The organization of monoamine-containing neurons in the brain of the salamander, Necturus maculosus

Molecular biology of serotonergic systems in avian brains

Serotonin (5-hydroxytryptamine, 5-HT) is a phylogenetically conserved neurotransmitter and modulator. Neurons utilizing serotonin have been identified in the central nervous systems of all vertebrates. In the central serotonergic system of vertebrate species examined so far, serotonergic neurons have been confirmed to exist in clusters in the brainstem. Although many serotonin-regulated cognitive, behavioral, and emotional functions have been elucidated in mammals, equivalents remain poorly understood in non-mammalian vertebrates. The purpose of this review is to summarize current knowledge of the anatomical organization and molecular features of the avian central serotonergic system. In addition, selected key functions of serotonin are briefly reviewed. Gene association studies between serotonergic system related genes and behaviors in birds have elucidated that the serotonergic system is involved in the regulation of behavior in birds similar to that observed in mammals. The widespread distribution of serotonergic modulation in the central nervous system and the evolutionary conservation of the serotonergic system provide a strong foundation for understanding and comparing the evolutionary continuity of neural circuits controlling corresponding brain functions within vertebrates. The main focus of this review is the chicken brain, with this type of poultry used as a model bird. The chicken is widely used not only as a model for answering questions in developmental biology and as a model for agriculturally useful breeding, but also in research relating to cognitive, behavioral, and emotional processes. In addition to a wealth of prior research on the projection relationships of avian brain regions, detailed subdivision similarities between avian and mammalian brains have recently been identified. Therefore, identifying the neural circuits modulated by the serotonergic system in avian brains may provide an interesting opportunity for detailed comparative studies of the function of serotonergic systems in mammals.

Organization of serotonergic system in Sphaerotheca breviceps (Dicroglossidae) tadpole brain

The monoaminergic neurotransmitter 5-hydroxytryptamine (5-HT) is known to be involved in several physiological, behavioural and neuroendocrine functions in vertebrates. In this study, we investigated the distribution of 5-HT neuronal system in the central nervous system (CNS) of Sphaerotheca breviceps tadpoles at metamorphic climax stage. In the telencephalon, there was no 5-HT-immunoreactive (5-HT-ir) perikarya, but conspicuous fibres were observed in the olfactory bulb, pallium, subpallium and amygdala complexes. The preoptic area showed dense 5-HT-ir somata and cerebrospinal fluid contacting fibres, whereas a few varicose 5-HT-ir fibres were noticed in the suprachiasmatic nucleus. 5-HT-ir cells and fibres were found in the ventral, lateral dorsal subdivisions of the hypothalamus and in the nucleus tuberculi posterioris, but only 5-HT-ir fibres were localised in the periventricular area and pituitary gland. Numerous 5-HT-ir cells and/or fibres were detected in the thalamus, entopeduncular area and mesencephalic subdivisions. In the rhombencephalon, although 5-HT-ir cells and fibres were noticed in the subdivisions of the raphe nucleus and reticular formation, a moderate plexus of fibres was observed in the cerebellum, parabrachial nucleus and solitary tract. Distinct 5-HT-ir fibres, but no perikarya, were observed in the rostral spinal cord. Overall, extensively labelled 5-HT-ir cells and fibres in the CNS of the metamorphic tadpole suggest possible roles for the involvement of 5-HT in various somatosensory, behavioural and neuroendocrine functions during final stages of development.

Locus Coeruleus in Non-Mammalian Vertebrates

The locus coeruleus (LC) is a vertebrate-specific nucleus and the primary source of norepinephrine (NE) in the brain. This nucleus has conserved properties across species: highly homogeneous cell types, a small number of cells but extensive axonal projections, and potent influence on brain states. Comparative studies on LC benefit greatly from its homogeneity in cell types and modularity in projection patterns, and thoroughly understanding the LC-NE system could shed new light on the organization principles of other more complex modulatory systems. Although studies on LC are mainly focused on mammals, many of the fundamental properties and functions of LC are readily observable in other vertebrate models and could inform mammalian studies. Here, we summarize anatomical and functional studies of LC in non-mammalian vertebrate classes, fish, amphibians, reptiles, and birds, on topics including axonal projections, gene expressions, homeostatic control, and modulation of sensorimotor transformation. Thus, this review complements mammalian studies on the role of LC in the brain.

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Homeostatic and regenerative neurogenesis in salamanders

Large-scale regeneration in the adult central nervous system is a unique capacity of salamanders among tetrapods. Salamanders can replace neuronal populations, repair damaged nerve fibers and restore tissue architecture in retina, brain and spinal cord, leading to functional recovery. The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools. Recent technological progress, such as genome sequencing, transgenesis and genome editing provide new momentum for systematic interrogation of regenerative processes in the salamander central nervous system. Understanding central nervous system regeneration also entails designing the appropriate molecular, cellular, and behavioral assays. Here we outline the organization of salamander brain structures. With special focus on ependymoglial cells, we integrate cellular and molecular processes of neurogenesis during developmental and adult homeostasis as well as in various injury models. Wherever possible, we correlate developmental and regenerative neurogenesis to the acquisition and recovery of behaviors. Throughout the review we place the findings into an evolutionary context for inter-species comparisons.

Ventral tegmental area dopamine revisited: effects of acute and repeated stress

Aversive events rapidly and potently excite certain dopamine neurons in the ventral tegmental area (VTA), promoting phasic increases in the medial prefrontal cortex and nucleus accumbens. This is in apparent contradiction to a wealth of literature demonstrating that most VTA dopamine neurons are strongly activated by reward and reward-predictive cues while inhibited by aversive stimuli. How can these divergent processes both be mediated by VTA dopamine neurons? The answer may lie within the functional and anatomical heterogeneity of the VTA. We focus on VTA heterogeneity in anatomy, neurochemistry, electrophysiology, and afferent/efferent connectivity. Second, recent evidence for a critical role of VTA dopamine neurons in response to both acute and repeated stress will be discussed. Understanding which dopamine neurons are activated by stress, the neural mechanisms driving the activation, and where these neurons project will provide valuable insight into how stress can promote psychiatric disorders associated with the dopamine system, such as addiction and depression.

Connectivity and cytoarchitecture of the ventral telencephalon in the salamanderPlethodon shermani

The cytoarchitecture and axonal connection pattern of centers in the ventral telencephalon of the salamander Plethodon shermani were studied using biocytin for anterograde and retrograde labeling of cell groups, as well as by intracellular injections. Application of biocytin to the main and accessory olfactory bulbs identified the olfactory pallial regions and the vomeronasal portion of the amygdala, respectively. According to our results, the amygdala of Plethodon is divided into (1) a rostral part projecting to visceral and limbic centers and receiving afferents from the dorsal thalamus, and (2) a caudal part receiving accessory olfactory input. The striatopallial transition area (SPTA) lies rostrodorsally to the caudal (vomeronasal) amygdala and is similar in connections and possibly in function. The rostral striatum has few descending projections to the medulla, whereas the intermediate striatum sends strong projections to the tegmentum and medulla. The caudal striatum has strong ascending projections to the striatum and descending projections to the ventral hypothalamus. The dendritic trees of neurons labeled below the striatum and in the SPTA spread laterally from the soma, whereas dendrites of striatal neurons converge into the laterally situated striatal neuropil. In the caudal amygdala, three distinct types of neurons are found differing in dendritic arborization. It is concluded that, hodologically, the rostral part of the urodele amygdala corresponds to the central and basolateral amygdala and the caudal part to the cortical/medial amygdala of mammals. The urodele striatum is divided into a rostral striatum proper, an intermediate dorsal pallidum, and a caudal part, with distinct connections described here for the first time in a vertebrate.

Descending supraspinal pathways in amphibians. II. Distribution and origin of the catecholaminergic innervation of the spinal cord

Immunohistochemical studies with antibodies against tyrosine hydroxylase, dopamine, and noradrenaline have revealed that the spinal cord of anuran, urodele, and gymnophionan (apodan) amphibians is abundantly innervated by catecholaminergic (CA) fibers and terminals. Because intraspinal cells occur in all three orders of amphibians CA, it is unclear to what extent the CA innervation of the spinal cord is of supraspinal origin. In a previous study, we showed that many cell groups throughout the forebrain and brainstem project to the spinal cord of two anurans (the green frog, Rana perezi, and the clawed toad, Xenopus laevis), a urodele (the Iberian ribbed newt, Pleurodeles waltl), and a gymnophionan (the Mexican caecilian, Dermophis mexicanus). To determine the exact site of origin of the supraspinal CA innervation of the amphibian spinal cord, retrograde tracing techniques were combined with immunohistochemistry for tyrosine hydroxylase in the same sections. The double-labeling experiments demonstrated that four brain centers provide CA innervation to the amphibian spinal cord: 1.) the ventrolateral component of the posterior tubercle in the mammillary region, 2.) the periventricular nucleus of the zona incerta in the ventral thalamus, 3.) the locus coeruleus, and 4.) the nucleus of the solitary tract. This pattern holds for all three orders of amphibians, except for the CA projection from the nucleus of the solitary tract in gymnophionans. There are differences in the strength of the projections (based on the number of double-labeled cells), but in general, spinal functions in amphibians are controlled by CA innervation from brain centers that can easily be compared with their counterparts in amniotes. The organization of the CA input to the spinal cord of amphibians is largely similar to that described for mammals. Nevertheless, by using a segmental approach of the CNS, a remarkable difference was observed with respect to the diencephalic CA projections.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Serotonergic systems in the spinal cord of the amphibian urodele Pleurodeles waltl

The role of the monoamine serotonin (5-HT) in modulating the neural networks underlying axial locomotor movements was studied in an adult amphibian urodele, Pleurodeles waltl. 5-HT was applied to an in vitro brainstem-spinal cord preparation of P. waltl, which displayed fictive axial locomotor patterns following bath application of N-methyl-D-aspartate (5 microM) with D-serine (10 microM). Our results showed that 5-HT (1-25 microM) produces a reversible increase in the cycle duration and the duration of rhythmic bursting activity recorded extracellularly from ventral roots innervating the axial musculature. When applied alone, 5-HT does not trigger axial locomotor activity. The distribution pattern of 5-HT immunoreactive (5-HT-ir) cells along the spinal cord was investigated both in intact and in chronic spinal animals. The number of 5-HT-ir cell bodies is higher at brachial levels and decreases through crural levels. Sparse oval or fusiform 5-HT-ir somata are present within the gray matter, just ventrolateral to the central canal. Longitudinal fibers were detected throughout the entire white matter, except in the medial part of the dorsal funiculi. Two columns of intensely labeled and profusely branching thick and thin fibers associated with numerous varicosities run continuously along the ventrolateral surface of the spinal cord. Three weeks following full spinal cord transection at the level of the second spinal root, all longitudinal processes had disappeared, indicating their supraspinal origin, whereas the ventrolateral plexes remained, suggesting that they originated from intraspinal 5-HT-ir cell bodies. Our data showing that spinal 5-HT is organized according to a rostrocaudal gradient suggest that the 5-HT systems of P. waltl are not related to the presence of limb motor pools but more likely are related to axial central pattern generators (CPGs) networks down the length of the spinal cord. The possible involvement of these two sources (descending vs. intraspinal) of 5-HT innervation in the modulation of the axial CPGs is discussed.

Basal ganglia organization in amphibians: catecholaminergic innervation of the striatum and the nucleus accumbens

The aim of the present study was to determine the origin of the catecholaminergic inputs to the telencephalic basal ganglia of amphibians. For that purpose, retrograde tracing techniques were combined with tyrosine hydroxylase immunohistochemistry in the anurans Xenopus laevis and Rana perezi and the urodele Pleurodeles waltl. In all three species studied, a topographically organized dopaminergic projection was identified arising from the posterior tubercle/mesencephalic tegmentum and terminating in the striatum and the nucleus accumbens. Although essentially similar, the organization of the mesolimbic and mesostriatal connections in anurans seems to be more elaborate than in urodeles. The present study has also revealed the existence of a noradrenergic projection to the basal forebrain, which has its origin in the locus coeruleus. Additional catecholaminergic afferents to the striatum and the nucleus accumbens arise from the nucleus of the solitary tract, where catecholaminergic neurons appear to give rise to the bulk of the projections to the basal forebrain. In other regions, such as the olfactory bulb, the anterior preoptic area, the suprachiasmatic nucleus, and the thalamus, retrogradely labeled neurons (after basal forebrain tracer-applications) and catecholaminergic cells were intermingled, but none of these centers contained double-labeled cell bodies. It is concluded that the origin of the catecholaminergic innervation of the striatum and the nucleus accumbens in amphibians is largely comparable to that in amniotes. The present study, therefore, strongly supports the existence of a common pattern in the organization of the catecholaminergic inputs to the basal forebrain among tetrapod vertebrates.

Antibodies against different forms of GnRH distinguish different populations of cells and axonal pathways in a urodele amphibian, Taricha granulosa

Neurons immunoreactive to the peptide hormone gonadotropin-releasing hormone (GnRH) have been identified in the posterior diencephalon or anterior midbrain of diverse vertebrates. These cells are located caudal to the more well-characterized GnRH neurons in the nervus terminalis and septo-preoptic area, and are believed to express one or more of the nonmammalian forms of the GnRH. The present study utilized immunocytochemical techniques to determine whether the posterior GnRH group is present in a urodele amphibian, the newt Taricha granulosa. Antibodies directed against different molecular forms of GnRH were used to evaluate the immunological properties of GnRH-containing neurons in amphibians. An antibody selective for mammalian GnRH labeled perikarya in the nervus terminalis (terminal nerve) and septo-preoptic region, as described previously. Thick fibers that arise from terminal nerve and septo-preoptic neurons project mainly to the median eminence, medial pallium and habenula. An antibody selective for chicken GnRH II labeled cell bodies in the paraventricular organ and posterior tubercle of the caudal diencephalon, and thin fibers that project widely throughout the central nervous system. Region-specific staining with different GnRH antibodies supports the interpretation that different molecular forms of GnRH are expressed by neuroanatomically distinguishable systems.

Organization of the serotoninergic system in the brain of two amphibian species, Ambystoma mexicanum (Urodela) and Typhlonectes compressicauda (Gymnophiona)

An immunocytochemical investigation was made of the distribution of serotonin (5-HT) in the brain of larval and adult Ambystoma mexicanum and adult Typhlonectes compressicauda. Immunoreactive perikarya can be identified in the caudal diencephalon (paraventricular organ and infundibular nucleus), in the ventral mesencephalon (interpeduncular nucleus) and in the raphe of the rhombencephalon. Immunopositive fibers and terminal arborizations are widely distributed, extending from the whole telencephalon to the spinal lemniscus area. However, the retinorecipient structures of the thalamus and mesencephalon are either very weakly innervated (Ambystoma) or completely immunonegative (Typhlonectes). The habenular system also exhibits very few 5-HT-positive structures. The major serotoninergic neuron clusters, in both Urodela and Gymnophiona, tend to gather, from the paraventricular organ to the raphe, on both sides of the sagittal plane, showing no tendency to "lateralization". A new interpretation of the limited development of the serotoninergic system in amphibians is given.

Distribution of catecholaminergic and serotoninergic systems in forebrain and midbrain of the newt, Triturus alpestris (Urodela)

Mapping of monoaminergic systems in the brain of the newt Triturus alpestris was achieved with antisera against (1) thyrosine hydroxylase (TH), (2) formaldehyde-conjugated dopamine (DA), and (3) formaldehyde-conjugated serotonin (5-HT). In the telencephalon, the striatum was densely innervated by a large number of 5-HT-, DA- and TH-immunoreactive (IR) fibers; IR fibers were more scattered in the amygdala, the medial and lateral forebrain bundles, and the anterior commissure. In the anterior and medial diencephalon, TH-IR perikarya contacting the cerebrospinal fluid (CSF-C perikarya) were located in the preoptic recess organ (PRO), the organum vasculosum laminae terminalis and the suprachiasmatic nucleus. Numerous TH-IR perikarya, not contacting the CSF, were present in the posterior preoptic nucleus and the ventral thalamus. At this level, DA-IR CSF-C neurons were only located in the PRO. In the posterior diencephalon, large populations of 5-HT-IR and DA-IR CSF-C perikarya were found in the paraventricular organ (PVO) and the nucleus infundibularis dorsalis (NID); the dorsal part of the NID additionally presented TH-IR CSF-C perikarya. Most regions of the diencephalon showed an intense monoaminergic innervation. In addition, numerous TH-IR, DA-IR and 5-HT-IR fibers, originating from the anterior and posterior hypothalamic nuclei, extended ventrally and reached the median eminence and the pars intermedia of the pituitary gland. In the midbrain, TH-IR perikarya were located dorsally in the pretectal area. Ventrally, a large group of TH-IR cell bodies and some weakly stained DA-IR and 5-HT-IR neurons were observed in the posterior tuberculum. No dopaminergic system equivalent to the substantia nigra was revealed. The possible significance of the differences in the distribution of TH-IR and DA-IR neurons is discussed, with special reference to the CSF-C neurons.

Comparative analysis of dopamine and tyrosine hydroxylase immunoreactivities in the brain of two amphibians, the anuran Rana ridibunda and the urodele Pleurodeles waltlii

To gain more insight into the dopaminergic system of amphibians and the evolution of catecholaminergic systems in vertebrates in general, the distribution of dopamine and tyrosine hydroxylase immunoreactivity was studied in the brains of the anuran Rana ridibunda and the urodele Pleurodeles waltlii. In both species, dopamine-immunoreactive (DAi) cell bodies were observed in the olfactory bulb, the preoptic area, the suprachiasmatic nucleus, the nucleus of the periventricular organ and its accompanying cells, the nucleus of the posterior tubercle, the pretectal area, the midbrain tegmentum, around the solitary tract, in the ependymal and subependymal layers along the midline of the caudal rhombencephalon, and ventral to the central canal of the spinal cord. Tyrosine hydroxylase (TH) immunohistochemistry revealed a similar pattern, although some differences were noted. For example, with the TH antibodies, additional cell bodies were stained in the internal granular layer of the olfactory bulb and in the isthmal region, whereas the same antibodies failed to stain the liquor contacting cells in the nucleus of the periventricular organ. Both antisera revealed an almost identical distribution of fibers in the two amphibian species. Remarkable differences were observed in the forebrain. Whereas the nucleus accumbens in Rana contains the densest DAi plexus, in Pleurodeles the dopaminergic innervation of the striatum prevails. Moreover, cortical structures of the newt contain numerous DAi fibers, whereas the corresponding structures in the frog are devoid of immunoreactivity. The dopaminergic system in amphibians appears to share many features not only with other anamniotes but also with amniotes.

Hypothalamic and telencephalic catecholamine content in the brain of the teleost fish, Pygocentrus notatus, during the annual reproductive cycle

The catecholamines noradrenaline (NA), dopamine (DA), and adrenaline (A) were measured in hypothalamic and telencephalic extracts of the Venezuelan freshwater fish "caribe colorado," Pygocentrus notatus, at different stages of the reproductive cycle. The concentration of NA was found to be significantly higher in the telencephalon than in the hypothalamus, but that of DA was higher in the hypothalamus than in the telencephalon. Fluctuations depending upon the reproductive stage and environmental conditions occurred in both hypothalamus and telencephalon. In the hypothalamus, DA content was highest during the prespawning period (June) as compared to other periods of the cycle. Although the NA concentration was reduced during spawning there was no significant variation during any other period. DA concentrations in both telencephalon and hypothalamus showed a similar pattern of changes. In the telencephalon, NA levels increased between preparatory and prespawning periods but decreased sharply during spawning. No sex differences were observed in either area at any stage of reproduction.

Paraventricular organ of the lungfish Protopterus dolloi: morphology and projections of CSF-contacting neurons

The morphology and projections of neurons in the paraventricular organ (PVO) were studied by means of silver impregnation after intraocular application of cobaltous lysine in the lungfish Protopterus dolloi. Cobalt-labeled neurons were found exclusively in the PVO in the dorsal and infundibular hypothalamus. These bipolar neurons possess one CSF-contacting process that protrudes into the ventricular lumen with a club-shape ending and a thick, ramifying process directed into the hypothalamic neuropil; the ependymofugal processes form intra- and extrahypothalamic projections. Impregnated fibers from paraventricular neurons cross in infundibular and hypothalamic commissures, the commissure of the posterior tuberculum, the postoptic, the habenular, and the anterior commissures. Projections to the infundibulum and the median eminence are relatively sparse; no fibers are labeled in the pituitary gland. Ascending projections to the forebrain are extensive. Major targets include the dorsal hypothalamus, the periventricular preoptic nuclei, the habenula, the subhabenular region, the anterodorsal thalamus, and the medial telencephalic hemisphere (septum). Most ascending fibers follow the medial forebrain bundle; others course in the fasciculus retroflexus and terminate in rostral parts of the ipsilateral habenula. Descending fibers run caudally along the ventral floor of the brainstem. They terminate in the neuropil of the mesencephalic tegmentum, ventral tectum, isthmic region, ventral portions of the reticular formation throughout the rhombencephalon, and extend into the spinal cord. Intraocular application of cobaltous lysine results in selective impregnation of neurons in the PVO and their ascending and descending projections, presumably via uptake of tracer from vascular circulation. These projections do not represent retinofugal or retinopetal projections. We provide conclusive evidence for the existence of a PVO in Protopterus. On the basis of PVO location and acetylcholinesterase histochemistry, we propose subdivisions of the infundibular hypothalamus corresponding to those in amphibians. Ascending PVO projections appear to be particularly well developed in lungfish compared with other species and may be related to specialized endocrine mechanisms in this group of vertebrates.

Catecholaminergic systems in the brain of a gymnotiform teleost fish: an immunohistochemical study

The localization of catecholamines (CA) in the brain of Apteronotus leptorhynchus was studied with immunohistochemical techniques using antibodies to the enzymes tyrosine hydroxylase (TH), dopamine B-hydroxylase (DBH), phenylethanolamine-N-methyltransferase (PNMT), and the neurotransmitter dopamine (DA). Telencephalic TH and DA immunoreactive (ir) neurons were located in the following structures: olfactory bulb, area ventralis telencephali partes ventralis, centralis, dorsalis, and intermediate. Diencephalic TH ir neurons were distributed in: nucleus preopticus periventricularis pars anterior, floor of preoptic recess, n. suprachiasmaticus, n. preopticus periventricularis pars posterior, n. anterior periventricularis, area ventralis lateralis, rostral region of posterior periventricular nucleus (paraventricular organ of other authors), periventricular nucleus of posterior tuberculum, n. recessus lateralis, n. tuberis lateralis pars anterior, and n. tuberis posterior. Although most diencephalic TH ir structures were also DAir, the posterior periventricular nucleus, n. recessus lateralis pars medialis, n. recessus posterioris, and ventral region of nucleus lateralis tuberis pars anterior showed differences in the distribution of TH and DA immunoreactivity. The rhombencephalic structures contained cell groups with different combinations of catecholamines as follows: TH and DBH ir neurons in the isthmic tegmentum (locus coeruleus); TH and DBH ir cells in the rostral medullary tegmentum ventral to VIIth nerve; TH and PNMT ir cells in the sensory nucleus of the vagus nerve; TH, DBH, and PNMT ir cells in the dorsal medullary tegmentum, TH and DBH ir cells in the dorsomedian postobecular region, ventral to the descending trigeminal tract and lateral to the central canal at medullospinal levels. This study shows that: (1) with few exceptions TH and DA ir coincides, (2) gymnotiforms possess similar DBH ir rhombencephalic groups, but additional telencephalic and rhombencephalic TH ir groups, and PNMT ir cells that were not reported previously in teleosts, and (3) the presence of CAergic fibers in the electrosensory system supports findings of their modulatory function in communication and aggression.

Distribution of substance P-like, leucine-enkephalin-like, and bombesine-like immunoreactivity and acetylcholinesterase activity in the visual system of salamanders

In the urodele species Salamandra salamandra and Batrachoseps attenuatus, the distribution of the neuropeptides substance P (SP), leucine-enkephalin (LENK), and bombesine (BOMB) was investigated by means of immunohistochemistry in brain areas containing retinofugal projection sites (tectum mesencephali, praetectum, thalamus) as well as in brain regions postsynaptic to the tectum. The activity of acetylcholinesterase (AChE) was studied histochemically. Despite its simplified, two-layered morphology, the salamander tectum shows a high degree of neurochemical differentiation, characterized by a laminar organization of neuropeptide-like immunoreactivity and AChE-activity comparable to that found in the anuran tectum, which has a multi-layered morphology. SP-like immunoreactivity constituted four tectal laminae, two of them occurring in the stratum opticum. LENK-like immunoreactivity formed three laminae, one in the stratum opticum. BOMB-like immunoreactivity formed one lamina within the stratum opticum and one in the tectal efferent layers. Layers 1 and 2 of the stratum opticum revealed high AChE-activity, whereas low activity was found in deep fiber layers containing tectal efferents. The outer cellular layer also revealed AChE-activity. After enucleation of one eye, the contralateral tectum lacked neuropeptide-like immunoreactivity and AChE-activity in the layers containing retinofugal projection sites. No reduction of immunoreactivity was found in nuclei postsynaptic to the tectum. Our experiments suggest that the secondary simplification that has taken place in salamanders with respect to tectal morphology did not affect the neurochemical differentiation of the tectum.

Origins of descending projections to the medulla oblongata and rostral medulla spinalis in the urodele Salamandra salamandra (amphibia)

Descending projections to the medulla oblongata and rostral medulla spinalis have been examined in the urodele Salamandra salamandra with retrograde horseradish peroxidase tracing. Ipsilateral projections originate from the striatum and the nucleus ventrolateralis thalami and reach the medulla oblongata. The ipsilateral nucleus praeopticus magnocellularis reaches the medulla spinalis. The rostral part of the nucleus tuberculi posterioris projects to the ipsilateral medulla oblongata; its caudal part projects further caudally. Tectal efferents and the efferents of the nucleus praetectalis profundus project bilaterally, the nucleus praetectalis superficialis, nucleus mesencephalicus nervi trigemini, torus semicircularis, nucleus Darkschewitsch, and nucleus fasciculi longitudinalis medialis project ipsilaterally to the medulla oblongata. The nucleus mesencephalicus nervi trigemini, nucleus fasciculi longitudinalis medialis, and tectal efferents reach the rostral medulla spinalis. The nucleus ruber projects mainly via the contralateral dorsolateral funiculus to the medulla spinalis. A largely crossed medullary projection arises in the nucleus dorsalis tegmenti pars anterior, a bilateral projection arises in the nucleus dorsalis tegmenti pars posterior, and an ipsilateral projection arises in the nucleus ventralis tegmenti pars anterior. Cerebellar and statoacoustic efferents descend to the medulla spinalis. The nucleus reticularis isthmi, superior, medius and inferior as well as the nucleus raphes exhibit spinal trajectories. The nucleus vestibularis magnocellularis projects bilaterally, the nucleus vestibularis medialis projects ipsilaterally spinalward. The supposed nucleus descendens nervi trigemini descends mainly contralaterally. A small spinal projection arises in the nucleus tractus solitarii. The results indicate that salamander brains display elaborate descending connections which are similar to those in other vertebrates despite their scarcely differentiated neuronal cytoarchitecture.

Distribution of serotonin immunoreactivity in the forebrain and midbrain of the lizard Gekko gecko

The distribution of serotonin (5-hydroxytryptamine, 5-HT) in the forebrain and midbrain of the lizard Gekko gecko was studied by means of antibodies against serotonin. In the diencephalon, serotonin-immunoreactive (5-HTi) cell bodies were found in the hypothalamic periventricular organ and the ependymal wall of the infundibular recess. In the midbrain, 5-HTi cells were observed in the nucleus raphes superior and the lateral portion of the nucleus reticularis superior. In addition, 5-HTi cell bodies were found lateral to the ventral interpeduncular nucleus and around the ventral aspect of the medial longitudinal fasciculus. Serotonin-immunoreactive fibers and varicosities are present throughout the forebrain and the midbrain, but particularly in the nucleus accumbens, the septal area, the dorsal cortex, the dorsal thalamus, the lateral geniculate body, the ventromedial hypothalamic nucleus, the pretectal nucleus, and the basal optic nucleus. The medial habenular nucleus contains a dense 5-HTi plexus that shows a patchlike pattern. A laminar organization of 5-HTi fibers and varicosities is present in the midbrain tectum. When compared with data obtained in other vertebrates, the present study has confirmed that in the phylogenetic series fishes-amphibians-reptiles-birds-mammals there appears to be (1) a gradual decrease in the number of cerebrospinal-fluid-contacting serotoninergic cells in the hypothalamic periventricular layer and (2) a remarkable increase in number of serotoninergic cells in the midbrain tegmentum. As in mammals, a strong serotoninergic innervation of structures related to sensory, in particular visual, pathways could be recognized.

Catecholamine neurons in the brainstem of the reptile Caiman crocodilus

Immunohistochemical methods were used to map the distribution of neurons exhibiting tyrosine hydroxylase-like immunoreactivity (TH) in the brainstem of the reptile Caiman crocodilus. The results reveal that many catecholamine systems previously described in mammalian and avian species are present in the brainstem of the caiman. Within the medulla, many immunoreactive neurons surround the central canal. This neuronal field extends rostrally to the level of the dorsal motor nucleus of the vagus. Many TH neurons overlap the region of the solitary nucleus, and an extensive system of fibers derived from these neurons extends ventrally and laterally into the region immediately bordering the descending nucleus of the trigeminal nerve. Some TH neurons are also present in the ventrolateral tegmentum of the medulla at this level. A large number of TH cells are present in the pons and midbrain. These include the locus coeruleus, nucleus subcoeruleus ventralis, nucleus subcoeruleus dorsalis, substantia nigra (Brauth et al., '83), and area ventralis of Tsai. The subcoeruleus nuclei are considerably larger in the caiman than in other reptilian species including turtles and lizards and closely resemble the subcoeruleus nuclei of birds in terms of position and anterior-posterior extent. Within the diencephalon, numerous small, intensely staining, TH-immunoreactive and CSF-contacting neurons were observed within the preoptic recess and in close proximity to the ventricular wall at rostral hypothalamic and preoptic levels. Many intensely stained, immunoreactive cell bodies were observed in the medial hypothalamus similar in position to the A13 cell group of mammals. In the subthalamus, TH neurons completely surround the ventral peduncle of the forebrain bundle (which contains fibers of the ansa lenticularis) and extend into the ventromedial and ventrolateral thalamic areas. A rich plexus of TH-positive axons and terminals invests the external layer of the median eminence.

Immunocytochemical analysis of the dopamine system in the forebrain and midbrain of Raja radiata: evidence for a substantia nigra and ventral tegmental area in cartilaginous fish

The distribution of dopamine-containing cell somata and fibers in the forebrain and midbrain of a cartilaginous fish, Raja radiata, was investigated by means of antibodies directed against dopamine. Many small dopamine immunoreactive neurons are distributed throughout the telencephalon, including the olfactory bulbs. Within the diencephalon and particularly in the hypothalamus, i.e., in the nucleus preopticus, nucleus suprachiasmaticus, the paraventricular organ, lateral hypothalamic area, recessus mamillaris, and nucleus tuberculi posterioris, numerous cell somata stain for dopamine. In the mesencephalon, two distinct cell masses are found, which on the basis of their immunoreactivity for dopamine and their location, may be homologous to the substantia nigra and ventral tegmental area of other vertebrates. Dopamine immunoreactive fibers are found in the glomeruli of the olfactory bulbs, in ventral portions of the telencephalon, where a dense dopaminergic plexus innervates the area superficialis basalis and striatum, and in the diencephalon, where the inferior lobe is the most densely innervated structure. In the mesencephalon, the dopamine immunoreactive fibers are confined predominantly to the periventricular zone and lateral portions of the tectum. We conclude that much of the dopaminergic system in Raja radiata is strikingly similar to that seen in amniotes.

Ventral tegmental (A10) system: neurobiology. 1. Anatomy and connectivity

The VTA contains the A10 group of DA containing neurons. These neurons have been grouped into nuclei to be found on the floor of the midbrain tegmentum--Npn, Nif, Npbp and Nln rostralis and caudalis. The VTA is traversed by many blood vessels and nerve fibers. Close to its poorly defined borders are found DA (A8, A9, A11) and 5-HT containing neurons (B8). Efferent projections of the VTA can be divided into 5 subsystems. The mesorhombencephalic projects to other monoaminergic nuclei, the cerebellum and a fine projection descends to other tegmental nuclei as far as the inferior olive. Fibers to the spinal cord have not been demonstrated. The mesodiencephalic path projects to several thalamic and hypothalamic nuclei and possibly the median eminence. Functionally important examples are the anterior hypothalamic-preoptic area, N. medialis dorsalis and reuniens thalami. These two subsystems are largely non-dopaminergic. A minor mesostriatal projection is overshadowed by the large mesolimbic projection to the accumbens, tuberculum olfactorium, septum lateralis and n. interstitialis stria terminalis. There are also mesolimbic connections with several amygdaloid nuclei (especially centralis and basolateralis), the olfactory nuclei and entorhinal cortex. A minor projection to the hippocampus has been detected. The mesocortical pathway projects to sensory (e.g. visual), motor, limbic (e.g. retrosplenial) and polysensory association cortices (e.g. prefrontal). Prefrontal, orbitofrontal (insular) and cingulate cortices receive the most marked innervation from the VTA. A more widespread presence of DA in other cortices of rodents becomes progressively more evident in carnivores and primates. Most but not all projections are unilateral. Some neurons project to more than one area in mesodiencephalic, limbic and cortical systems. The majority of these fibers ascend in the MFB. Most areas receiving a projection from the VTA (DA or non-DA) project back to the VTA. The septohippocampal complex in particular and the limbic system in general provide quantitatively much less feedback than other areas. The role of the VTA as a mediator of dialogue with the frontostriatal and limbic/extrapyramidal system is discussed under the theme of circuit systems. The large convergence of afferents to certain VTA projection areas (prefrontal, entorhinal cortices, lateral septum, central amygdala, habenula and accumbens) is discussed under the theme of convergence systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization of tyrosine-hydroxylase immunopositive neurons in the brain of the crested newt, Triturus cristatus carnifex

The localization of neurons, fibers, and terminals containing tyrosine hydroxylase (TH)-like immunoreactivity was studied in the brain of the crested newt by using an antiserum to rat phaeochromocytoma tyrosine hydroxylase. Immunoreactive cells and fibers were found in the spinal cord, the medulla oblongata (lateral periventricular areas), and the acousticolateral area. In the tegmentum mesencephali, two bilateral clusters of labelled cells were localized in the ventrolateral periventricular gray extending toward the caudal hypothalamus. In the hypothalamic tuberal lobes, the TH-like reactive neurons, frequently of CSF-contacting type, lined the dorsal wall of the lateral infundibular recesses. A thick network of TH-like reactive nerve fibers and terminals was observed in the perivascular zone of the median eminence and in the adenohypophysial pars intermedia. A number of labelled cell bodies were also found in the dorsal thalamus (pars intercalaris diencephali), the paraventricular organ, and the ventral wall of the preoptic recess. In the telencephalon, immunoreactive innervation was identified in the striatum, together with immunopositive cell bodies in the olfactory bulbs. The pattern of organization of TH-immunoreactive systems in the newt showed, except for some peculiarities (e.g., the labelled cell bodies in dorsal thalamus), close similarities to the arrangement typical of mammals.

The development of serotonergic raphespinal projections in Xenopus laevis

The development of serotonin-immunoreactive neurons in the central nervous system of Xenopus laevis larvae has been studied with special emphasis on the development of the raphe nuclei and raphespinal projections. The first serotonergic neurons were observed in the rostral part of the brain stem at stage 25, only 28 hr after fertilization. By stage 28 some 20 serotonin-immunoreactive neurons were found in the rostral part of the brain stem, bearing small protrusions on the ventromedial side of the soma. These initial axonal outgrowths reach the rostral part of the spinal cord at stage 32. By stage 35/36 the growth cones of the descending serotonergic axons in the spinal cord have reached the level of the anus (10th to 15th myotome). Up to stage 45 the majority of the descending serotonergic axons was found in the dorsolateral part of the marginal zone of the spinal cord. After stage 45 some serotonergic axons were also found scattered over other parts of the spinal marginal zone. Collateral branches were first observed in the caudal part of the brain stem at stage 35/36. Later they occurred also in the rostral (stage 43) and caudal (stage 50) spinal cord, usually on fibers in the ventral half of the spinal cord. The number of serotonergic neurons in the central nervous system (brain stem and hypothalamus) increased steadily throughout development until stage 45. After that the total number of serotonergic neurons in the central nervous system increased about two times faster than the number of serotonergic neurons in the raphe nuclei, due to a massive increase of serotonergic neurons in the hypothalamus. The present study shows that young, just differentiated raphe neurons already contain serotonin. The generation of these neurons appears to take place in the ventricular zone (matrix) of the brain stem between the caudal border of the mesencephalon and the entrance of the nervus octavus. From here these neurons seem to migrate to their final destination. The distribution of serotonin-immunoreactive neurons in the brain stem suggests that a superior (not described so far in Anura) and an inferior raphe nucleus can be distinguished in Xenopus. A rostrocaudal gradient seems to be present in the production of serotonergic neurons which project to the spinal cord. Spinal projections from the raphe nuclei are particularly extensive from the nucleus raphes inferior and gradually decrease rostralwards. In the rostral part of the nucleus raphes superior almost no neurons projecting to the spinal cord are found.

The projection of three extrathalamic cell groups to the cerebral cortex of the turtle Pseudemys

Three extrathalamic subcortical inputs to the part of the cerebral cortex that is known to receive thalamic fibers in the turtle were examined in the present study. Direct projections from the locus coeruleus, the superior medial raphe nucleus, and a wide area of the basal telencephalon that lies ventromedial to the globus pallidus were demonstrated with the horseradish peroxidase method. Fluorescence histochemistry confirmed the presence of catecholamine-containing fibers in the rostral half of dorsal cortex and also demonstrated a dense network of serotoninergic fibers. Biochemical analysis showed the concentration of both monoamines to be relatively high; the norepinephrine concentration was 709 ng/g and the serotonin concentration was 1,750 ng/g. No evidence was found to suggest the existence of either a dopamine fiber projection to cortex comparable to that of mammalian neocortex or the presence of an epinephrine pathway to turtle cortex equivalent to the epinephrine-containing fibers in the pallium of amphibians. The coexistence of the projections from the thalamus with noradrenergic projections from the locus coeruleus, serotoninergic projections from the superior medial raphe nucleus, and presumably cholinergic projections from the basal telencephalon provide at least four distinct subcortical inputs to the reptilian dorsal cortex. Neither thalamic nor similar extrathalamic inputs have been demonstrated in the dorsal pallium of amphibia. Mammalian neocortex, in contrast, has even more elaborately differentiated inputs of both types. These results support the idea that thalamic and extrathalamic inputs to cortex appear at the same time in vertebrate evolution, and that both types of inputs are required for the normal development and function of neocortex.

Regional distribution of catecholaminergic terminals in the pigeon visual system

A glyoxilic acid histofluorescence technique was used in this study to determine the distribution of catecholaminergic (CA) terminals in the pigeon visual areas. Our results show that the main visual structures are under the influence of CA nuclei of the brain stem. In particular, the pigeon Wulst, like the mammalian visual cortex, is profusely innervated by CA terminals. In fact, dense CA afferents, most likely noradrenergic (NA) terminals, were found in the hyperstriatum intercalatus superior and the nucleus intercalatus hyperstriati accessorii; area which represent the terminal zone of the retino-thalamo-hyperstriatal pathway. These results suggest a possible convergence of NA terminals and visual fibers on common target cells in the Wulst.

Distribution of serotonin in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study

The distribution of serotonin-containing nerve cell bodies, fibers and terminals in the lizard Varanus exanthematicus was studied with the indirect immunofluorescence technique, using antibodies to serotonin. Most of the serotonin-containing cell bodies were found in the midline, in both of the raphe nuclei, i.e. the nuclei raphes superior and inferior. A considerable number of more laterally shifted serotonergic neurons was found particularly at three levels of the brain stem, viz. in the caudal mesencephalic tegmentum, at the isthmic level, and over a long distance in the medulla oblongata. These laterally situated serotonin-positive neurons were partly found within the confines of the substantia nigra, the nucleus reticularis superior and the lateral part of the nucleus reticularis medius and ventrolateral part of the nucleus reticularis inferior, respectively. No serotonergic cell bodies were found in the spinal cord. In the brain stem a dense serotonergic innervation was observed in all of the motor nuclei of the cranial nerves, in two layers of the tectum mesencephali, in the nucleus interpeduncularis pars ventralis, the nucleus profundus mesencephali pars rostralis, the periventricular grey, the nucleus parabrachialis, the vestibular nuclear complex, the nucleus descendens nervi trigemini, the nucleus raphes inferior, and parts of the nucleus tractus solitarii. Descending serotonergic pathways could be traced into the spinal cord via the dorsolateral, ventral and ventromedial funiculi, and were found to innervate mainly three parts of the spinal grey throughout the spinal cord, i.e. the dorsal part of the dorsal horn, the motoneuron area in the ventral horn, and the intermediate zone just lateral to the central canal. The results obtained in the present study suggest a close resemblance of the organization of the serotonergic system in reptiles and mammals, especially as to the serotonergic innervation of the spinal cord.

Early development of descending pathways from the brain stem to the spinal cord in Xenopus laevis

The early development of descending pathways from the brain stem to the spinal cord has been studied in Xenopus laevis tadpoles. The relatively protracted development of this permanently aquatic amphibian as well as its transparency during development make this animal particularly attractive for experimental studies. Between the 5th and 10th myotome the spinal cord was crushed with a thin needle and dry horseradish peroxidase (HRP) crystals were applied. After a survival time of one day the tadpoles were fixed and the brain and spinal cord were stained as a whole according to a modification of the heavy metal intensification of the DAB-reaction, cleared in cedarwood oil and examined as wholemounts. At stage 28 (the neural tube has just closed) the first brain stem neurons projecting to the spinal cord were found in what appear to be the nucleus reticularis inferior and -medius. At this stage of development the first, uncoordinated swimming movements can be observed. At stage 30/31 (the tailbud is visible) both Mauthner cells project to the spinal cord as well as the interstitial nucleus of the fasciculus longitudinalis medialis situated in the mesencephalon. Towards stage 35/36 (the tail is now clearly visible), a more extensive reticulospinal innervation of the spinal cord appears, now including cells of the nucleus reticularis superior. At this stage also the first vestibulospinal and raphespinal projections were found. At stage 43/44 (the tadpoles have now a well-developed tail) the pattern of reticulospinal projections appears to be completed with the presence of labeled neurons in the nucleus reticularis isthmi. From stage 43/44 on, the number of HRP-positive cells is steadily increasing. At stage 47/48, when the hindlimb buds appear, the descending projections to the spinal cord are comparable with the adult situation except for the absence of a rubrospinal and a hypothalamospinal projection. The observations demonstrate that already very early in development reticulospinal fibers and, somewhat later, Mauthner cell axons and vestibulospinal fibers innervate the spinal cord. Furthermore, a caudorostral gradient appears to exist with regard to the development of descending projections to the spinal cord. However, the interstitial nucleus of the fasciculus longitudinalis medialis forms an exception to this rule.

Distribution of catecholamines in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study based on the use of antibodies to tyrosine hydroxylase

Antibodies to tyrosine hydroxylase were used to study the distribution of nerve cells, fibers and terminals, containing catecholamines, in the lizard Varanus exanthematicus, by means of the indirect immunofluorescence technique. Tyrosine hydroxylase-containing cell bodies occurred in the hypothalamus, the ventral and dorsal tegmentum mesencephali, the substantia nigra, the isthmic reticular formation, in and ventrolaterally to the locus coeruleus, in the nucleus tractus solitarii and in a lateral part of the nucleus reticularis inferior. In addition tyrosine hydroxylase-containing cell bodies were found throughout the spinal cord, ventral to the central canal. Tyrosine hydroxylase-immunoreactive terminal areas in the brain stem were seen in the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus raphes superior, the locus coeruleus, several parts of the reticular formation and the nucleus descendens nervi trigemini. Ascending catecholaminergic pathways could be traced from the ventral mesencephalic tegmentum as well as from the dorsal isthmic tegmentum rostralwards, through the lateral hypothalamus. These pathways correspond to the mesostriatal and isthmocortical projections respectively, as described in mammals. Furthermore, ascending catecholaminergic fibers could be traced from the catecholaminergic cell groups in the medulla oblongata to the isthmus, where they intermingle with the locus coeruleus neurons. These pathways correspond to the medullohypothalamic projection and to the dorsal periventricular system in mammals. Descending catecholaminergic fibers to the spinal cord pass via the dorsomedial part of the lateral funiculus, and mainly terminate in the dorsal horn. The results obtained in the present study have been placed in a comparative perspective, which illustrates the constancy of catecholaminergic innervation throughout phylogeny.

Cerebellar afferents in the frogs, Rana esculenta and Rana temporaria

Afferents to the cerebellum in frogs (Rana esculenta, Rana temporaria) were studied by use of retrograde transport of horseradish peroxidase. Following injections restricted to the molecular layer of the cerebellum cell labelling was found in the contralateral inferior olive and the ventral portion of the caudal medullary raphe. Injections involving the granular layer resulted in labelling in the ventral horn of the cervical spinal cord, the caudal spinal trigeminal nucleus, the nucleus caudalis and the medial portion of the nucleus ventralis of the vestibular nerve, the inferior reticular nucleus and the nucleus of the fasciculus longitudinalis medialis. Following larger injections, which may have spread significantly into the cerebellar, secondary gustatory, trigeminal or vestibular nuclei, labelled cell bodies were also found in the nucleus ruber, nucleus solitarius, the rostral spinal trigeminal nucleus and the rostral rhombencephalic reticular formation. It is unclear whether the fibers from these latter areas innervate the cerebellum of the frog, as they do in mammals, or only reach the underlying areas. This situation emphasizes a limitation of the HRP technique when applied to small structures as is often the case in lower vertebrates.

Distribution of 5-hydroxytryptamine (serotonin) in the brain of the teleost Gasterosteus aculeatus L

The distributions of serotoninergic neurons in the brain of the three-spined stickleback was demonstrated with the indirect peroxidase-antiperoxidase (PAP) immunohistochemical method with antibodies against serotonin. Serotoninergic perikarya were demonstrated in the brainstem reticular formation (nucleus raphe dorsalis, nucleus raphe medialis, and nucleus tegmenti dorsalis lateralis) and in the periventricular ventral thalamus and hypothalamus (nucleus ventromedialis thalami, nucleus posterioris periventricularis, nucleus recessus lateralis, and nucleus recessus posterioris). After pharmacological pretreatment of the animals with a monoamine oxidase inhibitor, serotoninergic perikarya were also visualized in area praetectalis and in the medial brainstem, caudal to nucleus raphe medialis. Whereas the cell groups of the brainstem give rise to both ascending and descending pathways, it was not possible to analyze the distribution of efferent projections from the diencephalic cell groups. Distribution of serotoninergic axons showed marked regional differences. Only scattered varicose fibers were demonstrated in the cerebellum, the facial lobes, and the lateral line lobes. In the mesencephalon, the dorsal periventricular tegmentum and the central gray receive only small numbers of serotoninergic axons, while torus semicircularis and the visual layers of tectum opticum are profusely innervated. In the diencephalon, the hypothalamus and ventral thalamus generally display the highest density of serotoninergic axons. Exceptions are found in nucleus glomerulosus and the ventromedial portion of lobus inferioris, where densities are low. In the telencephalon, the density of serotoninergic axons is very high in area dorsalis pars medialis and pars lateralis dorsalis, but low in area dorsalis pars dorsalis and pars lateralis ventralis, and intermediate in area ventralis.

Immunohistochemical demonstration of the serotonin neuron system in the central nervous system of the bullfrog, Rana catesbeiana

The distribution of serotonin immunoreactivity in the brain of the bullfrog (Rana catesbeiana) was studied, using the peroxidase-antiperoxidase (PAP) immunohistochemical method with serotonin antiserum. The somata of the serotonin neurons were mainly located in the raphe regions of the brain stem from the level of the caudal mesencephalon to that of the spinomedullary junction. A small number of serotonin neurons were also distributed as cerebrospinal-fluid contacting neurons in the preoptic recess organ (PRO), the paraventricular organ (PVO), and the nucleus infundibularis dorsalis (Nid). In the raphe region, these serotonin neurons formed nearly-continuous bilaterally-symmetrical cell columns along the midline of the brain stem, divided into lateral and medial groups. The medial group was further subdivided into rostral and caudal parts. Processes of the serotonin neurons were widely distributed in the central nervous system, forming dense networks in various regions. The greatest concentrations of these fibers were in the nucleus medialis septi, lateral portion of striatum, nucleus corporis geniculi, nucleus entopeduncularis, periventricular gray of ventral hypothalamus, optic tectum, nucleus isthmi, nucleus interpeduncularis, dorsal edge of medulla oblongata, and fasciculus solitarius.