Central projections and motor nuclei of the facial, glossopharyngeal, and vagus nerves in the mormyrid fish Gnathonemus petersii

The knowns and many unknowns of CNS immunity in teleost fish

Many disease agents infect the central nervous system (CNS) of teleost fish causing severe losses for the fish farming sector. Yet, neurotropic fish pathogens remain poorly documented and immune responses in the teleost CNS essentially unknown. Previously thought to be devoid of an immune system, the mammalian CNS is now recognized to be protected from infection by diverse immune cells that mostly reside in the meningeal lymphatic system. Here we review the current body of work pertaining immune responses in the teleost CNS to infection. We identify important knowledge gaps with regards to CNS immunity in fish and make recommendations for rigorous experimentation and reporting in manuscripts so that fish immunologists can advance this burgeoning field.

Etv1 Controls the Establishment of Non-overlapping Motor Innervation of Neighboring Facial Muscles during Development

The somatotopic motor-neuron projections onto their cognate target muscles are essential for coordinated movement, but how that occurs for facial motor circuits, which have critical roles in respiratory and interactive behaviors, is poorly understood. We report extensive molecular heterogeneity in developing facial motor neurons in the mouse and identify markers of subnuclei and the motor pools innervating specific facial muscles. Facial subnuclei differentiate during migration to the ventral hindbrain, where neurons with progressively later birth dates—and evolutionarily more recent functions—settle in more-lateral positions. One subpopulation marker, ETV1, determines both positional and target muscle identity for neurons of the dorsolateral (DL) subnucleus. In Etv1 mutants, many markers of DL differentiation are lost, and individual motor pools project indifferently to their own and neighboring muscle targets. The resulting aberrant activation patterns are reminiscent of the facial synkinesis observed in humans after facial nerve injury.

Tenney et al. demonstrate that embryonic facial motor neurons are transcriptionally diverse as they establish somatotopic innervation of the facial muscles, a process that requires the transcription factor ETV1. Facial-motor axon-targeting errors in Etv1 mutants cause coordination of whisking and eyeblink evocative of human blepharospasm.

Central connections of the trigeminal motor command system in the weakly electric Elephantnose fish (Gnathonemus petersii)

The highly mobile chin appendage of Gnathonemus petersii, the Schnauzenorgan, is used to actively probe the environment and is known to be a fovea of the electrosensory system. It receives an important innervation from both the trigeminal sensory and motor systems. However, little is known about the premotor control pathways that coordinate the movements of the Schnauzenorgan, or about central pathways originating from the trigeminal motor nucleus. The present study focuses on the central connections of the trigeminal motor system to elucidate premotor centers controlling Schnauzenorgan movements, with particular interest in the possible connections between the electrosensory and trigeminal systems. Neurotracer injections into the trigeminal motor nucleus revealed bilateral, reciprocal connections between the two trigeminal motor nuclei and between the trigeminal sensory and motor nuclei by bilateral labeling of cells and terminals. Prominent afferent input to the trigeminal motor nucleus originates from the nucleus lateralis valvulae, the nucleus dorsalis mesencephali, the cerebellar corpus C1, the reticular formation, and the Raphe nuclei. Retrogradely labeled cells were also observed in the central pretectal nucleus, the dorsal anterior pretectal nucleus, the tectum, the ventroposterior nucleus of the torus semicircularis, the gustatory sensory and motor nuclei, and in the hypothalamus. Labeled terminals, but not cell bodies, were observed in the nucleus lateralis valvulae and the reticular formation. No direct connections were found between the electrosensory system and the V motor nucleus but the central connections identified would provide several multisynaptic pathways linking these two systems, including possible efference copy and corollary discharge mechanisms.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

Effect of estrogen on vagal afferent projections to the brainstem in the female

The effects of 17β-estradiol (E) on the distribution and density of brainstem projections of small or large diameter primary vagal afferents were investigated in Wistar rats using transganglionic transport of wheat germ agglutinin- (WGA; preferentially transported by non-myelinated afferent C-fibers; 2%), or cholera toxin B-subunit- (CTB, 5%; preferentially transported by large myelinated afferent A-fibers) conjugated horseradish peroxidase (HRP) in combination with the tetramethylbenzidine method in age matched ovariectomized (OVX) only or OVX and treated with E (OVX+E; 30 pg/ml plasma) females for 12 weeks. Additionally, these projections were compared to aged matched males. Unilateral microinjection of WGA-HRP into the nodose ganglion resulted in dense anterograde labeling bilaterally, with an ipsilateral predominance in several subnuclei of the nucleus of the solitary tract (NTS) and in area postrema that was greatest in OVX+E animals compared to OVX only and males. Moderately dense anterograde labeling was also observed in paratrigeminal nucleus (PAT) of the OVX+E animals. CTB-HRP produced less dense anterograde labeling in the NTS complex, but had a wider distribution within the brainstem including the area postrema, dorsal motor nucleus of the vagus, PAT, the nucleus ambiguus complex and ventrolateral medulla in all groups. The distribution of CTB-HRP anterograde labeling was densest in OVX+E, less dense in OVX only females and least dense in male rats. Little, if any, labeling was found within PAT in males using either WGA-or CTB-HRP. Taken together, these data suggest that small, non-myelinated (WGA-labeled) and large myelinated (CTB-labeled) diameter vagal afferents projecting to brainstem autonomic areas are differentially affected by circulating levels of estrogen. These effects of estrogen on connectivity may contribute to the sex differences observed in central autonomic mechanisms between gender, and in females with and without estrogen.

Catecholamines inhibit neuronal activity in the glossopharyngeal-vagal motor complex of the Japanese eel: significance for controlling swallowing water

To clarify neuronal networks controlling swallowing water, inhibitory neurotransmitters were searched on the glossopharyngeal-vagal motor complex (GVC) of the medulla oblongata (MO), which is proposed as a motor nucleus controlling swallowing. Spontaneous firing (20-30 Hz) in the GVC was inhibited by adrenaline (AD), noradrenaline (NA) and dopamine (DA). The inhibitory effects of these catecholamines (CAs) were dose-dependent, and the effects of AD and NA were completely blocked by phenoxybenzamine or yohimbine, indicating that at least these two CAs act on the same receptor, presumably on alpha(2)-adrenoceptor. Even after blocking the alpha(2)-adrenoceptor with yohimbine, the inhibitory effect of DA still remained, indicating separate action of DA from AD or NA. Although DA receptor type was not determined in the present study, these results suggest existence of CA receptors in the GVC neurons. Almost 70% GVC neurons were inhibited by CAs. The CA-sensitive neurons were specifically restricted in the middle part of the GVC area. There were many tyrosine hydroxylase (TH)-immunoreactive somata and fibers in the eel MO. Among these TH-immunoreactive nuclei, the area postrema (AP) and the commissural nucleus of Cajal (NCC) appeared to project to the GVC morphologically. Significance of the catecholaminergic inhibition in the GVC activity is discussed in relation to controlling swallowing water.

Water metabolism in the eel acclimated to sea water: from mouth to intestine

Eels seem to be a suitable model system for analysing regulatory mechanisms of drinking behavior in vertebrates, since most dipsogens and antidipsogens in mammals influence the drinking rate in the seawater eels similarly. The drinking behavior in fishes consists of swallowing alone, since they live in water and water is constantly held in the mouth for respiration. Therefore, contraction of the upper esophageal sphincter (UES) muscle limits the drinking rate in fishes. The UES of the eel was innervated by the glossopharyngeal-vagal motor complex (GVC) in the medulla oblongata (MO). The GVC neurons were immunoreactive to an antibody raised against choline acetyltransferase (ChAT), an acetylcholine (ACh) synthesizing enzyme, indicating that the eel UES muscle is controlled cholinergically by the GVC. The neuronal activity of the GVC was inhibited by adrenaline or dopamine, suggesting catecholaminergic innervation to the GVC. The AP and the commissural nucleus of Cajal (NCC) in the MO projected to the GVC and were immunoreactive to an antibody raised against tyrosine hydroxylase (TH), rate limiting enzyme to produce catecholamines from tyrosine. Therefore, it is likely that activation in the AP or the NCC may inhibit the GVC and thus relaxes the UES muscle, which allows for water to enter into the esophagus. During passing through the esophagus, the imbibed sea water (SW) was desalted to approximately 1/2 SW, which was further diluted in the stomach and arrived at the intestine as approximately 1/3 SW, almost isotonic to the plasma. Finally, from the diluted SW, the eel intestine absorbed water following the Na(+)-K(+)-2Cl(-) cotransport (NKCC2) system. The NaCl and water absorption across the intestine was regulated by various factors, especially by peptides such as atrial natriuretic peptide (ANP) and somatostatin (SS-25 II). During desalination in the esophagus, however, excess salt enters into the blood circulation, which is liable to raise the plasma osmolarity. However, the eel heart was constricted powerfully by the hyperosmolarity, suggesting that the hyperosmolarity enhances the stroke volume to the gill, where excess salt was extruded powerfully via Na(+)-K(+)-2Cl(-) cotransport (NKCC1) system.

Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

Glutamate is a major neurotransmitter of chemoreceptor and baroreceptor afferent pathways in mammals and therefore plays a central role in the development of cardiorespiratory reflexes. In fish, the gills are the major sites of these receptors, and, consequently, the terminal field (sensory area) of their afferents (glossopharyngus and vagus) in the medulla must be an important site for the integration of chemoreceptor and baroreceptor signals. This investigation explored whether fish have glutamatergic mechanisms in the vagal sensory area (Xs) that could be involved in the generation of cardiorespiratory reflexes. The locations of the vagal sensory and motor (Xm) areas in the medulla were established by the orthograde and retrograde axonal transport of the neural tract tracer Fast Blue following its injection into the ganglion nodosum. Glutamate was then microinjected into identified sites within the Xs in an attempt to mimic chemoreceptor- and baroreceptor-induced reflexes commonly observed in fish. By necessity, the brain injections were performed on anaesthetised animals that were fixed by 'eye bars' in a recirculating water system. Blood pressure and heart rate were measured using an arterial cannula positioned in the afferent branchial artery of the 3rd gill arch, and ventilation was measured by impedance probes sutured onto the operculum. Unilateral injection of glutamate (40-100 nl, 10 mmol l(-1)) into the Xs caused marked cardiorespiratory changes. Injection (0.1-0.3 mm deep) in different rostrocaudal, medial-lateral positions induced a bradycardia, either increased or decreased blood pressure, ventilation frequency and amplitude and, sometimes, an initial apnea. Often these responses occurred simultaneously in various different combinations but, occasionally, they appeared singly, suggesting specific projections into the Xs for each cardiorespiratory variable and local determination of the modality of the response. Response patterns related to chemoreceptor reflex activation were predominantly located rostral of obex, whereas patterns related to baroreceptor reflex activation were more caudal, around obex. The glutamate-induced bradycardia was N-methyl-D-aspartate (NMDA) receptor dependent and atropine sensitive. Taken together, our data provide evidence that glutamate is a putative player in the central integration of chemoreceptor and baroreceptor information in fish.

Cholinergic innervation to the upper esophageal sphincter muscle in the eel, with special reference to drinking behavior

To elucidate innervation in the upper esophageal sphincter (UES) muscle of the eel, a key muscle in swallowing, repetitive electrical field stimulation (EFS; 30 mA, 40 V, 300 micros, 10 Hz, 10 trains) was employed. Anatomically, the eel UES muscle consists of striated fibers. The EFS-induced contraction of the UES was completely blocked by tetrodotoxin and curare, and abolished in Ca2+ -free Ringer solution. These results suggest that the EFS stimulates nerve fibers specifically and releases acetylcholine as a neurotransmitter. In fact, acetylcholine and carbachol constricted the UES in a concentration-dependent manner. Even after blocking neuronal firing with tetrodotoxin, acetylcholine constricted the UES muscle, suggesting the existence of acetylcholine receptors on the UES muscle cells. Both EFS- and carbachol-evoked contractions of the UES were blocked by curare at a lower concentration than by atropine or hexamethonium, suggesting that the acetylcholine receptor is nicotinic. Even in Ca2+ -free Ringer solution, a direct current stimulus (2 s duration) constricted the UES muscle to an extent similar to that in the presence of Ca2+, indicating that the muscle contraction itself does not need extracellular Ca2+, i.e., the muscle can be constricted by a release of Ca2+ from the sarcoplasmic reticulum.

Distribution and quantification of corticotropin-releasing hormone (CRH) in the brain of the teleost fish Oreochromis mossambicus (tilapia)

The recent characterization of the corticotropin-releasing hormone (CRH) prehormone of the fish tilapia (Oreochromis mossambicus) showed that more variation exists between vertebrate CRH amino acid sequences than recognized before. The present study investigates whether the deviating composition of tilapia CRH coincides with an atypical distribution of CRH in the brain. For this purpose we applied immunohistochemistry, as well as radioimmunoassay (RIA) quantification in brain slices. The results are plotted in a new atlas and reconstruction of the tilapia brain. The largest population of CRH-immunoreactive (ir) neurons is present in the lateral part of the ventral telencephalon (Vl). Approximately tenfold less CRH-ir neurons are observed in the preoptic and tuberal region. The CRH-ir neurons observed in the preoptic region are parvocellular and do not, or hardly, display arginine-vasotocin (AVT) immunoreactivity. CRH-ir neurons are also present in the glomerular layer of the olfactory bulb, in the periventricular layer of the optic tectum, and caudal to the glomerular nucleus. A very dense plexus of CRH-ir terminals is located in the most rostral part of the dorsal telencephalon. This region has not been described in other teleosts and is in the present study subdivided into the anterior part of the dorsal telencephalon (Da) and the anterior part of the laterodorsal telencephalon (Dla). High densities of CRH-ir terminals were observed in and around Vl, in the tuberal region, around the rostral part of the lateral recess, and in the caudal part of the vagal lobe. In the pituitary, CRH-ir terminals are concentrated in the neuro-intermediate lobe. Overall, the immunohistochemical and quantitative data correlated well, as the RIA CRH profile in serial 160-microm slices revealed four peaks, which corresponded with major ir-cell groups and terminal fields. Our results strongly suggest that the CRH-ir cells of Vl project to the rostro-dorsal telencephalon. Consequently, they may not be primarily involved in regulation of pituitary cell types but may subserve other functions. The presence of a CRH-containing Vl-Da/Dla projection seems to be restricted to the most modern group of teleosts, i.e., the Acanthopterygians. Further anatomic indications for non-pituitary-related functions of CRH are found in the vagal lobe and the optic tectum of tilapia. Although the low CRH content of the preoptic region reported here for tilapia may be typical for unstressed fish, the fact remains that remarkably few CRH-ir neurons are involved in regulating the pituitary. Overall, the CRH distribution in the brain of tilapia is more widespread than previously reported for other teleosts.

Development of immunoreactivity to neuropeptide Y in the brain of brown trout (Salmo trutta fario)

The development of neuropeptide Y-immunoreactive (NPY-ir) neurons in the brain of the brown trout, Salmo trutta fario, was studied by using the streptavidin-biotin immunohistochemical method. Almost all NPY-ir neurons found in the brain of adults already appeared in embryonic stages. The earliest NPY-ir neurons were observed in the laminar nucleus, the locus coeruleus, and the vagal region of 9-mm-long embryos. In the lateral area of the ventral telencephalon, habenula, hypothalamus, optic tectum, and saccus vasculosus, NPY-ir cells appeared shortly after (embryos 12-14 mm in length). The finding of NPY-ir cells in the saccus vasculosus and the vagal region expand the NPY-ir structures known in teleosts. Among the regions of the trout brain most richly innervated by NPY-ir fibers are the hypothalamus, the isthmus, and the complex of the nucleus of the solitary tract/area postrema, suggesting a correlation of NPY with visceral functions. Two patterns of development of NPY-ir populations were observed: Some populations showed a lifetime increase in cell number, whereas, in other populations, cell number was established early in development or even diminished in adulthood. These developmental patterns were compared with those found in other studies of teleosts and with those found in other vertebrates. J. Comp. Neurol. 414:13-32, 1999.