The distribution and morphological characteristics of serotonergic cells in the brain of monotremes

Cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations in the brain of the lesser hedgehog tenrec (Echinops telfairi)

The current study provides an analysis of the cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations, or nuclei, in the brain of the lesser hedgehog tenrec, as revealed with immunohistochemical techniques. For all four of these neuromodulatory systems, the nuclear organization was very similar to that observed in other Afrotherian species and is broadly similar to that observed in other mammals. The cholinergic system shows the most variation, with the lesser hedgehog tenrec exhibiting palely immunopositive cholinergic neurons in the ventral portion of the lateral septal nucleus, and the possible absence of cholinergic neurons in the parabigeminal nucleus and the medullary tegmental field. The nuclear complement of the catecholaminergic, serotonergic and orexinergic systems showed no specific variances in the lesser hedgehog tenrec when compared to other Afrotherians, or broadly with other mammals. A striking feature of the lesser hedgehog tenrec brain is a significant mesencephalic flexure that is observed in most members of the Tenrecoidea, as well as the closely related Chrysochlorinae (golden moles), but is not present in the greater otter shrew, a species of the Potomogalidae lineage currently incorporated into the Tenrecoidea. In addition, the cholinergic neurons of the ventral portion of the lateral septal nucleus are observed in the golden moles, but not in the greater otter shrew. This indicates that either complex parallel evolution of these features occurred in the Tenrecoidea and Chrysochlorinae lineages, or that the placement of the Potomogalidae within the Tenrecoidea needs to be re-examined.

Nuclear organization of serotonergic neurons in the brainstems of a lar gibbon and a chimpanzee

In the current study, we detail, through the analysis of immunohistochemically stained sections, the morphology and nuclear parcellation of the serotonergic neurons present in the brainstem of a lar gibbon and a chimpanzee. In general, the neuronal morphology and nuclear organization of the serotonergic system in the brains of these two species of apes follow that observed in a range of Eutherian mammals and are specifically very similar to that observed in other species of primates. In both of the apes studied, the serotonergic nuclei could be readily divided into two distinct groups, a rostral and a caudal cluster, which are found from the level of the decussation of the superior cerebellar peduncle to the spinomedullary junction. The rostral cluster is comprised of the caudal linear, supralemniscal, and median raphe nuclei, as well as the six divisions of the dorsal raphe nuclear complex. The caudal cluster contains several distinct nuclei and nuclear subdivisions, including the raphe magnus nucleus and associated rostral and caudal ventrolateral (CVL) serotonergic groups, the raphe pallidus, and raphe obscurus nuclei. The one deviation in organization observed in comparison to other primate species is an expansion of both the number and distribution of neurons belonging to the lateral division of the dorsal raphe nucleus in the chimpanzee. It is unclear whether this expansion occurs in humans, thus at present, this expansion sets the chimpanzee apart from other primates studied to date.

Distribution of two isoforms of tryptophan hydroxylase in the brain of rainbow trout (Oncorhynchus mykiss). An in situ hybridization study

Serotonin (5-HT) is one of the principal neurotransmitters in the nervous system of vertebrates. It is initially synthesized by hydroxylation of tryptophan (Trp) by means of tryptophan hydroxylase or TPH which is the rate-limiting enzyme in the production of 5-HT. In most vertebrates, there are two isoforms of TPH present, TPH1 and TPH2, which exhibit different catalytic or substrate specificity as well as different expression domains. Studies carried out in mammals show that only tph2 is expressed in the brain whereas tph1-mRNA is primarily localized in the enterochromaffin cells and pineal gland. A large number of neurons are also considered to be serotonergic or "pseudo-serotonergic" as they accumulate and release 5-HT yet do not produce it as no amine-synthetic enzymes are expressed, yet a combination of 5-HT transporters is observed. Therefore, tph expression is considered to be the only specific marker of 5-HT-producing neurons that can discriminate true 5-HT from pseudo-serotonergic neurons. This work examined in situ hybridization to study the mRNA distribution of one paralogue for tph1 and tph2 in the central nervous system of rainbow trout. Results show a segregated expression for both paralogues that predominantly match previous immunocytochemical studies. This study thus adds valuable information to the scarce analyses focusing on the central distribution of the expression of serotonergic markers, particularly tphs, in the vertebrate brain thus characterizing the true serotonergic brain territories.

Neuroanatomical Distribution of the Serotonergic System in the Brain and Retina of Holostean Fishes, The Sister Group to Teleosts

Among actinopterygian fishes, holosteans are the phylogenetically closest group to teleosts but they have been much less studied, particularly regarding the neurochemical features of their central nervous system. The serotonergic system is one of the most important and conserved systems of neurotransmission in all vertebrates. By means of immunohistochemistry against serotonin (5-hydroxytryptamine), we have conducted a comprehensive and complete description of this system in the brain and retina of representative species of the 3 genera of holostean fishes, belonging to the only 2 extant orders, Amiiformes and Lepisosteiformes. Serotonin-immunoreactive cell groups were detected in the preoptic area, the hypothalamic paraventricular organ, the epiphysis, the pretectal region, the long and continuous column of the raphe, the spinal cord, and the inner nuclear layer of the retina. Specifically, the serotonergic cell groups in the preoptic area, the epiphysis, the pretectum, and the retina had never been identified in previous studies in this group of fishes. Widespread serotonergic innervation was observed in all main brain regions, but more abundantly in the subpallium, the hypothalamus, the habenula, the optic tectum, the so-called cerebellar nucleus, and the area postrema. The comparative analysis of these results with those in other groups of vertebrates reveals some extremely conserved features, such as the presence of serotonergic cells in the retina, the pineal organ, and the raphe column, while other characteristics, like the serotonergic populations in the preoptic area, the paraventricular organ, the pretectum, and the spinal cord are generally present in all fish groups, but have been lost in most amniotes.

Serotonin systems in three socially communicating teleost species, the grunting toadfish (Allenbatrachus grunniens), a South American marine catfish (Ariopsis seemanni), and the upside-down catfish (Synodontis nigriventris)

We investigated immunohistochemically the distribution of serotonergic cell populations in three teleost species (one toadfish, Allenbatrachus grunniens, and two catfishes, Synodontis nigriventris and Ariopsis seemanni). All three species exhibited large populations of 5-HT positive neurons in the paraventricular organ (PVO) and the dorsal (Hd) and caudal (Hc) periventricular hypothalamic zones, plus a smaller one in the periventricular pretectum, a few cells in the pineal stalk, and - only in catfishes - in the preoptic region. Furthermore, the rhombencephalic superior and inferior raphe always contained ample serotonergic cells. In each species, a neuronal mass extended into the hypothalamic lateral recess. Only in the toadfish, did this intraventricular structure contain serotonergic cells and arise from Hd, whereas in the catfishes it emerged from medially and represents the dorsal tuberal nucleus seen in other catfishes as well. Serotonergic cells in PVO, Hd and Hc were liquor-contacting. Those of the PVO extended into the midline area of the periventricular posterior tubercular nucleus in both catfishes. Dopaminergic, liquor-contacting neurons were additionally investigated using an antibody against tyrosine hydroxylase (TH) in S. nigriventris showing that TH was never co-localized with serotonin. Because TH antibodies are known to reveal mostly or only the TH1 enzyme, we hypothesize that th1-expressing dopamine cells (unlike th2-expressing ones) do not co-localize with serotonin. Since the three investigated species engage in social communication using swim bladder associated musculature, we investigated the serotonergic innervation of the hindbrain vocal or electromotor nuclei initiating the social signal. We found in all three species serotonergic fibers seemingly originating from close-by serotonergic neurons of inferior raphe or anterior spinal cord. Minor differences appear to be rather species-specific than dependent on the type of social communication.

Melanin-concentrating hormone (MCH) neurons in the developing chick brain

The present study was undertaken because no previous developmental studies exist on MCH neurons in any avian species. After validating a commercially-available antibody for use in chickens, immunohistochemical examinations first detected MCH neurons around embryonic day (E) 8 in the posterior hypothalamus. This population increased thereafter, reaching a numerical maximum by E20. MCH-positive cell bodies were found only in the posterior hypothalamus at all ages examined, restricted to a region showing very little overlap with the locations of hypocretin/orexin (H/O) neurons. Chickens had fewer MCH than H/O neurons, and MCH neurons also first appeared later in development than H/O neurons (the opposite of what has been found in rodents). MCH neurons appeared to originate from territories within the hypothalamic periventricular organ that partially overlap with the source of diencephalic serotonergic neurons. Chicken MCH fibers developed exuberantly during the second half of embryonic development, and they became abundant in the same brain areas as in rodents, including the hypothalamus (by E12), locus coeruleus (by E12), dorsal raphe nucleus (by E20) and septum (by E20). These observations suggest that MCH cells may play different roles during development in chickens and rodents; but once they have developed, MCH neurons exhibit similar phenotypes in birds and rodents.

Delineation of the central melanocortin circuitry controlling the kidneys by a virally mediated transsynaptic tracing study in transgenic mouse model

To examine if brain neurons involved in the efferent control of the kidneys possess melanocortin-4 receptor (MC4-R) and/or tryptophan hydroxylase (TPH). Retrograde tracing pseudorabies virus (PRV)-614 was injected into the kidneys in adult male MC4R-green fluorescent protein (GFP) transgenic mice. After a survival time of 3-7 days, spinal cord and brain were removed and sectioned, and processed for PRV-614 visualization. The neurochemical phenotype of PRV-614-positive neurons was identified using double or triple immunocytochemical labeling against PRV-614, MC4R, or TPH. Double and triple labeling was quantified using microscopy. The majority of PRV-614 immunopositive neurons which also expressed immunoreactivity for MC4R were located in the ipsilateral intermediolateral cell column (IML) of the thoracic spinal cord, the paraventricular nucleus (PVN) of the hypothalamus, and raphe pallidus (RPa), nucleus raphe magnus (NRM) and ventromedial medulla (VMM) of the brainstem. Triple-labeled MC4R/PRV-614/TPH neurons were concentrated in the PVN, RPa, NRM and VMM. These data strongly suggest that central MC4R and TPH are involved in the efferent neuronal control of the kidneys.

Serotonergic activation during courtship and aggression in the brown anole, Anolis sagrei

The role of serotonin (5-hydroxytryptamine, 5-HT) in social behavior regulation is not fully understood. While 5-HT release in nuclei of the social behavior network has generally been associated with inhibition of aggressive behavior across multiple classes of vertebrates, less is known about its effects on sexual, especially non-copulatory courtship display behaviors. Furthermore, most research has examined effects at 5-HT release sites, while studies examining the behavioral relevance of source cell populations have generated contradictory findings. This study utilized immunohistochemistry to examine the colocalization of 5-HT with Fos, an immediate early gene product and marker of neural activity, in the raphe and superior reticular nuclei of male brown anoles (Anolis sagrei) exposed to either aggression, courtship, or control social interactions. Supporting previous research, copulation was associated with a decrease in 5-HT activity, while a novel link between 5-HT activity and latency to non-copulatory courtship was also found. Within the aggression group, intensity and frequency of behavior were both associated with decreased 5-HT activity. An effect of social context was also seen, with anoles exposed to either courtship or aggression encounters showing decreased 5-HT activity in certain raphe and superior reticular nuclei populations compared to controls. Interestingly, context effects and behavioral effects were seen at separate brain nuclei, suggesting the presence of separate systems with distinct functional roles.

Organization of cholinergic, catecholaminergic, serotonergic and orexinergic nuclei in three strepsirrhine primates: Galago demidoff, Perodicticus potto and Lemur catta

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Cholinergic, catecholaminergic, serotonergic and orexinergic systems in the brains of strepsirrhine primates are described.

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All species show a similar global pattern of nuclear organization of these systems.

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For these systems there appears to be a primate-typical organization.

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Certain variations indicate a phylogenetic relationship between primates and megachiropterans.

The nuclear organization of the cholinergic, catecholaminergic, serotonergic and orexinergic systems in the brains of three species of strepsirrhine primates is presented. We aimed to investigate the nuclear complement of these neural systems in comparison to those of simian primates, megachiropterans and other mammalian species. The brains were coronally sectioned and immunohistochemically stained with antibodies against choline acetyltransferase, tyrosine hydroxylase, serotonin and orexin-A. The nuclei identified were identical among the strepsirrhine species investigated and identical to previous reports in simian primates. Moreover, a general similarity to other mammals was found, but specific differences in the nuclear complement highlighted potential phylogenetic interrelationships. The central feature of interest was the structure of the locus coeruleus complex in the primates, where a central compactly packed core (A6c) of tyrosine hydroxylase immunopositive neurons was surrounded by a shell of less densely packed (A6d) tyrosine hydroxylase immunopositive neurons. This combination of compact and diffuse divisions of the locus coeruleus complex is only found in primates and megachiropterans of all the mammalian species studied to date. This neural character, along with variances in a range of other neural characters, supports the phylogenetic grouping of primates with megachiropterans as a sister group.

Nuclear organization of cholinergic, catecholaminergic, serotonergic and orexinergic systems in the brain of the Tasmanian devil (Sarcophilus harrisii)

This study investigated the nuclear organization of four immunohistochemically identifiable neural systems (cholinergic, catecholaminergic, serotonergic and orexinergic) within the brains of three male Tasmanian devils (Sarcophilus harrisii), which had a mean brain mass of 11.6g. We found that the nuclei generally observed for these systems in other mammalian brains were present in the brain of the Tasmanian devil. Despite this, specific differences in the nuclear organization of the cholinergic, catecholaminergic and serotonergic systems appear to carry a phylogenetic signal. In the cholinergic system, only the dorsal hypothalamic cholinergic nucleus could be observed, while an extra dorsal subdivision of the laterodorsal tegmental nucleus and cholinergic neurons within the gelatinous layer of the caudal spinal trigeminal nucleus were observed. Within the catecholaminergic system the A4 nucleus of the locus coeruleus complex was absent, as was the caudal ventrolateral serotonergic group of the serotonergic system. The organization of the orexinergic system was similar to that seen in many mammals previously studied. Overall, while showing strong similarities to the organization of these systems in other mammals, the specific differences observed in the Tasmanian devil reveal either order specific, or class specific, features of these systems. Further studies will reveal the extent of change in the nuclear organization of these systems in marsupials and how these potential changes may affect functionality.

Genetic dissection of midbrain dopamine neuron development in vivo

Midbrain dopamine (MbDA) neurons are partitioned into medial and lateral cohorts that control complex functions. However, the genetic underpinnings of MbDA neuron heterogeneity are unclear. While it is known that Wnt1-expressing progenitors contribute to MbDA neurons, the role of Wnt1 in MbDA neuron development in vivo is unresolved. We show that mice with a spontaneous point mutation in Wnt1 have a unique phenotype characterized by the loss of medial MbDA neurons concomitant with a severe depletion of Wnt1-expressing progenitors and diminished LMX1a-expressing progenitors. Wnt1 mutant embryos also have alterations in a hierarchical gene regulatory loop suggesting multiple gene involvement in the Wnt1 mutant MbDA neuron phenotype. To investigate this possibility, we conditionally deleted Gbx2, Fgf8, and En1/2 after their early role in patterning and asked whether these genetic manipulations phenocopied the depletion of MbDA neurons in Wnt1 mutants. The conditional deletion of Gbx2 did not result in re-positioning or distribution of MbDA neurons. The temporal deletion of Fgf8 did not result in the loss of either LMX1a-expressing progenitors nor the initial population of differentiated MbDA neurons, but did result in a complete loss of MbDA neurons at later stages. The temporal deletion and species specific manipulation of En1/2 demonstrated a continued and species specific role of Engrailed genes in MbDA neuron development. Notably, our conditional deletion experiments revealed phenotypes dissimilar to Wnt1 mutants indicating the unique role of Wnt1 in MbDA neuron development. By placing Wnt1, Fgf8, and En1/2 in the context of their temporal requirement for MbDA neuron development, we further deciphered the developmental program underpinning MbDA neuron progenitors.

Nuclear organization of cholinergic, putative catecholaminergic, serotonergic and orexinergic systems in the brain of the African pygmy mouse (Mus minutoides): organizational complexity is preserved in small brains

This study investigated the nuclear organization of four immunohistochemically identifiable neural systems (cholinergic, catecholaminergic, serotonergic and orexinergic) within the brain of the African pygmy mouse (Mus minutoides). The African pygmy mice studied had a brain mass of around 275 mg, making these the smallest rodent brains to date in which these neural systems have been investigated. In contrast to the assumption that in this small brain there would be fewer subdivisions of these neural systems, we found that all nuclei generally observed for these systems in other rodent brains were also present in the brain of the African pygmy mouse. As with other rodents previously studied in the subfamily Murinae, we observed the presence of cortical cholinergic neurons and a compactly organized locus coeruleus. These two features of these systems have not been observed in the non-Murinae rodents studied to date. Thus, the African pygmy mouse displays what might be considered a typical Murinae brain organization, and despite its small size, the brain does not appear to be any less complexly organized than other rodent brains, even those that are over 100 times larger such as the Cape porcupine brain. The results are consistent with the notion that changes in brain size do not affect the evolution of nuclear organization of complex neural systems. Thus, species belonging to the same order generally have the same number and complement of the subdivisions, or nuclei, of specific neural systems despite differences in brain size, phenotype or time since evolutionary divergence.

The serotonergic system in fish

Neurons using serotonin (5-HT) as neurotransmitter and/or modulator have been identified in the central nervous system in representatives from all vertebrate clades, including jawless, cartilaginous and ray-finned fishes. The aim of this review is to summarize our current knowledge about the anatomical organization of the central serotonergic system in fishes. Furthermore, selected key functions of 5-HT will be described. The main focus will be the adult brain of teleosts, in particular zebrafish, which is increasingly used as a model organism. It is used to answer not only genetic and developmental biology questions, but also issues concerning physiology, behavior and the underlying neuronal networks. The many evolutionary conserved features of zebrafish combined with the ever increasing number of genetic tools and its practical advantages promise great possibilities to increase our understanding of the serotonergic system. Further, comparative studies including several vertebrate species will provide us with interesting insights into the evolution of this important neurotransmitter system.

Nuclear organization of cholinergic, putative catecholaminergic and serotonergic nuclei in the brain of the eastern rock elephant shrew, Elephantulus myurus

The organization of the nuclear subdivisions of the cholinergic, putative catecholaminergic and serotonergic systems of the brain of the elephant shrew (Elephantulus myurus) were determined following immunohistochemistry for choline acetyltransferase, tyrosine hydroxylase and serotonin, respectively. This was done in order to determine if differences in the nuclear organization of these systems in comparison to other mammals were evident and how any noted differences may relate to specialized behaviours of the elephant shrew. The elephant shrew belongs to the order Macroscelidea, and forms part of the Afrotherian mammalian cohort. In general, the organization of the nuclei of these systems resembled that described in other mammalian species. The cholinergic system showed many features in common with that seen in the rock hyrax, rodents and primates; however, specific differences include: (1) cholinergic neurons were observed in the superior and inferior colliculi, as well as the cochlear nuclei; (2) cholinergic neurons were not observed in the anterior nuclei of the dorsal thalamus as seen in the rock hyrax; and (3) cholinergic parvocellular nerve cells forming subdivisions of the laterodorsal and pedunculopontine tegmental nuclei were not observed at the midbrain/pons interface as seen in the rock hyrax. The organization of the putative catecholaminergic system was very similar to that seen in the rock hyrax and rodents except for the lack of the rodent specific C3 nucleus, the dorsal division of the anterior hypothalamic group (A15d) and the compact division of the locus coeruleus (A6c). The nuclear organization of the serotonergic system was identical to that seen in all eutherian mammals studied to date. The additional cholinergic neurons found in the cochlear nucleus and colliculi may relate to a specific acoustic signalling system observed in elephant shrews expressed when the animals are under stress or detect a predator. These neurons may then function to increase attention to this type of acoustic signal termed foot drumming.

Organization of cholinergic, putative catecholaminergic and serotonergic nuclei in the diencephalon, midbrain and pons of sub-adult male giraffes

The current study describes the nuclear organization and neuronal morphology of the cholinergic, putative catecholaminergic and serotonergic systems within the diencephalon, midbrain and pons of the giraffe using immunohistochemistry for choline acetyltransferase, tyrosine hydroxylase and serotonin. The giraffe has a unique phenotype (the long neck), a large brain (over 500 g) and is a non-domesticated animal, while previous studies examining the brains of other Artiodactyls have all been undertaken on domesticated animals. The aim of the present study was to investigate possible differences in the nuclear organization and neuronal morphology of the above-mentioned systems compared to that seen in other Artiodactyls and mammals. The nuclear organization of all three systems within the giraffe brain was similar to that of other Artiodactyls. Some features of interest were noted for the giraffe and in comparison to other mammals studied. The cholinergic neuronal somata of the laterodorsal tegmental nucleus were slightly larger than those of the pedunculopontine tegmental nucleus, a feature not described in other mammals. The putative catecholaminergic system of the giraffe appeared to lack an A15 dorsal nucleus, which is commonly seen in other mammals but absent in the Artiodactyls, had a large and expanded substantia nigra pars reticulata (A9 ventral), a small diffuse portion of the locus coerueleus (A6d), an expansive subcoeruleus (A7sc and A7d), and lacked the A4 nucleus of the locus coeruleus complex. The nuclear organization of the serotonergic system of the giraffe was identical to that seen in all other eutherian mammals studied to date. These observations in the giraffe demonstrate that despite significant changes in life history, phenotype, brain size and time of divergence, species within the same order show the same nuclear organization of the systems investigated.

Distribution of tryptophan hydroxylase-immunoreactive neurons in the brainstem and diencephalon of the pigeon (Columba livia)

The distribution of tryptophan hydroxylase (TPH)-containing perikarya and processes in the brainstem and diencephalon of the pigeon (Columba livia) were investigated using single-labeling chromogenic and double-labeling fluorescence immunohistochemical methods for TPH and 5-HT. TPH-immunoreactive (TPH-ir) perikarya were seen extending from the caudal medulla to mid-hypothalamic levels, located in brainstem regions previously described as containing 5-HT-ir somata. Brainstem TPH-ir cell clusters (the midline raphe, and the dorsolateral and ventrolateral serotonergic cell groups) and the circumventricular cerebrospinal fluid-contacting neurons in the taenia choroidea (in the caudal brainstem), recessus infundibuli and paraventricular organ (in the hypothalamus) were shown to co-express 5-HT immunoreactivity. However, heavily labeled TPH-ir cell clusters were observed in the nucleus premamillaris (PMM), in the stratum cellulare internum (SCI), in the nucleus paraventricularis magnocellularis (PVN) and in the medial border of the nucleus dorsomedialis anterior thalami (DMA). Double-labeling experiments indicated that none of these medial hypothalamic TPH-ir cells were immunoreactive to 5-HT. These cells correspond to dopamine- and melatonin-containing neurons previously found in the avian hypothalamus, and appear to be comparable to the mammalian TPH-ir hypothalamic A11-A13 catecholaminergic somata, suggesting that they may be a conserved attribute in the amniote medial hypothalamus.

Axonal projections originating from raphe serotonergic neurons in the developing and adult zebrafish, Danio rerio, using transgenics to visualize raphe-specific pet1 expression

Serotonin is a major central nervous modulator of physiology and behavior and plays fundamental roles during development and plasticity of the vertebrate central nervous system (CNS). Understanding the developmental control and functions of serotonergic neurons is therefore an important task. In all vertebrates, prominent serotonergic neurons are found in the superior and inferior raphe nuclei in the hindbrain innervating most CNS regions. In addition, all vertebrates except for mammals harbor other serotonergic centers, including several populations in the diencephalon. This, in combination with the intricate and wide distribution of serotonergic fibers, makes it difficult to sort out serotonergic innervation originating from the raphe from that of other serotonergic cell populations. To resolve this issue, we isolated the regulatory elements of the zebrafish raphe-specific gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.

The thalamus of the monotremes: cyto- and myeloarchitecture and chemical neuroanatomy

Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytochemically for calcium-binding proteins, gamma aminobutyric acid (GABA) or other antigens. Cyto- and myeloarchitecture revealed thalami that are fundamentally mammalian in organization, with the three principal divisions of the thalamus (epithalamus, dorsal thalamus and ventral thalamus) identifiable as in marsupials and eutherian mammals. The dorsal thalamus exhibits more nuclear parcellation than hitherto described, but lack of an internal medullary lamina, caused by splaying out of afferent fibre tracts that contribute to it in other mammals, makes identification of anterior, medial and intralaminar nuclear groups difficult. Differentiation of the ventral nuclei is evident with the ventral posterior nucleus of the platypus enormously expanded into the interior of the cerebral hemisphere, where it adopts a relationship to the striatum not seen in other mammals. Other nuclei such as the lateral dorsal become identifiable by expression of patterns of calcium-binding proteins identical to those found in other mammals. GABA cells are present in the ventral and dorsal thalamic nuclei, and in the ventral thalamus form a remarkable continuum with GABA cells of the two segments of the globus pallidus and pars reticulata of the substantia nigra.

Distribution and morphology of putative catecholaminergic and serotonergic neurons in the brain of the greater canerat, Thryonomys swinderianus

The distribution, morphology and nuclear subdivisions of the putative catecholaminergic and serotonergic systems within the brain of the greater canerat (sometimes spelt cane rat) were identified following immunohistochemistry for tyrosine hydroxylase and serotonin. The aim of the present study was to investigate possible differences in the complement of nuclear subdivisions of these systems when comparing those of the greater canerat with reports of these systems in other rodents. The greater canerat was chosen for investigation as it is a large rodent (around 2.7kg body mass) and has an average brain mass of 13.75g, more than five times larger than that of the laboratory rat. The greater canerats used in the present study were caught from the wild, which is again another contrast to the laboratory rat. While these differences, especially that of size, may lead to the prediction of significant differences in the nuclear complement of these systems, we found that all nuclei identified in both systems in the laboratory rat and other rodents in several earlier studies had direct homologs in the brain of the greater canerat. Moreover, there were no additional nuclei in the brain of the greater canerat that are not found in the laboratory rat or other rodents. It is noted that the locus coeruleus of the laboratory rat differs in appearance to that reported for several other rodent species. The greater canerat is phylogenetically distant from the laboratory rat, but still a member of the order Rodentia. Thus, changes in the nuclear organization of these systems appears to demonstrate a form of constraint related to the phylogenetic level of the order.

Distribution and morphology of cholinergic, catecholaminergic and serotonergic neurons in the brain of Schreiber's long-fingered bat, Miniopterus schreibersii

The current study describes the nuclear parcellation and neuronal morphology of the cholinergic, catecholaminergic and serotonergic systems within the brain of a representative species of microbat. While these systems have been investigated in detail in the laboratory rat, and examined in several other mammalian species, no chiropterans, to the author's knowledge, have been examined. Using immunohistochemical stains for choline-acetyltransferase, tyrosine hydroxylase and serotonin, we were able to observe and document these systems in relation to the cytoarchitecture. The majority of cholinergic nuclei typically found in mammals were evident in the microbat, however we could not find evidence for choline-acetyltransferase immunopositive neurons in the Edinger-Westphal nucleus, parabigeminal nucleus, and the medullary tegmental field, as seen in several other mammalian species. A typically mammalian appearance of the catecholaminergic nuclei was observed, however, the anterior hypothalamic groups (A15 dorsal and ventral), the dorsal and dorsal caudal subdivisions of the ventral tegmental area (A10d and A10dc), and the ventral (pars reticulata) substantia nigra (A9v) were not present. The serotonergic nuclei were similar to that reported in all eutherian mammalian species studied to date. The overall complement of nuclei of these systems in the microbat, while different to the species examined in other orders of mammals, resembles most closely the complement seen in earlier studies of insectivore species, and is clearly distinguished from that seen in rodents, carnivores and primates. This data is discussed in terms of the phylogenetic relationships of the chiropterans.

Distribution and morphology of putative catecholaminergic and serotonergic neurons in the medulla oblongata of a sub-adult giraffe, Giraffa camelopardalis

The current study details the nuclear parcellation and appearance of putative catecholaminergic and serotonergic neurons within the medulla oblongata of a sub-adult giraffe, using immunohistochemistry for tyrosine hydroxylase and serotonin. We hypothesized that the unusual phenotype of the giraffe, this being the long neck and potential axonal lengthening of these neurons, may pose specific problems in terms of the efficient functioning of these systems, as several groups of catecholaminergic and serotonergic neurons, especially of the medulla, are known to project to the entire spinal cord. This specific challenge may lead to observable differences in the nuclear parcellation and morphology of these systems in the giraffe. Our personal observations in the giraffe reveal that, as with other Artiodactyls, the spinal cord extends to the caudal end of the sacral vertebrae. Within the giraffe medulla we found evidence for five putative catecholaminergic (neurons containing tyrosine hydroxylase) and five serotonergic nuclei. In terms of both morphological appearance of the neurons and nuclear parcellation we did not find any evidence for features that may be considered affected by the phenotype of the giraffe. The nuclear parcellation and appearance of both the putative catecholaminergic and serotonergic systems in the medulla of the giraffe studied are strikingly similar to that seen in previous studies of other Artiodactyls. We interpret these findings in terms of a growing literature detailing order specific phylogenetic constraints in the evolution of these neuromodulatory systems.

Distribution and morphology of cholinergic, putative catecholaminergic and serotonergic neurons in the brain of the Egyptian rousette flying fox, Rousettus aegyptiacus

Over the past decade much controversy has surrounded the hypothesis that the megachiroptera, or megabats, share unique neural characteristics with the primates. These observations, which include similarities in visual pathways, have suggested that the megabats are more closely related to the primates than to the other group of the Chiropteran order, the microbats, and suggests a diphyletic origin of the Chiroptera. To contribute data relevant to this debate, we used immunohistochemical techniques to reveal the architecture of the neuromodulatory systems of the Egyptian rousette (Rousettus aegypticus), an echolocating megabat. Our findings revealed many similarities in the nuclear parcellation of the cholinergic, putative catecholaminergic and serotonergic systems with that seen in other mammals including the microbat. However, there were 11 discrete nuclei forming part of these systems in the brain of the megabat studied that were not evident in an earlier study of a microbat. The occurrence of these nuclei align the megabat studied more closely with primates than any other mammalian group and clearly distinguishes them from the microbat, which aligns with the insectivores. The neural systems investigated are not related to such Chiropteran specializations as echolocation, flight, vision or olfaction. If neural characteristics are considered strong indicators of phylogenetic relationships, then the data of the current study strongly supports the diphyletic origin of Chiroptera and aligns the megabat most closely with primates in agreement with studies of other neural characters.

Distribution and morphology of catecholaminergic and serotonergic neurons in the brain of the highveld gerbil, Tatera brantsii

The distribution, morphology and nuclear subdivisions of the putative catecholaminergic and serotonergic systems within the brain of the highveld gerbil were identified following immunohistochemistry for tyrosine hydroxylase and serotonin. The aim of the present study was to investigate possible differences in the complement of nuclear subdivisions of these systems when comparing those of the highveld gerbil with those of the laboratory rat. The highveld gerbil was chosen as it is relatively closely related to the laboratory rat, but the Gerbillinae and Murinae lineages diverged over 20 million years ago. Moreover, even though brain sizes are similar, the life history and phenotypes between these two species are substantially different. The gerbils used in the present study were caught from the wild, which is again another contrast to the laboratory rat. While these differences may lead to the prediction of significant differences in the nuclear complement of these systems, we found that all nuclei identified in both systems in the laboratory rat in several earlier studies had direct homologs in the brain of the highveld gerbil. Moreover, there were no additional nuclei in the brain of the highveld gerbil that are not found in the laboratory rat. The only discernable difference between the two species was a greater density and number of catecholaminergic neurons in the olfactory bulb of the highveld gerbil. Thus, the evolution of nuclear parcellation in these systems appears to demonstrate a form of phylogenetic constraint related to the order Rodentia.

Nuclear organization and morphology of serotonergic neurons in the brain of the Nile crocodile, Crocodylus niloticus

The present study describes the location and nuclear organization of the serotonergic system in a representative of the order Crocodylia, the Nile crocodile (Crocodylus niloticus). We found evidence for serotonergic neurons in three regions of the brain, including the diencephalon, rostral and caudal brainstem, as previously reported in several other species of reptile. Within the diencephalon we found neurons in the periventricular organ of the hypothalamus, but not in the infundibular recess as noted in some other reptilian species. In addition we found serotonergic neurons in the pretectal nucleus, this being the first description of these neurons in any species. Within the rostral brainstem we found medial and lateral divisions of the superior raphe nucleus and a widely dispersed group of neurons in the tegmentum, the superior reticular nucleus. In the caudal brainstem we observed the inferior raphe nucleus and the inferior reticular nucleus. While much of the serotonergic system of the Nile crocodile is similar to that seen in other reptiles the entire suite of features appears to distinguish the crocodile studied from the members of the Squamate (lizards and snakes) and Testudine (turtles, tortoises and terrapins) reptiles previously studied. The observations are suggestive of order-specific patterns of nuclear organization of this system in the reptiles, reflecting potential evolutionary constraints in the mutability of the nuclear organization as seen for similar systems in mammals.

The hypothalamic supraoptic and paraventricular nuclei of the echidna and platypus

The monotremes are an intriguing group of mammals that have major differences in their reproductive physiology and lactation from therian mammals. Monotreme young hatch from leathery skinned eggs and are nourished by milk secreted onto areolae rather than through nipples. Parturition and lactation are in part controlled through the paraventricular and supraoptic nuclei of the hypothalamus. We have used Nissl staining, enzyme histochemistry, immunohistochemistry for tyrosine hydroxylase, calbindin, oxytocin, neurophysin and non-phosphorylated neurofilament protein, and carbocyanine dye tracing techniques to examine the supraoptic and paraventricular nuclei and the course of the hypothalamo-neurohypophysial tract in two monotremes: the short-beaked echidna (Tachyglossus aculeatus) and the platypus (Ornithorhynchus anatinus). In both monotremes, the supraoptic nucleus consisted of loosely packed neurons, mainly in the retrochiasmatic position. In the echidna, the paraventricular nucleus was quite small, but had similar chemoarchitectural features to therians. In the platypus, the paraventricular nucleus was larger and appeared to be part of a stream of magnocellular neurons extending from the paraventricular nucleus to the retrochiasmatic supraoptic nucleus. Immunohistochemistry for non-phosphorylated neurofilament protein and carbocyanine dye tracing suggested that hypothalamo-neurohypophysial tract neurons in the echidna lie mainly in the retrochiasmatic supraoptic and lateral hypothalamic regions, but most neurophysin and oxytocin immunoreactive neurons in the echidna were found in the paraventricular, lateral hypothalamus and supraoptic nuclei and most oxytocinergic neurons in the platypus were distributed in a band from the paraventricular nucleus to the retrochiasmatic supraoptic nucleus. The small size of the supraoptic nucleus in the two monotremes might reflect functional aspects of monotreme lactation.

Establishing order at the systems level in mammalian brain evolution

The present paper presents a new view of mammalian brain evolution based upon the finding of a level of neural organization at which phylogenetic constraints appear to play a channeling role. It is proposed that the subdivisions of a neural system exhibit the same complement (i.e. the same number of homologous subdivisions) within all species of a particular mammalian order, irrespective of the brain size, phenotype or life history. Specific examples from monotremes, cetaceans, rodents, carnivores and primates are given to provide an empirical basis for the presented hypothesis. The conclusion reached is that the presented evolutionary pattern shows a far higher relative frequency of occurrence than do other potential evolutionary explanations of systems level evolution in the mammalian nervous system.

Nuclear parcellation of certain immunohistochemically identifiable neuronal systems in the midbrain and pons of the Highveld molerat (Cryptomys hottentotus)

The present paper details our findings following immunohistochemical examination of the midbrain and pons of the Highveld molerat (Cryptomys hottentotus) using antibodies for vesicular acetylcholine transporter (cholinergic neurons), tyrosine hydroxylase (dopaminergic and noradrenergic neurons), and serotonin (serotonergic neurons). The aim was to see if, in this microphthalmic rodent that lacks a distinct circadian rhythm, the nuclei involved in aspects of visual processing and the sleep-wake cycle exhibited specific loss or morphological alteration. For all of the neural systems investigated we found nuclei that can be considered direct homologues of those found in the laboratory rat. There was no specific loss of any nuclear group of any of the systems investigated, but there was significant reduction (as judged qualitatively) in the number of neurons in the visual associated nuclei. The fact that we could identify all nuclear groupings from three systems in this species is suggestive of an evolutionary constraint acting at the level of the organization of the neural system.

Cyto- and chemoarchitecture of the amygdala of a monotreme, Tachyglossus aculeatus (the short-beaked echidna)

We have examined the cyto- and chemoarchitecture of the temporal and extended amygdala in the brain of a monotreme (the short-beaked echidna Tachyglossus aculeatus) using Nissl and myelin staining, enzyme histochemistry for acetylcholine esterase and NADPH diaphorase, immunohistochemistry for calcium binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase. While the broad subdivisions of the eutherian temporal amygdala were present in the echidna brain, there were some noticeable differences. No immunoreactivity for parvalbumin or calretinin for somata was found in the temporal amygdala of the echidna. The nucleus of the lateral olfactory tract could not be definitively identified and the medial nucleus of amygdala appeared to be very small in the echidna. Calbindin immunoreactive neurons were most frequently found in the ventrolateral part of the lateral nucleus, intraamygdaloid parts of the bed nucleus of the stria terminalis and the lateral part of the central nucleus. Neurons strongly reactive for NADPH diaphorase with filling of the dendritic tree were found mainly scattered through the cortical, central and lateral subnuclei, while neurons showing only somata reactivity for NADPH diaphorase were concentrated in the basomedial and basolateral subnuclei. Most of the components of the extended amygdala of eutherians could also be identified in the echidna. Volumetric analysis indicated that the temporal amygdala in both the platypus and echidna is small compared to the same structure in both insectivores and primates, with the central and medial components of the temporal amygdala being particularly small.

Cyto- and chemoarchitecture of the sensory trigeminal nuclei of the echidna, platypus and rat

We have examined the cyto- and chemoarchitecture of the trigeminal nuclei of two monotremes using Nissl staining, enzyme reactivity for cytochrome oxidase, immunoreactivity for calcium binding proteins and non-phosphorylated neurofilament (SMI-32 antibody) and lectin histochemistry (Griffonia simplicifolia isolectin B4). The principal trigeminal nucleus and the oralis and interpolaris spinal trigeminal nuclei were substantially larger in the platypus than in either the echidna or rat, but the caudalis subnucleus was similar in size in both monotremes and the rat. The numerical density of Nissl stained neurons was higher in the principal, oralis and interpolaris nuclei of the platypus relative to the echidna, but similar to that in the rat. Neuropil immunoreactivity for parvalbumin was particularly intense in the principal trigeminal, oralis and interpolaris subnuclei of the platypus, but the numerical density of parvalbumin immunoreactive neurons was not particularly high in these nuclei of the platypus. Neuropil immunoreactivity for calbindin and calretinin was relatively weak in both monotremes, although calretinin immunoreactive somata made up a large proportion of neurons in the principal, oralis and interpolaris subnuclei of the echidna. Distribution of calretinin immunoreactivity and Griffonia simplicifolia B4 isolectin reactivity suggested that the caudalis subnucleus of the echidna does not have a clearly defined gelatinosus region. Our findings indicate that the trigeminal nuclei of the echidna do not appear to be highly specialized, but that the principal, oralis and interpolaris subnuclei of the platypus trigeminal complex are highly differentiated, presumably for processing of tactile and electrosensory information from the bill.

Cyto- and chemoarchitecture of the dorsal thalamus of the monotreme Tachyglossus aculeatus, the short beaked echidna

We have examined the cyto- and chemoarchitecture of the dorsal thalamus of the short beaked echidna (Tachyglossus aculeatus), using Nissl and myelin staining, immunoreactivity for parvalbumin, calbindin, calretinin and non-phosphorylated neurofilament protein (SMI-32 antibody), and histochemistry for acetylcholinesterase and NADPH diaphorase. Immunohistochemical methods revealed many nuclear boundaries, which were difficult to discern with Nissl staining. Parvalbumin immunoreactive somata were concentrated in the ventral posterior, reticular, posterior, lateral and medial geniculate nuclei, while parvalbumin immunoreactivity of the neuropil was present throughout all but the midline nuclei. Large numbers of calbindin immunoreactive somata were also found within the midline thalamic nuclei, and thalamic sensory relay nuclei. Immunoreactivity for calretinin was found in many small somata within the lateral geniculate "a" nucleus, with other labelled somata found in the lateral geniculate "b" nucleus, ventral posterior medial and ventral posterior lateral nuclei. Immunoreactivity with the SMI-32 antibody was largely confined to somata and neuropil within the thalamocortical relay nuclei (ventral posterior medial and lateral nuclei, lateral and medial geniculate nuclei and the posterior thalamic nucleus). In broad terms there were many similarities between the thalamus of this monotreme and that of eutheria (e.g. disposition of somatosensory thalamus, complementarity of parvalbumin and calbindin immunoreactive structures), but there were some unique features of the thalamus of the echidna. These include the relatively small size of the thalamic reticular nucleus and the preponderance of calbindin immunoreactive neurons over parvalbumin immunoreactive neurons in the ventral posterior nucleus.