The caudal prethalamus: Inhibitory switchboard for behavioral control?

Prethalamic nuclei in the mammalian brain include the zona incerta, the ventral lateral geniculate nucleus, and the intergeniculate leaflet, which provide long-range inhibition to many targets in the midbrain, hindbrain, and thalamus. These nuclei in the caudal prethalamus can integrate sensory and non-sensory information, and together they exert powerful inhibitory control over a wide range of brain functions and behaviors that encompass most aspects of the behavioral repertoire of mammals, including sleep, circadian rhythms, feeding, drinking, predator avoidance, and exploration. In this perspective, we highlight the evidence for this wide-ranging control and lay out the hypothesis that one role of caudal prethalamic nuclei may be that of a behavioral switchboard that-depending on the sensory input, the behavioral context, and the state of the animal-can promote a behavioral strategy and suppress alternative, competing behaviors by modulating inhibitory drive onto diverse target areas.

Pax6 loss alters the morphological and electrophysiological development of mouse prethalamic neurons.

Pax6 is a well-known regulator of early neuroepithelial progenitor development. Its constitutive loss has a particularly strong effect on the developing prethalamus, causing it to become extremely hypoplastic. To overcome this difficulty in studying the long-term consequences of Pax6 loss for prethalamic development, we used conditional mutagenesis to delete Pax6 at the onset of neurogenesis and studied the developmental potential of the mutant prethalamic neurons in vitro. We found that Pax6 loss affected their rates of neurite elongation, the location and length of their axon initial segments, and their electrophysiological properties. Our results broaden our understanding of the long-term consequences of Pax6 deletion in the developing mouse forebrain, suggesting that it can have cell-autonomous effects on the structural and functional development of some neurons.

Single-cell analysis of early chick hypothalamic development reveals that hypothalamic cells are induced from prethalamic-like progenitors.

The hypothalamus regulates many innate behaviors, but its development remains poorly understood. Here, we used single-cell RNA sequencing (RNA-seq) and hybridization chain reaction (HCR) to profile multiple stages of early hypothalamic development in the chick. Hypothalamic neuroepithelial cells are initially induced from prethalamic-like cells. Two distinct hypothalamic progenitor populations then emerge and give rise to tuberal and mammillary/paraventricular hypothalamic cells. At later stages, the regional organization of the chick and mouse hypothalamus is highly similar. We identify selective markers for major subdivisions of the developing chick hypothalamus and many previously uncharacterized candidate regulators of hypothalamic induction, regionalization, and neurogenesis. As proof of concept for the power of the dataset, we demonstrate that prethalamus-derived follistatin inhibits hypothalamic induction. This study clarifies the organization of the nascent hypothalamus and identifies molecular mechanisms that may control its induction and subsequent development.

Flexible inhibitory control of visually evoked defensive behavior by the ventral lateral geniculate nucleus.

Animals can choose to act upon, or to ignore, sensory stimuli, depending on circumstance and prior knowledge. This flexibility is thought to depend on neural inhibition, through suppression of inappropriate and disinhibition of appropriate actions. Here, we identified the ventral lateral geniculate nucleus (vLGN), an inhibitory prethalamic area, as a critical node for control of visually evoked defensive responses in mice. The activity of vLGN projections to the medial superior colliculus (mSC) is modulated by previous experience of threatening stimuli, tracks the perceived threat level in the environment, and is low prior to escape from a visual threat. Optogenetic stimulation of the vLGN abolishes escape responses, and suppressing its activity lowers the threshold for escape and increases risk-avoidance behavior. The vLGN most strongly affects visual threat responses, potentially via modality-specific inhibition of mSC circuits. Thus, inhibitory vLGN circuits control defensive behavior, depending on an animal's prior experience and its anticipation of danger in the environment.

LacZ-reporter mapping of Dlx5/6 expression and genoarchitectural analysis of the postnatal mouse prethalamus.

We present here a thorough and complete analysis of mouse P0-P140 prethalamic histogenetic subdivisions and corresponding nuclear derivatives, in the context of local tract landmarks. The study used as fundamental material brains from a transgenic mouse line that expresses LacZ under the control of an intragenic enhancer of Dlx5 and Dlx6 (Dlx5/6-LacZ). Subtle shadings of LacZ signal, jointly with pan-DLX immunoreaction, and several other ancillary protein or RNA markers, including Calb2 and Nkx2.2 ISH (for the prethalamic eminence, and derivatives of the rostral zona limitans shell domain, respectively) were mapped across the prethalamus. The resulting model of the prethalamic region postulates tetrapartite rostrocaudal and dorsoventral subdivisions, as well as a tripartite radial stratification, each cell population showing a characteristic molecular profile. Some novel nuclei are proposed, and some instances of potential tangential cell migration were noted.

Histogenetic Radial Models as Aids to Understanding Complex Brain Structures: The Amygdalar Radial Model as a Recent Example.

The radial dimension expands during central nervous system development after the proliferative neuroepithelium is molecularly patterned. The process is associated with neurogenesis, radial glia scaffolding, and migration of immature neurons into the developing mantle stratum. Radial histogenetic units, defined as a delimited neural polyclone whose cells share the same molecular profile, are molded during these processes, and usually become roughly stratified into periventricular, intermediate, and superficial (subpial) strata wherein neuronal cell types may differ and be distributed in various patterns. Cell-cell adhesion or repulsion phenomena together with interaction with local intercellular matrix cues regulate the acquisition of nuclear, reticular, or layer histogenetic forms in such strata. Finally, the progressive addition of inputs and outputs soon follows the purely neurogenetic and radial migratory phase. Frequently there is heterochrony in the radial development of adjacent histogenetic units, apart of peculiarities in differentiation due to non-shared aspects of the respective molecular profiles. Tangential migrations may add complexity to radial unit cytoarchitecture and function. The study of the contributions of such genetically controlled radial histogenetic units to the emerging complex neural structure is a key instrument to understand central nervous system morphology and function. One recent example in this scenario is the recently proposed radial model of the mouse pallial amygdala. This is theoretically valid generally in mammals (Garcia-Calero et al., 2020), and subdivides the nuclear complex of the pallial amygdala into five main radial units. The approach applies a novel ad hoc amygdalar section plane, given the observed obliquity of the amygdalar radial glial framework. The general relevance of radial unit studies for clarifying structural analysis of all complex brain regions such as the pallial amygdala is discussed, with additional examples.

Topological atlas of the hypothalamus in adult rhesus monkey.

The prosomeric model explains the embryological development of the central nervous system (CNS) shared by all vertebrates as a Bauplan. As a primary event, the early neural plate is patterned by intersecting longitudinal plates and transverse segments, forming a mosaic of progenitor units. The hypothalamus is specified by three prosomeres (hp1, hp2, and the acroterminal domain) of the secondary prosencephalon with corresponding alar and basal plate parts, which develop apart from the diencephalon. Mounting evidence suggests that progenitor units within alar and basal plate parts of hp1 and hp2 give rise to distinct hypothalamic nuclei, which preserve their relative invariant positioning (topology) in the adult brain. Nonetheless, the principles of the prosomeric model have not been applied so far to the hypothalamus of adult primates. We parcellated hypothalamic nuclei in adult rhesus monkeys (Macaca mulatta) using various stains to view architectonic boundaries. We then analyzed the topological relations of hypothalamic nuclei and adjacent hypothalamic landmarks with homology across rodent and primate species to trace the origin of adult hypothalamic nuclei to the alar or basal plate components of hp1 and hp2. We generated a novel atlas of the hypothalamus of the adult rhesus monkey with developmental ontologies for each hypothalamic nucleus. The result is a systematic reinterpretation of the adult hypothalamus whose prosomeric ontology can be used to study relationships between the hypothalamus and other regions of the CNS. Further, our atlas may serve as a tool to predict causal patterns in physiological and pathological pathways involving the hypothalamus.

Zic4-Lineage Cells Increase Their Contribution to Visual Thalamic Nuclei during Murine Embryogenesis If They Are Homozygous or Heterozygous for Loss of Pax6 Function.

Our aim was to study the mechanisms that contribute to the development of discrete thalamic nuclei during mouse embryogenesis (both sexes included). We characterized the expression of the transcription factor coding gene Zic4 and the distribution of cells that expressed Zic4 in their lineage. We used genetic fate mapping to show that Zic4-lineage cells mainly contribute to a subset of thalamic nuclei, in particular the lateral geniculate nuclei (LGNs), which are crucial components of the visual pathway. We observed that almost all Zic4-lineage diencephalic progenitors express the transcription factor Pax6 at variable location-dependent levels. We used conditional mutagenesis to delete either one or both copies of Pax6 from Zic4-lineage cells. We found that Zic4-lineage cells carrying either homozygous or heterozygous loss of Pax6 contributed in abnormally high numbers to one or both of the main lateral geniculate nuclei (LGNs). This could not be attributed to a change in cell production and was likely due to altered sorting of thalamic cells. Our results indicate that positional information encoded by the levels of Pax6 in diencephalic progenitors is an important determinant of the eventual locations of their daughter cells.

Foxd1 is required for terminal differentiation of anterior hypothalamic neuronal subtypes.

Although the hypothalamus functions as a master homeostat for many behaviors, little is known about the transcriptional networks that control its development. To investigate this question, we analyzed mice deficient for the Forkhead domain transcription factor Foxd1. Foxd1 is selectively expressed in neuroepithelial cells of the prethalamus and hypothalamus prior to the onset of neurogenesis, and is later restricted to neural progenitors of the prethalamus and anterior hypothalamus. During early stages of neurogenesis, we observed that Foxd1-deficient mice showed reduced expression of Six3 and Vax1 in anterior hypothalamus, but overall patterning of the prethalamus and hypothalamus is unaffected. After neurogenesis is complete, however, a progressive reduction and eventual loss of expression of molecular markers of the suprachiasmatic, paraventricular and periventricular hypothalamic is observed. These findings demonstrate that Foxd1 acts in hypothalamic progenitors to allow sustained expression of a subset of genes selectively expressed in mature neurons of the anterior hypothalamus.

An ontologically consistent MRI-based atlas of the mouse diencephalon.

In topological terms, the diencephalon lies between the hypothalamus and the midbrain. It is made up of three segments, prosomere 1 (pretectum), prosomere 2 (thalamus), and prosomere 3 (the prethalamus). A number of MRI-based atlases of different parts of the mouse brain have already been published, but none of them displays the segments the diencephalon and their component nuclei. In this study we present a new volumetric atlas identifying 89 structures in the diencephalon of the male C57BL/6J 12 week mouse. This atlas is based on an average of MR scans of 18 mouse brains imaged with a 16.4T scanner. This atlas is available for download at www.imaging.org.au/AMBMC. Additionally, we have created an FSL package to enable nonlinear registration of novel data sets to the AMBMC model and subsequent automatic segmentation.

Celsr3 and Fzd3 Organize a Pioneer Neuron Scaffold to Steer Growing Thalamocortical Axons.

Celsr3 and Fzd3 regulate the development of reciprocal thalamocortical projections independently of their expression in cortical or thalamic neurons. To understand this cell non autonomous mechanism further, we tested whether Celsr3 and Fzd3 could act via Isl1-positive guidepost cells. Isl1-positive cells appear in the forebrain at embryonic day (E) 9.5-E10.5 and, from E12.5, they form 2 contingents in ventral telencephalon and prethalamus. In control mice, corticothalamic axons run in the ventral telencephalic corridor in close contact with Isl1-positive cells. When Celsr3 or Fzd3 is inactivated in Isl1-expressing cells, corticofugal fibers stall and loop in the ventral telencephalic corridor of high Isl1 expression, and thalamic axons fail to cross the diencephalon-telencephalon junction (DTJ). At E12.5, before thalamic and cortical axons emerge, pioneer projections from Isl1-positive cells cross the DTJ from both sides in control but not mutant embryos. These early projections appear to act like a bridge to guide later growing thalamic axons through the DTJ. Our data suggest that Celsr3 and Fzd3 orchestrate the formation of a scaffold of pioneer neurons and their axons. This scaffold extends from prethalamus to ventral telencephalon and subcortex, and steers reciprocal corticothalamic fibers.