The comparative distribution of peptidergic neural systems in the brain of the euryhaline, viviparous teleost Poecilia latipinna (green molly) was examined by immunohistochemistry. Topographically distinct, but often overlapping, systems of neurons and fibres displaying immunoreactivity (ir) related to a range of neuropeptides were found in most brain areas. Neurosecretory and hypophysiotrophic hormones were localized to specific groups of neurons mostly within the preoptic and tuberal hypothalamus, giving fibre projections to the neurohypophysis, ventral telencephalon, thalamus, and brain stem. Separate vasotocin (AVT)-ir and isotocin (IST)-ir cells were located in the nucleus preopticus (nPO), but many AVT-ir nPO neurons also displayed growth hormone-releasing factor (GRF)-like-ir, and in some animals corticotrophin-releasing factor (CRF)-like-ir. The main group of CRF-ir neurons was located in the nucleus recessus anterioris, where coexistence with galanin (GAL) was observed in some cells. Enkephalin (ENK)-like-ir was occasionally present in a few IST-ir cells of the nPO and was also found in small neurons in the posterior tuberal hypothalamus and in a cluster of large cells in the dorsal midbrain tegmentum. Thyrotrophin-releasing hormone (TRH)-ir cells were found near the rostromedial tip of the nucleus recessus lateralis. Gonadotrophin-releasing hormone (GnRH)-ir cells were present in the nucleus olfactoretinalis, ventral telencephalon, preoptic area, and dorsal midbrain tegmentum. Molluscan cardioexcitatory peptide (FMRF-amide)-ir was colocalized with GnRH-ir in the ganglion cells and central projections of the nervus terminalis. Melanin-concentrating hormone (MCH)-ir neurons were restricted to the tuberal hypothalamus, mostly within the nucleus lateralis tuberis pars lateralis, and somatostatin (SRIF)-ir neurons were numerous throughout the periventricular areas of the diencephalon. A further group of SRIF-ir neurons extending from the ventral telencephalon into the dorsal telencephalon pars centralis also contained neuropeptide Y (NPY)-, peptide YY (PYY)-, and NPY flanking peptide (PSW)-like-ir. These immunoreactivities were, however, also observed in non-SRIF-ir cells and fibres, particularly in the mesencephalon. Calcitonin gene-related peptide (CGRP)-like-ir had a characteristic distribution in cells grouped in the isthmal region and fibre tracts running forward into the hypothalamus, most strikingly into the inferior lobes. Antisera to cholecystokinin (CCK) and neurokinin A (NK) or substance P (SP) stained very extensive, separate systems throughout the brain, with cells most consistently seen in the ventral telencephalon and periventricular hypothalamus. Broadly similar, but much more restricted, distributions of cells and fibres were seen with antisera to neurotensin (NT) and vasoactive intestinal peptide (VIP).(ABSTRACT TRUNCATED AT 400 WORDS)

Comparative distribution of neuropeptide-immunoreactive systems in the brain of the green molly, Poecilia latipinna.

Modulation of ionic channel properties and synaptic functions by neurotransmitters and hormones is called neuro-modulation and may be the basis for many long-lasting changes in animal behavior, for example, changes in the arousal or motivational states. We have previously shown in a teleost, the dwarf gourami, that the terminal nerve (TN) cells are a major component of the gonadotropin-releasing hormone (GnRH) system and are structurally independent from the preoptic/hypophysial-GnRH system, which projects to the pituitary and facilitates gonadotropin release from the gonadotropes. While GnRH fibers originating from TN cells are distributed widely throughout the brain, they do not project to the pituitary. Thus, TN-GnRH system does not function as a hypophysiotropic hormone but probably as a neuro-modulator, capable of affecting widespread regions of the brain. In the present study, we used a whole-brain in vitro preparation to examine the spontaneous electrical activities of TN-GnRH cells and to determine the morphology of individual cells by intracellular injections of either neurobiotin or biocytin. The recorded cells were clearly identified as GnRH-immunoreactive TN cells by using combined intracellular injection and GnRH immunocytochemistry. Most of the TN cells showed endogenous slow (1-7 Hz), regular beating discharges. The intrinsic nature of this activity was demonstrated by the voltage dependency of the beating frequency, rhythm resetting, and persistence of rhythmicity after synaptic isolation. Only a small number of TN cells showed either irregular or bursting discharge patterns. Anatomical observation of intracellularly labeled cells showed that, regardless of discharge patterns, all the TN cells had multiple axonal branches that project to those areas where we had previously demonstrated dense GnRH-immunoreactive fibers. From the present results, we propose a hypothesis that may be relevant to the peptidergic and monoaminergic neuromodulatory systems in general. The modulator neurons have endogenous rhythmic activities that vary according to the animal's hormonal or environmental conditions, and they regulate the excitability of target neurons in a wide variety of brain regions simultaneously via multiple axonal branches.

https://www.jneurosci.org/content/jneuro/13/5/2161.full.pdf

Gonadotropin-releasing hormone (GnRH)-immunoreactive terminal nerve cells have intrinsic rhythmicity and project widely in the brain

The class of bony fishes or Osteichthyes is usually subdivided into two subclasses, the Actinopterygii or ray-finned fishes and the Sarcopterygii or lobe-finned fishes. The subclass last mentioned, the telencephalon of which will be treated in the next chapter, can further be subdivided into the Dipnoi or lungfishes and the Crossopterygii or tassel-finned fishes. The Actinopterygii constitute by far the most abundant group of recent vertebrates. They are usually subdivided into three superorders, the Chondrostei, Holostei, and Teleostei. Interestingly, these three superorders may be considered as representing three subsequent stages or gradations of actinopterygian evolution. The Chondrostei are considered as descendants of the Palaeoniscoidei, a primitive group of paleozoic fishes. During the early and middle Mesozoic times, the Chondrostei were supplanted by the Holostei, which in turn were largely replaced in the last phase of the Mesozoic era and the Cenozoic era by the expanding and now-abundant Teleostei.

The Telencephalon of Actinopterygian Fishes

Fish commonly exhibit reproductive responses to waterborne steroids, prostaglandins (PGs), and their metabolites (hormonal pheromones) – a logical result of a long evolutionary history spent in a medium that often reduces visual information but readily exposes the olfactory organ to information-rich chemicals released by conspecifics. Although fish from major taxa, including carps, catfishes, salmon, and gobies, respond with great sensitivity and specificity to a variety of hormonal pheromones, in no species are hormonal pheromones well understood. The phenomenon appears best characterized in goldfish, where steroid and PG pheromones released sequentially by periovulatory females dramatically affect male behavior, physiology, and reproductive fitness. Goldfish steroidal pheromones appear to involve only specializations of the receiving individual, a situation which these authors have termed chemical spying, whereas in other fish, where the sending individual is also specialized, hormonal pheromones evidently function in true communication. By demonstrating that sex steroids and PGs synchronize reproductive functions not only by endogenous hormonal actions but also by exogenous pheromonal actions, the relatively recent discovery of hormonal pheromones has considerably expanded our concepts of fish reproductive function while providing valuable model systems for future study of olfactory function and pheromone evolution.

Hormonal Pheromones in Fish

We have explored the structure and actions of terminal nerve (TN) fibers in the teleostean retina, the most accessible of TN projections. Using immunocytochemistry we have shown that the goldfish TN contains neuropeptides related to the molluscan cardioexcitatory peptide (FMRFamide) as well as luteinizing hormone-releasing hormone (LHRH). Retinal TN terminals were found upon major dendrites in the distal inner plexiform layer and neuronal cell bodies in the amacrine cell layer. Electron-microscopic double-labeling revealed TN terminals applied to the surface of [3H]-dopamine-, glycine-, and gamma-aminobutyric acid (GABA)-accumulating cells. Synthetic LHRH and FMRFamide at less than 1 microM modified spontaneous and light-evoked activity of ganglion cells in isolated superfused goldfish retina, especially during the active breeding season. Salmon(I)-LHRH was 10-30 times as potent as mammalian LHRH and caused rapid, prolonged desensitization. We conclude that LHRH- and FMRFamide-like peptides may be released by retinal TN endings, probably in concert with reproductive activity, and that they act independently through horizontal and/or amacrine cell pathways to modify visual information processing in the retina.

Functional-anatomical studies on the terminal nerve projection to the retina of bony fishes.

Central projections of the nervus terminalis (n.t.) in the goldfish were investigated using cobalt- and horseradish peroxidase-tracing techniques. Single n.t. fibers were identified after unilateral application of cobalt chloride-lysine to the rostral olfactory bulb. The central course and branching patterns of individual n.t. fibers were studied in serial sections. Eight types of n.t. fibers are differentiated according to pathways and projection patterns. Projection areas of the n.t. include the contralateral olfactory bulb, the ipsilateral periventricular preoptic nucleus, both retinae, the caudal zone of the periventricular hypothalamus bilaterally, and the rostral optic tectum bilaterally. N.t. fibers cross to contralateral targets in the anterior commissure, the optic chiasma, the horizontal commissure, the posterior commissure, and possibly the habenular commissure. We propose criteria that differentiate central n.t. fibers from those of the classical secondary olfactory projections. Branching patterns of eight n.t. fiber types are described. Mesencephalic projections of the n.t. and of secondary olfactory fibers are compared and discussed with regard to prior reports on the olfactory system of teleosts. Further fiber types for which the association with the n.t. could not be established with certainty were traced to the torus longitudinalis, the torus semicircularis, and to the superior reticular nucleus on the ipsilateral side.

Tracing of single fibers of the nervus terminalis in the goldfish brain.

Innervation of the cirri in three teleost species (Hypsoblennius gilberti, Hypsoblennius gentilis, Oxylebius pictus) was investigated with the use of HRP- and cobalttracing techniques. All projections were found to be ipsilateral. Labeled cells were demonstrated in both portions of the trigeminal ganglion and in the facial ganglion. Cirrus nerve fibers running in the trigeminal nerve project to terminal fields in an isthmic sensory trigeminal nucleus, to areas adjacent to the descending trigeminal root in the brainstem, and to the medial funicular nucleus in the medulla. Distribution of labeled cells in the trigeminal ganglion complex suggests a functional distinction of the two ganglion portions. Cirrus nerve fibers belonging to the facial nerve terminate in a circumscribed part of of the facial lobe, indicating a somatotopic projection. Pathways were principally the same in all three species investigated. Findings of facial innervation of teleost cirri suggest a suspected gustatory function of teleost head appendages.

Trigeminal and facial innervation of cirri in three teleost species

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Tracing of single fibers of the nervus terminalis in the goldfish brain.

Trigeminal and facial innervation of cirri in three teleost species