Neurogenesis in the olfactory bulb of the frog Xenopus laevis shows unique patterns during embryonic development and metamorphosis.

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog, Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.

Neural pathway for aggressive display in Betta splendens: midbrain and hindbrain control of gill-cover erection behavior.

Horseradish peroxidase (HRP) was used to identify parts of the presumptive neural pathway for gill cover erection, a behavioral display pattern performed by Siamese fighting fish (Betta splendens) during aggressive interactions. Motor, motor integration and sensory areas were identified in the medulla and mesencephalon. Motor neurons of the dilator operculi muscle, the effector muscle for gill cover erection, are located in the lateral and medial parts of the caudal trigeminal motor nucleus. Iontophoretic injections of HRP into the lateral trigeminal motor nucleus resulted in labeled cell bodies in two motor areas (medial part of the trigeminal motor nucleus, anterior part of the motor nucleus of cranial nerve IX-X), two parts of the reticular formation (medial and inferior reticular areas), and two nuclei of the octavolateralis system (nucleus medialis, magnocellular octaval nucleus). The HRP injections in the medial part of the caudal trigeminal motor nucleus resulted in labeled cells in the lateral part of the nucleus and in the medial reticular nucleus. Discrete injections of HRP into nucleus medialis revealed a strong axonal projection that terminated in the torus semicircularis. The medial reticular area and both of the octavolateralis nuclei received projections from their contralateral counterparts. Connections between motor areas, and between parts of the reticular formation, may coordinate the performance of gill cover erection with other behavioral patterns used during aggressive display. Connections with the octavolateralis system may provide information on the strength of an opponent's tail beats via the lateral-line system, as well as vestibular information about the fish's own orientation during aggressive display. The organization of inputs to the trigeminal motor nucleus in Betta, a perciform fish, was found to differ from that reported in the common carp, a cypriniform fish. These differences may underlie the different behavioral capabilities of the two groups of fish.

Motor innervation of respiratory muscles and an opercular display muscle in Siamese fighting fish Betta splendens.

Horseradish peroxidase was used to identify motor neurons projecting to the adductor mandibulae, levator hyomandibulae, levator operculi, adductor operculi, and dilator operculi muscles in Siamese fighting fish, Betta splendens. These muscles participate in the production of respiratory and feeding movements in teleost fishes. The dilator operculi is also the effector muscle for gill-cover erection behavior that is part of Betta's aggressive display. The motor innervation of these muscles in Betta was compared to that previously described for carp. Motor neurons of the adductor mandibulae, levator hyomandibulae, and dilator operculi are located in the trigeminal motor nucleus, and motor neurons of the adductor operculi and levator operculi are located in the facial motor nucleus in Betta and in carp. The trigeminal motor nucleus in both species is divided into rostral and caudal subnuclei. However, there are substantial differences in the organization of the subnuclei, and in the distribution of motor neurons within them. In Betta, the rostral trigeminal subnucleus consists of a single part but the caudal subnucleus is divided into two parts. Motor neurons for the dilator operculi and levator hyomandibulae muscles are located in the lateral part of the caudal subnucleus; the medial part of the caudal subnucleus contains only dilator operculi motor neurons. The single caudal subnucleus in carp is located laterally, and contains motor neurons of both the dilator operculi and levator hyomandibulae muscles. Differences in the organization of the trigeminal motor nucleus may relate to the use of the dilator operculi muscle for aggressive display behavior by perciform fishes such as Betta but not by cypriniform fishes such as carp. Five species of perciform fishes that perform gill-cover erection behavior had a Betta-like pattern of organization of the caudal trigeminal nucleus and a similar distribution of dilator operculi motor neurons. Goldfish, which like carp are cypriniform fish and do not display, had a carp-like trigeminal organization and dilator operculi motor neuron distribution.

Neurogenesis in the vocalization pathway of Xenopus laevis.

We examined possible contributions of neurogenesis to sex differences in the vocalization pathway of the South African clawed frog, Xenopus laevis. Birthdates of neurons were obtained from autoradiograms of animals receiving tritiated thymidine from gastrulation through 1 month after metamorphosis. Thymidine availability studies showed that 80% of the [3H]-thymidine injected into embryos and tadpoles was incorporated into the DNA of dividing cells within 3 hours. We observed 3 patterns of neurogenesis: late-short, a short burst of proliferation occurred late in development in the anterior preoptic area, the ventromedial nucleus of the thalamus, and the pretrigeminal nucleus of the dorsal tegmental area of the medulla; protracted-bimodal, a prolonged period of proliferation with an early and a late peak in the number of labeled cells occurred in the ventral striatum and in the ventrolateral and posterior nuclei of the thalamus; protracted-unimodal, a prolonged period of proliferation with a single early peak occurred in the inferior reticular formation and in the medial and lateral nucleus IX-X (containing laryngeal motor neurons). There were no differences between sexes in the number of tritiated thymidine labeled cells in any nucleus. The difference in nucleus IX-X neuron number in adults does not appear to result from sex differences in the proliferation of these cells during development. Since neurons in the vocalization pathway do not exhibit androgen receptors until after neurogenesis is complete, we also conclude that androgen probably does not regulate the genesis of these cells.

The ontogeny of androgen receptors in the CNS of Xenopus laevis frogs.

Androgenic steroids have been implicated in the development of sex differences in Xenopus laevis frogs. In order to determine when neurons first acquire the ability to concentrate androgen, we prepared autoradiograms of CNS in developing frogs following injection of tritiated dihydrotestosterone (DHT). X. laevis tadpoles and juveniles from stage 60 to 2 months post-metamorphosis (PM) were injected with [3H]DHT. Brain and spinal cord autoradiograms from these animals were examined for the presence of labelled cells. The pattern of [3H]DHT labelling in stage-64 tadpoles and in PM juveniles was similar but not identical to that seen in adults. Heavily labelled cells were seen in the motor nucleus of cranial nerves IX and X, medullary reticular formation, a presumed sensory nucleus of cranial nerve V, pretrigeminal nucleus of the dorsal tegmental area of the medulla, laminar nucleus of the torus semicircularis, anterior pituitary, ventral thalamus and anterior spinal cord. The vestibular sensory nucleus of cranial nerve VIII was the only area that concentrates DHT in adults but did not contain labelled cells in young animals. No [3H]DHT-labelled cells were found in stage-60 tadpoles. The onset of androgen concentrating capability in X. laevis CNS thus probably occurs between stages 60 and 64.