Distribution of vesicular glutamate transporter 2 in auditory and song control brain regions in the adult zebra finch (Taeniopygia guttata)

Expression patterns of prosaposin and neurotransmitter-related molecules in the chick paratympanic organ

The paratympanic organ (PTO) is a small sense organ in the middle ear of birds that contains hair cells similar to those found in vestibuloauditory organs and receives afferent fibers from the geniculate ganglion. To consider the histochemical similarities between the PTO and vestibular hair cells, we examined the expression patterns of representative molecules in vestibular hair cells, including prosaposin, G protein-coupled receptor (GPR) 37 and GPR37L1 as prosaposin receptors, vesicular glutamate transporter (vGluT) 2 and vGluT3, nicotinic acetylcholine receptor subunit α9 (nAChRα9), and glutamic acid decarboxylase (GAD) 65 and GAD67, in the postnatal day 0 chick PTO and geniculate ganglion by in situ hybridization. Prosaposin mRNA was observed in PTO hair cells, supporting cells, and geniculate ganglion cells. vGluT3 mRNA was observed in PTO hair cells, whereas vGluT2 was observed in a small number of ganglion cells. nAChRα9 mRNA was observed in a small number of PTO hair cells. The results suggest that the histochemical character of PTO hair cells is more similar to that of vestibular hair cells than that of auditory hair cells in chicks.

A neural hub for holistic courtship displays

Courtship displays often involve the concerted production of several distinct courtship behaviors. The neural circuits that enable the concerted production of the component behaviors of a courtship display are not well understood. Here, we identify a midbrain cell group (A11) that enables male zebra finches to produce their learned songs in concert with various other behaviors, including female-directed orientation, pursuit, and calling. Anatomical mapping reveals that A11 is at the center of a complex network including the song premotor nucleus HVC as well as brainstem regions crucial to calling and locomotion. Notably, lesioning A11 terminals in HVC blocked female-directed singing but did not interfere with female-directed calling, orientation, or pursuit. In contrast, lesioning A11 cell bodies strongly reduced and often abolished all female-directed courtship behaviors. However, males with either type of lesion still produced songs when in social isolation. Lastly, imaging calcium-related activity in A11 terminals in HVC showed that during courtship, A11 signals HVC about female-directed calls and during female-directed singing, about the transition from simpler introductory notes to the acoustically more complex syllables that depend intimately on HVC for their production. These results show how a brain region important to reproduction in both birds and mammals enables holistic courtship displays in male zebra finches, which include learning songs, calls, and other non-vocal behaviors.

Molecular identity of the lateral lemniscus nuclei in the adult mouse brain

Introduction

The dorsal (DLL), intermediate (ILL), and ventral (VLL) lateral lemniscus nuclei are relay centers in the central auditory pathway of the brainstem, commonly referred to as the lateral lemniscus nuclei (LLN). The LLN are situated in the prepontine and pontine hindbrain, from rhombomeres 1 to 4, extending from the more rostral DLL to the caudal VLL, with the ILL lying in between. These nuclei can be distinguished morphologically and by topological and connectivity criteria, and here, we set out to further characterize the molecular nature of each LLN.

Methods

We searched in situ hybridization studies in the Allen Mouse Brain Atlas for genes differentially expressed along the rostrocaudal axis of the brainstem, identifying 36 genes from diverse functional families expressed in the LLN.

Results

Available information in the databases indicated that 7 of these 36 genes are either associated with or potentially related to hearing disorders.

Discussion

In conclusion, the LLN are characterized by specific molecular profiles that reflect their rostrocaudal organization into the three constituent nuclei. This molecular regionalization may be involved in the etiology of some hearing disorders, in accordance with previous functional studies of these genes.

Glutamatergic pathways in the brains of turtles: A comparative perspective among reptiles, birds, and mammals

Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.

Expression of vesicular glutamate transporter 2 and 3 and glutamate receptor 1 and 2 mRNAs in the retina of adult laughing doves (Streptopelia senegalensis): An in situ hybridization study

The retina possesses few types of neurons so; it is considered an excellent model for understanding the neural mechanisms underlying basic neural information processing in the brain. Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system and retina. The present study was carried out to characterize the expression pattern of vesicular glutamate transporter 2 (Vglut2) and 3 (Vglut3) and glutamate receptor 1 (GluR1) and 2 (GluR2) mRNAs in the retina of adult laughing dove (Streptopelia senegalensis) by RT-PCR and in situ hybridization histochemistry. The cerebellum of adult laughing dove was used as a positive control in this study. Vglut2 mRNA was highly expressed only in the granular layer of the cerebellum while Vglut3 mRNA was weakly expressed only in the Purkinje cells layer. In the retina, Vglut2 mRNA was highly expressed in the ganglion cell layer and moderately expressed in the inner nuclear layer while Vglut3 mRNA was moderately expressed only in the inner nuclear layer. GluR1 mRNA was intensely expressed in the Purkinje cells layer while GluR2 mRNA signals were highly detectable in both granular and Purkinje cells layers of the cerebellum. In the retina, moderate expression of GluR1 and intense expression of GluR2 was found in both ganglion cell layer and the internal half of inner nuclear layer mostly amacrine cells. These results suggest that some retinal neuronal cells in the adult laughing dove are glutamatergic. Therefore, GluR1 and 2 are suggested as useful markers for glutamatergic retinal neuronal cells in the adult laughing doves.

Localization of AMPA, kainate, and NMDA receptor mRNAs in the pigeon cerebellum

In the present study, we investigated the location of mRNAs for three types of ionotropic glutamate receptors (iGluRs) in the pigeon cerebellum and then compared the results with those of mammals. The following nine iGluRs subunits were analyzed by in situ hybridization: AMPA receptors (GluA1, GluA2, GluA3, and GluA4), kainate receptors (GluK1, GluK2, and GluK4), and NMDA receptors (GluN1 and GluN2A). Subunit hybridization revealed expression in different cell types of the cerebellar cortex: Purkinje cells expressed most subunits, including AMPA receptors (GluA1, GluA2, and GluA3), kainate receptors (GluK1 and GluK4), and NMDA receptors (GluN1); granule cells expressed four subunits of kainate (GluK1 and GluK2) and NMDA receptors (GluN1 and GluN2A); stellate and basket cells expressed GluK1, GluK2, and GluN1; Golgi cells expressed GluA1, GluA3, and GluN1; and Bergmann glial cells expressed only AMPA receptors (GluA2 and GluA4). Cerebellar nuclei showed no AMPA subunit signals, whereas kainate and NMDA receptors were observed in the five cerebellar nuclei divisions (CbL, CbMic, CbMim, CbMin, and CbMvm). The five divisions showed weak expression of GluK1, GluK2, and GluN2A; moderate to intense expression of GluK4; and intense expression of GluN1. These results demonstrate that in pigeons the cerebellar cortex expresses AMPA, kainate, and NMDA receptors, while the cerebellar nuclei express kainate and NMDA receptors. Taken together, these findings provide anatomical data for further analysis of the functions of iGluR-expressing neurons in glutamatergic circuits of the avian cerebellum.

Inhibitory Neural Circuits in the Mammalian Auditory Midbrain

The auditory midbrain is the critical integration center in the auditory pathway of vertebrates. Synaptic inhibition plays a key role during information processing in the auditory midbrain, and these inhibitory neural circuits are seen in all vertebrates and are likely essential for hearing. Here, we review the structure and function of the inhibitory neural circuits of the auditory midbrain. First, we provide an overview on how these inhibitory circuits are organized within different clades of vertebrates. Next, we focus on recent findings in the mammalian auditory midbrain, the most studied of the vertebrates, and discuss how the mammalian auditory midbrain is functionally coordinated.

Prox1 mRNA expression in the medial cortex of the turtle (Pseudemys scripta elegans)

The medial cortex of the cerebrum in reptiles is thought to be homologous to the mammalian dentate gyrus, based on cytoarchitectures, fiber connections, and neurochemical profiles. To support this hypothesis, we examined the mRNA expression of vesicular glutamate transporter 1 (vGluT1), a glutamatergic gene marker, and Prox1, a selective gene marker for granule cells of the dentate gyrus, in the turtle medial cortex (zone 2). Reverse transcription-polymerase chain reaction revealed the presence of both mRNAs in the turtle cerebrum. In situ hybridization of zone 2, which is a layer of densely packed neurons in Nissl stains, intensely expressed vGluT1 and Prox1. In zone 3, which is a loosely packed layer, vGluT1 was intensely expressed, whereas Prox1 signals gradated from strong to negative toward zone 4. These findings demonstrate that zone 2 contains glutamatergic neurons and expresses Prox1 mRNA and suggest that zone 2 in the turtle cerebrum is homologous to the mammalian dentate gyrus.

Distribution of vesicular glutamate transporters in the brain of the turtle (Pseudemys scripta elegans)

The distribution of glutamatergic neurons has been extensively studied in mammalian and avian brains, but its distribution in a reptilian brain remains unknown. In the present study, the distribution of subpopulations of glutamatergic neurons in the turtle brain was examined by in situ hybridization using probes for vesicular glutamate transporter (VGLUT) 1-3. Strong VGLUT1 expression was observed in the telencephalic pallium; the mitral cells of the olfactory bulb, the medial, dorsomedial, dorsal, and lateral parts of the cerebral cortex, pallial thickening, and dorsal ventricular ridge; and also, in granule cells of the cerebellar cortex. Moderate to weak expression was found in the lateral and medial amygdaloid nuclei, the periventricular cellular layer of the optic tectum, and in some brainstem nuclei. VGLUT2 was weakly expressed in the telencephalon but was intensely expressed in the dorsal thalamic nuclei, magnocellular part of the isthmic nucleus, brainstem nuclei, and the rostral cervical segment of the spinal cord. The cerebellar cortex was devoid of VGLUT2 expression. The central amygdaloid nucleus did not express VGLUT1 or VGLUT2. VGLUT3 was localized in the parvocellular part of the isthmic nucleus, superior and inferior raphe nuclei, and cochlear nucleus. Our results indicate that the distribution of VGLUTs in the turtle brain is similar to that in the mammalian brain rather than that in the avian brain.

Thalamostriatal and cerebellothalamic pathways in a songbird, the Bengalese finch

The thalamostriatal system is a major network in the mammalian brain, originating principally from the intralaminar nuclei of thalamus. Its functions remain unclear, but a subset of these projections provides a pathway through which the cerebellum communicates with the basal ganglia. Both the cerebellum and basal ganglia play crucial roles in motor control. Although songbirds have yielded key insights into the neural basis of vocal learning, it is unknown whether a thalamostriatal system exists in the songbird brain. Thalamic nucleus DLM is an important part of the song system, the network of nuclei required for learning and producing song. DLM receives output from song system basal ganglia nucleus Area X and sits within dorsal thalamus, the proposed avian homolog of the mammalian intralaminar nuclei that also receives projections from the cerebellar nuclei. Using a viral vector that specifically labels presynaptic axon segments, we show in Bengalese finches that dorsal thalamus projects to Area X, the basal ganglia nucleus of the song system, and to surrounding medial striatum. To identify the sources of thalamic input to Area X, we map DLM and cerebellar-recipient dorsal thalamus (DTCbN ). Surprisingly, we find both DLM and dorsal anterior DTCbN adjacent to DLM project to Area X. In contrast, the ventral medial subregion of DTCbN projects to medial striatum outside Area X. Our results suggest the basal ganglia in the song system, like the mammalian basal ganglia, integrate feedback from the thalamic region to which they project as well as thalamic regions that receive cerebellar output.

Differential projections of the densocellular and intermediate parts of the hyperpallium in the pigeon (Columba livia)

The visual Wulst in birds shows a four-layered structure: apical part of the hyperpallium (HA), interstitial part of HA (IHA), intercalated part of hyperpallium (HI), and densocellular part of hyperpallium (HD). HD also connects with the hippocampus and olfactory system. Because HD is subjacent to HI, the two have been treated as one structure in many studies, and the fiber connections of HD have been examined by afferents and efferents originating outside HD. However, to clarify the difference between these two layers, they need to be treated separately. In the present study, the fiber connections of HD and HI were analyzed with tract-tracing techniques using a combination of injections of cholera toxin subunit B (CTB) for retrograde tracing and biotinylated dextran amine (BDA) for anterograde tracing. When the two tracers were bilaterally injected in HD, a major reciprocal connection was seen with the dorsolateral subdivision (DL) of the hippocampal formation. When CTB and BDA were bilaterally injected in HI, strong reciprocal connections were found between HI and HA. Next, projection neurons in HD and HI were examined by double staining for CTB combined with vesicular glutamate transporter 2 (vGluT2) mRNA in situ hybridization. After CTB was injected in DL or HA, many neurons revealed CTB+/vGluT2+ in HD or HI, respectively. Furthermore, in situ hybridization showed that DL and HA contained neurons expressing various subunits of ionotropic glutamate receptors: AMPA, kainate, and NMDA types. These results suggest that glutamatergic neurons in HD and HI project primarily to DL and HA, respectively.

Distribution of Vesicular Glutamate Transporter 2 and Ionotropic Glutamate Receptors in the Auditory Ganglion and Cochlear Nuclei of Pigeons (Columba livia)

Glutamate is a principal excitatory neurotransmitter in the auditory system. Our previous studies revealed localization of glutamate receptor mRNAs in the pigeon cochlear nuclei, suggesting the existence of glutamatergic input from the auditory nerve to the brainstem. This study demonstrated localization of mRNAs for vesicular glutamate transporter 2 (vGluT2) and ionotropic glutamate receptors (AMPA, kainate and NMDA) in the auditory ganglion (AG) and cochlear nuclei (magnocellular, angular and laminar nuclei). VGluT2 mRNA was intensely expressed in AG and intensely or moderately in the cochlear nuclei. The AG and cochlear nuclei showed intense-to-moderate mRNA signals for GluA2, GluA3, GluA4, GluK4 and GluN1. These results suggest that the pigeon AG neurons receives glutamatergic input from hair cells and in turn projects to the magnocellular and angular nuclei. Glutamate may play a pivotal role in the excitatory synapse transmission in the peripheral auditory pathway of birds.

Glutamatergic thalamopallial projections in the pigeon identified by retrograde labeling and expression of vGluT2 mRNA

A neocortical hypothesis as to homology of certain nuclear components of the avian brain proposes that the entopallium and field L2 are homologous to layer 4 of mammalian extrastriate and auditory neocortex, respectively. However, the hypothesis lacks support from the neurochemistry of thalamopallial projections. We investigated whether these projections are glutamatergic by injecting cholera toxin B into either the entopallium or field L2 in combination with in situ hybridization. Retrogradely labeled neurons in nucleus rotundus and nucleus ovoidalis were found to express vesicular glutamate transporter 2 mRNA, showing that the thalamopallial projections are glutamatergic. The results are consistent with the neocortical hypothesis.