Ultrastructural and immunohistochemical study of the telencephalo-habenulo-interpeduncular connections of the goldfish

Brain regions controlling courtship behavior in the bluehead wrasse

Bluehead wrasses (Thalassoma bifasciatum) follow a socially controlled mechanism of sex determination. A socially dominant initial-phase (IP) female is able to transform into a new terminal-phase (TP) male if the resident TP male is no longer present. TP males display an elaborate array of courtship behaviors, including both color changes and motor behaviors. Little is known concerning the neural circuits that control male-typical courtship behaviors. This study used glutamate iontophoresis to identify regions that may be involved in courtship. Stimulation of the following brain regions elicited diverse types of color change responses, many of which appear similar to courtship color changes: the ventral telencephalon (supracommissural nucleus of the ventral telencephalon [Vs], lateral nucleus of the ventral telencephalon [Vl], ventral nucleus of the ventral telencephalon [Vv], and dorsal nucleus of the ventral telencephalon [Vd]), parts of the preoptic area (NPOmg and NPOpc), entopeduncular nucleus, habenular nucleus, and pretectal nuclei (PSi and PSm). Stimulation of two regions in the posterior thalamus (central posterior thalamic [CP] and dorsal posterior thalamic [DP]) caused movements of the pectoral fins that are similar to courtship fluttering and vibrations. Furthermore, these responses were elicited in female IP fish, indicating that circuits for sexual behaviors typical of TP males exist in females. Immunohistochemistry results revealed regions that are more active in fish that are not courting: interpeduncular nucleus, red nucleus, and ventrolateral thalamus (VL). Taken together, we propose that the telencephalic-habenular-interpeduncular pathway plays an important role in controlling and regulating courtship behaviors in TP males; in this model, in response to telencephalic input, the habenular nucleus inhibits the interpeduncular nucleus, thereby dis-inhibiting forebrain regions and promoting the expression of courtship behaviors.

Cloning and expression of tachykinins and their association with kisspeptins in the brains of zebrafish

The tachykinins are a family of neuropeptides, including substance P (SP), neurokinin A (NKA), and neurokinin B (NKB), that are encoded by the tac1 (SP and NKA) or tac2/3 (NKB) genes. Tachykinins are widely distributed in the central nervous system and have roles as neurotransmitters and/or neuromodulators. Recent studies in mammals have demonstrated the coexpression of NKB and kisspeptin and their comodulatory roles over the control of reproduction. We have recently identified two kisspeptin-encoding genes, kiss1 and kiss2, in teleosts. However, such relationship between tachykinins and kisspeptins has not been demonstrated in non-mammalian species. To determine the involvement of tachykinins in the reproduction in teleosts, we identified tac1 and two tac2 (tac2a and tac2b) sequences in the zebrafish genome using in silico data mining. Zebrafish tac1 encodes SP and NKA, whereas the tac2 sequences encode NKB and an additional peptide homologous to NKB (NKB-related peptide). Digoxigenin in situ hybridization in the brain of zebrafish showed tac1 mRNA-containing cells in the olfactory bulb, telencephalon, preoptic region, hypothalamus, mesencephalon, and rhombencephalon. The zebrafish tac2a mRNA-containing cells were observed in the preoptic region, habenula, and hypothalamus, whereas the tac2b mRNA-containing cells were predominantly observed in the dorsal telencephalic area. Furthermore, we examined the coexpression of tachykinins and two kisspeptin genes in the brain of zebrafish. Dual fluorescent in situ hybridization showed no coexpression of tachykinins mRNA with kisspeptins mRNA in hypothalamic nuclei or the habenula. These results suggest the presence of independent pathways for kisspeptins and NKB neurons in the brain of zebrafish.

Selective asymmetry in a conserved forebrain to midbrain projection

How the left and right sides of the brain acquire anatomical and functional specializations is not well understood. The zebrafish has proven to be a useful model to explore the genetic basis of neuroanatomical asymmetry in the developing forebrain. The dorsal diencephalon or epithalamus consists of the asymmetric pineal complex and adjacent paired nuclei, the left and right medial habenulae, which in zebrafish larvae, exhibit differences in their size, neuropil density and patterns of gene expression. In all vertebrates, axons from the medial habenular nuclei project within a prominent fiber bundle, the fasciculus retroflexus, to a shared midbrain target, the interpeduncular nucleus of the ventral tegmentum. However, in zebrafish, projections from the left habenula innervate the dorsal and ventral regions of the target nucleus, whereas right habenular efferents project only to the ventral region. A similar dorsoventral difference in habenular connectivity is found in another teleost species, the highly derived southern flounder, Paralichthys lethostima. In this flatfish, directional asymmetry of the habenular projection appears to be independent of the left-right morphology and orientation that an individual adopts post-metamorphosis. Comparative anterograde labeling of the brains of salamanders, frogs and mice reveals that axons emanating from the left and right medial habenulae do not project to different domains, but rather, they traverse the target nucleus in a complementary mirror image pattern. Thus, although the habenulo-interpeduncular conduction system is highly conserved in the vertebrate brain, the stereotypic dorsoventral topography of left-right connections appears to be a feature that is specific to teleosts.

Topography and connections of the telencephalon in a chondrostean,Acipenser baeri: An experimental study

Sturgeons belong to an ancient group of the extant actinopterygian fishes. Accordingly, the study of their brain connections is important to understand brain evolution in the line leading to teleosts. We examined the topography and connections of the various telencephalic regions of the Siberian sturgeon (Acipenser baeri). The telencephalic regions were characterized on the basis of acetylcholinesterase histochemistry and calbindin-D28k and calretinin immunohistochemistry. The telencephalic connections were investigated by using the fluorescent dye DiI (1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate) in fixed brains. Application of DiI to different areas of the pallial (dorsal) regions of the telencephalic lobes showed that they have mostly intratelencephalic connections. A posterior pallial region is characterized by its similar hodology to that of the posterior zone of the teleosts dorsal telencephalon and those described in other ancient groups. Extratelencephalic connections of the pallium are scarce, although a few afferent and efferent connections with the diencephalon, mesencephalon, and rostral rhombencephalon were observed. DiI application to subpallial regions showed both intratelencephalic connections and connections with different brain regions. Afferents to the subpallium originate from the olfactory bulbs, preoptic area, thalamus, posterior tuberculum, hypothalamus, secondary gustatory nucleus, and raphe nuclei. Some of these connections are quite similar to those described for other vertebrates.

Directional asymmetry of the zebrafish epithalamus guides dorsoventral innervation of the midbrain target

The zebrafish epithalamus, consisting of the pineal complex and flanking dorsal habenular nuclei, provides a valuable model for exploring how left-right differences could arise in the vertebrate brain. The parapineal lies to the left of the pineal and the left habenula is larger, has expanded dense neuropil, and distinct patterns of gene expression from the right habenula. Under the influence of Nodal signaling, positioning of the parapineal sets the direction of habenular asymmetry and thereby determines the left-right origin of habenular projections onto the midbrain target, the interpeduncular nucleus (IPN). In zebrafish with parapineal reversal, neurons from the left habenula project to a more limited ventral IPN region where right habenular axons would normally project. Conversely, efferents from the right habenula adopt a more extensive dorsoventral IPN projection pattern typical of left habenular neurons. Three members of the leftover-related KCTD (potassium channel tetramerization domain containing) gene family are expressed differently by the left and right habenula, in patterns that define asymmetric subnuclei. Molecular asymmetry extends to protein levels in habenular efferents, providing additional evidence that left and right axons terminate within different dorsoventral regions of the midbrain target. Laser-mediated ablation of the parapineal disrupts habenular asymmetry and consequently alters the dorsoventral distribution of innervating axons. The results demonstrate that laterality of the dorsal forebrain influences the formation of midbrain connections and their molecular properties.

Cholinergic elements in the zebrafish central nervous system: Histochemical and immunohistochemical analysis

Recently, the zebrafish has been extensively used for studying the development of the central nervous system (CNS). However, the zebrafish CNS has been poorly analyzed in the adult. The cholinergic/cholinoceptive system of the zebrafish CNS was analyzed by using choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry in the brain, retina, and spinal cord. AChE labeling was more abundant and more widely distributed than ChAT immunoreactivity. In the telencephalon, ChAT-immunoreactive (ChAT-ir) cells were absent, whereas AChE-positive neurons were observed in both the olfactory bulb and the telencephalic hemispheres. The diencephalon was the region with the lowest density of AChE-positive cells, mainly located in the pretectum, whereas ChAT-ir cells were exclusively located in the preoptic region. ChAT-ir cells were restricted to the periventricular stratum of the optic tectum, but AChE-positive neurons were observed throughout the whole extension of the lamination except in the marginal stratum. Although ChAT immunoreactivity was restricted to the rostral tegmental, oculomotor, and trochlear nuclei within the mesencephalic tegmentum, a widespread distribution of AChE reactivity was observed in this region. The isthmic region showed abundant AChE-positive and ChAT-ir cells in the isthmic, secondary gustatory and superior reticular nucleus and in the nucleus lateralis valvulae. ChAT immunoreactivity was absent in the cerebellum, although AChE staining was observed in Purkinje and granule cells. The medulla oblongata showed a widespread distribution of AChE-positive cells in all main subdivisions, including the octavolateral area, reticular formation, and motor nuclei of the cranial nerves. ChAT-ir elements in this area were restricted to the descending octaval nucleus, the octaval efferent nucleus and the motor nuclei of the cranial nerves. Additionally, spinal cord motoneurons appeared positive to both markers. Substantial differences in the ChAT and AChE distribution between zebrafish and other fish species were observed, which could be important because zebrafish is widely used as a genetic or developmental animal model.

The adult central nervous cholinergic system of a neurogenetic model animal, the zebrafish Danio rerio

The central nervous cholinergic system of the zebrafish (Danio rerio), a model animal for neurogenetics, is documented here using immunohistochemical methods for visualizing choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme. Neuronal cell bodies containing ChAT are present in the telencephalon (lateral nucleus of ventral telencephalic area), preoptic region (anterior/posterior parvocellular and magnocellular preoptic nuclei), diencephalon (habenula, dorsal thalamus, posterior tuberculum), mesencephalon (Edinger-Westphal (EW) nucleus, oculomotor nerve nucleus, rostral tegmental nucleus, tectal type XIV neurons), isthmic region (nucleus lateralis valvulae, secondary gustatory-viscerosensory nucleus, nucleus isthmi (NI), perilemniscal nucleus, superior reticular nucleus (SRN)) and rhombencephalon (trochlear, trigeminal, abducens, facial, glossopharyngeal-vagal motor nerve nuclei, rostral and caudal populations of octavolateralis efferent neurons). In addition, some ChAT positive neurons are present in the rhombencephalic reticular formation, the central gray, and in cells accompanying the descending trigeminal tract. Obvious ChAT positive terminal fields are present in the supracommissural nucleus of area ventralis telencephali and the medial zone of area dorsalis telencephali, parvocellular superficial pretectal nucleus, torus semicircularis, medial octavolateralis nucleus, facial, glossopharyngeal, and vagal lobes, and in the inferior lobe (around the periventricular nucleus of the lateral recess and in the diffuse nucleus). The identification of all central nervous cholinergic systems provided here in this model system is pivotal for future detailed studies of their development and maintenance, e.g., with regard to the zebrafish ventral telencephalic and isthmic superior reticular neuronal populations, likely representing the homologues of at least part of the cholinergic basal forebrain and pedunculopontine/laterodorsal tegmental ascending activating systems of mammals, respectively.

A novel monoclonal antibody recognizes a previously unknown subdivision of the habenulo-interpeduncular system in zebrafish

The habenulo-interpeduncular system is an evolutionarily conserved structure found in the brain of almost all vertebrates. We prepared a monoclonal antibody (6G11) which very specifically recognizes only a part of this system. 6G11 is a monoclonal antibody prepared from a neuronal membrane protein in adult zebrafish brain. In western blot analysis of the adult zebrafish brain, the antibody recognized a 95 kDa protein, and the class of the antibody was determined to be IgM. The 6G11 antigen was not detected in zebrafish muscle, intestine, testis or ovary. A group of neurons stained by the 6G11 antibody was located in the caudomedial part of the zebrafish habenula. The 6G11-immunopositive neurons extended their axons into the fasciculus retroflexus (FR). One group of immunopositive neurons projected toward the interpeduncular nucleus (IPN), especially to the intermediate and the central subnucleus (type 1 neuron). The other group projected to the ventral midline at the level of the raphe nucleus; these axons passed ipsilaterally beside the IPN and converged in the ventral midline under the raphe nucleus (type 2 neuron). Both type 1 and type 2 fibers are relatively minor components of the FR. Little has previously been known about this topological pattern in any species. The 6G11 monoclonal antibody could be a useful tool for expanding knowledge of the habenulo-interpeduncular system.

Performance of appetitive or consummatory components of male sexual behavior is mediated by different brain areas: a 2-deoxyglucose autoradiographic study

The in vivo autoradiographic deoxyglucose method was used to identify the functional brain circuits that are involved in the performance of appetitive and consummatory components of male sexual behavior in Japanese quail (Coturnix japonica). Two groups of castrated, testosterone-treated male quail were trained during 12 sessions to associate the view of a female behind a window with the opportunity to interact freely and to copulate with her. They developed, as a consequence, a social proximity response (staying close and looking through the window providing a view of the female) that has been used in previous experiments to measure appetitive sexual behavior. A third control group (also castrated and treated with testosterone) was allowed to view the female but not to copulate with her and therefore did not develop this proximity response. 2-14C-deoxyglucose was then injected i.p. to these birds and they were allowed to either copulate freely with a female (consummatory sexual behavior group) or express the social proximity response (appetitive sexual behavior group). The control group was provided a view of the female but these birds, although they were exposed to the same stimuli as birds in the appetitive group, did not express the social proximity response because they had never learned the association with the opportunity to copulate. Birds were killed 45 min after the deoxyglucose injection and their brains were processed for autoradiography. Densitometric analyses of the autoradiograms revealed that the expression of appetitive or consummatory aspects of male sexual behavior was associated with significant increases by comparison with the control group in the deoxyglucose incorporation in the nucleus mesencephalicus lateralis, pars dorsalis and in the nucleus leminsci lateralis. In addition, an increase in the deoxyglucose incorporation was specifically observed in the paleostriatum primitivum, rostral preoptic area, nucleus intercollicularis, nucleus interpeduncularis and third nerve but a decrease was observed in the dorsomedial part of the hippocampus and in the nucleus nervi oculomotori in birds of the consummatory sexual behavior group by comparison with controls. By contrast, in the appetitive sexual behavior group, significant increases in deoxyglucose incorporation were observed in two telencephalic areas, the intermediate hyperstriatum ventrale and neostriatum caudolaterale by comparison with the controls, but decreases were detected in the stratum griseum et fibrosum superficiale of optic tectum by comparison with the consummatory behavior group. These studies demonstrate that the performance of appetitive or consummatory components of male sexual behavior affects in a specific manner the deoxyglucose uptake and accumulation in specific regions of the quail brain. Changes in metabolic activity were observed in steroid-sensitive areas, in auditory, visual and vocal brain regions, and in brain nuclei related to motor behavior but also in association telencephalic and limbic structures. These changes in oxidative metabolism overlap to some extent with metabolic changes as revealed by immunocytochemistry for the immediate early gene products Fos and Zenk, but many specific reactions are also detected indicating that these techniques are not necessarily redundant and, together, they can provide a more complete picture of the brain circuits that are implicated in the control and performance of complex behaviors.

Asymmetry in the left and right habenulo-interpeduncular tracts in the frog

In the frog Rana esculenta the left dorsal habenula includes a lateral and a medial component, whereas the right dorsal habenula is only represented by one nucleus. The efferents of the habenular nuclei to the interpeduncular nucleus were herein investigated with the retrograde horseradish peroxidase tracing. Injections of cobaltic-lysine complex in the interpeduncular nucleus were also performed. Intensely labeled fibers of the fasciculus retroflexus on the right and left sides of the brain were found to reach the interpeduncular nucleus from the habenular nuclei running prevalently in two routes--one through the medial, and the other through the lateral region of the diencephalon. On the right side, these fibers originated from the entire dorsal habenula. On the left side, the fibers of the medial route derived from the medial habenular subnucleus, while those of the lateral route derived from the lateral habenular subnucleus. In the dorsal habenulae of both sides, a large number of neurons displayed a Golgi-like labeling, while few such cells were detected in the ventral habenulae. Labeled neurons in the right dorsal habenula resembled those labeled in the lateral subnucleus of the left dorsal habenula, while larger and ramified neurons were detected in the left medial subnucleus. The present findings provide the first description of the pathway originating from the medial and the lateral subnucleus of the left dorsal habenula in the frog and point out that projection neurons of the medial habenular subnucleus are morphologically different from those of the other habenular nuclei. The present data indicate that in the frog the habenular asymmetry could underlie distinct functional correlates of the left and right habenulae.

Afferent and efferent connections of the habenula in the rainbow trout (Oncorhynchus mykiss): an indocarbocyanine dye (DiI) study

The habenula is a conserved structure in the brain of vertebrates. With the aim of further understanding of the evolution of the habenular system in vertebrates, we studied the afferent and efferent connections of the habenula of the rainbow trout. Experiments included application of the carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) into the habenula, telencephalon, pineal organ, posterior tubercle, and interpeduncular nucleus (IPN). The results obtained reveal a consistent pattern of habenular connections. Most afferents originate from three nuclei, one extending from the preoptic region to the rostral thalamus (the entopeduncular nucleus), the second located in the region of the hypothalamus-posterior tubercle and consisting of large bipolar cells (tuberculohabenular nucleus), and the third in the preoptic region (preoptic nucleus). A few large neurons of the locus coeruleus appeared to be labeled in some cases. The trout habenula also receives pineal and parapineal projections. Small labeled glial cells were observed in the thalamus around the fasciculus retroflexus and, sometimes, around the IPN. The most conspicuous efferents coursed in the fasciculus retroflexus to the IPN, the isthmal raphe, and the central gray. The existence of olfactohabenular or habenulotelencephalic projections is discussed.

Nitric oxide synthase in the brain of a urodele amphibian (Pleurodeles waltl) and its relation to catecholaminergic neuronal structures

The neuronal structures with NADPH-diaphorase activity and nitric oxide synthase (NOS) immunoreactivity have been studied in the brain of the urodele amphibian Pleurodeles waltl by means of histochemical and immunocytochemical techniques. Both approaches resulted in the selective labeling of the same neurons and fiber tracts in the brain, except for the primary olfactory fibers that did not stain for NOS but were positive for NADPH-diaphorase. NOS-containing neurons were found in the olfactory bulbs, pallial regions, septum, caudal striatum, amygdala and preoptic area. Only a few diencephalic cells were labeled in the posterior tubercle and ventral hypothalamus. In the brainstem, abundant cells were labeled in the tectum, mesencephalic tegmentum and isthmic region. The most conspicuous cell population was found in the isthmic-pretrigeminal region. Particularly well stained cells were distributed throughout the rhombencephalon in areas related to the descending trigeminal tract, solitary tract, raphe nucleus and the mid-caudal reticular formation. In the cervical spinal cord, NOS-containing cells were present in the dorsal, intermediate and ventral grey fields. Cells in the preoptic, postotic and dorsal root ganglia were also labeled. Double labeling techniques revealed an extensive codistribution of neurons with NOS and catecholamines in the urodele brain but actual colocalization in the same cells was never observed. The organization of the central systems in urodeles with NOS appears to share many features not only with other anamniotes but also with amniotes.