Trigeminal primary afferent projections to the spinal cord of the frog, Rana ridibunda

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.

Contribution of the submentalis muscle to feeding mechanics in the leopard frog,Rana pipiens

This study investigated the functional contributions of the submentalis muscle to the coordination of feeding behavior in the leopard frog, Rana pipiens. Additionally, the anatomical origins of the motor neurons innervating this muscle are identified and described. The m. submentalis is a small muscle connecting the distal mandibular tips. Depending upon the anuran species studied, this muscle contributes to mandibular bending and the degree to which the tongue is protracted, or has little or no role in feeding biomechanics. High-speed videography was used to quantify feeding attempts before versus after bilateral denervation of the m. submentalis. Additionally, the terminal branch of the trigeminal nerve prior to innervating the m. submentalis was retrogradely labeled to identify the origins of motor neurons innervating the muscle. For the kinematic analyses, denervation of the submentalis resulted in significant increases in the time to maximum tongue protrusion, and the duration of tongue protrusion. Neither mandibular bending, nor tongue length variables differed significantly between normal conditions and deafferented conditions. However, when unsuccessful feeding attempts were quantified following the denervation, failed attempts were nearly always due to the tongue not reaching the prey. None of the unsuccessful feedings prior to denervation were due to inadequate tongue protrusion. Anatomical data show a much larger rostral-caudal distribution of the trigeminal motor neurons than previously described for anurans. These data suggest a larger role for the submentalis muscle in Rana than in previously studied anurans with long protrusible tongues, and suggests a feedback mechanism from the trigeminal nerve to the nerves coordinating tongue protraction and retraction.

Distribution of adrenomedullin-like immunoreactivity in the central nervous system of the frog

Adrenomedullin (AM) is a recently discovered peptide widely distributed in the mammalian brain. By using an antiserum specific for human AM, we have analyzed the localization of AM-like immunoreactivity in the brain and spinal cord of the anuran amphibian Rana perezi. Cell bodies immunoreactive (AMi) for AM were located in the dorsal, lateral and medial pallial regions, diagonal band of Broca, medial septum, and above and rostral to the anterior commissure. A large population of AMi neurons was located in the anterior preoptic area, suprachiasmatic nucleus and in the infundibular hypothalamus. The processes of these latter cells are part of the hypothalamo-hypophysial pathway to the neural and intermediate lobes. Labeled cells were observed in the pretectal region, posterior tubercle and the mesencephalic anteroventral tegmental nucleus. Strikingly, Purkinje cells in the cerebellum also showed AM immunoreactivity, albeit not all of these cells were equally stained. Additional cells were located in the parabrachial region, principal trigeminal sensory nucleus, reticular nuclei medius and inferior, and the intermediolateral gray of the spinal cord. Immunolabeled fibers were widespread throughout the brain and spinal cord of the frog. They were particularly abundant in the medial amygdala, hypothalamus, mesencephalic tectum, periventricular gray and spinal cord. The distribution pattern of AM-like immunoreactivity in the brain of the frog is very selective and does not correspond with the pattern observed for any other transmitter or neuroactive molecule. The wide distribution of this peptide strongly suggests that it may play a significant role in the multiple neuronal functions in the amphibian brain.

Primary and secondary somatosensory projections in direct-developing plethodontid salamanders

In three species of plethodontid salamanders (Plethodon jordani, Hydromantes italicus, and Bolitoglossa subpalmata), primary and secondary somatosensory pathways were investigated by means of tract-tracing in vivo and in vitro using biocytin, horseradish peroxidase, and neurobiotin. Afferent sensory fibers of cranial nerves V, VII, and X and the brachial nerve run in the dorsal funiculus of the medulla oblongata and spinal cord. Fibers ascend to the level of, but do not enter, the cerebellum. In the caudal medulla oblongata, sensory tracts of the cranial nerves descend in a dorsal and a dorsolateral bundle and reach the level of the fourth spinal nerve. Two bundles are likewise formed by spinal afferent fibers, which descend to the level of the seventh spinal nerve. Secondary somatosensory projections ascend in contralateral ventral, contralateral lateral, and ipsilateral lateral tracts, the latter two corresponding to the spinal lemniscal tracts of Herrick. These tracts reach the cerebellum, mesencephalic, and diencephalic targets (tegmentum, torus, tectum, tuberculum posterius, pretectum, and ventral thalamus) ipsi- and contra-laterally. The projection to the tectum is confined to fiber layer 4. Fibers of the ascending tracts cross in the cerebellar and tectal commissure. Our study demonstrates that the ascending secondary somatosensory pathways of plethodontid salamanders differ remarkably from those of other amphibians.

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.

Evidence for an anuran homologue of the mammalian spinocervicothalamic system: an in vitro tract-tracing study in Xenopus laevis

Evidence is presented for an anuran homologue of the mammalian spinocervicothalamic system. In vitro tract-tracing experiments with biotinylated dextran amine Xenopus laevis show that ascending spinal fibres from all levels of the spinal cord, passing via the dorsolateral funiculus, terminate in a cell area ventrolateral to the dorsal column nucleus. This cell area can be considered a possible homologue of the mammalian lateral cervical nucleus. After tracer applications to the ventral thalamus or to the torus semicircularis (both targets for somatosensory projections), the anuran lateral cervical nucleus was retrogradely labelled contralateral to the application sites. Tracer applications to the dorsolateral funiculus at the obex level and rostral spinal cord resulted in labelling of the cells of origin of the spinocervical tract. These were found, mainly ipsilaterally, in the ventral part of the dorsal horn, and were rather evenly distributed throughout the spinal cord. These data suggest the presence of an anuran homologue of the mammalian spinocervicothalamic system. A brief survey of the literature shows that such a system is much more common in vertebrates than previously thought.

Anuran dorsal column nucleus: organization, immunohistochemical characterization, and fiber connections in Rana perezi and Xenopus laevis

As part of a research program on the evolution of somatosensory systems in vertebrates, the dorsal column nucleus (DCN) was studied with (immuno)histochemical and tract-tracing techniques in anurans (the large green frog, Rana perezi, and the clawed toad, Xenopus laevis). The anuran DCN contains some nicotinamide adenine dinucleotide phosphate diaphorase-positive neurons, very little calbindin D-28k, and a distinct parvalbumin-positive cell population. The anuran DCN is innervated by primary and non-primary spinal afferents, by primary afferents from cranial nerves V, VII, IX, and X, by serotonin-immunoreactive fibers, and by peptidergic fibers. Non-primary DCN afferents from the spinal cord appear to arise throughout the spinal cord, but particularly from the ipsilateral dorsal gray. The present study focused on the efferent connections of the DCN, in particular the targets of the medial lemniscus. The medial lemniscus could be traced throughout the brainstem and into the diencephalon. Along its course, the medial lemniscus gives off collaterals to various parts of the reticular formation, to the octavolateral area, and to the granular layer of the cerebellum. At mesencephalic levels, the medial lemniscus innervates the lateral part of the torus semicircularis as well as various tegmental nuclei. A striking difference between the two species studied is that while in R. perezi medial lemniscal fibers do not reach the tectum mesencephali, in X. laevis, intermediate and deep tectal layers are innervated. Beyond the midbrain, both dorsal and ventral thalamic areas are innervated by the medial lemniscus. The present study shows that the anuran "lemniscal pathway" is basically similar to that of amniotes.