Immunohistochemical study of the telencephalon of the spiny dogfish, Squalus acanthias

Ontogenetic changes in the tyrosine hydroxylase immunoreactive preoptic area in the small-spotted catshark Scyliorhinus canicula (L., 1758) females: catecholaminergic involvement in sexual maturation

Introduction

The catecholaminergic component of the brain-pituitary-gonadal axis, which mediates the influence of external and internal stimuli on the central nervous system and gonad development in vertebrates, is largely unexplored in Chondrichthyes. We considered Scyliorhinus canicula (L., 1758) females as a model for this vertebrate's class, to assess the involvement of the catecholaminergic system of the brain in its reproduction. Along the S. canicula reproductive cycle, we characterized and evaluated differences in somata morphometry and the number of putative catecholaminergic neurons in two brain nuclei: the periventricular preoptic nucleus, hypothesized to be a positive control for ovarian development, and the suprachiasmatic nucleus, examined as a negative control.

Materials and methods

16 S. canicula wild females were sampled and grouped in maturity stages (immature, maturing, mature, and mature egg-laying). The ovary was histologically processed for the qualitative description of maturity stages. Anti-tyrosine hydroxylase immunofluorescence was performed on the diencephalic brain sections. The immunoreactive somata were investigated for morphometry and counted using the optical fractionator method, throughout the confocal microscopy.

Results and discussions

Qualitative and quantitative research confirmed two separate populations of immunoreactive neurons. The modifications detected in the preoptic nucleus revealed that somata were more numerous, significantly smaller in size, and more excitable during the maturing phase but decreased, becoming slightly bigger and less excitable in the egg-laying stage. This may indicate that the catecholaminergic preoptic nucleus is involved in the control of reproduction, regulating both the onset of puberty and the imminent spawning. In contrast, somata in the suprachiasmatic nucleus grew in size and underwent turnover in morphometry, increasing the total number from the immature-virgin to maturing stage, with similar values in the more advanced maturity stages. These changes were not linked to a reproductive role. These findings provide new valuable information on Chondrichthyes, suggesting the existence of an additional brain system implicated in the integration of internal and environmental cues for reproduction.

CRISPR/Cas9-based QF2 knock-in at the tyrosine hydroxylase (th) locus reveals novel th-expressing neuron populations in the zebrafish mid- and hindbrain

Catecholaminergic neuron clusters are among the most conserved neuromodulatory systems in vertebrates, yet some clusters show significant evolutionary dynamics. Because of their disease relevance, special attention has been paid to mammalian midbrain dopaminergic systems, which have important functions in motor control, reward, motivation, and cognitive function. In contrast, midbrain dopaminergic neurons in teleosts were thought to be lost secondarily. Here, we generated a CRISPR/Cas9-based knock-in transgene at the th locus, which allows the expression of the Q-system transcription factor QF2 linked to the Tyrosine hydroxylase open reading frame by an E2A peptide. The QF2 knock-in allele still expresses Tyrosine hydroxylase in catecholaminergic neurons. Coexpression analysis of QF2 driven expression of QUAS fluorescent reporter transgenes and of th mRNA and Th protein revealed that essentially all reporter expressing cells also express Th/th. We also observed a small group of previously unidentified cells expressing the reporter gene in the midbrain and a larger group close to the midbrain-hindbrain boundary. However, we detected no expression of the catecholaminergic markers ddc, slc6a3, or dbh in these neurons, suggesting that they are not actively transmitting catecholamines. The identified neurons in the midbrain are located in a GABAergic territory. A coexpression analysis with anatomical markers revealed that Th-expressing neurons in the midbrain are located in the tegmentum and those close to the midbrain-hindbrain boundary are located in the hindbrain. Our data suggest that zebrafish may still have some evolutionary remnants of midbrain dopaminergic neurons.

Identifying Amygdala-Like Territories in Scyliorhinus canicula (Chondrichthyan): Evidence for a Pallial Amygdala

To identify the putative amygdalar complex in cartilaginous fishes, our first step was to obtain evidence that supports the existence of a pallial amygdala in the catshark Scyliorhinus canicula, at present the prevailing chondrichthyan model in comparative neurobiology and developmental biology. To this end, we analyzed the organization of the lateral walls of the telencephalic hemispheres of adults, juveniles, and early prehatching embryos by immunohistochemistry against tyrosine hydroxylase (TH), somatostatin (SOM), Pax6, serotonin (5HT), substance P (SP), and Met-enkephalin (MetEnk), calbindin-28k (CB), and calretinin (CR), and by in situ hybridization against regulatory genes such as Tbr1, Lhx9, Emx1, and Dlx2. Our data were integrated with those available from the literature related to the secondary olfactory projections in this shark species. We have characterized two possible amygdalar territories. One, which may represent a ventropallial component, was identified by its chemical signature (moderate density of Pax6-ir cells, scarce TH-ir and SOM-ir cells, and absence of CR-ir and CB-ir cells) and gene expressions (Tbr1 and Lhx9 expressions in an Emx1 negative domain, as the ventral pallium of amniotes). It is perhaps comparable to the lateral amygdala of amphibians and the pallial amygdala of teleosts. The second was a territory related to the pallial-subpallial boundary with abundant Pax6-ir and CR-ir cells, and 5HT-ir, SP-ir, and MetEnk-ir fibers capping dorsally the area superficialis basalis. This olfactory-related region at the neighborhood of the pallial-subpallial boundary may represent a subpallial amygdala subdivision that possibly contains migrated cells of ventropallial origin.

Neuroanatomical organization of methionine-enkephalinergic system in the brain of the Mozambique tilapia Oreochromis mossambicus

Enkephalins are a class of opioid peptides implicated in several physiological and neuroendocrine responses in vertebrates. In this study, using immunocytochemical or immunofluorescence technique, we examined the neuroanatomical distribution of methionine enkephalin (M-ENK) immunoreactivity in the central nervous system (CNS) of the cichlid fish Oreochromis mossambicus. In the telencephalon, no M-ENK-like-immunoreactive (M-ENK-L-ir) perikarya, but sparsely distributed fibres were detected in the glomerular layer and the granule cell layer of the olfactory bulb. Although intensely labeled M-ENK-L-ir cells and fibres were found in the pallium, no M-ENK immunoreactivity was observed in the subpallium. The preoptic area showed a few M-ENK-L-ir somata and dense innervations of fibres. In the hypothalamic area, M-ENK-L-ir cells and fibres were located in magnocellular and parvocellular subdivisions of the nucleus preopticus, and medial and lateral subdivisions of the nucleus lateralis tuberis. Surrounding the recessus lateralis of the third ventricle, several intensely stained and packed M-ENK-L-ir cells and fibres were seen in dorsal, lateral and ventral subdivisions of the nucleus recessus lateralis. In the diencephalon, M-ENK immunoreactivity was restricted to the habenula, the thalamus, the pretectal area and the nucleus posterior tuberis. Dense aggregations of M-ENK-L-ir fibres were seen in the mesencephalic subdivisions, the optic tectum and the torus semicircularis, whereas a few fusiform M-ENK-L-ir cells and fibres were scattered in the midbrain tegmentum. In the rhombencephalon, different populations of ovoid or spindle shaped M-ENK-L-ir cells were observed in the secondary gustatory nucleus, the sensory trigeminal nerve nucleus, the nucleus reticularis medialis and the vagal motor nucleus, whereas bands of fibres were seen in the rostral spinal cord. Collectively, the widespread distribution of M-ENK immunoreactivity in the CNS suggests a role for this opioid peptide in regulation of neuroendocrine control of reproduction and modulation of sensorimotor functions in fish.

Leucine-enkephalin-immunoreactive neurons in the brain of the cichlid fish Oreochromis mossambicus

Enkephalins are the pentapeptides involved in pain relief and neuroendocrine responses with high affinity for delta opioid receptors in vertebrates. In the present investigation, we studied the distribution of leucine-enkephalin-immunoreactive (L-ENK-ir) neurons in the brain of the cichlid fish Oreochromis mossambicus. Application of the antisera against L-ENK revealed the presence of numerous L-ENK-ir perikarya and fibres in subdivisions of the dorsal and the ventral telencephalon, the medial olfactory tract and the nucleus entopeduncularis, whereas intensely labelled L-ENK-ir fibres were noticed in the olfactory bulb. Furthermore, the presence of L-ENK-ir cells and dense accumulations of fibres in the preoptic area and its subdivisions, the nucleus preopticus pars magnocellularis and the nucleus preopticus pars parvocellularis suggested a role for this peptide in regulation of reproduction. While intensely labelled cells and fibres were found in the nucleus lateralis tuberis pars lateralis as well as the nucleus lateralis tuberis pars medialis, some L-ENK-ir fibres were seen at the hypothalamo-hypophyseal tract indicating the possible hypophysiotrophic role for this peptide. Numerous L-ENK-ir cells and dense network of fibres were observed in the subdivisions of the nucleus recess lateralis and the pretectal area, whereas intensely labelled thick network of L-ENK- fibres were found in the ventromedial thalamic nucleus, the sub-layers of the optic tectum and the rostral spinal cord. The widespread distribution of L-ENK-immunoreactivity in the olfactory bulb, the telencephalon, the diencephalon and the mesencephalon regions of the brain as well as the spinal cord suggests the possible involvement of this peptide in the regulation of diverse functions such as neuroendocrine, antinociceptive, visual and olfactory responses in O. mossambicus.

Differential expression of five prosomatostatin genes in the central nervous system of the catshark Scyliorhinus canicula

Five prosomatostatin genes (PSST1, PSST2, PSST3, PSST5, and PSST6) have been recently identified in elasmobranchs (Tostivint et al., General and Comparative Endocrinology, 2019, 279, 139-147). In order to gain insight into the contribution of each somatostatin to specific nervous systems circuits and behaviors in this important jawed vertebrate group, we studied the distribution of neurons expressing PSST mRNAs in the central nervous system (CNS) of Scyliorhinus canicula using in situ hybridization. Additionally, we combined in situ hybridization with tyrosine hydroxylase (TH) immunochemistry for better characterization of PSST1 and PSST6 expressing populations. We observed differential expression of PSST1 and PSST6, which are the most widely expressed PSST transcripts, in cell populations of many CNS regions, including the pallium, subpallium, hypothalamus, diencephalon, optic tectum, midbrain tegmentum, and rhombencephalon. Interestingly, numerous small pallial neurons express PSST1 and PSST6, although in different populations judging from the colocalization of TH immunoreactivity and PSST6 expression but not with PSST1. We observed expression of PSST1 in cerebrospinal fluid-contacting (CSF-c) neurons of the hypothalamic paraventricular organ and the central canal of the spinal cord. Unlike PSST1 and PSST6, PSST2, and PSST3 are only expressed in cells of the hypothalamus and in some hindbrain lateral reticular neurons, and PSST5 in cells of the region of the entopeduncular nucleus. Comparative data of brain expression of PSST genes indicate that the somatostatinergic system of sharks is the most complex reported in any fish.

Comparative Analysis of the Organization of the Catecholaminergic Systems in the Brain of Holostean Fishes (Actinopterygii/Neopterygii)

Living holosteans, comprising 8 species of bowfins and gars, form a small monophyletic group of actinopterygian fishes, which are currently considered as the sister group to the enormously numerous teleosts and have largely been neglected in neuroanatomical studies. We have studied the catecholaminergic (CAergic) systems by means of antibodies against tyrosine hydroxylase (TH) and dopamine (DA) in the brain of representative species of the 3 genera included in the 2 orders of holostean fishes: Amia calva (Amiiformes) and Lepisosteus platyrhincus, Lepisosteus oculatus, and Atractosteus spatula (Lepisosteiformes). Different groups of TH/DA-immunoreactive (ir) cells were observed in the olfactory bulb, subpallium, and preoptic area of the telencephalon. Hypothalamic groups were labeled in the suprachiasmatic nucleus, tuberal (only in A. calva), retrotuberal, and retromamillary areas; specifically, the paraventricular organ showed only DA immunoreactivity. In the diencephalon, TH/DA-ir groups were detected in the prethalamus, posterior tubercle, and pretectum. In the caudal hindbrain, the solitary tract nucleus and area postrema presented TH/DA-ir cell groups, and also the spinal cord and the retina. Only in A. calva, particular CAergic cell groups were observed in the habenula, the mesencephalic tegmentum, and in the locus coeruleus. Following a neuromeric analysis, the comparison of these results with those obtained in other classes of fishes and tetrapods shows many common traits of CAergic systems shared by most vertebrates and in addition highlights unique features of actinopterygian fishes.

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.

Evolution of the Chordate Telencephalon

The dramatic evolutionary expansion of the neocortex, together with a proliferation of specialized cortical areas, is believed to underlie the emergence of human cognitive abilities. In a broader phylogenetic context, however, neocortex evolution in mammals, including humans, is remarkably conservative, characterized largely by size variations on a shared six-layered neuronal architecture. By contrast, the telencephalon in non-mammalian vertebrates, including reptiles, amphibians, bony and cartilaginous fishes, and cyclostomes, features a great variety of very different tissue structures. Our understanding of the evolutionary relationships of these telencephalic structures, especially those of basally branching vertebrates and invertebrate chordates, remains fragmentary and is impeded by conceptual obstacles. To make sense of highly divergent anatomies requires a hierarchical view of biological organization, one that permits the recognition of homologies at multiple levels beyond neuroanatomical structure. Here we review the origin and diversification of the telencephalon with a focus on key evolutionary innovations shaping the neocortex at multiple levels of organization.

Evolutionary and developmental contributions for understanding the organization of the basal ganglia

Herein we take advantage of the evolutionary developmental biology approach in order to improve our understanding of both the functional organization and the evolution of the basal ganglia, with a particular focus on the globus pallidus. Therefore, we review data on the expression of developmental regulatory genes (that play key roles in patterning, regional specification and/or morphogenesis), gene function and fate mapping available in different vertebrate species, which are useful to (a) understand the embryonic origin and basic features of each neuron subtype of the basal ganglia (including neurotransmitter/neuropeptide expression and connectivity patterns); (b) identify the same (homologous) subpopulations in different species and the degree of variation or conservation throughout phylogeny, and (c) identify possible mechanisms that may explain the evolution of the basal ganglia. These data show that the globus pallidus of rodents contains two major subpopulations of GABAergic projection neurons: (1) neurons containing parvalbumin and neurotensin-related hexapetide (LANT6), with descending projections to the subthalamus and substantia nigra, which originate from progenitors expressing Nkx2.1, primarily located in the pallidal embryonic domain (medial ganglionic eminence), and (2) neurons containing preproenkephalin (and possibly calbindin), with ascending projections to the striatum, which appear to originate from progenitors expressing Islet1 in the striatal embryonic domain (lateral ganglionic eminence). Based on data on Nkx2.1, Islet1, LANT6 and proenkephalin, it appears that both cell types are also present in the globus pallidus/dorsal pallidum of chicken, frog and lungfish. In chicken, the globus pallidus also contains neurons expressing substance P (SP), perhaps originating in the striatal embryonic domain. In ray-finned and cartilaginous fishes, the pallidum contains at least the Nkx2.1 lineage cell population (likely representing the neurons containing LANT6). Based on the presence of neurons containing enkephalin or SP, it is possible that the pallidum of these animals also includes the Islet1 lineage cell subpopulation, and both neuron subtypes were likely present in the pallidum of the first jawed vertebrates. In contrast, lampreys (jawless fishes) appear to lack the pallidal embryonic domain and the Nkx2.1 lineage cell population that mainly characterize the pallidum in jawed vertebrates. In the absence of data in other jawless fishes, the ancestral condition in vertebrates remains to be elucidated. Perhaps, a major event in telencephalic evolution was the novel expression of Nkx2.1 in the subpallium, which has been related to Hedgehog expression and changes in the regulatory region of Nkx2.1.

Organization of afferents to the striatopallidal systems in the fire-bellied toad Bombina orientalis

The cerebral hemispheres of amphibians display paired dorsal and ventral striatum (commonly referred to as striatum proper and nucleus accumbens, respectively). Each striatal region is proposed to be closely associated with a pallidal structure located caudal to it to form a striatopallidal system. In the present study, afferents to the dorsal and ventral striatopallidal systems of the fire-bellied toad (Bombina orientalis) were investigated using the neuronal tracer biocytin. A quantitative analysis of the topographical distribution of afferent neurons from the thalamus and posterior tubercle/ventral tegmentum was emphasised. The main results show that inputs to the two striatopallidal systems originate from distinct dorsal thalamic nuclei, with dorsal and ventral striatopallidal afferent neurons favouring strongly the lateral/central and anterior thalamic nuclei, respectively. However, afferent neuron distribution in the dorsal thalamus does not differ in the rostrocaudal axis of the brain. Afferent neurons from the posterior tubercle and ventral tegmentum, on the other hand, are organised topographically along the rostrocaudal axis. About 85 % of afferent neurons to the dorsal striatopallidal system are located rostrally in the posterior tubercle, while 75 % of afferent neurons to the ventral striatopallidal system are found more caudally in the ventral tegmentum. This difference is statistically significant and confirms the presence of distinct mesostriatal pathways in an amphibian. These findings demonstrate that an amphibian brain displays striatopallidal systems integrating parallel streams of sensory information potentially under the influence of distinct ascending mesostriatal pathways.

New insights into the neuroanatomical distribution and phylogeny of opioids and POMC-derived peptides in fish

This review re-evaluates the use of immunological probes to map enkephalinergic, dynorphinergic, and endorphinergic circuits in the CNS of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes in light of the characterization of proenkephalin, prodynorphin, and POMC sequences from representatives of these groups of fish over the past 20 years. The use of α-MSH specific antisera is a reliable method for detecting POMC immunopositive cell bodies and fibers. Since α-MSH and β-endorphin are co-localized in the same neurons, these studies also reveal the distribution of endorphinergic networks. Met-enkephalin specific antisera can be used to detect enkephalinergic circuits in the CNS of gnathostomes because of the ubiquitous presence of this pentapeptide in the proenkephalin sequences of gnathostomes. However, the use of leu-enkephalin specific antisera to detect enkephalinergic networks is more problematic. While this immunological probe is appropriate for analyzing enkephalinergic networks in mammals and perhaps teleosts, for the lungfishes and cartilaginous fishes this probe is more likely able to detect dynorphinergic circuits. In this regard, there is a need to re-examine dynorphinergic networks in non-mammalian gnathostomes by using species specific antisera directed against dynorphin end-products.

Evolutionary conservation of the basal ganglia as a common vertebrate mechanism for action selection

BACKGROUND

Although the basal ganglia are thought to play a key role in action selection in mammals, it is unknown whether this mammalian circuitry is present in lower vertebrates as a conserved selection mechanism. We aim here, using lamprey, to elucidate the basal ganglia circuitry in the phylogenetically oldest group of vertebrates (cyclostomes) and determine how this selection architecture evolved to accommodate the increased behavioral repertoires of advanced vertebrates.

RESULTS

We show, using immunohistochemistry, tract tracing, and whole-cell recordings, that all parts of the mammalian basal ganglia (striatum, globus pallidus interna [GPi] and externa [GPe], and subthalamic nucleus [STN]) are present in the lamprey forebrain. In addition, the circuit features, molecular markers, and physiological activity patterns are conserved. Thus, GABAergic striatal neurons expressing substance P project directly to the pallidal output layer, whereas enkephalin-expressing striatal neurons project indirectly via nuclei homologous to the GPe and STN. Moreover, pallidal output neurons tonically inhibit tectum, mesencephalic, and diencephalic motor regions.

CONCLUSIONS

These results show that the detailed basal ganglia circuitry is present in the phylogenetically oldest vertebrates and has been conserved, most likely as a mechanism for action selection used by all vertebrates, for over 560 million years. Our data also suggest that the mammalian basal ganglia evolved through a process of exaptation, where the ancestral core unit has been co-opted for multiple functions, allowing them to process cognitive, emotional, and motor information in parallel and control a broader range of behaviors.

Immunohistochemical Organization of the Forebrain in the White Sturgeon, Acipenser transmontanus

The distribution of substance P (SP), leucine-enkephalin (LENK), serotonin (5HT), dopamine (DA), and tyrosine hydroxylase (TH) was examined in the forebrain of the white sturgeon in order to evaluate several anatomical hypotheses based on cytoarchitectonics, and to gain a better understanding of the evolution of the forebrain in ray-finned fishes. The subpallium of the telencephalon has the highest concentration of the neuropeptides SP and LENK, allowing the pallial-subpallial border to be easily distinguished. The distribution of dopamine is similar to that of serotonin in the subpallium, fibers positive for these transmitters are particularly dense in the dorsal and ventral divisions of the subpallium. In addition, a small population of DA- and 5HT-positive cell bodies--which appear to be unique to sturgeons--was identified at the level of the anterior commissure. The internal granular layer of the olfactory bulbs had large numbers of TH-positive cell bodies and fibers, as did the rostral subpallium. The occurrence of cell bodies positive for LENK in the dorsal nucleus of the rostral subpallium supports the hypothesis that this nucleus is homologous to the striatum in other vertebrates. This is further reinforced by the apparent origin of an ascending dopaminergic pathway from cells in the posterior tubercle that are likely homologous to the ventral tegmental area/substantia nigra in land vertebrates. Finally, the differential distribution of SP and TH in the pallium supports the hypothesis that the pallium, or area dorsalis, can be divided medially into a rostral division (Dm), a caudal division (Dp) that is the main pallial target of secondary olfactory projections, and a narrow lateral division (Dd+Dl) immediately adjacent to the attachment of the tela choroidea along the entire rostrocaudal length of the telencephalic hemisphere.

Expression patterns of developmental regulatory genes show comparable divisions in the telencephalon of Xenopus and mouse: insights into the evolution of the forebrain

In this study, we review data on the existence of comparable divisions and subdivisions in the telencephalon of different groups of tetrapods based on expression of some developmental regulatory genes, having a particular focus in the comparison of the anuran amphibian Xenopus and the mouse. The available data on Xenopus, mouse, chick and turtle indicate that apparently all tetrapod groups possess the same molecularly distinct divisions and subdivisions in the telencephalon. This basic organization was likely present in the telencephalon of stem tetrapods. Each division/subdivision is characterized by expression of a unique combination of developmental regulatory genes, and appears to represent a self-regulated and topologically constant histogenetic brain compartment that gives rise to specific groups of cells. This interpretation has an important consequence for searching homologies, since a basic condition for cell groups in different vertebrates to be considered homologous is that they originate in the same compartment. However, evolution may allow individual cell groups derived from comparable (field homologous) subdivisions to be either similar or dissimilar across the vertebrate groups, giving rise to several possible scenarios of evolution, which include both the evolutionary conservation of similar (homologous) cells or the production of novel cell groups. Finally, available data in the lamprey, a jawless fish, suggest that not all telencephalic subdivisions were present at the origin of vertebrates, raising important questions about their evolution.

Chemoarchitectonic subdivisions of the songbird septum and a comparative overview of septum chemical anatomy in jawed vertebrates

Available data demonstrate that the avian septal region shares a number of social behavior functions and neurochemical features in common with mammals. However, the structural and functional subdivisions of the avian septum remain largely unexplored. In order to delineate chemoarchitectural zones of the avian septum, we prepared a large dataset of double-, triple-, and quadruple-labeled material in a variety of songbird species (finches and waxbills of the family Estrildidae and a limited number of emberizid sparrows) using antibodies against 10 neuropeptides and enzymes. Ten septal zones were identified that were placed into lateral, medial, caudocentral, and septohippocampal divisions, with the lateral and medial divisions each containing multiple zones. The distributions of numerous immunoreactive substances in the lateral septum closely match those of mammals (i.e., distributions of met-enkephalin, vasotocin, galanin, calcitonin gene-related peptide, tyrosine hydroxylase, vasoactive intestinal polypeptide, substance P, corticotropin-releasing factor, and neuropeptide Y), enabling detailed comparisons with numerous chemoarchitectonic zones of the mammalian lateral septum. Our septohippocampal and caudocentral divisions are topographically comparable to the mammalian septohippocampal and septofimbrial nuclei, respectively, although additional data will be required to establish homology. The present data also demonstrate the presence of a medial septal nucleus that is histochemically comparable to the medial septum of mammals. The avian medial septum is clearly defined by peptidergic markers and choline acetyltransferase immunoreactivity. These findings should provide a useful framework for functional and comparative studies, as they suggest that many features of the septum are highly conserved across vertebrate taxa.

Revised nomenclature for avian telencephalon and some related brainstem nuclei

The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names. Revisions for the brainstem focused on vocal control, catecholaminergic, cholinergic, and basal ganglia-related nuclei. For example, the Forum recognized that the hypoglossal nucleus had been incorrectly identified as the nucleus intermedius in the Karten and Hodos (1967) pigeon brain atlas, and what was identified as the hypoglossal nucleus in that atlas should instead be called the supraspinal nucleus. The locus ceruleus of this and other avian atlases was noted to consist of a caudal noradrenergic part homologous to the mammalian locus coeruleus and a rostral region corresponding to the mammalian A8 dopaminergic cell group. The midbrain dopaminergic cell group in birds known as the nucleus tegmenti pedunculopontinus pars compacta was recognized as homologous to the mammalian substantia nigra pars compacta and was renamed accordingly; a group of gamma-aminobutyric acid (GABA)ergic neurons at the lateral edge of this region was identified as homologous to the mammalian substantia nigra pars reticulata and was also renamed accordingly. A field of cholinergic neurons in the rostral avian hindbrain was named the nucleus pedunculopontinus tegmenti, whereas the anterior nucleus of the ansa lenticularis in the avian diencephalon was renamed the subthalamic nucleus, both for their evident mammalian homologues. For the basal (i.e., subpallial) telencephalon, the actual parts of the basal ganglia were given names reflecting their now evident homologues. For example, the lobus parolfactorius and paleostriatum augmentatum were acknowledged to make up the dorsal subdivision of the striatal part of the basal ganglia and were renamed as the medial and lateral striatum. The paleostriatum primitivum was recognized as homologous to the mammalian globus pallidus and renamed as such. Additionally, the rostroventral part of what was called the lobus parolfactorius was acknowledged as comparable to the mammalian nucleus accumbens, which, together with the olfactory tubercle, was noted to be part of the ventral striatum in birds. A ventral pallidum, a basal cholinergic cell group, and medial and lateral bed nuclei of the stria terminalis were also recognized. The dorsal (i.e., pallial) telencephalic regions that had been erroneously named to reflect presumed homology to striatal parts of mammalian basal ganglia were renamed as part of the pallium, using prefixes that retain most established abbreviations, to maintain continuity with the outdated nomenclature. We concluded, however, that one-to-one (i.e., discrete) homologies with mammals are still uncertain for most of the telencephalic pallium in birds and thus the new pallial terminology is largely devoid of assumptions of one-to-one homologies with mammals. The sectors of the hyperstriatum composing the Wulst (i.e., the hyperstriatum accessorium intermedium, and dorsale), the hyperstriatum ventrale, the neostriatum, and the archistriatum have been renamed (respectively) the hyperpallium (hypertrophied pallium), the mesopallium (middle pallium), the nidopallium (nest pallium), and the arcopallium (arched pallium). The posterior part of the archistriatum has been renamed the posterior pallial amygdala, the nucleus taeniae recognized as part of the avian amygdala, and a region inferior to the posterior paleostriatum primitivum included as a subpallial part of the avian amygdala. The names of some of the laminae and fiber tracts were also changed to reflect current understanding of the location of pallial and subpallial sectors of the avian telencephalon. Notably, the lamina medularis dorsalis has been renamed the pallial-subpallial lamina. We urge all to use this new terminology, because we believe it will promote better communication among neuroscientists. Further information is available at http://avianbrain.org

Immunohistochemistry and neural connectivity of the Ov shell in the songbird and their evolutionary implications

The neuropeptide immunohistochemistry and neural connectivity of areas surrounding the thalamic auditory nucleus (the nucleus ovoidalis [Ov]), as well as the areas to which it is connected, were investigated in a songbird, the Bengalese finch. The results showed that met-enkephalin was present in the Ov shell and most of the areas connected to it, but not in the Ov core. Anterograde and retrograde tracing studies showed that the Ov shell was more widely connected than the Ov core. The Ov shell was mainly connected to: 1). areas flanking the primary telencephalic auditory field (i.e., fields L2b, L1, and L3) and areas surrounding the robust nucleus of the archistriatum (RA); 2). several hypothalamic areas such as the nucleus ventromedialis hypothalami (VMN) and the nucleus anterior medialis hypothalami (AM). Some of these areas connected to the Ov shell are thought to be involved in auditory mediated neurosecretory activities. These results, which are similar to those reported previously in non-songbirds, suggest that the Ov shell and other surrounding areas of auditory and song-control nuclei are conserved in birds. These findings are discussed in terms of the evolution of the core-and-surround organization of auditory and song-control nuclei.

Distribution of tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) immunoreactivity in the central nervous system of two chondrostean fishes (Acipenser baeri and Huso huso)

To obtain a better understanding of the evolution of the brain catecholaminergic systems of fishes, we have examined the distribution of catecholamine-synthesizing enzymes in two species of sturgeon (Acipenser baeri and Huso huso) using antibodies against tyrosine hydroxylase (TH) and dopamine-beta -hydroxylase (DBH; only analyzed in Acipenser). Both sturgeons showed TH-immunoreactive (THir) neurons widely distributed in most regions of the brain, the highest number of THir cells being located in the forebrain (olfactory bulb, preoptic area, and posterior tuberculum). THir cells were also seen in other forebrain areas (retrobulbar area, dorsal and ventral telencephalic areas, hypothalamus, ventral thalamus, pretectal area) and in the brainstem (locus coeruleus, viscerosensory area, caudal reticular formation, and area postrema). Immunoreactive fibers and varicosities showed a wide distribution, being particularly abundant in the diencephalon and mesencephalon. DBH-immunoreactive (DBHir) cells were observed in the anterior tuberal nucleus, where these cells were TH-negative, and in the locus coeruleus and the caudal rhombencephalon (vagal reticular formation), where the DBHir cells were also THir. DBHir fibers were scarce in the telencephalon and very abundant in the diencephalon, mesencephalon, and rhombencephalon. The comparative analysis of the catecholaminergic systems of chondrosteans and those observed in other groups of fishes and tetrapods indicate a similar organization of many nuclei, as well as characteristics that are probably primitive, such as the presence of a large number of forebrain catecholaminergic groups.

Telencephalo-habenulo-interpeduncular connections in the brain of the shark Chiloscyllium arabicum

The telencephalo-habenulo-interpeduncular connections have been studied in the brain of the cartilaginous fish Chiloscyllium arabicum. In addition to telencephalic and diencephalic projections to the habenula and to habenulo-interpeduncular projection, the presence of a direct telencephalo-interpeduncular connection confirmed some homologies of this system in cartilaginous fishes with actinopterigian fishes and land vertebrates.

Topography and topology of the teleost telencephalon: a paradox resolved

Analysis of vasculature in the telencephalic pallium of a teleost allows the considerable depth of the sulcus externus, which lies at the lateral extent of the ependymal attachment, to be appreciated. The depth of this sulcus is compelling evidence for a simple eversion process (an outfolding of the pallial wall of each hemisphere) during telencephalic development in all ray-finned fishes that is not complicated in teleosts by secondary migration of pallial cell groups. A simple eversion process is known to occur in some ray-finned fishes with relatively simple telencephalic cytoarchitecture but has been disputed in teleosts based on the pattern of olfactory tract projections. A resolution to the conflicting hypotheses of pallial relationships across ray-finned fishes and in comparison with other craniate radiations is presented here, based on a re-examination of hodological and histochemical data mandated by this sulcal anatomy.

Identification of the anterior nucleus of the ansa lenticularis in birds as the homolog of the mammalian subthalamic nucleus

In mammals, the subthalamic nucleus (STN) is a glutamatergic diencephalic cell group that develops in the caudal hypothalamus and migrates to a position above the cerebral peduncle. By its input from the external pallidal segment and projection to the internal pallidal segment, STN plays a critical role in basal ganglia functions. Although the basal ganglia in birds is well developed, possesses the same major neuron types as in mammals, and plays a role in movement control similar to that in mammals, it has been uncertain whether birds possess an STN. We report here evidence indicating that the so-called anterior nucleus of the ansa lenticularis (ALa) is the avian homolog of mammalian STN. First, the avian ALa too develops within the mammillary hypothalamic area and migrates to a position adjacent to the cerebral peduncle. Second, ALa specifically receives input from dorsal pallidal neurons that receive input from enkephalinergic striatal neurons, as is true of STN. Third, ALa projects back to avian dorsal pallidum, as also the case for STN in mammals. Fourth, the neurons of ALa contain glutamate, and the target neurons of ALa in dorsal pallidum possess AMPA-type glutamate receptor profiles resembling those of mammalian pallidal neurons. Fifth, unilateral lesions of ALa yield behavioral disturbances and movement asymmetries resembling those observed in mammals after STN lesions. These various findings indicate that ALa is the avian STN, and they suggest that the output circuitry of the basal ganglia for motor control is similar in birds and mammals.

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.

Evolution of the basal ganglia: new perspectives through a comparative approach

The basal ganglia (BG) have received much attention during the last 3 decades mainly because of their clinical relevance. Our understanding of their structure, organisation and function in terms of chemoarchitecture, compartmentalisation, connections and receptor localisation has increased equally. Most of the research has been focused on the mammalian BG, but a considerable number of studies have been carried out in nonmammalian vertebrates, in particular reptiles and birds. The BG of the latter 2 classes of vertebrates, which together with mammals constitute the amniotic vertebrates, have been thoroughly studied by means of tract-tracing and immunohistochemical techniques. The terminology used for amniotic BG structures has frequently been adopted to indicate putative corresponding structures in the brain of anamniotes, i.e. amphibians and fishes, but data for such a comparison were, until recently, almost totally lacking. It has been proposed several times that the occurrence of well developed BG structures probably constitutes a landmark in the anamniote-amniote transition. However, our recent studies of connections, chemoarchitecture and development of the basal forebrain of amphibians have revealed that tetrapod vertebrates share a common pattern of BG organisation. This pattern includes the existence of dorsal and ventral striatopallidal systems, reciprocal connections between the striatopallidal complex and the diencephalic and mesencephalic basal plate (striatonigral and nigrostriatal projections), and descending pathways from the striatopallidal system to the midbrain tectum and reticular formation. The connectional similarities are paralleled by similarities in the distribution of chemical markers of striatal and pallidal structures such as dopamine, substance P and enkephalin, as well as by similarities in development and expression of homeobox genes. On the other hand, a major evolutionary trend is the progressive involvement of the cortex in the processing of the thalamic sensory information relayed to the BG of tetrapods. By using the comparative approach, new insights have been gained with respect to certain features of the BG of vertebrates in general, such as the segmental organisation of the midbrain dopaminergic cell groups, the occurrence of large numbers of dopaminergic cell bodies within the telencephalon itself and the variability in, among others, connectivity and chemoarchitecture. However, the intriguing question whether the basal forebrain organisation of nontetrapods differs essentially from that observed in tetrapods still needs to be answered.

Distribution of choline acetyltransferase immunoreactivity in the brain of an elasmobranch, the lesser spotted dogfish (Scyliorhinus canicula)

Although the distribution of cholinergic cells is remarkably similar across the vertebrate species, no data are available on more primitive species, such as cartilaginous fishes. To extend the evolutionary analysis of the cholinergic systems, we studied the distribution of cholinergic neurons in the brain and rostral spinal cord of Scyliorhinus canicula by immunocytochemistry using an antibody against the enzyme choline acetyltransferase (ChAT). Western blot analysis of brain extracts of dogfish, sturgeon, trout, and rat showed that this antibody recognized similar bands in the four species. Putative cholinergic neurons were observed in most brain regions, including the telencephalon, diencephalon, cerebellum, and brainstem. In the retrobulbar region and superficial dorsal pallium of the telencephalon, numerous small pallial cells were ChAT-like immunoreactive. In addition, tufted cells of the olfactory bulb and some cells in the lateral pallium showed faint immunoreactivity. In the preoptic-hypothalamic region, ChAT-immunoreactive (ChAT-ir) cells were found in the preoptic nucleus, the vascular organ of the terminal lamina, and a small population in the caudal tuber. In the epithalamus, the pineal photoreceptors were intensely positive. Many cells of the habenula were faintly ChAT-ir, but the neuropil of the interpeduncular nucleus showed intense ChAT immunoreactivity. In the pretectal region, ChAT-ir cells were observed only in the superficial pretectal nucleus. In the brainstem, the somatomotor and branchiomotor nuclei, the octavolateral efferent nucleus, and a cell group just rostral to the Edinger-Westphal (EW) nucleus contained ChAT-ir neurons. In addition, the trigeminal mesencephalic nucleus, the nucleus G of the isthmus, some locus coeruleus cells, and some cell populations of the vestibular nuclei and of the electroreceptive nucleus of the octavolateral region exhibited ChAT immunoreactivity. In the reticular areas of the brainstem, the nucleus of the medial longitudinal fascicle, many reticular neurons of the rhombencephalon, and cells of the nucleus of the lateral funiculus were immunoreactive to this antibody. In the cerebellum, Golgi cells of the granule cell layer and some cells of the cerebellar nucleus were also ChAT-ir. In the rostral spinal cord, ChAT immunoreactivity was observed in cells of the motor column, the dorsal horn, the marginal nucleus (a putative stretch-receptor organ), and in interstitial cells of the ventral funiculus. These results demonstrate for the first time that cholinergic neurons are distributed widely in the central nervous system of elasmobranchs and that their cholinergic systems have evolved several characteristics that are unique to this group.

Distribution of serotonin in the brain of the Senegalese sole, Solea senegalensis: an immunohistochemical study

We report the distribution of serotonin immunoreactive (5-HT-ir) structures in the brain of the adult Senegalese sole, Solea senegalensis, using the streptavidin-biotin-peroxidase complex immunohistochemical method. We have found a wide distribution of immunoreactive fibers throughout the entire brain. 5-HT-ir cell bodies appeared restricted to some periventricular nuclei associated with the diencephalic recesses, and in the rhombencephalic reticular formation and inferior olivary region. Specifically, cerebrospinal fluid-contacting serotoninergic cells were found within the pars dorsalis and pars ventralis of the nucleus recessus lateralis, in the paraventricular organ and in the nucleus recessus posterioris. In the brainstem, 5-HT-ir perikarya appear within the superior and inferior raphe, the nucleus reticularis superioris, the nucleus interpeduncularis and the inferior olive. Although positive fibers were not found in the neurohypophysis, a few 5-HT-ir cells were identified in the adenohypophysis. This distribution is compared with those found in other fishes and discussed in the context of putative roles of 5-HT as a neuroendocrine factor and neurotransmitter in the Senegalese sole.

Structural and functional evolution of the basal ganglia in vertebrates

While a basal ganglia with striatal and pallidal subdivisions is 1 clearly present in many extant anamniote species, this basal ganglia is cell sparse and receives only a relatively modest tegmental dopaminergic input and little if any cortical input. The major basal ganglia influence on motor functions in anamniotes appears to be exerted via output circuits to the tectum. In contrast, in modern mammals, birds, and reptiles (i.e., modern amniotes), the striatal and pallidal parts of the basal ganglia are very neuron-rich, both consist of the same basic populations of neurons in all amniotes, and the striatum receives abundant tegmental dopaminergic and cortical input. The functional circuitry of the basal ganglia also seems very similar in all amniotes, since the major basal ganglia influences on motor functions appear to be exerted via output circuits to both cerebral cortex and tectum in sauropsids (i.e., birds and reptiles) and mammals. The basal ganglia, output circuits to the cortex, however, appear to be considerably more developed in mammals than in birds and reptiles. The basal ganglia, thus, appears to have undergone a major elaboration during the evolutionary transition from amphibians to reptiles. This elaboration may have enabled amniotes to learn and/or execute a more sophisticated repertoire of behaviors and movements, and this ability may have been an important element of the successful adaptation of amniotes to a fully terrestrial habitat. The mammalian lineage appears, however, to have diverged somewhat from the sauropsid lineage with respect to the emergence of the cerebral cortex as the major target of the basal ganglia circuitry devoted to executing the basal ganglia-mediated control of movement.

Dopaminergic modulation of mitral cell activity in the frog olfactory bulb: a combined radioligand binding-electrophysiological study

Dopamine content in the amphibian olfactory bulb is supplied by interneurons scattered among mitral cells in the external plexiform/mitral cell layer. In mammals, dopamine has been found to be involved in various aspects of bulbar information processing by influencing mitral cell odour responsiveness. Dopamine action in the bulb depends directly on the localization of its receptor targets, found to be mainly of the D2 type in mammals. The present study assessed, in the frog, both the anatomical localization of D2-like, radioligand-labelled receptors of dopamine and the in vivo action of dopamine on unitary mitral cell activity in response to odours delivered over a wide range of concentrations. The [125I]iodosulpride-labelled D2 binding sites were visualized on frozen sagittal sections of frog brains by film radioautography. The sites were found to be restricted to the external plexiform/mitral cell layer; other layers of the olfactory bulb were devoid of specific labelling. Electrophysiological recordings of mitral unit activity revealed that dopamine or its agonist apomorphine induced a drastic reduction of spontaneous firing rate of mitral cells in most cases without altering odour intensity coding properties of these cells. Moreover, pre-treatment with the D2 antagonist eticlopride blocked the dopamine-induced reduction of mitral cell spontaneous activity. In the frog olfactory bulb, both anatomical localization of D2-like receptors and functional data on dopamine involvement in information processing differ from those reported in mammals. This suggests a phylogenetic evolution of dopamine action in the olfactory bulb. In the frog, anatomical data perfectly corroborate electrophysiological results, together strongly suggesting a direct action of dopamine on mitral cells. In a physiologically operating system, such an action would result in a global improvement of signal-to-noise ratio.

Characterization of the RFamide-like neuropeptides in the nervus terminalis of the goldfish (Carassius auratus)

FMRFamide-immunoreactivity has been demonstrated in the CNS of many vertebrate species. We sought to further characterize this immunoreactivity in nervus terminalis retinal efferents of the goldfish using an antiserum raised against a bovine morphine modulating peptide (A18Famide). This antiserum robustly labels nervus terminalis efferents to the retina, as well as a sub-population of retinal amacrine cells. Under immunocytochemical conditions the antiserum cross-reacted with neuropeptide Y-like as well as A18Famide-like peptides, but under conditions of radioimmunoassay it was highly specific for A18Famide-like peptides. High pressure liquid chromatography, gel permeation chromatography and radioimmunoassay showed that at least two different RFamide-like peptides, approximately the same size as the bovine RFamide-like peptides, are present in the goldfish nervus terminalis.

Raphe nuclei in three cartilaginous fishes, Hydrolagus colliei, Heterodontus francisci, and Squalus acanthias

The vertebrate reticular formation, containing over 30 nuclei in mammals, is a core brainstem area with a long evolutionary history. However, not all reticular nuclei are equally old. Nuclei that are widespread among the vertebrate classes are probably ones that evolved early. We describe raphe nuclei in the reticular formation of three cartilaginous fishes that diverged from a common ancestor over 350 million years ago. These fishes are Hydrolagus colliei, a holocephalan, Squalus acanthias, a small-brained shark, and Heterodontus francisci, a large-brained shark. Nuclear identification was based on immunohistochemical localization of serotonin and leu-enkephalin, on brainstem location, and on cytoarchitectonics. Raphe nuclei are clustered in inferior and superior cell groups, but within these groups individual nuclei can be identified: raphe pallidus, raphe obscurus, and raphe magnus in the inferior group and raphe pontis, raphe dorsalis, raphe centralis superior, and raphe linearis in the superior group. Hydrolagus lacked a dorsal raphe nucleus, but the nucleus was present in the sharks. The majority of immunoreactive cells are found in the superior group, especially in raphe centralis superior, but immunoreactive cells are present from spinal cord to caudal mesencephalon. The distribution and cytoarchitectonics of serotoninergic and enkephalinergic cells are similar to each other, but raphe nuclei contain fewer enkephalinergic than serotoninergic cells. The cytoarchitectonics of immunoreactive raphe cells in cartilaginous fishes are remarkably similar to those described for raphe nuclei in mammals; however, the lack of a raphe dorsalis in Hydrolagus indicates that either it evolved later than the other raphe nuclei or it was lost in holocephalan fishes.

Organization of the avian "corticostriatal" projection system: a retrograde and anterograde pathway tracing study in pigeons

Birds have well-developed basal ganglia within the telencephalon, including a striatum consisting of the medially located lobus parolfactorius (LPO) and the laterally located paleostriatum augmentatum (PA). Relatively little is known, however, about the extent and organization of the telencephalic "cortical" input to the avian basal ganglia (i.e., the avian "corticostriatal" projection system). Using retrograde and anterograde neuroanatomical pathway tracers to address this issue, we found that a large continuous expanse of the outer pallium projects to the striatum of the basal ganglia in pigeons. This expanse includes the Wulst and archistriatum as well as the entire outer rind of the pallium intervening between Wulst and archistriatum, termed by us the pallium externum (PE). In addition, the caudolateral neostriatum (NCL), pyriform cortex, and hippocampal complex also give rise to striatal projections in pigeon. A restricted number of these pallial regions (such as the "limbic" NCL, pyriform cortex, and ventral/caudal parts of the archistriatum) project to such ventral striatal structures as the olfactory tubercle (TO), nucleus accumbens (Ac), and bed nucleus of the stria terminalis (BNST). Such "limbic" pallial areas also project to medialmost LPO and lateralmost PA, while the hyperstriatum accessorium portion of the Wulst, the PE, and the dorsal parts of the archistriatum were found to project primarily to the remainder of LPO (the lateral two-thirds) and PA (the medial four-fifths). The available evidence indicates that the diverse pallial regions projecting to the striatum in birds, as in mammals, are parts of higher order sensory or motor systems. The extensive corticostriatal system in both birds and mammals appears to include two types of pallial neurons: 1) those that project to both striatum and brainstem (i.e., those in the Wulst and the archistriatum) and 2) those that project to striatum but not to brainstem (i.e., those in the PE). The lack of extensive corticostriatal projections from either type of neuron in anamniotes suggests that the anamniote-amniote evolutionary transition was marked by the emergence of the corticostriatal projection system as a prominent source of sensory and motor information for the striatum, possibly facilitating the role of the basal ganglia in movement control.

Posthatching development of preproenkephalin mRNA-expressing cell populations in the pigeon telencephalon

Enkephalin peptides are highly expressed in the vertebrate telencephalon. Our previous investigations in the pigeon and in the chicken [26] suggested that the cellular distribution of these peptides is conserved in phylogenetically 'old' telencephalic regions (e.g. the basal ganglia), while it has species-specific organizations in areas (e.g. dorsomedial forebrain and bulbus olfactorius) that are likely to play important roles in species-specific behaviors. In the present study, we investigated the posthatching development of preproenkephalin (PPE) mRNA-containing cells in the pigeon forebrain using in situ hybridization histochemistry. These cells are densely distributed in the paleostriatal complex (corresponding to the mammalian caudate-putamen) at hatching, and their density progressively decreases during the first 9 days posthatching, when it is similar to that of adult pigeons. In the dorsomedial forebrain (corresponding to the mammalian hippocampus), PPE mRNA-expressing cells are present at hatching, and their density reaches a peak around the 6th day posthatching. In the bulbus olfactorius, the first PPE mRNA-containing cells are observed after 9 days posthatching. The developmental profile of PPE mRNA expression in these areas of the pigeon telencephalon shows remarkable similarities with the development of enkephalinergic cells in corresponding brain areas of mammals. As in the mammalian caudate-putamen, the developmental expression of enkephalin peptides in the paleostriatal complex is likely to be related to neuronal withdrawal from the mitotic cycle. The developmental pattern of expression of PPE mRNA in the dorsomedial forebrain suggests that enkephalin peptides contribute to the maturation of the behavioral functions of this area.

Distribution of calcitonin gene-related peptide-like immunoreactivity in the brain of the small-spotted dogfish, Scyliorhinus canicula L

The distribution of neuropeptides has been useful in comparing neuronal aggregates of elasmobranchs with those in other vertebrates. The distribution of calcitonin gene-related peptide (CGRP)-like immunoreactivity in the brain of the dogfish was examined with an antiserum to rat alpha-CGRP. Western blot analysis confirms that our antiserum recognizes a single peptide in the dogfish brain very similar to mammalian CGRP. CGRP-like immunoreactivity was located in discrete neuronal groups. CGRP-like-immunoreactive (CGRP-ir) neurons were found in the motor nuclei III, IV, V, VI, VII, IX, and X of the brainstem motor column and in the octavolateral efferent neurons. In the isthmal region, two groups of CGRP-ir neurons appeared in the parabrachial region and reticular substance. Three other CGRP-ir cell groups were observed in the mesencephalon: in the ventral tegmental area, in the substantia nigra, and one widely scattered but numerous population in superficial layers of the optic tectum. In the diencephalon, CGRP-ir cells were observed in the magnocellular preoptic nucleus and the organon vasculosum hypothalami. A population of CGRP-ir cells was also observed in the entopeduncular nucleus in the impar telencephalon. CGRP-ir fibers of central origin were widely distributed in the brain, but the most conspicuous areas were found in the ventral telencephalon, the hypothalamus, the mesencephalic lateral reticular area, and the dorsolateral isthmal region. The neurointermediate lobe of the hypophysis was also richly innervated by CGRP-ir fibers. CGRP-ir sensory fibers of cranial nerves IX and X and of dorsal spinal roots formed very conspicuous terminal fields in the lobus vagi and Cajal's nucleus commissuralis and in the dorsal region of the substantia gelatinosa, respectively. Comparison of the distribution of fibers and perikarya in dogfish and other vertebrates suggests that this CGRP-ir system has been well conserved during evolution.

Distribution of preproenkephalin mRNA in the chicken and pigeon telencephalon

Bioassay and immunological studies have detected the presence of opioid peptides in the nervous system of representatives of all classes of vertebrates. The present study evaluates the expression and localization of preproenkephalin (PPE) mRNA to determine the sites of synthesis of the enkephalin peptides in the adult chicken and pigeon telencephalon using in situ hybridization histochemistry. We used a 500-base-pair chicken RNA probe corresponding to chicken PPE cDNA. In both the chicken and the pigeon telencephalon, the highest concentration of PPE mRNA-containing cells was observed in the lobus parolfactorius, paleostriatum augmentatum, nucleus accumbens, and septum. Distinct populations of labeled cells were also detected in the hyperstriatum accessorium, hippocampus, area parahippocampalis, nucleus of the diagonal band, cortex dorsolateralis, and cortex piriformis. Differences in PPE mRNA expression between chicken and pigeon were observed in several telencephalic regions. For instance, the bulbus olfactorius was heavily labeled in the pigeon, but was not labeled in the chicken, and numerous PPE mRNA-containing cells were present in the area parahippocampalis of pigeons but not of chickens. In contrast, in the hyperstriatum dorsale and hyperstriatum ventrale, numerous PPE mRNA-expressing cells were detected in the chicken but not in the pigeon. Overall, PPE mRNA-expressing cells were more numerous than enkephalin-immunoreactive cells described in previous studies. In addition, our results suggest that the general pattern of enkephalin expression in the avian telencephalon is similar to that found in other vertebrates. Finally, the results of the present study illustrate some differences in the pattern of PPE mRNA distribution between closely related species, indicating the existence of species-specific neurochemical pathways, which may influence and perhaps mediate different behaviors characteristics of these species.

Immunocytochemical evidence for the presence of Met-enkephalin and Leu-enkephalin in distinct neurons in the brain of the elasmobranch fish Scyliorhinus canicula

Immunohistochemical methods have been used to investigate the distribution of various opioid peptides derived from mammalian proenkephalin in the central nervous system of Scyliorhinus canicula. The results indicate that both Leu- and Met-enkephalin-immunoreactive peptides are present in the dogfish brain. In contrast, enkephalin forms similar to Met-enkephalin-Arg-Phe or Met-enkephalin-Arg-Gly-Leu, and mammalian alpha-neo-endorphin, dynorphin A (1-8), dynorphin A (1-13), and dynorphin A (1-17) were not detected. Met- and Leu-enkephalin immunoreactivities were found in distinct neurons of the telencephalon and hypothalamus. In particular, cell bodies reacting only with the Met-enkephalin antiserum were localized in the preoptic nucleus and in the suprachiasmatic region of the hypothalamus. Conversely, cell bodies reacting only with the Leu-enkephalin antiserum were localized in the pallium and the nucleus lobi lateralis hypothalami. Several areas of the telencephalon and diencephalon exhibited both Met- and Leu-enkephalin-like immunoreactivity, but the two immunoreactive peptides were clearly contained in distinct perikarya. The overall distribution of Met-enkephalin-immunoreactive elements in the dogfish exhibited similarities to the distribution of proenkephalin-derived peptides previously reported for the brain of tetrapods. The fact that Met- and Leu-enkephalin-like peptides were detected in distinct neurons, together with the absence of dynorphin-related peptides, suggests the existence of a novel Leu-enkephalin-containing precursor in the dogfish brain.