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

Subdivisions of the mesencephalon and isthmus in the lizard Gekko gecko as revealed by ChAT immunohistochemistry

The distribution of cholinergic cell bodies and fibers was examined in the mesencephalon and isthmus of Gekko gecko. Distinct groups with prominent labeled cells were observed in the cranial nerve motor nuclei and isthmic nuclei, and weak labeled cell bodies and fibers were observed in the mesencephalic nucleus of the trigeminal nerve and the central nucleus of the torus semicircularis. After discussing the topological relationships within the tectum and isthmus, we unify the nomenclature of the caudal deep mesencephalic nucleus in lizards and the rostral magnocellular nucleus isthmi in turtles that is similar in terms of the preisthmic position, nontopographic connections with the tectum, and the same midbrain origin to the magnocellular preisthmic nucleus in birds, and may be homologous to the superficial cuneiform nucleus in mammals. None of them belong to the cholinergic nucleus isthmi, as the latter has isthmus origin and topographic reciprocal connections with the tectum. We also discuss the origin and intrinsic function of the inner longitudinal tract of the thick ChAT-ir fibers that course through the mesencephalon and diencephalon. We review the subdivisions of the mesencephalon and isthmus of Gekko gecko as revealed by ChAT immunohistochemistry, as well as the limits of the diencephalo-mesencephalic, mesencephalic-isthmo, and isthmo-rhombocephalic by the ChAT-ir cell- and fiber-poor distribution, and discuss the caudal limit of the isthmus. Our research on the subdivisions of the mesencephalon and isthmus in G. gecko as revealed by ChAT immunohistochemistry will serve as the neuroanatomical basis for subsequent relevant studies of Gekko gecko.

Gustatory and general visceral centers and their connections in the brain of adult zebrafish: a carbocyanine dye tract-tracing study

The central connections of the gustatory/general visceral system of the adult zebrafish (Danio rerio) were examined by means of carbocyanine dye tracing. Main primary gustatory centers (facial and vagal lobes) received sensory projections from the facial and vagal nerves, respectively. The vagal nerve also projects to the commissural nucleus of Cajal, a general visceral sensory center. These primary centers mainly project on a prominent secondary gustatory and general visceral nucleus (SGN/V) located in the isthmic region. Secondary projections on the SGN/V were topographically organized, those of the facial lobe mainly ending medially to those of the vagal lobe, and those from the commissural nucleus ventrolaterally. Descending facial lobe projections to the medial funicular nucleus were also noted. Ascending fibers originating from the SGN/V mainly projected to the posterior thalamic nucleus and the lateral hypothalamus (lateral torus, lateral recess nucleus, hypothalamic inferior lobe diffuse nucleus) and an intermediate cell- and fiber-rich region termed here the tertiary gustatory nucleus proper, but not to a nucleus formerly considered as the zebrafish tertiary gustatory nucleus. The posterior thalamic nucleus, tertiary gustatory nucleus proper, and nucleus of the lateral recess gave rise to descending projections to the SGN/V and the vagal lobe. The connectivity between diencephalic gustatory centers and the telencephalon was also investigated. The present results showed that the gustatory connections of the adult zebrafish are rather similar to those reported in other cyprinids, excepting the tertiary gustatory nucleus. Similarities between the gustatory systems of zebrafish and other fishes are also discussed. J. Comp. Neurol. 525:333-362, 2017. © 2016 Wiley Periodicals, Inc.

Living without DAT: Loss and compensation of the dopamine transporter gene in sauropsids (birds and reptiles)

The dopamine transporter (DAT) is a major regulator of synaptic dopamine (DA) availability. It plays key roles in motor control and motor learning, memory formation, and reward-seeking behavior, is a major target of cocaine and methamphetamines, and has been assumed to be conserved among vertebrates. We have found, however, that birds, crocodiles, and lizards lack the DAT gene. We also found that the unprecedented loss of this important gene is compensated for by the expression of the noradrenaline transporter (NAT) gene, and not the serotonin transporter genes, in dopaminergic cells, which explains the peculiar pharmacology of the DA reuptake activity previously noted in bird striatum. This unexpected pattern contrasts with that of ancestral vertebrates (e.g. fish) and mammals, where the NAT gene is selectively expressed in noradrenergic cells. DA circuits in birds/reptiles and mammals thus operate with an analogous reuptake mechanism exerted by different genes, bringing new insights into gene expression regulation in dopaminergic cells and the evolution of a key molecular player in reward and addiction pathways.

Sociosexual investigation in sexually experienced, hormonally manipulated male leopard geckos: relation with phosphorylated DARPP-32 in dopaminergic pathways

Dopaminergic activity is both associated with sociosexual exposure and modulated by sexual experience and hormonal state across vertebrate taxa. Mature leopard geckos, a reptile with temperature-dependent sex determination, have dopaminoceptive nuclei that are influenced by their embryonic environment and sensitive to adult hormonal manipulation. In this study, we exposed hormonally manipulated male leopard geckos from different incubation temperatures to conspecifics and measured their sociosexual investigation, as well as phosphorylated DARPP-32 at Threonine 34 (pDARPP-32) immunoreactivity as a marker for D1 dopamine receptor activity in the nucleus accumbens, striatum, and preoptic area. Social investigation time by males of different incubation temperatures was modulated in opposite directions by exogenous androgen treatment. Males exposed to novel stimuli spent a greater proportion of time investigating females of different incubation temperatures. The time spent investigating females was positively correlated to pDARPP-32 immunoreactivity in the preoptic area. This is the first study quantifying pDARPP-32 in a lizard species, and suggests the protein as a potential marker to measure differences in the dopaminergic pathway in a social setting with consideration of embryonic environment and hormonal state.

Distribution of neuropeptide FF-like immunoreactivity in the brain of the lizardGekko gecko and its relation to catecholaminergic structures

The present study provides a detailed description of the distribution of neuropeptide FF (NPFF)-like immunoreactivity in the brain of the lizard Gekko gecko. NPFF is found to be involved in nociception, cardiovascular regulation, and endocrine function. Because of its known relationship with catecholamines in mammals, double staining with tyrosine hydroxylase (TH) antibodies was used to corroborate this for reptiles. The present study revealed that NPFF-like-immunoreactive (NPFF-ir) cells and fibers were widely distributed throughout the brain. Major NPFF-ir cell groups were observed in the diagonal band nucleus of Broca, hypothalamus, and dorsal horn of the spinal cord. Additional cells were found in the anterior olfactory nucleus, lateral and dorsal cortices, dorsolateral septum, and diencephalic intergeniculate leaflet formation. Dense plexuses of NPFF-ir fibers were identified in the diagonal band nucleus of Broca, septum, preoptic and hypothalamic areas, isthmic region, ventrolateral tegmentum, solitary tract nucleus, and dorsolateral funiculus of the spinal cord. Extensive fiber staining also occurred in the nucleus accumbens and the midbrain tectum. Although an intimate relationship between NPFF-ir and TH-ir structures was obvious at many places in the brain, colocalization of these two substances was not observed. In conclusion, the distribution of NPFF in the brain of Gekko shares more features with anamniotes in terms of number of cell groups, more elaborate networks of fibers, and lack of colocalization with catecholamines than with mammals, suggesting a decrease in the distribution of this peptide in the latter vertebrate group.

Daily and circadian rhythms of neurotransmitters and related compounds in the hypothalamic suprachiasmatic nuclei of a diurnal vertebrate

By using immunocytochemistry we tested whether neurotransmitters, and enzymes specific to neurotransmitters synthesis are rhythmically expressed in the suprachiasmatic nuclei of the hypothalamus of Ruin lizards Podarcis sicula either kept in light-dark cycles or constant darkness. Within the suprachiasmatic nuclei, prominent 24 h rhythms under 12:12 light-dark cycles were found for vasoactive intestinal polypeptide (VIP) and for tyrosine hydroxylase (TH). Peaks of both VIP and TH fell in the light phase of the cycle. Rhythmic expression of TH persisted under constant temperature and darkness, demonstrating the existence of circadian rhythms of TH in the suprachiasmatic nuclei. No rhythmic expression of neurotransmitters and related compounds was found in the periventricular nuclei, the supraoptic nuclei, and the rest of the hypothalamus. Our data are the first demonstration of rhythmic expression of neurotransmitters and related compounds in the suprachiasmatic nuclei of a non-mammalian vertebrate. The demonstration of a diurnal peak of VIP in a diurnal reptile-vs. nocturnal peak of VIP typical of nocturnal mammals-provides new information for comparative studies on the circadian physiology of the suprachiasmatic nuclei across vertebrate classes and their adaptation strategies to different temporal niches.

Catecholaminergic innervation of the septum in the frog: a combined immunohistochemical and tract-tracing study

In the present study, we have investigated the distribution and the origin of the catecholaminergic innervation of the septal region in the frog Rana perezi. Immunohistochemistry for dopamine and two enzymes required for the synthesis of catecholamines, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) revealed a complex pattern of catecholaminergic (CA) innervation in the anuran septum. Dopaminergic fibers were primarily present in the dorsal portion of the lateral septum, whereas noradrenergic (DBH immunoreactive) fibers predominated in the medial septum/diagonal band complex. Catecholaminergic cell bodies were never observed within the septum. To determine the origin of this innervation, applications of dextran amines, both under in vivo and in vitro conditions, into the septum were combined with immunohistochemistry for TH. Results from these experiments demonstrated that four catecholaminergic cell groups project to the septum: (1) the group related to the zona incerta in the ventral thalamus, (2) the posterior tubercle/mesencephalic group, (3) the locus coeruleus, and (4) the nucleus of the solitary tract. While the two first groups provide dopaminergic innervation to the septum, the locus coeruleus provides the major noradrenergic projection. Noradrenergic fibers most likely arise also in the nucleus of the solitary tract. The results obtained in Rana perezi are readily comparable to those in mammals suggesting that the role of catecholamines in the septum is well conserved through phylogeny and that the CA innervation of the amphibian septum may be involved in functional circuits similar to those in 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.

Nucleus accumbens in the lizard Psammodromus algirus: chemoarchitecture and cortical afferent connections

To better understand the organization and evolution of the basal ganglia of vertebrates, in the present study we have analyzed the chemoarchitecture and the cortical input to the nucleus accumbens in the lacertid lizard Psammodromus algirus. The nucleus accumbens contains many gamma-aminobutyric acid (GABA)-positive neurons and calbindin-positive neurons, the majority of which may be spiny projection neurons, and a few dispersed neuropeptide Y-positive neurons that likely represent aspiny interneurons. The nucleus accumbens contains two chemoarchitectonically different fields: a rostromedial field that stains heavily for substance P, dopamine, GABA(A) receptor, and a caudolateral field that stains only lightly to moderately for them, appearing more similar to the adjacent striatum. Injections of biotinylated dextran amine were placed in either the medial, dorsomedial, or dorsal cortices of Psammodromus. The medial and the dorsal cortices project heavily to the rostromedial field of the accumbens, whereas they project lightly to moderately to the caudolateral field. Cortical terminals make asymmetric, presumably excitatory, synaptic contacts with distal dendrites and the head of spines. Our results indicate that the hippocampal-like projection to the nucleus accumbens is similar between mammals and reptiles in that cortical terminals make mainly excitatory synapses on spiny, putatively projection neurons. However, our results and results from previous investigations indicate that important differences exist between the nucleus accumbens of mammals and reptiles regarding local modulatory interactions between cortical, dopaminergic, and cholinergic elements, which suggest that the reptilian nucleus accumbens may be as a whole comparable to the shell of the mammalian nucleus accumbens.

Heterogeneity of electrically evoked dopamine release and reuptake in substantia nigra, ventral tegmental area, and striatum

Somatodendritic dopamine (DA) released in substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) may mediate extrasynaptic neuronal signaling. The concentration of extracellular DA ([DA]o) attained during somatodendritic activation will be governed by the density of release sites and properties of DA uptake. We evaluated these factors in SNc, VTA, and dorsal striatum with carbon-fiber microelectrodes and fast-scan cyclic voltammetry to monitor [DA]o during local electrical stimulation (10 Hz, 5 s) in guinea pig brain slices. Stimulated DA efflux was site specific, with significantly higher [DA]o in caudal (0.48 +/- 0.03 microM, mean +/- SE) than rostral SNc (0.16 +/- 0.01 microM), averaged over their mediolateral extents, and higher [DA]o in VTA (0.74 +/- 0.07 microM) than in medial (0.43 +/- 0.04 microM) or lateral SNc (0.29 +/- 0.05 microM), averaged rostrocaudally. Throughout SNc, evoked [DA]o correlated positively (r = 0.91) with the density of tyrosine-hydroxylase-immunoreactive cells. Modulation of evoked [DA]o by uptake was also site specific. The selective DA uptake inhibitor GBR 12909 significantly increased evoked [DA]o in caudal SNc (to 185 +/- 27%) and striatum (408 +/- 24%), but had no effect in rostral SNc or VTA. Conversely, the norepinephrine (NE) uptake inhibitor desipramine did not alter stimulated [DA]o in caudal SNc or striatum, but caused significant enhancement in rostral SNc (196 +/- 17%) and VTA (126 +/- 12%). Paroxetine, a selective 5-hydroxytryptamine uptake inhibitor had little effect in any region tested. Site-specific sensitivity to desipramine mandated evaluation of dopamine-beta-hydroxylase immunoreactivity (D beta H-ir) in midbrain. The density of filaments positive for D beta H-ir was greater in rostral SNc and VTA than in caudal SNc, suggesting DA clearance via the NE transporter in these regions. Importantly, D beta H-ir was most dense in sections rostral to SNc where no catecholamine signal was detected and no enhancement was observed with desipramine, indicating a lack of NE contribution to evoked release in any region examined. Taken together, these data confirmed that evoked somatodendritic [DA]o depends on DA cell density and on local uptake properties. Uptake was less efficient in SNc and VTA than in striatum. Moreover, enhancement of stimulated [DA]o by GBR 12909 demonstrated that evoked release from dendrites is not via reversal of the DA transporter. Lastly, the heterogeneous patterns of DA uptake within SNc and VTA were consistent with the pattern of degeneration in Parkinson's disease; less vulnerable DA cells, e.g., those in VTA, have less DA uptake than the more vulnerable cells of caudal SNc.

Noradrenergic system in the chicken brain: immunocytochemical study with antibodies to noradrenaline and dopamine-beta-hydroxylase

A light microscopic immunocytochemical study, using antisera against noradrenaline (NA) and dopamine-beta-hydroxylase (DBH), revealed the noradrenergic system in the brain of the chicken (Gallus domesticus). NA- and DBH-immunoreactive (ir) elements showed a similar distribution throughout the whole brain. The neurons immunoreactive for the monoamine were confined to the lower brainstem, the pons, and the medulla. In the pons, a rather dense group of cells was found in the dorsal, most posterior part of the locus coeruleus and in the caudal nucleus subcoeruleus ventralis. A few labeled cells appeared in and around the nucleus olivaris superior in the most caudal part of the metencephalic tegmentum. In the medulla oblongata, noradrenergic cells could be visualized at the level of the nucleus of the solitary tract and in a ventrolateral complex. Virtually all regions of the brain contained a rather dense innervation by NA- and DBH-immunopositive varicose fibers. Noradrenergic fibers and terminals were especially abundant in the ventral forebrain and in the periventricular hypothalamic regions. DBH-ir and NA-ir fibers, varicosities, and punctate structures could be observed in close association with immunonegative perikarya in several brain regions, more specifically in the ventral telencephalon, in the mid- and tuberal hypothalamic region, and in the dorsal rostral pons. Some perikarya in these brain areas were completely surrounded by noradrenergic structures that formed pericellular arrangements around the cells. The present study on the distribution of the noradrenergic system in the brain of the chicken combined with the results of a previous report on the distribution of L-Dopa and dopamine in the same species (L. Moons, J. van Gils, E. Ghijsels, and F. Vandesande, 1994, J. Comp. Neurol. 346:97-118) offers the opportunity to differentiate between the various catecholamines in the brain of this vertebrate. The results are discussed in relation to catecholaminergic systems previously reported in avian species and in the mammalian brain.

The septal complex of the telencephalon of the lizard Podarcis hispanica. I. Chemoarchitectonical organization

In this paper we study the septal complex architecture in the lizard Podarcis hispanica (Lacertidae). Histochemical and immunohistochemical techniques were used to define the distribution of zinc (Timm stain), acetyl cholinesterase (AChase), gamma-aminobutyric acid (GABA), tyrosine hydroxylase (TH), dopamine (DA), serotonin (5-HT), and two neuropeptides: leu-enkephalin (L-ENK) and substance P (SP). These reactions delineate a coherent map of nine septal nuclei that are named with a topographical nomenclature: anterior, lateral, ventromedial, medial, dorsolateral, ventrolateral, and dorsal septal nuclei, nucleus septalis impar, and nucleus of the posterior pallial commissure. The anterior septal nucleus is characterized by intense reaction for zinc and the presence of fibers immunoreactive for GABA, 5-HT, and L-ENK, which form pericellular nests. The lateral septal nucelus shows intense reaction for zinc, a high density of GABA-immunoreactive cells, and L-ENK-immunoreactive fibers forming basketlike figures around unstained somata. The ventromedial septal nucleus shows intense AChase reactivity, a dense network of 5-HT-immunoreactive fibers, and virtually no labeling for the other histochemical stains. The medial septal nucleus is defined by heavy reactivity for zinc, dense DA/TH and L-ENK innervations, and the presence of L-ENK-immunoreactive cells. The dorsolateral septal nucleus shows intense AChase staining in the neuropile and a dense network of fibers immunoreactive for 5-HT and DA/TH, but it shows low staining for zinc. The ventrolateral septal nucleus shows L-ENK-immunoreactive cells and a dense L-ENK innervation, but low reactivity for zinc. The dorsal septal nucleus, intermingled with the fimbrial fibers, shows a dense population of GABA-immunoreactive cells and terminals, but it is unreactive for zinc. Two subdivisions can be established in this dorsal septal nucleus: the dorsal part, intensely reactive for AChase and innervated by 5-HT fibers, and the central part, which shows L-ENK-immunoreactive neurons and fibers without reactivity for either AChase or 5-HT. The nucleus septalis impar, traversed by the fibers of the anterior pallial commissure (mildly reactive for zinc), shows reaction for AChase but low (if present) reactivity for the remaining markers. The nucleus of the posterior pallial commissure shows a generally low reactivity for the histochemical reactions employed. The distribution of these markers is similar to that found in other squamate reptiles and allows for a direct comparison with the septal formation of mammals. Such a comparison reinforces the view that the limbic system has undergone a conservative evolution within vertebrates.

Cerebrospinal fluid-contacting neurons in the paraventricular organ and in the spinal cord of the quail embryo: a fluorescence-histochemical study

Although the cerebrospinal fluid-contacting neurons of the avian paraventricular organ exhibit considerable amounts of catecholamines, they show no tyrosine hydroxylase immunoreactivity. In the quail embryo, the development of these neurons has been studied using the paraformaldeyde-glutaraldeyde method for the fluorescence-histochemical localization of catecholamines. The timing of the appearance of catecholamine fluorescence in cerebrospinal fluid-contacting neurons and that in catecholamine-containing neurons of the brainstem have been compared. The first neurons displaying catecholamine fluorescence are found within the locus coeruleus and the nucleus subcoeruleus ventralis on the 5.5th day of incubation. Catecholaminergic neuronal groups of the medulla and mesencephalon can be identified by embryonic day 7, and fluorescent cerebrospinal fluid-contacting neurons of the hypothalamic paraventricular organ can be first recognized at the 8th day of incubation. If the catecholamine content of cerebrospinal fluid-contacting neurons that lack tyrosine hydroxylase depends upon an uptake mechanism, it may be significant that, in fluorescence-histochemical preparations, these neurons can be identified 1-3 days later than those in which catecholamines are synthesized and from which catecholamines are released at an earlier developmental stage. Moreover, cerebrospinal fluid-contacting neurons that have previously been shown to be tyrosine-hydroxylase immunoreactive, and that lie at the spinal-medullary junction display a different developmental pattern. By fluorescence histochemistry, they can be detected only by embryonic day 10.5. The chemical, developmental and topographical differences suggest that the catecholamine-containing cerebrospinal fluid-contacting elements of the paraventricular organ and those of the spinal cord represent two different subsets of cerebrospinal fluid-contacting neurons whose respective functional roles remain to be investigated.

Noradrenergic and adrenergic systems in the brain of the urodele amphibian, Pleurodeles waltlii, as revealed by immunohistochemical methods

The distribution of noradrenaline and adrenaline in the brain of the urodele amphibian Pleurodeles waltlii has been studied with antibodies raised against noradrenaline and the enzymes dopamine-beta-hydroxylase and phenylethanolamine-N-methyltransferase. Noradrenaline-containing cell bodies were found in the anterior preoptic area, the hypothalamic nucleus of the periventricular organ, the locus coeruleus and in the solitary tract/area postrema complex at the level of the obex. Noradrenergic fibers are widely distributed throughout the brain innervating particularly the ventrolateral forebrain, the medial amygdala, the lateral part of the posterior tubercle, the parabrachial region and the ventrolateral rhombencephalic tegmentum. Putative adrenergic cell bodies were found immediately rostral to the obex, ventral to the solitary tract. Whereas the cell bodies and their dendrites were Golgi-like stained, axons were more difficult to trace. Nevertheless, some weakly immunoreactive fibers could be traced to the basal forebrain. A comparison of these results with data previously obtained in anurans reveals not only several general features, but also some remarkable species differences.

Efferent connections of the lateral cortex of the lizard Gekko gecko: evidence for separate origins of medial and lateral pathways from the lateral cortex to the hypothalamus

The lateral cortex of the lizard Gekko gecko is composed of three parts: a dorsal and ventral part located rostrally and a posterior part located caudally. In order to obtain detailed information about the efferent connections of these lateral cortex subdivisions, iontophoretic injections of the anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran were made in the various parts. The main projection from the dorsal part terminates in the caudal part of the medial cortex. Other cortical projections were noted to the ipsi- and contralateral lateral cortex, the large-celled part of the medial cortex, and the dorsal cortex. Additional fibers were found bilaterally in the anterior olfactory nucleus and the external amygdaloid nucleus. The ventral part of the lateral cortex projects mainly to the ipsilateral, posterior part of the dorsal ventricular ridge and the external amygdaloid nucleus. Minor contralateral projections to these nuclei were also found. Other projections were observed to travel to the caudal part of the medial cortex, to the nucleus sphericus, and bilaterally to the lateral cortex and the anterior olfactory nucleus. The posterior part of the lateral cortex has similar efferent connections as the dorsal part and should be regarded as the caudal continuation of the dorsal part. Because previous studies have shown that the medial cortex and the amygdaloid complex project to different hypothalamic areas, we conclude that the dorsal and ventral parts of the lateral cortex transmit olfactory information to separate hypothalamic areas that are probably involved with different types of behavior.

NADPH diaphorase-positive neurons in the lizard hippocampus: a distinct subpopulation of GABAergic interneurons

We analyzed the distribution and light-microscopic features of the NADPH diaphorase-containing structures in the lizard hippocampus, likely to correspond to nitric oxide synthase-containing cells and fibers, and thus likely to release nitric oxide. We also studied co-localization of NADPH diaphorase with the neurotransmitter GABA, the calcium-binding protein parvalbumin, and the neuropeptide somatostatin, in order to examine whether putative nitric oxide-synthesizing neurons represent a different subpopulation of GABA cells, on which the authors recently reported in lizards. We also studied co-localization of NADPH diaphorase with parvalbumin or somatostatin in mice to ascertain whether the characteristics of this population in reptiles parallel the situation in mammals. Most of the positive NADPH diaphorase neurons were stained in a Golgi-like manner and were in the plexiform layers of the lizard hippocampus with morphologies ranging from bipolar to multipolar. Co-localization with GABA was 100%, and NADPH diaphorase-positive neurons in the lizard hippocampus did not contain parvalbumin or somatostatin. The results indicate that putative nitric oxide-synthesizing neurons represent a distinct subpopulation of GABA interneurons in the lizard hippocampus. Two different types of fibers were described in the plexiform layers: one type bearing thick varicosities, and the other thinner ones. We discuss the possibility that at least part of the positive fibers arise from a hypothalamic aminergic nucleus contacting the third ventricle, the periventricular hypothalamic organ. Most radial glia were stained almost completely and formed typical end-feet both at the pia and around capillaries. The results of this study confirm that the capacity for synthesizing nitric oxide is linked to a determined set of neuronal markers depending on the specific brain region, and they provide new resemblances between hippocampal regions in different classes of vertebrates.

Light and electron microscopic characterization of cholinergic and dopaminergic structures in the striatal complex and the dorsal ventricular ridge of the lizard Gekko gecko

The purpose of the present study was to visualize the morphological substrate underlying acetylcholine-dopamine interactions in the striatal complex of the lizard Gekko gecko and to compare the results with data obtained by others in mammals. The results are also discussed in the light of data obtained previously by us on neurochemical aspects of acetylcholine-dopamine interactions in Gekko and in rats. The study is part of a large research program in which the cholinergic and dopaminergic elements of the striatum of rats and reptiles are studied at morphological and neurochemical levels. We employed light microscopic immunocytochemistry, using single-label staining with antibodies against choline acetyltransferase (ChAT) and dopamine (DA) and double-staining with antibodies against ChAT and tyrosine hydroxylase (TH). A detailed analysis of ultrastructural characteristics of ChAT- and DA-immunolabeled striatal tissue was undertaken. The morphology and synaptic relations of the ChAT-immunopositive neurons in the basal forebrain of the lizard Gekko gecko are very similar to those of the cholinergic cells in the striatum of mammals. Probably, the cholinergic cells are in both mammals and reptiles interneurons that receive inputs of intrinsic or extrinsic origin and project upon output neurons. The location of ChAT-immunopositive somata outside the patches of high TH- or DA-immunoreactivity is at odds with the situation in the striatum of mammals and suggests the possibility of axoaxonal or axodendritic contacts at the level of these patches. We found no essential differences between the synaptic relations of the dopaminergic fibers in the striatal complex of Gekko and the conditions described for rats. In conclusion, we found little evidence for the presence of synaptic interaction between the cholinergic and dopaminergic systems in the striatum of this reptile. The possibility of nonsynaptic interaction, however, remains open.

Tyrosine hydroxylase-immunoreactive cell groups in the brain of the teleost fish Gnathonemus petersii

Different antibodies against tyrosine hydroxylase (TH) were used to obtain detailed information about the distribution, morphology and chemical differentiation of catecholaminergic neurons in the highly differentiated brain of the electric mormyrid fish Gnathonemus petersii. The results show that the distribution of catecholaminergic neurons is much more widespread than was previously thought on the basis of dopamine and noradrenaline immunohistochemistry. Tyrosine hydroxylase-immunoreactive neurons were observed not only in clearly dopaminergic regions (the suprachiasmatic nucleus, the magnocellular hypothalamic nucleus and the area postrema) and noradrenergic cell groups (the locus coeruleus and inferior reticular cell group), but also in regions that do not, or only fragmentarily, display dopamine or noradrenaline immunoreactivity, including the ventral and intermediate telencephalon, the anterior and posterior preoptic cell group, the ventromedial thalamus, the pretectal region and the nucleus of the solitary tract, suggesting that they either represent depleted dopaminergic cell groups or L-dihydroxy phenylalanine-producing nuclei. Most TH-immunoreactive neurons are rather small (< 10 microns) and have only a few slender processes, but neurons in the magnocellular hypothalamic nucleus and the inferior reticular formation are multipolar and larger (10-20 microns), while those of the locus coeruleus are even more than 20 microns in diameter. The hypothalamic paraventricular organ, which is strongly dopamine and noradrenaline immunoreactive, displays minimal TH immunoreactivity, suggesting that its cerebrospinal fluid-contacting neurons do not synthesize catecholamines, but acquire them from external sources. Comparison with other teleosts shows that the catecholaminergic system in the brain of Gnathonemus is similarly organized as in Carassius, Gasterosteus, Anguilla and Aperonotus, with some variations that may partly be due to technical reasons, and partly reflect true species differences. However, TH-immunoreactive neurons in the midbrain tegmentum were not observed, confirming previous conclusions that a major difference between teleosts and mammals concerns the absence of dopaminergic midbrain groups and correlated mesencephalo-telencephalic projections in teleosts.

Noradrenaline in the brain of the South African clawed frog Xenopus laevis: a study with antibodies against noradrenaline and dopamine-beta-hydroxylase

To obtain insight into the noradrenergic system of amphibians, the distribution of noradrenaline was studied immunohistochemically with antibodies against noradrenaline (NA) and dopamine-beta-hydroxylase (DBH) in the brain of the South African clawed frog Xenopus laevis. Noradrenaline-containing cell bodies are found in the hypothalamic periventricular organ, the isthmic region, and in an area ventral and medial to the solitary tract. Noradrenaline-immunoreactive (NAi) fibers are widely, but not uniformly, distributed throughout the brain and spinal cord. In the telencephalon, dense plexuses of NAi fibers are present dorsomedial to the nucleus accumbens, in the nucleus of the diagonal band, the dorsolateral part of the striatum, the medial amygdala, and in an area that encompasses the lateral forebrain bundle. In the diencephalon, dense plexuses are found ventrolateral to the periventricular organ, in the posterior tubercle, and in the intermediate lobe of the hypophysis. Compared to the forebrain, the brainstem and spinal cord are less densely innervated by NAi fibers. The distribution of DBHi cell bodies and fibers resembles the pattern revealed with the NA antibodies. An exception is formed by the liquor contacting cells of the hypothalamic periventricular organ, which are immunonegative for the DBH antiserum. It is suggested that these cells accumulate rather than metabolize catecholamines. The present study combined with the results of a previous report in Xenopus on the distribution of dopamine (González, Tuinhof, Smeets, '93, Anat. Embryol. 187:193-201) offers the opportunity to differentiate between the two catecholamines. For example, it is now shown that both dopaminergic and noradrenergic fibers innervate the intermediate lobe of the hypophysis and that, therefore, both catecholamines are likely involved in background adaptation.

Distribution of noradrenaline-immunoreactivity in the brain of the mormyrid teleost Gnathonemus petersii

The distribution of noradrenaline-immunoreactivity in the brain of the mormyrid fish Gnathonemus petersii was studied in order to evaluate the noradrenergic innervation of a number of specialized mormyrid brain regions, including electrosensory centers and a gigantocerebellum. Noradrenaline-immunoreactive (NAi) neurons occur in the hypothalamic paraventricular organ (PVO), the locus coeruleus, and the caudal rhombencephalon. In the PVO, NAi cerebrospinal fluid (CSF)-contacting neurons are located in the same regions where dopamine- and serotonin-containing CSF contacting neurons occur. The locus coeruleus consists, on each side, of at least 30 rather large NAi neurons with ventrolaterally directed dendrites and dorsolaterally coursing axons. In the caudal rhombencephalon, NAi neurons are located in the transition region between the ventromedial motor zone and the dorsolateral sensory zone. The density of NAi fibers is very high in the efferent tract of the locus coeruleus, the medial forebrain bundle, and two telencephalic, one preoptic, and one rhombencephalic subependymal axonal plexus. A marked NAi innervation is present in the dorsomedial and ventral telencephalon, the preoptic region, periventricular hypothalamic and thalamic regions, the midbrain tectum, cerebellar granular layers, the electrosensory lateral line lobe, the rhombencephalic transition region between the sensory and motor zones, and the area postrema. Other regions are more sparsely innervated by NAi fibers, but regions completely devoid of NAi fibers were not observed. Interestingly, NAi fibers form large club endings in some subdivisions of the precerebellar nucleus lateralis valvulae, and parallel fibers in the cerebellar granular layer. Comparison with the distribution of NAi or dopamine-beta-hydroxylase-immunoreactivity in other species shows that all teleosts studied to date have noradrenergic cells in the locus coeruleus and the caudal rhombencephalon. However, NAi CSF-contacting PVO cells have been described only in the teleost Gnathonemus petersii and the lizard Gekko gecko (Smeets and Steinbusch: J. Comp. Neurol. 285:453-466, '89). It is possible that they might pick up catecholamines as well as serotonin from the CSF, into which monoamines might be released by telencephalic and preoptic subependymal axonal plexuses.

Catecholaminergic cells and fibers in the brain of the lizard Anolis carolinensis identified by traditional as well as whole-mount immunohistochemistry

Using traditional as well as whole-mount immunohistochemistry, we described the location of tyrosine hydroxylase- and dopamine beta hydroxylase-positive cells and fibers in the brain of the lizard Anolis carolinensis. Major catecholaminergic cell groups were in the ependyma in certain ventricular regions, along the periventricular floor in the preoptic region, within the anterior hypothalamic and lateral hypothalamic areas, and in the mesencephalic tegmental region, locus coeruleus, nucleus of the solitary tract, vagal motor nucleus, and rhombencephalic reticular formation. Major catecholaminergic fibers, tracts and varicosities included tuberohypophysial, mesolimbic, nigrostriatal, isthmocortical, medullohypothalamic, and coeruleospinal systems. Although the catecholaminergic systems in A. carolinensis are similar to those in the brains of other lizards studied, there are a few species differences. Our information about A. carolinensis will be used to help localize the hypothalamic asymmetry in catecholamine metabolism previously described in this lizard.

Asymmetric metabolism of hypothalamic catecholamines alternates with side of ovulation in a lizard (Anolis carolinensis)

We determined levels of monoamines and their metabolites in 2 hypothalami dissected from the right and left hemibrains of 15 females during the right-left alternating ovulatory cycle of Anolis carolinensis. Tissue contents of the following were measured using HPLC and electrochemical (coulometric) detection: dopamine (DA) and its metabolite 2,4-dihydroxyphenylacetic acid (DOPAC), norepinephrine (NE) and its metabolites 3-methoxy-4-hydroxyphenylglycol (MHPG) and 3,4-dihydroxyphenylglycol (DHPG), and serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). An asymmetry ratio (AR) was determined by subtracting hypothalamic content (pM/mg) on the larger ovary (LO) side from that on the smaller ovary (SO) side, divided by the sum of the 2 sides (AR = SO - LO/SO+LO). The Ar of MHPG and DHPG both decreased as the largest follicle in the LO grew during the cycle, from greater than 0 (content higher on the SO side) at the beginning of the cycle to less than 0 (content higher on the LO side). The average content of MHPG in the 2 sides significantly increased during the cycle. There were no significant asymmetric changes in hypothalamic DA or DOPAC. The average content of DA increased during the cycle, whereas the content of DOPAC, as well as DOPAC/DA, did not change. The average content of 5-HT increased, and the average metabolite ratio of 5-HIAA/5-HT decreased during the cycle without significant asymmetries. The metabolite ratios of NE and DA, but not 5-HT, were asymmetric on the same side in a given female.(ABSTRACT TRUNCATED AT 250 WORDS)

An intriguing pitfall in chemical neuroanatomy: specific populations of unspecifically immunoreactive neurons in the brain of the mormyrid fish Gnathonemus petersii

The present paper describes the location, morphology, ultrastructure and immunocytochemical properties of neurons in the brain of the mormyrid fish Gnathonemus petersii, that appear to be unspecifically immunoreactive to a number of secondary or tertiary antibodies used in immunohistochemical procedures, including rabbit-anti-mouse immunoglobulins (IGGs), rabbit peroxidase-anti-peroxidase IGGs, and rabbit-anti-sheep or sheep peroxidase-anti-peroxidase IGGs. Unspecifically immunoreactive (UI) cells have typically neuronal morphological and ultrastructural characteristics, and occur at four specific locations in the mormyrid brain. A small rhombencephalic group is located rostrolateral to the efferent octaval nucleus, between the fasciculus longitudinalis medialis and the decussation of the lateral lemniscus. A mesencephalic cluster of cells is located in the dorsal midbrain tegmentum against the tractus telencephalo-mesencephalicus. In addition, dispersed UI neurons were observed in the nucleus lateralis of the torus semicircularis and in the preoptic region above the optic chiasm. Remarkably, UI cells are clearly present in a substantial number of brains investigated, but not detectable in others. The present findings point to a curious pitfall in chemical neuroanatomy, the functional significance of which is unknown at present. In several previous studies using the brain of G. petersii, UI cells were abusively included in the description of monoaminergic cell groups. Similar cells have until now not been reported in other vertebrate brains.

Ultrastructural localization of tyrosine hydroxylase immunoreactivity in the rat diagonal band of Broca

The present study sought to establish the cellular basis for the catecholaminergic (i.e., noradrenaline and dopamine) modulation of neurons in the horizontal limb of the diagonal band of Broca (HDB) in the rat brain. The light and electron microscopic localization of antigenic sites for a polyclonal antibody directed against the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), were examined in the HDB using a double-bridged, peroxidase-antiperoxidase method. By light microscopy, numerous punctate, varicose processes with intense TH-immunoreactivity (TH-I) were detected in the HDB. Additionally, a few small, bipolar, or multipolar TH-immunoreactive neurons were observed. Ultrastructural analysis of single sections revealed that the TH-labeled processes were axons and axon terminals. Axons (n = 134) with TH-I were primarily unmyelinated. Terminals with TH-I (n = 169) were 0.3-1.4 microns in diameter and contained many small, clear vesicles and 0-5 larger dense-core vesicles. The types of associations (i.e., asymmetric synapses, symmetric synapses, and appositions which lacked a membrane specialization in the plane of section analyzed) formed by the TH-labeled terminals were quantitatively evaluated. The TH-labeled terminals: (1) formed associations with unlabeled perikarya and dendrites (134 out of 169), (2) were closely apposed without glial intervention to unlabeled and TH-labeled terminals (11 out of 169), or (3) had no neuronal associations in the plane of section analyzed (24 out of 169). The relatively rare (n = 4) associations with unlabeled perikarya were mostly characterized by symmetric synaptic specializations. The majority of the TH-labeled terminals were associated with the shafts of small dendrites (66% of 134). Moreover, most of the associations on dendrites and dendritic spines were further characterized by asymmetric synaptic specializations; however, many were also appositions without any apparent glial intervention in the plane of section analyzed. Additionally, the TH-labeled terminals were often associated with only one dendrite, which, in the same plane of section, was sparsely innervated by other terminals. Astrocytic processes usually surrounded the portions of the terminals and dendrites not involved in the region of association. The TH-immunoreactive perikarya were small (7-12 microns), ovoid, and had an indented nucleus with some heterochromatin. Their scant cytoplasm contained mitochondria, Golgi complexes, and endoplasmic reticulum. A few immunoreactive dendrites, presumably derived from the local neurons, were also detected. Both TH-immunoreactive perikarya and dendrites were associated primarily with unlabeled terminals, although a few terminals with TH-I also contacted them.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of neuropeptide Y (NPY) in the cerebral cortex of the lizards Psammodromus algirus and podarcis hispanica: co-localization of NPY, somatostatin, and GABA

The aim of the present study was to analyze the distribution and characteristics of NPY immunoreactive structures in the cerebral cortex of lizards and to investigate the degree of co-existence of this neuropeptide with somatostatin and GABA. The immunoperoxidase method was applied to vibratome sections as well as to semithin sections. NPY neurons are multipolar or fusiform and were unevenly distributed throughout the brain cortex. Within the medial, dorsomedial and dorsal cortices, most NPY perikarya were located in the plexiform layers, especially in the deep one. This suggests that these cells could be regarded as interneurons. In the lateral cortex, NPY neurons were found throughout all layers. The dorsomedial cortex displayed the highest NPY cell density. Here, neuronal perikarya projected many immunoreactive processes toward two distinct zones: the deep plexiform layer of the medial cortex and the superpositio medialis. The NPY neurons of the dorsomedial cortex differed from the other NPY cortical immunoreactive cells in that the latter displayed very few immunoreactive processes. A high degree of co-existence among NPY, somatostatin, and GABA (approx. 80%) was found. This co-existence rate is very similar to that reported in mammals and suggests that co-localization is a phylogenetically ancient phenomenon.

Localization of serotonin, tyrosine hydroxylase, and leu-enkephalin immunoreactive cells in the brainstem of the horn shark, Heterodontus francisci

In previous studies on reptiles and elasmobranchs, we determined that some reticular groups are either absent or may be displaced compared to their locations in mammals. For example, nucleus raphe dorsalis, the largest serotoninergic cell group in mammals, is not present in rays, skates, or guitarfish. In the present study, we chose heterodontid sharks, a sister group to these batoids, for an out-group comparison of this and other characters. We identified cells in the brainstem of Heterodontus francisci by use of antibodies against tyrosine hydroxylase, serotonin, or leu-enkephalin and compared the distribution of these nuclei to descriptions in mammals and other elasmobranchs. The majority of tyrosine hydroxylase-positive cells were found in the midbrain tegmentum (A8-A10) and the hypothalamus. In addition, putative A1, A2, A5, A7 (noradrenergic) groups were found in the metencephalon and myelencephalon. Serotonin-positive cells were found in raphe nuclei and scattered lateral to the raphe. We identified probable homologues to raphe pallidus, raphe obscurus, raphe magnus, and raphe centralis superior (B8) cell groups, which have been described in mammals. A cluster of cells dorsomedial to the medial longitudinal fasciculus was identified as raphe dorsalis. The distributions of leu-enkephalin and serotonin immunoreactive cells were similar to each other, but the tyrosine-hydroxylase immunoreactive cells rarely intermingle with the former two immunoreactive cell types. Other reticular groups that contained both serotonin- and leu-enkephalin-positive cells included reticularis (r.) ventralis, r. magnocellularis, r. paragigantocellularis lateralis, r. pontis caudalis, and r. pontis oralis medialis and lateralis. Thus, this shark contains many of the major brainstem raphe and catecholaminergic cell groups described for rats, but the relative distribution of the immunopositive cell groups differs in mammals and cartilaginous fish.

Afferent and efferent connections of the olfactory bulbs in the lizard Podarcis hispanica

The connections of the olfactory bulbs of Podarcis hispanica were studied by tract-tracing of injected horseradish peroxidase. Restricted injections into the main olfactory bulb (MOB) resulted in bilateral terminallike labeling in the medial part of the anterior olfactory nucleus (AON) and in the rostral septum, lateral cortex, nucleus of the lateral olfactory tract, and ventrolateral amygdaloid nucleus. Bilateral retrograde labeling was found in the rostral lateral cortex and in the medial and dorsolateral AON. Ipsilaterally the dorsal cortex, nucleus of the diagonal band, lateral preoptic area, and dorsolateral amygdala showed labeled cell bodies. Retrogradely labeled cells were also found in the midbrain raphe nucleus. Results from injections into the rostral lateral cortex and lateral olfactory tract indicate that the mitral cells are the origin of the centripetal projections of the MOB. Injections in the accessory olfactory bulb (AOB) produced ipsilateral terminallike labeling of the ventral AON, bed nucleus of the accessory olfactory tract, central and ventromedial amygdaloid nuclei, medial part of the bed nucleus of the stria terminalis, and nucleus sphericus. Retrograde labeling of neurons was observed ipsilaterally in the bed nucleus of the accessory olfactory tract and stria terminalis, in the central amygdaloid nucleus, dorsal cortex, and nucleus of the diagonal band. Bilateral labeling of somata was found in the ventral AON, the nucleus sphericus (hilus), and in the mesencephalic raphe nucleus and locus coeruleus. Injections into the dorsal amygdala showed that the mitral neurons are the cells of origin of the AOB centripetal projections. Reciprocal connections are present between AOB and MOB. To our knowledge, this is the first study to address the afferent connections of the olfactory bulbs in a reptile. On the basis of the available data, a discussion is provided of the similarities and differences between the reptilian and mammalian olfactory systems, as well as of the possible functional role of the main olfactory connections in reptiles.

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

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

Organization of tyrosine hydroxylase-immunoreactive neurons in the di- and mesencephalon of the American bullfrog (Rana catesbeiana) during metamorphosis

We examined the immunocytochemical distribution of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis, in the di- and mesencephalon of developing bullfrog tadpoles. Special attention was given to catecholaminergic innervation of the median eminence and pituitary. In premetamorphic tadpoles, tyrosine hydroxylase-immunoreactive neurons were visualized in the suprachiasmatic and infundibular hypothalamus, the ventral thalamus, and midbrain tegmentum by Taylor-Kollros stage V. The number of labeled neurons in all these areas increased as metamorphosis progressed. By mid-prometamorphosis, labeled neurons appeared in the preoptic recess organ as well as in the posterior thalamic nucleus. The majority of cells in the preoptic recess organ, as well as occasional neurons in the suprachiasmatic nucleus, exhibited labeled processes which projected through the ependymal lining of the preoptic recess to contact cerebrospinal fluid. The modified CSF-contacting neurons of the nucleus of the periventricular organ were devoid of specific staining. By late prometamorphosis, labeled fibers from the suprachiasmatic nucleus were observed projecting caudally to enter the hypothalamo-hypophysial-tract en route to innervating the median eminence and pituitary. Labeled fibers arising from the dorsal infundibular nucleus projected ventrolaterally to contribute to catecholaminergic innervation of the median eminence and pituitary. Immunoperoxidase staining of tyrosine hydroxylase-immunoreactive fibers and terminal arborizations in the median eminence were restricted to non-ependymal layers, while labeled fibers in the pituitary were observed in the pars intermedia and pars nervosa. Staining of tyrosine hydroxylase-immunoreactive fibers in the median eminence and pituitary was sparse or absent in premetamorphic tadpoles, but became increasingly more intense as metamorphosis progressed.

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

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