The postembryonic development of somatostatin immunoreactivity in the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: a double-labelling study

Neuronal regeneration in the goldfish telencephalon following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) insult

The constitutive regenerative ability of the goldfish central nervous system makes them an excellent model organism to study neurogenesis. Intraperitoneal injection of neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was used to deplete tyrosine hydroxylase-positive neurons in the adult goldfish telencephalon. We report novel information on the ability of the goldfish to regenerate (∼3–4 d post-MPTP insult) damaged neurons in telencephalic tissue by observing the rapid incorporation of bromodeoxyuridine into newly generated cells, which precedes the recovery of motor function in MPTP-treated animals. Specifically, the telencephalon area telencephali pars dorsalis in female goldfish, which is associated with fish motor activity, regenerates following MPTP toxicity. The remarkable ability of goldfish to rapidly regenerate damaged neurons provides insight into their use as model organisms to study neuroregenerative abilities within a few days following injury. We provide evidence that goldfish are able to regenerate neurons in ∼3–4 d to both replenish and recover baseline catecholaminergic levels, thus enabling the fish to reestablish basic activities such as swimming. The study of neuron regeneration in the damaged goldfish brain will increase our understanding of vertebrate neurogenesis and regeneration processes following central nervous system injury.

Neurogenesis and neuronal regeneration in the adult fish brain

Fish are distinctive in their enormous potential to continuously produce new neurons in the adult brain, whereas in mammals adult neurogenesis is restricted to the olfactory bulb and the hippocampus. In fish new neurons are not only generated in structures homologous to those two regions, but also in dozens of other brain areas. In some regions of the fish brain, such as the optic tectum, the new cells remain near the proliferation zones in the course of their further development. In others, as in most subdivisions of the cerebellum, they migrate, often guided by radial glial fibers, to specific target areas. Approximately 50% of the young cells undergo apoptotic cell death, whereas the others survive for the rest of the fish's life. A large number of the surviving cells differentiate into neurons. Two key factors enabling highly efficient brain repair in fish after injuries involve the elimination of damaged cells by apoptosis (instead of necrosis, the dominant type of cell death in mammals) and the replacement of cells lost to injury by newly generated ones. Proteome analysis has suggested well over 100 proteins, including two dozen identified ones, to be involved in the individual steps of this phenomenon of neuronal regeneration.

From oscillators to modulators: behavioral and neural control of modulations of the electric organ discharge in the gymnotiform fish, Apteronotus leptorhynchus

The brown ghost (Apteronotus leptorhynchus) is a weakly electric gymnotiform fish that produces wave-like electric organ discharges distinguished by their enormous degree of regularity. Transient modulations of these discharges occur both spontaneously and when stimulating the fish with external electric signals that mimic encounters with a neighboring fish. Two prominent forms of modulations are chirps and gradual frequency rises. Chirps are complex frequency and amplitude modulations lasting between 20 ms and more than 200 ms. Based on their biophysical characteristics, they can be divided into four distinct categories. Gradual frequency rises consist of a rise in discharge frequency, followed by a slow return to baseline frequency. Although the modulatory phase may vary considerably between a few 100 ms and almost 100 s, there is no evidence for the existence of distinct categories of this type of modulation signal. Stimulation of the fish with external electric signals results almost exclusively in the generation of type-2 chirps. This effect is independent of the chirp type generated by the respective individual under non-evoked conditions. By contrast, no proper stimulation condition is known to evoke the other three types of chirps or gradual frequency rises in non-breeding fish. In contrast to the type-2 chirps evoked when subjecting the fish to external electric stimulation, the rate of spontaneously produced chirps is quite low. However, their rate appears to be optimized according to the probability of encountering a conspecific. As a result, the rate of non-evoked chirping is increased during the night when the fish exhibit high locomotor activity and in the time period following external electric stimulation. These, as well as other, observations demonstrate that both the type and rate of modulatory behavior are affected by a variety of behavioral conditions. This diversity at the behavioral level correlates with, and is likely to be causally linked to, the diversity of inputs received by the neurons that control chirps and gradual frequency rises, respectively. These neurons form two distinct sub-nuclei within the central posterior/prepacemaker nucleus in the dorsal thalamus. In vitro tract-tracing experiments have elucidated some of the connections of this complex with other brain regions. Direct input is received from the optic tectum. Indirect input arising from telencephalic and hypothalamic regions, as well as from the preoptic area, is relayed to the central posterior/prepacemaker nucleus via the preglomerular nucleus. Feedback loops may be provided by projections of the central posterior/prepacemaker nucleus to the preglomerular nucleus and the nucleus preopticus periventricularis.

Spatio-temporal distribution of microglia/macrophages during regeneration in the cerebellum of adult teleost fish, Apteronotus leptorhynchus: a quantitative analysis

In contrast to mammals, adult teleost fish exhibit an enormous capacity to replace damaged neurons with newly generated ones after injuries in the central nervous system. In the present study, the role of microglia/macrophages, identified by tomato lectin binding, was examined in this process of neuronal regeneration in the corpus cerebelli of the teleost fish Apteronotus leptorhynchus. In the intact corpus cerebelli, or after short survival times following application of a mechanical lesion to this cerebellar subdivision, microglia/macrophages were virtually absent. Conversely, approximately 3 days after application of the lesion, the areal density of microglia/macrophages started to increase at and near the lesion site in the ipsilateral hemisphere, as well as in the contralateral hemisphere, and reached maximum levels at approximately 10 days post lesion. The density remained elevated until it reached background levels approximately one month after the injury. By comparing the time course of the appearance of microglia/macrophages with that of other regenerative events occurring within the first few weeks of wound healing in this model system, we hypothesize that one possible function of microglia/macrophages might be to remove debris of cells that have undergone apoptotic cell death at the lesion site.

Potential role of radial glia in adult neurogenesis of teleost fish

Persistence of radial glia within the adult central nervous system is a widespread phenomenon among fish. Based on a series of studies in the teleost species Apteronotus leptorhynchus, we propose that one function of this persistence is the involvement of radial glia in adult neurogenesis, i.e., the generation and further development of new neurons in the adult central nervous system. In particular, evidence has been obtained for the involvement of radial glia in the guidance of migrating young neurons in both the intact and the regenerating brain; for a possible role as precursor cells from which new neurons arise; and for its role as a source of trophic substances promoting the generation, differentiation, and/or survival of new neurons. These functions contribute not only to the potential of the intact brain to generate new neurons continuously, and of the injured brain to replace damaged cells by newly generated ones, but they also provide an essential part of the cellular substrate of behavioral plasticity.

Connections between the central posterior/prepacemaker nucleus and hypothalamic areas in the weakly electric fish Apteronotus leptorhynchus: evidence for an indirect, but not a direct, link

The central posterior/prepacemaker nucleus (CP/PPn) of the weakly electric fish Apteronotus leptorhynchus consists of a few thousands of neurons in the dorsal thalamus. Subpopulations of this complex play a crucial role in neural control of transient modulations of the otherwise extremely constant electric organ discharges. Because both the propensity to execute these modulations and the type of modulations produced may vary enormously with the behavioral situation, it has been hypothesized that this behavioral plasticity is, partially, mediated by peptidergic neuromodulators originating from hypothalamic areas. To define the structural basis of this proposed modulatory input, we have in the present study examined the connections between the CP/PPn proper and hypothalamic areas by employing an in vitro tract-tracing technique. Neither anterograde nor retrograde tracing experiments could provide evidence for the existence of a direct link between the CP/PPn proper and hypothalamic areas. However, the results of our investigation suggest an indirect connection between the CP/PPn proper and two hypothalamic regions, the hypothalamus ventralis and the hypothalamus lateralis, with the preglomerular nucleus serving as a relay station.

Neuronal regeneration in the cerebellum of adult teleost fish, Apteronotus leptorhynchus: guidance of migrating young cells by radial glia

In contrast to mammals, adult fish exhibit an enormous potential to replace injured brain neurons by newly generated ones. In the present study, the role of radial glia, identified by immunostaining against fibrillary acidic protein (GFAP), was examined in this process of neuronal regeneration. Approximately 8 days after application of a mechanical lesion to the corpus cerebelli in the teleost fish Apteronotus leptorhynchus, the areal density of radial glial fibers increased markedly in the ipsilateral dorsal molecular layer compared to shorter survival times, or to the densities found in the intact brain or in the hemisphere contralateral to the lesion. This density remained elevated throughout the time period of up to 100 days examined. The increase in fiber density was followed approximately 2 days later by a rise in the areal density of young cells, characterized by labeling with the nuclear dye DAPI, in the ipsilateral dorsal molecular layer. Based on this remarkable spatio-temporal correlation, and the frequently observed close apposition of elongated young cells to radial glial fibers, we hypothesize that radial glia play an important role in the guidance of migrating young cells from their proliferation zones to the site of lesion where regeneration takes place.

Proliferation zones in the adult brain of a sequential hermaphrodite teleost species (Sparus aurata)

Teleost sex change is an important model to understand general principles of sexual differentiation and plasticity in the adult brain. The present study is the first to examine the proliferation zones in the adult brain of males, females and sex-changing individuals of a protandrous teleost species (Sparus aurata), by means of 5-bromo-2-deoxyuridine immunocytochemistry. Postnatal neurogenesis in the marine teleost brain was found in ventricular and subventricular areas of the brain that in most cases coincided with the embryonic proliferation zones. The molecular layer of corpus and valvula cerebelli exhibited the highest mitotic activity in the adult brain. High mitotic activity was observed in the hypothalamic, thalamic and telencephalic ventricular areas, as well as the dorsal and ventral rim of the optic tectum. Most of the labeled cells were elongated, indicating the initiation of migratory activity. There were no qualitative differences in the distribution of proliferation zones between the sex phases studied with the exception of the ventricular region of the dorsal hypothalamic area. Volume fraction analysis of the area occupied by the labeled cells suggested that this region included higher densities of newborn cells in the female animals. The proliferation pattern in the adult gilthead sea bream brain is in agreement with the hypothesis of the continuous generation of new cells in the teleost brain. Moreover, our data propose that cell proliferation differences possibly existing in the ventricular region of the dorsal hypothalamus between sexual phases, might be involved in central mechanisms of sexual plasticity in protandrous hermaphrodite teleosts.

Social status controls somatostatin neuron size and growth

Many animal species show flexible behavioral responses to environmental and social changes. Such responses typically require changes in the neural substrate responsible for particular behavioral states. We have shown previously in the African cichlid fish, Haplochromis burtoni, that changes in social status, including events such as losing or winning a territorial encounter, result in changes in somatic growth rate. Here we demonstrate for the first time that changes in social status cause changes in the size of neurons involved in the control of growth. Specifically, somatostatin-containing neurons in the hypothalamus of H. burtoni increase up to threefold in volume in dominant and socially descending animals compared with cell sizes in subordinate and socially ascending fish. Because somatostatin is known to be an inhibitor of growth hormone release, the differences in cell size suggest a possible mechanism to account for the more rapid growth rates of subordinate and socially ascending animals compared with those of dominant or socially descending fish. These results reveal possible mechanisms responsible for socially induced physiological plasticity that allow animals to shift resources from reproduction to growth or vice versa depending on the social context.

Cell proliferation after lesions in the cerebellum of adult teleost fish: time course, origin, and type of new cells produced

In contrast to mammals, fish exhibit an enormous capacity to replace damaged neurons following injuries to the adult central nervous system. As the mechanisms controlling this so-called neuronal regeneration are unknown, we have, in the present study, examined the role of cell proliferation in this process. Lesions were applied to one subdivision of the cerebellum, the corpus cerebelli, in the teleost fish Apteronotus leptorhynchus. Proliferative activity was monitored through incorporation of the thymidine analogue 5-bromo-2'-deoxyuridine into replicating DNA. Cerebellar lesions induce high proliferative activity especially in areas in close vicinity to the injury, although the number of cells produced is also increased in other regions of the corpus cerebelli. Many of the cells generated in these areas become, after migration, specifically incorporated at the site of the lesion. The vast majority of them is dividing between 1 and 10 days following the lesion, with the maximum proliferative activity occurring at 5 days. Remarkably, also cells dividing 2 days prior to applying a lesion participate, at a significant number, in the regenerative process. Combination of 5-bromo-2'-deoxyuridine labeling with retrograde tract-tracing techniques demonstrated that at least some of the new cells that replace damaged neurons are cerebellar granule cells. This ability to generate new neurons, together with the previously described occurrence of apoptosis to remove damaged cells, is likely to form the basis for the enormous capacity of teleost fish to perform neuronal regeneration.

Molecular cloning and pharmacological characterization of a somatostatin receptor subtype in the gymnotiform fish Apteronotus albifrons

The actions of the various forms of somatostatin (SRIF), including those of the tetradecapeptide SRIF(14), are mediated by specific receptors. In mammals, five subtypes of SRIF receptors, termed sst(1-5), have been cloned. Using a combination of reverse transcriptase-polymerase chain reaction and genomic library screening in the gymnotiform fish Apteronotus albifrons, a gene encoding the first-known nonmammalian SRIF receptor has been isolated. The deduced amino acid sequence displays 59% identity with the human sst(3) receptor protein; hence, the gene is termed "Apteronotus sst(3)." The predicted protein consists of 494 amino acid residues exhibiting a putative seven-transmembrane domain topology typical of G protein-coupled receptors. A signal corresponding to the Apteronotus sst(3) receptor was detected in brain after amplification of poly(A)(+)-RNA by reverse transcriptase-polymerase chain reaction, but not by Northern blot analysis or in situ hybridization, suggesting a low level of expression. Membranes prepared from CCL39 cells stably expressing the Apteronotus sst(3) receptor gene bound [(125)I][Leu(8),d-Trp(22), (125) I-Tyr(25)]SRIF(28) with high affinity and in a saturable manner (B(max) = 4470 fmol/mg protein; pK(D) = 10.5). SRIF(14) and various synthetic SRIF receptor agonists produced a dose-dependent inhibition of radioligand binding, with the following rank order of potency: SRIF(14) approximately SRIF(28) > BIM 23052 > octreotide > BIM 23056. Under low stringency conditions, an Apteronotus sst(3) probe hybridized to multiple DNA fragments in HindIII or EcoRI digests of A. albifrons DNA, indicating that the Apteronotus sst(3) receptor is a member of a larger family of Apteronotus SRIF receptors.

Neurogenesis, cell death and regeneration in the adult gymnotiform brain

Gymnotiform fish, like all teleosts examined thus far, are distinguished by their enormous potential for the production of new neurons in the adult brain. In Apteronotus leptorhynchus, on average 10(5) cells, corresponding to approximately 0.2 % of the total population of cells in the adult brain, are in S-phase within any period of 2 h. At least a portion of these newly generated cells survive for the rest of the fish's life. This long-term survival, together with the persistent generation of new cells, leads to a continuous growth of the brain during adulthood. Zones of high proliferative activity are typically located at or near the surface of the ventricular, paraventricular and cisternal systems. In the central posterior/ prepacemaker nucleus, for example, new cells are generated, at very high rates, in areas near the wall of the third ventricle. At least some of these cells differentiate into neurons, express immunoreactivity against the neuropeptide somatostatin and migrate into more lateral areas of this complex. Approximately 75 % of all new brain cells are generated in the cerebellum. In the corpus cerebelli and the valvula cerebelli, they are produced in the molecular layers, whereas in the eminentia granularis the newborn cells stem from proliferation zones in the pars medialis. Within the first few days of their life, these cells migrate towards specific target areas, namely the associated granule cell layers. At least some of them develop into granule neurons. The high proliferative activity is counterbalanced by apoptosis, a mechanism that resembles the processes known from embryonic development of the vertebrate brain. Apoptosis also appears to be used as an efficient mechanism for the removal of cells damaged through injury in the brain of adult Apteronotus leptorhynchus. Since apoptosis is not accompanied by the side effects known from necrosis, this ‘clean’ type of cell death may, together with the enormous proliferative activity in the brain, explain, at least partially, the tremendous capability of teleost fish to replace damaged neurons with newly generated ones. One factor that appears to play a major role in the generation of new cells and in their further development is the neuropeptide somatostatin. In the caudal cerebellum of the gymnotiform brain, somatostatin-binding sites are expressed, at extremely high densities, at sites corresponding to the areas of origin, migration and differentiation of the newborn cells. This pattern of expression resembles the expression pattern in the rat cerebellum, where somatostatin immunoreactivity and somatostatin-binding sites are transiently expressed at the time when the granule cells of the cerebellum are generated. Moreover, after mechanical lesions of the corpus cerebelli, the expression of somatostatin-like immunoreactivity is tremendously increased in several cell types (presumably astrocytes, microglia and granule cell neurons) near the path of the lesion; the time course of this expression coincides with the temporal pattern underlying the recruitment of new cells incorporated at the site of the lesion.

Apoptosis after injuries in the cerebellum of adult teleost fish

In contrast to mammals, all teleost fish examined thus far exhibit an enormous potential to regenerate not only neuronal processes (axonal regeneration), but even whole neurons (neuronal regeneration) after injuries in the central nervous system. By application of lesions to one subdivision of the cerebellum, the corpus cerebelli, the role of apoptosis in neuronal regeneration was examined in the gymnotiform fish, Apteronotus leptorhynchus. Apoptotic cells were identified by examination of cryosections with the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL) reaction, an in situ technique employed for detection of nuclear DNA fragmentation. Additional evidence for the apoptotic nature of dying cells was obtained through analysis of morphologies displayed at both the light microscopic and the ultrastructural level. The first TUNEL-positive cells at the site of the lesion appeared as rapidly as 5 min following mechanical damage of the tissue. Thirty minutes after stab wound lesion, their number reached maximum levels. Starting with 2 days of postlesioning survival time, a gradual decline in the number of TUNEL-positive cells was evident, until this process reached background levels 20 days after the lesion. We hypothesize that apoptosis is used in A. leptorhynchus as an efficient mechanism for the removal of cells damaged through injury in the central nervous system. Since apoptosis is not accompanied by the side-effects known from necrosis (which is predominant after injuries in the mammalian central nervous system), this "clean" type of cell death may, at least partially, explain the tremendous regenerative capability of teleosts.

A distinct population of neurons in the central posterior/prepacemaker nucleus project to the nucleus preopticus periventricularis in the weakly electric gymnotiform fish, Apteronotus leptorhynchus

The central posterior/prepacemaker nucleus of weakly electric gymnotiform fish is a cell cluster in the dorsal thalamus involved in neural control of electric behaviors. By employing anterograde and retrograde tract-tracing techniques, we examined the neural connection between this complex and the preoptic area in Apteronotus leptorhynchus. Unilateral application of biocytin restricted to the region defined by the somata of the central posterior/prepacemaker nucleus revealed a network of fibers and terminals bilaterally in the anterior and posterior subdivisions of the nucleus preopticus periventricularis. Application of biocytin to the nucleus preopticus periventricularis demonstrated that these fibers arise from a small population of cell bodies located predominantly in the central and medial portions of the central posterior/prepacemaker nucleus. These somata were distinguished from the remaining cells in this complex not only by their pattern of connectivity, but also by their position within the cluster and by the relatively large size. The projection from the central posterior/prepacemaker nucleus to the nucleus preopticus periventricularis may provide a feedback loop complementing a recently described connection projecting from the preoptic area to the central posterior/prepacemaker nucleus with one synaptic link in the preglomerular nucleus.

Reciprocal connections between the preglomerular nucleus and the central posterior/prepacemaker nucleus in the diencephalon of weakly electric fish, Apteronotus leptorhynchus

The central posterior/prepacemaker nucleus of gymnotiform fish is a bilateral cell group located in the dorsal thalamus. This complex consists of approximately 10,000 neurons which can be divided into several subpopulations. One subpopulation comprised of a few hundreds of neurons projects to the pacemaker nucleus in the medulla oblongata, thus constituting the prepacemaker nucleus portion of this complex. By employing in vitro tract-tracing techniques, we have, in the present investigation, examined the pattern of connectivity formed by the central posterior/prepacemaker nucleus with a diencephalic cell group, the preglomerular nucleus. As demonstrated by anterograde and retrograde tracing, a subpopulation of several hundreds of neurons located in the central posterior/prepacemaker nucleus project to the ipsi- and contralateral preglomerular nucleus. Double-labelling experiments revealed that at least a fraction of these neurons also innervate the pacemaker nucleus. In the preglomerular nucleus, a large number of neurons give rise to projections that terminate in the ipsilateral central posterior/prepacemaker nucleus. The reciprocal connection between the central posterior/prepacemaker nucleus and the preglomerular nucleus may be used to relay sensory information directly conveyed to one of the two nuclei indirectly to the other nucleus. The existence of at least some central posterior/prepacemaker nucleus neurons projecting to both the preglomerular nucleus and the pacemaker nucleus may provide the morphological basis for the transmission of an efference copy of electromotor information produced by neurons in the central posterior/prepacemaker nucleus to the preglomerular nucleus.

Expression of somatostatin in neurons of the central posterior/prepacemaker nucleus projecting to the preglomerular nucleus: immunohistochemical evidence for a non-synaptic function

In the diencephalon of the weakly electric gymnotiform fish Apteronous leptorhynchus, part of the central posterior/prepacemaker nucleus innervates the preglomerular nucleus. A minor population of these neurons expresses immunoreactivity against somatostatin, as has been shown by combining peptide immunohistochemistry with an in vitro tract-tracing technique. In contrast to the expectation, however, this neuropeptide does not appear to be transported along the axons to the projection site, as somatostatin-like immunoreactivity could not be detected in the preglomerular nucleus. It is, therefore, likely that somatostatin expressed in these neurons exerts a non-synaptic function in the region of the central posterior/prepacemaker nucleus itself.

Long-term survival of postembryonically born cells in the cerebellum of gymnotiform fish, Apteronotus leptorhynchus

The gymnotiform fish Apteronotus leptorhynchus is, like all teleosts examined thus far, distinguished by its enormous potential for the production of new neurons in the adult brain. In the cerebellum, cells are generated continuously and at high rate in discrete proliferation zones. From there, they migrate into specific target areas comprised of granule cell layers in the four cerebellar subdivisions. The long-term fate of these cells was followed through labelling with 5-bromo-2'-deoxyuridine. Employment of survival times of up to 440 days after the administration of this thymidine analogue revealed that the newborn cells survive for extremely long periods of time, spanning most of the fish's adult life, without exhibiting a decline in their number. This long-term survival, together with the permanent addition of new cells to the population of older cells, forms the basis for the continuous growth of the cerebellum during adulthood.

Apoptosis in the cerebellum of adult teleost fish, Apteronotus leptorhynchus

While involvement of programmed cell death (apoptosis) in embryogenesis is well established, only very little is known about this phenomenon in later stages of development. Based primarily on indirect evidence, it has been proposed that during postembryonic development of fish cell death does not occur. We have re-addressed this issue by examining the gymnotiform fish Apteronotus leptorhynchus. This teleost exhibits a high degree of proliferative activity in the brain during adulthood. Most of these cells are born in the cerebellum, where they differentiate, migrate into specific target regions, and are added to the population of already existing cerebellar cells. By applying morphological criteria and an in situ technique for the detection of DNA fragmentation (a feature characteristic of apoptotic cells), we show here that a large number of cerebellar cells undergo apoptosis. The density of apoptotic cells is significantly higher in the granule cell layers of the subdivisions of the cerebellum than in the corresponding molecular layers. This finding is consistent with previous observations indicating a drastic reduction in areal density of newborn cells within these granule cell layers in a period 4-7 weeks after their generation. In the granule cell layers of two cerebellar subdivisions, the corpus cerebelli and the valvula cerebelli pars medialis, the areal density of apoptotic cells displays a significant negative correlation with body weight, thus pointing to a decrease in the number of apoptotic events with age. The results of our investigation provide clear evidence for the existence of apoptosis during adulthood in fish and underline the significance of this process in the postembryonic development of the brain.

Salvage pathway of pyrimidine synthesis: divergence of substrate specificity in two related species of teleostean fish

For nucleotide synthesis, cells use purine and pyrimidine nucleosides generated either through de novo synthesis or through utilization of salvage pathways. In the pyrimidine salvage pathway, thymidine is taken up by transport proteins and phosphorylated by the enzyme thymidine kinase to thymidine monophosphate. So far, all vertebrates analyzed are able to use radioactively labeled thymidine for the biosynthesis of nucleotides in brain tissue. However, when standard autoradiographic, immunohistochemical and biochemical procedures were applied for the detection of the incorporation of tritiated thymidine and the thymidine analogue 5-bromo-2'-deoxyuridine into DNA to two species of gymnotiform fish, a divergence in substrate specificity has been revealed. Although brain cells of the two species, Apteronotus leptorhynchus and Eigenmannia sp., can utilize 5-bromo-2'-deoxyuridine for pyrimidine synthesis, only Eigenmannia sp. is able to incorporate tritiated thymidine into DNA during the S phase of the cell cycle. We hypothesize that this inability to use thymidine for nucleotide synthesis is caused either by a defect in the transport system mediating the uptake of thymidine or by a deficiency in the thymidine kinase of A. leptorhynchus.

Apoptosis as a regulator of cell proliferation in the central posterior/prepacemaker nucleus of adult gymnotiform fish, Apteronotus leptorhynchus

Like many species of teleost fish, the gymnotiform Apteronotus leptorhynchus displays a high degree of proliferative activity in a large number of brain regions during adulthood. One of these regions is the central posterior/prepacemaker nucleus (CP/PPn) in the diencephalon. By applying in situ techniques for the detection of DNA fragmentation, a feature characteristic of apoptotic cells, we show in the present study that the high proliferative activity in the CP/PPn is counterbalanced by programmed cell death. Most of the apoptotic events occur in the ventricular and subventricular zones of this thalamic complex, where the generation of the cells and their differentiation into neurons take place. The demonstration of apoptosis in the CP/PPn provides strong evidence against the hypothesis that animals in which neurogenesis continues beyond embryonic stages of development lack cell death.