A histological and diceCT-derived 3D reconstruction of the avian visual thalamofugal pathway

Amniotes feature two principal visual processing systems: the tectofugal and thalamofugal pathways. In most mammals, the thalamofugal pathway predominates, routing retinal afferents through the dorsolateral geniculate complex to the visual cortex. In most birds, the thalamofugal pathway often plays the lesser role with retinal afferents projecting to the principal optic thalami, a complex of several nuclei that resides in the dorsal thalamus. This thalamic complex sends projections to a forebrain structure called the Wulst, the terminus of the thalamofugal visual system. The thalamofugal pathway in birds serves many functions such as pattern discrimination, spatial memory, and navigation/migration. A comprehensive analysis of avian species has unveiled diverse subdivisions within the thalamic and forebrain structures, contingent on species, age, and techniques utilized. In this study, we documented the thalamofugal system in three dimensions by integrating histological and contrast-enhanced computed tomography imaging of the avian brain. Sections of two-week-old chick brains were cut in either coronal, sagittal, or horizontal planes and stained with Nissl and either Gallyas silver or Luxol Fast Blue. The thalamic principal optic complex and pallial Wulst were subdivided on the basis of cell and fiber density. Additionally, we utilized the technique of diffusible iodine-based contrast-enhanced computed tomography (diceCT) on a 5-week-old chick brain, and right eyeball. By merging diceCT data, stained histological sections, and information from the existing literature, a comprehensive three-dimensional model of the avian thalamofugal pathway was constructed. The use of a 3D model provides a clearer understanding of the structural and spatial organization of the thalamofugal system. The ability to integrate histochemical sections with diceCT 3D modeling is critical to better understanding the anatomical and physiologic organization of complex pathways such as the thalamofugal visual system.

Src-NADH dehydrogenase subunit 2 complex and recognition memory of imprinting in domestic chicks

Src is a non-receptor tyrosine kinase participating in a range of neuronal processes, including synaptic plasticity. We have recently shown that the amounts of total Src and its two phosphorylated forms, at tyrosine-416 (activated) and tyrosine-527 (inhibited), undergoes time-dependent, region-specific learning-related changes in the domestic chick forebrain after visual imprinting. These changes occur in the intermediate medial mesopallium (IMM), a site of memory formation for visual imprinting, but not the posterior pole of the nidopallium (PPN), a control brain region not involved in imprinting. Src interacts with mitochondrial genome-coded NADH dehydrogenase subunit 2 (NADH2), a component of mitochondrial respiratory complex I. This interaction occurs at brain excitatory synapses bearing NMDA glutamate receptors. The involvement of Src-NADH2 complexes in learning and memory is not yet explored. We show for the first time that, independently of changes in total Src or total NADH2, NADH2 bound to Src immunoprecipitated from the P2 plasma membrane-mitochondrial fraction: (i) is increased in a learning-related manner in the left IMM 1 h after the end of training; (ii), is decreased in the right IMM in a learning-related way 24 h after training. These changes occurred in the IMM but not the PPN. They are attributable to learning occurring during training rather than a predisposition to learn. Learning-related changes in Src-bound NADH2 are thus time- and region-dependent.

Functional neurogenomic responses to acoustic threats, including a heterospecific referential alarm call and its referent, in the auditory forebrain of red-winged blackbirds

In animal communication, functionally referential alarm calls elicit the same behavioral responses as their referents, despite their typically distinct bioacoustic traits. Yet the auditory forebrain in at least one songbird species, the black-capped chickadee Poecile atricapillus, responds similarly to threat calls and their referent predatory owl calls, as assessed by immediate early gene responses in the secondary auditory forebrain nuclei. Whether and where in the brain such perceptual and cognitive equivalence is processed remains to be understood in most other avian systems. Here, we studied the functional neurogenomic (non-) equivalence of acoustic threat stimuli perception by the red-winged blackbird Agelaius phoeniceus in response to the actual calls of the obligate brood parasitic brown-headed cowbird Molothrus ater and the referential anti-parasitic alarm calls of the yellow warbler Setophaga petechia, upon which the blackbird is known to eavesdrop. Using RNA-sequencing from neural tissue in the auditory lobule (primary and secondary auditory nuclei combined), in contrast to previous findings, we found significant differences in the gene expression profiles of both an immediate early gene, ZENK (egr-1), and other song-system relevant gene-products in blackbirds responding to cowbird vs. warbler calls. In turn, direct cues of threats (including conspecific intruder calls and nest-predator calls) elicited higher ZENK and other differential gene expression patterns compared to harmless heterospecific calls. These patterns are consistent with a perceptual non-equivalence in the auditory forebrain of adult male red-winged blackbirds in response to referential calls and the calls of their referents.

A Purkinje cell to parabrachial nucleus pathway enables broad cerebellar influence over the forebrain

In addition to its motor functions, the cerebellum is involved in emotional regulation, anxiety and affect. We found that suppressing the firing of cerebellar Purkinje cells (PCs) rapidly excites forebrain areas that contribute to such functions (including the amygdala, basal forebrain and septum), but that the classic cerebellar outputs, the deep cerebellar nuclei, do not directly project there. We show that PCs directly inhibit parabrachial nuclei (PBN) neurons that project to numerous forebrain regions. Suppressing the PC-PBN pathway influences many regions in the forebrain and is aversive. Molecular profiling shows that PCs directly inhibit numerous types of PBN neurons that control diverse behaviors that are not involved in motor control. Therefore, the PC-PBN pathway allows the cerebellum to directly regulate activity in the forebrain, and may be an important substrate for cerebellar disorders arising from damage to the posterior vermis.

Neural circuit-wide analysis of changes to gene expression during deafening-induced birdsong destabilization

Sensory feedback is required for the stable execution of learned motor skills, and its loss can severely disrupt motor performance. The neural mechanisms that mediate sensorimotor stability have been extensively studied at systems and physiological levels, yet relatively little is known about how disruptions to sensory input alter the molecular properties of associated motor systems. Songbird courtship song, a model for skilled behavior, is a learned and highly structured vocalization that is destabilized following deafening. Here, we sought to determine how the loss of auditory feedback modifies gene expression and its coordination across the birdsong sensorimotor circuit. To facilitate this system-wide analysis of transcriptional responses, we developed a gene expression profiling approach that enables the construction of hundreds of spatially-defined RNA-sequencing libraries. Using this method, we found that deafening preferentially alters gene expression across birdsong neural circuitry relative to surrounding areas, particularly in premotor and striatal regions. Genes with altered expression are associated with synaptic transmission, neuronal spines, and neuromodulation and show a bias toward expression in glutamatergic neurons and Pvalb/Sst-class GABAergic interneurons. We also found that connected song regions exhibit correlations in gene expression that were reduced in deafened birds relative to hearing birds, suggesting that song destabilization alters the inter-region coordination of transcriptional states. Finally, lesioning LMAN, a forebrain afferent of RA required for deafening-induced song plasticity, had the largest effect on groups of genes that were also most affected by deafening. Combined, this integrated transcriptomics analysis demonstrates that the loss of peripheral sensory input drives a distributed gene expression response throughout associated sensorimotor neural circuitry and identifies specific candidate molecular and cellular mechanisms that support the stability and plasticity of learned motor skills.

Vocal changes in a zebra finch model of Parkinson's disease characterized by alpha-synuclein overexpression in the song-dedicated anterior forebrain pathway

Deterioration in the quality of a person's voice and speech is an early marker of Parkinson's disease (PD). In humans, the neural circuit that supports vocal motor control consists of a cortico-basal ganglia-thalamo-cortico loop. The basal ganglia regions, striatum and globus pallidus, in this loop play a role in modulating the acoustic features of vocal behavior such as loudness, pitch, and articulatory rate. In PD, this area is implicated in pathogenesis. In animal models of PD, the accumulation of toxic aggregates containing the neuronal protein alpha-synuclein (αsyn) in the midbrain and striatum result in limb and vocal motor impairments. It has been challenging to study vocal impairments given the lack of well-defined cortico-basal ganglia circuitry for vocalization in rodent models. Furthermore, whether deterioration of voice quality early in PD is a direct result of αsyn-induced neuropathology is not yet known. Here, we take advantage of the well-characterized vocal circuits of the adult male zebra finch songbird to experimentally target a song-dedicated pathway, the anterior forebrain pathway, using an adeno-associated virus expressing the human wild-type αsyn gene, SNCA. We found that overexpression of αsyn in this pathway coincides with higher levels of insoluble, monomeric αsyn compared to control finches. Impairments in song production were also detected along with shorter and poorer quality syllables, which are the most basic unit of song. These vocal changes are similar to the vocal abnormalities observed in individuals with PD.

Responses to Song Playback Differ in Sleeping versus Anesthetized Songbirds

Vocal learning in songbirds is mediated by a highly localized system of interconnected forebrain regions, including recurrent loops that traverse the cortex, basal ganglia, and thalamus. This brain-behavior system provides a powerful model for elucidating mechanisms of vocal learning, with implications for learning speech in human infants, as well as for advancing our understanding of skill learning in general. A long history of experiments in this area has tested neural responses to playback of different song stimuli in anesthetized birds at different stages of vocal development. These studies have demonstrated selectivity for different song types that provide neural signatures of learning. In contrast to the ease of obtaining responses to song playback in anesthetized birds, song-evoked responses in awake birds are greatly reduced or absent, indicating that behavioral state is an important determinant of neural responsivity. Song-evoked responses can be elicited during sleep as well as anesthesia, and the selectivity of responses to song playback in adult birds is highly similar between anesthetized and sleeping states, encouraging the idea that anesthesia and sleep are similar. In contrast to that idea, we report evidence that cortical responses to song playback in juvenile zebra finches (Taeniopygia guttata) differ greatly between sleep and urethane anesthesia. This finding indicates that behavioral states differ in sleep versus anesthesia and raises questions about relationships between developmental changes in sleep activity, selectivity for different song types, and the neural substrate for vocal learning.

Theta-gamma coupling emerges from spatially heterogeneous cholinergic neuromodulation

Theta and gamma rhythms and their cross-frequency coupling play critical roles in perception, attention, learning, and memory. Available data suggest that forebrain acetylcholine (ACh) signaling promotes theta-gamma coupling, although the mechanism has not been identified. Recent evidence suggests that cholinergic signaling is both temporally and spatially constrained, in contrast to the traditional notion of slow, spatially homogeneous, and diffuse neuromodulation. Here, we find that spatially constrained cholinergic stimulation can generate theta-modulated gamma rhythms. Using biophysically-based excitatory-inhibitory (E-I) neural network models, we simulate the effects of ACh on neural excitability by varying the conductance of a muscarinic receptor-regulated K+ current. In E-I networks with local excitatory connectivity and global inhibitory connectivity, we demonstrate that theta-gamma-coupled firing patterns emerge in ACh modulated network regions. Stable gamma-modulated firing arises within regions with high ACh signaling, while theta or mixed theta-gamma activity occurs at the peripheries of these regions. High gamma activity also alternates between different high-ACh regions, at theta frequency. Our results are the first to indicate a causal role for spatially heterogenous ACh signaling in the emergence of localized theta-gamma rhythmicity. Our findings also provide novel insights into mechanisms by which ACh signaling supports the brain region-specific attentional processing of sensory information.

Local Axonal Conduction Shapes the Spatiotemporal Properties of Neural Sequences

Sequential activation of neurons has been observed during various behavioral and cognitive processes, but the underlying circuit mechanisms remain poorly understood. Here, we investigate premotor sequences in HVC (proper name) of the adult zebra finch forebrain that are central to the performance of the temporally precise courtship song. We use high-density silicon probes to measure song-related population activity, and we compare these observations with predictions from a range of network models. Our results support a circuit architecture in which heterogeneous delays between sequentially active neurons shape the spatiotemporal patterns of HVC premotor neuron activity. We gauge the impact of several delay sources, and we find the primary contributor to be slow conduction through axonal collaterals within HVC, which typically adds between 1 and 7.5 ms for each link within the sequence. Thus, local axonal "delay lines" can play an important role in determining the dynamical repertoire of neural circuits.

Metabolic adaptations to hypoxia in the neonatal mouse forebrain can occur independently of the transporters SLC7A5 and SLC3A2

Neonatal encephalopathy due to hypoxia-ischemia is associated with adverse neurodevelopmental effects. The involvement of branched chain amino acids (BCAAs) in this is largely unexplored. Transport of BCAAs at the plasma membrane is facilitated by SLC7A5/SLC3A2, which increase with hypoxia. We hypothesized that hypoxia would alter BCAA transport and metabolism in the neonatal brain. We investigated this using an organotypic forebrain slice culture model with, the SLC7A5/SLC3A2 inhibitor, 2-Amino-2-norbornanecarboxylic acid (BCH) under normoxic or hypoxic conditions. We subsequently analysed the metabolome and candidate gene expression. Hypoxia was associated with increased expression of SLC7A5 and SLC3A2 and an increased tissue abundance of BCAAs. Incubation of slices with 13C-leucine confirmed that this was due to increased cellular uptake. BCH had little effect on metabolite abundance under normoxic or hypoxic conditions. This suggests hypoxia drives increased cellular uptake of BCAAs in the neonatal mouse forebrain, and membrane mediated transport through SLC7A5 and SLC3A2 is not essential for this process. This indicates mechanisms exist to generate the compounds required to maintain essential metabolism in the absence of external nutrient supply. Moreover, excess BCAAs have been associated with developmental delay, providing an unexplored mechanism of hypoxia mediated pathogenesis in the developing forebrain.

Adult-like neural representation of species-specific songs in the auditory forebrain of zebra finch nestlings

Encoding of conspecific signals during development can reinforce species barriers as well as set the stage for learning and production of species-typical vocalizations. In altricial songbirds, the development of the auditory system is not complete at hatching, so it is unknown the degree to which recently hatched young can process auditory signals like birdsong. We measured in vivo extracellular responses to song stimuli in a zebra finch (Taeniopygia guttata) secondary auditory forebrain region, the caudomedial nidopallium (NCM). We recorded from three age groups between 13 days post-hatch and adult to identify possible shifts in stimulus encoding that occur before the opening of the sensitive period of song motor learning. We did not find differences in putative cell type composition, firing rate, response strength, and selectivity across ages. Across ages narrow-spiking units had higher firing rates, response strength, accuracy, and trial-by-trial reliability along with lower selectivity than broad-spiking units. In addition, we showed that stimulus-specific adaptation, a characteristic of adult NCM, was also present in nestlings and fledglings. These results indicate that most features of secondary auditory processing are already adult-like shortly after hatching. Furthermore, we showed that selectivity for species-specific stimuli is similar across all ages, with the greatest fidelity in temporal coding in response to conspecific song and domesticated Bengalese finch song, and reduced fidelity in response to owl finch song, a more ecologically relevant heterospecific, and white noise. Our study provides the first evidence that the electrophysiological properties of higher-order auditory neurons are already mature in nestling songbirds.

Cholinergic Deep Brain Stimulation for Memory and Cognitive Disorders

Memory and cognitive impairment as sequelae of neurodegeneration in Alzheimer's disease and age-related dementia are major health issues with increasing social and economic burden. Deep brain stimulation (DBS) has emerged as a potential treatment to slow or halt progression of the disease state. The selection of stimulation target is critical, and structures that have been targeted for memory and cognitive enhancement include the Papez circuit, structures projecting to the frontal lobe such as the ventral internal capsule, and the cholinergic forebrain. Recent human clinical and animal model results imply that DBS of the nucleus basalis of Meynert can induce a therapeutic modulation of neuronal activity. Benefits include enhanced activity across the cortical mantle, and potential for amelioration of neuropathological mechanisms associated with Alzheimer's disease. The choice of stimulation parameters is also critical. High-frequency, continuous stimulation is used for movement disorders as a way of inhibiting their output; however, no overexcitation has been hypothesized in Alzheimer's disease and lower stimulation frequency or intermittent patterns of stimulation (periods of stimulation interleaved with periods of no stimulation) are likely to be more effective for stimulation of the cholinergic forebrain. Efficacy and long-term tolerance in human patients remain open questions, though the cumulative experience gained by DBS for movement disorders provides assurance for the safety of the procedure.

Circuit and synaptic organization of forebrain-to-midbrain pathways that promote and suppress vocalization

Animals vocalize only in certain behavioral contexts, but the circuits and synapses through which forebrain neurons trigger or suppress vocalization remain unknown. Here, we used transsynaptic tracing to identify two populations of inhibitory neurons that lie upstream of neurons in the periaqueductal gray (PAG) that gate the production of ultrasonic vocalizations (USVs) in mice (i.e. PAG-USV neurons). Activating PAG-projecting neurons in the preoptic area of the hypothalamus (POAPAG neurons) elicited USV production in the absence of social cues. In contrast, activating PAG-projecting neurons in the central-medial boundary zone of the amygdala (AmgC/M-PAG neurons) transiently suppressed USV production without disrupting non-vocal social behavior. Optogenetics-assisted circuit mapping in brain slices revealed that POAPAG neurons directly inhibit PAG interneurons, which in turn inhibit PAG-USV neurons, whereas AmgC/M-PAG neurons directly inhibit PAG-USV neurons. These experiments identify two major forebrain inputs to the PAG that trigger and suppress vocalization, respectively, while also establishing the synaptic mechanisms through which these neurons exert opposing behavioral effects.

Disruption of Nrxn1α within excitatory forebrain circuits drives value-based dysfunction

Goal-directed behaviors are essential for normal function and significantly impaired in neuropsychiatric disorders. Despite extensive associations between genetic mutations and these disorders, the molecular contributions to goal-directed dysfunction remain unclear. We examined mice with constitutive and brain region-specific mutations in Neurexin1α, a neuropsychiatric disease-associated synaptic molecule, in value-based choice paradigms. We found Neurexin1α knockouts exhibited reduced selection of beneficial outcomes and impaired avoidance of costlier options. Reinforcement modeling suggested that this was driven by deficits in updating and representation of value. Disruption of Neurexin1α within telencephalic excitatory projection neurons, but not thalamic neurons, recapitulated choice abnormalities of global Neurexin1α knockouts. Furthermore, this selective forebrain excitatory knockout of Neurexin1α perturbed value-modulated neural signals within striatum, a central node in feedback-based reinforcement learning. By relating deficits in value-based decision-making to region-specific Nrxn1α disruption and changes in value-modulated neural activity, we reveal potential neural substrates for the pathophysiology of neuropsychiatric disease-associated cognitive dysfunction.

Experience, circuit dynamics, and forebrain recruitment in larval zebrafish prey capture

Experience influences behavior, but little is known about how experience is encoded in the brain, and how changes in neural activity are implemented at a network level to improve performance. Here we investigate how differences in experience impact brain circuitry and behavior in larval zebrafish prey capture. We find that experience of live prey compared to inert food increases capture success by boosting capture initiation. In response to live prey, animals with and without prior experience of live prey show activity in visual areas (pretectum and optic tectum) and motor areas (cerebellum and hindbrain), with similar visual area retinotopic maps of prey position. However, prey-experienced animals more readily initiate capture in response to visual area activity and have greater visually-evoked activity in two forebrain areas: the telencephalon and habenula. Consequently, disruption of habenular neurons reduces capture performance in prey-experienced fish. Together, our results suggest that experience of prey strengthens prey-associated visual drive to the forebrain, and that this lowers the threshold for prey-associated visual activity to trigger activity in motor areas, thereby improving capture performance.

Neurotelemetry Reveals Putative Predictive Activity in HVC during Call-Based Vocal Communications in Zebra Finches

Premotor predictions facilitate vocal interactions. Here, we study such mechanisms in the forebrain nucleus HVC (proper name), a cortex-like sensorimotor area of songbirds, otherwise known for being essential for singing in zebra finches. To study the role of the HVC in calling interactions between male and female mates, we used wireless telemetric systems for simultaneous measurement of neuronal activity of male zebra finches and vocalizations of males and females that freely interact with each other. In a non-social context, male HVC neurons displayed stereotypic premotor activity in relation to active calling and showed auditory-evoked activity to hearing of played-back female calls. In a social context, HVC neurons displayed auditory-evoked activity to hearing of female calls only if that neuron showed activity preceding the upcoming female calls. We hypothesize that this activity preceding the auditory-evoked activity in the male HVC represents a neural correlate of behavioral anticipation, predictive activity that helps to coordinate vocal communication between social partners.SIGNIFICANCE STATEMENT Most social-living vertebrates produce large numbers of calls per day, and the calls have prominent roles in social interactions. Here, we show neuronal mechanisms that are active during call-based vocal communication of zebra finches, a highly social songbird species. HVC, a forebrain nucleus known for its importance in control of learned vocalizations of songbirds, displays predictive activity that may enable the male to adjust his own calling pattern to produce very fast sequences of male female call exchanges.

Rest-task modulation of fMRI-derived global signal topography is mediated by transient coactivation patterns

Recent resting-state functional MRI (fMRI) studies have revealed that the global signal (GS) exhibits a nonuniform spatial distribution across the gray matter. Whether this topography is informative remains largely unknown. We therefore tested rest-task modulation of GS topography by analyzing static GS correlation and dynamic coactivation patterns in a large sample of an fMRI dataset (n = 837) from the Human Connectome Project. The GS topography in the resting state and in seven different tasks was first measured by correlating the GS with the local time series (GSCORR). In the resting state, high GSCORR was observed mainly in the primary sensory and motor regions, whereas low GSCORR was seen in the association brain areas. This pattern changed during the seven tasks, with mainly decreased GSCORR in sensorimotor cortex. Importantly, this rest-task modulation of GSCORR could be traced to transient coactivation patterns at the peak period of GS (GS-peak). By comparing the topography of GSCORR and respiration effects, we observed that the topography of respiration mimicked the topography of GS in the resting state, whereas both differed during the task states; because of such partial dissociation, we assume that GSCORR could not be equated with a respiration effect. Finally, rest-task modulation of GS topography could not be exclusively explained by other sources of physiological noise. Together, we here demonstrate the informative nature of GS topography by showing its rest-task modulation, the underlying dynamic coactivation patterns, and its partial dissociation from respiration effects during task states.

Recent resting-state fMRI studies have shown that the global signal exhibits a nonuniform spatial distribution across gray matter, but is this informative? This neuroimaging study reveals novel insights into the informative nature of global signal by rest-task modulation of the global signal topography.

The neurobiology of innate, volitional and learned vocalizations in mammals and birds

Vocalization is an ancient vertebrate trait essential to many forms of communication, ranging from courtship calls to free verse. Vocalizations may be entirely innate and evoked by sexual cues or emotional state, as with many types of calls made in primates, rodents and birds; volitional, as with innate calls that, following extensive training, can be evoked by arbitrary sensory cues in non-human primates and corvid songbirds; or learned, acoustically flexible and complex, as with human speech and the courtship songs of oscine songbirds. This review compares and contrasts the neural mechanisms underlying innate, volitional and learned vocalizations, with an emphasis on functional studies in primates, rodents and songbirds. This comparison reveals both highly conserved and convergent mechanisms of vocal production in these different groups, despite their often vast phylogenetic separation. This similarity of central mechanisms for different forms of vocal production presents experimentalists with useful avenues for gaining detailed mechanistic insight into how vocalizations are employed for social and sexual signalling, and how they can be modified through experience to yield new vocal repertoires customized to the individual's social group. This article is part of the theme issue 'What can animal communication teach us about human language?'

Fos-CreER-based genetic mapping of forebrain regions activated by acupuncture

Acupuncture increasingly is accepted as a potential therapy for many diseases in the Western world. However, the mechanism of acupuncture is not well understood mechanistically. We have established that manual acupuncture (MA) at the Neiguan (P6) acupoint inhibits excitatory cardiovascular reflex responses through modulation of the autonomic nervous system in the brainstem. It is unclear whether P6 MA activates neurons in the brain regions beyond the brainstem. Thus, we mapped P6 specific neural activation by MA in the forebrain using the Fos-CreER; Ai9 mice model, which allows for enhanced sensitivity and efficiency compared to conventional immunohistochemical staining. Compared to sham-MA control without manual stimulation, we find that MA at P6 markedly increases c-Fos positive neurons in a number of the forebrain regions (n = 5 in each group). These activated regions include accumbens nucleus, caudate putamen, claustrum, bed nucleus of the stria terminalis, amygdaloid nucleus, ventral posterior division of the thalamic nucleus, paraventricular hypothalamic nucleus, arcuate hypothalamic nucleus, primary and secondary somatosensory cortex, ectorhinal cortex, and dorsolateral entorhinal cortex. As MA at P6 activates neurons in relatively broad brain networks beyond the brainstem, our data suggest that acupuncture at this acupoint has the potential to influence physiological functions associated with autonomic and non-autonomic nervous systems through its effects on multiple brain regions.

GABARAPs dysfunction by autophagy deficiency in adolescent brain impairs GABAA receptor trafficking and social behavior

Dysfunctional mTOR signaling is associated with the pathogenesis of neurodevelopmental and neuropsychiatric disorders. However, it is unclear what molecular mechanisms and pathogenic mediators are involved and whether mTOR-regulated autophagy continues to be crucial beyond neurodevelopment. Here, we selectively deleted Atg7 in forebrain GABAergic interneurons in adolescent mice and unexpectedly found that these mice showed a set of behavioral deficits similar to Atg7 deletion in forebrain excitatory neurons. By unbiased quantitative proteomic analysis, we identified γ-aminobutyric acid receptor-associated protein-like 2 (GABARAPL2) to differentially form high-molecular weight species in autophagy-deficient brains. Further functional analyses revealed a novel pathogenic mechanism involving the p62-dependent sequestration of GABARAP family proteins, leading to the reduction of surface GABA_A receptor levels. Our work demonstrates a novel physiological role for autophagy in regulating GABA signaling beyond postnatal neurodevelopment, providing a potential mechanism for the reduced inhibitory inputs observed in neurodevelopmental and neuropsychiatric disorders with mTOR hyperactivation.

Reliability of human cortical organoid generation

The differentiation of pluripotent stem cells in three-dimensional cultures can recapitulate key aspects of brain development, but protocols are prone to variable results. Here we differentiated multiple human pluripotent stem cell lines for over 100 d using our previously developed approach to generate brain-region-specific organoids called cortical spheroids and, using several assays, found that spheroid generation was highly reliable and consistent. We anticipate the use of this approach for large-scale differentiation experiments and disease modeling.

Infralimbic prefrontal cortex structural and functional connectivity with the limbic forebrain: a combined viral genetic and optogenetic analysis

The medial prefrontal cortex is critical for contextual appraisal, executive function, and goal-directed behavior. Additionally, the infralimbic (IL) subregion of the prefrontal cortex has been implicated in stress responding, mood, and fear memory. However, the specific circuit mechanisms that mediate these effects are largely unknown. To date, IL output to the limbic forebrain has been examined largely qualitatively or within a restricted number of sites. To quantify IL presynaptic input to structures throughout the forebrain, we utilized a lentiviral construct expressing synaptophysin-mCherry. Thus, allowing quantification of IL efferents that are specifically synaptic, as opposed to fibers of passage. Additionally, this approach permitted the determination of IL innervation on a sub-structural level within the multiple heterogeneous limbic nuclei. To examine the functional output of the IL, optogenetic activation of IL projections was followed by quantification of neuronal activation throughout the limbic forebrain via fos-related antigen (Fra). Quantification of synaptophysin-mCherry indicated that the IL provides robust synaptic input to a number of regions within the thalamus, hypothalamus, amygdala, and bed nucleus of the stria terminalis, with limited input to the hippocampus and nucleus accumbens. Furthermore, there was high concordance between structural connectivity and functional activation. Interestingly, some regions receiving substantial synaptic input did not exhibit significant increases in Fra-immunoreactivity. Collectively, these studies represent a step toward a comprehensive and quantitative analysis of output circuits. This large-scale efferent quantification or 'projectome' also opens the door for data-driven analyses of the downstream synaptic mechanisms that mediate the integrative aspects of cortico-limbic interactions.

Neural blastocyst complementation enables mouse forebrain organogenesis

Genetically modified mice are commonly generated by the microinjection of pluripotent embryonic stem (ES) cells into wild-type host blastocysts1, producing chimeric progeny that require breeding for germline transmission and homozygosity of modified alleles. As an alternative approach and to facilitate studies of the immune system, we previously developed RAG2-deficient blastocyst complementation2. Because RAG2-deficient mice cannot undergo V(D)J recombination, they do not develop B or T lineage cells beyond the progenitor stage2: injecting RAG2-sufficient donor ES cells into RAG2-deficient blastocysts generates somatic chimaeras in which all mature lymphocytes derive from donor ES cells. This enables analysis, in mature lymphocytes, of the functions of genes that are required more generally for mouse development3. Blastocyst complementation has been extended to pancreas organogenesis4, and used to generate several other tissues or organs5-10, but an equivalent approach for brain organogenesis has not yet been achieved. Here we describe neural blastocyst complementation (NBC), which can be used to study the development and function of specific forebrain regions. NBC involves targeted ablation, mediated by diphtheria toxin subunit A, of host-derived dorsal telencephalic progenitors during development. This ablation creates a vacant forebrain niche in host embryos that results in agenesis of the cerebral cortex and hippocampus. Injection of donor ES cells into blastocysts with forebrain-specific targeting of diphtheria toxin subunit A enables donor-derived dorsal telencephalic progenitors to populate the vacant niche in the host embryos, giving rise to neocortices and hippocampi that are morphologically and neurologically normal with respect to learning and memory formation. Moreover, doublecortin-deficient ES cells-generated via a CRISPR-Cas9 approach-produced NBC chimaeras that faithfully recapitulated the phenotype of conventional, germline doublecortin-deficient mice. We conclude that NBC is a rapid and efficient approach to generate complex mouse models for studying forebrain functions; this approach could more broadly facilitate organogenesis based on blastocyst complementation.

[Central cholinergic system innervates song control system of songbirds and regulates song behavior]

Songbird has become an ideal model for studying motor learning due to its unique learned song behavior. It has been proved that song behavior is directly regulated by song control system in the forebrain of songbirds. There are lines of evidence to show that cholinergic transmitters and their receptors are distributed in song control system, and vocal control nuclei in song control system are innervated by cholinergic nerves from the central cholinergic system in basal forebrain, which can affect activities of vocal control nuclei through cholinergic transmitters, and then affect song behavior. Studies in mammals have confirmed that the central cholinergic system is involved in the regulation of motor behavior and neural process of motor learning. Elucidation of regulation of songbirds' song behavior by central cholinergic system would shed light on the neural mechanism of song motor control and song learning and memory in songbirds, and provide theoretical insights for researches on other animals' sensorimotor processes and human language learning. This review summarized recent progresses, including the research work of our laboratory, in the studies on the selectivity of cholinergic transmitters to their receptors and their effects on neuronal activities in vocal control nuclei of songbirds and provided valuable clues for revealing the regulation mechanism of central cholinergic system on songbirds' song behavior.

Simulation of the song motor pathway in birds: A single neuron initiates a chain of events that produces birdsong with realistic spectra properties

Birdsong is a complex learned behavior regulated by Neuromuscular coordination of different muscle sets necessary for producing relevant sounds. We developed a heterogeneous and stochastically connected neural network representing the pathway from the high vocal center (HVC) to the robust nucleus of the arcopallium (RA) neurons that drive the muscles to generate sounds. We show that a single active neuron is sufficient to initiate a chain of spiking events that results to excite the entire network system. The network could synthesize realistic bird sounds spectra, with spontaneous generation of intermittent sound bursts typical of birdsong (song syllables). This study confirms experiments on animals and on humans, where results have shown that single neurons are responsible for the activation of complex behavior or are associated with high-level perception events.

Neurons with diverse phenotypes project from the caudal to the rostral nucleus of the solitary tract

The nucleus of the solitary tract is a potential site for taste-visceral interactions. Connections from the caudal, visceral area of the nucleus (cNST) to the rostral, gustatory zone (rNST) have been described, but the phenotype of cells giving rise to the projection(s) and their distribution among rNST subdivisions are unknown. To determine these characteristics of the intrasolitary pathway, we injected pan-neuronal and floxed AAV viruses into the cNST of mice expressing cre in glutamatergic, GABAergic, or catecholaminergic neurons. Particular attention was paid to the terminal field distribution in rNST subdivisions by simultaneously visualizing P2X2 localized to gustatory afferent terminals. All three phenotypically identified pathways terminated in rNST, with the density greatest for glutamatergic and sparsest for catecholaminergic projections, observations supported by retrograde tracing. Interestingly, cNST neurons had more prominent projections to rNST regions medial and ventral to P2X2 staining, i.e., the medial and ventral subdivisions. In addition, GABAergic neurons projected robustly to the lateral subdivision and adjacent parts of the reticular formation and spinal trigeminal nucleus. Although cNST neurons also projected to the P2X2-rich central subdivision, such projections were sparser. These findings suggest that cNST visceral signals exert stronger excitatory and inhibitory influences on local autonomic and reflex pathways associated with the medial and ventral subdivisions compared to weaker modulation of ascending pathways arising from the central subdivision and ultimately destined for the forebrain.

Transcriptomic analysis of left-right differences in human embryonic forebrain and midbrain

Left-right asymmetry is subtle but pervasive in the human central nervous system. This asymmetry is initiated early during development, but its mechanisms are poorly known. Forebrains and midbrains were dissected from six human embryos at Carnegie stages 15 or 16, one of which was female. The structures were divided into left and right sides, and RNA was isolated. RNA was sequenced with 100 base-pair paired ends using Illumina Hiseq 4000. After quality control, five paired brain sides were available for midbrain and forebrain. A paired analysis between left- and right sides of a given brain structure across the embryos identified left-right differences. The dataset, consisting of Fastq files and a read count table, can be further used to study early development of the human brain.

Palatable Food Affects HPA Axis Responsivity and Forebrain Neurocircuitry in an Estrous Cycle-specific Manner in Female Rats

Eating palatable foods can provide stress relief, but the mechanisms by which this occurs are unclear. We previously characterized a limited sucrose intake (LSI) paradigm in which twice-daily access to a small amount of 30% sucrose (vs. water as a control) reduces hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress and alters neuronal activation in stress-regulatory brain regions in male rats. However, women may be more prone to 'comfort feeding' behaviors than men, and stress-related eating may vary across the menstrual cycle. This suggests that LSI effects may be sex- and estrous cycle-dependent. The present study therefore investigated the effects of LSI on HPA axis stress responsivity, as well as markers of neuronal activation/plasticity in stress- and reward-related neurocircuitry in female rats across the estrous cycle. We found that LSI reduced post-restraint stress plasma ACTH in female rats specifically during proestrus/estrus (P/E). LSI also increased basal (non-stress) FosB/deltaFosB- and pCREB-immunolabeling in the basolateral amygdala (BLA) and central amygdala specifically during P/E. Finally, Bayesian network modeling of the FosB/deltaFosB and pCREB expression data identified a neurocircuit that includes the BLA, nucleus accumbens, prefrontal cortex, and bed nucleus of the stria terminalis as likely being modified by LSI during P/E. When considered in the context of our prior results, the present findings suggest that palatable food reduces stress responses in female rats similar to males, but in an estrous cycle-dependent manner. Further, the BLA may contribute to the LSI effects in both sexes, whereas the involvement of other brain regions appears to be sex-dependent.

Monoaminergic levels at the forebrain and diencephalon signal for the occurrence of mutualistic and conspecific engagement in client reef fish

Social interactions are commonly found among fish as in mammals and birds. While most animals interact socially with conspecifics some however are also frequently and repeatedly observed to interact with other species (i.e. mutualistic interactions). This is the case of the (so-called) fish clients that seek to be cleaned by other fish (the cleaners). Clients face an interesting challenge: they raise enough motivation to suspend their daily activities as to selectively visit and engage in interactions with cleaners. Here we aimed, for the first time, to investigate the region-specific brain monoaminergic level differences arising from individual client fish when facing a cleaner (interspecific context) compared to those introduced to another conspecific (socio-conspecific context). We show that monoaminergic activity differences occurring at two main brain regions, the diencephalon and the forebrain, are associated with fish clients' social and mutualistic activities. Our results are the first demonstration that monoaminergic mechanisms underlie client fish mutualistic engagement with cleanerfish. These pathways should function as a pre-requisite for cleaning to occur, providing to clients the cognitive and physiological tools to seek to be cleaned.

Inositol 1,4,5-trisphosphate 3-kinase A overexpressed in mouse forebrain modulates synaptic transmission and mGluR-LTD of CA1 pyramidal neurons

Inositol 1,4,5-trisphosphate 3-kinase A (IP₃K-A) regulates the level of the inositol polyphosphates, inositol trisphosphate (IP₃) and inositol tetrakisphosphate to modulate cellular signaling and intracellular calcium homeostasis in the central nervous system. IP₃K-A binds to F-actin in an activity-dependent manner and accumulates in dendritic spines, where it is involved in the regulation of synaptic plasticity. IP₃K-A knockout mice exhibit deficits in some forms of hippocampus-dependent learning and synaptic plasticity, such as long-term potentiation in the dentate gyrus synapses of the hippocampus. In the present study, to further elucidate the role of IP₃K-A in the brain, we developed a transgenic (Tg) mouse line in which IP₃K-A is conditionally overexpressed approximately 3-fold in the excitatory neurons of forebrain regions, including the hippocampus. The Tg mice showed an increase in both presynaptic release probability of evoked responses, along with bigger synaptic vesicle pools, and miniature excitatory postsynaptic current amplitude, although the spine density or the expression levels of the postsynaptic density-related proteins NR2B, synaptotagmin 1, and PSD-95 were not affected. Hippocampal-dependent learning and memory tasks, including novel object recognition and radial arm maze tasks, were partially impaired in Tg mice. Furthermore, (R,S)-3,5-dihydroxyphenylglycine-induced metabotropic glutamate receptor long-term depression was inhibited in Tg mice and this inhibition was dependent on protein kinase C but not on the IP₃ receptor. Long-term potentiation and depression dependent on N-methyl-d-aspartate receptor were marginally affected in Tg mice. In summary, this study shows that overexpressed IP₃K-A plays a role in some forms of hippocampus-dependent learning and memory tasks as well as in synaptic transmission and plasticity by regulating both presynaptic and postsynaptic functions.

Somatostatin Neurons in the Basal Forebrain Promote High-Calorie Food Intake

Obesity has become a global issue, and the overconsumption of food is thought to be a major contributor. However, the regulatory neural circuits that regulate palatable food consumption remain unclear. Here, we report that somatostatin (SOM) neurons and GABAergic (VGAT) neurons in the basal forebrain (BF) play specific roles in regulating feeding. Optogenetic stimulation of BF SOM neurons increased fat and sucrose intake within minutes and promoted anxiety-like behaviors. Furthermore, optogenetic stimulation of projections from BF SOM neurons to the lateral hypothalamic area (LHA) selectively resulted in fat intake. In addition, activation of BF VGAT neurons rapidly induced general food intake and gnawing behaviors. Whole-brain mapping of inputs and outputs showed that BF SOM neurons form bidirectional connections with several brain areas important in feeding and regulation of emotion. Collectively, these results suggest that BF SOM neurons play a selective role in hedonic feeding.

A Membrane G-Protein-Coupled Estrogen Receptor Is Necessary but Not Sufficient for Sex Differences in Zebra Finch Auditory Coding

Estradiol acts as a neuromodulator in brain regions important for cognition and sensory processing. Estradiol also shapes brain sex differences but rarely have these concepts been considered simultaneously. In male and female songbirds, estradiol rapidly increases within the auditory forebrain during song exposure and enhances local auditory processing. We tested whether G-protein-coupled estrogen receptor 1 (GPER1), a membrane-bound estrogen receptor, is necessary and sufficient for neuroestrogen regulation of forebrain auditory processing in male and female zebra finches (Taeniopygia guttata). At baseline, we observed that females had elevated single-neuron responses to songs vs males. In males, narrow-spiking (NS) neurons were more responsive to conspecific songs than broad-spiking (BS) neurons, yet cell types were similarly auditory responsive in females. Following acute inactivation of GPER1, auditory responsiveness and coding were suppressed in male NS yet unchanged in female NS and in BS of both sexes. By contrast, GPER1 activation did not mimic previously established estradiol actions in either sex. Lastly, the expression of GPER1 and its coexpression with an inhibitory neuron marker were similarly abundant in males and females, confirming anatomical similarity in the auditory forebrain. In this study, we found: (1) a role for GPER1 in regulating sensory processing and (2) a sex difference in auditory processing of complex vocalizations in a cell type-specific manner. These results reveal sex specificity of a rapid estrogen signaling mechanism in which neuromodulation accounts and/or compensates for brain sex differences, dependent on cell type, in brain regions that are anatomically similar in both sexes.

Enhanced fear and altered neuronal activation in forebrain limbic regions of CX3CR1-deficient mice

Mounting evidence supports immune dysfunction in psychiatric conditions such as post-traumatic stress disorder (PTSD). The association of immunomodulatory mechanisms with PTSD-relevant behavior and physiology is not well understood. Communication between neurons and microglia, resident immune cells of the central nervous system, is crucial for optimal regulation of behavior and physiology. In this regard, the fractalkine CX3CL1, secreted from neurons and its target, the microglial CX3CR1 receptor represent a primary neuron-microglia inter-regulatory system important for synaptic plasticity and function. The current study investigated the impact of CX3CR1 deficiency on behaviors relevant to PTSD, such as fear acquisition and memory, acoustic startle response and anxiety-like behavior. Morphological analysis of microglia and neuronal activation within PTSD-relevant forebrain nuclei regulating stress and fear behaviors was also conducted. CX3CR1-deficient (CX3CR1-/-) mice elicited increased fear acquisition as well as reinstatement of fear as compared to wild type (CX3CR1+/+) mice. Conditioned fear and extinction were not significantly different between genotypes. No significant differences were observed in unconditioned acoustic startle response between genotypes. CX3CR1-/- mice showed reduced anxiety-like behaviors as compared with CX3CR1+/+ mice. Morphological assessment of microglia showed region-selective effects of CX3CR1 deficiency, primarily within hypothalamic and cortical areas. Lastly, CX3CR1-/- mice elicited elevated neuronal activity in the PVN and the ventral tegmental-interpeduncular area following reinstatement of fear. Collectively, our data suggest that impaired CX3CR1 function may evoke region-selective alterations in forebrain circuits regulating stress, anxiety and fear, impacting behaviors relevant to disorders such as PTSD.

Temperature manipulation of neuronal dynamics in a forebrain motor control nucleus

Different neuronal types within brain motor areas contribute to the generation of complex motor behaviors. A widely studied songbird forebrain nucleus (HVC) has been recognized as fundamental in shaping the precise timing characteristics of birdsong. This is based, among other evidence, on the stretching and the “breaking” of song structure when HVC is cooled. However, little is known about the temperature effects that take place in its neurons. To address this, we investigated the dynamics of HVC both experimentally and computationally. We developed a technique where simultaneous electrophysiological recordings were performed during temperature manipulation of HVC. We recorded spontaneous activity and found three effects: widening of the spike shape, decrease of the firing rate and change in the interspike interval distribution. All these effects could be explained with a detailed conductance based model of all the neurons present in HVC. Temperature dependence of the ionic channel time constants explained the first effect, while the second was based in the changes of the maximal conductance using single synaptic excitatory inputs. The last phenomenon, only emerged after introducing a more realistic synaptic input to the inhibitory interneurons. Two timescales were present in the interspike distributions. The behavior of one timescale was reproduced with different input balances received form the excitatory neurons, whereas the other, which disappears with cooling, could not be found assuming poissonian synaptic inputs. Furthermore, the computational model shows that the bursting of the excitatory neurons arises naturally at normal brain temperature and that they have an intrinsic delay at low temperatures. The same effect occurs at single synapses, which may explain song stretching. These findings shed light on the temperature dependence of neuronal dynamics and present a comprehensive framework to study neuronal connectivity. This study, which is based on intrinsic neuronal characteristics, may help to understand emergent behavioral changes.

The study of the neuronal mechanisms that give rise to the complex behavior of singing in birds has been hotly debated lately. Many models have been tested and novel tools have been developed to try to understand the role of a key brain nucleus in the song pathway: HVC. It is believed that it is highly responsible for generating the precise timing of songs, and this has been tested by manipulating it with temperature. Results showed that cooling can stretch, but that it can also restructure or “break” the song syllables. However, single neuronal mechanisms are not yet described. To better understand this, we cooled HVC in canaries and measured spontaneous activity electrophysiologically. We found three effects: spike shape widening, spike rate reduction and changes in inter-spike-interval (ISI) distributions. To explain them, we built a computational model with a detailed description of ion channel conductances and temperature dependency. We could explain the first effect with a single neuron model. The second, could be explained adding single synapses. Finally, we showed similar ISI modifications of one of the timescales present by means of multiple stochastic inputs. In addition, we found that excitatory neurons show natural bursting behavior at normal brain temperatures and that synaptic delays are the main candidates to explain song stretching at low temperatures.

Region specific changes in nonapeptide levels during client fish interactions with allopatric and sympatric cleaner fish

Social relationships are crucially dependent on individual ability to learn and remember ecologically relevant cues. However, the way animals recognize cues before engaging in any social interaction and how their response is regulated by brain neuromodulators remains unclear. We examined the putative involvement of arginine vasotocin (AVT) and isotocin (IT), acting at different brain regions, during fish decision-making in the context of cooperation, by trying to identify how fish distinguish and recognize the value of other social partners or species. We hypothesized that the behavioural responses of cleaner fish clients to different social contexts would be underlain by changes in brain AVT and IT levels. We have found that changes in AVT at the level of forebrain and optic tectum are linked with a response to allopatric cleaners (novel or unfamiliar stimuli) while those at cerebellum are associated with the willingness to be cleaned (in response to sympatric cleaners). On the other hand, higher brain IT levels that were solely found in the diencephalon, also in response to allopatric cleaners. Our results are the first to implicate these nonapeptides, AVT in particular, in the assessment of social cues which enable fish to engage in mutualistic activities.

Addressing sufficiency of the CB1 receptor for endocannabinoid-mediated functions through conditional genetic rescue in forebrain GABAergic neurons

Genetic inactivation of the cannabinoid CB1 receptor gene in different cell types in the brain has previously revealed necessary functions for distinct synaptic plasticity processes and behaviors. Here, we sought to identify CB1 receptor expression sites that are minimally required to reconstruct normal phenotypes. In a CB1-null background, we re-expressed endogenous CB1 receptors in forebrain GABAergic neurons, thereby assessing the sufficiency of CB1 receptors. Depolarization-induced suppression of inhibitory, but not excitatory, transmission was restored in hippocampal and amygdalar circuits. GABAergic CB1 receptors did not convey protection against chemically induced seizures, but prevented the spontaneous mortality observed in CB1 null mutants. Rescue of GABAergic CB1 receptors largely restored normal anxiety-like behavior but improved extinction of learned fear only marginally. This study illustrates that the approach of genetic reconstruction of complex behaviors is feasible. It also revealed distinct degrees of modulation for different emotional behaviors by the GABAergic population of CB1 receptors.

Neuromolecular correlates of cooperation and conflict during territory defense in a cichlid fish

Cooperative behavior is widespread among animals, yet the neural mechanisms have not been studied in detail. We examined cooperative territory defense behavior and associated neural activity in candidate forebrain regions in the cichlid fish, Astatotilapia burtoni. We find that a territorial male neighbor will engage in territory defense dependent on the perceived threat of the intruder. The resident male, on the other hand, engages in defense based on the size and behavior of his partner, the neighbor. In the neighbor, we find that an index of engagement correlates with neural activity in the putative homolog of the mammalian basolateral amygdala and in the preoptic area, as well as in preoptic dopaminergic neurons. In the resident, neighbor behavior is correlated with neural activity in the homolog of the mammalian hippocampus. Overall, we find distinct neural activity patterns between the neighbor and the resident, suggesting that an individual perceives and processes an intruder challenge differently during cooperative territory defense depending on its own behavioral role.

Singing modulates parvalbumin interneurons throughout songbird forebrain vocal control circuitry

Across species, the performance of vocal signals can be modulated by the social environment. Zebra finches, for example, adjust their song performance when singing to females ('female-directed' or FD song) compared to when singing in isolation ('undirected' or UD song). These changes are salient, as females prefer the FD song over the UD song. Despite the importance of these performance changes, the neural mechanisms underlying this social modulation remain poorly understood. Previous work in finches has established that expression of the immediate early gene EGR1 is increased during singing and modulated by social context within the vocal control circuitry. Here, we examined whether particular neural subpopulations within those vocal control regions exhibit similar modulations of EGR1 expression. We compared EGR1 expression in neurons expressing parvalbumin (PV), a calcium buffer that modulates network plasticity and homeostasis, among males that performed FD song, males that produced UD song, or males that did not sing. We found that, overall, singing but not social context significantly affected EGR1 expression in PV neurons throughout the vocal control nuclei. We observed differences in EGR1 expression between two classes of PV interneurons in the basal ganglia nucleus Area X. Additionally, we found that singing altered the amount of PV expression in neurons in HVC and Area X and that distinct PV interneuron types in Area X exhibited different patterns of modulation by singing. These data indicate that throughout the vocal control circuitry the singing-related regulation of EGR1 expression in PV neurons may be less influenced by social context than in other neuron types and raise the possibility of cell-type specific differences in plasticity and calcium buffering.

Forebrain-Specific Transgene Rescue of 11β-HSD1 Associates with Impaired Spatial Memory and Reduced Hippocampal Brain-Derived Neurotrophic Factor mRNA Levels in Aged 11β-HSD1 Deficient Mice

Mice lacking the intracellular glucocorticoid-regenerating enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) are protected from age-related spatial memory deficits. 11β-HSD1 is expressed predominantly in the brain, liver and adipose tissue. Reduced glucocorticoid levels in the brain in the absence of 11β-HSD1 may underlie the improved memory in aged 11β-HSD1 deficient mice. However, the improved glucose tolerance, insulin sensitisation and cardioprotective lipid profile associated with reduced peripheral glucocorticoid regeneration may potentially contribute to the cognitive phenotype of aged 11β-HSD1 deficient mice. In the present study, transgenic mice with forebrain-specific overexpression of 11β-HSD1 (Tg) were intercrossed with global 11β-HSD1 knockout mice (HSD1KO) to examine the influence of forebrain and peripheral 11β-HSD1 activity on spatial memory in aged mice. Transgene-mediated delivery of 11β-HSD1 to the hippocampus and cortex of aged HSD1KO mice reversed the improved spatial memory retention in the Y-maze but not spatial learning in the watermaze. Brain-derived neurotrophic factor (BDNF) mRNA levels in the hippocampus of aged HSD1KO mice were increased compared to aged wild-type mice. Rescue of forebrain 11β-HSD1 reduced BDNF mRNA in aged HSD1KO mice to levels comparable to aged wild-type mice. These findings indicate that 11β-HSD1 regenerated glucocorticoids in the forebrain and decreased levels of BDNF mRNA in the hippocampus play a role in spatial memory deficits in aged wild-type mice, although 11β-HSD1 activity in peripheral tissues may also contribute to spatial learning impairments in aged mice.

PERK Regulates Working Memory and Protein Synthesis-Dependent Memory Flexibility

PERK (EIF2AK3) is an ER-resident eIF2α kinase required for memory flexibility and metabotropic glutamate receptor-dependent long-term depression, processes known to be dependent on new protein synthesis. Here we investigated PERK’s role in working memory, a cognitive ability that is independent of new protein synthesis, but instead is dependent on cellular Ca²⁺ dynamics. We found that working memory is impaired in forebrain-specific Perk knockout and pharmacologically PERK-inhibited mice. Moreover, inhibition of PERK in wild-type mice mimics the fear extinction impairment observed in forebrain-specific Perk knockout mice. Our findings reveal a novel role of PERK in cognitive functions and suggest that PERK regulates both Ca²⁺ -dependent working memory and protein synthesis-dependent memory flexibility.

Neurobiology of the Sleep-Wake Cycle: Sleep Architecture, Circadian Regulation, and Regulatory Feedback

This mini-review article presents the remarkable progress that has been made in the past decade in our understanding of the neural circuitry underlying the regulation of sleep-wake states and circadian control of behaviors. Following a brief introduction to sleep architecture and physiology, the authors describe the neural circuitry and neurotransmitters that regulate sleep and cortical arousal (i.e., wakefulness). They next examine how sleep and wakefulness are regulated by mutual inhibition between sleep-and arousal-promoting circuitry and how this interaction functions analogously to an electronic “flip-flop” switch that ensures behavioral state stability. The authors then discuss the role of circadian and homeostatic processes in the consolidation of sleep, including the physiologic basis of homeostatic sleep drive (i.e., wake-dependent increase in sleep propensity) and the role of the SCN in the circadian regulation of sleep-wake cycles. Finally, they describe the hypothalamic circuitry for the integration of photic and nonphotic environmental time cues and how this integration allows organisms to sculpt patterns of rest-activity and sleep-wake cycles that are optimally adaptive.

The Forebrain Song System Mediates Predictive Call Timing in Female and Male Zebra Finches

The dichotomy between vocal learners and non-learners is a fundamental distinction in the study of animal communication. Male zebra finches (Taeniopygia guttata) are vocal learners that acquire a song resembling their tutors', whereas females can only produce innate calls. The acoustic structure of short calls, produced by both males and females, is not learned. However, these calls can be precisely coordinated across individuals. To examine how birds learn to synchronize their calls, we developed a vocal robot that exchanges calls with a partner bird. Because birds answer the robot with stereotyped latencies, we could program it to disrupt each bird's responses by producing calls that are likely to coincide with the bird's. Within minutes, the birds learned to avoid this disruptive masking (jamming) by adjusting the timing of their responses. Notably, females exhibited greater adaptive timing plasticity than males. Further, when challenged with complex rhythms containing jamming elements, birds dynamically adjusted the timing of their calls in anticipation of jamming. Blocking the song system cortical output dramatically reduced the precision of birds' response timing and abolished their ability to avoid jamming. Surprisingly, we observed this effect in both males and females, indicating that the female song system is functional rather than vestigial. We suggest that descending forebrain projections, including the song-production pathway, function as a general-purpose sensorimotor communication system. In the case of calls, it enables plasticity in vocal timing to facilitate social interactions, whereas in the case of songs, plasticity extends to developmental changes in vocal structure.

[Method of Calculating the Distance Between the Classes of the Structural Components of the Forebrain Birds]

The method of calculating the distance between the classes of the structural components of the brain of birds. Compared interclass distances of glia, neurons and neuroglial complexes in the forebrain hooded crow (Corvus cornix) (a bird with a highly rational activity) and common crossbill (Loxia curvirostra) (birds with a medium level of rational activity).

The impact of hippocampal lesions on trace-eyeblink conditioning and forebrain-cerebellar interactions

Behavioral Neuroscience published a pivotal paper by Moyer, Deyo, and Disterhoft (1990) 25 years ago that described the impaired acquisition of trace-eyeblink conditioning in rabbits with complete removal of the hippocampus. As part of the Behavioral Neuroscience celebration commemorating the 30th anniversary of the journal, we reflect upon the impact of that study on understanding the role of the hippocampus, forebrain, and forebrain-cerebellar interactions that mediate acquisition and retention of trace-conditioned responses, and of declarative memory more globally. We discuss the expansion of the conditioning paradigm to species other than the rabbit, the heterogeneity of responses among hippocampal neurons during trace conditioning, the responsivity of hippocampal neurons following consolidation of conditioning, the role of awareness in conditioning, how blink conditioning can be used as a translational tool by assaying potential therapeutics for cognitive enhancement, how trace and delay classical conditioning may be used to investigate neurological disorders including Alzheimer's disease and schizophrenia, and how the 2 paradigms may be used to understand the relationship between declarative (explicit) and nondeclarative (implicit) memory systems.

Molecular mechanisms of memory in imprinting

Converging evidence implicates the intermediate and medial mesopallium (IMM) of the domestic chick forebrain in memory for a visual imprinting stimulus. During and after imprinting training, neuronal responsiveness in the IMM to the familiar stimulus exhibits a distinct temporal profile, suggesting several memory phases. We discuss the temporal progression of learning-related biochemical changes in the IMM, relative to the start of this electrophysiological profile. c-fos gene expression increases <15 min after training onset, followed by a learning-related increase in Fos expression, in neurons immunopositive for GABA, taurine and parvalbumin (not calbindin). Approximately simultaneously or shortly after, there are increases in phosphorylation level of glutamate (AMPA) receptor subunits and in releasable neurotransmitter pools of GABA and taurine. Later, the mean area of spine synapse post-synaptic densities, N-methyl-D-aspartate receptor number and phosphorylation level of further synaptic proteins are elevated. After ∼ 15 h, learning-related changes in amounts of several synaptic proteins are observed. The results indicate progression from transient/labile to trophic synaptic modification, culminating in stable recognition memory.

Overexpression of the NR2A subunit in the forebrain impairs long-term social recognition and non-social olfactory memory

Animals must recognize and remember conspecifics and potential mates, and distinguish these animals from potential heterospecific competitors and predators. Despite its necessity, aged animals are known to exhibit impaired social recognition memory. As the brain ages, the ratio of NR2A:NR2B in the brain increases over time and has been postulated to underlie the cognitive decline observed during the aging process. Here, we test the hypothesis that an increased NR2A:NR2B subunit ratio underlies long-term social recognition memory. Using transgenic overexpression of NR2A in the forebrain regions, we investigated the ability of these mice to learn and remember male and female conspecifics, mice of another strain and animals of another rodent species, the rat. Furthermore, due to the importance of olfaction in social recognition, we tested the olfactory memory in the NR2A transgenic mice. Our series of behavioral experiments revealed significant impairments in the NR2A transgenic mice in long-term social memory of both male and female conspecifics. Additionally, the NR2A transgenic mice are unable to recognize mice of another strain or rats. The NR2A transgenic mice also exhibited long-term memory impairments in the olfactory recognition task. Taken together, our results provide evidence that an increased NR2A:NR2B ratio in the forebrain leads to reduced long-term memory function, including the ethologically important memories such as social recognition and olfactory memory.

Social information embedded in vocalizations induces neurogenomic and behavioral responses

Social cues facilitate relationships within communities. Zebra finches form long-term stable mate pairs and produce offspring within a multi-family, multi-generational community that can include hundreds of birds. Males use song to communicate in this complex environment. Males sing as part of their courtship display but also abundantly throughout each day, suggesting a role for their vocal signature outside of a reproductive context. One advantage of a vocal social cue is that it can be exchanged when birds are out of visual contact, as regularly occurs in a zebra finch community. Previous works have demonstrated that females hearing song are affected by their social relationship to the bird singing it, and the immediate social context. Here, we probed the question of whether or not the song itself carried social information, as would be expected from the situations when males sing outside of view of the female. We quantified behavioral and neurogenomic responses to two songs we predicted would have distinct “attractive” qualities in adult females housed in either mixed sex or female-only social communities. Our results show that only mixed sex-housed females show distinctive behavioral and neurogenomic responses to attractive songs. These data are consistent with the idea that the acoustic properties of song carry social information, and that the current social situation modulates the neural and behavioral responses to these signals.

The role of p75NTR in cholinergic basal forebrain structure and function

The role of the p75 neurotrophin receptor (p75(NTR)) in adult cholinergic basal forebrain (cBF) neurons is unclear due to conflicting results from previous studies and to limitations of existing p75(NTR)-knock-out mouse models. In the present study we used a novel conditional knock-out line (ChAT-cre p75(in/in)) to assess the role of p75(NTR) in the cBF by eliminating p75(NTR) in choline acetyl-transferase-expressing cells. We show that the absence of p75(NTR) results in a lasting increase in cBF cell number, cell size, and cholinergic innervation to the cortex. Analysis of adult ChAT-cre p75(in/in) mice revealed that mutant animals show a similar loss of cBF neurons with age to that observed in wild-type animals, indicating that p75(NTR) does not play a significant role in mediating this age-related decline in cBF neuronal number. However, the increased cholinergic axonal innervation of the cortex, but not the hippocampus, corresponded to alterations in idiothetic but not allothetic navigation. These findings support a role for p75(NTR)-mediated regulation of cholinergic-dependent cognitive function, and suggest that the variability in previous reports of cBF neuron number may stem from limited spatial and temporal control of p75(NTR) expression in existing knock-out models.

NCAM deficiency in the mouse forebrain impairs innate and learned avoidance behaviours

The neural cell adhesion molecule (NCAM) has been implicated in the development and plasticity of neural circuits and the control of hippocampus- and amygdala-dependent learning and behaviour. Previous studies in constitutive NCAM null mutants identified emotional behaviour deficits related to disturbances of hippocampal and amygdala functions. Here, we studied these behaviours in mice conditionally deficient in NCAM in the postmigratory forebrain neurons. We report deficits in both innate and learned avoidance behaviours, as observed in elevated plus maze and passive avoidance tasks. In contrast, general locomotor activity, trait anxiety or neophobia were unaffected by the mutation. Altered avoidance behaviour of the conditional NCAM mutants was associated with a deficit in serotonergic signalling, as indicated by their reduced responsiveness to (±)-8-hydroxy-2-(dipropylamino)-tetralin-induced hypothermia. Another serotonin-dependent behaviour, namely intermale aggression that is massively increased in constitutively NCAM-deficient mice, was not affected in the forebrain-specific mutants. Our data suggest that genetically or environmentally induced changes of NCAM expression in the late postnatal and mature forebrain determine avoidance behaviour and serotonin (5-HT)1A receptor signalling.

Status-appropriate singing behavior, testosterone and androgen receptor immunolabeling in male European starlings (Sturnus vulgaris)

Vocalizations convey information about an individual's motivational, internal, and social status. As circumstances change, individuals respond by adjusting vocal behavior accordingly. In European starlings, a male that acquires a nest site socially dominates other males and dramatically increases courtship song. Although circulating testosterone is associated with social status and vocal production it is possible that steroid receptors fine-tune status-appropriate changes in behavior. Here we explored a possible role for androgen receptors. Male starlings that acquired nest sites produced high rates of courtship song. For a subset of males this occurred even in the absence of elevated circulating testosterone. Immunolabeling for androgen receptors (ARir) was highest in the medial preoptic nucleus (POM) in males with both a nest site and elevated testosterone. For HVC, ARir was higher in dominant males with high testosterone (males that sang longer songs) than dominant males with low testosterone (males that sang shorter songs). ARir in the dorsal medial portion of the nucleus intercollicularis (DM) was elevated in males with high testosterone irrespective of dominance status. Song bout length related positively to ARir in POM, HVC and DM, and testosterone concentrations related positively to ARir in POM and DM. Results suggest that the role of testosterone in vocal behavior differs across brain regions and support the hypothesis that testosterone in POM underlies motivation, testosterone in HVC relates to song quality, and testosterone in DM stimulates vocalizations. Our data also suggest that singing may influence AR independent of testosterone and that alternative androgen-independent pathways regulate status-appropriate singing behavior.