Preservation of segmental hindbrain organization in adult frogs

Abstract

To test for possible retention of early segmental patterning throughout development, the cranial nerve efferent nuclei in adult ranid frogs were quantitatively mapped and compared with the segmental organization of these nuclei in larvae. Cranial nerve roots IV-X were labeled in larvae with fluorescent dextran amines. Each cranial nerve efferent nucleus resided in a characteristic segmental position within the clearly visible larval hindbrain rhombomeres (r). Trochlear motoneurons were located in r0, trigeminal motoneurons in r2-r3, facial branchiomotor and vestibuloacoustic efferent neurons in r4, abducens and facial parasympathetic neurons in r5, glossopharyngeal motoneurons in r6, and vagal efferent neurons in r7-r8 and rostral spinal cord. In adult frogs, biocytin labeling of cranial nerve roots IV-XII and spinal ventral root 2 in various combinations on both sides of the brain revealed precisely the same rostrocaudal sequence of efferent nuclei relative to each other as observed in larvae. This indicates that no longitudinal migratory rearrangement of hindbrain efferent neurons occurs. Although rhombomeres are not visible in adults, a segmental map of adult cranial nerve efferent nuclei can be inferred from the strict retention of the larval hindbrain pattern. Precise measurements of the borders of adjacent efferent nuclei within a coordinate system based on external landmarks were used to create a quantitative adult segmental map that mirrors the organization of the larval rhombomeric framework. Plotting morphologically and physiologically identified hindbrain neurons onto this map allows the physiological properties of adult hindbrain neurons to be linked with the underlying genetically specified segmental framework.

Spinal Efference Copy Signaling and Gaze Stabilization during Locomotion in JuvenileXenopusFrogs

“…Although locomotion of terrestrial tetrapods with flexible necks generally requires relatively complex 3D eye movements to stabilize retinal images, the undulatory tail-based swimming of most aquatic vertebrates, such as larval Xenopus (Hoff and Wassersug, 1986;Azizi et al, 2007), necessitates only relatively simple paired (conjugate) ocular movements to counteract head rotational movements that are mainly in the horizontal plane. Indeed, such an eye motion profile has been recently observed during swimming in Xenopus tadpoles (Lambert et al, 2009(Lambert et al, , 2012, consistent with alternate burst discharge elicited in bilateral synergistic pairs of MR and LR motor nerves during passively induced, semicircular canal-derived horizontal angular vestibulo-ocular reflexes (aVOR) (Straka and Dieringer, 2004;Angelaki and Cullen, 2008).…”

“…All lesions were made with a razor blade fragment or with micro-scissors (Superfine Vannas Scissors, World Precision Instruments) depending on the extent of the lesion. In the case of the restricted lesion to rhombomere 5, in which the abducens nucleus is located (Straka et al, 2001) and wherein abducens internuclear neurons traverse the brainstem midline to ascend in the medial longitudinal fascicle toward the oculomotor nucleus (Straka and Dieringer, 1991), the incision was guided using external anatomical landmarks. In particular, these included the VIIIth and IXth cranial nerves, which have been shown to delineate, respectively, the rostral and caudal borders of rhombomere 5 (Straka et al, 2006).…”

“…Accordingly, a restricted sagittal midline section was made in the hindbrain of adult preparations at rhombomere 5 ( Fig. 4; see Materials and Methods) where the abducens nuclei and their internuclear neurons are located (Straka et al, 2001). In the tadpole, these midline crossing interneurons project to the oculomotor nucleus on the opposite side to excite contralateral MR motoneurons, thereby coupling synergistic MR and LR motoneurons in a manner required for conjugate movements of the two eyes.…”

“…Such gaze-stabilizing adjustments, in correspondence with the specific locomotor strategy used, are traditionally attributed to the transformation of movement-detecting sensory inputs into appropriate extraocular motor commands (Angelaki and Hess, 2005). Accordingly, visuo-vestibular and proprioceptive reflexes generate eye movements that counteract body/head motion, thereby minimizing retinal image drift (Straka and Dieringer, 2004). Recently, however, an alternative gaze-stabilizing mechanism has been reported in swimming larval Xenopus, whereby classical reflexdriven eye movements were found to be supplanted by an intrinsic predictive mechanism that uses efference copy signals arising from the locomotor CPG itself (Lambert et al, 2012), and thus with potentially novel implications for vertebrate gaze control in general.…”

“…In this respect, the Xenopus frog, with its employment of fundamentally different propulsive mechanisms before and after metamorphosis, offers a unique opportunity to study within a single species the adaptive plasticity in gaze control and the role of spinal efference copy signaling in an organism that lies at the interface between two principal locomotor strategies in the animal kingdom. In contrast to tadpole axial swimming, postmetamorphic froglets produce body displacements by rhythmic, bilaterally synchronous hindlimb kicking (Combes et al, 2004;Beyeler et al, 2008), with an accompanying switch from horizontal angular head rotations (Lambert et al, 2012) to linear forward acceleration and then deceleration in each locomotor cycle (Straka and Dieringer, 2004). Consequently, instead of conjugate left-right eye oscillations during larval locomotion, the gaze-stabilizing process in the swimming adult is substantially different, now requiring binocular movements appropriate for offsetting the disruptive visual effects of longitudinal body/head motion (Rohregger and Dieringer, 2002).…”