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

Abstract

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.

A descending dopamine pathway conserved from basal vertebrates to mammals

“…After 10 min, the brain was rinsed thoroughly and transferred to a chamber perfused with oxygenated Ringer's solution for 3-4 h at room temperature to allow retrograde transport of the tracers. The injection sites were chosen based on previous studies on the MLR (5, 7, 59), middle reticular nucleus (mRN) (7,60,61), and striatum (15,62) and verified by histology. The preparations were then transferred to a fixative solution according to the immunofluorescence procedure to follow.…”

“…Fluorescence recordings were performed with a fluorescence microscope (Olympus BX50WI) equipped with a custom LED light source. The RS cells of the mRN were easily identified under the microscope (7,60,61). An optimal focal plane was chosen for imaging RS cells in the mRN.…”

Interfacing a salamander brain with a salamander-like robot: Control of speed and direction with calcium signals from brainstem reticulospinal neurons

“…The stimulation site was identified on the basis of previous studies describing the distribution of MLR neurons [30], [31]. An optimal focus was chosen for imaging the RS neurons of the middle reticular nucleus (mRN) on the basis of previous studies describing the distribution of RS neurons in salamanders [33], [31]. The changes in fluorescence in RS cells were measured in regions of interests manually delineated around the RS cells bodies filled with the Ca 2+ green dextran amine.…”

“…Further increase induced a transition to swimming. The MLR does so by sending bilaterally symmetrical descending glutamatergic input to RS neurons [31], which in turn project to the spinal CPG [32], [33], [34]. RS are active during locomotion [35], [34], and their activity can be precisely controlled by the strength of MLR stimulation [36], [37], [31].…”

Role of midbrain in the control of breathing in anuran amphibians

“…According to Sánchez-Camacho et al (27), the main differences between amphibians and others anamniotes concern the presence of extensive pretectospinal projections. These projections are absent in amniotes.…”