Heinrich Münz

Centrifugal innervation of the retina by luteinizing hormone releasing hormone (LHRH)-immunoreactive telencephalic neurons in teleostean fishes

SummaryIn cichlid, poecilid and centrarchid fishes luteinizing hormone releasing hormone (LHRH)-immunoreactive neurons are found in a cell group (nucleus olfactoretinalis) located at the transition between the ventral telencephalon and olfactory bulb. Processes of these neurons project to the contralateral retina, traveling along the border between the internal plexiform and internal nuclear layer, and probably terminating on amacrine or bipolar cells. Horseradish peroxidase (HRP) injected into the eye or optic nerve is transported retrogradely in the optic nerve to the contralateral nucleus olfactoretinalis where neuronal perikarya are labeled. Labeled processes leave this nucleus in a rostral direction and terminate in the olfactory bulb. The nucleus olfactoretinalis is present only in fishes, such as cichlids, poecilids and centrarchids, in which the olfactory bulbs border directly the telencephalic hemispheres. In cyprinid, silurid and notopterid fishes, in which the olfactory bulbs lie beneath the olfactory epithelium and are connected to the telencephalon via olfactory stalks, the nucleus olfactoretinalis or a comparable arrangement of LHRH-immunoreactive neurons is lacking. After retrograde transport of HRP in the optic nerve of these fishes no labeling of neurons in the telencephalon occurred. It is proposed that the nucleus olfactoretinalis anatomically and functionally interconnects and integrates parts of the olfactory and optic systems.

LHRH systems in the brain of platyfish

The luteinizing hormone-releasing hormone (LHRH) system of the platyfish Xiphophorus has been studied using immunohistochemistry and retrograde transport of horseradish peroxidase (HRP). Three different populations of LHRH-positive cell bodies are present in the brain, one in the ventral telencephalon at the border to the olfactory bulb (nucleus of olfactoretinalis), one lateral to the n. preopticus (nucleus preopticus basalis lateralis) and one in the midbrain. LHRH neurons from the nucleus olfactoretinalis project via the medial olfactory tract to the olfactory bulb and to olfactory nerves. A second projection from this nucleus enters the optic tract, crosses in the optic chiasm, and courses rostrally in the outer layer of the optic nerve to the retina, where LHRH-positive nerve fibers terminate near amacrine and bipolar cells. HRP injections into the eye or into the cut optic nerve result in retrograde transport of the enzyme to the contralateral LHRH nucleus olfactoretinalis. Projections from LHRH neurons in the lateral preoptic region can be followed medially to surround the interhemispheric ventricle and laterally to border the optic tract. At the level of the postoptic commissure, LHRH fibers condense to form a fascicle which reaches the pituitary stalk to arborize throughout the hypophysis. LHRH fibers, probably in part from the midbrain LHRH neurons, project to the optic tectum, torus semicircularis, corpus and valvula of the cerebellum, as well as to the medulla oblongata. Associations of LHRH projections with sensory systems and with endocrine-autonomic systems in hypothalamus-pituitary and lower brain stem suggest a role in the modulation and integration of sensory, autonomic, behavioral and hypophyseotrophic functions.

Centrifugal innervation of the retina in cichlid and poecilid fishes. A horseradish peroxidase study

Abstract After injecting horseradish peroxidase (HRP) solution into the eye or optic nerve in cichlid or poecilid fishes, neurons in the olfactory bulb, ventral telencephalon and pretectal area of the diencephalon contain retrogradely transported HRP. The dendrites of the telencephalic neurons project into the olfactory bulb. The dendrites of the pretectal neurons project into the ventricular gray on the border between ventral and dorsal thalamus and are detectable up to the caudal end of the nucleus glomerulosus. We consider these neurons to be the origin of centrifugal innervation of the retina.

Morphology and innervation of the lateral line system inSarotherodon niloticus (L.) (cichlidae, teleostei)

SummaryTopography, morphology, and innervation of superficial neuromasts and canal neuromast in adult bony fishes ofSarotherodon niloticus (L.) were studied and compared by light and electron microscopical methods. Apart from certain other morphological differences the two neuromast types also differ in innervation. They can be distinguished by the number, course, and ending of the myelin sheath of their corresponding nerve fibers. All superficial neuromasts arranged in one row are interconnected by a so-called connecting strand. This tissue, unlike the epidermis, consists of tightly packed cells the external membranes of which are considerably meshed. The tissue of the connecting strand does not contain neuronal structures.

Directional sensitivity of lateral line units in the clawed toadXenopus laevis Daudin

SummaryFollowing lateral line stimulation with surface waves single unit activity was recorded from the periphery, torus semicircularis, and tecturn opticum ofXenopus laevis. The reaction of units to varying stimulus directions was examined.The directional specificity (DS) was calculated on the basis of spike counts per stimulus using circular statistics. It was expressed as the length of the mean vector.Discharges of primary afferents of the ramus supraorbitalis and ramus infraorbitalis were phase locked to the stimulus to a varying degree depending on the location of the corresponding groups of neuromasts. Their DS was not better than 0.26.Lemniscal fibers, representing the ascending output of the medulla and units of the torus semicircularis reached a DS of 0.10–0.24 and 0.11–0.36 respectively. Neurons in the tectum opticum were the most sharply tuned with DS ranging between 0.81 and 0.96.The surroundings were represented by the best directions of two arrays of tectal units forming a map which is in register with the representation of the corresponding visual field of the animal.

Electroreceptive and mechanoreceptive units in the lateral line of the axolotlAmbystoma mexicanum

SummaryThe properties of electroreceptive units (ampullary organs) and mechanoreceptive units (neuromasts) in the head lateral line system of the axolotlAmbystoma mexicanum were compared by recording single unit activity in the afferent fibers:1.Electroreceptive units respond with excitation to cathodal (outward) and with inhibition to anodal (inward) current. Ordinary lateral line units react in the opposite way: they are excited by anodal and inhibited by cathodal current.2.Electroreceptive units react to electric field stimuli (square pulses) down to a voltage gradient parallel to the skin of 10 μV cm−1 with 10% change of the mean resting activity. Mechanoreceptive lateral line units show this response at a stimulus strength of about 10 mV cm−1. Sinusoidal stimulation of electroreceptive units in the best frequency range with amplitudes down to 5 μV cm−1 resulted in 10% modulation response.3.Electroreceptive units respond only to rough mechanical stimulation with water jets, while mechanoreceptive lateral line units have thresholds to local water displacement below 1 μm and show direction sensitivity.4.The statistical distribution of the resting activity impulse intervals in electroreceptive units has a median between 5 and 25 imp s−1. The value of the median of mechanoreceptive units resting activity is between 5 and 80 imp s−1. Electroreceptive units have a more symmetrical interspike interval distribution (modus — median: ca. 5 imp s−1) than mechanoreceptive units (modus — median: 0–80 imp s−1) under present experimental conditions.5.Electroreceptive units reduce their resting activity after application of 200 μl aliquots of 10 mmol/l MgCl2 solution to the receptor sites. However, in mechanoreceptive lateral line units, the same stimulus elicits either a weak increase in activity or no reaction at all.6.The electroreceptive units were tested with sinusoidal electric field stimuli from 0.05 to 100 Hz. The gain curve has its maximum around 10 Hz. At the low frequency end the slope of the curve is 2.7 dB/oct. Above 20 Hz the gain decreases with a slope of 3–4 dB/oct. The mechanoreceptive lateral line units are most sensitive to local sinusoidal water displacements of 20–50 Hz. The gain curve increases with a slope of 12 dB/oct.

Physiology of Lateral-Line Mechanoreceptors in a Teleost with Highly Branched, Multiple Lateral Lines

The physiological aspects of the lateral line of Xiphister atropurpureus--a teleost with multiple, highly branched lateral-line canals on each body side--were investigated. In terms of sensitivity and frequency response, the lateral line of Xiphister is similar to simple, unbranched lateral-line canals. For both the dorsal and the medial trunk canal the lowest displacement threshold is in the frequency range of 100-150 Hz. Within this range, a peak-to-peak displacement of 0.2-1.0 microns is sufficient to evoke a neural response. For the stimuli applied, both the dorsal and the medial lateral-line canals had well-defined, but largely overlapping, receptive fields. A comparison of the experimental results with theoretical calculations showed that the trunk lateral line of Xiphister responds in proportion to the sum of the radial and angular water displacement components caused by a vibrating sphere. Our calculations further predict that multiple lateral lines cause a substantial increase in receptive-field size if nearby objects create water displacements that are barely above threshold and/or if a stimulus is masked by background noise.

Common efferents to lateral line and labyrinthine hair cells in aquatic vertebrates

Abstract The axonal arborization of lateral line efferent neurones of fishes and urodeles is demonstrated using retrogradely transported horseradish peroxidase. Axon collaterals can be traced into the lateral line nerve and the sensory epithelia of the labyrinth. In fishes efferent somata and axon collaterals are restricted to the ipsilateral side, but they are bilaterally distributed in urodeles. Because of the widespread axonal branching the efferents are considered to have a more general effect, rather than to influence single maculae selectively.

The Terminal Nerve and Its Development in the Teleost Fishes

The name “nervus terminalis” ( N T ) ’ was given at the turn of the century to a cranial nerve first identified in sharks by Fritsch’ and in lungfish by Pinkus.’ Since then, this “supernumerary” cranial nerve has been found in nearly all vertebrate classes.” In recent years several studies demonstrated that N T neurons in different species or even in the same species are of heterogeneous morphology and arrangement.8.9 The course and the targets of their peripheral and central processes and their neuropeptide content also show considerable differences.6.’.1° In summary, the published anatomical data indicated that the N T system is more complex than a typical cranial nerve. In cartilagenous fish, lungfish and some reptiles the N T can be easily identified on the basis of its extracerebral fiber tract which is completely separated from the olfactory nerve. In teleosts as in higher vertebrates, the most rostral N T fibers are closely associated with the fila olfactoria and the central parts of the olfactory system proper.”-” The heterogeneity of the N T in different teleostean species and the proximity to the olfactory system proper sometimes makes it difficult to distinguish the central projections of the two systems with classical histologic methods. In the past the presence of projections to areas rostral to the olfactory bulb and the termination of fibers caudal to the olfactory bulb were used as criteria for identifying a neuron as belonging to the NT. But, since phylogenetic development of the NT may be accompanied by variation in these characters, a clear distinction between NT and secondary olfactory neurons is almost impossible in many cases. More recently, fiber tracing techniques have revealed additional characters of NT neurons. These include the projection of teleostean NT neurons to the retina”” and to hypothalamic and mesencephalic centers.” Furthermore, the presence of gonadotropin hormone-releasing hormone (GnRH) and other neuropeptide-like s u b ~ t a n c e s ~ ~ ’ ~ ~ ’ ’ ~ ’ ~ in NT was found. In the present study the reactivity of NT neurons to GnRH antibodies was used to reinvestigate the distribution of cell bodies and fibers making up the teleostean NT. The data provide a basis to discuss the development of the NT during the phylogeny of teleostean fishes.

Projection of lateral line afferents in a teleost's brain.

After injection of horseradish peroxidase solution into the lateral line nerves of a poeciliid fish the afferent fibres can be followed up to their termination fields in the medulla, lobus vestibulolateralis, corpus cerebelli and valvula cerebelli. Labelling of all fibres innervating individual neuromasts reveals a well-ordered branching pattern of afferents in the CNS and a projection to all termination areas.