary afferents, which were not labeled for CB. Double immunohistofluorescence demonstrated distinct cell populations in the dorsal horn labeled only for CB or nitric oxide synthase, whereas in the dorsal part of the ventral horn colocalization of nitric oxide synthase was found in about 6% of the CB-immunoreactive cells in this region. Choline acetyltransferase immunohistochemistry revealed that only about 2% of the neurons in the dorsal part of the ventral horn colocalized CB, whereas motoneurons were not CB-immunoreactive. The involvement of CB-containing neurons in ascending spinal projections to the thalamus, tegmentum, and reticular formation was demonstrated combining the retrograde transport of dextran amines and immunohistochemistry. Similar experiments demonstrated supraspinal projections from CB-containing cells mainly located in the reticular formation but also in the thalamus and the vestibular nucleus. The revealed organization of the neurons and fibers containing CB in the spinal cord of the turtle shares distribution and developmental features, colocalization with other neuronal markers, and connectivity with other tetrapods and, in particular with mammals.

Calbindin-D28k and calretinin immunoreactivity in the spinal cord of the lizard Gekko gecko: Colocalization with choline acetyltransferase and nitric oxide synthase

The distribution of the calcium-binding proteins calbindin-D28k (CB) and calretinin (CR) was investigated in the spinal cord of the lizard Gekko gecko, by means of immunohistochemical techniques. Abundant cell bodies and fibers immunoreactive for either CB or CR were widely distributed throughout the spinal cord. Most neurons and fibers were labeled in the superficial dorsal horn, but numerous cells were also located in the intermediate gray and ventral horn. Distinct CB- and CR-containing cell populations were observed, although double immunohistochemistry revealed that 17-20% of the single-labeled cells for CB or CR in the dorsal horn contained both proteins. In addition, nitric oxide synthase was immunodetected in about 6% of the CB-positive neurons in the dorsal horn and in 10% in the ventral horn, whereas nitric oxide synthase was present in 9-13% of CR-positive cells in the dorsal horn and in 14% in the ventral horn. These doubly immunoreactive cells were restricted to areas IV, VII and VIII. Similar colocalization experiments revealed that 18-24% of the cholinergic cells in the ventral horn contained CB and 21-30% CR, with some variations throughout the length of the spinal cord. The pattern of distribution for CB and CR immunoreactivity in the spinal cord of the lizard, reported in the present study, is largely comparable to those reported for mammals, birds and anuran amphibians suggesting a high degree of conservation of the spinal systems modulated by these calcium-binding proteins.

Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract-tracing study

The connections of the cerebellum of the rainbow trout were studied by experimental methods. The pretectal paracommissural nucleus has reciprocal connections with the cerebellum. Three additional pretectal nuclei project to both the corpus and valvula cerebelli, and seem to receive cerebellar afferents. A large number of cells of the lateral nucleus of the valvula project to wide regions of the cerebellum, including the valvula, the corpus, the granular eminences, and the caudal lobe, whereas the contralateral inferior olive and scattered reticular cells project only to the corpus and valvula cerebelli. Afferents to the corpus were also observed from the ventral tegmental nucleus, the "paraisthmic nucleus," the perilemniscal nucleus, the central gray, and the octavolateral area. Valvular afferents were also observed from the torus semicircularis and the midbrain tegmental areas. In most cases of cerebellar application, labeled fibers were seen in the thalamus, the pretectum, the torus longitudinalis and torus semicircularis, the nucleus of the medial longitudinal fascicle, and midbrain and rhombencephalic reticular areas. From the corpus cerebelli some fibers also project to the posterior tubercle and the hypothalamus. Moreover, the granular eminences project to the cerebellar crest. DiI application to most of the areas showing labeled fibers after cerebellar tracer application led to the labeling of characteristic eurydendroid cells, mainly in the valvula cerebelli and the caudal lobe. A few putative eurydendroid cells were labeled from the octavolateralis regions. These results in a teleost with a generalized brain indicate several differences with respect to the cerebellar connections reported in other teleost fishes that have specialized brains.

Monoaminergic systems in the brainstem and spinal cord of the turtlePseudemys scripta elegansas revealed by antibodies against serotonin and tyrosine hydroxylase

With the aim of gaining more insight into the monoaminergic regulation of spinal motor systems in the turtle, we have studied the distribution of 5-HT (5-HTir) and tyrosine hydroxylase immunoreactivity (THir) in the brainstem and spinal cord of Pseudemys scripta elegans. 5-HTir cell bodies were located in the midline in nucleus raphe inferior, nucleus raphe superior, and laterally in nuclei reticularis superior and inferior and nucleus reticularis isthmi. THir cell bodies were located in the commissural nucleus, nucleus tractus solitarii, the locus coeruleus-subcoeruleus complex, nuclei reticularis superior and inferior, the pretectal area, and substantia nigra. 5-HTir and THir tracts were found in lateral and ventral bundles superficially in the brainstem. 5-HTir fibers in the spinal cord were located in a large dorsolateral and a smaller ventrolateral tract. In the gray matter, a high concentration of 5-HTir fibers were observed in areas I-IV and in the lateral motor column of cervical and lumbar enlargements. Areas V-VIII and area X were less intensively innervated, with the lowest fibre concentration in areas VII-VIII and area X. Throughout the spinal cord, THir nerve fibres were located in the same areas but with a lower density. Small bipolar 5-HTir and THir cell bodies were found ventromedially to the central canal especially in cervical and lumbosacral segments. Large THir cells were found in area IX in the caudal sacral and coccygeal spinal cord. THir cerebrospinal fluid-contacting cells were also found in the most caudal part of the brainstem and the upper cervical spinal cord. The well developed spinal 5-HT system and the less developed THir system provides an anatomical explanation for the monoaminergic modulation of turtle motoneuron membrane properties, which has been observed in electrophysiological experiments.

Evolutionary significance of different neurochemical organisation of the internal and external regions of auditory centres in the reptilian brain: an immunocytochemical and reduced NADPH-diaphorase histochemical study in turtles

An immunocytochemical and histochemical study was undertaken of the torus semicircularis and nucleus reuniens, the mesencephalic and diencephalic auditory centres, in two chelonian species, Testudo horsfieldi and Emys orbicularis. The nucleus centralis of the torus semicircularis receives few 5-HT-, TH-, substance P-, and menkephalin-immunoreactive fibres and terminals, in marked contrast to the external nucleus laminaris of the torus semicircularis, in which 5-HT-, TH-, substance P-, and menkephalin-immunoreactive elements and cell bodies show a laminar distribution. Dense NPY-positive terminal-like profiles and cell bodies were observed in both the nuclei centralis and laminaris, and many NADPH-d-positive cell bodies were observed in the cell layers of the latter. In the nucleus reuniens, the distribution of 5-HT-, TH-, substance P-, and menkephalin-immunolabelling resembles that seen in the torus semicircularis, but at a lower density. The dorsorostral regions of the nucleus reuniens, as in the nucleus centralis, is insignificantly labelled, in contrast to the ventrocaudal regions in which labelled elements abound. NPY-positive elements are uniformly distributed throughout the nucleus, but no labelled cell bodies were observed. NADPH-d-positive fibres and terminals were observed in both dorsal and ventral regions of the nucleus reuniens, but the few labelled cell bodies to be observed were located in the peripheral regions of the nucleus. These findings are discussed in terms of the evolution of the core-and-belt organisation of sensory nuclei observed in other vertebrate species.

Descending supraspinal pathways in amphibians. I. A dextran amine tracing study of their cells of origin

The present study is the first of a series on descending supraspinal pathways in amphibians in which hodologic and developmental aspects are studied. Representative species of anurans (the green frog, Rana perezi, and the clawed toad, Xenopus laevis), urodeles (the Iberian ribbed newt, Pleurodeles waltl), and gymnophionans (the Mexican caecilian, Dermophis mexicanus) have been used. By means of retrograde tracing with dextran amines, previous data in anurans were largely confirmed and extended, but the studies in P. waltl and D. mexicanus present the first detailed data on descending pathways to the spinal cord in urodeles and gymnophionans. In all three orders, extensive brainstem-spinal pathways were present with only minor representation of spinal projections originating in forebrain regions. In the rhombencephalon, spinal projections arise from the reticular formation, several parts of the octavolateral area, the locus coeruleus, the laterodorsal tegmental nucleus, the raphe nucleus, sensory nuclei (trigeminal sensory nuclei and the dorsal column nucleus), and the nucleus of the solitary tract. In all species studied, the cerebellar nucleus and scattered cerebellar cells innervate the spinal cord, predominantly contralaterally. Mesencephalic projections include modest tectospinal projections, torospinal projections, and extensive tegmentospinal projections. The tegmentospinal projections include projections from the nucleus of Edinger-Westphal, the red nucleus, and from anterodorsal, anteroventral, and posteroventral tegmental nuclei. In the forebrain, diencephalospinal projections originate in the ventral thalamus, posterior tubercle, the pretectal region, and the interstitial nucleus of the fasciculus longitudinalis medialis. The most rostrally located cells of origin of descending spinal pathways were found in the suprachiasmatic nucleus, the preoptic area and a subpallial region in the caudal telencephalic hemisphere, probably belonging to the amygdaloid complex. Our data are discussed in an evolutionary perspective.

Afferent and efferent connections of the torus semicircularis in the sea lamprey: an experimental study

The afferent and efferent connections of the torus semicircularis (TS) of larval sea lampreys were studied with horseradish peroxidase, carbocyanine dye (DiI) and fluorescein-coupled or Texas-Red-coupled dextran amine tract-tracing methods. Application of tracers to the TS or to the octavolateral area revealed the presence of bilateral projections from the octavolateral area to the torus semicircularis, mainly from the mechanoreceptive regions (medial and ventral octavolateral nuclei) though also from the electroreceptive (dorsal octavolateral nucleus) region. The nucleus of the descending root of the trigeminal nerve projects to the contralateral TS, mostly from neurons located rostral to the obex. Fairly numerous reticular cells of the rhombencephalon project to the torus semicircularis. In the mesencephalon, scattered cells in the tegmentum, and some in the tectum, have toral projections, mostly ipsilateral. Numerous thalamic neurons, as well as fairly numerous neurons of the posterior tubercle, hypothalamus and preoptic region, and a few neurons in the ventral telencephalon (striatum, septum), were labeled after tracer application to the TS. The torus semicircularis mainly projects to the thalamus, the hypothalamus and the reticular rhombencephalic nuclei. Our results reveal for the first time a complex pattern of connections of the lamprey TS, which suggests that it is a multisensory center integrating head cutaneous sensitivity with mechano- and electrosensory information from the octavolateral area and with visual information. A number of afferents from the forebrain also appear to contribute to TS function.

Descending neural projections to the spinal cord in the channel catfish, Ictalurus punctatus

Retrograde transport of horseradish peroxidase was used to determine the descending projections to the spinal cord in an otophysan fish, the channel catfish, Ictalurus punctatus. The majority of cells projecting to the spinal cord are located in the reticular formation, which is organized into rhombomeric segments. Vestibulospinal neurons are located in the descending, magnocellular, and tangential octaval nuclei, as well as in the medial octavolateralis nucleus of the lateral line system. Cells in the facial lobe project to the spinal cord. Additionally, axons of cells of the trigeminal system and the nucleus of the lateral lemniscus project caudally into the spinal cord. In the midbrain, descending spinal projections arise from cells of the medial longitudinal fasciculus and the red nucleus. More rostrally, cells of the ventrolateral thalamus, dorsal periventricular hypothalamus, central pretectal and magnocellular preoptic nuclei also project to the cord. The results of this study indicate that there are a number of homologies in the descending systems of bony fishes and other vertebrate taxa, including tetrapods. We also provide further evidence that a red nucleus is present in the brains of bony fishes and is therefore a primitive vertebrate character antedating the evolution of tetrapods.

A tract-tracing study of the central projections of the mesencephalic nucleus of the trigeminus in the guppy (Lebistes reticulatus, teleostei), with some observations on the descending trigeminal tract

We studied the central projections of the mesencephalic nucleus of the trigeminal nerve (MesV) in the guppy (Lebistes reticulatus), after application of horseradish peroxidase or fluorescein dextran amine into the eye orbit. A small number (1 to 13) of large mesencephalic trigeminal neurons were solid labeled in the ipsilateral rostral mesencephalon. At the level of the trigeminal nerve entrance, the united process of each mesencephalic trigeminal cell bifurcates, giving rise to a peripheral branch that exits in the trigeminal nerve and a descending branch that runs caudally in a medial bundle separated from the descending trigeminal tract. This bundle passes close to the visceromotor nuclei of the medulla oblongata. Descending processes give rise to short collaterals to the descending nucleus of the trigeminus and the ventrolateral reticular area. Most MesV descending fibres terminate in this ventrolateral field at the transition of the medulla to the spinal cord, but one or two fibres could be followed to the C6 level, where they give rise to collaterals to the dorsal funicular nucleus. No collaterals directed to the trigeminal motor nucleus, the cerebellum, or the mesencephalic tegmentum were observed. These projections were also compared with those of the descending trigeminal tract.

Organisation of the cerebellar nucleus of the dogfish, Scyliorhinus canicula L.: a light microscopic, immunocytochemical, and ultrastructural study

Elasmobranchs possess a well-developed cerebellum with an associated cerebellar nucleus. To determine whether the organization of this nucleus is comparable with that of the deep cerebellar nuclei of mammals, we studied the dogfish cerebellar nucleus with light microscopic methods (Nissl stain, Golgi method, reduced silver stain, NADPH-diaphorase histochemistry and immunocytochemistry) and with electron microscopy. We found the dogfish cerebellar nucleus to consist of about 1,050 large neurons, the ratio of Purkinje cells to cerebellar nucleus neurons being about 17:1. Immunocytochemistry showed large glutamatergic neurons in the main portions of the nucleus and small glutamate- and/or alpha-aminobutyric acid (GABA)-immunoreactive cells in the subventricular region of the nucleus. Large glutamatergic neurons corresponded to bipolar or triangular cells revealed by Golgi methods. Application of horseradish peroxidase to the cerebellar cortex produced the labelling of beaded fibres of Purkinje cells in the cerebellar nucleus. Unlike in mammals, GABAergic innervation of the cerebellar nucleus was scare: Purkinje cell axon terminals in the cerebellar nucleus did not appear to be GABA-immunoreactive, most GABAergic fibres being found in the subventricular neuropile. Some fibres immunoreactive to serotonin and somatostatin were also observed in the subventricular neuropile of the cerebellar nucleus. Three neuron types were distinguished with electron microscopy (types A to C). Type A cells were abundant and smooth-surfaced, and appeared to correspond to Golgi-impregnated neurons and large glutamate-immunoreactive cells. Type B neurons were scarce and possessed dendrites covered by sessile or stalked spines. Type C neurons were small cells located mainly in the medialmost region of the nucleus and corresponded to subventricular glutamate- and GABA-immunoreactive cells. Six types of synaptic bouton were observed (types I to VI). The most abundant (type I boutons) made symmetrical contacts and appeared to correspond to Purkinje cell axons. Type I boutons were the only type observed on perikarya and initial axon segments of type A cells. Type IV and type V boutons made complex glomerular-like asymmetrical contacts with spines of type B cells. Type VI boutons appeared to correspond to peptidergic and/or monoaminergic axons. The functional significance of these results is discussed.

Pre- and postmetamorphic organization of the vestibular nuclear complex in the turbot examined by retrograde tracer substances

During metamorphosis of flatfish larvae, eye migration leads to a 90 degrees misalignment of the visual and vestibular frames of reference. In order to maintain vestibular eye stabilization, the vestibulo-ocular (V-O) pathways have to be radically reorganized. Here, we have examined the vestibular projections in turbot larvae and juveniles by means of conventional neurohistological techniques using horseradish peroxidase and fluorescent dextranamines as tracers. We have found that the vestibular projections to the rostral eye motor nuclei consist of five densely clustered groups of neurons projecting to the rostral eye motor nuclei, some through the ipsilateral, others through the contralateral medial longitudinal fascicle (MLF). In addition, there are three groups of vestibulo-spinal neurons. The most prominent of these gives rise to the ipsilateral vestibulo-spinal tract. The other two project contralaterally, one descending in the MLF, the other more laterally in the anterior funiculus of the spinal cord. These subnuclei of the vestibular complex are easily identifiable in larvae before metamorphosis, as well as in juvenile turbots. The number of projection neurons in each of the subnuclei is approximately doubled over the period of metamorphosis. Applying different tracers to rostrally and caudally projecting pathways, we found no double-labeled neurons, indicating that the V-O and vestibulo-spinal groups are distinct entities. However, by applying the two tracers ipsi- and contralaterally in the terminal fields in the rostral eye motor nuclei after metamorphosis, we found many double-labeled neurons in all the V-O subgroups. In contrast, we found only a small fraction of double-labeled vestibular neurons when the same strategy was applied to larval preparations. We conclude that 1) the basic organization of the vestibular nuclei of the turbot is similar to that of other teleosts, in larvae as well as juveniles; 2) there is a substantial increase in projection neurons over the period of metamorphosis in all the subgroups of the vestibular nuclear complex; and 3) many more of the V-O neurons project bilaterally to the rostral eye motor nuclei in juvenile than in larval turbots.

Connections of the basal telencephalic areas c and d in the turtle brain

Tracer substances were injected into the basal telencephalic areas c and d of the turtle brain. These areas (Acd) have recently been shown to be connected reciprocally with the dorsal spino-medullary region, though the particular subregions involved in these projections remained unclear. We demonstrated that the efferent projections of area d terminate predominantly within or immediately adjacent to the trigeminal nuclear complex and in the high cervical spinal gray. The dendritic domain of the vagus-solitarius complex and the dorsal column nuclear complex might also receive some basal telencephalic efferents. The afferent projections to Acd, on the other hand, arise predominantly in the dorsal column nuclei as defined according to cytoarchitectural and hodological criteria. A few retrogradely labeled cells were found in the vagus-solitarius complex, the principal trigeminal nucleus and the high cervical spinal cord. Numerous labeled cells were found in the dorsolateral isthmo-rhombencephalic tegmentum, especially the n. visceralis secundarius, the n. vestibularis superior and parts of the lateral lemniscal complex. Aminergic cell populations projecting to Acd were the n. raphes inferior and superior, the locus coeruleus, the substantia nigra, pars compacta and the ventral tegmental area. Other meso-diencephalic cell groups were the griseum centrale (including the n. laminaris of the torus semicircularis), the n. interpeduncularis dorsalis, the nucleus of the fasciculus longitudinalis medialis, the nucleus and the nucleus interstitialis of flm, the n. interstitialis commissuralis posterior and then n. caudalis. Several hypothalamic regions, the reuniens complex and the perirotundal region of the thalamus also appeared to project heavily to Acd. Telencephalic areas retrogradely labeled after injection of tracer into Acd and its immediate surroundings were the rostral part of the lateral (olfactory) cortex, adjacent regions of the basal dorsal ventricular ridge and the n. centralis amygdalae, the n. tractus olfactorius lateralis as well as the areas g and h. The data suggest that areas c and d may correlate best with the 'extended' amygdala in mammals; further correlation with structures similar to the ventral striopallidum, however, cannot be excluded. Homostrategies are discussed with regard to the processing of higher-order somatovisceral information in turtles, birds and mammals.

A telencephalospinal projection in the Tegu lizard (Tupinambis teguixin)

Tegu lizards (Tupinambis teguixin) were studied to determine the presence of a homologue of the mammalian corticospinal tract. The sources of telencephalic efferent projections to the spinal cord were determined by evaluating the localization of retrogradely transported horseradish peroxidase applied in the cervical spinal cord. Labeled cells were present in subtelencephalic sites reported previously by other authors and, in addition, were found in the principal sensory and motor nuclei of the trigeminal nerve and in the nucleus of the posterior commissure. A telencephalospinal projection was identified, originating in the ventral caudal telencephalon. Histochemical staining revealed a high concentration of acetylcholinesterase in cells and neuropil in the same area. This tract is suggested to be homologous to the mammalian amygdalospinal tract. No reptilian homologue of the corticospinal tract was identified.

Descending pathways to the spinal cord: II. Quantitative study of the tectospinal tract in 23 mammals

To study the early evolution of the mammalian motor systems, we have collected quantitative data on the nuclear origins of tracts descending into the spinal cord in 99 individuals representing 23 species of mammals and one species of reptile. In each individual, the spinal cord was hemisected at the C1-C2 junction and raw HRP immediately applied to the cut fibers. After a 3-day survival period, brain and spinal cord sections were treated with conventional tetramethylbenzidine procedures. In every case, this procedure resulted in heavy retrograde labeling of neural somata throughout the neuraxis from coccygeal cord to cerebral neocortex. Many thousands of supraspinal neurons were vividly labeled within at least 27 discrete cell groups in every mammal (Nudo and Masterton, '88). Despite the vast number and wide diversity of heavily labeled neurons, however, relatively few labeled somata were found in the superior colliculus. The total number of labeled cells in the tectum contralateral to the hemisection was highest in the cat (909) and second highest in the raccoon (628). In the remaining animals, the number was considerably less--averaging only 243 in the 23 mammalian species, 193 in the 21 noncarnivores, and 95 in the iguana. In 7 species of primates the average was 220, and in 3 species of Old World monkeys the average was 142. This wide variation in the number of tectospinal neurons is not related to body size, brain size, or absolute and relative tectum size. Arranging the animals in order of their kinship or recency-of-last-common-ancestor with Man, the average number of labeled tectal cells tends to decrease slightly, whereas arranging the same animals in order of their kinship with the cat or raccoon shows a marked and statistically reliable increase. Neither the evolutionary increase in the tectospinal tract along the Carnivora lineage nor the slight decrease along Man's lineage is altered by mathematical corrections for allometric or scaling factors. Of an array of morphological, visual, motor, and ecological traits tested statistically as a possible source of the variation in size of the tectospinal tract, only a primarily carnivorous feeding preference was found to be reliably related. The relatively small number of tectospinal fibers in most mammals in our sample, including the primates, suggests that the tectospinal tract in Man may be quite small, perhaps far too small to warrant continuing description as a "major descending tract."

Origins of the descending spinal projections in petromyzontid and myxinoid agnathans

The origins of the descending spinal pathways in sea lampreys (Petromyzon marinus), silver lampreys (Ichthyomyzon unicuspis), and Pacific hagfish (Eptatretus stouti) were identified by the retrograde transport of horseradish peroxidase (HRP) placed in the rostral spinal cord. In lampreys, the majority of HRP-labeled cells were located along the length of the brainstem reticular formation in the inferior, middle, and superior reticular nuclei of the medulla, mesencephalic tegmentum, and nucleus of the medial longitudinal fasciculus. Labeled reticular cells included the Mauthner and Müller cells. Horseradish-peroxidase-filled cells were also present in the descending trigeminal tract, intermediate and posterior octavomotor nuclei, and a diencephalic cell group, the nucleus of the posterior tubercle. As in lampreys, the reticular formation of the Pacific hagfish was the largest source of descending afferents to the spinal cord. Labeled cells were found in the dorsolateral and ventromedial reticular nuclei, the dorsal tegmentum at the juncture of the medulla and midbrain, and the nucleus of the medial longitudinal fasciculus. Additional medullary cells projecting to the cord were located in the perivagal nucleus, the central gray, and the anterior and posterior magnocellular octavolateralis nuclei. The existence of reticulospinal and possible vestibulo-, trigemino-, and solitary spinal projections in lampreys and hagfishes and the wide distribution of these pathways in jawed vertebrates suggest that they evolved in the common ancestor of gnathostomes and both groups of jawless fishes. However, descending spinal pathways from the cerebellum, red nucleus, and telencephalon appear to be gnathostome characters.

Descending pathways to the spinal cord: a comparative study of 22 mammals

In order to estimate the qualitative commonalities and range of variation among major descending spinal pathways relevant to mankind's ancestral lineage, the supraspinal cell groups originating fibers descending directly to the spinal cord were examined in 22 mammalian species. In a standardized retrograde tract-tracing procedure, flakes of raw HRP were applied directly to the freshly cut fibers of the spinal cord after it had been hemisected at the C1-C2 junction. After a 72-hour survival period, brain and spinal cord tissues were processed by conventional HRP-processing techniques. This procedure was performed on 94 individual animals. Of this total, 41 individual cases were eliminated by a rigorous culling procedure. The results are based on 53 individuals representing 15 species selected for their successive kinship with mankind and seven species in two other lineages selected for the convergence of their visual or sensorimotor systems with anthropoids. The 22 species represent 19 genera, 14 families, eight orders, and two subclasses of Mammalia. The results show that at least 27 supraspinal cell groups, each containing intensely labeled cells, can be readily identified in each of the species. Despite vast quantitative differences in cell number and cell size, this qualitative uniformity among the relatively large number of diverse taxa suggests that the same pathways were probably present in the extinct ancestors throughout mankind's mammalian lineage and are probably still present in extant viviparous mammals as well. If so, these pathways are as old in phylogenetic history as the last common ancestor of marsupial and placental mammals--dating from the late Jurassic to early Cretaceous, perhaps 145-120 million years ago. Further comparison of the results with similar experimental findings in members of other vertebrate classes supports the notion that several of these same pathways can be traced to even more remote ancestry, with some possibly as old as the entire vertebrate subphylum--dating from the early Devonian or before, perhaps 430 million years ago. Within mankind's ancestral lineage, from the appearance of vertebrates to the appearance of mammals, there seems to have been an irregular stepwise augmentation of the set of descending pathways until the full mammalian complement was finally attained with the appearance of the corticospinal tract.

Afferent projections of the trigeminal nerve in the goldfish, Carassius auratus

The horseradish peroxidase (HRP) histochemical technique was used to examine the peripheral distribution and afferent projections of the trigeminal nerve in the goldfish, Carassius auratus. Sensory fibers of the trigeminal nerve distribute over the head via four branches. The ophthalmic branch distributes fibers to the region above the eye and naris. The maxillary and mandibular branches innervate the regions of the upper and lower lip, respectively. A fourth branch of the trigeminal nerve was demonstrated to be present in the hyomandibular trunk. Upon entering the medulla the trigeminal afferent fibers divide into a rostromedially directed bundle and a caudally directed bundle. The rostromedially directed bundle terminates in the sensory trigeminal nucleus (STN) located within the rostral medulla. The majority of fibers turn caudally, forming the descending trigeminal tract. Fibers of the descending trigeminal tract terminate within three medullary nuclei: the nucleus of the descending trigeminal tract (NDTV), the spinal trigeminal nucleus (Spv), and the medial funicular nucleus (MFn). All projections, except for those to the MFn, are ipsilateral. Contralateral projections were observed at the level of the MFn following the labeling of the ophthalmic and maxillomandibular branches. All branches of the trigeminal nerve project to all four of the trigeminal medullary nuclei. Projections to the STN and MFn were found to be topographically organized such that the afferents of the ophthalmic branch project onto the ventral portion of these nuclei, while the afferents of the maxillo- and hyomandibular branches project to the dorsal portion of these nuclei. Cells of the mesencephalic trigeminal nucleus were retrogradely labeled following HRP application to the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve. In addition to demonstrating the ascending mesencephalic trigeminal root fibers, HRP application to the above-mentioned branches also revealed descending mesencephalic trigeminal fibers. The descending mesencephalic trigeminal fibers course caudally medial to the branchiomeric motor column and terminate in the ventromedial portion of the MFn.

Origins of descending projections to the medulla oblongata and rostral medulla spinalis in the urodele Salamandra salamandra (amphibia)

Descending projections to the medulla oblongata and rostral medulla spinalis have been examined in the urodele Salamandra salamandra with retrograde horseradish peroxidase tracing. Ipsilateral projections originate from the striatum and the nucleus ventrolateralis thalami and reach the medulla oblongata. The ipsilateral nucleus praeopticus magnocellularis reaches the medulla spinalis. The rostral part of the nucleus tuberculi posterioris projects to the ipsilateral medulla oblongata; its caudal part projects further caudally. Tectal efferents and the efferents of the nucleus praetectalis profundus project bilaterally, the nucleus praetectalis superficialis, nucleus mesencephalicus nervi trigemini, torus semicircularis, nucleus Darkschewitsch, and nucleus fasciculi longitudinalis medialis project ipsilaterally to the medulla oblongata. The nucleus mesencephalicus nervi trigemini, nucleus fasciculi longitudinalis medialis, and tectal efferents reach the rostral medulla spinalis. The nucleus ruber projects mainly via the contralateral dorsolateral funiculus to the medulla spinalis. A largely crossed medullary projection arises in the nucleus dorsalis tegmenti pars anterior, a bilateral projection arises in the nucleus dorsalis tegmenti pars posterior, and an ipsilateral projection arises in the nucleus ventralis tegmenti pars anterior. Cerebellar and statoacoustic efferents descend to the medulla spinalis. The nucleus reticularis isthmi, superior, medius and inferior as well as the nucleus raphes exhibit spinal trajectories. The nucleus vestibularis magnocellularis projects bilaterally, the nucleus vestibularis medialis projects ipsilaterally spinalward. The supposed nucleus descendens nervi trigemini descends mainly contralaterally. A small spinal projection arises in the nucleus tractus solitarii. The results indicate that salamander brains display elaborate descending connections which are similar to those in other vertebrates despite their scarcely differentiated neuronal cytoarchitecture.

Evolution of the red nucleus and rubrospinal tract

A red nucleus, defined by its relative position in the tegmentum mesencephali, its contralateral rubrospinal or rubrobulbar projections and by crossed cerebellar afferents, is found in terrestrial vertebrates and certain rays. A crossed rubrospinal tract occurs in anurans, limbed urodeles and reptiles, birds and mammals, but is apparently absent in boid snakes, caecilians and sharks. A distinct rubrospinal tract is found in certain rays which use their enlarged pectoral fins for locomotion. A crossed tegmentospinal tract, possibly a rubrospinal tract, is found in lungfishes. Although evidence was presented for a rubrospinal tract in more advanced snakes, the available experimental data in lower vertebrates suggest that the presence of a rubrospinal tract is related to the presence of limbs or limb-like structures. In the connectivity of the red nucleus in terrestrial vertebrates, 'levels' of complexity can be distinguished, paralleled by the development of the cerebellum. These 'grades of organization' are probably related to the type of motor performance the particular terrestrial vertebrates are capable of.

Identification of the teleost Edinger-Westphal nucleus by retrograde horseradish peroxidase labeling and by electrophysiological criteria

A homolog of the Edinger-Westphal nucleus of other vertebrates is described in two species of serranid basses of the genus Paralabrax, a group possessing a wide range of ocular accommodation but lacking a pupillary reflex to light. The nucleus was found by retrograde labeling from the ciliary ganglion and lies dorsolateral to the ipsilateral oculomotor nucleus. The nucleus consists of 60 to 100 neurons with an average soma diameter of about 20 microns in animals weighing 70 to 150 g. Electrophysiological experiments support the identification. Microstimulation of the nucleus evokes contraction of the ipsilateral lens retractor muscle and slight constriction of the caudal ipsilateral iris. Multi- and single-unit recordings in the nucleus reveal spontaneous firing (about 30 spikes/s in single units), the rate of which decreases during visually-evoked lens retractor relaxations (accommodation to near stimuli). Recordings of muscle fiber activity in the lens retractor show essentially the same behavior, which suggests that the ciliary ganglion and neuromuscular junctions simply relay impulses with little if any synaptic integration. The existence of a discrete Edinger-Westphal nucleus devoted largely to accommodation makes Paralabrax a good model system for the further tracing of central accommodation control pathways.

Descending projection neurons to the spinal cord of the goldfish, Carassius auratus

The sources of descending spinal tracts in the goldfish, Carassius auratus, were visualized by retrograde transport of horseradish peroxidase (HRP) administered to the hemisected spinal cord. In the diencephalon, HRP-positive neurons were identified in the nucleus preopticus magnocellularis pars magnocellularis and ventromedial nucleus of the thalamus of the ipsilateral side. In the mesencephalic tegmentum, a few somata of the contralateral nucleus ruber and several ipsilateral neurons of the nucleus of the median longitudinal fasciculus were labeled. The reticular formation of the rhombencephalon was the major source of descending afferents to the spinal cord. A larger number of neurons were retrogradely labeled in the ipsilateral superior, middle, and inferior nuclei than in the contralateral nuclei. A few raphe neurons and the contralateral Mauthner neuron were also HRP-positive. The octaval area showed retrogradely labeled neurons in the anterior, magnocellular, descending, and posterior octaval nuclei of the ipsilateral side. A large number of neurons in the facial lobe and a few somata located adjacent to the descending trigeminal tract were labeled on the ipsilateral side. The pattern of descending spinal projections in goldfish is comparable to that of tetrapods and suggests that the spinal tracts have originated quite early in the course of vertebrate evolution.

Hypothalamo-spinal projections in the golden hamster

The distribution of hypothalamic projections to the spinal cord in hamsters was determined using the retrograde tracers horseradish peroxidase (HRP) and wheat germ agglutinin-HRP (WGA-HRP). Large injections of HRP or WGA-HRP were made into the thoracic spinal cord of adult male golden hamsters. HRP-labeled neurons were observed primarily in the parvocellular division of the paraventricular nucleus and in the lateral hypothalamus. The organization of hypothalamo-spinal connections appears to be highly conserved in mammalian species.

Collateralization of descending pathways from the brainstem to the spinal cord in a lizard, Varanus exanthematicus

With the multiple fluorescent retrograde tracer technique, the collateralization in the spinal cord of descending supraspinal pathways was studied in a lizard, Varanus exanthematicus. Fast Blue (FB) gels were implanted unilaterally in the spinal gray matter of the cervical enlargement and Nuclear Yellow (NY) gels were implanted ipsilaterally in two series of experiments in all spinal funiculi of the lumbar enlargement or in midthoracic spinal segments, respectively. All brainstem nuclei known to project to the spinal cord in reptiles were found to give rise to branching axons that may influence widely separate levels of the spinal cord. The number of double-labeled FB-NY cells varied in these brainstem nuclei from none to half the number of neurons projecting to the cervical enlargement. Highly collateralizing projections (expressed as the percentage of double-labeled neurons, DL) were found to arise from the nucleus raphes inferior, the contralateral nucleus reticularis superior pars lateralis, the contralateral nuclei vestibulares ventromedialis and descendens, and the ipsilateral nucleus reticularis inferior pars ventralis. A lower percentage of DL neurons was noted for the contralateral nucleus ruber and bilaterally for the nucleus reticularis medius and nucleus reticularis inferior. Extensive brainstem projections directed to cervical and high thoracic spinal levels originate from the area lateralis hypothalami, the nucleus of the fasciculus longitudinalis medialis, the contralateral nucleus cerebellaris medialis, and from the nucleus tractus solitarii. Projections preferentially directed to midthoracic or lower levels of the spinal cord were found to arise from the ipsilateral locus coeruleus, the contralateral nucleus reticularis superior pars lateralis, the nucleus reticularis inferior pars ventralis, the nucleus reticularis inferior, and the nucleus raphes inferior. In contrast to findings in mammals, in Varanus exanthematicus the red nucleus, the nucleus vestibularis ventrolateralis, and certain parts of the reticular formation did not display a clear-cut somatotopic organization. In general two different patterns of collateralization can grossly be discerned: a gradual decrease of spinal collaterals caudalward, which can be interpreted as a certain specificity of such projections; and a constant number of collateral nerve fibers throughout the spinal cord that can be interpreted as either a nonspecific or, in contrast, a highly specific system, focussed exclusively on the cervical and lumbar enlargements.

Projections from the cochlear nuclear complex to rhombencephalic auditory centers and torus semicircularis in the turtle

The projections arising from the cochlear nucleus complex in the turtle were investigated using [35S]methionine as a tracer. Commissural connections between the auditory tubercles terminate in both the laminar and the magnocellular nuclei. The cochlear projections to the superior olivary complex, the lateral lemniscus complex and the torus semicircularis were predominantly contralateral. Terminal labeling in the torus semicircularis was found in the peritoral region (TSP) and in the central nucleus (TSC), particularly its caudoventral and rostromedial portions. The caudal dorsolateral portion of TSC, which had previously been shown to receive ascending spinal fibers was not labeled following injections of tracer into the cochlear nuclear complex. Ascending spinal and cochlear projections, however, appeared to overlap in the rostral portions of TSC and in TSP.

Distribution of some peptides (substance P, [Leu]enkephalin, [Met]enkephalin) in the brain stem and spinal cord of a lizard, Varanus exanthematicus

The distribution of substance P-like and [Leu]- and [Met]enkephalin-immunoreactive cell bodies, fibers and terminal structures in the brain stem and spinal cord of a lizard, Varanus exanthematicus, was studied with the indirect immunofluorescence technique, using antibodies to these peptides. Substance P-like immunoreactive cell bodies were found in the hypothalamus, in a periventricular cell group in the rostral mesencephalon, in the interpeduncular nucleus, in and ventral to the descending nucleus of the trigeminal nerve, in and directly ventral to the nucleus of the solitary tract, scattered in the brain stem reticular formation and in the trigeminal and spinal ganglia. A rather widespread distribution of substance P-like immunoreactivity was found in the brain stem and spinal cord, mainly concentrated in striatotegmental projections related to visceral and/or taste information (nucleus of the solitary tract, parabrachial region), in the descending nucleus of the trigeminal nerve and in the dorsal horn of the spinal cord (areas I and II). In the spinal cord also around the central canal (area X and adjacent parts of area V-VI) a distinct substance P innervation was found. The ventral horn receives only a very sparse substance P innervation. The distribution of [Leu]- and [Met]enkephalin in the brain stem and spinal cord of Varanus exanthematicus is less impressive than that of substance P. Enkephalinergic cell bodies were found particularly in the caudal hypothalamus. Small populations of enkephalinergic cell bodies were found in the vestibular nuclear complex, in the nucleus of the solitary tract, in and around the descending nucleus of the trigeminal nerve and throughout the rhombencephalic reticular formation. Enkephalins are likely to be present in efferent projections of the striatum, in projections related to taste and/or visceral information (nucleus of the solitary tract, parabrachial region) and in descending pathways to the spinal cord. Enkephalinergic fibers are present in the lateral funiculus and enkephalin-immunoreactive cell bodies are found in the reticular formation, particularly the inferior reticular nucleus which is known to project to the spinal cord. In the spinal cord enkephalinergic terminal structures were found especially in the superficial layer of the dorsal horn (areas I and II) and around the central canal. The ventral horn including the motoneuron area receives only a relatively sparse enkephalinergic innervation.

The cerebellar and vestibular nuclear complexes in the turtle. II. Projections to the prosencephalon

Prosencephalic projections from the cerebellar and vestibular nuclear complexes in the turtle Pseudemys scripta elegans were investigated with anterograde tracing. Following injections of 35S-methionine at various locations within the cerebellar and vestibular nuclear complexes, labeled ascending fibers were found to arise from the lateral cerebellar and the rostral (superior and/or dorsolateral) vestibular nuclei. The great majority of these fibers coursed within the ipsilateral ascending periventricular tract. There were possible terminations in the hypothalamosuprapeduncular region, the ovalis-complex, and the nucleus commissuralis anterior, but scarcely any indication of terminal labeling within the dorsal thalamus. The labeled fibers, however, continued rostralward, entered the lateral forebrain bundle, and terminated in the anterior dorsal ventricular ridge--in all but one case, exclusively ipsilaterally. The terminal area within the lateral division (referred to as area L) of the anterior dorsal ventricular ridge was sharply delimited, being situated ventrolateral to the visually oriented area D of the anterior dorsal ventricular ridge (Balaban and Ulinski, '81), medial to the lateral cortex, and ventral to the pallial thickening (motor pallium of Johnston, '16). The findings are compared with related ones in mammals, particularly those pertaining to telencephalic somatosensorimotor regions and their interactions with the vestibular nuclear complex and the cerebellum.

The origins of descending spinal projections in lepidosirenid lungfishes

The origins of descending spinal projections in the lepidosirenid lungfishes were identified by retrograde transport of horseradish peroxidase (HRP) introduced into the rostral spinal cords of juvenile African (Protopterus annectans and Protopterus amphibians) and South American (Lepidosiren paradoxa) lungfishes. Standard HRP histochemistry revealed retrogradely labeled neurons in the nucleus of the medial longitudinal fasciculus, midbrain tegmentum, red nucleus, optic tectum, mesencephalic trigeminal nucleus, granule cell layer of the cerebellum, superior, middle, and inferior medullary reticular nuclei, magnocellular and descending octaval nuclei, region of the descending trigeminal tract, solitary complex, and the margins of the spinal gray matter anterior to the spinal HRP implant. A small number of retrogradely labeled neurons were also present in the ventral thalamus of Protopterus. A descending spinal projection from the forebrain was not evident in either genus of lepidosirenid lungfishes. The presence of projections to the spinal cord from the diencephalon, medial reticular formation of the midbrain and medulla, octaval (vestibular) nuclei, solitary complex, and probable nucleus of the descendin trigeminal tract in lungfishes and their overall similarity to comparable projections in other vertebrates suggest that these pathways are among those representative of the primitive pattern of descending spinal projections in vertebrates.

Horseradish peroxidase evidence for a spinal projection from the preoptic area of the goldfish, a light and electron microscopic study

Horseradish peroxidase (HRP) was applied to the transected spinal cord of goldfish labeled neurons in the preoptic area. Since leakage of HRP into the blood could produce the labeling of neurosecretory cells, intraperitoneal (i.p.) injections of HRP were made with a wide range of dosages in order to intentionally label preoptic neurosecretory cells. The distribution of preoptic neurons labeled after spinal HRP application was far more restricted than the labeling via uptake of HRP from the blood, even when cells in the spinal cord-transected fish were intensely labeled. Furthermore, in HRP electron microscopic material, morphological differences were observed between neurons labeled by the two procedures. Large numbers of dense core vesicles and well-developed stacks of rough endoplasmic reticulum, features typical of cells projecting to the pituitary, were not observed in cells labeled via the spinal cord. These findings indicate that in goldfish a direct projection exists from the preoptic area to the spinal cord which could be homologous to one arising from the paraventricular nucleus of mammals. Both i.p. injection and spinal transection also produced labeling of more caudal periventricular diencephalic cells which resemble preoptic cells in efferent projections as well as ultrastructural features.

The fasciculus longitudinalis medialis in the lizard Varanus exanthematicus. 2. Vestibular and internuclear components

In the present study the vestibular components of the fasciculus longitudinalis medialis (flm) were investigated in the lizard Varanus exanthematicus with various tracing techniques: anterograde transport of horseradish peroxidase to study vestibulo-oculomotor and vestibulospinal projections, the multiple retrograde fluorescent tracer technique for the cells of origin of such projections. Internuclear projections between the oculomotor and abducens nuclei could also be studied in this way. Rather extensive vestibulo-ocular projections passing via the flm were demonstrated. Mainly ipsilateral ascending projections arise in the dorsolateral vestibular nucleus, mainly contralateral ascending projections in the ventromedial vestibular nucleus and adjacent parts of the ventrolateral and descending vestibular nuclei. Furthermore, distinct bilateral ascending projections of the nucleus prepositus hypoglossi were demonstrated. Extensive vestibulospinal projections pass via the flm and form the medial vestibulospinal tract. This largely contralateral descending pathway arises predominantly in the ventromedial and descending vestibular nuclei. Terminal structures presumably arising in the ventromedial and descending vestibular nuclei were found on contralateral neurons, probably motoneurons innervating neck muscles. Vestibular neurons with both ascending (presumably to extra-ocular motoneurons) and descending projections to the spinal cord are present in all vestibular nuclei, although preferentially in the ventromedial vestibular nucleus and adjacent parts of the ventrolateral and descending vestibular nuclei. However, also in the dorsolateral vestibular nucleus a substantial number of double labeled neurons were found. These vestibular neurons with both vestibulomesencephalic and vestibulospinal projections are probably involved in combined movements of eyes and head. Evidence for reciprocal internuclear connections between the oculomotor and abducens nuclei was found. Neurons in the dorsal part of the oculomotor nucleus probably project to the ipsilateral abducens nucleus, while neurons in the abducens nucleus most likely project to the contralateral oculomotor nucleus. These reciprocal internuclear connections between the oculomotor and abducens nuclei probably play an important role in conjugate horizontal eye movements.

The nucleus dorsalis myelencephali of snakes: relay nucleus between the spinal cord and the posterior colliculus (paratorus)

Nucleus dorsalis myelencephali is in the dorsolateral area of the caudal medulla in snakes. The parvocellular area projects bilaterally to the paratorus and receives ipsilateral projections from the spinal cord. The magnocellular area projects bilaterally to the spinal cord. This nucleus has been only briefly described in snakes but not in any other reptilian group.

Tectoreticular pathways in the turtle, Pseudemys scripta. II. Morphology of tectoreticular cells

The morphology of tectoreticular neurons in turtles was examined with serial section reconstructions of neurons retrogradely filled with HRP. Six classes of tectal neurons project into the three tectobulbar pathways characterized in the preceding paper (Sereno, '85). (1) Large multipolar neurons with somata in the central gray layers, and with moderately branched dendrites sometimes spanning over a millimeter, project into the dorsal tectobulbar pathway, TBd. Their dendrites are covered with fine spicules and tend to arborize in the lower third of the superficial gray layers. (2) Medium-sized neurons with multiple radial dendrites and somata in the central white and upper periventricular layers probably project into the ipsilateral intermediate tectobulbar pathway, TBi. Their dendrites also bear fine spicules and usually reach the tectal surface. (3) Small radial cells in the periventricular layers, and (4) small bitufted radial cells in the superficial gray project into the small caliber component of the ipsilateral ventral tectobulbar pathway, TBv(sm). (5) Medium-sized central gray neurons with stratified dendrites, and (6) medium-sized central gray neurons with horizontal dendrites probably project into the medium caliber component of the ventral tectobulbar pathway, TBv(med). In contrast to TBd and TBi neurons, these last four classes emit a spray of long, filamentous dendritic appendages in the central gray and have dendritic arbors near the top of the superficial gray. The morphology of the neurons described in this and the preceding paper is briefly discussed in light of current ideas about tectally mediated sensorimotor transformations.

Distribution of serotonin in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study

The distribution of serotonin-containing nerve cell bodies, fibers and terminals in the lizard Varanus exanthematicus was studied with the indirect immunofluorescence technique, using antibodies to serotonin. Most of the serotonin-containing cell bodies were found in the midline, in both of the raphe nuclei, i.e. the nuclei raphes superior and inferior. A considerable number of more laterally shifted serotonergic neurons was found particularly at three levels of the brain stem, viz. in the caudal mesencephalic tegmentum, at the isthmic level, and over a long distance in the medulla oblongata. These laterally situated serotonin-positive neurons were partly found within the confines of the substantia nigra, the nucleus reticularis superior and the lateral part of the nucleus reticularis medius and ventrolateral part of the nucleus reticularis inferior, respectively. No serotonergic cell bodies were found in the spinal cord. In the brain stem a dense serotonergic innervation was observed in all of the motor nuclei of the cranial nerves, in two layers of the tectum mesencephali, in the nucleus interpeduncularis pars ventralis, the nucleus profundus mesencephali pars rostralis, the periventricular grey, the nucleus parabrachialis, the vestibular nuclear complex, the nucleus descendens nervi trigemini, the nucleus raphes inferior, and parts of the nucleus tractus solitarii. Descending serotonergic pathways could be traced into the spinal cord via the dorsolateral, ventral and ventromedial funiculi, and were found to innervate mainly three parts of the spinal grey throughout the spinal cord, i.e. the dorsal part of the dorsal horn, the motoneuron area in the ventral horn, and the intermediate zone just lateral to the central canal. The results obtained in the present study suggest a close resemblance of the organization of the serotonergic system in reptiles and mammals, especially as to the serotonergic innervation of the spinal cord.

Cerebellar efferents in the lizard Varanus exanthematicus. II. Projections of the cerebellar nuclei

The projections of the cerebellar nuclei have been studied in the lizard Varanus exanthematicus with various experimental anatomical techniques. In anterograde degeneration experiments (lesions of the cerebellar peduncle) both ascending and decending contralateral projections were found. Ascending fibers which could be traced from the cerebellar commissure ventralward decussated at the level of the trochlear and oculomotor nuclei. These fibers coursed rostralward to the mesodiencephalic junction. With anterograde tracing techniques (3H-leucine and HRP) this tract was found to terminate in the nucleus ruber and the interstitial nucleus of the fasciculus longitudinalis medialis. Moreover, retrograde tracer studies (HRP, "Fast Blue") showed that this tract appeared to arise mainly in the lateral cerebellar nucleus. With both anterograde degeneration and tracing techniques (3H-leucine and HRP) a bundle of fibers could be followed, which decussates in the basal part of the cerebellum and passes dorsally around the contralateral medial cerebellar nucleus to the lateral side of the brainstem. This contralaterally descending projection system was found, lateral to the vestibular nuclear complex, and as far caudally as the descending vestibular nucleus, to terminate on various vestibular nuclei. Horseradish peroxidase studies showed that this contralaterally descending projection system originates mainly in the medial cerebellar nucleus, but ipsilaterally descending projections were also found. With the fluorescent double labeling technique ("Fast Blue" and "Nuclear Yellow") the projections of the cerebellar nuclei described above were confirmed. Furthermore, double labeling revealed neurons in both cerebellar nuclei (especially the medial nucleus) that project to both the mesencephalon and the cervical spinal cord. The present results indicate that the efferent connections of the cerebellar nuclei in the lizard Varanus exanthematicus are organized as two main projections, an ascending projection comparable to the mammalian brachium conjunctivum arising in the lateral cerebellar nucleus, and a descending projection comparable to the mammalian hook bundle (fasciculus uncinatus), originating mainly in the medial cerebellar nucleus. Such projections are common for terrestrial vertebrates.

Early development of descending pathways from the brain stem to the spinal cord in Xenopus laevis

The early development of descending pathways from the brain stem to the spinal cord has been studied in Xenopus laevis tadpoles. The relatively protracted development of this permanently aquatic amphibian as well as its transparency during development make this animal particularly attractive for experimental studies. Between the 5th and 10th myotome the spinal cord was crushed with a thin needle and dry horseradish peroxidase (HRP) crystals were applied. After a survival time of one day the tadpoles were fixed and the brain and spinal cord were stained as a whole according to a modification of the heavy metal intensification of the DAB-reaction, cleared in cedarwood oil and examined as wholemounts. At stage 28 (the neural tube has just closed) the first brain stem neurons projecting to the spinal cord were found in what appear to be the nucleus reticularis inferior and -medius. At this stage of development the first, uncoordinated swimming movements can be observed. At stage 30/31 (the tailbud is visible) both Mauthner cells project to the spinal cord as well as the interstitial nucleus of the fasciculus longitudinalis medialis situated in the mesencephalon. Towards stage 35/36 (the tail is now clearly visible), a more extensive reticulospinal innervation of the spinal cord appears, now including cells of the nucleus reticularis superior. At this stage also the first vestibulospinal and raphespinal projections were found. At stage 43/44 (the tadpoles have now a well-developed tail) the pattern of reticulospinal projections appears to be completed with the presence of labeled neurons in the nucleus reticularis isthmi. From stage 43/44 on, the number of HRP-positive cells is steadily increasing. At stage 47/48, when the hindlimb buds appear, the descending projections to the spinal cord are comparable with the adult situation except for the absence of a rubrospinal and a hypothalamospinal projection. The observations demonstrate that already very early in development reticulospinal fibers and, somewhat later, Mauthner cell axons and vestibulospinal fibers innervate the spinal cord. Furthermore, a caudorostral gradient appears to exist with regard to the development of descending projections to the spinal cord. However, the interstitial nucleus of the fasciculus longitudinalis medialis forms an exception to this rule.

Ascending and descending axon collaterals efferent from the brainstem reticular formation. A retrograde fluorescent tracer study in the lizard, Varanus exanthematicus

The existence of divergent axon collaterals of neurons in the reticular formation has been studied with fluorescent tracers in a lizard. It appeared that ascending and descending projections arise in at least partially overlapping fields. However, only few reticular or raphe neurons were found with both ascending and descending projections.

The efferent connections of the nuclei of the descending trigeminal tract in the mallard (Anas platyrhynchos L.)

The efferent and intranuclear connections of the nuclei of the descending trigeminal tract of the mallard have been studied with lesion methods, and by axonal transport techniques following injections of tritiated leucine, and of horseradish peroxidase. The large subnucleus oralis neurons, including those belonging to the nucleus of the ascending glossopharyngeal tract, have proven to be the sole origin of trigeminocerebellar connections. The cerebellar afferents are of the mossy fiber type, and terminate predominantly in lobules V, VI and VII, and possibly, lobule IV. Trigeminocerebellar projections are ipsilateral except for the vermal area. Subnucleus interpolaris is the main source of intratrigeminal fibers that terminate in subnucleus oralis and the ventral part of the main sensory nucleus. These intranuclear connections are bilateral, but the medium-celled caudal part of subnucleus interpolaris in particular contains the majority of bi- and/or contralaterally projecting neurons. Additionally, the small cells in the rostral part of subnucleus interpolaris project ipsilaterally upon the parabrachial region, and upon the lateral reticular formation. Projections upon the parabrachial region furthermore emanate bilaterally from layer I of the rostral subnucleus caudalis. A minor part of layer I neurons sends its axons contralaterally along with those of the dorsal column nuclei toward the thalamic nucleus dorsolateralis posterior. Associated with the medial lemniscus, contralateral termination is also present in the lateral part of the ventral lamella of oliva caudalis, in the marginal zone of nucleus mesencephalicus lateralis, pars dorsalis and immediately surrounding intercollicular grey and, finally, in the nucleus intercalatus thalami. Furthermore, a bilaterally descending projection from subnucleus caudalis upon layers I and II of the rostral cervical cord was observed. Close to their origin subnucleus caudalis neurons project upon the adjoining caudal part of the lateral reticular formation.

Distribution of catecholamines in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study based on the use of antibodies to tyrosine hydroxylase

Antibodies to tyrosine hydroxylase were used to study the distribution of nerve cells, fibers and terminals, containing catecholamines, in the lizard Varanus exanthematicus, by means of the indirect immunofluorescence technique. Tyrosine hydroxylase-containing cell bodies occurred in the hypothalamus, the ventral and dorsal tegmentum mesencephali, the substantia nigra, the isthmic reticular formation, in and ventrolaterally to the locus coeruleus, in the nucleus tractus solitarii and in a lateral part of the nucleus reticularis inferior. In addition tyrosine hydroxylase-containing cell bodies were found throughout the spinal cord, ventral to the central canal. Tyrosine hydroxylase-immunoreactive terminal areas in the brain stem were seen in the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus raphes superior, the locus coeruleus, several parts of the reticular formation and the nucleus descendens nervi trigemini. Ascending catecholaminergic pathways could be traced from the ventral mesencephalic tegmentum as well as from the dorsal isthmic tegmentum rostralwards, through the lateral hypothalamus. These pathways correspond to the mesostriatal and isthmocortical projections respectively, as described in mammals. Furthermore, ascending catecholaminergic fibers could be traced from the catecholaminergic cell groups in the medulla oblongata to the isthmus, where they intermingle with the locus coeruleus neurons. These pathways correspond to the medullohypothalamic projection and to the dorsal periventricular system in mammals. Descending catecholaminergic fibers to the spinal cord pass via the dorsomedial part of the lateral funiculus, and mainly terminate in the dorsal horn. The results obtained in the present study have been placed in a comparative perspective, which illustrates the constancy of catecholaminergic innervation throughout phylogeny.

Cerebellar efferents in the lizard Varanus exanthematicus. I. Corticonuclear projections

The organization of the cerebellar corticonuclear projections, i.e., the projections from the Purkinje cell layer to the cerebellar nuclei and the vestibular nuclear complex, was investigated with the horseradish peroxidase (HRP) technique in the lizard Varanus exanthematicus. After HRP slow-release gels were implanted in the cerebellar nuclei or various parts of the vestibular nuclear complex, the following longitudinally oriented zones of labeled Purkinje cells could be distinguished: a medial zone projecting to the medial cerebellar nucleus; an intermediate zone projecting to the vestibular nuclear complex, especially the ventrolateral vestibular nucleus, but probably also the dorsolateral vestibular nucleus; a caudolaterally located area of the cerebellar cortex projecting to the lateral cerebellar nucleus; and the flocculus and the adjacent lateral part of the Purkinje cell layer with projections to the middle and caudal parts of the vestibular nuclear complex, i.e., the descending and ventromedial vestibular nuclei. All projections of the Purkinje cells appeared to be strictly ipsilateral. It can be concluded that in reptiles a longitudinal organization of cerebellar corticonuclear projections exists, which may be basic for terrestrial vertebrates.

The organization of motoneurons in the turtle lumbar spinal cord

The distribution of motoneurons in the lumbosacral spinal cord of the turtle Pseudemys scripta elegans was studied by using the technique of retrograde transport of horseradish peroxidase. A total of 19 different hindlimb muscles were injected with varying amounts of horseradish peroxidase. The resulting distribution of labeled motoneurons was studied in both longitudinal and transverse sections of spinal cord. Motoneurons innervating a particular hindlimb muscle are clustered in longitudinally arranged motorpools. Motorpools of different muscles can show considerable overlap in both the rostrocaudal and transverse planes. The distribution of the various motorpools demonstrates a somatotopic organization of motoneurons within the lumbar spinal cord. Motoneurons innervating more distally positioned muscles are generally found in the more caudal segments, while motoneurons supplying proximal muscles are distributed throughout almost the whole lumbosacral intumescence. Motoneurons innervating anterodorsally positioned muscles are found in the ventrolateral part of area IX in the ventral horn, while more dorsomedially positioned motoneurons innervate the posteroventral muscles. These features are consistent with observations in other tetrapods, although the somatotopic representation of motoneurons is more evident in higher vertebrates such as chicken and cat. The observed motorpool distribution is discussed in relation to the presumed ontogeny of the spinal cord and hindlimb muscles and also in relation to the functions of the investigated muscles.

The fasciculus longitudinalis medialis in the lizard Varanus exanthematicus

With the horseradish peroxidase (HRP) technique the various descending components of the medial longitudinal fasciculus (flm) have been studied in the lizard Varanus exanthematicus. After wheat germ agglutinin conjugated HRP injections at the spinomedullary border, retrogradely labeled fibers passing via the flm could be traced to various parts of the magnocellular rhombencephalic reticular formation, the descending and ventromedial vestibular nuclei and the interstitial nucleus of the flm. By implanting HRP slow-release gels into the flm the trajectory and site of termination of various components of the flm have been analysed. The interstitiospinal tract passes via the dorsal part of the flm. Reticulospinal fibers arising in the nucleus reticularis superior and nucleus reticularis medius take a position ventral to the interstitiospinal fibers. Vestibulospinal projections via the flm are found in its ventral part and arise mainly in the contralateral ventromedial and descending vestibular nuclei. A strong vestibulocollic projection to cervical motoneurons should be noted. The positional relations of the various fiber components within the flm found in a lower vertebrate such as the lizard Varanus exanthematicus are comparable to those in mammals.