The structure of the diencephalic prepacemaker nucleus revisited: light microscopic and ultrastructural studies

The effect of urethane and MS-222 anesthesia on the electric organ discharge of the weakly electric fish Apteronotus leptorhynchus

Urethane and MS-222 are agents widely employed for general anesthesia, yet, besides inducing a state of unconsciousness, little is known about their neurophysiological effects. To investigate these effects, we developed an in vivo assay using the electric organ discharge (EOD) of the weakly electric fish Apteronotus leptorhynchus as a proxy for the neural output of the pacemaker nucleus. The oscillatory neural activity of this brainstem nucleus drives the fish's EOD in a one-to-one fashion. Anesthesia induced by urethane or MS-222 resulted in pronounced decreases of the EOD frequency, which lasted for up to 3 h. In addition, each of the two agents caused a manifold increase in the generation of transient modulations of the EOD known as chirps. The reduction in EOD frequency can be explained by the modulatory effect of urethane on neurotransmission, and by the blocking of voltage-gated sodium channels by MS-222, both within the circuitry controlling the neural oscillations of the pacemaker nucleus. The present study demonstrates a marked effect of urethane and MS-222 on neural activity within the central nervous system and on the associated animal's behavior. This calls for caution when conducting neurophysiological experiments under general anesthesia and interpreting their results.

Local vasotocin modulation of the pacemaker nucleus resembles distinct electric behaviors in two species of weakly electric fish

The neural bases of social behavior diversity in vertebrates have evolved in close association with hypothalamic neuropeptides. In particular, arginine-vasotocin (AVT) is a key integrator underlying differences in behavior across vertebrate taxa. Behavioral displays in weakly electric fish are channeled through specific patterns in their electric organ discharges (EODs), whose rate is ultimately controlled by a medullary pacemaker nucleus (PN). We first explored interspecific differences in the role of AVT as modulator of electric behavior in terms of EOD rate between the solitary Gymnotus omarorum and the gregarious Brachyhypopomus gauderio. In both species, AVT IP injection (10μg/gbw) caused a progressive increase of EOD rate of about 30%, which was persistent in B. gauderio, and attenuated after 30min in G. omarorum. Secondly, we demonstrated by in vitro electrophysiological experiments that these behavioral differences can be accounted by dissimilar effects of AVT upon the PN in itself. AVT administration (1μM) to the perfusion bath of brainstem slices containing the PN produced a small and transient increase of PN activity rate in G. omarorum vs the larger and persistent increase previously reported in B. gauderio. We also identified AVT neurons, for the first time in electric fish, using immunohistochemistry techniques and confirmed the presence of hindbrain AVT projections close to the PN that might constitute the anatomical substrate for AVT influences on PN activity. Taken together, our data reinforce the view of the PN as an extremely plastic medullary central pattern generator that not only responds to higher influences to adapt its function to diverse contexts, but also is able to intrinsically shape its response to neuropeptide actions, thus adding a hindbrain target level to the complexity of the global integration of central neuromodulation of electric behavior.

Large-scale identification of proteins involved in the development of a sexually dimorphic behavior

Sexually dimorphic behaviors develop under the influence of sex steroids, which induce reversible changes in the underlying neural network of the brain. However, little is known about the proteins that mediate these activational effects of sex steroids. Here, we used a proteomics approach for large-scale identification of proteins involved in the development of a sexually dimorphic behavior, the electric organ discharge of brown ghost knifefish, Apteronotus leptorhynchus. In this weakly electric fish, the discharge frequency is controlled by the medullary pacemaker nucleus and is higher in males than in females. After lowering the discharge frequency by chronic administration of β-estradiol, 2-dimensional difference gel electrophoresis revealed 62 proteins spots in tissue samples from the pacemaker nucleus that exhibited significant changes in abundance of >1.5-fold. The 20 identified protein spots indicated, among others, a potential involvement of astrocytes in the establishment of the behavioral dimorphism. Indeed, immunohistochemical analysis demonstrated higher expression of the astrocytic marker protein GFAP and increased gap-junction coupling between astrocytes in females compared with males. We hypothesize that changes in the size of the glial syncytium, glial coupling, and/or number of glia-specific potassium channels lead to alterations in the firing frequency of the pacemaker nucleus via a mechanism mediating the uptake of extracellular potassium ions from the extracellular space.

Fiber connections of the diencephalic nucleus tuberis anterior in the weakly electric fish,Gymnotus cf. carapo: An in vivo tract-tracing study

Transport of biotinylated dextran amine shows the spatial segregation of mechanosensory afferents in the nucleus tuberis anterior (TA) of a gymnotiform fish, Gymnotus cf. carapo. Only the intermediate subdivision of this nucleus receives projections from the lateral region of the ventral torus semicircularis (TSv), which represents the principal midbrain center for mechanosensory information processing, and from the ventral nucleus praeeminentialis, which receives collaterals of ascending second order mechanosensory fibers that emerge from the mechanosensory lateral line lobe. Considering this aspect, a rostrocaudal subdivision of the TA is proposed. The TA also receives input from regions subserving other sensory modalities, suggesting a role in multisensory interaction. Another important finding of this work consisted in the demonstration of reciprocal connections between the TA and the inferior lobe of the hypothalamus, which is known to receive gustatory, visual, and electrosensory input and is therefore considered a multisensory integration center involved in feeding and aggressive behavior. Furthermore, reciprocal connections between the TA and the preelectromotor central-posterior/prepacemaker complex may provide an access for the processed mechanosensory information to interact with the transient modulations of the electric organ discharge that accompany different behaviors.

Potential output pathways for agonistic-like responses resulting from the GABAA blockade of the torus semicircularis dorsalis in weakly electric fish, Gymnotus carapo

The purpose of this study is to examine the pathways involved in the electromotor (electric organ discharge interruptions) and skeletomotor responses (defense-like) observed by blockade of GABAergic control of the torus semicircularis dorsalis (TSd) of the awake weakly electric fish Gymnotus carapo, described in a former study. Microinjection of NMDA (5 mM) into the pacemaker nucleus (PM) through a guide cannula previously implanted caused a prolonged interruption of the electric organ discharge (EOD) intermingled with reduction in frequency, similar to that described for TSd GABA(A) blockade, but without noticeable skeletomotor effects. The EOD alterations elicited by bicuculline microinjections (0.245 mM) into the TSd could be blocked or attenuated by a previous microinjection of AP-5 (0.5 mM), an NMDA antagonist, into the PM. Labeled terminals are found in the nucleus electrosensorius (nE) after injection of the biotinylated dextran amine (BDA) tracer into the TSd and into the sublemniscal prepacemaker nucleus (SPPn) subsequent to the tracer injection into the nE. Defense-like responses but not EOD interruptions are observed after microinjections of NMDA (5 mM) into the rhombencephalic reticular formation (RF), where labeled terminals are seen after BDA injection into the TSd and somata are filled after injection of the tracer into the spinal cord. In this last structure, marked fibers are seen subsequent to injection of BDA into the RF. These results suggest that two distinct pathways originate from the torus: one for EOD control, reaching PM through nE and SPPn, and the other one for skeletomotor control reaching premotor reticular neurons. Both paths could be activated by toral GABA(A) blockade.

Walter Heiligenberg: the jamming avoidance response and beyond

Walter Heiligenberg (1938-1994) was an exceptionally gifted behavioral physiologist who made enormous contributions to the analysis of behavior and to our understanding of how the brain initiates and controls species-typical behavioral patterns. He was distinguished by his rigorous analytical approach used in both behavioral studies and neuroethological investigations. Among his most significant contributions to neuroethology are a detailed analysis of the computational rules governing the jamming avoidance response in weakly electric fish and the elucidation of the principal neural pathway involved in neural control of this behavior. Based on his work, the jamming avoidance response is perhaps the best-understood vertebrate behavior pattern in terms of the underlying neural substrate. In addition to this pioneering work, Heiligenberg stimulated research in a significant number of other areas of ethology and neuroethology, including: the quantitative assessment of aggressivity in cichlid fish; the ethological analysis of the stimulus-response relationship in the chirping behavior of crickets; the exploration of the neural and endocrine basis of communicatory behavior in weakly electric fish; the study of cellular mechanisms of neuronal plasticity in the adult fish brain; and the phylogenetic analysis of electric fishes using a combination of morphology, electrophysiology, and mitochondrial sequence data.

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.

Interruption of pacemaker signals by a diencephalic nucleus in the African electric fish, Gymnarchus niloticus

The African electric fish Gymnarchus niloticus rhythmically emits electric organ discharges (EODs) for communication and navigation. The EODs are generated by the electric organ in the tail in response to the command signals from the medullary pacemaker complex, which consists of a pacemaker nucleus (PN), two lateral relay nuclei (LRN) and a medial relay nucleus (MRN). The premotor structure and its modulatory influences on the pacemaker complex have been investigated in this paper. A bilateral prepacemaker nucleus (PPn) was found in the area of the dorsal posterior nucleus (DP) of the thalamus by retrograde labeling from the PN. No retrogradely labeled neurons outside the pacemaker complex were found after tracer injection into the LRN or MRN. Accordingly, anterogradely labeled terminal fibers from PPn neurons were found only in the PN. Iontophoresis of L-glutamate into the region of the PPn induced EOD interruptions. Despite the exclusive projection of the PPn neurons to the PN, extracellular and intracellular recordings showed that PN neurons continue their firing while MRN neurons ceased their firing during EOD interruption. This mode of EOD interruption differs from those found in any other weakly electric fishes in which EOD cessation mechanisms have been known.

Sex and species differences in neuromodulatory input to a premotor nucleus: A comparative study of substance P and communication behavior in weakly electric fish

Many electric fish species modulate their electric organ discharges (EODs) to produce transient social signals that vary in number and structure. In Apteronotus leptorhynchus, males modulate their EOD more often than females, whereas in Apteronotus albifrons, males and females produce similar numbers of modulations. Sex differences in the number of EOD modulations in A. leptorhynchus are associated with sex differences in substance P in the diencephalic nucleus that controls transient EOD modulations, the CP/PPn. These sex differences in substance P have been hypothesized to regulate sex differences in the production of EOD modulations. To comparatively test this hypothesis, we examined substance P immunoreactivity in the CP/PPn of male and female A. leptorhynchus and A. albifrons. Because the number of EOD modulations is sexually monomorphic in A. albifrons, we predicted no sex difference in substance P in the CP/PPn of this species. Contrary to this prediction, male A. albifrons had significantly more substance P in the CP/PPn than females. This suggests that sex differences in substance P are not sufficient for controlling sex differences in the number of EOD modulations. Modulation structure (frequency excursion and/or duration), however, is also sexually dimorphic in A. leptorhynchus and is another possible behavioral correlate of the sexually dimorphic distribution of substance P. The present study found pronounced sex differences in the structure of EOD modulations in A. albifrons similar to those in A. leptorhynchus. Thus, sex differences in substance P may influence sex differences in the structure, rather than the number, of EOD modulations.

Reciprocal neural connections between the central posterior/prepacemaker nucleus and nucleus G in the gymnotiform fish, Apteronotus leptorhynchus

The central posterior nucleus of teleost fish is a cluster of neurons in the dorsal thalamus that plays an important role in controlling social behaviors. In the weakly electric gymnotiform fish, Apteronotus leptorhynchus, this nucleus forms a larger complex together with the prepacemaker nucleus, hence called central posterior/prepacemaker nucleus (CP/PPn). This complex is crucially involved in neural control of transient modulations of the electric organ discharge, which occur both spontaneously and in the context of social interactions. This control function is intimately linked to its pattern of connectivity with other brain regions. By employing an in vitro neuronal tract-tracing technique, we have, in the present study, identified a novel reciprocal connection between the CP/PPn and a cell group situated in the region between the ventral thalamus and the inferior lobe. Despite the previous interpretation by other authors of this cell group as the glomerular nucleus, the lack of a projection of this nucleus to the hypothalamus, as also demonstrated in the present investigation, makes such a homology unlikely. We, therefore, interpret this nucleus as a brain structure of unknown homology in other teleosts and suggest 'nucleus G' to identify it.

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.

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.

Neural circuitry for communication and jamming avoidance in gymnotiform electric fish

Over the past decade, research on the neural basis of communication and jamming avoidance in gymnotiform electric fish has concentrated on comparative studies of the premotor control of these behaviors, on the sensory processing of communication signals and on their control through the endocrine system, and tackled the question of the degree to which these behaviors share neural elements in the sensory-motor command chain by which they are controlled. From this wealth of investigations, we learned, first, how several segregated premotor pathways controlling a single central pattern generator, the medullary pacemaker nucleus, can provide a large repertoire of behaviorally relevant motor patterns. The results suggest that even small evolutionary modifications in the premotor circuitry can yield extensive changes in the behavioral output. Second, we have gained some insight into the concerted action of the brainstem, the diencephalon and the long-neglected forebrain in sensory processing and premotor control of communication behavior. Finally, these studies shed some light on the behavioral significance of multiple sensory brain maps in the electrosensory lateral line lobe that long have been a mystery. From these latter findings, it is tempting to interpret the information processing in the electrosensory system as a first step in the evolution towards the ‘distributed hierarchical’ organization commonly realized in sensory systems of higher vertebrates.

Effect of drugs that alter alertness and emotionality on the novelty response of a weak electric fish, Gymnotus carapo

Weak field electric fish respond to alerting stimuli with a transient increase in the frequency of electric organ discharge (novelty response). In an attempt to demonstrate the influence of different degrees of alertness and of emotionality on the novelty response of Gymnotus carapo, we studied the variations in the magnitude of this response induced by the application of an electric stimulus to the water of the experimental box using a pair of electrodes, before and after intramuscular injections of d-amphetamine (1-2 and 4 mg/kg), sodium pentobarbital (10-20 and 30 mg/kg), diazepam (1-2 and 4 mg/kg), beta-carboline (2 mg/kg), and saline. After d-amphetamine injection the animal presented increased somatic motility but no changes in electric organ baseline firing rate or in response to the alerting electric stimulus. Sodium pentobarbital induced a partial loss of posture and a reduction of fin and operculum movements, as well as a reduction of baseline firing rate and of the response to the alerting electric stimulus, with frequent interruptions in electric organ firing. Beta-carboline caused increased motility, but no changes in basal firing rate or in response to the alerting stimulus. Diazepan-injected fishes remained still throughout the experiment, and some of those threated with the higher dose (4 mg/kg) presented interruptions on electric organ discharges in response to stimulation but no change on baseline firing rate. The data suggest that a reduction of the degree of alertness by the barbiturate and a decrease in emotionality and/or stress by the benzodiazepine interfere with the novelty response. The possible site of action of the drugs is discussed.

An in vitro technique for tracing neuronal connections in the teleost brain

The availability of neuronal tract-tracing techniques has been fundamental to the development of the neurosciences. While most of the previously described methods are performed in vivo, in the present paper, detailed protocols are reported for tracing neuronal connections in an in vitro preparation. This technique, tested in various neural systems of the teleost brain, allows precise application of tracer substance(s) under visual control. After the isolation of the brain, the tissue is kept alive by superfusion with oxygenated artificial cerebrospinal fluid in a slice chamber. Neuronal connections are traced by the application of crystals of biocytin or dextran-tetramethylrhodamine to the region of interest. Following intracellular transport over 8-18 h, the tissue is fixed and processed histochemically for visualization of structures filled with the tracer substance. This method can readily be modified for double labelling. Step-by-step procedures are outlined for (a) the simultaneous detection of two tracer substances in the same tissue sample, (b) the combination of tract tracing with the immunohistochemical identification of various biochemical markers such as 'classical' transmitters and neuropeptides, and (c) the visualization of both traced structures and mitotically active cells labelled with the thymidine analogue 5-bromo-2'-deoxyuridine. By exhibiting a high degree of efficiency, the described in vitro tract-tracing technique represents also a significant contribution towards a reduction of living animals in neurobiological experimentation.

Novelty response in the weakly electric fish Gymnotus carapo: seasonal differences and the participation of the telencephalon in its modulation

The arousal or novelty response (increased frequency of electric organ discharge in the presence of an electric stimulus delivered by a pair of electrodes placed in the water of the experimental chamber) and its habituation were studied in Gymnotus carapo, a weak field electric fish, in different seasons (September and March) and in animals with extensive or restricted telencephalic lesions. The novelty response was reduced but not abolished by repeated stimuli at fixed intervals in normal animals both in September (pre-mating season) and in March (beginning of the dry season). The magnitude of the novelty response did not differ between sham-operated animals and animals with extensive (detelencephalization) or restricted telencephalic lesions (dorsocentral area) within their respective groups (September or March). The novelty response to the first applied stimulus was significantly greater in normal, sham-operated, and derelencephalated animals in September compared to March, although baseline firing rate did not differ. The maximum extent of response reduction occurred after the 18th stimulus in September for all three experimental groups, whereas in the 4 groups tested in March the maximum reduction occurred between the 16th and 72nd stimulus, indicating that in March there is a greater fluctuation in the arousal level.

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.

Neurons of the posterior subdivision of the nucleus preopticus periventricularis project to the preglomerular nucleus in the weakly electric fish, Apteronotus leptorhynchus

By using an in vitro tract-tracing technique, the neural connections between two diencephalic cell groups, the posterior subdivision of the nucleus preopticus periventricularis (PPp) and the preglomerular nucleus (PG), was examined in the weakly electric gymnotiform fish Apteronotus leptorhynchus. Neurons of the PPp project to one area within PG, the ventromedial cell group of the medial subdivision of the preglomerular nucleus (PGm-vmc). Axons of these cells reach the ipsilateral PGm-vmc via the basic hypothalamic tract, while collaterals decussate via the postoptic commissure to innervate the contralateral PGm-vmc. We hypothesize that those neurons within PPp that project to the PGm-vmc are homologous to neurons of the medial preoptic area of mammals. As part of an elaborate circuit, PPp and PG may participate, as in mammals, in the control of complex social behavior patterns.

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.

The preglomerular nucleus of gymnotiform fish: relay station for conveying information between telencephalon and diencephalon

The preglomerular nucleus of teleost fishes, believed to be a lateral part of the posterior tuberculum in the diencephalon, receives input from several sensory areas. By employing an in vitro technique, the pattern of connectivity between this cell group and the telencephalon has been explored through retrograde and anterograde tracing in the gymnotiform fish Apteronotus leptorhynchus. Neurons of the preglomerular nucleus project to the following telencephalic areas: central division of dorsal forebrain, dorsal subdivision of dorsolateral telencephalon, posterior subdivision of dorsolateral telencephalon, dorsal posterior telencephalon, and probably, also subdivision 2 of dorsomedial telencephalon. Experiments in which tracer application was restricted to the lateral subdivision of the preglomerular nucleus, or in which tracer substance was placed into various regions of the telencephalon revealed a differential projection pattern of cells in the lateral and the medial subdivision of the preglomerular nucleus. Neurons in the central division of the dorsal forebrain, the dorsal posterior telencephalon, and likely, also in the subdivision 2 of the dorsomedial telencephalon and the ventricular zone of the intermediate subdivision of the ventral telencephalon project back to the preglomerular nucleus. Thus, a major function of the preglomerular nucleus appears to be to act as a relay station for conveying information between diencephalon and telencephalon.

Afferent and efferent connections of the diencephalic prepacemaker nucleus in the weakly electric fish, Eigenmannia virescens: interactions between the electromotor system and the neuroendocrine axis

The afferent and efferent connections of the gymnotiform central posterior nucleus of the dorsal thalamus and prepacemaker nucleus (CP/PPn) were examined by retrograde and anterograde transport of the small molecular weight tracer, Neurobiotin. The CP/PPn was identified by physiological assay and received a local iontophoretic injection of Neurobiotin. Retrogradely labeled somata were observed in the ventral telencephalon, hypothalamus, and the pretectal nucleus electrosensorius. Anterogradely labeled fibers were traced from the CP/PPn to the ventral telencephalon, the hypothalamus, the neuropil immediately adjacent to the most rostral subdivision of the nucleus electrosensorius, the optic tectum, and the pacemaker nucleus. Retrograde transport of tracer following injections into the ventral telencephalon, preoptic area, lateral hypothalamus, tectum, and pacemaker nucleus confirmed these efferent targets. A rostromedial subarea of the CP/PPn can be identified that projects to basal forebrain regions and to a lateral region of the CP/PPn that contains afferents to the pacemaker. Many of the targets, which are connected with the CP/PPn, have been linked to reproductive behavior or neuroendocrine control in other fishes. A comparative analysis reveals that the efferent pathways of the CP/PPn appear similar and may be homologous to efferent pathways of some components of the auditory thalamus among tetrapods.

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.

Tectal input to the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: an in vitro tract-tracing study

The weakly electric fish Apteronotus leptorhynchus produces electric organ discharges which are highly stable in waveform and frequency. Short-term modulations of these discharges, typically displayed during social interactions, are controlled by the prepacemaker nucleus (PPn). Neurons of this thalamic cell group intermingle with cells of the central posterior nucleus (CP) to form a complex called 'CP/PPn'. By employing in vitro tract-tracing techniques, we have, in the present investigation, demonstrated that this complex receives input from the tectum opticum. The tectal input is mediated by varicose fibers forming an elongated stripe at the ventral rim of the CP/PPn. As suggested by retrograde tracing from the CP/PPn, this projection is likely to arise from 'multipolar cells with an ascending axon' previously characterized in a Golgi study [14]. As this tectal cell type has been shown to be predominantly driven by electrosensory stimuli [6], information arising from these cells may be used in controlling modulations of the electric organ discharges.

Motor control of the jamming avoidance response of Apteronotus leptorhynchus: evolutionary changes of a behavior and its neuronal substrates

The two closely related gymnotiform fishes, Apteronotus and Eigenmannia, share many similar communication and electrolocation behaviors that require modulation of the frequency of their electric organ discharges. The premotor linkages between their electrosensory system and their medullary pacemaker nucleus, which controls the repetition rate of their electric organ discharges, appear to function differently, however. In the context of the jamming avoidance response, Eigenmannia can raise or lower its electric organ discharge frequency from its resting level. A normally quiescent input from the diencephalic pre-pacemaker nucleus can be recruited to raise the electric organ discharge frequency above the resting level. Another normally active input, from the sublemniscal pre-pacemaker nucleus, can be inhibited to lower the electric organ discharge frequency below the resting level (Metzner 1993). In contrast, during a jamming avoidance response, Apteronotus cannot lower its electric organ discharge frequency below the resting level. The sublemniscal pre-pacemaker is normally completely inhibited and release of this inhibition allows the electric organ discharge frequency to rise during the jamming avoidance response. Further inhibition of this nucleus cannot lower the electric organ discharge frequency below the resting level. Lesions of the diencephalic pre-pacemaker do not affect performance of the jamming avoidance response. Thus, in Apteronotus, the sublemniscal pre-pacemaker alone controls the changes of the electric organ discharge frequency during the jamming avoidance response.

The distribution of Met-enkephalin like immunoreactivity in the brain of Apteronotus leptorhynchus, with emphasis on the electrosensory system

The distribution of Met-enkephalin-like immunoreactivity in the brain of the electric fish, Apteronotus leptorhynchus was analysed by immunohistochemistry. The majority of Met-enkephalin immunoreactive neurons were found in the hypothalamus. Dense Met-enkephalin immunoreactive fiber plexuses were seen in the hypothalamus and ventral forebrain. In the dorsal telencephalon an olfacto-recipient region (ventral subdivision of the dorsolateral forebrain) was specifically and densely innervated. Regions of the brain known to be involved in electrocommunication also received a substantial innervation by Met-enkephalin-like immunoreactive fibers. This distribution of immunoreactive fibers in the brain of this gymnotiform fish indicates that Met-enkephalin may be generally involved in the regulation of sensory, neuroendocrine and reproductive functions and specifically in the regulation of electrocommunication.

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

The neuropeptide somatostatin (SS) is widely distributed in both the central and peripheral nervous system of vertebrates. Its widespread distribution is paralleled by a large variety of diverse functions. While embryonic and perinatal development of SS-like immunoreactivity have been well examined, little is known about the postnatal development of this neuropeptide. Since, in teleosts, neurogenesis persists in many brain regions during adulthood, these vertebrates are well suited to investigate this phenomenon. In the present study, we have, therefore, examined the development of somatostatinergic cells born during adulthood in the central posterior/prepacemaker nucleus (CP/PPn) of Apteronotus leptorhynchus, a weakly electric gymnotiform fish. This was achieved by labelling proliferating cells with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) and by simultaneous immunocytochemical detection of SS-like immunoreactivity. SS-like immunoreactivity is adopted in a period between 2 days and 3.5 days after birth. While the number of BrdU-labelled cells in the CP/PPn decreases 10 days after birth, the percentage of double-labelled cells among the BrdU-labelled cells remains with 1.0-7.6% in the period between 3.5 days and 100 days after birth rather constant. This percentage matches well the fraction of SS-positive cells in the total population of cells present in the CP/PPn.

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.

Somatostatin in the prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: evidence for a nonsynaptic function

Neuropeptides are widely distributed throughout the nervous system and exert a large number of heterogeneous functions. While they are synthesized in the soma, release is thought to take place in axonal terminals of neurons. A good model system to investigate the role of peptides in the nervous system is provided by the central posterior/prepacemaker nucleus (CP/PPn) of pacemaker nucleus (Pn), a medullary cell group controlling the electric organ discharge (EOD). Previous immunocytochemical and in situ-hybridization studies employing topographical criteria indicated that PPn neurons may express the neuropeptide somatostatin (SS). In the present study, we unambiguously identified PPn neurons by in vitro tract tracing. By combining this technique with SS immunocytochemistry, we found that a large portion of retrogradely labelled PPn neurons exhibited SS-like immunoreactivity (72-89%, n = 708 cells in 10 fish examined). Surprisingly, however, neither the proximal PPn axons nor anterogradely labelled terminals innervating the Pn displayed significant amounts of SS-like immunolabelling (n = 10 fish examined in each experiment). These results and the lack of SS binding sites in the Pn [82] suggest that SS expressed by PPn cells is not synaptically released at the target site of their axons, the Pn, but acts via a nonsynaptic mechanism in the CP/PPn proper.

Proliferation zones in the brain of adult gymnotiform fish: a quantitative mapping study

Whereas in mammals postnatal neurogenesis, gliogenesis, and angiogenesis appear to be kept at low rates, in fish the capability for the production of new brain cells during adulthood is very pronounced. Many of the newly generated cells originate from germinal layers that maintain their proliferative activity during adulthood. By employing incorporation of the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) into mitotic active cells, we have quantitatively mapped such proliferation zones in the brain of adult Apteronotus leptorhynchus (Gymnotiformes, Teleostei). In the telencephalon, diencephalon, mesencephalon, and rhombencephalon, the total number of BrdU-labelled cells was low, making up approximately 25% of all mitotic active cells in the brain. Many of these cells were scattered over wide areas. Otherwise, zones of high proliferative activity were typically located at or near the surface of ventricular, paraventricular, and cisternal systems. Approximately 75% of all BrdU-labelled cells found in the brain of adult Apteronotus leptorhynchus were situated in the cerebellum. Zones displaying proliferative activity were restricted to small areas, such as narrow stripes around the midline of corpus cerebelli and valvula cerebelli, the boundary between corpus and valvula, and a large portion of the area covered by the eminentia granularis medialis. Counts indicate that, on average, 100,000 cells, corresponding to approximately 0.2% of the total population of cells in the brain of adult Apteronotus leptorhynchus, are in S-phase within a period of 2 hours. At least part of these newly generated cells is added to the population of already existing cells. This leads to a permanent growth of the brain with increasing size of the fish, a process that appears to slow down only in individuals of relatively advanced age.

Connections of the olfactory bulb in the gymnotiform fish, Apteronotus leptorhynchus

This work examines the connectivity of the olfactory bulb in the gynmotiform fish Apteronotus leptorhynchus. Wheat germ agglutinin conjugated horseradish peroxidase was iontophoresed in different areas and depths of the bulb in order to define its efferent and afferent connections. The olfactory bulb projects bilaterally via the medial (medial and centromedial fascicles) and lateral olfactory (lateral and centrolateral fascicles) tracts. The nervus terminalis courses through the ventromedial aspect of the bulb to terminate in parts of the medial subpallium and hypothalamus. Its telencephalic component could be identified by a nonpreadsorbable substance P-like immunoreactivity. Fibers within the medial olfactory tract form four telencephalic terminal fields: peduncular, medial, intermediate and posterior fields. The diencephalic terminal fields in the habenula, preoptic, and hypothalamic areas appear to correspond to some of the nervus terminalis fibers (von Bartheld and Meyer [1986] Cell Tissue Res. 245:143-158, Krishna et al. [1992] Gen. Comp. Endocrinol. 85:111-117), and to axons of telencephalic bulbopetal cells of area dorsalis posterior. The terminal fields of the medial olfactory tract and nervus terminalis partially overlap in the ventral telencephalic areas partes ventralis, supracommissuralis, and rostral preoptic region. The lateral olfactory tract forms a lateral terminal field and contributes to the intermediate and posterior terminal fields. Olfactory fibers cross in the interbulbar, anterior, and habenular commissures and tuberal decussation. Consistent differences were noted between the medial and lateral olfactory bulb, with respect to their cytoarchitectonics, immunohistochemistry, and connections. In addition to the olfactory nerve, bulbar afferents are predominantly ipsilateral, with minor inputs originating from the contralateral bulb and telencephalic area dorsalis posterior, nucleus raphe centralis, and locus ceruleus.

Birth and migration of neurons in the central posterior/prepacemaker nucleus during adulthood in weakly electric knifefish (Eigenmannia sp.)

In contrast to mammals, fish maintain their capacity to generate neurons in the central nervous system even during adulthood for prolonged periods of life. By employing immunohistochemical, autoradiographic, and electron microscopic techniques, we studied such a postnatal neurogenesis within the complex of the central posterior/prepacemaker nucleus (CP/PPn) in knifefish (Eigenmannia sp.), a weakly electric teleost. The CP/PPn is a bilateral cluster of neurons in the thalamus. It controls frequency modulations of the electric organ discharge as they are used during social interactions. In the CP/PPn region adjacent to the wall of the third ventricle ("ventricular zone"), cells are born continuously and at high rates. They undergo multiple cell divisions before differentiating into neurons. Concomitant with this development, the newborn neurons migrate toward lateral regions of the CP/PPn. In the course of this lateral migration, they appear to acquire immunological and morphological characteristics that are typical for mature CP/PPn neurons. We hypothesize that at least some of the newly generated cells develop finally into functional CP/PPn neurons.