The origin of trochlear motoneurons in the larval sea lamprey, Petromyzon marinus L. An HRP study

Ontogeny of gamma-aminobutyric acid-immunoreactive neurons in the rhombencephalon and spinal cord of the sea lamprey

The development of neurons expressing gamma-aminobutyric acid (GABA) in the rhombencephalon and spinal cord of the sea lamprey (Petromyzon marinus) was studied for the first time with an anti-GABA antibody. The earliest GABA-immunoreactive (GABAir) neurons appear in late embryos in the basal plate of the isthmus, caudal rhombencephalon, and rostral spinal cord. In prolarvae, the GABAir neurons of the rhombencephalon appear to be distributed in spatially restricted cellular domains that, at the end of the prolarval period, form four longitudinal GABAir bands (alar dorsal, alar ventral, dorsal basal, and ventral basal). In the spinal cord, we observed only three GABAir longitudinal bands (dorsal, intermediate, and ventral). The larval pattern of GABAir neuronal populations was established by the 30-mm stage, and the same populations were observed in premetamorphic and adult lampreys. The ontogeny of GABAergic populations in the lamprey rhombencephalon and spinal cord is, in general, similar to that previously described in mouse and Xenopus.

Distribution of choline acetyltransferase-immunoreactive structures in the lamprey brain

The distribution of cholinergic neurons and fibers was studied immunohistochemically in the brain of two species of lampreys (Petromyzon marinus and Lampetra fluviatilis), by using an antiserum against choline acetyltransferase (ChAT). The results obtained in both species were similar, but there appeared some interspecies differences. In the forebrain, cholinergic cells were present in the striatum, preoptic region, paraventricular nucleus, pineal and parapineal organs, habenula, and pretectum. The cranial nerve motoneurons (III, IV, V, VI, VII, IX, and X), the first and second spino-occipital nerves (so), and the ventral horn of the spinal cord showed a strong ChAT immunoreactivity. Additional cholinergic neurons were observed: the mesencephalic M5 nucleus of Schober, two different cell populations in the isthmic region, the efferent component of the eighth nerve, putative preganglionic parasympathetic cells, cells in the solitary tract nucleus, and the rhombencephalic reticular formation. Cholinergic fibers were widely distributed in the brain. Comparison with previous studies in other vertebrates suggests that major cholinergic pathways, like tectal innervation from the isthmic region, are also present in lampreys. Of particular interest was the prominent projection to the neurohypophysis from cholinergic neurons in the preoptic region and paraventricular nucleus. Present data were analyzed within the segmental paradigm, as was previously done in other vertebrates. Our results reveal that the organization of many cholinergic systems in the lamprey as, for example, in the striatal, preoptic, and isthmic regions, comprises features of the anamniote brain that remain common to all living amniotes studied so far, thus being conservative to a surprisingly high degree. Therefore, the distribution of ChAT-immunoreactive structures in the lamprey brain is, in general, comparable to that previously described in other vertebrate species.

GABA-immunoreactive internuclear neurons in the ocular motor system of lampreys

The presence of internuclear neurons in the abducens and oculomotor nuclei of lampreys [González, M.J., Pombal, M.A., Rodicio, M.C. and Anadón, R., Internuclear neurons of the ocular motor system of the larval sea lamprey, J. Comp. Neurol. 401 (1998) 1-15] indicates that coordination of eye movements by internuclear neurons appeared early during the evolution of vertebrates. In order to investigate the possible involvement of inhibitory neurotransmitters in internuclear circuits, the distribution of gamma-aminobutyric acid (GABA) in the extraocular motor nuclei of the lamprey was studied using immunocytochemical techniques. Small GABA-immunoreactive (GABAir) neurons were observed in the three ocular motor nuclei. Numerous GABAir neurons were observed in the group of internuclear neurons of the dorsal rectus oculomotor subnucleus. A second group of GABAir neurons was observed among and below the trochlear motoneurons. Two further groups of GABAir interneurons, periventricular and lateral, were located in the abducens nucleus among the cells of the caudal rectus and the ventral rectus motor subnuclei, respectively. In addition to the presence of GABAir neurons, in all the ocular motor nuclei the motoneurons were contacted by numerous GABAir boutons. Taken together, these results suggest that GABA is involved as a neurotransmitter in internuclear pathways of the ocular motor system of lampreys.

Secondary vestibulo-oculomotor projections in larval sea lamprey: anterior octavomotor nucleus

The ventral octavolateral area of lampreys contains three nuclei: the anterior, intermediate and posterior octavomotor nuclei, formed of large neurons that are contacted by thick primary vestibular fibres. We used horseradish peroxidase (HRP) or fluorescein-dextran-amine (FDA) labelling to study the projections of the anterior octavomotor nucleus (AON) in the larval sea lamprey, Petromyzon marinus. The tracers were injected either in the AON, the oculomotor nucleus or the rostralmost spinal cord. HRP injection in the AON labelled thick axons that coursed to the basal mesencephalic tegmentum, where most decussate and project to the oculomotor nucleus and the third Müller cell. Electron microscopy confirmed that AON axons contact with the contralateral third Müller cell and with oculomotor neurons. Some AON axons run in the mesencephalic tegmentum and the ventral diencephalon. An AON axon was observed to run close to the axon of the contralateral third Müller cell, establishing what appeared to be en passant contacts. HRP injection in the AON also revealed commissural fibres projecting to the contralateral octavolateral area. HRP or FDA injections in the oculomotor nucleus labelled both large and small neurons of the AON, mostly contralateral to the injection site, as well as of cells in the intermediate octavomotor nucleus, mainly ipsilateral. HRP injection in the AON or in the rostral spinal cord did not reveal any projections from the AON to the spinal cord. Our results indicate that the pattern of octavo-oculomotor connections in the lamprey is different from that observed in other vertebrates.

Developmental increases in expression of neurofilament mRNA selectively in projection neurons of the lamprey CNS

Neurofilaments of the sea lamprey are unique in being homopolymers of a single subunit (NF-180). Digoxigenin-labeled RNA probes complementary to NF-180 were used to determine the distribution and timing of expression of neurofilament message in the brain and spinal cord of the lamprey. In the brainstem, detection of NF-180 mRNA was restricted to neurons with axons projecting to the spinal cord or the periphery. The majority of brainstem neurons, whose axons project locally, did not express NF-180 within the detection limits of this technique. NF-180-positive neurons included cells with a wide range of axon diameters, suggesting neurofilament mRNA expression was linked to axon length rather than caliber. To further evaluate this hypothesis, expression was studied in animals of different developmental stages between larvae and adults. In younger (shorter) larvae, the large Mauthner and rhombencephalic Müller cells did not express NF-180 mRNA, even though their axons are among the largest caliber in the animal and extend the entire length of the spinal cord. In contrast, many other reticulospinal neurons, whose axons are smaller in diameter than those of the Müller and Mauthner cells, expressed NF-180 message throughout larval development. Furthermore, neurons of the cranial motor nuclei did not express NF-180 until later developmental stages and the extraocular motor neurons did not label until metamorphosis. Therefore, while detectable neurofilament mRNA expression in the lamprey is restricted to neurons with long axons, its expression in this population of neurons appears to be developmentally regulated by factors still not determined. It is postulated that need for NF message is determined by a balance between the volume of axon to be filled and the rate of turnover of NF in that axon.

Development and organization of the ocular motor nuclei in the larval sea lamprey, Petromyzon marinus L.: an HRP study

Retrograde transport of horseradish peroxidase (HRP) after its application into the orbit was used to investigate the development of the different ocular motor nuclei in larvae of the sea lamprey (Petromyzon marinus) and to identify their regions of origin. In the smallest larvae studied (10-19 mm in length), the oculomotor and abducens neurons were ipsilateral to the site of HRP application, whilst trochlear neurons were contralateral. These motoneurons did not have dendritic processes. In larvae more than 19 mm in length, both ipsilateral and contralateral components were found in the oculomotor and trochlear nuclei; dendrites were present, and their length and branching increased with larval age. An adult-like pattern of topographic organization and dendritic arborization was reached in larvae of about 45-60 mm in length. In oculomotor neurons, medial dendrites appear first, then dorsolateral dendrites, and finally ventral dendrites. Similarly, in trochlear neurons ventral and ventrolateral dendrites develop first, followed by dorsal dendrites that course either to the caudal optic tectum or to the terminal fields of the octaval and lateral line nerves in the cerebellar plate. Dorsal and ventral dendrites of the abducens neurons arise at the same time, but dorsal dendrites attain an adult-like morphology earlier. A few motoneurons showed ventricular attachments in larvae longer than 40 mm. The significance of these processes and their possible usefulness as a marker for the regions of origin of the ocular motor nuclei are discussed. Finally, the results presented here indicate that differentiation of the ocular motor nuclei in larval lampreys precedes and is independent of the maturation of the eye at transformation.

Origin and migration of trochlear, oculomotor and abducent motor neurons in Petromyzon marinus L

The development of the ocular motor system was studied in 3- to 6-year old larval lampreys with two different retrograde tracers. Motor neurons of the oculomotor and trochlear nuclei are situated closely to one another in younger larvae. Cases in which only trochlear neurons were labelled revealed trochlear motor neurons scattered from the midbrain tegmentum through the anterior medullary velum. We believe this distribution reflects the place of final mitosis (midbrain tegmentum) and subsequent migration (anterior medullary velum) of lamprey trochlear motor neurons. Evidence is also presented for contralateral migration of oculomotor motor neurons and for ventrolateral migration of abducent motor neurons. The distances covered by migrating ocular motor neurons range from 100 to 150 microns in small larvae; these are distances that could be covered easily during the several years duration of larval development in lampreys.