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

Organization of the gymnotiform fish pallium in relation to learning and memory: II. Extrinsic connections

This study describes the extrinsic connections of the dorsal telencephalon (pallium) of gymnotiform fish. We show that the afferents to the dorsolateral and dorsomedial pallial subdivisions of gymnotiform fish arise from the preglomerular complex. The preglomerular complex receives input from four clearly distinct regions: (1) descending input from the pallium itself (dorsomedial and dorsocentral subdivisions and nucleus taenia); (2) other diencephalic nuclei (centroposterior, glomerular, and anterior tuberal nuclei and nucleus of the posterior tuberculum); (3) mesencephalic sensory structures (optic tectum, dorsal and ventral torus semicircularis); and (4) basal forebrain, preoptic area, and hypothalamic nuclei. Previous studies have implicated the majority of the diencephalic and mesencephalic nuclei in electrosensory, visual, and acousticolateral functions. Here we discuss the implications of preglomerular/pallial electrosensory-associated afferents with respect to a major functional dichotomy of the electric sense. The results allow us to hypothesize that a functional distinction between electrocommunication vs. electrolocation is maintained within the input and output pathways of the gymnotiform pallium. Electrocommunication information is conveyed to the pallium through complex indirect pathways that originate in the nucleus electrosensorius, whereas electrolocation processing follows a conservative pathway inherent to all vertebrates, through the optic tectum. We hypothesize that cells responsive to communication signals do not converge onto the same targets in the preglomerular complex as cells responsive to moving objects. We also hypothesize that efferents from the dorsocentral (DC) telencephalon project to the dorsal torus semicircularis to regulate processing of electrocommunication signals, whereas DC efferents to the tectum modulate sensory control of movement.

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.

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.

Re-evaluation of the afferent connections of the pituitary in the weakly electric fish Apteronotus leptorhynchus: an in vitro tract-tracing study

The pituitary plays a key role in the interaction between the brain and the endocrine system. We re-examined the afferent connections of the pituitary in the weakly electric fish Apteronotus leptorhynchus using the in vitro application of dextran-tetramethylrhodamine to the pituitary. The resultant retrograde labeling was analyzed. Application of the tracer to the rostral part, but not the caudal part, of the pituitary labels hypothalamic cells in the anterior division of the periventricular nucleus, the suprachiasmatic nucleus, and the nucleus tuberis lateralis pars anterior. Application of the tracer to either the rostral or caudal parts of the pituitary labels hypothalamic cells in the posterior division of the periventricular nucleus (RPPp), the nucleus hypothalamus caudalis (Hc), the nucleus hypothalamus anterioris, the ventral hypothalamic nucleus, and the central nucleus of the inferior lobe. Furthermore, cells in the rostral two-thirds of the brainstem reticular formation (RF) project to the entire rostrocaudal extent of the pituitary. The largest projections to the pituitary are from Hc, PPp, and RF. Of the cells in Hc that project to the pituitary, almost all (96%) are small and the remainder are medium-sized. Of the cells in PPp that project to the pituitary, about half are small or medium-sized (44% and 56%, respectively). In Hc and PPp, about one-third to one-half of the cells that project to the pituitary are markedly elongated. The cells in RF that project to the pituitary are small (4%), medium-sized (89%), or large (7%) and about four-fifths of these cells are markedly elongated. With regard to the RF projections, the pituitary may receive copies of motor instructions and sensory information supplied by collaterals of the descending and ascending projection systems of RF cells. Thus, the ongoing motor activity of the animal and the ensuing sensory feedback from this activity could directly influence the pituitary.

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.

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.

Corticotropin releasing factor in the brain of the gymnotiform fish, Apteronotus leptorhynchus: immunohistochemical studies combined with neuronal tract tracing

The expression of corticotropin-releasing factor (CRF) has been studied by immunohistochemistry in the brain of the gymnotiform fish, Apteronotus leptorhynchus. Labeled somata were found exclusively in the posterior subdivision of the nucleus preopticus periventricularis and in the hypothalamus anterioris, where these cells form a continuous cluster of neurons. Combination of anti-peptide immunohistochemistry with an in vitro tract-tracing technique confirmed that at least some of these neurons project to the pituitary. Additional terminal fields were present in the following areas of the telencephalon and the diencephalon: ventral subdivision of the ventral telencephalon, supracommissural subdivision of the ventral telencephalon, anterior subdivision of the nucleus preopticus periventricularis, inferior subdivision of the nucleus recessus lateralis, central posterior/prepacemaker nucleus, hypothalamus dorsalis and lateralis, medial subdivision 2 of the nucleus recessus lateralis, and in the region between the dorsal edge of the nucleus tuberis anterior on the one side and both the glomerular nucleus and the central nucleus of the inferior lobe on the other side. It is likely that the projection of CRF-expressing neurons of the posterior subdivision of the nucleus preopticus periventricularis/hypothalamus anterioris to the pituitary provides, similarly as in other fishes, the neural substrate for the activation of the hypothalamo-pituitary adrenal axis through CRF. In addition to this function, CRF may be involved in the regulation of several other processes, including neural control of communicatory behavior exerted by neurons of the central posterior/prepacemaker nucleus.

Distribution of calcium/calmodulin-dependent kinase 2 in the brain of Apteronotus leptorhynchus

Antibodies directed against the mammalian alpha and beta subunits of calcium/calmodulin-dependent kinase 2 (CaMK2) and brain dissection were used for immunoblot analysis of these proteins in various brain regions of Apteronotus leptorhynchus. Western blots revealed that the CaMK2alpha antibody labeled a single band of the expected molecular mass (approximately 50 kDa) for this enzyme in rat cortex and electric fish brain. CaMK2alpha was enriched in fish forebrain and hypothalamus and also strongly expressed in midbrain sensory areas. Western blots revealed that CaMK2beta antibodies labeled bands in an appropriate molecular mass range (approximately 58-64 kDa) for this enzyme in mammalian cortex and electric fish brain. However, a higher molecular mass band (approximately 80 kDa) was also labeled; because all these bands were eliminated by preadsorbtion with the CaMK2-derived peptide antigen, they may all represent CaMK2beta-like isoforms. We mapped the brain distribution of CaMK2 isoforms with emphasis on the electrosensory system. CaMK2alpha was present at high density in dorsal forebrain, hypothalamic nuclei, torus semicircularis, and tectum. It was also enriched in discrete fiber tracts in forebrain, diencephalon, and rhombencephalon. CaMK2beta-like isoforms were enriched in ventral forebrain, hypothalamic nuclei, torus semicircularis and the reticular formation. Unlike CaMK2alpha, CaMK2beta -like isoforms were predominantly present in cell bodies and rarely found in fiber tracts or neuropil. In the electrosensory lateral line lobe, CaMK2alpha was restricted to specific feedback fibers, i.e., tractus stratum fibrosum and its terminal field in the ventral molecular layer. In contrast, CaMK2beta-like isoforms were enriched in somata and dendrites of pyramidal cells and granular interneurons.

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.

Reciprocal connections between the preglomerular complex and the dorsolateral telencephalon in the weakly electric fish, Gymnotus carapo

The diencephalic preglomerular complex of gymnotiform fish receives inputs from several sensory areas. By employing anterograde and retrograde tracing techniques, we studied the afferent and efferent connections of the dorsolateral area (dorsal subdivision) of the telencephalon with the preglomerular nuclei in the weakly electric fish, Gymnotus carapo. Neurons of the medial preglomerular nucleus project to intermediate and deep portions of the middle (commissural) level of the dorsolateral telencephalon, and neurons located in the lateral preglomerular nucleus project to superficial portions of the middle levels of the dorsolateral telencephalon. Therefore, we observed a spatial distribution pattern of connectivity between dorsolateral telencephalon and preglomerular complex.

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.

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.

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.

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.