Acetylcholine receptors in spider peripheral mechanosensilla

Peripherally located parts of spider mechanosensory neurons are modulated by several neurotransmitters released from apposed efferent fibers. Activities of acetylcholine (ACh) synthesizing enzyme choline acetyltransferase (ChAT) and ACh degrading enzyme acetylcholine esterase (AChE) were previously found in some efferent fibers. ChAT activity was also present in all the mechanosensory neurons, while AChE activity was only found in some. We show that spider mechanosensory neurons and probably some efferent neurons are immunoreactive to a monoclonal antibody against muscarinic ACh receptors (mAChRs). However, application of muscarinic agonists did not change the physiological responses or membrane potentials of neurons in the lyriform organ VS-3. Similarly, the sensitivities of the neurons of trichobothria (filiform hairs) remained unchanged after application of these agonists. Therefore, activation of mAChRs may only modulate the function of spider mechanosensory neurons indirectly, for example, by affecting the release of other transmitter(s). However, a subgroup of VS-3 neurons was inhibited by ACh, which also depolarized the membrane similar to these neurons' responses to GABA, suggesting that ACh activates anion channels in these neurons. Interestingly, all of the neurons responding to ACh were the rapidly adapting Type A neurons that were previously shown to express AChE activity.

Putative inhibitory collicular boutons contact large neurons and their dendrites in the dorsal cochlear nucleus of the rat

Within the circuits of the acoustic nuclei, the inferior colliculus sends descending (collicular) terminals to control with a feedback mechanism, part of the activity of the dorsal cochlear nucleus (DCN). It is not known whether this descending projection is prevalently excitatory or inhibitory. Using the neuronal tracer Wheat Germ Agglutinin conjugated to Horse Radish Peroxidase (WGA-HRP) the connections between the inferior colliculus and the DCN of the rat have been investigated. By far most retrograde labelled large neurons were glycine and GABA negative (pyramidal and giant neurons) and rare medium-size cells were glycine positive. The ultrastructural immunocytochemical analysis for glycine and GABA shows that mainly large, excitatory, neurons innervate the inferior colliculus. Rare medium-size glycine-positive cells with intermediate characteristics between pyramidal and cartwheel cells, seem also to project to the colliculus. Few WGA-HRP labelled boutons contact the large cells or their dendrites, have symmetric pre- and post-synaptic thickenings, contain pleomorphic and/or flat vesicles, and are labelled for GABA or glycine. Since no GABA labelled cells in both the dorsal and ventral cochlear nucleus were retrograde labelled from the colliculus, the source of these intrinsic anterograde labelled boutons must be external to the cochlear nucleus. GABA positive neurons are both present in the inferior colliculus (injected with the tracer) and superior olivary complex (not injected with the tracer). This suggests that the double labelled boutons (WGA-HRP and GABA) are inhibitory GABA-ergic collicular terminals contacting the excitatory neurons of the DCN. Other few boutons or mossy fibers containing round vesicles and immunonegative for both glycine and GABA, were also seen contacting the large neurons and their dendrites in the DCN. As the round vesicles boutons may be derived from other retrograde cells of the cochlear nucleus (pyramidal and stellate cells) and those glycine positive from the glycinergic neurons in paraolivary nuclei, it is more likely that only the WGA-HRP and GABA labelled boutons are true collicular terminals.

P2X(2) receptor immunoreactivity in the dorsal vagal complex and area postrema of the rat

Adenosine 5'-triphosphate (ATP) can function as a fast synaptic transmitter through its actions on ionotropic (P2X) and metabotropic (P2Y) receptors in neuronal tissue. The ionotropic receptors have been classified into seven subtypes (P2X(1)-P2X(7)) by molecular cloning. However, they are difficult to distinguish pharmacologically owing to an absence of specific agonists and antagonists. In this study we used neuroanatomical methods to determine the origin and neurochemical phenotype of the P2X(2) subtype of purinoceptor in the dorsal medulla of the rat. Using immunohistochemistry we observed dense networks of P2X(2) receptor immunoreactive labelled fibres and terminals in the dorsal vagal complex and area postrema, as well as labelled cell bodies in the dorsal vagal nucleus and the area postrema. The P2X(2) receptor was localized presynaptically in vagal afferent fibres and terminals in the nucleus tractus solitarius at the ultrastructural level by combining injections of an anterograde tracer (biotin dextran amine) into the nodose ganglion with pre-embedding immunogold visualization of P2X(2) immunoreactivity. Terminals immunoreactive for the P2X(2) receptor in the nucleus tractus solitarius were found to contain glutamate, but not GABA immunoreactivity by post-embedding immunogold-labelling techniques. In cell bodies in the area postrema, dual immunofluorescence also indicated that P2X(2) receptor immunoreactive cells are glutamatergic but not GABAergic. The P2X(2) receptor was localized to vagal preganglionic neurons in the dorsal vagal nucleus that were identified by prior intraperitoneal injections of the retrograde tracer FluoroGold. No cells immunoreactive for the P2X(2) receptor were observed in the nucleus tractus solitarius. The localization of P2X(2) receptor immunoreactivity presynaptically in vagal afferent terminals indicates that the receptor may be involved in modulating transmitter release from vagal afferent fibres. Furthermore, the presence of the P2X(2) receptor in vagal preganglionic cells and in glutamatergic cells of the area postrema implies that it may, respectively, play a role in regulation of vagal efferent cell activity and modulation of excitatory outputs from the area postrema to other brain regions.

Muscarinic receptor-mediated calcium signaling in spiral ganglion neurons of the mammalian cochlea

Using indo-1 microspectrofluorometry, we examined the effects of cholinergic agonists on the concentration of intracellular Ca(2+) ions ([Ca(2+)](i)) in spiral ganglion neurons, isolated from rat cochleae at different stages of post-natal development (from P3 to P30). Extracellular application of acetylcholine (ACh) or carbamylcholine generated a rapid and transient increase in [Ca(2+)](i). The ACh concentration-response curve indicated an apparent dissociation constant (K(d)) of 8 microM and a Hill coefficient of 1.0. Removing extracellular free Ca(2+) did not suppress the ACh-induced Ca(2+) responses suggesting an intracellular Ca(2+)-release mechanism. When we compared the cholinergic response at different stages of postnatal development, there were no significant differences on the aspect of the Ca(2+) response and the percentage of responsive neurons, which ranged between 50 and 65% per cochlear preparation. The application of muscarine triggered reversible Ca(2+) responses similar to those observed with ACh, with an apparent K(d) of 10 microM and a Hill coefficient of 1.0. The cholinergic-induced Ca(2+)pirenzepine. Nicotine (10 to 100 microM) did not evoke Ca(2+) responses and the nicotinic antagonist curare (10 microM) did not block the ACh-evoked responses. The present study is the first direct demonstration of functional muscarinic receptors (mAChRs) in spiral ganglion neurons. These mAChRs activated by the cholinergic lateral efferent system may participate in the regulation of the electrical activity of the afferent auditory fibers contacting the inner hair cells.

Corticocortical associative neurons expressing latexin: specific cortical connectivity formed in vivo and in vitro

Latexin, a carboxypeptidase A inhibitor, is expressed in a subset of neurons in the infragranular layers of the lateral cortex in the rat. We here show that latexin-expressing neurons exhibit ultrastructural features common to cortical pyramidal neurons. We show in combined retrograde tracing and immunofluorescent experiments that latexin-expressing neurons contribute to specific corticocortical pathways. Thus, injections of the retrograde tracer fluorogold into either the primary somatosensory (SI) or the primary motor (MI) cortical area labeled many latexin-expressing neurons in the infragranular layers of the secondary somatosensory (SII) and visceral sensory (Vi) areas. In contrast, tracer injections involving the thalamus, striatum, or contralateral SII and Vi exclusively labeled latexin-nonexpressing neurons in both the SII and Vi. Finally, we show that the correct corticocortical projections can be formed in organotypic slice cultures in vitro from latexin-expressing neurons: when slices of developing SII were cocultured with those from the SI and the thalamus, latexin-immunoreactive neurons in the SII projected preferentially to their normal SI target. The specific connectivity formed in vivo and in vitro by this molecularly distinct neuronal population reveals its characteristic manner of cortical organization and provides a unique model system to analyze mechanisms underlying the formation of precise corticocortical pathways.

Neurons containing hypocretin (orexin) project to multiple neuronal systems

The novel neuropeptides called hypocretins (orexins) have recently been identified as being localized exclusively in cell bodies in a subregion of the tuberal part of the hypothalamus. The structure of the hypocretins, their accumulation in vesicles of axon terminals, and their excitatory effect on cultured hypothalamic neurons suggest that the hypocretins function in intercellular communication. To characterize these peptides further and to help understand what physiological functions they may serve, we undertook an immunohistochemical study to examine the distribution of preprohypocretin-immunoreactive neurons and fibers in the rat brain. Preprohypocretin-positive neurons were found in the perifornical nucleus and in the dorsal and lateral hypothalamic areas. These cells were distinct from those that express melanin-concentrating hormone. Although they represent a restricted group of cells, their projections were widely distributed in the brain. We observed labeled fibers throughout the hypothalamus. The densest extrahypothalamic projection was found in the locus coeruleus. Fibers were also seen in the septal nuclei, the bed nucleus of the stria terminalis, the paraventricular and reuniens nuclei of the thalamus, the zona incerta, the subthalamic nucleus, the central gray, the substantia nigra, the raphe nuclei, the parabrachial area, the medullary reticular formation, and the nucleus of the solitary tract. Less prominent projections were found in cortical regions, central and anterior amygdaloid nuclei, and the olfactory bulb. These results suggest that hypocretins are likely to have a role in physiological functions in addition to food intake such as regulation of blood pressure, the neuroendocrine system, body temperature, and the sleep-waking cycle.

Electrophysiological characterization of GABAergic neurons in the ventral tegmental area

GABAergic neurons in the ventral tegmental area (VTA) play a primary role in local inhibition of mesocorticolimbic dopamine (DA) neurons but are not physiologically or anatomically well characterized. We used in vivo extracellular and intracellular recordings in the rat VTA to identify a homogeneous population of neurons that were distinguished from DA neurons by their rapid-firing, nonbursting activity (19.1 +/- 1.4 Hz), short-duration action potentials (310 +/- 10 microseconds), EPSP-dependent spontaneous spikes, and lack of spike accommodation to depolarizing current pulses. These non-DA neurons were activated both antidromically and orthodromically by stimulation of the internal capsule (IC; conduction velocity, 2.4 +/- 0.2 m/sec; refractory period, 0.6 +/- 0.1 msec) and were inhibited by stimulation of the nucleus accumbens septi (NAcc). Their firing rate was moderately reduced, and their IC-driven activity was suppressed by microelectrophoretic application or systemic administration of NMDA receptor antagonists. VTA non-DA neurons were recorded intracellularly and showed relatively depolarized resting membrane potentials (-61.9 +/- 1.8 mV) and small action potentials (68.3 +/- 2.1 mV). They were injected with neurobiotin and shown by light microscopic immunocytochemistry to be multipolar cells and by electron microscopy to contain GABA but not the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Neurobiotin-filled dendrites containing GABA received asymmetric excitatory-type synapses from unlabeled terminals and symmetric synapses from terminals that also contained GABA. These findings indicate that VTA non-DA neurons are GABAergic, project to the cortex, and are controlled, in part, by a physiologically relevant NMDA receptor-mediated input from cortical structures and by GABAergic inhibition.

Calcitonin gene-related peptide immunoreactivity in the rat efferent vestibular system during development

The organization of the efferent fiber network during postnatal development was investigated by immunocytochemical detection of the calcitonin gene-related peptide (CGRP) in rat vestibular receptors from postnatal day 0 (PD 0) to adulthood. CGRP was detected at birth in a few efferent fibers below the sensory epithelia of cristae and maculae. Thereafter, the nerve fibers in the cristae progressively invaded the epithelia with an apex to base gradient from PD 2 to PD 4. There was also a rearrangement of the fibers during maturation of the efferent innervation, such that after reaching the surface of the epithelium, they turned back and moved towards the base of the sensory cells, producing numerous synaptic contacts. Analysis of surface preparations of utricules showed the irregular and asymmetric topographic organization of the efferent fiber network and the extensive, complex distribution of this innervation. The presence and broad distribution of CGRP in the epithelium at critical stages of development and synaptogenesis suggests that it is involved in the maturation of vestibular receptors.

Three-dimensional spatial relationship of neuropeptides and receptors in the rat dorsal vagal complex

Retrograde tracing, multi-label fluorescence immunohistochemistry, confocal microscopy and three-dimensional (3-D) reconstruction techniques were combined to examine the spatial relationship of immunoreactive nerve terminals containing either calcitonin gene-related polypeptide (CGRP) or substance P (SP) to identified gastric efferent neurons in the dorsal motor nucleus of the vagus (DMV) in the brainstem of the rat. The availability of an antibody to the receptor for SP (NK-1r) permitted observation of the association between peptide and receptor. Although both SP-IR and CGRP-IR nerve fibres came in close spatial proximity to identified gastric efferent neurons, few discrete contacts between these fibres and the neuronal membrane were observed. In addition, NK-1r-IR was localized to the somatic and dendritic membranes of a subpopulation of neurons within the DMV, with the majority of receptor labelling not in close spatial proximity to SP-IR nerve fibres. The methodology described in this study permitted the simultaneous observation of the spatial relationship between neuropeptide and an identified neuron (and the corresponding receptor in the case of SP) in 3-D, which is something that cannot be achieved using conventional microscopic techniques

Axonal transport of neurotrophins by visceral afferent and efferent neurons of the vagus nerve of the rat

The receptor-mediated axonal transport of [125I]-labeled neurotrophins by afferent and efferent neurons of the vagus nerve was determined to predict the responsiveness of these neurons to neurotrophins in vivo. [125I]-labeled neurotrophins were administered to the proximal stump of the transected cervical vagus nerve of adult rats. Vagal afferent neurons retrogradely transported [125I]neurotrophin-3 (NT-3), [125I]nerve growth factor (NGF), and [125I]neurotrophin-4 (NT-4) to perikarya in the ipsilateral nodose ganglion, and transganglionically transported [125I]NT-3, [125I]NGF, and [125I]NT-4 to the central terminal field, the nucleus tractus solitarius (NTS). Vagal afferent neurons showed minimal accumulation of [125I]brain-derived neurotrophic factor (BDNF). In contrast, efferent (parasympathetic and motor) neurons located in the dorsal motor nucleus of the vagus and nucleus ambiguus retrogradely transported [125I]BDNF, [125I]NT-3, and [125I]NT-4, but not [125I]NGF. The receptor specificity of neurotrophin transport was examined by applying [125I]-labeled neurotrophins with an excess of unlabeled neurotrophins. The retrograde transport of [125I]NT-3 to the nodose ganglion was reduced by NT-3 and by NGF, and the transport of [125I]NGF was reduced only by NGF, whereas the transport of [125I]NT-4 was significantly reduced by each of the neurotrophins. The competition profiles for the transport of NT-3 and NGF are consistent with the presence of TrkA and TrkC and the absence of TrkB in the nodose ganglion, whereas the profile for NT-4 suggests a p75 receptor-mediated transport mechanism. The transport profiles of neurotrophins by efferent vagal neurons in the dorsal motor nucleus of the vagus and nucleus ambiguus are consistent with the presence of TrkB and TrkC, but not TrkA, in these nuclei. These observations describe the unique receptor-mediated axonal transport of neurotrophins in adult vagal afferent and efferent neurons and thus serve as a template to discern the role of specific neurotrophins in the functions of these visceral sensory and motor neurons in vivo.

Engrailed protein is expressed in interneurons but not motor neurons of the dorsal unpaired median group in the adult grasshopper

We report that the homeodomain protein Engrailed (En) is differentially expressed by neuronal type. Expression was examined within identified midline neurons in T3, A1, and A2 neuromeres of the adult grasshopper by using immunohistochemistry. All save a few neurons in the adult dorsal unpaired median (DUM) group arise embryonically from a single precursor, the median neuroblast. DUM neurons are efferent neurons, local interneurons, or intersegmental interneurons, recognizable as such by their distinct morphologies and neurotransmitter phenotypes. We show that interneurons are En-positive, whereas efferents are En-negative. In the T3 DUM group, the 70 or so interneurons contained cytoplasmic immunoreactivity for gamma-aminobutyric acid (GABA) and glutamate decarboxylase. In double-labeling experiments, all GABA-immunoreactive neurons were also En-positive, and all En-positive neurons contained GABA immunoreactivity. In complementary experiments, the 20 or so efferents in the T3 DUM group, which are octopaminergic, were selectively labeled with a histological marker and then processed to reveal En immunoreactivity. No efferents in the group were En-positive. The abdominal DUM groups contain fewer neurons, but the same dichotomy of labeling was found. The En pattern is established during embryogenesis, with the type-specific pattern apparent by stage 90% of development, the earliest stage examined here. The differential expression of En in the embryo and its continued expression in the adult nervous system suggest a role in the development and maintenance of neuronal phenotype. Morphological differences between efferents and interneurons are discussed in light of a hypothesis that En mediates differential expression of cell adhesion or cell-affinity molecules.

Distribution of BDNF and trkB mRNA in the otic region of 3.5 and 4.5 day chick embryos as revealed with a combination of in situ hybridization and tract tracing

We have used a recently developed technique which combines fluorescent tract tracing and in situ hybridization to study co-localization of neurotrophin mRNA and neurotrophin receptor mRNA expression simultaneously with the pattern of innervation in the developing chick ear. Efferent and afferent fibersfrom the VII/VIIIth cranial nerves were retrogradely and anterogradely filled with Dextran amines conjugated to Texas red and the brain stem was incubated for 2 hours in tissue culture medium. The tissue was subsequently fixed, sectioned frozen, mounted and subjected to in situ hybridization analysis using probes for brain-derived neurotrophic factor (BDNF) and its tyrosine kinase receptor, trkB. The results show that afferent and efferent fibers to the ear innervate areas of the developing otocyst which express BDNF mRNA. We also found that neurons in the stato-acoustic ganglion express high levels of trkB mRNA whereas the subset of facial motor neurons that is efferent to the ear only had no or very low levels of trkB mRNA. From our results we conclude that chicken otic efferent fibers preferentially project to areas with BDNF mRNA expression. The very low levels of trkB mRNA in these motor neurons compared to afferent neurons innervating the same region suggest that other factors, perhaps co-expressed with BDNF, may support efferents. A possible involvement of afferents in guiding efferents to specific areas of the ear is suggested.

Lack of neurotrophin 3 causes losses of both classes of spiral ganglion neurons in the cochlea in a region-specific fashion

Essential functions of neurotrophin 3 (NT-3) in regulating afferent and efferent innervation of the cochlea have been characterized by comparison of normal and NT-3 mutant mice. NT-3 deficiency has striking, region-specific effects, with complete loss of sensory neurons in the basal turn and dramatic but incomplete neuronal loss in the middle and apical turns. The sensory innervation of inner and outer hair cells was reorganized in mutant animals. Instead of a strictly radial pattern of innervation, the axons of remaining sensory neurons projected spirally along the row of inner hair cells to innervate even the most basal inner hair cells. Innervation of outer hair cells was strongly reduced overall and was not detected in the basal turn. The presence of fibers extending to both inner and outer hair cells suggests that subsets of types I and II sensory neurons survive in the absence of NT-3. Likewise, projections of the cochlea to auditory nuclei of the brainstem were attenuated but otherwise present. Equally striking changes in efferent innervation were observed in mutant animals that closely mimicked the abnormal sensory innervation pattern. Despite these impressive innervation deficiencies, the morphology of the organ of Corti and the development of inner and outer hair cells appeared comparatively normal.

Distribution of vasopressin and vasoactive intestinal polypeptide (VIP) fibers in the human hypothalamus with special emphasis on suprachiasmatic nucleus efferent projections

The human suprachiasmatic nucleus (SCN) is located in the basal part of the anterior hypothalamus and is considered as the biological clock that generates circadian rhythms and synchronizes the daily activity pattern with the environmental light-dark cycle. However, the mechanisms and pathways by which the SCN transmits its information to the other brain areas are unknown. Therefore, in the present study, we investigated the efferent projections of the SCN by the immunocytochemical staining of two major peptidergic SCN neurotransmitters: vasopressin (VP) and vasoactive intestinal polypeptide (VIP). It confirmed that these peptides are present in different subdivisions of the SCN. The results of this investigation show that VP and VIP fibers arising from the SCN were detected to branch extensively and hence seem to innervate the SCN itself and the central and medial part of the anteroventral hypothalamic area (AVH), the area below the paraventricular nucleus (sub-PVN), the ventral part of the paraventricular nucleus (PVN), and the dorsomedial nucleus of the hypothalamus (DMH). There appeared to be substantial congruity between the presumptive human SCN projections and those as observed by tracing in rat or hamster. Regarding the anatomical organization of the human SCN projections, the main projection areas appeared to be the AVH, the sub-PVN, the ventral part of the PVN, and the DMH. The observation that VIP and in particular VP fibers pass between the SCN and the PVN suggests that the human SCN and the PVN may have a direct anatomical connection. In addition, VP and VIP fibers were detected in several other hypothalamic areas that are not known to have clear direct connections to the SCN. The possible origin of these VP and VIP fibers is discussed.

Extrinsic innervation of the cat prostate gland: a combined tracing and immunohistochemical study

PURPOSE

The purpose of the present study was to determine the peripheral neural pathways, spinal distribution, sizes, and peptide transmitter content of primary afferent and autonomic efferent neurons that innervate the prostate gland.

METHODS

Retrograde transport of the fluorescent dye "fast blue" (injected into the prostate gland) was combined with neurotransmitter immunohistochemistry. Lesions of the pelvic and pudendal nerve were used to determine the peripheral neural pathways.

RESULTS

The majority of the afferent innervation arose from the sacral dorsal root ganglia (DRG) and was equally comprised of small, substance P- and calcitonin gene-related peptide-immunoreactive (IR) neurons and large, non-IR neurons. The majority (70%) of the afferent axons traversed the pelvic nerve with the remainder traversing the pudendal nerve. Fewer afferent neurons were located in lumbar DRG; nearly all of these were small, peptidergic neurons. Efferent autonomic neurons were located in the inferior mesenteric ganglia (IMG), sympathetic chain ganglia (SCG), and pelvic plexus ganglia (PPG). Nearly all efferent neurons in the IMG and SCG, but only 2/3 of the PPG neurons, contained dopamine-beta-hydroxylase. Substantial neuropeptide Y innervation was derived from the SCG but not the IMG or PPG.

CONCLUSIONS

First, clinical reports suggested that sensory innervation of the prostate would be purely nociceptive in nature (implied by small, peptide-IR neurons). However, the present study suggests that there may also be a substantial, presumably non-nociceptive, afferent innervation (implied by findings of large, non-IR neurons). Second, 3 sources of autonomic efferent innervation exist, each being different in the distribution of transmitter phenotypes. Understanding the physiological role of putative non-nociceptive primary afferent neurons, and the differential roles of the various autonomic neurons, is likely to be important in developing therapies for the treatment of prostatic diseases, such as benign prostatic hyperplasia and prostatodynia.

Choline acetyltransferase, glutamate decarboxylase, tyrosine hydroxylase, calcitonin gene-related peptide and opioid peptides coexist in lateral efferent neurons of rat and guinea-pig

The lateral efferent (olivocochlear) innervation of the cochlea originates in the brainstem lateral superior olive. It is likely to use acetylcholine, gamma-aminobutyric acid, dopamine and various neuropeptides as neurotransmitters and/or neuromodulators. In order to determine the different coexistence patterns of these molecules in lateral efferent perikarya, we have used double and triple immunofluorescence co-localization techniques to colocalize choline acetyltransferase, glutamate decarboxylase, tyrosine hydroxylase, calcitonin gene-related peptide and enkephalins in single sections of the lateral superior olive. We also used a non-radioactive in situ hybridization technique onto serial sections of this nucleus to confirm the immunofluorescence co-localization data at the mRNA level. Whatever the pair or triplet of primary antibodies tested was, a high ratio of coexistence was observed in the immunofluorescence experiments. In triple co-localization experiments, 90-93% of the choline acetyltransferase-like immunoreactive neurons were also immunoreactive to the two other antigens investigated. The in situ hybridization co-localization data, based on the use of biotin-labelled oligoprobes, qualitatively confirmed these immunofluorescence data. In conclusion, it can be postulated that acetylcholine, gamma-aminobutyric acid, dopamine, calcitonin gene-related peptide, enkephalins and dynorphins (whose coexistence with choline acetyltransferase and enkephalins has been previously described immunocytochemically) coexist in lateral efferent neurons. Based on these results, it is tempting to propose the lateral efferent innervation as a useful model with which the functional implications of the coexistence of neurotransmitters/neuromodulators can be investigated in vivo.

Projection neurons of the mormyrid electrosensory lateral line lobe: morphology, immunohistochemistry, and synaptology

This paper describes the morphological, immunohistochemical, and synaptic properties of projection neurons in the highly laminated medial and dorsolateral zones of the mormyrid electrosensory lateral line lobe (ELL). These structures are involved in active electrolocation, i.e., the detection and localization of objects in the nearby environment of the fish on the basis of changes in the reafferent electrosensory signal generated by the animal's own electric organ discharge. Electrosensory, corollary electromotor command-associated signals (corollary discharges), and a variety of other inputs are integrated within the ELL microcircuit. The organization of ELL projection neurons is analyzed at the light and electron microscopic levels based on Golgi impregnations, intracellular labeling, neuroanatomical tracer techniques, and gamma-aminobutyric acid (GABA), gamma-aminobutyric acid decarboxylase (GAD), and glutamate immunohistochemistry. Two main types of ELL projection neurons have been distinguished in mormyrids: large ganglionic (LG) and large fusiform (LF) cells. LG cells have a multipolar cell body (average diameter 13 microns) in the ganglionic layer, whereas LF cells have a fusiform cell body (on average, about 10 x 20 microns) in the granular layer. Apart from the location and shape of their soma, the morphological properties of these cell types are largely similar. They are glutamaterigic and project to the midbrain torus semicircularis, where their axon terminals make axodendritic synaptic contacts in the lateral nucleus. They have 6-12 apical dendrites in the molecular layer, with about 10,000 spines contacted by GABA-negative terminals and about 3,000 GABA-positive contacts on the smooth dendritic surface between the spines. Their somata and short, smooth basal dendrites, which arborize in the plexiform layer (LG cells) or in the granular layer (LF cells), are densely covered with GABA-positive, inhibitory terminals. Correlation with physiological data suggests that LG cells are I units, which are inhibited by stimulation of the center of their receptive fields, and LF cells are E units, excited by electric stimulation of the receptive field center. Comparison with the projection neurons of the ELL of gymnotiform fish, which constitute another group of active electrolocating teleosts, shows some striking differences, emphasizing the independent development of the ELL in both groups of teleosts.

Ultrastructural localization of sympathetic axons in experimental rat sciatic nerve neuromas

The ultrastructural localization of sympathetic axons was investigated in normal rat sciatic nerves and experimental sciatic nerve neuromas. The best ultrastructural localization of noradrenaline in the dense-cored vesicles of sympathetic axons was accomplished following pretreatment of rats with nialamide and 5-hydroxy dopamine, followed by fixation according to the modified chromaffin technique of Tranzer and Richards (1976). After such preparation, sympathetic axons containing 5-hydroxy dopamine-labelled dense-cored vesicles could be identified in normal sciatic nerve. Large accumulations of labelled dense-cored vesicles were also found in acute neuromas, up to 1 week after nerve section. Much smaller numbers of dense-cored vesicles could be identified in chronic neuromas from 2 to 3 weeks following nerve section. Sympathetic axons could also be identified following electron probe X-ray microanalysis of the tissue sections, using chromium detection as the marker for the noradrenaline-containing dense-cored vesicles. Unusual configurations of Schwann cell subunits, which enclosed myelinated fibres and sympathetic axon sprouts within the same basal lamina, were identified in the acute neuromas, 3-7 days after nerve section. Such configurations may be of relevance to the pathophysiological interaction which develops between sympathetic efferent and sensory fibres in peripheral nerve neuromas.

Olivocochlear innervation of inner and outer hair cells during postnatal maturation: an immunocytochemical study

Changes in the expression of several neurochemical markers associated with either axonal growth (GAP-43), synaptic vesicles (synaptophysin), or the cholinergic population of lateral olivocochlear (OC) efferents were investigated in the postnatal cochlea of hamsters. Growth-associated protein was expressed in the neonatal cochlea but not in the adult; immunoreactivity was found below inner hair cells (IHCs) from postnatal day (P) 2 through P14 and below outer hair cells (OHCs) from P5 to P14. In contrast, synaptophysin was expressed in both the neonate and adult cochlea; immunoreactivity was found below IHCs around P4 and below OHCs at P5. Both GAP-43 and synaptophysin immunoreactivities occurred first below IHCs in basal regions of the cochlea. Efferent fibers containing calcitonin gene-related peptide (CGRP) immunoreactivity were identified as early as P4 within the cochlear nerve but were not observed underneath IHCs until P7. Similar to GAP-43 and synaptophysin immunoreactivity, CGRP expression followed a basal to apical gradient; however, expression below OHCs appeared restricted to apical regions. These data raise the possibility that efferents expressing growth proteins and efferents expressing synaptic vesicle proteins co-exist during the first postnatal week. Furthermore, it is hypothesized that CGRP-containing lateral OC neurons form part of a later, secondary innervation to the cochlea.

Nervus terminalis projection to the retina in the 'four-eyed' fish, Anableps anableps

The eye of the surface dwelling 'four-eyed' fish, Anableps possesses an aquatic and an aerial optical system. The aerial system is strongly hyperopic when the animal dives, i.e. during mating, and the dorsal pupil is submerged. We studied the retino-petal nervus terminals projection to the aerial and to the aquatic retina by Phe-Met-Arg-Phe-NH2 (FMRF) immunocytochemistry and found both to be equally innervated. This finding sheds doubt on the proposed functional significance of this projection for reproductive behaviour.

Association of substance P and its receptor with efferent neurons projecting to the greater curvature of the rat stomach

Retrograde tracing and immunocytochemistry were used to identify and map the distribution of substance P (SP) and its receptor (NK-1r) associated with gastric motor neurons in the dorsal motor nucleus of the vagus (DMV) in the rat brain stem. The presence of peptide and receptor in surrounding regions within the dorsal vagal complex were also observed. Injection of the retrograde tracer Fluorogold (FG) into the greater curvature of the stomach produced bilateral labelling of neurons within the DMV. The majority of the NK-1r immunoreactivity appeared as an intricate lattice of fibres with a small number of immunoreactive cell bodies. The NK-1r-labelled fibres were detected within the DMV in close association with FG-labelled neurons and in the region between the DMV and nucleus of the solitary tract (NTS). A proportion of FG-labelled neuronal cell bodies were also labelled with NK-1r (7% of total). The greatest density of NK-1r-labelled fibres was observed at the rostral end of the FG-labelled neuron columns in the DMV (close to the IV ventricle) in the region where gastric vagal afferents terminate. Little NK-1r labelling was observed at the caudal end of the FG-labelled neuron tracts adjacent to the central canal. In the coronal plane, the NK-1r-labelled fibres were seen at the edges of the DMV extending into overlying NTS. Substance P was visualized as a dense network of fibres spanning the entire length of the DMV and in close association with FG-labelled neurons. Substance P staining was also detected in the NTS and in the ventral AP. Most of the association between SP/NK-1r immunoreactive fibres was observed within the DMV and at the border between the DMV and NTS. These findings suggest that SP directly regulates a subpopulation of efferent neurons in the DMV which project to the greater curvature of stomach.

Enkephalin-like immunoreactivity in the chick brainstem: possible relation to the cochlear efferent system

Mammalian lateral olivocochlear (LOC) neurons that are immunoreactive for choline acetyltransferase (ChAT) are also immunoreactive for enkephalin (Enk). To determine whether cochlear efferent neurons in birds might also contain Enk-like immunoreactivity (Enk-LI), we studied the auditory brainstem of the domestic chicken using antisera to ChAT, leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk). Enk-LI terminals are found around, but not within, the superior olivary nucleus (SO) and the nucleus of the lateral lemniscus, pars intermedia (LLi). A moderate concentration of Enk-LI terminals is found ventromedial to the ventral facial nucleus (VIIv) where the ventrolateral group of ChAT-I cochlear efferent neurons is located. After colchicine injections into the lateral ventricle, a population of intensely stained Enk-LI perikarya was found in the nucleus of the lateral lemniscus, pars ventralis (LLv) with scattered cells in the LLi and the nucleus subceruleus ventralis (SCv). The distribution of Enk-LI and ChAT-I somata, however, never overlapped, even in adjacent sections. Thus, in the chick, Enk-LI perikarya are not distributed in areas where cochlear efferent neurons are found. Instead, a dense concentration of Enk-I terminals can be found in areas containing ChAT-I cochlear efferent neurons. The source of these enkephalinergic terminals may be a population of Enk-LI cells in the LLv.

Abundant distribution of locustatachykinin-like peptide in the nervous system and intestine of the cockroach Leucophaea maderae

An antiserum raised to the locust neuropeptide locustatachykinin I (LomTK I) was used for analysis of the distribution of tachykinin-related peptide in the cockroach Leucophaea maderae. Extracts of dissected brains, suboesophageal ganglia, thoracic ganglia and midguts were separated by high performance liquid chromatography and the fractions analysed in enzyme-linked immunosorbent assay with use of the LomTK antiserum. Each of the tissues was found to contain LomTK-like immunoreactive (LomTK-LI) components with retention times corresponding approximately to synthetic LomTK I and II and callitachykinins I and II. The LomTK antiserum was also used for immunocytochemical mapping of peptide in the nervous system and intestine of L. maderae. A large number of LomTK-LI interneurons were detected in the proto-, deuto- and tritocerebrum of the brain and in the suboesophaegeal ganglion. The immunoreactive neurons supply processes to most parts of the brain: the central body, protocerebral bridge, mushroom body calyces, antennal lobes, optic lobe and most regions of the non-glomerular neuropil. A few protocerebral neurons send LomTK-LI processes to the glandular lobe of the corpora cardiaca. In each of the thoracic ganglia there are six LomTK-LI interneurons and in each of the unfused abdominal ones there are two interneurons. The fused terminal ganglion contains some additional cell bodies in the posterior neuromers. LomTK-LI cell bodies were detected in the frontal ganglion and fibres were seen in this ganglion as well as in the hypocerebral ganglion. The frontal ganglion supplies LomTK-LI processes to the muscle layer of the pharynx. The muscle layer of the midgut is innervated by LomTK-LI fibres from the stomatogastric system (oesophageal nerve and associated ganglia). Additionally the midgut contains numerous LomTK-LI endocrine cells. A number of the pharyngeal dilator muscles were also found to be innervated by LomTK-LI fibres, probably derived from cell bodies in the suboesophageal ganglion. All the LomTK-LI neurons of the central nervous system appear to be interneurons, suggesting a neuromodulatory role of the endogenous tachykinins. The tachykinin-like peptides from peripheral ganglia may be involved in the control of foregut and midgut contractility and possibly the peptide of the endocrine cells in the midgut has additional actions related to intestinal function.

GABA and glutamate-like immunoreactivity at synapses received by dorsal unpaired median neurones in the abdominal nerve cord of the locust

Dorsal unpaired median (DUM) neurones in the abdominal ganglia of the locust were impaled with microelectrodes and some were injected intracellularly with horseradish peroxidase so that their synapses could be identified in the electron microscope. Simultaneous recordings from DUM neurones in different abdominal ganglia revealed that they received common postsynaptic potentials from descending interneurones. Post-embedding immunocytochemistry using antibodies against GABA and glutamate was carried out on ganglia containing HRP-stained neurones. GABA-like immunoreactivity was found in 39% (n = 82) of processes presynaptic to abdominal DUM neurones and glutamate-like immunoreactivity in 21% (n = 42) of presynaptic processes. Output synapses from the DUM neurites were rarely observed within the neuropile. Structures resembling presynaptic dense bars but not associated with synaptic vesicles, were seen in some large diameter neurites.

Expression of GAP-43 in growing efferent fibers during cochlear development

The development of olivocochlear efferent fibers has been studied by means of the immunocytochemical detection of the growth-associated protein GAP-43. This study has been carried out in pre- and postnatal, developing, pigmented rats. Results indicate that olivocochlear efferents reach the developing auditory epithelium from embryonic day 18 on. GAP-43-like immunoreactivity persists in the organ of Corti until the beginning of the second postnatal month. The relationships between the expression of GAP-43 in olivocochlear efferent fibers and other plasticity-related phenomena are discussed.

Characterization of peptidergic efferents from the lateral parabrachial nucleus to identified neurons in the rat dorsal raphe nucleus

The peptidergic content of the lateral parabrachial nucleus (LPB) efferents to the dorsal raphe nucleus (DRN) was studied by combining visualization of the anterogradely transported tracer Phaseolus vulgaris leucoagglutinin within fibers that were immunocytochemically stained for neurotensin (NT), calcitonin gene-related peptide (CGRP) or galanin (GAL). The identity of DRN target neurons was determined with simultaneous immunocytochemical labelling for serotonin, the major transmitter within the nucleus. Within the DRN, we estimated that about two-thirds of the anterogradely labelled fibers arising from the LPB also showed peptidergic immunoreactivity. NT was the most commonly observed neuropeptide in LPB neuronal efferents directed to the DRN, followed by CGRP and GAL. The peptidergic afferents in the DRN were oriented preferentially in the dorsoventral plane. Peptidergic fibers from the LPB possessed varicosities (diameters not exceeding 3 microns) and were apposed on serotoninergic neuronal somata. Some of the anterogradely labelled peptidergic fibers were not associated with cells showing immunoreactivity for serotonin. The present results suggest that NT-ergic, CGRP-ergic and GAL-ergic neurons within the LPB are in contact with serotoninergic and non-serotoninergic neurons within the DRN. Since the DRN is known to project to the LPB, it is likely that bi-directional interconnections between these nuclei exist. Such linkages may provide anatomical substrates for coordinated autonomic responses.

The efferent connections of the lateral septal nucleus in the guinea pig: projections to the diencephalon and brainstem

The anterograde Phaseolus vulgaris-leucoagglutinin (PHA-L) tracing technique was used to determine the distribution of efferent fibers originating in the lateral septal nucleus of the guinea pig. For complementary detection of the chemical identity of the target neurons, double-labeling immunocytochemistry was performed with antibodies to PHA-L and to vasopressin, oxytocin, vasoactive intestinal polypeptide, serotonin or dopamine beta-hydroxylase, respectively. The hypothalamus received the majority of the PHA-L-stained septofugal fibers. Here, a specific topography was observed. (1) The medial and lateral preoptic area, (2) the anterior, lateral, dorsal, posterior hypothalamic and retrochiasmatic area, (3) the supraoptic, paraventricular, suprachiasmatic, dorsomedial, caudal ventromedial and arcuate nuclei, and (4) the tuberomammillary, medial and lateral supramammillary, dorsal and ventral premammillary nuclei always contained PHA-L-labeled fibers. The rostral portion of the ventromedial nucleus and the medial and lateral mammillary nucleus only occasionally showed weak terminal labeling. In other diencephalic areas, termination of PHA-L-labeled fibers was observed in the epithalamus and the nuclei of the midline region of the thalamus. In the mesencephalon, terminal varicosities occurred in the ventral tegmental area, interfascicular and interpeduncular nucleus, and periaqueductal gray. In addition, the dorsal and medial raphe nuclei of the metencephalon, together with the locus coeruleus and the dorsal tegmental nucleus, received lateral septal efferents.

Ontogeny of glutamate decarboxylase and gamma-aminobutyric acid immunoreactivities in the rat cochlea

gamma-Aminobutyric acid (GABA)-ergic efferent nerve fibers were studied during the postnatal development of the rat cochlea, using light microscopic immunocytochemical techniques. Antibodies against GABA and its synthesizing enzyme, glutamate decarboxylase (GAD), were used. Immunoreactivity to GAD is already present at birth (postnatal day 1) and could be found below the inner hair cells of the basal turn. Immunoreactivity progressively extends toward the apical turn until day 3. GAD-like immunoreactivity appears under the outer hair cells on postnatal day 15 and is only found in the upper part of the second turn and in the apical turn. The distribution of GABA-like immunoreactivity closely corresponds to that observed with the anti-glutamate decarboxylase antibody. However, the GABA-like immunoreactivity appears about 1-2 days after GAD-like immunoreactivity. At the beginning of the 3rd postnatal week, an adult pattern of GABA- and GAD-like immunoreactivity is established. These results suggest that GABA, which appears under the inner hair cells largely before the onset of hearing, may play a neurotrophic function during cochlear maturation and participate in the regulation of the first cochlear potentials as soon as they appear.