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L Maler

Publications and source records attributed to L Maler.

At least 37 records · Page 2Linked to original sources
PubMed1998

Interaction of GABAB-mediated inhibition with voltage-gated currents of pyramidal cells: computational mechanism of a sensory searchlight.

Interaction of GABAB-mediated inhibition with voltage-gated currents of pyramidal cells: computational mechanism of a sensory searchlight. J. Neurophysiol. 80: 3197-3213, 1998. This study examines, in the in vitro electrosensory lateral line lobe (ELL) slice preparation, mono- and disynaptic inhibition in pyramidal cells evoked by stimulation of the direct descending pathway from nucleus praeminentialis (Pd). The pathway forms the stratum fibrosum (StF) in the ELL and consists of excitatory fibers from Pd stellate cells that make monosynaptic contact with pyramidal cells and disynaptic inhibitory contacts via local interneurons and of GABAergic inhibitory fibers from Pd bipolar cells. Single or tetanic stimulation (physiological rates of 100-200 Hz) of the StF produced excitatory postsynaptic potentials (EPSPs) or compound EPSPs in ELL pyramidal cells. Slow (>600 ms) and fast inhibitory postsynaptic potentials (IPSPs; 5-50 ms) also were evoked. Application of gamma-aminobutyric acid-A (GABAA) antagonists blocked the fast inhibition and dramatically increased the firing rate response to StF tetanic stimuli. GABAA antagonists also increased the amplitude of the slow IPSP. The slow IPSP was reduced by GABAB antagonists. Blockade of excitatory amino acid (EAA) synaptic transmission allowed the monosynaptic bipolar-cell-mediated inhibition to be studied in isolation: EAA antagonists blocked most of the EPSP response to StF stimulation leaving fast and (an increased amplitude) slow IPSP components. The bipolar-cell IPSPs were mediated by GABAA and GABAB receptors as they were sensitive to GABAA and GABAB antagonists. The bipolar-cell IPSPs scaled with stimulation rate (20-400 Hz), reaching a maximum amplitude at 200 Hz. Inhibitory efficacy of bipolar-cell slow IPSPs were tested by their ability to reduce spiking in the face of sustained or brief current pulses. Established spike trains (by sustained injected current) were little affected by the onset of the slow IPSP. Weak brief currents injected during the slow IPSP were strongly inhibited. Strong brief currents could overcome the slow IPSP inhibitory effect. Inhibition was observed to interact with the intrinsic IA-like K+ currents to produce a complex control of cell spiking. Hyperpolarizing inhibition removes inactivation of IA to prevent subsequent inputs from driving the cell to threshold. Established depolarizing inputs, having allowed IA to inactivate, enable the cell to be highly sensitive to further depolarizing input. The term "conditional inhibition" is proposed to describe the general phenomenon where synaptic inhibition interacts with voltage-sensitive intrinsic currents.

Animals↗
PubMed1998

Distal versus proximal inhibitory shaping of feedback excitation in the electrosensory lateral line lobe: implications for sensory filtering.

Distal versus proximal inhibitory shaping of feedback excitation in the electrosensory lateral line lobe: implications for sensory filtering. J. Neurophysiol. 80: 3214-3232, 1998. The inhibition controlling the indirect descending feedback (parallel fibers originating from cerebellar granule cells in the eminentia posterior pars granularis) to electrosensory lateral line lobe (ELL) pyramidal cells was studied using intracellular recording techniques in vitro. Parallel fibers (PF) contact stellate cells and dendrites of ventral molecular layer (VML) GABAergic interneurons. Stellate cells provide local input to pyramidal cell distal dendrites, whereas VML cells contact their somata and proximal dendrites. Single-pulse stimulation of PF evoked graded excitatory postsynaptic potentials (EPSPs) that were blocked by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl--aspartate (NMDA) antagonists. The EPSPs peaked at 6.4 +/- 1.8 ms (mean +/- SE; n = 11) but took >50 ms to decay completely. Tetanic stimulation (100 ms, 100 Hz) produced a depolarizing wave with individual EPSPs superimposed. The absolute amplitude of the individual EPSPs decreased during the train. Spike rates, established by injected current, mostly were increased, but in some cells were decreased, by tetanic stimulation. Global application of a gamma-aminobutyric acid-A (GABAA) antagonist to the recorded cell's soma and apical dendritic region increased the EPSP peak and decay phase amplitudes. Tetanic stimulation always increased current-evoked spike rates after GABAA blockade during, and for several hundred milliseconds after, the stimulus. Application of a GABAB antagonist did not have any significant effects on the PF-evoked response. This, and the lack of any long hyperpolarizing inhibitory postsynaptic potentials, suggests that VML and stellate cell inhibition does not involve GABAB receptors. Focal GABAA antagonist applications to the dorsal molecular layer (DML) and pyramidal cell layer (PCL) had contrasting effects on PF-evoked EPSPs. DML GABAA blockade significantly increased the EPSP peak amplitude but not the decay phase of the EPSP, whereas PCL GABAA-blockade significantly increased the decay phase, but not the EPSP peak, amplitude. The order of antagonist application did not affect the outcome. On the basis of the known circuitry of the ELL, we conclude that the distal inhibition originated from GABAergic molecular layer stellate cells and the proximal inhibition originated from GABAergic cells of the ventral molecular layer (VML cells). Computer modeling of distal and proximal inhibition suggests that intrinsic differences in IPSP dynamics between the distal and proximal sites may be amplified by voltage-dependent NMDA receptor and persistent sodium currents. We propose that the different time courses of stellate cell and VML cell inhibition allows them to act as low- and high-pass filters respectively on indirect descending feedback to ELL pyramidal cells.

Algorithms↗
PubMed1997

N-methyl-D-aspartate receptor 1 mRNA distribution in the central nervous system of the weakly electric fish Apteronotus leptorhynchus.

We have isolated a partial cDNA for the N-methyl-D-aspartate (NMDA) receptor 1 (NMDAR1) subunit from an Apteronotus leptorhynchus brain cDNA library. The A. leptorhynchus cDNA fragment, which corresponds to nucleotides 135-903 within the 5' region of the rat NR1 mRNA, encodes 252 amino acids that are >80% identical to the homologous segments of the rat, human, and duck NR1 proteins. RNAse protection assays revealed that the A. leptorhynchus NR1 mRNA was highly enriched in the forebrain and hypothalamus, with lesser amounts in the brainstem, and very low levels in the cerebellum. In situ hybridization also demonstrated that neurons in the pallial forebrain were highly enriched in NR1 transcripts. High levels of NR1 mRNA were found in pyramidal cells within the optic tectum and octavolateral regions. Pyramidal cells of the electrosensory lateral line lobe had the highest levels of expression, and the NR1 mRNA was found to be selectively enriched in their apical dendrites.

Amino Acid Sequence↗
PubMed1997

Excitatory amino acid receptors at a feedback pathway in the electrosensory system: implications for the searchlight hypothesis.

The electrosensory lateral line lobe (ELL) of the South American gymnotiform fish Apteronotus leptorhynchus has a laminar structure: electroreceptor afferents terminate ventrally whereas feedback input distributes to a superficial molecular layer containing the dendrites of the ELL principle (pyramidal) cells. There are two feedback pathways: a direct feedback projection that enters the ELL via a myelinated tract (stratum fibrosum, StF) and terminates in the ventral molecular layer (VML) and an indirect projection that enters as parallel fibers and terminates in the dorsal molecular layer. It has been proposed that the direct feedback pathway serves as a "searchlight" mechanism. This study characterizes StF synaptic transmission to determine whether the physiology of the direct feedback projection is consistent with this hypothesis. We used field and intracellular recordings from the ELL to investigate synaptic transmission of the StF in an in vitro slice preparation. Stimulation of the StF produced field potentials with a maximal negativity confined to a narrow band of tissue dorsal to the StF. Current source density analysis revealed two current sinks: an early sink within the StF and a later sink that corresponded to the anatomically defined VML. Field potential recordings from VML demonstrated that stimulation of the StF evoked an excitatory postsynaptic potential (EPSP) that peaked at a latency of 4-7 ms with a slow decay ( approximately 50 ms) to baseline. Intracellular recordings from pyramidal cells revealed that StF-evoked EPSPs consisted of at least two components: a fast gap junction mediated EPSP (peak 1.2-1.8 ms) and a chemical synaptic potential (peak 4-7 ms) with a slow decay phase ( approximately 50 ms). The amplitudes of the peak and decay phases of the chemical EPSP were increased by depolarizing current injection. Pharmacological studies demonstrated that the chemical EPSP was mainly due to ionotropic glutamate receptors with bothN-methyl--aspartate (NMDA) and non-NMDA components. NMDA receptors contributed substantially to both the early and late phase of the EPSP, whereas non-NMDA receptors contributed mainly to the early phase. Stimulation of the StF at physiological rates (100-200 Hz, 100 ms) produced an augmenting depolarization of the membrane potential of pyramidal cells. Temporal summation and a voltage-dependent enhancement of later EPSPs in the stimulus train permitted the compound EPSP to reach spike threshold. The nonlinear behavior of StF synaptic potentials is appropriate for the putative role of the direct feedback pathway as part of a searchlight mechanism allowing these fish to increase the electrodetectability of scanned objects.

Animals↗
PubMed1997

In vitro plasticity of the direct feedback pathway in the electrosensory system of Apteronotus leptorhynchus.

We have used field and intracellular recording from pyramidal cells in an in vitro preparation of the electrosensory lateral line lobe (ELL) of Apteronotus leptorhynchus to investigate synaptic plasticity of a direct feedback pathway: the (StF). Tetanic stimulation of the StF enhanced the StF-evoked synaptic response by 145% in field and the excitatory postsynaptic potential (EPSP) 190% in intracellular recordings. Maximal enhancement occurred at 5 s and lasted for approximately 120 s. Tetanic frequencies of 100-300 Hz produced enhancement; lower or higher frequencies failed to produce statistically significant changes in EPSP amplitude. Rates of 100-200 Hz occur in vivo in the cells of origin of the StF, suggesting that this form of plasticity may be operative under natural conditions. We could not elicit either long-term potentiation or depression by any stimulation protocol of the StF; in the case of long-term potentiation, this held even when excitatory transmission was enhanced by application of bicuculline, a gamma-aminobutyric acid-A antagonist. When tetanic stimulation of the StF was paired with hyperpolarization of pyramidal cells, subsequent StF-evoked EPSPs were increased by 146% (5 min posttetanus); this anti-Hebbian synaptic enhancement lasted for approximately 10 min. Neither tetanic stimulation alone, hyperpolarization alone, nor tetanic stimulation paired with pyramidal cell depolarization altered StF-evoked EPSP amplitudes on this time scale. Anti-Hebbian synaptic enhancement was not blocked by the N-methyl--aspartate-receptor antagonist D. L-aminophosphovalerate. The in vitro demonstration of anti-Hebbian plasticity at StF synapses replicates similar in vivo results. Anti-Hebbian synaptic plasticity of the StF may be responsible in part for the ability of gymnotiform fish to reject redundant electrosensory signals.

Animals↗
PubMed1996

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.

Animals↗
PubMed1996

Oscillatory and burst discharge across electrosensory topographic maps.

1. Three parallel maps of the distribution of tuberous electroreceptor inputs are found in the medullary electrosensory lateral line lobe (ELL) of weakly electric fish. Pyramidal cells in each map are known to respond differentially to the frequency of amplitude modulations (AMs) of external electric fields in vivo. We used an in vitro ELL slice preparation of Apteronotus leptorhynchus to compare the characteristics of spontaneously active single units across the three tuberous maps. It was our objective to determine whether spontaneous bursting activity of pyramidal cells in each map correlates with the known AM frequency selectivities of pyramidal cells in vivo. 2. Single-unit discharges were recorded from the pyramidal cell layer of the centromedial segment (CMS), centrolateral segment (CLS), and lateral segment (LS) of the ELL. Stochastic analysis of interspike intervals (ISIs) was used to identify bursting and nonbursting unit activity, and to separately analyze intra- and interburst ISIs. Four ISI patterns were identified as 1) bursting, 2) regular spiking, 3) irregular spiking, and 4) highly irregular spiking. This work focuses primarily on the characteristics of bursting units across the ELL segments. 3. Spontaneous bursting discharge was identified in all three maps (68 of 97 units), with several characteristics changing in a gradual manner across the maps. The coefficient of variation (CV) of ISIs and intraburst ISIs decreased significantly from the CMS to the LS, whereas the CV of burst periods increased significantly from the CMS to the LS. Autocorrelations and power spectral density analysis identified units discharging in an oscillatory manner with the following ratio: CMS, 75%; CLS, 4%; LS, 8%. 4. The mean period of spike bursts decreased significantly across the segments (CMS, 2.7 s; CLS, 1.2 s; LS, 1.1 s) primarily because of a shortening of mean burst duration (CMS, 1.0 s; CLS, 0.1 s; LS, 0.05 s). The average number of spikes per burst decreased significantly across the maps (CMS, 61; CLS, 8; LS, 8), whereas the average frequency of spikes per burst increased (CMS, 90 Hz; CLS, 130 Hz; LS, 178 Hz), mainly through an increase in the maximal frequencies attained by units within each map. 5. Bursts in the CMS were unstructured in that the intraburst ISIs were serially independent, whereas for many units in the CLS and especially the LS there were serial dependencies of successive spikes, with alternating short and long ISIs during the burst. 6. These data reveal that the characteristics of bursting unit activity differ between the CMS, CLS, and LS maps in vitro, implying a modulation of the factors underlying burst discharge across multiple sensory maps. Because the pattern of change in burst activity between these maps parallels that of pyramidal cell AM frequency selectivity in vivo, oscillatory and burst discharge may represent the cellular mechanism used to tune these cells to specific frequencies of afferent input during electrolocation and electrocommunication.

Action Potentials↗
PubMed1995

Inositol 1,4,5-trisphosphate receptor localization in the brain of a weakly electric fish (Apteronotus leptorhynchus) with emphasis on the electrosensory system.

Inositol 1,4,5-trisphosphate is a widespread intracellular second messenger that mobilizes intracellular Ca2+ stores. The inositol 1,4,5-trisphosphate receptor involved is associated with the endoplasmic reticulum in neurons. In mammalian brain, inositol 1,4,5-trisphosphate receptor-containing neurons are found in many diverse regions, with cerebellar Purkinje cells containing the highest density of these receptors. We used immunohistochemical methods to identify the distribution of inositol 1,4,5-trisphosphate receptor-containing neurons in the brain of the weakly electric fish and Western blotting to confirm that a protein similar to the inositol 1,4,5-trisphosphate receptor of mammalian brain was recognized in the fish brain. In the telencephelon, the dorsal forebrain regions had low amounts of inositol 1,4,5-trisphosphate receptor. In the diencephalon, only the nucleus tuberis posterior was moderately immunoreactive. In the mesencephalon, only the optic tectum contained cells with intense immunoreactivity, similar to our findings for the ryanodine receptor (G.K.H. Zupanc, J.A. Airey, L. Maler, J. Sutko, and M.H. Ellisman, 1992, J. Comp. Neurol. 325:135-151), which also mobilizes intracellular calcium. In the rhombencephalon, a subset of the pyramidal cells of the electrosensory lateral line lobe contained inositol 1,4,5-trisphosphate receptor. These cells have been shown to contain ryanodine receptor (Zupanc et al., 1992). However, unlike the ryanodine receptor, the distribution of inositol 1,4,5-trisphosphate receptor in these cells is constrained to the soma and proximal dendrites. This compartmentalization may indicate the limit of the range of second-messenger action. Other regions containing immunoreactive cells were the nucleus praeminentialis dorsalis (multipolar and boundary cells), nucleus medialis and crista cerebellaris, and the cerebellum, whose Purkinje cells were the most intensely labeled. The functional implications of inositol 1,4,5-trisphosphate receptor localization in the electrosensory lateral line lobe are discussed.

Animals↗
PubMed1995

Androgen-induced changes in electrocommunicatory behavior are correlated with changes in substance P-like immunoreactivity in the brain of the electric fish Apteronotus leptorhynchus.

The hormonal regulation of sex differences in electrocommunicatory behavior and brain substance P-like immunoreactivity (SPI-ir) were examined in the weakly electric fish, Apteronotus leptorhynchus. This animal modulates its electric organ discharge (EOD) to produce discrete electric social signals (chirps), which function in aggressive and reproductive displays. Males readily chirp in response to electrosensory stimuli that mimic the presence of a conspecific; females also chirp in response to such stimuli, but do so at much lower rates than males. We have recently demonstrated that androgen treatment enhances chirping behavior in females and may also lead to a change in chirp quality or structure. In this study, we quantified androgen-induced changes in chirp structure and simultaneously examined whether androgens alter the sexually dimorphic pattern of SPI-ir in a brain region (prepacemaker nucleus, PPn) known to control chirping. Our results demonstrate that, in females, androgens cause both the induction of chirping and an alteration of chirp structure; chirps recorded from androgen-implanted females had longer durations and more dramatic frequency and amplitude modulations compared to controls, and appear similar to those reported to be produced during spawning. Moreover, androgen-induced changes in chirping are correlated with increased expression of SPI-ir within specific brain nuclei of females. These changes may underly behavioral changes in chirping, since treated females showed a male-like pattern of SPI-ir in the PPn. However, alterations in SPI-ir were not restricted to the PPn, but also occurred in diencephalic regions related to pituitary function and reproductive behavior. The results suggest that androgens modulate chirping activity and cause both specific and wide-spread changes in SPI-ir that may relate to a functional system that interrelates pituitary function, reproductive behavior, and chirping.

Androgens↗
PubMed1994

The immunocytochemical localization of glutamate in the electrosensory system of the gymnotiform fish, Apteronotus leptorhynchus.

The first processing station of the electrosensory system is the medullary electrosensory lateral line lobe. The electrosensory lateral line lobe of gymnotiform fish is a laminated structure with three anatomically distinct excitatory inputs: primary afferents from electroreceptors, the direct feedback from n. praeminentialis and indirect feedback from n. praeminentialis via a cerebellar lobe--the eminentia granularis posterior. We utilized immunocytochemistry on L-R White-embedded semithin sections, and immunogold on thin sections to localize glutamate and aspartate in the electrosensory lateral line lobe, its afferent inputs and efferent targets. The excitatory inputs and the electrosensory lateral line lobe projection neurons are immunopositive for glutamate only, suggesting that glutamate rather than aspartate is the excitatory transmitter of the electrosensory system.

Animals↗
PubMed1994

Correlating gamma-aminobutyric acidergic circuits and sensory function in the electrosensory lateral line lobe of a gymnotiform fish.

Electric fish generate an electric field, which they sense with cutaneous electroreceptors. Electroreceptors project topographically onto the medullary electrosensory lateral line lobe (ELL). The ELL of gymnotiform electric fish is divided into four segments specialized to detect different aspects of the electrosensory input; it is also laminated with separate laminae devoted to electroreceptive input, interneurons, projection neurons, and feedback input. We have utilized antisera to glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) to map the distribution of GABAergic cells and fibers in the ELL of the gymnotiform fish, Apteronotus leptorhynchus. Six types of GABAergic interneurons are found in ELL: Type 2 granular cells (granular layer) project to pyramidal cells; polymorphic cells (pyramidal cell layer) project to the non-GABAergic type 1 granular cells; ovoid cells (deep neuropil layer) project bilaterally upon basilar dendrites of pyramidal cells; multipolar cells (deep neuropil layer) project bilaterally, probably to dendrites and neurons within the deep neuropil layer; and neurons of the ventral molecular layer and stellate cells (molecular layer) project to apical dendrites of pyramidal cells. GABAergic bipolar cells in the nucleus praeminentialis, a rhombencephalic structure devoted to feedback in the electrosensory system, project in relatively diffuse fashion to pyramidal cells. We hypothesize that the various GABAergic circuits of the ELL can be correlated with specific functions: type 2 granular cells with adaptation, size of receptive field center, and gain; polymorphic cells and type 1 granular cells with regulation of surround inhibition; ovoid cells with common mode rejection; and neurons of the ventral molecular layer with adaptive gain control. The feedback GABAergic input from bipolar cells of n. praeminentialis to pyramidal cells may be part of a searchlight mechanism similar to the one postulated for thalamocortical systems.

Animals↗
PubMed1994

The distribution of somatostatin binding sites in the brain of gymnotiform fish, Apteronotus leptorhynchus.

The neuropeptide somatostatin (SS) and its binding sites display a wide distribution in the central nervous system of vertebrates. By employing semi-quantitative autoradiography, we identified such binding sites in the brain of the weakly electric fish Apteronotus leptorhynchus (Gymnotiformes, Teleostei). Whereas (SS1) binding sites for the octapeptide analogue Tyr3-SMS-201-995 appear to be absent in the gymnotiform brain, (SS2) binding sites for the analogue [Tyr0-D-Trp8]-somatostatin-14 were found in many brain regions and showed a similar distribution to that observed by other authors in the amphibian and mammalian central nervous system. Telencephalon While binding in the ventral telencephalon was typically low, all cell groups of the dorsal portion displayed a high degree of binding. The highest density of binding sites was found in the dorsal and caudal subdivision 2 of the dorsomedial telencephalon. Diencephalon Many cell groups of the diencephalon showed a medium to high degree of binding density. The highest level was seen in the habenula. Mesencephalon All layers of the optic tectum contained a medium number of binding sites, except the stratum marginale. In the torus semicircularis, the different layers displayed distinct binding density. While laminae 7-8 showed the highest degree of binding, the lowest density was found in lamina 6. Rhombencephalon Binding was generally low or absent in the tegmentum. Low levels of binding density were observed in the electrosensory lateral line lobe. Cerebellum Extremely high levels of binding were found in the eminentia granularis medialis and the eminentia granularis posterior. Throughout most regions of the brain, the relative density of binding sites and the relative amount of somatostatin immunoreactivity in fibres, as determined in previous studies, were in good agreement.

Animals↗
PubMed1994

The distribution of tachykinin binding sites in the brain of an electric fish (Apteronotus leptorhynchus).

We mapped the distribution of tachykinin binding sites utilizing quantitative autoradiography of iodinated substance P and eledoisin as prototypic ligands for neurokinin-1 (NK1) and neurokinin-3 (NK3) receptors, respectively. The two ligands produced highly heterogenous and quantitatively different patterns of specific binding, suggesting that they revealed different tachykinin receptor subtypes. Although [125I]substance P and [125I]eledoisin binding were correlated in most brain regions, the binding of substance P was usually denser. [125I]substance P binding and substance P-like immunoreactivity were reasonably correlated in most brain areas, although discrepancies were found in some nuclei. Dense [125I]substance P binding was found in most areas of the subpallium and in parts of the pallium related to the olfactory system, as well as in the glomerular layer of the olfactory bulb. Moderate to dense binding of both ligands was observed in preoptic area, hypothalamus, habenula, parts of the thalamus and preglomerular complex. Especially noteworthy was the presence of [125I] substance P binding in the diencephalic prepacemaker nucleus, a region involved in the control of electroncommuncatory behavior. Substance P-like immunoreactivity is sexually dimorphic in certain diencephalic nuclei, including the prepacemaker nucleus (Weld and Maler, 1992); no obvious difference was seen between [125I]substance P or [125I]eledoisin binding in the brains of male versus female fish. In the mesencephalon striking laminar patterns of binding were seen in the torus semicircularis dorsalis and the optic tectum. Dense binding was also noted in the raphé nuclei, the locus ceruleus and the sensory nucleus of the vagus. Although binding of substance P in the electrosensory lateral line lobe and nucleus preeminentialis was light, it was distributed in a discrete fashion, suggesting a role of substance P in electrosensory processing.

Amino Acid Sequence↗
PubMed1994

TTX-sensitive dendritic sodium channels underlie oscillatory discharge in a vertebrate sensory neuron.

Immunocytochemical and electrophysiological techniques were used to localize TTX-sensitive sodium channels (NaChs) over the soma-dendritic axis of basilar and nonbasilar pyramidal cells of the electrosensory lateral line lobe (ELL) of weakly electric fish (Apteronotus leptorhynchus). Dense NaCh-like immunolabel was detected on the membranes of basilar and nonbasilar pyramidal cell somata. Punctate regions of immunolabel (approximately 15 microns) were separated by nonlabeled expanses of membrane over the entire extent of basal dendrites. Similar punctate immunolabel was observed over the apical dendrites, and frequently on membranes of afferent parallel fiber boutons in the distal apical dendritic region. Intracellular recordings from pyramidal cell somata or proximal apical dendrites (75-200 microns) were obtained using an in vitro ELL slice preparation. TTX-sensitive potentials were identified by focal pressure ejection of TTX. Somatic recordings demonstrated both TTX-sensitive fast spike discharge and a slow prepotential; similar but lower amplitude potentials were recorded in apical dendrites. Dendritic spikes were composed of at least two active components triggered by a fast prepotential (FPP) generated by the somatic spike. TTX-sensitive spikes propagated in a retrograde fashion over at least the proximal 200 microns of the apical dendrites, as determined by the conduction of an antidromic population spike and focal TTX ejections. Somatic spikes were followed by a depolarizing afterpotential (DAP) that was similar in duration and refractory period to that of proximal dendritic spikes. During repetitive spike discharge, the DAP could increase in amplitude and attain somatic spike threshold, generating a high-frequency spike doublet and a subsequent hyperpolarization that terminated spike discharge. Repetition of this process gave rise to an oscillatory burst discharge (2-6 spikes/burst) with a frequency of 40-80 Hz. Both the DAP and oscillatory discharge were selectively blocked by TTX ejections restricted to the proximal apical dendritic region. The present study demonstrates an immunolocalization of NaChs over somatic and dendritic membranes of a vertebrate sensory neuron that correlates with the distribution of TTX-sensitive potentials. The interaction of somatic and dendritic action potentials is further shown to underlie an oscillatory discharge believed to be important in electrosensory processing.

Animals↗
PubMed1993

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.

Afferent Pathways↗
PubMed1993

Collateral sprouting in the electrosensory lateral line lobe of weakly electric teleosts (gymnotiformes) following ricin ablation.

Sprouted collateral axons were observed in the electrosensory lateral line lobe (ELL) of gymnotiform teleosts (Apteronotus leptorhynchus) following the ablation of the supraorbital branch of the anterior lateral line nerve. Ablation was accomplished by using microinjections of the toxic lectin ricin. Sprouted axons were followed for up to 26 weeks postablation. Ricin exposure severely reduced axonal numbers and the peripheral electroreceptors in the region innervated by these fibers. To visualize sprouted fibers, intact lateral line afferent nerve branches were anterogradely labelled with the neuronal tract tracers horseradish peroxidase or cobalt chloride, or the monoclonal antibody Q26A3. Within the four somatotopically organized ELL segments, sprouted collaterals were first observed two weeks after ricin injection in the medial and centromedial segments, and four weeks postinjection in the centrolateral and lateral segments. Sprouting involved intrasegmental, horizontally directed axons from adjacent nerve branch terminal fields, and mixed intra- and extrasegmental, dorsally directed axons from the ELL deep fiber layer. The sprouting response was robust but variable in its timing, peaking between 6 and 12 weeks. Subsequently, the intrasegmental, horizontally directed fibers were retained but the mixed dorsally directed fibers, including all extrasegmental axons, were retracted. Therefore, this sprouting response appears to consist of a collateral overproduction followed by a selective axonal retraction. In our view, the most likely explanation for this axonal retraction is that the descending inputs from the isthmus and the cerebellum, as well as commissural fibers from the contralateral ELL, maintain established somatotopic relationships by eliminating somatotopically mismatched sprouted collaterals.

Animals↗
PubMed1992

Immunohistochemical localization of ryanodine binding proteins in the central nervous system of gymnotiform fish.

The ryanodine receptor, an integral membrane protein of the sarcoplasmic reticulum in muscle, embodies a high conductance channel permeable to calcium ions. Recent studies have identified ryanodine-binding proteins in avian and mammalian central nervous systems. These neuronal ryanodine receptors appear to function as Ca2+ channels which may gate the release of Ca2+ from caffeine-sensitive intracellular pools in neurons. In the present investigation, we employed monoclonal antibodies against ryanodine-binding proteins of avian muscle cells to the brain of weakly electric gymnotiform fish. Immunoprecipitation and Western blot analysis revealed two isoforms in the fish brain, with molecular weights comparable to those of avian and fish muscle ryanodine-binding proteins. By employing immunohistochemical techniques, we mapped these proteins in fish brain. Ryanodine receptor-like immunoreactivity was found in nerve cell bodies as well as dendrites and axonal processes. The ryanodine-binding protein is distributed throughout the neuraxis in specific cell types of the gymnotiform brain. In the telencephalon, immunoreactive cells were found in the glomerular layer of the olfactory bulb, in the supracommissural subdivision of the ventral telencephalon, and in the intermediate rostral subdivision of the ventral telencephalon. In the diencephalon, immunoreactive cells or fibers were observed in the nucleus prethalamicus and the habenula, within the nucleus at the base of the optic tract and the adjacent dorsal tegmental nucleus, the pretectal nuclei A and B, and the nucleus electrosensorius. In addition, immunopositive cells were seen in several nuclei of the hypothalamus, with the inferior and lateral subdivision of the nucleus recessus lateralis displaying the highest concentration of neurons. In the mesencephalon, the optic tectum contained the greatest number of immunopositive cells. In the rhombencephalon, labelling was seen in the nucleus of the lateral valvula, central gray, lateral tegmental nucleus, in boundary cells of the nucleus praeminentialis, efferent octavolateral nucleus, an area adjacent to the medial edge of the lateral reticular nucleus, nucleus medialis, and electrosensory lateral line lobe. As in avian brain, cerebellar Purkinje cells were positive for ryanodine-binding protein, although only subsets of Purkinje cells were labelled.

Animals↗