Parallel organization of the avian sensorimotor arcopallium: Tectofugal visual pathway in the pigeon (Columba livia)

The sensory–motor division of the avian arcopallium receives parallel inputs from primary and high-order pallial areas of sensory and vocal control pathways, and sends a prominent descending projection to ascending and premotor, subpallial stages of these pathways. While this organization is well established for the auditory and trigeminal systems, the arcopallial subdivision related to the tectofugal visual system and its descending projection to the optic tectum (TeO) has been less investigated. In this study, we charted the arcopallial area displaying tectofugal visual responses and by injecting neural tracers, we traced its connectional anatomy. We found visual motion-sensitive responses in a central region of the dorsal (AD) and intermediate (AI) arcopallium, in between previously described auditory and trigeminal zones. Blocking the ascending tectofugal sensory output, canceled these visual responses in the arcopallium, verifying their tectofugal origin. Injecting PHA-L into the visual, but not into the auditory AI, revealed a massive projection to tectal layer 13 and other tectal related areas, sparing auditory, and trigeminal ones. Conversely, CTB injections restricted to TeO retrogradely labeled neurons confined to the visual AI. These results show that the AI zone receiving tectofugal inputs sends top-down modulations specifically directed to tectal targets, just like the auditory and trigeminal AI zones project back to their respective subpallial sensory and premotor areas, as found by previous studies. Therefore, the arcopallium seems to be organized in a parallel fashion, such that in spite of expected cross-modal integration, the different sensory–motor loops run through separate subdivisions of this structure.     

The ventromedial hypothalamic nucleus in the zebra finch (Taeniopygia guttata): Afferent and efferent projections in relation to the control of reproductive behavior

Sex‐specific mating behaviors occur in a variety of mammals, with the medial preoptic nucleus (POM) and the ventromedial hypothalamic nucleus (VMH) mediating control of male and female sexual behavior, respectively. In birds, likewise, POM is predominantly involved in the control of male reproductive behavior, but the degree to which VMH is involved in female reproductive behavior is unclear. Here, in male and female zebra finches, a combination of aromatase immunohistochemistry and conventional tract tracing facilitated the definition of two separate but adjacent nuclei in the basal hypothalamus: an oblique band of aromatase‐positive (AR+) neurons, and ventromedial to this, an ovoid, aromatase‐negative (AR–) nucleus. The AR– nucleus, but not the AR+ nucleus, was here shown to receive a projection from rostral parts of the thalamic auditory nucleus ovoidalis and from the nucleus of the tractus ovoidalis. The AR– nucleus also receives an overlapping, major projection from previously uncharted regions of the medial arcopallium and a minor projection from the caudomedial nidopallium. Both the AR– and the AR+ nuclei project to the intercollicular nucleus of the midbrain. No obvious sex differences in either the pattern of AR immunoreactivity or of the afferent projections to the AR– nucleus were observed. The significance of these results in terms of the acoustic control of avian reproductive behavior is discussed, and a comparison with the organization of VMH afferents in lizards suggests a homologous similarity of the caudal telencephalon in sauropsids.     

Cholinergic innervation of principal neurons in the cochlear nucleus of the Mongolian gerbil

Principal neurons in the ventral cochlear nucleus (VCN) receive powerful ascending excitation and pass on the auditory information with exquisite temporal fidelity. Despite being dominated by ascending inputs, the VCN also receives descending cholinergic connections from olivocochlear neurons and from higher regions in the pontomesencephalic tegmentum. In Mongolian gerbils, acetylcholine acts as an excitatory and modulatory neurotransmitter on VCN neurons, but the anatomical structure of cholinergic innervation of gerbil VCN is not well described. We applied fluorescent immunohistochemical staining to elucidate the development and the cellular localization of presynaptic and postsynaptic components of the cholinergic system in the VCN of the Mongolian gerbil. We found that cholinergic fibers (stained with antibodies against the vesicular acetylcholine transporter) were present before hearing onset at P5, but innervation density increased in animals after P10. Early in development cholinergic fibers invaded the VCN from the medial side, spread along the perimeter and finally innervated all parts of the nucleus only after the onset of hearing. Cholinergic fibers ran in a rostro‐caudal direction within the nucleus and formed en‐passant swellings in the neuropil between principal neurons. Nicotinic and muscarinic receptors were expressed differentially in the VCN, with nicotinic receptors being mostly expressed in dendritic areas while muscarinic receptors were located predominantly in somatic membranes. These anatomical data support physiological indications that cholinergic innervation plays a role in modulating information processing in the cochlear nucleus.     

Kölliker–Fuse GABAergic and glutamatergic neurons project to distinct targets

The Kölliker‐Fuse nucleus (KF) is known primarily for its respiratory function as the “pneumotaxic center” or “pontine respiratory group.” Considered part of the parabrachial (PB) complex, KF contains glutamatergic neurons that project to respiratory‐related targets in the medulla and spinal cord (Yokota, Oka, Tsumori, Nakamura, & Yasui, 2007). Here we describe an unexpected population of neurons in the caudal KF and adjacent lateral crescent subnucleus (PBlc), which are γ‐aminobutyric acid (GABA)ergic and have an entirely different pattern of projections than glutamatergic KF neurons. First, immunofluorescence, in situ hybridization, and Cre‐reporter labeling revealed that many of these GABAergic neurons express FoxP2 in both rats and mice. Next, using Cre‐dependent axonal tracing in Vgat‐IRES‐Cre and Vglut2‐IRES‐Cre mice, we identified different projection patterns from GABAergic and glutamatergic neurons in this region. GABAergic neurons in KF and PBlc project heavily and almost exclusively to trigeminal sensory nuclei, with minimal projections to cardiorespiratory nuclei in the brainstem, and none to the spinal cord. In contrast, glutamatergic KF neurons project heavily to the autonomic, respiratory, and motor regions of the medulla and spinal cord previously identified as efferent targets mediating KF cardiorespiratory effects. These findings identify a novel, GABAergic subpopulation of KF/PB neurons with a distinct efferent projection pattern targeting the brainstem trigeminal sensory system. Rather than regulating breathing, we propose that these neurons influence vibrissal sensorimotor function.     

The expression of tyrosine hydroxylase and DARPP-32 in the house crow (Corvus splendens) brain

Birds of the family Corvidae which includes diverse species such as crows, rooks, ravens, magpies, jays, and jackdaws are known for their amazing abilities at problem-solving. Since the catecholaminergic system, especially the neurotransmitter dopamine, plays a role in cognition, we decided to study the distribution of tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of catecholamines in the brain of house crows (Corvus splendens). We also studied the expression of DARPP-32 (dopamine and cAMP-regulated phosphoprotein), which is expressed in dopaminoceptive neurons. Our results demonstrated that as in other avian species, the expression of both TH and DARPP-32 was highest in the house crow striatum. The caudolateral nidopallium (NCL, the avian analogue of the mammalian prefrontal cortex) could be differentiated from the surrounding pallial regions based on a larger number of TH-positive “baskets” of fibers around neurons in this region and greater intensity of DARPP-32 staining in the neuropil in this region. House crows also possessed distinct nuclei in their brains which corresponded to song control regions in other songbirds. Whereas immunoreactivity for TH was higher in the vocal control region Area X compared to the surrounding MSt (medial striatum) in house crows, staining in RA and HVC was not as prominent. Furthermore, the arcopallial song control regions RA (nucleus robustus arcopallialis) and AId (intermediate arcopallium) were strikingly negative for DARPP-32 staining, in contrast to the surrounding arcopallium. Patterns of immunoreactivity for TH and DARPP-32 in “limbic” areas such as the hippocampus, septum, and extended amygdala have also been described.     

Wiring patterns from auditory sensory neurons to the escape and song-relay pathways in fruit flies

Many animals rely on acoustic cues to decide what action to take next. Unraveling the wiring patterns of the auditory neural pathways is prerequisite for understanding such information processing. Here, we reconstructed the first step of the auditory neural pathway in the fruit fly brain, from primary to secondary auditory neurons, at the resolution of transmission electron microscopy. By tracing axons of two major subgroups of auditory sensory neurons in fruit flies, low-frequency tuned Johnston's organ (JO)-B neurons and high-frequency tuned JO-A neurons, we observed extensive connections from JO-B neurons to the main second-order neurons in both the song-relay and escape pathways. In contrast, JO-A neurons connected strongly to a neuron in the escape pathway. Our findings suggest that heterogeneous JO neuronal populations could be recruited to modify escape behavior whereas only specific JO neurons contribute to courtship behavior. We also found that all JO neurons have postsynaptic sites at their axons. Presynaptic modulation at the output sites of JO neurons could affect information processing of the auditory neural pathway in flies.     

The brain of the African wild dog. III. The auditory system

The large external pinnae and extensive vocal repertoire of the African wild dog (Lycaon pictus) has led to the assumption that the auditory system of this unique canid may be specialized. Here, using cytoarchitecture, myeloarchitecture, and a range of immunohistochemical stains, we describe the systems-level anatomy of the auditory system of the African wild dog. We observed the cochlear nuclear complex, superior olivary nuclear complex, lateral lemniscus, inferior colliculus, medial geniculate body, and auditory cortex all being in their expected locations, and exhibiting the standard subdivisions of this system. While located in the ectosylvian gyri, the auditory cortex includes several areas, resembling the parcellation observed in cats and ferrets, although not all of the auditory areas known from these species could be identified in the African wild dog. These observations suggest that, broadly speaking, the systems-level anatomy of the auditory system, and by extension the processing of auditory information, within the brain of the African wild dog closely resembles that observed in other carnivores. Our findings indicate that it is likely that the extraction of the semantic content of the vocalizations of African wild dogs, and the behaviors generated, occurs beyond the classically defined auditory system, in limbic or association neocortical regions involved in cognitive functions. Thus, to obtain a deeper understanding of how auditory stimuli are processed, and how communication is achieved, in the African wild dog compared to other canids, cortical regions beyond the primary sensory areas will need to be examined in detail.     

Extrastriate connectivity of the mouse dorsal lateral geniculate thalamic nucleus

The mammalian visual system is one of the most well-studied brain systems. Visual information from the retina is relayed to the dorsal lateral geniculate nucleus of the thalamus (LGd). The LGd then projects topographically to primary visual cortex (VISp) to mediate visual perception. In this view, the VISp is a critical network hub where visual information must traverse LGd–VISp circuits to reach higher order “extrastriate” visual cortices, which surround the VISp on its medial and lateral borders. However, decades of conflicting reports in a variety of mammals support or refute the existence of extrastriate LGd connections that can bypass the VISp. Here, we provide evidence of bidirectional extrastriate connectivity with the mouse LGd. Using small, discrete coinjections of anterograde and retrograde tracers within the thalamus and cortex, our cross-validated approach identified bidirectional connectivity between LGd and extrastriate visual cortices. We find robust reciprocal connectivity of the medial extrastriate regions with LGd neurons distributed along the “ventral strip” border with the intergeniculate leaflet. In contrast, LGd input to lateral extrastriate regions is sparse, but lateral extrastriate regions return stronger descending projections to localized LGd areas. We show further evidence that axons from lateral extrastriate regions can overlap onto medial extrastriate-projecting LGd neurons in the ventral strip, providing a putative subcortical LGd pathway for communication between medial and lateral extrastriate regions. Overall, our findings support the existence of extrastriate LGd circuits and provide novel understanding of LGd organization in rodent visual system.     

Innervation of the syrinx of the zebra finch (Taeniopygia guttata)

In songbirds, the learning and maintenance of song is dependent on auditory feedback, but little is known about the presence or role of other forms of sensory feedback. Here, we studied the innervation of the avian vocal organ, the syrinx, in the zebra finch. Using a combination of immunohistochemistry, immunofluorescence and neural tracing with subunit B of cholera toxin (CTB), we analysed the peripheral and central endings of the branch of the hypoglossal nerve that supplies the syrinx, the tracheosyringeal nerve. In the syringeal muscles, we show the presence of numerous choline acetyl transferase‐like immunoreactive en plaque motor endplates and substance P‐like immunoreactive, thin and varicose free nerve endings. Substance P‐like immunoreactive free nerve endings were also present in the luminal syringeal tissues, especially in the luminal epithelium of the trachea and pessulus. Also, by a combination of immunofluorescence and transganglionic tracing following injections of CTB in the tracheosyringeal nerve, we identified as central targets of the syringeal receptors the caudolateral part of the interpolaris subnucleus of the descending trigeminal tract, a caudolateral region of the nucleus tractus solitarius, and a lateral band of the principal sensory trigeminal nucleus. Further studies are required to determine the sensory modalities of these receptors and the connections of their specific synaptic targets.     

Thalamic connections of the core auditory cortex and rostral supratemporal plane in the macaque monkey

In the primate auditory cortex, information flows serially in the mediolateral dimension from core, to belt, to parabelt. In the caudorostral dimension, stepwise serial projections convey information through the primary, rostral, and rostrotemporal (AI, R, and RT) core areas on the supratemporal plane, continuing to the rostrotemporal polar area (RTp) and adjacent auditory‐related areas of the rostral superior temporal gyrus (STGr) and temporal pole. In addition to this cascade of corticocortical connections, the auditory cortex receives parallel thalamocortical projections from the medial geniculate nucleus (MGN). Previous studies have examined the projections from MGN to auditory cortex, but most have focused on the caudal core areas AI and R. In this study, we investigated the full extent of connections between MGN and AI, R, RT, RTp, and STGr using retrograde and anterograde anatomical tracers. Both AI and R received nearly 90% of their thalamic inputs from the ventral subdivision of the MGN (MGv; the primary/lemniscal auditory pathway). By contrast, RT received only ～45% from MGv, and an equal share from the dorsal subdivision (MGd). Area RTp received ～25% of its inputs from MGv, but received additional inputs from multisensory areas outside the MGN (30% in RTp vs. 1–5% in core areas). The MGN input to RTp distinguished this rostral extension of auditory cortex from the adjacent auditory‐related cortex of the STGr, which received 80% of its thalamic input from multisensory nuclei (primarily medial pulvinar). Anterograde tracers identified complementary descending connections by which highly processed auditory information may modulate thalamocortical inputs.     

Dorsal pallidal neurons directly link the nidopallium and midbrain in the zebra finch (Taeniopygia guttata)

The dorsal pallidum in birds is considered similar, if not homologous, to the globus pallidus (GP) of mammals. The dorsal pallidum projects to both thalamic and midbrain targets similar to the direct and indirect pathways arising from the internal and external segments of the GP. In the present study, retrograde and anterograde tracing studies revealed a previously undescribed projection of the avian dorsal pallidum. This arises from a specific dorsomedial component, which terminates in the intercollicular nucleus and partly surrounds the avian equivalent of the central nucleus of the inferior colliculus. The respiratory‐vocal dorsomedial nucleus of the intercollicular complex, however, does not receive these projections. The somata of the pallidal neurons retrogradely labeled from injections in the intercollicular nucleus were large and generally multipolar and had extensive, sparsely branching central processes (presumptive dendrites) that together extended up to 2 mm dorsally into the intermediate and caudomedial nidopallium. The size and morphology of these neurons were similar to those of large pallidal neurons labeled by calretinin immunoreactivity, which could be co‐localized to the same cells. Thus, rather than being directly involved in the control of movement, the large dorsomedial neurons of the caudal dorsal pallidum may be involved in sensory processing, in that they provide an unusual direct link between sensory (auditory/somatosensory) regions of the nidopallium and sensory regions of the intercollicular nucleus of the midbrain. J. Comp. Neurol. 525:1731–1742, 2017. © 2016 Wiley Periodicals, Inc.     

Differential innervation within a transverse plane of spinal gray matter by sensorimotor cortices,with special reference to the somatosensory cortices

The corticospinal (CS) neurons projecting to the cervical cord distribute not only in motor-related cortical areas, but also in somatosensory areas, including the primary somatosensory cortex (S1). The exact functions of these widely distributed CS neurons are largely unknown, however. In this study, we injected mice with adeno-associated virus encoding membrane-binding fluorescent proteins to investigate the distribution of axons from CS neurons in different regions within a broad cortical area. We found that CS axons from the primary motor cortex (M1), the rostral part of S1 (S1r), and the caudal part of S1 (S1c) differentially project to specific compartments within the spinal gray matter of the seventh cervical cord segment: (a) M1 projects mainly to intermediate and ventral areas, (b) S1r to the mediodorsal area, and (c) S1c to the dorsolateral area. We also found that the projection from S1r, which corresponds to the forelimb area, largely overlaps the cutaneous afferent terminals from the forepaw (hand) in the dorsal horn, and we detected a similar relation between S1c and the trunk. Our findings suggest the existence of considerably fine somatotopic compartments within the dorsal horn that process somatosensation and descending information, which is provided mainly by S1 CS neurons and contribute to delicate control of sensory information in generation of movement.     

Differential deletion of GDNF in the auditory system leads to altered sound responsiveness

Glial-derived neurotrophic factor (GDNF) has been proposed as a potent neurotrophic factor with the potential to cure neurodegenerative diseases. In the cochlea, GDNF has been detected in auditory neurons and sensory receptor cells and its expression is upregulated upon trauma. Moreover, the application of GDNF in different animal models of deafness has shown its capacity to prevent hearing loss and promoted its future use in therapeutic trials in humans. In the present study we have examined the endogenous requirement of GDNF during auditory development in mice. Using a lacZ knockin allele we have confirmed the expression of GDNF in the cochlea including its sensory regions during development. Global inactivation of GDNF throughout the hearing system using a Foxg1-Cre line causes perinatal lethality but reveals no apparent defects during formation of the cochlea. Using TrkC-Cre and Atoh1-Cre lines, we were able to generate viable mutants lacking GDNF in auditory neurons or both auditory neurons and sensory hair cells. These mutants show normal frequency-dependent auditory thresholds. However, mechanoelectrical response properties of outer hair cells (OHCs) in TrkC-Cre GDNF mutants are altered at low thresholds. Furthermore, auditory brainstem wave analysis shows an abnormal increase of wave I. On the other hand, Atoh1-Cre GDNF mutants show normal OHC function but their auditory brainstem wave pattern is reduced at the levels of wave I, III and IV. These results show that GDNF expression during the development is required to maintain functional hearing at different levels of the auditory system.     

Clustered organization and region‐specific identities of estrogen‐producing neurons in the forebrain of Zebra Finches (Taeniopygia guttata)

A fast, neuromodulatory role for estrogen signaling has been reported in many regions of the vertebrate brain. Regional differences in the cellular distribution of aromatase (estrogen synthase) in several species suggest that mechanisms for neuroestrogen signaling differ between and even within brain regions. A more comprehensive understanding of neuroestrogen signaling depends on characterizing the cellular identities of neurons that express aromatase. Calcium‐binding proteins such as parvalbumin and calbindin are molecular markers for interneuron subtypes, and are co‐expressed with aromatase in human temporal cortex. Songbirds like the zebra finch have become important models to understand the brain synthesis of steroids like estrogens and the implications for neurobiology and behavior. Here, we investigated the regional differences in cytoarchitecture and cellular identities of aromatase‐expressing neurons in the auditory and sensorimotor forebrain of zebra finches. Aromatase was co‐expressed with parvalbumin in the caudomedial nidopallium (NCM) and HVC shelf (proper name) but not in the caudolateral nidopallium (NCL) or hippocampus. By contrast, calbindin was not co‐expressed with aromatase in any region investigated. Notably, aromatase‐expressing neurons were found in dense somato‐somatic clusters, suggesting a coordinated release of local neuroestrogens from clustered neurons. Aromatase clusters were also more abundant and tightly packed in the NCM of males as compared to females. Overall, this study provides new insights into neuroestrogen regulation at the network level, and extends previous findings from human cortex by identifying a subset of aromatase neurons as putative inhibitory interneurons.     

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 (DT_CbN). Surprisingly, we find both DLM and dorsal anterior DT_CbN adjacent to DLM project to Area X. In contrast, the ventral medial subregion of DT_CbN 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.     

Projections to the putamen from neurons located in the white matter and the claustrum in the macaque

Continuing investigations of corticostriatal connections in rodents emphasize an intricate architecture where striatal projections originate from different combinations of cortical layers, include an inhibitory component, and form terminal arborizations which are cell-type dependent, extensive, or compact. Here, we report that in macaque monkeys, deep and superficial cortical white matter neurons (WMNs), peri-claustral WMNs, and the claustrum proper project to the putamen. WMNs retrogradely labeled by injections in the putamen (four injections in three macaques) were widely distributed, up to 10 mm antero-posterior from the injection site, mainly dorsal to the putamen in the external capsule, and below the premotor cortex. Striatally projecting labeled WMNs (WMNsST) were heterogeneous in size and shape, including a small GABAergic component. We compared the number of WMNsST with labeled claustral and cortical neurons and also estimated their proportion in relation to total WMNs. Since some WMNsST were located adjoining the claustrum, we wanted to compare results for density and distribution of striatally projecting claustral neurons (ClaST). ClaST neurons were morphologically heterogeneous and mainly located in the dorsal and anterior claustrum, in regions known to project to frontal, motor, and cingulate cortical areas. The ratio of ClaST to WMNsST was about 4:1 averaged across the four injections. These results provide new specifics on the connectional networks of WMNs in nonhuman primates, and delineate additional loops in the corticostriatal architecture, consisting of interconnections across cortex, claustralstriatal and striatally projecting WMNs.     

Postnatal developmental trajectory of dopamine receptor 1 and 2 expression in cortical and striatal brain regions

Healthy brain function requires a balance between the activity of dopamine receptor 1 (D1) and dopamine receptor 2 (D2). Alterations in this balance increase the risk for numerous developmental brain disorders. Indeed, D1 and D2 expression fluctuates throughout maturation, although there is conflicting evidence regarding the precise changes that occur. Here, we used stereology to investigate the developmental changes in the number of D1- or D2-expressing neurons in the prelimbic cortex, infralimbic cortex (IL), insula cortex, dorsal striatum, and ventral striatum of female and male mice with green fluorescent protein-tagged D1 or D2. Postnatal day 17, 25, 35, 49, and 70 were examined to cover juvenility to adulthood. In all regions, analysis of D1 density compared to D2 density within each sex seldom detected effects or interactions involving age. However, D1:D2 density ratio changed across age depending on sex. In the IL, D1:D2 density ratio increased in females from adolescence, whereas it was stable in males. In the insula cortex, D1:D2 ratio initially increased in males but decreased in females from juvenility to preadolescence. The ratio then increased in males and females from adolescence to adulthood, with males showing a more dramatic increase. In both the dorsal and ventral striatum, the ratio increased from adolescence. In all regions, females had a higher ratio compared to males throughout maturation except in the insula cortex at P25. These comprehensive observations are novel, and highlight how the maturational changes in the expression of these receptors may contribute to developmental disorders.     

Anatomical characterization of subcortical descending projections to the inferior colliculus in mouse

Descending projections from the thalamus and related structures to the midbrain are evolutionarily highly conserved. However, the basic organization of this auditory thalamotectal pathway has not yet been characterized. The purpose of this study was to obtain a better understanding of the anatomical and neurochemical features of this pathway. Analysis of the distributions of retrogradely labeled cells after focal injections of retrograde tracer into the inferior colliculus (IC) of the mouse revealed that most of the subcortical descending projections originated in the brachium of the IC and the paralaminar portions of the auditory thalamus. In addition, the vast majority of thalamotectal cells were found to be negative for the calcium‐binding proteins calbindin, parvalbumin, or calretinin. Using two different strains of GAD‐GFP mice, as well as immunostaining for GABA, we found that a subset of neurons in the brachium of the IC is GABAergic, suggesting that part of this descending pathway is inhibitory. Finally, dual retrograde injections into the IC and amygdala plus corpus striatum as well into the IC and auditory cortex did not reveal any double labeling. These data suggest that the thalamocollicular pathway comprises a unique population of thalamic neurons that do not contain typical calcium‐binding proteins and do not project to other paralaminar thalamic forebrain targets, and that a previously undescribed descending GABAergic pathway emanates from the brachium of the IC. J. Comp. Neurol. 525:885–900, 2017. © 2016 Wiley Periodicals, Inc.     

Neuronal activation in zebra finch parents associated with reintroduction of nestlings

Recent studies of the brain mechanisms of parental behaviors have mainly focused on rodents. Using other vertebrate taxa, such as birds, can contribute to a more comprehensive, evolutionary view. In the present study, we investigated a passerine songbird, the zebra finch (Taeniopygia guttata), with a biparental caring system. Parenting-related neuronal activation was induced by first temporarily removing the nestlings, and then, either reuniting the focal male or female parent with the nestlings (parental group) or not (control group). To identify activated neurons, the immediate early gene product, Fos protein, was labeled. Both parents showed an increased level of parental behavior following reunion with the nestlings, and no sexual dimorphism occurred in the neuronal activation pattern. Offspring-induced parental behavior-related neuronal activation was found in the preoptic, ventromedial (VMH), paraventricular hypothalamic nuclei, and in the bed nucleus of the stria terminalis. In addition, the number of Fos-immunoreactive (Fos-ir) neurons in the nucleus accumbens predicted the frequency of the feeding of the nestlings. No difference was found in Fos expression when the effect of isolation or the presence of the mate was examined. Thus, our study identified a number of nuclei involved in parental care in birds and suggests similar regulatory mechanisms in caring females and males. The activated brain regions show similarities to rodents, while a generally lower number of brain regions were activated in the zebra finch. Furthermore, future studies are necessary to establish the role of the apparently avian-specific neuronal activation in the VMH of zebra finch parents.